Mapping Photoelectrochemical Current ... - ACS Publications

Letter pubs.acs.org/JPCL

Mapping Photoelectrochemical Current Distribution at Nanoscale Dimensions on Morphologically Controlled BiVO4 Pongkarn Chakthranont, Linsey C. Seitz, and Thomas F. Jaramillo* Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford, California 94305, United States

Downloaded by FLORIDA STATE UNIV on September 8, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acs.jpclett.5b01587

S Supporting Information *

ABSTRACT: We develop a method that can be used to qualitatively map photocurrent on photoelectrode surfaces, and show its utility for morphologically controlled W-doped BiVO4. The method is based on the deliberate photoinduced sintering of Au NPs, a photon-driven process that indicates oxidation with nanoscale-resolution. This strategy allows us to identify the active regions on W-doped BiVO4 photoelectrodes, and we observe a strong dependence of photoactivity on the electrode morphology, controlled by varying the relative humidity during the sol−gel fabrication process. We find that photoelectrode morphologies that exhibit the most evenly distributed Au sintering are those that yield the highest photoelectrochemical (PEC) activity. Understanding the correlation between electrode morphology and PEC activity is essential for designing structured semiconductors for PEC water splitting. field transmission electron microscopy, conducting atomic force microscopy, and computational modeling to correlate the type of grain boundaries with charge transport in the nanoparticle domains.22 However, a diagnostic method that allows for spatial mapping of electrode performance without advanced characterization techniques and intensive sample preparation has not yet been developed. We present a direct diagnostic strategy to map the photocurrent distribution across highly active nanostructured BiVO4 electrodes using gold nanoparticles (Au NPs) as an indicator. Au NPs supported on metal oxides have been widely studied due to their high catalytic activity for low temperature CO oxidation.23 It has been established that Au interactions with metal oxides are typically weaker than those in Au−Au bonds. Hence, diffusion of Au NPs usually occurs at elevated temperature or under oxidation reaction conditions,24 resulting in particle sintering or the formation of Au dendrites.25 Exploiting this phenomenon, we can qualitatively map the photocurrent distribution by observing the sintering of Au NPs on nanostructured BiVO4 photoelectrodes, which allows us to correlate electrode morphology with PEC activity. This simple diagnostic strategy offers tens of nanometers spatial resolution and can easily be generalized for other nanostructured metal oxide electrodes. To the best of our knowledge, this is the first report of utilizing Au NPs as an indicator of photocurrent distribution. We synthesized highly active W-doped BiVO4 electrodes by the sequential drop-casting of metal organic precursors in a humidity-controlled environment. This sol−gel synthesis technique is simple, scalable, reproducible, and yields high

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ince the early demonstration of photoelectrochemical (PEC) water splitting on a TiO2 surface by Fujishima and Honda in 1972,1 metal oxide semiconductors have become one of the most studied classes of materials for a PEC water splitting cell. Many metal oxides have the advantages of earth abundance, low cost, low toxicity, and relatively high stability in aqueous environments.2 One of the most promising metal oxide candidates is bismuth vanadate (BiVO4). Monoclinic BiVO4 is an n-type semiconductor with a band gap of ∼2.5 eV.3 It has a conduction band position close to the H2/H+ redox potential,4 leading to a photocurrent onset as early as 0.2 V versus the reversible hydrogen electrode (RHE) when decorated with oxygen evolution catalysts.5 A known performance limitation of BiVO4 involves the poor charge transport for both majority and minority carriers. Recent studies show that the majority carrier transport can be greatly improved by introducing donor-type dopants such as tungsten (W) and molybdenum (Mo), resulting in higher PEC performance.6−8 For the minority carrier, although BiVO4 has a relatively long minority carrier lifetime (40 ns)7 compared to other popular metal oxides such as TiO2 (25 ns),9 WO3 (1−9 ns),7,10 and Fe2O3 (3 ps),11 the carrier mobility of BiVO4 is low. This results in a short carrier diffusion length, which greatly limits the charge extraction depth of this material.1213 A promising strategy to mitigate the short charge extraction depth of BiVO4 is nanostructuring, which minimizes photogenerated charge transport distance while maintaining high optical density.14 Recent studies on nanoporous,5,15 nanowire,16−19 and inverse opal20,21 architectures have proven the success of the nanostructuring strategy. However, the greatest challenge in studying these nanostructured electrodes is identifying the active sites and the photocurrent distribution across the surface of such complex architectures. A method of identifying active nanoparticle domains has been developed for nanostructured Fe2O3 electrodes using a combination of dark© XXXX American Chemical Society

Received: July 23, 2015 Accepted: September 2, 2015

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DOI: 10.1021/acs.jpclett.5b01587 J. Phys. Chem. Lett. 2015, 6, 3702−3707

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The Journal of Physical Chemistry Letters

Figure 1. Top down SEM images (a−d) and cross sectional SEM images (e−h) of W-doped BiVO4 samples synthesized at RH < 20% (a,e), RH ∼ 30% (b,f), RH ∼ 50% (c,g), and RH ∼ 80% (d,h) show variations in film morphologies.

distribution leads to nonuniformity in the film thickness. As shown in Figure 1g−h, the W-doped BiVO4 particles agglomerated into micron-size clusters, forming macroscopically rough films. In the lowest humidity system, RH < 20%, the hydrolysis and polycondensation reactions occurred for only a short period of time due to the low ambient water content and the fast evaporation rate, forming only small W-doped BiVO4 particles. The high solvent evaporation rate also caused high stress and cracks in the film28 as seen in Figure 1e where several separated layers can be observed. Increasing the relative humidity increased the water adsorption on the films, resulting in a higher rate of hydrolysis and polycondensation. Higher humidity also suppresses the solvent evaporation rate, which lengthens the gelation process and allows for a crack-free porous network to form26 as seen in the RH ∼ 30% sample (Figure 1b). However, at very high humidity, the hydrolysis reaction continued to nucleate more W-doped BiVO4 nanoparticles. This resulted in a broader particle size distribution as seen in the RH ∼ 50% and RH ∼ 80% samples. Having determined how humidity controls the morphology of the Wdoped BiVO4 films, we next examined their optical, structural, and chemical properties. In short, UV−vis spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) each show similar features for all samples, irrespective of the relative humidity employed during synthesis. The UV−vis absorbance spectra confirm that all W-doped BiVO4 photoelectrodes synthesized at various humidities exhibited a direct band gap at ∼2.5 eV (Figure S1). The grazing incidence XRD patterns reveal that all W-doped BiVO4 samples had a monoclinic scheelite structure with similar crystallite sizes (Figure S2 and Table S2). The XPS spectra suggest that the surface elemental compositions of all samples were similar and Bi-rich (Figure S3). A small degree of Bi2O3 phase segregation was observed on the samples synthesized at RH ∼ 30%, RH ∼ 50%, and RH ∼ 80% (Figure S3). Overall, the W-doped BiVO4 films synthesized at different relative humidities are all quite similar to one another in terms of their optical absorption, band gap, crystal structure, crystallite size, and chemical compositions. The samples primarily differ in their morphology as described previously.

performance photoanodes. We found that varying the relative humidity allowed us to control the morphologies of the Wdoped BiVO4 electrodes, similar to previously reported sol−gel processes of TiO2,26 SiO2,27 and WO3.28 One cycle of the drop-casting technique for W-doped BiVO4 deposition consisted of three steps: depositing 40 μL of the BiV0.99W0.01 precursor on the tin oxide-coated FTO substrates (10 mm × 30 mm), letting the solvent evaporate in a humiditycontrolled environment, and annealing on a heating plate at a surface temperature of 425 °C for 5 min. Eight drop-casting cycles were used to grow each film. Four sets of films were synthesized at four different relative humidities (RH < 20%, 30 ± 5%, 50 ± 5%, and 80 ± 5%). The W-doped BiVO4 films were then calcined in a tube furnace at 450 °C in air for 2 h (additional details in the Supporting Information (SI)). The formation of W-doped BiVO4 photoelectrodes prepared by this sol−gel technique is governed by four important steps. (1) The sol formation, a dispersion of colloidal particles in liquid, occurs by the hydrolysis reaction of metal nitrate and alkoxide precursors to form hydrated metal oxides. This process is followed by a polycondensation reaction to make metal oxide nanoparticles. (2) The gelation process occurs when hydrolysis and polycondensation proceed at a high rate, forming an interconnected 3D network of oxides when the drop-casted precursor is left to dry in the chamber. (3) The drying process removes the solvent from the gel, leaving a porous network behind. (4) The dehydration process by heat treatment converts the films to a stable metal oxide. The ambient humidity affects the rate of both hydrolysis and polycondensation reactions and solvent evaporation.29 Scanning electron microscopy (SEM) images show that the drop-casted W-doped BiVO4 photoelectrodes consist of coagulated particulates with varying size distributions depending on the relative humidity of the growth environment (Figure 1 and Table S1 in the SI). The W-doped BiVO4 synthesized at low humidity (RH < 20%) consists of small closely packed particles with a characteristic size of 160 ± 32 nm, forming a fairly flat film with low porosity (Figure 1e). In contrast, synthesis at higher humidities resulted in highly porous networks of larger particle sizes with broader distributions from 204 ± 44 nm at RH ∼ 30% to 196 ± 56 nm at RH ∼ 50% and 208 ± 92 nm at RH ∼ 80%. The larger particle size 3703


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oxidation and reduction reactions. We further investigated the potential window of this process and found that under the potential cycling window from 0.2 V to any potential greater than 1.6 V vs RHE, Au NPs on conductive FTO slides exhibit an irreversible sintering process observable in SEM (Figure S5). This process occurs at low current densities, even as low as 0.01 mA/cm2. Hence, Au NP sintering can be used to map the oxidizing power of the photogenerated holes on metal oxide semiconductor surfaces with high sensitivity (see discussion in the SI). The Au NPs were synthesized on a similarly prepared set of W-doped BiVO4 photoelectrodes by depositing 3 nm of Au by physical vapor deposition (PVD) onto their top surfaces. Films were subsequently examined for PEC water oxidation by applying 5 potential cycles from 0.2−2.4 V vs RHE under constant illumination from the back side in a KPi electrolyte (see J−E plots in Figure S6). SEM was used to examine asdeposited (Figure 3a−d) and PEC tested (Figure 3e−h) portions of each sample. Figure 3a−d reveals that, for the as-deposited samples, PVD Au spontaneously forms Au NPs (10 ± 6 nm), closely packed and uniformly coated across the top surface of each sample. The SEM images in Figure 3e−h, measured after PEC testing, reveal various degrees of Au sintering, ranging from the RH < 20% sample, which showed no noticeable change in the Au NPs to the RH ∼ 30−80% samples, which showed significant sintering (see Figure S7 for higher resolution images). Among the RH ∼ 30−80% W-doped BiVO4 films, the trend in Au NP sintering correlates directly with the macroscopic roughness of each sample. The RH ∼ 30% film is the flattest of the three, and also shows the most even sintering of Au NPs across its surface. The RH ∼ 50% film shows greater variation in film topology; consequently, the sintering was not uniform. The thinner regions of this particular film clearly exhibited a higher degree of sintering than on the thicker portions. The RH ∼ 80% film exhibits the greatest variation in film topology of all, and correspondingly the greatest variation in Au sintering across its surface. As seen in Figure 3h, the thinner regions of the RH ∼ 80% film exhibit significant Au nanoparticle sintering while for the thicker regions of the film, no sintering was observed. Recognizing that Au NPs tend to sinter under potential cycling between oxidation and reduction reactions,24,25 the sintering effects observed in Figure 3a−h can shed light onto the relationship between morphology and PEC performance among the W-doped BiVO4 photoelectrodes. The absence of Au NP sintering at the topmost layer of the RH < 20% sample suggests that the topmost layer was inactive for water oxidation. Negligible sintering was observed even at a higher anodic potential of 3.1 V vs RHE (Figure S8). This suggests that the top portion of the film is electrically isolated from the ohmic contact at the back. Hence, a large portion of the film is a “dead layer”, which does not experience the applied potential and thus does not contribute to the overall photocurrent density. The “dead layer” could be caused by the high film resistivity arising from incomplete gelation process during the sol−gel synthesis due to low humidity and increased number of interparticle boundaries due to the smaller size of the W-doped BiVO4 particles. As a result, only the W-doped BiVO4 particles deeper within the porous electrode, closer to the FTO surface, would be photoactive. Hence, the photocurrent density of such a thin photoactive region would be expected to reach saturation, exactly as we had observed in Figure 2a.


The PEC performance of the W-doped BiVO4 samples was measured in a three-electrode configuration using a Pt wire counter electrode and a Ag/AgCl reference electrode (SI). Water oxidation was performed in 0.1 M potassium phosphate buffer (KPi) and the chopped AM1.5G current−voltage (J−E) plots measured with back-side illumination (via FTOs) are shown in Figure 2a. All potentials are reported on the RHE scale.

Figure 2. Chopped AM1.5G J−E measurements of W-doped BiVO4 electrodes in 0.1 M KPi (a), and in 0.1 M Na2SO3 + 0.1 M KPi (b). IPCE (c) and APCE (d) at 1.23 V vs RHE in 0.1 M Na2SO3 + 0.1 M KPi. The illumination for all the measurements was done on the back side.

Figure 2a shows a clear trend in photocurrent density for water oxidation: [RH ∼ 30%] > [RH ∼ 50%] > [RH ∼ 80%] > [RH < 20%]. The RH ∼ 30% sample, which was the most active film, achieved a water oxidation photocurrent of 1 mA/ cm2 at 1.23 V vs RHE. Two features in the J−E measurements Figure 2a are particularly intriguing: (1) The RH < 20% sample was the only one whose photocurrent density reached a saturation value (∼0.8 mA/cm2) within the potential range investigated, whereas the photocurrent density of the RH ∼ 30−80% samples continued to rise as a function of higher applied anodic potentials. (2) The photocurrent onset of the RH < 20% film was approximately +0.2 V more anodic than that of the RH ∼ 30−80% samples. Upon further investigation by varying the thickness of the RH < 20% film, we discovered that the optimal thickness for these films was much thinner than that for films synthesized at RH ∼ 30−80%. Irrespective of the thickness, the performance of each RH < 20% film was inferior to those of the RH ∼ 30% films (Figure S4). As described earlier, controlling relative humidity during synthesis primarily contributes to differences in film morphology among the W-doped BiVO4 samples. Thus, to explore the relationship between morphology and PEC activity, we developed a method to map the photocurrent distribution across the nanostructured surface. The method involves deliberate sintering of Au NPs under potential cycling. We discovered that Au NPs sinter when undergoing alternating 3704


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Figure 3. Top down SEM images of Au nanoparticle coated W-doped BiVO4 samples synthesized at RH < 20% (a,e), RH ∼ 30% (b,f), RH ∼ 50% (e,g), and RH ∼ 80% (d,h). The as-deposited surfaces were evenly coated with compact layer of Au NPs (a-b). After water oxidation, different degrees of Au NP sintering were exhibited on each sample (e−h). According to the Au NP sintering shown above, the PEC activity distribution on each sample is illustrated by the cross sectional schematics of RH < 20% (i), RH ∼ 30% (j), RH ∼ 50% (k), and RH ∼ 80% (l).

2a, where the onset of the RH < 20% sample was shifted by +0.2 V versus the others. This implies that the origin of the +0.2 V shift in onset is not due to differences in the electronic structure of the band edges, but rather to differences in surface catalysis, also consistent with a thin photoactive layer as described previously. The limited PEC active surface area for the RH < 20% sample would require larger overpotentials to drive the water oxidation reaction compared to the RH ∼ 30− 80% samples. The incident photon to current efficiency (IPCE) and the absorbed photon to current efficiency (APCE) were also measured for sulfite oxidation on all samples to further probe their PEC behavior (Figure 2c,d). The absorption coefficients (α) for short wavelength photons are much higher than for longer wavelength photons near the band edge (Figure S1c). Thus, the short wavelength photons are expected to be absorbed closer to the back of the samples, the direction from which the light was illuminated, whereas longer wavelength photons are expected to penetrate deeper through the samples closer to the front-face of the electrode. Recall that all samples exhibited similar optical absorption properties; yet, Figure 2c,d shows that IPCE and APCE data exhibit differences among samples. The RH ∼ 30% and RH ∼ 50% samples show onsets close to the band edge of 520 nm, while the onsets of the RH < 20% and RH ∼ 80% samples occurred at shorter wavelengths, closer to 500 nm. The low photon conversion efficiency in the 500−520 nm range indicates that relative to the RH ∼ 30% and RH ∼ 50% photoelectrodes, much less PEC activity occurred from the topmost layers of the RH < 20% and RH ∼ 80% photoelectrodes, consistent with the PEC mapping results with Au NPs. However, unlike the RH < 20% photoelectrode where

Significant sintering of Au NPs on the topmost surfaces of RH ∼ 30−80% samples indicates that a larger portion of the W-doped BiVO4 film was utilized, from the topmost layer to deeper below, leading to higher PEC performance than the RH < 20% sample. The RH ∼ 30% achieved the highest photocurrent density of all the samples, as might be expected given the even distribution of Au NP sintering in Figure 3f. Figure 3g,h shows that the RH ∼ 50% and RH ∼ 80% samples exhibited nonuniform sintering of Au NPs, a map indicating nonuniform PEC response across those surfaces, consistent with lower photocurrent densities compared to the RH ∼ 30% sample. To elucidate the PEC performance of the W-doped BiVO4 photoelectrodes without involving the kinetics of water oxidation, we performed PEC sulfite oxidation on the Au-free samples as a diagnostic tool (Figure 2b). Due to the facile kinetics of sulfite oxidation compared to water oxidation, it can be assumed that the majority of the photogenerated holes that reach the surface can participate in the sulfite oxidation reaction.30,31 Therefore, the photocurrent onset potential in sulfite photo-oxidation is only limited by the location of the Wdoped BiVO4 flat band potential.32 In general, the trends in PEC activity for sulfite oxidation were similar to those observed for water oxidation (Figure 2a,b). The RH < 20% sample also exhibited saturation in the sulfite oxidation photocurrent density just as it did in water oxidation, as expected based on the thin photoactive layer discussed previously. However, one key difference is that in the case of sulfite oxidation for the RH < 20% sample, the photocurrent onset potential is the same as with the other three samples, RH ∼ 30−80%. This result is different from that of the PEC water oxidation study of Figure 3705


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edges support from the DARE Doctoral Fellowship supported by the Vice Provost for Graduate Education at Stanford University. Characterizations were performed at Stanford Nano Shared Facilities (SNSF). The authors would like to thank Tom Carver for Au deposition and Dr. Jakob Kibsgaard, Dr. Arturas Vailionis, and Dr. Claude Reichard for helpful discussions.

the low activity arises from electrically isolated particles deep within the film, the thick portion of the films synthesized at higher humidity (RH ∼ 50−80%) can be activated by applying a higher anodic potential. Figure S9 in the SI shows that with an applied potential of 3.1 V vs RHE, the Au NPs on the surface of the RH ∼ 50% film had completely sintered, indicating photoactivity of the top-surface. In summary, the highest performance films consist of larger W-doped BiVO4 particles with flatter topologies. The PEC mapping studies show that such a surface yields uniform Au NP sintering, indicating an even distribution of photoactivity, which is a desirable situation. Films consisting of smaller, densely packed particles are less photoactive, as the topmost surface is unable to fully participate in the photoelectrochemistry, likely due to charge transfer limitations across interparticle boundaries. For the same reason, films consisting of larger particles might not have full PEC access to the topmost surface if the film topology is macroscopically rough, leading to regions that are too thick. These concepts are illustrated in Figure 3i− l.The combination of the PEC mapping methods developed in this work along with conventional PEC measurements has provided deeper insights into understanding how morphology plays a crucial role in PEC performance of sol−gel-derived Wdoped BiVO4 photoelectrodes. The PEC mapping study allows one to directly visualize regions of activity of photogenerated holes across the photoelectrode surfaces at different applied potentials, providing a one-to-one correlation between PEC performance and morphology. The Au NP mapping demonstrated that for a highly resistive oxide semiconductor such as W-doped BiVO4 presented in this work, the most active particles in the film are those closest to the substrate, requiring the least amount of applied potential to drive water oxidation. The electrode that consists of thick resistive particles always suffers from the resistance losses, resulting in a photoelectrode with lower photocurrent and a lower fill factor. Overall, the PEC mapping methods developed in this work provide a new means to understand the performance of photoelectrodes, particularly those with complex morphologies as shown in this study of W-doped BiVO4. PEC mapping by means of photoinduced Au NP sintering is potentially extendable to other materials, providing new information that would allow for the development of improved photoelectrodes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01587. Additional details on the synthesis and the characterization methods. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.C. was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy under contract number DE-AC02-05CH11231 through subcontract 7058299 from Lawrence Berkeley National Laboratory. L.C.S. acknowl3706


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