Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2017, 8, 6099−6105
Photocurrent Enhancement by a Rapid Thermal Treatment of Nanodisk-Shaped SnS Photocathodes Malkeshkumar Patel,†,‡ Mohit Kumar,†,‡ Joondong Kim,*,†,‡ and Yu Kwon Kim*,§ †
Photoelectric and Energy Device Application Lab (PEDAL), Multidisciplinary Core Institute for Future Energies (MCIFE), Incheon National University, 119 Academy Road, Yeonsu, Incheon 406772, Republic of Korea ‡ Department of Electrical Engineering, Incheon National University, 119 Academy Road, Yeonsu, Incheon 406772, Republic of Korea § Department of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea S Supporting Information *
ABSTRACT: Photocathodes made from the earth-abundant, ecofriendly mineral tin monosulfide (SnS) can be promising candidates for p/n-type photoelectrochemical cells because they meet the strict requirements of energy band edges for each individual photoelectrode. Herein we fabricated SnS-based cell that exhibited a prolonged photocurrent for 3 h at −0.3 V vs the reversible hydrogen electrode (RHE) in a 0.1 M HCl electrolyte. An enhancement of the cathodic photocurrent from 2 to 6 mA cm−2 is observed through a rapid thermal treatment. Mott−Schottky analysis of SnS samples revealed an anodic shift of 0.7 V in the flat band potential under light illumination. Incident photon-to-current conversion efficiency (IPCE) analysis indicates that an efficient charge transfer appropriate for solar hydrogen generation occurs at the −0.3 V vs RHE potential. This work shows that SnS is a promising material for photocathode in PEC cells and its performance can be enhanced via simple postannealing.
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tion,22,23,26,28 successive ionic layer adsorption and reaction (SILAR),24 and chemical bath deposition (CBD).27,30 These results generally revealed that SnS photocathodes can provide the enhanced photocurrents, as summarized in Table 1, which compares the performances of various SnS photocathodes. The measured photocurrents are in the range of 0.6 to 0.7 mA cm−2 with stable cathodic operation in an aqueous solution with pH of ∼1, and they can be further improved by a proper preparation method for surface modification. In this study, SnS photocathodes obtained using a recently developed fabrication method have been subject to a post-thermal process that can induce surface modification. Our analysis revealed that the post-process enhances the photocurrent of our SnS photocathodes in PEC operation. Herein, high-quality thickness-controlled SnS films were fabricated on a fluorine-doped tin-oxide-coated glass (FTO/ glass) substrate using radio frequency (RF) magnetron sputtering of the amorphous SnS2 target following the procedure described in our previous reports.15,31 Thick (100−200 nm) SnS photocathodes exhibited photocurrents that were approximately double those of the ultrathin (10−20 nm) SnS samples at 0 V vs the reversible hydrogen electrode (RHE) in an aqueous solution (0.1 M HCl) and in seawater. Photoinduced Mott−Schottky characteristics were also correlated with the enhanced PEC cell performances. In addition to
hotoelectrochemical (PEC) water splitting for hydrogen production has attracted much attention because it is considered to be environmentally friendly and cost-effective. A typical PEC cell for a two-step water splitting is composed of a photocathode and a photoanode that are connected to each other because it can relax the strict requirement for each individual photoelectrode in relation to the conduction band minimum and valence band maximum.1−5 Inorganic semiconductor thin films used as photocathodes produce highly pure molecular hydrogen via the half-cell reaction involving the reduction of protons.2,6−8 The semiconductor materials used for solar cells have been commonly used as the photoelectrode materials in PEC hydrogen production because of their high absorption of the sunlight spectrum. Cost-competitive photocathode materials would be desired. Most of all, however, they need to provide a stable photocurrent (∼10 mA cm−2) with long-term stability for use in solar-driven hydrogen generation under the reaction condition.9,10 Thus to ensure chemical stability under the harsh PEC reaction condition, protective overlayers for the photoabsorbing materials are typically required to prevent PEC corrosion.5,11−14 Earth-abundant nontoxic tin monosulfide (SnS, mineral Herzenbergite) is a potential candidate for such an application due to its optical band gap (1.4 to 1.7 eV), appropriate for solar energy absorption and high absorption coefficient (>104 cm−1).15−20 In addition, intrinsic p-type nature of SnS makes it promising for use as a photocathode material for PEC cells.21−30 Recently, SnS photocathodes have been obtained from various fabrication methods such as solution-phase deposition,29 spray pyrolysis,21,25 electrochemical deposi© XXXX American Chemical Society
Received: November 10, 2017 Accepted: December 6, 2017 Published: December 6, 2017 6099
DOI: 10.1021/acs.jpclett.7b02998 J. Phys. Chem. Lett. 2017, 8, 6099−6105
Letter
The Journal of Physical Chemistry Letters
Table 1. Summary of the Performances of SnS Photocathodes, Including the Cathodic Photocurrent, Choice of Electrolyte, and Fabrication Methodsa year/ref 21
2014/ 2014/29 2015/22 2016/23
2016/24 2016/25 2017/26 2017/27 2017/28 2017/30 present study
photocathode structure FTO/Cu:SnS FTO/SnS FTO/Sb:SnS FTO/In:SnS FTO/SnS FTO/SnS/Nb2O5 FTO/SnS/TiO2 FTO/SnS/Ta2O5 FTO/SnS/TiO2 FTO/SnS/CdS Sn/SnS FTO/SnS FTO/SnS/CdS/Pt FTO/SnS FTO/SnS/pt FTO/SnS/CdS/TiO2/Pt FTO/SnS (160 nm) FTO/SnS@ 300 °C FTO/SnS@400 °C FTO/SnS@450 °C
photocurrent
electrolyte
potential
3.2 mA cm 0.2 mA cm−2 0.3 mA cm−2 (Sb:SnS)
0.1 M K4Fe(CN)6 0.1 M Eu(NO3)3 0.1 M Na2S2O3
−0.5 V vs Ag/AgCl −0.7 V vs SCE −0.2 V vs Ag/AgCl
spray pyrolysis solution-phase deposition electrochemical
0.183 mA cm−2 0.620 mA cm−2 0.547 mA cm−2 0.320 mA cm−2 80 μA cm−2 10 μA cm−2 8 μA cm−2 32 μA cm−2 0.7 mA cm−2 0.2 mA cm−2 1 mA cm−2 2.4 mA cm−2 2.0 mA cm−2 3.8 mA cm−2 4.0 mA cm−2 3.0 mA cm−2
0.1 M Na2SO4
−0.8 V vs Ag/AgCl
electrodeposition
Na2SO3 + Na2S 0.5 M H2SO4 0.1 M Na2SO4 0.5 M H2SO4
0 V vs Ag/AgCl −0.4 V vs RHE −0.5 V −0.4 V vs Ag/AgCl
SILAR spray pyrolysis electrochemical CBD
0.1 M H2SO4 0.5 M H2SO4
−0.7 V vs SCE 0 V vs RHE
electrodeposition CBD
0.1 M HCl
−0.3 V vs RHE
sputtering (phase structural transition)
−2
fabrication
a
These data are summarized in chronological order. FTO, SCE, SILAR, and CBD are abbreviations of F:SnO2, saturated calomel electrode, successive ionic layer adsorption and reaction, and chemical bath deposition, respectively.
Figure 1. Thickness-dependent surface morphology of nanodisk-shaped SnS photocathodes fabricated on FTO/glass substrates for a deposition time of (a) td = 100 s, tSnS = 9 nm, (b) td = 500 s, tSnS = 42 nm, (c) td = 1000 s, tSnS = 85 nm, and (d) td = 2000 s, tSnS = 170 nm (td and tSnS are the deposition time and the thickness of the SnS layer, respectively) before and after water splitting. Current−potential plots obtained from the SnScontaining photoelectrochemical (PEC) cells, showing the (e) effect of thickness in 0.1 M HCl electrolyte, (f) effect of pH (from 0.9 to 1.6), and (g) cathodic and (h) anodic behaviors of PEC cells in seawater under a pulsed-light condition. The linear sweep voltammetry scan was recorded at 20 mV s−1 from a higher to a lower potential.
the thickness dependency, the post-treatment resulted in surface-modified SnS photocathodes, which exhibited a photocurrent enhancement of 300% at the potential range of −0.4 to 0.2 V vs RHE compared with the values obtained from
untreated electrodes. In addition, the incident photon-tocurrent conversion efficiency (IPCE) of an optimized SnS photocathode was close to 100% at −0.3 V vs RHE under an irradiation with a photon wavelength of
