Article pubs.acs.org/Langmuir
Morphology Controlled Synthesis of SnS2 Nanomaterial for Promoting Photocatalytic Reduction of Aqueous Cr(VI) under Visible Light Chanchal Mondal, Mainak Ganguly, Jaya Pal, Anindita Roy, Jayasmita Jana, and Tarasankar Pal* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India S Supporting Information *
ABSTRACT: A mild, template free protocol has been demonstrated for SnS2 nanoflake formation at the gram level from SnCl2 and thioacetamide (TAA). The SnS2 nanoflakes congregate to nanoflowers and nanoyarns with variable TAA concentrations. BET measurements reveal that the synthesized nanomaterials are highly porous having very high surface area, and the nanoflower has higher surface area than the nanoyarn. The synthesized nanomaterial finds application for promoting photoreduction of extremely toxic and lethal Cr(VI) under visible light irradiation due to their porous nature. The nanoflowers photocatalyst is proved to be superior to nanoyarn due to the increased surface area and higher pore volume. It was also inferred that increased pH decreased the reaction rate. The present result suggests that the morphology-dependent photoreduction of Cr(VI) by SnS2 nanomaterial under visible light exposure will endorse a new technique for harvesting energy and purification of wastewater.
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INTRODUCTION Chromium compounds exist in two common oxidation states, Cr(III) and Cr(VI).1 Among them Cr(VI) is tremendously toxic and a confirmed carcinogen, whereas Cr(III) is less poisonous and can readily be precipitated out of a solution as Cr(OH)3. Cr(VI) compounds find its innumerable applications in electroplating, leather tanning, metal finishing, textile production, and dyeing.2 As a result, large quantities of aqueous chromium wastes are discharged and they are highly lethal and harmful once introduced into the environment without pretreatment. For example, presence of Cr(VI) in drinking water causes bladder, liver, kidney, and skin cancer.3 Therefore, development of effective technologies to reduce Cr(VI) to Cr(III) and remediation of Cr(VI) contaminations are of paramount importance in the wastewater treatment and for water consumption.4 Various pioneering works have been anticipated for the reduction of Cr(VI) to Cr(III) and its removal, including reduction by magnetic polyaniline,5 microbial reduction,6 photocatalytic reduction,7,8 and adsorption technique.9 For example, Song et al. reported the reduction of Cr(VI) induced by UV−visible light in the presence of aqueous medium contained TiO2 nanosheets.10 Huang et al. have reported reduction and removal of aqueous Cr(VI) induced by glow discharge plasma that occurs at the gas/solution interface.11 Wu et al. reported photoreduction of Cr(VI) induced by NH2-mediated zirconium metal organic framework.12 Zhang et al. reported visible-light-driven photocatalytic reduction of Cr(VI) in the presence of SnS2/SnO2 nanoheterojunction.13 Among these above-mentioned methods, photocatalytic reduction of Cr(VI) using semiconductors is © 2014 American Chemical Society
promising as it is efficient, cost-effective, and does not use or discharge any perilous chemicals. The semiconducting metal sulfides have the capability to harvest light in the visible and short-wavelength near-infrared regions, which makes them a class of promising sensitizers for wide band gap semiconductors14 or visible-light-driven photocatalysts.15 Among them, CdS, which has a band gap of about 2.4 eV, received significant attention as a visible-light-driven photocatalyst.16 However, due to its acute toxicity CdS itself is highly detrimental to human health and the environment. Among the semiconductor metal sulfides, SnS2 has received significant attention due to its optical and electronic properties.17 It is a layered semiconductor material and it bears the CdI2-type structure with a bandgap of 2.2 eV, which is slightly less than that of CdS. It is harmless, chemically stable, and hence effective as a visible light-driven photocatalyst.18 To date, various morphologies of SnS2 nanostructures, such as fullerenelike nanoparticles,19 nanobelts,20 nanosheets,21 and nanoflakes,22 have been synthesized successfully adopting a variety of methods, including laser ablation, template-assisted solvothermal thief processes, hydrothermal treatments, and the thio sol−gel synthetic route.23 However, systematic studies on the morphology-controlled synthesis of SnS2 nanoflakes in the gram level and their unique congregation-dependent photocatalytic reduction of Cr(VI) is not reported so far. Herein, for the first time, we report the synthesis of gram quantity of SnS2 nanoflakes, adopting modified hydrothermal Received: February 6, 2014 Revised: March 21, 2014 Published: March 21, 2014 4157
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light of 15 W, impinging 6.95 × 1010 no. of photons/(cm2 sec) in the reaction vessel. The high photocatalytic activity of SnS2 was evaluated, individually probing 50 mL of 2 × 10−4 M aqueous solution of the dichromate in the presence of various dosages of the catalysts (nanoflower and nanoyarn) at various pH values. The exposed dye solution and nanomaterial were separated by centrifugation (5000 rpm for 5 min) at various time intervals to monitor the reduction of dichromate.
protocol. The SnS2 nanoflakes aggregate as yarns or flowers by fine-tuning the procedure. Then it becomes a material for the reduction of harmful Cr(VI) under visible light. We foreword a comparative account and superiority of SnS2 nanoflowers over nanoyarns toward the reduction of Cr(VI) in terms of surface area and pore volume.
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EXPERIMENTAL SECTION
Materials and Analytical Instrument. The relevant information are depicted in the Supporting Information. Synthesis of SnS2 Nanomaterial. In a typical preparation of SnS2 nanomaterial, 4 mL of 0.1 M SnCl2 was taken in a screw-capped test tube and to it, various amounts of thioacetamide (TAA) (Table 1)
RESULT AND DISCUSSION Figure 1a depicts the XRD patterns of the as-synthesized nanoflower and nanoyarn. All the diffraction peaks confirm the hexagonal phase SnS2, and it coincides well with the JCPDS card no. 23-677. No trace of impurities could be detected in the XRD pattern, indicating the phase purity of the obtained nanomaterial under hydrothermal condition. There occurs no variation in the diffraction pattern due to change in morphology of SnS2 nanomaterial.25 Figure 1b displays optical absorption abilities of SnS2 nanoflower and nanoyarn nearly in the entire UV and visible light spectrum, in the wavelength range of 250− 700 nm. The absorption maximum of the materials is at around 385 nm. The broad spectrum in the visible region suggests that both the SnS2 nanomaterials are capable of harvesting visible light and hence act as a photocatalyst under visible light.26,27 The Tauc approach was invoked to determine the band gap energy (Eg) of the SnS2 nanoflowers. The relationship of absorption coefficient (α) on the photon energy equation is given as follows:
Table 1. Reaction Conditions for the Growth of a Couple of SnS2 Diverse Nanoarchitectures Sl no.
vol of SnCl2 in mL (0.1 M)
amount of TAA (g)
morphology
1 2
4 4
0.06 0.1
nanoflower nanoyarn
were added and stirred for 15 min. Then it was made acidic by adding 0.5 mL of concd HCl. Finally, the sealed screw-capped test tube was subjected to heating hydrothermally at about 100 °C for 6 h using a tungsten bulb (100W) illumination in a closed wooden box. The experimental set up for the preparation of SnS2 nanomaterial is virtually known as modified hydrothermolysis (MHT).24 Yellow precipitate of the nanomaterial was found at the bottom of the screwcapped test tube. It was isolated and washed many times with water and ethanol, and finally it was dried under vacuum for characterization and application. Photocatalytic Activity of SnS2. The dichromate reduction was carried out at room temperature where the solution containing the catalysts (nanoflower, nanoyarn) was first stirred under the dark condition for 15 h to reach adsorption−desorption equilibrium. Then the solution containing the catalyst was illuminated by a fluorescent
αEp = K (Ep − Eg )1/2
where K is a constant, α is the absorption coefficient, Ep is the discrete photoenergy, and Eg is the band gap energy. Figure 1c shows the plot of (αEp)2 versus Ep, which is based on the direct transition, and the extrapolated value of Ep at α = 0 gives an
Figure 1. (a) XRD pattern, (b) DRS spectrum of the SnS2 nanoflower and nanoyarn, and (c) corresponding band gap of SnS2 nanomaterial. 4158
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analysis reported in the literature.28 FESEM analysis as shown in Figure 3 (panels a and b) confirmed the large-scale production of the novel flower like morphology of the asprepared SnS2 nanomaterial at a relatively higher concentration of TAA. FESEM displayed panoramic images of the product, which are composed of interconnected nanoflakes (Figure 4a). The pore size between the sheets was observed in a wide range of 100−800 nm. The thickness of the nanoflakes was found to be 50−60 nm (Figure S1 of the Supporting Information). On increasing the concentration of thioacetamide SnS2 nanoyarns (Figure 3, panels c and d) are formed. These nanoyarns composed of several interconnected nanosheets, and they are also porous in nature (Figure 4b). The images revealed that the pore size of the nanoyarn is much lower than the material with flowerlike congregated rarefied morphology. This indicates that specific surface area of the nanoflowers should be greater than the specific surface area of the nanoyarns, which is again confirmed from BET analysis. Therefore, nanoflowers reduce Cr(VI) at a much faster rate than the SnS2 nanoyarns. An oriented growth mechanism could be a potential explanation for the formation of such hierarchical architecture, in the presence/absence of some surfactant-assisted synthetic routes.29 However, the surfactant is absent in the present synthetic manipulation, which indicates an untraditional growth mechanism different from the template or surfactant-assisted process. The Ostwald ripening process may be the plausible explanation for the anisotopic growth of the nanomaterial for generation of hierarchical morphology.30 Herein, TAA was employed as the sulfur source, which decomposed slowly,
absorption edge energy correspondence to the band gap of synthesized SnS2 nanomaterial (Eg) = 2.12 eV. The lower band gap of the SnS2 nanomaterial suggests its capability of exhibiting visible light driven photocatalytic activity. The obtained DRS spectra and the calculated band gap of both the nanoflower and nanoyarn were found to be the same. Raman spectrum as displayed in Figure 2 of the assynthesized SnS2 nanoflower exhibits an intense peak at
Figure 2. Raman spectrum of the SnS2 nanoflower.
about 312 cm−1, which is assigned to the A1g mode of SnS2 nanoflower. This is in accordance with the group theory
Figure 3. FESEM images. The SnS2 nanoflower: (a) medium magnification and (b) high magnification. The nanoyarn: (c) medium magnification and (d) high magnification. 4159
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Figure 4. FESEM images. The SnS2 nanoflower: (a) low magnification (inset showing magnified image of the nanoflower). The SnS2 nanoyarn: (b) low magnification (inset showing magnified image of the nanoyarn).
Figure 5. TEM images of the (a) SnS2 nanoflower and (b) nanoyarn. (c) SAED pattern of the SnS2 nanoflower and (d) fringe spacing of the SnS2 nanoflower.
generate flakelike morphology. Subsequently, the flakes formed on the primary nuclei and orient in different fashion in various S2− concentrations under an MHT condition for the generation of various 3d architectures (e.g., nanoflowers and nanoyarns to minimize surface energy). There are huge amounts of unreacted S2− ions in the reaction medium in the forms of CH3CSNH2 at the primary stage of the reaction. With increase
leading to a lower degree of super saturation in solution. Under acidic conditions, the hydrolysis of NH2CSNH2 releases H2S (S2−) at 100 °C, which results in the initial nucleation of SnS2 from hydrothermal treatment in acidic medium. FESEM images (Figure S2 of the Supporting Information) clearly depicts that initially particles are obtained which come in contact with each other with prolonged reaction time. Then the agglomerates 4160
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Figure 6. XPS spectrum of the SnS2 nanoflower and nanoyarn for the elements (a) Sn and (b) S.
Figure 7. Nitrogen adsorption−desorption isotherm of the SnS2 nanomaterial (a) nanoflower and (b) nanoyarn.
Figure 8. Photocatalytic reduction of 50 mL of 2 × 10−4 M Cr (VI) in the presence of 50 mg catalyst of (a) SnS2 nanoflower and (b) SnS2 nanoyarn.
of reaction time and reaction temperature, there occurs release of S2− ions to form SnS2. The aggregated flakes form flower- or yarnlike morphologies at various S2− concentrations during the surface-controlled kinetics process. Additionally, crystal growth of the SnS2 nanoflower/nanoyarn via Ostwald ripening process takes place due to prolonged reaction time and with an increase of reaction temperature. Thus larger size SnS2 nanocrystals are obtained at the expense of dissolution of smaller ones with the progress of the reaction. Excess S2− and H+ in the solution is responsible for the formation of SnS2 nuclei under the hydrothermal process.31
Composition of the as-synthesized SnS2 nanoflower was analyzed by EDX measurement (Figure S3 of the Supporting Information). It reveals the coexistence of Sn and S in the nanomaterial. The Sn:S atomic percentage was found to be 35.16% and 65.84%, which is close to the ideal value 1:2 of the SnS2 nanomaterial. TEM images are also in good agreement with the FESEM images. Figure 5a shows the nanoflower, while Figure 5b shows the nanoyarn morphology of SnS2. TEM images also reveal that the as-synthesized materials are highly porous in nature. SAED pattern (Figure 5c) of SnS2 indicates that the as-synthesized material is single crystalline in nature. Fringe spacing (Figure 5d) provides the interlayer spacing 4161
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Figure 9. Photocatalytic reduction of 50 mL of 2 × 10−4 M aqueous solution of dichromate under visible light in the presence of various dosage of SnS2 nanomaterial: (a) nanoflower (0.025 g), (b) nanoflower (0.01 g), (c) nanoyarn (0.025 g), and (d) nanoyarn (0.01 g).
visible light photocatalyst for the reduction of Cr(VI) into Cr(III). Figure 8 (panels a and b) shows the photocatalytic reduction of aqueous Cr(VI) under visible light irradiation in the presence of the as-synthesized SnS2 nanoflower and nanoyarn, respectively. Blank test was carried out in the presence of the catalyst without visible light exposure, and there occurs almost no change of Cr(VI) concentration, as displayed in Figure S4a of the Supporting Information. Again the experiment was also conducted in the absence of a catalyst, under visible light irradiation (Figure S4b of the Supporting Information). However, no change was observed in the concentration of Cr(VI). This indicates that the direct photolysis could not drive the reduction of Cr(VI). It is also inferred that the removal of aqueous Cr(VI) is entirely photocatalyst-driven reduction of Cr(VI), rather than a simple physical adsorption of Cr(VI). Figure 8 shows that as-synthesized SnS2 nanoflowers exhibit enhanced photocatalytic activity. The main absorption band of Cr(VI) centered at 365 nm notably decreases with the exposure time. The result is also in agreement with the gradual color change of the solution from yellow to laurel-green with irradiation time (inset in Figure 8a).7 It is observed that the asprepared SnS2 architecture inherits remarkable photochemical reducing capability, and Cr2O72− ions are reduced within 90 min of irradiation. Figure 9 (panels a and b) shows Cr(VI) reduction under various catalyst (nanoflower) dosages. It authenticates that with the increase in catalyst dosage, the rate of photocatalytic reduction increases. The photocatalytic reduction of Cr(VI) to Cr(III) is mainly facilitated by photoexcited electrons, and therefore, better electron−hole
between the planes, and it is in accordance with the interlayer spacing of the (001) plane. XPS analysis was carried out for both SnS2 nanoflower and SnS2 nanoyarn. Figure 6a shows the presence of sharp peaks at 487.2 and 495.6 eV, which were assigned to Sn 3d5/2 and Sn 3d3/2, respectively. The highresolution S 2p core level analysis exhibits (Figure 6b) the presence of peak at 162.3 eV for S2− in the SnS2 nanocomposite. The observed binding energies of Sn4+ and S2− are in good agreement with the literature report of SnS 2 nanoparticles.26 The XPS study also reveals that no obvious change in XPS spectrum occurs due to change in morphology. Surface area of the materials is a crucial factor of the photocatalytic reduction rate, which is mainly dependent on surface contacts and, hence, surface area measurements were carried out taking both flowerlike and yarnlike nanostructures. Figure 7 (panels a and b) show that both the materials are mesoporous in nature. The BET surface areas for nanoflower and nanoyarn are estimated to be 131.08 and 100.70 m2 g−1, respectively. The pore volumes for both materials are 0.51 and 0.12 cc/g, respectively. The surface area and pore volume of the flowerlike nanostructure were found to be greater than that of the yarnlike nanostructure. Visible-Light Driven Reduction of Cr(VI). Cr(VI) is highly hazardous to living organisms and, therefore, it should be rapidly removed before it is released into the environment. Scientists have developed several approaches to remove Cr(VI) from the polluted waters. The flowerlike and flakelike SnS2 architectures have good high surface area, large pore volume, and photochemical activities, which makes the sample a perfect 4162
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catalysis. The high surface area and high pore volume of flowerlike morphology compared to the yarn makes nanoflower a superior material over nanoyarn for the photocatalytic reduction of Cr(VI). The reusability of the photocatalysts has always been vital for its industrial application, and therefore, it is imperative to study the stability of the as-synthesized SnS2 nanocrystals in photocatalytic reduction of aqueous Cr(VI). The stability of the as-prepared photocatalysts is further examined by monitoring the photocatalytic activity during fourth cycles of operation. The results reveal that the photocatalytic activity of the SnS2 nanomaterials decreased slightly after fourth cycles of the photochemical reduction of Cr2O72− ions. The slight decrease of the photocatalytic reduction ability may be attributed to the deposition of small amounts of Cr(III) after each cycle. After each reuse cycle, the photocatalyst was separated from the aqueous suspension by centrifugation and washed with 1 mol/L HNO3 aqueous solution to diminish the amount of greenish Cr(OH)3 deposited on the surface of the SnS2 nanomaterial, dried under vacuum for further use. The XRD pattern of the materials was carried out after photocatalytic reduction, and no change in XRD pattern occurs after the reduction process (Figure S6 of the Supporting Information). It signifies that the obtained three-dimensional archietectures of SnS2 nanomaterials have high stability and do not photocorrode during the photocatalytic reduction of Cr(VI), which shows its potentiality in its promising applications for wastewater treatment.
separation is achieved in this reduction process where nanoflakes are truly ratified.32 The reason for photocatalytic Cr(VI) reduction under visible light by the as-synthesized SnS2 nanomaterial can be described in the succeeding lines. The as-prepared SnS2 nanomaterial is a p-block metal sulfide, which has a central metal ion with a d10 electronic configuration. It was reported that the d10 electronic configuration favors the separation of photogenerated electron/ hole pairs due to the highly dispersive conduction band.33,34 After that, photoexcited conduction band electron capture takes place. The separated electron takes care of the one-electron reduction reaction of Cr(VI) to generate Cr(III). The proposed reaction pathway for the photocatalytic reduction of Cr(VI) under visible light over SnS2 in aqueous solution is as follows: SnS2 + hv → SnS2 (e− + h+) H 2O + 2h+ → 1/2O2 + 2H+ CrO4 2 − + 8H+ + 3e− → Cr 3 + + 4H 2O
Effect of Initial pH. The effect of initial pH on the visible light-driven photocatalytic reduction of Cr(VI) in the presence of SnS2 was examined by varying the initial pH from 1.5 to 7, keeping the photocatalyst dosage constant. As depicted in Figure S5 of the Supporting Information, photocatalytic reduction of Cr(VI) retarded with increased pH. Under visible light exposure, the color of Cr(VI) ions containing solution gradually fades and turns into lauryl green, indicating photocatalytic reduction of Cr(VI) to Cr(III) ions. It is observed that the photocatalytic reduction of Cr(VI) in low pH condition was much higher than that in high pH condition. It is presumed that lower concentration of H+ along with increased dissolution of SnS2 in high pH is responsible for the decreased photocatalytic activity.35,36 In high pH, the deposition of Cr(OH)3 on the surface of photocatalyst causes a decrease in photocatalytic activity. Effect of Morphology. It is observed that the flowerlike nanostructure is more efficient in photocatalytic reduction of Cr(VI) than the yarnlike morphology (Figure 8b). The rates of photocatalytic reduction of Cr(VI) of nanoflower and nanoyarn were also compared, employing various dosages of catalyst (Figure 9). In every case, nanoflowers show superior photocatalytic activity than the yarnlike material. The discrepancy in the photocatalytic activity of both the SnS2 nanomaterial may be elucidated by the combined action of several factors, such as specific surface area, particle size, band gap, crystal phase, morphology, surface chemical state and composition, crystallinity, and crystal defects. Since almost all of the aforementioned factors were the same as revealed by the characterization techniques except the specific surface area, it is can be said that the difference in photocatalytic activity of SnS2 nanocrystals is solely influenced by a single parameter, specific surface area. The main differences in their catalytic activity attributes to differences in their surface chemistry and morphology. As we know, the photocatalytic process is governed mainly by the adsorption and desorption of molecules on the surface of catalysts. Thus, the higher surface area of the porous SnS2 nanoflower bears more unsaturated surface coordination sites exposed to the solution than the yarnlike morphology. Moreover, the presence of the open porous structure helps more efficient transportation for the reactant molecules into the active sites and, thereby, enhances the efficiency of photo-
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CONCLUSION In conclusion, a facile, cost-effective, template or surfactant-free approach has been adopted for the synthesis of the SnS2 nanomaterial at the gram level. A couple of diverse structures (i.e., nanoyarn and nanoflower) have been synthesized simply by varying the concentration of TAA. The as-synthesized materials were employed for the reduction of aqueous Cr(VI) under visible light (λ > 420 nm) irradiation. It was observed that the photocatalytic activity of SnS2 nanoflowers in aqueous suspension is better than nanoyarn due to its higher surface area and is promising in the efficient utilization of solar energy for the treatment of Cr(VI)-containing wastewater.
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ASSOCIATED CONTENT
* Supporting Information S
Details about material and analytical instrument, XRD pattern, FESEM images, and photoreduction of dichromate in the absence of catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors are thankful to IIT Kharagpur, BRNS, and CSIR, New Delhi, for financial assistance. 4163
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dx.doi.org/10.1021/la500509c | Langmuir 2014, 30, 4157−4164
