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Adsorption-Driven Catalytic and Photocatalytic Activity of Phase Tuned In2S3 Nanocrystals Synthesized via Ionic Liquids Rahul Kumar Sharma, Yogendra Nath Chouryal, Sushmita Chaudhari, Jeganathan Saravanakumar, Suhash Ranjan Dey, and Pushpal Ghosh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01092 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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ACS Applied Materials & Interfaces

Adsorption-Driven Catalytic and Photocatalytic Activity of Phase Tuned In2S3 Nanocrystals Synthesized via Ionic Liquids Rahul Kumar Sharma,a Yogendra Nath Chouryal,a Sushmita Chaudhari,b Jeganathan Saravanakumar,a Suhash Ranjan Dey b and Pushpal Ghosha* a

School of Chemical Science and Technology, Department of Chemistry, Dr. Hari Singh Gour University (A Central University) Sagar, M.P-470003, India b

Department of Materials Science and Engineering, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana-502285, India E-mail: [email protected]

ABSTRACT Phase tuned quantum confined In2S3 nanocrystals are accessible solvothermally using taskspecific ionic liquids (ILs) as structure directing agents. Selective tuning of size, shape, morphology and, most importantly, crystal phase of In2S3 is achieved by changing the alkyl side chain length, the H-bonding and aromatic π-stacking ability of the 1-alkyl-3methylimidazolium bromide ILs, [Cnmim]Br (n=2,4,6,8 and 10). It is observed that crystallite size is significantly less when ILs are used compared to the synthesis without ILs keeping the other reaction parameters same. At 150oC, when no IL is used, pure tetragonal form of β-In2S3 appears however in presence of [Cnmim]Br [n=2,4], at the same reaction condition, a pure cubic phase crystallizes. However in case of methylimidazolium bromides with longer pendant alkyl chains such as hexyl (C6), octyl (C8) or decyl (C10), nanoparticles of the tetragonal polymorph form. Likewise, judicious choice of reaction temperature and precursors has a profound effect to obtain phase pure and morphology controlled nanocrystals. Furthermore, the adsorption driven catalytic and photocatalytic activity of as-prepared nanosized indium sulphide is confirmed by studying the degradation of crystal violet (CV) dye in presence of dark and visible light. Maximum 94.8 % catalytic efficiency is obtained for the In2S3 nanocrystals using tetramethylammonium bromide (TMAB) ionic liquid. 1 ACS Paragon Plus Environment


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KEYWORDS: phase transition, indium sulphide, ionic liquids, band gap, photocatalysis. 1. INTRODUCTION Semiconducting nanomaterials are extensively used in different photonic and biophotonic applications ranging from photovoltaics, photocatalysis, sensing, imaging etc. due to their size dependent optoelectronic properties.1-3 But it is very important to look beyond the conventional semiconducting materials like CdS, CdSe etc. due to their inherent toxicity which causes substantial environmental and biological issues. In this context, metal oxide semiconductors like ZnO, TiO2; chalcogenides like ZnSe, ZnTe, InS, In2S3 have drawn tremendous attention to the researcher.4-6 On the other hand, removal of organic pollutants especially different dye indicators like methyl orange (MO), rhodamine B, crystal violet (CV) etc. which are extensively used in dyeing, foodstuff, pulp, paper and textile industries through the use of semiconductor-based photocatalysts has been a very active research topic in recent years.4,5,7-10 However metal oxide semiconductors especially TiO2, ZnO etc. are active only under ultraviolet (UV) lighting irradiation and not responding to visible light which is the major component in solar light. In this context, chalcogenides devoid of toxic metal ions like Cd2+ ions etc. like ZnSe, ZnTe, In2S3 etc. can be useful.9,10 As a non toxic chalcogenides indium sulphide draws a huge interest to the researcher specially after its successful replacements as a buffer layer in lieu of environmentally unfriendly CdS, in Cu(In,Ga)Se2 (CIGS) thin film based solar cells, where it acts as an n-type semiconductor to form a heterojunction with p-type CIGS.5,11-12 Indium sulphide is regarded as non-toxic III-VI group of the semiconductor. It normally exists in three polymorphic forms at atmospheric pressure: α-In2S3 (cubic, defective structure, stable upto 693 K), β-In2S3 (defective spinel structure and stable upto 1027 K), γ- In2S3 (hexagonal layered structure and stable above 1027 K).13 Among these, β-In2S3 having band gap in the range of 2.0-2.3 eV, is more stable in ambient condition. Normally β phase occurs in either cubic or tetragonal crystal structure and is gaining a centre of attention due to its distinctive applications in various fields like optoelectronic,13,14-15 electrochemical,16 and photocatalytic degradation of dyes.4,9,15,17 There are numerous methods for β-In2S3 including spray pyrolysis,14,

18-19

physical vapour deposition,20 sonochemical,21

hydrothermal,13,22 solvothermal,12,23 spray ion layer gas,24 atomic layer deposition,25 chemical bath and vapour deposition,26-29 single source precursor method,30-31 and electrodeposition etc.32 Though various morphologies of indium sulphide like nanosheet,33 hollow microsphere,4,16,17 nanosphere,12 nanorods,30 nanowires,29-30 nanobelts,15 cage-like structure21

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etc. are reported but the interplay between its crystal phases is not reported to the best of our knowledge. Recently special emphasis is given on environmentally benign synthesis to prepare efficient materials with tunable property/ property combinations. In this context, ionic liquids (ILs) often described as “green” and “designer solvents” draw considerable attention since their properties can be tailored by judicious selection of cation-anion combination and changing the alkyl chain length etc. Because of having various physical and chemical properties for instances high viscosity, large electrochemical window, negligible vapour pressure, nonflammable in nature, catalyzing behavior, ILs are extensively used in organic catalysis, electrochemistry, f-element separation and many other applications.34-37 As ILs can efficiently stabilize nanoparticles and shield them against agglomeration on the base of their size and their charge; these can be useful for morphology and also for a phase control of inorganic nanomaterials.6,38-52 But the synthesis of phase tuned indium sulphide on the nanoscale using ionic liquid is not reported to the best of our knowledge. Herein we present an even easy, yet highly phase selective solvothermal synthesis of indium sulphide nanocrystals within band gap in the range 2.01-2.41 eV using task-specific ILs such as [Cnmim]Br (1-alkyl-3-methylimidazolium bromide). The main motivation of this work is to study the effect of tunable ILs based on varying alkyl chain length, H-bonding and aromatic π-stacking ability, on changing the crystal phase, size and morphology of indium sulphide nanocrystals. In addition, different other factors like the effect of sulphur precursors, reaction temperature, are also investigated. Moreover, growth mechanism of the nanocrystals is understood from atomistic origin using microscopic studies like SEM, TEM and HRTEM. Adsorption based catalysis and photocatalysis is studied for all the In2S3 samples. Maximum catalytic and photocatalytic efficiency in visible light is observed for the sample prepared in presence of tetramethyl ammonium bromide (TMAB) IL. However, a significant degradation of crystal violet dye is also noticed if the same sample is simply kept in dark for the entire reaction time. 2. EXPERIMENTAL SECTION Chemicals: Details and purity of the chemicals used are given in supplementary section. Synthesis of ionic liquids: Details of the synthesis of ionic liquids used in this work are given in the supplementary section (Scheme S1 and Table S1).53-54 Synthesis of indium sulphide nanoparticles: In a novel solvothermal approach, indium sulphide nanocrystals are prepared using imidazolium-based ILs with tunable alkyl chain



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length, reaction temperature etc. Here ILs act as a nanosynthetic template. In a typical synthesis, 1% (w/v) IL was dissolved in a mixture of distilled water and ethanol (1:3) and stirred for 5 minutes. 0.00307 moles of InCl3 was dissolved in distilled water and was mixed with IL solution, and stirred for 30 minutes. Now aqueous solution of thiourea/ thioacetamide (0.0046 moles) was added to the IL-containing solution and stirred for a while. Then this solution was poured into a Teflon lined autoclave with a capacity of 100 mL and kept in hot air oven at 120oC, 150°C and 180oC respectively with varying reaction time (Table 1). The obtained nanocrystals were centrifuged and washed several times with methanol, ethanol and acetone and dried in an oven at 80°C. Fourier transformed infrared (FTIR) spectra (Figure S4) of the samples prepared with [Cnmim]Br (1-alkyl-3-methylimidazolium bromide) IL show no stretching vibration of the C(2)–H in the imidazole ring or no imidazole ring skeleton stretching vibration bands, suggesting complete removal of IL. The absence of bending or stretching vibrational bands for –OH in the FTIR spectra also suggests that the as-prepared sample is free from hydroxide or water contamination. Characterization: PXRD (Powder X-ray diffraction) were carried out on a D8 Advance BRUKER, equipped with Cu Kα (1.54060 Å) as the incident radiation. The crystallite size was calculated using Scherer equation D = Kλ / βcosθ , where K = 0.9, D represents crystallite size (Å), λ is the wavelength of Cu-Kα radiation, and β is the corrected half width of the diffraction peak. TEM (transmission electron microscopy; FEI Tecnai STWIN-T30 using 300 kV electron beam source) was used to map the shape, size and lattice structure of the nanocrystals dispersed on a carbon coated copper grid from acetone solution. Morphological characterization was also carried out by Field Emission Gun Scanning Electron Microscopy (Carl Zeiss Germany, Model Supra-40). FT-IR results were measured using FTIR-8400S SHIMADZU. UV-Vis spectra were measured using Agilent Cary 100 UVVisible DRS spectrophotometer. 1H NMR spectra were recorded on Bruker Advance 400 (400 MHz) spectrometer at 295 K in CDCl3; chemical shifts (δ ppm) and coupling constants (J in Hz) are reported in standard fashion with reference to either internal standard tetramethylsilane (TMS) (δH =0.00 ppm) or CHCl3 (δH = 7.25 ppm).13C NMR spectra were recorded on Bruker Advance 400 (100 MHz) spectrometer at room temperature in CDCl3; chemical shifts (δ in ppm) are reported relative to CHCl3 [δC = 77.00 ppm (central line of the triplet)]. In the 13C NMR, the nature of carbons (C, CH, CH2 and CH3) was determined by recording the DEPT-135. The surface area of the as-prepared indium sulphide nanomaterials was investigated through BET (Bellsorp MR6, Japan). Intermediates of the photo-degraded


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Crystal violet by-products were carried out using the GC-MS/EI (JEOL JMS-T100GCV, AccuTOF GCv 4G, Japan). The emission spectra were recorded on luminescence spectrometer (Shimadju,RF-5301, Japan). Photocatalytic reaction: Photocatalytic reaction of as-prepared indium sulphides was done under compact fluorescence lamp (Philips) of 28 watt as a visible light source, in which initially 80 ml of 6 ppm (initial concentration) crystal violet (CV) solution was prepared in deionized water and 2M sodium nitrite solution is used to cut off the UV light. The fresh solution was prepared for each study. Before irradiating to light, 6 mg of indium sulphide as a catalyst was added to this solution and kept it for vigorous stirring in dark for one hour to achieve adsorption/desorption equilibrium condition. Later solution was exposed to visible light (28 watts) and the sample was taken out at an interval of time. The 5 ml solution was taken out from the reactor and it was centrifuged at 4000 rpm for 5 minutes and then absorbance was measured using the UV-Vis spectrophotometer (Systronic Double Beam UVVis spectrophotometer 2201) in order to study the degradation of CV dye. 3. RESULTS AND DISCUSSION 3.1. Structural characterization by powder-X ray diffraction (PXRD) and phase evolution 3.1.1. Effect of sulphur precursor The PXRD patterns of as prepared indium sulphide nanomaterials prepared in different reaction conditions are analyzed. During the synthesis, once thiourea (TU) is used as precursor while thioacetamide (TA) is used for other case keeping all the reaction parameters same (Figure 1 and Table 1). The cubic β-In2S3 (JCPDS card no. 32-456) appears for both the cases.55 However indium hydroxide (JCPDS card no. 85-1338) impurity appears (indexed by *) in addition when thiourea is used (Figure 1a). Appearance of indium hydroxide peak can be attributed to slow release of sulphur at 150oC on solvothermal condition. Due to less reactivity and higher decomposition temperature of TU compared to TA, more chances of reaction to be occurred between oxygen and In3+ in the previous. It is known that the reactivity of precursors also governs the nucleation and growth of nanocrystals, so the crystallite size of the nano-size particles is less (ca. 8.17 nm) in case of more reactive precursors i.e. TA compared to TU (ca. 12.1 nm) (Table 1).56 The effect of concentration of precursors (both In3+ and S2-) and reaction time on phase evolution is also studied (Figure S5 , S6 and S7) and there is no formidable change in crystal phase noticed (Details in Supporting information). 3.1.2. Effect of reaction temperature 5 ACS Paragon Plus Environment


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To understand the effect of reaction temperature on phase evolution, indium sulphide nanoparticles are solvothermally prepared at different temperatures 120oC, 150oC, and 180oC in the presence and absence of ionic liquid ([C2mim] Br) under similar reaction conditions. When no IL is used and reaction temperature is maintained at 120oC for 7 hours (IS10), pure cubic phase with high crystallinity is obtained (JCPDS card no. 32-456), however in presence of [C2mim]Br IL under similar reaction conditions, cubic phase appears with less crystallinity (Figure. 2a) indicating a significant effect played by IL on size of particles. But there is a drastic change in phase if the simply reaction temperature is increased from 120oC to 150oC without IL. Pure tetragonal phase (JCPDS card no. 25-390) appears when no IL is used at 150oC (Figure2a (iii)). But when reaction temperature is increased more for example at 180oC, tetragonal phase appears in both the cases: by the presence and absence of IL indicating reaction temperature is determining factor in phase evaluation not the IL at such high reaction temperature (Figure 2a (v and vi)). Despite of phase tuning, ILs played a tremendous role in controlling particle size. For example, at 120oC reaction temperature, average particle size without IL is ca. 15.88 nm whereas in presence of IL it is ca. 7.89 nm (Table 1) considering the cubic (440) plane. Similar observation is noticed for 150oC and 180oC. Particle size is also increased due to Ostwald ripening when the reaction temperature is increased significantly for example from 120oC to 180oC (Table 1) in presence of IL. 3.1.3. Effect of aromatic π-system of the IL on the phase evolution Figure 2b depicts the effect of different ILs on phase evolution of the resultant products. During the growth of nanoparticles, ILs can be bound at the particular site of nucleation and can govern the crystal growth by three probable ways: a) by π-π interaction between aromatic ring systems b) hydrogen bonding between the initial nuclei facets and the H atom in C(2) position of [Cnmim]+ and c) steric crowding due to alkyl chain length of the ILs on either of nitrogen in imidazolium ring system.6-7 From Figure 1b and 2b(i), it is seen that cubic phase occurs at 150oC with 7 hours reaction time in presence of [C2mim]Br IL. To understand the effect of hydrogen bonding on phase evolution, 1-ethyl-2,3-dimethylimidazolium bromide [C2dmim]Br, in which acidic C(2)-H is replaced by methyl is used in similar synthesis conditions. Here also the same phase appears (Figure 2b(ii)) suggesting that the evolution of the phase is not affected by H-bonding rather [C2mim]+ ions are anchored on to a particular plane via the π-system. It is also seen that size of crystallite was nearly comparable in cubic phase (Table 1) for both the ILs. To understand the effect of aromatic π-system on phase and size, a quaternary ammonium IL ([Me4N]Br), tetramethylammonium bromide (TMAB) which 6 ACS Paragon Plus Environment

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lacks aromatic π- system, is used. In case of TMAB, the pure tetragonal phase is evolved (Figure 2b (v)). Unlike imidazolium cation, TMAB is highly symmetrical and less bulky and does not cause much steric hindrance. So it can be effectively united to the surface of growing nanoparticles and cover the surface more effectively. Result of this, diffusion of sulphide anion might be faster leading to decrease in crystallite size as noticed in Table 1. Another imperative factor that plays a key role in tuning the phase of nano-size materials is the substitution of alkyl chain length on position 1-N of imidazolium ring. Previously, Singh et. al proposed that alkyl chain length is a prerequisite for modifying the assembly of imidazolium ring during the formation of reverse micelles like organization.57 This assembly does not only control the morphology of resultant products but also abets in governing the phase of material by governing the diffusion of ions from bulk solution to the nucleation centre and in attaching to the specific facet via the π-ring system. On increasing the alkyl chain length, the grain size of as-prepared indium sulphide nanoparticles is increasing. Similar result was also noticed by Chauvin and co-workers.47 As the alkyl chain length of IL is increased, the orientation attachment of cationic part of ILs about developing crystallite is different and this is perhaps due to the increase in interaction between hydrophobic parts and steric hindrance. The strength of binding of cation at the site of growth is contrariwise to alkyl chain length. This can be nicely shown through the PXRD patterns in which up to four carbon alkyl chain length substituted at position 1-N, cubic phases was obtained. But the shifting of the plane (211) towards lowers 2θ angle is taking place. This can be attributed to the strength of attachment of IL at the (211) plane that is gradually reducing with alkyl chain length of ILs. But a drastic and noteworthy change can be seen in crystal phase evolution and tetragonal phase is obtained above the four carbon long alkyl chain length as shown in (Figure 2b (iv)) and (Figure S8). By careful analysis, it is revealed that the crystallite size of the nanoparticles of tetragonal phase is bigger compared to cubic which clearly indicates that capping by IL cation with the initial nuclei is less when ILs with pendant longer alkyl chains such as hexyl (C6), octyl (C8) or decyl (C10) are used (Table 1). Keeping the reaction condition same, if the concentration of the IL for examples [C2mim]Br is increased 10 folds, no change in crystal phase is noticed (Figure S9) though the size of the crystallite is decreased (Table 1). 3.2. Structural characterizations by scanning electron microscope (SEM) and transmission electron microscope the (TEM) Figure 3a,b shows the SEM images of indium sulphide nanoparticles prepared by the absence and presence of an IL, [C2mim]Br respectively. Though in both the cases,



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microflower shaped morphologies with various sizes obtained, better contrast in the image obtained from IL (Figure 3b) indicating templating tendencies of IL cation. Likewise, microflowers/microspheres are obtained when [C4mim]Br is employed (Figure 3c). Here it can be anticipated that ILs can be attached at the specific facet of nanoparticles through π-π interaction between imidazolium ring systems of ILs. To understand the effect of these above two factors, an IL, (TMAB) without having aromatic π-system is used (Figure 3d) and a clear change in morphology is noticed compared to the use of imidazolium-based ILs (Figure 3b,c,e,&f). Appearance of clear nanoflake like structure indicates a difference in growth mechanism when an IL without aromatic π-system is used. Keeping the same IL [C2mim]Br, if sulphur concentration is increased to 9 times (IS14) no noteworthy changes is noticed (Figure 3e) however if concentration of In3+ ion is increased 3 times in as-prepared IS15 (Figure 3f), highly dispersible and regular shape morphology compared to stoichiometry composition (in Figure 3b) is observed. To understand the formation of microflower/microspheres from the atomistic origin, TEM images are analyzed in detail. Panel a, c, d and f in Figure 4 shows the morphology of asprepared indium sulphide using the [C2mim]Br, [C2dmim]Br, [C4mim]Br and TMAB respectively and panel b and e show the HRTEM images of as-prepared indium sulphide sample using [C2mim]Br and [C4mim]Br ILs respectively. From low-resolution TEM, nanoribbon like structure is observed for imidazolium-based ILs (Figure 4a-e), however, distinctly different structure is observed when TMAB is used (Figure 4d) indicating the importance of aromatic π-system in the growth mechanism. The average diameter of nanoribon is ~25 nm (shown in the inset of Figure 4a) and it is clearly understood that these are making microspheres/microflowers which are observed through SEM images by self-organization of nanoribbons/ nanoflakes. To get more insight, HRTEM of the sample obtained from [C2mim]Br was analyzed in detail. From the area marked with a circle (Figure 4b), it is noticed that all the lattice planes belong to the (222) cubic indium sulphide with inter-lattice spacing about 0.30 nm. Due to having similar surface energy, these planes are attached to each other through imperfect oriented attachment via twin boundaries indicated by the arrowhead. Mӧrre patterns like attachment can also be seen due to the overlapping of two different planes of comparable energy. Nanoflakes obtained in the presence of tetramethylammonium bromide are highly diverse in size and shape as these are agglomerated. There might be two key reasons behind this: first, there is no π-π interaction occurring between molecules of TMAB due to the absence of 8 ACS Paragon Plus Environment

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aromatic system and the second is a less bulky (-Me) group is attached with ammonium cation. Growth mechanism of indium sulphide The growth mechanism of indium sulphide microsphere can be divided into two steps: Nucleation of indium sulphide nanoparticles involving the interaction between indium ions and sulphide ions in the presence of ILs and then the formation of indium sulfide microspheres through the self-assembly of nanoribbons/nanoflakes. Initially, IL was dissolved in water and ethanol solution carefully at ambient condition. The white solution appears after adding of indium chloride solution indicating the formation of indium hydroxide which will be now surrounded by IL molecules. As the thioacetamide solution was added to above solution, HS- was begun to release in this slightly acidic medium (pH >2).26 It is already reported that in acidic medium, dissociation constant of hydrogen sulphide to HS- and later HS- to S2- is 1.12 x 10-7 and 1.2x 10-13 respectively.26 Thus dissociation constant of S2- is much less than HS-, in other words, it can be said that concentration of HS- ions in solution is much greater than S2-. But the diffusion of HS- to the core of reaction is dependent on alkyl chain length which is also known as Kirkendall effect.58 Greater the alkyl chain length, the minimum would be the chance of chemical interaction between In3+ and HS- which is clearly evidenced in the phase modulation of the indium sulphide nanocrystal. In the same medium, conversion of indium hydroxide to indium sulphide increases as H2S comes in solution resulting to decrease of pH of the medium which facilitates the formation of bare indium ion (In3+). Due to this, the possibility of reaction between HS- ion and bare In3+ would be high and this step is called dissolution. Therefore, in excess of water in reaction medium leads to the formation of In2S3 at the nucleation site.26 Consequently, as the temperature of the reaction medium was elevated, the growth of indium sulphide nanoparticles is also increased which might be due to Ostwald ripening. The growth of indium sulphide nanosized particles is also dependent on the IL used. When ILs with small alkyl chain length (n=2 and 4) were used, microsphere like indium sulphide was obtained. These microspheres were mainly consisted of nanoflakes like structures (Figure 2C). These flowers like morphology are attributed to self-organization of nanoflakes/nanoribbons that are fundamentally consisted of wide range nanocrystallite. However, on using the IL with long alkyl chain (n=6, 8 and 10), well-dispersed sheet-like structure forms rather than microsphere like structure (Figure S10 a,b,c). Such morphology can be illustrated on the basis of two key factors a) steric hindrance on the surface of nanoparticles and b) facet selective attachment of ILs to the nanoparticles. When short alkyl 9 ACS Paragon Plus Environment


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chain length IL was employed, it causes less steric hindrance at the surface of particles resulting particles of well organized and definite shape such as a microsphere. However, when IL with long alkyl chain was used, sheet-like morphology is found. Thus it can be concluded that morphology of as-prepared indium sulphide nanoparticles are highly influenced by nature of IL used during the course of synthesis. HRTEM images of the samples prepared without IL at 120oC and 180oC are shown in Figure S10d and f respectively and their low magnification TEM images are shown in Figure S10e and g. 3.3 Optical properties Optical studies of these products were done by using the UV-vis Diffuse reflectance spectrophotometer. First absorbance of the prepared samples was recorded and thereafter band gap of indium sulphide was calculated by plotting a graph between (abs*hν(eV))2 vs hν (eV). By this, band gap was revealed within the range of 2.01-2.41 eV corresponding to the absorption edge (Table S2).59 This substantial band gap is attributed to quantum confinement as the size of particles is very smaller compared to the Bohr exciton radius of bulk indium sulphide ( 33.8 nm). Engineering of band gap by tunable ILs and changing reaction temperature Band gap of as-obtained indium sulphide nanoparticles can be nicely correlated with alkyl chain length of ILs. Binding of IL at the site of nucleation steadily decreases on increasing the alkyl chain length. Due to feeble attachment of IL at the growing site of nanoparticles, the size of particles increases with increasing the alkyl length. As a result, the band gap of the nanoparticles is subsequently decreased (Figure 5a). Besides the effect of alkyl chain length, the effect of temperature in both conditions whether in the presence or absence of IL was also investigated. In this case, band gap steeply decreases with rising of temperature. Due to increasing the temperature, Ostwald ripening is taken place leading to increasing crystallinity and size of resultant indium sulphide nanoparticles as shown in Figure 2. However, in the presence of IL, band gap was found to be greater as compared to the sample prepared without IL. For instance, in the presence of [C2mim]Br at 120oC, its band gap was found 2.32 eV while in absence of IL at same reaction conditions its band gap was obtained 2.23 eV (Figure 5b). So it can be inferred that judicious choice of IL could be useful as a good capping agent which finally can tune the band gap of semiconductors. Measurement of surface area Specific surface area of the resultant indium sulphide products was measured through Brunauer-Emmett–Teller surface area analyzer under the inert liquid N2 environment at -194K as shown in Table S2. It is seen that surface area increases significantly when IL is used 10 ACS Paragon Plus Environment

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keeping the same reaction condition. For example, at 120oC reaction temperature, when [C2mim]Br IL is used surface area is 109 m2/g . However, the surface area was found less (44.82 m2/g) when IL is not used at the same reaction temperature. Similar observation for indium sulphide products that were synthesized using different ILs and specific surface area can be seen Table S2. 3.4 Photocatalytic activity The photocatalytic activity of as-prepared indium sulphide samples is pointed up by analyzing the photodegradation of CV under the visible light. CV dye is considered as one of the model dye for understanding the photocatalytic/catalytic activity of indium sulphide products. Furthermore, photocatalysis of dye can be partitioned into two stages: adsorption of dye and photocatalysis in light. Adsorption of dye (Dark phase) and photocatalysis Adsorption of dye is an essential step in analysis of the degradation of dye using the nanomaterials. It is become indispensable to investigate the adsorption capacity of nanoparticles in solution. Adsorption capacity (qe / mg gm-1) of as-prepared indium sulphide samples were quantitatively measured using the following expression: 60 qe =

(Co − Ce ) ×V W

(1)

Where Co and Ce are the initial concentration and concentration of dye after adsorption in dark period respectively, V is the volume of the dye solution and W is the weight of adsorbent (catalyst). It is seen in Table 2 and Figure 6c, that as prepared samples using TMAB (IS5), 120oC without IL(IS10), [C2mim]Br (IS1) and[C4mim]Br (IS3) have high adsorption capacity (qe) than the [C6mim]Br (IS4), [C8mim]Br (IS6) and [C10mim]Br (IS7). Interestingly, degradation of dye was predominantly occurred in dark period (> 40% dye) through adsorption process. The adsorption of dye onto the surface of as-prepared samples may be attributed to the strong electrostatic interaction between the positively charged dye and n-type of indium sulphide nanomaterials. For example, maximum and minimum adsorption capacity is noticed for the sample prepared by TMAB (IS5) (68.4 mg gm-1) and the samples prepared at 180oC without IL (IS11) (39.2 mg gm-1). Eventually photocatalytic efficiency (80.27%) is noticed higher for IS5 and lowest for IS11 (30.79%) (as shown in Table 2). Figure 6a is showing the photocatalytic activity of CV dye in presence of In2S3 nanocrystals prepared in presence of TMAB. Gross adsorption capacity (68.4 mg gm-1), photocatalytic efficiency (80.27%) and overall catalytic efficiency (η=94.8%) is noticed along with rate 11 ACS Paragon Plus Environment


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constant of (0.00965 min-1) which is found highest for all the as-prepared samples (Table 2). However, a significant catalytic efficiency (67.89%) though less compared to the presence of light as mentioned above is also noticed if the sample is simply kept for dark for entire reaction time (Figure S12a) . It is also noticed that IS5 is showing 2.5 fold more rate constant (Klight=0.00965 min-1) in a light reaction including dark phase than that of completely dark reaction (Kdark=0.0038 min-1) as shown in Figure 6b. Similarly, the samples prepared at 120oC without IL (IS10) have shown significant overall catalytic efficiency (89.09%) (Figure S12b). In contrary, the lowest value of all these parameters is reported for sample IS11 e.g. overall catalytic efficiency (η = 63.96%), the rate constant (0.00268 min-1) and the gross adsorption capacity (39.2 mg gm-1). This can be attributed to the presence of preferential facet or substantially exposed site for the strong interaction between dye molecules and photocatalyst and the size of nanoparticles. For example, facet dependent enhancement of photocatalytic activity is already reported in ZnO and TiO2. In Titania, high energy facet (001) was the preferential site for the photocatalytic activity.61-62 On the other hand, hydrogen evolution reaction (HER) using heterostructure TiO2@MoS2 and TiO2 TiO2@MoS2@CdS nanoparticles was also studied in which structure-dependent photocatalytic activity of hierarchical nanoarchitectures was illustrated63-64. But in our case, as both the high and low photocatalytic efficiency is observed for the samples which have a tetragonal majority, facet dependent efficiency can be nullified. However, the size of the nanoparticles can play a major role in photocatalytic efficiency. As the size of the nanoparticle is either too big or very small, both these conditions are not significant for better photocatalytic activity. If the particle is very small then the defects on the surface of nanomaterials will be more which may provoke the non-radiative emission and aid to the recombination of exciton leading to a reduced photocatalytic efficiency of nanomaterials.61-62 However when the size is very big then the active sites would be covered up due to agglomeration and site dependent activity of the catalyst would not be seen. In our study, the lowest efficiency is observed for the sample (IS11) which has the highest crystallite size of 21.58 nm (Table 1) and the samples with intermediate crystallite size have shown the better photocatalytic efficiency (Table 1 and Table 2). Since the indium sulphide is an n-type of nanomaterial and crystal violet is a cationic dye, there will be strong interaction between the dye and catalyst during the dark phase. Therefore, the degradation of dye through adsorption process is appearing more in the dark phase. Moreover, from the Table 2, another point can be figured out that greater the magnitude of adsorption capacity higher would be photocatalytic activity and vice-versa. To understand this adsorption process more effectively, an anionic dye, methyl orange (MO) is 12 ACS Paragon Plus Environment

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used keeping the other parameters same. Very less adsorption based degradation is noticed compared to the CV which indicates that adsorption plays a crucial role in the degradation of dye in the dark phase for CV (Figure S13). The analysis also reveals that specific surface area dependent photocatalytic activity is not found in this case. Mechanism of Photocatalysis Figure 6d shows the fundamental illustration for photo-degradation of CV dye in the presence of visible light. When a flash of light was irradiated upon the as-synthesized indium sulphide samples, the electron is excited from valance band (VB) to conduction band (CB). Consequently positively charged hole is created in VB and electron is excited in CB (step a). Meanwhile, adsorbed oxygen molecules are converted into superoxide free radical by accepting the electrons from CB (step b). Simultaneously adsorbed water molecules on the particle surface also dissociated into H+ and OH-, later hydroxyl ion further interacts with hole situated in VB and get converted into hydroxyl radical (.OH) through transferring the electron to VB (step c). Later this hydroxyl radical reacts with CV and converts it into degraded products (step d). Liao et. al. .

65

proposed the photo-degradation of CV in the presence of

OH/O2.- radicals. They have illustrated the two pathways for photo-degradation of CV in

aqueous medium: (i) N-de-methylation and (ii) destruction of chromophore structure. It can be anticipated that similar kind of photo-degradation of CV is happening in present case in the presence of hydroxyl radicals.65 In order to understand, what kind of by-products are forming during the photocatalysis reaction, obtained intermediates are analyzed using GC-MS/EI in which N-methylaminobenzene (m/z =107, 77, 51), aminobenzene (m/z =93, 66), 1-hydroxy-2propanone (m/z =74, 43) and acetic acid (m/z =60, 43) peaks are found. Their peak positions are encircled by red oval rings (as shown in Figure S14). Similar intermediates using GC-MS were also obtained by Fan et.al.

66

The GC-MS/EI spectra of the degraded dye in dark

condition is given as Figure S15. To confirm the photocatalytic activity, terephthalic acid (TA) is used as a hydroxyl radical scavenger. In the presence of light, terephthalic acid forms hydroxyl terephthalic acid (HTA) in photocatalysis reaction.4 Normally hydroxyl radical is generated only in the presence of light.4 So terephthalic acid was added to dye solution with catalyst and kept in the dark period for 1 hour, and then photoluminescence (PL) of the sample was measured by excited at λex= 315nm. There was no emission peak of HTA at λem= 425nm was observed. But when the solution was irradiated with light, HTA peak was found and intensity of the peak at 425 nm kept increasing with time (Figure S16) which confirms the photocatalysis. 4. CONCLUSIONS 13 ACS Paragon Plus Environment


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In summary, we have succeeded in preparing efficient quantum confined and phase tuned In2S3 nanocrystals useful for catalysis/photocatalysis using ILs as “nanosynthetic templates”. Nanoparticles of ~10 nm size are obtained in presence of IL which are less compared to the synthesis without taking IL, confirms the feasibility of ILs as a templating agent in inorganic synthesis. Except for size control, for the first time, it is noticed that by varying the properties of ILs like alkyl side chain length and aromatic π-stacking ability, the crystal phase of In2S3 nanocrystals can be tuned. For example, cubic β-In2S3 is observed up to [C4mim]Br IL, however tetragonal β-In2S3 polymorph is obtained when ILs of higher chain length (n≥ 6) is used. Judicious choice of reaction temperature and reaction precursors aside from the ILs is also mandatory, as it not only controls the size and morphology of the nanoparticle but also tunes the crystal phase, which is essential for band gap engineering. Very good catalytic and photocatalytic performance was obtained from TMAB mediated In2S3 samples. The analysis suggests that the strong interaction occurs between the n-type nanomaterials and cationic dye which results the degradation of the later through adsorption process even in dark. ASSOCIATED CONTENT Supporting Information Syntheses of ionic liquids, chemical structures and their chemical names; 1H NMR, 13C NMR, of [C2mim] Br and [C4mim] Br; FTIR of [C2mim] Br ionic liquid; PXRD, TEM images, UVvis spectra of indium sulphide nanoparticles; surface area and band gap of as-prepared indium sulphide nanoparticles; photocatalytic activity of methyl orange, GC-MS spectra of photodegraded products of crystal violet dye and PL spectra of 2-Hydroxy terephthalic acid. This material is available free of charge via internet at http://pubs.acs.org. ACKNOWLEDGEMENTS The authors would like to acknowledge support from the Science and Engineering Research Board (Start Up Research Grant for Young Scientist) and UGC Start-Up Grant, Govt of India. RKS acknowledge Dr. H. S. Gour University for financial support through a graduate fellowship. Authors acknowledge the support from Sophisticated Instrumentation Centre (SIC) and Department of Chemistry of Dr. H. S. Gour University for SEM, TEM and other characterizations. NOTES AND REFERENCES 1. van Veggel, F. C. J. M., Near-Infrared Quantum Dots and Their Delicate Synthesis, Challenging Characterization, and Exciting Potential Applications. Chem. Mater. 2014, 26, 111-122. 14 ACS Paragon Plus Environment

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Table 1. Phase composition, crystallite size and reaction conditions to obtain In2S3 nanocrystals using ILs. The crystallite size is estimated from the (440) diffraction peak of the cubic and (109) plane of the tetragonal material. Sample Code

Sample Name

Sulphur source

Reaction Time (hr)

Reaction Temperature (OC)

Phase/Crystallite Size (nm) Cubic (440)

Tetragonal (109)

1S0 IS1

(1%)[C2mim]Br-S (1%)[C2mim]Br-S

TU TA

7 7

150 150

12.1(±0.2)[a] 8.17(±0.3)

IS2

(1%)[C2dmim]Br-S

TA

7

150

7.96(±0.3)

IS3

(1%)[C4mim]Br-S

TA

7

150

8.30(±0.2)

IS4

(1%)[C6mim]Br-S

TA

7

150

IS5

(1%)TMAB-S

TA

7

150

13.8(±0.2)

IS6 IS7

(1%)[C8mim]Br-S (1%)[C10mim]Br-S

TA TA

7 7

150 150

14.63(±0.2) 10.93(±0.2)

IS8

(1%)[C2mim]Br-S

TA

7

120

10.80(±0.2)

7.89(±0.2)

IS9

(1%)[C2mim]Br-S

TA

7

180

IS10 IS11

Without IL-S Without IL-S

TA TA

7 7

120 180

IS12

Without IL-S

TA

7

150

IS13

(1%)[C2mim]Br-3S

TA

7

150

8.01(±0.2)

IS14 IS15 IS16 IS17 IS18

(1%)[C2mim]Br-9S (1%)[C2mim]Br-S-3In (1%)[C2mim]Br-S (10%)[C2mim]Br-S (1%)[C2mim]Br-S

TA TA TA TA TA

7 7 14 7 4

150 150 150 150 150

6.51(±0.2) 8.86(±0.2) 8.21(±0.2) 7.61(±0.3) 7.86(±0.2)

22.77(±0.2) 15.88(±0.2) 21.58(±0.3) 14.68(±0.2)

[a] Estimated deviation

[C 2 m im ] B r-T A (400)

(111)

C C

C

10

20

30

* 40

*

C

* 50

(422)

(440)

C

(222)

(411)

(222)

C C

(400)

C

(311)

(211)

*

(420)

[C 2 m im ] B r-T U

(220)

*

(a )

C

C C

(200)

C

(440)

(b )

Counts (a.u.)

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


* 60

2θ / o

Figure 1. PXRD patterns of as prepared indium sulphide nanoparticles using [C2mim]Br IL at 150oC for 7 hours using different sulphur precursors a) thiourea and b) thioacetamide.



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

Figure 2. (a) PXRD patterns of as-prepared indium sulphide nanocrystals using IL ([C2mim] Br) and without IL in similar reaction conditions: i) without IL at 120oC, ii) with [C2mim]Br IL at 120oC iii) without IL at 150oC, iv) with [C2mim]Br at 150oC, v) without IL at 180oC and vi) with [C2mim]Br at 180oC. (b) PXRD patterns of as-prepared indium sulphide using different ILs in same reaction conditions: i) [C2mim]Br, ii) [C2dmim]Br, iii) [C4mim]Br, iv) [C6mim]Br and v) TMAB respectively. (c) Schematic representation for synthesis of indium sulphide microsphere in the presence of ionic liquids.


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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


Figure 3. SEM images of indium sulphide nanoparticles under similar reaction conditions-a) without IL b) [C2mim]Br c) [C4mim]Br d) TMAB e) [C2mim]Br -9 times sulphur f) [C2mim] Br with 3 times indium. All the syntheses are done solvothermally at 150oC for 7 hours.



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Figure 4. TEM images of as-prepared indium sulphide-a) and b) are Low and high-resolution images of indium sulphide samples in the presence of [C2mim] Br respectively, nanorods like structure is shown in inset of image a, c) low-resolution TEM images of [C2dmim] Br, d) and


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e) are the Low resolution and HRTEM of [C4mim] Br respectively f) Low resolution TEM image of as-prepared indium sulphide using TMAB.

(a)

0

1.6

1.8

2.0

2.2

2.4

20

(b)

10

[C2mim]Br-120 C (2.32 eV) o [C2mim] Br-180 C (2.24 eV) o (2.23 eV) 0IL-120 C o (2.21 eV) 0IL-150 C o 0IL-180 C (2.01 eV)

2.6

2.8

3.0

0

3.2

1.6

1.8

2.0

2.2

2.4

Band gap

o

2 (hv*abs)2 (eV)

(2.41 eV) (2.37 eV) (2.34 eV) (2.33 eV) [C6mim] Br (2.32 eV) [C8mim]Br (2.28 eV) 10 [C10mim] Br (2.23 eV) (2.21 eV) (0% IL) TMAB [C2mim] Br [C4mim] Br [C2dmim] Br

Alkyl chain length Band gap

20

(hv*abs)2(eV)2

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


2.6

2.8

3.0

3.2

hv (eV)

hv (eV)

Figure 5. UV-vis absorbance spectra of indium sulphides a) change in band gap with alkyl chain length of IL under similar reaction conditions b) change in band gap with reaction temperature.



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

Figure 6. a) UV-vis spectra of the photocatalytic activity of indium sulphide (IS5) prepared using TMAB in light (using CV dye). b) Comparison between the rate constant of photocatalytic activity of as-prepared indium sulphide in 7 hrs reaction, using TMAB ionic liquid in dark and in the light. c) Histogram representation of catalytic efficiency and adsorption capacity of the as prepared indium sulphide nanoparticles. d) Schematic representation of proposed mechanism for the photocatalytic activity of indium sulphide nanoparticles in visible light.


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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


Table 2. Adsorption capacity, catalytic/photocatalytic efficiency and rate constant of as prepared indium sulphide samples. S.No.

Catalyst (Indium sulphide nanoparticles)

Adsorption Capacity (mg/ g)

Photocatalytic efficiency (η%)

Rate constant (k)(min-1)

Overall catalytic efficiency (Dark + Light) (ηo%)

1

TMAB (1%) (Light)

68.4

80.26

0.00965

94.8

2

TMAB (1%) (Dark)

36.79

-

0.0038

67.89*

3

[C2mim] Br (1%)

55.44

48.07

0.0042

84.06

4

[C4mim] Br (1%)

61.27

38.23

0.0031

85.59

5

[C6mim] Br (1%)

48.12

55.13

0.00529

82.14

6

[C8mim] Br (1%)

50.83

56.22

0.00575

84.19

7

[C10mim] Br (1%)

42.86

36.37

0.0029

70.27

8

IL(0%) -120oC

62.61

51.49

0.00542

89.09

9

IL(0%) -180oC

39.2

30.79

0.00268

63.96

(qe)

(*) This reaction is completely performed in dark condition with catalyst.



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Table of Content Figure


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Adsorption-Driven Catalytic and Photocatalytic ... - ACS Publications

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