ZnSe nanorods as a visible-light-absorber for photocatalytic and photoelectrochemical H2 evolution from water

A precious metaland Cd-free photocatalyst system for efficient H2 evolution from water with a performance comparable to Cd-based quantum dots is presented. Rod-shaped ZnSe nanocrystals (nanorods, NRs) with a Ni(BF4)2 co-catalyst suspended in aqueous ascorbic acid evolve H2 with up to 54±2 mmolH2 gZnSe –1 h – 1 under visible light illumination (λ > 400 nm, AM 1.5G, 100 mW cm – 2 ), and 50±4 % quantum yield (λ = 400 nm). Under full-spectrum simulated solar irradiation (AM 1.5G, 100 mW cm –2 ), up to 149±22 mmolH2 gZnSe –1 h –1 are generated. Significant photocorrosion was not observed within 40 h and activity was even observed without added co-catalyst. The ZnSe NRs can also be employed to construct an inexpensive Delafossite-based photocathode, which does not rely on a sacrificial electron donor. Immobilized ZnSe-NRs on CuCrO2 generate photocurrents of ~10 μA cm –2 in aqueous electrolyte solution (pH 5.5), with a photocurrent onset potential at ~+0.75 V vs. RHE. This work establishes ZnSe as a state-of-the-art light-absorber for photocatalytic and photoelectrochemical H2 generation. Artificial photosynthesis, in which solar energy is stored in chemical fuels is a promising strategy for overcoming the temporal mismatch between renewable energy supply and demand. H2 is the most prominent example of a solar fuel as it can be generated by photoreduction of aqueous protons by a broad range of photocatalysts. Among the most active materials are chalcogenide nanocrystals based on CdS and CdSe. Despite the remarkable activities and stabilities shown by these materials, the toxicity and carcinogenic nature of cadmium represents a considerable obstacle for their widespread application. Carbon-based materials, such as carbon nitride, carbon dots, and conjugated organic polymers have recently been introduced as benign alternatives. While these materials are inexpensive and non-toxic, their performances have yet to match those of Cd-based photocatalysts to achieve high quantum yields for aqueous H2 production without precious and carcinogenic metals. Here, we report ZnSe nanorods (NRs) as an inexpensive, Cd-free light-absorber for efficient H2 evolution under visible-light irradiation. The ZnSe NRs exhibit activity approaching that of Cd-based materials, even without an added co-catalyst. Furthermore, we demonstrate that the superior activity of suspended ZnSe nanocrystals under sacrificial conditions can be translated to heterogeneous conditions by assembling a simple, precious-metal-free photoelectrode from ZnSe nanocrystals immobilized on p-type CuCrO2. Figure 1. Schematic representation of the reported ZnSe nanorod photocatalyst system and its application for the construction of a noble-metalfree photocathode (CB = conduction band, VB = valence band). ZnSe is a stable and inexpensive semiconductor with a direct bulk band gap of 2.7 eV, which enables absorption of near-UV and some visible light. The conduction band (CB) is located at around –1.1 V vs. NHE (pH 0), providing ample driving force for the reduction of aqueous protons. Despite these favorable properties, ZnSe has received surprisingly little attention for solar fuel generation, unlike its cadmium analogues CdS and CdSe. Domen and co-workers reported ZnSecopper indium gallium selenide (CIGS) solid solution-based photocathodes for H2 evolution [11] with photocurrents up to 12 mA cm at 0 V vs. RHE and onset potentials of +0.89 V vs. RHE. However, the complex photocathode assembly required a CdS extraction layer and a Pt proton reduction catalyst. While a number of reports have demonstrated the application of ZnSe-based nanomaterials for photocatalytic dye degradation, and water oxidation, only a few examples of Cd-free ZnSe particles for photocatalytic H2 generation have been reported, all of which show low activity. We prepared ZnSe NRs by injecting trioctylphosphine/Se into an octadecane solution of zinc stearate at 300 °C, followed by a 25 min growth period. Surface modification of asprepared stearate-capped ZnSe NRs (ZnSe-St) was achieved by ligand exchange with mercaptopropionic acid to give watersoluble NRs (ZnSe-MPA), and by reactive ligand removal with [Me3O][BF4] to give ligand-free NRs (ZnSe-BF4). [16] Independent of the surface capping, the NRs are 5.2±0.6 nm in diameter and 30.0±4.8 nm long (aspect ratio 5.8±0.9), as determined from transmission electron microscopy (TEM, Fig. S1). Powder X-ray diffraction (Fig. S1F) shows that the ZnSe NRs are obtained as a mixture of the zinc blende and wurtzite polymorphs, as has ‡ M. F. Kuehnel, C. E. Creissen and C. D. Sahm contributed equally. [*] Dr. M. F. Kuehnel, C. E. Creissen, C. D. Sahm, D. Wielend, A. Schlosser, Dr. K. L. Orchard, Prof. E. Reisner Christian Doppler Laboratory for Sustainable Syngas Chemistry Department of Chemistry, University of Cambridge Lensfield Road, Cambridge CB2 1EW, UK E-mail: reisner@ch.cam.ac.uk http://www-reisner.ch.cam.ac.uk Dr. M. F. Kuehnel Department of Chemistry, Swansea University Singleton Park, Swansea SA2 8PP, UK Supporting information and raw data for this article is given via a link at the end of the document.

. Schematic representation of the reported ZnSe nanorod photocatalyst system and its application for the construction of a noble-metalfree photocathode (CB = conduction band, VB = valence band).
ZnSe is a stable and inexpensive semiconductor with a direct bulk band gap of 2.7 eV, [8] which enables absorption of near-UV and some visible light. The conduction band (CB) is located at around -1.1 V vs. NHE (pH 0), [9] providing ample driving force for the reduction of aqueous protons. Despite these favorable properties, ZnSe has received surprisingly little attention for solar fuel generation, unlike its cadmium analogues CdS and CdSe. [10] Domen and co-workers reported ZnSecopper indium gallium selenide (CIGS) solid solution-based photocathodes for H2 evolution [11] with photocurrents up to 12 mA cm -2 at 0 V vs. RHE and onset potentials of +0.89 V vs. RHE. [11b] However, the complex photocathode assembly required a CdS extraction layer and a Pt proton reduction catalyst. While a number of reports have demonstrated the application of ZnSe-based nanomaterials for photocatalytic dye degradation, [12] and water oxidation, [13] only a few examples of Cd-free ZnSe particles for photocatalytic H2 generation have been reported, all of which show low activity. [14] We prepared ZnSe NRs by injecting trioctylphosphine/Se into an octadecane solution of zinc stearate at 300 °C, followed by a 25 min growth period. [15] Surface modification of asprepared stearate-capped ZnSe NRs (ZnSe-St) was achieved by ligand exchange with mercaptopropionic acid to give watersoluble NRs (ZnSe-MPA), and by reactive ligand removal with [Me3O][BF4] to give ligand-free NRs (ZnSe-BF4). [16] Independent of the surface capping, the NRs are 5.2±0.6 nm in diameter and 30.0±4.8 nm long (aspect ratio 5.8±0.9), as determined from transmission electron microscopy (TEM, Fig. S1). Powder X-ray diffraction (Fig. S1F) shows that the ZnSe NRs are obtained as a mixture of the zinc blende and wurtzite polymorphs, as has been previously observed with ZnSe nanorods synthesized by hot injection. [17] ZnSe NRs show UV-visible light absorption up to ~440 nm (Fig. S2A) and two emission maxima separated by 0.097 eV in their photoluminescence (PL) spectra that can be attributed to differences in the band gaps of the two ZnSe polymorphs (Fig. S2B). [18] Additional emissions at longer wavelengths likely result from trap states as previously observed with ZnSe nanocrystals. [16a] PL is reductively quenched by adding ascorbic acid (AA, Fig. S2C-D).  Table S1 and Fig. S3 for optimization details), ZnSe-BF4 produced up to 33.6±2.0 mmolH2 gZnSe -1 h -1 ( Fig. 2A). To further enhance the photocatalytic activity of ZnSe NRs, Fe(BF4)2, Co(BF4)2, Ni(BF4)2 and K2PtCl4 were tested as cocatalysts (Fig. 2B). Ni showed the highest performance increase to 54.3±1.9 mmolH2 gZnSe -1 h -1 at 20 μM, whereas K2PtCl4 quenched the photocatalytic activity almost completely. We speculate that ligand-free particles suppress the deposition of Pt particles on the ZnSe surface, as previously observed with ligand-free CdS; [19] pre-formed Pt nanoparticles showed a higher activity, but still lower than without co-catalyst. Under the same conditions, ligand-capped ZnSe-MPA and ZnSe-St NRs showed a lower H2 generation activity of 45.9±1.4 and 12.1±2.7 mmolH2 gZnSe -1 h -1 , respectively (Fig. 2B). This observation agrees with our previous studies, demonstrating enhanced HER activity of CdS nanocrystals upon ligand removal. [20] Under full-spectrum simulated solar irradiation (AM 1.5G, 100 mW cm -2 ), ZnSe-BF4 generates up to 149±22 mmolH2 gZnSe -1 h -1 and 95±27 mmolH2 gZnSe -1 h -1 in the presence and absence of Ni(BF4)2, respectively (Fig. 2C). The internal quantum yield (IQE) under 400 nm monochromatic light was 50.2±3.6 % (35.9±2.6 % external quantum yield, EQE, Table S2).
Long-term experiments using ZnSe-BF4 showed that H2 production is sustained over more than 40 h with a gradual decrease in rate (Fig. 2D). This decreasing activity is likely due to accumulation of dehydroascorbic acid (DHA) in solution. Photodegradation of ZnSe is only marginal as separating ZnSe-BF4 NRs after 20 h and re-dispersing them in a fresh AA solution largely restored activity (some material is lost during separation). In contrast, adding fresh ZnSe NRs had no effect on the activity (Fig. S4). Previous work has shown that the AA oxidation product DHA can inhibit photocatalytic H2 production. [21] UV-vis spectra before and after prolonged irradiation show no degradation apart from an increase in scattering resulting from particle aggregation (Fig. S5). Post-catalysis TEM confirms the formation of aggregates with aspherical nanocrystalline features (Fig. S6). Inductively-coupled plasma optical emission spectroscopy (ICP-OES) of ZnSe-BF4/Ni isolated after 3 h irradiation showed incorporation of 8.5±2.3 Ni atoms per ZnSe NR (<1 % of total added Ni), suggesting in-situ formation of a heterogeneous Ni-based catalyst on the NR surface. [22] No H2 was generated without ZnSe, in the dark or without electron donor (Table S3).
Having established good performance and stability of ZnSe nanorods for photocatalytic H2 production, even in the absence of added co-catalyst, we aimed to eliminate the sacrificial electron donor AA. The production of low-value H2 gas at the expense of a sacrificial electron donor is not sustainable unless the electron donor is freely available, for example by photoreforming waste. [19,36] Instead, a nanocrystal-sensitized photocathode can be assembled, where the nanocrystal provides electrons for photocatalysis, and a p-type semiconductor accepts photogenerated holes, replacing the electron donor. Such systems enable overall water splitting through coupling with a photoanode for water oxidation. [37] To this end, we immobilized ZnSe-BF4 NRs on a CuCrO2 electrode. CuCrO2 is a wide-bandgap semiconductor (Eg~3.1 eV), which crystallizes in a Delafossite-type structure. Previous work has shown that modification of CuCrO2 with an organic dye and a nickel bis(diphosphine) catalyst enabled visible light driven proton reduction in aqueous solution. [38] The characteristic high hole mobility, p-type conductivity, and straightforward synthesis from abundant materials using solution processing techniques, position CuCrO2 as a suitable candidate for coupling with ZnSe in a hydrogen-generating photocathode.
ZnSe nanorods were immobilized by drop casting (8 μL cm -2 , 1.66 mg mL -1 , acetonitrile) directly on CuCrO2 electrodes (thickness approx. 300 nm, Fig. S7; 13.4 µg ZnSe cm -2 ), with EDX spectra confirming an even distribution over the electrode surface (Fig. S8). UV-vis spectra of ZnSe-modified CuCrO2 feature the characteristic absorptions of both CuCrO2 and ZnSe (Fig. S9). Linear sweep voltammograms and chronoamperograms of ZnSe-modified electrodes show enhanced photocurrents over the bare CuCrO2 electrode with an onset potential of approximately +0.75 V vs. RHE (Fig. 3), indicating the ability of photoexcited ZnSe nanorods to inject holes (EVB, ZnSe = 1.6 V vs. RHE) into the valence band of CuCrO2 (EVB, CuCrO2 = 1.0 V vs. RHE). [38] Controlled potential photoelectrolysis (CPPE, Fig. S10) confirmed that the highly reducing CBZnSe electrons are used to reduce aqueous protons to H2. CPPE with a CuCrO2|ZnSe electrode maintained at Eapp = 0 V vs. RHE and illuminated from the front side (100 mW cm -2 , AM 1.5G, λ > 400 nm) produced 35±7 nmol H2 over the course of 4 h with a Faradaic efficiency (FE) of 7±2 % (Table S5). Bare CuCrO2 produced no detectable H2, confirming the essential role of ZnSe in this system. The high dark current, as previously reported for CuCrO2, [38] and dissolved H2 that is not sufficiently accounted for in low current-generating systems, [39] both contribute to the modest FE. Adding Ni 2+ as a cocatalyst increases the overall H2 production yield, corresponding well with photocatalysis results (Fig. S11, Table S5). Incident photon-to-current efficiency measurements showed an increased current in the 400-440 nm region for CuCrO2|ZnSe electrodes compared to bare CuCrO2 confirming the role of ZnSe-NRs in this photocathode (Fig. S12). H2-generating QD-sensitized photocathodes in the absence of a co-catalyst have reported photocurrents of -60 µA cm -2 at 0.3 V vs. RHE with mercaptoacetic acid modified CdSe on NiO, [40] and -180 µA cm -2 at 0.5 V vs. RHE using a phenothiazine hole-accepting ligand with CdSe on NiO were observed. [41] CuCrO2|ZnSe photoelectrodes generated -10 µA cm -2 at 0 V vs. RHE, comparable to photocurrents observed with a molecular dye/catalyst assembly. [38] The low photocurrent can be partly attributed of low light absorption, but the dominant limiting factor is likely a non-ideal interface between CuCrO2 and ZnSe NRs. This results in high charge recombination, limiting the number of electrons available for catalysis. Adding a HER co-catalyst therefore only results in a small activity enhancement. Although this performance does not yet match that of the corresponding Cd-based systems, it does demonstrate that the ZnSe NR photocatalyst can operate in the absence of a sacrificial reagent and in a photoelectrochemical cell. We expect future improvements for the integration of ZnSe into electrodes from CuCrO2 nanostructuring and ligand engineering to improve the CuCrO2/ZnSe interface, [40][41][42] alternative assembly methods, [43] and integration of molecular catalysts, [44] especially for CO2 reduction, [16a] making use of the highly reducing CB of ZnSe.
In summary, we have demonstrated that ZnSe nanorods are highly efficient light-absorbers for solar-driven H2 production, even without added hydrogen-evolution co-catalyst. Their performance already approaches that of Cd-containing quantum dots without exhibiting their carcinogenicity, highlighting the potential of designing novel inorganic materials for efficient photocatalysis. We showed that the ZnSe nanorods can also be integrated into photoelectrochemical cells, which paves the way to closed-cycle solar fuel synthesis and we also envision its use in organic photoredox catalysis and photorefoming of waste and pollutants in future development.