Z-scheme electron transfer mode in natural photosynthesis is well-known for combining two photosynthetic systems and separating the oxidation and reduction reactions spatially to avoid undesirable charge recombination [1-5]. Learning from nature's Z-scheme, the artificial Z-scheme solar water splitting system, combining two photocatalysts responsible for the independent oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) through an electron transfer assisted by electron mediators, has been extensively studied for the solar hydrogen production [6-9]. As the redox potential of the shuttle mediator is located between the conduction band (CB) of the OER photocatalyst and the valance band (VB) of the HER photocatalyst, the forward electron transfer reaction is easily hindered by the redox reactions (so-called "backward reactions") of the shuttle mediators, resulting in a lower efficiency of the solar-to-hydrogen conversion [10-14]. Avoiding the backward reaction therefore becomes one of the crucial issues in constructing an efficient Z-scheme water splitting system. By making channels for gas production and diffusion [10], constructing selective cocatalysts [11], and modifying the surface of photocatalyst [13], the backward reactions are inhibited to some extent; however, such approaches are still limited.

The redox couple Fe(CN)₆^4-/3- is an efficient electron mediator frequently employed in photosystem Ⅱ (PSII)-integrated biotic-abiotic hybrid Z-scheme water splitting systems, which have received much attention owing to the application of the excellent natural water oxidation enzyme (PSII) in solar energy conversion, and also for the research on the basic scientific issues in water splitting [15-17]. Fe(CN)₆^3-, as an efficient exogenous electron acceptor of PSII, is capable of extracting the photogenerated electrons from the reducing side of PSII during the light-driven water oxidation process without triggering any undesirable side reactions, and the produced Fe(CN)₆^4- then transfers the electrons to the artificial component for water reduction. However, the backward reactions caused by the shuttle mediators have also been encountered in the Fe(CN)₆^4-/3--mediated biotic-abiotic hybrid systems. Even though the OER is almost unaffected owing to the highly selective active sites of PSII for water oxidation and Fe(CN)₆^3- reduction [18-20], the inhibition of HER on the semiconductor photocatalyst happens easily, which limits the usability of the combination of PSII and artificial materials for solar water splitting.

In the present study, ZrO₂/TaON was employed as the HER photocatalyst to construct a PSII-ZrO₂/TaON hybrid Z-scheme water splitting system, and PtCrO_x particles were loaded on ZrO₂/TaON as HER cocatalysts. We found that the backward reaction of hydrogen oxidation by Fe(CN)₆^3- can be catalyzed and accelerated remarkably on the metallic Pt cocatalyst; however, it was not catalyzed on the oxide PtCrO_x due to a weak adsorption and activation of H₂. The higher oxidation states of Pt^Ⅱ and Pt^Ⅳ in PtCrO_x were well-sustained during the HER, and might be stabilized by the coordination of the surrounding CrO_x species that protect Pt^Ⅱ and Pt^Ⅳ from being reduced into metallic Pt even on aggregation of the photogenerated electrons. Therefore, a long-term hydrogen evolution took place on PtCrO_x-loaded ZrO₂/TaON, with Fe(CN)₆^4- as the electron donor. Finally, Z-scheme overall water splitting was achieved successfully through the Fe(CN)₆^3-/4--assisted electron transfer from PSII to PtCrO_x-loaded ZrO₂/TaON. This work paves a new way to inhibit the backward reaction caused by the electron mediator, Fe(CN)₆^3-/4-, for an efficient electron transfer between PSII and the artificial photocatalyst in the Z-scheme water splitting system.

ZrO₂/TaON was synthesized as reported previously [21]. Typically, Ta₂O₅ and ZrO(NO₃)₂ were mixed with a 0.1 molar ratio of Zr/Ta in a small amount of methanol, dried at 70 ℃ for 1 h, and subsequently heated in air at 800 ℃ for 2 h to yield a ZrO₂/Ta₂O₅ composite. ZrO₂/TaON was obtained by nitridation of the as-prepared ZrO₂/Ta₂O₅ powder at 900 ℃ under a flow of NH₃ (20 mL min^-1) for 20 h.

Metallic Pt particles (1 wt% Pt) and the mixed oxide, PtCrO_x (1 wt% Pt, Cr: according to the different Cr/Pt mass ratios), were loaded on the surface of ZrO₂/TaON by photodeposition. ZrO₂/TaON (0.2 g) was dispersed in a 100 mL aqueous solution containing 20 vol% methanol as the electron donor, to which the calculated volume of concentrated (NH₄)₂PtCl₆ and K₂CrO₄ solution was added as precursors. The photodeposition process was carried out under the full-spectrum irradiation of a Xenon lamp for 6 h at 15 ℃ in a Pyrex-top irradiation type reaction vessel connected to a closed gas circulation system. After photodeposition, the resultant powder was washed with distilled water and dried overnight at 65 ℃ in an oven.

ZrO₂/TaON/FTO electrodes were prepared by the electrophoresis method. ZrO₂/TaON powder (20 mg) was dispersed in 25 mL acetone containing 15 mg I₂ under an ultrasonic treatment for 10 min. FTO plates (1 cm × 2 cm) were used as substrates after they were ultrasonically washed in acetone, isopropanol, ethanol, and water successively. Two FTO plates were connected to a two-electrode system with their conductive layers face to face, and the as-prepared ZrO₂/TaON/I₂ solution was used as the electrolyte. A pulsing voltage of 20 V was applied 10 times, each time lasting for 10 s with an interval of 10 s, upon which, ZrO₂/TaON powder got deposited onto the FTO anode. The electrode was dried at 65 ℃ in an oven for 2 h. The cocatalysts, Pt and PtCrO_x, were then photo-deposited onto the ZrO₂/TaON/FTO surface. First, the ZrO₂/TaON/FTO plate was placed at the bottom of a beaker and 25 mL (NH₄)₂PtCl₆-methanol mixed aqueous solution, with the mass of the Pt element being 0.25 mg, or 25 mL (NH₄)₂PtCl₆/K₂CrO₄-methanol mixed aqueous solution, with the mass of the Pt and Cr elements being 0.25 and 0.125 mg, respectively, was added for the respective deposition of Pt or PtCrO_x. The volume proportion of methanol in the mixed solutions is 20%. Photodeposition was carried out under the light of a Xenon lamp (λ ≥ 420 nm) for 4 h at 15 ℃. The electrode was carefully washed with distilled water and then dried at 65 ℃ in an oven for 2 h.

PSII core complex was isolated from the thermophilic cyanobacterium T. vulcanus as described previously [22, 23], and finally dispersed in an MES solution (30 mM MES, 20 mM NaCl, 3 mM CaCl₂, pH = 6.0). The concentration of Chl a (denoted as [Chl a]) in the PSII sample was obtained according to the equation [Chl a] = A665 × 13.4 × dilution factor, in which A665 is the UV-Vis absorbance of the sample at 665 nm in methanol [24]. [Chl a] was employed for calculating the quantity of the PSII centers by using 35 Chl a/PSII [25]. Steady-state O₂ evolution of the prepared PSII sample was measured with a Clark-type electrode at 30 ℃ with 0.5 mM phenyl-p-benzoquinone and 0.5 mM K₃Fe(CN)₆ as the electron acceptors, and the O₂ evolution rate exceeded 3000 μmol O₂ (mg of Chl)^-1 h^-1 under a saturated illumination.

Morphologies of the cocatalysts were examined by high resolution transmission electron microscopy (HR-TEM, JEOL JEM-2000EX) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM, JEM-ARM200F). Electronic states of the cocatalysts were examined by X-ray photoelectron spectroscopy (XPS) measurements on a VG ESCALAB MK2 spectroscope under a monochromatized Al Kα excitation.

For the photocatalytic half reactions and the overall water splitting reactions, a homemade airtight vessel with a top quartz window was utilized. Before irradiation, the vessel containing the reaction solution was connected to a vacuum/inert gas line to replace the air in the headspace with the Ar gas, and then placed in a thermostatic water bath with the temperature kept at 25 ℃ for the subsequent photocatalytic reaction. The gas produced during the reaction was monitored by injecting the headspace samples into a gas chromatography instrument (Agilent GC 7890, USA, 5A zeolite column and Ar carrier gas).

The dark backward reaction between hydrogen and Fe(CN)₆^3- was carried out in the same homemade airtight vessel. For each reaction, either no photocatalyst or 40 mg of it was dispersed in 20 mL of the PBS buffer (pH = 6) containing 2 mM Fe(CN)₆^3-. The vessel containing the reaction solution was filled with Ar gas and sealed when quantitative hydrogen gas was injected in for the backward reaction; however, it was not sealed when the reaction solution was bubbled with hydrogen gas.

The electrochemical performances of the ZrO₂/TaON/FTO and the cocatalyst-loaded ZrO₂/TaON/FTO electrodes were measured in a three-electrode setup, where Pt electrode and the saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. The PBS buffer (100 mM phosphate, 15 mM NaCl, pH = 6) containing 5 mM Fe(CN)₆^3- was used as the electrolyte. Linear sweep voltammetry (LSV) was carried out in the range of 0.2 to -1.2 V vs. SCE at a scan rate of 50 mV s^-1 on a CHI 760D electrochemical workstation (Shanghai Chenhua Instruments, China). Electrochemical impedance spectroscopy (EIS) was carried out at the potential of -0.2 V vs. SCE, with the AC potential frequency ranging from 0.1 Hz to 200 kHz on a PARSTAT 2273 workstation (Princeton Applied Research).

ZrO₂-modified TaON (ZrO₂/TaON), with the band gap of 2.5 eV, has been previously demonstrated as a good photocatalyst for hydrogen production [26-28]. In the present study, ZrO₂/TaON was employed to collaborate with the natural PSII core complex to construct a bio-artificial Z-scheme water splitting system. The redox mediator couple Fe(CN)₆^4-/3- was used to transfer the electrons from PSII to the semiconductor photocatalyst. Pt and PtCrO_x particles with different Cr/Pt mass ratios were loaded on ZrO₂/TaON as cocatalysts through the photodeposition method described above. Fig. 1(a) depicts the time courses of hydrogen evolution on the cocatalyst-loaded ZrO₂/TaON samples. Hydrogen evolution on the Pt-loaded sample was negligible, whereas the activities increased notably on the PtCrO_x-loaded samples, reaching a maximum value at a Cr/Pt ratio of 0.5. Furthermore, the PtCrO_x (Cr/Pt = 0.5)-loaded sample was able to display a long-term hydrogen evolution for 9 h until the electron donor, Fe(CN)₆^4-, was consumed completely (Fig. 1(b)). As a control experiment, when only CrO_x was loaded on ZrO₂/TaON without addition of the Pt precursor during photodeposition, no hydrogen was detected in the HER with Fe(CN)₆^4- as the donor on this Pt-absent sample, which indicates that the Pt species in PtCrO_x is indispensable for hydrogen production.

To investigate the relationship between the photocatalytic activity and the composition and structure of the cocatalysts, the electronic states and morphologies of the Pt and Cr species on ZrO₂/TaON were characterized by XPS and HR-TEM. Fig. 2(a) shows the XPS spectra of Pt 4f of the as-prepared samples with different Cr/Pt ratios. For the Pt-only loaded sample (Cr/Pt = 0), the spectrum is mainly attributed to the Pt⁰ species at the binding energies of ca. 70.2 (4f_7/2) and 73.6 eV (4f_5/2), and the small fitted peaks centered at ca. 72.3 (4f_7/2) and 75.7 eV (4f_5/2) are attributed to the Pt^Ⅱ species. With the addition of the Cr species, the XPS peaks due to Pt^Ⅱ and Pt^Ⅳ (ca. 74.5 eV for 4f_7/2, 77.9 eV for 4f_5/2) became dominant, whereas those due to Pt⁰ decreased remarkably. There is a gradual change in the spectra with the increasing Cr/Pt ratio; the Pt⁰ peaks at the low binding energy disappear, whereas the Pt^Ⅳ peaks at the higher binding energy grow up, until reaching a stable proportion of Pt^Ⅱ and Pt^Ⅳ after the Cr/Pt ratio is higher than 0.5. The Cr 2p spectra in all of the PtCrO_x-loaded samples show the same peaks at the binding energies of ca. 576.5 and 586.4 eV, corresponding to the 2p_3/2 and 2p_1/2 electrons, respectively, of the Cr₂O₃ species (Fig. S1). Accordingly, it could be speculated that the chemical state of Pt was regulated by the CrO_x species during the photodeposition process, and the HER activity of PtCrO_x-ZrO₂/TaON is sensitive to the chemical state of Pt.

XPS spectra after the HER with Fe(CN)₆^4- as the electron donor were also recorded. For Pt-ZrO₂/TaON, the proportion of the Pt peak at the high binding energy increased slightly after the photoreaction (Fig. S2(a)), probably due to the adsorption of hexacyanoferrate on Pt through the highly polar CN^-, as also found by some researchers that a surface species containing Pt-CN was formed after the redox reaction of Fe(CN)₆^3-/Fe(CN)₆^4- on the Pt electrode [29-31]. For PtCrO_x(0.5)-ZrO₂/TaON, the electronic states of Pt and Cr were well-sustained after the photoreaction (Fig. S2(b)), indicating the stable configuration of PtCrO_x during the proton reduction process, which is consistent with the stable HER performance. It is well-known that the photogenerated electrons aggregate on the HER cocatalyst for proton production, but the high oxidation states of Pt^Ⅱ and Pt^Ⅳ in the PtCrO_x cocatalyst could still be well-sustained without being reduced into metallic Pt during the HER process. The coordination function of the CrO_x species to Pt^Ⅱ and Pt^Ⅳ might contribute to the good stability of PtCrO_x.

Morphologies of the cocatalysts were observed by HR-TEM. In the absence of Cr, metallic Pt particles with a diameter of ~5 nm and a lattice fringe of Pt (111) were observed on the ZrO₂/TaON surface (Fig. 2(b)). Upon addition of Cr, amorphous PtCrO_x clusters without any distinct lattice fringe formed on the substrate, with a size of ~2 nm, much smaller than the Pt particles (Fig. S3). It was noted that on the sample with a low Cr/Pt ratio of 0.2, a spot of the metallic Pt particles could still be found (inset of Fig. S3(a)), in agreement with the small proportion of the Pt⁰ XPS peaks. This suggests that during the photodeposition process, the redundant Pt precursor that did not form PtCrO_x was finally reduced into metallic Pt particles. Considering the limitation of resolution of HR-TEM, the small PtCrO_x particles on PtCrO_x(0.5)-ZrO₂/TaON were further confirmed by HAADF-STEM, and small bright dots were found to distribute in the PtCrO_x clusters, which are considered as the Pt species with a heavy atomic weight (Fig. 2(c)). Compared with the metallic Pt particles, the PtCrO_x clusters are able to provide more active sites for the proton reduction due to the larger surface area of smaller particles.

When Fe(CN)₆^4- acts as an electron donor in the HER, hydrogen evolution is accompanied by the production of Fe(CN)₆^3-. The forward activity might be hindered by the backward reactions shown in Eqs. (1) and (2).

(1)

(2)

It can be noted that both the backward reactions involve the reduction of Fe(CN)₆^3- and ultimately affect hydrogen production. In the case of Eq. (1), Fe(CN)₆^3- competitively reacts with the photogenerated electrons which are expected to reduce protons for hydrogen production, while in the case of Eq. (2), the produced hydrogen gas might be consumed due to hydrogen oxidation by Fe(CN)₆^3-.

Some control experiments were carried out to investigate the backward reactions in the HER. Hydrogen evolution on Pt-ZrO₂/TaON with methanol as an electron donor was measured both in the absence and presence of Fe(CN)₆^3-. As shown in Fig. S4(a), the activity declined by seven times after adding 1 mM Fe(CN)₆^3-, and Fe(CN)₆^3- was reduced to Fe(CN)₆^4-, as indicated by the UV-Vis absorption spectrum (Fig. S4(b)). Subsequently, the decline in hydrogen evolution on PtCrO_x-ZrO₂/TaON as a function of Fe(CN)₆^3- concentration was investigated. To avoid light being blocked by the Fe(CN)₆^3- solution, the lamp was equipped with a long-pass filter (λ ≥ 480 nm). As shown in Fig. S5, among the three samples with different Cr/Pt mass ratios (0.2, 0.35, and 0.5), PtCrO_x(0.5)-ZrO₂/TaON showed the slowest activity decline along with the increasing Fe(CN)₆^3- concentration, while hydrogen production on PtCrO_x(0.2)-ZrO₂/TaON, which contained a spot of the metallic Pt cocatalyst, was almost quenched in 5 mM Fe(CN)₆^3-. These results indicate that the presence of Fe(CN)₆^3- indeed lowers the hydrogen production activity, especially for the photocatalyst loaded with metallic Pt. PtCrO_x(0.5)-ZrO₂/TaON containing only the Pt^Ⅱ and Pt^Ⅳ species shows the best capability to block the backward reaction.

Furthermore, the redox reaction between hydrogen and Fe(CN)₆^3- was investigated in the dark in the presence and absence of photocatalysts. The corresponding changes in the amount of hydrogen and the Fe(CN)₆^3- concentration are shown in Fig. 3(a) and Fig. S6, respectively. It was found that both, the amount of hydrogen and the concentration of Fe(CN)₆^3-, remained unchanged if there was no photocatalyst in the solution, indicating that the redox reaction did not take place. Even in the presence of PtCrO_x-ZrO₂/TaON (PtCrO_x-ZrO₂/TaON hereafter represents the sample with a Cr/Pt mass ratio of 0.5, unless otherwise noted), the redox reaction did not take place. However, in the presence of Pt-ZrO₂/TaON, the amount of hydrogen decreased obviously and Fe(CN)₆^3- was transformed into Fe(CN)₆^4-. To ensure sufficient contact between hydrogen, Fe(CN)₆^3-, and the photocatalyst, hydrogen was bubbled into the reaction solution. Similarly, as shown in Fig. 3(b), Fe(CN)₆^3- remained unchanged in the absence of the photocatalyst or in the presence of PtCrO_x-ZrO₂/TaON, whereas almost all Fe(CN)₆^3- in the solution was reduced in 15 min in the presence of Pt-ZrO₂/TaON. These results indicate that the kinetically controlled backward reaction of hydrogen oxidation by Fe(CN)₆^3- was remarkably accelerated via the catalytic action of the metallic Pt particles, resulting in the remarkable hydrogen loss in the HER. On the contrary, PtCrO_x oxide did not catalyze this backward reaction; thus, hydrogen could evolve even in the presence of Fe(CN)₆^3-.

Subsequently, surface interactions between the cocatalysts (Pt and PtCrO_x) and the reactants (Fe(CN)₆^3- and hydrogen) were investigated to understand their different catalytic behavior. Zeta potentials of Pt-ZrO₂/TaON and PtCrO_x-ZrO₂/TaON in the reaction buffer solution (PBS buffer, pH = 6) were found to be about -24 and -19 mV, respectively, suggesting that there are no electrostatic attractions between Fe(CN)₆^3- ions and the electronegative photocatalyst surface. The adsorption tests showed that there is no chemical or physical adsorption of Fe(CN)₆^3- on the surface of both Pt-ZrO₂/TaON and PtCrO_x-ZrO₂/TaON. Moreover, it has been well-convinced that hydrogen molecules can be chemisorbed on the metallic Pt surface strongly and dissociate into highly active atomic H* [32-35]. In contrast, the adsorption and activation of hydrogen molecules on the metal oxide surface was reported to be much weaker [35]. Therefore, it is reasonable to speculate that the difference in the catalytic behavior of Pt and PtCrO_x in hydrogen oxidation by Fe(CN)₆^3- is mainly due to the different adsorption and activation performances of hydrogen on their surfaces.

To obtain further insights into the function of cocatalysts on the forward and backward reactions, the electrochemical behavior of proton and Fe(CN)₆^3- reduction was measured for ZrO₂/TaON, Pt-ZrO₂/TaON, and PtCrO_x-ZrO₂/TaON-covered FTO electrodes. In Fig. 4(a), the LSV curves show that the reduction peaks of Fe(CN)₆^3- emerge at more positive potentials than those of hydrogen evolution for all the samples. The Pt-loaded sample exhibits the highest hydrogen evolution current, but the most positive onset potential for Fe(CN)₆^3- reduction. For the PtCrO_x-loaded sample, the proton reduction performance is not so good, but it possesses the most negative onset potential for Fe(CN)₆^3- reduction. To further clarify the charge transfer of Fe(CN)₆^3- reduction at the solution-electrode interface, EIS was performed at an applied potential of -0.2 V vs. SCE, which is negative enough for Fe(CN)₆^3- reduction, but not for proton reduction. The Nyquist curves are shown in Fig. 4(b) and the values of the charge transfer resistance (Rct) are given in Table S1. The order of Rct (PtCrO_x > bare > Pt) also confirms the favorable kinetics of Fe(CN)₆^3- reduction on metallic Pt and the unfavorable kinetics of Fe(CN)₆^3- reduction on PtCrO_x. As a consequence, even though PtCrO_x is not as good as Pt in catalyzing proton reduction, it conducts the slowest Fe(CN)₆^3- reduction, which is more important for achieving sustainable hydrogen evolution.

Finally, a PSII-integrated hybrid Z-scheme system was constructed for the overall water splitting reaction with PtCrO_x-ZrO₂/TaON as the HER photocatalyst and Fe(CN)₆^4-/Fe(CN)₆^3- as the electron mediator. The water oxidation rate of PSII has been demonstrated to be greatly dependent on the concentration of the exogenous electron acceptor, Fe(CN)₆^3-, in the previous work [16]. Benefiting from the high selectivity of the Mn₄CaO₅ cluster and Q_B pocket of PSII for water oxidation and Fe(CN)₆^3- reduction, no backward reactions or other side reactions were observed.

During the overall water splitting process, the initial H₂ and O₂ evolution rates are kinetically controlled and strongly dependent on the concentration of the electron donor or acceptor. As shown in Fig. 5(a), when 100% Fe(CN)₆^3- (10 mM Fe(CN)₆^3-) was used as the initial electron mediator with no Fe(CN)₆^4- supplied, O₂ evolution by PSII is considerably high, whereas H₂ evolution by PtCrO_x-ZrO₂/TaON is much less, which is comprehensible as the concentration of transformed Fe(CN)₆^4- is definitely low. On the contrary, O₂ is much more slowly produced than H₂ when 100% Fe(CN)₆^4- was used initially. It is predictable that the stoichiometric ratio of H₂ to O₂ would approach 2:1 at some point, along with the concentration change of the mediator ions during the water splitting process. However, taking into consideration the gradual photodegradation of PSII, it is better to offer an optimal concentration of the shuttle ions at the initial stage for the stoichiometric water splitting. To simulate the mutual transformation of Fe(CN)₆^4-/3- during the water splitting process, the total concentration of the shuttle ions was fixed at 10 mM, and their relative proportions were modulated to several possible points with the percentage of Fe(CN)₆^4- being 80%, 70%, and 20%. As expected, hydrogen evolution decreased and oxygen evolution increased regularly with the decline of the Fe(CN)₆^4- proportion, and the stoichiometric splitting of water into ca. 20 μmol H₂ h^-1 and 10 μmol O₂ h^-1 was achieved at 8 mM Fe(CN)₆^4- and 2 mM Fe(CN)₆^3-. The corresponding time courses of gas evolution are shown in Fig. 5(b).

In this PSII- PtCrO_x-ZrO₂/TaON Z-scheme system, due to the original favorable water oxidation performance of PSII, the key point to realize a successful overall water splitting in the presence of both Fe(CN)₆^3- and Fe(CN)₆^4- lies in the improved HER on PtCrO_x-ZrO₂/TaON without any major backward reactions. According to the results and analyses above, the mechanisms of the generation and suppression of Fe(CN)₆^3- reduction involved in the backward reactions on metallic Pt and oxide PtCrO_x cocatalysts can be proposed reasonably. Owing to the negligible interactions between Fe(CN)₆^3- and the surfaces of the two photocatalysts, hydrogen oxidation by Fe(CN)₆^3- (Eq. (2)) is considered as the major backward reaction to block the forward HER. As shown in Fig. 6(a), metallic Pt loaded on ZrO₂/TaON not only catalyzes proton reduction to produce hydrogen, but also catalyzes the backward reaction that results in hydrogen oxidation by Fe(CN)₆^3- due to the activation of adsorbed hydrogen. Therefore, molecular H₂ and H* intermediate can be oxidized by Fe(CN)₆^3- rapidly, with a negligible amount of hydrogen gas being evolved. However, for the PtCrO_x-loaded ZrO₂/TaON sample, hydrogen adsorption on PtCrO_x is much weaker so that the backward reaction would not be catalyzed (Fig. 6(b)). Although the proton reduction on PtCrO_x is not so efficient as that on metallic Pt, the net hydrogen production performance on PtCrO_x-ZrO₂/TaON becomes much better than that on Pt-ZrO₂/TaON due to the relatively better suppressed backward reactions. In addition, stable PtCrO_x during the photocatalytic reduction process with no reduction of the Pt^Ⅱ and Pt^Ⅳ species helps to maintain sustainable hydrogen production. As a consequence, Z-scheme overall water splitting into H₂ and O₂ can be realized by combining PSII with PtCrO_x-ZrO₂/TaON via the Fe(CN)₆^3-/4--mediated forward electron transfer (Fig. 6(c)).

In summary, the present work has demonstrated the suppression of the backward hydrogen oxidation reactions caused by the electron shuttle, Fe(CN)₆^3-, in a PSII-ZrO₂/TaON hybrid system for Z-scheme water splitting. Serious hydrogen oxidation by Fe(CN)₆^3- took place under the catalysis of the metallic Pt cocatalyst loaded on ZrO₂/TaON and blocked the forward HER. By introducing the oxide PtCrO_x which contains stable Pt^Ⅱ and Pt^Ⅳ species to replace metallic Pt as the cocatalyst, hydrogen oxidation was greatly suppressed owing to the lack of adsorption and activation of hydrogen on PtCrO_x, thus leading to a remarkable enhancement in the hydrogen producing activity. This work provides new opportunities for the inhibition of the backward reaction when using Fe(CN)₆^3-/4- as the electron shuttle to build the PSII-semiconductor hybrid water splitting system.