Tin doping speeds up hole transfer during light-driven water oxidation at hematite photoanodes †

Numerous studies have shown that the performance of hematite photoanodes for light-driven water splitting is improved substantially by doping with various metals, including tin. Although the enhanced performance has commonly been attributed to bulk effects such as increased conductivity, recent studies have noted an impact of doping on the efficiency of the interfacial transfer of holes involved in the oxygen evolution reaction. However, the methods used were not able to elucidate the origin of this improved efficiency, which could originate from passivation of surface electron–hole recombination or catalysis of the oxygen evolution reaction. The present study used intensity-modulated photocurrent spectroscopy (IMPS), which is a powerful small amplitude perturbation technique that can de-convolute the rate constants for charge transfer and recombination at illuminated semiconductor electrodes. The method was applied to examine the kinetics of water oxidation on thin solution-processed hematite model photoanodes, which can be Sn-doped without morphological change. We observed a significant increase in photocurrent upon Sn-doping, which is attributed to a higher transfer efficiency. The kinetic data obtained using IMPS show that Sn-doping brings about a more than tenfold increase in the rate constant for water oxidation by photogenerated holes. This result provides the first demonstration that Sn-doping speeds up water oxidation on hematite by increasing the rate constant for hole transfer.


Introduction
The photoelectrochemical splitting of water into hydrogen and oxygen under solar irradiation holds the promise of providing a vital fuel for a future low-carbon energy economy. In order to reach competitive efficiencies for hydrogen production, tandem cell architectures will be required, 1 for example by connecting appropriate n-type and p-type semiconductors in optical series. 2 This has sparked intensive research into semiconductor materials able to perform one of the half-reactions in water splitting. Metal oxide semiconductors are promising materials for this application owing to their relative low cost, ease of preparation and stability.. 3 However, the sluggish kinetics of the light-driven oxygen and hydrogen evolution reactions (OER and HER, respectively) compared with recombination of electrons and holes typically limit the efficiency of metal oxide photoelectrodes. The problem of recombination is evident from the characteristic "spike and overshoot" in the transient photocurrent response to chopped illumination, which has been observed in Fe 2 O 3 , BiVO 4 , Cu 2 O and WO 3 photoelectrodes during HER and OER. [4][5][6][7] Hematite, or α-Fe 2 O 3, is one of the most widely studied photoanode materials for the OER, owing to its chemical stability in basic media, abundance, visible light absorption and suitable valence band energy. Significant improvements in the performance of hematite photoanodes have been achieved through nanostructuring, which has helped to overcome the trade-off that exists between sufficient light absorption and carrier collection (due to hematite's indirect bandgap and poor hole-mobility). 8,9 Doping 1 of hematite with additives such as Sn, [10][11][12][13][14] Si, 8,[15][16][17][18] Ti, 10,[19][20][21][22][23] Pt, 24,25 Cr, 26 Mo, 26 Zn 27 and I, 28 enhances the performance of hematite for the light-driven OER. Several studies have attributed the effect of such dopants to changes in bulk hematite properties such as conductivity, 10,11,13,16,18,19,29,30 or crystallinity, 8 but very few studies 1 The term 'doping' is widely used in the water splitting literature, and we therefore use it here whilst noting that the levels of inclusion of 'dopants' are generally many orders of magnitude higher than those encountered in classical semiconductor physics.

Page 3 of 32 Physical Chemistry Chemical Physics
Physical Chemistry Chemical Physics Accepted Manuscript 4 have explored other possible beneficial roles that dopant atoms may play in the processes involved in light-driven oxygen evolution. However, recent work by Zandi et al 31

and
Chemelewski et al. 17 has not only shown that Ti and Si do not act as an electrical dopant in their materials, but also that the efficiency of interfacial hole transfer to take part in the OER (or "transfer efficiency") is improved by doping.
The hole transfer efficiency, which is of the order of just 25 % under standard operating conditions (at 1.23 V vs. RHE under AM 1.5 illumination) for benchmark hematite photoanodes, 32 can be assessed by comparing the photocurrent of a photoanode in the presence and absence of a hole scavenger 32 or by analysis of photocurrent transients to chopped illumination. 7 While these methods are equivalent in the evaluation of the transfer efficiency, only the latter allows the distinction between a catalytic effect and a passivation of surface recombination (this is discussed in detail below). The sluggish kinetics of the multistep (4electron) oxygen evolution reaction leads to a large build-up of photogenerated holes that are vulnerable to recombination with electrons. The competition between recombination and transfer thus lowers the efficiency of hole-transfer to the solution phase. The hole transfer efficiency can therefore be improved by speeding up interfacial hole transfer by catalysis [33][34][35] and by suppressing surface recombination. 7,36 In spite of the obvious importance of the kinetics of interfacial charge transfer in this context, only a few studies have measured the rate constants for reactions involving photogenerated holes. 7 The lack of kinetic information arises from the fact that methods commonly used to measure rate constants for electron transfer at metal electrodes are not applicable to reactions involving photogenerated minority carriers at illuminated semiconductor electrodes. The reason for this is that the rate constants cannot be changed simply by altering the applied potential.

5
Variations in potential appear predominantly across the space charge region in the semiconductor rather than across the Helmholtz layer, so in the ideal case at least, the rate constants remain unperturbed. The rate (rather than rate constant) of reactions involving photogenerated holes can be changed by altering the illumination intensity. This gives rise to a family of experimental methods that are analogues of conventional electrochemical techniques, with potential perturbation replaced by perturbation of light intensity. 37 So, for example, the potential step method to determine rate constants at metal electrodes corresponds to the light step method for determining rate constants at illuminated photoelectrodes. The electrochemical impedance method for metal electrodes corresponds to intensity modulated photocurrent spectroscopy (IMPS), which is based on a small ac perturbation of the light intensity. 38-40 Although these methods have been known for some time, surprisingly few studies of light-driven water splitting have made use of them to understand the influence of interfacial kinetics on efficiency. Here we illustrate the power of the approach by an IMPS study of the influence of tin-doping on lightdriven water splitting at hematite electrodes.
In practice, one of the most widely encountered effects of doping of hematite layers is a modification of the nanostructure morphology, leading in many cases to a reduction in feature size and thus to an increase in surface area. 8,11,13,14,16,20,23,26,29,41 17 In principle, the improvement could be explained either by a higher rate constant for the transfer of holes across the interface, or by suppression of surface electron-hole recombination. In order to discover which explanation holds in the present case, the rate constants for surface recombination and charge transfer were de-convoluted using intensity-modulated photocurrent spectroscopy (IMPS), which showed that Sn-doping increases the rate constant for hole-transfer by more than an order of magnitude. The material composition was characterized by analytical transmission electron microscopy (TEM), which probes the degree of Sn-incorporation in the bulk of the hematite crystallites and provides insights into the spatial distribution of Sn. It emerged that Sn-atoms are incorporated into the hematite structure without phase separation or formation of tin oxide clusters, but that the tin atoms are preferentially distributed in the near-surface regions of the hematite nanoparticles, resulting in a core-shell type structure. The results of our study thus provide the first proof that doping 7 hematite can speed up the interfacial reaction of photogenerated holes, which is one of the fundamental limitations of hematite and other photoelectrodes for light driven water splitting.

Theory
The external quantum efficiency, EQE(λ), of light-driven water oxidation taking place at bulk semiconductor electrolyte junctions depends on the product of the efficiencies of light harvesting, η LH (λ), charge separation, η sep (λ), and hole-transfer, ηtrans, to the electrolyte, the first two being functions of the light wavelength, λ.
In the absence of light scattering and internal reflection, the light harvesting efficiency can be calculated from the wavelength-dependent absorption coefficient, α(λ), and the film thickness, d.
For a planar electrode geometry, the electron-hole separation efficiency η sep (λ) can be calculated using the Gärtner equation 42 (see below) if the width of the space charge region, W sc , and the hole diffusion length, L p , are known. However, this calculation will not be correct if substantial recombination takes place in the space charge region (in which case η sep is lower) or if the electrode is nanostructured (see supporting information). For this reason, we derive η sep (λ) from the measured external quantum efficiency using the light harvesting efficiency calculated from the absorption spectrum and the transfer efficiency derived using the rate constants for hole transfer and recombination obtained from IMPS or photocurrent transient measurements .
Hematite photoanodes respond to chopped illumination with a characteristic "spike and overshoot" photocurrent transient. [43][44][45] This transient response is typical for systems with a large degree of surface electron-hole recombination. 7,45 When the light is switched on, holes generated In principle, the exponential decay of the current towards the steady state, which is characterized by the time constant (k trans + k rec ) -1 , can be analysed, and then k trans and k rec can be separated using equation (0). In practice, however, it is more convenient to determine the time constant using small amplitude frequency-resolved measurements such as IMPS 7 or photoelectrochemical impedance spectroscopy (PEIS). 46 The IMPS method involves small amplitude (< 10%) variable frequency sinusoidal modulations of the light intensity about a dc value. The resulting phase and amplitude of the photocurrent are recorded as a function of frequency, and the results are displayed in the complex plane. 47,48 The imaginary component of the photocurrent reaches a maximum when the frequency, ω max , matches the characteristic relaxation constant of the system, i.e. the same time constant (k trans + k rec ) -1 seen in the exponential decay of the transient photocurrent.  . IMPS response predicted for k rec = k trans = 10 s -1 , C sc = 1 µF cm -2 , R ser = 20 Ω. The response is normalized to the hole current, qJ h , generated by collection of holes in the space charge region. The radial frequency corresponding to the maximum of the upper recombination semicircle is equal to k trans + k rec , and the normalized low frequency intercept is equal to k trans /(k trans + k rec ), which corresponds to the ratio of the steady state current to the instantaneous current in Figure 1.
As noted above, this interpretation of photocurrent transients and IMPS is valid for semiconductors with a well-defined depletion layer at the interface with the electrolyte. To be applicable to structured semiconductors, such as those studied here, W SC should be smaller than the average feature size. One method to determine the width of the depletion region is through the measurement of the electrode capacitance. The flat band potential, V fb , and donor density, N d , derived from the Mott Schottky relationship 50 (equation (0)), are then used to calculate values of W SC as a function of applied potential (equation (0)), where ε r is the relative permittivity, ε 0 is the permittivity of free space, A is the electrode area, V is the applied potential, q is the elementary charge, k B is Boltzmann's constant and T is the temperature.
( ) However, in the case of the light driven OER, some fraction of the holes reaching the surface is lost by surface recombination so that the EQE is lower than predicted by equation (0). If we take the non-unity transfer efficiency into account, the simplified Gärtner equation can be rearranged to give the width of the space charge region.

Synthetic route for the incorporation of Sn into mesoporous hematite electrodes
Hematite precursor solutions were prepared according to the following procedure. For the preparation of the Sn-containing hematite, Sn(OAc) 4 was added to the solution described above, see Table 1 in the supporting information for further details. The desired amounts of Sn(OAc) 4 were first dispersed under vigorous stirring for 5 h followed by 15 min sonication in the above mixture of Pluronic P123 and tert-butanol. The remaining steps of the synthesis then followed those described above for pure hematite. It is important to note that

Photoelectrochemical characterization
Hematite photoelectrodes were masked with a PTFE-coated glass fibre adhesive tape leaving a circular area of 1 cm in diameter exposed to a 0.
The light intensity was measured at the position of the electrode inside the cell using a 4 mm 2 photodiode, which had been calibrated against a certified Fraunhofer ISE silicon reference cell equipped with a KG5 filter. Intensity modulated photocurrent spectroscopy (IMPS) was carried out using a PGSTAT302N Autolab (Metrohm), equipped with an FRA32M frequency response analyser, connected to an LED driver kit which powered a 470 nm high-power LED. The light intensity was modulated by 10 % between 100 kHz and 0.1 Hz.    Improvement in the PEC performance of hematite upon doping are typically attributed to improvements in conductivity. 10,11,13,16,18,19,29,30 In n-type semiconductors, the conductivity is given by the product ‫ܰߤݍ‬ ௗ , where ߤ is the electron mobility. In the case of compact planar electrodes, the doping density (and hence conductivity) can be obtained from Mott Schottky plots according to equation (0). Rough semiconductor layers, such as the ones studied here, are not ideal for quantitative Mott Schottky analysis, due to the poorly defined surface area, and the possible presence of exposed FTO providing parallel charge transfer pathways. However, assuming the electron mobility, surface area and dielectric constants of the hematite do not change significantly upon doping, a qualitative increase in doping density would be clearly

Physical Chemistry Chemical Physics Accepted Manuscript
19 apparent as decrease in the slope of the Mott-Schottky plot. Since there is no significant change in the slopes of the Mott-Schottky plots (see Figure SI 9), we conclude that enhanced conductivity is not responsible for the observed improvements in performance in this system.
In order to elucidate the role of Sn-incorporation in improving the performance of the hematite photoelectrodes, photocurrent transients were recorded, as illustrated in Figure 5a. In order to ensure that the theoretical treatment outlined above is applicable, the condition that the depletion layer should be narrower than the nanostructure feature size was tested (see supporting information for a detailed discussion of the methods used to evaluate W SC ). Calculation of the width of the space charge region requires knowledge of the relative permittivity, ε r , of hematite.
Values of ε r for hematite in the literature vary considerably. For example Glasscock et al. 55 measured values of ε r between 31 and 57, whereas Lunt et al. 56  This lower value is likely to be due to a reduction of band bending arising from the build-up of holes at the surface, which results in more potential being dropped across the Helmholtz layer rather than across the depletion layer. It follows that -under illumination at least -the condition of a well-defined depletion layer with W sc smaller than the feature size (≥ 30 nm) should be satisfied. This reduction in W sc under illumination highlights why small amplitude perturbation methods such as IMPS are preferable to large amplitude ones such as photocurrent transients.
Further evidence for the assumption that W sc is smaller than the feature size is provided by the suggesting that the "bulk" properties of the material improve. Possible reasons for improved charge separation include a reduction in space charge recombination losses, or an enhanced hole diffusion length. Interestingly, j (t=0) decreases with further addition of Sn. This is most pronounced for the 30 % Sn doped sample, which has a significantly less well-defined morphology compared to the rest of the series, see Figure 3. We attribute the slight decrease in j (t=0) for the 20 % sample to a the lower extinction coefficient (and hence light harvesting efficiency) of these samples, see Figure SI 4d. This lower extinction coefficient is an artefact related to the film preparation, and is discussed in detail in the supporting information.
Most remarkably, the transfer efficiencies, obtained from the ratio j ss /j (t=0) according to equation ( 57 On the other hand, suppression of surface recombination can also significantly enhance performance. 7,36 The objective of the present study was a clear distinction between these two possibilities. Although the analysis of photocurrent transients demonstrates that adding tin improves the hole transfer efficiency, further the quantitative analysis was not attempted, since "on-off" illumination is likely to change the band bending as a consequence of the build-up of holes at the  Since these measurements did not probe the distribution of Sn within the individual hematite particles, scanning transmission electron microscopy (STEM) was employed in combination with EDX to probe the Sn-content with a step size of approximately 2 nm. To this end, an electron beam with a diameter of less than 1 nm and, consequently low intensity, was used. Line scans across the width of a particle can reveal inhomogeneities between surface and bulk compositions, as illustrated in the inset of Figure 8b. Close to the edge of the particle, when the STEM beam is tangent to the particle edge, the electron beam probes primarily the surface.
In contrast, when the beam is incident normal to the surface of the particle, the X-ray generated due to inelastic scattering of the electron beam is dominated by the bulk (X-rays stemming from the surface contribute to a lesser extent to the total signal, due to the shorter path length through the surface with respect to the bulk). Six particles were probed along lines approximately perpendicular to the surface, such as the one depicted in Figure 8a. Due to an insufficient signalto-noise-ratio, background subtraction could not be performed, and therefore the local atomic ratio could not be quantified. However, the ratio of the intensity of the signals attributable to Sn and that obtained from both Sn and Fe (including the background), revealed a substantial Snenrichment at all measured surfaces. The Sn-content strongly decays towards the middle of the particle, where very little signal attributable to Sn was detected. The enrichment of Sn at the surface is also visible in the HAADF-STEM images as a white brim. Since the signal in 26 HAADF-STEM images is approximately proportional to the square of the atomic number for a given thickness, 58 this bright rim is attributed to a higher average atomic number, which is consistent with the inclusion of Sn in the surface atomic columns. Thus, we conclude from our EDX line scan and HAADF STEM results that the introduction of a Sn-precursor into the hematite synthesis leads to a gradient doping with preferential incorporation of Sn-atoms near the surface of the hematite nanoparticles. Due to the small overall content of Sn, an investigation of the mode of tin incorporation into the hematite structure proved difficult. However, in HRTEM images such as those shown in Figure 8c and 8d, the lattice planes of several dozen undoped and doped particles extend to the surface, and there is no evidence for newly formed separate phases such as SnO 2 or SnO at the surface. We therefore conclude that the Sn incorporates into the hematite structure without substantial structural changes. Our experimental evidence therefore reveals a structure-function relationship between preferential Sn-doping near the surface of hematite, and an increased hole transfer efficiency (i.e. a higher proportion of photogenerated holes taking part in the OER). This result is in very good  17,31 In this work, the power of dynamic lightperturbation techniques further allows us to provide the first demonstration that the increased transfer efficiency brought about by Sn-doping hematite is due to enhanced catalysis of the OER rather than a passivation of surface recombination.
While it is not clear at present how the Sn dopants beneficially impact the OER kinetics, the insight that dopants can speed up the sluggish OER is an important milestone in the optimisation of photoanodes for water oxidation. Indeed, if the specific role of different dopants (and other surface treatments) can be unambiguously identified by methods such as IMPS, the virtues of each could potentially be combined to further improve the efficiency of state of the art hematite photoanodes for water oxidation. Our observations may support recent theoretical predictions that mixed metal sites at the hematite surface (i.e. surface doping) could play a role in balancing the intermediate energetic barriers involved in the OER. 59,60 Although these studies considered many common hematite dopants such as Si and Ti, Sn was not included. Given the phenomenological nature of the rate constants obtained here, it is plausible that Sn atoms at the hematite surface may affect the OER intermediates, which would, in turn lead to an increased k trans . A parallel can be drawn between our findings and recent work by Riha et al., 35 who suggest that a sub-monolayer Co-coating also catalyses the OER on hematite photoanodes.
Interestingly, the authors note that the existence of neighbouring Fe and Co sites may be crucial to this catalytic activity, which could be an example of the behaviour predicted by Busch and Carter. 59,60  Sn-enrichment at the surface of the nanoparticles, indicating a structure-function relationship between the surface nature of the Sn-doping, and the improved catalytic properties at the surface.
While all dopants may not affect hematite in this way, catalysis due to surface doping could be a more widespread effect than realized currently. Application of techniques such as IMPS to distinguish between changes in the rates of hole transfer and surface recombination brought about by inclusion of dopant atoms in other cases would therefore be useful. The combination of dopants playing complimentary roles in the enhancement of hematite photoanodes for water oxidation could be a useful strategy towards significant future improvements in performance.

Supporting information available:
Further synthetic details, powder XRD, SEM images, absorbance, dark cyclic voltammetry data, cyclic voltammetry of 20 % Sn sample under AM 1.5, electron diffraction data obtained in TEM,