Boosting CO2 to Alcohol Conversion: Powerful Photocatalysts Based on TiO2-Cu(I)-Iodine-Pyridine 1D Coordination Polymers

Boosting CO2 to Alcohol Conversion: Powerful Photocatalysts Based on TiO2-Cu(I)-Iodine-Pyridine 1D Coordination Polymers | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 23 July 2025 V1 Latest version Share on Boosting CO2 to Alcohol Conversion: Powerful Photocatalysts Based on TiO2-Cu(I)-Iodine-Pyridine 1D Coordination Polymers Authors : Julián Ávila Duran 0009-0007-0888-3994 , Jon Napal , Fernando Aguilar-Galindo , Oscar Castillo , and Pilar Amo Ochoa 0000-0002-1952-1020 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.175329927.75955482/v1 235 views 144 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Coordination polymers (CPs) are promising materials for environmental applications, particularly in catalysis, due to their flexible structures, tunable electronic properties, and adaptable surface chemistry. This study reports the one-step, room-temperature synthesis of five 1D Cu(I)-iodide-pyridine-based coordination polymers with the general formula [CuI(L)]ₙ, where L represents different pyridine derivatives: pyridine (CP1), 3-methylpyridine (CP2), 4-methylpyridine (CP3), 2-amino-4-methylpyridine (CP4), and 2-chloro-4-methylpyridine (CP5). All the compounds exhibit band gap energies around 3 eV, making them suitable candidates for photocatalytic applications. Indeed, the study investigates the photoreduction of CO₂ to alcohols using a heterogeneous photocatalytic system consisting of TiO₂ and varying proportions of CPs. The reactor design enables the rapid removal of produced alcohols, preventing them from being oxidized by TiO₂ as sacrificial materials, thus achieving near-zero net alcohol production. The optimal TiO₂@CP mixture, TiO₂@5%CP4, demonstrated higher chemical stability due to the amine substituent on the pyridine, which facilitates hydrogen bonding between CP chains, and an enhanced ability to interact with CO₂, as confirmed by adsorption experiments and DFT calculations. The optimized mixture achieved selective methanol production of 894 μg·g cat⁻¹·h⁻¹, significantly surpassing the benchmark photocatalytic system TiO₂@3%CuO (318 μg·g cat⁻¹·h⁻¹). Furthermore, TiO₂@5%CP4 maintained stable photocatalytic performance over 10 hours without noticeable degradation. Article category: Full paper Subcategory: catalysis, Environmental remediation, Clean air, Carbon capture, storage, and utilization Boosting CO 2 to Alcohol Conversion: Powerful Photocatalysts Based on TiO 2 -Cu(I)-Iodine-Pyridine 1D Coordination Polymers Julián Avila-Duran 1 , Jon Napal 2 , Fernando Aguilar-Galindo 3, 4 Oscar Castillo 2* , Pilar Amo-Ochoa 1, 4* To Lucía Garrote de la Fuente J. Avila-Duran, Prof. P. Amo-Ochoa 1 Inorganic Chemistry Department, Faculty of Sciences, Autonomous University of Madrid (UAM), 28049 Madrid, Spain. J. Napal, Prof. O. Castillo 2 Department of Organic and Inorganic Chemistry, University of the Basque Country (UPV/EHU), P.O. 644, E-48080 Bilbao, Spain Dr. F. Aguilar-Galindo 3 Chemistry Department, Faculty of sciences, Autonomous University of Madrid (UAM), 28049 Madrid, Spain. Prof. P. Amo-Ochoa, Dr. F. Aguilar-Galindo 4 Institute for Advanced Research in Chemical Sciences (IAdChem). Autonomous University of Madrid (UAM), 28049 Madrid, Spain E-mail: [email protected] / [email protected] Keywords: photocatalysis, Copper coordination polymer, TiO 2 , CO 2 photoreduction, alcohols. Coordination polymers (CPs) are promising materials for environmental applications, particularly in catalysis, due to their flexible structures, tunable electronic properties, and adaptable surface chemistry. This study reports the one-step, room-temperature synthesis of five 1D Cu(I)-iodide-pyridine-based coordination polymers with the general formula [CuI(L)]ₙ, where L represents different pyridine derivatives: pyridine (CP1), 3-methylpyridine (CP2), 4-methylpyridine (CP3), 2-amino-4-methylpyridine (CP4), and 2-chloro-4-methylpyridine (CP5). All the compounds exhibit band gap energies around 3 eV, making them suitable candidates for photocatalytic applications. Indeed, the study investigates the photoreduction of CO₂ to alcohols using a heterogeneous photocatalytic system consisting of TiO₂ and varying proportions of CPs. The reactor design enables the rapid removal of produced alcohols, preventing them from being oxidized by TiO₂ as sacrificial materials, thus achieving near-zero net alcohol production. The optimal TiO₂@CP mixture, TiO₂@5%CP4, demonstrated higher chemical stability due to the amine substituent on the pyridine, which facilitates hydrogen bonding between CP chains, and an enhanced ability to interact with CO₂, as confirmed by adsorption experiments and DFT calculations. The optimized mixture achieved selective methanol production of 894 μg·g cat⁻¹·h⁻¹, significantly surpassing the benchmark photocatalytic system TiO₂@3%CuO (318 μg·g cat⁻¹·h⁻¹). Furthermore, TiO₂@5%CP4 maintained stable photocatalytic performance over 10 hours without noticeable degradation. 1. Introduction Carbon dioxide (CO 2 ) is the primary contributor to global climate change, accounting for over 64% of greenhouse gas emissions into the atmosphere. [1] Excessive CO 2 release has severely disrupted the natural carbon cycle, leading to major environmental consequences, most notably the intensification of the greenhouse effect. [2] Consequently, CO 2 capture and transformation technologies are being developed at a rapid pace to mitigate these issues. However, CO 2 exhibits remarkable chemical stability, primarily due to the strength of its C=O double bonds. As a result, any attempt to transform CO 2 requires considerable energy input to overcome this thermodynamic barrier. [3] Since carbon in CO 2 is in its highest oxidation state, reducing it can yield a wide range of valuable products with lower oxidation states, including carbon monoxide, methane, and oxygenated liquid compounds such as methanol, ethanol, and formic acid. [4] Various strategies have been developed for CO 2 conversion, including chemical, enzymatic, thermochemical, electrochemical, biochemical, and photochemical methods. [5] Among these, photocatalytic CO 2 reduction is regarded as a sustainable and cost-effective approach, enabling the transformation of CO 2 and H 2 O into hydrocarbon fuels using solar energy as the sole energy input. [6,7] The first reports on photocatalytic CO 2 reduction emerged almost simultaneously: Halmann (1978) demonstrated the formation of formic acid using a p-type gallium phosphide photoelectrode after 18 hours of reaction, [8] and Inoue (1979) reported the production of methanol and formaldehyde using semiconductors such as TiO 2 , ZnO, CdS, GaP, and SiC in aqueous suspensions. [9] After that, TiO 2 has been extensively studied in CO 2 photoreduction, due to its favourable optoelectronic characteristics, high thermal stability, low cost, and strong photocatalytic activity. [10,11] Among its various crystalline forms, the anatase phase exhibits superior photoconductivity, with a bandgap of around 3.2 eV. [12] Despite its advantages, TiO 2 has certain limitations, primarily due to its relatively low quantum efficiency. This is largely due to a high recombination rate of up to 90% of the photogenerated electron-hole pairs during the photocatalytic process. [13] To enhance the quantum efficiency of TiO 2 and suppress charge recombination, various modification strategies have been explored, such as doping and the forming heterojunctions with different cocatalysts. [14] These approaches promote more efficient charge separation by facilitating the transfer of photogenerated electrons to the cocatalyst, thereby reducing the likelihood of electron-hole recombination. The cocatalyst also increases affinity towards the CO 2 molecule and largely determines the selectivity of the reaction towards different products (CO, HCOOH, CH 4 , CH 3 OH, CH 3 CH 2 OH, etc.). [15–17] A wide array of cocatalysts has been investigated for this purpose, including different metal sulfides, [18] metal oxides, [19–24] graphitic carbon nitride (g-C 3 N 4 ), [25] and metal halides. [26] Among these, copper and copper oxide co-catalysts have garnered particular attention due to their notable efficiency in facilitating alcohol production during CO 2 photoreduction. [27–29] However, the production efficiency and selectivity remain low by industry standards, prompting extensive research to identify the most appropriate TiO 2 cocatalyst. The application of coordination compounds (CCs) to CO 2 photoreduction is emerging as their flexible and dynamic structures, tunable electronic properties, and adaptable surface chemistry are yielding highly promising results. [30] Studies have been primarily focused on porous coordination polymers (PCPs), such as metal-organic frameworks (MOFs), [31–33] as the source of e - /h + pairs upon light irradiation. [34,35] However, the most efficient MOFs for CO 2 reduction rely on their doping or combination with nanoparticles of an inorganic cocatalyst (RuO x , Cu, Au, etc.). [36,37] These inorganic cocatalysts are usually based on soft metals that can interact better with CO 2 molecules and facilitate the electron transfer to them more readily. Water-stable MOFs are usually based on a combination of secondary building units (SBUs) involving hard transition metal ions and carboxylate ligands. Therefore, they require a second cocatalyst or on a postsynthetic modification of the MOF to introduce a CO 2 interaction site that interacts better with CO 2 , often by doping with softer metal atoms, ions or complexes. [38,39] The use of non-porous coordination polymers (NPCs) is less studied, but offers a greater material diversity, including many based on soft metal ions. The presence of these soft metal ions in collaboration with well-selected ligands, can increase their capacity to interact with CO 2 compared to their inorganic counterparts. This allows the formation of efficient photocatalytic system by combining well-known inorganic e - /h + pair generators with CPs based on soft metal ions. However, this approach is less studied and usually involves expensive or toxic metals. [40–43] Therefore, it is desirable to develop photocatalysts based on more abundant and environmentally benign metals. Another crucial factor to consider for potential industrial applications is the simplicity of fabrication. However, few investigations are focus on facile room temperature synthesis. [44] This study explores the synergy that can be achieved by combining copper iodide (CuI) with organic ligands derived from pyridines. This approach allows air-stable CPs to be formed in one step at room temperature. These Cu(I)-I-based CPs are insoluble in water and exhibit flexible and dynamic bonding, semiconductor properties and are capable of absorbing visible light. They also have well-established electrical and catalytic properties. [45–48] Specifically, five 1D-CPs were designed and selected for this study. Each incorporates a Cu(I)-I ladder chain motif along with a pyridine derivative coordinated to the metal centre ( Scheme 1 ). The presence of aromatic rings and various functional groups imparts distinct polarities and enables diverse supramolecular interactions, which are expected to enhance CO 2 adsorption and interaction. These CPs exhibit semiconducting behaviour and will be used as cocatalysts alongside TiO 2 reference catalysts. The aim is to improve the latter photocatalytic reduction of CO 2 in aqueous media to produce alcohols without using any sacrificial agent. Scheme 1 2. Results and discussion 2.1. Chemical and morphological characterization The synthesis of these CPs as suitable single crystals useful for SCXRD, implies the diffusion at room temperature between the corresponding ligand (pyridine ( CP1 ), 3-methylpyrdine ( CP2 ), 4-methylpyrdine ( CP3 ), 2-amino-4-methylpyrdine ( CP4 ), and 2-chloro-4-methylpyrdine ( CP5 )) dissolved in ethanol, and CuI dissolved in a saturated aqueous solution of KI. The crystal structures of CP1 , CP2 and CP3 have been previously reported [49–51] and the crystallographic data for CP4 and CP5 are provided in Table S1 . All of them present a one-dimensional structure featuring a ladder-like motif along the Cu(I)-I chains, in which copper(I) centers are interconnected via μ 3 -I bridges. [52] The copper(I) binds to the nitrogen of a pyridine ligand adopting a local tetrahedral geometry. The major change between these compounds relies on whether the pyridinic ligand presents a substitution in ortho positions. CP1-3 , which do not have ortho substituents, provide very similar structural features for their 1D coordination polymer, as can be seen in Table 1 . However, CP4 and CP5 with ortho substituents have a less regular 1D coordination polymer in which short and long Cu···Cu distances alternate ( CP4 : 2.778 and 3.517 Å; CP5 : 2.696 and 3.517 Å). The shortest Cu···Cu distances are below the sum of the van der Waals radii, approximately 2.8 Å, and indicate a meaningful interaction between these Cu centers that can play a significant role in their photocatalytic properties. [53,54] There are also changes on the dihedral angle between consecutive Cu 2 I 2 planar segments that are now more acute and also the pyridinic ring is more twisted with respect to the step of the ladder. Another interesting feature related to CP4 is the N-H···I hydrogen bond taking place within the ladder chain, and probably related to the latter, its short Cu-N bond distance which is the shortest one within this family of compounds. Figure 1. Views of the 1D CPs of CP3 and CP4 highlighting the differences in the alternation pattern. Table 1. More relevant structural parameters of compound CP1-5 . Cu-N (Å) 2.047 2.063 2.051 2.039 2.073 Cu-I step (Å) 2.640 2.661 2.638 2.656 2.673 Cu-I upper (Å) 2.689 2.694 2.693 2.745 2.742 Cu-I lower (Å) 2.644 2.651 2.653 2.661 2.648 Cu···Cu (Å) 2.874 2.911 2.855 2.778, 3.401 2.696, 3.517 I···I (Å) 4.461 4.469 4.479 4.196, 4.534 4.119, 4.588 Cu-I-Cu (º) 65.9, 65.3 66.5, 66.8 65.3, 64.7 63.0, 78.0 60.9, 81.0 I-Cu-I (º) 113.7-115.2 113.1-114.6 114.3-115.7 102.0, 117.0 99.0, 119.1 Dihedral Cu 2 I 2 (º) 119.9 120.8 119.9 110.9 114.1 Torsion I step -Cu-N-C (º) 32.7 31.5 32.9 44.9 49.3 The CP1-3 coordination chains are held together by van der Waals type interactions without evidence of π-stacking interaction between the aromatic rings of the organic ligand. CP5 presents chloro···aromatic interactions [55] within the chain but the interactions holding the chains together are again van der Waals type. CP4 presents stronger supramolecular interactions with hydrogen bonds (N-H···N: 3.28 Å, 158º) being established between the amino groups of adjacent chains and involving an intrachain hydrogen bond involving the amino group and the bridging iodide anions (N-H···I: 3.80 Å, 132º), providing additional stabilization to the crystal structure ( Figure 2b ). In fact, TGA data on these CPs indicate that CP4 exhibits the highest thermal stability (ca. 180ºC), whereas CP1-3 are stable up to 115-140ºC and CP5 is only stable up to 100ºC ( Figure S4 ). Figure 2 . Crystal packing of compounds CP3 (a) and CP4 (b) viewed along the chain propagation direction. Dashed lines indicate hydrogen bonds. These CPs can also be synthesized as microcrystalline powders within minutes in a one-step reaction at room temperature using water as the only solvent and stoichiometric amounts of the building blocks. This new synthesis route is easily scalable and fulfills the green chemistry criteria. The reaction yields range from 40% for CP5 to 75% for CP3 . Elemental analysis, FTIR-ATR and PXRD confirms that these powder samples correspond to CP1-5 , respectively ( Figure S1 and S2 ). The most representative IR signals include ν(C=C + C=N) stretching bands between 1650 and 1550 cm -1 , δ(C-H) bending bands between 1450 and 1350 cm -1 . More specific bands corresponding to the stretching ν(C-C) in CP2-5 in the range 785-880 cm -1 , ν(C-N) in CP4 at 1245 cm -1 and ν(C-Cl) in CP5 as a very intense band at 825 cm -1 . Likewise, in CP4 , v(N-H) stretching bands are identified in the range of 3200 to 3400 cm -1 . SEM micrographs of the microcrystalline powder ( Figures 3a and S3 ) reveal a microrod morphology, in all cases, consistent with the 1D crystalline structures of these materials. Statistical analysis of the rod lengths yields average values ± standard deviation of: CP1 21 ± 8 µm, CP2 20 ± 8 µm, CP3 18 ± 8 µm, CP4 16 ± 6 µm and CP5 19 ± 9 µm. 2.2 CO 2 photoreduction experiments Initial CO 2 photoreduction experiments did not provide evidence of alcohol production for any of these CPs. Therefore, it was decided to employ a mixture of TiO 2 nanoparticles in combination with grinded CPs to create heterojunctions that could boost the CO 2 reduction capability. SEM micrographs reveal that the grinding procedure transforms the initial rod-like particles of CPs (e.g., CP4 with an aspect ratio of 5-20 x 1-2 µm) into more isotropic granules with sizes reduced to 100–300 nm ( Figures 3b and S5 ) maintaining their initial crystalline phase as is confirmed by FTIR-ATR and PXRD analyses ( Figures S6 and S7 ). In addition, the mixing with TiO 2 nanoparticles upon grinding provides a uniform distribution of the co-catalysts, as shown by the mixed color patterns of the SEM-EDX mapping, in which the TiO 2 nanoparticles (~20 nm) ( Figure S8 ) coat the larger CP particles ( Figures 3c, 3d , S9 and S10 ). Again, FTIR-ATR and PXRD analyses confirm the chemical integrity of the CPs upon mixing with anatase ( TiO 2 @CP ) ( Figures S11 and S12 ). Figure 3. SEM micrographs of the CP4 : a) microcrystals before grinding, b) after grinding, c) ground TiO 2 @CP4 , and d) ground TiO 2 @CP4 SEM-EDX. DRS spectra were recorded before and after grinding to estimate the optical band gaps via the Kubelka-Munk function ( Table 2, Figures S13 - S16 ). There is a slight increase in the band gap after grinding, probably related to the reduction of the particle size into the submicrometric regime altogether with probably an increased amount of defects. [56] Table 2. Band gap values (in eV) of CPs and TiO 2 , before and after grinding. Before grinding 2.91 2.94 2.37 3.50 2.96 3.36 After grinding 3.01 3.00 3.11 3.55 3.01 3.36 To ensure reliable and comparable results, the photocatalytic CO 2 reduction process involves multiple experiments conducted under identical conditions (reactor, experimental setup, light source, and reaction parameters). [57] An arbitrary 50:50 wt initial Ti/CP ratio ( TiO 2 @50%CP ) is used for the photocatalytic studies. The results obtained for the different photocatalytic systems are compared and the one exhibiting the highest chemical stability, determined by PXRD and FTIR, and highest alcohol production is selected for an optimization of the cocatalysts mixture ratio. To perform the photocatalytic experiments, the catalyst mixture is placed in a Schlenk flask containing an aqueous potassium bicarbonate solution and sealed with a septum. CO 2 is continuously bubbled through the system at ~25 °C ( Figure 4 ) while irradiated with a UV-lamp (λ = 365 nm). The gas outlet is directed to a cold-water trap to condense the produced alcohols. The experiment was carried out during 10 h over 5 cycles of 2h. The first three cycles were carried out continuously, while the remaining two took place after 12 hours of rest. Figure 4 . Scheme of experimental configuration employed for the CO 2 photoreduction to alcohol studies . First, the photocatalytic activity of pure anatase is evaluated. This is followed by tests using CuO and Cu 2 O as cocatalysts with TiO 2 , [23,24] as these materials have shown promising results in CO 2 photoreduction to alcohols and can be useful as references in our measurement conditions. Given that the studied CPs contain CuI chains, the photocatalytic performance of anatase combined with CuI ( TiO 2 @CuI ) is also investigated. Finally, the CO 2 photoreduction capacity of anatase mixed with the synthesized CPs is assessed. The introduction of 50 wt% Cu 2 O as a cocatalyst ( TiO 2 @50%Cu 2 O ) significantly enhances methanol and ethanol production compared to pure anatase, but it is the TiO 2 @3%CuO catalytic mixture which outstands among the reference cocatalytic systems, reaching 318 μg·g cat -1 ·h -1 of methanol selectively ( Figure 5 ). The TiO 2 @50%CuI system achieved a methanol yield of 84 μg·g cat -1 ·h -1 , also with 100% selectivity ( Figure 5 ). All TiO 2 @50%CPs systems produce, at some point, amounts of alcohol rivaling with those of TiO 2 @3%CuO ( Figure 5 ). The stability of the photocatalytic systems was studied upon five consecutive CO 2 photoreduction cycles, each lasting 2 hours. The alcohol production, along the 10 h run is quite irregular except for CP4 . The total accumulated alcohol production ranges from 2640 μg·g -1 for TiO 2 @50%CP2 to 3640 μg·g -1 for TiO 2 @50%CP4 , with methanol selectivity varying from 50% for TiO 2 @50%CP5 to 72% for TiO 2 @50%CP4 ( Figure 5 ). In addition, there is a decay on alcohol production over time for TiO 2 @50%CP1-3 ( Figure 5 ). TiO 2 @50%CP4 shows no clear evidence of decay ( Figure 5 ). Figure 5 . Alcohol production rates for TiO 2 , and TiO 2 @50%CP systems. Methanol and ethanol production are presented in blue and orange color, respectively. After the photocatalysis experiments, the particles of the cocatalysts mixture were recovered and analyzed by FTIR-ATR spectroscopy, PXRD, SEM and SEM-EDX microscopy ( Figures S17-S20 ). The FTIR-ATR spectra do not provide information on the inorganic co-catalyst (TiO 2 ), but they do allow characterization of the CP co-catalyst. CP1 - CP4 do not undergo appreciable alterations after the photocatalytic process, with the characteristic signals of the pyridinic ligand being retained ( Figure S17 ). However, the FTIR-ATR of CP5 shows the disappearance of the ligand bands, indicating its transformation after 10 hours of photocatalytic reaction ( Figure S17 ). The PXRD obtained after the CO 2 photoreduction process show no variation in the patterns of TiO 2 @50%CP1-CP4 ( Figure S18 ); however, in TiO 2 @50%CP5 the CP5 associated diffraction peaks disappear ( Figure S18 ). This suggests that the crystal structures of CP1 - CP4 are unchanged, while CP5 does not withstand the photocatalytic conditions releasing the organic ligand. SEM and SEM-EDX images clearly indicate that the homogeneity of the initial TiO 2 @50%CP catalytic mixtures is only retained, after the photoreduction reaction, for TiO 2 @50%CP4 ( Figures 6, S19 and S20 ). The remaining mixtures, TiO 2 @50%CP1-3, show a growth of the CP particles into the micrometric regime again with the segregation of the TiO 2 nanoparticles. This growth of the CP1-3 particles seems to be due to a solubilization/recrystallization process and this, although not clearly seen in the 10 h photocatalytic run, could have a negative impact on the alcohol production because of the surface area reduction. The apparent lower solubility of CP4 could be related to the presence of stronger supramolecular interactions (N-H···N hydrogen bonds) holding the coordination chains together. Figure 6 . SEM and SEM-EDX micrographs of the TiO 2 @50%CP1 (a and b) and TiO 2 @50%CP4 (c and d), after 10 h of CO 2 photoreduction The reason for the good performance of the TiO 2 @50%CP catalytic systems can be attributed to several reasons but perhaps the two most important ones would be interaction with the CO 2 molecule [58] and the energy arrangement of the CB and VB in both cocatalysts. In fact, the photocatalytic reduction of CO 2 on the surface of a semiconductor involves five fundamental steps: (1) CO 2 adsorption, (2) light absorption, (3) charge separation and transfer, (4) surface redox reactions, and (5) desorption of the reaction products. [59] Therefore, it is important to determine if the incorporation of CP cocatalysts enhances the CO 2 adsorption, which can occur via the carbon atom, oxygen atoms, or a mixed mode. [60–62] To this end, N 2 and CO 2 adsorption isotherms were measured at 77 K and 273 K, respectively ( Figures S21 - S24 ). Table 3 lists the specific area values, determined by the BET method, together with the maximum adsorption capacity of CO 2 . The N 2 isotherms of the CPs ( Figure S22 ) present a type II profile and exhibit very low surface areas, confirming its non-porous character. In contrast, the isotherms corresponding to TiO 2 and the TiO 2 @50%CP ( Figures S21 and S23 ) are of type IV and show similar hysteresis loops, evidencing the presence of mesopores in both samples probably coming from the interparticle space. The calculated BET surface area of TiO 2 nanoparticles (83.3 m 2 /g) is diluted when mixed with the CP particles. Interestingly the BET surface area is more than halved with respect to TiO 2 which can be understood as a sign of good adhesion between both types of particles as shared surfaces will not compute as external surface. The mixtures with the lower surface area values are TiO 2 @50%CP3-4. The CO 2 adsorption capacity of TiO 2 @50%CPs is also reduced with respect to TiO 2 , but normalizing these values with respect to the surface area provides also an insight into the CO 2 interaction with these CPs, with TiO 2 @50%CP4 providing the higher CO 2 capture per surface. Table 3 . Textural characterisation of TiO 2 , CP1-4 and TiO 2 @50%CP1-4 . S BET (m 2 /g) a 83.3 5.16 2.45 4.29 2.69 Mixture of TiO 2 /CP 50 wt.% S BET (m 2 /g) a 83.3 31.18 26.85 24.36 25.59 C CO2 273 K (mmol/g) b 0.153 0.062 0.066 0.064 0.097 C CO2 * 273K (10 -3 mmol/m 2 ) c 1.85 1.99 2.46 2.63 3.79 a Surface area BET. b CO 2 adsorbed capacity. c CO 2 adsorbed capacity normalised to surface area. XPS studies were carried out to determine the processes taking place at the surface level of TiO 2 @50%CP4 during the photocatalytic reaction. Spectra were recorded for both pure CP4 ( Figure S25 ) and TiO 2 @50%CP4 after the photoreduction reaction ( Figure S26 ). The Ti 4+ characteristic Ti 2p 3/2 (458.2 eV) and Ti 2p 1/2 (463.9 eV) signals agree with those reported for commercial pure TiO 2 nanopowder (Degussa P-25) ( Table S2 ). [63] However, the characteristic signals of Cu and I, in TiO 2 @50%CP4 , are displaced toward smaller binding energies (0.3 eV for copper and 0.6 eV for iodine) with respect to CP4 ( Figure 7a and 7b ). These variations are attributed to a higher electron density on CP4 particles due to formation of heterojunctions with TiO 2 . The absence of characteristic Cu 2+ satellite peaks at higher binding energies (BE) indicates that the copper is mostly in the Cu + state, in agreement with what has been reported for similar systems. [64] Two main components are identified in the C 1s spectrum, with bond energies of 284.6 eV and 286.0 eV, assignable to C-C/C-H and C-N bonds, [65] respectively. Additionally, the TiO 2 @50%CP4 sample exhibits a third component at 288.2 eV, compatible with N-C=O bonds, which confirms the presence of carbonyl species derived from CO 2 adsorbed on the surface after the photoreduction test ( Figure 7c ). [66] The N 1s spectrum shows a main peak at 399.2 eV in CP4 and at a slightly lower binding energy of 398.8 eV in TiO 2 @50%CP4 , which are characteristic of the pyridine nitrogen in the pyridine ring. [36] In addition, a secondary component at 400.4 eV appears in TiO 2 @50%CP4 , which is attributed to the interaction of pyridine with CO 2 ( Figure 7d ). [67] The O 1s region in TiO 2 @50%CP4 is resolved into two components: a main peak at 529.4 eV, attributed to oxygen in the TiO 2 lattice, and a second at 530.9 eV, corresponding to carboxylate groups. [68] Figure 7. XPS spectra of CP4 (bottom) and TiO 2 @50%CP4 (top) in Cu 2p region (a), I 3d region (b), C 1s region (c), and N 1s region (d). XPS measurements in the low binding energy for TiO 2 and TiO 2 @50%CP4 give an insight on the relative energy of the valence band (VB) upper edge of TiO 2 and CP4 . These results indicate that the VBE ( CP4 ) is 2.16 eV above that of TiO 2 ( Figure S27 ). Combining this information with the band gap values obtained from the DRS measurements allow us to depict the conduction band (CB) and the energy diagram present in the TiO 2 @50%CP4 photocatalytic system ( Figure 8 ), which strongly suggest the presence of a Z-scheme process that would explain the good performance of this mixture. Figure 8. Normal Hydrogen Electrode (NHE) potential values of the TiO 2 and CP4 VB and CB edges based on the experimental DRS and XPS low binding energy measurements. 2.3 Computational calculations The XPS and DRS provided insight on the electronic structure of CP4 was complemented with theoretical Density Functional Theory (DFT) calculations (see computational details in the “Experimental Section” part). These calculations provide not only approximate values of the VBE/CBE and the band gap energies, which can be compared to the experimental ones, but also the orbitals contributing to the energy levels located at these band edges. The procedure implies an optimization of the crystal structure of CP1-4 , which produced deviations lower than 4% in the lattice constants with respect to the crystallographic ones. On the top of these optimized cells, the Projected Density of States (PDOS) were computed, obtaining the contributions of the different angular momenta of each atom ( Figure 9 and S28 ). Figure 9. Density of states of a) CP1 , b) CP2 , c) CP3 and d) CP4 with projection on the ligand atoms and on the Cu and I unit atoms. In all the computed CPs, the valence band energy (VBE) is largely located on the Cu and I atoms. However, the conductive band energy (CBE) is located mainly on the organic ligand atoms, pointing to a metal to ligand charge transfer transition. The computed band gap is 2.8 eV for CP1 and CP2 , 2.7 eV for CP3 , and 3.4 eV for CP4 , in good agreement with the experimentally measured values for the non-grinded CPs ( Table 2 ). However, to explain the favourable effect of heterogeneous mixing between TiO 2 and the CPs on the CO 2 photoreduction, the relative positions of the VB and CB band edges of the cocatalysts used needs to be considered. The absolute CB and VB edges energy values, referred to the vacuum, corresponding to the CPs have been calculated for all CPs, TiO 2 and CuI. The latter ones to establish the accuracy of the theoretical method. To this, we have calculated the difference between the Fermi level and the asymptotic (Hartree) potential in vacuum ( Figure S29 ), which allows us to determine the work function of the material ( Table S3 ). The remaining data for other cocatalysts have been extracted from the literature ( Table S4 ). [69,70] These energy levels translated into NHE potentials, together with the reduction potentials of CO 2 to methanol and ethanol at pH 7.35, which corresponds to the value reached when bubbling CO 2 in a 0.5 M potassium bicarbonate solution, are presented in Figure 10 . There is a significant mismatch between the DFT theoretical and experimental band edges values, but the same trend observed on the experimental data is reproduced. TiO 2 anatase provides the deepest energy levels, CuI raises these values probably because of the most reduced nature of its components, and the incorporation of the pyridines in the CPs increases these levels even more, approaching their VBE of CPs to the CBE of TiO 2 . The VBE of CPs gets closer to the CBE of TiO 2 reinforcing a Z-scheme mechanism that provides efficient electron-hole spatial separation ( Figure 8 ). Figure 10. Energy values of the CB and VB edges of the photocatalysts as reported in the literature and calculated for each CP and selected inorganic cocatalysts. Orange values correspond to DFT provided values ( Table S3 ) and blue values are derived from the literature and, in the case of CP4, experimental DRS and XPS low binding energy measurements ( Table S4 ). Theoretical calculations also allow the study of CO 2 adsorption, which point to a higher (more negative, thus more favorable) adsorption energy in CP2-4 , which are the compounds with substituents in the pyridine ring ( Figure 11 ). The presence of these groups increases the van der Waals type interactions that CO 2 molecule can establish with the surface of the CP particles, although the preferential adsorption site is close to the iodine atom of the Cu(I)-I unit due to its higher electron density. Specifically, CP3 and CP4 show the highest theoretical affinities for CO 2 ( Figure 11 ). Although computational models do not differentiate between them, already described CO 2 experimental adsorption isotherms confirm that CP4 adsorbs significantly more CO 2 than the other compounds ( Figure S24 ). This difference is attributed to the presence of an amino group at position 2 of the pyridine ring, adjacent to the Cu(I)-I chain, which locally increases the electron density of the iodine atom and thereby enhances CO 2 interaction. This increase in electron density has also been observed in the XPS results, where iodine exhibits lower BE in TiO 2 @50%CP4 ( Table S2 and Figure S26 ) compared to pure CP4 ( Table S2 and Figure S25 ). This suggests a mechanism in which CO₂ is attracted to and stabilized by the iodine atom of the Cu(I)-I chain, while the pyridine ring acts as an electron donor site, transferring the excited electron to CO 2 . On the other hand, geometry optimizations reveal that, although the interaction is stronger, the O 2 C-I distances in CP4 , are larger than for CP1-3 . This is not a problem for the electron transfer process, since the excited electron is located at the organic part, as previously discussed. However, this larger distance in CP4 , may difficult the adsorption of the intermediates that should be stabilized to evolve to C 2+ products, such as ethanol, which could explain the higher selectivity of TiO 2 @50%CP4 towards methanol compared to the other TiO 2 @50%CPs . Figure 11. Preferred CO 2 adsorption site on the most stable surface of each CP, computed adsorption energy values, and C CO2 ···I distance. 2.4 Optimization of the TiO 2 /CP4 ratio The alcohol production has been maximized by the optimization of the ratio between the two catalysts in the TiO 2 @CP4 mixture. The photoreduction experiments were performed by varying the proportion of CP4 (5, 25, 50 and 75% by mass), keeping the total mass constant at 20 mg ( Figure 12 ). The results show a progression on the alcohol production when increasing the TiO 2 proportion in the photocatalytic mixture with values that change from null alcohol production for TiO 2 @75%CP4 to 894 ± 80 μg·g cat -1 ·h -1 and 100% selectivity towards methanol for TiO 2 @5%CP4 , with no evidence of decay during the 10 h run. This yield far exceeds the 318 μg·g cat -1 ·h -1 of methanol produced by the benchmark TiO 2 @3%CuO ( Figure 4 ) and emphasizes the relevance of having abundant electron transfer from the TiO 2 nanoparticles to CP4 . Figure 12 . Alcohol production rates for TiO 2 @CP4 at different cocatalyst ratios keeping the total mass of the photocatalytic system at 20 mg. Five consecutive CO 2 photoreduction cycles, each lasting 2 hours, were performed for each TiO 2 @CP4 system. Methanol and ethanol production are presented in blue and orange color, respectively. Red dot line corresponds to the total alcohol production value of the reference mixture TiO 2 @3%CuO . The selectivity toward methanol of TiO 2 @5%CP4 contrasts with the methanol/ethanol mixtures obtained for the less TiO 2 rich mixtures. The reason for this selectivity toward methanol, which cannot be attributed entirely to the chemical features of CP4 , must be understood as a competition between the two reduction pathways, as illustrated in Figure 13 . The mechanism that produces methanol is favoured under highly reductive conditions, whereas the pathway that produces ethanol requires the formation of a C-C bond. This implies that the latter would be favoured under conditions in which the initial intermediate single-carbon species have enough time to wait for a second CO 2 molecule to be adsorbed at an adjacent position or to migrate along the surface of the cocatalyst, meet another single carbon species, and evolve into two carbon species before being further reduced. [71,72] Figure 13. The proposed mechanisms for the different-selectivities shown by a) the catalytic mixture under TiO 2 -rich conditions ( TiO 2 @5%CP4 ) and b) more TiO 2 -poor conditions. 3. Conclusion These one-dimensional CPs in which Cu(I)-I ladder-like chain is stabilized by the coordination of pyridines have their VBE and CBE high in energy with band-gaps of 2.87 - 3.54 eV, for ground CP. These electronic features do not produce a stand-alone efficient photocatalyst for CO 2 reduction as the experimental results indicate and the high energy VBE anticipates (above the potential required for H 2 O oxidation). However, its combination with the ubiquitous and commercially available TiO 2 anatase nanoparticles provides photocatalytic systems that generate a high alcohol output when compared with referential materials under the same photoreactor conditions. It has been explained based on several facts. First, TiO 2 anatase has a band gap similar to these CPs and therefore both can be activated using the same UV-lamp (365 nm). Second, the relative VBE/CBE energy values for the two cocatalysts habilitate a Z-scheme mechanism in which the highly reducing photoexcited electrons and the highly oxidizing holes are separated. The photoexcited electron is located in the CB of CPs, high in energy, and the electron-hole is placed in the VB of TiO 2 , deep in energy. This arrangement maximizes the reductive capacity of the photoexcited electrons and the oxidative capacity of the electron-holes. The experimental setup for the photocatalytic reaction uses a CO 2 flux of 100 mL·min -1 bubbling in water containing the particles of the two cocatalysts and a cold trap connected to the gas-outlet of the reactor. This reactor design allows the fast withdrawal of the produced alcohols before they could be employed as oxidative sacrificial material by TiO 2 instead of water and leading to zero, or close to zero, net production of alcohol. The best performing TiO 2 @CP mixture corresponds to that of CP4 which brings together higher chemical stability because of the amine substituent of the pyridine establishing hydrogen bonds linking together the CP chains and greater capability to interact with CO 2 molecule as proved by experimental adsorption measurements and DFT calculations. The optimization of the mixture of both cocatalysts leads to selective methanol production of 894 μg·g cat -1 ·h -1 for TiO 2 @5%CP4, which is considerably higher than that of TiO 2 @3%CuO (318 μg·g cat -1 ·h -1 ; a benchmark photocatalytic system). In addition, TiO 2 @5%CP4 withstands 10 h run of photocatalytic reaction without evidence of decay. To conclude, it is important to emphasize that this photocatalytic system is built from cheap chemicals, avoiding the use of noble metal or rare earth elements. TiO 2 nanoparticles are available at high quantities and low prices, while Cu(I)-I based CPs are synthesized under room conditions, using water as solvent and following a easily scalable soft chemistry procedure. 4. Experimental Section 4.1 Materials All reagents were used as obtained from the suppliers. Ethanol (≥99.9%) was purchased from Scharlau. Pyridine (≥99.8%) was supplied by Prolabo. The supplier for 3-methylpyridine (≥99%), 2-amino-4-methylpyridine (≥99%), 2-chloro-4-methylpyridine (≥98%), copper(I) iodide (≥99%), titanium(IV) oxide, anatase (<25nm, ≥99.7%) and potassium bicarbonate (≥99.5%) is Sigma Aldrich. Potassium iodide (≥99.5%) was purchased from Fluka. 4-Methylpyridine (≥99%) was supplied by Thermo scientific. 4.2 Synthesis 4.2.1 [CuI(L)] n polycrystals ( L= pyridine (CP1), [49] 3-methylpyridine (CP2), [50] 4-methylpyridine (CP3), [50] 2-amino-4-methylpyridine (CP4), and 2-chloro-4-methylpyridine (CP5). Solutions of CuI (190.5 mg, 1 mmol) in concentrated aqueous KI (4 g, 4mL) are prepared under magnetic stirring (800 rpm) at room temperature. After that, and according to the compound being prepared, ligand of pyridine (79,1 mg, 1 mmol), or 3-methylpyridine (93.1 mg, 1 mmol), or 4-methylpyridine (93.1 mg, 1 mmol), or 2-amino-4-methylpyridine (108 mg, 1 mmol), or 2-chloro-4-methylpyridine (128 mg, 1 mmol) are added. Immediately white precipitates are formed on all synthesis, and the reactions are stirred at 25 °C for 15 minutes. The obtained precipitates are filtered off under vacuum and washed with concentrated aqueous KI (10 mL × 2) and H 2 O (15 mL × 3) respectively. Finally, the precipitates are dried under vacuum for 24 h. Yields: ( CP1 ) 175 mg, yield: 65%, ( CP2 ) 196 mg, yield: 69%; ( CP3 ) 213 mg, yield: 75% ( CP4 ) 203 mg, yield: 68%, and ( CP5 ) 127 mg, yield: 40%, based on the monomer of each CP. ( CP1 ) Anal. calcd for [CuI(C 5 H 5 N)] n : C, 22.28; H, 1.87; N, 5.20; found: C, 22.12; H, 2.08; N, 5.23. IR (cm -1 ): 1597 (w), 1481 (w), 1442 (m), 1212 (w), 1146 (w), 1068 (w), 1038 (w), 1007 (w), 748 (m), 694 (s) and 632 (w). ( CP2 ) Anal. calcd for [CuI(C 6 H 7 N)] n : C, 25.41; H, 2.49; N, 4.94; found: C, 25.30; H, 2.60; N, 4.95. IR (cm -1 ): 1601 (w), 1478 (w), 1451 (w), 1416 (w), 1369 (w), 1188 (w), 1123 (w), 1100 (w), 1045 (w), 1034 (w), 791 (m), 698 (s) and 648 (w). ( CP3 ) Anal. calcd for [CuI(C 6 H 7 N)] n : C, 25.41; H, 2.49; N, 4.94; found: C, 25.46; H, 2.57; N, 4.94. IR (cm -1 ):1611 (m), 1495 (m), 1420 (m), 1382 (w), 1329 (w), 1221 (m), 1208 (m), 1067 (w), 1013 (m), 805 (s), 718 (m) and 672 (w). ( CP4 ) Anal. calcd for [CuI(C 6 H 8 N 2 )] n : C, 24.14; H, 2.70; N, 9.38; found: C, 24.05; H, 2.79; N, 9.27. IR (cm -1 ): 3406 (w), 3314 (w), 3209 (w), 1630 (s), 1592 (m), 1556 (m), 1488 (w), 1436(m), 1376 (w), 1311 (m), 1242 (w), 1182 (w), 1000 (w), 952 (m), 799 (s), 770 (w), 746 (w) and 638 (w). ( CP5 ) Anal. calcd for [CuI(C 6 H 6 NCl)] n : C, 22.66; H, 1.90; N, 4.40; found: C, 22.48; H, 1.86; N, 4.20. IR (cm -1 ): 1596 (w), 1541 (w), 1472 (w), 1380 (w), 1284 (w), 1220 (w), 1132 (m), 1084 (m), 1004 (w), 880 (w), 828 (s) and 720 (w). PXRD and IR of the CPs are in Figures S1 and S2. 4.2.2 CP3-CP5, single crystals. Single crystals of compounds CP4 and CP5 were obtained by slow diffusion or layering technique at room temperature. In this respect, three phases were prepared and carefully deposited one on top of the other in a sample tube. The upper phase is a solution of 4-methylpyridine (47 mg, 0.5 mmol), 2-amino-4-methylpyridine (54 mg, 0.5 mmol) or 2-chloro-4-methylpyridine (64 mg, 0.5 mmol) dissolved in 2 ml of ethanol, the middle phase is pure ethanol (1 mL), and the lower phase is a solution of CuI (95 mg, 0.5 mmol) in 2 mL of concentrated aqueous KI solution. Both sample tubes were capped with Parafilm© and allowed to stand in a vertical position. After 24 hours, suitable colorless crystals needle-shaped were obtained at the interface between the EtOH layer and the concentrated aqueous KI solution layer. These crystals were useful for single-crystal X-ray diffraction studies. 4.3 Preparation of TiO 2 /CP (CP1-CP5) catalysts 20 mg of 50:50 mixture of TiO 2 and the corresponding CP as cocatalyst was weighed. They were ground for 5 minutes in an agate mortar until a homogeneous mixture was obtained. It was then suspended in 5 mL of a 0.5 M potassium bicarbonate solution with a pH of 8.5 and sonicated for 15 minutes to break up the agglomerates. Finally, this suspension was used to carry out the photoreduction tests. 4.4 Methods and equipment’s Infrared Spectrum (FTIR-ATR). The infrared spectra were recorded on a PerkinElmer spectrophotometer equipped with a MIRacle Universal Attenuated Total Reflectance accessory (ATR). Powder X-ray Diffraction (PXRD) data were collected on a Rigaku Miniflex 600 HyPix-400 MF 2D together with 600W X-ray source diffractometer with geometry θ/2θ (Cu-Kα radiation; λ = 1.5418 Å). The samples were scanned over a range of 3° to 50° (θ/2θ), using an angular increment of 0.03° and a counting time of 10 º per minute. Elemental Analysis (EA) measurements were carried out using a LECO CHNS-93217 elemental analyzer. Single-crystal X-ray diffraction (SCXRD) single-crystal diffraction X-ray diffraction data for structure determination were collected on XtaLAB Synergy, Single source at home/near, HyPix diffractometer ( λ MoK α = 0.71073 Å at 295 K). Data reduction was done with the CrysAlisPro program (Rigaku) [73] and refined by full-matrix least-squares on F 2 including all reflections (SHELXL). [74] All calculations for these structures were performed using the OLEX2 crystallographic software package programs. [75] Details of the structure determination and refinement of all compounds are summarized in Table S1 of Supporting Information. Diffuse Reflectance (DRS). Measurements were carried out on a Varian Cary 500 spectrophotometer fitted with an integrating sphere accessory. The resulting diffuse reflectance data were transformed using the Kubelka–Munk function, [76] which enables the conversion of reflectance values to an equivalent absorbance unit. This transformation provides a basis for plotting the Tauc expression, [77] often used to analyze optical band gaps. [78] Thermal analysis (TGA) was performed on a METTLER TOLEDO TGA/SDTA851 thermal analyzer in synthetic air (80% N 2 , 20% O 2 ) flux of 50 cm 3 .min –1 , from room temperature to 800 °C with heating rate of 5 °C min –1 and a sample size of about 10–20 mg per-run. Scanning Electron Microscopy ( SEM ). Images were acquired on a Hitachi S-3000N SEM-EDX electron microscope, operating at an accelerating voltage of 5.0 kV under a chamber pressure of 10 -9 Pa. Prior to imaging, the samples were coated with a thin chromium layer at a pressure of 10 -3 Pa. N 2 and CO 2 adsorption isotherms were measured using a Micromeritics 3Flex volumetric instrument under static adsorption conditions. Samples were previously activated at 50 °C overnight and outgassed to 10 −6 bar. According to IUPAC recommendations, the specific surface area by the BET method can be estimated in the range of P/P 0 =0.05 to 0.30 for non-porous and macroporous solids (Type II isotherms), as well as for mesoporous materials (Type IV isotherms). [79] However, for microporous materials (Type I isotherms) this range may be affected; in such cases, it is appropriate to use lower relative pressures following the Rouquerol criteria. [80] These criteria require that the BET constant ‘C’ is positive, that the n(1-P/P 0 ) term increases continuously with P/P 0 over the chosen range, and that the value of P/P 0 that determines the monolayer capacity is within the range. In general, for Type I isotherms, the applicable range is usually P/P 0 <0.08. X-ray Photoelectron Spectroscopy ( XPS ) measurements were performed on a Versaprobe III AD Physical Electronics (ULVAC) system with a monochromatic Al Kα (1486.7 eV) radiation source. An initial analysis was carried out to determine the elements present (wide scan: step energy 0.2 eV, pass energy 224 eV) and detailed analysis of the detected elements (detail scan: step energy 0.05 eV, pass energy 27 eV, time per step 20 ms, (pass energy 13 eV and step energy 0.025 eV for BV region)) was performed with an electron output angle of 45°. The spectrometer was previously calibrated with Ag (Ag 3d 5/2 , 368.26 eV). The XPS data was processed using CasaXPS 2.3.26 software. CO 2 photoreduction experiments were carried out using a mixture of TiO 2 /CP in different proportions, by manual grinding in mortar agata for 5 minutes. Additionally, a PhotoRedOX Box reactor equipped with a monochromatic UV lamp (Hepatochem, λ_exc = 365 nm) of 10 W power was used. For the reaction, 20 mg of a heterogeneous catalyst mixture was placed in a Schlenk tube containing 5 mL of 0.5 M potassium bicarbonate solution with a pH of 8.5. The tube was sealed with a septum crossed by a syringe connected to a CO 2 cylinder, with a flow rate of approximately 100 mL·min -1 . To the side arm of the Schlenk tube was attached an 8 mm internal diameter conduit to evacuate the carrier gas. A 15 cm hypodermic needle was attached to the opposite end and inserted into the septum of a test tube containing 6 mL of distilled water. This test tube was immersed in an ice/water bath, at a temperature between 0 and 4 °C, so that the long needle ensured bubbling of the gas to the bottom. Another needle, 3 cm long and located above the water level, was used as a final escape route for the carrier gas. With this arrangement, the produced alcohol circulates through the 15 cm needle into the test tube, which acts as a trap for condensable species, while the carrier gas exits through the 3 cm needle ( Figure 4 ). The experiment is conducted for 10 h in 5 cycles of 2 hours each. The first three trials were carried out continuously, while the remaining two took place after 12 hours of rest. The liquid phase retained in the cold trap was analyzed with a gas chromatograph (Shimadzu Nexis GC-2030) equipped with a 30 m Shimadzu SH-Wax column (He as a carrier gas, flow rate 1.54 mL.min -1 ) and a dielectric barrier discharge ionization detector (BID). The operating temperature was set at 35 °C and injections (of 5 μL in splitless mode) were performed in triplicate to assess reproducibility. A calibration curve was prepared for methanol and ethanol in the range 0 to 100 ppm, and limits of detection and quantification were established from the signal-to-noise ratio. Any measurement below these values was discarded. Theoretical calculations. All the simulations were performed in the frame of the Density Functional Theory (DFT) with the Vienna Ab initio Simulation Package (VASP), [81] which considers periodic boundary conditions for a proper description of the materials. The electronic structure was expanded in a plane-wave basis, with an energy cutoff of 400 eV, and the interaction between electrons and nuclei described using the Projected Augmented Wave (PAW) pseudopotentials, as provided by the VASP source. Reciprocal space was sampled with the Monkhorst-Pack scheme using a grid of 3-5-1 points. For geometry optimizations the optPBE functional [82] was used. Convergence threshold for electronic energies and forces were 10 -5 eV and 0.01 eV/Å respectively. On the top of the optimized structures, single point calculations with the hybrid HSE06 functional [83] were performed, to obtain more accurate band gaps. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. The supplementary information contains detailed experimental procedures and computational details. CCDC 2472938 and 2472939 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Acknowledgements This work has been supported by MCINN/AEI/10.13039/5011000011033 under the National Program of Sciences and Technological Materials (PID2022-138968NB-C21, PID2022-138968NB-C22, PID2022-138470NB-I00) and by the Basque Government (T1722-22). Julián Ávila Duran thanks for his predoctoral fellowship (PREP2022-000268), and Jon Napal thanks UPV/EHU for his predoctoral fellowship (PIF22/139). The authors thank to the Single Crystal XRD laboratory, to the Universidad Autónoma de Madrid (UAM), and to the Servicio Interdepartamental de Investigación (SIdI), for the use of their scientific equipment. The authors also thank the allocation of computer time at the Centro de Computación Científica at the Universidad Autónoma de Madrid (CCC-UAM) and for technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF). Received: (will be filled in by the editorial staff) Revised: (will be filled in by the editorial staff) Published online: (will be filled in by the editorial staff) References [1] M.-J. Choi, D.-H. Cho, CLEAN – Soil, Air, Water 2008 , 36 , 426.[2] J. Fu, K. Jiang, X. Qiu, J. Yu, M. Liu, Mater. Today, 2020 , 32 , 222.[3] S. Xie, Q. Zhang, G. Liu, Y. Wang, Chem. Commun. 2015 , 52 , 35.[4] X. Chang, T. Wang, J. Gong, Energy Environ. Sci. 2016 , 9 , 2177.[5] A. Saravanan, P. Senthil kumar, D.-V. N. Vo, S. Jeevanantham, V. Bhuvaneswari, V. Anantha Narayanan, P. R. Yaashikaa, S. Swetha, B. Reshma, Chem. Eng. Sci. 2021 , 236 , 116515.[6] K. Li, B. Peng, T. Peng, ACS Catal. 2016 , 6 , 7485.[7] Y. Cui, P. Ge, M. Chen, L. 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Supporting Information Boosting CO 2 to Alcohol Conversion: Powerful Photocatalysts Based on TiO 2 -Cu(I)-Iodine-Pyridine 1D Coordination Polymers Julian Avila-Duran 1 , Jon Napal 2 , Fernando Aguilar-Galindo 3,4 Oscar Castillo 2* , Pilar Amo-Ochoa 1, 4* To Lucia Garrote de la Fuente J. Avila-Duran, Prof. P. Amo-Ochoa 1 Inorganic Chemistry Department, Faculty of Sciences, Autonomous University of Madrid (UAM), 28049 Madrid, Spain. J. Napal, Prof. O. Castillo 2 Department of Organic and Inorganic Chemistry, University of the Basque Country (UPV/EHU), P.O. 644, E-48080 Bilbao, Spain F. Aguilar-Galindo 3 Chemistry Department, Faculty of sciences, Autonomous University of Madrid (UAM), 28049 Madrid, Spain. Prof. P. Amo-Ochoa 4 Institute for Advanced Research in Chemical Sciences (IAdChem). Autonomous University of Madrid (UAM), 28049 Madrid, Spain E-mail: [email protected] / [email protected] Experimental details SCXRD of CPs Table S1 . Crystallographic parameters of compounds CP4 and CP5 . Formula C 6 H 8 CuI N 2 C 6 H 6 ClCuIN D calc / g.cm -3 2.374 2.489 μ / mm -1 6.236 6.448 Formula Weight 298.58 318.01 Colour Clear colourless Clear colourless Shape Needle Needle Size/ mm 3 0.18x0.11x0.05 0.252x0.058x0.038 T/ K 295 295 Crystal system Monoclinic Triclinic Space group P 2 1 /n P -1 a/ Å 11.4046(3) 4.30550(10) b / Å 4.2166(1) 10.0703(2) c / Å 17.9223(5) 11.0726(2) α/ º 90 113.967(2) β/ º 104.244(3) 93.021(2) γ/ º 90 101.841(2) V/ Å 3 835.36(4) 424.357(17) Z 4 2 Measured Ref. 15051 15591 Independent Refl. 1521 1554 GooF 0.961 1.063 w R 2 (all data) 0.0421 0.0518 w R 2 0.0415 0.0510 R 1 (all data) 0.0175 0.0208 R 1 0.0166 0.0196 FTIR-ATR of CPs Figure S1. FTIR-ATR spectrum of microcrystalline CP powders: CP1 (black line), CP2 (red line), CP3 (blue line), CP4 (pink line) and CP5 (green line). PXRD of CPs Figure S2. PXRD of theoretical (black line) and experimental (red line) of a) CP1 , b) CP2 , c) CP3 , d) CP4 and e) CP5 . Morphology characterization of CPs Figure S3. SEM micrographs of a) CP1 , b) CP2 , c) CP3 , d) CP4 and e) CP5 microcrystals. Thermal characterization of CPs Figure S4. TGA analysis of CP1 (black line), CP2 (red line), CP3 (blue line), CP4 (pink line) and CP5 (green line), under synthetic air (80% N 2 , 20% O 2 ). Morphology characterization of ground CPs Figure S5. SEM micrographs of a) CP1 , b) CP2 , c) CP3 , d) CP4 and e) CP5 obtained after grinding for 5 min. FTIR-ATR of ground CPs Figure S6. FTIR-ATR spectrum of ground CP: CP1 (black line), CP2 (red line), CP3 (blue line), CP4 (pink line) and CP5 (green line). PXRD of ground CPs Figure S7. Experimental PXRD of microcrystalline CP (black line) and ground CP (red line) of a) CP1 , b) CP2, c) CP3 , d) CP4 and e) CP5 . Morphology of TiO 2 nanoparticles Figure S8. SEM micrographs of TiO 2 nanoparticles. Morphology of ground TiO 2 @50%CP Figure S9. SEM micrographs of TiO 2 @50%CP systems with a) CP1 , b) CP2 , c) CP3 and d) CP4 obtained after grinding for 5 min. Elemental distribution of ground TiO 2 @50%CP Figure S10. Elemental distribution by SEM-EDX of TiO 2 @50%CP systems with a) CP1 , b) CP2 , c) CP3 and d) CP4 obtained after grinding for 5 min. FTIR-ATR of ground TiO 2 @50%CP Figure S11. FTIR-ATR spectra of TiO 2 @50%CP systems with each CP: CP1 (black line), CP2 (red line), CP3 (blue line), CP4 (pink line) and CP5 (green line). PXRD of ground TiO 2 @50%CP Figure S12. PXRD of experimental TiO 2 @50%CP systems with each CP of a) CP1 , b) CP2 , c) CP3 , d) CP4 and e) CP5 . The red triangles are the peaks corresponding to the TiO 2 . Optical characterization Figure S13 . Experimental diffuse reflectance spectra of the TiO 2 and microcrystalline CP: CP1 (black), CP2 (red), CP3 (blue), CP4 (pink), CP5 (green) and TiO 2 (brown). Figure S14 . Tauc plots obtained with the Kubelka-Munk function and the linear fit for optical band gaps of the TiO 2 and microcrystalline CP: CP1 (black), CP2 (red), CP3 (blue), CP4 (pink) and CP5 (green) and TiO 2 (brown). Figure S15 . Experimental diffuse reflectance spectra of the ground TiO 2 and ground CP: CP1 (black), CP2 (red), CP3 (blue), CP4 (pink) and CP5 (green) and TiO 2 (brown). Figure S16 . Tauc plots obtained with the Kubelka-Munk function and the linear fit for optical band gaps of the ground TiO 2 and ground polymer: CP1 (black), CP2 (red), CP3 (blue), CP4 (pink) and CP5 (green) and TiO 2 (brown). FTIR-ATR of ground TiO 2 @50%CP after CO 2 photoreduction Figure S17 . FTIR-ATR spectra of the TiO 2 @50%CP systems after 10h of CO 2 photoreduction with each CP: CP1 (black line), CP2 (red line), CP3 (blue line), CP4 (pink line) and CP5 (green line). PXRD of ground TiO 2 @50%CP after CO 2 photoreduction Figure S18. PXRD of the experimental TiO 2 @50%CP systems with each polymer (black line) and after 10 h of CO 2 photoreduction (red line) of a) CP1 , b) CP2 , c) CP3 , d) CP4 and e) CP5 . The red triangles are the peaks corresponding to TiO 2 . Morphology of ground TiO 2 @50%CP after CO 2 photoreduction Figure S19. SEM micrographs of TiO 2 @50%CP systems with a) CP1 , b) CP2 , c) CP3 and d) CP4 after 10 h of CO 2 photoreduction. Elemental distribution of ground TiO 2 @50%CP after CO 2 photoreduction Figure S20. Elemental distribution by SEM-EDX of TiO 2 @50%CP systems with a) CP1 , b) CP2 , c) CP3 and d) CP4 after 10 h of CO 2 photoreduction. Surface area and CO 2 adsorption Figure S21. N 2 adsorption-desorption isotherms for TiO 2 . Figure S22. N 2 adsorption-desorption isotherms for ground CP: CP1 (black line), CP2 (red line), CP3 (blue line) and CP4 (pink line). Figure S23. N 2 adsorption-desorption isotherms of TiO 2 @50%CP : CP1 (black line), CP2 (red line), CP3 (blue line) and CP4 (pink line). Figure S24. CO 2 adsorption isotherms of TiO 2 (green line) and TiO 2 @50%CP : CP1 (black line), CP2 (red line), CP3 (blue line) and CP4 (pink line). Surface characterization Figure S25 . XPS survey spectrum of CP4. Figure S26 . XPS survey spectrum of TiO 2 @50%CP4 after photocatalytic reduction. Table S2 . Assignment of the peaks obtained in XPS. Element Assignment Position (eV) Element Assignment Position (eV) C C 1s (C-C, C-H) 284.6 C C 1s (C-C, C-H) 284.6 C 1s (C-N) 286.0 C 1s (C-N) 286.0 - - C 1s (C=O) 288.2 Cu Cu 2p 3/2 (I) 932.5 Cu Cu 2p 3/2 (I) 932.2 Cu 2p 1/2 (I) 952.3 Cu 2p 1/2 (I) 951.9 I I 3d 5/2 619.5 I I 3d 5/2 618.9 N N 1s (pyridine) 399.2 N N 1s (pyridine) 398.8 - - N 1s (N-C=O) 400.4 Ti Ti 2p 3/2 (IV) 458.2 Ti 2p 1/2 (IV) 463.9 O O 1s (Ti-O) 529.4 O 1s (O-C=O) 530.9 Figure S27. Representation of XPS low binding energy region of TiO 2 (black line) and TiO 2 @50%CP4 (red line). Energy values referent to the Fermi level. Density Functional Theory (DFT) calculations Figure S28. Density of states of: a) TiO 2 with projection on Ti and O unit atoms and b) CuI with projection on the Cu and I unit atoms. Figure S29. Z-averaged Hartree potential as for CP4 surface. Dashed line indicates Fermi level. Work function is calculated as difference of the asymptotic potential and the Fermi level. Table S3. Computationally calculated values of work function, valence band edge (referred to the vacuum and NHE), theoretical band gap and conduction band edge (NHE). TiO 2 7.820 -8.22 3.72 3.20 0.52 CuI 6.740 -7.04 2,54 3.60 -1.06 CP1 3.778 -4.18 -0.32 2.80 -3.12 CP2 3.638 -4.04 -0.46 2.80 -3.26 CP3 3.597 -4.00 -0.50 2.70 -3.20 CP4 3.972 -4.37 -0.13 3.40 -3.53 Table S4. Experimental values obtained from the literature of the valence band edge (referred to the vacuum and NHE), band gap and conduction band edge (NHE). Data source for TiO 2 @50%CP4 : low binding energy XPS and DRS. TiO 2 [69] -7.41 2.91 3.20 -0.29 Cu 2 O [69] -6.42 1.92 2.20 -0.28 CuO [69] -6.66 2.16 1.70 0.46 CuI [70] -5.84 1.34 3.02 -1.68 TiO 2 @50%CP4 -5.25 0.75 3.55 -2.80 Supplementary Material File (image15.png) Download 424.19 KB File (image18.png) Download 334.49 KB File (image20.png) Download 435.33 KB File (image25.png) Download 413.94 KB File (image27.png) Download 380.81 KB File (image28.png) Download 423.10 KB File (image29.png) Download 363.66 KB File (image30.png) Download 370.14 KB File (image31.png) Download 423.29 KB File (image35.png) Download 314.99 KB File (image36.png) Download 372.62 KB File (image37.png) Download 408.12 KB File (image38.png) Download 408.61 KB File (image41.png) Download 475.58 KB Information & Authors Information Version history V1 Version 1 23 July 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords alcohols co2 photoreduction copper coordination polymer photocatalysis Authors Affiliations Julián Ávila Duran 0009-0007-0888-3994 Autonomous University of Madrid View all articles by this author Jon Napal University of the Basque Country View all articles by this author Fernando Aguilar-Galindo Autonomous University of Madrid View all articles by this author Oscar Castillo University of the Basque Country View all articles by this author Pilar Amo Ochoa 0000-0002-1952-1020 [email protected] Universidad Autónoma de Madrid View all articles by this author Metrics & Citations Metrics Article Usage 235 views 144 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Julián Ávila Duran, Jon Napal, Fernando Aguilar-Galindo, et al. Boosting CO2 to Alcohol Conversion: Powerful Photocatalysts Based on TiO2-Cu(I)-Iodine-Pyridine 1D Coordination Polymers. Authorea . 23 July 2025. 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