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• Overcoming the current challenge of energy crisis and climate change resulting from excessive fossil fuels combustion along with CO₂ emissions has attracted great attention. Semiconductor photo-catalysis, especially solar-driven photo-catalytic CO₂ reduction, was regarded as one of promising approaches for environmentally friendly converting CO₂ into hydrocarbon fuels. To achieve such an artificial photosynthesis conversion, the development of a high-active photo-catalyst towards CO₂ photo-reduction reaction is a prerequisite issue.

  Since Inoue et al.[1] reported the conversion of CO₂ to small amounts of hydrocarbon fuels in the presence of photosensitive semiconductor powders suspended in water as catalysts, various semiconductor photo-catalysts, including oxides [2, 3], sulfides [4, 5], perovskite [6], carbon nitride [7], metal/covalent organic framework [8-10], etc., had been widely employed to study for the CO₂ photo-reduction. Among those semiconductor photo-catalysts, TiO₂-based materials are still at the center of attention due to their remarkable stability and suitable band structure [11]. However, pure TiO₂ materials usually suffered from the drawback of the rapid recombination of photo-induced electron-hole pairs, which prohibited the transfer of charge carriers and accordingly slowed the photo-catalytic CO₂ reduction reaction down, resulting in a low photo-catalytic activity.

  As a matter of fact, the CO₂ photo-reduction reaction contains three main processes. Firstly, the semiconductor photo-catalysts absorb the light and produce electron-pairs. Then, the electrons and holes would be separated and transfer to the surface of the semiconductor. Lastly, CO₂ reduction and H₂O oxidation reactions occur with involving of electrons and holes, respectively. In other words, under the condition of thermodynamic equilibrium, the photo-catalytic CO₂ reduction performance is determined by the kinetics of the above three processes.[11] Therefore, it is reasonable to promote the capability of CO₂ photo-reduction through enhancing the efficiency of one or more of the above three processes during the photo-catalysis reaction.

  The past few years, a tremendous flurry of research interest have been devoted onto the surface, interface, and composition engineering of TiO₂-based photo-catalysts for enhancing their relatively photo-catalytic activity [12, 13]. Especially, continued breakthroughs have been made in the co-catalyst effect [14-17], oxygen vacancy involving [18-22], hetero-junction construction [23-28], etc. Among them, coupling with metal co-catalyst [29, 30], as one of promising approaches, has attracted more interest thus it can enhance the photo-catalytic performance through promoting the electron–hole separation and migration. For example, Xie et al.[31] examined the effect of noble metal co-catalysts and found that the rate of CH₄ formation increased in the sequence of Ag < Rh < Au < Pd < Pt, corresponding well to the increase in the efficiency of electron-hole separation. Although most of the published papers revealed that Pt is the most effective co-catalyst to extract photo-generated electrons for CO₂ eduction [32-35], Pt element is too rare and expensive to be used in large-scale solar fuels production. Therefore, it is of significance to develop earth-abundant and nonprecious materials based co-catalysts for achieving high efficiency of photo-catalytic CO₂ reduction.

  Based on the above backgrounds and inspired by the challenges, the present study focuses on the solar-driven CO₂ photo-reduction activity of nonprecious Cu particle anchored TiO₂. Combining with previous work on preparation of hetero-phase TiO₂[36] and Cu-MO_x (M = W, Ti and Ce)[37] nano-composites, a facile one-pot polyol-mediated solvo-thermal approach was employed to successfully prepare TiO₂-Cu nano-hybrids. The corresponding photo-catalytic performance was evaluated through CO₂ reduction under simulated sunlight irradiation. The contribution of the Cu particles anchored on the TiO₂ nano-aggregates to its superior photo-catalytic reduction capability was also studied.

  Fig. 1 shows the preparation process of TiO₂-Cu composites, which contains the formation of mesoporous TiO₂ nanostructure and simultaneous decoration of Cu particle by polyol reduction strategy. The X-ray diffraction (XRD) patterns of the as-synthesized products were displayed in Fig. 2a. It can be found the diffraction peak of the material synthesized without involving of Cu²⁺ precursor could be well indexed with the standard anatase phase TiO₂ (JCPDS No. 1-562). After introducing the Cu²⁺ precursor into the reaction system, it was found the as prepared materials were composed of TiO₂ and Cu from the XRD diffraction peaks, which correspond well with standard patterns of JCPDS No. 1-562 and 1-1242. Furthermore, with increasing of Cu²⁺ precursor concentration from TiO₂-Cu-2.5% to TiO₂-Cu-7.5%, the diffraction peak of Cu became stronger. This result suggested that the TiO₂-Cu nano-hybrids had been successfully synthesized. As shown in the UV–vis diffuse reflectance spectrum (UV-DRS) of the as-prepared materials (Fig. 2b), an obvious absorption tail could be detected in the visible light region, suggesting that the involving of copper could tailor the light absorption of the materials. As shown in Fig. S1a (Supporting information), when further increased the copper loading amounts, the XRD diffraction peaks of copper in the composites continue growing. At the meantime, as shown in Fig. S1b (Supporting information), the absorption tail of those composites obviously shifted to the visible light region, but the absorption peak intensity below 350 nm become weaker due to the decreased TiO₂ amount.

  Figure 1

  

  Figure 1.  Illustration for the fabrication process and formation mechanism of TiO₂-Cu hybrids.

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  Figure 2

  

  Figure 2.  (a) XRD patterns and (b) UV-DRS spectra of TiO₂ and TiO₂-Cu products. (c) Whole survey, (d) Ti 2p, (e) O 1s and (f) Cu 2p XPS spectra of TiO₂-Cu-5%.

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  X-ray photoelectron spectroscopy (XPS) was further used to confirm the intrinsic characteristics and chemical states of the as-prepared materials. Take TiO₂-Cu-5% as an example, the survey XPS spectrum in Fig. 2c suggests the sample consists of the elements of Ti, O, and Cu. Figs. 2d-f show high-resolution XPS spectra of Ti 2p, O 1s, and Cu 2p, respectively. The binding energy peaks located at 458.5, 459.5, and 464.2 eV belong to the valence states of Ti 2p. The peaks at 529.7 and 531.2 eV of O 1 s spectrum (Fig. 2e) are attributed to the formation of Ti-O bonds in TiO₂. As displayed in Fig. 2f, the two main peaks at 932.5 and 952.4 eV correspond to the metallic Cu(0) 2p_3/2 and Cu 2p_1/2 state [38], respectively.

  The morphology and structure of the as-obtained TiO₂ and TiO₂-Cu-5% products were shown in Fig. 3a. The TiO₂ sample shows morphology of aggregated spherical nanoparticles. With involving of Cu precursor into the reaction system, the obtained TiO₂-Cu-5% sample keeps the same morphology (Fig. 3b). However, the energy dispersive X-ray spectroscopy (EDX)-mapping in Figs. 3c-f suggests the material contains the elements of Ti, O, and Cu. TEM images in Figs. 3g-h indicate that both the TiO₂ and TiO₂-Cu-5% samples comprise many small nano-crystals, leading to a pseudo-porous structure. As shown in HRTEM image in Fig. 3i, the lattice fringes of the nanoparticles can be easily identified to be 0.35 and 0.21 nm apart, which is in good agreement with the (101) plane of TiO₂ and the (111) plane of Cu, respectively. The above results confirm the successful decoration of Cu nanoparticles on the surfaces of the TiO₂ nano-aggregates. The N₂ adsorption–desorption isotherms (Fig. S2 in Supporting information) of the TiO₂-Cu-5% sample display a distinct Type II hysteresis loop and reveal the typical characteristics of porous materials. The calculated BET surface area is 159.9 m²/g, which corresponds to the average pore sizes of the 9.1 nm.

  Figure 3

  

  Figure 3.  SEM images of TiO₂ (a) and TiO₂-Cu-5% (b). (c-f) EDX elemental mapping images of TiO₂-Cu-5%. TEM image of TiO₂ (g). TEM (h) and HRTEM (i) images of TiO₂-Cu-5%.

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  The photo-catalytic activities of the as-synthesized TiO₂ and TiO₂-Cu nano-composites were evaluated through solar-driven CO₂ reduction in the presence of H₂O vapor under continuous artificial sunlight irradiation. Figs. 4a and b show the CO₂ photo-reduction activity and the corresponding evolution rates of the above materials. It was found pure TiO₂ could convert CO₂ into CH₄ and CO products under light irradiation. Furthermore, the involving of proper amounts of copper could enhance the evolution rates of CH₄ and CO. When the TiO₂-Cu-5% was used as the catalyst, the evolution rates of CH₄ and CO were 25.73 and 0.42 µmol g^-1 h^-1, respectively, which were significantly promoted comparing with that of pure TiO₂ (11.67 and 0.14 µmol g^-1 h^-1 for CH₄ and CO, respectively). It is clear that the TiO₂-Cu-5% present a 2.2 times higher yield of CH₄ and 3 times higher CO yield compared with pure TiO₂. When further increase the Cu content (TiO₂-Cu-7.5%), the CO₂ photo-reduction activity decreased and it was attributed to the decrease of TiO₂ in such hybrid (Fig. S3 in Supporting information). As shown in Fig. S3, when the loading amount of Cu was 50%, no CO was produced and the CH₄ yield of TiO₂-Cu-50% was only one tenth of pure TiO₂. This is due to that too much Cu could shield light absorption of TiO₂ (Fig. S1b). In addition, the XRD pattern of the sample after CO₂ photo-reduction was also collected and displayed in Fig. S4 (Supporting information). It was found the composites still showed the main constitute of TiO₂ and Cu. But the diffraction peak of Cu₂O also appeared due to the oxidation of Cu by the generated O₂ during the photo-catalysis reaction.

  Figure 4

  

  Figure 4.  Photo-reduction activity towards the conversion of CO₂ into CH₄ (a) and CO (b) upon TiO₂ and TiO₂-Cu nano-composites under simulated solar light irradiation for 4 h. (c) Calculated band gap from UV-DRS spectra of TiO₂. (d) Valence band XPS spectra of TiO₂.

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  It is well-known that proper matching of valence band (E_VB) and conduction band (E_CB) sites is important for CO₂ photo-reduction. As shown in Fig. 4c, the band gap (E_g) of TiO₂ was calculated to be 2.89 eV from the Kubelka-Munk function. At the same time, the valence band extreme of TiO₂ on the basis of Valence band XPS spectra in Fig. 4d was 2.30 eV. Then, the E_CB level from (E_VB - E_g) was -0.59 eV. Previous studies have pointed out that the potential for reducing CO₂ to CH₄ and CO in water at a pH value of 7 is -0.24 V (E_CO2/CH4 = -0.24 V vs. NHE) and -0.53 V (E_CO2/CO = -0.53 V vs. NHE)[39-39], respectively. In this work, the E_CB value of TiO₂ (corresponding to -0.59 V) was more negative than those values. This indicates that CH₄ and CO could be the preferred product.

  To better understand the improvement of photo-catalytic activities for the TiO₂-Cu nano-composites, the transient photocurrent responses of the as-prepared products were performed to characterize the generation, migration, and recombination of photo-induced electrons and holes. It was clearly observed in Fig. 5a that the photocurrent density of the TiO₂-Cu-5% sample electrode was much higher than that of pure TiO₂, suggesting a higher separation and lower recombination rate of photo-generated electron-hole pairs in such a hybrid during the photo-catalysis process [41-45]. Photoluminescence (PL) spectra were further used to study the electron-hole separation of the photo-catalyst. As shown in Fig. 5b, the TiO₂-Cu-5% sample displayed a lower fluorescence intensity than that of pure TiO₂, which indicates a higher separation rate of photo-generated electron and hole pairs during CO₂ photo-reduction. Considering the Cu has higher work function than that of TiO₂ [14, 46, 47], the photo-induced electrons in TiO₂ would be transferred to the Cu. Meanwhile, the formation of Schottky barrier between TiO₂ and Cu resulted from the strong interfacial interaction in the TiO₂-Cu nano-composite could promote the transfer and separation of photo-generated electrons.

  Figure 5

  

  Figure 5.  (a) Photocurrent response and (b) PL spectra of TiO₂ and the TiO₂-Cu-5% nano-composites. (c) Schematic illustration of the charge transfer paths in the TiO₂-Cu nano-composite towards CO₂ photo-reduction.

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  On the basis of the above analysis, the impact of the Cu particles anchoring on the photo-catalytic CO₂ reduction in the TiO₂ material was illustrated in Fig. 5c. As mentioned in the photo-reduction test, the photo-catalysts firstly would adsorb a certain amount of CO₂ molecules before initiated the Xe lamp, as the adsorption is a prerequisite for the occurrence of a photo-catalysis reaction [48-48]. Upon the light irradiation, the TiO₂ could absorb the light photons to generate photo-induced electrons and holes. Then, the electrons in the conduction band of TiO₂ would be transferred to the surface to engage in the photo-catalytic CO₂ reduction for yielding products. In particular, with involving of Cu particles, it would accept the photo-generated electrons from TiO₂ and accelerate the rapid charge separation and transfer. As a result, more efficient electron-hole separation in the TiO₂-Cu nano-composite was achieved. Finally, the photo-catalytic CO₂ reduction to CH₄ and CO is significantly enhanced by more photo-generated electrons participating in the photo-catalysis process.

  In summary, the TiO₂-Cu nano-composite was successfully prepared by a simple polyol approach, which combines the formation of TiO₂ nano-aggregates with reduction of Cu²⁺ to Cu. The UV–vis diffuse reflectance and Valence band XPS spectra suggested the prepared TiO₂ nano-aggregates had suitable band edge alignment with respect to the CO₂/CH₄ and CO₂/CO redox potential. Under simulated sunlight irradiation, an enhanced CH₄ and CO yield was achieved in the photo-reduction of CO₂ using the TiO₂-Cu nano-composite. The TiO₂-Cu-5% sample exhibits 2.2 times higher CH₄ yield and 3 times higher CO yield compared with pure TiO₂. This performance enhancement is realized because efficient separation of photo-generated charges was achieved with involving of Cu particles into the TiO₂ nano-aggregates. It is expected this work could provide a rational reference for designing efficient and low cost photo-catalysts towards CO₂ reduction.

  Declaration of competing interest

  The authors declared that they have no conflicts of interest to this work.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 22102122), the Hubei Provincial Natural Science Foundation (No. 2019CFB386) and the Central Committee Guides Local Science and Technology Development Special Project of Hubei Province (No. 2019ZYYD073).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.07.052.

  
  
• 1. [1]

     T. Inoue, A. Fujishima, S. Konishi, K. Honda, Nature 277(1979) 637-638. doi: 10.1038/277637a0

  2. [2]

     T. Zhang, J. Low, X. Huang, et al., Chem. Cat. Chem. 9(2017) 3054-3062.

  3. [3]

     F. Chen, Z. Ma, L. Ye, et al., Adv. Mater. 32(2020) 1908350. doi: 10.1002/adma.201908350

  4. [4]

     M. Zhou, S. Wang, P. Yang, et al., ACS Catal. 8(2018) 4928-4936. doi: 10.1021/acscatal.8b00104

  5. [5]

     X. Jiao, X. Li, X. Jin, et al., J. Am. Chem. Soc. 139(2017) 18044-18051. doi: 10.1021/jacs.7b10287

  6. [6]

     R. Shi, G.I.N. Waterhouse, T. Zhang, Sol. RRL 1(2017) 1700126. doi: 10.1002/solr.201700126

  7. [7]

     J. Liang, Z. Jiang, P.K. Wong, C.S. Lee, Sol. RRL 5(2020) 2000478.

  8. [8]

     R. Li, W. Zhang, K. Zhou, Adv. Mater. 30(2018) 1705512. doi: 10.1002/adma.201705512

  9. [9]

     W. Zhong, R. Sa, L. Li, et al., J. Am. Chem. Soc. 141(2019) 7615-7621. doi: 10.1021/jacs.9b02997

  10. [10]

      Z.B. Fang, T.T. Liu, J. Liu, et al., J. Am. Chem. Soc. 142(2020) 12515-12523. doi: 10.1021/jacs.0c05530

  11. [11]

      Y. Ma, X. Wang, Y. Jia, et al., Chem. Rev. 114(2014) 9987-10043. doi: 10.1021/cr500008u

  12. [12]

      T. Kong, Y. Jiang, Y. Xiong, Chem. Soc. Rev. 49(2020) 6579-6591. doi: 10.1039/C9CS00920E

  13. [13]

      Z. Xiong, Z. Lei, Y. Li, et al., J. Photochem. Photobiol. C 36(2018) 24-47. doi: 10.1016/j.jphotochemrev.2018.07.002

  14. [14]

      X. Li, J. Yu, M. Jaroniec, X. Chen, Chem. Rev. 119(2019) 3962-4179. doi: 10.1021/acs.chemrev.8b00400

  15. [15]

      C. Dong, M. Xing, J. Zhang, Mater. Horizons 3(2016) 608-612. doi: 10.1039/C6MH00210B

  16. [16]

      A. Meng, L. Zhang, B. Cheng, J. Yu, Adv. Mater. 31(2019) 1807660.

  17. [17]

      J. Ran, M. Jaroniec, S.Z. Qiao, Adv. Mater. 30(2018) 1704649. doi: 10.1002/adma.201704649

  18. [18]

      W. Fang, L. Khrouz, Y. Zhou, et al., Phys. Chem. Chem. Phys. 19(2017) 13875-13881. doi: 10.1039/C7CP01212H

  19. [19]

      L. Liu, S. Wang, H. Huang, et al., Nano Energy 75(2020) 104959. doi: 10.1016/j.nanoen.2020.104959

  20. [20]

      S. Bai, N. Zhang, C. Gao, Y. Xiong, Nano Energy 53(2018) 296-336. doi: 10.1016/j.nanoen.2018.08.058

  21. [21]

      M. Ai, J.W. Zhang, Y.W. Wu, et al., Chem. Asian J. 15(2020) 3599-3619. doi: 10.1002/asia.202000889

  22. [22]

      Z. Xiu, M. Guo, T. Zhao, et al., Chem. Eng. J. 382(2020) 123011. doi: 10.1016/j.cej.2019.123011

  23. [23]

      L. Yuan, S.F. Hung, Z.R. Tang, et al., ACS Catal. 9(2019) 4824-4833. doi: 10.1021/acscatal.9b00862

  24. [24]

      Z. Xiong, Z. Lei, C.C. Kuang, et al., Appl. Catal. B 202(2017) 695-703. doi: 10.1016/j.apcatb.2016.10.001

  25. [25]

      K. Li, B. Peng, J. Jin, et al., Appl. Catal. B 203(2017) 910-916. doi: 10.1016/j.apcatb.2016.11.001

  26. [26]

      J. Zhou, H. Wu, C.Y. Sun, et al., J. Mater. Chem. A 6(2018) 21596-21604. doi: 10.1039/C8TA08091G

  27. [27]

      S. Jiang, K. Zhao, M. Al-Mamun, et al., Inorg. Chem. Front. 6(2019) 1667-1674.

  28. [28]

      W. He, X. Wu, Y. Li, et al., Chin. Chem. Lett. 31(2020) 2774-2778. doi: 10.1016/j.cclet.2020.07.019

  29. [29]

      Q. Liu, Q. Chen, T. Li, et al., J. Mater. Chem. A 7(2019) 27007-27015. doi: 10.1039/C9TA09938G

  30. [30]

      X. Cai, J. Wang, R. Wang, et al., J. Mater. Chem. A 7(2019) 5266-5276. doi: 10.1039/C9TA00172G

  31. [31]

      S. Xie, Y. Wang, Q. Zhang, et al., ACS Catal. 4(2014) 3644-3653. doi: 10.1021/cs500648p

  32. [32]

      L.Y. Lin, S. Kavadiya, X. He, et al., Chem. Eng. J. 389(2020) 123450. doi: 10.1016/j.cej.2019.123450

  33. [33]

      M. Tasbihi, F. Fresno, U. Simon, et al., Appl. Catal. B 239(2018) 68-76. doi: 10.1016/j.apcatb.2018.08.003

  34. [34]

      C. Wang, X. Liu, W. He, et al., J. Catal. 389(2020) 440-449. doi: 10.1016/j.jcat.2020.06.026

  35. [35]

      X. Wu, C. Wang, Y. Wei, et al., J. Catal. 377(2019) 309-321. doi: 10.1016/j.jcat.2019.07.037

  36. [36]

      J. Xiong, M. Zhang, G. Cheng, J. Colloid Interface Sci. 579(2020) 872-877. doi: 10.1016/j.jcis.2020.06.103

  37. [37]

      M. Zhang, J. Xiong, H. Yang, et al., Ind. Eng. Chem. Res. 59(2020) 15454-15463. doi: 10.1021/acs.iecr.0c02663

  38. [38]

      J. Zhu, M. Zhang, J. Xiong, et al., Chem. Eng. J. 375(2019) 121902. doi: 10.1016/j.cej.2019.06.030

  39. [39]

      X. Chang, T. Wang, J. Gong, Energy Environ. Sci. 9(2016) 2177-2196. doi: 10.1039/C6EE00383D

  40. [40]

      R. Wang, C. He, W. Chen, et al., Chin. Chem. Lett. 32(2021) 3821-3824. doi: 10.1016/j.cclet.2021.05.024

  41. [41]

      Y. Wei, G. Cheng, J. Xiong, et al., J. Energy Chem. 32(2019) 45-56. doi: 10.1016/j.jechem.2018.05.013

  42. [42]

      Z. Zhou, Y. Li, M. Li, et al., Chin. Chem. Lett. 31(2020) 2698-2704. doi: 10.1016/j.cclet.2020.07.003

  43. [43]

      B. Li, L.C. Nengzi, R. Guo, et al., Chin. Chem. Lett. 31(2020) 2705-2711. doi: 10.1016/j.cclet.2020.04.026

  44. [44]

      L. Jiang, Y. Xie, F. He, et al., Chin. Chem. Lett. 32(2021) 2187-2191. doi: 10.1016/j.cclet.2020.12.010

  45. [45]

      L. Fu, R. Wang, C. Zhao, et al., Chem. Eng. J. 414(2021) 128857. doi: 10.1016/j.cej.2021.128857

  46. [46]

      S. Xiao, P. Liu, W. Zhu, et al., Nano Lett. 15(2015) 4853-4858. doi: 10.1021/acs.nanolett.5b00082

  47. [47]

      X.H. Li, M. Antonietti, Chem. Soc. Rev. 42(2013) 6593-6604. doi: 10.1039/c3cs60067j

  48. [48]

      M. Zhang, G. Cheng, Y. Wei, et al., J. Colloid Interface Sci. 572(2020) 306-317. doi: 10.1016/j.jcis.2020.03.090

  49. [49]

      H. Yang, C. He, L. Fu, et al., Chin. Chem. Lett. 32(2021) 3202-3206. doi: 10.1016/j.cclet.2021.03.038

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  Figure 1  Illustration for the fabrication process and formation mechanism of TiO₂-Cu hybrids.

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  Figure 2  (a) XRD patterns and (b) UV-DRS spectra of TiO₂ and TiO₂-Cu products. (c) Whole survey, (d) Ti 2p, (e) O 1s and (f) Cu 2p XPS spectra of TiO₂-Cu-5%.

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  Figure 3  SEM images of TiO₂ (a) and TiO₂-Cu-5% (b). (c-f) EDX elemental mapping images of TiO₂-Cu-5%. TEM image of TiO₂ (g). TEM (h) and HRTEM (i) images of TiO₂-Cu-5%.

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  Figure 4  Photo-reduction activity towards the conversion of CO₂ into CH₄ (a) and CO (b) upon TiO₂ and TiO₂-Cu nano-composites under simulated solar light irradiation for 4 h. (c) Calculated band gap from UV-DRS spectra of TiO₂. (d) Valence band XPS spectra of TiO₂.

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  Figure 5  (a) Photocurrent response and (b) PL spectra of TiO₂ and the TiO₂-Cu-5% nano-composites. (c) Schematic illustration of the charge transfer paths in the TiO₂-Cu nano-composite towards CO₂ photo-reduction.

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