Article pubs.acs.org/JPCC
Horizontal Attenuated Total Reflectance Fourier Transform Infrared and X‑ray Photoelectron Spectroscopy Measurements of Water Adsorption on Oxidized Tin(II) Sulfide (SnS) Surfaces Courtney D. Hatch,*,† Matthew J. Christie,† Robert M. Weingold,† Chia-Ming Wu,‡ David M. Cwiertny,§ and Jonas Baltrusaitis∥,⊥ †
Department of Chemistry, Hendrix College, 1600 Washington Avenue, Conway, Arkansas, United States Department of Chemistry, University of South Dakota, 414 E Clark Street, Vermillion, South Dakota 57069, United States § Department of Civil and Environmental Engineering, University of Iowa, 4105 Seamans Center, Iowa City, Iowa 52242, United States ∥ Departments of Chemistry and Chemical and Biochemical Engineering, University of Iowa, EMRB 76, Iowa City, Iowa 52242, United States ‡
ABSTRACT: Tin(II) sulfide (SnS) is considered to be a promising optoelectronic material due to its narrow band gap, strong optical absorption, low cost and nontoxic and chemically inert characteristics. As an inherently stable compound, SnS surfaces are expected to be hydrophobic by nature. However, exposure of pristine SnS surfaces to air inevitably leads to surface oxidation which can affect the mineral’s dissolution, reactivity, optical and electronic properties as well as hydrophobicity. In the present study, water adsorption measurements on oxidized SnS thin films were performed using horizontal attenuated total reflection Fourier transform infrared (HATR-FTIR) spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis allowed for characterization of the SnS surface composition before water vapor exposure and identification of any changes that occurred to the surface after water vapor exposure. XPS results are consistent with water adsorption occurring on SnS surfaces containing hydroxyl and carbonate groups. Additionally, XPS analysis showed that exposure of SnS to water vapor resulted in no significant changes to the original surface composition. Quantitative water adsorption measurements using HATR-FTIR spectroscopy show that the oxidized SnS surface exhibits a slightly hydrophilic nature, demonstrating multilayer water adsorption at high relative humidity (RH) values. Experimental water adsorption data were fit using the Brunauer−Emmett−Teller (BET) and Freundlich adsorption models. From these model fits, details of monolayer water adsorption and the water adsorption mechanisms were extracted to provide a better understanding of gas/ surface adsorption on oxidized SnS surfaces. Results suggest that water adsorption on SnS powder occurs in three distinct regimes, including sub-monolayer water adsorption up to monolayer coverage at 13% RH, followed by filling of mesopores (13− 76% RH) and finally multilayer water adsorption (>76% RH) via filling of macropores. This study represents the first report of in situ water adsorption measurements on SnS as a function of relative humidity, illustrating how oxidized surface species can alter the hydrophobic nature of SnS surfaces.
1. INTRODUCTION Tin(II) sulfide (SnS), also known naturally as herzenbergite, is a rare mineral in its natural form, yet its chemical precursors are abundant in nature. SnS can be found in two crystalline structures: zinc blende and orthorhombic.1 The latter is currently considered to be a promising optoelectronic material for use in efficient energy conversion photovoltaic materials,2 in infrared photon transducer devices3 and as heat mirrors in solar control coatings.4 Orthorhombic SnS has a distorted sodium chloride (NaCl) structure that is isostructural with germanium sulfide (GeS) having cation layers separated by van der Waals forces yielding a chemically inert surface devoid of dangling bonds.5 Most importantly, these structural properties result in the absence of intermediate states in the band gap and Fermi © 2012 American Chemical Society
level pinning at the semiconductor surface, thus rendering the material inherently chemically and environmentally stable; both are important characteristics of promising semiconductor materials. Additionally, this low-cost, nontoxic and chemically inert material has a narrow band gap (∼1.3 eV)6−8 and strong optical absorption (α > 104 cm−1)8,9 in the visible and nearinfrared spectral regions.10 Although significant advances in the study of SnS electronic and optical properties7,11,12 and synthetic methods10,13−15 have recently been made, current Received: October 30, 2012 Revised: December 14, 2012 Published: December 17, 2012 472
dx.doi.org/10.1021/jp310726t | J. Phys. Chem. C 2013, 117, 472−482
The Journal of Physical Chemistry C
Article
uptake properties of oxidized SnS surfaces and the effect of the presence of water vapor on the chemical composition of the SnS surface. X-ray photoelectron spectroscopy (XPS) was used to probe the surface composition of SnS while horizontal attenuated total reflection Fourier transform infrared (HATRFTIR) spectroscopy was used to measure water adsorption on the oxidized SnS surface. In addition to quantitative FTIR measurements of water adsorption, theoretical adsorption models, including the Brunauer−Emmett−Teller (BET) and Freundlich adsorption isotherms, were applied to the experimental data to extract adsorption parameters and provide a fundamental understanding of the gas/surface adsorption mechanisms on oxidized SnS surfaces.
knowledge of fundamental adsorptive properties of SnS surfaces is lacking in the scientific literature. Despite the potential for commercial use and widespread application of IV−VI group semiconductors, such as SnS, research and development in this area is still in the stages of optimizing synthetic methods for thin film manufacturing in terms of cost, yield and conversion efficiency. Currently, several methods have been developed to synthesize SnS thin films, including chemical vapor deposition (CVD),16,17 vacuum thermal evaporation,8,18 molecular beam epitaxy (MBE)19 and wet chemical synthetic methods, such as chemical bath deposition (CBD)20−22 and spray23 pyrolysis. Many of the current methods of SnS synthesis involve heterogeneous gas/ surface interactions. However, very little is known regarding the adsorptive properties of these surfaces. Furthermore, gas/ surface adsorption is known to alter the physical, chemical, electronic and optical properties of semiconductors, including incorporation of chemical impurities and alteration of surface physical characteristics. Many previous studies have established that exposure of all metal sulfide minerals to air and water (liquid or vapor) results in the formation of surface oxide/hydroxide species and can facilitate formation of higher oxidation products, such as surface sulfite and sulfate, thus affecting the physical and chemical properties of the SnS surface.24−30 Oxidation of metal sulfide surfaces in the presence of air and water vapor can be generally represented by the following reaction mechanism.24,31 MS + (1/2)nO2 + nH 2O → M1 − nS + n M(OH)2
2. EXPERIMENTAL METHODS 2.1. SnS Surface Characterization. Crystalline SnS was obtained from American Elements (#SN-S-05-P, 99.39% assay, density = 5.22 g/cm3). As grinding has been shown to maximize the occurrence of surface oxide/hydroxide sites,24 the crystalline material was mechanically ground using a mortar and pestle to produce a fine powder material for analysis of water adsorption properties of an oxidized SnS surface. The surface area, pore volume and pore size of the powder material were measured using N2 physisorption. The N2 adsorption and desorption isotherms were measured at 77 K using a Quantachrome NOVA 2200e series surface area analyzer following degassing of the SnS sample at 353 K for three hours. Figure 1a shows the N2 adsorption/desorption
(R1)
The above reaction shows that surface oxidation in the presence of water vapor results in the formation of surface hydroxyl species. However, cumulative evidence from previous studies has shown that the oxidation mechanisms of metal sulfide minerals are not so simple.24 For example, galena (PbS) surfaces oxidize by initially forming lead oxide/hydroxide and lead carbonate surface species.28,32,33 Many factors influence the rate of formation of oxidized surface species. Previous studies have shown that surface oxide/ hydroxide and carbonate groups form fairly quickly upon exposure to air while the formation of higher oxidation products, such as sulfate, occurs more slowly.24 Additionally, sulfate formation rates on pyrrhotite are significantly enhanced by grinding in air; reducing the sulfate formation time to less than 10 min.29 It is well-known that the presence of oxidized species on metal sulfide surfaces affects the nature and stability of the mineral surface, including its optical and electronic properties, reactivity, dissolution and hydrophobicity.24 Furthermore, once formed, these species are difficult to remove from the mineral surface. As these materials are used not only in photovoltaics but also as photoelectrocatalysts where contact with water, be it liquid or vapor, is common, it is important to understand both the nature of the oxidized surface species and their effects on the properties of the metal sulfide mineral. Although these processes have been studied on many metal sulfide surfaces, currently the literature lacks information on the surface oxidation of SnS and its subsequent implications for SnS physical and chemical properties. While pristine metal sulfide surfaces are known to be hydrophobic by nature, the presence of oxidized surface species is likely to reduce the hydrophobicity of the surface. As a result, the water adsorptive properties of the surface would be enhanced, possibly facilitating further oxidation of the surface. To this end, the aim of the present work is to probe the water
Figure 1. (a) (left) The N2 adsorption/desorption isotherms in terms of the volume of N2 adsorbed to SnS as a function of the relative N2 pressure (P/Po) and (b) (right) the pore size distribution of the SnS sample.
isotherms in terms of the volume of N2 adsorbed as a function of the relative pressure (P/Po) of N2 gas. At low P/Po values, the adsorption (solid circles) and desorption (open circles) isotherms coincide, and thus very little hysteresis was observed for N2 adsorption on the SnS powder. At high values of P/Po, the amount of N2 adsorbed increases steeply with increasing relative pressure values. The increase at high relative pressures is attributed to the filling of macropores (i.e., the microstrucuture that occurs between interparticle aggregates as opposed to being intrinsic micropores within a single SnS particle) that are evident in the pore size distribution plot, shown in Figure 1b. BET surface area analysis of the N2 adsorption isotherm in the relative pressure range of 0.05 to 0.30 resulted in a relatively low surface area of 2.4 m2/g for the SnS powder. The pore volume, determined from the volume of N2 adsorbed at P/Po values close to unity, was calculated to be 0.006 cm3/g. The average pore diameter (42.5 Å) was 473
dx.doi.org/10.1021/jp310726t | J. Phys. Chem. C 2013, 117, 472−482
The Journal of Physical Chemistry C
Article
separated from the reaction chamber by a hand valve (Granville-Philips Co.), consists of a rotary pump, a foreline trap (both BOC Edwards) and an EXT75DX turbomolecular pump with 60 L/s pumping capacity. The water vapor source was an Ultrapure water reservoir that had been degassed by multiple freeze−pump−thaw cycles. Water vapor was introduced into the reaction chamber and allowed to equilibrate with the SnS sample for 1 h at 10 Torr of pressure. After exposure, water vapor was removed until the residual pressure reached ∼5 × 10−5 Torr. The reacted sample was then transferred back to the surface analysis chamber via the transfer antechamber for postreaction XPS surface characterization. The surface analysis chamber is equipped with monochromatic radiation at 1486.6 eV from an aluminum Kα source using a 500 mm Rowland circle silicon single crystal monochromator. The X-ray gun was operated using a 15 mA emission current at an accelerating voltage of 15 kV. Low energy electrons were used for charge compensation to neutralize the sample. High resolution spectra were acquired in the region of interest using the following experimental parameters: 20 to 40 eV energy window; pass energy of 20 eV; step size of 0.1 eV and dwell time of 1000 ms. One sweep was used to acquire all regions. The absolute energy scale was calibrated to the Cu 2p3/2 peak binding energy of 932.6 eV using an etched copper plate. CasaXPS software35 was used to process the XPS data, and all XPS spectra were calibrated using the C1s peak at 285.0 eV. A Shirley-type background was subtracted from each spectrum to account for inelastically scattered electrons that contribute to the broad background. Transmission corrected relative sensitivity factor (RSF) values from the Kratos library were used for elemental quantification. The S2p transition was fit to two peaks with a ratio of 2:1 for the 2p3/2 and 2p1/2 transitions, respectively. The S2p doublet was constrained to a separation energy of 1.2 eV with equivalent full width at half-maximum (fwhm). The components of the peaks contain a Gaussian/ Lorentzian product with 30% Lorentzian and 70% Gaussian character. An error of ±0.2 eV is reported for all peak binding energies. Results of XPS measurements of the SnS surface before and after exposure to water vapor are discussed in further detail in section 3.1. 2.3. HATR-FTIR Water Adsorption Measurements. 2.3.1. Justification for Water Adsorption Measurements Using HATR-FTIR Spectroscopy. HATR-FTIR spectroscopy was specifically chosen for experimental measurements of the water adsorptive properties of SnS surfaces. Previous studies have shown that HATR-FTIR spectroscopy is a valuable tool for probing the chemical processing of semiconductor crystalline surfaces. In many cases, the semiconductor crystals have been used directly as the internal reflection element (IRE) with the caveat that the angle of incidence to the IRE exceeds the critical angle for total internal reflection,36−39 where the critical angle is defined by θc = sin−1(1/ns), where ns is the refractive index of the substrate.38 Here, room temperature experimental measurements of water adsorption on SnS powder films deposited on a Ge IRE with a 45° angle of incidence were performed using HATR-FTIR spectroscopy. To our knowledge, this study represents the first report of using HATR-FTIR spectroscopy for these purposes. Based on ATR theory, total internal reflection is required to obtain an infrared (IR) spectrum of the sample material in intimate contact with the IRE. For the Ge IRE (n = 4.0)40 used in the present study with an incident angle of 45°, Snell’s law
calculated by applying the Barrett−Joyner−Halenda (BJH) equation, which assumes cylindrical capillaries, to the desorption isotherm. As observed from the pore size distribution plot in Figure 1b, the SnS powder largely exhibits mesoporous structure, yet it also contains a fairly large number of macropores, indicating a fairly heterogeneous surface morphology. The surface morphology of the SnS powder was observed at high magnification using a Hitachi S-3400N scanning electron microscope (SEM) equipped with a backscatter electron detector and operating in variable pressure mode. SnS particles were adhered to carbon tape mounted on an aluminum stub using a similar deposition method as was used in the water adsorption measurements (discussed below) prior to imaging. Figure 2 shows a representative SEM image of the SnS powder.
Figure 2. A representative SEM image of the SnS sample. The scale bar represents 5 μm.
As observed in the SEM image, the SnS powder exhibited a distribution of particle sizes. The average particle diameter, determined by averaging the diameters extracted from SEM images of over 300 individual SnS particles, was found to be ∼300 nm. Together with the relatively low surface area and results from pore size measurements, the SEM image confirms the heterogeneous sizes and morphology of the SnS powder. 2.2. XPS Surface Analysis. A custom-designed Kratos Axis Ultra X-ray photoelectron spectroscopy system was used to monitor the surface composition of SnS before and after exposure to water vapor. Details of the experimental design and analysis have been reported previously in the literature.34 Briefly, the XPS instrument used in this study has three chambers, namely (i) an ultrahigh vacuum (UHV) surface analysis chamber, (ii) a sample transfer antechamber and (iii) a reaction chamber. The transfer antechamber was used to transfer samples between the analysis chamber and the reaction chamber. For a typical XPS experiment, SnS powder was pressed into indium foil and mounted onto a copper stub. The sample was then introduced into the transfer antechamber under ambient conditions and pumped to ∼5 × 10−7 Torr, effectively removing any physisorbed species. Once this pressure was achieved, samples were introduced into either the surface analysis chamber or the reaction chamber. The reaction chamber (∼3 L) was fabricated from stainless steel and was retrofit with a leak valve, a pressure transducer (BOC Edwards WRG-S-NW35), a pumping system (Boc Edwards TIC) and two Pyrex glass windows. The pumping system, 474
dx.doi.org/10.1021/jp310726t | J. Phys. Chem. C 2013, 117, 472−482
The Journal of Physical Chemistry C
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Figure 3. An illustration of the use of HATR-FTIR spectroscopy to monitor surface adsorption on high refractive index materials, such as SnS.
layer and will be adequate for quantifying adsorbed water on the surface of the SnS thin film. 2.3.2. HATR-FTIR Flow Cell. Water adsorption on oxidized SnS powder was measured using a Nicolet 6700 FTIR spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector and a multiple-reflection HATR flow cell for Spectra-Tech ARK (Thermo Fisher Scientific Inc.). The in situ FTIR flow system has been described previously in the literature.44 Briefly, dry air (75-62NA FTIR purge gas generator,
