Article pubs.acs.org/Langmuir
Controlling the Photocorrosion of Zinc Sulfide Nanoparticles in Water by Doping with Chloride and Cobalt Ions Philipp Weide,† Katharina Schulz,† Stefan Kaluza,†,∥ Markus Rohe,‡ Radim Beranek,§ and Martin Muhler*,†,⊥ †
Laboratory of Industrial Chemistry, Ruhr-University Bochum, 44780 Bochum, Germany Huntsman Pigments and Additives, Dr. Rudolf-Sachtleben Str. 4, 47198 Duisburg, Germany § Institute of Electrochemistry, Ulm University, Albert-Einstein-Allee 47, 89069 Ulm, Germany ∥ Fraunhofer UMSICHT, 46047 Oberhausen, Germany ⊥ Max Planck Institute for Chemical EnergyConversion, Stiftstraße 34-36, 45470 Mülheim an der Ruhr, Germany ‡
S Supporting Information *
ABSTRACT: Photodegradation under UV light irradiation is a major drawback in photocatalytic applications of sulfide semiconductors. ZnS nanoparticles were doped with very low amounts of chloride or cobalt ions in the ppm range and codoped with chloride and cobalt ions during their synthesis by precipitation in aqueous solution followed by calcination. The high-temperature wurtzite phase annealed at 800 °C had a high susceptibility to UV irradiation in water, while the low-temperature zincblende phase annealed at 400 °C was found to be stable. Chlorine doping increased the rate of photocorrosion in water, whereas cobalt doping led to a stabilization of the ZnS nanoparticles. Based on photochemical and spectroscopic investigations applying UV/vis, X-ray photoelectron, and photoluminescence spectroscopy, the increased susceptibility of Cl-doped ZnS is ascribed to a higher number of surface point defects, whereas the stabilization by Co2+ is caused by additional recombination pathways for the charge carriers in the bulk, thus avoiding photocorrosion processes at the surface. Additional doping of Cl-doped ZnS with cobalt ions was found to counteract the detrimental effect of the chloride ions efficiently.
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INTRODUCTION Inorganic semiconductors based on transition metal oxides and sulfides are of great interest for photocatalytic and photoelectrochemical hydrogen production and water splitting.1−3 Zinc sulfide (ZnS) is a II−VI semiconductor with a typical bandgap energy of about 3.6 eV (∼340 nm), making it a suitable photocatalyst for various UV light-driven photochemical applications such as pollutant degradation,4,5 hydrogen evolution,6,7 photoreduction of CO2,8,9 or selective photocatalytic organic transformations.7,10−14 Furthermore, ZnS is stable in a large pH range and nontoxic and can be synthesized from earth-abundant components. However, similar to other sulfides, ZnS suffers from photocorrosion, i.e., the oxidative degradation under UV irradiation.15 Based on the historical industrial use as a white pigment, the photocorrosion of ZnS and especially the related darkening of such paints under sunlight irradiation have been observed already at the turn of the 19th to the 20th century.16−18 In early reports the darkening of the pigments was ascribed to the formation of metallic Zn,19 and ZnSO4 and H2 were postulated as other photocorrosion products.19,20 It has been reported that the occurring degradation reactions strongly depend on the experimental conditions, e.g., the presence of water or oxygen, and on the synthesis temperature of the material and its © 2016 American Chemical Society
structural properties. Additionally, the photocorrosion of ZnS was reported to be significantly increased when salts such as NaCl had been present during the annealing process.18,20 However, except the study of Dunstan et al.21 on the photochemistry of ZnS colloids, these studies mainly provide phenomenological observations without detailed explanations and conclusive experimental evidence. More recent investigations on metal sulfide photocorrosion including surfacesensitive characterization techniques such as X-ray photoelectron spectroscopy (XPS) focused on CdS, which has been widely studied due to its narrow bandgap and ability to absorb visible light.22,23 Metallic Cd, sulfate, elemental sulfur, and H2 were found as photocorrosion products. As a consequence of the industrial use as a white pigment, various efforts have been made to enhance the stability of ZnS nanoparticles. Notably, a remarkable enhancement of the photostability of commercial ZnS pigments was observed upon doping ZnS with transition metal ions such as Fe2+, Cu2+, Ni2+, and Co2+.24 The origin of this stability enhancement has remained unclear, although understanding its mechanism should allow for the development Received: September 19, 2016 Revised: November 7, 2016 Published: November 8, 2016 12641
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from 800 to 200 nm with a resolution of 1 nm and 0.4 s integration time. BaSO4 was used as the internal 100% reflectance standard. ZnS powder samples were measured by diluting 25 mg of ZnS in 500 mg of BaSO4. For the bandgap determination Tauc plots assuming a direct bandgap transition were constructed from the absorption spectra. Photoluminescence Spectroscopy. PL emission spectra were measured at room temperature using a Fluorolog-3 photoluminescence spectrometer (Horiba) with an excitation wavelength of 320 nm. The resulting photoemission was recorded from 330 to 620 nm with 1 nm step size and 0.1 s integration time, resulting in an irradiation time of 29 s for each spectrum and a total irradiation time of 203 s for the seven spectra recorded consecutively. ZnS powder samples were measured in a quartz tube after ultrasonication for 5 min to minimize effects of the particle packing. N2 Physisorption Measurements and Elemental Analysis. The specific surface area of the samples was determined by N2 physisorption experiments carried out in a BELSORP-max sorption analyzer (BEL JAPAN) at 78 K. The samples were dried for 2 h at 200 °C prior to the physisorption measurements. The BET equation was applied to derive specific surface areas. The composition of the original and the annealed ZnS samples with respect to the Co2+ and/or Cl− amounts was investigated by elemental analysis. The Co2+ content in the materials was quantified by atomic absorption spectroscopy (AAS). Because of the ubiquitous nature of NaCl, it was not possible to determine the Cl− content reliably in the few 100 ppm range. Therefore, the Cl-doped ZnS samples were labeled according to the relative amount of ZnCl2 used in the precipitation substituting ZnSO4. For the ZnS sample synthesized by substituting 50% of ZnSO4 by ZnCl2, a Cl− content of about 2000 ppm was detected, which decreased to about 1000 ppm due to calcination. Roughly the same trend was observed for the 30% ZnCl2−ZnS sample, which had a lower, but not proportional, Cl− content, and for samples with lower amounts of Cl− the elemental analysis results were erratic. X-ray Photoelectron Spectroscopy. The chemical composition in the near-surface region of the ZnS photocatalysts before and after the photocorrosion experiments was investigated by XPS measurements performed in an ultrahigh vacuum setup equipped with a monochromatic Al Kα X-ray source (1486.6 eV, anode operating at 14.5 kV and 30.5 mA) and a high-resolution Gammadata-Scienta SES 2002 analyzer. The base pressure in the measurement chamber was maintained at about 6 × 10−10 mbar. The XP spectra were recorded in the fixed transmission mode with 200 eV pass energy, resulting in an overall resolution of about 0.6 eV. Charging effects were compensated applying a flood gun, and binding energies were calibrated based on the adventitious carbon C 1s peak at 284.8 eV. Analysis and fitting of the recorded spectra were performed using the CasaXPS software with Gaussian−Lorentzian peak functions and Shirley-type backgrounds.
of photocorrosion mitigation strategies for sulfide-based semiconductors for various applications. In the present study, photochemical experiments in the aqueous phase using UV light and postreaction characterization by X-ray diffraction (XRD) and XPS combined with photoluminescence (PL) spectroscopy were applied to investigate the effect of anionic and cationic doping demonstrating that doping with Cl− ions enhances photocorrosion, whereas Co2+ ions increase the photostability of ZnS.
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EXPERIMENTAL SECTION
Synthesis of Pure and Doped ZnS. ZnS nanoparticles were synthesized by precipitation: In a homogeneously stirred batch reactor, 1 M ZnSO4·7H2O (≥99%, Sigma-Aldrich) and 1 M Na2S·9H2O (≥98%, Sigma-Aldrich) aqueous solutions were added dropwise simultaneously to pure HPLC water (HiPerSolvChromanorm, VWR Chemicals, also used for all solutions and post-treatment steps), immediately leading to the formation of a white precipitate. During the reaction the pH of the suspension was kept constant at pH 6 by the use of an autotitrator (TitroLine 7000, SI Analytics), and the temperature was maintained at 60 °C. The precipitate was filtrated, washed with HPLC water to remove reaction byproducts, and subsequently dried at 80 °C overnight. For the synthesis of Co2+doped or Cl−-doped ZnS, a corresponding amount of a 1 M CoSO4· 7H2O (≥99%, Sigma-Aldrich) solution was added to the zinc sulfate solution, or a zinc salt solution with a suitable ratio of ZnSO4 and ZnCl2 (≥98%, Sigma-Aldrich) was used as the reactant. Accordingly, codoped Cl/Co:ZnS was obtained by adding CoSO4 to the ZnSO4/ ZnCl2 solution. The dried precipitate was annealed for 30 min at 400 and 800 °C in a N2-flushed tube furnace leading to ZnS crystallized in the zincblende and wurtzite structure, respectively. After annealing, an ultrasonication post-treatment using aqueous H2SO4 (pH 3) was applied to remove residual ZnO formed during the annealing process due to the presence of traces of O2. Photochemical Experiments. The UV-light-triggered photocorrosion of the ZnS materials was studied in a homemade photocatalytic test setup consisting of a double-walled stirred semicontinuous glass reactor equipped with a 500 W Hg lamp (Peschl UV-Consulting) as UV light source and connected to an X-STREAM online multichannel product gas analyzer (Emerson Process Consulting). The setup is described in detail elsewhere.25−27 Highpurity N2 (99.9999%, Air Liquide) was used as inert carrier and purge gas. For each experiment 500 mg of the photocatalyst was suspended and ultrasonicated in 600 mL of HPLC water and subsequently irradiated with UV light for 2.5 h. Samples of the suspension were taken from the reactor every 30 min, separated from the suspended solids by centrifugation and investigated by ion chromatography. After the experiment the suspension was filtrated, washed, dried, and further characterized. Ion chromatographic sulfate quantification was conducted using a Dionex ICS-1100 IC system with a DionexIonPac AS22 capillary column. X-ray Diffraction. For the characterization of the crystallographic properties of the materials XRD patterns were collected with a PANalytical Empyrean Bragg−Brentano θ−θ diffractometer using Cu Kα radiation (λ = 0.154 06 nm, 45 kV, 40 mA) and a PIXCel detector. The measurements were conducted at room temperature and atmospheric pressure in the 5°−80° 2θ range with a step size of 0.013° 2θ and a fixed divergence slit of 0.38 mm. The qualitative analysis of the observed patterns was carried out using the PANalytical HighScore Plus software together with reference diffraction data from the Inorganic Crystal Structure Database (ICSD). UV/Vis Diffuse Reflectance Spectroscopy (UV/Vis-DRS). UV/ vis diffuse reflectance spectra were recorded to determine the absorption characteristics and the bandgap of the pure and doped ZnS materials. The spectra were measured using a Lambda 650 spectrophotometer (PerkinElmer) equipped with a praying-mantis array for solid samples and a R955 photomultiplier detector together with the UV WinLab software (PerkinElmer). Spectra were recorded
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RESULTS Structural Properties. Pure, chlorine-doped, and cobaltdoped as well as Cl/Co-codoped ZnS samples were synthesized by substituting ZnSO4 with appropriate amounts of ZnCl2 or/ and CoSO4 in the precipitation reaction. After drying, freshly precipitated ZnS was obtained as a pure white powder regardless of the Cl or Co content. According to the XRD shown in Figure 1, the zincblende structure (ICSD 92-0077082) is identified in ZnS nanoparticles, which is retained after annealing at 400 °C. The patterns are mainly characterized by three broad diffraction peaks at 29.0°, 48.2°, and 57.0°, indicating very small crystallite sizes. Annealing at 800 °C led to the appearance of additional diffraction peaks of which the most prominent ones are located at 27.0°, 30.6°, 39.7°, and 51.9°. These reflections originate from ZnS in the hightemperature wurtzite structure (ICSD 98-006-7453). The weaker diffraction peaks observed at 31.9°, 34.5°, and 36.3° in the pattern of the 800 °C sample before the acidic posttreatment indicate the presence of ZnO (ICSD 98-015-4486). 12642
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g−1 were obtained, respectively. Additionally, SEM images were recorded to illustrate the shape and size of the obtained ZnS materials. Figure S4 reveals very small spherical particles existing in the as-prepared ZnS material and the sample annealed at 400 °C and an increased particle size after annealing at 800 °C in agreement with the XRD and physisorption results. Electronic Absorption Properties. The light absorption properties of the ZnS materials and the influence of Cl and Co doping were investigated using UV/vis diffuse reflectance spectroscopy. All pure, Cl-doped, and Co-doped ZnS materials exhibit one distinct absorption edge originating from the bandgap absorption around 350 nm corresponding to a bandgap energy of 3.5 eV (Figure S5). Hence, neither Cl doping nor Co doping changed the bandgap energy of the ZnS materials significantly. For all samples obtained at different temperatures the bandgap energies are in the range of 3.3−3.6 eV matching typical bandgap energies of ZnS. However, additional absorption bands are observed in the visible range between 600 and 800 nm for all Co-containing samples, which are not detected in the spectrum of pure ZnS as shown in Figure 2. The absorption intensity scales with the Co content and originates from d−d* transitions within the tetrahedrally coordinated d7 Co2+ ions incorporated in the ZnS lattice. Although diffuse reflectance spectra are not entirely quantifiable, this observation reflects different levels of Co-doping in ZnS. This additional Co-related visible-light absorption is also observed in the Cl/Co-doped samples, confirming the incorporation of Co2+ in the ZnS lattice and the different levels of Co doping (Figure S6). Emission Characteristics. When excited with light of a suitable wavelength, ZnS exhibits a luminescence emission, which mainly originates from emission recombination at surface defects such as sulfur and zinc vacancies or interstitial atoms.28 The luminescence spectra of the Cl-doped ZnS samples shown in Figure 3 feature two main peaks at 450 and 518 nm originating mainly from VS → VB and VS → IS transitions, VS and IS denoting sulfur vacancies and interstitial sulfur atoms, respectively. The luminescence intensity is significantly increasing as a function of Cl− content, while the emission shape remains unchanged implying a correlation between the Cl− content and the generated number of point defects in ZnS for a given annealing temperature. Figure 3b shows consecutive spectra of the 50% ZnCl2−ZnS sample. The luminescence
Figure 1. XRD patterns of ZnS with 10% ZnCl2 substitution after drying and after annealing at 400 and 800 °C and acidic posttreatment.
These peaks are close to the detection limit after the posttreatment demonstrating an essentially complete removal of ZnO. Analogous results were obtained regardless of the level of ZnCl2 substitution as shown in Figure S1. Thus, no chloridecontaining phases were formed corresponding to a very low amount of Cl− ions present in the samples. Similar diffraction patterns were obtained for the Co- and Cl/Co-doped samples (Figures S2 and S3). As indicated by the decrease of the FWHMs of the diffraction peaks, the size of the ZnS nanoparticles increased as a function of temperature. Correspondingly, the specific surface area of the Cl-doped samples decreased from 47.9 m2 g−1 to 33.8 and 3.4 m2 g−1 for the as-prepared and the samples obtained at 400 and 800 °C, respectively. An analogous decrease in specific surface area was observed for the Co- and Cl/Co-doped samples, for which areas of 55.5, 40.7, and 5.3 m2 g−1 and 99.5, 69.5, and 3.0 m2
Figure 2. Full range (a) and enlarged (b) UV/vis diffuse reflectance spectra of differently Co-doped ZnS samples annealed at 800 °C. 12643
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Figure 3. Photoluminescence emission spectra of differently Cl-doped ZnS (a) and consecutive spectra of 50% ZnCl2−ZnS annealed at 800 °C (b).
Figure 4. Photoluminescence emission spectra of differently Cl/Co-codoped ZnS (a) and consecutive spectra of 30% ZnCl2−ZnS with 54 ppm of Co annealed at 800 °C (b).
Figure 5. Photochemical H2 evolution and SO42− formation during irradiation of differently Cl-doped ZnS annealed at 800 °C. mcat = 500 mg, t = 2.5 h, Vsusp = 580 mL, 500 W Hg lamp.
emission intensity is drastically reduced with increasing number of spectra pointing to a strongly decreasing number of emission centers as a function of irradiation time. After doping ZnS with Co2+ instead of Cl−, the shape and the position of the emission maximum are significantly altered (Figure S7). Whereas an emission comparable to pure ZnS is observed for the samples doped with 108 ppm of Co, the position of the luminescence maximum is shifted toward lower wavelength with increasing Co content. The shift of the emission maximum and the decrease in luminescence intensity
as a function of Co doping imply different nonradiative recombination pathways including Co2+ centers to occur in Codoped ZnS. As a result of such transfer paths, the amount of charge carriers available for photochemical surface reactions should be lower affecting the photochemical properties. Figure 4 shows the luminescence emission spectra of the ZnS codoped with both Co2+ and Cl−. While the luminescence intensity of the sample with only 54 ppm of Co is still similar to the 30% ZnCl2 sample without Co, increasing the Co content to 118 and 286 ppm leads to a drastic decrease of the emission 12644
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Figure 6. Photocatalytic H2 evolution and SO42− formation during irradiation of differently Co-doped ZnS annealed at 800 °C. mcat = 500 mg, t = 2.5 h, Vsusp = 580 mL, 500 W Hg lamp. 5% ZnCl2−ZnS (no Co) is shown for comparison.
Figure 7. Photocatalytic H2 evolution and SO42− formation during irradiation of differently Cl/Co-codoped ZnS annealed at 800 °C. mcat = 500 mg, t = 2.5 h, Vsusp = 580 mL, 500 W Hg lamp.
intensity. The 54 ppm of Co sample still features a decrease of the emission intensity of consecutive spectra similar to ZnS containing only Cl, which is, however, not observed for the samples containing higher amounts of Co (118 and 286 ppm, see Figure S8). This observation clearly demonstrates faster nonradiative charge recombination to occur in ZnS codoped with Co2+ and Cl− by alternative recombination pathways as already observed in the ZnS samples doped with Co2+ only. Photochemical Experiments. The photochemical properties of the differently doped ZnS materials were investigated by irradiation of a ZnS suspension with UV light in pure water in the absence of any sacrificial agents. Experiments were carried out using ZnS annealed at 400 and 800 °C to additionally study the influence of the structural properties on the photochemical properties. Figure 5 shows the H2 evolution rates and the sulfate formation measured during the irradiation of the photocatalyst suspension of Cl-doped ZnS with different Cl contents. A remarkable enhancement in H2 evolution is observed with increasing Cl content. For pure ZnS an almost stationary H2evolution rate of about 0.5 mmol h−1 is observed, whereas all Cl-containing samples exhibit an intense H2 evolution rate maximum after the lamp had been switched on, which then decreased with time. Maximum H2 evolution rates of 2.07, 3.43, and 5.97 mmol h−1 were observed for Cl-doped ZnS with a ZnCl2 substitution of 5%, 20%, and 50%, respectively, together with a comparable increase in the total amount of evolved H2 from 1.35 mmol for pure ZnS to 6.58 mmol for the 50% ZnCl2
sample. Maximum H2 evolution rates, the rates after 2.5 h time on stream (TOS), and the total formed H2 amounts for all Cldoped samples are summarized in Table S1. In contrast to the observed H2 evolution, no significant O2 formation was detected for all the samples. This observation implies that H2 evolution is not based on overall water splitting, but on proton reduction with simultaneous oxidation of the sulfide anions in ZnS, which is preferred over water oxidation, according to eq 1: ZnS + 4H 2O → ZnSO4 + 4H 2
(1)
Thus, the observed H2 evolution is exclusively ascribed to the photocorrosion of ZnS in water. Figure 5 (right) shows the sulfate concentrations measured in the suspension by sampling every 30 min and subsequent ion chromatographic analysis. Samples at t = 0 h and t = 3.0 h were taken before the lamp was switched on and after 30 min stirring with the lamp being switched off, respectively. For all samples the SO 4 2− concentration is increasing until the lamp was switched off after 2.5 h TOS, indicating that SO42− formation is correlated with photocorrosion under UV light. Additionally, the SO42− concentration is increasing strongly as a function of the Cl− content, which is in agreement with the corresponding H2 evolution results. The evolved amounts of H2 and SO42− during the photochemical experiments with differently Cl-doped ZnS are shown in Figure S9 and summarized in Table S1. H2 and SO42− formation clearly increase as a function of the ZnCl2 substitution during the precipitation, and the molar H2/SO42− ratio is close to 4, matching the stoichiometric ratio according 12645
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Figure 8. (a) XRD patterns of Cl-doped ZnS (30% ZnCl2) annealed at 800 °C before and after the photochemical experiments. (b) Normalized intensities of the Zn (111) and S (222) reflections relative to the main ZnS peak.
Figure 9. (a) XRD patterns of Cl/Co-codoped ZnS (30% ZnCl2, 54 ppm of Co) annealed at 800 °C before and after photochemical experiments. (b) Normalized intensities of the Zn (111) and S (222) reflections after reaction relative to the main ZnS peak.
800 °C no significant H2 and SO42− formation is observed for samples obtained at 400 °C (Figure S11). Analogous to the individually Cl- and Co-doped ZnS, the photochemical properties of the Cl/Co-codoped ZnS were investigated to elucidate the stabilizing effect of Co on Clcontaining ZnS. Figure 7 (left) shows a significant decrease of the H2 evolution rate achieved by doping higher amounts of Co. In addition, the Co-doped samples evolve H2 at almost steady state, while a distinct maximum followed by a drastic decrease is observed for the 30% ZnCl2 sample without Co doping. As a consequence of Co codoping, the observed H2 evolution rate for the 286 ppm of Co sample is even lower than that for pure ZnS despite the presence of Cl− in the sample. The observed H2 evolution originates from photocorrosion reactions exclusively, as no O2 evolution was detected for any of the samples. In line with the lower rate of H2 evolution, also the SO42− formation is diminished significantly as a function of the codoped Co amount. Investigations after Reaction. After reaction, a gray to black coloring of the ZnS suspensions suggests other photocorrosion reactions such as the formation of highly dispersed metallic Zn particles to occur in additional to the oxidation to ZnSO4. In the case of Cl-doped ZnS this coloring
to eq 1. The H2 evolution rates and SO42− concentrations for selected Cl-doped ZnS samples annealed at 400 °C, which contain ZnS mainly in the zincblende structure, are shown in Figure S10. No significant H2 evolution or SO42− formation is observed for any of the investigated samples pointing to a strong dependence of the photocorrosion on the ZnS structure. Figure 6 shows the measured H2 and SO42− formation for the Co-doped ZnS materials annealed at 800 °C. Both H2 and SO42− evolution are significantly lower as compared to pure and Cl-doped ZnS. Moreover, the decrease is more pronounced for samples with higher Co contents. In contrast to the Cl-containing samples, moderately Codoped (108 ppm of Co) samples show a rather stable H2 evolution rate during irradiation comparable to pure ZnS. Again, no O2 evolution was detected, confirming the measured H2 to originate exclusively from the photocorrosion of ZnS and not from water splitting. In accordance with the measured H2 evolution rate, the SO42− concentration in the suspension is increasing as a function of time for all investigated samples. The amounts of both H2 and SO42− decrease significantly as a function of Co doping, with the amounts of H2 and SO42− for ZnS doped with at least 262 ppm of Co being approximately 2.5 and 20 times lower than those observed for pure and 5% ZnCl2−ZnS, respectively. In contrast to the samples annealed 12646
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Figure 10. XP S 2p (a) and Zn LMM Auger (b) spectra of 30% ZnCl2−ZnS annealed at 800 °C before and after UV irradiation in ambient air.
was more pronounced for the samples with higher Cl− content, which is in line with the enhanced H2 and SO42− evolution. Therefore, the phase composition and the presence of additional photocorrosion products were investigated using XRD and XPS. In the diffraction pattern of a ZnS sample with 30% ZnCl2 substitution after irradiation shown in Figure 8a, a number of diffraction peaks appear in addition to those related to the two ZnS crystal structures. The reflections at 36.5°, 39.2°, and 43.3° originate from the (002), (010), and (011) crystal planes of metallic Zn. A number of other additional peaks correspond to elemental sulfur, of which the most intense ones are located at 23.2° (222) and 26.0° (026) 2θ. Figure 8b shows the normalized intensities of the Zn (011) and S (222) reflections relative to the main ZnS peak. Both Zn- and Srelated peaks are significantly enhanced by increasing the Cl− content of ZnS. For high Cl− contents the metallic Zn-related diffraction peaks even become the dominant peaks in the pattern, being more intense than the ZnS-related peaks. In contrast to the samples obtained at 800 °C, those annealed at 400 °C remain almost as white as before the experiment, and almost no peak intensities related to elemental zinc or sulfur are observed in the corresponding diffraction patterns. The coloring of the Co-doped samples after the photochemical experiments is less pronounced as for the pure and the Cl-doped materials. The diffraction pattern of the 118 ppm of Co sample shown in Figure S12 displays diffraction peaks at 36.4°, 39.1°, and 43.4° originating from metallic Zn after irradiation. However, these reflections are less pronounced compared with the Cl-doped samples. Moreover, diffraction peaks originating from elemental sulfur are hardly observed. Increasing the level of Co doping leads to a further decrease of the peaks relative to the main combined zincblende/wurtzite ZnS reflection, indicating a less pronounced formation of photocorrosion products with increasing Co content. In case of the Cl/Co-co-doped ZnS, the coloring of the samples is less pronounced as a function of Co content pointing to diminished
formation of metallic Zn as a photocorrosion product. In the corresponding diffraction pattern shown in Figure 9a, reflections from elemental sulfur and metallic Zn at 23.1° and 36.3°, 39.0° and 43.3°, respectively, are clearly observed after irradiation. Increasing the level of Co doping leads to a significant decrease in intensity of these diffraction peaks (Figure S13b), which is further demonstrated by the relative peak intensities shown in Figure 9b. The formation of photocorrosion products in the nearsurface region of ZnS was further investigated using XPS. The corresponding S 2p, Zn 2p, and Zn LMM Auger spectra before and after irradiation of the Cl-doped ZnS (30% ZnCl2) are shown in Figure S14. Both S and Zn peaks are not altered significantly by the irradiation. The S 2p3/2 peak at 162.0 eV corresponds to S2− in ZnS. No sulfate peak is detected around 169 eV due to the high solubility of sulfate in water. For the Zn 2p peak, a distinction between Zn2+ and Zn0 is hardly possible due to the very similar binding energies. Even from the Auger LMM lines, no significant difference is observed before and after irradiation. To overcome the solubility issue of sulfates, the irradiation was additionally carried out in ambient air to avoid ZnSO4 removal from the surface by dissolution and to enable the surface spectroscopic detection of the photooxidation of ZnS to ZnSO4. The sample was prepared on the XPS holder, irradiated for 0.5, 5, and 60 min and subsequently investigated by XPS. In addition to the ZnS-related 2p3/2 and 2p1/2 peaks at 162.0 and 163.1 eV, an additional doublet from sulfur in ZnSO4 is observed in the S 2p spectrum after irradiation as shown in Figure 10. The ratio of sulfide to sulfate is significantly changing with increasing irradiation time as confirmed by the quantification of the surface atomic concentrations summarized in Table S2. While no sulfate is observed for the initial sample the amounts are almost equal (ZnS/ZnSO4 = 1.3) after 60 min of UV exposure. Moreover, the oxygen atomic concentration simultaneously increased due to irradiation and is not correlated with a 12647
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were presumably formed during the annealing process as a consequence of Cl− ions partially desorbing from the sample. These additional defects within the ZnS lattice clearly act as centers for photochemical reactions such as photocorrosion. The enhanced susceptibility of Cl-doped ZnS to photocorrosion can be therefore ascribed to the higher amounts of surface defects. In Co-doped ZnS, the measured emission intensity was found to decrease with increasing Co2+ content. The electronic absorption spectra revealed that Co doping led to additional cobalt ion-induced energy levels within the ZnS bandgap. These energy levels act as recombination centers providing alternative nonradiative recombination pathways for the generated charge carriers. As a consequence of charge transfer from ZnS to Co states, the characteristic ZnS emission is strongly decreased. These observations explain the increased photostability of Co-doped ZnS in photochemical experiments. This stabilizing effect was further demonstrated to be very effective also in the case of ZnS samples codoped with both Cl− and Co2+ ions. The decreased rate of formation of photocorrosion products with increasing Co2+content correlates well with the quenching of luminescence. Thus, the stabilizing effect of Co doping is obviously fast enough to counteract the detrimental effects of surface defects induced by Cl doping. These results clearly demonstrate the significant impact of dopants in the ppm range on the photochemical properties of ZnS, which should also be taken into account for other photochemical applications using semiconductor photocatalysts, especially when using commercial materials which may contain anionic and cationic heteroatoms as impurities.
change in the adventitious carbon concentration, indicating the formation of ZnSO4. Because of the low level of Cl doping, XPS was not sensitive enough to detect Cl− traces in the Cl 2p region. While no significant change in the line shape is observed in the Zn 2p spectra, the Auger LMM peak shown in Figure 10b is changed significantly. After 0.5 min the line shape of the initial ZnS sample with the main peak at 989.2 eV (2) and a shoulder at 992.7 eV is retained. In contrast, longer irradiation leads to the occurrence of a shoulder at 986.8 eV (1) and a new peak at 996.5 eV (4). Additionally, the shape of the shoulder (3) is becoming more distinct. These features are ascribed to ZnSO4 (1), metallic Zn (4), and the superposition of the ZnS shoulder at 992.7 eV and the main peak of metallic Zn (3), respectively. Hence, XP and Auger spectroscopy applied subsequent to UV exposure in ambient air confirm the surface formation of ZnSO4 and metallic Zn as a result of ZnS photocorrosion.
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DISCUSSION X-ray diffraction identified the zincblende structure in the ZnS samples synthesized by precipitation, which was retained after annealing at 400 °C. Annealing at 800 °C was found to convert zincblende into wurtzite and to increase the particle size. For both Cl and Co doping no changes of the diffraction patterns were observed in agreement with the low amount of dopants in the ppm range. It was possible to determine the Co content reliably by AAS measurements, but not the Cl content. However, the presence of traces of Cl− ions was detected indirectly by the photocatalytic measurements and PL spectroscopy. While Cl doping had no significant influence on the ZnS absorption properties and bandgap energy, Co doping led to additional visible light absorption bands caused by internal d−d* transitions within the Co centers. The intensity of these absorption bands was found to match the Co concentrations determined by AAS. The photochemical experiments in combination with postexperiment characterization demonstrated a significant change of the photochemical stability of ZnS due to Cl and Co doping. The formation of H2, SO42−, and elemental S and Zn as photocorrosion products in water was found to increase drastically upon Cl doping, whereas the photocorrosion was to a large extent inhibited by Co doping. Hence, Cl− and Co2+ have opposing effects on the photocorrosion of ZnS: While Cl− increased the light susceptibility, Co2+ effectively stabilizes the material against photocorrosion. This counteracting effect was confirmed by the codoped samples containing both Cl− and Co2+ ions, in which increasing amounts of Co2+ were demonstrated to compensate for the negative effect of Cl−. Moreover, the formation of photocorrosion products was negligible in the case of ZnS nanoparticles obtained by annealing temperatures at 400 °C. The low-temperature samples containing the zincblende form of ZnS were stable against photocorrosion, whereas samples containing both zincblende and wurtzite were prone to photochemical degradation. In addition to the photochemical properties also the emission characteristics of the ZnS nanoparticles were found to be altered significantly by doping. In the case of Cl doping, an enhanced luminescence emission was observed, while the shape of the signal remained unchanged. Since the emission is known to be caused by surface defects such as zinc and sulfur vacancies or interstitial atoms, the increased emission indicates a higher number of defect sites in Cl−-containing samples. These defects
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CONCLUSIONS
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ASSOCIATED CONTENT
Cl-doped, Co-doped, and Cl/Co-codoped ZnS nanoparticles were synthesized by precipitation and subsequent annealing at 400 and 800 °C resulting in almost phase-pure zincblende or mixed zincblende/wurtzite, respectively. Photochemical experiments in the aqueous phase using UV light and postreaction characterization by XRD and XPS were applied to investigate the effect of anionic and cationic doping on the photocorrosion of ZnS. The UV irradiation experiments with the aqueous ZnS nanoparticles dispersed in water demonstrated that wurtzite has a much more pronounced susceptibility to photocorrosion than zincblende and that doping with Cl− ions enhanced photocorrosion, whereas doping with Co2+ ions increased the photostability of ZnS. Using Cl/Co-codoped ZnS, Co doping was confirmed to efficiently counteract the destabilizing effect of Cl doping. By applying photoluminescence spectroscopy, these effects were found to be related to an increased amount of surface defects for Cl doping and to fast alternative charge transfer and recombination pathways for Co doping.
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03385. Additional XRD patterns, UV/vis spectra, photoluminescence spectra, SEM images, and data from photochemical experiments (PDF) 12648
DOI: 10.1021/acs.langmuir.6b03385 Langmuir 2016, 32, 12641−12649
Article
Langmuir
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]; phone +49 234 32 28754 (M.M.). ORCID
Martin Muhler: 0000-0001-5343-6922 Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.langmuir.6b03385 Langmuir 2016, 32, 12641−12649
