Insight into the Roles of Metal Loading on CO2 Photocatalytic Reduction Behaviors of TiO2

The photocatalytic reduction of carbon dioxide (CO2) into value-added chemicals is considered to be a green and sustainable technology, and has recently gained considerable research interest. In this work, titanium dioxide (TiO2) supported Pt, Pd, Ni, and Cu catalysts were synthesized by photodeposition. The formation of various metal species on an anatase TiO2 surface, after ultraviolet (UV) light irradiation, was investigated insightfully by the X-ray absorption near edge structure (XANES) technique. CO2 reduction under UV-light irradiation at an ambient pressure was demonstrated. To gain an insight into the charge recombination rate during reduction, the catalysts were carefully investigated by the intensity modulated photocurrent spectroscopy (IMPS) and photoluminescence spectroscopy (PL). The catalytic behaviors of the catalysts were investigated by density functional theory using the self-consistent Hubbard U-correction (DFT+U) approach. In addition, Mott–Schottky measurement was employed to study the effect of energy band alignment of metal-semiconductor on CO2 photoreduction. Heterojunction formed at Pt-, Pd-, Ni-, and Cu-TiO2 interface has crucial roles on the charge recombination and the catalytic behaviors. Furthermore, it was found that Pt-TiO2 provides the highest methanol yield of 17.85 µmol/gcat/h, and CO as a minor product. According to the IMPS data, Pt-TiO2 has the best charge transfer ability, with the mean electron transit time of 4.513 µs. We believe that this extensive study on the junction between TiO2 could provide a profound understanding of catalytic behaviors, which will pave the way for rational designs of novel catalysts with improved photocatalytic performance for CO2 reduction.


Introduction
The severe adverse effects of global warming resulting from excessive carbon dioxide (CO 2 ) emission arouses the need for urgent research into CO 2 reduction. CO 2 conversion to valued-added chemicals or fuels has gained enormous research interest as a gamechanging technology for sustainable development [1][2][3][4][5]. Artificial photosynthesis, which mimics natural photosynthesis using renewable solar energy and water to convert CO 2 to manageable chemicals while leaving oxygen as a by-product, has been considered as one of the most green and sustainable technologies [6][7][8]. This method is also particularly attractive due to its ability to convert CO 2 to value-added hydrocarbons using ambient temperature and pressure [2,9]. Several semiconductors, including TiO 2 [10][11][12], CuO [13], g-C 3 N 4 [14], Bi 2 WO 6 [15], ZnO [16], SrTiO 3 [17], and CeO 2 [18], have been applied for photocatalytic reduction of CO 2 . Among these semiconductors, anatase TiO 2 appears to be one of the most utilized catalysts due to its high performance, nontoxicity, high stability, and low cost [19][20][21]. However, rapid charge recombination is one of the important drawbacks, limiting the performance of TiO 2 , and its large band gap also results in low CO 2 reduction efficiency [22].
Various strategies, such as surface modification, forming heterojunction and band alignment, and doping with metals and non-metals, have been reported as effective strategies to overcome these limitations and promote photocatalytic CO 2 reduction performance [3,[23][24][25][26]. In particular, metal-TiO 2 composites have been shown to promote overall photocatalytic activity by reducing the recombination rate of the photogenerated charges and increasing light harvesting efficiency [27][28][29][30][31]. Generally, after irradiating the photocatalyst by incident light, the photogenerated electrons can transfer from the conduction band (CB) of photocatalysts across potential barriers to the contacting metal [32]. Therefore, metal acts as an electron sink for retarding the charge recombination rate. This can align the energy band between the metal and the semiconductor by shifting the Fermi level of the semiconductor to the metal located below the CB states of the semiconductor, and generating the semiconductor-metal heterojunction; namely the Schottky barriers [33]. The Schottky barrier effectively traps electrons, reducing the flow of electrons back to the semiconductor [23]. For example, Su et al. [3] studied the effect of Pd-loaded TiO 2 on CO 2 photoreduction. They found that the presence of Pd could enhance the CO 2 -tomethane conversion by around two orders of magnitude compared to the bare TiO 2 . It is well known that metal loading on semiconductors can enhance the photocatalytic CO 2 reduction performance [3,22,[34][35][36][37], however the roles and underlying mechanisms of metals remain unclear. Some intrinsic challenges and critical factors, including surface molecular structures, charge transfer behaviors, and charge recombination rate during reduction, are also debatable. Moreover, insights into the interaction of adsorbed CO 2 with the semiconductor-modified surface as the catalytic sites are still expected to be further explored as the structure and the cation sites on the modified Ti surface composition are also involved in the catalytic pathways and selectivity of products. They can lower the reaction barrier to activate CO 2 , and stabilize CO 2 intermediates to enhance CO 2 photoreduction.
In the present study, the roles of loading metals, including Pt, Pd, Ni, and Cu, prepared by photodeposition on TiO 2 towards the photocatalytic reduction of CO 2 are investigated in many aspects simultaneously (i.e., band alignment, plasmonic effects, charge recombination, charge transfer, and surface chemistry), in order to gain an insight into true catalytic behavior. The plasmonic metal-TiO 2 nanostructures and their compositions are extensively characterized by various techniques, including X-ray absorption near edge structure (XANES), X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectra (UV-Vis), and inductively coupled plasma-optical emission spectroscopy (ICP-OES). The influences of energy band alignment of different heterojunctions, charge recombination behaviors, and photonic efficiency of metal-TiO 2 are insightfully studied by intensity-modulated photocurrent spectroscopy (IMPS) and photoluminescence spectroscopy (PL), compared to the pristine anatase TiO 2 . The interactions of CO 2 on TiO 2 and Pt-TiO 2 photocatalysts were studied by CO 2 -TPD, combined with theoretical simulation by density function theory (DFT). Interestingly, Pt-TiO 2 showed the best photocatalytic CO 2 -to-methanol performance among the metals studied (Pt, Pd, Ni, and Cu) with a methanol production rate of 17.85 µmol/g cat /h, which is among the top unassisted photocatalysts that have been reported for CO 2 -to-methanol conversion. The impressive performance is attributed to suppressed charge recombination, suitable band alignment, and appropriate surface chemistry.

Characterization of Photocatalysts
Transmission electron microscopy (TEM) (HF-3300, Hitachi, Japan) was used to observe the morphologies of the as-synthesized catalysts. UV-visible diffuse reflectance spectra (UV-DRs) (UV-3101PC, Shimadzu, Japan) was used to analyze the band gap energy of the samples. The oxidation states and species of the metals deposited on the surface of anatase TiO 2 were investigated by X-ray absorption near edge structure (XANES). XANES measurements were carried out with the fluorescent mode at the beamline 1.1 W and beamline 8, Synchrotron Light Research Institute (SLRI), Nakhon Ratchasima, Thailand. The data reduction of XANES spectra was performed using ATHENA program. CO 2 temperature programmed desorption (CO 2 -TPD, Chemisorption analyzer; ChemStar TPX, Quantachrome Instruments, Boynton Beach, FL, USA) was carried out to investigate the interaction of CO 2 and the catalyst. Photoluminescence (PL) (Avaspec-2048TEC-USB2-2, Apeldoorn, The Netherlands) and Intensity-modulated photocurrent spectroscopy (IMPS) (Metrohm Autolab, Utrecht, The Netherlands) were used to determine the charge dynamics and recombination. IMPS was obtained using a Metrohm Autolab PGSTAT12. The modulation frequency ranged from 120 to 500 kHz. The Mott-Schottky technique was used to classify the semiconductor types, and also used to estimate the flat band potential V fb and band alignment of the composites. The Mott-Schottky was obtained using the frequency response analyzer (Metrohm Autolab PGSTAT204, Utrecht, The Netherlands) with an applied bias ranging from 1.5 to −1.0 V (vs Ag/AgCl), and the frequency of impedance was fixed at 1 kHz with the RMS amplitude of 10 mV.
The Mott-Schottky equation (Equation (1)) involves the relationship between the capacitance and the biased voltage across the semiconductor/electrolyte interface. The derived Mott-Schottky plots were fitted by using the simple linear regression method.
where C 2 SC is the space charge capacitance density (F). V is the applied potential (V). ε 0 (F·m −1 ) and ε r are the vacuum permittivity and relative permittivity of TiO 2 , respectively [38]. e and k B are electron charge (C) and Boltzmann's constant (m 2 ·kg·s −2 ·K −1 ), respectively. A and T are the active surface area (cm 2 ) and the absolute temperature (K), respectively. N D is the electron carrier density or donor concentration (cm −1 ). Corresponding to this equation, the V f b can be extracted from the intercept between the extrapolated linear line and x-axis. In addition, N D was also evaluated from the slope of the equation [39]. The position of the Fermi level (E F ) relative to the conduction energy (E C ) can be calculated by using Equations (2) and (3): where N c is the effective density of state in the conduction band (cm −3 ). h is Plank's constant (m 2 ·kg·s −1 ). The N c was calculated by setting m * e as 10m 0 [40]. Where the m * e and m 0 are the density of state effective mass for the electrons of anatase TiO 2 and the mass of the free electron (kg), respectively.

CO 2 Photoreduction
The CO 2 photoreduction was carried out in a closed system under UV-light (Hg-125 W). In a typical procedure, 0.1 g of the photocatalyst was dispersed in 50 mL of DI water. Prior to starting the reaction, N 2 (Linde, UHP 99.999%) gas was first purged for 30 min to remove air, then CO 2 was subsequently flowed into the system for 30 min to ensure that all oxygen and N 2 were removed. The pressure in the reactor was kept at 1 atm, and the UV-light was irradiated to start the reaction. The resulting products from the photocatalytic CO 2 reduction were measured by a gas chromatograph (GC-14 Shimadzu, Kyoto, Japan) equipped with a flame ionization detector (FID, Porapak Q mesh 50/80 Column) and a thermal conductivity detector (TCD, GC-SCI 310C) to identify and quantify the products. The product selectivity is calculated as Equation (4): where Xi is product yield, including CH 3 OH and CO.

Density Functional Theory (DFT) Calculations
The reduced TiO 2 surface was modeled by creating an oxygen vacancy on the surface. The simulations were carried out by an efficient density functional theory using the selfconsistent Hubbard U-correction (DFT+U) approach [41,42] implemented in the Vienna Ab initio Simulation Package (VASP) [43][44][45][46]. The DFT+U methodology has been known as an ad hoc method that improves the description of d-states of the transition metals (3d-orbital in case of Ti) by implementing U-correction, solving the underestimated electronic interactions problems and providing a more accurate estimation than the standard DFT method [47,48]. The applied U value of 3.5 eV for Ti atoms was selected based on other works, which have performed the calculations of CO 2 adsorbed on reduced TiO 2 surface with different values of U and showed comparable calculated results to the experiments [47,49]. The effective Projector Augmented Wave (PAW) pseudopotentials [50] were constructed to describe the electron exchange and correlation effects. The calculations were performed within the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) functional [51]. The self-consistent (SCF) field tolerance and the ionic force convergence threshold were 1.0 × 10 −5 eV and −0.01 eV/Å, respectively. The kinetic energy cut-off for the plane wave basis set was set to 500 eV. The Monkhorst-Pack mesh sampling [52] k-points of 2 × 2 × 1 was used. The Methfessel-Paxton scheme of order two with a value of the smearing parameter σ of 0.03 eV was employed and the spin-polarized calculations were carried out. Bader charge analysis was performed using VASP-VTST [53][54][55]. For geometry optimization, the coordinates of the atoms in the two bottom layers were kept fixed while the rest of the atoms were allowed to relax. A vacuum space between slabs of 15 Å was set. To construct a reduced surface, an O atom at the bridge site (2c-O) was removed [47,56,57]. The optimized metal clusters of tetramers Pt, Pd, Ni and Cu were located on the optimized reduced TiO 2 surface, following the configurations proposed by the literature [46,[58][59][60][61]. The 3 × 1 supercell of the anatase TiO 2 (101) was constructed with six layers of the (101) surface (Ti 36 O 72 ). To study the CO 2 adsorption on M 4 -TiO 2 surfaces, both bent and linear CO 2 molecules were considered.

Characterization of Photocatalysts
The particle sizes and morphologies of Pt, Pd, Ni, and Cu-loaded TiO 2 prepared by photodeposition method and measured by Transmission Electron Microscopy (TEM) are shown in Figure 1a-e. As seen in the TEM images, both TiO 2 and metal nanoparticles are in a spherical shape. The metal nanoparticles are in good distribution (Supplement Information, Figure S1). The average particle size of Pt, Pd, and Ni was approximately 4-5 nm, while Cu-TiO 2 sample showed a larger particle size of (~12 nm). Moreover, there are also metal signals distributing throughout the whole samples, suggesting that there could be metals in other forms such as ions, clusters, or single atoms existing in the samples. The XRD patterns of the samples with different types of metal loaded on anatase TiO 2 are displayed in Figure 1f. We found that the characteristic peaks of TiO 2 (anatase, PDF 71-1167) were clearly observed, while the characteristic peaks belonging to Pt, Pd, Ni, and Cu species were invisible, due to the low concentration of the metals. The optical properties of the as-synthesized photocatalysts characterized by UV-Vis spectrophotometer are shown in Figure 1g. Their band gap energies were calculated using Kubelka-Munk equation derived from UV-visible diffuse reflectance (UV-vis DRs) spectra. According to the diffuse reflectance spectra, it was found that all samples have quite similar light absorption edges of around 400 nm, which corresponds to the similar band gap of 3.2 eV based on the Kubelka-Munk equation (inset of Figure 1g). with a value of the smearing parameter σ of 0.03 eV was employed and the spin-polarized calculations were carried out. Bader charge analysis was performed using VASP-VTST [53][54][55]. For geometry optimization, the coordinates of the atoms in the two bottom layers were kept fixed while the rest of the atoms were allowed to relax. A vacuum space between slabs of 15 Å was set. To construct a reduced surface, an O atom at the bridge site (2c-O) was removed [47,56,57]. The optimized metal clusters of tetramers Pt, Pd, Ni and Cu were located on the optimized reduced TiO2 surface, following the configurations proposed by the literature [46,[58][59][60][61]. The 3 × 1 supercell of the anatase TiO2 (101) was constructed with six layers of the (101) surface (Ti36O72). To study the CO2 adsorption on M4-TiO2 surfaces, both bent and linear CO2 molecules were considered.

Characterization of Photocatalysts
The particle sizes and morphologies of Pt, Pd, Ni, and Cu-loaded TiO2 prepared by photodeposition method and measured by Transmission Electron Microscopy (TEM) are shown in Figure 1a-e. As seen in the TEM images, both TiO2 and metal nanoparticles are in a spherical shape. The metal nanoparticles are in good distribution (Supplement Information, Figure S1). The average particle size of Pt, Pd, and Ni was approximately 4-5 nm, while Cu-TiO2 sample showed a larger particle size of (~12 nm). Moreover, there are also metal signals distributing throughout the whole samples, suggesting that there could be metals in other forms such as ions, clusters, or single atoms existing in the samples. The XRD patterns of the samples with different types of metal loaded on anatase TiO2 are displayed in Figure 1f. We found that the characteristic peaks of TiO2 (anatase, PDF 71-1167) were clearly observed, while the characteristic peaks belonging to Pt, Pd, Ni, and Cu species were invisible, due to the low concentration of the metals. The optical properties of the as-synthesized photocatalysts characterized by UV-Vis spectrophotometer are shown in Figure 1g. Their band gap energies were calculated using Kubelka-Munk equation derived from UV-visible diffuse reflectance (UV-vis DRs) spectra. According to the diffuse reflectance spectra, it was found that all samples have quite similar light absorption edges of around 400 nm, which corresponds to the similar band gap of 3.2 eV based on the Kubelka-Munk equation (inset of Figure 1g). first order derivative of Pt-, Pd-, Ni-, and Cu-loaded TiO 2 samples were compared to the standards illustrated in Figure 2. Figure 2a,b demonstrate the XANES spectra and the first order derivative of platinum samples (Pt L 3 -edge), compared to the spectra of H 2 Cl 6 Pt precursor and the reference standard materials, including Pt foil and PtO 2 (representing the oxidation states of 0 and +4, respectively). Normally, Pt 0 exhibits absorption edges at 11,567.9, while Pt 4+ provides the edge energy at 11,567.4 eV (Table S2). We observed that the absorption edges of the fresh and spent Pt-TiO 2 revealed the edge energy was close to that of the Pt 0 , indicating that both the fresh and spent Pt-TiO 2 catalysts were mainly metallic [62]. As shown in Table 1, which tabulates the linear combination fit of the XANES spectra, the metallic form of Pt in the fresh sample was 72.4%, while that of the spent Pt-TiO 2 was 100%. This evidence suggests that some of Pt 4+ could be further reduced to form metallic Pt during the CO 2 reduction. Considering the Pd-decorated TiO 2 samples, the Pd L 3 -edge XANES spectra and their first order derivative were compared with Pd foil, PdO, and PdCOCl 2 (Figure 2c,d). The edge energies of the fresh and spent Pd-TiO 2 were found to be 3175.4 and 3175.5 eV, which are significantly close to that of Pd foil, confirming the metallic Pd 0 oxidation state [63]. This result is also consistent with the result from linear combination analysis, which shows 82.2% and 90.5% of Pd 0 in the fresh and spent Pd-TiO 2 samples, respectively. Figure 2e,f display the XANES spectra and the first order derivative of the Ni-loaded TiO 2 samples, compared with the nickel standard references; namely, Ni foil, NiO, Ni(OH) 2 , and Ni(NO 3 ) 2 which have the edge energies of 8340.9, 8350.4, 8349.9, and 8350.4 eV, respectively [64]. The pre-edge of both fresh and spent Ni-TiO 2 were found at 8341.2 eV and 8339.9 eV, respectively. The linear combination fitting of Ni-TiO 2 samples shows that the Ni species in the fresh and the spent Ni-loaded TiO 2 were close to Ni foil, confirming the existence of Ni 0 in these samples.   Figure 2h. We observed that Cu 1+ and Cu 2+ components existed in both samples while the metallic copper disappeared. According to the investigation of the metal oxidation states by XANES from the linear combination analysis as tabulated in Table 1, it can be noted that the metal nanoparticles resulting from photodeposition in the fresh and spent Pt-, Pd-, and Ni-loaded TiO 2 were mostly in metallic form. However, Cu-TiO 2 favorably formed Cu 1+ species with a ratio of 60.1 and 93.2% for fresh and spent Cu-TiO 2 , respectively.
Previous research has explored the metallic behaviors on the photodeposition and photo-oxidation of propanol using the photodeposition methodology [65]. The proposed mechanism for metal photodeposited TiO 2 is given in Equations (5)-(7). H + is a proton produced by the photo-oxidation of propanol with holes, as shown in Equation (6). Metal ions were reduced over TiO 2 by reacting with photogenerated electrons, resulting in the formation of metallic particles, as validated by XANES: CO 2 temperature programmed desorption (CO 2 -TPD) was carried out to investigate the interaction of CO 2 reactant on the catalyst as shown in Figure 3 for the adsorption temperature from 50 to 900 • C. The main spectra can be assigned to the molecularly adsorbed bidentate carbonates (b-HCO 2− 3 ) (380-550 • C) and monodentate carbonate (m-HCO 2− 3 ) (550-760 • C), corresponding to strong basic sites of catalysts [66]. The peak intensity of b-HCO 2− 3 and m-HCO 2− 3 over Pt-TiO 2 was lower than TiO 2 , suggesting that more medium and strong basic sites were formed over TiO 2 . However, as compared to pristine TiO 2 , the chemical desorption peaks of Pt-deposited TiO 2 have shifted to a higher temperature, implying the stronger basicity of its adsorption sites [67]. The b-HCO 2− 3 and m-HCO 2− 3 species were generated from CO 2 molecules combined with oxygen atoms or metal atoms of the cocatalyst [68].

CO2 Photocatalytic Reduction
The photocatalytic reduction of CO2 was performed with liquid H2O under UV-light irradiation using various metal-loaded TiO2 samples as a photocatalyst. For the control experiment, the photocatalytic CO2 reduction was performed without photocatalysts and

CO 2 Photocatalytic Reduction
The photocatalytic reduction of CO 2 was performed with liquid H 2 O under UV-light irradiation using various metal-loaded TiO 2 samples as a photocatalyst. For the control experiment, the photocatalytic CO 2 reduction was performed without photocatalysts and light irradiation. There was no reduced CO 2 detected during the reaction, indicating that the reduced CO 2 products were generated by photocatalytic reactions. The influence of the metals (Pt, Pd, Ni, and Cu) on CO 2 photoreduction was carefully investigated, and the results are shown in Figure 4. The amount of metal deposited on TiO 2 -based photocatalyst was fixed at around 0.1 wt%, and this was confirmed by ICP-OES (Supplement Information, Table S1). Methanol and CO were the major and minor products, respectively ( Figure S2). After 2 h photoreaction, methanol was produced approximately 3.12 µmol/g cat /h over the pristine TiO 2 . We found that metal loading can significantly promote the generation of the reduced CO 2 products, compared to the pristine TiO 2 . Different dopants (Pt, Pd, Ni, and Cu) on TiO 2 resulted in different degrees of enhancement on the photocatalytic CO 2 reduction performance. The highest amount of methanol yield was found over the Pt-TiO 2 catalyst with the production rate of 17.85 µmol/g cat /h with selectivity of 96.41%, following by Cu, Pd, and Ni-TiO 2 , which can produce methanol of 9.98, 9.35, and 6.09 µmol/g cat /h, respectively ( Figure 4). The CO production over all catalysts seems negligible, as no higher than 2.5 µmol/g cat /h of CO was detected in any samples. The possible products in the liquid phase, such as formaldehyde, were also undetectable, as analyzed by a UV-Vis spectrophotometer. Moreover, 0.1 wt% Pt-TiO 2 was carried out on the photoreaction without the involvement of CO 2 to verify methanol and CO produced by CO 2 photoreduction over metal loaded TiO 2 . The result showed that no product was detected (Supplement Information, Figure S3). Furthermore, CHN analysis of 0.1 wt% Pt-TiO 2 catalyst showed that the amount of carbon on the catalyst was negligible (Table S3). This evidence indicated that methanol and CO were produced by the photoreduction of CO 2 . Pt-TiO 2 also showed excellent stability, as the catalyst can maintain >90% of its original reactivity after running for three cycles (Supplement Information, Figure S4 and Table S8).
Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 18 and 6.09 µ mol/gcat/h, respectively ( Figure 4). The CO production over all catalysts seems negligible, as no higher than 2.5 µ mol/gcat/h of CO was detected in any samples. The possible products in the liquid phase, such as formaldehyde, were also undetectable, as analyzed by a UV-Vis spectrophotometer. Moreover, 0.1 wt% Pt-TiO2 was carried out on the photoreaction without the involvement of CO2 to verify methanol and CO produced by CO2 photoreduction over metal loaded TiO2. The result showed that no product was detected (Supplement Information, Figure S3). Furthermore, CHN analysis of 0.1 wt% Pt-TiO2 catalyst showed that the amount of carbon on the catalyst was negligible (Table S3). This evidence indicated that methanol and CO were produced by the photoreduction of CO2. Pt-TiO2 also showed excellent stability, as the catalyst can maintain >90% of its original reactivity after running for three cycles (Supplement Information, Figure S4 and Table  S8). To attain a greater understanding of CO2 adsorption and reaction on TiO2 supported catalysts loading with Pt, Pd, Ni, and Cu, we further computationally evaluated the structural and electronic characteristics of CO2 adsorption on the supported tetramer metal clusters, including Pt, Pd, Ni, and Cu on the reduced surface of anatase TiO2 (101). The anatase TiO2 (101) surface consists of five-fold (5c-Ti) and six-fold (6c-Ti) coordinated Ti To attain a greater understanding of CO 2 adsorption and reaction on TiO 2 supported catalysts loading with Pt, Pd, Ni, and Cu, we further computationally evaluated the structural and electronic characteristics of CO 2 adsorption on the supported tetramer metal clusters, including Pt, Pd, Ni, and Cu on the reduced surface of anatase TiO 2 (101). The anatase TiO 2 (101) surface consists of five-fold (5c-Ti) and six-fold (6c-Ti) coordinated Ti atoms and two-fold (2c-O) and three-fold (3c-O) coordinated O atoms at the surface (see Figure S6). These tetrameric metal clusters well represent both two-and three-dimensional deposited clusters. Three possible CO 2 adsorption sites found in this study could be categorized as: (i) CO 2 binding on the metal cluster; (ii) CO 2 binding at the interface between the metal cluster and the TiO 2 ; and (iii) linear CO 2 adsorbed on the TiO 2 surface (Supplement Information, Figures S5-S9 and Tables S4-S7).
To analyze the CO 2 adsorption systems, four properties of CO 2 were analyzed, including the adsorption energy of CO 2 , the angle of O-C-O of adsorbed CO 2 , the charge accumulation on the adsorbed CO 2 molecule, and the vibrational frequencies of the adsorbed CO 2 . The adsorption energy (E ads ) of CO 2 was calculated according to Equation (8): where E Co 2 /M 4 −TiO 2 is the total energy of CO 2 adsorbed system, E M 4 −TiO 2 and E CO 2 (g) are the total energy of the metal clusters located on the reduced TiO 2 surfaces and the energy of isolated CO 2 , respectively. The results are illustrated in Figure 5. We found that the more negative E ads indicated the more stable CO 2 adsorption configuration. The difference in Bader charge (∆e) of CO 2 were the changes of CO 2 atomic charges upon adsorption. The negative ∆e implied electron accumulation. The more negative ∆e indicated the adsorbed CO 2 molecule gains more electrons. To confirm the key bond characteristics of adsorbed CO 2 anion, the vibration frequency calculations were obtained. Three key vibrational modes of CO 2 including symmetric (ν1), bending (ν2), and asymmetric (ν3) stretching modes were agreed to for other calculations for CO 2 anion adsorption on M 4 -TiO 2 surfaces [47,59,69].  These experimental results reveal that the reaction mechanism of CO2 photoreaction over various metal-deposited TiO2 photocatalysts could proceed through the carbene pathway. This research fits well with the work reported by Habisreutinger and co-workers [70]. CO2 photoreaction could be initiated through the chemisorbed CO2 molecules on the heterogeneous catalyst and form the adsorbed (a) The adsorption energy ranges (b) CO 2 charge ranges for different CO 2 adsorption sites. The colors represent the different metals; green, purple, gray, and orange are Pt, Pd, Ni, and Cu, respectively. The light and dark shades of color represent CO 2 adsorption sites at metal sites and at metal-TiO 2 interface sites, respectively.
These experimental results reveal that the reaction mechanism of CO 2 photoreaction over various metal-deposited TiO 2 photocatalysts could proceed through the carbene pathway. This research fits well with the work reported by Habisreutinger and co-workers [70]. CO 2 photoreaction could be initiated through the chemisorbed CO 2 molecules on the heterogeneous catalyst and form the adsorbed CO •− 2 species on the surface [71], confirmed by computational DFT result. This reaction is more likely to occur at the metal-TiO 2 interfaces rather than on the pure TiO 2 or the metal surfaces, as indicated by the more negative E ads values (Tables S4-S7). The simultaneous photogenerated holes react with adsorbed water or hydroxide ions OH − ads to generate oxygen and proton. Subsequently, the CO •− 2 reacts with the adsorbed H • , which is produced by the reduction of H + before cleavage to form carbon monoxide and hydroxide ion (OH − ) [67]. The adsorbed CO can desorb from the catalyst sites to produce CO as a product due to the weak CO adsorption of catalyst surface [72]. The adsorbed CO can also combine with an additional two electrons to form carbon residue on the surface, and then react with three H • radicals to form˙CH radical, carbene, and methyl radicals. Methyl radicals can further react with hydroxyl radical to form methanol [73]. As shown in Figure 5a, Pt and Cu provide moderately negative E ads, which could facilitate both the adsorption and desorption processes of CO 2 molecules, while Ni provides very negative E ads . In principle, Cu should also show good CO 2 -tomethanol conversion performance, as it has suitable E ads and good charge accumulation on CO 2 molecules. However, our experiment found that Cu can easily turn to copper oxide and, hence, Pt appears to be the most promising among the plasmonic metals studied in this work. The enhancement of methanol yield from the modified TiO 2 with metal loading could be explained by the formation of a Schottky barrier that could reduce e − /h + recombination [3,34,[74][75][76]. To gain insight into the charge recombination kinetics, intensitymodulated photocurrent spectroscopy (IMPS) and photoluminescence spectra (PL) were implemented to study the behaviors of charges. The IMPS results clarify the photogenerated charge transfer, as shown in Figure 6a. The frequency minimum in the complex plane of the IMPS plot can be used to calculate the mean transit time of the photogenerated e − according to Equation (9) [77]: where f c is the minimum point frequency (Hz) of the IMPS response. The smaller τ c indicates the better charge transfer [77]. The results showed that the τ c values of Pt-, Cu-, Pd-, and Ni-loaded TiO 2 were 4.513, 4.613, 4.665, and 4.668 µs, respectively. Obviously, Pt-decorated TiO 2 showed the lowest value of τ c , indicating enhanced charge transfer ability compared to other samples. In contrast, Ni-TiO 2 has the highest values of τ c , indicating the poor charge transfer kinetics. These results are in good agreement with the actual photocatalytic CO 2 reduction performance. Furthermore, PL was applied to further examine the charge transfer characteristics of Pt-TiO 2 compared to the pristine TiO 2 , as shown in Figure 6b. The spectra of pure TiO 2 showed higher PL intensity of the peak emission, indicating the higher charge recombination rate [78]. It can be noted that the presence of metals decorated on the TiO 2 surface can enhance the photocatalytic reduction of CO 2 by inhibiting the recombination rate [76].
catalytic CO2 reduction performance. Furthermore, PL was applied to further examine the charge transfer characteristics of Pt-TiO2 compared to the pristine TiO2, as shown in Figure  6b. The spectra of pure TiO2 showed higher PL intensity of the peak emission, indicating the higher charge recombination rate [78]. It can be noted that the presence of metals decorated on the TiO2 surface can enhance the photocatalytic reduction of CO2 by inhibiting the recombination rate [76].   Table 2, Pt-TiO2 has the lowest EF (−4.47 eV), which is more negative than the unmodified TiO2 (−4.17 eV). Furthermore, it was found that the EF of the catalysts decreased as the work function (ΦM) of the deposited metals increased. It was believed that the higher work function of metal (low EF) can cause the more downward shifting of EF in TiO2.   Table 2, Pt-TiO 2 has the lowest E F (−4.47 eV), which is more negative than the unmodified TiO 2 (−4.17 eV). Furthermore, it was found that the E F of the catalysts decreased as the work function (Φ M ) of the deposited metals increased. It was believed that the higher work function of metal (low E F ) can cause the more downward shifting of E F in TiO 2 .  The electron carrier density (ND) of Pt-TiO2 was calculated to be 3.27 × 10 20 cm −3 , which was higher than that of pristine TiO2. It is well known that a higher donor density implies a higher material conductivity, which facilitates the charge transport process [79]. Interestingly, Cu-TiO2 has low EF (−4.47 eV), which is comparable to Pt-TiO2 even though the work function (ΦM) of Cu is the lowest value (−4.65 eV) compared to the other  The electron carrier density (N D ) of Pt-TiO 2 was calculated to be 3.27 × 10 20 cm −3 , which was higher than that of pristine TiO 2 . It is well known that a higher donor density implies a higher material conductivity, which facilitates the charge transport process [79]. Interestingly, Cu-TiO 2 has low E F (−4.47 eV), which is comparable to Pt-TiO 2 even though the work function (Φ M ) of Cu is the lowest value (−4.65 eV) compared to the other inter-ested metals. This might be due to the effect of Cu 2 O, which was the main component of Cu-TiO 2 (see the linear combination analysis of XANES results). According to the report of Aguirre et al. [80], the E F of TiO 2 can shift downward when it intimately contacted Cu 2 O to equilibrate the E F between two materials. Although the E F of Cu-TiO 2 was equal to the E F of Pt-TiO 2 , the N D of Cu-TiO 2 was lower (2.64 × 10 20 cm −3 ). This could be the reason why Cu-TiO 2 has lower CO 2 reduction efficiency than that of Pt-TiO 2 .
Therefore, the relationship between the metal's work function obtained from the Mott-Schottky results and the CH 3 OH yield is further analyzed as shown in Figure 7b. Normally, noble metals are known as efficient cocatalysts due to their large work functions. The metal's work function (Φ M ) is the energy needed to bring the electron from the metal's Fermi energy to a vacuum level [33,81]. The larger metal work function, the better electron trapping ability. Band bending is formed when noble metals and semiconductors make intimate contact [32]. Pt-TiO 2 showed the highest amount of methanol production, followed by Pd and Ni, respectively. This trend follows the order of the work function of the metals (see Figure 7b). The Schottky junction between Pt-decorated TiO 2 with energy band alignment is shown in Figure 7c. The work function of Pt is −5.65 eV versus E vacuum , which is more positive than the conduction band of anatase TiO 2 (−4.12 eV vs. vacuum) [82]. Therefore, electrons can easily transfer from the CB of TiO 2 to Pt sites, which act as an electron sink [83]. On the other hand, metal with a smaller work function, such as Ni, causes a weaker driving force of electron migration [84]. As a result, the Pt-deposited on TiO 2 sample showed significantly higher photocatalytic activity than the unloaded TiO 2 due to the higher charge separation efficiency, which is also higher than the other metals [75,85]. However, the production of methanol was not in the trend over Cu-loaded TiO 2. According to the XANES result, Cu loading on TiO 2 was in a form of complex oxide. Therefore, the energy band alignment of Cu x O-TiO 2 heterojunction was a semiconductor-semiconductor heterojunction, as revealed in Figure 7d. When Cu x O with lower CB level contacts with TiO 2 , which has higher level of CB, electrons in the CB of TiO 2 can be transferred to that of Cu x O. The electrons and holes are transferred to the CB of Cu x O and the VB of TiO 2 , respectively [33]. As a result, the photoexcited electron-hole pairs can be separated by the electric field.

Conclusions
TiO 2 loaded with various metals (Pt, Pd, Ni, and Cu) was successfully synthesized by photodeposition method. We found that all samples are active for CO 2 photoreduction. On the other hand, various insightful characterizations reveal that Pt is the most promising metal, as it provides the largest work function when formed in heterojunction with TiO 2 , and provides the most appropriate CO 2 adsorption and charge accumulation energies for methanol formation. which is also in good agreement with the experimental results. The large metal work function could enhance the charge transfer ability and suppress charge recombination. Electrons can transfer from the conduction band of a semiconductor to the metal surface and thus promote the photocatalytic reduction activity. When compared to Cu, Pd, and Ni-loaded TiO 2 photocatalysts, Pt-loaded TiO 2 photocatalyst also has the fastest charge transfer of 4.513 µs. Hence, Pt-TiO 2 can generate methanol (major product) with the rate of 17.85 µmol/g cat /h, which is the highest photocatalytic CO 2 reduction activity among the M-TiO 2 that have been studied in this work, and among the top TiO 2 -based photocatalysts that have been reported for photocatalytic CO 2 -to-methanol conversion.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/nano12030474/s1, Table S1: The actual amount of the metals in the samples at the theoretical content of 0.1% wt. measured by ICP-OES, Table S2: Edge energy of the samples and the standards obtained from XANES spectra, Table S3: Element of C, H, and N over Pt loaded TiO 2 , Table S4: Calculated properties of adsorption energy, O-C-O angle, CO 2 charge accumulation a and vibrational frequencies b for CO 2 adsorption on the V-Pt 4 -TiO 2 structure, Table S5: Calculated properties of adsorption energy, O-C-O angle, CO 2 charge accumulation a and vibrational frequencies b for CO 2 adsorption on the V-Pd 4 -TiO 2 structure, Table S6: Calculated properties of adsorption energy, O-C-O angle, CO 2 charge accumulation a and vibrational frequencies b for CO 2 adsorption on the V-Ni 4 -TiO 2 structure, Table S7: Calculated properties of adsorption energy, O-C-O angle, CO 2 charge accumulation a and vibrational frequencies b for CO 2 adsorption on the V-Cu 4 -TiO 2 structure, Table S8: Comparison of the photocatalysts for CO 2 -to-methanol conversion with this work [16,31,[86][87][88][89][90], Figure S1: TEM-EDS result of photocatalysts, Figure S2: (a) GC-FID profile of the liquid products (b) GC-TCD profile of the gaseous products produced during the reaction, Figure S3: (a) GC-FID profile of the liquid products (b) GC-TCD profile of the gaseous products produced during the reaction without involvement of CO 2 as reactant, Figure S4: CO 2 photoreduction of the reuse catalyst of 0.1 wt% Pt-TiO 2 , Figure S5