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• INTRODUCTION

  Photocatalytic reduction of CO₂ into chemical fuels provided an attractive option to ameliorate global warming and help solve the energy crisis.^[1-3] One of the challenges is the development of efficient photocatalysts. TiO₂ has received significant attention for photocatalytic CO₂ reduction due to its chemical stability, nontoxicity, and low-cost.^[4] However, its wide band gap and rapid recombination of photo-induced carriers limited the practical application of TiO₂ photocatalysts.^[5] Moreover, the complex catalytic processes and products cannot be ignored, considering the multiple-electron reduction of CO₂ and the interaction between reactant molecules and surface sites.

  In order to overcome the above problems, ongoing attempts have been made to such as nanostructure engineering, element doping, heterojunction construction and surface modification, cocatalyst loading and so on.^[6-10] The construction of low dimensional structures such as nanorods, nanoplates and nanotubes could enhance separation and transfer of photoinduced electrons and holes in TiO₂ based catalyst.^[6,11-13] D. Regonini et al. reported that the electron transport property of electrospun TiO₂ fibers was higher than that of TiO₂ nanoparticles.^[14] Meanwhile, element doping could widen the light-absorption range of TiO₂ based materials and influence the arrangement of surface atoms, leading to the change of catalytic process.^[4,11] H. Y. He et al. proved that the N doping narrowed the band gap of N-doped TiO₂ nanorods and nanotubes, resulting in enhanced photocatalytic activities.^[15] Meanwhile, the influence of the CO₂ and H₂O adsorption on the selectivity of catalytic products was never ignored. C. Wang et al. reported that the formation of W(Ⅴ)-O-Ti(Ⅲ) active sites on TiO₂ surface significantly promoted CO₂ reduction efficiency and selectivity for CH₄ over CO), which exceeds those of pristine TiO₂ by more than one order of magnitude.^[16] Therefore, the construction of surface adsorbed sites for CO₂ and H₂O molecules was further studied for CO₂ conversion. Among multitudinous kinds of doped impurities, the effect of Cu atom of semiconductor surface on CO₂ reduction was never neglected.^[17-20] M. Park et al. proved that photoexcited electrons by the Cu(Ⅰ) ions of Cu doped TiO₂ were captured efficiently and transferred to the adsorbed CO₂ molecules^[21]. Co-doping method gradually became the focus of TiO₂ based materials for CO₂ photoreduction.^[4] The Cu and N co-doped TiO₂ catalysts exhibited superior photocatalytic performances for the degradation of methylene blue, 4-nitrophenol, oxytetracycline or sulfamethoxazole.^[22-26] Therefore, the Cu and N co-doped TiO₂ nanorod may become an ideal visible-light-driven catalyst for CO₂ reduction.

  Photothermal catalysis is another promising technology to improve catalytic performance of CO₂ reduction due to synergetic effect of photocatalysis and thermalcatalysis.^[27-29] The thermal energy promotes the photocatalytic process by reducing the apparent activation energy of photocatalysis, promoting the transfer of charge carriers and mass transfer of reactant.^[30] Besides, the reaction rate increases with increasing temperature based on the Arrhenius equation^[31]. Stable CO₂ molecules can be activated using additional thermal energy, accelerating the reduction rate of adsorbed CO₂ molecules on the catalyst's surface.^[27,28,31] Z. Liu et al. employed the TiO₂ photonic crystals with photothermal effect achieving high efficiency for CO₂ reduction^[32]. X. Zhang et al. reported the modification of TiO₂ with cocatalyst and oxygen vacancies for photothermalcatalytic reduction of CO₂ with H₂O steam.^[33] C. Wang et al. synthesized TiO₂ catalyst with rich oxygen vacancies followed by hydrogen doping exhibiting improved activity for photo- and photothermal-catalytic CO₂ reduction.^[34] Based on the above considerations, the development of doped TiO₂ catalyst is feasible for efficient photothermal CO₂ conversion.

  In this work, Cu and N co-doped TiO₂ nanorods were synthesized by the electrospinning-calcination method for catalytic CO₂ reduction with water vapor. The influence of Cu and N doping on the inherent property of TiO₂ was studied. Photocatalytic and photothermal catalytic activities of CO₂ reduction were investigated over the Cu and N co-doped TiO₂ nanorods. Finally, the surface mode of TiO₂ (1 0 1) facet was built, and the absorbed mechanism of CO₂ and H₂O molecules on the surface was discussed.

  RESULTS AND DISCUSSION

  Structure and Morphology. The phase structures of the obtained samples were measured by powder XRD techniques, and the results are shown in Figure 1a. The obvious peaks at 25.3, 37.8, 48.1, 53.9 and 55.1° were corresponding to (1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 1 1) facets of anatase phase TiO₂. The peak intensity in all doped samples obviously weakened in the existence of Cu dopant, which was the influence of impurity ions on the crystal structure. Although no other diffraction peak of co-doped sample appeared, the (1 0 1) and (2 0 0) peak location of Cu/N codoped TiO₂ evidently moved in high-angle region and the (0 0 4) peak location reversely shifted, compared with the undoped TiO₂ samples. Based on the Bragg's Law and the formula for a tetragonal unit cell, the lattice parameters of the as-prepared samples are summarized in Table S1. The related lattice parameters (a and c) of Cu-TiO₂ reached 3.826 and 9.617 Å, which are slightly bigger than those of undoped TiO₂ (a = 3.815 and c = 9.611 Å). It was reasonable since the radius of Cu(Ⅰ/Ⅱ) (~0.73 Å) was greater than that of Ti(Ⅳ) (0.61 Å), resulting in the extension of bond lengths and the expansion of lattice.^[35] Besides, the lattice parameter c value of N-TiO₂ increased by about 0.03 Å and other parameter are never distinctly changed, compared with that of the undoped TiO₂. Combined with the above results, it was consistent with swelling of the unit cell caused by the interstitial N and substitutional Cu in anatase phase TiO₂ structure^[35,36].

  Figure 1

  

  Figure 1.  XRD patterns of the obtained samples (degree range: a. 10-80° and b. 23-50°).

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  The surface chemical states of the materials were detested by the XPS measurement. As shown in Figure 2a, the strong peaks at 458.7 and 464.5 eV were assigned to Ti(Ⅳ), while the shoulder peaks at 457.4 and 463.1 eV were interpreted as Ti(Ⅲ) in the 8-Cu/N-TiO₂ sample.^[37,38] In the Cu 2p XPS spectrum (Figure 2b), the strong peaks at 934.7 and 954.3 eV were assigned to the Cu(Ⅱ) in 8-Cu/N-TiO₂.^[25] In the spectrum of N 1s (Figure 2c), the characteristic peak of N-O bond appeared at 400.1 eV, indicating that the N impurity was interstitially doped in the lattice of TiO₂.^[25,39] Based on the above results, the N and Cu impurities were successfully co-doped into the crystal structure of TiO₂.

  Figure 2

  

  Figure 2.  XPS spectra of the 8-Cu/N-TiO₂ sample (a. Ti2p, b. Cu 2p and c. N 1s).

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  The morphologies of TiO₂, N-TiO₂, Cu-TiO₂ and 8-Cu/N-TiO₂ samples were detected by SEM measurement. The SEM image in Figure 3a revealed that TiO₂ exhibited a fibrous morphology. In the images of doped TiO₂ samples (Figure 3b-d), the short rod-like structure appeared due to the impurity doping. The morphology of 8-Cu/N-TiO₂ sample was further elucidated using TEM measurement. As shown in Figure 3e, the rod-like morphology is made up of nanoparticles. According to the N₂ adsorption-desorp-tion measurements, the surface areas of TiO₂, N-TiO₂, Cu-TiO₂ and 8-Cu/N-TiO₂ samples reached 35.6, 38.6, 36.5 and 45.9 m²/g, respectively, indicating the Cu and N co-doping promoted the specific surface area of TiO₂ catalyst. In the inset image of Figure 3e, the lattice fringes of 0.351 and 0.244 nm corresponded to the (1 0 1) and (1 0 3) lattice planes of anatase TiO₂. Additionally, the EDX mapping in Figure 3f showed that the N, Cu and Ti elements were uniformly distributed of the rod surface. Combined with the analysis of XRD and XPS, it can be deduced that Cu and N impurities were successfully doped into the rod-like TiO₂ materials.

  Figure 3

  

  Figure 3.  SEM images of the TiO₂ (a), N-TiO₂ (b), Cu-TiO₂ (c) and 8-Cu/N-TiO₂ (d) samples; TEM image (e) and EDX mapping (f) of 8-Cu/N-TiO₂.

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  The DRS and VB-XPS measurements were performed to investigate the band structure of the as-prepared TiO₂, N-TiO₂, Cu-TiO₂ and 8-Cu/N-TiO₂ samples. As shown in Figure 4a, the doped TiO₂ samples exhibited red shift of the absorption edges compared with that of pristine TiO₂. The optical absorption intensity of 8-Cu/N-TiO₂ obviously improved in the wavelength region of 550-800 nm, which was possibly due to the formation of Ti(Ⅲ) or oxygen defect. In the inset of Figure 4a, the band gap (Eg) values of TiO₂, N-TiO₂, Cu-TiO₂ and 8-Cu/N-TiO₂ were calculated by the Kubelka-Munk function to be 2.98, 2.79, 2.87 and 2.64 eV, respectively. The decreased Eg values were induced by Cu and N doping. In the VB-XPS spectra (Figure 4b), the VB values of TiO₂, N-TiO₂, Cu-TiO₂ and 8-Cu/N-TiO₂ samples reached 2.80, 2.68, 2.47 and 2.35 eV, respectively. Combined with Eg, the corre-sponding CB values were respectively calculated to be -0.18, -0.19, -0.31 and -0.29 eV. Based on the above results, N and Cu doping promoted the CB position of TiO₂-based catalysts, which was beneficial for CO₂ reduction.

  Figure 4

  

  Figure 4.  DRS (a) and VB-XPS (b) spectra of the as-prepared samples (A. TiO₂, B. Cu-TiO₂, C. N-TiO₂ and D. 8-Cu/N-TiO₂).

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  Photocatalytic and Photothermal Catalytic Performance for CO₂ Reduction. The photocatalytic activities of the as-prepared samples were evaluated for CO₂ reduction with H₂O vapor (λ > 400 nm) at room temperature. Firstly, no products were detected in the blank test without catalyst or illumination (Figure S1). As shown in Figure 5a and S3, CO and CH₄ products were found in the gas phase for all catalysts after 3 h of illumination. The photocatalytic activities of the doped TiO₂ catalysts were higher than that of pristine TiO₂, and the CH₄/CO ratio was increased by Cu doping. With increasing the doped Cu content, the yields and CH₄ selectively were all enhanced and the 8-Cu/N-TiO₂ sample exhibited higher activity than other samples (Figure S2a). Although, due to the high overpotential for H₂O oxidation, the practical O₂ yields were lower than the theoretical values, the difference between them became small with the co-existence of illumination and heating (Figure S2b). Thermal catalytic performance (Figure 5b) at 160 ℃ for all the as-prepared samples showed obvious decreased CO and CH₄ production yields compared with those of photocatalysis. Photothermal catalytic performance in Figure 5c revealed that the CO, CH₄ and O₂ yields of 8-Cu/N-TiO₂ at 160 ℃ with illumination reached 34.4, 541.4 and 1087.7 μmol/g_cat, respectively, and the CH₄ selectivity reached more than 95%. Moreover, the photothermal catalytic activities of all the doped TiO₂ samples were promoted compared with those of undoped TiO₂ and commercial P25, indicating the positive influence of Cu and N co-doping on the catalytic activity. Figure 5d showed the comparison of catalytic performance of Cu/N-TiO₂ in the above different catalytic systems. Catalytic activity and CH₄ selectivity in the photothermal catalytic system were obviously superior to those in thermal and photothermal catalytic systems for the 8-Cu/N-TiO₂ sample, indicating the synergistic effect of heating and illumination for CO₂ reduction. Photocatalytic or photothermalcatalytic performances of TiO₂ based materials for CO₂ reduction have been summarized in Table S2 based on previous studies. The CO, CH₄ and O₂ yields over Cu/N-TiO₂ were obviously higher than those of the reported materials. Additionally, the photothermal catalytic performance under different light conditions in Table S3 showed that high CH₄ selectivity was kept, indicating its independence of wavelength ranges and intensities. Additionally, the difference between the theoretical and practical O₂ yields (Figure S3) of Cu/N-TiO₂ observably fallen off under 160 ℃ temperature. The influence of catalytic temperature on the photothermal catalytic activity of the 8-Cu/N-TiO₂ sample was further studied and the results are shown in Figure S4. The CO and CH₄ yields firstly increased with the increase of temperature from room temperature (RT) to 160 ℃, beyond which the yields gradually decreased (Figure S5a), which may be due to the decrease of CO₂ or H₂O adsorption capacity at high temperature. The practical O₂ yields increased with increasing temperature (Figure S4b) and reached the maximum at 160 ℃, indicating that the relatively high temperature had a positive effect on the O₂ generation from H₂O molecules. To further confirm the speculation, H₂O-TPD measurement was recorded. In Figure S5, the adsorbed water interacting with catalyst surface would be desorbed at relatively high temperature (> ~167 ^oC), thus leading to the decreased catalytic performance of CO₂ conversion.

  Figure 5

  

  Figure 5.  (a) Production yields of photocatalytic (a, visible-light illumination), (b) thermal catalytic (b, 160 ℃) and photothermal catalytic (c, 160 ℃ and visible-light illumination) for CO₂ reduction over different samples after 3 h; (d) comparison of catalytic performance of 8-Cu/N-TiO₂ in the above different catalytic systems.

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  The photothermalcatalytic activity of 8-Cu/N-TiO₂ was further evaluated by the time-dependent CO₂ reduction behaviors. As shown in Figure 6a, the CO and CH₄ yields increased linearly by prolonging the irradiation-heating time. After 9 h of continuous reaction, the CO, CH₄ and O₂ yields reached 49.7, 1455.1 and 2910.2 μmol/g_cat, respectively. The stability of the photocatalyst played an important role in practical application. In this study, cycling experiments for photothermal catalytic CO₂ reduction were carried out with each period lasting 5 h and the result is shown in Figure 6b. The CO, CH₄ and O₂ yields still reached 28.4, 870.5 and 1735.7 μmol/g_cat in the 6^th cycling experiment. To further investigate the stability, XRD patterns of 8-Cu/N-TiO₂ before and after seven cycles is shown in Figure S6. No apparent change was found after cycling experiments, suggesting that 8-Cu/N-TiO₂ possesses an excellent stability without structural deterioration during the CO₂ reduction process.

  Figure 6

  

  Figure 6.  (a) CO, CH₄ and O₂ yields as a function of time over 8-Cu/N-TiO₂; (b) cycling test of photothermalcatalytic CO₂ reduction for the 8-Cu/N-TiO₂ catalyst (5 h for one cycle).

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  XPS analysis was conducted to investigate the surface chemical states of the 8-Cu/N-TiO₂ catalyst in the cycling test, and the results are presented in Figure 7 and Table S4. The area ratio of characteristic peak for Cu(Ⅰ) and Cu(Ⅱ) was related to the content ratio of Cu(Ⅰ) and Cu(Ⅱ) on the surface. The calculated Cu(Ⅰ)/Cu(Ⅱ) ratios after three cycling tests increased as compared with that of the fresh sample, and the ratio slightly decreased after the 4^th cycling experiment. The variation trend of Cu(Ⅰ)/Cu(Ⅱ) ratio was consistent with that of photothermal catalytic activity. It was indicated that the enhanced catalytic activity was mainly caused by the existence of Cu(Ⅰ) compared with the Cu(Ⅱ) impurity in the doped TiO₂ sample.^[19,40] Meanwhile, the Ti(Ⅲ)/Ti(Ⅳ) ratio on the surface decreased after the cycling tests. There were no obvious differences of N 1s characteristic peaks between the used and fresh samples (Figure S7). Hence, the Cu(Ⅰ) and Ti(Ⅲ) impurities evidently influenced the catalytic activity of CO₂ conversion.

  Figure 7

  

  Figure 7.  XPS spectra of (a) Cu 2p and (b) Ti 2p for the 8-Cu/N-TiO₂ catalyst before and after cycling tests of photothermalcatalytic CO₂ reduction.

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  Catalytic Mechanism. The photocurrent response curves of TiO₂, Cu-TiO₂, N-TiO₂ and 8-Cu/N-TiO₂ samples are presented in Figure 8a. The Cu/N-TiO₂ sample exhibited the highest photocurrent intensity under light illumination, which was about 1.5, 15.2 and 1.5 times those of TiO₂, Cu-TiO₂ and N-TiO₂, indicating that Cu and N co-doping improved charge separation of the 8-Cu/N-TiO₂^[41]. The radius of the semicircle in the EIS spectra (Figure 8b) followed the order: TiO₂ > Cu-TiO₂ > N-TiO₂ > Cu/N-TiO₂. This suggested that Cu/N-TiO₂ sample had a smaller resistance, facilitating charge transfer at the interface^[42]. Therefore, Cu and N co-doping facilitated transfer of photo-induced carriers, leading to the enhanced photocatalytic performance. Combined with XRD results, the exposed (1 0 1) surface of anatase phase TiO₂ was selected, and several models of Cu and N mono-doped TiO₂ have been optimized (Figure S8). According to the formation energy, lattice structure and chemical composition (Figure S8e), two structure modes of N@gap and Cu@Ti_5c were adopted for the N and Cu mono-doped TiO₂ samples, and then multi-modes of surface structure for co-doped TiO₂ (Figure S9) were built and optimized. According to enhanced catalytic performance and the chemical composition, the influence of Cu/N impurities and Ti(Ⅲ) site of TiO₂ surface on the CO₂ conversion was of significance. The co-doped surface mode with the interstitial N and lower state Cu impurities (Cu@Ti_5c & N@gap TiO₂ mode) was selected due to the lower formation energy and structural feature (the existence of Ti(Ⅲ) and N-O bond). As shown in Figure 9 and Figure S10, the adsorption energies of CO₂ (-11.96 kcal/mol) and H₂O (-21.40 kcal/mol) for the Cu and N mono-doped TiO₂ sample decreased compared with those of pristine TiO₂ (-9.82 and -11.50 kcal/mol). Similarly, the above adsorption energies for Cu-TiO₂ noticeably decreased. In the system containing CO₂ and H₂O molecules, the doped Cu and N impurities facilitated adsorption of reactant molecules. In mode of codoped TiO₂ surface (Figure 9), the CO₂ molecule was adsorbed around the doped Cu and N impurities, and its adsorption energy (-13.29 kcal/mol) noticeably decreased compared with that of pristine TiO₂. Similarly, the adsorption energy of H₂O molecule on Cu/N-TiO₂ surface (-22.74 kcal/mol) was obviously lower than that of TiO₂ (-11.50 kcal/mol). Additionally, it is notable that the O=C=O angle in CO₂ and the H-O-H angle in H₂O on the Cu/N co-doped TiO₂ surface reduced to about 178.6° and 100.8°, indicating that the energy barrier for CO₂ and H₂O activation was obviously lower than that of TiO₂. To further confirm the above speculation, the adsorption abilities of undoped and codoped TiO₂ samples were measured, with the results shown in Figure S11. The CO₂ and H₂O adsorption capacities of the co-doped TiO₂ sample were higher than those of undoped TiO₂, which agreed with the theoretical results of surface mode. Considering the simultaneous process of CO₂ reduction and H₂O oxidization, the adsorption of co-existed CO₂ and H₂O molecules was studied on the catalyst surface. H₂O and CO₂ molecules were adsorbed around the Ti(Ⅲ) site on surface, and their distance was distinctly shortened (Figure S12). The adsorbed energy (-33.42 kcal/mol) in the CO₂ and H₂O co-existed system was lower than that of undoped TiO₂ (-22.29 kcal/mol). Additionally, the angle of H-O-H in reactant molecules on the Cu/N-TiO₂ surface still exhibited a descent in the co-existence of CO₂ and H₂O by comparison with that of pristine TiO₂ surface. Based on the above results, the substitutional doped Cu and N impurities could effectively promote the adsorption ability for CO₂ and H₂O molecules, leading to the enhanced catalytic performance.

  Figure 8

  

  Figure 8.  I-t curves (a) and EIS spectra (b) of the obtained samples.

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  Figure 9

  

  Figure 9.  DFT optimized CO₂ adsorption configurations on (1 0 1) facets of pristine TiO₂ and Cu/N-TiO₂. (Bond distance units: Å; Color scheme: Ti, gray; O, red; H, white; C, dark gray; Cu, green; N, blue).

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  According to the enhanced catalytic performance for CO₂ conversion, the doped Cu and N impurities of TiO₂ catalyst could promote the adsorption ability for H₂O and CO₂ molecules. Meanwhile, the symmetry of adsorbed reactant molecules on the surface of Cu/N-TiO₂ surface dropped off, compared with that of the pristine TiO₂, resulting in the enhanced catalytic activity for CO₂ reduction. For further exploring the CO₂ conversion on the surface, the in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) were recorded. In Figure 10a, several characteristic peaks of CO₃^2- (1318 and 1365 cm^-1), CO₂^- (1650, 1516 and 1222 cm^-1), HCO₃^- (1626 and 1422 cm^-1) and HCOO^- (1541 and 1339 cm^-1) species appeared, and the peak intensities gradually increased with increasing the light irradiation time^[43-48]. It was demonstrated that HCOO^- is an important intermediate during the process of photocatalytic CO₂ reduction to CO, consistent with previous reports^[47,48]. The above peaks also appeared with 8-Cu/N-TiO₂ sample and their peak intensity is higher than that of undoped TiO₂, resulting in higher CO₂ conversion efficiency over Cu/N codoped TiO₂. Particularly, three new peaks of ·CH₃ (1458 and 1389 cm^-1) and ·CO₂^- (1679 cm^-1) were identified for the 8-Cu/N-TiO₂ sample compared with the undoped TiO₂^[49,50]. The characteristic peaks of ·CH₃ indicated the production of CH₄, which agreed well with the product selectivity of 8-Cu/N-TiO₂. Additionally, the peaks of partial species (·CH₃, HCOO^-, HCO₃^- and so on) slightly shifted due to the variation of surface charge for the catalysts^[47]. According to the DRIFTS results (Figure 10) and catalytic performance, the activation of CO₂ to •CO₂^- was achieved by shoving excessive photo-induced electrons to the adsorbed CO₂ molecules at the surface defect sites^[51]. HCOO^- was taken as an intermediate to generate CO selectively in photocatalytic CO₂ conversion^[52]. HCO₃^- species, generated from CO₂ and OH groups, served as a possible intermediate for the production of CO and C1 fuels associated with the emergence of dissociative H atoms^[52]. Both CO₃^2- and HCO₃^- were important intermediates, which were subsequently transformed into CO products^[53,54]. The characteristic peaks of methyl radicals (•CH₃) indicated the production of CH₄^[50]. Hence, the possible mechanism of CO₂ conversion with water vapor was summarized as follows:

  $ {\text { catalyst }}+{\mathrm{E}} \rightarrow {\mathrm{h}}^{+}+{\mathrm{e}}^{-} $

  (1)

  $ {\mathrm{H}}_2 {\mathrm{O}}+{\mathrm{h}}^{+} \rightarrow \cdot {\mathrm{OH}}+{\mathrm{H}}^{+} $

  (2)

  $ {\mathrm{H}}_2 {\mathrm{O}}+2 {\mathrm{~h}}^{+} \rightarrow {\mathrm{H}}_2 {\mathrm{O}}_2+2 {\mathrm{H}}^{+} $

  (3)

  $ {\mathrm{H}}_2 {\mathrm{O}}_2+{\mathrm{E}} \rightarrow 2 {\mathrm{H}}_2 {\mathrm{O}}+{\mathrm{O}}_2({\mathrm{~g}}) $

  (4)

  $ {\mathrm{CO}}_2+{\mathrm{e}}^{-} \rightarrow \cdot {\mathrm{CO}}_2^{-} $

  (5)

  $ \cdot {\mathrm{CO}}_2{ }^{-}+\cdot {\mathrm{OH}} \rightarrow {\mathrm{HCO}}_3^{-} $

  (6)

  $ {\mathrm{HCO}}_3^{-}+{\mathrm{H}}^{+}+2 {\mathrm{e}}^{-} \rightarrow 2 {\mathrm{OH}}^{-}+{\mathrm{CO}}({\mathrm{g}}) $

  (7)

  $ {\mathrm{CO}}_3{ }^{2-}+2 {\mathrm{H}}^{+}+2 {\mathrm{e}}^{-} \rightarrow 2 {\mathrm{OH}}^{+}+{\mathrm{CO}}({\mathrm{g}}) $

  (8)

  $ \cdot {\mathrm{CO}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} \rightarrow \cdot \operatorname{COOH} $

  (9)

  $ \cdot {\mathrm{COOH}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} \rightarrow \cdot {\mathrm{CO}}+{\mathrm{H}}_2 {\mathrm{O}} $

  (10)

  $ \cdot {\mathrm{CO}} \rightarrow {\mathrm{CO}}({\mathrm{g}}) $

  (11)

  $ \cdot {\mathrm{COOH}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} \rightarrow \cdot {\mathrm{HCOOH}} $

  (12)

  $ \cdot {\mathrm{HCOOH}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} \rightarrow \cdot {\mathrm{CHO}}+{\mathrm{H}}_2 {\mathrm{O}} $

  (13)

  Figure 10

  

  Figure 10.  DRIFTS spectra of the undoped TiO₂ (a) and 8-Cu/N-TiO₂ samples.

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  CONCLUSION

  The rod-like Cu/N co-doped TiO₂ samples were synthesized by the electrospinning-calcination method. The visible-light absorption ability of catalyst was enhanced by the substitutional Cu and interstitial N doping and the Ti(Ⅲ) formation was discovered in co-doped sample. The synergistic effect between the photocatalysis and thermalcatalysis was discovered for CO₂ conversion over 8-Cu/N-TiO₂. After constant catalytic conduction for 9 h, the CO, CH₄ and O₂ yield can reach 49.7, 1455.1 and 2910.2 μmol/g_cat. In the 7^th cycling experiment, the yields of two main products (CH₄ and O₂) were slightly down by less than 11.5%. The surface mode of the main (1 0 1) crystal face over Cu/N co-doped TiO₂ was constructed. Based on the experimental and theoretical results, Cu/N co-doping not only was in favor of the CO₂ and H₂O adsorption, but also reduced activation energy for CO₂ reduction process.

  EXPERIMENTAL

  Synthesis of Cu/N-TiO₂. The rod-like TiO₂ samples were synthesized by the electrospinning-calcination method. Typically, 3.4 mL butyl titanate and 3.0 g polyvinyl pyrrolidone were dissolved into 15 mL ethyl alcohol. The TiO₂ precursor was obtained by electrostatic spinning under the voltage of 18.5 kV with a distance of 10 cm between the needle and acceptor. And then, the undoped TiO₂ sample was obtained after calcination at 580 ℃ for 1 h. Cu doped TiO₂ was synthesized by the above method with the addition of 0.2 g Cu(NO₃)₂·3H₂O, which was named as Cu-TiO₂. N doped TiO₂ (N-TiO₂) was synthesized similarly by adding 15 mL dimethyl formamide instead of ethyl alcohol. N and Cu co-doped TiO₂ was synthesized similarly with the addition of Cu(NO₃)₂·3H₂O and 15 mL dimethyl formamide, which was finally named as x-Cu/N-TiO₂ (x was on behalf of ideal amount-of-substance ratio of Cu/Ti in raw materials). The contents of metal ions for all of the obtained samples were detected by ICP-AES measurements, and the real Cu/Ti ratios (Table S5) in the composites were basically consistent with the initial inventory ratio.

  Characterization. The structure and composition characterization of the obtained samples were explored using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectra (DRS), photoelectrochemical test, in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) and so on. The processes of the above measurements and tests were detailed in Supporting Information S1.

  Catalytic Test for CO₂ Reduction. Photocatalytic, thermal and photothermal catalytic performances for CO₂ reduction were measured in a 250 mL pressure-proof reactor (CEL-HPR250T, Beijing China Education Au-Light Co., LTD, China). The dispersion containing 15 mg catalyst and 5 mL H₂O drop onto a hydrophilic porous quatz plate (2.8cm × 2.8cm). After vacuum drying at 110 ℃, the plate loaded with sample was put into the reactor and subsequently vacuum-treated. Before illumination, high purity CO₂ (99.995%) as the purging gas was injected into the reactor for several times, and the reactant mixture (CO₂ and H₂O vapor) was generated by the flowing CO₂ gas and water bubbler. Thermal catalytic CO₂ reduction was conducted at the predetermined temperature (160 ℃). Photothermal catalytic performance for CO₂ reduction was investigated at 160 ℃, and a 300 W Xe lamp with an ultraviolet cutoff filter (λ > 400 nm) was adopted as the light source. For comparison, photocatalytic performance for CO₂ reduction was evaluated at low temperature (3 ℃), and all the other conditions remained unchanged. The gaseous products (CO, O₂ and CH₄) from the reactor were quantifiably identified for off-line analysis using a gas chromatograph (GC-7890Ⅱ Beijing China Education Au-Light Co., LTD, China) with FID and TCD detectors, and the other hydrocarbon was further analyzed by GC-MS (Agilent 7890A-5975C) equipped with a DB-FFAP capillary column.

  The selectivity of CO (S_CO) and CH₄ (S_CH4) products and the theoretical O₂ yield (Y _{ideal O2}) were obtained according to Equation (14), (15) and (16), respectively:

  $ S_{{\mathrm{CO}}}=2 \times n_{{\mathrm{CO}}} /\left(2 \times n_{{\mathrm{CO}}}+8 \times n_{{\mathrm{CH}} 4}\right) \times 100 \% $

  (14)

  $ S_{{\mathrm{CH}} 4}=8 \times n_{{\mathrm{CH}} 4} /\left(2 \times n_{{\mathrm{CO}}}+8 \times n_{{\mathrm{CH}} 4}\right) \times 100 \% $

  (15)

  $ Y_{{\text {ideal }} \;{\mathrm{O}} 2}=0.5 \times n_{{\mathrm{CO}}}+2 \times n_{{\mathrm{CO}}} $

  (16)

  Theoretical Model and Calculation Method. A 48-atom anatase supercell of pure TiO₂ with the I41/AMD space group was constructed and optimized by using the first principle pseudopotential plane wave algorithm of Koho-Sham self-consistent density functional theory with the Castep package of Materials Studio. The geometrically optimized lattice parameters of pure TiO₂ super cell were a = b = 3.904 Å, c = 9.878 Å, α = β = γ = 90º, respectively. Based on the optimized geometrical structure of pure TiO₂ supercell, the TiO₂ (1 0 1) surface was modeled with eighteen atomic layers. The terminal atoms of TiO₂ (1 0 1) surface were dicoordinated O atoms. Considering the role of defective sites on the TiO₂ (1 0 1) surface, we also considered six co-doped-TiO₂ models (Figure S9). A vacuum layer of 15 Å was set in the direction perpendicular to the (1 0 1) surface for all the models. Eight atomic layers on the surface were subjected for coordinate relaxation using the Castep package of Materials Studio. The ultra-soft pseudopotential as well as the generalized gradient approximation of Perdew-Burke-Ernzerhof parameterization (GGA-PBE) were applied to approximately describe the interaction between valence electrons and ions. In this work, the cut-off energy of plane wave was set to be 400 eV, and the Monkhorst-Pack scheme with a 2×3×1 k-point grid was used to divide the Brillouin zone. During the processes of atomic relaxation calculations, the specific optimization parameters were selected as follows: the convergence value of the total energy of the system was 2.0 × 10^-5 eV/atom; The force on each atom was less than 0.05 eV/Å, and the internal stress deviation was less than 0.1 GPa; The maximum displacement between atoms was less than 0.002 Å. The standard DFT could underestimate the band gap of the semiconductors. The DFT+U formalism was thus employed, and the values of parameter U were set to be 8.0 eV for the Ti 3d electrons and 7.5 eV for the Cu 3d electrons.

  In order to study the effect of Cu and N doping on the CO₂ and H₂O adsorption capacities, the grand canonical Monte Carlo (GCMC) method via Adsorption Locator was used to adsorb CO₂ and H₂O molecules onto the TiO₂ (1 0 1) surface, and the best adsorption site and configuration were obtained by the simulated annealing methods.

  
  ACKNOWLEDGEMENTS: This work was supported by the National Natural Science Foundation of China (No. 51802082), Key Scientific and Technological Project of Henan Province (No. 222102320100) and Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 21HATIT016). COMPETING INTERESTS
  The authors declare no competing interests.
  ADDITIONAL INFORMATION
  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0191
  For submission: https://www.editorialmanager.com/cjschem
  
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• 

  Figure 1  XRD patterns of the obtained samples (degree range: a. 10-80° and b. 23-50°).

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  Figure 2  XPS spectra of the 8-Cu/N-TiO₂ sample (a. Ti2p, b. Cu 2p and c. N 1s).

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  Figure 3  SEM images of the TiO₂ (a), N-TiO₂ (b), Cu-TiO₂ (c) and 8-Cu/N-TiO₂ (d) samples; TEM image (e) and EDX mapping (f) of 8-Cu/N-TiO₂.

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  Figure 4  DRS (a) and VB-XPS (b) spectra of the as-prepared samples (A. TiO₂, B. Cu-TiO₂, C. N-TiO₂ and D. 8-Cu/N-TiO₂).

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  Figure 5  (a) Production yields of photocatalytic (a, visible-light illumination), (b) thermal catalytic (b, 160 ℃) and photothermal catalytic (c, 160 ℃ and visible-light illumination) for CO₂ reduction over different samples after 3 h; (d) comparison of catalytic performance of 8-Cu/N-TiO₂ in the above different catalytic systems.

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  Figure 6  (a) CO, CH₄ and O₂ yields as a function of time over 8-Cu/N-TiO₂; (b) cycling test of photothermalcatalytic CO₂ reduction for the 8-Cu/N-TiO₂ catalyst (5 h for one cycle).

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  Figure 7  XPS spectra of (a) Cu 2p and (b) Ti 2p for the 8-Cu/N-TiO₂ catalyst before and after cycling tests of photothermalcatalytic CO₂ reduction.

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  Figure 8  I-t curves (a) and EIS spectra (b) of the obtained samples.

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  Figure 9  DFT optimized CO₂ adsorption configurations on (1 0 1) facets of pristine TiO₂ and Cu/N-TiO₂. (Bond distance units: Å; Color scheme: Ti, gray; O, red; H, white; C, dark gray; Cu, green; N, blue).

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  Figure 10  DRIFTS spectra of the undoped TiO₂ (a) and 8-Cu/N-TiO₂ samples.

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