Ternary Facet Junction Engineered Cu2O Photocathode for Efficient Urea Synthesis via CO2 and Nitrate Co-reduction

Ternary Facet Junction Engineered Cu2O Photocathode for Efficient Urea Synthesis via CO2 and Nitrate Co-reduction | 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 Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed.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. 18 November 2025 V1 Latest version Share on Ternary Facet Junction Engineered Cu2O Photocathode for Efficient Urea Synthesis via CO2 and Nitrate Co-reduction Authors : Hong Liang , Min Li 0009-0008-5415-9259 , Zhiheng Li , Xiaowen Liu , Shixin Yu , Wenfu Xie , Tianyu Zhang , Haohong Duan 0000-0002-9241-0984 , and Qiang Wang [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.176349284.48005533/v1 283 views 169 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Facet junction engineering offers a promising route to accelerate charge separation in photoelectrocatalysis (PEC), yet its application in CO 2 and NO 3 - co-reduction for urea synthesis remains unexplored. Herein, we synthesized a polyhedral Cu 2 O photocathode exposing coexisting {100}, {110}, and {111} facets via a pH controlled precipitation method. Density functional theory (DFT) calculations and selective photodeposition experiments reveal that these anisotropic facets form ternary facet junctions within the polyhedral Cu 2 O, creating built-in electric fields that drive directional migration of photogenarated carriers. This spatial charge separation significantly suppresses electron-hole recombination and accelerates interfacial charge transfer, as confirmed by comprehensive photoelectrochemical measurements. Benefiting from these optimized charge dynamics, the ternary facet Cu 2 O achieves a urea Faradaic efficiency (FE) of 15.35% and a yield of 0.97 mmol·g cat −1 ·h −1 , outperforming single and binary facet Cu 2 O samples. In situ infrared spectroscopy (in situ FTIR) identifies *OCO, *CO, and *OCNO intermediates, revealing that the ternary facet junction facilitate C–N coupling between activated CO 2 and NO 3 - derived species. This study not only elucidates the underlying mechanism of facet junction enhanced charge dynamics, but also highlights the potential of facet engineered materials for sustainable carbon utilization. Ternary Facet Junction Engineered Cu 2 O Photocathode for Efficient Urea Synthesis via CO 2 and Nitrate Co-reduction Hong Liang, Min Li * , Zhiheng Li, Xiaowen Liu, Shixin Yu, Wenfu Xie, Tianyu Zhang * , Haohong Duan * , Qiang Wang * H. Liang, M. Li, Z. H. Li, X. W. Liu, W. F. Xie, T. Y. Zhang, Q. Wang College of Environmental Science and Engineering Beijing Forestry University, Beijing 100083, China Email: [email protected] ; [email protected] math_shortcuts Q. Wang State Key Laboratory of Efficient Production of Forest Resources Beijing Forestry University, Beijing 100083, China S. X. Yu Beijing Key Laboratory of Lignocellulosic Chemistry Beijing Forestry University, Beijing 100083, China. H. H. Duan Department of Chemistry Tsinghua University, Beijing 100084, China Abstract Facet junction engineering offers a promising route to accelerate charge separation in photoelectrocatalysis (PEC), yet its application in CO 2 and NO 3 ⁻ co-reduction for urea synthesis remains unexplored. Herein, we synthesized a polyhedral Cu 2 O photocathode exposing coexisting {100}, {110}, and {111} facets via a pH controlled precipitation method. Density functional theory (DFT) calculations and selective photodeposition experiments reveal that these anisotropic facets form ternary facet junctions within the polyhedral Cu 2 O, creating built-in electric fields that drive directional migration of photogenarated carriers. This spatial charge separation significantly suppresses electron-hole recombination and accelerates interfacial charge transfer, as confirmed by comprehensive photoelectrochemical measurements. Benefiting from these optimized charge dynamics, the ternary facet Cu 2 O achieves a urea Faradaic efficiency (FE) of 15.35% and a yield of 0.97 mmol·g cat −1 ·h −1 , outperforming single and binary facet Cu 2 O samples. In situ infrared spectroscopy (in situ FTIR) identifies *OCO, *CO, and *OCNO intermediates, revealing that the ternary facet junction facilitate C–N coupling between activated CO 2 and NO 3 ⁻ derived species. This study not only elucidates the underlying mechanism of facet junction enhanced charge dynamics, but also highlights the potential of facet engineered materials for sustainable carbon utilization. Keywords: Cu 2 O, ternary facet junction, photoelectrocatalysis, urea, CO 2 and nitrate co-reduction 1. Introduction Amid escalating climate change and environmental pollution, the simultaneous transformation of greenhouse gases such as CO 2 and nitrogen-containing pollutants like nitrate (NO 3 − ) into value-added chemicals has emerged as a promising strategy for achieving carbon neutrality and resource circularity. [1-2] Among the target products, urea, a key nitrogen-containing compound, is extensively used in fertilizers and the chemical industry, playing a vital role in ensuring food security and driving sustainable development . [3-4] However, industrial urea synthesis primarily relies on the energy-intensive Haber–Bosch and urea-carbonation processes, which operate under harsh conditions ( 150~200 o C and 150~200 bar), consume large amounts of fossil resources, and generate significant carbon emissions . [5] Therefore, developing a green, mild and sustainable route to synthesize urea from CO 2 and NO 3 − remains an urgent scientific challenge . Electrocatalytic CO 2 and nitrate co-reduction under ambient conditions has recently emerged as a feasible approach toward green urea synthesis. [6-8] For example, Fe–Ni dual-atom catalyst achieved a FE of 17.8% for urea production at −1.5 V vs. RHE. [9] The copper single atoms decorated on a CeO 2 support exhibits an average urea yield rate of 52.84 mmol g cat. −1 h −1 at −1.6 V vs. RHE [10] . Nonetheless, it still sufers from the drawbacks of large external bias and low energy efficiency. [11-13] Photoelectrocatalysis, which synergistically couples solar energy and electrical bias, offers significant advantages. By integrating photoelectrodes with a small applied potential, it enables efficient separation of photogenerated charge carriers, enhances C–N coupling efficiency, and reduces energy consumption. [14] Moreover, the built-in electric field within photoelectrodes can modulate interfacial charge distribution and reaction pathways, providing new insights for selective urea formation. Despite these advantages, PEC performance is still limited by sluggish interfacial kinetics and inefficient charge separation in the photoelectrode material. [15] Facet engineering has attracted growing interest as a strategy to tailor the surface electronic structure and carrier dynamics. [16] The anisotropic band structure and surface states between different crystal facets can induce directional migration of charge carriers, forming facet homojunctions that promote charge separation. [17-18] For instance, {001}/{101} binary facet junction in TiO 2 promotes the migration of photogenerated electrons and holes, enhancing photocatalytic CO 2 reduction. [19] Except binary surface heterojunctions, our previous work also demonstrated that 18-faceted BiOCl single crystals with {001}/{102}/{112} ternary junction significantly enhanced photo-redox activity. [20] However, the application of ternary facet junctions in photoelectrocatalytic co-reduction of CO 2 and NO 3 − remains largely unexplored. Cu 2 O, a p-type semiconductor with a moderate band gap (~2.0 eV), strong visible light absorption, and inherent catalytic activity, has emerged as a promising candidate for photoelectrocatalysis. [21] Different crystal facets exhibit distinct electronic structures, surface charge distributions, and active sites, providing an ideal platform for facet engineering. [22] In addition, Cu-based sites are known to facilitate CO 2 activation and C–N coupling. [23-24] For example, a TiO 2 -supported single-atom copper catalyst was designed, in which the dynamic Cu⁺/Cu 2 ⁺ redox cycle significantly accelerated the extraction of photogenerated electrons, while the electron-rich Cu sites further promoted C–N coupling, ultimately achieving a urea yield of 432.12 μg·g cat −1 . [25] Sun et al. reported a CuWO₄ catalyst with intrinsic bimetallic sites, in which the alternately distributed W and Cu sites possessed relatively positive formation potentials and suitable adsorption capacities for *NO 2 and *CO intermediates. [26] This configuration enhanced the probability of C–N coupling and lowered the reaction barrier, thereby achieving high urea synthesis performance with a yield of 98.5 ± 3.2 μg·h −1 ·mg cat. −1 . In our previous study, Cu 2 O was demonstrated to enable urea synthesis via PEC co-reduction of CO 2 and NO 3 − . However, single-facet Cu 2 O photocathodes suffers from limited charge separation efficiency, restricting the utilization of photogenerated electrons in the reduction reaction. Herein, we construct facet engineered Cu 2 O photocathode by tuning the pH during synthesis to control the exposure of {100}, {110} and {111} facets. The resulting polyhedral Cu 2 O with co-exposured {100}, {110} and {111} facets exhibits a well-defined facet junction architecture that facilitates the formation of cascade band alignments, enabling spatial separation and directional transport of charge carriers. Compared with conventional single-facet and dual facets structures, the Cu 2 O with ternary facet junction shows significantly enhanced efficiency of charge separation and migration, and improved PEC urea synthesis performance via CO 2 and NO 3 − co-reduction. As a result, the ternary facet Cu 2 O delivers a urea FE of 15.35% and a yield of 0.97 mmol·g cat −1 ·h −1 . Furthermore, in situ FTIR reveals the formation of *OCO, *CO, and *OCNO intermediates, confirming that the ternary facet junction facilitates efficient C–N coupling between CO 2 and NO 3 ⁻-derived species. This work not only offers a strategy for designing efficient photocathode materials via facet engineering to enhance photogenerated charge separation, but also opens up a novel pathway for urea synthesis through the co-utilization of carbon and nitrogen resources. 2. Results and Discussion 2.1. Structural Characterization Polyhedral Cu 2 O crystal with different facets were synthesized using a room-temperature chemical synthesis method by varying hydroxide ion concentration (0.6 M to 3.96 M) during synthesis process. Scanning electron microscopy (SEM) images clearly reveal uniform Cu 2 O particles with an average size of 1 ~ 2 μm (Figure 1a). Without hydroxide, Cu 2 O adopts a cubic shape exposing only single facet. Increasing hydroxide concentration gradually generates new crystal facets at the edges and vertices, eventually forming octahedral structures. Cu 2 O, with cubic, edge-truncated cubic, edge and corner truncated octahedral, truncated octahedral and octahedral structure were remarked as Cu 2 O-c, Cu 2 O-etc, Cu 2 O-ecto, Cu 2 O-to and Cu 2 O-o, respectively. Then, high-resolution transmission electron microscopy (HR-TEM) was conducted to analyze the exposed crystal facet. It confirms that Cu 2 O-c and Cu 2 O-o exclusively expose {100} and {111} facets, respectively (Figure 1b and c, S1). Intermediate morphologies, Cu 2 O-etc ({100}/{110}) and Cu 2 O-to ({100}/{111}), display two facets, while Cu 2 O-ecto simultaneously exposes {100}, {110}, and {111} three facets. The phase structure of the prepared was examined by X-ray diffraction (XRD) characterization. Figure 1d presents the detailed XRD patterns of the samples with different morphologies. The synthesized Cu 2 O exhibits six distinct diffraction peaks at 2θ = 29.5°, 36.4°, 42.3°, 61.3°, 73.5°and 77.3°, corresponding to the (110), (111), (200), (220), (311) and (222) planes of Cu 2 O (JCPDS NO. 05-0667). The sharp and intense diffraction peaks of Cu 2 O indicate high crystallinity, and no impurity peaks were observed, further confirming the high purity of the samples. Moreover, the X-ray photoelectron spectroscopy (XPS) measurements are consistent with the XRD results, further confirming the successful synthesis of Cu 2 O with different exposed crystal facets (Figure S2). Figure 1. a) SEM images and corresponding geometrical diagrams of five different Cu 2 O catalysts; TEM images of b) Cu 2 O-c and c) Cu 2 O-o; d) XRD spectra of the Cu 2 O catalysts with different morphologies and crystal structures. 2.2. Band structure and charge migration pathways To further confirm the facet junction in Cu 2 O samples and explore the charge separation and migration behavior of catalysts, we first determined their band positions. The optical absorption properties and bandgap energies were investigated by UV–vis diffuse reflectance spectroscopy (DRS). In Figure 2a and Figure S3, the strong absorption peaks range from 450 nm to 700 nm are observed in five different Cu 2 O catalysts. It can be clearly found that Cu 2 O-ecto displays a blue-shift of light adsorption compared to other samples. In addition, the bandgap energies (E g ) of Cu 2 O-c, Cu 2 O-ecto and Cu 2 O-o are calculated from the Tauc plots are 1.88 eV, 1.94 eV, and 1.92 eV, respectively. (Figure 2b). According to the VB-XPS results in Figure S4, the value from the valence band maximum to the Fermi level positions of Cu 2 O-c, Cu 2 O-ecto, and Cu 2 O-o are 0.29 eV, 0.33 eV, and 0.38 eV, respectively. By combining the flatband potentials obtained from Mott-Schottky curve (Figure S5), we can calculated the conduction band (E CB ) and valence band (E VB ) positions of Cu 2 O-c, Cu 2 O-ecto and Cu 2 O-o in Figure 2b, respectively. The E CB of Cu 2 O-c is higher than that of the Cu 2 O-o, whereas the E VB of the Cu 2 O-c is lower than Cu 2 O-o. Cu 2 O-ecto has the most negative conduction band and the most positive valence band position. The result demonstrates that the crystal facet exposure markedly affects their electronic energy level distribution. The band structure of {100}, {110} and {111} facets are further calculated by DFT calculations, as displayed in Figure 2c-e. Among these facets, the CB position of the {110} facet is more negative than that of the {100} facet, and the {100} facet is more negative than the {111} facet. Conversely, the trend for the VB positions is opposite: the {111} facet exhibits the most positive valence band position, followed by the (100) facet and {110} facet. Thus, a {100}/{110} binary facet junction can be formed in Cu 2 O-etc and a {111}/{100} binary facet junction can be formed in Cu 2 O-to, allowing charge separation. However, a {100}/{110}/{111} ternary facet junction is formed in Cu 2 O-etco.(Figure 2f). Electrons migrate from the {110} facet to the {100} facet and subsequently to the {111} facet, whereas holes migrate from the {111} facet to the {100} facet and further toward the {110} facet. This bidirectional migration pathway effectively promotes spatial separation and directional transport of electrons and holes. These results are consistent with most facet junction reports in the literature. [27-29] To verify the charge transfer pathways, we performed photodeposition of Pt and MnO x on Cu 2 O-c, Cu 2 O-ecto and Cu 2 O-o using H 2 PtCl 6 and MnSO 4 as precursors. In the precursor solutions, photogenerated electrons reduce Pt 4 ⁺ to Pt nanoparticles, while photogenerated holes oxidize Mn 2 ⁺ to MnO x [17, 30-31] . As shown in Figure 2g−i, MnO x uniformly covers the {100} facet of Cu 2 O-c, whereas Pt nanoparticles are randomly distributed on the {111} facet of Cu 2 O-o and also observed on the {111} facet of Cu 2 O-ecto. The corresponding EDS patterns further confirmed the deposition of Pt and MnO x (Figure S6). Notably, Pt nanoparticles are absent on Cu 2 O-c, and MnO x nanosheets are also absent on Cu 2 O-o (Figure S7). These results suggest that electrons preferentially accumulate on the {111} facet, while holes mainly enrich on the {100} facet in Cu 2 O-ecto, in accordance with the results obtained by the above DFT calculation. Electrons and holes are spatially and efficiently separated via steering the cascade charge flow. Figure 2 a) UV–vis and b) Tauc plot (illustration) for Cu 2 O-c, Cu 2 O-ecto and Cu 2 O-o; DFT for band structure of c) Cu 2 O(110), d) Cu 2 O(100) and e) Cu 2 O(111) crystal facet; f) Schematic diagram of electron transfer on Cu 2 O-ecto surface; SEM images of MnO x deposited on Cu 2 O-c (g), Pt deposited on Cu 2 O-o and MnO x and Pt co-deposited on Cu 2 O-ecto (i). 2.3. Photoelectrochemical Properties Based on the above experimental results and theoretical calculations, we confirmed the formation of ternary facet junctions in Cu 2 O-ecto, enabling directional migration of electrons and holes. Building on the result, a series of photoelectrochemical measurements were conducted to evaluate its charge separation and transfer efficiency. Transient photocurrent responses under AM 1.5G illumination demonstrate that Cu 2 O-ecto delivers the highest photocurrent density, indicating superior charge separation efficiency (Figure 3a and Figure S8). This observation is further supported by electrochemical impedance spectroscopy (EIS) results. EIS Nyquist plots reveal that Cu 2 O-ecto exhibits the smallest arc radius, means the smallest charge transfer resistance, corresponding with its transient photocurrent data (Figure 3b and Figure S9). To further quantify the surface charge transfer efficiency of Cu 2 O-ecto, we validated its efficiency by comparing the current densities with and without the addition of an electron sacrificial agent (K 2 S 2 O 8 ). As shown in Figure 3c, the average charge transfer efficiency of Cu 2 O-ecto reaches 47.3%. Mott-Schottky analysis (Figure 3d) shows p-type semiconductor characteristics for all samples, with flat band potentials of −0.15 V (Cu 2 O-c), −0.03 V (Cu 2 O-o), and 0.07 V (Cu 2 O-ecto) vs. Ag/AgCl. These differences in flat-band potential reflect variations in charge carrier concentration and charge transfer behavior. Based on the slope of the Mott-Schottky plot, the carrier densities of different Cu 2 O catalysts were calculated. Among these catalysts, Cu 2 O-ecto exhibits the highest carrier density, reaching 4.29×10 21 cm −3 (Figure 3e and Figure S10). Additionally, the applied bias photon-to-current efficiency (ABPE) for urea synthesis (Figure 3f and Figure S11) further confirmes that Cu 2 O-ecto delivers the highest energy conversion efficiency, highlighting its promising potential for practical PEC applications. Figure 3 a) Transient photocurrent density curves and b) EIS Nyquist plots of Cu 2 O-c, Cu 2 O-ecto and Cu 2 O-o; c) charge separation efficiency of the Cu 2 O-ecto catalyst during the reaction; d) Mott-Schottky plots for Cu 2 O-c, Cu 2 O-ecto and Cu 2 O-o; e) The carrier densities of Cu 2 O-c, Cu 2 O-ecto and Cu 2 O-o; f) the applied bias photon-to-current efficiency for urea synthesis of Cu 2 O-c, Cu 2 O-ecto and Cu 2 O-o. 2.4. Photoelectrocatalytic urea synthesis performance To validate the role of facet junction in promoting photocharge separation, PEC urea synthesis tests were carried out via the co-reduction of CO 2 and NO 3 − in a three electrode configuration using a H-type quartz cell, with Ag/AgCl and Pt foil serving as the reference and counter electrodes, respectively, and 0.1 M KNO 3 as electrolyte. Urea was quantified by urease decomposition method, while gaseous and liquid phase byproducts (NH 3 and NO 2 − ) were identified via gas chromatography and UV-vis spectroscopy (Figure S12-S13). Notably, no CO, H 2 , or other C 2 + gaseous byproducts were detected. The photoelectrochemical performance of the Cu 2 O catalyst was evaluated by linear sweep voltammetry (LSV). In 0.1 M KNO 3 solution saturated with either CO 2 or Ar, the LSV curves of the Cu 2 O catalysts show that, under visible-light irradiation, the current density in both atmospheres is significantly higher than in the dark, indicating that light illumination effectively enhances the generation and participation of photogenerated charge carriers, thereby promoting CO 2 RR and NO 3 ⁻ RR. Notably, the photocurrent density under CO 2 is consistently higher than that under Ar, further confirming that CO 2 participates in the reaction and contributes to the overall catalytic activity. Among the tested samples, Cu 2 O-ecto exhibits the highest photoelectrocatalytic activity in both atmospheres (Figure 4a, Figure S14 and S15). Moreover, we systematically evaluated the effect of catalyst loading on the performance of the photoelectrode. When the loading was 0.05 mg·cm −2 , the FE and production rate of urea reached the maxinum values of 13.63% and 0.81 mmol·g cat −1 ·h −1 , respectively (Figure S16). Insufficient loading results in a lack of active sites, whereas excessive loading can hinder light absorption and charge transport, both of which lead to a decline in urea synthesis performance. To further clarify the effect of crystal facet effects on C−N coupling for PEC urea synthesis, repeated experiments were conducted using these catalysts. As shown in Figure 4b, the urea synthesis performance exhibits a volcanic trend with the emergence of the {110} and {111} crystal facets and the disappearance of the {100} facets, Cu 2 O transforms from a cubic to an octahedral morphology, and both the FE and R urea initially increase and then decrease. Among the catalysts, Cu 2 O-ecto achieves the highest FE of urea and R urea . Morever, compared with other catalysts, Cu 2 O-ecto produces the least amount of by-products (Figure S17). Subsequently, the applied potential for urea synthesis was optimized. As displayed in Figure 4c, the performance of Cu 2 O-ecto in PEC urea synthesis was evaluated over a potential range of +0.23 to −0.17 V vs. RHE. Both R urea and FE display a volcano-type curve, with optimal values of 0.97 ± 0.16 mmol·g cat −1 ·h −1 and 15.35 ± 1.52% at 0.03 V vs. RHE. Increasing the applied potential enhances NO 3 ⁻ reduction, leading to a moderate increase in the formation of by-products NO 2 − and NH 3 (Figure S18). In addition to the excellent electrocatalytic activity, the stability of the catalyst is also pivotal to the practical application. During the reaction process, the current density was maintained at approximately −0.2 mA cm⁻ 2 and exhibited stable performance over 20 h of continuous cycling (Figure 4d). After 10 cycles, no significant loss of activity was observed, further confirming the excellent stability of the Cu 2 O-ecto catalyst. Figure 4 a) LSV curves under: Ar dark/light, CO 2 dark/light for Cu 2 O-ecto; b) urea FE and formation rate of five different Cu 2 O catalysts at +0.03 V vs. RHE; c) urea FE and formation rate of Cu 2 O-ecto at various applied potentials; d) cycling stability test of the Cu 2 O-ecto catalyst. 2.5. Mechanism of C−N coupling to urea To elucidate the intrinsic advantages of photoelectrocatalysis in urea synthesis, we conducted single-variable-controlled experiments, systematically comparing PEC with photocatalysis (PC) and electrocatalysis (EC) under identical conditions. As shown in Figure 5a, the R urea from PEC reached 0.97 mmol·g cat −1 ·h −1 , which is 1.9 times than that of EC (0.51 mmol·g cat −1 ·h −1 ) and nearly 24 times than that of PC (0.04 mmol·g cat −1 ·h −1 ). These results clearly demonstrate the synergistic enhancement effect between photogenerated carriers and applied bias in PEC systems. To verify the source of carbon and nitrogen in the urea product, control experiments were carried out in the absence of CO 2 and NO 3 ⁻. No urea was produced in the absence of carbon and nitrogen sources, indicating that the carbon and nitrogen elements originate from CO 2 and NO 3 ⁻, respectively (Figure 5b). Additionally, when NO 2 ⁻ was reduced as a nitrogen source instead of NO 3 ⁻, only trace amounts of urea were detected. This observation suggests that the desorption of NO 2 ⁻ inhibits urea synthesis, implying that *NO 2 may be a key intermediate in C−N coupling. To elucidate the mechanism of PEC urea synthesis, In situ-FTIR were conducted to identify the reaction intermediates at different applied potentials. At 0.03 V vs. RHE, infrared spectra were collected in the range of 1350 to 3250 cm −1 over a scanning period of 0 to 60 minutes (Figure 5c). In the range of 3150 to 3250 cm −1 , a distinct band at 3174 cm −1 was observed, corresponding to N−H bending vibrations, which can be attributed to urea molecules or nitrogen-containing intermediates. [32] In the range of 1350 to 1450 cm −1 , the infrared band at 1396 cm −1 can be attributed to the *OCO species after CO 2 activation. [33] Additionally, in the range of 1950 to 2150 cm −1 , bands at 1988 cm −1 and 2032 cm −1 were detected, corresponding to *CO intermediate, a key species in CO 2 reduction. [34] Notably, a stretching band at 1420 cm −1 , attributed to C−N stretching vibrations, was identified and its intensity progressively increased over time, suggesting the accumulation of C–N coupling intermediates. [9] Additionally, a band at 2110 cm −1 was also observed and is assigned to the *OCNO intermediate, a proposed key coupling species in the C–N coupling step leading to urea formation. [10] These results provide strong evidence for the C–N coupling pathway during the PEC urea synthesis process. Based on our previous studies, we propose the following reaction mechanism occurring on the surface of the ternary facet junction Cu₂O photocathode. First, CO 2 and NO 3 − are adsorbed on the catalyst surface. Under light irradiation, the presence of facet junctions induces directional separation of photogenerated electron–hole pairs. The electrons preferentially accumulate on the {111} facet, where they drive the reduction of CO 2 and NO 3 ⁻ into *CO and *NO 2 intermediates, respectively. These intermediates undergo the first C–N coupling step to form *CO–NH species. Subsequently, through successive proton–electron transfer processes, *CO–NH further couples with *NO 2 in a second C–N coupling step, eventually leading to the formation of urea molecules. math_shortcuts Figure 5 a) Performance comparison of PC, EC and PEC technologies; b) Products in different electrolytes (CO 2 RR: 0.1 M KHCO 3 , NO 3 ⁻ RR: 0.1 M KNO 3 , NO 3 ⁻+CO 2 RR: 0.1 M KNO 3 , NO 2 ⁻ +CO 2 RR: 0.1 M KNO 2 ); c) Infrared signals during photoelectrocatalytic co-reduction of CO 2 and NO 3 − by Cu 2 O-ecto at different times. 3. Conclusion In summary, we systematically research the influence of crystal facet on the PEC performance of polyhedral Cu 2 O catalysts for urea synthesis. By synthesizing and comparing Cu 2 O catalysts with different exposed crystal facets, it was found that the Cu 2 O-ecto with {100}, {110}, and {111} facets, exhibits the best performance in PEC urea synthesis. Specifically, the Cu 2 O-ecto catalyst achieved a FE of 15.35% and a urea yield of 0.97 mmol·g cat −1 ·h −1 , significantly outperforming other Cu 2 O samples with single facet and binerary facet junction. Mechanistic studies revealed that the presence of facet junction in Cu 2 O-ecto induces a built-in electric field and a cascade charge transfer pathway, which significantly promotes the separation and directional migration of photogenerated charge carriers. This, in turn, facilitates efficient C–N coupling between CO 2 and NO 3 − . Overall, this study not only highlights the critical role of facet engineering in modulating charge dynamics but also provides a promising approach for the sustainable and selective synthesis of urea through carbon–nitrogen co-reduction. math_shortcuts Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was supported by the National Natural Science Foundation of China (52300125, 52225003, 52202360, 22208021), the 5·5 Engineering Research & Innovation Team Project of Beijing Forestry University (BLRC2023B04), the Fundamental Research Funds for the Central Universities (QNTD202506) and the Energy Revolution S&T Program of Yulin Innovation Institute of Clean Energy (No. E411080705). Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. 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] C. S. Gerke, Y. Xu, Y. Yang, G. D. Foley, B. Zhang, E. Shi, N. M. Bedford, F. Che, V. S. Thoi, J. Am. Chem. Soc. 2023 , 145, 26144.[2] J. Geng, S. Ji, M. Jin, C. Zhang, M. Xu, G. Wang, C. Liang, H. Zhang, Angew. Chem. Int. Ed. 2023 , 62, e202318589.[3] X. 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Keywords photoelectrocatalysis ternary facet junction urea Authors Affiliations Hong Liang Beijing Forestry University School of Environmental Science and Engineering View all articles by this author Min Li 0009-0008-5415-9259 Beijing Forestry University School of Environmental Science and Engineering View all articles by this author Zhiheng Li Beijing Forestry University School of Environmental Science and Engineering View all articles by this author Xiaowen Liu Beijing Forestry University School of Environmental Science and Engineering View all articles by this author Shixin Yu Beijing Forestry University Beijing Key Laboratory of Lignocellulosic Chemistry View all articles by this author Wenfu Xie Beijing Forestry University School of Environmental Science and Engineering View all articles by this author Tianyu Zhang Beijing Forestry University School of Environmental Science and Engineering View all articles by this author Haohong Duan 0000-0002-9241-0984 Tsinghua University Department of Chemistry View all articles by this author Qiang Wang [email protected] Beijing Forestry University School of Environmental Science and Engineering View all articles by this author Metrics & Citations Metrics Article Usage 283 views 169 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Hong Liang, Min Li, Zhiheng Li, et al. Ternary Facet Junction Engineered Cu2O Photocathode for Efficient Urea Synthesis via CO2 and Nitrate Co-reduction. Authorea . 18 November 2025. DOI: https://doi.org/10.22541/au.176349284.48005533/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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