Photothermal Catalytic Reduction of CO 2 by Cobalt Silicate Heterojunction Constructed from Clay Minerals

: The coupled utilization of solar and thermal energy is considered an efﬁcient way to improve the efﬁciency of CO 2 reduction. Herein, palygorskite (Pal) clay is as a silicon source, while Co 2+ is introduced to prepare two-dimensional Co 2 SiO 4 nanosheets, and the excess of Co 2+ leads to the growth of Co 3 O 4 on the surface of Co 2 SiO 4 to obtain an S-scheme Co 2 SiO 4 /Co 3 O 4 − x heterojunction, which facilitates the charge transfer and maintains higher redox potentials. Beneﬁting from black color and a narrow band gap, the cobalt oxide on the surface can increase the light absorption and produce a local photothermal effect. Under proper thermal activation conditions, the photoelectrons captured by the abundant oxygen vacancies can obtain a secondary leap to the semiconductor conduction band (CB), suppressing the recombination of electron-hole pairs, thus favoring the electron transfer on Co 2 SiO 4 /Co 3 O 4 − x. The composites not only have abundant oxygen vacancies, but also have a large speciﬁc surface area for the adsorption and activation of CO 2 . The yields of CH 3 OH on Co 2 SiO 4 /Co 3 O 4 − 5% reach as high as 48.9 µ mol · g − 1 · h − 1 under simulated sunlight irradiation. In situ DRIFTS is used to explore the photocatalytic reduction CO 2 mechanism. It is found that the thermal effect facilitates the generation of the key intermediate COOH* species. This work provides a new strategy for photothermal catalytic CO 2 reduction by taking advantage of natural clay and solar energy.


Introduction
With the rapid development of the fossil energy economy, CO 2 emissions are enormous, which disrupts the balance of nature and causes climate change and serious ecological damage in human society [1].Therefore, research on CO 2 reduction using renewable fuel technology has received widespread attention [2,3].
Inspired by photosynthetic organisms in nature, visible light reduction is used to convert CO 2 into chemicals with high added value and solar fuels (e.g., CO, HCOOH, CH 3 OH, CH 4 , etc.) [4].For example, Lin [5] et al. decomposed CO 2 into CO with an apparent quantum yield of 0.25% at 420 nm by using graphitic carbon nitride (g-CN) semiconductors.However, the yield of conventional photocatalytic reduction of CO 2 is generally low, and the difficulties are mainly related to the limited utilization of solar energy and the low efficiency of photogenerated electron/hole separation [6,7].Recent studies have shown that the introduction of heat in some photocatalytic reactions can effectively enhance reaction performance [8].
Cobalt-based catalysts with full-spectrum absorption performance and multiple valence states are widely used in CO 2 reduction [9].The modification of Pal clay can significantly change its physical and chemical properties, such as acidification, alkalization, and ion exchange [10].In our previous work, a unique solar driven catalysis system was developed to convert cellulose into lactic acid using Cu-modified natural palygorskite (Pal) catalyst [11].Liu [12] et al. reported on one-dimensional Pal treated with acid to remove most of the metal ions from the silica framework, and bismuth ions were incorporated to grow a two-dimensional (2D) bismuth silicate (Bi 12 SiO 20 ) nanosheet under a microwavehydrothermal process.Metal silicates can be prepared from silica and are often used in cement and glass processing, as well as in manufacturing [13].
Herein, acid modification of 1D Pal was performed to remove most of the metal ions in the Pal skeleton and retain its unique silicon-oxygen tetrahedral structure.A 2D cobalt silicate Co 2 SiO 4 was synthesized by the microwave hydrothermal method, in which onedimensional rod silicate was converted to two-dimensional flake silicate.Cobalt oxide particles were grown on the surface of Co 2 SiO 4 by adjusting the amount of Co 2+ in Co 2 SiO 4 .Co 2 SiO 4 /Co 3 O 4 catalysts not only have a large specific surface area, but they also have abundant oxygen vacancies for the adsorption and activation of CO 2 , which effectively achieves full-spectrum absorption and light-induced heat under solar light.1b shows the magnified view of Figure 1a from 36.5 • to 39.5 ion exchange [10].In our previous work, a unique solar driven catalysis system was developed to convert cellulose into lactic acid using Cu-modified natural palygorskite (Pal) catalyst [11].Liu [12] et al. reported on one-dimensional Pal treated with acid to remove most of the metal ions from the silica framework, and bismuth ions were incorporated to grow a two-dimensional (2D) bismuth silicate (Bi12SiO20) nanosheet under a microwavehydrothermal process.Metal silicates can be prepared from silica and are often used in cement and glass processing, as well as in manufacturing [13].

Result and Discussion
Herein, acid modification of 1D Pal was performed to remove most of the metal ions in the Pal skeleton and retain its unique silicon-oxygen tetrahedral structure.A 2D cobalt silicate Co2SiO4 was synthesized by the microwave hydrothermal method, in which onedimensional rod silicate was converted to two-dimensional flake silicate.Cobalt oxide particles were grown on the surface of Co2SiO4 by adjusting the amount of Co 2+ in Co2SiO4.Co2SiO4/Co3O4 catalysts not only have a large specific surface area, but they also have abundant oxygen vacancies for the adsorption and activation of CO2, which effectively achieves full-spectrum absorption and light-induced heat under solar light.

XRD
Figure 1a shows the XRD spectra of the Pal, SiO2 Co3O4, Co2SiO4, and Co2SiO4/Co3O4 composites.The peaks located at 8.5°, 20.1°, 28.0°, and 35.8°correspond to the (110), ( 040), (400), and (161) crystal planes of the original Pal.Compared with the original Pal, the characteristic peaks of acid-treated Pal basically disappeared, indicating that the metal ions in Pal were largely dissolved after a long treatment with high concentrations of hydrochloric acid, corresponding to the SiO2.The diffraction peaks appearing at 18.8°, 31.0°, and 36.5°, are ascribed to (111), (220), and (311), crystal planes of Co2SiO4 (PDF#29-0508).The characteristic peaks at 31.3°, 36.8°,44.8°, and 65.2° are consistent with the characteristic peaks of cubic spinel Co3O4 (PDF#42-1467).It is noteworthy that the diffraction peaks of Co3O4 are very close to Co2SiO4. Figure 1b shows the magnified view of Figure 1a from 36.5° to 39.5°.The peaks of Co2SiO4/Co3O4 composites are obviously shifted to a higher angle compared with Co3O4 and Co2SiO4, indicating a stronger interaction between Co2SiO4 and Co3O4 [12,14].2a, the morphology of Pal is a one-dimensional nanofiber structure with a diameter of about 30-50 nm.After a high concentration of acid treatment, the structure of Pal is destroyed, and the rod-like structure tends to break and assemble into sheets.Figure 2b,f indicates TEM and HRTEM of Co 2 SiO 4 .The microstructure of Co 2 SiO 4 is a two-dimensional flake, which may be due to the preservation of the silicon-oxygen tetrahedron structure in the skeleton after the destruction of Pal [12].After the addition of Co 2+ , the chain-like structure of the silicon-oxygen tetrahedron regularly grows into two-dimensional cobalt silicate flakes under alkaline hydrothermal conditions.The lattice spacing of the Co 2 SiO 4 is 0.09 nm, corresponding to the (400) crystal plane of Co 2 SiO 4 .Figure 2c  Figure 2 shows the TEM picture of the Pal, Co3O4, Co2SiO4, and Co2SiO4/Co3O4 com posites.As shown in Figure 2a, the morphology of Pal is a one-dimensional nanofibe structure with a diameter of about 30-50 nm.After a high concentration of acid treatmen the structure of Pal is destroyed, and the rod-like structure tends to break and assembl into sheets.Figure 2b,f indicates TEM and HRTEM of Co2SiO4.The microstructure o Co2SiO4 is a two-dimensional flake, which may be due to the preservation of the silicon oxygen tetrahedron structure in the skeleton after the destruction of Pal [12].After th addition of Co 2+ , the chain-like structure of the silicon-oxygen tetrahedron regularly grow into two-dimensional cobalt silicate flakes under alkaline hydrothermal conditions.Th lattice spacing of the Co2SiO4 is 0.09 nm, corresponding to the (400) crystal plane o Co2SiO4. Figure 2c,g displays TEM and HRTEM of Co2SiO4/Co3O4−1%, respectively.It ca be seen that, on the surface of Co2SiO4, the Co3O4 particles have a diameter of about 1 nm. Figure 2d,h shows TEM and HRTEM of Co2SiO4/Co3O4−5%; Co3O4 particles on th surface of Co2SiO4 become more stacked.However, when the amount of Co reaches 10% it is difficult to distinguish Co2SiO4 and Co3O4, as shown in Figure 2e.The HRTEM image (Figure 2g,h) show that the Co3O4 nanoparticles are uniformly loaded on the surface o the Co2SiO4 2D nanosheets.The lattice spacing of Co3O4 on the surface is 0.32 nm and 0.3 nm, corresponding to the (311) and (400) planes of Co3O4, suggesting the existence of heterogeneous structure between Co2SiO4 and Co3O4.Most of the cobalt-based compounds appear as black or dark green, which can absorb the full range of the spectrum and are ideal photothermal materials [8,15].The band gaps of Co 2 SiO 4 and Co 3 O 4 in Figure 3b are estimated by the formula of (ahv) n/2 = A (hv − Eg); where a, h, v, A, and Eg represent the optical absorption coefficient, Planck constant, optical frequency, a constant, and band gap.The values n = 1 and n = 4 represent the indirect and direct band gap semiconductors, respectively [16,17].As shown in Figure 3b, the corresponding band gap energies of Co 2 SiO 4 and Co 3 O 4 are estimated to be 3.39 and 1.53 eV, respectively.The narrow band gap is more favorable for capturing the energy of sunlight, thus promoting the generation of e − /h + pairs [18].

UV-vis-NIR
Catalysts 2023, 13, x FOR PEER REVIEW 4 of 12 Figure 3a shows the UV-Vis-NIR absorption spectra of the Pal, Co3O4, Co2SiO4, and Co2SiO4/Co3O4 composites.Pal has a weak UV-Vis-NIR absorption edge of about 380 nm, while Co2SiO4 and Co2SiO4/Co3O4 composites obtained by modifying Pal have fullspectrum absorption to achieve the conversion of UV-responsive natural silicate minerals to visible-responsive silicates.Most of the cobalt-based compounds appear as black or dark green, which can absorb the full range of the spectrum and are ideal photothermal materials [8,15].The band gaps of Co2SiO4 and Co3O4 in Figure 3b are estimated by the formula of (ahv) n/2 = A (hv − Eg); where a, h, v, A, and Eg represent the optical absorption coefficient, Planck constant, optical frequency, a constant, and band gap.The values n = 1 and n= 4 represent the indirect and direct band gap semiconductors, respectively [16,17].As shown in Figure 3b, the corresponding band gap energies of Co2SiO4 and Co3O4 are estimated to be 3.39 and 1.53 eV, respectively.The narrow band gap is more favorable for capturing the energy of sunlight, thus promoting the generation of e − /h + pairs [18].

XPS
Figure 4a shows the XPS survey of Co3O4, Co2SiO4, and Co2SiO4/Co3O4−x, where the presence of Co and O elements can be determined; Si elements are present in the Co2SiO4 and Co2SiO4/Co3O4−x.Figure 4b shows the Co 2p spectra of Co3O4, Co2SiO4, and Co2SiO4/Co3O4−x.Co can be divided into four peaks, the peaks with binding energies at 794.59 eV and 795.96 eV correspond to Co 2p1/2.The peaks with binding energies at 779.77 eV and 781.68 eV correspond to Co 2p3/2, suggesting the presence of +2 valence and +3 valence of Co3O4, and also appear in the Co2SiO4 and Co2SiO4/Co3O4−x [19].The peaks near the binding energy of 781.06 eV and 797.23 eV in (Figure 4b) correspond to Co 2p1/2 and Co 2p3/2, respectively.Nevertheless, the binding energy of Co2SiO4/Co3O4−x is higher than Co2SiO4, which indicates that Co2SiO4 has a strong interaction with the additional generated Co3O4, increasing the electron cloud density around Co [20].The studies show that the binding energy of Co 2p in the Co2SiO4 structure is 781.3 eV, while the binding energy of Co 2p in Co3O4 is 779.6 eV.Obviously, the binding energy of cobalt is closer to the former, indicating that the product is Co2SiO4, rather than Co3O4, which is consistent with the results of the XRD analysis.In addition, the energy separation between Co 2p1/2 and Co 2p3/2 of the product is 15.96 eV, which is close to Co2SiO4 (ΔCo 2p = 15.5 eV) [21].Figure 4c shows the O 1s spectra of Co3O4, Co2SiO4, and Co2SiO4/Co3O4−x.The peaks around the binding energy of 529.85 eV and 532.82 eV correspond to the lattice oxygen (OL) and adsorbed oxygen (OA), respectively.The ratio of OA to OL in Co2SiO4/Co3O4−x is higher than that of Co3O4, indicating the composite Co2SiO4/Co3O4−x has higher content of oxygen vacancy than that of Co3O4 [22].4b) correspond to Co 2p 1/2 and Co 2p 3/2 , respectively.Nevertheless, the binding energy of Co 2 SiO 4 /Co 3 O 4 −x is higher than Co 2 SiO 4 , which indicates that Co 2 SiO 4 has a strong interaction with the additional generated Co 3 O 4 , increasing the electron cloud density around Co [20].The studies show that the binding energy of Co 2p in the Co 2 SiO 4 structure is 781.3 eV, while the binding energy of Co 2p in Co 3 O 4 is 779.6 eV.Obviously, the binding energy of cobalt is closer to the former, indicating that the product is Co 2 SiO 4 , rather than Co 3 O 4 , which is consistent with the results of the XRD analysis.In addition, the energy separation between Co 2p 1/2 and Co 2p 3/2 of the product is 15.96 eV, which is close to Co 2 SiO 4 (∆Co 2p = 15.5 eV) [21].Figure 4c    Figure 4d shows the Si 2p spectra of Co2SiO4 and Co2SiO4/Co3O4−x.The Si 2p of Co2SiO4 is located at the binding energy of 102.75.The Si 2p characteristic peaks of Co2SiO4/Co3O4 composites exhibit lower binding energy and weaker peaks than Co2SiO4, which may be due to the formation of Si-O-Co bonds on the surface [23].

EIS, Mott-Schottky Plot, and VB-XPS
Figure 5a indicates that the radius of the electrochemical impedance spectroscopy (EIS) spectrum of the Co2SiO4/Co3O4−5% sample is far less than other counterparts, indicating that the value of resistance is lowest, and the transfer of surface charges is fastest [24,25].Figure 5b-d    The flat band potentials (E fb ) are 0.79, −0.32, and 0.51 V (vs.Ag/AgCl, pH = 7), respectively, suggesting that their Fermi energy levels are 0.99, −0.12, and 0.71 V (vs.normal hydrogen electrode (NHE), pH = 7) [26].Generally, valence band (VB) XPS represents the distance between the valence band and the Fermi level [27]

In-Situ DRIFTS
The reaction pathway of Co2SiO4/Co3O4-x was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).The samples are exposed to simulated solar light irradiation (300 W Xe lamp) for 45 minutes, and exporting spectrograms every 5 min.The peak at 2,360 cm −1 can be attributed to the symmetric stretching vibration modes of CO2 (Figure 6a) [28].Figure 6b shows the magnified view of Figure 6a from 1,750 cm −1 to 1,250 cm −1 .The peaks from 1,600 to 1,750 cm −1 correspond to the stretching vibration of C-O.The peaks at 1,560 and 1,540 cm −1 can be ascribed to COOH* species.The peaks at 1,455 and 1,340 cm −1 relate to the O-H bending and C-O

In-Situ DRIFTS
The reaction pathway of Co 2 SiO 4 /Co 3 O 4 −x was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).The samples are exposed to simulated solar light irradiation (300 W Xe lamp) for 45 minutes, and exporting spectrograms every 5 min.The peak at 2360 cm −1 can be attributed to the symmetric stretching vibration modes of CO 2 (Figure 6a) [28].Figure 6b shows the magnified view of Figure 6a from 1750 cm −1 to 1250 cm −1 .The peaks from 1600 to 1750 cm −1 correspond to the stretching vibration of C-O.The peaks at 1560 and 1540 cm −1 can be ascribed to COOH* species.The peaks at 1455 and 1340 cm −1 relate to the O-H bending and C-O stretching vibration of COOH* groups, respectively [29,30].It is observed that the peak of COOH* species becomes stronger with increasing light-irradiation time, indicating that the COOH* is one of the key intermediate species for CO 2 -to-CH 3 OH conversion [31].Moreover, the signal of CH 2 * is detected at 1385 cm −1 , confirming the hydrogenation of HCHO* [12].The IR results show that the photocatalytic reduction of CO 2 on the Co 2 SiO 4 /Co 3 O 4 −x surface goes through a sequential process in which COO− and HCHO act as reaction intermediates to generate methanol groups [32].According to the DRIFTS analysis, a possible CO 2 reduction pathway is summarized in the following reactions [33]:  [31].Moreover, the sign of CH2* is detected at 1,385 cm −1 , confirming the hydrogenation of HCHO* [12].The results show that the photocatalytic reduction of CO2 on the Co2SiO4/Co3O4-x surface go through a sequential process in which COO− and HCHO act as reaction intermediates generate methanol groups [32].According to the DRIFTS analysis, a possible CO2 redu tion pathway is summarized in the following reactions [33]:

CO2 Reduction Performance
The photothermal catalytic reduction of CO2 performance was performed under s lar light.The photothermal catalytic CH3OH generation rate of Co3O4, Co2SiO4, an Co2SiO4/Co3O4−x is shown in Figure 7a.For Co2SiO4/Co3O4−x photocatalysts, the CH3O generation rate increases with the increase in Co3O4, but excessive Co3O4 may inhibit t photocatalytic reduction activity.Co2SiO4/Co3O4−5% expresses the best CO2 reductio rates of CH3OH and CH4, corresponding to 48.9 and 14.1 μmol•g −1 •h −1 , which is better th what has been recently reported.After 4 h of sunlight exposure each time, the sampl were washed with distilled water for the next test.The samples could maintain conside able photoactivity without significant reduction in activity after five cycling tests (Figu 7b).

CO 2 Reduction Performance
The photothermal catalytic reduction of CO

Schematic Mechanism of Photothermal Catalytic Conversion
Figure 8 demonstrates the photothermal catalytic CO2 reduction mechanism Co2SiO4/Co3O4−x composites.Figure 8a shows the before-contact configuration of t band structures of Co2SiO4 and Co3O4−x.Co2SiO4 has a lower Fermi level than Co3O4−x = 0.99), and the free electrons of Co3O4−x can be transferred to Co2SiO4 until the Fer reaches equilibrium (Ef = −0.12).After contact, the band edge of Co2SiO4 with a high Fermi level bends downward.In comparison, the band edge of Co3O4−x with a high Fermi level bends upward, thus forming a built-in electric field, facilitating the recom nation of electrons in Co2SiO4 and holes in Co3O4−x.The Co2SiO4/Co3O4−x catalyst has ultrathin two-dimensional structure with abundant sites for physisorption of CO2, a abundant oxygen vacancies for chemisorption and activation of CO2, which facilitates t reduction reaction.Benefiting from a black color and narrow band gap, Co3O4−x can crease light absorption and produce a local photothermal effect.Under proper heat ac vation, the photoelectrons captured by the abundant oxygen vacancies can obtain a s ondary leap to the semiconductor conduction band (CB), suppressing the recombinati of electron-hole pairs to a certain extent; thus, the energy-rich environment favors t electron transfer on Co2SiO4/Co3O4−x.Under solar light, Co2SiO4/Co3O4−x has full-sp trum absorption and can be stimulated to produce e − and h + pairs.The photogenerated in the CB of Co2SiO4 and the photogenerated h + in the VB of Co3O4−x recombines, fac tated by the built-in electric field, thus establishing an S-scheme process.Since ECB Co3O4 is more negative than CO2/CH3OH (−0.38 eV), the e − in the CB position of Co3O is kept converting CO2 to methanol.Moreover, Co2SiO4 has a more positive VB potent energy than H2O/H + (1.23 eV).The photogenerated holes in the VB of Co2SiO4 oxidize H to O2 and H + ions (Figure 8b).

Preparation of SiO2
An amount of 10 g of Pal was mixed with 30 g of ammonium sulfate and placed in a ceramic crucible.The mixtures were calcined in a muffle furnace at 500 °C for 2 h.An amount of 200 mL 0.5 M HCl was added with refluxing and stirring in a water bath at 80 °C for 6 h.After washing and drying, the SiO2 powders were obtained.

Preparation of Co2SiO4 and Co2SiO4/Co3O4
An amount of 0.3 g modified Pal was mixed with 10 mmol Co(NO3)2•6H2O in a beaker; 20 mmol NH4Cl was added after 2 h, and then 1 mL ammonia was added drop by drop to adjust the pH.The solution was transferred into a 100 mL microwave hydrothermal reactor for hydrothermal treatment at 160 °C for 90 min.After the reaction, the samples were centrifuged, washed, dried overnight, and the Co2SiO4 was obtained.
The preparation method of Co2SiO4/Co3O4−x composite material was the same as Co2SiO4, except the quantity of Co(NO3)2•6H2O was adjusted to deposit Co3O4.The different mass ratios in the composite material were denoted as Co2SiO4 /Co3O4−x; where x represents the theoretical mass fraction of Co3O4 in the composite from 1% to 10%.The preparation of pure Co3O4 was the same as this method without the addition of SiO2.

Catalyst Characterizations
The XRD was used to measure the phase structure of materials on a Rigaku D/max 2500PC (Rigaku Corporation, Tokyo, Japan) diffractometer using Cu-Kα radiation (λ = 1.5406Å) with a scanning range of 2θ from 5° to 80°.The TEM was operated on a JEOL-2100 (Japan Electronics Co., Ltd., Tokyo, Japan) with 200 kV working voltage.The UVvis-NIR was carried out to measure the optical properties of materials with an integrating sphere (UV-3600, DRS, SHIMADZU, Kyoto, Japan).The XPS was performed on a Quantum 2000 Scanning ESCA Microprobe instrument (Thermo Nicolet Evolution, Thermo Fisher, Waltham, MA, USA).The photocurrent response and Mott-Schottky measured the optical properties of materials using an electrochemical workstation equipped with a 300 W Xe lamp and standard three electrodes (Auto Lab 302N, CH

Preparation of SiO 2
An amount of 10 g of Pal was mixed with 30 g of ammonium sulfate and placed in a ceramic crucible.The mixtures were calcined in a muffle furnace at 500 • C for 2 h.An amount of 200 mL 0.5 M HCl was added with refluxing and stirring in a water bath at 80 • C for 6 h.After washing and drying, the SiO 2 powders were obtained.

Catalyst Characterizations
The XRD was used to measure the phase structure of materials on a Rigaku D/max 2500PC (Rigaku Corporation, Tokyo, Japan) diffractometer using Cu-Kα radiation (λ = 1.5406Å) with a scanning range of 2θ from 5 • to 80 • .The TEM was operated on a JEOL-2100 (Japan Electronics Co., Ltd., Tokyo, Japan) with 200 kV working voltage.The UV-vis-NIR was carried out to measure the optical properties of materials with an integrating sphere (UV-3600, DRS, SHIMADZU, Kyoto, Japan).The XPS was performed on a Quantum 2000 Scanning ESCA Microprobe instrument (Thermo Nicolet Evolution, Thermo Fisher, Waltham, MA, USA).The photocurrent response and Mott-Schottky measured the optical properties of materials using an electrochemical workstation equipped with a 300 W Xe lamp and standard three electrodes (Auto Lab 302N, CH Instruments, Shanghai, China).In situ DRIFTS was conducted by in situ FT-IR spectrometer (Nicolet iS20 FT-IR, ThermoFisher, USA), with a specific designed reaction cell and MCT detector with 120 scans and a resolution of 4 cm −1 .

Photothermal Catalytic CO 2 Reduction Experiments
The photothermocatalytic activity of the samples was carried out in a closed photochemical reactor coupled with a condensation system and gas chromatography.First, 0.1 g of catalysts were added into 100 mL of water in the bottle in the reactor.After sealing, the reactor was infiltrated with CO 2 gas (99.99%) for 45 min to ensure that the reactor was filled with pure CO 2 and to remove the impurity gasses.Then, at room temperature, a xenon lamp was irradiated through the transparent window (diameter 30~40 mm) on the top of the visible reactor.During the reaction process, the temperature in the reactor was monitored, the reaction sampling interval was 30 min, while the mixture product was obtained from the photochemical reactor with a syringe, and the gaseous product was taken out with a gastight needle.The reaction product was detected by a gas chromatograph (GC-7860 Plus, Shanghai Xinuo Instrument Co., Ltd., Shanghai, China) with a flame ionization detector (FID) and a thermal conductivity detector (TCD).

Conclusions
In summary, a novel cobalt silicate nanosheet composite (Co 2 SiO 4 /Co 3 O 4 −x) was prepared using Pal clay as a silicon source.The acid-modified Pal removed most of the metal ions and retained its unique silicon-oxygen tetrahedral structure, then Co 2+ were incorporated to acquire Co 2 SiO 4 nanosheet and Co 2 SiO 4 /Co 3 O 4 −x heterojunction, which realized the conversion of one-dimensional rod silicate to two-dimensional flake silicate.Co 2 SiO 4 /Co 3 O 4 −x not only have abundant oxygen vacancies, but also have large specific surface areas for the adsorption and activation of CO 2 .The yields of CH 3 OH on Co 2 SiO 4 /Co 3 O 4 −5% were as high as 48.9 µmol•g −1 •h −1 under simulated sunlight irradiation.In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to explore the photocatalytic reduction CO 2 mechanism.Co 2 SiO 4 /Co 3 O 4 −x effectively and stably achieved full-spectrum absorption and light-induced heat under solar light.The photothermal synergistic effect facilitated the generation of key intermediate COOH* species to enhance methanol production.This work provides a new approach for photothermal catalytic CO 2 reduction by taking advantage of natural clay and solar energy.
,g displays TEM and HRTEM of Co 2 SiO 4 /Co 3 O 4 −1%, respectively.It can be seen that, on the surface of Co 2 SiO 4 , the Co 3 O 4 particles have a diameter of about 10 nm.
Figure 2d,h shows TEM and HRTEM of Co 2 SiO 4 /Co 3 O 4 −5%; Co 3 O 4 particles on the surface of Co 2 SiO 4 become more stacked.However, when the amount of Co reaches 10%, it is difficult to distinguish Co 2 SiO 4 and Co 3 O 4 , as shown in Figure 2e.The HRTEM images (Figure 2g,h) show that the Co 3 O 4 nanoparticles are uniformly loaded on the surface of the Co 2 SiO 4 2D nanosheets.The lattice spacing of Co 3 O 4 on the surface is 0.32 nm and 0.33 nm, corresponding to the (311) and (400) planes of Co 3 O 4 , suggesting the existence of a heterogeneous structure between Co 2 SiO 4 and Co 3 O 4 .

2. 3 .
Figure 3a shows the UV-Vis-NIR absorption spectra of the Pal, Co 3 O 4 , Co 2 SiO 4 , and Co 2 SiO 4 /Co 3 O 4 composites.Pal has a weak UV-Vis-NIR absorption edge of about 380 nm, while Co 2 SiO 4 and Co 2 SiO 4 /Co 3 O 4 composites obtained by modifying Pal have fullspectrum absorption to achieve the conversion of UV-responsive natural silicate minerals to visible-responsive silicates.Most of the cobalt-based compounds appear as black or dark green, which can absorb the full range of the spectrum and are ideal photothermal materials [8,15].The band gaps of Co 2 SiO 4 and Co 3 O 4 in Figure3bare estimated by the formula of (ahv) n/2 = A (hv − Eg); where a, h, v, A, and Eg represent the optical absorption coefficient, Planck constant, optical frequency, a constant, and band gap.The values n = 1 and n = 4 represent the indirect and direct band gap semiconductors, respectively[16,17].As shown in Figure3b, the corresponding band gap energies of Co 2 SiO 4 and Co 3 O 4 are estimated to be 3.39 and 1.53 eV, respectively.The narrow band gap is more favorable for capturing the energy of sunlight, thus promoting the generation of e − /h + pairs[18].
Figure 4a shows the XPS survey of Co 3 O 4 , Co 2 SiO 4 , and Co 2 SiO 4 /Co 3 O 4 −x, where the presence of Co and O elements can be determined; Si elements are present in the Co 2 SiO 4 and Co 2 SiO 4 /Co 3 O 4 −x.Figure 4b shows the Co 2p spectra of Co 3 O 4 , Co 2 SiO 4 , and Co 2 SiO 4 /Co 3 O 4 −x.Co can be divided into four peaks, the peaks with binding energies at 794.59 eV and 795.96 eV correspond to Co 2p 1/2 .The peaks with binding energies at 779.77 eV and 781.68 eV correspond to Co 2p 3/2 , suggesting the presence of +2 valence and +3 valence of Co 3 O 4 , and also appear in the Co 2 SiO 4 and Co 2 SiO 4 /Co 3 O 4 −x [19].The peaks near the binding energy of 781.06 eV and 797.23 eV in (Figure4b) correspond to Co 2p 1/2 and Co 2p 3/2 , respectively.Nevertheless, the binding energy of Co 2 SiO 4 /Co 3 O 4 −x is higher than Co 2 SiO 4 , which indicates that Co 2 SiO 4 has a strong interaction with the additional generated Co 3 O 4 , increasing the electron cloud density around Co[20].The studies show that the binding energy of Co 2p in the Co 2 SiO 4 structure is 781.3 eV, while the binding energy of Co 2p in Co 3 O 4 is 779.6 eV.Obviously, the binding energy of cobalt is closer to the former, indicating that the product is Co 2 SiO 4 , rather than Co 3 O 4 , which is consistent with the results of the XRD analysis.In addition, the energy separation between Co 2p 1/2 and Co 2p 3/2 of the product is 15.96 eV, which is close to Co 2 SiO 4 (∆Co 2p = 15.5 eV)[21].Figure4cshows the O 1s spectra of Co 3 O 4 , Co 2 SiO 4 , and Co 2 SiO 4 /Co 3 O 4 −x.The peaks around the binding energy of 529.85 eV and 532.82 eV correspond to the lattice oxygen (O L ) and adsorbed oxygen (O A ), respectively.The ratio of O A to O L in Co 2 SiO 4 /Co 3 O 4 −x is higher than that of Co 3 O 4 , indicating the composite Co 2 SiO 4 /Co 3 O 4 −x has higher content of oxygen vacancy than that of Co 3 O 4[22].
Figure5aindicates that the radius of the electrochemical impedance spectroscopy (EIS) spectrum of the Co2SiO4/Co3O4−5% sample is far less than other counterparts, indicating that the value of resistance is lowest, and the transfer of surface charges is fastest[24,25].Figure5b-d exhibits the Mott-Schottky patterns of Co2SiO4, Co3O4, and Co2SiO4/Co3O4−x.The flat band potentials (Efb) are 0.79, −0.32, and 0.51 V (vs.Ag/AgCl, pH = 7), respectively, suggesting that their Fermi energy levels are 0.99, −0.12, and 0.71 V (vs.normal hydrogen electrode (NHE), pH = 7)[26].Generally, valence band (VB) XPS represents the distance between the valence band and the Fermi level[27].The VB-XPS of Co2SiO4 and Co3O4 are shown in Figure5e,f, which are 1.34 and 0.43 eV.Therefore, the valence bands (EVB) of Co2SiO4 and Co3O4 are 2.33 and 0.31 V, respectively.According to ECB = EVB − Eg, the conduction bands (CBs) of Co2SiO4 and Co3O4 are calculated to be −1.06 and −1.22 V, respectively.

Figure
Figure 4d shows the Si 2p spectra of Co 2 SiO 4 and Co 2 SiO 4 /Co 3 O 4 −x.The Si 2p of Co 2 SiO 4 is located at the binding energy of 102.75.The Si 2p characteristic peaks of Co 2 SiO 4 /Co 3 O 4 composites exhibit lower binding energy and weaker peaks than Co 2 SiO 4 , which may be due to the formation of Si-O-Co bonds on the surface [23].2.5.EIS, Mott-Schottky Plot, and VB-XPSFigure5aindicates that the radius of the electrochemical impedance spectroscopy (EIS) spectrum of the Co 2 SiO 4 /Co 3 O 4 −5% sample is far less than other counterparts, indicating that the value of resistance is lowest, and the transfer of surface charges is fastest[24,25].Figure5b-dexhibits the Mott-Schottky patterns of Co 2 SiO 4 , Co 3 O 4 , and Co 2 SiO 4 /Co 3 O 4 −x.The flat band potentials (E fb ) are 0.79, −0.32, and 0.51 V (vs.Ag/AgCl, pH = 7), respectively, suggesting that their Fermi energy levels are 0.99, −0.12, and 0.71 V (vs.normal hydrogen electrode (NHE), pH = 7) [26].Generally, valence band (VB) XPS represents the distance between the valence band and the Fermi level [27].The VB-XPS of Co 2 SiO 4 and Co 3 O 4 are shown in Figure 5e,f, which are 1.34 and 0.43 eV.Therefore, the valence bands (E VB ) of Co 2 SiO 4 and Co 3 O 4 are 2.33 and 0.31 V, respectively.According to E CB = E VB − Eg, the conduction bands (CBs) of Co 2 SiO 4 and Co 3 O 4 are calculated to be −1.06 and −1.22 V, respectively.
2 performance was performed under solar light.The photothermal catalytic CH 3 OH generation rate of Co 3 O 4 , Co 2 SiO 4 , and Co 2 SiO 4 /Co 3 O 4 −x is shown in Figure 7a.For Co 2 SiO 4 /Co 3 O 4 −x photocatalysts, the CH 3 OH generation rate increases with the increase in Co 3 O 4 , but excessive Co 3 O 4 may inhibit the photocatalytic reduction activity.Co 2 SiO 4 /Co 3 O 4 −5% expresses the best CO 2 reduction rates of CH 3 OH and CH 4 , corresponding to 48.9 and 14.1 µmol•g −1 •h −1 , which is better than what has been recently reported.After 4 h of sunlight exposure each time, the samples were washed with distilled water for the next test.The samples could maintain considerable photoactivity without significant reduction in activity after five cycling tests (Figure 7b).

Figure 8
Figure 8 demonstrates the photothermal catalytic CO 2 reduction mechanism of Co 2 SiO 4 / Co 3 O 4 −x composites.Figure 8a shows the before-contact configuration of the band structures of Co 2 SiO 4 and Co 3 O 4 −x.Co 2 SiO 4 has a lower Fermi level than Co 3 O 4 −x (E f = 0.99), and the free electrons of Co 3 O 4 −x can be transferred to Co 2 SiO 4 until the Fermi reaches equilibrium (E f = −0.12).After contact, the band edge of Co 2 SiO 4 with a higher Fermi level bends downward.In comparison, the band edge of Co 3 O 4 −x with a higher Fermi level bends upward, thus forming a built-in electric field, facilitating the recombination of electrons in Co 2 SiO 4 and holes in Co 3 O 4 −x.The Co 2 SiO 4 /Co 3 O 4 −x catalyst has an ultrathin two-dimensional structure with abundant sites for physisorption of CO 2 , and abundant oxygen vacancies for chemisorption and activation of CO 2 , which facilitates the reduction reaction.Benefiting from a black color and narrow band gap, Co 3 O 4 −x can increase light absorption and produce a local photothermal effect.Under proper heat activation, the photoelectrons captured by the abundant oxygen vacancies can obtain a secondary leap to the semiconductor conduction band (CB), suppressing the recombination of electron-hole pairs to a certain extent; thus, the energy-rich environment favors the electron transfer on Co 2 SiO 4 /Co 3 O 4 −x.Under solar light, Co 2 SiO 4 /Co 3 O 4 −x has full-spectrum absorption and can be stimulated to produce e − and h + pairs.The photogenerated e − in the CB of Co 2 SiO 4 and the photogenerated h + in the VB of Co 3 O 4 −x recombines, facilitated by the built-in electric field, thus establishing an S-scheme process.Since E CB of Co 3 O 4 is more negative than CO 2 /CH 3 OH (−0.38 eV), the e − in the CB position of Co 3 O 4 −x is kept converting CO 2 to methanol.Moreover, Co 2 SiO 4 has a more positive VB potential energy than H 2 O/H + (1.23 eV).The photogenerated holes in the VB of Co 2 SiO 4 oxidize H 2 O to O 2 and H + ions (Figure 8b).

Figure 8 .
Figure 8.(a) Band structure and transfer route of photogenerated carriers before and after contact between Co3O4 and Co2SiO4; (b) schematic mechanism of photocatalytic conversion of CO2 over Co2SiO4/Co3O4−x heterostructure under sunlight.

Figure 8 .
Figure 8.(a) Band structure and transfer route of photogenerated carriers before and after contact between Co 3 O 4 and Co 2 SiO 4 ; (b) schematic mechanism of photocatalytic conversion of CO 2 over Co 2 SiO 4 /Co 3 O 4 −x heterostructure under sunlight.

3. 3 .
Preparation of Co 2 SiO 4 and Co 2 SiO 4 /Co 3 O 4 An amount of 0.3 g modified Pal was mixed with 10 mmol Co(NO 3 ) 2 •6H 2 O in a beaker; 20 mmol NH 4 Cl was added after 2 h, and then 1 mL ammonia was added drop by drop to adjust the pH.The solution was transferred into a 100 mL microwave hydrothermal reactor for hydrothermal treatment at 160 • C for 90 min.After the reaction, the samples were centrifuged, washed, dried overnight, and the Co 2 SiO 4 was obtained.The preparation method of Co 2 SiO 4 /Co 3 O 4 −x composite material was the same as Co 2 SiO 4 , except the quantity of Co(NO 3 ) 2 •6H 2 O was adjusted to deposit Co 3 O 4 .The different mass ratios in the composite material were denoted as Co 2 SiO 4 /Co 3 O 4 −x; where x represents the theoretical mass fraction of Co 3 O 4 in the composite from 1% to 10%.The preparation of pure Co 3 O 4 was the same as this method without the addition of SiO 2 .
• , 44.8 • , and 65.2 • are consistent with the characteristic peaks of cubic spinel Co 3 O 4 (PDF#42-1467).It is noteworthy that the diffraction peaks of Co 3 O 4 are very close to Co 2 SiO 4 .Figure [12,14]peaks of Co 2 SiO 4 /Co 3 O 4 composites are obviously shifted to a higher angle compared with Co 3 O 4 and Co 2 SiO 4 , indicating a stronger interaction between Co 2 SiO 4 and Co 3 O 4[12,14].
4a shows the XPS survey of Co 3 O 4 , Co 2 SiO 4 , and Co 2 SiO 4 /Co 3 O 4 −x, where the presence of Co and O elements can be determined; Si elements are present in the Co 2 SiO 4 and Co 2 SiO 4 /Co 3 O 4 −x. Figure 4b shows the Co 2p spectra of Co 3 O 4 , Co 2 SiO 4 , and Co 2 SiO 4 /Co 3 O 4 −x.Co can be divided into four peaks, the peaks with binding energies at 794.59 eV and 795.96 eV correspond to Co 2p 1/2 .The peaks with binding energies at 779.77 eV and 781.68 eV correspond to Co 2p 3/2 , suggesting the presence of +2 valence and +3 valence of Co 3 O 4 , and also appear in the Co 2 SiO 4 and Co 2 SiO 4 /Co 3 O 4 −x [19].The peaks near the binding energy of 781.06 eV and 797.23 eV in (Figure . The VB-XPS of Co 2 SiO 4 and Co 3 O 4 are shown in Figure 5e,f, which are 1.34 and 0.43 eV.Therefore, the valence bands (E VB ) of Co 2 SiO 4 and Co 3 O 4 are 2.33 and 0.31 V, respectively.According to E CB = E VB − Eg, the conduction bands (CBs) of Co 2 SiO 4 and Co 3 O 4 are calculated to be −1.06 and −1.22 V, respectively.