Electrocatalytic Activity of Au/tio2 Nanocomposites in Oxygen Electroreduction Reaction

Electrocatalytic Activity of Au/tio2 Nanocomposites in Oxygen Electroreduction Reaction | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Electrocatalytic Activity of Au/tio 2 Nanocomposites in Oxygen Electroreduction Reaction E. A. Ovodok, M. I. Ivanovskaya, S. K. Poznyak, A. M. Mal'tanova, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8246742/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 4 You are reading this latest preprint version Abstract Herein, Au NPs/TiO 2 composites were obtained by adding an aqueous solution of HAuCl 4 or pre-synthesized gold nanoparticle (Au NP) sols to the TiO 2 sol, followed by annealing the films deposited on a substrate. The size of Au NPs, structural features of Au/TiO 2 composites, their surface modification with different functional groups and the interaction between Au NPs and TiO 2 matrix were studied by TEM, XPS, and FTIR. The activity of bare TiO 2 and Au NPs/TiO 2 films was evaluated in electrochemical oxygen reduction reaction (ORR). The influence of Au NPs size, concentration and chemical state of gold on improving electrocatalytic properties of Au/TiO 2 films in ORR is shown. Doping the TiO 2 matrix with tetrazole-stabilized Au NPs of about 2 nm size leads to a significant reduction in overvoltage and the appearance of an additional Au-assisted wave on cyclic voltammograms. The high activity of these Au/TiO 2 electrodes can be explained by the small size of Au NPs and the excessive negative charge on them. titanium dioxide gold nanoparticles electrocatalysis oxygen reduction XPS FTIR spectroscopy Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The ever-increasing demand for energy requires the development of high-performance energy conversion and storage devices, which is important but remains a major scientific challenge [ 1 ]. Energy conversion often involves the oxygen electroreduction reaction (ORR), in which oxygen molecules are reduced at the cathode of fuel cells or metal-air batteries [ 2 , 3 ]. The sluggish kinetics of ORR require the use of large amounts of expensive Pt-based electrocatalysts to achieve practical power densities [ 4 , 5 ]. Therefore, the search for efficient catalysts is a crucial scientific issue that holds significant implications for advancing sustainable energy solutions and improving industrial processes. Titanium dioxide in the form of films and powders with anchored noble metal nanoparticles is a promising catalytic material for various types of chemical reactions and electrochemical processes [ 6 – 10 ]. Significant interest in titanium dioxide is due to its unique physicochemical properties, including a large surface area, availability, high stability in both acidic and alkaline environments, and non-toxicity [ 11 ]. Additionally, noble metal nanoparticles supported on semiconductive oxides can significantly enhance catalytic activity due to the strong metal-support interaction (SMSI) effect, as demonstrated by studies of Pt/TiO 2 and Au/TiO 2 interfaces [ 12 – 14 ]. Catalytic activity of Au/TiO 2 composites is extensively studied in CO oxidation reactions and electrochemical oxygen reduction [ 12 , 15 – 20 ]. It has been shown that the adsorption, catalytic, and electrocatalytic properties of semiconductor materials with anchored gold nanoparticles (Au NPs) depend on the size of the Au particles and the nature of their interaction with the oxide support [ 12 , 15 – 20 ]. Moreover, the methods of synthesis of Au/TiO 2 composites, the surface area and the doping level of semiconductors can significantly alter their activity [ 21 ]. With respect to oxygen electroreduction, smaller Au nanoparticles generally exhibit higher activity compared to larger ones [ 18 , 19 , 22 – 24 ]. This is attributed to the increased surface area and higher proportion of low-coordinated surface atoms in smaller nanoparticles, which facilitates oxygen adsorption and its subsequent reduction [ 25 , 26 ]. For example, in the study [ 18 ], it was shown that the lowest ORR overpotential was observed for the Au/TiO 2 composite with 5 nm Au NPs compared to 15 and 25 nm. Also the ORR activity of carbon-supported Au NPs is 2.5 times higher when the particle size is reduced from 7 to 3 nm [ 23 ]. Other comprehensive data suggest that Au/TiO 2 composites with Au particle sizes of about 5 nm are of particular interest for study, since the electron properties of gold nanoparticles with sizes 2 − 5 nm differ significantly compared to the bulk metal, and the strongest interaction between Au and TiO 2 is observed when the Au particle sizes do not exceed 5 nm [ 15 , 16 , 27 ]. Mesoporous materials are widely used as catalysts and catalyst supports due to their large surface area, controllable pore size, and ease of functionalization, which together help suppress the migration and agglomeration of gold nanoparticles within the pore structure [ 21 ]. Moreover, encapsulation of Au NPs in mesoporous TiO 2 is potentially attractive owing to the combination of the catalytic properties of gold and the semiconducting and structural advantages of mesoporous titania. This configuration enhances the dispersion and thermal stability of Au NPs, and also improves mass transport and reactant accessibility [ 21 , 28 ]. Moreover, the synergistic interaction between Au NPs and TiO 2 support can modulate the electronic structure of active sites, thereby enhancing the catalytic activity and selectivity [ 29 ]. Furthermore, the plasmonic properties of gold extend the light absorption range of TiO 2 into the visible region, making such composites promising for photocatalytic applications such as pollutant degradation, O 2 reduction, and water splitting [ 21 , 30 ]. The structural tunability of mesoporous TiO 2 also enables the design of multifunctional catalysts by incorporating additional active sites or co-catalysts, further expanding their potential in advanced catalytic systems. The goal of this study is to obtain Au/TiO 2 films with Au particle sizes of 5 nm or less, and to determine the influence of the size and chemical state of gold on the electrocatalytic properties of titanium dioxide films in the oxygen reduction reaction in an alkaline medium. Experimental Materials. All chemicals were of analytical grade and were used as received without any further purification. All solutions were prepared with triple distilled water. Synthesis procedures . The TiO 2 · n H 2 O sol was synthesized via sol-gel method according to the procedure described in [ 31 ]. Titanium tetrachloride was slowly added to a 0.65 M HCl solution in an ice bath (0°C) under constant stirring for 30 minutes until it was completely dissolved. An aqueous solution of NH 4 OH (12 wt.%) was then added dropwise to the solution of TiCl 4 in HCl under vigorous stirring at 0°C until pH 4. Then, the resulting suspension was centrifuged, and the precipitate was rinsed with distilled water to remove chloride ions. After adding 0.9 mL of concentrated HNO 3 (65 wt. %) as a sol stabilizer, the precipitate was stirred to give a movable suspension. Then, the suspension was ultrasonically treated (frequency – 22 kHz) with an ultrasonic horn placed into a beaker with the suspension to obtain a transparent opalescent sol. The prepared sol contains 7–8 wt. % of TiO 2 and is stable for several months at room temperature. According to TEM measurement, the average size of TiO 2 nanoparticles was about 5 nm (SI, Fig. S1 ). A series of Au NPs/TiO 2 composites were prepared to elucidate the effects of the Au NPs size and concentration on the catalytic activity of Au NPs/TiO 2 electrodes in ORR and to provide different types of interaction between Au NPs and the oxide support. Colloidal Au NPs with an average diameter of 4.5 nm were prepared by HAuCl 4 reduction using sodium borohydride as a reducing agent and sodium citrate as a stabilizer [ 18 ]. First, 2 mL of 5 mM HAuCl 4 aqueous solution was mixed with 2 mL of 10 mM Na 3 Cit aqueous solution and diluted to 100 mL with distilled water. Next, 270 mL of 0.5 M NaBH 4 solution was added dropwise to the resultant solution under vigorous stirring, giving rise to a red colored Au hydrosol. The synthesis was carried out at 18 ºC. Colloidal 5-(2-mercaptoethyl)tetrazole stabilized Au NPs with an average size 1.9 nm were synthesized via the method described in [ 32 ]. The stabilizing ligand precursor, namely1,2-bis(2-(1H-tetrazol-5-yl)ethyl)disulfane, was prepared as described [ 33 ]. Initially, 20 mL of 30 µM aqueous solution of the ligand precursor was prepared, followed by the addition of 100 µL of 1 M NaOH solution to adjust the pH to 8–9. In the next step, 200 µL of 50 mM HAuCl 4 solution was mixed with 20 mL of 30 µM of aqueous 1,2-bis(2-(1H-tetrazol-5-yl)ethyl)disulfane solution. Then, a freshly prepared aqueous solution of sodium borohydride (0.4 mL, 0.05 M) was added dropwise to the resultant solution under vigorous stirring, giving rise to a brown colored Au hydrosol. At the same time, the disulfide bond of 1,2-bis(2-(1H-tetrazol-5yl)ethyl)disulfane is also reduced with the formation of 5-(2-mercaptoethyl)tetrazole ligand. The synthesis was carried out at 18 ºC. After stirring for 1 h, the reaction mixture was concentrated under vacuum and the obtained residue was precipitated three times with isopropanol and finally redispersed in distilled water. TEM results of synthesized Au NPs are presented in SI (Figs. S2 and S3). Combined sols, TiO 2 · n H 2 O + Au NPs, were prepared by mixing individual sols, TiO 2 · n H 2 O and Au NPs stabilized by citrate ions, TiO 2 · n H 2 O and Au NPs stabilized by 5-(2-mercaptoethyl)tetrazole, as well as TiO 2 · n H 2 O and HAuCl 4 solution, to obtain the final TiO 2 /Au composite with gold content of 0.5, 1.0 and 2.0 wt. %. To obtain powdered samples, TiO 2 · n H 2 O and TiO 2 · n H 2 O + Au NPs sols were dried at 50 °С and then annealed at 400 °С during 1 h. Thin film samples of TiO 2 and Au/TiO 2 were deposited onto glass and FTO substrates by spin coating of the corresponding sols. To obtain high-quality films, the procedure of sol application onto the substrate was repeated five times; each time after application, the sample was heated at 100 °С. The resulting thin films were finally annealed at 400°C for 1 h. The samples prepared by doping TiO 2 ·nH 2 O sol with HAuCl 4 contain Au NPs formed via thermal decomposition of HAuCl 4 within the TiO 2 matrix. The simultaneous in situ dehydration of TiO 2 ·nH 2 O and the Au NPs formation can facilitate strong interfacial interactions at the Au/TiO 2 heterojunction. Such interactions are critical for efficient charge transfer across the Au/TiO 2 interface, which plays a pivotal role in enhancing the electrocatalytic performance. Table 1 presents the data for the synthesized TiO 2 /Au samples. In sample 4, Au NPs were undetectable by TEM. The size of the Au nanoparticles (NPs) in this sample was estimated indirectly through UV-vis spectroscopy. Table 1 The characterization of Au/TiO 2 samples Number of the sample Composition The size of Au NPs, nm Doping method 1 TiO 2 - - 2 Au NPs/TiO 2 5 ± 0,3 Addition of Au NPs sol stabilized by sodium citrate 3 Au NPs /TiO 2 2 ± 0,2 Addition of Au NPs sol stabilized by 5-(2-mercaptoethyl)tetrazole 4 Au NPs/TiO 2 < 2 nm* Addition of HAuCl 4 solution * Estimated size based on UV-vis spectroscopy Characterization. FTIR- and optical spectroscopies, XPS, TEM were used to study the size and the chemical state of Au, and the type of interaction between components at Au/TiO 2 interface. XPS analysis was performed on a Kratos DLD Ultra spectrometer using Al Kα monochromatized radiation (E = 1486.6 eV). For survey spectra, a pass energy (PE) of 160 eV was used while for regions the PE was 20 eV. XPS spectra were recorded before and after Ar + etching at different sputtering time (sputter rate 8 nm/min calibrated on Ta 2 O 5 ). All binding energies (BEs) were referenced to the C 1s peak (284.8 eV) of surface adventitious carbon. A LEO-906E transmission electron microscope and a Hitachi HD2700D scanning transmission electron microscope were utilized for TEM measurements. Fourier transform infrared (FTIR) spectra were collected on an AVATAR-330 (Thermo Nicolet) spectrometer supplied with a diffuse reflectance accessory in the wavenumber range from 400 to 4000 cm − 1 . A Shimadzu UV 2550 spectrophotometer was used to record optical spectra. The electrocatalytic activity of TiO 2 and Au/TiO 2 films on FTO substrates in the oxygen reduction reaction was studied by the cyclic voltammetry (CV) method using an Autolab PGSTAT 302N potentiostat-galvanostat (the Netherlands) in a 0.1 M KOH solution saturated with oxygen during 1 h. Electrochemical experiments were performed in a standard three-electrode electrochemical cell, where an Hg/HgO electrode filled with 1 M KOH (Radiometer Analytical) was applied as a reference electrode (all potentials in this study are given relative to this reference electrode), and platinum foil was applied as an auxiliary electrode. The potential sweep rate was 10 mV/s. Results and Discussion X-ray photoelectron spectroscopy. In the XPS spectrum of sample 1, a single, fairly narrow (FWHM = 1 eV) peak of Ti 2p 3/2 with a maximum of 458.8 eV is recorded, which can be attributed to the Ti(IV) state in TiO 2 . The asymmetric O 1s peak is split into two peaks with the binding energy (BE) equal to 530.2 eV (84%) and 531.8 eV (16%). The first peak is attributed to oxygen in the TiO 2 lattice, while the second peak is most likely associated with oxygen in hydroxyl groups. The O/Ti ratio (1.96) is less than the stoichiometric composition of TiO 2 , which may confirm the presence of OH groups on the surface of the films and oxygen vacancies. The Au 3d peaks in the spectra of samples 2–4 are low-intensity and wide (FWHM Au 3d 5/2 ~ 5.6 eV in sample 3 and FWHM Au 3d 5/2 ~ 6.9 eV in sample 4). The Au 4f peaks are more informative (Fig. 1). They are narrower, more intense, and are separated into two components in the XPS spectra of samples 3 and 4. According to the spectrum of Au 4f (Fig. 1, a), sample 2 contains only Au 0 . The spectra of samples 3 and 4 indicate the presence of gold in two different states. In sample 3, the binding energy of one of the Au states is lower, while in sample 4 it is higher than the BE of the metal (Fig. 1, b, c). The Au 4f binding energies and the quantitative ratio of gold in these states are shown in Table 2. According to XPS data, gold in sample 4 is present in both the zero oxidation state and the oxidized state (Fig. 1, c and Table 2). Gold was introduced during sample 4 preparation as AuCl 4- ions, not in a colloidal state. Thermal treatment of the sample simultaneously leads to dehydration and crystallization of the titanium oxide phase and a thermal transformation from Au(III) to Au(0), which contributes to a strong mutual influence of the components on each other's electronic and valence states. These processes may result in the presence of gold in the oxidized state (Au 3+ ). The positive shift in the binding energy of Au 4f 7/2 is approximately 3 eV relative to Au(0), which presumably indicates the presence of Au 3+ (Table 2). It should be noted that the Au 4f 7/2 and Au 4f 5/2 peaks in sample 4 are very broad, especially the peak of the oxidized state (FWHM = 4.5 eV). Given the large width of this peak, the presence of gold in the Au 1+ state cannot be ruled out. In sample 3, along with Au(0), the Au 4f XPS spectrum also shows gold with an excess negative charge (Fig. 1 and Table 2). The negative chemical shift of the Au 4f electron spectrum observed in sample 3 generally indicates the appearance of excess electron density on the Au nanoparticles, which may be due to its transfer from the oxide carrier to the surface of the gold particles: Au δ - ←Ti δ + TiO 2–x . An alternative explanation for this shift may be the quantum size effect, which is characterized by an increased population of p-AO compared to the d10s1p0 configuration [34]. In Au clusters, a partial transition of electron density from the d-orbital to the p-orbital can occur: [d10s1] → [d8s1p2], which causes a change in the electron binding energy in the clusters relative to metallic gold. A change in the state of Au d-orbitals with a decrease in electron density can contribute to increased activity of Au clusters in the adsorption of molecules and the catalysis of reactions involving them. In [16] it is noted that even a small transfer of electron density can cause a change in the state of Au–Au bonds, a change in the distances and symmetry of d-orbitals, which will affect the nature of oxygen adsorption. In sample 3, the Au δ − content accounts for just over half of the surface gold atoms. This fact, regardless of the mechanism by which excess electron density arises on gold particles in sample 3, indicates a significant change in the energy state of its orbitals. It should be noted that the gold phase is not detectable by X-ray diffraction in samples 2-4. This is likely due to the low Au content (0.5 wt%) in the samples. In the optical spectrum of sample 2, the maximum of the surface plasmon resonance (SPR) peak is observed at 514 nm. The optical spectra of films of samples 3 and 4 show a low-intensity and broad SPR peak in the range of 500–600 nm. This shape of the SPR peak confirms the high dispersion of gold particles; their size may be less than 2 nm [27, 35]. In the XPS spectrum of sample 3, the Ti 2p 3/2 peak has an asymmetric shape. In addition to the Ti(IV) state in TiO 2 (458.8 eV), it contains a state with a higher binding energy (459.8 eV; approximately 30%). This value of the Ti 2p 3/2 energy may correspond to a more ionic state of Ti atoms than in titanium dioxide. Since an S-containing substance (5-(2-mercaptoethyl)-tetrazole) was used to stabilize the gold particles in this sample, the presence of Ti-O–SO x groups on the titanium dioxide surface can be expected. The presence of sulfur in sample 3 is also indicated by the XPS spectrum (Fig. 1, g), which contains a peak of unresolved S 2p 3/2 and S 2p 1/2 lines with a maximum BE of 168.4 eV. According to reference data [36], the BE of 168.4 eV of S 2p-level electrons can be assigned to SO 3 2– ions, as well as –SO 2 CH 3 and other more complex groups, for example, (H 2 C) 2 SO – . The binding energy of the O 1s level in such groups is 532–533 eV. Characteristically sample 3 has a significantly higher oxygen content with an increased O 1s binding energy of 531.9 eV (up to 80%) compared to the BE of 530.0 eV (20%). At the same time, in sample 1 (pure TiO 2 ), the oxygen content with the BE of 531.9 eV is only 16%. The S-containing groups can arise in sample 3 during the degradation and oxidation of the stabilizer, 5-(2-mercaptoethyl)-tetrazole, during thermal treatment. They may participate in the attachment of gold particles to TiO 2 surface. According to the thermal analysis data presented in [32], –CN 4 and –СН 3 fragments are released from the sol of Au nanoparticles stabilized by5-(2-mercaptoethyl)-tetrazole upon heating to 400 °C. The removal of S-containing fragments begins only above 500 °C and is completed at 600 °C. These results indicate the presence of S-containing groups in sample 3 and the formation of a non-stoichiometric TiO 2–x structure. A strong interaction with the transfer of electron density to Au nanoparticles can be observed between such titanium dioxide and highly dispersed gold particles. Table 2. Comparison of Au 4f XPS spectra for Au/TiO 2 samples Sample BE Au 4f 7/2 , eV Au content, at. % BE Au 4f 5/2 , eV O/Ti ratio, at. % Au state #2, TiО 2 + Au NPs stabilized by citrate 83,85 100 86,7 2,40 Au 0 #3, TiО 2 + Au NPs stabilized by tetrazole 82,9 83,9 54 45 86,5 87,3 2,14 Au δ - (Au←Ti) Au 0 #4, TiО 2 + HAuCl 4 83,9 87,2 68 32 87,0 90,2 1,96 Au 0 Au 3+ (Au 3+ –O–Ti) Infrared spectroscopy . IR spectroscopy data confirm the conclusions drawn from the XPS spectra. In the IR spectrum of sample 1, in addition to a broad absorption band of the characteristic vibrations of Ti–O (400–800 cm–1), intense absorption bands are recorded related to the stretching d(O−H, H 2 O) at 3425 cm –1 and the deformation d(H–O–H) at 1630 cm –1 vibrations (Fig. 2, a). Weak absorptions at 1125 and 1295 cm –1 can be attributed to the d(Ti–O–H) vibrations. (Fig. 2, a). These data evidence the presence of structural water and surface hydroxyl groups in sol-gel derived TiO 2 . For samples 2-4, additional bands are observed in the IR spectra in the region 990–1530 cm –1 (Fig. 2, b ). The spectrum of sample 2 displays absorption bands related to bond vibrations in carbonate-carboxylate groups. Typically, carboxyl groups bound to metals exhibit two characteristic vibrational series, related to the asymmetric n а s (COO − ) stretching vibrations and the symmetric n s (COO − ) stretching vibrations of the C–O bonds. The appearance of such oscillations in the spectrum of sample 2 is entirely expected, since citrate ions were used to stabilize gold particles, the thermal transformation products of which contain –COO – groups. The spectrum of sample 2 shows the following vibrations: n а s (COO − ) = 1531 cm –1 ; n s (COO − )= 1375 cm –1 ; n а s (COO − ) = 1435 cm –1 (monodentate carbonate CO 3 2– ). The IR spectrum of sample 3 shows a weak absorption band at 990 cm –1 and fairly intense bands at 1078, 1135 and 1350 cm –1 . These bands can be attributed to vibrations of S=O bonds, which are in the region of 990 – 1420 cm –1 [37]. These results confirm the XPS data on the retention of sulfur in the sample 3 in the form of a compound with oxygen, which arises as a result of thermal destruction and oxidation of the stabilizer – 5-(2-mercaptoethyl)-tetrazole. These can be either individual sulfite groups or sulfoxide groups >S=O. The IR spectrum of sample 4, obtained by introducing HAuCl 4 into the TiO 2 ·nH 2 O sol, shows a reduced content of hydroxyl groups on the surface compared to sample 1. The d(Ti–O–H) vibrations at 1125 and 1295 cm –1 are not detected, while the n(O–H, H 2 O) at 3425 cm –1 and d(H–O–H) at 1630 cm –1 decrease in intensity. The introduction of acid apparently promotes dehydroxylation of the titanium dioxide surface. It can be assumed that the AuCl 4 – ions in the sol are coordinated around the positively charged nuclei of hydrated titanium dioxide particles. During subsequent drying and heating of the composite TiO 2 ·nH 2 O + HAuCl 4 sol, the processes of HAuCl 4 decomposition and crystallization of Au particles slow down as a result of strong dilution in the formed xerogel. Thus, using different synthesis methods, Au/TiO 2 samples were prepared that differed in the size of gold particles, the nature of their interaction with titanium dioxide, and modification of its surface with different functional groups. Electrocatalytic measurements. The performance of TiO 2 electrode and Au/TiO 2 electrodes (Au 0.5, 1.0 and 2.0 % wt.%) as electrocatalysts for ORR was examined by cyclic voltammetry in 0.1 M KOH saturated with oxygen. To compare the CV curves for different electrodes, in the further consideration, we chose CV curves related to the ninth cycle (hereafter referred to as quasi-steady-state CVs), since after the ninth cycle the change in the CV curves basically stopped. The electrocatalytic activity of electrodes was evaluated via analysis of the ORR peak potential ( E p ), the half-wave potential ( Е 1/2 ) and the current density at the ORR peak potential ( j p ) and at the half-wave potential ( j 1/2 ). Figure 3 shows typical quasi-stationary CV curves for bare TiO 2 and different Au/TiO 2 electrodes with the same Au content (0.5 wt.%). The bare TiO 2 electrode demonstrates an irreversible wave of oxygen reduction current with j p at -0.78 V and Е 1/2 at -0.70 V. For Au/TiO 2 electrodes, the TiO 2 -assisted ORR wave remains unchanged (as for sample 3) or shifts insignificantly (by 20 mV) in the positive direction for samples 2 and 4. Additionally, the j 1/2 becomes slightly higher, with the maximum current density for sample 2 (Fig. 3 and Table 3). It should be noted that the shoulder arises at the leading edge of the current wave after gold doping, probably due to the ORR on the surface of gold nanoparticles. As a result, the ORR overvoltage decreases approximately by 92–100 mV for samples 2 and 3 and by 36 mV for sample 4 (Fig. 3 and Table 3). For sample 3, additional waves corresponding to ORR on the surface of Au NPs appear at less negative potentials ( Е > -0.6 V). It is worth mentioning that the minimum ORR overvoltage is observed for Au/TiO 2 electrodes at lower current densities (Table 3). Table 3. Parameters of electrochemical oxygen reduction on thin-film TiO 2 and Au/TiO 2 electrodes Sample E p , V j p , mA cm -2 E 1/2 , V j 1/2 , mA cm -2 E at j =0.04 mA cm -2 , V #1, TiО 2 -0.780 -0.248 -0.700 -0.124 -0.590 #2, TiО 2 + Au NPs stabilized by citrate -0.757 -0.305 -0.600 -0.152 -0.444 #3, TiО 2 + Au NPs stabilized by tetrazole -0.785 -0.278 -0.608 -0.139 -0.370 #4, TiО 2 + HAuCl 4 -0.759 -0.271 -0.664 -0.135 -0.519 To evaluate the effect of the Au NPs concentration on the efficiency of Au/TiO 2 electrodes in ORR, electrodes (sample 3) with a final gold concentration of 0.5, 1.0 and 2.0 wt. %. were investigated. We observed a significant decrease in the ORR overvoltage and an increase of current density with increasing the concentration of Au NPs. The TiO 2 electrode with an Au NPs concentration of 2 wt. % demonstrates a decrease in overvoltage by 220 mV at a current density of 0.11 mA cm -2 and by 350 mV at 0.035 mA cm -2 (Fig. 4). It is worth mentioning that a well-defined Au-catalyzed cathodic current wave was developed starting from 1 wt. % of Au NPs. The Au/TiO 2 composite with 2 wt.% Au NP S also demonstrates a shift of TiO 2 -assisted wave in the negative direction. The presented experimental data demonstrate that the TiO 2 doped with Au NPs from sols have a higher catalytic activity in ORR compared to the composite obtained by thermal decomposition of HAuCl 4 within the TiO 2 matrix. Although sample 4 contains small Au NPs, the decrease of ORR activity may be related to the oxidized state of Au. This occurs due to incomplete reduction of Au(III) to Au(0) during annealing of the HAuCl 4 -doped TiO 2 sol-gel matrix at 400°С. Also, the surface of citrate-stabilized Au NPs contains adsorbed carbonate-carboxylated groups with donor properties. This may lead to higher oxygen adsorption efficiency and facilitate charge transfer to the adsorbed oxygen molecules, which is favorable for oxygen activation 3 . Notably, sample 3 with high Au NP content exhibits an additional ORR wave at more positive potentials (see Fig. 3 and Fig. 4). This wave can be explained by the different Au NPs sizes appeared after thermal treatment of such electrodes. At high Au NPs concentrations, small Au NPs can aggregate to form larger particles. It has been previously shown that Au NP particles of different sizes can produce an ORR wave at different potentials [18, 19]. The observed effect may also be due to the increased activity of gold particles, since there is excessive electron density on the Au NPs of sample 3, as evidenced by the XPS data (see Fig. 1, c ). Conclusion Au/TiO 2 electrocatalysts have been prepared by doping of TiO 2 sol with Au NPs and characterized using TEM, XPS, and FTIR. The results show that the prepared samples differ in Au NPs size, the mode of interaction between the gold nanoparticles and the oxide, and surface modification by various functional groups and ions. Electrocatalytic activity of Au/TiO 2 electrodes toward oxygen reduction reaction (ORR) has been examined by cyclic voltammetry (CV) in an alkaline medium. The electrocatalytic efficiency of Au NPs-doped TiO 2 electrodes in ORR was found to depend on various factors such as the size of Au NPs, their loading amount, the state of Au and the nature of stabilizing ligands used in the preparation of Au sols. The highest efficiency was demonstrated for the Au/TiO 2 sample prepared by doping the TiO2 sol with 5-(2-mercaptoethyl)tetrazole-stabilized Au NPs. Moreover, the ORR overvoltage for such electrodes was found to decrease with increasing the Au NPs concentration. The high activity of such composite can be explained by small size of Au NPs (2 nm) and the excessive negative charge on gold particles. Declarations Conflict of interest The authors declare no competing financial interest. Ethical approval This declaration is not applicable. Funding This project is funded by the Belarusian Republican Foundation for Fundamental Research (grants № T23RNFM-35) and Russian Science Foundation (Grant No. 24-49-10012). Competing Interests: The authors have no competing interests to declare that are relevant to the content of this article. Author Contribution E.A. Ovodok: Conceptualization, Data curation, Visualization, Investigation. M.I. Ivanovskaya: Conceptualization, Supervision, Writing-Original draft preparation, Editing, Methodology. S.K. Poznyak: Investigation, Supervision, Writing-Reviewing. A.M. 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J Catal 144:175–192. https://doi.org/10.1006/jcat.1993.1322. Tsubota S, Cunningham DAH, Bando Y, Haruta M (1995) Preparation of nanometer gold strongly interacted with TiO2 and the structure sensitivity in low-temperature oxidation of CO. Stud Surf Sci Catal 91:227–235. https://doi.org/10.1016/S0167-2991(06)81759-3. Beard BC, Ross PN (1986) Characterization of a Titanium‐Promoted Supported Platinum Electrocatalyst. J Electrochem Soc 133:1839. DOI 10.1149/1.2109033. Haruta M (1997) Size- and support-dependency in the catalysis of gold. Catal Today 36:153–166. https://doi.org/10.1016/S0920-5861(96)00208-8. Cosandey F., Madey TE (2001) Growth, morphology, interfacial effects and catalytic properties of Au on TiO2. Surface Review and Letters 8:73-93. DOI: 10.1016/S0218-625X(01)00088-4. Guerin S, Hayden BE, Pletcher D et al (2006) A combinatorial approach to the study of particle size effects on supported electrocatalysts: Oxygen reduction on gold. J Comb Chem 8:679–686. https://doi.org/10.1021/cc060041c. Maltanava H, Poznyak S, Starykevich M, Ivanovskaya M (2016) Electrocatalytic activity of Au nanoparticles onto TiO2 nanotubular layers in oxygen electroreduction reaction: size and support effects. Electrochim Acta 222:1013–1020. https://doi.org/10.1016/j.electacta.2016.11.070. Maltanava H, Mazheika S, Starykevich M et al (2021) UV-assisted anchoring of gold nanoparticles into TiO2 nanotubes for oxygen electroreduction. J Electroanal Chem 904:115844. DOI:10.1016/j.jelechem.2021.115844. Shao M (2013) Electrocatalysis in fuel cells: a non-and low-platinum approach. 9. Springer Science & Business Media. DOI: 10.1007/978-1-4471-4911-8. Sankar M, He Q, Engel RV et al (2020) Role of the support in gold-containing nanoparticles as heterogeneous catalysts. Chemical reviews 120:3890-3938. DOI: 10.1021/acs.chemrev.9b00662. Zhang GR, Xu BQ, Tang W et al (2013) Nano-size effect of Au catalyst for electrochemical reduction of oxygen in alkaline electrolyte. Chinese J Catal 34:942–948. https://doi.org/10.1016/S1872-2067(12)60546-4. Tang W, Lin H, Kleiman-Shwarsctein A et al (2008) Size-dependent activity of gold nanoparticles for oxygen electroreduction in alkaline electrolyte. J Phys Chem C 112:10515–10519. https://doi.org/10.1021/jp710929n. Wang L, Tang Z, Yan W et al (2016) Porous carbon-supported gold nanoparticles for oxygen reduction reaction: effects of nanoparticle size. ACS Appl Mater Interfaces 8:20635–20641. https://doi.org/10.1021/acsami.6b02223. Ishida T, Murayama T, Taketoshi A, Haruta M (2020) Importance of size and contact structure of gold nanoparticles for the genesis of unique catalytic processes. Chemical reviews 120:464-525. DOI: 10.1021/acs.chemrev.9b00551. Xiao C, Lu BA, Xue P et al (2020) High-index-facet-and high-surface-energy nanocrystals of metals and metal oxides as highly efficient catalysts. Joule 4:2562-2598. DOI: 10.1016/j.joule.2020.10.002. Daniel MC, Astruc D (2004) Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem Rev 104:293–346. https://doi.org/10.1021/cr030698+. Hou J, Jang W, Yun J (2021) Systematic incorporation of gold nanoparticles onto mesoporous titanium oxide particles for green catalysts. Catalysts 11:451. https://doi.org/10.3390/catal11040451. Jang W, Yun J, Ludwig L et al Comparative catalytic properties of supported and encapsulated gold nanoparticles in homocoupling reactions. Front Chem 8:834. DOI:10.3389/fchem.2020.00834. Ishaq T, Ehsan Z, Qayyum A et al Recent strategies to improve the photocatalytic efficiency of TiO2 for enhanced water splitting to produce hydrogen. Catalysts 14:674. DOI:10.3390/catal14100674. Poznyak SK, Kokorin AI, Kulak AI (1998) Effect of electron and hole acceptors on the photoelectrochemical behaviour of nanocrystalline microporous TiO2 electrodes. J Electroanal Chem 442:99–105. https://doi.org/10.1016/S0022-0728(97)00458-0. Guhrenz C, Wolf A, Adam M et al (2017) Tetrazole-stabilized gold nanoparticles for catalytic applications. Zeitschrift fur Phys. Chemie 231:51–62. DOI: 10.1515/zpch-2016-0879. Voitekhovich SV, Wolf A, Guhrenz C et al (2016) 5‐(2‐Mercaptoethyl)‐1H‐tetrazole: Facile Synthesis and Application for the Preparation of Water Soluble Nanocrystals and Their Gels. Chem. - A Eur. J. 22:14746–14752. https://doi.org/10.1002/chem.201602980. Ermolov LV, Slinkin AA (1991) Strong metal-carrier interaction and its role in catalysis. Russ. Chem. Rev. 60:331–344. DOI 10.1070/RC1991v060n04ABEH000983. Jain PK, El-Sayed IH, El-Sayed MA (2007) Au nanoparticles target cancer. Nano today 2:18-29. DOI: 10.1016/S1748-0132(07)70016-6. Briggs D, Seah MP (1983) Practical surface analysis by Auger and X-ray photoelectron spectroscopy. John Wiley and Sons Ltd, Chichester. https://doi.org/10.1002/sia.740060611. Berger F, Beche E, Berjoan R et al (1996) An XPS and FTIR study of SO2 adsorption on SnO2 surfaces. Appl Surf Sci 93:9–16. https://doi.org/10.1016/0169-4332(95)00319-3. Additional Declarations No competing interests reported. Supplementary Files SupportingInformation.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 09 Dec, 2025 Editor assigned by journal 08 Dec, 2025 Submission checks completed at journal 08 Dec, 2025 First submitted to journal 01 Dec, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8246742","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":557725466,"identity":"4f93bbfd-4025-42fa-8ce3-87a0050e9005","order_by":0,"name":"E. A. Ovodok","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAx0lEQVRIiWNgGAWjYJACCYYDDAz8IFZCASlaJBtAWgxI0WJwAMQkRgu/RPLBGx/O2Mgbn1+d+OGBAYM8v9gB/FokZ6QlW864kWa47cbbzRJAhxnOnJ2AX4vBmTNm0jwfDjNuu3F2A0hLgsFt4rT8t9884+zmH8RpOd4D1HLjQOIG/t5txNki2d4G9MuZ5OQZN3i3WSQYSBD2Cz8zMzDEjtnZ9vef3XzzR4WNPL80AS0IIAFWKUGscrB9B0hRPQpGwSgYBSMJAABaaUg2GrhbfAAAAABJRU5ErkJggg==","orcid":"","institution":"Belarusian State University","correspondingAuthor":true,"prefix":"","firstName":"E.","middleName":"A.","lastName":"Ovodok","suffix":""},{"id":557725468,"identity":"0d418c82-707d-44e9-94ba-7db442da1c0a","order_by":1,"name":"M. I. Ivanovskaya","email":"","orcid":"","institution":"Belarusian State University","correspondingAuthor":false,"prefix":"","firstName":"M.","middleName":"I.","lastName":"Ivanovskaya","suffix":""},{"id":557725470,"identity":"a48596fc-24be-47d3-9351-9e7b087d48b9","order_by":2,"name":"S. K. Poznyak","email":"","orcid":"","institution":"Belarusian State University","correspondingAuthor":false,"prefix":"","firstName":"S.","middleName":"K.","lastName":"Poznyak","suffix":""},{"id":557725471,"identity":"1d9b960c-1ae6-46ac-b4bb-2e038fb704c3","order_by":3,"name":"A. M. Mal'tanova","email":"","orcid":"","institution":"Belarusian State University","correspondingAuthor":false,"prefix":"","firstName":"A.","middleName":"M.","lastName":"Mal'tanova","suffix":""},{"id":557725472,"identity":"34c1e8b8-0f2d-463a-9e1b-b953d4bb037c","order_by":4,"name":"S. V. Voitekhovich","email":"","orcid":"","institution":"Belarusian State University","correspondingAuthor":false,"prefix":"","firstName":"S.","middleName":"V.","lastName":"Voitekhovich","suffix":""},{"id":557725473,"identity":"1adaadd3-892b-47b9-bac0-5d9c3ca58bf1","order_by":5,"name":"T. V. Gaevskaya","email":"","orcid":"","institution":"Belarusian State University","correspondingAuthor":false,"prefix":"","firstName":"T.","middleName":"V.","lastName":"Gaevskaya","suffix":""},{"id":557725474,"identity":"c298e801-fa40-40ef-8d03-70eca735ff84","order_by":6,"name":"A. E. Seleznev","email":"","orcid":"","institution":"Moscow State University of Technology","correspondingAuthor":false,"prefix":"","firstName":"A.","middleName":"E.","lastName":"Seleznev","suffix":""}],"badges":[],"createdAt":"2025-12-01 06:38:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8246742/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8246742/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":100362473,"identity":"22546084-c373-485e-a670-dc89db631918","added_by":"auto","created_at":"2026-01-16 07:46:50","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":377273,"visible":true,"origin":"","legend":"\u003cp\u003eAu 4f XPS spectra of Au/TiO\u003csub\u003e2\u003c/sub\u003e samples 2 (a), 3 (b), 4 (c) and S 2p spectrum of sample 3 (d)\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8246742/v1/bdfab2fd1a374a0b710fb08a.png"},{"id":100028214,"identity":"ebe1790a-e47e-42ea-a209-806bb00b4dad","added_by":"auto","created_at":"2026-01-12 09:02:40","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":188045,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cem\u003eа\u003c/em\u003e) FTIR spectrum of TiO\u003csub\u003e2\u003c/sub\u003e sample 1; (\u003cem\u003eb\u003c/em\u003e) fragments of FTIR spectra of TiO\u003csub\u003e2 \u003c/sub\u003esample 1 (1), Au/TiO\u003csub\u003e2\u003c/sub\u003e samples 2(2), 3(3) and 4(4)\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8246742/v1/df5763c5941ec7bd6f68fd67.png"},{"id":100028212,"identity":"0b92ec9e-fec2-43f7-b0f3-0e2f638c8a99","added_by":"auto","created_at":"2026-01-12 09:02:40","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":186296,"visible":true,"origin":"","legend":"\u003cp\u003eCyclic voltammograms for TiO\u003csub\u003e2\u003c/sub\u003e electrode (sample 1 (1)) and Au/TiO2 electrodes\u003c/p\u003e\n\u003cp\u003e(samples 2 (2), 3 (3), and 4 (4))\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8246742/v1/1cc2d4525891b782760a63e4.png"},{"id":100028215,"identity":"528de559-8556-46e2-8095-e07f77027544","added_by":"auto","created_at":"2026-01-12 09:02:40","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":205063,"visible":true,"origin":"","legend":"\u003cp\u003eCyclic voltammograms for TiO\u003csub\u003e2\u003c/sub\u003e electrode (sample 1 (1)) and Au/TiO\u003csub\u003e2\u003c/sub\u003e electrodes with different concentrations of Au NPs stabilized by 5-(2-mercaptoethyl)tetrazole: 0.5 wt.% (2), 1 wt.% (3), 2 wt.% (4)\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8246742/v1/e3ab7e211dc576e558d96a3f.png"},{"id":100405914,"identity":"64fbad25-ea39-4a1b-a1a0-ee6478df1dd0","added_by":"auto","created_at":"2026-01-16 12:30:03","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1453886,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8246742/v1/cb5ff41c-e30e-48fd-9f4d-4993b1561151.pdf"},{"id":100028216,"identity":"8775f63c-9f28-49c1-9ce9-51d245b90e14","added_by":"auto","created_at":"2026-01-12 09:02:41","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":2525436,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-8246742/v1/e6cd715ab852067d6d84ac90.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eElectrocatalytic Activity of Au/tio\u003csub\u003e2\u003c/sub\u003e Nanocomposites in Oxygen Electroreduction Reaction\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe ever-increasing demand for energy requires the development of high-performance energy conversion and storage devices, which is important but remains a major scientific challenge [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Energy conversion often involves the oxygen electroreduction reaction (ORR), in which oxygen molecules are reduced at the cathode of fuel cells or metal-air batteries [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The sluggish kinetics of ORR require the use of large amounts of expensive Pt-based electrocatalysts to achieve practical power densities [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Therefore, the search for efficient catalysts is a crucial scientific issue that holds significant implications for advancing sustainable energy solutions and improving industrial processes.\u003c/p\u003e \u003cp\u003eTitanium dioxide in the form of films and powders with anchored noble metal nanoparticles is a promising catalytic material for various types of chemical reactions and electrochemical processes [\u003cspan additionalcitationids=\"CR7 CR8 CR9\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Significant interest in titanium dioxide is due to its unique physicochemical properties, including a large surface area, availability, high stability in both acidic and alkaline environments, and non-toxicity [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Additionally, noble metal nanoparticles supported on semiconductive oxides can significantly enhance catalytic activity due to the strong metal-support interaction (SMSI) effect, as demonstrated by studies of Pt/TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e interfaces [\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Catalytic activity of Au/TiO\u003csub\u003e2\u003c/sub\u003e composites is extensively studied in CO oxidation reactions and electrochemical oxygen reduction [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan additionalcitationids=\"CR16 CR17 CR18 CR19\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. It has been shown that the adsorption, catalytic, and electrocatalytic properties of semiconductor materials with anchored gold nanoparticles (Au NPs) depend on the size of the Au particles and the nature of their interaction with the oxide support [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan additionalcitationids=\"CR16 CR17 CR18 CR19\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Moreover, the methods of synthesis of Au/TiO\u003csub\u003e2\u003c/sub\u003e composites, the surface area and the doping level of semiconductors can significantly alter their activity [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWith respect to oxygen electroreduction, smaller Au nanoparticles generally exhibit higher activity compared to larger ones [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. This is attributed to the increased surface area and higher proportion of low-coordinated surface atoms in smaller nanoparticles, which facilitates oxygen adsorption and its subsequent reduction [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. For example, in the study [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], it was shown that the lowest ORR overpotential was observed for the Au/TiO\u003csub\u003e2\u003c/sub\u003e composite with 5 nm Au NPs compared to 15 and 25 nm. Also the ORR activity of carbon-supported Au NPs is 2.5 times higher when the particle size is reduced from 7 to 3 nm [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Other comprehensive data suggest that Au/TiO\u003csub\u003e2\u003c/sub\u003e composites with Au particle sizes of about 5 nm are of particular interest for study, since the electron properties of gold nanoparticles with sizes 2\u0026thinsp;\u0026minus;\u0026thinsp;5 nm differ significantly compared to the bulk metal, and the strongest interaction between Au and TiO\u003csub\u003e2\u003c/sub\u003e is observed when the Au particle sizes do not exceed 5 nm [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMesoporous materials are widely used as catalysts and catalyst supports due to their large surface area, controllable pore size, and ease of functionalization, which together help suppress the migration and agglomeration of gold nanoparticles within the pore structure [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Moreover, encapsulation of Au NPs in mesoporous TiO\u003csub\u003e2\u003c/sub\u003e is potentially attractive owing to the combination of the catalytic properties of gold and the semiconducting and structural advantages of mesoporous titania. This configuration enhances the dispersion and thermal stability of Au NPs, and also improves mass transport and reactant accessibility [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Moreover, the synergistic interaction between Au NPs and TiO\u003csub\u003e2\u003c/sub\u003e support can modulate the electronic structure of active sites, thereby enhancing the catalytic activity and selectivity [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Furthermore, the plasmonic properties of gold extend the light absorption range of TiO\u003csub\u003e2\u003c/sub\u003e into the visible region, making such composites promising for photocatalytic applications such as pollutant degradation, O\u003csub\u003e2\u003c/sub\u003e reduction, and water splitting [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. The structural tunability of mesoporous TiO\u003csub\u003e2\u003c/sub\u003e also enables the design of multifunctional catalysts by incorporating additional active sites or co-catalysts, further expanding their potential in advanced catalytic systems.\u003c/p\u003e \u003cp\u003eThe goal of this study is to obtain Au/TiO\u003csub\u003e2\u003c/sub\u003e films with Au particle sizes of 5 nm or less, and to determine the influence of the size and chemical state of gold on the electrocatalytic properties of titanium dioxide films in the oxygen reduction reaction in an alkaline medium.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cp\u003e \u003cb\u003eMaterials.\u003c/b\u003e All chemicals were of analytical grade and were used as received without any further purification. All solutions were prepared with triple distilled water.\u003c/p\u003e \u003cp\u003e \u003cb\u003eSynthesis procedures\u003c/b\u003e. The TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;\u003cem\u003en\u003c/em\u003eH\u003csub\u003e2\u003c/sub\u003eO sol was synthesized via sol-gel method according to the procedure described in [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Titanium tetrachloride was slowly added to a 0.65 M HCl solution in an ice bath (0\u0026deg;C) under constant stirring for 30 minutes until it was completely dissolved. An aqueous solution of NH\u003csub\u003e4\u003c/sub\u003eOH (12 wt.%) was then added dropwise to the solution of TiCl\u003csub\u003e4\u003c/sub\u003e in HCl under vigorous stirring at 0\u0026deg;C until pH 4. Then, the resulting suspension was centrifuged, and the precipitate was rinsed with distilled water to remove chloride ions. After adding 0.9 mL of concentrated HNO\u003csub\u003e3\u003c/sub\u003e (65 wt. %) as a sol stabilizer, the precipitate was stirred to give a movable suspension. Then, the suspension was ultrasonically treated (frequency \u0026ndash; 22 kHz) with an ultrasonic horn placed into a beaker with the suspension to obtain a transparent opalescent sol. The prepared sol contains 7\u0026ndash;8 wt. % of TiO\u003csub\u003e2\u003c/sub\u003e and is stable for several months at room temperature. According to TEM measurement, the average size of TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles was about 5 nm (SI, Fig.\u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eA series of Au NPs/TiO\u003csub\u003e2\u003c/sub\u003e composites were prepared to elucidate the effects of the Au NPs size and concentration on the catalytic activity of Au NPs/TiO\u003csub\u003e2\u003c/sub\u003e electrodes in ORR and to provide different types of interaction between Au NPs and the oxide support.\u003c/p\u003e \u003cp\u003eColloidal Au NPs with an average diameter of 4.5 nm were prepared by HAuCl\u003csub\u003e4\u003c/sub\u003e reduction using sodium borohydride as a reducing agent and sodium citrate as a stabilizer [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. First, 2 mL of 5 mM HAuCl\u003csub\u003e4\u003c/sub\u003e aqueous solution was mixed with 2 mL of 10 mM Na\u003csub\u003e3\u003c/sub\u003eCit aqueous solution and diluted to 100 mL with distilled water. Next, 270 mL of 0.5 M NaBH\u003csub\u003e4\u003c/sub\u003e solution was added dropwise to the resultant solution under vigorous stirring, giving rise to a red colored Au hydrosol. The synthesis was carried out at 18 \u0026ordm;C.\u003c/p\u003e \u003cp\u003eColloidal 5-(2-mercaptoethyl)tetrazole stabilized Au NPs with an average size 1.9 nm were synthesized via the method described in [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The stabilizing ligand precursor, namely1,2-bis(2-(1H-tetrazol-5-yl)ethyl)disulfane, was prepared as described [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Initially, 20 mL of 30 \u0026micro;M aqueous solution of the ligand precursor was prepared, followed by the addition of 100 \u0026micro;L of 1 M NaOH solution to adjust the pH to 8\u0026ndash;9. In the next step, 200 \u0026micro;L of 50 mM HAuCl\u003csub\u003e4\u003c/sub\u003e solution was mixed with 20 mL of 30 \u0026micro;M of aqueous 1,2-bis(2-(1H-tetrazol-5-yl)ethyl)disulfane solution. Then, a freshly prepared aqueous solution of sodium borohydride (0.4 mL, 0.05 M) was added dropwise to the resultant solution under vigorous stirring, giving rise to a brown colored Au hydrosol. At the same time, the disulfide bond of 1,2-bis(2-(1H-tetrazol-5yl)ethyl)disulfane is also reduced with the formation of 5-(2-mercaptoethyl)tetrazole ligand. The synthesis was carried out at 18 \u0026ordm;C. After stirring for 1 h, the reaction mixture was concentrated under vacuum and the obtained residue was precipitated three times with isopropanol and finally redispersed in distilled water.\u003c/p\u003e \u003cp\u003eTEM results of synthesized Au NPs are presented in SI (Figs. S2 and S3).\u003c/p\u003e \u003cp\u003eCombined sols, TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;\u003cem\u003en\u003c/em\u003eH\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;Au NPs, were prepared by mixing individual sols, TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;\u003cem\u003en\u003c/em\u003eH\u003csub\u003e2\u003c/sub\u003eO and Au NPs stabilized by citrate ions, TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;\u003cem\u003en\u003c/em\u003eH\u003csub\u003e2\u003c/sub\u003eO and Au NPs stabilized by 5-(2-mercaptoethyl)tetrazole, as well as TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;\u003cem\u003en\u003c/em\u003eH\u003csub\u003e2\u003c/sub\u003eO and HAuCl\u003csub\u003e4\u003c/sub\u003e solution, to obtain the final TiO\u003csub\u003e2\u003c/sub\u003e/Au composite with gold content of 0.5, 1.0 and 2.0 wt. %. To obtain powdered samples, TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;\u003cem\u003en\u003c/em\u003eH\u003csub\u003e2\u003c/sub\u003eO and TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;\u003cem\u003en\u003c/em\u003eH\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;Au NPs sols were dried at 50 \u0026deg;С and then annealed at 400 \u0026deg;С during 1 h.\u003c/p\u003e \u003cp\u003eThin film samples of TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e were deposited onto glass and FTO substrates by spin coating of the corresponding sols. To obtain high-quality films, the procedure of sol application onto the substrate was repeated five times; each time after application, the sample was heated at 100 \u0026deg;С. The resulting thin films were finally annealed at 400\u0026deg;C for 1 h.\u003c/p\u003e \u003cp\u003eThe samples prepared by doping TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;nH\u003csub\u003e2\u003c/sub\u003eO sol with HAuCl\u003csub\u003e4\u003c/sub\u003e contain Au NPs formed via thermal decomposition of HAuCl\u003csub\u003e4\u003c/sub\u003e within the TiO\u003csub\u003e2\u003c/sub\u003e matrix. The simultaneous \u003cem\u003ein situ\u003c/em\u003e dehydration of TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;nH\u003csub\u003e2\u003c/sub\u003eO and the Au NPs formation can facilitate strong interfacial interactions at the Au/TiO\u003csub\u003e2\u003c/sub\u003e heterojunction. Such interactions are critical for efficient charge transfer across the Au/TiO\u003csub\u003e2\u003c/sub\u003e interface, which plays a pivotal role in enhancing the electrocatalytic performance.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e presents the data for the synthesized TiO\u003csub\u003e2\u003c/sub\u003e/Au samples. In sample 4, Au NPs were undetectable by TEM. The size of the Au nanoparticles (NPs) in this sample was estimated indirectly through UV-vis spectroscopy.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThe characterization of Au/TiO\u003csub\u003e2\u003c/sub\u003e samples\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of the sample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eComposition\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eThe size of Au NPs, nm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDoping method\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAu NPs/TiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5\u0026thinsp;\u0026plusmn;\u0026thinsp;0,3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAddition of Au NPs sol stabilized by sodium citrate\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAu NPs /TiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2\u0026thinsp;\u0026plusmn;\u0026thinsp;0,2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAddition of Au NPs sol stabilized by 5-(2-mercaptoethyl)tetrazole\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAu NPs/TiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;2 nm*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAddition of HAuCl\u003csub\u003e4\u003c/sub\u003e solution\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003e* Estimated size based on UV-vis spectroscopy\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eCharacterization.\u003c/b\u003e FTIR- and optical spectroscopies, XPS, TEM were used to study the size and the chemical state of Au, and the type of interaction between components at Au/TiO\u003csub\u003e2\u003c/sub\u003e interface. XPS analysis was performed on a Kratos DLD Ultra spectrometer using Al Kα monochromatized radiation (E\u0026thinsp;=\u0026thinsp;1486.6 eV). For survey spectra, a pass energy (PE) of 160 eV was used while for regions the PE was 20 eV. XPS spectra were recorded before and after Ar\u003csup\u003e+\u003c/sup\u003e etching at different sputtering time (sputter rate 8 nm/min calibrated on Ta\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e). All binding energies (BEs) were referenced to the C 1s peak (284.8 eV) of surface adventitious carbon. A LEO-906E transmission electron microscope and a Hitachi HD2700D scanning transmission electron microscope were utilized for TEM measurements. Fourier transform infrared (FTIR) spectra were collected on an AVATAR-330 (Thermo Nicolet) spectrometer supplied with a diffuse reflectance accessory in the wavenumber range from 400 to 4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. A Shimadzu UV 2550 spectrophotometer was used to record optical spectra.\u003c/p\u003e \u003cp\u003eThe electrocatalytic activity of TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e films on FTO substrates in the oxygen reduction reaction was studied by the cyclic voltammetry (CV) method using an Autolab PGSTAT 302N potentiostat-galvanostat (the Netherlands) in a 0.1 M KOH solution saturated with oxygen during 1 h. Electrochemical experiments were performed in a standard three-electrode electrochemical cell, where an Hg/HgO electrode filled with 1 M KOH (Radiometer Analytical) was applied as a reference electrode (all potentials in this study are given relative to this reference electrode), and platinum foil was applied as an auxiliary electrode. The potential sweep rate was 10 mV/s.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e\u003cstrong\u003eX-ray photoelectron spectroscopy.\u0026nbsp;\u003c/strong\u003eIn the XPS spectrum of sample 1, a single, fairly narrow (FWHM = 1 eV) peak of Ti 2p\u003csub\u003e3/2\u0026nbsp;\u003c/sub\u003ewith a maximum of 458.8 eV is recorded, which can be attributed to the Ti(IV) state in TiO\u003csub\u003e2\u003c/sub\u003e. The asymmetric O 1s peak is split into two peaks with the binding energy (BE) equal to 530.2 eV (84%) and 531.8 eV (16%). The first peak is attributed to oxygen in the TiO\u003csub\u003e2\u003c/sub\u003e lattice, while the second peak is most likely associated with oxygen in hydroxyl groups. The O/Ti ratio (1.96) is less than the stoichiometric composition of TiO\u003csub\u003e2\u003c/sub\u003e, which may confirm the presence of OH groups on the surface of the films and oxygen vacancies.\u003c/p\u003e\n\u003cp\u003eThe Au 3d peaks in the spectra of samples 2\u0026ndash;4 are low-intensity and wide (FWHM Au 3d\u003csub\u003e5/2\u003c/sub\u003e ~ 5.6 eV in sample 3 and FWHM Au 3d\u003csub\u003e5/2\u0026nbsp;\u003c/sub\u003e~ 6.9 eV in sample 4). The Au 4f peaks are more informative (Fig. 1). They are narrower, more intense, and are separated into two components in the XPS spectra of samples 3 and 4. According to the spectrum of Au 4f (Fig. 1, a), sample 2 contains only Au\u003csup\u003e0\u003c/sup\u003e. The spectra of samples 3 and 4 indicate the presence of gold in two different states. In sample 3, the binding energy of one of the Au states is lower, while in sample 4 it is higher than the BE of the metal (Fig. 1, b, c). The Au 4f binding energies and the quantitative ratio of gold in these states are shown in Table 2.\u003c/p\u003e\n\u003cp\u003eAccording to XPS data, gold in sample 4 is present in both the zero oxidation state and the oxidized state (Fig. 1, c and Table 2). Gold was introduced during sample 4 preparation as AuCl\u003csup\u003e4-\u003c/sup\u003e ions, not in a colloidal state. Thermal treatment of the sample simultaneously leads to dehydration and crystallization of the titanium oxide phase and a thermal transformation from Au(III) to Au(0), which contributes to a strong mutual influence of the components on each other\u0026apos;s electronic and valence states. These processes may result in the presence of gold in the oxidized state (Au\u003csup\u003e3+\u003c/sup\u003e). The positive shift in the binding energy of Au 4f\u003csub\u003e7/2\u003c/sub\u003e is approximately 3 eV relative to Au(0), which presumably indicates the presence of Au\u003csup\u003e3+\u003c/sup\u003e (Table 2). It should be noted that the Au 4f\u003csub\u003e7/2\u003c/sub\u003e and Au 4f\u003csub\u003e5/2\u003c/sub\u003e peaks in sample 4 are very broad, especially the peak of the oxidized state (FWHM = 4.5 eV). Given the large width of this peak, the presence of gold in the Au\u003csup\u003e1+\u003c/sup\u003e state cannot be ruled out.\u003c/p\u003e\n\u003cp\u003eIn sample 3, along with Au(0), the Au 4f XPS spectrum also shows gold with an excess negative charge (Fig. 1 and Table 2). The negative chemical shift of the Au 4f electron spectrum observed in sample 3 generally indicates the appearance of excess electron density on the Au nanoparticles, which may be due to its transfer from the oxide carrier to the surface of the gold particles: Au\u003csup\u003e\u0026delta;\u003c/sup\u003e\u003csup\u003e-\u003c/sup\u003e\u0026larr;Ti\u003csup\u003e\u0026delta;\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003eTiO\u003csub\u003e2\u0026ndash;x\u003c/sub\u003e. An alternative explanation for this shift may be the quantum size effect, which is characterized by an increased population of p-AO compared to the d10s1p0 configuration [34]. In Au clusters, a partial transition of electron density from the d-orbital to the p-orbital can occur: [d10s1] \u0026rarr; [d8s1p2], which causes a change in the electron binding energy in the clusters relative to metallic gold. A change in the state of Au d-orbitals with a decrease in electron density can contribute to increased activity of Au clusters in the adsorption of molecules and the catalysis of reactions involving them. In [16] it is noted that even a small transfer of electron density can cause a change in the state of Au\u0026ndash;Au bonds, a change in the distances and symmetry of d-orbitals, which will affect the nature of oxygen adsorption.\u003c/p\u003e\n\u003cp\u003eIn sample 3, the Au\u003csup\u003e\u0026delta;\u003c/sup\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e content accounts for just over half of the surface gold atoms. This fact, regardless of the mechanism by which excess electron density arises on gold particles in sample 3, indicates a significant change in the energy state of its orbitals.\u003c/p\u003e\n\u003cp\u003eIt should be noted that the gold phase is not detectable by X-ray diffraction in samples 2-4. This is likely due to the low Au content (0.5 wt%) in the samples. In the optical spectrum of sample 2, the maximum of the surface plasmon resonance (SPR) peak is observed at 514 nm. The optical spectra of films of samples 3 and 4 show a low-intensity and broad SPR peak in the range of 500\u0026ndash;600 nm. This shape of the SPR peak confirms the high dispersion of gold particles; their size may be less than 2 nm [27, 35].\u003c/p\u003e\n\u003cp\u003eIn the XPS spectrum of sample 3, the Ti 2p\u003csub\u003e3/2\u003c/sub\u003e peak has an asymmetric shape. In addition to the Ti(IV) state in TiO\u003csub\u003e2\u003c/sub\u003e (458.8 eV), it contains a state with a higher binding energy (459.8 eV; approximately 30%). This value of the Ti 2p\u003csub\u003e3/2\u003c/sub\u003e energy may correspond to a more ionic state of Ti atoms than in titanium dioxide. Since an S-containing substance (5-(2-mercaptoethyl)-tetrazole) was used to stabilize the gold particles in this sample, the presence of Ti-O\u0026ndash;SO\u003csub\u003ex\u003c/sub\u003e groups on the titanium dioxide surface can be expected. The presence of sulfur in sample 3 is also indicated by the XPS spectrum (Fig. 1, g), which contains a peak of unresolved S 2p\u003csub\u003e3/2\u003c/sub\u003e and S 2p\u003csub\u003e1/2\u003c/sub\u003e lines with a maximum BE of 168.4 eV. According to reference data [36], the BE of 168.4 eV of S 2p-level electrons can be assigned to SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026ndash;\u003c/sup\u003e ions, as well as \u0026ndash;SO\u003csub\u003e2\u003c/sub\u003eCH\u003csub\u003e3\u003c/sub\u003e and other more complex groups, for example, (H\u003csub\u003e2\u003c/sub\u003eC)\u003csub\u003e2\u003c/sub\u003eSO\u003csup\u003e\u0026ndash;\u003c/sup\u003e. The binding energy of the O 1s level in such groups is 532\u0026ndash;533 eV. Characteristically sample 3 has a significantly higher oxygen content with an increased O 1s binding energy of 531.9 eV (up to 80%) compared to the BE of 530.0 eV (20%). At the same time, in sample 1 (pure TiO\u003csub\u003e2\u003c/sub\u003e), the oxygen content with the BE of 531.9 eV is only 16%. The S-containing groups can arise in sample 3 during the degradation and oxidation of the stabilizer, 5-(2-mercaptoethyl)-tetrazole, during thermal treatment. They may participate in the attachment of gold particles to TiO\u003csub\u003e2\u003c/sub\u003e surface.\u003c/p\u003e\n\u003cp\u003eAccording to the thermal analysis data presented in [32], \u0026ndash;CN\u003csub\u003e4\u003c/sub\u003e and \u0026ndash;СН\u003csub\u003e3\u003c/sub\u003e fragments are released from the sol of Au nanoparticles stabilized by5-(2-mercaptoethyl)-tetrazole upon heating to 400 \u0026deg;C. The removal of S-containing fragments begins only above 500 \u0026deg;C and is completed at 600 \u0026deg;C. These results indicate the presence of S-containing groups in sample 3 and the formation of a non-stoichiometric TiO\u003csub\u003e2\u0026ndash;x\u003c/sub\u003e structure. A strong interaction with the transfer of electron density to Au nanoparticles can be observed between such titanium dioxide and highly dispersed gold particles.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2. Comparison of Au 4f XPS spectra for Au/TiO\u003csub\u003e2\u003c/sub\u003e samples\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv align=\"\"\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"567\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 123px;\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003eBE Au 4f\u003csub\u003e7/2\u003c/sub\u003e,\u0026nbsp;eV\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003eAu content,\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eat. %\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003eBE Au 4f\u003csub\u003e5/2\u003c/sub\u003e,\u0026nbsp;eV\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 85px;\"\u003e\n \u003cp\u003eO/Ti ratio,\u003c/p\u003e\n \u003cp\u003eat. %\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 132px;\"\u003e\n \u003cp\u003eAu state\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 123px;\"\u003e\n \u003cp\u003e#2, TiО\u003csub\u003e2\u003c/sub\u003e +\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eAu NPs stabilized by citrate\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e83,85\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003e86,7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 85px;\"\u003e\n \u003cp\u003e2,40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 132px;\"\u003e\n \u003cp\u003eAu\u003csup\u003e0\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 123px;\"\u003e\n \u003cp\u003e#3, TiО\u003csub\u003e2\u003c/sub\u003e +\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eAu NPs stabilized by tetrazole\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e82,9\u003cbr\u003e\u0026nbsp;83,9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e54\u003cbr\u003e\u0026nbsp;45\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003e86,5\u003cbr\u003e\u0026nbsp;87,3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 85px;\"\u003e\n \u003cp\u003e2,14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 132px;\"\u003e\n \u003cp\u003eAu\u003csup\u003e\u0026delta;\u003c/sup\u003e\u003csup\u003e-\u003c/sup\u003e (Au\u0026larr;Ti)\u003cbr\u003e Au\u003csup\u003e0\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 123px;\"\u003e\n \u003cp\u003e#4, TiО\u003csub\u003e2\u003c/sub\u003e + HAuCl\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e83,9\u003cbr\u003e\u0026nbsp;87,2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e68\u003cbr\u003e\u0026nbsp;32\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 76px;\"\u003e\n \u003cp\u003e87,0\u003cbr\u003e\u0026nbsp;90,2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 85px;\"\u003e\n \u003cp\u003e1,96\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 132px;\"\u003e\n \u003cp\u003eAu\u003csup\u003e0\u003cbr\u003e\u0026nbsp;\u003c/sup\u003eAu\u003csup\u003e3+\u003c/sup\u003e (Au\u003csup\u003e3+\u003c/sup\u003e\u0026ndash;O\u0026ndash;Ti)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInfrared spectroscopy\u003c/strong\u003e\u003cem\u003e.\u0026nbsp;\u003c/em\u003eIR spectroscopy data confirm the conclusions drawn from the XPS spectra. In the IR spectrum of sample 1, in addition to a broad absorption band of the characteristic vibrations of Ti\u0026ndash;O (400\u0026ndash;800 cm\u0026ndash;1), intense absorption bands are recorded related to the stretching\u0026nbsp;d(O\u0026minus;H, H\u003csub\u003e2\u003c/sub\u003eO) at 3425 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e and the deformation d(H\u0026ndash;O\u0026ndash;H) at 1630 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e vibrations (Fig. 2, a). Weak absorptions at 1125 and 1295 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e can be attributed to the d(Ti\u0026ndash;O\u0026ndash;H) vibrations. (Fig. 2, a). These data evidence the presence of structural water and surface hydroxyl groups in sol-gel derived TiO\u003csub\u003e2\u003c/sub\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFor samples 2-4, additional bands are observed in the IR spectra in the region 990\u0026ndash;1530 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e (Fig. 2, \u003cem\u003eb\u003c/em\u003e). The spectrum of sample 2 displays absorption bands related to bond vibrations in carbonate-carboxylate groups. Typically, carboxyl groups bound to metals exhibit two characteristic vibrational series, related to the asymmetric\u0026nbsp;n\u003csub\u003eа\u003c/sub\u003e\u003csub\u003es\u003c/sub\u003e(COO\u003csup\u003e\u0026minus;\u003c/sup\u003e) stretching vibrations and the symmetric\u0026nbsp;n\u003csub\u003es\u003c/sub\u003e(COO\u003csup\u003e\u0026minus;\u003c/sup\u003e) stretching vibrations of the C\u0026ndash;O bonds. The appearance of such oscillations in the spectrum of sample 2 is entirely expected, since citrate ions were used to stabilize gold particles, the thermal transformation products of which contain \u0026ndash;COO\u003csup\u003e\u0026ndash;\u003c/sup\u003e groups. The spectrum of sample 2 shows the following vibrations: n\u003csub\u003eа\u003c/sub\u003e\u003csub\u003es\u003c/sub\u003e(COO\u003csup\u003e\u0026minus;\u003c/sup\u003e) = 1531 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e;\u0026nbsp;n\u003csub\u003es\u003c/sub\u003e(COO\u003csup\u003e\u0026minus;\u003c/sup\u003e)= 1375 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e;\u0026nbsp;n\u003csub\u003eа\u003c/sub\u003e\u003csub\u003es\u003c/sub\u003e(COO\u003csup\u003e\u0026minus;\u003c/sup\u003e) = 1435 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e (monodentate carbonate CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026ndash;\u003c/sup\u003e). \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe IR spectrum of sample 3 shows a weak absorption band at 990 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e and fairly intense bands at 1078, 1135 and 1350 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e. These bands can be attributed to vibrations of S=O bonds, which are in the region of 990 \u0026ndash; 1420 cm\u003csup\u003e\u0026ndash;1\u0026nbsp;\u003c/sup\u003e[37]. These results confirm the XPS data on the retention of sulfur in the sample 3 in the form of a compound with oxygen, which arises as a result of thermal destruction and oxidation of the stabilizer \u0026ndash; 5-(2-mercaptoethyl)-tetrazole. These can be either individual sulfite groups or sulfoxide groups \u0026gt;S=O.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe IR spectrum of sample 4, obtained by introducing HAuCl\u003csub\u003e4\u003c/sub\u003e into the TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;nH\u003csub\u003e2\u003c/sub\u003eO sol, shows a reduced content of hydroxyl groups on the surface compared to sample 1. The\u0026nbsp;d(Ti\u0026ndash;O\u0026ndash;H) vibrations at 1125 and 1295 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e are not detected, while the n(O\u0026ndash;H, H\u003csub\u003e2\u003c/sub\u003eO) at 3425 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e and d(H\u0026ndash;O\u0026ndash;H) at 1630 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e decrease in intensity. The introduction of acid apparently promotes dehydroxylation of the titanium dioxide surface. It can be assumed that the AuCl\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e ions in the sol are coordinated around the positively charged nuclei of hydrated titanium dioxide particles. During subsequent drying and heating of the composite TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;nH\u003csub\u003e2\u003c/sub\u003eO + HAuCl\u003csub\u003e4\u003c/sub\u003e sol, the processes of HAuCl\u003csub\u003e4\u003c/sub\u003e decomposition and crystallization of Au particles slow down as a result of strong dilution in the formed xerogel.\u003c/p\u003e\n\u003cp\u003eThus, using different synthesis methods, Au/TiO\u003csub\u003e2\u003c/sub\u003e samples were prepared that differed in the size of gold particles, the nature of their interaction with titanium dioxide, and modification of its surface with different functional groups.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrocatalytic measurements.\u003c/strong\u003e The performance of TiO\u003csub\u003e2\u003c/sub\u003e electrode and Au/TiO\u003csub\u003e2\u003c/sub\u003e electrodes (Au 0.5, 1.0 and 2.0 % wt.%) as electrocatalysts for ORR was examined by cyclic voltammetry in 0.1 M KOH saturated with oxygen. To compare the CV curves for different electrodes, in the further consideration, we chose CV curves related to the ninth cycle (hereafter referred to as quasi-steady-state CVs), since after the ninth cycle the change in the CV curves basically stopped. The electrocatalytic activity of electrodes was evaluated via analysis of the ORR peak potential (\u003cem\u003eE\u003csub\u003ep\u003c/sub\u003e\u003c/em\u003e), the half-wave potential (\u003cem\u003eЕ\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e) and the current density at the ORR peak potential (\u003cem\u003ej\u003csub\u003ep\u003c/sub\u003e\u003c/em\u003e) and at the half-wave potential (\u003cem\u003ej\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 3 shows typical quasi-stationary CV curves for bare TiO\u003csub\u003e2\u003c/sub\u003e and different Au/TiO\u003csub\u003e2\u003c/sub\u003e electrodes with the same Au content (0.5 wt.%).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe bare TiO\u003csub\u003e2\u003c/sub\u003e electrode demonstrates an irreversible wave of oxygen reduction current with \u003cem\u003ej\u003csub\u003ep\u003c/sub\u003e\u003c/em\u003e at -0.78 V and \u003cem\u003eЕ\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e at -0.70 V. For Au/TiO\u003csub\u003e2\u003c/sub\u003e electrodes, the TiO\u003csub\u003e2\u003c/sub\u003e-assisted ORR wave remains unchanged (as for sample 3) or shifts insignificantly (by 20 mV) in the positive direction for samples 2 and 4. Additionally, the \u003cem\u003ej\u003c/em\u003e\u003csub\u003e1/2\u0026nbsp;\u003c/sub\u003ebecomes slightly higher, with the maximum current density for sample 2 (Fig. 3 and Table 3). It should be noted that the shoulder arises at the leading edge of the current wave after gold doping, probably due to the ORR on the surface of gold nanoparticles. As a result, the ORR overvoltage decreases approximately by 92\u0026ndash;100 mV for samples 2 and 3 and by 36 mV for sample 4 (Fig.\u0026nbsp;3 and Table 3). For sample 3, additional waves corresponding to ORR on the surface of Au NPs appear at less negative potentials (\u003cem\u003eЕ\u003c/em\u003e \u0026gt; -0.6 V). It is worth mentioning that the minimum ORR overvoltage is observed for Au/TiO\u003csub\u003e2\u003c/sub\u003e electrodes at lower current densities (Table 3).\u003c/p\u003e\n\u003cp\u003eTable 3.\u003cem\u003e\u0026nbsp;\u003c/em\u003eParameters of electrochemical oxygen reduction on thin-film TiO\u003csub\u003e2\u003c/sub\u003e and Au/TiO\u003csub\u003e2\u003c/sub\u003e electrodes\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"588\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 134px;\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cem\u003eE\u003csub\u003ep\u003c/sub\u003e\u003c/em\u003e, V\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 95px;\"\u003e\n \u003cp\u003e\u003cem\u003ej\u003csub\u003ep\u003c/sub\u003e\u003c/em\u003e, mA cm\u003csup\u003e-2\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 61px;\"\u003e\n \u003cp\u003e\u003cem\u003eE\u003csub\u003e1/2\u003c/sub\u003e\u003c/em\u003e, V\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 104px;\"\u003e\n \u003cp\u003e\u003cem\u003ej\u003csub\u003e1/2\u003c/sub\u003e\u003c/em\u003e, mA cm\u003csup\u003e-2\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 137px;\"\u003e\n \u003cp\u003e\u003cem\u003eE\u003c/em\u003e at\u003c/p\u003e\n \u003cp\u003e\u003cem\u003ej\u003c/em\u003e=0.04 mA cm\u003csup\u003e-2\u003c/sup\u003e,\u0026nbsp;V\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 134px;\"\u003e\n \u003cp\u003e#1, TiО\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e-0.780\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e-0.248\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 61px;\"\u003e\n \u003cp\u003e-0.700\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 104px;\"\u003e\n \u003cp\u003e-0.124\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 137px;\"\u003e\n \u003cp\u003e-0.590\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 134px;\"\u003e\n \u003cp\u003e#2, TiО\u003csub\u003e2\u003c/sub\u003e +\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eAu NPs stabilized by citrate\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e-0.757\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e-0.305\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 61px;\"\u003e\n \u003cp\u003e-0.600\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 104px;\"\u003e\n \u003cp\u003e-0.152\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 137px;\"\u003e\n \u003cp\u003e-0.444\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 134px;\"\u003e\n \u003cp\u003e#3, TiО\u003csub\u003e2\u003c/sub\u003e +\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eAu NPs stabilized by tetrazole\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e-0.785\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e-0.278\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 61px;\"\u003e\n \u003cp\u003e-0.608\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 104px;\"\u003e\n \u003cp\u003e-0.139\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 137px;\"\u003e\n \u003cp\u003e-0.370\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 134px;\"\u003e\n \u003cp\u003e#4, TiО\u003csub\u003e2\u003c/sub\u003e + HAuCl\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 57px;\"\u003e\n \u003cp\u003e-0.759\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 95px;\"\u003e\n \u003cp\u003e-0.271\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 61px;\"\u003e\n \u003cp\u003e-0.664\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 104px;\"\u003e\n \u003cp\u003e-0.135\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 137px;\"\u003e\n \u003cp\u003e-0.519\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eTo evaluate the effect of the Au NPs concentration on the efficiency of Au/TiO\u003csub\u003e2\u003c/sub\u003e electrodes in ORR, electrodes (sample 3) with a final gold concentration of 0.5, 1.0 and 2.0 wt. %. were investigated. We observed a significant decrease in the ORR overvoltage and an increase of current density with increasing the concentration of Au NPs. The TiO\u003csub\u003e2\u003c/sub\u003e electrode with an Au NPs concentration of 2 wt. % demonstrates a decrease in overvoltage by 220 mV at a current density of 0.11 mA cm\u003csup\u003e-2\u003c/sup\u003e and by 350 mV at 0.035 mA cm\u003csup\u003e-2\u003c/sup\u003e (Fig. 4). It is worth mentioning that a well-defined Au-catalyzed cathodic current wave was developed starting from 1 wt. % of Au NPs. The Au/TiO\u003csub\u003e2\u003c/sub\u003e composite with 2 wt.% Au NP\u003csub\u003eS\u003c/sub\u003e also demonstrates a shift of TiO\u003csub\u003e2\u003c/sub\u003e-assisted wave in the negative direction.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe presented experimental data demonstrate that the TiO\u003csub\u003e2\u003c/sub\u003e doped with Au NPs from sols have a higher catalytic activity in ORR compared to the composite obtained by thermal decomposition of HAuCl\u003csub\u003e4\u003c/sub\u003e within the TiO\u003csub\u003e2\u003c/sub\u003e matrix. Although sample 4 contains small Au NPs, the decrease of ORR activity may be related to the oxidized state of Au. This occurs due to incomplete reduction of Au(III) to Au(0) during annealing of the HAuCl\u003csub\u003e4\u003c/sub\u003e-doped TiO\u003csub\u003e2\u003c/sub\u003e sol-gel matrix at 400\u0026deg;С. Also, the surface of citrate-stabilized Au NPs contains adsorbed carbonate-carboxylated groups with donor properties. This may lead to higher oxygen adsorption efficiency and facilitate charge transfer to the adsorbed oxygen molecules, which is favorable for oxygen activation \u003csup\u003e3\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eNotably, sample 3 with high Au NP content exhibits an additional ORR wave at more positive potentials (see Fig. 3 and Fig. 4). This wave can be explained by the different Au NPs sizes appeared after thermal treatment of such electrodes. At high Au NPs concentrations, small Au NPs can aggregate to form larger particles. It has been previously shown that Au NP particles of different sizes can produce an ORR wave at different potentials [18, 19]. The observed effect may also be due to the increased activity of gold particles, since there is excessive electron density on the Au NPs of sample 3, as evidenced by the XPS data (see Fig. 1, \u003cem\u003ec\u003c/em\u003e).\u0026nbsp;\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eAu/TiO\u003csub\u003e2\u003c/sub\u003e electrocatalysts have been prepared by doping of TiO\u003csub\u003e2\u003c/sub\u003e sol with Au NPs and characterized using TEM, XPS, and FTIR. The results show that the prepared samples differ in Au NPs size, the mode of interaction between the gold nanoparticles and the oxide, and surface modification by various functional groups and ions. Electrocatalytic activity of Au/TiO\u003csub\u003e2\u003c/sub\u003e electrodes toward oxygen reduction reaction (ORR) has been examined by cyclic voltammetry (CV) in an alkaline medium. The electrocatalytic efficiency of Au NPs-doped TiO\u003csub\u003e2\u003c/sub\u003e electrodes in ORR was found to depend on various factors such as the size of Au NPs, their loading amount, the state of Au and the nature of stabilizing ligands used in the preparation of Au sols. The highest efficiency was demonstrated for the Au/TiO\u003csub\u003e2\u003c/sub\u003e sample prepared by doping the TiO2 sol with 5-(2-mercaptoethyl)tetrazole-stabilized Au NPs. Moreover, the ORR overvoltage for such electrodes was found to decrease with increasing the Au NPs concentration. The high activity of such composite can be explained by small size of Au NPs (2 nm) and the excessive negative charge on gold particles.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eConflict of interest\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis declaration is not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis project is funded by the Belarusian Republican Foundation for Fundamental Research (grants № T23RNFM-35) and Russian Science Foundation (Grant No. 24-49-10012).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests:\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no competing interests to declare that are relevant to the content of this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eE.A. Ovodok: Conceptualization, Data curation, Visualization, Investigation. M.I. Ivanovskaya: Conceptualization, Supervision, Writing-Original draft preparation, Editing, Methodology. S.K. Poznyak: Investigation, Supervision, Writing-Reviewing. A.M. Mal\u0026apos;tanova: Investigation, Writing-Reviewing . T. V. Gaevskaya: Investigation, Editing. S.V. Voitekhovich: Investigation. A.E. Seleznev: Investigation. All authors reviewed the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eNjema GG, Ouma RBO, Kibet JK (2024) A review on the recent advances in battery development and energy storage technologies. J Renew Energy 2329261. https://doi.org/10.1155/2024/2329261.\u003c/li\u003e\n\u003cli\u003eHelsel N, Choudhury P (2025) Non-Platinum Group Metal Oxygen Reduction Catalysts for a Hydrogen Fuel Cell Cathode: A Mini-Review. 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DOI: 10.1016/S1748-0132(07)70016-6.\u003c/li\u003e\n\u003cli\u003eBriggs D, Seah MP (1983) Practical surface analysis by Auger and X-ray photoelectron spectroscopy. John Wiley and Sons Ltd, Chichester. https://doi.org/10.1002/sia.740060611.\u003c/li\u003e\n\u003cli\u003eBerger F, Beche E, Berjoan R et al (1996) An XPS and FTIR study of SO2 adsorption on SnO2 surfaces. Appl Surf Sci 93:9\u0026ndash;16. https://doi.org/10.1016/0169-4332(95)00319-3.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"electrocatalysis","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ecat","sideBox":"Learn more about [Electrocatalysis](http://link.springer.com/journal/12667)","snPcode":"12678","submissionUrl":"https://submission.nature.com/new-submission/12678/3","title":"Electrocatalysis","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"titanium dioxide, gold nanoparticles, electrocatalysis, oxygen reduction, XPS, FTIR spectroscopy","lastPublishedDoi":"10.21203/rs.3.rs-8246742/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8246742/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHerein, Au NPs/TiO\u003csub\u003e2\u003c/sub\u003e composites were obtained by adding an aqueous solution of HAuCl\u003csub\u003e4\u003c/sub\u003e or pre-synthesized gold nanoparticle (Au NP) sols to the TiO\u003csub\u003e2\u003c/sub\u003e sol, followed by annealing the films deposited on a substrate. The size of Au NPs, structural features of Au/TiO\u003csub\u003e2\u003c/sub\u003e composites, their surface modification with different functional groups and the interaction between Au NPs and TiO\u003csub\u003e2\u003c/sub\u003e matrix were studied by TEM, XPS, and FTIR. The activity of bare TiO\u003csub\u003e2\u003c/sub\u003e and Au NPs/TiO\u003csub\u003e2\u003c/sub\u003e films was evaluated in electrochemical oxygen reduction reaction (ORR). The influence of Au NPs size, concentration and chemical state of gold on improving electrocatalytic properties of Au/TiO\u003csub\u003e2\u003c/sub\u003e films in ORR is shown. Doping the TiO\u003csub\u003e2\u003c/sub\u003e matrix with tetrazole-stabilized Au NPs of about 2 nm size leads to a significant reduction in overvoltage and the appearance of an additional Au-assisted wave on cyclic voltammograms. The high activity of these Au/TiO\u003csub\u003e2\u003c/sub\u003e electrodes can be explained by the small size of Au NPs and the excessive negative charge on them.\u003c/p\u003e","manuscriptTitle":"Electrocatalytic Activity of Au/tio2 Nanocomposites in Oxygen Electroreduction Reaction","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-12 09:02:31","doi":"10.21203/rs.3.rs-8246742/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-09T15:08:36+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-08T23:25:10+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-08T23:23:51+00:00","index":"","fulltext":""},{"type":"submitted","content":"Electrocatalysis","date":"2025-12-01T06:21:57+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"electrocatalysis","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ecat","sideBox":"Learn more about [Electrocatalysis](http://link.springer.com/journal/12667)","snPcode":"12678","submissionUrl":"https://submission.nature.com/new-submission/12678/3","title":"Electrocatalysis","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d1bb0f03-c66c-4f13-ba80-ebc1bdc076ac","owner":[],"postedDate":"January 12th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-01-12T09:02:31+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-12 09:02:31","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8246742","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8246742","identity":"rs-8246742","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}