A Coating Strategy for Coevolving Photocatalysis to Stabilise Visible-Light Absorbing Semiconductors

Semiconductors of narrow bandgaps and high quantum e�ciency have not been successfully utilised for coevolving photocatalysis despite the widely demonstrated protective coating schemes. Herein, we showcase a general strategy of using conformal coatings and cocatalysts energetic properties to transform CdS powders and GaInP 2 �lms into stable and e�cient photocatalysts for coevolution of H 2 and reversible redox couples. A scalable redox-mediated solar water-splitting reactor was constructed, regenerating the redox mediators while evolving O 2 in a separate compartment. Distinct from the single direction of charge transfer found with conventional photoelectrode stabilisation, the coating herein allows both photo-excited electrons and holes to spatially separate and inject simultaneously to the respective reductive and oxidative sites. With TiO 2 stabilisation, CdS particles produced H 2 continuously for 150 hours. Under simulated sunlight, solar-to-hydrogen (STH) e�ciency of 5.9% and 9.4% can be achieved for the CdS and GaInP 2 panels, respectively, by stacking multiple panels and matching the rate of redox regeneration to that of H 2 production.


Main Text
3][14] For example, cadmium sulphide (CdS) and gallium-indium-phosphide (GaInP 2 ) photoabsorbers have bandgaps of 2.42 eV and 1.87 eV, respectively, promising >10% STH e ciency. 1,15However, these semiconductors suffer from poor photo-stability in water, and their few-hour stability is much shorter than the thousands of hours operation by oxides. 5Hole scavengers leverage their fast and irreversible kinetics to compete against photooxidation. 16These scavengers provide the chemical bias to improve H 2 -evolution rates, but they cannot be regenerated to sustain continuous photocatalysis.
Existing coatings were shown to protect photoelectrodes for either reductive or oxidative reactions, but not for hosting both reactions simultaneously. 17Herein, we found the oxide coatings form a heterojunction, not a Schottky barrier, with the underlying protected semiconductors.Therefore, we developed a general approach to improve the stability and at the same time, to facilitate e cient charge separation and to transfer electrons and holes simultaneously during coevolving photocatalysis.Corroborated by both energetic measurements and numerical calculations, the charge separation was found driven by a semiconductor/coating/cocatalyst structure with locally varying barrier heights, rather than the speci c morphology or crystal facets of the photocatalyst.As illustrated in Figure 1a, CdS particulate lms and GaInP 2 epitaxial lms were coated by TiO 2 , the surfaces of which were subsequently loaded with Rh cocatalysts (see Methods).This simple two-step procedure transformed narrow-bandgap semiconductors into stable photocatalysts: the CdS/TiO 2 /Rh@CrO x panel continuously evolved H 2 at 50.4 μmol h −1 cm −2 , and showed an internal quantum yield (IQY) of 44.3% at 438 nm in a Na 2 S solution.
The GaInP 2 /TiO 2 /Rh@CrO x panel achieved an H 2 -evolution rate of 144.7 μmol h −1 cm −2 , equivalent to a photocurrent density of 7.8 mA cm −2 .While the TiO 2 surface oxidised the reversible sulphide/polysulphide (S 2− /S n 2− ) redox couples, the Rh-core and CrO x -shell, i.e.Rh@CrO x , cocatalysts evolved H 2 and suppressed the re-reduction of redox mediators.The CrO x shell also prevented performance degradation and sulphur poisoning of the Rh cocatalysts.

Stability of the coating-stablised photocatalysts
A lm of drop-casted CdS particles was encapsulated by TiO 2 via atomic-layer deposition (ALD) (Fig. 1a, inset).Then, the ALD TiO 2 was decorated with Rh@CrO x core-shell cocatalysts via photodeposition (see Methods).This cocatalyst-decorated coating not only suppresses photocorrosion of the CdS semiconductor but also allows coevolution of H 2 and reversible redox mediators.In this case, photoexcited electrons and holes simultaneously inject to the active sites of Rh cocatalysts and bare TiO 2 surfaces, respectively, to drive the hydrogen evolution reaction (HER) and sulphide oxidation reaction (SOR) along the photocatalyst-liquid interface.It is necessary to capture hole charges by the reversible redox mediators (A/A − ) to sustain overall water splitting, but it is also challenging due to their reversible kinetics.[20] CdS photocatalysts are known to rapidly degrade due to photooxidation of lattice sulphides, which forms soluble sulphates or insulating sulphur. 21Typical CdS photocatalysts were reported to evolve H 2 that lasted for only a few hours. 22,23Therefore, we tested the stability of TiO 2 -stablised CdS panel by measuring the cumulative amount of H 2 evolved in 50 mM Na 2 S solutions (pH 13.0) over time (see Supplementary Fig. S1, Note 1, and Video 1).As shown in Fig. 1b, the CdS/3-nm TiO 2 /Rh@CrO x panel produced 1-atm H 2 for 150 hours continuously.To simplify the testing procedure, the redox solution was replenished by a new Na 2 S solution every 8 -9 hours without disturbing the measurement.In comparison, the bare CdS panel produced H 2 at a much lower rate while the activity dropped to almost zero after 10 hours.The lack of TiO 2 coating made the CdS powder lm delaminate even during the Rh photodeposition.The X-ray photoelectron spectroscopy (XPS) Cd 3d and S 3p core-level spectra showed little changes after 150 hours (Supplementary Fig. S2).The absence of SO x signature suggested that the TiO 2 coating eliminated the primary failure mode of CdS photocorrosion.The H 2 -production activity decreased by 20% and 30% after the rst 100 h, and 150 h, respectively, which was not due to the loss of CdS particles (Supplementary Fig. S3 and Note 1). 10 In another compartment, (photo-)electrochemical redox-mediator reduction and water oxidation are paired with the H 2 photocatalysis to complete overall water splitting (see the Photocatalytic reactor Section).

Design for e cient charge separation and product coevolution
The introduction of coatings and cocatalysts improved the photocatalytic activity of CdS and GaInP 2 .
The commercial CdS powder lm showed a 1-atm H 2 -evolution rate of only 2.2 μmol h −1 cm −2 in a 100 mM Na 2 S solution.The rate increased by a factor of 1.3 and 5.8, respectively, after coating 3 nm of TiO 2 only and after loading 1 wt% Rh on the TiO 2 (Fig. 2a).Upon growth of CrO x shells on individual Rh nanoparticles, a signi cant boost for the activity to 50.4 μmol h −1 cm −2 was achieved.Similar improvement was observed with the GaInP 2 panel.With 3-nm TiO 2 and Rh@CrO x deposition, the n-type GaInP 2 lm evolved 1-atm H 2 at 144.7 μmol h −1 cm −2 in the Na 2 S solution, 11 times of the as-grown GaInP 2 (Fig. 2b).This H 2 -evolution rate was stable, while the bare GaInP 2 dropped to 28.8% of the initial rate within the rst 1.5 hours.To our knowledge, this is the rst demonstration of coevolving GaInP 2 photocatalysts.The equivalent photocurrent density of 2.7 and 7.8 mA cm −2 was achieved by the TiO 2stablised CdS and GaInP 2 panels, respectively.Besides S 2− /S n 2− , coevolution of H 2 and [Fe(CN) 6 ] 3−/4− redox couples by the CdS/TiO 2 /Rh@CrO x panel was also observed in a 50 mM K 4 Fe(CN) 6 aqueous solution at pH 7.0 (Supplementary Fig. S4), showing the versatility of this coating strategy.The rate can be further improved by tuning the coating energetics and making facile charge transfer to redox mediators in future studies. 24,25cient charge separation, leading to the simultaneous injection of electrons and holes to the liquid, can be realised through the varying barrier heights along the same photocatalyst-liquid interface.Cocatalyst deposition introduces two types of sites, i.e., semiconductor/TiO 2 and semiconductor/TiO 2 /Rh@CrO x , when in contact with the redox-mediator solution.We investigated their barrier-height energetics through light-intensity dependent OCP measurements by using CdS particle-assembled electrodes and GaInP 2 planar electrodes (see Methods). 26The ohmic back contact for the CdS and GaInP 2 electrodes allows for probing of the averaged electrochemical potentials of photo-excited electrons.Fig. S5a shows that as the light intensity increased, the OCPs of the CdS/TiO 2 electrode measured in a 10 mM Na 2 S solution (pH 12) became increasingly negative and reached a steady-state value of 0.34 V vs. RHE, denoted as OCP light .
The OCP light shifted to −0.09 V vs. RHE after loading the CdS/TiO 2 electrode with Rh@CrO x (Fig. 3a), indicating a reducing potential for the Rh@CrO x sites to drive H + reduction.The OCP light of the CdS/TiO 2 /RhCrO x electrode was consistent with the conduction band (CB) edge position of the CdS/TiO 2 sites, which was determined from the Mott-Schottky analysis independently (Supplementary Fig. S5b and Note 2).Therefore, both sites are able to coevolve HER and SOR, according to their band edge positions (Supplementary Fig. S6).
Purging the redox solution with H 2 resembles the local environment during H 2 -evolving photocatalysis.
When multiple charge transfer pathways coexist at the photocatalyst-liquid interface, e.g., a two-redox liquid junction, the OCP is the potential at the detailed balance of the kinetic rates for all the forward and backward charge-transfer pathways. 26Unlike the almost unaltered OCPs of the CdS/TiO 2 electrode after H 2 purging (Supplementary Fig. S5a), the OCP dark of CdS/TiO 2 /Rh@CrO x approached 0 V vs. RHE, while the OCP light remained at − 0.10 V vs. RHE (Fig. 3a).In other words, the potential of the Rh cocatalysts (inside CrO x shells) shifted upwards to align with E 0 (H + /H 2 ).Meanwhile, the CdS CB position stayed at − 0.10 V vs. RHE.Instead of a xed value, the barrier height at those CdS/TiO 2 /Rh@CrO x sites was reduced to 0.11 V, much smaller than the CdS/TiO 2 barrier height of 0.60 V (Supplementary Note 2).The variable barrier height of Rh nanoparticles is not speci c but rather general for many nanoparticulate cocatalysts: they were typically photodeposited over a hydrous or electrolyte-wetted TiO 2 surface, thus applicable to our coevolution coating strategy. 26,27Following the same procedure, we obtained electron barrier heights of 0.68 V and 0.14 V for the GaInP 2 /TiO 2 and GaInP 2 /TiO 2 /Rh@CrO x liquid interface, respectively (Supplementary Fig. S5c,d and Note 2).
Our strategy achieves local charge separation without leveraging the barrier-height difference speci c to crystal facets. 6,28Both photocatalysts feature a barrier-height difference, or asymmetric barrier energetics. 29As illustrated in Fig. 3b and 3c, the barrier-height difference induced an electric eld between the HER and SOR sites.This eld made photogenerated electrons transport to the Rh@CrO x sites for H 2 evolution, whereas holes transport to and accumulated at the bare TiO 2 surface to oxidise the redox mediator.The detailed calculation for the band diagram is explained in the Supplementary Note 2 and Fig. S7, and the various energetic parameters are summarised in Supplementary Table S1.
However, only decorating Rh cocatalysts but without regulating the re-reduction of redox mediators cannot achieve su cient barrier-height difference.The OCP light of CdS/TiO 2 /Rh electrodes, i.e., the Rh potential, was not raised to more negative potentials (Supplementary Fig. S8).As a two-redox liquid junction, 26 the Rh site could also transfer electrons to S n 2− /S 2− : this process was fast enough to compete with the desirable pathway of H + reduction.The resulting 0.45 V barrier height of the Rh sites was comparable to that of the CdS/TiO 2 sites.Therefore, this lack of energetic asymmetry adversely affected the charge separation e ciency for semiconductor/TiO 2 /Rh panels without the CrO x selectivity shell layer (Fig. 2).Besides, the selective Rh@CrO x cocatalysts ensure the high activities regardless in 1-atm Ar, air, or H 2 atmospheres (Supplementary Fig. S9).Such high selectivity against O 2 reduction or S n 2− reduction is consistent with the unchanged OCP light at ca. − 0.10 V vs. RHE for the semiconductor/TiO 2 /Rh@CrO x panels.The continuous H 2 production eventually replaces the headspace with 1-atm H 2 .Essentially, the observed activity trends highlighted the dual functionality of the Rh@CrO x cocatalysts: i) improving kinetics and ii) creating barrier-height asymmetry to facilitate charge separation.

Modelling of steady-state charge separation
So far, the HER and SOR sites have been proven to bear different barrier heights.Therefore, we choose the CdS photocatalyst to build a semiconductor model and to simulate a steady-state distribution of photo-excited electrons and holes in two dimensions, i.e., inside the CdS particle and along the TiO 2 -liquid interface.The materials and junction properties used are discussed in Supplementary Note 3 and listed in Table S2.The CdS/TiO 2 solid-solid interface is modelled as a heterojunction.The barrier-height energetics vary along the liquid junction dynamically during H 2 photocatalysis: for simplicity, xed Schottky barriers of 0.11-V and 0.60-V barrier height were set to simulate those HER and SOR sites, respectively.Their geometric parameters used in the simulation were estimated from Fig. S10 -S11.
The gradient of electron potential energy within the CdS indicated the steady-state electron-hole separation.A uniform carrier generation rate of ca.cm −3 s −1 was calculated under 1-sun illumination (see Methods).Fig. 3d shows that the electron potential energy reached the maximum at the SOR sites and the minimum at the HER sites.Driven by this potential gradient, the simulated electron and hole transport directions, indicated by the red and black arrows, respectively, were consistent with the 1D schematic in Fig. 3b.From the HER and SOR site at the liquid interface into the CdS bulk, ca.200 nm or 20% of the absorber volume was under an electric eld (Supplementary Fig. S12a − 12c).Within this volume, either the electron or hole current dominated the total currents (Supplementary Fig. S12d).Away from the surface, electron diffusion current became comparable to and eventually higher than the electron drift current (Supplementary Fig. S12e).This spatial distribution indicated a steady-state electron accumulation at the Rh@CrO x site (Supplementary Fig. S12f).Essentially, this diffusion-controlled charge separation is distinctive to the drift-dominated regime typically observed in solid-state energy conversion: 30 even a barrier-height difference much less than the bandgap enables e cient charge separation during coevolution.
Too thick TiO 2 coatings may reduce the hole injection rates, and thus, affect charge separation e ciency and photocatalytic activity.Experimentally, we observed that H 2 rates of the CdS photocatalyst decreased dramatically when the TiO 2 thickness reached 10 nm (Supplementary Fig. S13).This thicknessdependent behaviour should not always occur but was attributed to the energetic mismatch during hole transport from CdS to the redox mediator.Through > 3-nm thick TiO 2 , the hole hopping transport was slow as compared to the direct tunnelling transport, 31 because the Ti 3+ -defect band does not match with the energy level of CdS valence bands.In the modelling, different charge injection currents can be simulated by modulating the hole surface recombination velocity (SRV) at the 10-nm TiO 2 .The modelling outcome is consistent with the measured activity: reducing the hole SRV by 10 times decreased the photocurrent, or the H 2 -evolution rate, to a third of the 3-nm TiO 2 coated photocatalyst (Supplementary Fig. S13).The calculated effective resistivity of ~ 10 8 Ω for the 10-nm TiO 2 layer also agrees with the literature report. 31The low SRV led to signi cant hole accumulation up to 10 21 cm −3 at the 10-nm TiO 2 surface (Supplementary Fig. S14).Therefore, more carrier recombination occurred inside the CdS than with 3-nm TiO 2 coatings, reducing the photocurrents adversely.Insu cient hole transport that causes activity degradation is irrespective to CdS carrier lifetime or radiative e ciency.
Improving quantum yields of CdS photocatalysts With 3-nm TiO 2 , 1 wt% of Rh, and 1 wt% Cr loading, one CdS photocatalytic panel produced H 2 at a rate of 50.4 μmol h −1 cm −2 in a 100 mM Na 2 S solution (pH 13.5) (see Methods).We measured an apparent quantum yield (AQY) of 24.6% at 438 nm for the same panel in the 100 mM Na 2 S solution.Considering 55.5% of the light absorbed at that wavelength (Supplementary Fig. S15), the internal quantum yield (IQY) of the panel was calculated to be 44.3%.Stacking multiple panel devices can take full advantage of the incident light.With a three-panel stacking, the H 2 evolution rate increased to 90.6 μmol h −1 cm −1 , and the AQY reached 44.2%, approaching the IQY (Supplementary Fig. S16 and Video 2).This H 2 rate promises STH e ciency of 5.9% in a redox-mediated solar H 2 generator.
The numerical simulation also helped us discover the property that limited the measured IQY to be 44.3%.Experimentally, we obtained a CdS carrier lifetime of 0.1 ns (Supplementary Fig. S17a), which is short.Using the experimental lifetime, we introduced Shockley-Read-Hall recombination in the model (see Methods) to show that 44.0% of photo-excited electrons are collected as photocurrents.Varying the modelling parameter further showed the possibility of reaching 90% IQY, as long as the carrier lifetime of commercial CdS powders could be longer than 0.5 ns (Supplementary Fig. S17b).Such sensitivity to the carrier lifetime is consistent with the diffusion-controlled charge separation driven by a small barrierheight asymmetry.Therefore, increasing the carrier diffusion length or the depletion region volume is desirable (Supplementary Note 4), 29 and can follow various reported strategies. 32

Photocatalytic reactor
We constructed a solar-fuel generator, which allows for H 2 and O 2 evolution at two separated reactor chambers continuously, or at two separated times.As illustrated in Fig. 4a, a photocatalysis (PC) reactor was integrated with a photovoltaic-driven electrolysis (PVE).The reversible S 2− /S n 2− redox mediators shuttle photogenerated hole charges between the PC reactor and PVE device.A triple-junction (3-J) amorphous Si (a-Si) solar cell of 1.7 − 2.0 eV bandgaps absorbs the light transmitted through the CdS panel, to power the redox regeneration at a matching current. 33This proof-of-concept redox-mediated water-splitting reactor was constructed for simplicity. 34,35For the redox-mediator regeneration, the PVE component can be essentially replaced by a suspension or a panel of particulate photocatalysts: 36 e.g., the iron-, cobalt-, and iodine-mediated O 2 evolution by BViO 4 and WO 3 photocatalysts. 20,37,38More generally, the GaInP 2 /TiO 2 /Rh@CrO x panel can also operate inside the H 2 -evolving chamber when a GaInP 2 epi-layer is lifted-off from reusable GaAs substrates and mounted on a glass panel. 39The light transmitted through the GaInP 2 layer can drive redox regeneration.prevents the O 2 crossover, while replenishing the redox solution with the protons needed for H 2 evolution.
In the PC step, 115.3 μmol of H 2 was evolved under 1-sun illumination at 1 atm, equivalent to 22.2 C of charges received by the reversible S 2− /S n 2− redox mediators.In the second PVE step, the same amount of charge was passed to the electrolysis device.This step generated 57.7 μmol of O 2 , i.e., a 2:1 molar ratio of H 2 and O 2 .For this demonstration, the 25.6 μmol h -1 cm -2 rate of H 2 evolution under 1-sun illumination indicated STH conversion e ciency of 1.7%, equivalent to a photocurrent of 1.4 mA cm −2 .
The H 2 rate of 25.6 μmol h −1 cm −2 after two consecutive cycles was kept the same as in the rst cycle, and the potential of the redox mediator solution also returned to the original open-circuit potential after each PVE regeneration (Supplementary Note 5 and Fig. S18).Both indicated complete regeneration of the redox mediators.
The rates of oxygen evolution and redox regeneration by the electrolysis device are further scalable.Thus, it is also reasonable to assume that all remaining sunlight transmitted through the CdS panel can be utilised to power the S 2− /S n 2− regeneration at a rate matching to the H 2 production.In this case, the reactor should achieve STH conversion e ciency as high as 5.9% by stacking three CdS panels to maximise the light capture.Besides, the redox mediators buffer the photo-excited charges, such that the reactor operation can be exibly tailored by using short-term energy storage, even with intermittent solar irradiation or diurnal solar cycles.A similar water-splitting reactor based on the TiO 2 -stabilised GaInP 2 panel can pair with e cient O 2 -evolving photocatalysts, such as stabilised GaAs or Si, 29 S3).The current photocatalytic activity was limited by the intrinsic properties of CdS powders, not by the coating approach.E cient charge separation and simultaneous injection of both types of carriers were induced by the asymmetric barrier heights between the local HER and SOR sites, which were nanometres apart and distributed along the liquid-junction interface.This simple design is expected to initialise new development in water-splitting reactors, which produced stoichiometric and separated H 2 and O 2 gases.Future directions include (i) improving carrier lifetime of CdS particles, (ii) employing other reversible redox mediators and O 2 -evolving photocatalysts to optimise the redox regeneration processes, and (iii) improving hole transport through >10-nm coatings.

Methods
Chemicals and materials.CdS powders were purchased from Nanoshel LLC (Delaware, USA).N-type GaInP 2 (100) with 500 nm thickness and N p = 1 -2 x 10 Fabrication of 2 panels.CdS powders were treated by CdCl 2 before constructing photocatalytic panels.The CdCl 2 surface modi cation improved the optoelectronic properties of CdS.CdS powders (5 mg/mL) and CdCl 2 (10 mM) were mixed in methanol and sonicated for 10 min.Then the powders were dried by ltering through lter paper and evaporating the residue methanol at 80 ˚C to obtain CdCl 2 treated CdS.On a pre-cleaned (1 M HCl and DI water) frosted glass substrate (1.8x2.5 cm 2 ), 10 mg of CdCl 2 treated CdS powders were added with 30 μL of methanol to form a slurry.A uniform lm of powders was made via the roll press method with a glass rod.For the TiO 2 coating, CdS photocatalyst panels were placed in the ALD chamber and maintained at 150 ˚C.H 2 O and TDMAT were pulsed alternatively into the chamber as the oxidising and titanium precursors, respectively.Once the desired number of cycles were reached, the ALD process was terminated, and the TiO 2 -coated CdS panels were cooled down to room temperature for use.
Fabrication of GaInP 2 /TiO 2 panels.The commercial GaInP 2 of 500 nm was grown on lattice-matched n + -GaAs (100) wafers with electron concentration of 1 -2 x 10 17 cm -3 .A multi-layer metal contact was sputtered in the order of Ni/Au-Ge/Ag/Au with thickness of 5 nm/50 nm/50 nm/30 nm on the backside of the n + -GaAs.The substrate was annealed at 350 ˚C for 1 min to form ohmic contacts.A 2-min treatment in HF buffer was applied to remove surface oxide.Immediately following the HF treatment, the TiO 2 ALD is performed in the identical way as described above.Finally, copper wires were soldered to the back contact by indium and encapsulated by epoxy.
Photodeposition of Rh@CrO x on CdS/TiO 2 .For both photodeposition and photocatalytic reactions, we utilised a home-built photoreactor, which consists of a closed-loop glass manifold connected to a Gas Chromatograph (GC; SRI 8610C #3) via automatic sampling valves.The GC quanti ed the amount of gas produced with a molecular sieve column (MS-13X) and a thermal conductivity detector.Argon (Ar) was used as a carrier gas for the GC.The glass manifold was strictly sealed and airtight to ensure the vacuum level of 1x10 −2 Torr.A top-irradiation reaction vessel containing the photocatalyst and reactant solution was connected to the glass manifold with circulating cooling water to keep the reaction temperature at 10 ˚C.The system was degassed and purged with Ar to remove air; and a background pressure (Ar + water vapour) was adjusted to 100 Torr before each reaction.The irradiation at the sample was ~340 mW cm -2 generated from a 1000 W Mercury-Xenon Arc Lamp with an optical cutoff (λ≥ 395 nm).
For photodeposition of cocatalysts, a TiO 2 -coated CdS panel was placed in a degassed mixture of 8 mL of DI water and 2 mL of methanol with 1 wt% of Rh from Na 3 RhCl 6 .During the photodeposition, the amount of H 2 evolved was monitored by the GC every 30 min.The deposition was stopped after 1 -1.5 hours as the H 2 -evolution rate became steady-state.Following the Rh deposition, the CrO x shell was deposited in a similar approach, where 1 wt% of Cr from K 2 CrO 4 was added to the mixture of DI water and methanol.Photodepositon of CrO x typically lasted 2.5 -3 hours depending on when the H 2 -evolution rate became steady.The 1 wt% of Rh and 1 wt% of Cr were found to be the optimal loading condition that yielded the highest H 2 -evolution rate.
Electrodeposition Rh@CrO x on 2 /TiO 2. To deposit Rh, the GaInP 2 /TiO 2 photoelectrode was cyclically scanned three times from -0.3 V to 0.1 V vs. Ag/AgCl in an N 2 -purged aqueous Na 3 RhCl 6 solution (5 mg mL -1 ).Then the CrO x deposition was conducted in a 10 mg mL -1 K 2 CrO 4 solution in DI water by applying a constant potential of -1.0 V vs. Ag/AgCl for 5 hours under N 2 purging.
Photocatalytic reactions in the Na 2 S solution.For a typical reaction, a CdS/TiO 2 panel or GaInP 2 /TiO 2 photoelectrode loaded with Rh@CrO x cocatalysts was immersed in 10 mL of Na 2 S aqueous solution.An optimal activity was observed when the Na 2 S concentration was 100 mM (pH 13.5), which was then used for all rate measurements.The effect of Na 2 S concentration is still under investigation.The background pressure in the photoreactor was adjusted to be close to 760 Torr before the reaction began.A solar simulator (AAA-grade, Abet Technologies) with an AM 1.5 G lter was used as the illumination source.1sun illumination intensity was calibrated by a certi ed photodiode.
Quantum yield measurements.Photocatalytic H 2 evolution was performed twice with a 425 nm and a 450 nm long-pass lter (Edmund Optics) under AM1.5 G illumination, respectively.The corresponding H 2 evolution rates were obtained as r H2,425 nm and r H2,450 nm.The rate at 438 ± 12.5 nm was then determined by r H2,438 nm = r H2,425 nm -r H2,450 nm.AQY was calculated according to AQY = 2x r H2,438 nm xN A /A-I 438 /hv, where N A is Avogadro's number, A is the illuminated area, I 438 nm is light intensity at 438 nm, h is Planck's constant and is the photon's frequency at 438 nm.AM 1.5 G spectrum was used to calculate the I 438 nm by integrating the light intensity from 425 to 450 nm to count for spectral dependence.
Open-circuit potential (OCP) measurements.CdS powder-based photoelectrodes were made through the particle transfer method. 41 and 350 mM K 4 Fe(CN) 6 ) in the complete dark.The impedance was measured from 10 kHz to 1 Hz and tted with a Randles circuit to extract the capacitance of the liquid junction.In the Mott−Schottky plot, the intercept to the x-axis provided the at-band potential, and the slope was used to calculate the doping concentration of the semiconductor.
Photovoltaic-electrolysis (PVE) device.Flex-stak PEM fuel cell (purchased from the Fuel Cell Store) was utilised to assemble the PVE device.Both the anodic and cathodic compartment were composed of a plastic end plate, a graphite plate and a home-made acrylic frame.A CEM separated the two compartments.Pt on carbon cloth (the Fuel Cell Store) was used as cathodic catalysts.IrO x nanoparticles were synthesised and electrodeposited on carbon paper following a reported method. 42A 3-J a-Si solar panel placed in tandem with the photoreactor powered the PVE cell.Faradic e ciency measurements.Faradaic e ciency (FE) was measured in an airtight cell with a threeelectrode setup and two additional ports for carrier gas.The gas outlet was connected to the GC for analysing the gas products.Counter electrode (Pt wire) was separated from the cell by a Na on 117 membrane.Ar carrier gas was continuously purged through the cell to the GC.The ow rate was kept constant (10 sccm) using a mass ow controller (MFC), which was calibrated using a bubble meter.The FE was determined by the ratio between the produced O 2 and the amount of passed charges on the OER catalysts.Following this method, we measured a ~ 100% FE for the IrO x /C catalyst under chronopotentiometry at 1 mA.Simulation of photocatalyst electrostatics Using the COMSOL Multiphysics Semiconductor Module, a 2D model was built for the CdS/TiO 2 /Rh@CrO x structure.The CdS was modelled as a semi-sphere of 1 μm in radius, which was conformally coated with a 3-nm TiO 2 layer.The Rh/CrO x sites of 100 nm in length were distributed periodically on the surface.The length scale of Rh sites is such that their local barrier heights are not in uenced by CdS/TiO 2 surroundings. 43The energetics were simulated by solving the Poisson's Equation, drift-diffusion current, and continuity equations under open-circuit conditions.A heterojunction boundary condition was used for the CdS/TiO 2 interface.For liquid-junction interfaces, Schottky contacts were set for the TiO 2 -and Rh@CrO x -liquid sites with the barrier heights of 0.60 and 0.11 V for SOR and HER sites, respectively.The absorption coe cient of CdS is ~ 10 5 cm −1 .Light can penetrate ~ 1 μm deep in the CdS.As a simpli cation, a generation rate of cm −3 s −1 was introduced uniformly in the 1-μm CdS to mimic 1-sun illumination.Shockley-Read-Hall recombination processes were imitated by setting electron trap states with 0.25 eV below the conduction band of CdS 44 for the energy level and cm −3 for the trap density in the model.For the IQY simulation, the generation rate was reduced to cm −3 s −1 , corresponding to the light intensity of 438 ± 12.5 nm spectra.The surface recombination velocity at the TiO 2 -liquid junction was varied to achieve a thermionic current that matches with the experimental result.Materials characterisations.The SEM, AFM and TEM images were taken by a Hitachi SU8230 UHR system, a Cypher ES Environmental AFM system and an FEI Tecnai Osiris system (200 kV) equipped with EDS, respectively.X-ray photoelectron spectroscopy (XPS) measurements were conducted on the PHI VersaProbeII.Diffused re ectance spectroscopy (DRS) of CdS panel was obtained on a UV-visible spectrometer (UV-2600, SHIMADZU).Time-resolved photoluminescence (TRPL) was collected using timecorrelated single-photon counting (PicoHarp 300).The optical excitation was provided by a 480-nm pulsed laser, and the detection wavelength was centred at 550 nm with a 500 nm long-pass lter.

Fig. 4b shows
Fig. 4b shows a separated production of stoichiometric H 2 and O 2 by alternating the PC and the PVE processes.The amount of H 2 evolved was directly measured by an online gas chromatograph (GC), while the amount of O 2 was quanti ed based on the charge passed and the 100% Faradaic e ciency of IrO x catalysts (see Methods).The membrane-electrode electrolysis device consists of a Pt/C catalyst-loaded cathode and an IrO x /C catalyst-loaded anode.The cathode reduces S n 2− ions to regenerate S 2− , whereas

Figure 2 Charge
Figure 2

Figure 3 Local
Figure 3 500 nm of Ti and 200 nm of Au were sputtered onto the CdS lm as conductive back contacts.Carbon tape attached to a glass substrate was then used to peel off the CdS particles covered by the metal layers.The electrode was then loaded into the ALD tool for TiO 2 deposition.Finally, the electrode was completed by contacting copper wires with the carbon tape and encapsulated by epoxy.The OCP measurements were conducted on a Bio-Logic S200 potentiostat with Ag/AgCl used as a reference electrode and a carbon rod as a counter electrode.All OCPs of the CdS powder electrode and GaInP 2 electrode were measured in 10 mM Na 2 S solutions.