Single-Step Fabrication of Visible-Light-Active ZnO-GaN:ZnO

May 22, 2018 - Designing an efficient photoanode for water splitting is a promising way to produce hydrogen fuel for a green and sustainable future. H...
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Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Single-Step Fabrication of Visible-Light-Active ZnO-GaN:ZnO Branched Nanowire Arrays as Photoanode for Efficient Water Splitting Yasir Abbas,† Zareen Zuhra,† Naeem Akhtar,‡ and Shafqat Ali*,§ †

Key Laboratory of Carbon Fiber and Functional Polymers, Beijing University of Chemical Technology, Ministry of Education, Beijing 100029, China ‡ Interdisciplinary Research Center in Biomedical Materials (IRCBM), COMSATS Institute of Information Technology, Lahore 54000, Pakistan § The Key Laboratory of Advanced Materials of Ministry of Education, School of Material Science and Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Designing an efficient photoanode for water splitting is a promising way to produce hydrogen fuel for a green and sustainable future. Herein, we reported a facile single-step fabrication of ZnOGaN:ZnO three-dimensional branched nanowire (3D B-NW) arrays photoanode for efficient visible-light-driven solar water splitting. The ZnO-GaN:ZnO 3D B-NW arrays were synthesized through chemical vapor deposition route via simultaneous growth of ZnO 1D NW arrays followed by the diffusion of gallium (Ga) and nitrogen (N) vapors into pregrown ZnO 1D NW arrays. The ZnO-GaN:ZnO 3D B-NW arrays were highly crystalline, and branches extended in all possible directions along the ⟨100⟩ orientation. The 3D geometry of the as-constructed photoanode offers supreme light harvesting ability, visible-light response due to N-doping, and enhanced electrical conductivity as a consequence of Ga insertion. A type II energy band diagram was evaluated for photogenerated electron−hole transport. The 3D B-NW arrays of ZnO-GaN:ZnO photoanode yield remarkably higher JPEC of 1.81 mA/cm2 leading to 37.12% enhanced photocurrent over ZnO 1D NW arrays photoanode. This growth route will open a new avenue for synthesis of 3D B-NW arrays of other semiconductors to improve the efficiency of solar-based devices. KEYWORDS: branched nanowires, ZnO-GaN:ZnO, photoanode, co-catalyst-free, water splitting

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lowering electron−hole recombination.9,10 Moreover, electron−hole recombinations are further mitigated, and resultantly charge utilization is enhanced significantly in the case of photoelectrode tandem design and type II band alignments.11 (2) Various co-catalysts can be decorated on photoelectrode surfaces for improving the kinetics of the water redox reaction.12−18 These advances have considerably improved the PEC water splitting efficiency. However, further efforts must be devoted to reduce the fabrication cost of the synthetic process for photoelectrode tandem design and suppress deactivation of co-catalyst in corrosive electrolyte. In tandem design, it is of great importance that the photoelectrode work efficiently without any assistance of co-catalyst. It relies on fabrication of highly efficient photoanode materials. Solid solution of bimetallic oxynitride (GaN:ZnO) has emerged as an attractive photocatalytic material for water

hoto-electrochemical (PEC) water splitting on semiconductor photoelectrodes has grabbed significant attention for the generation of the clean hydrogen (H2) fuel to overcome the energy and environmental crisis, simultaneously.1,2 Fundamentally, when sunlight illuminates semiconductor photoelectrodes, electrons and holes are generated possessing sufficient energy to carry out redox reaction to split water into O2 and H2. A long-standing issue with PEC water splitting is poor efficiency and the high cost of photoelectrodes fabrication.3−7 Basically, PEC water splitting efficiency has been restricted during any of the energy transferring processes, that is, photon absorption, electron−hole pair generation, and separation, and their utilization (redox reaction). Conversely, efficiency of energy transferring processes can be enhanced via the following. (1) Nanostructures of semiconducting materials8 can be programmed to enchance the processes, significantly through the use of three-dimensional branched nanowire (3D B-NW) arrays due to their enhanced surface area for the utmost absorption of incident photons, vectorial charge separation, and transport perpendicular to the charge collecting substrate for © XXXX American Chemical Society

Received: March 5, 2018 Accepted: May 22, 2018 Published: May 22, 2018 A

DOI: 10.1021/acsaem.8b00346 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials splitting19,20 as it exhibited several important properties, for instance, strong resistance against photocorrosion, visible-light absorption, appropriate band edge positions allowing overall water splitting with a small onset potential. Domen’s group, inventors of ZnO:GaN, have made massive efforts to fabricate stable and efficient GaN:ZnO photocatalyts20−24 and have demonstrated stable photocatalytic water splitting over a period of 6 months with the assistance of the co-catalyst (Rh2−yCryO3).25 However, it is still challenging to achieve even higher water splitting efficiency of GaN:ZnO than that reported for its sub-micrometer nanoparticles. Most likely lower efficiency is due to unwanted recombinations between electron−hole pairs as well as poor absorption by them. One possible approach to overcome these problems is fabrication of high crystal quality GaN:ZnO 3D branched nanostructure. Furthermore, the arrays of GaN:ZnO onto a conductive substrate favor the use of external bias to further enhance the charge separation and discourage their recombination at the electrolyte/GaN:ZnO interface for even higher water splitting efficiency. Herein, we report a facile single-step fabrication of conductive and visible-light-active 3D B-NW arrays of ZnOGaN:ZnO via a chemical vapor deposition (CVD) growth route. The as-constructed photoanodes demonstrated an efficient PEC water oxidation current of 1.8 mA/cm2 and higher solar to hydrogen efficiency (0.81%) during water splitting reaction without assistance of any co-catalyst. The ZnO-GaN:ZnO photoanodes compared to their counterparts (ZnO 1D NW arrays photoanodes) have exhibited enhanced water splitting which is attributed to their high surface area and insertion of Ga and N into a ZnO lattice for efficient absorption of visible light and reduced charge recombination due to type II band alignment in 3D geometry of NW arrays. The morphology of the as-prepared 3D B-NW arrays was investigated using field emission scanning electron microscopy (JEOL Model 6500). By adjusting the working voltage at 10 kV, we captured the image at our desired magnification. FESEM requires the use of an electrically conductive specimen to prohibit the electrostatic charge accumulation which can cause scanning faults. In our case, the NWs samples were conductive enough for proper operation of FE-SEM and we did not require coating of the metallic conducting layer to enhance the sample’s conductivity. To perform elemental analysis of the NWs’ samples, energy dispersive X-ray spectroscopy (EDX) coupled with FE-SEM was used at an accelerating voltage of 20 kV. Using this technique, atomic or weight percentages were found out for constituent elements at desired points through the NWs’ surfaces. Also, mapping of the lateral distribution of elements from the selected imaged area was performed. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were collected through a FEI Cscorrected Titan 80−300 kV microscope with an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were obtained through a Bruker D8 Advance apparatus using an operating current of 40 mA and a voltage of 40 kV to analyze the crystal structures of the as-prepared NW arrays photoanodes at a scan rate of 10°/min with monochromated Cu Kα X-radiation (λ = 1.54178 Å). A spectrophotometer (Lambda 750 UV/visible/NIR) was employed to collect the UV−visible diffuse reflectance spectra with wavelengths in the range of 250−800 nm at room temperature using BaSO4 as a reference. X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI Quantera SXM (ULVAC-PHI) apparatus

(Perkin−Elmer Co., USA) comprising Al Kα as an X-ray source for excitation (1.5 mm × 0.1 mm, 15 kV, 50 W) under a pressure of 4 × 10−8 Pa. XPS data were used to measure the chemical states, valence band (VB) position, and surface compositions of the samples. The PEC water splitting properties were investigated through a conventional three electrodes setup via current density measurements with well-known 0.5 M Na 2 SO 4 electrolyte solution prepared in NaOH solution at pH ∼ 13, analogous to conditions reported elsewhere.26 The total exposed photoanode area under solar-light irradiation was 0.2 cm2. We used platinum wire as counter electrode, Ag/AgCl (saturated KCl) as reference electrode, and the as-synthesized ZnO 1D NW and 3D B-NW arrays of ZnO-GaN:ZnO as working anodes. Electrolyte solution was purged with highpurity N2 gas and high-vacuum treatment prior to each measurement. A Xe lamp of 500 W was used as the illumination source. The power densities of the lamp were 140 and 450 mW/cm2 respectively during photocurrent density and solar to hydrogen efficiency measurements. IPCE was recorded at 0.8 V vs Ag/AgCl. These measurements were carried at room temperature with back side illumination of working electrodes. Finally, the potential values calculated against Ag/AgCl were transformed to reversible hydrogen electrode (RHE) by using the Nernst equation (eq 1). o VRHE = VAg/AgCl + 0.059pH + V Ag/AgCl

(1)

o V Ag/AgCl = (0.199 V)(25 °C)

The ZnO 1D NW arrays and ZnO-GaN:ZnO 3D B-NW arrays were fabricated through a CVD growth route. In a typical process, first of all growth of the ZnO 1D NW arrays was done on a glass coated conductive aluminum doped zinc oxide (AZO) substrate via CVD growth route in a two heating zone (Z1 and Z2) furnace according to modified conditions reported elsewhere.27 The as-grown ZnO 1D NW arrays were later used as reference photoanodes during water splitting measurements. Moreover, ZnO-GaN:ZnO 3D B-NW arrays were grown on a AZO substrate though a single-step process in the same CVD furnace (147 cm long) used for growth of ZnO 1D NW arrays. Our growth route is novel and shown in Figure 1a. The reaction was carried out in a one end closed tube (90 cm long) placed inside the CVD furnace. In a typical procedure, AZO

Figure 1. Experimental setup (a), growth of ZnO 1D NW arrays and ZnO-GaN:ZnO 3D B-NW growth via single-step CVD process (b), and scheme for electron and hole transfer during water splitting reaction (c). B

DOI: 10.1021/acsaem.8b00346 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials substrate was sonicated in acetone and isopropyl alcohol and rinsed with deionized water to remove organic pollutants. Two physical mixtures, (i) ZnO:graphite and (ii) GaN:ZnO, were used as precursors and placed at the center and near the end of the first heating zone, respectively (Figure 1a). The distance between the two precursor boats was kept at 5 cm while the distance between the GaN:ZnO precursor boat and the substrate was set at 8−9 cm. Ammonia (NH3) and argon (Ar) gases were used as a source of nitrogen and vapor carrier, respectively. The AZO substrate was placed downstream at 8− 9 cm away from second boat to collect the ZnO-GaN:ZnO 3D B-NW arrays product. Moreover, we have repeated several experiments to find out a proper position for the boat containing the ZnO:graphite precursor mixture in order to evaporate it first to fabricate ZnO 1D NW arrays followed by the evaporation of GaN:ZnO precursor mixture to get branches of GaN:ZnO in 3D B-NW arrays of ZnO-GaN:ZnO (Figure 1b). Also, such a process guaranteed the vapor-phase-diffusion reaction of both Ga and N into the as-grown ZnO 1D NW arrays.26 This special design/arrangement shown in Figure 1a allows us to fabricate ZnO-GaN:ZnO 3D B-NW arrays in a single-step process (Figure 1b). The temperatures of the first and second heating zones were raised to 1140 and 700 °C at the ramp up rates of 40 and 25 °C min−1, respectively. Initially, Ar at 30 cm3(STP) min−1 is given to carry ZnO vapors for 1D ZnO NW array fabrication until the temperature reaches 700 °C (Figure 1a, gray arrows). The NH3 and Ar flow was started at 50 cm3(STP) min−1, when the temperature of the first zone reached 1140 °C. The growth time was set for 30 min for branch fabrication and diffusion of Ga and N (Figure 1a, red arrow). After the completion of the reaction, the supply of NH3 and Ar was cut off at 800 °C (first heating zone temperature) and the CVD furnace was cooled naturally without opening the lid. The 3D B-NW arrays’ growth mechanism is still under investigation in our laboratory. During the growth process, very careful control of the fabrication conditions is obligatory to get visible-light-active single crystalline ZnO-GaN:ZnO 3D B-NW arrays on a large area. Different from the reported synthesis techniques for the ZnO:GaN sub-micrometer nanoparticles fabricated via annealing of ZnO powder and Ga 2 O 3 powder in an NH 3 environment, a single-step CVD growth route is used to fabricate single crystalline GaN:ZnO 3D B-NW arrays on asgrowth ZnO 1D NW arrays because of two reasons: (1) the atomic sizes of Zn and O are similar to those of Ga and N, respectively, and (2) both ZnO and GaN hold the same wurtzite crystal structure and analogous crystal lattice parameters. ZnO 1D NW arrays serve as a template providing a pathway for photogenerated charge transport, therefore attaining good conductivity for photoanode. GaN:ZnO acts as an anticorrosive and visible-light-active photocatalyst material for PEC water oxidation. The 3D NW arrays’ geometry tremendously increases the interface surface area at the photoanode/electrolyte junction for the endorsement of water oxidation during water splitting reaction. Moreover, ZnO and GaN:ZnO have suitable alignments of valence and conduction bands10,28 to thermodynamically increase the migration of photogenerated holes toward the nanowire/ electrolyte interface for water oxidation reaction (Figure 1c). Figure 2a exhibited the top view of a FE-SEM image of the successful as-grown ZnO-GaN:ZnO photocatalyst at large area, and 3D B-NW arrays were uniformly distributed over the AZO substrate. The nanobranches are grown in all possible

Figure 2. Top view SEM images over large area with inset for closer view (a), field emission SEM-EDS elemental spectrum (b), and elemental mapping (c) of a single 3D B-NW of the ZnO-GaN:ZnO shown in inset of panel a.

directions from pregrown ZnO 1D NW array (inset of Figure 2a). The average length and diameter of ZnO 1D NW were calculated to be about 10 and 1 μm, respectively, while the lengths of GaN:ZnO branches were around 50−60 nm. Such long 3D branched nanostructures enhance the surface area of the photoelectrodes by a factor of about 100, and compared with its planar film counterpart, promising a higher NW/ electrolyte contact area for PEC water splitting. Figure 2b exhibited the EDS spectrum of the 3D B-NW (shown in Figure 2a inset), having a curved configuration for all four constituent elements (Ga, Zn, N, and O). Herein, we observed a higher peak intensity for ZnO while a lower one for GaN revealing ZnO as the host material and GaN as the guest material in ZnO-GaN:ZnO photocatalyst. Quantitative analysis also reveals a higher ratio of ZnO over GaN (atomic %: ZnO, 64; GaN, 36). FE-SEM-EDS mapping (Figure 2c) of the selected area from the inset of Figure 2a clearly indicates concentrated spots for ZnO at the central part and light spots for GaN:ZnO at the peripheral part of the 3D B-NW of ZnO-GaN:ZnO. All of these findings reinforce our claim that ZnO evaporated first and formed ZnO 1D NW arrays followed by the formation of GaN:ZnO branches. Moreover, these ZnO-GaN:ZnO 3D BNW arrays are directly rooted in films of AZO coated substrate. This film composed of the same material underneath the NW arrays will provide a direct path for the electrons to flow, therefore, supporting the development of conductive NW arrays for an efficient photoelectrode. Figure 3 showed the microstructure and growth direction of the as-fabricated 3D B-NW arrays. Herein, the central trunk with the lattice spacing of 0.278 nm was grown along the ⟨100⟩ direction. The lattice spacing corresponds to a single hexagonal wurtzite crystal phase which supported our XRD results (Figure 5a). Also, the branches were grown along the ⟨100⟩ direction having lattice spacing of 0.273 nm. Moreover, the atoms were arranged regularly at the trunk−branch interface with lattice spacing of 0.275 nm. These findings indicated that the lattice spacing subsequently reduced from branch to interface and toward the bulk of the trunk. It is ascribed to a higher ratio of GaN in branches, and its contents decreased gradually from the branch surface to the bulk of the trunk. It is ascribed to the smaller ionic radius of Ga3+ (0.61 Å) rather than Zn2+ (0.74 Å), and the lattice constants of GaN (a = b = 3.19 Å; c = 5.19 Å) are also lower than those of ZnO (a = b = 3.25; c = 5.21 Å), which causes the bond distortion;29 therefore, the lattice spacing is varied.30 Furthermore, TEM observation depicted that the branches were extended along all the possible directions from the trunk (Figure 3, inset panel a). The FFT diffraction points at the interface reinforced the lattice matched C

DOI: 10.1021/acsaem.8b00346 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

binding energy peaks of the Ga, N, Zn, and O constituent elements of GaN:ZnO are identified in the XPS spectrum of the ZnO−GaN:ZnO 3D B-NW array sample. This is obvious evidence for the formation of GaN:ZnO solid solution on the surface of ZnO 1D NW arrays upon successful vapor phase doping of Ga and N in pregrown ZnO 1D NW arrays.26 Moreover, we also recognize the chemical state of the constituent elements of GaN:ZnO in ZnO-GaN:ZnO NW arrays. It is previously reported that Ga can be easily doped into ZnO crystal lattice forming ZnGa2O4 or Ga doped ZnO via CVD process.31 We further found that in the XPS spectrum (Figure 4a) peak positions of Ga 2p, Zn 2p, and O 1s were consistent with the chemical states of Ga3+, Zn2+, and O2−, respectively, evidencing the successful doping of Ga into ZnO. However, the case of N-doping in the ZnO crystal lattice has always been a challenging task that was successfully overcome through co-doping of N and Ga simultaneously via controlled CVD process, and consequently the bandgap of ZnO was reduced without affecting its electrical conductively. Figure 4b revealed the high-resolution Zn, O, and Ga peaks at 1022.2, 532.1, and 1118.3 eV, respectively; and these peak positions are well-matched with reported ZnO:GaN solid solution.24 However, in the deconvoluted N pattern, the binding energies were observed at 395−397 eV, which is in good agreement with the N3 chemical state in most of the metal nitrides, for instance, Zn−N (∼396.2 eV) and Ga−N (∼396.5 eV),24,32−34 where N atoms attained significant charge from metal atoms in the vicinity. It is also worth noting that N having chemical bonding in nitrate (binding energy of ∼406.8 eV), N2 diatomic nitrogen (∼404.9 eV), nitrite (∼404.5 eV), and N−H amines (∼399.6 eV) always rendering the optoelectronic quality of GaN:ZnO solid solution have not been detected in as-grown ZnOGaN:ZnO 3D B-NW arrays. Therefore, these data strengthen the fact of chemically doping or incorporation of Ga and N atoms rather than any physical bonding with ZnO. The crystal quality of the as-grown samples was analyzed through XRD characterization, which is herein favorable due to two major reasons: (1) we can determine crystal quality over large (centimeter) scale for as-prepared photoanodes, and (2) XRD is very precise in measuring small lattice mismatches, therefore revealing differences in individual diffraction peaks of ZnO and ZnO-GaN:ZnO samples that might be observed. Figure 5a showed characteristic diffraction peaks’ patterns for ZnO and ZnO-GaN:ZnO NW arrays samples indicating these samples are single crystalline with wurtzite crystal phase along the (002) out-of-plane orientation. Peak shifts were detected toward higher angle that is consistent with previously reported GaN:ZnO solid solution; that is, chemical doping of Ga and N into a ZnO crystal lattice decreases the lattice constant of ZnO and therefore peaks are shifted toward higher 2θ angle.25 Additionally, the crystallites’ size is inversely proportional to the full width at half-maximum (fwhm) of the XRD peak (Scherer’s formula) and therefore a small crystal produces border XRD peaks.35 In the as-fabricated ZnO-GaN:ZnO 3D B-NW arrays, the GaN:ZnO branches promised obviously smaller crystals (Figures 2a and 3), so their fwhm was broader than that for ZnO 1D NW arrays. Due to the broader fwhm or smaller crystal size of the branches, the overall crystallinity of the ZnOGaN:ZnO 3D B-NW arrays was increased. Ultravoilet−visible diffused spectroscopy was performed to elucidate the absorption response of samples, and results are shown in Figure 5b. Despite a direct or an indirect bandgap energy (Eg), herein we simply calculated bandgaps via

Figure 3. Microstructural investigation. HRTEM indicating the lattice spacing and growth directions of trunk and branch with TEM (inset a) and FFT pattern (inset b).

and single-crystal-like nature of the photocatalyst (Figure 3, inset panel b). A significant difference in the chemical states as well as constituent compositions in the ZnO and ZnO−GaN:ZnO NW arrays were proved by XPS data analysis. In Figure 4a, XPS wide-scan survey spectra are given for comparison of the ZnO 1D NW arrays and ZnO−GaN:ZnO 3D B-NW arrays. Herein, we observed the binding energy peaks of Zn and O in the XPS spectrum of pure ZnO 1D NW arrays.26 On the contrary,

Figure 4. XPS wide-range spectrum (a) and high-resolution spectra of Zn, O, Ga, and deconvoluted N peak (b) of ZnO-GaN:ZnO. D

DOI: 10.1021/acsaem.8b00346 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

(Figure 6). Figure 6a exhibited JPEC of the as-synthesized ZnO 1D NW arrays and 3D B-NW arrays of ZnO-GaN:ZnO

Figure 5. XRD (a), UV−visible diffused spectrum (b), XPS and calculated valence band position (c, d), energy bands diagram (e) of ZnO 1D NW arrays and ZnO-GaN:ZnO 3D B-NW arrays.

Figure 6. PEC water splitting and electron−hole transportation. Photocurrent density and dark current (a), photocurrent density and solar to hydrogen efficiency (b), IPCE (c), EIS (d), and PL (e) of ZnO 1D NW arrays and ZnO-GaN:ZnO 3D B-NW arrays.

extrapolation of the absorption edges.30 A near bandgap absorption edge at 383.9 nm (3.23 eV, black dotted line) by extrapolation of the absorption edges was observed corresponding to the bandgap of pure ZnO, whereas 3D B-NW arrays of ZnO-GaN:ZnO exhibited two absorption edges with an approximately 2:1 ratio of their intensities (Figure 5b red line). The first edge at about 409.2 nm (3.03 eV, blue dotted line) corresponds to the doping of N atoms into crystal lattice of ZnO causing a red shift in its absorption edge, while second at 500 nm (2.48 eV, red dotted line) is attributed to the bandgap absorption of GaN:ZnO solid solution. The visible absorption of the GaN:ZnO solid solution governed when the N 2p orbital occupies a state above the O 2p orbital or a strong repulsion between N 2p and Zn 3d orbitals.26 These absorption edge values are in good agreement with those of reported values for GaN:ZnO solid solution. Furthermore, extrapolation of the XPS edge is widely used to find the valence band (VB) position,30,36 while the position of the conduction band (CB) can be determined by subtracting the VB from Eg.28 Therefore, herein, we performed extrapolation of UV−visible absorption edge to calculate Eg (Figure 5b) and XPS to determine the VB (Figure 5c,d), while the position of CV with the energy band diagram is illustrated in Figure 5e. The VBs of ZnO and ZnOGaN:ZnO are positioned at 2.71 and 1.89 eV, while CBs are at −0.52 and −0.59 eV, respectively. Resultantly, a type II band alignment is obtained (Figure 5e). During water splitting, when sunlight is illuminated on the photoanode, the photogenerated holes were generated and transferred from the VB of ZnO to the VB of ZnO-GaN:ZnO and performed water oxidation (Figures 5e and 1c) whereas the photogenerated electrons moved from CB of ZnO-GaN:ZnO to the CB of ZnO and propagate to the Pt cathode for water reduction (Figures 5e and 1c). These findings are also matched to the previous literature.37 Moreover, the VB of ZnO-GaN:ZnO was 0.82 eV higher than that of the pure ZnO trunk, which has been attributed to the p−d repulsion of Zn 3d and N 2p; that is the main reason for bandgap narrowing.29,38 Finally, we evaluated water splitting performance, charge transportation, and recombinations of the as-fabricated NW arrays photoanodes

photoanode, without any assistance of co-catalyst. The JPEC of ZnO 1D arrays was 1.32 mA/cm2 at 1.23 V Vs RHE, while the ZnO-GaN:ZnO 3D B-NW arrays photoanode yields remarkably higher JPEC of 1.81 mA/cm2 leading to 37.12% enhanced photocurrent over that of the ZnO 1D arrays photoanode. This increase in JPEC of 3D B-NW arrays of ZnO-GaN:ZnO over ZnO 1D NW arrays is ascribed to a red shift or visible response of ZnO-GaN:ZnO. We know that the JPEC is the measure of Vbias and Jabsorb,39 where Vbias (V) is the applied external potential and Jabsorb is the number of photons absorbed through the surface of the photoelectrode. So, JPEC is directly associated with the number of photons absorbed. Meanwhile, a red shift describes the absorption of a large number of incident photons through the photoelectrode. For example, a semiconductor with a bandgap of 2.7 eV can absorb 5.8% of incident photons, while another one with a bandgap of 2.2 eV can absorb 15.3%.30 So, the later semiconductor attained a red shift of 0.5 eV which enhanced 2.6 times the absorption of incident photons.30 Thus, an observed red shift of 0.75 eV (3.23−2.48 eV) in the asfabricated ZnO-GaN:ZnO 3D NW arrays samples increased the fraction of photons absorbed by a factor of 3.9, and consequently more photogenerated electron and hole pairs will be produced. Consequently, the efficiency of the redox reaction will be increased resulting an improvement of JPEC. Moreover, the dark current value was zero along the whole applied potential range vs RHE. To further quantify the performance of these photoanodes, conversion efficiency, which is a valuable tool for insight analysis of the PEC properties’ nanomaterials, was evaluated at +0.4 V vs Ag/AgCl as a function of incident light. Figure 6b showed the conversion efficiency of 3D B-NW arrays photoanodes. A very low current was observed in the start of scans up to 0.37 V vs RHE; afterward a sharp increase was detected with bias ranging from 0.37 to 0.75 V vs RHE and ultimately reaches at plateau of 1.8 mA/cm2 at applied bias of 1.23 mA/cm2. The initiation of the anodic photocurrent at 0.37 V vs RHE points out the onset potential of the ZnO-GaN:ZnO photoanode in current−potentiometry measurements. A slow E

DOI: 10.1021/acsaem.8b00346 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

photogenerated recombination and prolonged lifetime owing to the Ga-doping and type II band alignment (Figure 5e). Additionally, ZnO-GaN:ZnO 3D B-NW arrays have assured a red shift that corresponded to more absorption in the visible region because of N-doping.29 These results also strengthen the red shift in UV−visible absorption results (Figure 5b). Compared to most of the oxynitride efficiencies reported in the literature, the high efficiency of 0.81% in the present study is ascribed to the development of the conductive and visiblelight-active photoanode comprising high surface area nanostructure. We believe further enhancement of efficiency through the as-fabricated photoanode could be achieved by surface decoration with efficient water oxidation co-catalysts, by further increasing water oxidation kinetics. In summary, we documented a facile single-step fabrication of ZnO-GaN:ZnO 3D B-NW arrays on a transparent and conductive substrate for efficient solar water splitting performance without decorating any co-catalyst. The single-crystal-like ZnO-GaN:ZnO 3D B-NW arrays were oriented along the ⟨100⟩ direction, and branches originated in all possible directions from the central ZnO 1D NW arrays. The considerable enhanced water splitting performance of the asconstructed photoanode attributed to the superior 3D branched geometry is due to the high surface area, enhanced conductivity, and good crystal quality of the as-fabricated oxynitride nanostructure. Nitrogen and gallium were chemically incorporated in the crystal lattice of ZnO while forming ZnOGaN:ZnO bimettalic oxynitride. The visible response of the ZnO-GaN:ZnO comes from incorporation of nitrogen that results in bandgap narrowing, while gallium incorporation enhances the conductivity. These advances result in more absorption in the visible region, better charge transportation, and lower charge recombination losses that elevate the performance of the photoanodes. Under standard sunlight irradiation, a photocurrent density of 1.8 mA/cm2 and a solar to hydrogen efficiency of 0.81% were achieved. This fabrication route can be extended for the growth of other powerful photocatalysts for highly efficient energy devices.

increment in the photocurrent at the applied bias of 0.37 V vs RHE involves sluggish kinetics for water oxidation reaction; that is, applied bias is close to the band potential of the ZnOGaN:ZnO photoanode.40 Most prominently, the photoexcited electron−hole pairs might recombine before their utilization at ZnO-GaN:ZnO photoanode below the applied bias of 0.37 V, and thereby mitigated photocurrent density was obtained. However, upon steadily increasing bias from 0.37 to 1.23 V a swift enhancement of photocurrent density from 0 to 1.8 mA/ cm2 was observed. This big increase in the photocurrent density is ascribed to the extreme enhancement in the water oxidation reaction at the interface of the photoanode/electrolyte. It seems that a significant amount of photogenerated holes might be transported across the photoanode/electrolyte interface and have carried out water oxidation reaction specifically at a higher positive bias. Notably, the plateau of the photocurrent density is an indication for saturation behavior, which reinforces effective photogenerated holes transport toward the photoanode/ electrolyte interface for their utilization instead of their nonsaturated photocurrent density behavior for electron−hole recombination. Such a saturation trend is a significant characteristic, suggesting a good quality ZnO-GaN:ZnO photoanode. Moreover, a continuous increase in photoceuurent density on increasing bias was observed that appeared as consequence of even lower electron−hole recombination. Moreover, we calculated the solar to hydrogen efficiency (η) from the current−potentiometry data by using eq 2: η = J(1.23 − VRHE)/Psunlight

(2)

wherein J presents the measured photocurrent density, VRHE is the applied voltage V vs RHE, and Psunlight is the incident irradiations. A maximum solar to hydrogen efficiency of 0.81% is attain with the ZnO-GaN:ZnO photoanode (Figure 6b) without loading of any co-catalysts. Moreover, we performed IPCE to understand the relationship between the photoanode capability to the harvest incident photons and the water oxidation photocurrent. The ZnO-GaN:ZnO 3D B-NW arrays photoanode showed enhanced IPCE over ZnO 1D NW arrays photoanode (Figure 6c). This can be attributed to the significant absorption of lower energy photons.28 However, ZnO 1D NW arrays depicted very low IPCE beyond λ ≥ 400 nm that ultimately approaches zero at higher absorption wavelength (Figure 6c) owing to the larger bandgap of ZnO or poor absorption in the visible region of the solar spectrum. We also examined the photogenerated charge carrier transport resistance through EIS with DC potential of 0.23 V vs Ag/AgCl under simulated AM 1.5G sunlight irradiations at a frequency range of 120 kHz to 120 mHz. Normally, the Nyquist plot’s (EIS data) semicircle diameter is associated with recombination losses and charge transportation resistance.10 Herein, Nyquist plots consisted of a single semicircle and its diameter is related to charge transport resistance at the photoanode/electrolyte interface. The ZnO-GaN:ZnO 3D BNW arrays photoanode depicted a smaller diameter that reinforced the lower recombination losses or efficient charge transportation compared to the ZnO 1D NW arrays photoanode (Figure 6d). The PL analysis is performed to provide insight into the charge carrier lifetime at an excitation source of argon ion at 488 and 300 nm of a tunable laser (Figure 6e). Basically, the PL peak intensity is related to the magnitude of the charge carrier lifetime or charge recombination losses.21 The PL intensity of ZnO-GaN:ZnO 3D B-NW arrays was lower than that of ZnO 1D NW arrays which showed their less

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-64421693. Fax: +86-10-6442-1693. ORCID

Shafqat Ali: 0000-0003-4603-1841 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We dedicate this research work to our 20th friendship anniversary (Dr. Yasir Abbas and Dr. Shafqat Ali) and Tsinghua University (Grant 2017660685).

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REFERENCES

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DOI: 10.1021/acsaem.8b00346 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX