Indium Doped Gan Porous Micro‐Rods Enhanced CO2 Reduction Driving By Solar Light

Photocatalytic CO2 reduction plays an important role in solar energy storage and carbon balance. GaN as an III‐V semiconductor material has received extensive attention in the field of photocatalysis. In this study, the porous indium‐doped GaN (In/GaN) micro‐rods are synthesized successfully by a facile hydrothermal method and subsequent controlled atmosphere heat treatment. Photocatalytic CO2 reduction performance of In/GaN is higher than that of pure GaN due to the In doping improves the light absorption efficiency. The primary products are CO and CH4. Among the samples, 3%‐In/GaN exhibits the highest catalytic activity as well as stability and reusability. The yield rates of CO and CH4 can be reached 50.2 and 14.6 µmol · g−1 · h−1, respectively. Furthermore, density functional theory (DFT) calculations reveal that the bandgap is narrowed and the adsorption energy of CO2 molecules is improved by In doping. Moreover, the N‐vacancy detected by electron paramagnetic resonance (EPR) increases with the In doping, resulting in an increase in the number of unpaired electrons, which is conducive to carrier transport. This work provides a new study In/GaN prepared by simple method for the light‐driven photocatalytic conversion of CO2 to high‐value‐added products.


Introduction
The rapid increase in atmospheric CO 2 concentrations has attracted significant attention due to its adverse impact on global climate through the greenhouse effect, leading to serious planetary issues.[3] Nowadays, metal oxides [4][5][6][7][8] including TiO 2 , WO 3 , and ZnO have emerged as the predominant focus of research in the realm of photocatalytic CO 2 conversion.These materials exhibit notable photocatalytic capabilities for reducing CO 2 .
Moreover, some nitrides have been paid attention in the field of photocatalytic CO 2 reduction due to the more negative potential of N 2p orbitals than O 2p orbitals. [9]Among these materials, GaN as typical III-V semiconductor has gained attention for its excellent physiochemical stability and electrical properties. [10,11]n 2015, Bandar et al. [12] reported GaN nanowires grown on silicon wafers by molecular beam epitaxial (MBE) method, where the Rh/Cr 2 O 3 was used as the cocatalyst facilitate to GaN reduction CO 2 to CH 4 .In 2016, Zhang et al. [13] demonstrated that the introduction of oxygen vacancies into hollow GaN spheres enhanced CO 2 reduction performance.In 2018, Ren et al. [14] delved into the crystal surface structures of (GaN) 1-x (ZnO) x nanowires annealed at different temperatures.Notably, nanowires sintered at 900 °C displayed a positive Ga/Zn crystal surface (101 ± ) and negative N/O crystal surface (101± ), which was conducive to the rapid separation of carriers and improved the efficiency of photocatalytic CO 2 reduction.In 2021, Huang et al. [15] modified GaN with plasmodium Ag to form a Schottky barrier, which was not only facilitated the separation of electron-hole pairs but also extended the light absorption band through surface plasmon resonance (SPR).The integration of Ag into GaN significantly enhanced the performance of photocatalytic CO 2 reduction.Furthermore, Li et al. [16] reported that the photocatalytic reduction of CO 2 to CO could be significantly improved by modifying GaN surface with Au/Al 2 O 3 .These advancements indicate GaN exceptional catalytic activity in the field of photocatalytic CO 2 reduction.
Nonetheless, the bandgap of wurtzite structure GaN is 3.39 eV, corresponding to the violet spectrum, which limits the absorption of the visible and infrared solar spectrum.Consequently, there has been a recent surge of interest in In x Ga 1-x N [17][18][19] as highly promising photocatalysts for solar fuel applications.It is known that the energy bandgap of In x Ga 1-x N can be tuned in the range of 0.7-6.2eV.Moreover, its conduction and valence band edges align with the redox potentials of water, [20,21] rendering it an attractive candidate for photocatalytic applications.Despite these advantages, the utilization of In x Ga 1-x N for the photocatalytic reduction of CO 2 is rarely explored.In 2016, AlOtaibi et al. [22] reported remarkable CO 2 photocatalytic reduction performance using multilayer InGaN/GaN nanowire arrays prepared by MBE method.In 2017, J.K. Sheu et al. [23] studied the InGaN/GaN grown on sapphire substrate through metal-organic vapor phase epitaxy (MOVPE) method, where CO 2 was mainly reduced to HCOOH by photoelectrocatalytic reduction.It is worth noting, the MBE and MOVPE methods require complex process at high cost, which could potentially hinder the widespread application of In x Ga 1-x N photocatalysts.
Herein, porous In/GaN micro-rods were synthesized by a hydrothermal method at low cost with simple preparation facility.The investigation focused on evaluating the impact of varying In doping ratios on the photocatalytic CO 2 reduction performance of GaN micro-rods.The mechanism analysis revealed that In/GaN enhanced the light absorption comparing with pure GaN, and the porous structure facilitated the diffusion and adsorption of CO 2 molecules.Meanwhile, the density functional theory (DFT) analysis shows that the adsorption energy of CO 2 molecules can be effectively improved by In doping.The photocatalysts were analyzed with various characterization methods.In this work, 3%-In/GaN displayed the most efficient CO 2 reduction performance ascribed to the fast carrier transfer rate and low carrier recombination.The primary products were CH 4 and CO, with yield rates of 14.6 and 50.2 μmol • g −1 • h −1 , respectively.Additionally, 3%-In/GaN reveals outstanding chemical stability and repeatability.These findings highlight the potential of porous In/GaN microrods as highly effective and stable photocatalysts for CO 2 reduction, underscoring their promise in addressing environmental and energy challenges.

Structure and Morphology Characterization
The preparation of porous GaN micro-rods has been reported in our previous work.Figure 1 illustrates the preparation process for the GaN.The obtained GaOOH powder was sintered at 500 °C in air atmosphere for 2 h and the Ga 2 O 3 was achieved.To verify this transformation, the X-ray diffraction (XRD) patterns of GaOOH and Ga Obviously, the sample surface becomes more porous and rough with increasing In doping ratios.Some agglomeration is observed in 5%-In/GaN micro-rods, and the surface of 5%-In/GaN appears denser compared to the other three samples.Notably, there are distinct color differences between pure GaN and the In/GaN, as presented in Figure S3 (Supporting Information).The color of the sample gradually changes from yellow to black with the increase of In doping.
Figure 3a,b displays the transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images of pure GaN, and Figure 3c,d displays the TEM and HRTEM images of 3%-In/GaN.A comparative analysis of the HRTEM images reveals that both pure GaN and 3%-In/GaN exhibit excellent crystallinity and minimal differences in lattice spacing.The lattice fringes measuring 0.279 nm (Figure 3b) and 0.281 nm (Figure 3d) correspond to the crystal (100) plane of pure GaN and 3%-In/GaN, respectively.As shown in Figure 3e-h, the highly uniform distributions of Ga, N, and In elements were observed through the energy dispersive spectrometer (EDS) elemental mappings, which confirms that In has been successfully introduced into GaN.
Figure 4a displays the XRD patterns of pure GaN and In/GaN micro-rods.The peaks can be well indexed to wurtzite structure GaN (JCPDS No. 50-0792). [24,25]The diffraction angle 2 = 32.38°,34.58°, 36.82°,48.12°, 57.74°, 63.43°, 69.07°, and 70.50°a re indexed to the diffraction plane (100), (002), (101), (102), (110), (103), (112), and (201), respectively.No visible peak corresponding to In or Indium compound was observed in the XRD patterns, attributed to the low concentration of In on the sample surface.The average grain sizes of GaN products calculated by Scherrer's formula [26] from the (101) plane are ≈10.3,11.8, 13.7, and 16.5 nm, corresponding to pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN, respectively.The increase in grain size can be attributed to the indium atoms larger than gallium atoms.This observation proves the successful doping of In into the GaN micro-rods from another point of view.Raman spectra for the four samples are shown in Figure 4b.The spectra are dominated by strong E 2 (high) and A 1 (LO), which are in agreement with Raman selection rules for wurtzite GaN. [27,28]A blue Raman shift has been observed as the increase of In doping ratio [28] and the Raman intensity of the In/GaN samples enhanced accordingly.The increase in scattering intensity of In/GaN can be attributed to physical surface roughening, which enhances the efficient out-coupling of scattered radiation from the crystal.Additionally, non-zone center zone boundary (ZB) phonon mode is observed at 422.2, 415.9, 414.8, and 413.1 cm −1 for pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN, respectively.This mode arises due to the finite size of the GaN micro-rods. [29]E 2 peaks at 570.7 cm −1 , 566.0 cm −1 , 563.1 cm −1 and 562.1 cm −1 are detected for pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN, respectively.Obviously, a decrease in the E2 photon frequency was observed with increasing In doping ratio, which can be ascribe to the larger size of In atoms compared to Ga atoms leads to an enhancement of tensile strain. [30]Moreover, the presence of A 1 (LO) mode in the Raman pattern indicates that the obtained sample has good crystallinity, which is consistent with the results of XRD pattern.The X-ray photoelectron spectroscopy (XPS) was used to further determine the presence of In shown in Figure 4c,d.In order to calibrate the test data obtained from XPS, the peak value of C 1s was corrected to 284.8 eV. Figure 4c reveals that only Ga and N elements are present in pure GaN, while In is clearly detected in 3%-In/GaN.Figure 4d displays In 3d high-resolution spectra of 3%-In/GaN.The In 3d is composed of In 3d 3/2 and In 3d 5/2 , which located at 452.3 and 444.6 eV, respectively [26] The Ga 2p and N 1s high-resolution spectra of pure GaN and 3%-In/GaN were displayed in Figure S4 (Supporting Information).
The UV-vis DRS of pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN is displayed in Figure 5a.The absorption wavelength range of the sample is redshift with the increasing of In doping ratio, which is beneficial to enhance the absorption efficiency of light.The inset of Figure 5a depicts the calculated bandgap energies (E g ) using Tauc plots.E g of 2.94, 2.89, 2.77, and 2.71 eV are for pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN, respectively, with the following Tauc equation: [33] Surface texture analysis is a vital study to understand the activity of photocatalyst. Figure 5b illustrates the adsorption/desorption isotherms of pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN.The samples feature IV isotherms with type H2 hysteresis, [34] which exhibits the porous of sample.

The Performance of Photocatalytic Reduction CO 2
CO 2 photoreduction with water over pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN have been studied (TEOA in the solution as an electron donor).Figure 6a,b shows the production rates of CO and CH 4 from CO 2 reduction as a function of illumination time for different catalysts, respectively.Within 13 hours of light illumination, the CO evolution of pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN is 488.4,598.1, 653.5, and 553.9 μmol g −1 , respectively.Simultaneously, the CH 4 evolution of pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN is 74.6, 129.5, 190.0, and 93.5 μmol g −1 , respectively.These results demonstrate that the photocatalytic CO 2 reduction efficiency is enhanced by the In doping, which ascribe to narrows the bandgap and broadens the absorption spectrum of the samples.However, as seen in Figure 6a,b, the evolution of 3%-In/GaN is the highest.In a liquid phase system, the CO 2 reduction reaction occurs simultaneously with the hydrogen evolution reaction, resulting in the generation of H 2 as a by-product.Figure 6c illustrates a comparative analysis of the yield rates of CH 4 , CO, and H 2 for the samples.Specifically, for the 3%-In/GaN sample, the measured yield rates are 14.6 μmol•g − ¹•h − ¹ for CH 4 , 50.2 μmol•g − ¹•h − ¹ for CO, and 6.5 μmol•g − ¹•h − ¹ for H 2 .These results indicate that the predominant reaction pathway primarily involves the reduction of CO 2 .Furthermore, the CO and CH 4 selectivity of 3%-In/GaN is determined to be 43.6% and 50.7%, respectively.37][38] The stability of photocatalytic CO 2 reduction can be attributed to the inherently stable physical and chemical properties of GaN, which are less prone to photo-corrosion.To further confirm the repeatability and stability of the GaN, post-reaction samples were collected and subjected to repeated photocatalytic CO 2 reduction.Figure S5 (Supporting Information) illustrates the measurement for 3%-In/GaN, which shows the excellent reusability.Moreover, the liquid products were analyzed through 1 H spectrum using the NMR equipment, with DMSO serving as an internal standard for quantification.In this study, extremely small amounts of methanol were detected in liquid products, [39] as depicted in Figure S6 (Supporting Information).The photochemical reduction CO 2 performance of our sample is compared with the references in Table 1.To ascertain the source of the gaseous products, the comparative experiment detailed in Figure S7 (Supporting Information) effectively excludes the role of TEOA in the catalytic process.
Photoluminescence (PL) spectra and transient photocurrent response were measured to investigate the charge separation.PL spectra of the photocatalyst at the excitation wavelength of 325 nm are presented in Figure 7a.Clearly, all the samples exhibit PL spectra with two emission bands, a narrow ultraviolet band, and a broad visible band spanned from 500 to 800 nm.The emission located at ≈370 nm should be attributed to the near band edge emission of GaN.[49] The emission intensity of 3%-In/GaN is lowest among the samples, which indicates the carrier lifetime of 3%-In/GaN is the longest in four samples.The low PL intensity of 3%-In/GaN resulted in the rapid charge transfer, enhancing the electron-hole pair separation rate.The time-resolved photoluminescence (TRPL) analysis gives detailed insights on charge transfer mobility dynamics for all samples, [50] as shown in Figure S9 (Supporting Information).There is a tiny difference in  the average fluorescence lifetime of the four samples.The average lifetime (Table S1, Supporting Information) of 3%-In/GaN (2.17 ns) is the shortest than the other samples that can be ascribed to the lower recombination of charge carriers and fast charge transfer.In addition, the transient photocurrent response reflects 3%-In/GaN shows an obviously higher photocurrent response than the other catalysts (Figure 7b), it is worth noting that the photocurrent density of 3%-In/GaN > 1%-In/GaN > 5%-In/GaN ≅ pure GaN.This trend indicates that the introduction of In indeed enhances light absorption.The band-edge potential of semiconductor materials plays a crucial role in the redox reactions occurring on the photocatalyst surface during the photocatalytic process.The electrochemical impedance spectroscopy (EIS) measurements of the pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN photoanodes in the light shown in Figure 8a.The diameter of the semi-circle in the Nyquist plot gives the charge transfer resistance, which is associated with the involved charge transfer limited processes.These are the charge transfer through the surface from the electrode to the reactant, contributing to the catalytic surface reaction activity.Compared to other samples, 3%-In/GaN exhibited the smallest arc radius of the impedance spectrum.The arc radii of the impedance spectra of the other samples are consistent with the change in transient photocurrent.It can be seen from the Nyquist curve that 3%-In/GaN exhibits the highest separation efficiency of the photogenerated charge carriers in four samples.By PL and electrochemical   analysis, the optimal reduction characteristics of 3%-In/GaN should be due to the rapid transport and separation of carriers.Mott-Schottky [32,51] curve can be used to estimate the energy band position (E CB ) as shown in Figure 8b and Figure S9 (Supporting Information).The pure GaN and In/GaN are n-type semiconductors with the positive slop.By fitting the curves, the flat band potential (E fb ) values of pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN are −0.78,−0.74, −0.70, and −0.65 V versus Ag/AgCl, respectively.It is known that the E CB of n-type semiconductor is more negative 0.1 or 0.2 V than E fb . [52]Here, -  Therefore, the valence band (E VB ) potentials of pure GaN, 1%-In/GaN, 3%-In/GaN, and 5%-In/GaN are 2.28, 2.27 2.19, and 2.18 V (versus NHE, pH = 7), respectively.In order to further confirm the accuracy of the band structure, E VB was detected using XPS as shown in Figure S10 (Supporting Information).The corresponding E VB of pure GaN and 3%-In/GaN is measured to be 2.30 and 2.20 eV, respectively.That is consistent with above experimental results.

DFT Study
To reveal the origin of the enhanced CO 2 reduction performance, theoretical calculations were conducted using density functional theory (DFT) calculation.The adsorption energies of CO 2 for the GaN (100) and In/GaN (100) surfaces were calculated to analyze the absorption ability of CO 2 (Figure 9a,c).GaN (100) exhibited a lower adsorption energy (−1.451 eV) than In/GaN (100) (−2.072 eV), indicating that the incorporation of In doping improves the adsorption capacity of CO 2 on GaN.In Figure S11 (Supporting Information), the adsorption energies of H 2 O on the GaN (100) and In/GaN (100) surfaces were calculated as −0.876 and −0.807 eV, respectively.The minor difference in the adsorp-tion energies indicates that In doping will not increase the adsorption ability of water on GaN (100) surface.The comparison of adsorption energy of CO 2 and H 2 O on GaN (100) and In/GaN (100) surfaces is presented in Table S2 (Supporting Information).
To obtain a deeper understanding of the electronic structure of GaN and In/GaN, we have analyzed the contribution of each atomic character in a series of bands by decomposing the total density of states (DOS) into contributions of s, p and d orbitals (Figure 9b,d).For GaN, the valence band near fermi level is mainly contributed by 4p, 3d states of Ga, and 2p states of N.
There is a strong covalent orbital hybridization between Ga and N between −7.5 and 0 eV.After In doping, the bandgap of In/GaN is narrower than pure GaN.The valence band of In/GaN near the fermi level is primarily shaped by the hybridization of the 4p and 3d states of Ga, along with the 2p states of N. Additionally, the interaction between the 4d and 5p states of In with the 2p states of N also plays a significant role.These results provide a theoretical basis for the promotion of electron transfer and enhancement of CO 2 adsorption capacity on the surface of the In/GaN.

Photocatalytic Reduction Mechanism
In order to reveal the mechanism for the enhanced photocatalytic activity, it is essential to explore the charge transfer mechanism in the In doped GaN micro-rods, which was studied by EPR measurements.Figure 10a displays the EPR spectra of N-vacancy created within the samples.Notably, all samples exhibit a symmetric derivative peak near the g value of 2.004, with the intensity markedly increasing upon the introduction of In.This observation signifies the presence of unpaired electrons, which would be increase the electron concentration and improve the conductivity.Thus, the reduction CO 2 efficiency of In/GaN is enhanced can be attribute to the increase of N-vacancy. [53,54]Figure 8b shows the EPR spectra of the DMPO adduct of •CH 3 O radical over 3%-In/GaN micro-rods under light or in dark condition. [55]Obviously, no free radicals detected in the dark.Conversely, upon light exposure, •CH 3 O radicals are detected along with the •OH. [56]The emergence of •OH radicals predominantly originates from the existence of N-vacancy in GaN.Electrons could be excited from the VB to the defects donor state under visible light illumination.The lifetime of electrons in the defects donor state is much longer than that on the CB, which facilitates the formation of superoxide radicals by attachment of electrons to oxygen.Meantime, the holes left in the VB accelerate the generation of free OH radicals. [57]And the formation of •CH 3 O radicals is ascribed to photocatalytic reduction CO 2 process.Therefore, the possible reaction pathways for the formation of CO and CH 4 from CO 2 photoreduction with H 2 O on GaN with N-vacancy maybe as the following (Figure 11). [58]onclusively, the mechanism of CO 2 photocatalytic reduction shown in Figure 12.The photocatalytic reduction mechanism may be as follows:Given the porous nature of GaN micro-rods, CO 2 molecules can be sufficiently diffused and interact with GaN.[61][62] Thus, CO 2 molecules become activated on GaN due to the lowered lowest unoccupied molecular orbital (LUMO) level of CO 2 after bending. [59]The GaN absorbs light energy greater than the bandgap width, valence band  electrons transition to the conduction band, generating free electrons (e − ) and leaving behind holes (h + ) in the valence band.Subsequently, e − diffuse to the GaN surface and react with activated CO 2 − to further dissociate it into CO.Concurrently, photogenerated h + on GaN dissociate water to produce protons, which combine with the photogenerated e − to form hydrogen atoms that subsequently hydrogenate CO, yielding CH 4 .The remaining hydrogen atoms combine to produce H 2 , which escapes from the GaN surface.In this study, the bandgap is reduced by introduction of In into GaN, resulting in enhanced light absorption.Additionally, DFT calculations demonstrated that the adsorption energy of CO 2 molecules for In/GaN is improved.Besides, the N-vacancy detected by EPR increased effectively with the increase of In-doping content.That result in the number of unpaired electrons increased, thereby effectively increasing the electron concentration and conductivity, which is conducive to carrier transport.In four samples, 3%-In/GaN shows the best CO 2 reduction performance due to the fast carrier transfer rate and low carrier recombination.Consequently, the performance of photocatalytic CO 2 reduction using In/GaN is substantially improved.

Conclusion
In summary, the porous GaN and In/GaN micro-rods were successfully compounded by hydrothermal method, which is low cost and simple preparation.The porous GaN micro-rods display efficient photocatalytic reduction of CO 2 to CO and CH 4 .Importantly, the introduction of In into GaN resulted in the bandgap narrowing and improved light absorption.DFT calculations further revealed that In doping not only reduced the bandgap structure enhancing light absorption but also enhanced the CO 2 adsorption capacity of GaN.Moreover, the N-vacancy increases with the In doping, resulting in an increase in the number of unpaired electrons, which is conducive to carrier transport.Consequently, In/GaN proved to be effective in enhancing the photocatalytic CO 2 reduction properties.It is noteworthy that 3%-In/GaN exhibits highest photocatalytic CO 2 performance with the fast carrier transfer rate and low carrier recombination.The yield rates for CO and CH 4 reached 50.2 and 14.6 μmol•g −1 •h −1 , respectively.At the same time, 3%-In/GaN also reveals excellent stability and reusability of photocatalytic CO 2 reduction.These findings underscore the effectiveness of In-doped GaN in enhancing the photocatalytic CO 2 reduction performance.

Experimental Section
Chemicals: The pure GaN micro-rods have been prepared with a process similar to our previous work. [63]Briefly, in a typical hydrothermal procedure, 4 g of Ga(NO 3 ) 3 •9H 2 O (99.999%, Aldrich) was dissolved in 35 mL deionized (DI) water, and concentrated ammonia aqueous solution (NH 3 •H 2 O, 25%) was added dropwise under stirring to tune the pH to 9, as illustrated in Figure 1.Then the mixed solution was transferred into a 50 mL Teflon lined autoclave and kept at 150 °C for 5 h in the oven.Subsequently, the reacted solution was centrifuged with DI water and ethanol.After washing several times, the collected precipitate dried at 80 °C and placed into a tube furnace sintering in air at 500 °C for 1 h.Finally, the temperature of the furnace was ramped to 1000 °C and kept for another 1 h, during which ammonia (99.999%) was introduced at a flow rate of 35 sccm.
Characterization of GaN: The crystal structure and morphology of the samples were characterized by X-ray diffraction (XRD, Rigaku Smartlab) using Cu-K radiation ( = 0.15 418 nm) in the 2 range of 20-80°, field-emission scanning electron microscope (FE-SEM, Zeiss Gemini 300), and high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30).The Brunauer-Emmett-Teller (BET) surface area and pore-size distribution were measured on ASAP 2020 system using nitrogen (N 2 ) adsorption at 77 K.The UV-vis diffuse reflectance spectra (DRS) were measured using a UV-vis spectrophotometer (PerkinElmer, Lambda 750) equipped with an integrating sphere attachment.X-ray photoelectron spectroscopy (XPS) was measured using a Perkin-Elmer model PHI 5600 XPS system with a monochromatic aluminum radiation source (1486.6 eV), and the spectra were calibrated with the C 1s peak at 284.6 eV.Raman spectra were acquired using a Horiba Scientific LabRAM HR evolution Raman spectrometer with a 532 nm laser.Photoluminescence (PL) spectra were taken at room temperature with an excitation light of 325 nm laser.Time-resolved photoluminescence (TRPL) investigation was carried out using a lab-built optical microscope system.A picosecond pulsed diode laser (PicoQuant, LDH-P-FA-355) was employed to excite (focus = 40 × (NA = 0.6) objective (Nikon)) the sample with a wavelength of 355 nm (FWHM = 56 ps) and a 40 MHz repetition rate.Electron paramagnetic resonance (EPR) spectra were collected on the electron spin resonance spectrometer (JES FA200).5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was used as the capture agent to capture the free radicals.The reaction conditions were the same as the photocatalytic performance measurements except for the addition of the capture agent (concentration: 50 mmol L −1 ).
Photoelectrochemical Measurements: The photoelectrochemical experiment was carried out in Shanghai Chenhua CHI660E electrochemical workstation in ambient conditions under irradiation of a 300 W Xe lamp (CEL-HXF300, Beijing Gold).An electrolyte of 0.5 M Na 2 SO 4 aqueous solution was chosen for the test.Standard three-electrode setup was used with the ITO coated glass as photoelectrode, a Pt foil as counter electrode, and an Ag/AgCl electrode as reference electrode.For the photoelectrode: 5 mg of photo-catalysts were dispersed in a mixture of 200 μL ethanol, 100 μL DI water, 50 μL isopropanol, and 50 μL nafion solution to make a slurry, which was coated onto an indium tin oxide (ITO) glass with an exposed area of 1 × 1 cm 2 .The ITO electrode was then vacuum-dried at 60 °C for 1 h.The photoresponse of the prepared photoelectrodes (i.e., It) was operated by measuring the photocurrent densities under chopped light irradiation (light on/off cycles: 50 s) at a bias potential of 1 V versus Ag/AgCl for 600 s.The electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 10 −2 -10 6 Hz with an alternating current (AC) voltage amplitude of 5 mV at a bias potential of 1 V versus Ag/AgCl.
Photocatalytic Reduction CO 2 Activity Measurements: Photocatalytic reduction of CO 2 was measured in a 100 mL reaction cell connected to a closed gas circulation system (Perfect Light Company, Beijing).In a typical test, 5 mg of GaN micro-rods were placed on the bottom of the reaction vessel containing 10 mL of DI water with triethanolamine (TEOA, the volume ratio is 2%) as the sacrificial.Then, the reactor containing the photocatalyst was first evacuated with a vacuum pump and then purged with CO 2 (purity of 99.999%) to remove residual air.After sealing the reactor, the CO 2 pressure was adjusted to 80 kPa, and cooling water was introduced to circulate in the reactor.The amounts of gas products were determined using a gas chromatography (FULI INSTRUMENTS, GC-9790 II).The CO and CH 4 were determined using flame ionization detectors (FID) and the H 2 was determined using a thermal conductivity detector (TCD).The liquid phase products were analyzed by nuclear magnetic resonance spectroscopy (NMR, AVANCE III 400 M), the 400 μL D 2 O as solvent and 100 μL dimethyl sulfoxide (DMSO) as internal standard.
The selectivity for the CO 2 reduction product was obtained using the following formula: CH 4 selectivity = 8R CH4 ∕ (8R CH4 + 2R CO + 2R H2 ) 100% (2) where R CO , R CH4 and R H2 represent the yield rates of CH 4 , CO and H 2 , respectively.Density-Functional Theory (DFT) Calculations: All calculations of structure relaxation and electronic properties were performed by the DFT using the Vienna Ab-initio Simulation Package (VASP) package. [64]The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional were used to describe the electronic exchange and correlation effects. [60,65]Uniform G-centered k-points meshes with a resolution of 2*0.04Å −1 and Methfessel-Paxton electronic smearing were adopted for the integration in the Brillouin zone for geometric optimization.The simulation was run with a cut-off energy of 500 eV throughout the computations.These settings ensure convergence of the total energies to within 1 meV per atom.Structure relaxation proceeded until all forces on atoms were less than 1 meV/Å and the total stress tensor was within 0.01 GPa of the target value.The DFT-D2 Van der Walls correction by Grimme [66] was also considered in all calculations.Density of states was calculated by using the hybrid exchange-correlation functionals HSE06 [67] with the range-separation parameter in range-separated hybrid functionals equal to 0.28.The adsorption behavior of CO 2 and H 2 O molecules on the (100) surface of GaN and In/GaN were studied.The adsorption energy of CO 2 and H 2 O on GaN (100) and In/GaN (100) surfaces were calculated by the following equation: where E(total), E(surface), and E(molecule) represent the energy of adsorb CO 2 and H 2 O on GaN (100) and In/GaN (100) surfaces, the energy of clean surfaces, and the energy of the CO 2 and H 2 O molecules.

Figure 1 .
Figure 1.Schematic diagram of GaN precursor synthesis process.
2 O 3 are shown in Figure S1 (Supporting Information).The scanning electron microscope (SEM) images shown in Figure S2 (Supporting Information) comparing the sample of GaOOH and Ga 2 O 3 morphology before and after In doping.After the subsequent heat treatment in ammonia, the Ga 2 O 3 and In/Ga 2 O 3 were expected to transform to GaN and In/GaN, re-spectively.The SEM images of pure GaN a), 1%-In/GaN b), 3%-In/GaN c), and 5%-In/GaN d) are shown in Figure 2, respectively.

Figure 3 .
Figure 3. a) the TEM image and b) HRTEM image of pure GaN.The In doped GaN: c) the TEM image, d) HRTEM image and the corresponding elemental mapping of e) 3%-In/GaN, f) Ga, g) In, and h) N.

Figure 4 .
Figure 4. a) XRD patterns and b) Raman of pure GaN compared with different molar ratio of In doped GaN.c) XPS survey spectrum of pure GaN and 3%-In/GaN and d) In 3d high-resolution XPS spectra of 3%-In/GaN.

Figure 5 .
Figure 5. a) UV-vis absorbance and the inset is (hv) 1/2 versus hv curves and b) Nitrogen adsorption/desorption isotherms of pure GaN compared to GaN doped with different molar ratio of In.The corresponding pore size distribution is shown in the inset.

Figure 6 .
Figure 6.The comparison of production of a) CO and b) CH 4 by GaN photocatalytic CO 2 reduction.c) The yield rate of CH 4 , CO, and H 2 .d) The stability of 3%-In/GaN.

Figure 9 .
Figure 9.The adsorption energies of CO 2 adsorbed on a) GaN and c) In/GaN; Calculated density of states (DOS) of b) GaN and d) In/GaN.Ga: green; N: blue; In: purple; C: brown; O: red.

Figure 10 .
Figure 10.a) EPR spectra of N-vacancy in four samples and b) In situ EPR spectra for identifying the •OH radicals and CH 3 O radicals over 3%-In/GaN under light irradiation or in the dark (5,5-Dimethy1-1-pyrroline N-oxide (DMPO) was used as the capture agent).

Figure 11 .
Figure 11.The possible reaction pathways for the formation of CO and CH 4 .

Figure 12 .
Figure 12.Mechanism of GaN photocatalytic reduction of CO 2 .

Table 1 .
The comparison about photochemical reduction CO 2 performance.
Cr 2 O 3 /GaN 300 W Xe lamp CO 2 with H 2 O vapor