Design and properties of the photochemical diode

Schematically shown in Fig. 1a is the typical overall neutral pH water splitting on multi-band (GaN/InGaN) nanowire photocatalysts vertically aligned on the substrate (Si). Redox sites (and co-catalysts) in such axially symmetric nanostructures are randomly distributed on the surfaces. In contrast, gradient in p-type dopant (Mg) concentrations leads to a large work function difference (up to 300 meV) between the two parallel surfaces of a photochemical diode. The resulting p-p+ nanoscale lateral junction, schematically illustrated in Fig. 1b, induces unidirectional flow of photo-excited charge carriers, i.e., electrons and holes migrate toward the surfaces with a relatively small and large work function (Φ red and Φ ox ), respectively. Shown in Fig. 1a, the energy bandgap of the nanosheet structures can be further varied along the vertical direction, i.e. the photon absorption path. The resulting multi-band photocatalysts promise photocatalytic solar water splitting with the highest efficiency possible7,40. Figure 1b schematically illustrates the energy bands of the proposed InGaN nanosheet structures, which are grown directly on Si substrate using plasma-assisted molecular beam epitaxy (MBE) . During the epitaxy process, p-type dopants (Mg) are impingent primarily on one side of the nanosheet structure (see Methods). The resulting Mg-doping gradient along the lateral dimension of the nanosheet establishes a strong built-in electric field, schematically shown in Fig. 1b, which separates the photo-generated electrons and holes, and drives them towards opposite surfaces, thus reducing the probability of recombination. Figure 1c shows the typical bird’s-eye-view SEM images of InGaN nanostructures, which are comprised of axially asymmetric nanosheets with parallel non-polar surfaces. A simplistic view of the dynamic behaviors of charge carriers are depicted in Fig. 1d, which includes electron–hole pair generation upon photo-excitation, bulk recombination, carrier separation and migration towards laterally opposite direction. Consequently, the two catalytic surfaces are enriched with electrons and holes, respectively. The electron enriched surface (cathode) of the photochemical diode largely facilitates photo-deposition of proton reduction co-catalysts (Rh/Cr 2 O 3 core/shell nanoparticles) (see Methods), which in turn enhances the hydrogen evolution reaction (HER) significantly. Water oxidation reaction takes place on the hole enriched surface (anode). Details about conventional photochemical diode and junction engineering approach can be found in Supplementary Fig. 1 and Supplementary Note 1. One direct evidence for the efficient charge carrier separation and extraction of the presented InGaN nanosheet structures, compared to the conventional nanowires, is the significantly reduced photoluminescence intensity. Shown in the inset of Fig. 1e, the photoluminescence (PL) emission intensity of InGaN nanosheets is nearly 20 times smaller, compared to that of InGaN nanowires grown under similar conditions. Therefore, with significantly reduced charge carrier recombination, InGaN nanosheets are expected to exhibit noticeably higher photocatalytic activity than corresponding nanowire structures41 (Supplementary Figs. 2–3 and Supplementary Notes 2–3). By varying the epitaxy conditions, the energy bandgap of InGaN nanosheets, evident by the photoluminescence emission spectra, can be tuned over a large part of the visible spectral range, shown in Fig. 1e.

Fig. 1 Structural and optical properties of InGaN photochemical diode. a Schematic illustration of wafer-level unassisted photocatalytic overall water splitting on double-band nanowire arrays36, which are vertically aligned on a planar substrate and decorated with co-catalysts for hydrogen evolution reaction (HER). Unlike tandem PEC cells or photovoltaic (PV) devices66,67,68,69 this approach does not require any carrier recombination/transfer or current matching between the layers along vertical direction. Both water oxidation and proton reduction reaction occur on the radial non-polar surfaces of each layer. b Energy-band representation of the proposed photochemical diode (PCD) with radial thicknes “d” showing the built-in electric field (band-bending) that separates the charge carriers (electron and hole) and drives towards the opposite cathode and anode surfaces. In contrast to conventional p-n PCD (Supplementary Fig. 1 and Supplementary Note 1), only single photon absorption is required to generate one active electron–hole pair to participate in redox reaction (like Schottky-type photochemical diode). c A 45° tilted SEM image of InGaN:Mg PCD nanostructures, vertically aligned on Si substrate. Scale bar, 1 µm. The magnified image of the nanosheets is also presented in the inset for clarity. d Schematic (real space) depiction of the dynamic behaviors of charge carriers in a single-photon PCD upon photoexcitation. Electron enriched surface (cathode) of the PCD is largely decorated with photo-deposited HER co-catalysts (Rh/Cr 2 O 3 core/shell nanoparticles). e Room temperature photoluminescence (PL) spectrum from as-grown p-InGaN PCDs for different indium incorporations (correspond to different bandgaps, depicted using distinct colors). The inset shows ~20-fold reduction in PL intensity for the photochemical diodes compared to that of nanowires Full size image

Surface selectivity for oxidation and reduction

Scanning transmission electron microscopy (STEM) imaging, supported by the energy dispersive X-ray scanning (EDXS) analysis on p-type InGaN nanosheet photochemical diodes, decorated with Rh-nanoparticles, shows a significant difference in the number of nanoparticles loading between the two parallel surfaces. As shown in Fig. 2b, preferential photo-reduction of Rh-metal precursors to Rh-nanoparticles is facilitated on the reduction surface (cathode) due to its electron enrichment compared to that on the oxidation surface (anode) in Fig. 2a. Scanning transmission electron microscopy (STEM)-ZC/BF images further confirm Rh-nanoparticles’ deposition on the reduction sites of InGaN photochemical diode nanostructure (Supplementary Fig. 4). High-resolution STEM bright-field lattice image also depicts high crystalline quality of the defect-free In 0.22 Ga 0.78 N nanostructure surfaces, shown in Fig. 2c, d. For comparison, non-selective, rather random distribution of metal nanoparticles on the non-polar surfaces of conventional nanowires are also presented in Supplementary Fig. 5.

Fig. 2 Surface selectivity of InGaN photochemical diode for Rh-nanoparticle deposition. Comparison of STEM-SE and EDXS elemental mapping on two different surfaces of InGaN nanosheet (decorated with Rh-nanoparticles) shows that a very few Rh nanoparticles were deposited on the anode (outer) surface, whereas b significantly large number of Rh-nanoparticles get deposited on the cathode (inner) surface. Scale bars, 400 nm. HRSTEM-BF lattice fringe image from InGaN photochemical diode nanosheet, illustrating c defect-free single crystalline In 0.22 Ga 0.78 N anode surface, and d Rh nanoparticles on the crystalline cathode surface of photochemical diode. Scale bars, 5 nm. A radial density filter was used for Fig. 2c Full size image

To further gain a deep insight regarding the deviation in photo-deposition behavior, near-surface band-structure of as-grown p-InGaN nanowires and p-InGaN nanosheets were characterized using angle resolved X-ray photoelectron spectroscopy (ARXPS). Illustrated in Fig. 3a, the measured surface valence band maximum (E VS ) values between the two non-polar surfaces (relative to surface Fermi-level, E FS ) are significantly different, with E VS for the cathode surface being ~300 meV larger than that for the anode surface. This suggests the presence of a built-in potential ~300 meV (ΔE) along the lateral dimension of the nanosheet structure, as shown schematically in Fig. 1b. Subsequently, surface dependence of E VS was analyzed by measuring the valence spectra vs. radial scanning angle, α (Fig. 3b). Variations of E FS −E VS as a functional of scanning angle is illustrated in Fig. 3c for the entire range of α, further confirming the strong dependence of E VS on different surfaces. The sharp change in E FS -E VS (ΔE VS ) vs. scanning angle can be ascribed to the transition from one parallel surface to another, e.g., from anode to cathode surface, whereas the slow and gradual change (δE VS ) is attributed to the curvature and orientation of nanosheet arrays. In contrast, conventional InGaN nanowires exhibit nearly constant E VS at different scanning angles, also shown in Fig. 3c for comparison. Further comparative analysis between the surface potentials of nanowire and nanosheet structures is discussed in Supplementary Fig. 2 and Supplementary Note 3. A quantitative estimation for the band-diagram of InGaN nanosheet structures is shown in Supplementary Fig. 6a, which is derived from the XPS and TEM analysis performed on InGaN nanosheets with optimum bandgap for enhanced photocatalytic activity. It is evident that the large built-in potential leads to the spontaneous accumulation of electrons and holes on the cathode and anode surfaces, respectively36. This implies that the origin of preferential photo-deposition of noble metal nanoparticles on cathode surface of p-InGaN photochemical diode, as shown in Fig. 2a–d, is due to the reduction of noble metal precursors by photo-excited electrons enriched on that surface.

Fig. 3 Surface charge properties of In 0.22 Ga 0.78 N:Mg photochemical diode. a ARXPS valence spectrum for cathode and anode surface of p-InGaN photochemical diode nanosheets, depicting the offset in surface valence band maximum (E VS ) relative to surface Fermi-level (E FS ). b Schematic illustration of probing photochemical diode surfaces for valence spectra using ARXPS. Angles on the imaginary plane normal to c-axis (parallel to the substrate) are the radial scanning angles (α, clockwise), and “θ’“ denotes the angle of X-ray excitation relative to c-axis (See Methods). c E FS −E VS for Mg-doped In 0.22 Ga 0.78 N nanosheets and nanowire arrays, derived from ARXPS valence spectrum as a function of scanning angle. Periodic fluctuation is clearly observed for E FS position on the photochemical diode nanosheets relative to E VS . An error bar of ~±0.03 eV corresponds to uncertainties involved in measuring E VS and C 1s peak. d, 3D depiction of photochemical diode nanosheets with an arbitrary radial thickness ‘d’. Inner surface of the curved nanosheet is denoted as the cathode surface as per Fig. 2. e Neutral pH overall water splitting on the surfaces of photochemical diode nanostructures, presented schematically as a top view at the plane (X-X′) of cross-section in Fig. 3d. η a and η c represents the anodic and cathodic over-potentials for water oxidation and proton reduction reaction, respectively. With the directional (opposite) migration of electrons and holes, redox reactions can be coupled between parallel (cathode and anode) surfaces of vertically aligned adjacent photochemical diode nanosheets Full size image

It is worthwhile mentioning that anisotropic facet-dependent co-catalyst deposition had been reported previously to ensure spatial separation of oxygen evolution reaction (OER) and HER co-catalysts42,43,44,45,46,47,48, and thus to provide enhanced carrier separation in the near-surface region (Supplementary Note 4). However, bulk recombination remains a limiting factor for their low apparent quantum efficiency in water splitting. Unique to the presented photochemical diode nanostructure is the net lateral band-bending between two spatially separated redox surfaces. Water oxidation and proton reduction reactions occur at the two distinct reaction sites on photochemical diode nanosheets, and are coupled between the parallel anode and cathode surfaces49,50, schematically illustrated in Fig. 3d, e. Under concentrated sunlight, the band-bending can be reduced in the bulk, which can further lower the recombination probability by making the built-in electric field nearly linear and hence the flow/separation of charge carriers unidirectional (Supplementary Fig. 6b).

Characterization and performance analysis of double-band PCD

Double-band GaN:Mg/InGaN:Mg nanostructures were grown on Si wafer using plasma-assisted molecular beam epitaxy (See Methods)34,35,36. The nanosheet structures are vertically standing on the Si substrate, having an areal density in the range of ~1.5 × 1010 cm−2 (Fig. 1c). The photochemical diodes have an average height ~1.5–2 µm, and the thickness varies from ~50 to 120 nm. The PL spectra revealed an optical emission peak at ~485 nm, as shown in Fig. 4a, which can be attributed to InGaN bandgap of 2.56 eV. The average indium incorporation is estimated to be ~22% for the grown nanostructures. Detailed STEM and EDXS analysis confirms the existence of a continuous long InGaN segment, simultaneously showing the distribution of Rh/Cr 2 O 3 nanoparticles on the surface34,36 (Supplementary Note 5). The p-type behavior of Mg-doped crystalline In 0.22 Ga 0.78 N photochemical diodes is confirmed by photo-electrochemical characterization that includes open-circuit potential (OCP), Mott-Schottky and photocurrent measurements (Supplementary Fig. 7 and Supplementary Note 6).

Fig. 4 Enhanced STH efficiencies on double-band GaN:Mg/InGaN:Mg photochemical diodes. a Room temperature photoluminescence (PL) spectrum depicting optical emission peaks at ~365 nm (GaN) and at ~485 nm (In 0.22 Ga 0.78 N). The inset shows 15° tilted SEM image of the photochemical diodes. Scale bar, 1 µm. b H 2 evolution rate in overall neutral (pH ~ 7.0) water splitting for various photocatalyst samples under different excitation conditions. All the photocatalysts contain Rh/Cr 2 O 3 as HER co-catalyst, photo-deposited on the surface. Photochemical diodes provided two-fold enhancement in solar to hydrogen (STH) conversion efficiency compared to their nanowire counterparts. c Stoichiometric H 2 and O 2 evolution rate and the time course of overall water splitting, demonstrating balanced redox reaction and stability of nanowire photochemical diodes. d Comparative illustration of apparent quantum efficiency (AQE) and energy conversion efficiency (ECE) for different photocatalyst samples, derived under full arc using AM1.5 G filter (FA) and 400 nm long-pass filter (400LP) Full size image

The nanosheet arrays were tested for both hydrogen evolution reaction (HER) in aqueous methanol (CH 3 OH) solution, as well as neutral pH overall water splitting (OWS). A 300 W Xenon lamp was used as a concentrated irradiation source for photo-excitation, which has an intensity equivalent to ~32 suns when measured on the nanostructure substrate (Supplementary Fig. 8). Rh nanoparticles and Rh/Cr 2 O 3 core–shell nanostructures were photo-deposited as the co-catalysts for HER and OWS reactions, respectively. In the wavelength range of 200–485 nm (incident intensity of ~611 mW cm−2, see Supplementary Notes 7–8), stoichiometric gas evolution from neutral pH water splitting was measured at a rate of ~1.62 mmol h−1 cm−2 H 2 and ~0.784 mmol h−1 cm−2 O 2 , resulting in an AQE ~45.85%, which is more than two-fold higher than previously reported AQE of ~20% for double-band nanowire heterostructures36. Time evolution of photocatalytic hydrogen production from the photochemical diode arrays are shown in Supplementary Fig. 9. Under visible light irradiation (> 400 nm), the evolution rate was measured as ~0.5 mmol h−1 cm−2 for H 2 , and the AQE from the photochemical diode nanosheets (nanowires) was estimated to be ~19.93% (12.3%).

Evidently, significant enhancement in overall photocatalytic water splitting activity had been derived from photochemical diode nanosheets compared to that from nanowire heterostructures. For comparison, the amount of hydrogen evolution from the photochemical diodes is increased by more than a factor of two using full arc illumination with AM1.5 G filter. This, in turn, enhanced the energy conversion efficiency (ECE) from ~7.5 to ~17.5%. Moreover, an impressive ~3.3% of solar-to-hydrogen conversion efficiency has been measured in this study, which is significantly higher than that estimated from dual-band nanowire structures, as depicted in Fig. 4b. Repeated cycles for the stoichiometric hydrogen and oxygen evolution in neutral pH water splitting using AM1.5 G filter are demonstrated in Fig. 4c (Supplementary Movie 1 and 2). Illustrated in Fig. 4b, d are the comparative study of hydrogen evolution and corresponding AQE and ECE from neutral pH overall water splitting under full arc illumination using AM1.5 G optical filter and under visible light irradiation using a 400 nm long-pass optical filter. The photochemical diode nanostructures remain stable after the photocatalytic reactions, and negligible signs of degradation was observed after ~4 h of overall neutral pH water splitting and hydrogen evolution reaction from aqueous methanol solution (shown in Supplementary Fig. 10). The stability of the co-catalyst nanoparticles on photocatalyst surface was further confirmed from TEM analysis.