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Antimony(III) sulfide thin films as a photoanode material in photocatalytic water splitting Alexander D DeAngelis, Kingsley Christian Kemp, Nicolas Gaillard, and Kwang S. Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12178 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 25, 2016

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ACS Applied Materials & Interfaces

Antimony(III) sulfide thin films as a photoanode material in photocatalytic water splitting Alexander Daniel DeAngelis#1,3, Kingsley Christian Kemp*,#1,2,4, Nicolas Gaillard3, and Kwang S. Kim*1 1

Center for Superfunctional Materials, Department of Chemistry, Ulsan National Institute of

Science and Technology (UNIST), UNIST-gil 50, Ulsan 689-798, Korea 2

Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science

and Technology, Pohang 790-784, Korea 3

Hawaii Natural Energy Institute, University of Hawaii, 1680 East-West Rd POST 109

4

Current address: Department of Environmental Engineering, Pohang University of Science and

Technology, Pohang 790-784, Korea #

These authors contributed equally to this work.

*Corresponding author E-mail address: [email protected] or [email protected]

ACS Paragon Plus Environment


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Abstract For the first time, we present exploratory investigations on the performance of thermally evaporated Sb2S3 thin film photoanodes for solar-assisted water splitting applications. With a bandgap of 1.72 eV, a 250 nm thick Sb2S3 photoanode showed a saturation photocurrent density of ~600 µA cm-2 measured at 1.0 V vs. reversible hydrogen electrode (RHE) in 0.1 M Na2SO4 under 1-sun illumination, with an onset potential of ~0.25 V vs. RHE. However, subsequent photodegradation studies revealed that the material dissolves relatively quickly with the application of both illumination and bias. Nonetheless, Sb2S3 does have the advantage of having a relatively low optimal fabrication temperature of 300 °C and thus may have utility as a top cell absorber of a tandem device where the bottom cell is temperature sensitive, if protected from corrosion. Therefore, we characterized relevant aspects of the material in an attempt to explain the large difference between the theoretical maximum and measured current density. From our characterization it is believed that the photocatalytic efficiency of this material can be improved by modifying the surface to reduce optical reflection and addressing inherent issues such as high electrical resistivity and surface defects. Keywords: water splitting; antimony sulfide; photocatalytic; thin film; PEC


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1. Introduction Burning rapidly depleting carbon based fuels, as we do now, releases greenhouse gases (CO2 and acidic SO2/NOx)1 that enable acidic rain and anthropogenic global warming, which may foster an environment harmful to our health.2,3,4 Environmentally-friendly and renewably abundant, so-called next generation, fuels will need to be sourced to ultimately solve these issues. Of these fuels, hydrogen holds promise as it can be generated by splitting water using renewable energy sources such as solar and wind, this hydrogen can then be utilized in high-efficiency energy conversion devices, such as fuel cells, which produce water as the primary by-product. Photoelectrochemical (PEC) hydrogen production is one viable direct solar water-splitting process. However, the main predicament that the PEC water-splitting community finds itself in is that it cannot seem to find the perfect material for the task at hand, i.e. an efficient photocatalyst that is cheap and stable, with a band gap that allows a maximum energy conversion. When cheap and stable, they are inefficient (α-Fe2O3 with a bandgap of 2.1 eV),5 TiO2 (3.0 eV),6 ZnO (3.2 eV),7 WO3 (2.8 eV).8 When efficient, they are instable and expensive (InGaP2).9,10 As a workaround, some attempt to stabilize well-performing materials (Cu2O)11,12 while others try to enhance efficiency by fashioning nanostructures,13 doping,14 or modifying kinetics with surface catalysts.8,15,16 However, there are still many candidate materials yet to have been explored.

In this paper, we introduce antimony sulfide (Sb2S3) and discuss its potential, and shortcomings, as a photoanode for PEC water splitting. Our work points out a bandgap of 1.72 eV for this material, a value that practical theoretical calculations, based on best-case scenario overpotentials and maximum photocurrent density arguments, have shown to be optimal for PEC water splitting.17,18,19 We also demonstrate that photoactive Sb2S3 thin films can be synthesized at



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relatively low temperatures (250 ºC), a beneficial feature for both monolithic tandem PEC device integration and production cost. To the best of our knowledge, this is the first report on Sb2S3 for PEC water splitting. However, Sb2S3 has been reported for other applications including: photovoltaics,20,21,22 photosensitization of large band gap semiconductors,23

photocatalytic

environmental remediation,24,25 battery storage,26 and even in PEC solar cells.27,28,29 We reasoned, then, that it must be a very inactive photocatalyst for water splitting. However in our study, minimal optimization efforts with respect to film thickness and annealing temperature allowed us to achieve a saturation photo current-density of 600 µA cm-2. Although photocorrosion of the substance was observed, we reasoned that if Sb2S3 were protected from this corrosion then it still may be a viable material as a top cell absorber in a tandem device. This prompted us to characterize the material further to figure out how to improve it. Broadly speaking, we found three areas for improvement: high optical reflectance, high electrical resistivity, and Fermi-level pinning. 2. Materials and Methods 2.1. Materials Sb2S3 was purchased from Junsei Chemicals (32590-1501, minimum assay 95 %) and used without further purification. Argon gas (99.99 %) supplied by Linde was used as an inert atmosphere during annealing processes. Fluorine doped Tin Oxide (FTO) coated glass was supplied by Aldrich (surface resistivity 13 Ω.square-1). 2.2. Sample Fabrication Amorphous Sb2S3 thin film electrodes were fabricated by thermal evaporation onto cleaned FTO substrates. A Mo boat was filled to capacity with Sb2S3 powder and the system was


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pumped down to a base pressure of ~ 5 x 10-5 Torr. The electric current of evaporation was varied from 36 – 41 A to maintain a rate of evaporation about 10 to 20 Å s-1 measured by a quartz crystal monitor. There was no intentional heating of the samples throughout the deposition. After depositing to the desired thickness, the gate valve was closed and samples were then allowed to remain in high vacuum for 30 minutes before venting. All thermal evaporations were carried out using a Deki Hi Tech DKOL04-02-T thermal evaporator connected to a Hyundai Engineering Co. Cryopump HD-03A. The crystalline phase of the Sb2S3 material was obtained by annealing the as-deposited samples under an Ar atmosphere at temperatures of 250 to 400 °C for 1 hour. Electrodes were then fabricated by attaching an insulated copper wire with indium solder to the exposed FTO then insulating all exposed conductive areas with epoxy, see Figure S1. 2.3. Hall Effect sample preparation Solid state Hall Effect measurements were made on square shaped (6 x 6 mm) pellets of Sb2S3 ~1 mm in thickness. These pellets were prepared by compressing as-purchased Sb2S3 in a hydraulic press at 50 MPa. The compressed pellets were subsequently annealed at 500 °C under an Ar atmosphere. A higher annealing temperature, than applied for the thin films, was chosen to ensure crystallization as well as sinter the Sb2S3 particles to form a cohesive and compact pellet. Ohmic contact pads were made on the corners of the surface with silver paste, which yielded linear current-voltage behaviour for scans between adjacent pads. 2.4. PEC Measurements All PEC measurements were conducted using a standard three-electrode setup with Pt as a counter electrode, Ag/AgCl/sat. KCl as a reference electrode and the as-fabricated Sb2S3 thin



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film as a working electrode. The working electrodes were illuminated vertically from above with the Sb2S3 thin film facing upwards towards the incoming irradiation. In this paper, potentials are reported against the reversible hydrogen electrode (RHE), this is obtained by converting the experimental data by adding 0.2 + (0.059 x pH) V. The supporting electrolytes used during linear sweep voltammetry and chronoamperometry were 0.1 M Na2SO4 (pH = 5.5) or 0.5 M H2SO4 whereas for illuminated open-circuit potential measurements we employed 0.1 M HCl (pH = 1), as well as buffers of pH = 4 (potassium hydrogen phthalate, Hydrion Micro Essential Labs) and pH = 8 (sodium tetraborate decahydrate, Hydrion Micro Essential Labs), both buffers also contain dihydrogen phosphate and disodium hydrogen phosphate,. Fresh electrolyte was used for each test. All photoelectrochemical measurements were performed using a Princeton Applied Research VersaSTAT 3 potentiostat with a solar simulator (Newport 94082A Class ABA Solar Simulator) outputting 1-sun, which was calibrated by adjusting the input power until its measured illumination power density (ThorLabs, PM100, 400-1100 nm) matched the integrated power density of the AM1.5G spectrum (ASTM G173-03) in the same wavelength range. 2.5. Materials Characterization Raman spectra were obtained using a Bruker Senterra Raman Microscope. A 532 nm laser was used for preliminary material characterization while a 785 nm laser was used for the PEC photocorrosion studies. X-ray diffraction (XRD) patterns were obtained using a Rigaku, Japan, RINT 2500 V X-ray diffractometer using Cu Kα irradiation (λ = 1.5046 Å). Preliminary UV/Vis reflectance and transmittance measurements of films were obtained using an Agilent Cary 4000 spectrophotometer whereas UV/Vis transmittance measurements done for the photocorrosion studies were obtained using a Scinco S-3100 spectrophotometer. The y-axis of


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the Tauc plots (see Appendix) correspond to n=1/2 (allowed direct), n=3/2 (forbidden direct), n=2 (allowed indirect), and n=3 (forbidden indirect) in accordance with equation 1 shown below. (hνα)1/n = C (hν - Eg), where C is a constant of proportionality

(1)

The thickness of the as manufactured films was measured using a Tencor Alpha Step 500 profilometer. Hall Effect measurements were obtained using an ECOPIA Hall Effect Measurement System (HMS-5000 VER 5.6.1) equipped with a 0.570 T permanent magnet. SEM images of the thin films were obtained using an Agilent Technologies 8500 Field Emission Scanning Electron Microscope with a beam voltage of 1 kV. 3. Results and Discussion 3.1. Material Characterization Using XRD we were able to determine the temperature at which Sb2S3 changes phase from amorphous to crystalline. This transition takes place at 250 °C with the emergence of diffraction peaks, shown in Figure 1, at 2θ angles of 15.7, 17.6, 22.3, 25.0, 28.6, 29.2, 32.4, 35.6, 43.1, and 46.8 degrees, which match those for orthorhombic Sb2S3 (PDF 00-042-1393). Raman spectroscopy, shown in Figure 2, confirmed the presence of Sb2S3 in all samples indicated by the characteristic adsorption bands at ~170 and ~290 cm-1, which correspond to the Sb-Sb and Sb-S vibrations.30 The broad hump of the unannealed sample at 290 cm-1 begins to split into two peaks at 290 and 310 cm-1 at 250 °C, which is expected as the Sb and S form new bonds in transforming from the amorphous to crystalline phase. The full Raman spectrum for the samples is shown in Figure S2. With the exception of the FTO peaks, all peaks in XRD and Raman belonged to Sb2S3.



* = FTO

* *

*

*

Relative intensity (a.u.)

*

* *

500 400 300 250 200

unannealed 10

20

30

40

50

60

70

80

2θ

Figure 1. XRD patterns of the Sb2S3 films annealed at different temperatures. Crystallization occurs at 250 °C and at 500 °C the sample sublimes exposing the FTO substrate. * indicates FTO peaks.

Sb-S

Sb-Sb

500

Relative intensity (a.u.)

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400 300

250 200 unnanealed 100

200

300

400

500

Raman shift (cm-1)

Figure 2. Raman spectra from 100 to 500 cm-1 for the Sb2S3 thin films. As the film crystallizes the 290 cm-1 band, associated with Sb-S bonding, splits into two well defined peaks.


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The SEM images of Figure 3 and S3 show that the surface changes from rough and amorphous to smooth and crystalline when annealed at 250 °C, an phenomenon that was also noticed by Tigau and colleagues.31 The surface in Figures 3b (250 °C) and 3d (300 °C) both look smooth. However, the two separate images (from which the composite images in Figure 3 are made from) more clearly show that crystal imperfections exist in the 250 °C sample, indicated by the red circles in Figure S3(h-i), that do not exist in the 300 °C sample. We believe these imperfections inhibited electrical conduction and as such their removal may explain why our 300 °C samples had optimal PEC behaviour, as will be shown later. Lastly, the thin film starts to decompose when annealed at 400 °C, evidenced by the large cracks in Figure 3c, and eventually completely evaporates at 500 °C with no residual trace of Sb2S3 as measured by XRD and Raman spectroscopy. It should be noted that the annealing of the Sb2S3 pellet for the Hall Effect at 500 °C did not lead to a complete evaporation of the sample due to bulk effects, i.e. far greater thickness of the pellet (̴ 1 mm) than the thin film (250 nm).



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Figure 3. SEM images of the Sb2S3 thin films (a) as deposited, (b) annealed at 250 °C (c) 400 °C and (d) 300 °C. The film becomes smoother as it crystallizes at 250 °C, but eventually begins to crack at 400 °C. The absorption coefficient and Tauc plots shown in Figures 4 and S4 suggest that Sb2S3 has three bandgaps. Excluding the allowed direct transition, all plots suggest that the first bandgap is 1.72 eV. This value of 1.72 eV for the first bandgap falls within the range of that reported for crystalline Sb2S3, which is between 1.70-1.80 eV.31,32,33 The other observed bandgaps in the Tauc plot at, 1.90-2.20 eV and 2.60-2.90 eV can be attributed to electron transitions from: three distinct valence band maxima to a single conduction band minimum, a single valence band maximum to three distinct conduction band minima, or any combination


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thereof. We do not believe these bandgaps to be attributed to different phases as our XRD data show the existence of only one Sb2S3 phase. However, contrary to other groups who have claimed that the bandgap is an allowed direct transition,32 we found that a forbidden direct transition, plotted in Figure 4, yielded R2 values closer to unity for the energy range across which the data was fit to a linear regression. It is important to keep in mind though that the difference in the R2 values between the forbidden and allowed direct transitions is small. The works of both Fujita et al. and Shutov et al. yielded experimental and theoretical evidence that suggests that the fundamental band gap of 1.75 eV is indirect whereas the direct band gap is 1.88 eV.34,35 Being that these two band gaps are so close in energy to each other it is no surprise that they are commonly interpreted to be one single excitation However we found no mention in the literature of the two other possible band gaps that appear in our data. A cryogenictemperature photoluminescence study, also by Fujita et al, reveals the excitonic nature of the indirect fundamental band gap, but only includes measurements up to 1.9 eV.36 Photoluminescence measurements and theoretical calculations at higher energies may prove instrumental in definitively confirming the existence and determining the precise value of the optical excitations of Sb2S3.



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Figure 4. Tauc plot of Sb2S3 for the forbidden direct transition showing a bandgap of 1.72 eV. The other two observed bandgap values can be assigned to photocorrosion processes. Solid state Hall Effect measurements were conducted to assess the electrical properties of Sb2S3. We found that we could extract electrical information using compressed and annealed ~1 mm thick 1 x 1 cm Sb2S3 pellets. The results of Table 1 show that the resistivity of crystalline Sb2S3 was on the order of 105 Ω-cm, which corroborates the resistivity data obtained by Tigau and colleagues.37 More importantly we were able to determine, for the first time, the carrier concentration and hall mobility for Sb2S3, which were ~4 x 1012 cm-3 and ~7 cm2 V-1 s-1, respectively. We were however unable to determine if Sb2S3 was inherently n- or p-type, as repeated measurements of the Hall coefficient on a given sample resulted sometimes in negative and other times positive values. We are confident that the values obtained were not from systematic error as using the same apparatus consistently yielded a resistivity value of 1.95 x 10-4


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Ω-cm for an indium tin oxide (ITO) reference sample.38 As such, the data presented in Table 1 is a statistical analysis of only the positive values. However, we should note that the absolute value of the average of the negative values is within one standard deviation of the average of the positive values. This uncertainty in the inherent n- or p-type nature of Sb2S3 would suggest that the Fermi level lies in the middle of its bandgap and thus would explain why it is able to produce both photo-anodic and -cathodic current, as shown later. Table 1. Electrical properties of Sb2S3.

Resistivity

Carrier Conc.

Hall Mobility

ρ

N

µ

Mean

5.22 ͯ 105 Ω-cm

3.652 x1012 cm-3

7.096 cm2V-1s-1

Standard Deviation

2.84 ͯ 105 Ω-cm

1.725 x 1012 cm-3

3.191 cm2V-1s-1

Reference ITO

1.95 ͯ 10-4 Ω-cm

-

-

3.2. Device Performance and Analysis

The optimized crystalline Sb2S3 samples on which we performed linear sweep voltammetry (LSV) exhibited the overall behaviour seen in Figure 5, characterized by a single



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photocurrent onset potential at 0.25 V RHE with a cathodic and anodic photocurrent density (Jph). The fact that there is a negative and positive photocurrent implies, along with the Hall measurements, that the material is lightly doped as heavily doped materials tend to have only either a cathodic or anodic photocurrent. The maximum cathodic Jph was no more than 100 µA cm-2 prior to its dark current onset at 0.1 V RHE, which prompted us to shift our attention to studying the anodic Jph. Operating as a photoanode, the optimal annealing temperature and thickness were subsequently determined to be 300 °C and 250 nm (Figures S5 and S6). We should note that this temperature is slightly higher than needed for crystallization, yet affords optimal chronoamperometry (CA) results.31 Comparing the SEM images of the samples in Figures 3b and d, we observe that structural defects appear to be removed upon increasing the annealing temperature, i.e from 250 to 300 °C, which may have led to improved electrical mobility and thus conductivity. In contrast, the optimized amorphous Sb2S3 films showed a negligible photocurrent density (Jph), see Figure S7. At low anodic bias we observed that the photocurrent spikes and then rapidly decays upon illumination during the chopped-light LSV measurement indicating that reaction kinetics at the surface of Sb2S3 are possibly sluggish, which may be due to the high surface state density discussed below. Eventually this anodic Jph begins to plateau and eventually saturates, seen in Figure 5b, around 600 µA cm-2 at 1.0 V RHE. It should be noted that at 1.5 V RHE the transient spikes flatten out indicating that the kinetic barriers have been overcome. At 1.9 V RHE there exists yet another onset in Jph, which is then followed by the anodic dark current onset at 2.5 V RHE.


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Current Density (mA/cm2)

(a) 0.2

0.1

0.0

0.2

0.4

0.6

0.8

Voltage (V vs RHE) (b) 4.0


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3.5 3.0 2.5

Dark current Chopped photoresponse

2.0 1.5 1.0 0.5 0.0 0.0

0.5

1.0

1.5

2.0

2.5

Voltage (V vs RHE)

Figure 5. Chopped linear sweep voltammetry of Sb2S3 under 1-sun illumination showing a) the onset voltage at 0.25 V and b) a saturation photocurrent of 600 µA cm-2 at 1.0 V with a second onset at 1.9 V.



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The maximum theoretical photocurrent density (Jph,max) for a material of bandgap 1.72 eV was calculated to be 19 mA cm-2 using Equation 2, where Φ is the photon flux of the AM1.5G spectrum, and assuming an incident-photon-to-current efficiency of 100 %:

Jph,max = . = 19

(2) Dividing 0.6 by 19 mA cm-2 shows that the saturation Jph only accounts for 3 % of the Jph,max. This is obviously a major drawback for the application of pure Sb2S3 as 97 % of the incoming photons are not utilized in the PEC reaction. One of the first visual observations of the post-annealed Sb2S3 thin films was its mirrorlike sheen (Figure S1d), which led us to suspect it possessed a high overall reflectance that would consequently lower photocurrent. Indeed reflectance measurements, shown in Figure 6, confirmed that anywhere from 25 to 62 % of light is reflected for photon energies equal to or greater than the bandgap. Multiplying the integrand of equation 1 by the measured reflectance and recalculating Jph,max showed that 7 of the 19 mA cm-2 are simply lost from reflection, which accounts for 37 % of the Jph,max. The remaining photons that are not reflected are then either absorbed or transmitted by the thin film. Using the absorption coefficient, calculated from optical measurements, we calculated that only 1 mA cm-2 worth of photons is actually transmitted through a 250 nm thin film, which accounts for 5 % of Jph,max. Therefore, optical losses are largely responsible for the poor performance of Sb2S3 at moderate anodic bias accounting for 42 % of the Jph,max. This suggests that there is quite a bit of room for improvement of the PEC performance of Sb2S3 if these losses can be eliminated, for example, by modifying the surface morphology to reduce reflection.


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Figure 6. Reflectance measurement of Sb2S3, showing significant reflectance losses of at least 25 % at energies above the bandgap. The green and red squares are included as a visual aid. The remaining 58 % of photons that are actually absorbed are subject to internal device losses. The built-in voltage (Vbi), defined as the voltage that exists at the semiconductor/liquid interface at equilibrium, is the heart of the PEC device as it is responsible for separating photogenerated charges. Thus, illuminated open-circuit potential (IOCP) measurements at different pH were performed to determine the material’s ability to develop a Vbi. In this manuscript, Vbi is equal to the difference between the dark and saturated light open-circuit voltage measurements, as shown in figures S9 and S10, against a Ag/AgCl/sat KCl reference electrode. As Figure 7 shows, the Vbi of Sb2S3 was not strongly correlated with pH. Regardless of the sample (all from separate batches) the observed Vbi exhibited non-ideal behaviour with a maximum shift of ~9 mV pH-1, which is 15 % of the maximum theoretical Nernstian room temperature dependence of 59 mV pH-1. This high degree of non-ideality is most likely borne out by Fermi level pinning, a phenomenon where a high density of surface states trap charge carriers



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and thus weaken the electrical interaction between the semiconductor and electrolyte,39 which hinders charge separation and may negatively affect the reaction kinetics as well. It is possible that the Sb2S3 thin film surface could be transformed into SbO or other species in the aqueous phase under illumination. As such this transformation of the Sb2S3 surface species could be a major contributor towards the proposed Fermi level pinning. Regardless, by altering the surface of Sb2S3 one may be able to decrease the surface state density and allow for the realization of its maximum built-in voltage that will in turn improve overall charge separation and possibly surface kinetics. After being separated, charge carriers must be able to conduct through the bulk. As mentioned before, we measured the resistivity of Sb2S3 to be on the order of 105 Ω-cm. However this high resistivity is mainly due to a low carrier concentration (~1012) as the mobility of Sb2S3 was measured to be on the order of 10 cm2 V-1 s-1 (see Table 1). Thus, strategies focused on increasing the carrier concentration (e.g. impurity doping) should be more effective at increasing PEC performance than those that circumvent mobility (e.g. c-axis nanowire arrays).


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Figure 7. Built-in Voltage measurements of several Sb2S3 thin films showing that its dependence on pH is far from the ideal 59 mV pH-1 at 25 °C, presumably due to Fermi level pinning. Pearson’s correlation coefficients for the linear regressions shown in the figure are 0.48017 for Sample 1, 0.95317 for Sample 2 and 0.99186 for Sample 3. As a preliminary test for photocorrosion, un-wired samples were placed in aqueous solutions of varying pH. In acidic (0.1 M HCl) and neutral (0.1 M Na2SO4) solutions no corrosion was visually observed after 10 minutes whether in dark or 1-sun illumination. In contrast, samples placed in a basic (0.1 M NaOH) solution were visibly corroded within minutes in the dark, while illumination accelerated this process. To assess the long-term durability of Sb2S3 under light and bias we subjected samples to hour-long PEC CA measurements at 0.73 V RHE under 1-sun illumination (Figure S8). To the naked eye, there were no changes of physical appearance in any of the films as a result of the durability tests. As such, UV-VIS transmittance and Raman spectra were measured before and after each test to provide insight on any chemical changes that may have occurred, shown in figures 8 and 9 respectively.



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Figure 8. Transmittance measurements on Sb2S3 films before and after the durability test. Major changes in shape and uniformly increased magnitude, as would be expected from corrosion, were not observed. The increase in transmittance at 900 nm could be from minor structural changes.

Figure 9. Raman measurements of Sb2S3 films before and after an hour long PEC CA photocorrosion test. Major changes in shape and uniformly decreased magnitude, as would be expected from corrosion, were not observed. As the measurements were nearly identical before and after testing, we initially were led to believe that the material was fairly stable. However, performing the same PEC CA measurement at 1.0 V RHE on fresh samples led to a complete dissolution of the film all the way down to the FTO substrate within 30 to 60 minutes. This has led us to believe that the previous durability test was corroding at a rate slow enough such that we could not observe it after an hour.


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Repeating this measurement in 0.5 M H2SO4 also led to a complete dissolution of the film at an approximately equal rate. Using a Hofmann apparatus, we conducted a series of PEC CA measurements, with the addition of a surfactant and a stir bar, to determine the faradaic efficiency of the oxygen evolution reaction. However, we were unable to measure any appreciable generation of oxygen and thus concluded that the faradaic efficiency is very low. Additionally, the difficulty in measuring oxygen generation could be attributed to a low steady state photocurrent. To circumvent the hindrance of photocorrosion we have also performed LSV measurements with various amounts of H2O2 added to the electrolyte. With a much faster reaction rate than that of water oxidation, we reasoned that by adding H2O2 we may be able to suppress the corrosion by providing a sacrificial reaction pathway.40 We also suspected that we may be able to extract the large photocurrent seen at high anodic bias by applying this technique. However, this proved ineffective at doing either and after several full-range LSV scans (0 – 2.5 V RHE) we observed that the film was already heavily corroded with the underlying FTO being nearly completely exposed. This would seem to suggest that the second onset of photocurrent observed for high anodic bias is due to an alternate reaction pathway for the photocorrosion of Sb2S3. It is interesting to note here that the highest photocurrent that we have observed for high anodic bias is 8 mA/cm2. 4. Conclusions Crystalline Sb2S3 thin films, with a bandgap of 1.72 eV, deposited onto FTO substrates by vacuum evaporation exhibited high reflectance, low carrier concentration, and a high density of surface states. Despite this and without further optimization beyond annealing temperature and thickness (300 °C, 250 nm), we were still able to measure a saturation photocurrent of 600 µA



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cm-2 at an applied external bias of 1.0 V vs RHE. Our analysis shows that this is only 3% of the maximum theoretical photocurrent density and that the limitation is due to high reflectivity, high resistivity, and possible Fermi level pinning. However, photocorrosion tests show that they degrade immediately in basic solution, even in the dark, and within an hour in acidic and neutral media upon applying illumination and moderate anodic bias. For this reason we believe that further investigations into the application of Sb2S3 with regards to PEC water splitting should address, first and foremost, the issue of photocorrosion. If this is done, there is merit in investigating its application to tandem devices as it is an inexpensive material with a low fabrication temperature. Acknowledgements This work was supported by NRF (National Honor Scientist Program: 2010-0020414). Supporting Information Available Additional SEM images, Raman and Tauc plots are shown. Photographs of electrode preparation methods and PEC optimization data referenced in the manuscript are also available in the supplementary information. This material is available free of charge via the Internet at http://pubs.acs.org. References 1. Hu, Y.; Naito, S.; Kobayashi, N.; Hasatani, M. CO2, NOx and SO2 Emissions from the Combustion of Coal with High Oxygen Concentration Gases. Fuel 2000, 79, 1925-1932. 2. Tebaldi, C.; Strauss, B. H.; Zervas, C. E. Modelling Sea Level Rise Impacts on Storm Surges Along US Coasts. Environ. Res. Lett. 2012, 7, 014032.


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36. Fujita, T.; Kurita, K.; Takiyama, K.; Oda, T. The Fundamental Absorption Edge and Electronic structure in Sb2S3. J. Phys. Soc. Jpn. 1987, 56, 3734-3739. 37. Tigau, N.; Rusu, G. I.; Ciupina, V.; Prodan, G.; Vasile, E. Structural and Electrical Properties of Antimony Trisulfide Thin Films. J. Optoelectron. Adv. M. 2005, 7, 727-732. 38. Granqvist, C. G.; Hultaker, A. Transparent and Conducting ITO Films: New Developments and Applications. Thin Solid Films 2002, 411, 1-5. 39. Bard, A. J.; Bocarsly, A. B.; Fan, F.-R. F.; Walton, E. G.; Wrighton, M. S. The Concept of Fermi Level Pinning at Semiconductor/Liquid Junctions. Consequences for Energy Conversion Efficiency and Selection of Useful Solution Redox Couples in Solar Devices. J. Am. Chem. Soc. 1980, 102, 3671 – 3677 40. Abdi, F. F.; van de Krol, R. Nature and Light Dependence of Bulk Recombination in CoPiCatalyzed BiVO4 Photoanodes. J. Phys. Chem. C 2012, 116, 9398-9404.

Table of Contents Graphic



4.0


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3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0

0.5

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Voltage (V vs RHE)


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Antimony(III) Sulfide Thin Films as a Photoanode ... - ACS Publications

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