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Synthesis, Characterization, and Photocatalytic Properties of In2S3, ZnIn2S4, and CdIn2S4 Nanocrystals Sudip K. Batabyal,*,† Shu En Lu,‡ and Jagadese J. Vittal*,‡ †

Amrita Centre for Industrial Research and Innovation (ACIRI), Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham University, Tamil Nadu, India 641112 ‡ Department of Chemistry, National University of Singapore, Singapore 117543 S Supporting Information *

ABSTRACT: A one-pot method has been employed to synthesize the nanocrystals of In2S3, ZnIn2S4, and CdIn2S4. The single-source precursor [In(bipy)(SC{O}Ph)3] has been used for making In2S3 nanocrystals. On the other hand, [Zn(SC{O}Ph)2]·2H2O or [Cd(SC{O}Ph)2]·2H2O was reacted with [In(bipy)(SC{O}Ph)3] and decomposed to make the corresponding ternary metal indium sulfides. The nanocrystals have been characterized by Xray powder diffraction (XRPD), transmission (TEM) and scanning electron microscopies (SEM), selected area electron diffraction (SAED) patterns, and energy-dispersive X-ray analysis (EDX). Parameters such as temperature, molar ratio of precursor to surfactant, duration of reaction time, and surfactant type were varied to investigate their influence on the morphology and size of nanomaterial. It was found that In2S3 exhibits different morphologies under different reaction conditions whereas the effect of these reactions conditions on the morphological evolution is not very prominent for ZnIn2S4 and CdIn2S4. All the synthesized metal ternary In2S3 showed efficient photocatalytic degradation of dye under ultraviolet (UV) light irradiation. We observed that the degradation of dye is much faster in chloroform than in the aqueous solution as the dispersion of nanoparticles is more homogeneous in chloroform solution. A comparison of the photocatalytic activity of In2S3 with ZnIn2S4 and CdIn2S4 having similar morphology and size shows that the photocatalytic activity of the ternary chalcogenides of In2S3 is considerably enhanced.

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INTRODUCTION The control of size, shape, and structure of nanomaterials is fundamentally important due to the strong correlation between these parameters and their properties. Among the various strategies employed, the one-pot synthesis route has been proven to be effective in synthesizing nanomaterials with wellcontrolled sizes, shapes, and structures.1−3 Colloidal synthesis of high-quality nanocrystals of binary and ternary chalcogenide compounds by direct thermal decomposition of single-source or dual-source precursors attracts immense research interest because of the technological applications in diverse areas.4−7 Over many years, binary chalcogenide A2B3 (A = Al, Ga, In; B = S, Se, Te) and ternary chalcogenide AB2C4 (A = Ca, Mg, Zn, Cd; B = Al, Ga, In; C = S, Se, Te) have been the subject of intense research due to their unique optoelectronic, electronic, optical, and catalytic properties. Among these compounds, In2S3,7−22 ZnIn2S4,23−26 and CdIn2S427−31 have attracted considerable attention. Binary chalcogenide indium sulfide exists in three different forms, namely, α-In2S3 (defect cubic), βIn2S3 (defect spinel), and γ-In2S3 (layered structure). In particular, β-In2S3 is an n-type semiconductor and, having a band gap of 2.0−2.3 eV, has attracted research interest due to its promising applications in optical, photoconductive, and optoelectronic fields.4,6,9 Recently several reports have appeared © XXXX American Chemical Society

in the literature on the synthesis of different morphologies of In2S3 like nanospheres,10 nanosheets,11−14 nanowires,15 nanobelts,16 microspheres,17 and 3D flower-like structures.18 Previously reported synthesis methods include hydrothermal treatment,18,19 colloidal synthesis,10,13 sonochemical route,20 and precursor based approach.11,12,14,21 Recently it was observed that photoconducting and photocatalytic properties of In2S3 nanosheets were enhanced by incorporating dopant elements.12,22 Therefore, it would be of interest to investigate how the morphology and the photocatalytic properties of In2S3 nanosheets will be modified after incorporation of Zn(II) and Cd(II). Ternary chalcogenide ZnIn2S4 has been shown to have potential applications in charge storage,23 thermoelectricity,24 electrochemical recording,25 and photocatalytic activity.26 On the other hand, CdIn2S4 is a feasible material for use in solar cells,27 photoconductors,28 and light-emitting diodes.29 Much effort has been focused on the preparation of these ternary chalcogenide materials with interesting morphologies. ZnIn2S4 nano- and microstructures with controlled size were obtained Received: January 12, 2016 Revised: March 1, 2016

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via a facile solution route,26 while CdIn2S4 nanotubes and “marigold-like” nanostructures30 and hollow spheres31 were synthesized using a facile hydrothermal method. In this report, a one-pot synthesis method was employed to make the nanocrystals of In2S3, ZnIn2S4, and CdIn2S4 using single- and dual-source precursors. The synthesis route is also optimized through the variation of temperature, precursor-tosurfactant molar ratio, choice of surfactant, and duration of reaction time. The photocatalytic activity of these nanomaterials has been tested using degradation of organic dye under UV light. The results are described below.

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preparation was used for EDX experiments except that no coating of the platinum was required. Transmission electron microscopy (TEM) was performed on a JEOL JSM-2010 microscope operating at 200 kV. High resolution transmission electron microscopy (HRTEM) images and electronic diffraction patterns were obtained from a JEOL JSM-3010 instrument. The samples for TEM were prepared by placing a drop of dilute solution of the sample in toluene onto a carbon coated Cu grid (300 mesh), and then it was completely dried under vacuum. The SAED images were captured during the acquisition process of TEM images. Measurement of Optical Properties and Photocatalytic Activity. To investigate the optical properties, approximately 0.05 mg of the nanomaterials of In2S3, CdIn2S4, or ZnIn2S4 was dispersed in toluene (2 mL), and UV−vis absorption and photoluminescence spectra were recorded. The UV−vis absorption spectra were obtained at room temperature on a Shimadzu UV2450 UV−visible spectrophotometer. Room temperature photoluminescence spectra were recorded on a PerkinElmer LS55 luminescence spectrometer. To investigate the photocatalytic activity, the nanomaterials were used to decompose methylene blue (MB) dye at room temperature. Initially, 2 mL (2 × 10−5 mol/L) of MB in aqueous solution was poured into a standard quartz cuvette. Then ZnIn2S4 used as catalyst (0.04 mg) was dispersed in water (0.1 mL) and added to the cuvette. The cuvette was placed 10 cm away from a high-pressure mercury lamp and irradiated at room temperature, and the irradiated solution was subjected to UV−vis absorption spectra measurements at regular intervals of time.

EXPERIMENTAL SECTION

Materials and Methods. Sodium hydroxide pearls (Merck), thiobenzoic acid (90%, Alfa-Aesar), indium(III) chloride (98%, Aldrich), 2,2′-bipyridyl (bipy, Fluka), zinc nitrate hexahydrate (Fluka), cadmium nitrate tetrahydrate (Fluka), oleylamine (70%, Fluka) (OA), oleic acid (90%, Aldrich), 1-dodecanethiol (98+%, Aldrich) (DT), trioctylphosphine oxide (90%, Aldrich), ethanol (ARgrade), toluene (AR-grade), chloroform (AR-grade), and 16mercaptohexadecanoic acid (90%, Aldrich) were used as received. Synthesis of Precursors. The precursors [In(bipy)(SC{O}Ph)3], 1, [Zn(SC{O}Ph)2]·2H2O, 2, and [Cd(SC{O}Ph)2]·2H2O, 3 were synthesized according to the reported methods.2 The details of the syntheses are described in ESI 1. Synthesis of Nanocrystals. In the single-molecular synthesis route, the precursor [In(bipy)(SC{O}Ph)3], 1 (20 mg, 0.03 mmol), was added to oleylamine (OA, C18H37N, 1.5 mmol, 0.49 mL) and oleic acid (OLA, C18H34O2, 1.5 mmol, 0.48 mL) at room temperature of 25 °C (the molar ratio of precursor:OA:OLA = 1:50:50). The contents were then heated at 300 °C for 24 h with gentle stirring under nitrogen atmosphere via standard Schlenk techniques. The solution was cooled to room temperature before an excess of absolute ethanol was added. The precipitate obtained was separated by centrifugation, washed with ethanol several times, and dried in vacuum. The dried powder can be easily redispersed in nonpolar solvents like toluene and hexane. In the dual-source synthesis route, the precursors [Zn(SC{O}Ph)2]· 2H2O, 2 (20 mg, 0.053 mmol), and [In(bipy)(SC{O}Ph)3], 1 (72 mg, 0.106 mmol), were added to OA (5.3 mmol, 1.74 mL) and OLA (5.3 mmol, 1.69 mL) at room temperature (the molar ratio of precursor:OA:OLA = 1:100:100), and the contents were heated at 300 °C for 24 h with gentle stirring under nitrogen atmosphere. The products were separated as described for the single-molecular synthesis route before. Using a similar procedure described above, the precursors [Cd(SC{O}Ph)2]·2H2O, 3 (20 mg, 0.047 mmol), and [In(bipy)(SC{O}Ph)3], 1 (64 mg, 0.094 mmol), were added to OA (4.7 mmol, 1.55 mL) and oleic acid (4.7 mmol, 1.50 mL) at room temperature (the molar ratio of precursor:OA:oleic acid = 1:100:100), and the contents were heated at 300 °C for 24 h with gentle stirring under nitrogen atmosphere. The product was isolated by a procedure similar to that used in the single-molecular synthesis route. Characterization. X-ray powder diffraction (XRPD) patterns were obtained using a D5005 Bruker X-ray diffractometer equipped with Cu Kα radiation. The accelerating voltage and current were 40 kV and 40 mA respectively. Samples were prepared on glass slides. A concentrated toluene solution was slowly evaporated at room temperature in fume hood on a glass slide to obtain a sample for analysis. Scanning electron microscopy (SEM) images were taken using a JEOL JSM-6700F field emission scanning electron microscope operating at 5 kV and 10A. The samples were deposited over carbon tape attached on the metal holder and coated with a thin layer of platinum using JEOL JFC-1600 auto fine coater to increase conductivity. EDX was acquired using another model, JEOL JSM6701F field emission scanning electron microscope. A similar sample

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RESULTS AND DISCUSSION The precursor 1 was prepared by a simple reaction between a metal salt, thiobenzoate anion, and a chelating ligand bipy. Precursors 2 and 3 were synthesized using stoichiometric amounts of the corresponding metal nitrate and thiobenzoate anions in situ in EtOH solvent. After drying in vacuum, the compounds were found to be very stable, and no apparent decomposition occurred while they were left at room temperature in air. Formation of In2S3 Nanocrystals. Nanostructured In2S3 was formed from the thermolysis of 1 at 300 °C in the presence of different surfactants. The effect of molar ratio of precursor to surfactant and surfactant type was studied. In general, an alteration in the reaction environment has great impact on the morphology of the final products. Our experimental results indicated that the presence of an alkyl amine, in this case oleylamine, OA, is crucial for the formation of In2S3. Du et al. reported that alkyl amine assists the formation of In2S3 but not the morphology of the product.16 They reported when octadecylamine or oleylamine was used in place of hexadecylamine in the pyridine system, similar β-In2S3 belt-like nanostructures were obtained.16 We used double surfactant system for our synthesis method. When only oleic acid was used as surfactant, no product was obtained. As oleic acid is a kind of strong coordinating solvent, it would coordinate with In(III) to form a stable complex which is likely to suppress the formation of In3S3. Figure 1 shows the XRPD patterns of In2S3 prepared under different precursor-to-surfactant molar ratios and different surfactants. We prepared samples with surfactants OA and OLA in varying precursor-to-surfactant molar ratios, namely, 1:50:50 (IS1) and 1:100:0 (IS2) at 300 °C for 24 h. A combination of OA and DT was also studied (IS3) with precursor-to-surfactant molar ratio of 1:50:50 at 300 °C for 24 h. All peaks in the patterns reveal that the In2S3 product has the tetragonal structure and can be indexed to the standard data (JCPDS No.: 00-025-0390). No impurity peaks were detected. B

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hexagonal phase of ZnIn2S4 (JCPDS No.: 01-072-0773). The lower intensity and larger full width at half-maximum (fwhm) of the diffraction peaks can be attributed to the smaller sizes of the obtained ZnIn2S4. We noticed substantial shift in the peak position for the (101) and (102) planes of ZnInS2 (Figure 2) from the standard JCPDF data; probably it is due to the internal strain and compressive stress for particular that direction. We noted that the intensity of the peak corresponding to the (006) plane is strengthened with the increase in the amount of the surfactant used, in comparison with the intensities of the peaks for the (101), (102), and (110) planes. As from the TEM investigation, we observed that the size of the plate is reduced from ZIS1 to ZIS3. The HRTEM investigation also shows that the interplanar spacing along the width is identical with the (101) plane so this comparative peak intensity change in the XRPD patterns is quite consistent with the morphological studies. At molar ratios below 1:100:100, a mixture of In2S3 and ZnS was obtained. The growth process of ZnIn2S4 was monitored over a period of 6, 12, and 24 h. All peaks in the XRPD patterns can be indexed to the hexagonal phase of ZnIn2S4 (JCPDS No.: 01072-0773) (ESI 4). By comparing the intensities of the major peaks, we noticed that an increase in the reaction time increases the intensity of the peak of the (110) plane in comparison with the intensity of the peak of the (006) plane. This indicates that ZnIn2S4 preferentially grows in the (110) orientation over time, i.e., the width of the plates increases. The effect observed here is opposite to that when the amount for surfactant was increased. EDX analysis of all the samples reveals close proximity to the stoichiometric compound ZnIn2S4. One typical spectrum for ZIS1 is shown in ESI 5. The experimental atomic ratio of Zn to In to S is found to be 1:2.2:3.7. Formation of CdIn2S4 Nanocrystals. Nanostructured CdIn2S4 was formed from the thermolysis of precursors 1 and 3 at high temperature in the presence of surfactants. Parameters such as the effect of temperature, molar ratio of precursor to surfactant, and reaction time were studied. From the XRPD patterns of the obtained product at different temperatures (ESI 6), it was noted that CdIn2S4 can only be obtained at the optimum temperature of 300 °C. At 300 °C, the product can be verified to be in the cubic phase (JCPDS No.: 00-027-0060). Any lower or higher reaction temperature resulted in the formation of unidentified products. Comparing the synthesis of ZnIn2S4 and CdIn2S4 done at varying temperature, it appears that precursor 2 may have higher solubility than 3. This is because the inception temperature for the formation of ZnIn2S4 is as low as 200 °C while it is 300 °C for CdIn2S4. The effect of precursor to the molar ratio was varied and investigated by the XRPD experiments (Figure 3) as follows: 1:25:75 (CIS1) to 1:50:50 (CIS2) to 1:100:0 (CIS3) to 1:100:100 (CIS4) to 1:150:50 (CIS5). When only OLA was used, the reaction yielded an amorphous solid. This is similar to that observed in the synthesis of In2S3. The molar ratio of precursor to OA to oleic acid was then varied to observe any changes in the product formation. Interestingly, when the precursor-to-total surfactant ratio was kept at 1:100 or below, highly crystalline and tetragonal In2S3 phase (JCPDS No.: 00025-0390) was formed. Only when the precursor-to-total surfactant ratio was above 1:100 was CdIn2S4 formed. As observed from the XRPD patterns obtained at molar ratios of 1:100:100 and 1:150:50, the peaks are left-shifted by 2° and can be matched to the cubic phase of CdIn2S4 (JCPDS No.: 00-

Figure 1. XRPD patterns of In2S3 synthesized at 300 °C for 24 h with different precursor-to-surfactant molar ratios and surfactant systems. IS1 and IS2 for the surfactants OA and OLA with 1:50:50 and 1:100:0 and IS3 for the surfactants OA and DT with 1:50:50.

EDX analysis of all the samples reveals close proximity to the stoichiometric compound In2S3. One typical spectrum for IS2 is shown in ESI 2. The experimental atomic ratio of In to S is found to be 2:3.1. Formation of ZnIn2S4 Nanocrystals. Nanostructured ZnIn2S4 was formed by the thermolysis of the precursors 1 and 2 at high temperature in the presence of surfactants. Several parameters were studied, namely, the effect of temperature, molar ratio of precursor to surfactant, and reaction time. The XRPD pattern obtained from the product prepared at 200 °C can be indexed to the hexagonal phase of ZnIn2S4 (JCPDS No.: 01-072-0773). The major peaks originating from (006), (102), and (110) are observable (ESI 3). When the reaction temperature rose to 300 °C, these peaks became sharper and more distinct. Lower-intensity peaks corresponding to (105), (116), and (202) also became more prominent, which indicates increase of crystallinity of ZnIn2S4. Beyond 300 °C, the formation of ZnIn2S4 does not occur. As such, we believe that the optimum reaction temperature is 300 °C. Figure 2 shows the XRPD patterns of synthesized ZnIn2S4 at 300 °C for 24 h with varying precursor-to-surfactant molar ratios from 1:100:100 (ZIS1) to 1:300:300 (ZIS2) to 1:600:600 (ZIS3). All the peaks can be indexed to the known

Figure 2. XRPD patterns of ZnIn2S4 synthesized at 300 °C for 24 h when the molar ratio of precursor:OA:OLA = 1:100:100 (ZIS1), 1:300:300 (ZIS2), and 1:600:600 (ZIS3). C

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00-027-0060). There is no evidence of binary sulfides, oxides, or unreacted precursors being observed. Further, strong and sharp peaks indicate that the final product is of high purity and good crystallinity. EDX analysis of all the samples reveals close proximity to the stoichiometric compound CdIn2S4. One typical spectrum for CIS5 is shown in ESI 8. The experimental atomic ratio of Cd to In to S is found to be 1:1.8:3.8. Morphological Investigation. The morphology of the obtained In2S3 depends on the nature of the surfactants used. When equal amounts of OA and OLA were used, very thin In2S3 nanosheets of thickness 1.3 nm were formed. The width of nanosheets is estimated to be 80−300 nm. Due to the high flexibility of the nanosheets, they curled like a crumpled cloth or paper as shown in Figures 4a and 4b. When only OA is used as surfactant, In2S3 hexagonal plates of width 250−1000 nm are observed. The SAED pattern also implies that the In2S3 obtained is highly polycrystalline in nature with interplanar spacing of 3.249 Å, 2.694 Å, and 1.905 Å, which then correspond to the (1 0 9), (0 0 12) and (2 2 12) planes, respectively. The interplanar spacing obtained from HRTEM analysis is approximately 6.2 Å, which corresponds to the (1 0 3) plane of In2S3 (Figure 5). If

Figure 3. XRPD patterns of In2S3 and CdIn2S4 synthesized at 300 °C for 24 h when the molar ratio of precursor:OA:OLA = 1:25:75 (CIS1), 1:50:50 (CIS2), 1:100:0 (CIS3), 1:100:100 (CIS4), and 1:150:50 (CIS5). Impurity peaks are indicated as *.

027-0060). We propose that this observation is due to the difference in the solubility of the precursors [In(bipy)(SC{O}Ph)3] and [Cd(SC{O}Ph)2]·2H2O. When the amount of surfactant used is low, [In(bipy)(SC{O}Ph)3], which has a higher solubility, decomposes first in the reaction mixture, giving rise to the formation of In2S3. As the amount of surfactant used is increased, [Cd(SC{O}Ph)2]·2H2O is more insoluble and hence decomposes and reacts with In2S3 to give CdIn2S4. It is known that In2S3 can adopt either cubic or tetragonal form above 420 °C, hence a plausible formation mechanism of CdIn2S4 is proposed. In the initial stages, tetragonal phase In2S3 is formed. It is then transformed to cubic phase after insertion of Cd(II), leading to formation of the final product as cubic phase of CdIn2S4. The growth progress of CdIn2S4 was also monitored over a period of 6 and 12 h (CIS6), and 24 h (ESI 7). The bright yellow product synthesized via the dual-source precursor method is revealed to assume the cubic structure of CdIn2S4 as all major peaks can be indexed to standard data (JCPDS No.:

Figure 5. TEM images of ZIS1 (a), ZIS2 (b), and ZIS3 (c).

OLA is replaced to DT, the plate-like structure with irregular triangular or hexagonal shapes has been retained. The width of the plates is in the range of 100−600 nm (ESI 9). In the synthesis of ZnIn2S4 nanocrystals, the hexagonal shape of the plate has been retained when the amount of surfactants was increased. However, the dispersity of size of the hexagonal

Figure 4. TEM images of IS1 (a, b) and IS2 (c, d). SAED (e) and lattice fringe (f) patterns of IS2. D

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Optical Properties. Figures 8a and 8b show the UV and PL spectra of In2S3 nanoparticles. For the three samples of In2S3 synthesized under different reaction conditions, the absorption spectrum typically shows a broad shoulder with a tail in the longer wavelength region. Although there is no obvious absorption peak, the In2S3 nanoparticles have an absorption edge at around 290 nm, giving rise to a band gap of 4.3 eV (as found from the Tauc plot in ESI 10). This is significantly blueshifted from that of bulk In2S3 (λmax = 601 nm, Eg = 2.07 eV) and consistent with the quantum confinement effect. On the other hand, the PL spectrum shows emission peaks centered at 345 and 417 nm. Similar observations were observed in the literature. Du et al., for example, noted a steplike absorption edge of 260−430 nm and a broad emission peak at 380 nm for β-In2S3 nanobelts.16 We also realize that variation in morphology does not give rise to considerable change in absorption edge or the PL of In2S3. The absorption spectrum for the samples of ZnIn2S4 in Figure 8c shows a broad shoulder with a tail in the long wavelength region. Peng et al. reported a wide absorption range (2.18 eV to 3.28 eV) of the synthesized ZnIn2S4 nanocrystals depending upon the size and shape of the nanocrystals.26 We also observed a blue shift in the absorption edge from ZIS1 to ZIS3. When the amount of the surfactant is increased, the size of the ZnIn2S4 nanoparticles is reduced (as mentioned above), giving rise to a respective increase in the band gap. This corroborates our results observed from TEM. The sample ZIS3 gives an absorption edge at 330 nm and a band gap of 3.77 eV (ESI 11). This is significantly blue-shifted from that of bulk ZnIn2S4 (λmax = 540 nm, Eg = 2.30 eV) and is consistent with the quantum confinement effect. A red shift in absorption edge is observed when the reaction time is increased, leading to a corresponding decrease in band gap. This implies that the nanoparticles aggregate and form bigger particles with time. On the other hand, the PL spectrum shows emission peaks centered at 420, 460, and 484 nm. Figures 8e and 8f show the UV and PL spectra of CdIn2S4 nanoparticles. The absorption spectrum for the three samples of CdIn2S4 shows a broad shoulder with a tail in the longer

plates is reduced, from 20−500 nm to 40−400 nm, to 150−300 nm (for molar ratios 1:100:100, 1:300:300, and 1:600:600 respectively). The interplanar spacing, ∼3.3 Å, for ZnIn2S4 for the sample ZIS1 in Figure 6 corresponds to the crystallographic (101)

Figure 6. SAED (a) and lattice fringe pattern (b) of ZIS1.

plane. The SAED pattern implies that the ZnIn2S4 obtained takes the form of a single crystal with interplanar spacings of 3.304 Å, 1.925 Å, and 1.663 Å correspond to the (101), (110), and (201) planes, respectively. As for CdIn2S4 nanocrystals, when the molar ratio of precursor to surfactant is 1:100:100, hexagonal plates of size 175 nm have been observed for 24 h reaction. When the ratio is changed to 1:150:50, the morphology is still retained as hexagonal plates of size in the range 10−30 nm for 12 h of reaction. The size of the plates increases to 150 nm in 24 h. The sample obtained after 12 h of the reaction was investigated using HRTEM as we found an interesting self-assembly pattern of triangular plates. This leads us to believe that the formation of CdIn2S4 hexagonal plates takes place via a preorganization of six triangular plates. With reference to Figure 7e, the interplanar spacing is approximately 3.8 Å, which corresponds to the (220) plane of CdIn2S4. The SAED pattern reveals that the CdIn2 S4 obtained is highly polycrystalline with interplanar spacings of 3.270 Å, 2.213 Å, 1.916 Å, and 1.654 Å corresponding to the (311), (422), (440), and (533) planes, respectively.

Figure 7. TEM images (a, b) of CIS5 synthesized at 12 and 24 h, respectively. TEM image (c) of CIS4. SAED (d) and lattice fringe pattern (e) of CIS4. E

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To comply with increasingly stringent international environmental standards (ISO 14001, October 1996), technological advancements for their removal from the environment have been attempted.32 Titanium dioxide was the most widely used photocatalyst in the past because of its strong redox power and high photocorrosion resistance. However, the drawback is that it suffers from a relatively low quantum yield. Possible solutions include the modification of the TiO2 surface and the development of new materials.33 In recent years, heterogeneous photocatalytic oxidation (PCO) has gained popularity as “green” technology for the degradation of soluble dyes in wastewater.34 The development of non-titania semiconductor materials for the photodegradation of organic pollutants utilizing the solar energy has attracted considerable attention. Chen et al. reported the photocatalytic degradation of several dyes (methyl orange, Congo red, and rhodamine B) by ZnIn2S4 microspheres under visible light irradiation.35 In addition, Du et al. conducted a study on the photocatalytic performance of β-In2S3 nanobelts by monitoring the decrease in absorptive intensity of methylene blue (MB, Figure 9) with UV light irradiation with time.16

Figure 9. Molecular structures of (a) methylene blue, MB, and (b) leucomethylene blue, LMB.

MB is commonly used as an oxidation−reduction indicator in chemistry and biology. It is a cationic dye soluble in water and has a basic skeleton of a thiazine group.36 By losing 2 electrons, MB is easily reduced to the colorless hydrogenated molecule, leucomethylene blue (LMB). As measured from UV−vis spectroscopy, λmax of MB occurs at 655 nm. MB was employed for the first time to investigate the photocatalytic performance of ZnIn2S4 in an aqueous system. After 2.5 h of UV light irradiation, it was observed that the percentage reduction in absorbance of MB was approximately 60%. In the absence of the nanoparticles, no decrease in the absorbance of MB was detected under identical experimental conditions. This infers that ZnIn2S4 nanoparticles act as catalyst for the degradation of MB. After the reaction, the catalyst can be retrieved using centrifugation. Our results obtained for the photocatalytic degradation of MB by ZnIn2S4 are comparable to those reported in the literature. Chen et al. reported a 90% degradation of methyl orange after 3 h of visible light irradiation. The same percentage of degradation was achieved in Congo red and rhodamine B after 5 and 3 h of visible light irradiation, respectively.35 A photocatalytic degradation mechanism of dyes by ZnIn2S4 microspheres is postulated. Under the irradiation of visible or UV light, electron excitation occurs from the valence band to the conduction band of ZnIn2S4 leading to the generation of an electron hole in the valence band. Following this the molecular oxygen rapidly reacts with electrons at the photocatalyst surface to yield the superoxide radical anion, •O2−, which further combines with H+ to generate •HO2 radical. After formation of the •HO2 radical, the •OH radical is formed from the trapped electron. In the final step, the active oxygen species, such as

Figure 8. UV and PL spectra of (a, b) In2S3, (c, d) ZnIn2S4, and (e, f) CdIn2S4.

wavelength of the spectrum. Fan et al. reported an absorption edge of 572 nm and a red emission band centered at 862 nm for hollow CdIn2S4 microspheres.31 On the contrary, for the nanoparticles that we synthesized here, an absorption edge at 405 nm and a band gap of 3.07 eV have been observed for CIS6 (ESI 12). This is significantly blue-shifted from that of bulk CdIn2S4 (λmax = 540 nm, Eg = 2.30 eV), which is consistent with the quantum confinement effect. In addition, for the nanoparticles synthesized using long reaction time, a red shift in the absorption edge was observed and a corresponding decrease in band gap. This is consistent with our observations from TEM and reveals that the nanoparticles aggregate and form bigger particles over the period of time. While comparing the spectra of the samples CIS4 and CIS5, it is noticed that the latter gives a smaller λmax and larger Eg. This could be due to the different strengths of oleylamine and oleic acid as capping agents. When more oleylamine is used, the CdIn2S4 nanoparticles are stabilized more effectively against aggregation. On the other hand, the PL spectrum shows emission peaks centered at 485 and 530 nm. Photocatalytic Activity of Synthesized In2S3, ZnIn2S4, and CdIn2S4. In the past few decades, the focus has been on the global environmental issues related to harmful pollutants. Dyes and their degradation products such as aromatic amines are carcinogenic and form a major part of organic pollutants. F

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OH, •HO2, and •O2− radicals, or the electron hole attacks the dye, giving rise to its degradation. In our synthesis of ZnIn2S4 nanoparticles, oleylamine and oleic acid are used as surfactants. Both surfactants are characterized by long hydrocarbon chains which make them hydrophobic. As such, when the nanoparticles are surrounded by surfactants, they become highly insoluble in water. This results in a poor dispersion of nanoparticles in the solvent and a higher tendency for them to aggregate to form clusters. So the dispersion in water with a smaller reactive surface area decreases the efficiency of the photocatalyst. This explains the relatively long irradiation time needed for MB degradation in water. Taking this into consideration of similar photocatalytic activity measurements, the best dispersion for ZnIn2S4, In2S3, and CdIn2S4 was obtained in CHCl3 as solvent. A drop of EtOH was added to each system to assist in the radical formation. As shown in Figure 10a for ZnIn2S4 and Figure 10b Figure 11. UV absorption spectrum for the successive decomposition of MB in CHCl3 solution under UV irradiation in the presence of (a) IS1, (b) IS2, and (c) IS3. (d) Comparative graph shows the percentage degradation of MB in the presence of In2S3 under UV light irradiation.

Figure 10. UV absorption spectrum for the successive decolourization of MB in CHCl3 solution under UV light irradiation in the presence of (a) ZnIn2S4 and (b) CdIn2S4.

for CdIn2S4, the time for MB to degrade by 66% is merely 25 min. This is a drastic improvement of photocatalytic performance of ZnIn2S4 as compared to that in the aqueous system (ESI 13). Similar measurements were made for the synthesized In2S3 nanocrystals also. All three samples for In2S3 were investigated as it was interesting to find out the effect of morphology on their photocatalytic activity. From our results shown in Figure 11, we found that the photocatalytic activity of In2S3 increases in the order triangular plates < hexagonal plates < nanosheets. A plausible explanation may be attributed to the increase of the exposed surface area of the photocatalyst. The phtotcatalytic properties of the nanocrystals are dominated by the surfaces and hence the capping agents on the surface play a crucial role. We observed that nanoparticles synthesized with the OA−OLA system (IS1 and IS2) show better catalytic efficiency than the nanoparticles synthesized with the DT system (IS3). The difference in photocatalytic activities of In2S3, ZnIn2S4, and CdIn2S4 has also been investigated. The samples selected for this study have similar morphology (hexagonal plates) and approximately the same size. This was to ensure that the comparison would be done on a fair basis. From Figure 12a, we see that ZnIn2S4 and CdIn2S4 cause a significantly greater percentage degradation of MB compared to In2S3 over the same amount of UV irradiation time. Probably the catalytic sites on the surface of the nanocrystals in the ternary form of In2S3 (namely, ZnIn2S4 and CdIn2S4) are much more than the In2S3 nanocrystals which could be responsible for the overall higher catalytic activity of ZnIn2S4 and CdIn2S4 than the In2S3

Figure 12. (a) Comparative graph to show the percentage degradation of MB in the presence of In2S3, ZnIn2S4, and CdIn2S4 under UV light (CHCl3 as solvent). (b) UV−vis spectrum of as synthesized In2S3, ZnIn2S4, and CdIn2S4.

nanocrystals. Also the higher optical absorption width and smaller band gap of ZnIn2S4 and CdIn2S4 have a crucial role for higher catalytic activity as shown in Figure 12b. Overall photocatalytic activity depends on the efficiency of photoabsorption, reaction efficiency of photogenerated electron−hole pairs, and their recombination rate.37 Probably the extra coordination of the metal ion into the binary chalcogenide introduces more defects. These improve the photocatalytic activities of ZnIn2S4 and CdIn2S4 by enabling efficient light harvesting and provide much more efficient transport channels for the electrons to be excited to the conduction band.38

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CONCLUSION In2S3 nanocrystals and 2D nanosheets were synthesized from the single-molecular precursor [In(bipy)(SC{O}Ph)3]. The presence of oleylamine has been found to be crucial for the formation of In2S3 nanocrystas. The reaction conditions were optimized by varying the temperature, precursor-to-surfactant molar ratio, choice of surfactants, and duration of reaction time. Incorporating the dual source precursor either [Zn(SC{O}Ph)2]·2H2O or [Cd(SC{O}Ph)2]·2H2O in the reaction mixture along with the [In(bipy)(SC{O}Ph)3] successfully formed the ternary metal sulfide as ZnIn2S4 and CdIn2S4 G

DOI: 10.1021/acs.cgd.6b00050 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

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respectively. As synthesized In2S3 nanostructures exhibit promising photocatalytic properties for degradation of methylene blue dye. After incorporation of Zn(II) and Cd(II), the photocatalytic properties of ZnIn2S4 and CdIn2S4 enhance in comparisons to In2S3 nanocrystals.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00050. Details of the synthesis and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*S.K.B.: Amrita Centre for Industrial Research and Innovation (ACIRI), Amrita University, Coimbatore, Tamil Nadu, India 641112. E-mail: [email protected]. *J.J.V.: Department of Chemistry, National University of Singapore, Singapore 117543. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Education, Singapore, for financial support through NUS Tier 1 FRC Grant No. R-143-000-604112.

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DOI: 10.1021/acs.cgd.6b00050 Cryst. Growth Des. XXXX, XXX, XXX−XXX