Hydrogen Photoassisted Generation by Visible Light and an Earth

Article pubs.acs.org/JPCC

Hydrogen Photoassisted Generation by Visible Light and an Earth Abundant Photocatalyst: Pyrite (FeS2) Mariam Barawi,† Isabel J. Ferrer,* Eduardo Flores, Satoko Yoda, José R. Ares, and Carlos Sánchez MIRE Group, Dpto. de Física de Materiales, Universidad Autónoma de Madrid, Madrid 28049, Spain S Supporting Information *

ABSTRACT: N-Type pyrite (FeS2) thin films, deposited on titanium substrates, have been synthesized and photoelectrochemically characterized. Its flat band potential has been estimated to be −0.75 ± 0.05V vs Ag/AgCl by electrochemical impedance spectroscopy (EIS). The corresponding energy level diagram of the FeS2/electrolyte interface has been established. Profuse hydrogen flows have been produced under visible light illumination of FeS2 photoanode in a photoelectrochemical cell (PEC) at different bias potentials. Their values, measured by mass spectrometry (MS), were higher than 5 μmol H2/min·cm2. Hydrogen photogeneration efficiencies of ∼8% have been reached.

suitable for PV21,22 and PEC9,17,23,24 applications, even as photodetectors25 due to its band gap energy of 1.00 ± 0.15 eV11,24,26,27 and its high optical absorption,28 with absorption coefficients of ∼105 cm−1 in the visible range of the solar spectrum. Here, the suitability of pyrite thin films as photoanodes in a PEC for hydrogen photogeneration promoted by visible light absorption is investigated. Up to now, scarce information related with hydrogen evolution9,10,29 and transport30 in pyrite has been reported. More abundant are the papers dealing with the photoelectrochemical properties of pyrite both natural28,30−35 and synthetic28,29,36−38 samples. Photoelectrochemical properties of pyrite thin films39−42 have been also investigated, but reported results are incomplete and dispersed. Most of the papers deal with Iph−V curves in different electrolytes under several illumination conditions, showing low quantum yield and fill factors as well as efficiencies < 1%.11 Recently, two contributions have been published on hydrogen evolution by using FeS2: one of them investigates the use of pyrite thin films as a HER (hydrogen evolution reaction) electrocatalyst under dark conditions,10 and the second one faces the photocatalytic hydrogen evolution from FeS2/TiO2 composites.9 In the present investigation, the photoelectrochemical properties of n-doped FeS2 thin films grown on Ti substrates have been studied including both, the flat band potential and the photogenerated hydrogen flux under visible light illumination. Results are compared to those from FeS2/ TiO2 composites.9

1. INTRODUCTION Photoelectrochemical water splitting is a promising route for the renewable production of hydrogen fuel by solar energy. Since the discovery of the Fujishima−Honda effect in the early 1970s,1 photocatalytic and photoelectrochemical (PEC) water splitting with semiconducting materials has been extensively investigated. Metal oxides, as TiO2, possess suitable energy band levels for the thermodynamical splitting of water. Additionally, they are nontoxic and stable during the photocatalytic process, but most of them are only active under ultraviolet light, which only accounts for ∼4% of the solar spectrum.2 On the other hand, some metal sulfides have lower band gap energies than oxides, what awards them the capability to absorb visible light.3 In addition, they are nontoxic and cheap materials. CdS and ZnS are the most investigated sulfide photocatalysts4−7 due to their band gap energy (2.4 eV), which matches with the solar spectrum, and the adequate position of their energy bands to reduce H2O to H2. However, these sulfides are toxic and unstable in aqueous solution. Hence, other metal sulfides are being investigated to achieve this goal, among them earth abundant compounds like pyrite are pointed out.8−10 Pyrite has been proposed since the eighties decade of the past century, mainly by the research group led by Tributsch, as a promising material for different energy conversion applications: as a photovoltaic (PV) semiconductor,11−13 and as a photoelectrode in photoelectrochemical cells (PEC).13,14 After more than a decade of diminished interest in this material, it comes back to the scene with new concern.15−18 The resurgent interest in pyrite mainly derives from its low toxicity, high abundance, and the low cost of its raw materials.19 Iron and sulfur are some of the cheapest and available earth abundant elements.20 Once again, pyrite seems to be very © XXXX American Chemical Society

Received: November 24, 2015 Revised: April 19, 2016

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DOI: 10.1021/acs.jpcc.5b11482 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION Synthesis of Ti-Doped Pyrite. Pyrite thin films have been deposited on titanium discs by thermal evaporation of Fe (100 nm) and later sulfuration at 350 °C for 20 h under a sulfur pressure of about 0.2 bar. Before Fe evaporation, Ti substrates (Goodfellow 99.9%, ⌀ = 15 mm) had been etched in a HF/ HNO3 mixture (4 wt %: 30 wt %) to eliminate any impurity accumulated on their surfaces. During sulfuration, Ti from the substrate diffuses into the FeS2 film, leading to its n-type conductivity.43,44 Morphological and Structural Characterization. These films have been structurally and morphologically characterized by X-ray diffractometry (XRD) in a Panalytical X’pert Pro X-ray diffractometer (Cu Kα radiation (λ = 1.5406 Å) at a glancing angle of 1.7°) and by scanning electron (SEM) and field emission gun (FEG) microscopies (Hitachi S 3000N and Philips XL-30 S FEG, respectively). S/Fe and S/(Fe + Ti) atomic ratios in the films were obtained by energy dispersive Xray analyses (EDX, Oxford Instruments, INCA x-sight) at 10 keV electron beam energy. Electro- and Photoelectrochemical Measurements. Electrochemical measurements were performed in a threeelectrode cell with an aqueous solution of 0.5 M Na2SO3 (pH = 9.0) as electrolyte. Ti−FeS2 samples were used as working electrodes, a platinum sheet was the counter electrode, and a Ag/AgCl electrode was the reference one. Due to the design of the working electrode holder, the geometric electrode area in contact with the electrolyte and under illunination was about 1 cm2 (corresponding to ⌀ = 11 mm). Current density (at dark and under illumination) and impedance through the Ti−FeS2/ electrolyte junction as a function of the bias potential were measured with a potentiostat-galvanostat PGSTAT302N provided with an integrated impedance module FRAII. AC modulated cyclic voltage scans of 10 mV amplitude, from −1.0 V to 0.5 V vsAg/AgCl, in the range of frequencies between 100 Hz and 900 kHz, were used. A 650 W tungsten halogen lamp was used as visible light source. Total power density of illumination reaching the surface sample was 270 ± 20 mW/cm2 as determined via a Möll thermopile. A quadrupole mass spectrometer (QMS, Mod. Prisma, Balzers) coupled to the photoelectrochemical cell was used to quantify the photogenerated hydrogen flux. During the experiments, an argon flow of 20 sccm was passed through the top of the cell. Details are reported in ref 45.

Ti substrate (JCPDS 005-0682). Pyrite lattice parameter, estimated from the XRD pattern, is a0 = 5.42 ± 0.01 Å. Crystallite size, determined by applying the Scherrer equation46 to the height half wide of the most intense peak (200) of pyrite, is 20 ± 5 nm hinting its nanocrystalline character. S/Fe stoichiometric ratio of the films is 2.3 ± 0.1. S/Fe > 2.0 could be related with the presence of Ti atoms,43 since through the films the ratio S/(Fe+Ti) is ∼2. The low electron beam energy (10 keV) makes negligible the substrate contribution.47 SEM and FEG images of the films are shown in Figure 2, where their surface appears formed by crystallites of about tens nm size and whiskers which can reach up to 1 mm length.

Figure 2. (a) SEM and (b) FEG images of an FeS2 thin film deposited on Ti at two different magnifications.

The photovoltage sign confirms the n-type conductivity of the films, as expected from the doping of FeS2 with diffused Ti atoms from the substrate (Figure S1). This doping effect has been previously demonstrated by thermoelectric coefficient measurements.43,44 The Ti atom distribution across the film has been previously investigated by Rutherford Backscattering Spectroscopy (RBS).43 Photovoltage values (about tens mV) agrees well with previously reported data from n-type singlecrystals.32,38 In order to determine the energy levels at the Ti−FeS2/ electrolyte interface, the flat band potential, Vfb, has been estimated from the space charge layer capacitance (CSC) measured by potentiodynamic electrochemical impedance spectroscopy (EIS).48 The dependence of CSC on bias potential (V) is described by the Mott−Schottky equation:2,49 1 CSC

2

⎛ ⎞⎛ k T⎞ 2 =⎜ ⎟⎜[V − Vfb] − B ⎟ e0 ⎠ ⎝ εSCε0NDe0 ⎠⎝

(1)

where CSC is the measured differential capacitance per area unit, e0 is the elementary charge, εSC is the material dielectric constant (εFeS2 = 10.950), ε0 is the electrical permittivity of vacuum, ND is the semiconductor donor density, V is the applied bias potential in volts, kB is Boltzmann’s constant, and T is the temperature (298 K). Therefore, from CSC−2 vs V plots, Vfb can be easily determined from the interception of the obtained straight lines with the x-axis.51 Figure 3 shows the Mott−Schottky plots of Ti−FeS2 at different frequencies. From this figure, the estimated value of the flat band potential is, Vfb = −0.75 ± 0.02 V (Ag/ClAg) = −0.55V ± 0.02 V (NHE). The

3. RESULTS AND DISCUSSION XRD pattern of a Ti−FeS2 film is shown in Figure 1. Diffraction peaks corresponding to an unique crystalline phase, cubic Pyrite (JCPDS 042-1340), have been identified beside those from the

( ) has been neglected because of its low value. V

term −

kBT e0

fb

here obtained agrees well with that previously reported by Schoonen and Xu,52 −0.46 V (NHE). Finally, from the slope of (1/CSC)2 vs V (1.3 × 1010 cm4 F−2 V−1), a value of ND = 1020 cm−3 was estimated, which is close to that determined by Hall effect measurements in films of Ti-doped FeS2 deposited on glass.53

Figure 1. XRD pattern of a FeS2 thin film deposited on a Ti substrate. XRD peaks of pyrite can be seen beside those from the Ti substrate. B

DOI: 10.1021/acs.jpcc.5b11482 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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depicted in the energy level scheme shown at Figure 4. Figure 5 presents the hydrogen photogeneration flux at different applied

Figure 3. Mott−Schottky plot of Ti−FeS2 film in aqueous Na2SO3 (pH = 9.0) at different modulation frequencies. Figure 5. H2 photogeneration flux as a function of time at different bias potentials.

Figure 4 shows the energy level diagram at the Ti−FeS2/ electrolyte interface at pH = 9.0, depicted from our

potentials: 0.3, 0.4, and 0.5 V (Ag/AgCl) under our tungsten lamp illumination. As it can be observed, hydrogen flow goes up as bias potential is increased, reaching values of ∼1 μmolH2/ min·cm2 at 0.3 V (Ag/AgCl) and ∼5 μmolH2/min·cm2 at 0.5 V (Ag/AgCl). The total amount of photogenerated hydrogen over 1 h is measured and obtained values are included in Table 1, at different bias potentials. These amounts are close to those reported by Lee and Kang9 with FeS2/TiO2 composites based in anatase TiO2 loaded on synthetic pyrite particles used as photocatalyst in a methanol solution. There, hydrogen was quantified by gas chromatography. Assuming no significant differences between both methods, the values obtained by Lee and Kang (without bias potential and under UV illumination) are similar to those here obtained at 0.4−0.5 V (Ag/AgCl) under white light. That report shows the improvement of the H2 photogenerated flux by using TiO2 loaded with different amounts of FeS2. The compound with the optimal proportion produces flows four times higher than nude FeS2 and 20 times higher than pure TiO2. The main significant difference reported here is the use of white light with bare FeS2 although a bias potential is needed (0.3 to 0.5 V vs Ag/AgCl in a 3 electrode configuration cell) to enhance the hydrogen amount. It is worth noting that the use of solar illumination will increase the photogenerated hydrogen flow due to its characteristic spectrum whose maximum intensity is in the range of visible light (530 nm) vs the maximum of the halogen lamp used in this work (930 nm) (Figure S3). Finally, one of the most significant parameters on dealing with hydrogen photogeneration is the efficiency of light energy conversion into chemical energy (hydrogen). Solar and light energy conversion efficiency (STH and LTH respectively) are usually determined by eq 22,58,59 when an external electrical bias is needed:

Figure 4. Energy level diagram at the Ti−FeS2/Na2SO3 interface (pH = 9.0).

experimental data. Flat band potential (Vfb = −0.55 VNHE) is used to locate the semiconductor Fermi level (assuming zero charge situation). An energy band gap of Eg = 1.0 eV, estimated from the absorption optical spectral response (Figure S2), is considered. Reported value of FeS2 work function (Φ = 3.9 eV50) could also be used to determine the Fermi level energy and would agree well with the value here estimated from the flat band potential. As it can be observed in Figure 4 the half reaction of water reduction to generate hydrogen could be thermodynamically possible,54 although it is in the limit of suitability. In spite of this situation, H2 is photogenerated (as quantified by QMS), hinting a fast reaction rate. The hydrogen half reaction could be improved by decreasing the electrolyte pH, always in the range of pyrite stability versus acid dissolution. However, the water oxidation to oxygen would not be favorable. Then, the half oxidation reaction will be the oxidation of SO32− to SO42−. The depicted energy level scheme is useful to define futures strategies in order to improve the water splitting. For example, one of these possible alternatives is to widen the energy band gap of FeS2 by doping it with different metals (Sn55 and Zn56 have been recently investigated to this aim) or alloying it with oxygen.57 This is a strategy opposite to that used with TiO2, where the efforts are focused in reducing the energy band gap. The composition of the photogenerated gases under different bias potentials has been investigated “in situ” by QMS. Hydrogen was the unique identified element. No traces of oxygen were detected, confirming the unfavorable situation

η=

ΔG H° 2R H2 − VbI PS

× 100

(2)

where ΔG°H2 (237.2 kJ/mol, at 25 °C and 1 bar) is the standard Gibbs energy of water splitting, RH2 is the rate of production of hydrogen (mol/s·cm2), Vb is the external cell potential (in volts.), I is the current density (A/cm2), and PS is the power density of illumination (W/cm2) (270 mW/cm2 in our measurements), responsible for the generation of hydrogen at the flux, RH2. Here, efficiency is estimated by considering the cell potential, Vb, as proposed by the Liu approximation60 (used to convert the applied potential, V, between WE and RE in a C

DOI: 10.1021/acs.jpcc.5b11482 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Hydrogen Evolution Flux, Photogenerated Hydrogen in the First 60 min under Illumination, and Hydrogen Photogeneration Efficiency at Different Bias Potentials V (V vs Ag/AgCl)

Vb (V) estimated by Liu approx.

I (μA/cm2)

H2 flux (μmol H2/min·cm2)

H2 in 60 min (μmol H2/cm2)

LTH efficiency η (%) eq 2

0.3 0.4 0.5

1.057 1.157 1.257

2.80 5.02 5.74

0.91 ± 0.05 3.16 ± 0.05 5.28 ± 0.05

41.2 ± 0.2 172.8 ± 0.2 326.2 ± 0.2

1.3 ± 0.2 4.6 ± 0.2 7.7 ± 0.2

Notes

three electrode configuration in the cell potential, Vb, between WE and CE in a two electrode one). It is worthwhile to mention that the power density of illumination here used to estimate the efficiency includes the whole spectrum of the halogen lamp; therefore, if only the energies higher than Eg were considered, the calculated efficiency in our experiments would be higher (Figure S3). Table 1 shows the hydrogen photogeneration flux, the total amount of hydrogen photogenerated during 1 h, and the photogeneration efficiency (LTH), estimated at different applied voltages by means of eq 2. These results demonstrate the suitability of Ti-doped pyrite as photoanode in a PEC for hydrogen photogeneration. Compared with other sulfides investigated by us, such as PdS,45 and TiS361 under similar conditions of polarization and illumination (0.3 V vs Ag/AgCl, and power density of white light), FeS2 yields H2 fluxes 20 times higher than those obtained with PdS but only 50% of the TiS3 one.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Experimental support of F. Moreno is acknowledged, as well as financial support from Spanish MINECO under Contract MAT2011-22780. E.F. acknowledges to the Mexican National Council for Science and Technology (CONACyT) for providing the grant necessary to complete his Ph.D.

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4. CONCLUSIONS Ti-doped FeS2 films have been used as photoanode in a PEC for hydrogen photogeneration under white light illumination. Ti−FeS2 flat band potential was determined (Vfb = −0.55 ± 0.02 V vs NHE), which is in agreement with the previously reported FeS2 work function. This value is used to depict the energy level diagram at the FeS2/electrolyte interface, resulting in a suitable position for the hydrogen evolution reaction (HER). Fluxes of photogenerated hydrogen at different bias potentials have been quantified by quadrupole mass spectrometry. Photogenerated hydrogen amount for the first hour of illumination is similar to that reported from the best ratio FeS2/ TiO2 composite (0.3 mmol vs 0.4 mmol). Hydrogen photogeneration efficiencies up to ∼8% have been estimated. All these results make Ti-doped FeS2 a good candidate to be used as photoanode in a PEC device for solar hydrogen photogeneration.

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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.jpcc.5b11482. Figure S1, open circuit photovoltage transient of Ti− FeS2 in Na2SO3 (pH = 9.0); Figure S2, optical absorption spectrum of Ti- FeS2 deposited on glass; Figure S3, solar vs halogen lamp emission spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: 00 34 497 5027/8579. E-mail: isabel.j.ferrer@ uam.es. Present Address

† M.B.: Center for Biomolecular Nanotechnologies, Fondazione Istituto Italiano di Tecnologia. 73010 Arnesano (LE), Italy.

D

DOI: 10.1021/acs.jpcc.5b11482 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b11482 J. Phys. Chem. C XXXX, XXX, XXX−XXX