Template-Free Synthesis of Zn1–xCdxS Nanocrystals with Tunable

Apr 7, 2017 - Template-Free Synthesis of Zn1–xCdxS Nanocrystals with Tunable Band Structure for Efficient Water Splitting and Reduction of Nitroarom...
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Template-Free Synthesis of Zn1−xCdxS Nanocrystals with Tunable Band Structure for Efficient Water Splitting and Reduction of Nitroaromatics in Water Manjodh Kaur and C. M. Nagaraja* Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, 140001 Punjab, India S Supporting Information *

ABSTRACT: A facile, one-pot, solvothermal synthesis of nanocrystals (NCs) of Zn1−xCdxS (x = 0.1 (S1)−0.9 (S9)) solid solutions has been successfully carried out using 4,4′-dipyridyldisulfide (DPDS = (C5H4N)2S2)) as a new temperature-dependent in situ source of S2− ions. Powder XRD patterns of the samples revealed gradual phase transformation from cubic to hexagonal upon increasing the Cd content (x) in the solid solutions Zn1−xCdxS (0 ≤ x ≤ 1). FESEM analyses showed almost spherical morphology of the solid solutions, S2, S5, and S9. HR-TEM analyses of S3 and S9 unveiled the presence of small nanocrystals (NCs) of size 7 and 15 nm, respectively, and highlights the discontinuity in the pattern of lattice fringes. Optical measurements revealed that Zn1−xCdxS solid solutions exhibit precisely tunable band structure with varying the concentration of Cd content. Furthermore, visible-light-assisted photocatalytic investigation revealed very good activity of the Zn0.7Cd0.3S solid solution for water splitting with H2 generation rate of 750 μmol h−1 g−1. Interestingly, for the first time, the water splitting activity of the Zn1−xCdxS NCs has been applied for efficient reduction of nitroaromatic pollutants in water by utilizing water as a source of hydrogen. Remarkably, various substituted nitroaromatics containing both electron-donating and -withdrawing groups as well as dinitroaromatics can be efficiently reduced to their corresponding amines in high yield and selectivity. Also, the photocatalyst can be recycled and reused for several cycles without significant loss of the activity. The plausible mechanism for the reduction of nitroaromatics in water by Zn1−xCdxS solid solution has also been studied. Herein we demonstrate a unique approach wherein water acts as a source of reducing agent for the visible-light-assisted photocatalytic reduction of nitroaromatic pollutants in water by Zn1−xCdxS solid solutions. KEYWORDS: Zn1−xCdxS solid solutions, heterogeneous photocatalysis, visible-light-assisted water splitting, nitroaromatics, aminobenzenes

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and fine chemicals.29−31 The synthesis of aminobenzenes is usually carried out by the catalytic reduction of nitrobenzenes using various metal nanoparticle catalysts in the presence of reducing agents such as ammonium formate, sodium borohydride, hydrazine, and so on.32,33 However, these processes suffer from the limitations of generating large amounts of environmentally toxic waste and are not suitable for the large scale production. Hence, development of green catalytic routes for selective hydrogenation of nitroaromatics into their corresponding amines is highly desirable for industrial applications of these compounds. In this context, visible-lightdriven heterogeneous photocatalysts have drawn immense interest for the utilization of the abundant sunlight compared to those operate under UV light which constitutes only 4% of the solar energy reaching Earth.34,6,29

INTRODUCTION Visible-light-driven heterogeneous photocatalysis by semiconductor nanomaterials has attracted growing interest due to its promising potential for solar energy conversion.1−11 Literature study revealed that most of the photocatalytic studies using semiconductor nanomaterials are focused on the degradation of toxic organic pollutants and water splitting into H2 and O2,12−24 whereas the application of semiconductorbased photocatalysts in organic transformation is relatively uncommon.25−28 Hence, application of semiconducting nanomaterials for important industrial reactions has become extremely important to overcome the energy crisis and environmental challenges. In this regard, visible-light-assisted reduction of nitroaromatics into their corresponding amines has been considered as an important industrially relevant process from the point of environmental safety and national security. Nitroaromatics are common ingredients of explosive materials, while aminobenzenes are important intermediates in the synthesis of agrochemicals, pharmaceuticals, pigments, dyes, © 2017 American Chemical Society

Received: February 2, 2017 Revised: March 20, 2017 Published: April 7, 2017 4293

DOI: 10.1021/acssuschemeng.7b00325 ACS Sustainable Chem. Eng. 2017, 5, 4293−4303

Research Article

ACS Sustainable Chemistry & Engineering

Zn0.9Cd0.1S (S1) is given as follows. To 4 mL of an aqueous solution of Cd(NO3)2·4H2O (x = 0.1 mmol) and Zn(NO3)2·6H2O (1 − x = 0.9 mmol), ethanolic solution (4 mL) of DPDS (1 mmol) was added. To this solution, 175 μL (2.5 mmol) of MCE capping agent was added. The mixture with the solvent ratio of 1:1 was stirred for 30 min and then taken in a 23 mL PTFE-lined acid digestion bomb and heated at 200 °C for 12h. After being cooled to room temperature, product S1 was filtered and washed with methanol few times and dried under vacuum for 6 h. The syntheses of other solid solutions Zn1−xCdxS (x = 0.2 (S2) to x = 0.9 (S9)) were carried out following similar procedures with x varied from 0.2 to 0.9 mmol, respectively. Synthesis of ZnS and CdS was done by adding 4 mL (1 mmol) of an aqueous solution of respective metal salt into 4 mL of ethanolic solution of DPDS (1 mmol) along with 175 μL of MCE capping agent. The yield for all solid solutions S1−S9 was ∼75%, whereas for CdS and ZnS, it was found to be 80%. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a PANalytical’s X’PERT PRO diffractometer using Cu Kα radiation source (λ = 1.54056) with applied voltage and current of 45 kV and 40 mA respectively. The average crystallite size (d) was determined using Scherrer equation (d = 0.9λ/B cos θ, where B = fullwidth at half maxima (fwhm), λ is 0.154 nm, and θ is the Bragg’s diffraction angle. Field-emission scanning electron microscopy (FESEM) images were recorded on FE-SEM Supra 55 (Carl Zeiss, Germany with Energy Dispersive Spectroscopy (EDS) facility). Highresolution transmission electron microscopy (HR-TEM) and selectedarea electron diffraction (SAED) measurements were recorded on field-emission gun-transmission electron microscope (300 kV, ModelTecnai G2 F30). UV−vis spectra were recorded on Shimadzu UV2600 spectrophotometer using reflectance standard BaSO4. Fourier transform infrared spectroscopy was performed on IR-Affinity-1S Shimadzu S No-A22135300146. Microplasma-atomic emission spectroscopy (MP-AES) analysis was performed on Agilent technologies 4200 MP-AES. The amount of H2 produced in water splitting was quantified by gas chromatography (PerkinElmer, Clarus 580) equipped with TCD with nitrogen as the carrier gas and 5 Å molecular sieve column. Gas chromatography mass spectrometry (GCMS) analysis was performed on Shimadzu, GCMS-QP2010 Ultra Gas chromatograph mass spectrometer. Nitrogen adsorption studies were performed at 77 K using quantachrome Quadrasorb SI surface area analyzer with high-purity gases (99.995%). Photocatalytic Activity of Zn1−xCdxS for Water Splitting. The photocatalytic water splitting and hydrogen production activity of assynthesized solid solutions S1, S3, S5, and S9 along with CdS/ZnS were tested under inert conditions at ambient temperature and pressure in a 135 mL quartz cell. Typically, 10 mg of the photocatalyst was dispersed in 75 mL of water containing 0.25 M Na2S and 0.3 M Na2SO3 in a 135 mL quartz cell. Prior to irradiation, the dispersion was bubbled with N2 gas to remove any dissolved O2 and sonicated for 15 min. The reactions of photocatalytic water splitting were performed with a Xenon arc lamp (New Port, 6279NS ozone-free, working at 400 W) as the source of visible light which is equipped with 395 nm cut off filter to obtain light of wavelength λ > 395 nm. The suspensions were allowed to stir continuously for uniform dispersion. A 5 mL gastight syringe was used to abstract the gas from head space of the quartz flask. The amount of H2 was quantified by GC and apparent quantum yield (AQY) was calculated using the formula

It is well-known that CdS (cadmium sufide) with a band gap of 2.4 eV has been extensively studied as a visible light active photocatalyst. However, CdS suffers from the limitation of photocorrosion; generally, CdS is embedded in other materials to form hybrid photocatalysts which can remarkably overcome photocorrosion. It has been observed that the photocatalytic activity of CdS increases with doping of metal nanoparticles, such as Cu, Ni, Co, Rh, and others.35−38 In contrast, doping of ZnS to prepare Zn1−xCdxS (0 ≤ x ≤ 1) solid solutions has gained immense importance due to the easy formation of fine and uniform solid solutions and due to their potential applications.39−44 The high negative potential of ZnS has been found to be beneficial for enhancing the water splitting activity of Zn1−xCdxS solid solutions without the assistance of noble metal cocatalysts. More interestingly, the band gap width and band edge position of Zn1−xCdxS can be precisely tuned by varying the concentration of Cd2+ and Zn2+ ions. Hence, the Zn1−xCdxS ternary system possesses tunable optical and electrical properties and offers potential applications in photocatalysis, high density optical recording, and diodes.45−48 Keeping the above points in mind, we were interested to develop facile template-free route for the synthesis of Zn1−xCdxS solid solutions with tunable band structure by utilizing new organo sulfur compounds which can exhibit temperature-dependent release of S2− ions. Therefore, herein we report for the first time use of 4, 4′-dipyridyldisulfide (DPDS) as a new temperature-dependent in situ source of S2− ions for the template-free synthesis of Zn1−xCdxS (0 ≤ x ≤ 1) solid solutions. Furthermore, visible-light-assisted photocatalytic investigation of the solid solutions for water splitting in the presence of Na2S/Na2SO3 as sacrificial electron donors revealed very good photocatalytic activity of Zn0.7Cd0.3S (S3) with H2 generation of 750 μmol h−1 g−1. Interestingly, there are no reports in literature wherein water splitting activity of Zn1−xCdxS solid solution is utilized for organic transformations like reduction of nitroaromatics to aminoarenes. A literature survey reveals that the examples of visible-light-assisted photocatalytic reduction of nitroaromatics using water as a source of reducing agent are very rare. Therefore, we extended the photocatalytic activity of the Zn1−xCdxS solid solutions for reduction of nitro aromatics in water. Surprisingly, Zn0.1Cd0.9S (S9) solid solution showed higher catalytic activity over other solid solutions for reduction of nitro compounds to respective amines with high yield and selectivity. Furthermore, the photocatalyst can be recycled for three cycles without losing significant activity. The plausible mechanism of the photocatalytic reactions has also been discussed. Herein, we report the application of a new sulfur source, DPDS, for facile synthesis of Zn1−xCdxS solid solutions and their photocatalytic activity for nitroreduction in water for the first time.

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EXPERIMENTAL SECTION

(AQY)% = 2R /I × 100

Materials. All starting materials were commercially available and used as received without further purification. Cd(NO3)2·4H2O, Zn(NO3)2·6H2O, and 4,4′-dipyridyl disulfide (DPDS) were purchased from Sigma-Aldrich Chemical Co. Absolute ethanol was purchased from Merck Chemical Co. Millipore water was used for all the experiments. Mercaptoethanol (MCE) was purchased from Spectrochem and used as received. All the nitroaromatics were purchased from Sigma-Aldrich Chemical Co. Synthesis of CdS, ZnS, and Zn1−xCdxS Microspheres. Zn1−xCdxS (0 ≤ x ≤ 1) samples with different Cd/Zn molar ratios were synthesized by solvothermal method following the same conditions. As a representative example, the synthesis procedure of

where R is the evolution rate of H2 per second and I is the photon flux per second i.e. no. of photons per second per unit area (6.3 × 1017 photons s−1). Photocatalytic Reduction of Nitroaromatics by Zn1−xCdxS. The photocatalytic activity of Zn1−xCdxS solid solutions was investigated for the reduction of nitroaromatics, and the experiment was carried out in oxygen-free water at room temperature. Typically, 10 mL of aqueous solution containing the nitroaromatic substrate, e.g., nitrobenzene (0.2 mmol), the sacrificial reagents (Na2S, 0.25 M and Na2SO3, 0.3 M), and the photocatalyst (5 mg) in a 15 mL doublewalled water-circulated quartz jacket. The visible light source used was 4294

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ACS Sustainable Chemistry & Engineering two 15 W cool white LED light lamps (λ > 420 nm) under nitrogen atmosphere with constant stirring. Before visible light illumination, the above suspension was sonicated for 15 min, purged with N2 gas to remove any dissolved gases and left for 1 h stirring in dark to attain adsorption−desorption equilibrium. In the case of other nitroaromatics, benzotrifluoride (BTF) has been used for higher solubility of the nitroderivatives due to their low solubility in water. The aliquots were taken at different time intervals from the reaction mixture, extracted with ethyl acetate, and analyzed by GC-MS. The catalyst was recovered by centrifugation, washed with water and methanol, and dried at 100 °C for 4 h, then reused for two more runs.

The gradual phase transition from cubic to hexagonal with increase in Cd concentration suggest the formation of homogeneous solid solution obtained by varying the Cd/Zn ratio which is also found to be in agreement with the ratios calculated from Vegard’s law (Table 1).56,57 The value of x was determined using Vegard’s law, Cx = CCdS + (CZnS − CCdS)x where, Cx, CCdS, and CZnS are the c-axis lattice constants of Zn1−xCdxS, CdS, and ZnS, respectively, and the results are in line with the experimental values.58−61 Furthermore, careful examination of the PXRD pattern (Figure. 1) revealed shifting

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RESULTS AND DISCUSSIONS A series of Zn1−xCdxS (0 ≤ x ≤ 1) solid solutions, S1−S9, were synthesized by using DPDS ((C5H4N)2S2) as a new temperature-dependent in situ source of S2− ions in the presence of MCE capping agent by a solvothermal route. The formation of Zn1−xCdxS solid solutions has been explained in Scheme 1. It Scheme 1. Proposed Mechanism for the Formation of Zn1−xCdxS Microspheres

Figure 1. Powder XRD patterns for as-synthesized Zn1−xCdxS (S1− S9) with varying ratios of Cd (x = 0−1), ZnS, and CdS.

involves in situ generation of S2− ions at high temperature from DPDS followed by reaction with Cd/Zn ions.49−51 Powder Xray diffraction patterns of the Zn1−xCdxS solid solutions indicate the gradual phase change from cubic Zn1−xCdxS (x = 0) (indexed to planes (111), (220), and (311)) to hexagonal phase Zn1−xCdxS (x = 1) with planes indexed to (100), (002), (101), (102), (110), (103), and (112) with an increase in the Cd content. This phenomenon signifies the incorporation of Cd atoms into the lattice of ZnS influencing the positions of Zn atoms and subsequently changes the lattice structure of ZnS in the process of formation of Zn1−xCdxS solid solutions under high-temperature conditions. This observation is supported by the fact that in the case of ZnS zinc blende (cubic) phase is more stable than wurtzite (hexagonal) phase whereas in the case of CdS wurtzite (hexagonal) phase is more stable.52−55

of peaks to lower 2θ with increase in the Cd content. This is also accompanied by the gradual decrease in the lattice spacing (d-spacing) as given in the Table 1. The observed shift in the peak position of ZnS to lower 2θ is attributed due to the incorporation of larger Cd2+ ions (ionic radius = 0.97 Å) with the positions of Zn2+ ions (ionic radius = 0.74 Å) in the lattice of ZnS. The crystallite size of the Zn1−xCdxS solid solutions was determined by Scherrer equation using the peak of highest intensity from the diffraction pattern. As shown in Table 1, with increase in the Cd content there is a decrease in the crystallite size from 19.00 nm (ZnS) to 6.54 nm (S4), and further increase in the Cd content results in an increase in the crystallite size from 6.54 nm (S4) to 17.78 nm (CdS). This phenomenon indicates that the Zn2+ and Cd2+ ions restrain the growth of grain size of pure ZnS and CdS crystals, respectively;

Table 1. Experimental Conditions, Elemental Composition, and Properties of Zn1−xCdxS (0 ≤ x ≤ 1) Samples % wt calculated from molar ratio

s. no.

sample name

Cd/Zn/S molar ratio as used in the reactants

1 2 3 4 5 6 7 8 9 10 11

ZnS S1 S2 S3 S4 S5 S6 S7 S8 S9 CdS

Cd0.10Zn0.90S Cd0.20Zn0.80S Cd0.30Zn0.70S Cd0.40Zn0.60S Cd0.50Zn0.50S Cd0.60Zn0.40S Cd0.70Zn0.30S Cd0.80Zn0.20S Cd0.90Zn0.10S

% wt from EDS

Cd/Zn/S molar ratio as calculated from Vegard’s law

Cd/Zn/S molar ratio as calculated from MP-AES results

Zn

Cd

Zn

Cd

Cd0 02Zn0 98S Cd0.l4Zn0.86 s Cd0.22Zn0.78S Cd0.31Zn0.69S Cd0.42Zn0.58S Cd0.64Zn0.36S Cd0.68Zn0 32S Cd0.79Zn0.21S Cd0.82Zn0.18S

Cd0.13Zn0.86S Cd0.27Zn0.72S Cd0 37Zn0.63S Cd0.46Zn0.53S Cd0.55Zn0.44S Cd0.67Zn0.32S Cd0 66Zn0.33S Cd0.84Zn0.15S Cd0.91Zn0.09S

57.41 48.97 41.02 33.73 27.02 20.80 15.04 9.67 4.67

10.96 21.05 30.23 38.67 46.46 53.66 60.35 66.57 72.37

52.40 43.56 40.08 33.28 23.90 19.02 15.17 10.57 7.89

15.00 16.36 31.63 37.78 49.58 55.47 58.92 64.47 70.92

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band gap (eV)

crystallite size (nm)

d-spacing of highest intensity peak (Å)

3.52 3.23 2.88 2.65 2.60 2.55 2.48 2.37 2.39 2.33 2.26

20.00 13.35 11.24 7.57 6.54 8.09 9.00 12.38 13.17 14.84 17.78

3.11 3.13 3.16 3.18 3.20 3.25 3.28 3.29 3.32 3.33 3.35

DOI: 10.1021/acssuschemeng.7b00325 ACS Sustainable Chem. Eng. 2017, 5, 4293−4303

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ACS Sustainable Chemistry & Engineering hence, the crystallinity of the Zn1−xCdxS (0 ≤ x ≤ 1) solid solutions is poorer than those of ZnS (x = 0) and CdS (x = 1). Energy dispersive spectroscopy (EDS) analyses of the solid solutions, S1−S9, and ZnS/CdS confirms the presence of Zn, Cd, and S (Figure S1) and the percent compositions of Zn and Cd are given in Table 1. Furthermore, elemental mapping of the samples revealed the homogeneous distribution of Cd, Zn, and S elements in Zn1−xCdxS solid solutions (Figure S2). Furthermore, FT-IR spectra of S1, S3, S5, and S9 show stretching bands at 665/735, 1048, 1055, 2555, 2930, and 3370 cm−1 corresponding to C−S, C−C, C−O, S−H, C−H, and O− H stretching, respectively, highlighting the presence of MCE capping agent (Figure S3). FE-SEM analysis of S2, S5, and S9 samples show the presence of uniform solid spheres with diameters in the range of 0.5−2.5 μm as shown in Figure 2. Furthermore, the

Figure 3. HR-TEM images of S3 showing (a) the presence of nanocrystals and (b) lattice fringe of S3.

break in the lattice pattern in the same nanocrystal indicating polycrystalline nature of the material. The metal compositions of the as-synthesized samples was determined by MP-AES analysis and are given in the Table 1. The estimated values of Cd and Zn compositions are in good agreement with the values calculated based on the composition used for synthesis. The similarity in the values of wt % obtained from starting materials and the products rules out the possibility of formation of any side product. Furthermore, it highlights the efficiency of the synthesis which is difficult to achieve by other methods.62,63 To investigate the surface area of the as-prepared samples, N2 adsorption/desorption measurements were performed for the samples, ZnS, S1, S3, S5−S9, and CdS. Figure S4 shows the N2 adsorption/desorption isotherms and the corresponding surface area. According to the Brunauer−Deming−Deming− Teller (BDDT) classification of isotherms, all the samples show type-IV isotherms suggesting the presence of mesopores (2−50 nm).64,65 The estimated value of BET surface area of the samples ZnS, S1, S3, S5−S9, and CdS are, respectively, 15.69, 20.22, 17.80, 18.14, 41.67, 27.12, 39.38, 27.11, and 31.06 m2 g−1. The pore size distribution of the samples was found to be in the range of 4−15 nm suggesting the presence of meso pores (Figure S4).66,67 Optical Properties. The room-temperature solid state UV−vis absorption spectra of Zn1−xCdxS (S1−S9) solid solutions along with ZnS and CdS are shown in Figure 5. The spectra show a red-shift in the absorption edge from 410 to 550 nm with increase in the Cd content in the Zn1−xCdxS solid solution with absorption edges of ZnS and CdS at 350 and 560 nm, respectively. Thus, Zn1−xCdxS solid solutions show sharp band edge absorptions in the visible region. This is also accompanied by the color change from gray to dark orange for S1−S9, respectively, as shown in the inset of Figure 5. This implies that the samples S1−S9 are not mere mixtures of CdS and ZnS but are solid solutions of Zn1−xCdxS with varying compositions (0 ≤ x ≤ 1). The observed optical properties clearly suggest that the band gaps of the Zn1−xCdxS solid solutions can be precisely controlled with the variation in Zn/ Cd content. The value of direct band gap (Eg) for S1−S9 was calculated from the plot of (αhν)2 versus photon energy (hν) using the relationship: αhν = A(hν − Eg)n where hν = photon energy, A = constant, α = absorption coefficient, α = 4Πk/λ (wherein k is the absorption index and λ is the wavelength), and n = 1/2 for the allowed direct band gap. The values of the direct band gap calculated by extrapolating the absorption edge by a linear fitting method are listed in Table 1. The calculated band gap values of Zn1−xCdxS solid solutions varies from the

Figure 2. FE-SEM images showing the formation of microspheres composed of nanocrystals for S2 (a, b), S5 (c, d), and S9 (e, f).

magnified images show the presence of nanocrystals on the surface of the microspheres (Figure 2b,d,f). Thus, it is clear from the FE-SEM images that there is agglomeration of particles, so the actual size of the crystallites is not clear. Hence, HR-TEM analyses were carried out for precise determination of the crystallite’s size. Figure. 3 shows the HR-TEM images of Zn0.7Cd0.3S (S3) showing the presence of nanocrystals of size 10−15 nm. The lattice fringe shown in Figure 3b reveals the interplanar spacing of 0.318 nm which is in agreement with the d-spacing obtained from PXRD pattern (mentioned in Table 1). HR-TEM of S9 is shown in Figure. 4 showing the presence of nanocrystals in the size range of 15−20 nm. Lattice fringe are shown in Figure. 4b, and the interplanar spacing of 0.333 nm is in agreement with the d-spacing data obtained from pxrd pattern. The regions demarcated in Figure 4c highlight the 4296

DOI: 10.1021/acssuschemeng.7b00325 ACS Sustainable Chem. Eng. 2017, 5, 4293−4303

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Figure 4. HR-TEM images of S9 (a) showing the presence of nanocrystals and (b) lattice fringe of Zn1−xCdxS; (c) the circled regions show discontinuity in the lattice fringes.

negligible activity as compared to the solid solutions, Zn1−xCdxS.70 Interestingly, increase in the Cd content up to 0.5 wt % (S3) in the solid solution resulted in an increase in the amount of H2 evolution. The rate of H2 evolution is 378, 750, 60, 105, and 20 μmol h−1 g−1 for S1, S3, S5, S9, and CdS, respectively. Therefore, it is clear that the highest H 2 production rate is observed for Zn0.7Cd0.3S (S3) (750 μmol h−1 g−1) which is almost 40 times higher than that of CdS, and pure ZnS did not show hydrogen evolution. A literature survey revealed that the H2 generation activity of S3 is comparable or higher than the activity reported for ZnxCd1−xS solid solutions. For example, Guo et al. has reported photocatalytic activity of Zn0.16Cd0.62S with H2 production rate of 670 μmol h−1 g−1.71 Furthermore, the activity of S3 is found to be higher than that of CdS doped with noble metal cocatalyst (Pt/CdS) hybrid catalyst with a reported H2 generation activity of about 540 μmol h−1 g−1.72,73 The photocatalytic activity of Zn1−xCdxS solid solutions depends on the position of CB (conduction band edge) and the visible-light absorption capability. A semiconducting material with a more negative CB edge level will have higher reduction power for the H2 production by water splitting. Thus, ZnS although is capable of reducing the protons into H2 generation, it inefficiently absorbs visible light due to its large band gap of 3.52 eV, resulting in lower photocatalytic H2 evolution. However, CdS has an optimum band gap (2.26 eV) to absorb visible light, but its reduction potential is lower due to low negative CB edge potential. The solid solutions possess an intermediate band structure as pictorially shown in Scheme 2. The water splitting experiments of solid solutions revealed that an increase in the concentration of Cd up to the mole ratio of 0.37 results in an increase in the rate of H2 production, and

Figure 5. Solid state UV−vis absorption spectra of Zn1−xCdxS (S1− S9) compared to those of ZnS and CdS. Inset image showing the change in color from ZnS (gray) to CdS (orange) through intermediate colors for Zn1−xCdxS solid solutions (S1−S9).

value of 3.52 (ZnS) to 2.26 eV (CdS) with increase in the Cd content (see Figure S5 and Table 1). Photocatalytic Hydrogen Production. Chalcogenide nanomaterials have been extensively investigated for H 2 evolution by water splitting under visible light irradiation. In this direction, Zn1−xCdxS solid solutions have attracted considerable attention as visible-light-assisted photocatalysts for water splitting and H2 production.68,69 Therefore, we investigated the photocatalytic activity of Zn1−xCdxS solid solutions (S1, S3, S5, and S9) for water splitting and H2 production using Na2S and Na2SO3 as the sacrificial electron donors in the absence of noble metal cocatalysts. As shown in Figure. 6, the parent compounds, CdS and ZnS showed

Figure 6. (a) Water splitting and H2 evolution activity of solid solutions with time; (b) comparison of H2 production activity of Zn1−xCdxS (ZnS, x = 0.1 (S1), 0.3 (S3), 0.5 (S5), 0.9 (S9), and CdS) samples for the photocatalytic H2 production from 0.25 M Na2S and 0.3 M Na2SO3 mixed aqueous solution under visible light (>395 nm) irradiation. 4297

DOI: 10.1021/acssuschemeng.7b00325 ACS Sustainable Chem. Eng. 2017, 5, 4293−4303

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Scheme 2. Schematic Diagram Showing the Band Positions of CdS, Zn1−xCdxS and ZnS and the Formation of Intermediate Band Position for Zn1−xCdxS Solid Solutions

out in the absence of water using an aprotic solvent (thoroughly dried acetonitrile) as the reaction medium (Scheme 3). GC-MS analysis of the aliquots taken at regular time intervals revealed no conversion of nitrobenzene into aniline (Supporting Information). This observation unambiguously confirms that water serves as a source of protons required for the reduction of nitroaromatics catalyzed by ZnxCd1−xS solid solutions in the presence of sacrificial electron donors as depicted in Scheme 4. This observation clearly supports that the water acts as a source of reducing agent for the reduction of nitroaromatics. To confirm the two competing reactions of water splitting and nitroreduction, the water splitting experiment was repeated in the presence of added nitrobenzene. To our surprise, the GC-MS analysis shows no evolution of H2 from the reaction (Figure S6), suggesting that the protons generated during the photocatalytic water splitting process were utilized for the reduction of nitrobenzene. From Table 2 it is clear that the visible-light-assisted nitroreduction activity of S9 is higher than those of S1, S3, and S5 for the hydrogenation of nitrobenzene. In the case of S9, the complete conversion of nitrobenzene to aniline occurs within the time span of 4 h, whereas in the case of solid solutions S1, S3, and S5, the complete conversion of nitrobenzene to aniline occurs in the time span of 22, 13, and 6 h, respectively (Table 2). The activity of S9 was found to be much higher than those of pure ZnS and CdS which is in accordance with the higher water splitting activity of the solid solution over individual metal sulfides as explained before.78−80 Furthermore, the heterogeneity of photocatalyst S9 was examined to understand whether the reaction occurred on the photocatalyst or was catalyzed by the species leached from the catalyst into solution. To study this, a separate reaction was carried out in which the photocatalyst S9 was removed after 2 h of irradiation, and the conversion of nitrobenzene was found to be 58%. At this stage the photocatalyst was removed by centrifugation, and the reaction was continued with the irradiation for additional 4 h. GC-MS analysis of the aliquots taken at regular intervals of time did not show appreciable change in the conversion of nitrobenzene, suggesting heterogeneity of the photocatalyst S9 (Figure 7a). The time course of photocatalytic reduction of nitrobenzene has been shown in Figure 7b. As the reaction proceeds, the concentration of nitrobenzene decreases, and that of aniline increases. In addition, the formation of azobenzene and azoxybenzene intermediates en route to the formation of aniline were observed which suggests that the reaction proceeds through condensation pathway (Scheme S1).81−83

further increase in the Cd content led to fast deterioration in the rate of H2 production (Figure 6b). Therefore, solid solution Zn0.7Cd0.3S (S3) possesses a suitable reduction potential and visible-light absorption capacity which probably results in higher efficiency of photocatalytic H2 production (750 μmol h−1 g−1) over that of other solid solutions. Similar observations on the photocatalytic activity of ZnxCd1−xS solid solutions for H2 production has been reported before.74−77 Furthermore, the apparent quantum yield for S3 was estimated to be 0.40% as compared to the value of 0.01% for CdS. Photocatalytic Activity of Solid Solutions for Reduction of Nitroaromatics. For the first time, the photocatalytic water splitting activity of the Zn1−xCdxS solid solutions has been extended for the reduction of nitroaromatic compounds in water by utilizing hydrogen generated by water splitting as a reducing agent. Therefore, the visible-light-assisted photocatalytic nitroreduction activity of Zn1−xCdxS (ZnS, x = 0.1 (S1), 0.3 (S3), 0.5 (S5), 0.9 (S9), and CdS) samples were tested for the reduction of nitrobenzene in the presence of sacrificial electron donors (Na2S and Na2SO3) (Table 2). Table 2. Photocatalytic Reduction of Nitrobenzenea s. no.

conditions

time (h)

% conversionb

% selectivityb

1 2 3 4 5 6

Cd0.10Zn0.90S (S1) Cd0.30Zn0.70S (S3) Cd0.50Zn0.50S (S5) Cd090Zn0.10S (S9) pure CdS pure ZnS

4 4 4 4 4 4

NR 20 85 >99 50 NR

NR 100 100 100 100 NR

a

Reaction conditions: 0.2 mmol of nitrobenzene, 5 mg of photocatalyst, 10 mL of Millipore water, room temperature, visible light source (two 15 W cool white light LEDs). bDetermined by GC-MS.

Control experiments carried out to understand the role of the photocatalyst, sacrificial electron donors, visible light, and source of reducing agent (water) revealed that all the four components were vital for the reduction of the nitroaromatics and the absence of any one of the four components did not result in the formation of the product as shown in Scheme 3. However, use of two 15 W cool white LED lamps as visible light source, sacrificial electron donors (Na2S and Na2SO3), and S9 as photocatalyst in water led to complete reduction of nitrobenzene into aniline within the time span of 4 h (Scheme 3 and Table 2). To realize the role of water as a source of reducing agent for the nitroreduction reaction, a control experiment was carried 4298

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ACS Sustainable Chemistry & Engineering Scheme 3. Various Conditions Screened for the Reduction of Nitrobenzene

substituent groups. Nitroaromatics with both electron-donating (−CH3) and electron-accepting (−Cl and −Br) groups as well as dinitroarenes were converted to their corresponding amines in high yield and selectivity (Table 3). Furthermore, the photostability and recyclability of the catalyst was tested for three successive runs; remarkably, there was no significant change in the activity as well as structural rigidity of the catalyst (Figure 8). As it can be seen from Figure 8b, PXRD plot of the recycled sample clearly shows that the crystallinity of the catalyst is retained even after three cycles of photocatalysis. The plausible mechanism of photocatalytic water splitting and reduction of nitroaromatics is shown in Scheme 4, it involves photo excitation of electrons from the valence band (VB) of the photocatalyst (Zn1−xCdxS) into the conduction band (CB) leaving a hole in the VB, hence forming an electron−hole pair. In the case of water splitting (absence of nitroaromatics), the photoexcited electron reacts with water molecules giving H2 molecule and OH− ions (path (i), Scheme 4), whereas in the presence of nitrobenzene, the protons generated by water splitting are being utilized for the reduction process following path (ii). Furthermore, the mixture of Na2S/ Na2SO3 is used as sacrificial electron donors to react with the holes formed in the VB to form oxidized species as shown in path (iii). S2− are converted into S22− in the presence of holes,

Scheme 4. Schematic Representation of Visible-LightAssisted Photocatalytic Water Splitting and Nitroreduction by Zn1−xCdxS Photocatalyst

Having confirmed the photocatalytic nitroreduction activity of S9, the scope of the reaction was extended to the reduction of more challenging nitroaromatic compounds with diverse

Figure 7. (a) Leaching test for photocatalyst S9. (b) Time course of photocatalytic reduction of nitrobenzene with S9, indicating the formation of azobenzene and azoxybenzene intermediates. 4299

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ACS Sustainable Chemistry & Engineering Table 3. Photocatalytic Reduction of Nitroaromatics in Water with Photocatalyst S9a

a Reaction conditions: 0.2 mmol of nitroarene, 18 mol % S9, 10 mL of Millipore water, room temperature; >90% conversion in all the cases. Visible light source was two 15 W cool white light LEDs. bDetermined by GC-MS. cDue to the formation of azo and azoxy intermediates for pbromonitrobenzene. dDue to the formation of azo and azoxy intermediates for p-chloronitrobenzene.

Figure 8. (a) Bar diagram showing recyclability of photocatalyst S9 (b) Powder XRD pattern of photocatalyst S9 after each catalytic cycle.

which further react with SO32− to regenerate the S2− as shown in path (iv). The difference in the reactivity pattern of the solid solutions between the water splitting and nitroreduction could be arising due to the difference in the light sources used. Water splitting experiments were carried out using 400 W xenon lamp, whereas

two 15 W cool white LED lamps were used for the photocatalytic reduction of nitroaromatics. Furthermore, the nitroreduction reaction is kinetically a slow process and requires six protons and six electrons for complete reduction of a nitro group to the amino derivative (case ii, Scheme 4). Therefore, if the rate of hydrogen generation is faster, then the 4300

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ACS Sustainable Chemistry & Engineering ORCID

reduction step would be slower and vice versa. Since rate of H2 evolution is faster and highest with S3 as shown in Figure 7, there is faster conversion of generated protons into H2, and they are less likely to be available for nitroreduction. However, in the case of S9, the rate of H2 evolution is slower; hence, the protons are relatively available for reduction of nitrobenzene. In addition, the efficiency of nitroreduction step will depend on the adsorption of nitro aromatics on the surface of the Zn1−xCdxS photocatalyst. In this regard, the hexagonal phase of Zn0.1Cd0.9S (S9) possesses a relatively higher surface area (27.12 m2 g−1) than that of the cubic phase Zn0.7Cd0.3S (S3) (17.80 m2 g−1). Therefore, a greater number of nitroaromatic molecules will be able to interact with the surface of solid solution S9 in comparison to that with S3, supporting the higher nitroreduction activity of the former over the later catalyst. Furthermore, though in the case of pure CdS the surface area is relatively higher (31.06 m2 g−1), the water splitting activity (proton generation) is lower due to which the rate of nitrobenzene reduction is slower as compared to that of S9. Importantly, the nitroreduction experiments clearly highlight the role of water as a source of reducing agent which is otherwise added externally in most of the nitro reduction reactions.84,85

C. M. Nagaraja: 0000-0002-4271-6424 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Sandeep Singh Dhankhar for the adsorption measurements and SAIF, IIT Bombay for the TEM measuremnts. We are grateful to Prof. S. K. Das, Director, IIT Ropar, for the encouragement and support under the interdisciplinary project on H2 storage and generation.

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CONCLUSIONS A facile, one-pot, solvothermal synthesis of Zn1−xCdxS (0 ≤ x ≤ 1) solid solutions from a new sulfur source, DPDS, which causes temperature-dependent in situ release of S2− ions has been achieved for the first time. FE-SEM and HR-TEM analyses confirmed the presence of smaller nanocrystals of the solid solutions. The optical measurements revealed that Zn1−xCdxS solid solutions exhibit precisely tunable band structure with varying concentrations of Cd content. Visiblelight-assisted water splitting study unveiled the higher activity of Zn0.7Cd0.3S with H2 generation of 750 μmol h−1 g−1 in the absence of any noble metal cocatalyst. Interestingly, for the first time, the water splitting activity of the Zn1−xCdxS NCs has been applied for efficient reduction of nitroaromatics in water by utilizing water as a source of reducing agent. Remarkably, various substituted nitroaromatics containing both electrondonating and -withdrawing groups as well as dinitroaromatic compounds can be efficiently reduced to their corresponding amines in high yield and selectivity. Furthermore, the photocatalyst can be recycled and reused for several cycles without significant loss of the activity. Herein, we demonstrate a unique synthesis and an unprecedented application of Zn1−xCdxS solid solutions for visible-light-assisted photocatalytic reduction of nitroaromatics by utilizing water as a source of reducing agent.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00325. EDAX, Elemental mapping, FT-IR, N2 adsorption isotherms of solid solutions and GC-MS plots (PDF)

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*Tel.:+91-1881242229. E-mail: [email protected]. 4301

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