Shape-Dependent Photocatalytic Activity of Hydrothermally

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Shape-dependent Photocatalytic Activity of Hydrothermally Synthesized Cadmium Sulfide Nanostructures Joyjit Kundu, Santimoy Khilari, and Debabrata Pradhan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16456 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017

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ACS Applied Materials & Interfaces

Shape-dependent Photocatalytic Activity of Hydrothermally Synthesized Cadmium Sulfide Nanostructures Joyjit Kundu, Santimoy Khilari, and Debabrata Pradhan* Materials Science Centre, Indian Institute of Technology, Kharagpur, W. B. 721 302, India

Abstract The effective surface area of the nanostructured materials is known to play a prime role in catalysis. Here we demonstrate that the shape of the nanostructured materials plays equally important role in their catalytic activity. Hierarchical CdS microstructures with different morphologies such as microspheres assembled of nanoplates, nanorods, nanoparticles, and nanobelts are synthesized using a simple hydrothermal method by tuning the volume ratio of solvents i.e. water or ethylenediamine (en). With an optimum solvent ratio of 3:1 water:en, the role of other synthesis parameters such as precursor’s ratio, temperature, and precursor combinations are also explored and reported here. Four selected CdS microstructures are used as photocatalysts for the degradation of methylene blue and photoelectrochemical water splitting for hydrogen generation. In spite of smaller effective surface area of CdS nanoneedles/nanorods than that of CdS nanowires network, the former exhibits higher catalytic activity under visible light irradiation which is ascribed to the reduced charge recombination as confirmed from the photoluminescence study.

Keywords: CdS, nanoplates, nanobelts, nanorods, nanowires, photocatalyst, photoanode.

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1. INTRODUCTION The success of photocatalytic processes such as photoelectrochemical water splitting and photocatalytic degradation of organic contaminants primarily depends on the light absorption as well as low internal recombination of charge carriers in the semiconductor catalyst.1 ZnO and TiO2 have been widely used as semiconductor photocatalysts though their band gaps in the ultraviolet (UV) region make them less desirable.2 In recent years, chalcogenide materials are gaining attention as photocatalysts due to their lower band gaps than most of the oxides as well as improved performance.3,4,5 Among the chalcogenides, cadmium chalcogenides (CdS, CdSe, and CdTe) are widely studied because of their low cost and easy synthesis process.6,7 CdS is one of the important II-VI semiconducting materials with ~2.42 eV direct band gap that falls in the visible region of the electromagnetic spectrum.8 It is widely used in photovoltaics mostly as window material in the heterojunction solar cell.9,10 Apart from photovoltaics, CdS is extensively used as photocatalyst,11,12 photoconductor,13 metal-semiconductor field-effect transistor,14 and in DNA sequencing.15 Crystal structure of a photocatalyst plays a key role on its photochemical activity. Generally CdS exists in two different crystal structures: hexagonal (wurtzite, referred as α-CdS) and cubic (zinc blend, referred as β-CdS). Wurtzite is thermodynamically more stable form of CdS. However, the energy difference of these two phases of CdS is very small.16 It has been reported that low temperature solution-based synthesis produces cubic CdS phase whereas high temperature synthesis (>300ºC) produces hexagonal phase.17,18 Moreover, suitable solvent such as NaCl has been used to produce wurtzite CdS even at lower temperature.18 Instead of using NaCl, here we demonstrate the formation of wurtzite CdS at 150ºC using cadmium chloride as precursor. Synthesis of nanostructured CdS is desirable because of the improved properties than its bulk counterpart. In addition, properties of nanostructured catalysts vary with shape and 2


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size bringing new avenue of information. Several physical and chemical techniques such as laser ablation,19 microwave radiation,20 chemical vapor deposition (CVD)-template,21 CVD,22 colloidal,23 sonochemical,24 micro-emulsion25 and hydrothermal/solvothermal26,27 methods have been used to synthesize micro/nano structures of CdS in the shape of nanoparticles,28 nanorods,13 nanowires,22 nanotubes,21 flower-like,11 and dendrites.26 Among these shapes, microstructure self-assembled of nanostructure becomes a promising choice for photocatalyst design due to its ability to provide high active surface area as well as improved charge separation.11,29,30 Recently we reported that water or ethylene glycol or their mixtures as solvent can control the morphology of CuS.31 On the other hand, Gorai et al. reported that the ratio of ethylenediamine (en) and water has strong impact on the morphology of Cu2S.32 This suggests that the solvent composition plays an important role to control the morphology of chalcogenides in the solvothermal method. In the present work, by varying the solvent ratio, i.e., only water or en or their mixtures (3:1, 1:1 and 1:3), spherical CdS morphologies with different building blocks such as nanoplates, nanorods, nanoparticles, and nanobelts are synthesized. Additionally, variation of precursors is studied for the first time and spherical CdS microspheres composed of nanoneedles and nanowire networks are obtained by changing the sulfur source. The advantages of present work involve synthesis at a lower temperature (150ºC) and in a shorter duration (8 h) than those reported earlier.33,34 In particular, Xiong et al. and Phuruangrat et al. hydrothermally synthesized spherical CdS nanostructures at ≥180 °C for 24 h using water/ethylenediamine mixed solvent.33,34 A detail study on the role of different synthesis parameters for CdS microstructures is presented here. The as-prepared CdS microstructures are used as photocatalyst for the degradation of methylene blue (MB) dye under visible light. Furthermore, the as-prepared CdS microstructures are employed as photoanode for the photoelectrochemical water splitting under visible light irradiation for hydrogen generation. It 3



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is found that the photocatalytic properties of CdS microstructures depend majorly on their shapes than the effective surface area. The CdS microspheres composed of straight onedimensional (1D) structures (nanorods and nanoneedles) with smaller surface area (7.5 and 19.0 m2/g) showed superior photocatalytic activity than that of nanobelts and nanowires morphologies with larger surface area (32.8 and 83.9 m2/g). This is attributed to the reduced charge recombination and superior charge transport properties in the straight morphologies (nanorods and nanoneedles). 2. EXPERIMENTAL DETAILS 2.1. Chemicals. Cadmium chloride hydrate (CdCl2·H2O), cadmium nitrate hydrate (Cd(NO3)2·4H2O) and ethylenediamine (en) were purchased from Merck, India. Thiourea (CH4N2S) and polyvinylpyrrolidone (PVP) were purchased from Loba Chemie, India. Thioacetamide (C2H5NS) was purchased from Spectrochem, India. Terephthalic acid (C8H6O4), L-Cystein (C3H7NO2S) and methylene blue (MB) were purchased from SRL Chem., India. Polytetrafluoroethylene (PTFE) solution was procured from Sigma Aldrich, USA. All the reagents were analytical grade and used without further purification. 2.2. Synthesis. Typically, 40 mL of the solvent (only water or only en or their mixture in the V/V ratio of 3:1, 1:1 or 1:3) was taken in a 100 mL of beaker to which 1g of PVP was added and stirred for 10 min. 402 mg of CdCl2 (2 mmol) and 152 mg (2 mmol) of thiourea was added to the above solution and stirred for 20 min. The resulting solution was transferred to a 50 mL Teflon-lined autoclave, heated at 150ºC for 8 h, and cooled to room temperature. The precipitated product was filtered and washed with ethanol and water several times and dried at 60°C for 4 h prior to characterization. Taking solvent composition 3:1 water:en fixed, reactions were performed using precursors molar ratios i.e. 2:1, 1:1, and 1:2 of CdCl2:thourea at 150ºC for 8 h. Reactions were also performed at different temperature (100ºC, 150ºC, and 200ºC) for 8 h with 3:1 water:en and 1:1 CdCl2:thiourea. Effect of precursor’s combination 4


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was studied by performing four different experiments using (1) CdCl2 and thiourea, (2) Cd(NO3)2 and thiourea, (3) CdCl2 and thioacetamide, and (4) CdCl2 and L-Cystein as precursors at 1:1 molar ratio in 3:1 water:en solvent with other reaction parameters fixed (150ºC, 8 h). 2.3. Characterization. X-ray diffraction (XRD) measurement was carried out using a PANalytical X’pert Pro (PW 3040/60) diffractometer in the two-theta range of 20° to 70°. The surface morphology and elemental composition of the as-prepared CdS samples were examined with a Carl-Zeiss SUPRA 40 field emission scanning electron microscope (FESEM) and a FEI NOVA NANOSEM 450 FESEM coupled with BRUKER energy dispersive X-ray (EDX) detector, respectively. Transmission electron microscopy (TEM) study was performed with a FEI-TECNAI G2 instrument at an operating voltage of 200 kV. UV-vis absorption measurement was carried out using a PerkinElmer Lambda 750 and SHIMADZU UV-2600 UV-Vis spectrophotometer. Fourier-Transform Infra Red (FT-IR) measurement was performed with a BRUKER (Germany) TENSOR 27 FT-IR spectrophotometer. The surface composition of the samples was studied with a Thermo-VG Scientiﬁc ESCA Lab 250 Microprobe X-ray photoelectron spectroscopy (XPS) with a monochromatic Al Kα source (1486.6 eV). Raman measurement was carried out using Horiba Jobin Yvon T64000 Raman spectrometer using argon-krypton mixed ion gas laser as excitation source (Spectra Physics, USA). The photoluminescence (PL) measurements were performed with a Jobin Yvon-Spex Fluorolog-3 spectrofluorimeter with an excitation wavelength of 510 nm. The effective Brunauer–Emmett–Teller (BET) surface area of the assynthesized samples was measured with an Autosorb iQ2 BET analyzer (Quantachrome, USA). 2.4. Photocatalysis study. 10 mg of the as-prepared CdS was added to 40 mL of 10−5 (M) MB solution in a 50 mL beaker and stirred for 30 min at 600 rpm under dark to achieve 5



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adsorption-desorption equilibrium between CdS and MB solution. The resulting solution was irradiated for different durations with a 500 W Halogen lamp providing primarily visible light source with an intensity of 45 mW/cm2 at the surface of reaction vessel. 1 mL of solution was removed at different time intervals and solution was centrifuged to separate the CdS powder and the MB concentration was measured using UV-vis absorption spectroscopy. Distilled water was used as reference solution for UV-vis absorption study. 2.5. Photoelectrochemical Study. 5 mg of the CdS powder was added to 1 mL of ethanol and sonicated for 15 min. Then 20 µL of 10% (w/v) solution of PTFE was added and sonicated for 15 min. The resulting solution was then drop-casted on a pre-cleaned indiumtin-oxide (ITO)-coated glass (1 cm2) and dried at 100ºC for overnight under vacuum. Electrochemical measurements were carried out using a CHI 760D (CH Instruments, Inc., USA) electrochemical analyzer with a conventional three electrode system. CdS coated ITOglass, Pt wire, and saturated calomel electrode (SCE) were served as the working, counter, and reference electrode, respectively. A mixture of 1 M Na2SO3 and 0.1 M Na2S solution was used as electrolyte and a regular 300W Xenon lamp with a cut-off filter (λ> 420 nm) was used as the light source for the photoelectrochemical studies. The distance between light source and working electrode was maintained at 10 cm with irradiation intensity on sample 100 mW/cm2. To measure the hydrogen generation though the photoelectrochemical process, CdS coated ITO-glass, Pt wire, and SCE were placed in a 350 mL custom made closed electrochemical cell filled with 200 mL of 1 M Na2SO3 and 0.1 M Na2S mixed solution. The CdS coated ITO-glass was then irradiated for 30 min with the same light source as stated above. After irradiation, gas sample was collected by a gas tight syringe and hydrogen gas was measured quantitatively using a Gas Chromatograph (GC) (Agilent Technologies 7890B GC system) with TCD detector.

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3. RESULTS AND DISCUSSION 3.1. Structural Property. The crystal structure of the as-synthesized powder products was determined by the XRD. All the samples prepared in different solvents show similar XRD patterns as shown in Figure 1(A). Figure 1A(a) shows the XRD pattern of the sample prepared with water as solvent, which match hexagonal wurtzite CdS [JCPDS No. 01-0772306]. The measured lattice parameters a = 4.13 Å, c = 6.71 Å match the standard reference data a = 4.136 Å, c = 6.713 Å. All other samples synthesized by varying the solvents also show similar XRD patterns. No additional diffraction peak except for hexagonal CdS suggests phase-pure sample without any impurity. Moreover, XRD pattern of the samples prepared in only water [Figure 1A(a)] and with en [Figure 1A(b−e)] as solvent show maximum growth along (101) and (002) plane, respectively, as confirmed from their highest diffraction intensity. This suggests the role of solvent on the growth direction of CdS and thus morphology. The crystallite size of the as-synthesized CdS nanostructures was estimated using Scherer equation [eqn (1)].35 D = 0.9λ/βcosθ

(1)

where λ represents wavelength of the X-ray, β is the full width half maxima (radian) of (002) diffraction plane and θ is the Bragg diffraction angle. The measured average crystallite size (D) of the CdS nanostructures were estimated to be 11.33 nm, 13.89 nm, 12.93 nm, 13.12 nm and 12.38 nm for CdS samples synthesized with only water, 3:1 water:en, 1:1 water:en, 1:3 water:en, and only en, respectively. Raman scattering measurements were carried out to further evaluate the structure and purity of the prepared CdS microstructures. Figure 1(B) shows the Raman spectra of the samples prepared with five different solvents. Two intense peaks observed at 298.5 cm‒1 and 596 cm‒1 correspond to the first and second order longitudinal optical (LO) phonon modes of CdS crystal, respectively.36,37

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Figure 1: (A) XRD patterns and (B) Raman spectra of CdS microstructures prepared hydrothermally at 150ºC for 8 h using (a) only water (b) 3:1 water:en (c) 1:1 water:en (d) 1:3 water:en, and (e) only en, as solvent.

3.2. Surface Composition. The surface composition of a representative CdS sample was measured using XPS. Figure 2a shows the survey spectrum of the sample prepared with only water as solvent. The XPS survey spectrum shows the binding energy (BE) features of Cd, S, C, and O elements at 404.3 eV (Cd 3d), 160.8 eV (S 2p), 284.4 eV (C 1s), and 531.4 eV (O 1s), respectively. The C 1s and O 1s features are due to the surface carbon impurities and surface adsorbed oxygen or hydroxide, respectively. Figure 2b shows the region XPS spectrum for the Cd 3d level with two intense photoelectron peaks at BE of 404.3 eV (Cd 3d5/2) and 411.1 eV (Cd 3d3/2) that match for Cd2+.38 Moreover, the spin-orbit splitting of Cd 3d is measured to be 6.8 eV matching the standard value. The S 2p region XPS spectrum 8


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(Figure 2c) shows photoelectron peaks at BE of 160.8 eV and 161.9 eV assigned as S 2p3/2

Cd 3d5/2 S 2s S 2p

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160

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Binding Energy (eV)

Figure 2. (a) XPS survey spectrum of CdS prepared hydrothermally using only water as solvent at 150°C for 8 h. XPS region spectra of (b) Cd 3d and (c) S 2p.

3.3. Morphology of CdS microstructures 3.3a. Effect of solvent composition. The morphology of the CdS products synthesized using different solvent compositions was examined with FESEM (Figure 3). The other reaction parameters such as precursor concentration (CdCl2:thiourea = 1:1), temperature (150°C), and duration (8 h) were fixed. Figure 3a shows an FESEM image of the sphere-like CdS structures of diameter 250−300 nm obtained with only water as solvent. All the spheres were found to be uniform and well separated from each other. A magnified image of these 9



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spherical structures (inset of Figure 3a) clearly suggests that these spheres are composed of hexagonal plates of thickness ~10 nm. These plates are appeared to be merged with each other forming sphere-like clusters. Upon adding en to water (with 3:1 water:en solvent), spheres with a larger diameter (0.4−1 µm) were obtained (Figure 3b). A magnified FESEM image shows that these spheres are actually composed of nanorods (SNR) of ~20 nm diameter (Figure 3c). With an equal quantity of water and en (1:1 ratio) solvent, agglomerated nanoparticles were obtained as shown in Figure 3d. With a higher en content in the solvent, i.e., 1:3 water:en, again agglomerated nonuniform spheres were obtained (Figure 3e). A magnified image is shown in the inset of Figure 3e revealing nanobelt-like structures with a width 1µm) composed of nanoflakes using CdCl2 and thioacetamide as cadmium and sulfur source, respectively, in ethylene glycol through cyclic microwave radiation.20 Phuruangrat et al. and Liu et al. prepared CdS nanorods at 200°C and urchin-like CdS composed of nanorods at 180°C in 24 h using solvothermal technique, respectively.39,40 Thiruvengadathan and Regev reported synthesis of CdS nanowires array by using mesoporous silica as template.41 Gao and Lu synthesized CdS nanowires at 160°C in 24 h using wet chemical synthesis.42 The earlier reported similar CdS nanorods and nanowires morphology were obtained either at a higher temperature or a longer synthesis duration that that reported here.13,39,40,41,42 In addition to lower temperature and shorter duration synthesis, we report here CdS microspheres composed of building blocks with diverse shapes by varying the synthesis parameters. The effective surface area of the assynthesized CdS products obtained by varying the solvent was measured [Figure S1, 10


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Supporting Information (SI)]. The surface area of the samples obtained with higher en content was found to be larger than those prepared with lower en quantity (Table S1, SI). The measured surface area is found to be well-correlated to the morphology of the product shown in Figure 3.

Figure 3. FESEM images of hydrothermally synthesized CdS materials obtained with precursor mole ratio CdCl2:thiourea = 1:1 at 150°C for 8 h using (a) water, (b and c) 3:1 water:en, (d) 1:1 water:en, (e) 1:3 water:en, and (f) en, as solvent. Inset of (a) and (e) show the magnified image of the corresponding sample.

TEM analysis was performed to obtain further microstructural details of CdS spheres composed of nanorods and nanobelts. Figure 4a shows a TEM image of the sample prepared with 3:1 water:en solvent. The spherical morphology assembled of small sized nanorods was similar to the FESEM image (Figure 3b). The diameter of these nanorods was measured to be in the range of 15−20 nm from the magnified image (Figure 4b). Figure 4c shows a lattice resolved high resolution TEM (HRTEM) image from the edge of a nanorod with a lattice spacing of 0.32 nm for (101) plane of hexagonal CdS. Inset of Figure 4c shows a spot selected area electron diffraction (SAED) pattern suggesting single crystalline nature of the 11



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nanorods. Figure 4d and 4e show the TEM images of the sample prepared with 1:3 water:en as solvent. TEM images clearly reveal assembly of quasi 1D nanobelts-like structure. The width of these nanobelts is SNR (56%) > SNB (49.9%). Moreover, the photocatalytic activity per unit surface area follows the order of SNR > SNN > SNB > SNW, considering that surface area plays a crucial role in the heterogeneous catalysis (Table S3, SI). The MB degradation efficiency of as-synthesized SNN and SNW in the present work is found to be higher than that of several CdS-based chalcogenides (Table S4, SI). To check the advantages of the as-synthesized CdS photocatalysts, MB degradation performance of commercially available benchmark Degussa P25 (TiO2) photocatalyst was measured in the same conditions. Degussa P25 exhibits MB degradation efficiency of only 18.5% in 120 min of irradiation duration as measured from the UV-vis absorption spectra (Figure 9 and Figure S8, SI). The poor MB degradation performance of Degussa P25 is attributed to its band gap of ~3.2 eV which refers to absorption in the UV region and the use of visible light irradiation in the present study. The MB degradation rate constant was measured from the slope of straight line plot of ln(C0/Ct) vs. irradiation time (Figure 9b) and given in Table 1. The CdS spheres composed of either nanowires network or nanoneedles showed similar degradation efficiency and rate constant despite their large difference in effective surface area. It is well known that large surface area, crystallinity, and ability to light absorption are the main factors which control the efficiency of a photocatalyst.54 Photocatalytic activity can thus be a combined effect of these properties. Previously, there are reports where it is found that material with higher surface area shows lower photocatalytic activity compare to material 22


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with lower surface area.55,56,57 Fu et al. showed that the rhodamine-B degradation by ZnWO4 does not depends on the surface area of material.55 Ren et al. demonstrated that MB degradation does not depend on the surface area of TiO2.56 Xu et al. utilize CdWO4-GO composite for degradation of MB.57 Though CdWO4-5% GO exhibit higher surface area still it has lower degradation rate compare to CdWO4- 2% GO. In the present case, despite larger surface of CdS spheres composed of nanowires networks (SNW) or nanobelts (SNB), they showed poor photocatalytic activity per unit surface area as compared to spheres composed of either nanorods or nanoneedles (Table S3, SI). This clearly suggests that the morphology play a significant role on the photocatalytic activity, which is attributed to higher charge recombination in nanobelts/nanowires morphology than that of nanorods/nanoneedles (discussed later).

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The mechanism of the present photocatalytic process can be described as follows. Upon light irradiation with energy greater than the band gap of semiconductor catalyst, 23



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electrons (e−) and holes (h+) are created in the valance band (VB) and conduction band (CB), respectively (Step-1). The free electrons in CB of CdS (E0 = −0.85 V vs. NHE) have enough reduction potential to reduce oxygen molecule to superoxide radicals (O2/O2˙¯; E0 = −0.33 V vs. NHE) whereas holes in the VB (E0 = +1.55 V vs. NHE) is not enough to oxidize water or hydroxide ion to hydroxyl radical (OH˙, H+/H2O; E0 = +2.73 V vs. NHE and OH˙/OH¯; E0 = +1.90 V vs. NHE).58 So the free electrons react with oxygen molecule to form superoxide radicals (Step 2). These superoxides can be converted to hydroperoxyl radical (Step 3), which is then converted to hydrogen peroxide and hydroxyl radical (Step 4). Hydrogen peroxide can also give rise to hydroxyl radical in the system (Step 5). These hydroxyl radicals would react with the MB and degrade it to CO2 and H2O (Step 6). To confirm the hydroxyl radical formation in presence of CdS nanostructures under irradiation, terephthalic acid was taken as a probe molecule.59 Terephthalic acid reacts with hydroxyl radical to form 2-hydroxy terephthalic acid, which shows an intense luminescence peak in the visible region. The time dependent PL spectra of photocatalytically generated 2-hydroxy terephthalic acid are presented in Figure 10. Prior to irradiation, negligible luminescence intensity was detected, indicating the absence of 2-hydroxy terephthalic acid. With increase in the irradiation duration, the solution shows increased luminescence intensity. This is due to the increase in 2-hydroxy terephthalic acid content confirming formation of more number of hydroxyl radicals upon light irradiation in the aqueous solution containing CdS photocatalysts. On the other hand, holes in the VB could react with CdS causing photocorrosion under visible light illumination.37,60,61 A schematic diagram of dye degradation is shown in Scheme 2. CdS + hν → e¯CB + h+VB

(Step 1)

e¯CB + O2 → O2˙¯

(Step 2)

O2˙¯ + H2O → HOO˙+ OH¯

(Step 3)

HOO˙ + H2O → H2O2 + OH˙

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H2O2 → OH˙

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Scheme 2. Schematic representation of electron-hole pair generation in CdS, hydroxyl radical formation, and dye degradation.

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5. Photoelectrochemical performance of CdS microstructures The photocatalytic activity of the as-synthesized selected CdS microstructures (Table 1) was further studied by using those as photoanode for the electrochemical water-splitting under visible light irradiation. Figure 11a shows the linear sweep voltammograms (at scan rate of 20 mV/s) of different CdS microstructures as photoanode in an electrolyte containing 1 M Na2SO3 and 0.1 M Na2S where latter was used as hole scavenger in combination with SO32−. All the CdS samples show significant photocurrent under light irradiation indicating their role as photoanode for the water-splitting reaction. Upon irradiation, the electron and hole pairs are generated in the CdS photoanode. The holes at the photoanode surface are captured by hole scavenger (S2−/SO32−) whereas electrons are transferred to the electrode and subsequently to the Pt cathode via external circuit to reduce hydrogen ions to produce hydrogen at cathode as shown in Scheme 3. Thus the magnitude of current passed through the external circuit to the cathode depends on the effective formation and separation of electron/ hole pairs along with transport of electrons to the circuit from the catalyst. The photocurrent generation by different catalysts follow the order of CdS spheres composed of nanoneedles (SNN) > nanorods (SNR) > nanowires networks (SNW) > nanobelts (SNB). On the other hand, photocurrent per unit surface area follows the order of SNR > SNN > SNB > SNW, same order as that obtained for photocatalytic MB degradation (Table S3, SI). Further the transient photocurrent response of different photoanodes was studied at a fixed potential (0.5 V vs. SCE) by chronoamperometry (I-t measurements) with light ON and OFF mode. A sharp increase and decrease in photocurrent was observed on light illumination ON and OFF mode, respectively, as shown in Figure 11b. This suggests the superior photoresponse of all the CdS photoanodes. Moreover, the magnitude of photocurrent was found to follow the trend that of LSV study as expected. The combined LSV and chronoamperometry study reveal the inherent electron/hole pair generation and separation ability of different CdS 26


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photocatalysts. Furthermore, photoelectrochemically generated H2 was quantitatively measured using a gas chromatograph with thermal conductivity detector. Figure 12 shows the H2 generation rate for different CdS morphologies. The ITO-glass coated with SNN exhibits highest hydrogen generation rate (1774.1 µmol cm−2 h−1) among the four samples followed by SNR. The amount of H2 generation follows the same order as that of photocurrent

1.0 0.8 0.6

(iii)

(i) SNR (ii) SNB (iii) SNN (iv) SNW

(i) (iv)

0.4

(ii)

0.2

Dark

0.0 -0.3

0.88

2

(a)

Light

2

1.2

Current Density (mA/cm )

response.

Current Density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

0.3

0.6

0.86 0.84

(iii)

(b)

(i) (iv)

Photo Current

(ii) 0.82 0.80 0.78 Dark Current 0.76 200 250 300

Potential (V) vs. SCE

350

400

Time (s)

Figure 11. (a) Current density vs. potential plot of CdS spheres composed of (i) nanorods (ii) nanobelts (iii) nanoneedles, and (iv) nanowires network under light irradiation. For comparison, plot of current density was also measured under dark using CdS spheres composed of nanorods. (b) Time dependence of photocurrent density at an external bias of 0.5 V versus SCE with illumination switched ON and OFF for CdS spheres composed of (i) nanorods (ii) nanobelts (iii) nanoneedles and (iv) nanowires network.

27



Scheme 3. Schematics for photocurrent generation on CdS photoanode under visible light irradiation.

1800

−2

−1

Η2(µmol cm h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1500 1200 900 600 300 0

SNR

SNB

SNN

SNW

CdS Morphology Figure 12. Hydrogen generation rate of different CdS morphologies under visible light irradiation through photoelectrochemical process.

The photocurrent of the CdS microspheres composed of straight morphology such as nanoneedles (SNN) and nanorods (SNR) was found to be the higher than that of bent/curved 28


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morphologies such as nanobelts (SNB) and nanowires networks (SNW) despite the lower surface area of the former. Similar trend was obtained for MB degradation per unit surface area. In the heterogeneous catalysis process, in addition to the surface area, the other two major factors contribute to the photoactivity are (i) number of electron-hole pair generated and (ii) lower recombination rate of electron-hole pair. First factor depends on the band gap and absorption efficiency of the catalyst. On the other hand, second factor depends on the inherent electronic property of the catalyst material. The recombination rate of electron-hole for different photocatalysts is compared from the intensity of PL spectra (Figure S9, SI). The lower PL intensity suggests reduced recombination and higher carrier life time and vice versa.62 The straight CdS morphology (SNN and SNR) shows lower PL intensity than that of curved morphology (SNB and SNW) indicating slower recombination in the former and thus better photoactivity. The charge separation ability is found to be directly correlated to the morphology of CdS microstructures. Both the nanoneedles and nanorods are straight in nature and thus believed to have fewer surface defects that are recombination centers of electron and holes. On the other hand, nanobelts and nanowires with network morphology have several bends and curvatures which would not only hamper the charge movements but also have more recombination centers as confirmed from the high PL intensity. The mechanism of charge separation and movement in the photoelectrocatalytic hydrogen generation and photocatalytic MB degradation process is also schematically shown in Scheme S1 (SI) and Scheme S2 (SI), respectively. The present photoactivity behavior corroborates the earlier reports on the oriented TiO2 nanotubes exhibiting higher charge transfer properties and photoactivity compared to the randomly grown TiO2 nanotubes or nanoparticles.63,64,65

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5. CONCLUSIONS A simple hydrothermal technique is used to synthesize hierarchical CdS microstructures by varying the synthesis parameters such as solvent (water to en) ratio, precursor’s ratio, temperature, and precursor combination, which have important role on the product morphology. In particular, complete aqueous medium facilitates 2D growth whereas 1D nanoentities were formed upon adding en to the solvent. A set of selected CdS morphologies were further employed as photocatalysts for the degradation of MB and watersplitting reaction under visible light irradiation. The CdS spheres composed of nanorods and nanoneedles exhibited higher catalytic activity than nanowires morphology despite the larger surface area of latter. This is ascribed to higher charge recombination rate in nanowires/nanobelts than that of nanorods/nanoneedles morphology because of more number of defect centers in the former.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. N2 adsorption-desorption isotherm plots, SEM images, UV-vis absorption,

photoluminescence

spectra,

schematics

on

photocatalytic mechanism.

AUTHOR INFORMATION Corresponding author *[email protected] Notes The authors declare no competing financial interest. 30


photoelectrocatalytic

and

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ACKNOWLEDGMENTS This work was supported by Science and Engineering Research Board (SERB), New Delhi, India through the grant SB/S1/IC-15/2013. Authors acknowledge the Central Research Facility (CRF), IIT-Kharagpur for the XRD, SEM, and TEM characterization. J.K. is thankful to IIT-Kharagpur for a research fellowship.

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Table of Contents Graphic

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Shape-Dependent Photocatalytic Activity of Hydrothermally

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