Solar-Driven Syngas Production Using Al-Doped ZnTe Nanorod Photocathodes

Syngas, traditionally produced from fossil fuels and natural gases at high temperatures and pressures, is an essential precursor for chemicals utilized in industry. Solar-driven syngas production can provide an ideal pathway for reducing energy consumption through simultaneous photoelectrochemical CO2 and water reduction at ambient temperatures and pressures. This study performs photoelectrochemical syngas production using highly developed Al-doped ZnTe nanorod photocathodes (Al:ZnTe), prepared via an all-solution process. The facile photo-generated electrons are transferred by substitutional Al doping on Zn sites in one-dimensional arrays to increase the photocurrent density to −1.1 mA/cm2 at −0.11 VRHE, which is 3.5 times higher than that for the pristine ZnTe. The Al:ZnTe produces a minor CO (FE ≈ 12%) product by CO2 reduction and a major product of H2 (FE ≈ 60%) by water reduction at −0.11 VRHE. Furthermore, the product distribution is perfectly switched by simple modification of Au deposition on photocathodes. The Au coupled Al:ZnTe exhibits dominant CO production (FE ≈ 60%), suppressing H2 evolution (FE ≈ 15%). The strategies developed in this study, nanostructuring, doping, and surface modification of photoelectrodes, can be applied to drive significant developments in solar-driven fuel production.


Introduction
Synthetic gas (syngas), a mixture of carbon monoxide (CO) and hydrogen gas (H 2 ), is an essential precursor for the production of hydrocarbon fuels and value-added chemicals, such as alcohols and acetic acid [1,2]. Industrial syngas is typically produced from fossil fuels and natural gases at high temperatures and pressures [3,4]. However, employing photoelectrochemical (PEC) water and CO 2 reduction at ambient temperatures and pressures is a sustainable and environmentally friendly alternative to traditional syngas production [5,6].
PEC cells comprising semiconductor photoelectrodes can harvest solar energy to perform electrochemical catalytic reactions [7][8][9]. The standard reduction potential of CO 2 to CO (CO 2 + 2H + + 3e − →CO + H 2 O, E o = −0.11 V RHE ) is slightly more negative than that of water to H 2 (2H + + 2e − →H 2 , E o = 0.0 V RHE ). Therefore, ideally, any photocathode can be used in photoelectrochemical CO 2 reduction to produce H 2 in a thermodynamic manner. Moreover, CO 2 to CO conversion is kinetically poorer than water-to-H 2 conversion, primarily because of the slow CO 2 adsorption and intermediate formation [10][11][12]. This indicates that direct syngas production can be achieved using a single photoelectrochemical CO 2 reduction system. Furthermore, the ratio of H 2 /CO, which is the key factor determining the upgraded product selectivity, is controllable by catalysts [13][14][15].
ZnTe semiconductors are an interesting and useful class of photoelectrocatalysts and have received considerable attention owing to their narrow bandgap energy (2.26 eV) and band alignment. This allows harvesting of visible light and a suitable band alignment for syngas production. Specifically, the conduction band minimum (CBM) of ZnTe located at Materials 2022, 15, 3102 2 of 9 −1.63 V RHE , which is highly negative compared to CO 2 -to-CO reduction potential, can provide a strong driving force for photoexcited electron injection for syngas production [16][17][18]. However, ZnTe photocathodes typically exhibit poor photoexcited charge separation because of the rapid charge recombination and kinetically inactive catalytic reaction [16,19].
Therefore, this study proposes Al-doped ZnTe nanorod photocathodes for photoelectrochemical syngas production. The one-dimensional array ZnTes can provide a shortened carrier transport distance, which can facilitate their separation. Furthermore, ZnTes with substitutional Al dopants at Zn sites exhibit increased charges transport activity in the bulk compared with pristine ZnTe. The difference in valence states between Al (Al 3+ in lattice) and Zn (Zn 2+ in lattice) causes a significantly increasing major carrier concentration. In addition, PEC syngas production is demonstrated, enabling a change of the H 2 :CO ratio from 5:1 to 1:4, using surface-modified ZnTe-based photocathodes. The proposed method results in a significant enhancement in syngas production and H 2 /CO ratio changes. The photocathode preparation, results, and effects of catalyst modification are presented in this paper.

Physico and Chemical Characterizations
The crystal structures of Al-doped ZnO and Al-doped ZnTe were examined using X-ray diffraction (XRD) (Mac Science, Kanagawa, Japan, M18XHF using Cu Ka radiation, λ = 0.15406 nm). The morphologies of the electrodes were investigated using field-emission scanning electron microscopy (FESEM) (JEOL, Tokyo, Japan, JMS-7401F and Phillips Electron Optics B.V. XL30S FEG, operated at 10 keV) and high-resolution transmission electron microscopy (HR-TEM) (JEOL, Tokyo, Japan, JEM-2200FS), combined with an energy dispersive X-ray spectrometer operated at 200 kV. The elemental compositions and their oxidation states were investigated using X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, Waltham, MA, USA, ESCALAB 250Xi) and the binding energy of each element was calibrated with respect to the carbon 1 s peak at 284.8 eV. The absorbance of photoelectrodes was examined using UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) (Shimadzu, Kyoto, Japan, UV2501PC).

Photoelectrochemical Measurements
All photoelectrochemical CO 2 reduction experiments (J-V, J-t, and EIS) were conducted in a three electrodes configuration with ZnTe-based working electrodes, a graphite rod counter electrode, and an Ag/AgCl (4 M KCl) reference electrode, under simulated solar illumination in an undivided gas-tight cell. Simulated solar illumination was generated using a 300 W Xe lamp (Newport, CA, USA, Oriel, 91-160,) with an AM 1.5 G and an IR filter. The light intensity of 100 mW/cm 2 was calibrated using the guaranteed reference by National Renewable Energy Laboratories, US. A potentiostat (Gramry, Warminster, PA, USA, Reference 600TM) supplied bias to adjust the potential difference to electrodes connected in a circuit. The exposed comparable illuminating area (0.25-0.3 cm 2 for J-V measurements and 1-1.2 cm 2 for J-t measurements) of the photocathodes was masked with insulating epoxy. CO 2 -saturated 0.5 M KHCO 3 (Junsei chemicals, Tokyo, Japan) in water was used as the electrolyte.
J-V measurements were performed while cathodically sweeping the potentials at a scan rate of 10 mV/s. J-t measurements were recorded for 3 h at −0.11 V and −0.3 V with respect to RHE. While performing J-t measurements, the produced gases were analyzed using gas chromatography (Agilent, Santa Clara, CA, USA, Model 7890) with a Carboxen 1000 packed column and a thermal conductivity detector for gaseous products-H 2 , CO, CO 2 , and CH 4 . Electrochemical impedance spectroscopy (EIS) measurements were taken at a constant DC potential of −0.11 V vs RHE and an AC potential frequency range of 0.1-100,000 Hz with a 20 mV amplitude.
The faradaic efficiency (FE) is determined by dividing the number of charges used to produce the detected number of products, H 2 or CO, by the total charge passed during the photoelectrochemical measurement using the following equation, where n is the number of electrons required to produce one H 2 or CO molecule, which is 2, and F is Faraday's constant (96485.33 C/mol). FE (%) = (n × mol of product × F)/(Total charge passed) × 100

Synthesis of Al:ZnTe Nanorod Photocathodes
The Al-doped ZnTe (Al:ZnTe) nanorod photocathodes used in this work were prepared using a two-step, modified hydrothermal method and following the ion exchange reaction. The preparation method is depicted in the schematic diagram (Scheme S1, SI). The Aldoped ZnO (Al:ZnO) nanorods were hydrothermally grown on the FTO with Al-dopant concentrations in the range of 0-5 at% compared with the concentration of Zn precursors. The Al:ZnO nanorods served as an in situ template and precursors to prepare Al:ZnTe. As the solubility product constant (K sp ) for ZnTe (5.0 × 10 −34 ) is significantly lower than that for ZnO (6.8 × 10 −34 ), ZnO nanorods were transformed to ZnTe by spontaneous anion exchange reactions and thus ZnO core-ZnTe shell heterojunction type electrode was prepared [21].
ZnTe nanorods with different concentrations of Al dopants (Al:ZnTe X, where X = 0, 1, 3, and 5 represent the at% of Al compared with Zn) were prepared and their photoelectrochemical performance was investigated in Figure S1 in SI. Amo1ang the photocathodes, Al:ZnTe 3, containing 3 at% Al dopant, exhibited the highest photocurrent densities. The optimized Al:ZnTe 3 photocathodes were named Al:ZnTe and their physicochemical characteristics and photoelectrochemical activities were compared with those of pristine ZnTe photocathodes.
The XRD patterns of ZnTe and Al:ZnTe showed hexagonal wurtzite ZnO (JCPDS no. 01-089-0511) and zinc blend ZnTe (JCPDS no. 01-065-0385) ( Figure 1a). All prepared photocathodes showed two patterns of ZnTe and ZnO, regardless of the Al dopant concentration ( Figure S2, SI). Furthermore, neither peak splitting nor shifting was observed by Al doping. Moreover, no crystal structure change occurred by Al doping. The presence of Al 3+ in Al:ZnTe was confirmed by the XPS results for the Al 2p orbital ( Figure 1b). Additionally, the oxidation states of Zn and Te elements were analyzed to investigate physicochemical changes for ZnTe by Al doping. In the core level of the Zn 2p spectra, two peaks attributed to Zn 2+ from ZnO and ZnTe slightly positive shifted 0.5 eV by doping with the electron-deficient Al 3+ relative to Zn 2+ . However, no significant peak shifts were observed in the core level of the Te 3d results (Figure 1c,d) [22].
Another distinct difference in the morphologies for ZnO and ZnTe resulting from Al doping was observed. The pristine ZnO had a cylindrical nanorod, whereas Al:ZnO had a needle-shaped nanorod (Figure 2a,c). As the concentration of Al dopants increased, the shape transition became more apparent ( Figure S3 in SI). The Al:ZnO nanorods were formed by hydrothermal growth of Zn hydroxyl anions with substitutional doping of Al hydroxyl anions. The different valence states and sizes of the two anions induced unique needle shapes for Al:ZnO [23]. Both ZnO and Al:ZnO nanorods were spontaneously transformed to ZnTe and Al:ZnTe via Te 2− anion exchange reactions. The radii of ZnTe and Al:ZnTe became larger than those of ZnO and Al:ZnO, respectively, preserving the densities of nanorods in each film (Figure 2b  The presence of Al 3+ in Al:ZnTe was confirmed by the XPS results for the Al 2p orbital ( Figure 1b). Additionally, the oxidation states of Zn and Te elements were analyzed to investigate physicochemical changes for ZnTe by Al doping. In the core level of the Zn 2p spectra, two peaks attributed to Zn 2+ from ZnO and ZnTe slightly positive shifted 0.5 eV by doping with the electron-deficient Al 3+ relative to Zn 2+ . However, no significant peak shifts were observed in the core level of the Te 3d results (Figure 1c,d) [22].
Another distinct difference in the morphologies for ZnO and ZnTe resulting from Al doping was observed. The pristine ZnO had a cylindrical nanorod, whereas Al:ZnO had a needle-shaped nanorod (Figure 2a,c). As the concentration of Al dopants increased, the shape transition became more apparent ( Figure S3 in SI). The Al:ZnO nanorods were formed by hydrothermal growth of Zn hydroxyl anions with substitutional doping of Al hydroxyl anions. The different valence states and sizes of the two anions induced unique needle shapes for Al:ZnO [23]. Both ZnO and Al:ZnO nanorods were spontaneously transformed to ZnTe and Al:ZnTe via Te 2− anion exchange reactions. The radii of ZnTe and Al:ZnTe became larger than those of ZnO and Al:ZnO, respectively, preserving the densities of nanorods in each film (Figure 2b

Photoelectrocatalytic Syngas Production Using Al:ZnTe Nanorod Photocathode
Photoelectrochemical CO2 reductions were investigated in a conventional three-electrode configuration, using the developed ZnTe-based electrode as a working electrode, an Ag/AgCl reference electrode, and a graphite rod counter electrode, under simulated solar illumination (AM 1.5 G, 100 mW/cm 2 ). To estimate the activity under dark and light conditions simultaneously, the photocurrent-potential (J-V) was measured under chopped illumination in a CO2-saturated KHCO3 electrolyte (Figure 3a). The photovoltage gain for CO2-to-CO reduction, the difference between the photocurrent onset potential of photocathodes, and the thermodynamic CO2/CO redox potential (Eonset-E°CO2/CO) were clearly observed at more than 0.4 V for both ZnTe and Al:ZnTe. For the photocurrent density at the theoretical CO2/CO redox potential (E°CO2/CO), −0.11 VRHE of the Al:ZnTe photocathode exhibited −1.1 mA/cm 2 , which was at least 3.5 times higher than that of the ZnTe photocathode. In the potential range of −0.4 to 0.3 VRHE, the Al:ZnTe photocathode exhibited enhanced photocurrent densities compared to the ZnTe photocathode.

Photoelectrocatalytic Syngas Production Using Al:ZnTe Nanorod Photocathode
Photoelectrochemical CO 2 reductions were investigated in a conventional threeelectrode configuration, using the developed ZnTe-based electrode as a working electrode, an Ag/AgCl reference electrode, and a graphite rod counter electrode, under simulated solar illumination (AM 1.5 G, 100 mW/cm 2 ). To estimate the activity under dark and light conditions simultaneously, the photocurrent-potential (J-V) was measured under chopped illumination in a CO 2 -saturated KHCO 3 electrolyte (Figure 3a). The photovoltage gain for CO 2 -to-CO reduction, the difference between the photocurrent onset potential of photocathodes, and the thermodynamic CO 2 /CO redox potential (E onset -E • CO 2 /CO ) were clearly observed at more than 0.4 V for both ZnTe and Al:ZnTe. For the photocurrent density at the theoretical CO 2 /CO redox potential (E • CO 2 /CO ), −0.11 V RHE of the Al:ZnTe photocathode exhibited −1.1 mA/cm 2 , which was at least 3.5 times higher than that of the ZnTe photocathode. In the potential range of −0.4 to 0.3 V RHE , the Al:ZnTe photocathode exhibited enhanced photocurrent densities compared to the ZnTe photocathode.

Photoelectrocatalytic Syngas Production Using Al:ZnTe Nanorod Photocathode
Photoelectrochemical CO2 reductions were investigated in a conventional three-electrode configuration, using the developed ZnTe-based electrode as a working electrode, an Ag/AgCl reference electrode, and a graphite rod counter electrode, under simulated solar illumination (AM 1.5 G, 100 mW/cm 2 ). To estimate the activity under dark and light conditions simultaneously, the photocurrent-potential (J-V) was measured under chopped illumination in a CO2-saturated KHCO3 electrolyte (Figure 3a). The photovoltage gain for CO2-to-CO reduction, the difference between the photocurrent onset potential of photocathodes, and the thermodynamic CO2/CO redox potential (Eonset-E°CO2/CO) were clearly observed at more than 0.4 V for both ZnTe and Al:ZnTe. For the photocurrent density at the theoretical CO2/CO redox potential (E°CO2/CO), −0.11 VRHE of the Al:ZnTe photocathode exhibited −1.1 mA/cm 2 , which was at least 3.5 times higher than that of the ZnTe photocathode. In the potential range of −0.4 to 0.3 VRHE, the Al:ZnTe photocathode exhibited enhanced photocurrent densities compared to the ZnTe photocathode.  Let us consider the critical origin of the improvement of photocurrent densities due to Al doping on ZnTe. The first possibility is the enhancement of the number of photo-excited carriers resulting from the improvement of the light absorption properties of Al:ZnTe compared to ZnTe [24][25][26][27]. However, the light absorbance of Al:ZnTe was approximately similar (330-800 nm) to that of ZnTe. (Figure 3b) Considering the direct bandgap of Al:ZnTe (2.26 ± 0.02 eV) and ZnTe (2.24 ± 0.03 eV) determined by Kubelka-Munk relation using their absorbance, Al doping strategy had a negligible effect to increase the number of photo-generated carriers.
Another possibility is the enhancement of photo-excited carrier separation due to Al doping, resulting from the decrease of resistance to charge transfer [28][29][30]. The Nyquist plots, prepared by measuring EIS, show that Al:ZnTe has a smaller semicircular radius than ZnTe [31] (Figure 3c). As the radius in the Nyquist plots corresponds to the impedance value, it indicates that Al:ZnTe has a relatively reduced impedance and improved charge transfer ability compared to pristine ZnTe. Therefore, the increased majority carriers and surface area by substitutional Al doping are considered to be the two main origins of photocurrent enhancement.
To investigate the product distributions, photoelectrochemical CO 2 reduction was performed on the photocathodes at −0.11 V RHE which is thermodynamic CO 2 /CO redox potential. The ZnTe-based photocathodes produced H 2 and CO via two-electron transfer using protons or dissolved CO 2 in the electrolyte, but no additional byproduct was detected ( Figure 4). The total product amounts, the sum of H 2 and CO, of the Al:ZnTe photocathode were at least three times higher than those for ZnTe, which perfectly corresponded to increased photocurrents almost three times because of Al doping on ZnTe (Figure 3a). Let us consider the critical origin of the improvement of photocurrent densities due to Al doping on ZnTe. The first possibility is the enhancement of the number of photoexcited carriers resulting from the improvement of the light absorption properties of Al:ZnTe compared to ZnTe [24][25][26][27]. However, the light absorbance of Al:ZnTe was approximately similar (330-800 nm) to that of ZnTe. (Figure 3b) Considering the direct bandgap of Al:ZnTe (2.26 ± 0.02 eV) and ZnTe (2.24 ± 0.03 eV) determined by Kubelka-Munk relation using their absorbance, Al doping strategy had a negligible effect to increase the number of photo-generated carriers.
Another possibility is the enhancement of photo-excited carrier separation due to Al doping, resulting from the decrease of resistance to charge transfer [28][29][30]. The Nyquist plots, prepared by measuring EIS, show that Al:ZnTe has a smaller semicircular radius than ZnTe [31] (Figure 3c). As the radius in the Nyquist plots corresponds to the impedance value, it indicates that Al:ZnTe has a relatively reduced impedance and improved charge transfer ability compared to pristine ZnTe. Therefore, the increased majority carriers and surface area by substitutional Al doping are considered to be the two main origins of photocurrent enhancement.
To investigate the product distributions, photoelectrochemical CO2 reduction was performed on the photocathodes at −0.11 VRHE which is thermodynamic CO2/CO redox potential. The ZnTe-based photocathodes produced H2 and CO via two-electron transfer using protons or dissolved CO2 in the electrolyte, but no additional byproduct was detected ( Figure 4). The total product amounts, the sum of H2 and CO, of the Al:ZnTe photocathode were at least three times higher than those for ZnTe, which perfectly corresponded to increased photocurrents almost three times because of Al doping on ZnTe (Figure 3a). Furthermore, to investigate selectivity change resulting from Al doping, the ratio of H2/CO was compared using data collected from product distributions at −0.11 VRHE (Figure 4c). ZnTe had a ratio of 5.2, whereas Al:ZnTe had a ratio of 5.0. The close ratio values indicate that the H2 production reaction is approximately five times more favorable than the CO2-to-CO reduction reaction on the surface of photoelectrodes for both ZnTe and Al:ZnTe as reported previously [16,17]. This suggests that both photoelectrodes provide an identical catalytic nature for electrochemical reactions and that Al dopants were ineffective in altering favorable catalytic reactions.
To further study the possibility of switching major products during photoelectrochemical CO2 reduction, surface modification using Au electrocatalysts on Al:ZnTe (Al:ZnTe-Au) was conducted. The Au nanoparticles were physically deposited using an E-Beam evaporator and their atomic ratio was approximately 1% [13]. The J-V curve of Al:ZnTe-Au was identical to that of Al:ZnTe, indicating that the electron transfer rate remained unchanged after Au deposition (Figure 5a). Furthermore, to investigate selectivity change resulting from Al doping, the ratio of H 2 /CO was compared using data collected from product distributions at −0.11 V RHE (Figure 4c). ZnTe had a ratio of 5.2, whereas Al:ZnTe had a ratio of 5.0. The close ratio values indicate that the H 2 production reaction is approximately five times more favorable than the CO 2 -to-CO reduction reaction on the surface of photoelectrodes for both ZnTe and Al:ZnTe as reported previously [16,17]. This suggests that both photoelectrodes provide an identical catalytic nature for electrochemical reactions and that Al dopants were ineffective in altering favorable catalytic reactions.
To further study the possibility of switching major products during photoelectrochemical CO 2 reduction, surface modification using Au electrocatalysts on Al:ZnTe (Al:ZnTe-Au) was conducted. The Au nanoparticles were physically deposited using an E-Beam evaporator and their atomic ratio was approximately 1% [13]. The J-V curve of Al:ZnTe-Au was identical to that of Al:ZnTe, indicating that the electron transfer rate remained unchanged after Au deposition (Figure 5a). Materials 2022, 15, x FOR PEER REVIEW 7 of 9 In addition, to investigate product distribution differences due to Au coupling, the Al:ZnTe-Au photocathode underwent potential constant photoelectrolysis at −0.11 VRHE and faradaic efficiencies for CO and H2 were determined based on the quantification results (Figure 5b,c). The imperfect total faradaic efficiencies observed for all ZnTe based photocathode are mainly due to the instability of ZnTe in an aqueous solution [16]. Al:ZnTe-Au exhibited dominant CO production, with FECO = 60% and minor H2 production with FEH2 = 15%. By contrast, Al:ZnTe showed limited CO production (FECO = 12%) with robust H2 evolution (60%). The Au nanoparticles can provide active sites for CO production and suppress H2 production owing to their intrinsically superior electronic nature for CO2 adsorption, CO2-to-CO conversion, and CO desorption using transferred protons and electrons, resulting in the major product switch [32][33][34]. The results demonstrate the availability of photoelectrochemical syngas production. Furthermore, the composition of the syngas-H2/CO-can be altered by modifying the light absorber using an electrocatalyst.

Conclusions
Photoelectrochemical systems can provide an ideal method for direct syngas production via simultaneous CO2 and water reduction at ambient temperatures and pressures. This provides an alternative to the conventional process, requiring fossil fuels and natural gases at high temperatures and pressures. In this study, highly advanced Al-doped ZnTe nanorod photocathodes (Al:ZnTe) were developed for solar-energy-driven syngas production. This system has the following advantages: (1) a cost-effective solution process to prepare samples, (2) a nanorod array for facilitating photo-excited carrier separation, and (3) substitutional in situ Al doping on the site Zn while hydrothermally growing ZnO NW. Furthermore, a simple strategy was proposed for switching the major product to CO by suppressing competitive H2 evolution. Au electrocatalyst coupling can provide active sites for CO production. By the combination of all modifications, the advanced Al:ZnTe-Au photocathode represented −1.1 mA/cm 2 at −0.11 VRHE and the ratio of the H2:CO to 1:4, whereas pristine ZnTe exhibited −0.31 mA/cm 2 with the syngas production ratio to 5:1. With further research advances, the developed system can be applied for solar-driven value-added chemicals production in practical.
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Scheme S1: Schematic diagram for synthesis, Figure S1: J-V plots, Figure  S2: XRD patterns, Figure S3: SEM images.  In addition, to investigate product distribution differences due to Au coupling, the Al:ZnTe-Au photocathode underwent potential constant photoelectrolysis at −0.11 V RHE and faradaic efficiencies for CO and H 2 were determined based on the quantification results (Figure 5b,c). The imperfect total faradaic efficiencies observed for all ZnTe based photocathode are mainly due to the instability of ZnTe in an aqueous solution [16]. Al:ZnTe-Au exhibited dominant CO production, with FE CO = 60% and minor H 2 production with FE H2 = 15%. By contrast, Al:ZnTe showed limited CO production (FE CO = 12%) with robust H 2 evolution (60%). The Au nanoparticles can provide active sites for CO production and suppress H 2 production owing to their intrinsically superior electronic nature for CO 2 adsorption, CO 2 -to-CO conversion, and CO desorption using transferred protons and electrons, resulting in the major product switch [32][33][34]. The results demonstrate the availability of photoelectrochemical syngas production. Furthermore, the composition of the syngas-H 2 /CO-can be altered by modifying the light absorber using an electrocatalyst.

Conclusions
Photoelectrochemical systems can provide an ideal method for direct syngas production via simultaneous CO 2 and water reduction at ambient temperatures and pressures. This provides an alternative to the conventional process, requiring fossil fuels and natural gases at high temperatures and pressures. In this study, highly advanced Al-doped ZnTe nanorod photocathodes (Al:ZnTe) were developed for solar-energy-driven syngas production. This system has the following advantages: (1) a cost-effective solution process to prepare samples, (2) a nanorod array for facilitating photo-excited carrier separation, and (3) substitutional in situ Al doping on the site Zn while hydrothermally growing ZnO NW. Furthermore, a simple strategy was proposed for switching the major product to CO by suppressing competitive H 2 evolution. Au electrocatalyst coupling can provide active sites for CO production. By the combination of all modifications, the advanced Al:ZnTe-Au photocathode represented −1.1 mA/cm 2 at −0.11 V RHE and the ratio of the H 2 :CO to 1:4, whereas pristine ZnTe exhibited −0.31 mA/cm 2 with the syngas production ratio to 5:1. With further research advances, the developed system can be applied for solar-driven value-added chemicals production in practical.