Protagonists and spectators during photocatalytic solar water splitting with SrTaOxNy oxynitride

With a new photoelectrochemical cell we perform operando X-ray absorption spectroscopy on SrTaOxNy during photocatalytic solar water splitting. Operando characterisation proves to be an invaluable tool for the design and discovery of novel materials.


Introduction
Independence from the reliance on fossil and nuclear fuels requires the sustainable production of green energy and chemicals. As an alternative, utilisation of solar energy is possible by several means: solar thermal applications, 1 the direct conversion to electricity (photovoltaic), 2 or storage in the form of chemical energy as solar fuels. 3 One promising clean energy carrier and renewable fuel source is hydrogen harvested from photoelectrochemical (PEC) water splitting. 4,5,6 PEC water splitting requires the use of semiconductor (SC) photocatalysts that utilise solar light energy to generate electron/hole pairs. After reaching the surface, in contact with water, these charge carriers can be used to dissociate water molecules directly into molecular hydrogen and oxygen 7 . Perovskite oxynitrides are a promising class of SC materials for solar light driven water splitting. These materials have the general formula ABO 3-x N x (where A can be La, Sr, Ba, Ca, etc., and the B site: Ti, Ta, or Nb for example). The substitution of N into the O site of the precursor oxides affect the energy position of the band edges, reducing the band gap down to the visible light energy range. 8 Moreover, in this class of compounds, the photogenerated electrons and holes, both possess enough energy to promote the hydrogen and oxygen evolution reaction, respectively.
The majority of studies to date focus on the characterisation of the photocatalytic properties of these materials in the form of powders, 9,10,11 since powder development and optimisation is the primary way to device design and engineering. However, with powder samples, probing specific material properties is challenging. For example, the polycrystalline oxynitride powders do not provide well-defined surfaces to allow detailed studies of the solid-liquid interface, where the electrochemical reactions take place. The understanding of the catalytic process at the solid-liquid interface of oxynitride materials is therefore limited, as a result. A way to circumvent this limitation is the growth of thin films. By physical or chemical vapour deposition methods, it is possible to fabricate thin films with well-defined and atomically flat surfaces, therefore representing ideal model systems that allow the investigation of the surface and interface properties.
In this work, we present an operando study of the evolution of the photocatalyst semiconductor/water interface during photoelectrochemical solar water splitting using SrTaO x N y (STON) thin films. Grazing incidence X-ray absorption spectroscopy (GIXAS) coupled with modulation excitation (ME)-XAS were used to increase the surface sensitivity, whilst probing epitaxially grown STON thin films with a custom designed operando reactor cell. This technique and experimental setup allow us to probe, during operation conditions, the changes of the local chemical and geometric environment surrounding the A-and Bcations of perovskite oxynitrides near and at the solid-liquid interface. Due to the attenuation of X-rays by the aqueous electrolyte, it is not feasible to extend this study to the O and N anions, since the O 1s and N 1s edges are too low in energy (543.1 eV and 409.9 eV, respectively). Therefore, complimentary ex-situ X-ray photoelectron spectroscopy (XPS) has been included to observe all four As mentioned above, a number of oxynitride perovskite materials possess these characteristics, making them ideal candidates for PEC solar water splitting. 8,9,11,12,13 Thin films of these materials provide excellent model systems to probe the physical and chemical evolution of the surface of the SC, in contact with water.   light conditions minus the current response under dark conditions. More indepth experimental details are included in the corresponding methods section. Fig. 1c shows the PEC performance of the STON oxynitride thin films, the initial photocurrent reaches a photocurrent density of ca. 12 μA cm -1 at 1.5 V vs. reversible hydrogen electrode (RHE). This value is in line with previous reports on oxynitride thin films using the bare SC material without surface decoration with co-catalyst, which dramatically facilitate hole extraction. On the one hand, the N content of textured thin films is typically lower than the stoichiometric value that can be obtained with powder samples. 14 This reduces the thin films photo-response to visible light illumination. On the other, the thin films possess atomically flat surfaces compared to the corresponding oxynitride powders. Therefore, the surface area is ca. 20 times smaller than it would be in their powder forms. 15 However, after successive measurements (potentiodynamic sweeps) the material shows significant degradation in its initial performance.
The stabilised photocurrent density shows values ca. 40% of its initial value.
The sudden and large degradation in performance is a huge hindrance for the application of a material that initially seems quite promising. Some studies have looked at ways of improving the performance and long-term stability of STON 10,16,17 by doping constituent elements and/or decorating the surface with co-catalyst nanoparticles. However, probing the physicochemical evolution of the surface under operation conditions will provide insight into what physical and chemical processes occur at the surface during the OER and therefore, allow a rational design of stable STON.

Oxynitride Thin Films
The STON thin films were fabricated using a modified pulsed laser deposition technique described in previous works, 18

X-ray Photoelectron Spectroscopy
The XPS spectra for all four elements are shown in Fig 3. For all Sr spectra collected after PEC, the peaks shift to lower energy by ca. 0.10-0.15 eV, as can be seen in Fig. 3a and 3b, suggesting an increase in electron density surrounding Sr (formally 2+ in STON). This shift in binding energy is likely associated with the coupling of oxygen adsorbates on the Sr sites, which then desorb contributing to O 2 generation, leaving the once occupied site vacant on Sr.
Where deprotonation was calculated as the overpotential determining step  After PEC, we see an overall increase in peak area and intensity for the Sr XPS spectra. It has previously been shown, that an increase in XPS peak intensity can result from Sr segregation/accumulation 23, 24 as well as, increased doping concentrations. 25 Peak intensity/area increases are also observed due to Sr particle/Sr surface species formation. 23,24 In this work however, the increases in peak area and intensity are likely due to the increase in surface hydrophilicity  This then shifts the mid gap f band higher towards the CB or above, 26 leaving the 4d core states unchanged. The increase in intensity observed could be a result of apparent Ta enrichment due to the O/N vacancy generation exposing the subsurface B sites. 27 It could also be a result of Sr leaching, also exposing the subsurface Ta on average, Since Sr is more soluble than Ta and, it has been shown that Ta is passive in alkaline electrolytes with concentrations 2.5 x stronger than used in this work. 28 The Ta 4p 3/2 is observed as a broad peak at ca. 404-405 eV (Fig. 3e). However, the Ta 4p 3/2 spectrum overlaps with the N 1s signal in the energy region at ca. With respect to oxygen, after PEC the O 1s spectra (Fig. 3f)  Clearer evidence of NO x formation for the oxynitride LaTiO x N y can be seen by XPS in a previous work 19 , where Ta does not convolute the N 1s signal. We also observe evidence for the depletion of N states from the lattice structure using angle-resolved photoemission spectroscopy (ARPES). 30 A more detailed discussion on N 2 /NO x formation is included in the discussion section.
Overall, the XPS analyses of the initial and final state of the STON photocatalyst suggest that STON suffers from a surface degradation and reconstruction, which lead to a dramatic decrease in photocurrent (Fig. 1c). This process involves (a) loss of N from the structure (including interstitial N), where N remains partially chemisorbed as N 2 /NO x species, (b) an increase in electron density on Sr (c) Ta enrichment/exposure with an increase in disorder of its local environment due to changes in hybridisation with the N and O 2p states.
Next, we explore the effect of several external stimuli (applied potential and light) on the surface of the oxynitride SC under oxygen evolution reaction conditions by operando XAS in liquid phase.

Operando Reactor Cell
The operando XAS measurements were performed in a custom-built reactor cell   previously discussed, upon partial N loss in the surface layer, it will also result in a local loss of hybridisation between the Ta 5d and the N 2p states at the VB maximum (Fig. 5c). As a result, a more 'oxide-like' electronic structure forms at the surface as the potential is increased.
Iis reasonable to assume that Ta will tend to fulfil its full coordination sphere upon loss of N by replacing with O under OER conditions. 29 This would also explain why there is no significant change in the spectral shape for Ta. There is instead a slight shift to higher energy of the absorption edge, which is commonly assumed to be resulting from oxidation. 35 However, the degradation in the initial performance of STON (Fig. 1c) suggests this reduction on Sr is permanent and detrimental. This will be explored more in the next section. When changing the applied potential from 1.3 to 1.5 V, we see no significant change, except an overall further reductive shift albeit, at a reduced magnitude (Fig. 5d insert) within error. This observation could be explained by the earlier discussion regarding the deprotonation of Sr as the ODS. As the potential increases the majority of the Sr species reduce, followed by Sr/SrO/SrOH leaching. This disorder would also lead to defect Ta 5d states near the CBM which will then trap electrons, 44 leading to chargerecombination. 45 This, in conjunction with changes seen for Sr and the generation of N species in competition with O 2 generation, can explain why there is unusually large degradation in the initial photoelectrochemical performance of STON before the material stabilises.

Modulation Excitation X-ray Absorption Spectroscopy
In    induce small changes in the overall electronic system. This is reflected by the increase in intensity of the Sr K-edge whiteline under illumination (Fig. 6e) .
Both the A (Sr) and B (Ta) are active sites for the OER mechanism to proceed on. However, since the OER mechanism ( Fig. 5f) proceeds more sluggish on Sr intermediates that adsorb too weakly or too strongly will reduce the overall kinetics and, increase the overpotential of the OER. 29, 54 At 1.5 V, the applied potential overcomes the increased overpotentials, reflected in the increased photocurrent response. As the kinetics on Sr increase, we are no longer observing changes in electron density, similar for Ta. Since, the kinetics increases the excited state fraction, which is smaller at 1.5 V (Fig. 6d).
Suggesting that the remaining reduced Sr species that have not leached into the electrolyte, behaves more like Ta, contributing to O 2 evolution. Where, the system may not have time to either change as the generated charges are more energetic and consumed in the evolution of O 2 at faster rates.
The reduced intensity of changes shown on Sr with light modulation at 1.5 V may also relate to the changes suggested by the static XAS and XPS measurements in relation to the OER. The XAS measurements (Fig. 5e)  generation. However, as the potential increases, the oxidation change on Sr would change the lattice energy. Therefore, dissolution and leaching of Sr 2+ into the electrolyte would be expected. As the reactions proceed, the change of rate in reduction corresponds to the remaining non-reduced Sr species and increase in overpotentials. Surface reconstruction under OER conditions has previously been discussed for Sr and Co containing perovskite oxide catalysts 55,56,57,58 where, the OER proceeds under applied potential together with the lattice-oxygen evolution reaction. 55,59 This has been shown to have a beneficial effect with respect to the OER due to the formation of an enriched BOH/BO(OH) surface, increased hydrophilicity and an increase in catalytic activity. 55,60 For the oxynitride STON, we also observe surface enrichment of the B cation (Ta) and increased hydrophilicity. However, contrary to oxides, for oxynitrides this superficial BOH/BO(OH) enriched surface layer may have complications.

Discussion
We observe in the N 1s XPS spectra that there are N 2 /NO x chemisorbed species, which are formed in competitive reactions from the surface lattice N and the OER intermediates. We would like to focus our discussion on the PEC nitrogen evolution reactions at the surface of the oxynitride SC, since the literature on this topic is scarce. Although, this process seems to have important consequences on the oxygen and hydrogen evolution half reactions.
Photocatalytic nitrate formation on Ti dioxide surfaces irradiated by UV or sunlight irradiation under atmospheric N 2 and O 2 has been discussed by Yuan et al. 61 Where the photogenerated holes on the Ti SC surface catalyse the intermediary NO x species according to the following reactions given by equations (1) and (2) (3) and (4): Where the intermediate NO x species are oxidised given by equations (5) and (6): With the aid of theoretical calculations, the authors note that Co shows increased hybridisation with the O 2p states, which aids in the stabilisation of OHadsorption. This superficial CoO(OH) enriched surface layer has a beneficial effect on the OER for the oxides. 55,62 However, in the presence of surface nitrogen based species, the stabilised CoO(OH) contribute to both the NOR and OER. 62 Kato and Kudo 63 looked at numerous tantalate oxide photocatalysts, and the authors observed nitrate reduction under UV irradiation in an aqueous environment. Where nitrate forms intermediate nitrite, dinitrogen and ammonia, even in the absence of a co-catalyst or reducing agents. The authors proposed the following mechanisms shown in equations (7-10): 2 − + 7 + + 6 − → 3 + 2 2 (8) Where the photogenerated electrons in the CB reduce the nitrate and nitride species (equations7-8). Alternatively, the nitrate species can undergo photochemical decomposition (equation (9)). The photogenerated holes in the VB can then oxidise ammonia to form dinitrogen (equation (10)), which can form NO x species according to equations (1-4).
Wei et al. 64 recently reported increased photocatalytic activity for nitride reduction on Ni 2 P modified tantalum nitride and tantalum oxynitride (Ta 3 N 5 and TaON, respectively). Where the photogenerated holes in the VB of the two materials are involved in the water oxidation reaction (equation (12)) and, the photogenerated electrons in the CB transfer to the Ni 2 P states where they are involved in the photocatalytic reduction of NO 3 -. For a detailed overview on the role of N in electrocatalysis, the readers are directed to the work by Rosca et al. 65 The photocurrent is proportional to stoichiometric 2:1 H 2 /O 2 generation, according to the two half reactions of the water splitting process on a SC photocatalyst (equations (11)(12)).
2 2 → 4ℎ + + 2 + 4 + As the potential increases, the forward reactions for O 2 generation dominate since the nitrogen content at the surface is limited, compared to the high OHcontent in the alkaline electrolyte. Once the nitrogen species desorbs from the surface, the OER could proceed on the now vacant active site and/or the adsorbed intermediates would no longer contribute to N 2 /NO x formation in competition. As shown by the increase in photocurrent density as seen in Fig. 1c and 6b. However, the above-mentioned changes for STON result in a surface reconstruction during the initial stages of operation, reducing its long-term performance.
As previously discussed, the CB of STON is primarily comprised of Ta  The intermediates that adsorb too weakly or too strongly will reduce the overall kinetics and, increase the overpotential of the OER. 29,54 To conclude, OER and HER remain the dominant reactions, however the initial reconstruction due to Sr dissolution and the following competitive reactions, limit the overall efficiency of STON compared to its initial performance.

Conclusions
In this work, we employed operando grazing incidence and modulated excitation X-ray absorption spectroscopy to study the effect of several stimuli on STON oxynitride thin films during photoelectrochemical water splitting. A comparison with ex-situ XPS of the samples in their initial and final states is also included. Overall, the XPS analysis of the initial and final state of the STON photocatalyst suggests that STON suffers from a surface degradation and reconstruction as evidenced with respect to the decrease in initial photocurrent.
The XPS data suggests that STON undergoes a) slight loss of N from the structure where N remains bound as N 2 /NO x species. b) The increase in electron density and segregation of Sr. c) apparent Ta enrichment at the surface due to the changing O/N, d) Increased hydrophilicity.
Operando GIXAS corroborates the XPS findings, determining that the surface reconstruction occurs when the applied potential increases above 1.3 V vs.
RHE. ME-XAS-PSD measurements. With modulated light as an external stimulus we show that other than the generation of charge carriers, the light has minimal effect on the system, where it was previously shown for LaTiOxN y that light and the electrolyte do not contribute to the degradation of the material. 31 Whereas, in this work, Sr is an alkali earth element, more soluble at pH 13. That said, we have observed that Ta does not respond to the light on the time scales of the modulation period used in this work (120 s). However, we observe changes in the light response for Sr, with respect to its electronic structure and its evolution as a function of applied potential. Indicating that the kinetics of the OER are much faster on the Ta site than for Sr. However, when the potential is increased to 1.5V, the rate of the OER on the reduced Sr sites increase, where Sr behaves more like a transition metal and less ionic in nature.
Here we show that the nature of the A cation and the evolution of its electronic and geometric environment at the solid/liquid interface has large impacts on the overall stability and catalytic activity of the material during operation conditions. We also note that nitrogen determines the bulk electronic structure of STON and in turn, the light absorption properties of the photocatalyst.
However, at the surface the nitrogen species take on a more apparent antagonistic role, competing with the OER. To improve the performance of semiconductor photocatalysts, it is important to understand on a fundamental level what is occurring at the photocatalyst surface during operation. Future work plans to explore in operando, the effect of applied potential and the synergistic effects first row transition metal based co-catalysts have on the performance and stability of the oxynitride photocatalyst templates, where it has been shown that passivation layers and co-catalysts can prevent/minimise the detrimental surface reconstruction.

Thin Film Deposition
Three sets of films used in this work were grown using pulsed laser deposition (PLD). A KrF excimer laser (Lambda Physik LPX 300, 30 ns pulse width, λ = 248 nm) was used to ablate a target of Sr 2 Ta 2 O 7 fabricated in our laboratory.
The target to substrate distance was set at 5 cm. The laser was focused on a spot of 1.1 mm 2 with a laser fluence of ca. 3 J cm -2 and laser repetition rate of 10 Hz.
Commercially available (001)-oriented MgO was used as a substrate (10 x 10 x 0.5 mm). Platinum paste was applied between the substrates and heating stage to provide good thermal conductivity. The substrate temperature of 750°C was measured via a pyrometer. N 2 background partial pressure of 8.0 x 10 -4 mbar was set via a gas inlet line to the vacuum chamber. NH 3 gas was injected through a nozzle near the laser spot at the target. The titanium nitride current collector layer was grown in situ, prior to the deposition of the oxynitride film, by conventional PLD using a commercially available TiN target under vacuum with a base pressure of ca. 5 x 10 -6 mbar. The substrate to target distance, substrate temperature, laser repetition and fluence were the same as above.

Photoelectrochemical Characterisation
Photoelectrochemical (PEC) measurements were performed using a threeelectrode configuration in the operando reactor cell described in Fig. 3

Crystalline Properties
XRD measurements were performed using a Seifert X-ray Diffractometer with characteristic Cu Kα radiation 0.154 nm. Theta-2theta scans were performed to determine the out-of-plane orientations of the films.

X-ray Photoelectron Spectroscopy
The XPS measurements were performed at the SX-ARPES endstation 67  an X-ray grazing incidence angle of 20°. The energy resolution was set to 50 meV and the sample temperature was kept at 298 K. Spectra were calibrated using the C 1s signal due to adventitious carbon situated at 284.8 eV.

X-ray Absorption Spectroscopy
The time resolved XAS were measured using the quickXAS method 50