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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
RGO-ZnTe: A Graphene Based Composite for Tetracycline Degradation and Their Synergistic Effect Koushik Chakraborty,† Tanusri Pal,‡ and Surajit Ghosh*,† †
Department of Physics and Technophysics, Vidyasagar University, Midnapore 721102, India Department of Physics, Midnapore College, Midnapore 721101, India
‡
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S Supporting Information *
ABSTRACT: For a high-performance photocatalyst, efficient exciton formation and subsequent dissociation, effective interfacial charge separation, and transportation to the photocatalytic reduction active sites are highly desired. Here, the visible-lightresponsive reduced graphene oxide-zinc telluride (RGO-ZnTe) photocatalyst was synthesized by a single-pot one-step solvothermal process. Analysis of chemical compositions and structural and morphological characterization of the assynthesized RGO-ZnTe samples were carried out intensively by TGA, XRD, XPS, TEM, and SEM study. Efficient transport of photoinduced electrons from ZnTe to RGO through their interface is confirmed by photoluminescence (PL) study. It is observed that the RGO mats are well-decorated with ZnTe nanoparticles, where RGO acts as a solid support as well as a nucleation center of the ZnTe nanocrystal. The RGO-ZnTe composite exhibited higher (6 times compared to RGO and 2.6 times compared to ZnTe) photocatalytic efficiency toward the visible-light-driven photodegradation of tetracycline (TC) antibiotics. Efficient catalytic performance is ascribed to better interaction and synergy among RGO and ZnTe. In the RGO-ZnTe composite the 2D wrinkled surface of RGO has a vital role in receiving the enhanced performance of ZnTe nanoparticle by minimizing the recombination probabilities of the photoinduced electron−hole. The responsible reactive species for photocatalytic TC degradation are also investigated comprehensively, which confirms that both holes and oxygen radicals have a dominating role toward the degradation of TC. KEYWORDS: RGO-ZnTe composite, photocatalysis, tetracycline degradation, synergistic effect, reactive species
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INTRODUCTION In veterinary practice and aquaculture a widely used medicine is tetracycline (TC).1,2 However, TC is a broad spectrum antibiotic and is excreted via stool and urine and contaminates water. Being a broad spectrum antibiotics, in trace concentration it triggers development of a multidrug resistant strain of bacteria.3,4 These types of bacteria are very difficult to treat with antibiotics available to us. The TC residues present in water are not amenable to effective removal by our conventional biological purification technology.5 Thus, it has become a matter of concern to the researchers. It is vital to degrade or diminish its effectiveness before being release into the environment. Therefore, researchers have tried to build up an effectual method for quick degradation of antibiotics for the environmental interest. Different physical and chemical schemes like physical adsorption, electrochemical techniques, etc. are adopted to make the polluted groundwater uncontaminated.6−8 TC adsorption using a variety of adsorbents, for example, graphene oxide (GO), Al2O3, binary oxide of Fe−Mn, activated carbon, and carbon nanotubes (CNTs), is considered the easier treatment approach.9−13 It ought to be pointed out here that solar-light-responsive photocatalysis is one of the most effective and low-cost techniques, as sunlight is the most abundant energy source. © XXXX American Chemical Society
Thus, photocatalytic degradation under sunlight is a highly viable technique to get rid of antibiotics in the water by an environmentally friendly approach. Semiconductor materials have attracted intensive attention owing to their enormous prospects in catalytic applications.14−18 In a semiconductor photocatalyst exciton, a bound electron−hole pair is formed after the absorption of photon flux. The exciton dissociates at the interface or at defect states present in the composites and creates free charge carriers. Finally, it transfers to catalytically active sites where they reduce the electron acceptors or oxidize the donor species. However, electron−hole pairs have the tendency to recombine quickly before arriving at the active sites. The latest studies have revealed that formation of a composite with graphene or reduced graphene oxide (RGO), a two-dimensional honeycomb structure of carbon with outstanding electron conductivity, gets to the bottom of the problem.19,20 Moreover, the ultraflexible layers of RGO would provide greatly enhanced surface area to the whole system and offer to attach pollutants to their basal plane by the “π−π” electron interactions, and therefore promise extremely striking Received: February 21, 2018 Accepted: June 6, 2018
A
DOI: 10.1021/acsanm.8b00295 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials
ethylenediamine [EN, NH2CH2CH2NH2], and ethylene glycol [EG, C2H6O2] from Merck, India, were used in this work. Materials Preparation. The photocatalytic activity of RGO based composite materials could be altered by varying the fraction of RGO and its counterpart of attached optical materials. The single-step solvothermal process was adopted to synthesize the RGO-ZnTe composite. The solvothermal process is considered to be a most advantageous technique for synthesis of solution processable materials as it is a one-step, single-pot, easy to achieve, cost-effective method. In this process, dimension and morphology of the material can be engineered by simply changing solvents, surfactant, and precursors and by tuning reaction parameters31 like reaction time, temperature, pH, etc. Herein, reduction of GO, formation of ZnTe, and the decoration of ZnTe on top of the RGO mat takes place concurrently. Initially, preoxidized exfoliated graphite was oxidized, and it produced GO.32 For single-step, one-pot synthesis of RGO-ZnTe composite, GO (80 mg) was dissolved into a mixture of EG (24 mL) and ED (1 mL) solvent followed by 10 min of sonication to obtain a homogeneous solution of GO. Then Zn(CH3COO)2·2H2O [1 mM], Na2TeO3[1 mM], and PVP [0.013 mM] were added to the solution under magnetic stirring for 30 min to get a uniform solution. Then, the reaction mixture was transferred into a Teflon lined autoclave covered with a stainless steel jacket and filled to 80% of its capacity. Then, the sealed autoclave was kept at 180 °C for 12 h inside a preheated oven. Upon completion of the reaction, the furnace was kept aside to come down to normal temperature naturally. Then, the resultant precipitate was assembled together by centrifugation followed by reparative washing in ethanol and double distilled water (DW). Washed samples were dried (80 °C) for 5 h in a vacuum furnace. Additional annealing at 200 °C was done for 3 h to remove the amine related peaks in the composite. The composite thus prepared was marked as RGO-ZnTe. The other composites (RGO0.25ZnTe, RGO-0.5ZnTe, RGO-2ZnTe, and RGO-3ZnTe) were synthesized by a similar protocol with a varying molar ratio of GO and Zn(CH3COO)2·2H2O. The RGO and controlled-ZnTe were also synthesized by an identical experimental route except for the addition of GO (for controlled-ZnTe) and Zn(CH3COO)2·2H2O, Na2TeO3, and PVP (for RGO). The detailed synthesis of the RGO-ZnTe composite is schematically presented in Scheme 1. Materials Characterization. Thermal gravimetric analysis (TGA) was executed from room temperature to 760 °C in air using a thermogravimetric analyzer (PerkinElmer Pyris diamond TG/ DTA with 10 °C min−1 heating rate). Powder diffraction patterns of the synthesized RGO, controlled-ZnTe, and RGO-ZnTe samples
possible applications in photocatalysis. In these types of materials RGO has a dual role through photoinduced exciton dissociation at the interface while providing continuous passageways for the transformation of electrons. The ability to form free electrons and transport makes RGO an essential template on the way to the synthesis of hybrid materials with enhanced photocatalytic ability. So far, a wide range of inorganic optical nanomaterials, such as ZnS, ZnSe, CdS, ZnO, and TiO2, have been conjugated to RGO or other carbon based matrices to achieve better photocatalytic performance.19−26 All the catalysts were used for the degradation of organic dyes and 4-nitrophenol or other organic pollutants in the UV or violet-blue region. On the basis of the literature survey, there is no report about solar-light-responsive photocatalytic degradation of TC using an RGO based nanocomposite. Particularly, zinc telluride (ZnTe), an important direct band gap binary octet semiconductor with appropriate band gap energy (2.26 eV for bulk) for efficient solar energy harvesting, has been extensively investigated in the field of optoelectronics.27−29 It is also considered as a potential photocathode material for the photoelectrochemical (PEC) water splitting application.30 The nanostructuring of the materials is considered the prime approach toward obtaining better photocatalysis performance as it gives high surface-to-volume ratios. ZnTe nanomaterials with different nanostructure morphologies, such as nanotube, nanobelt, nanowire, microsphere, and tetrapod nanorod, have been studied extensively. Solution processable spherical morphology of ZnTe not only possesses a large surface area, low production cost, good environmental stability, and excellent reusability, but also demonstrates a prominent visible-light absorption. The above features make ZnTe a prospective solar-light-responsive photocatalyst. The present work demonstrates the potential of an RGOZnTe composite for the solar-light-induced photodegradation of TC in aqueous medium. The synthesis of RGO-ZnTe and attachment of the ZnTe nanoparticle with RGO sheet were supported by TGA, XRD, XPS, TEM, SEM, UV−vis, and PL studies. Here, RGO nanosheets function as the photoinduced charge separator and subsequently as a solid state electrontransfer moderator, which swiftly collects the photoinduced electrons from the conduction band of ZnTe and efficiently transfers to the reactive sites. The photocatalytic decomposition kinetics is also investigated. A synergistic effect among RGO and ZnTe is proposed, and the synergy factor in RGOZnTe system is also calculated. Furthermore, the quenching experiments elucidated the role of individual reactive species for the degradation of TC in an aqueous medium. Our findings promote the RGO-ZnTe composite as a promising photocatalyst for diverse prospective applications. Beside this, the present study offers several new insights into a synergistic effect based on high-efficiency photocatalytic materials and their performances.
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Scheme 1. Schematic Representation of the Synthesis of RGO-ZnTe Composite
EXPERIMENTAL SECTION
Materials. Analytical grade graphite powder, potassium persulfate [K2S2O8], sodium nitrate [NaNO3], zinc acetate dihydrate [Zn(CH3COO)2·2H2O], phosphorus pentoxide [P2O5], sodium tellurite [Na2TeO3], and polyvinylpyrrolidone [PVP] were from SigmaAldrich, and potassium permanganate [KMnO4], sulfuric acid [H2SO4], hydrogen peroxide [H2O2], hydrochloric acid [HCl], B
DOI: 10.1021/acsanm.8b00295 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 1. (A) XRD patterns of GO, RGO, ZnTe, and RGO-ZnTe composite. (B) TEM and (C) SEM image of RGO-ZnTe composite. HRTEM image and SAED pattern of ZnTe on a RGO sheet are shown in the upper and lower inset of part B, respectively. were recorded for 2θ angles between 3° and 60° from an X-ray diffractometer (XRD, Rigaku-Miniflex II using Cu Kα radiation, λ = 0.15418 nm) operated at 30 kV/10 mA. The crystalline structure of the RGO-ZnTe composite was studied with transmission electron microscope (TEM) imaging and highresolution (HR) TEM imaging (JEOL-JEM 2100F, accelerating voltage 200 kV). The surface micromorphology was studied with a scanning electron microscope (SEM; Zeiss Merlin). Successful reduction of GO and formation of RGO-ZnTe composite were confirmed by focus X-ray photoelectron spectroscopy (XPS) using ULVAC-PHI, Inc. A 5000 VersaProbe II spectrometer with Al Kα radiation (1486.6 eV) was operated at 25 W, 15 kV, with vacuum level below 1 × 10−10 Torr with pass energies for survey and highresolution spectra of 117.4 and 29.35 eV, respectively. The absorbance in the UV−vis range and the photoluminescence spectra of the samples (controlled-ZnTe and RGO-ZnTe composite) were recorded by an absorption spectrophotometer (Shimadzu UV-1700) and a fluorescence spectrophotometer (PerkinElmer LS 55), respectively. The optical band gap energy (Eg) of control-ZnTe was calculated by using the relation αhν = A(hν − Eg)1/2 where hν and α represent the photon energy and absorption coefficient, respectively. A is a material constant.33,34 Measurement of Photocatalytic Activity. The photocatalytic activity toward the degradation of TC in the presence of photocatalysts was evaluated under visible light. Throughout the study, photocatalyst (100 mg) was dispersed in 50 mL of aqueous stock solution of TC (10 mg/L) in a quartz photocatalytic reaction chamber equipped with a solar simulator (AM 1.5, 100 mW/cm2) at ambient temperature. Before starting the reaction, the solution was stirred for 1 h under darkness in the presence of photocatalyst, to attain a state of adsorption equilibrium. A 4 mL sample was collected at every 5 min interval to monitor the degradation phenomenon of TC using a UV−vis spectrophotometer. Degradation efficiency was estimated using C/C0, where C and C0 represent concentration of TC at each illumination time interval (t) and the concentration before irradiation, respectively. To ensure that there was no thermal effect at the time of degradation, a water jacket was fitted with the reaction chamber.
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The phase structure, the crystallinity, and the structural composition of the as prepared samples (GO, RGO, controlled-ZnTe, and RGO-ZnTe) were investigated by the XRD study, and the diffraction patterns are compared in Figure 1A. A signature peak emerged at 10.5° with interplanner spacing of 0.804 nm, which authenticates GO formation. After solvothermal treatment, a new and wide peak centered at 24.2° is clearly observed owing to the formation of RGO after GO reduction. XRD peaks of controlled-ZnTe are indexed as (111), (200), (220), (311), (222), and (400) planes. The indexed peaks are in good agreement with the structure of cubic zinc blende.35 The interplanner spacing of the ZnTe crystal is 0.2148 nm as calculated (using the Scherrer equation) from the XRD profile and assigned to the (220) lattice plane. Moreover, no shift of peaks of ZnTe is observed in RGO-ZnTe, indicating that the presence of RGO does not noticeably affect the crystallographic phase of ZnTe. The oxygenous peak of GO is completely diminished in the diffraction profile of RGO-ZnTe composite which confirms the formation of RGO. Microstructural information on the RGO-ZnTe composite was obtained from TEM and HRTEM imaging and is presented in Figure 1B. Here, well-spread attachment of ZnTe onto a 2D RGO sheet with many wrinkles has been seen. An intimate interfacial contact among ZnTe and RGO is clearly observable: expect a better charge transfer during the embodiment.36 The lattice fringe spacing is ∼0.21 nm as shown in the HRTEM image of ZnTe (inset, Figure 1B). This value is in good agreement with the value (0.2142 nm) calculated from the XRD pattern. No signature of the core− shell structure was found in the HRTEM image analysis of RGO-ZnTe, but accumulation of many layers of RGO in a small area gives the cause of nonuniform contrast. The distribution of ZnTe on RGO is observed through an SEM image (Figure 1C). Figure S2, in Supporting Information, displays the SEM images of GO (A), RGO (B), and controlled-ZnTe (C). The XPS spectra were analyzed to characterize further the chemical states of the RGO-ZnTe composites as well as to confirm the reduction of GO and formation of RGO and are shown in Figure 2. The considerable existence of Zn, Te, O, and C in the RGO-ZnTe composite was confirmed by the surface survey (0−1200 eV). Zn 3d, Zn 3s, Zn 3p, Zn 2p3/2, and Zn 2p1/2 have been observed at the binding energies 9, 137, 821, 1020, and 1043 eV, respectively, while Te gives signals at 574 and 585 eV corresponding to Te 3d5/2 and Te 3d3/2. Figure 3A displays the UV−vis absorption spectra of controlled-ZnTe and RGO-ZnTe composite. A characteristic
RESULTS AND DISCUSSION
Materials Characterization. The actual ratio of RGO and ZnTe was determined from the TGA study of the assynthesized RGO-ZnTe composite of different ratios, and the curves are displayed in Supporting Information as Figure S1. In the TGA curve of all RGO-ZnTe composites, a weight loss from 250 to 550 °C is observed, ascribed to the segregation of oxygen-containing groups and the decomposition of the carbon matrix present in the composites. Mass ratios of ZnTe in RGO-ZnTe composite as derived from the TGA curves are 16, 36, 50, 69, and 75 wt %, which corresponds to the composite mentioned earlier as RGO0.25ZnTe, RGO-0.5ZnTe, RGO-ZnTe, RGO-2ZnTe, and RGO-3ZnTe, respectively. C
DOI: 10.1021/acsanm.8b00295 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ZnTe was found to be ∼3.6 ns, which shortened to a faster time scale than the temporal resolution (∼90 ps) of our excitation lamp after addition of RGO. Solar-Light-Induced Photocatalytic Degradation of TC by RGO-ZnTe Composite. The photocatalytic performances of controlled-ZnTe, RGO, and RGO-ZnTe samples were evaluated under identical experimental conditions by the degradation of aqueous TC solution under simulated solarlight illumination. The degradation kinetics has been studied with the help of the UV−vis absorption study, and the absorption spectra of TC in different stages are presented in Figure S4, in Supporting Information: controlled-ZnTe (A), RGO (B), and RGO-ZnTe (C). Under solar-light illumination, no significant changes in the concentration of TC were observed in the absence of any catalyst (Figure S5A, in Supporting Information). However, in the dark a slight change of TC concentration was observed in the presence of RGOZnTe catalyst that can be attributed to dark adsorption by the surface of the composite (Figure S5B, in Supporting Information).38,39 In aqueous solution, TC demonstrates two major absorption peaks centered at 275 and 360 nm in the UV−vis spectra. The 360 nm peak was initiated from the aromatic B−D rings.6,34 In the presence of catalyst, this absorption peak decayed gradually with illumination time indicating the fragmentation of the phenolic group present in aromatic ring B6,40 which finally degrades to H2O, CO2, and NH4+.41 The reduction of the 270 nm absorbance peak is attributed to the formation of the acylamino group and the hydroxyl group.41 It is observed from Figure 4A that the
Figure 2. (A) XPS survey spectrum of RGO-ZnTe composite. (B) Peak deconvolution of C 1s. High-resolution XPS spectra of (C) Zn 2p and (D) Te 3d.
Figure 3. (A) Optical absorption spectra of controlled ZnTe and RGO−ZnTe composite. Plot of (αhν)2 vs photon energy for controlled ZnTe is shown in the inset of part A. (B) Photoluminescence spectra of controlled ZnTe and the RGO-ZnTe composite.
spectrum of controlled-ZnTe demonstrates the fundamental absorption edge of ZnTe rising at 300 nm. After RGO was loaded on the ZnTe, an improved absorption in the visible range (200−900 nm) can be observed, which matches closely to the solar spectrum. The valence band edge of ZnTe was observed at 0.89 eV from the XPS spectra,35 and the work function of the XPS instrument was 4.21 eV which gives the valence band (VB) position at −5.1 eV. The optical band gap of ZnTe was calculated at 2.36 eV (Figure 3A, inset). This estimates the conduction band (CB) position at −2.74 eV. Enhancement of optical absorption within the visible region makes this composite a promising material for the solar-lightresponsive catalytic application. The PL spectra of controlledZnTe and RGO-ZnTe composites with 460 nm excitation are presented in Figure 3B. Controlled-ZnTe exhibits a strong green emission assigned to the band to band emission of ZnTe with peak position at around 545 nm.37 This PL intensity is quenched fully after attachment on RGO. It indicates an additional pathway for charge transfer at the time of interactions between excited ZnTe and RGO sheets which is highly desirable for photocatalytic applications. To get a better insight into the charge-transfer mechanism, a time correlated single photon count (TCSPC) measurement was performed. The fluorescence lifetime of photogenerated charge carriers in
Figure 4. (A) Comparison of photodegradation efficiency as a function of time under simulated solar-light illumination for RGO, controlled-ZnTe, and RGO-ZnTe composite. (B) Plot of ln(C/C0) as a function of simulated solar-light irradiation time for the photocatalysis of an aqueous solution of TC containing RGO, controlledZnTe, and RGO-ZnTe composite.
controlled-ZnTe and RGO possessed somewhat low degradation efficiencies which are 41% and 22%, respectively, after irradiation for 45 min. Obviously, RGO-ZnTe nanocomposites presented a higher degradation efficiency (%), and the presence of RGO nanosheets could improve photocatalytic performance of ZnTe. Degradation percentage was analyzed from the normalizing curve, and it was exponential in nature. From the degradation curve of both controlled-ZnTe and RGO-ZnTe it is clear that RGO-ZnTe has superior photocatalytic skill compared to that of controlled-ZnTe. To further understand the degradation process, the photocatalytic decomposition kinetics was investigated. The photocatalytic degradation of sufficiently low-concentration aqueous TC solution exhibits a pseudo-first-order reaction42 which can be articulated as ln(C/C0) = −kt, where k is the degradation rate constant. Figure 4B compares the variation of ln(C/C0) D
DOI: 10.1021/acsanm.8b00295 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials with t, and corresponding k values are calculated from the slopes for different photocatalysts. The linear variation confirms the occurrence of pseudo-first-order reaction kinetics present in photocatalytic reaction in our case. For both controlled-ZnTe and RGO, relatively low degradation efficiency with k values of 0.013 and 0.005 min−1, respectively, is observed. When ZnTe are grafted on an RGO mat, a noticeably enhanced photocatalytic activity (k = 0.033 min−1) is observed for the RGO-ZnTe (1:1) composite. In order to compare the efficiency of RGO-ZnTe nanocomposites with different mass ratios of RGO and ZnTe as 1:0.25, 1:0.5, 1:3, and 1:2, TC degradation was also evaluated under identical experimental conditions, and the UV−vis absorption spectra are presented in Figure S6, Supporting Information. All the composites show pseudo-first-order kinetics, and the calculated k values are 0.021, 0.022, 0.033, 0.027, and 0.024 min−1 for different ratios of RGO and ZnTe as 1:0.25, 1:0.5, 1:1, 1:3, and 1:2 in the composites, respectively. The results are presented in Figure S7, in Supporting Information. It is found that all the RGO-ZnTe composites exhibit a relatively high photocatalytic rate constant compared to those of their counterparts due to the synergistic effect involving RGO and ZnTe. This is probably due to heterostructure formation among RGO and ZnTe which is advantageous for the photoinduced charge separation and finally boosts the photocatalytic activity. Maximum photocatalytic efficiency, obtained for the RGO-ZnTe composite, is 1.75 and 3.7 times better than those of controlled-ZnTe and RGO, respectively. Thus, it is extremely attractive and meaningful to explore the effective synergistic factor involved in the photocatalytic process. As observed in Figure 4B, both RGO and controlled-ZnTe follow pseudo-first-order kinetics toward the photodegradation of TC. This also shows that photocatalytic TC degradation by RGO and controlled-ZnTe are proportional to e−kRGOt and e−kZnTet, respectively. Here, kRGO and kZnTe represent the degradation rate constants of RGO and controlled-ZnTe, respectively, for the pseudo-first-order degradation mechanism. Considering the synergy of RGO and ZnTe in the heterostructure composite (RGO-ZnTe), the degradation of TC by the RGO-ZnTe photocatalyst should be proportional to the product of their individual counterparts (e−kRGOt × e−kZnTet). If the initial concentration is C0 at t = 0 min, and the concentration is C(t) at time t, then C(t) can be expressed as
R=
This yielded synergy factors of 1.91, 2.00, 2.82, 2.19, and 1.97 for the RGO-ZnTe composites with varying ratios of 1:0.25, 1:0.5, 1:1, 1:3, and 1:2, respectively (Figure S7C, in Supporting Information). To further reveal the photocatalytic mechanism and give significant insight into the identification of the responsible reactive species intended for the TC degradation, ethanol, isopropyl alcohol (IPA), ethylenediamine tetra acetate disodium (EDTA-Na2), and N2 atmosphere were used as scavengers.45−48 The UV−vis absorption spectra of aqueous solutions of TC and RGO-ZnTe in the presence of different scavengers are presented in Figure S8, in Supporting Information. The plotting of degradation efficiency of RGOZnTe composite and the variations of ln(C0/C) with t for different scavenger of specific reactive species are presented in Figure 5A,B, respectively. Moreover, the variations of the
Figure 5. (A) Comparison of photodegradation efficiency of the RGO-ZnTe photocatalyst in the presence of different scavengers. (B) Variation of ln(C/C0) with irradiation time.
degradation rate constant for different scavenger are presented in Figure S9, in Supporting Information. As seen from Figure 5A, the RGO-ZnTe composite was mildly inhibited with photocatalytic degradation efficiencies of 65% and 60%, respectively, in the presence of e− quencher (ethanol) or OH• quencher (IPA). These indicate e− and OH• have a mild effect on photocatalytic TC degradation using RGO-ZnTe catalyst, whereas the photocatalytic degradation efficiency of RGO-ZnTe reduces significantly to 12% by the introduction of EDTA-Na2, a quencher of h+, indicating that holes have a dominating role toward TC degradation. On the other hand, photocatalytic degradation efficiency was diminished moderately to 34% when the reaction was performed in N2 atmosphere, a quencher of O2•−. This suggests that the O2•− radical also acts as a dominating force toward photocatalytic TC degradation. The above findings indicate that although all the reactive species like e−, h+, O2•−, and OH• have some contribution toward the degradation of TC, h+ and O2•− radicals are the primary active species responsible for the photodegradation of TC using RGO-ZnTe catalysts. It is extremely intriguing as well as advisible to be looking into the prospective photocatalytic performance of a highly efficient RGO-ZnTe composite. Basically, ZnTe demonstrates very good efficiency toward degrading water pollutants because of its suitable reduction potential and oxidation power of photoinduced electrons and holes, respectively.35 Under visible-light illumination, excitons are generated in ZnTe, and dissociate into free electrons (e−) and holes (h+) in the CB and
C(t ) = C0e−kRGOt × e−k ZnTet = C0e−(kRGO+ k ZnTe)t
This leads to ln
C0 C(t )
= (kRGO + k ZnTe)t
The RGO-ZnTe composite also pursues identical degradation kinetics (with degradation rate constant kRGO‑ZnTe) like its counterparts toward the visible-light-assisted photocatalytic degradation of TC. Thus, the following relation is also acceptable. ln
kRGO ‐ ZnTe kRGO + k ZnTe
C0 = kRGO ‐ ZnTet C(t )
From the above two equations, the synergy factor (R)43,44 for the RGO-ZnTe composite can be defined by considering its controlled counterpart as E
DOI: 10.1021/acsanm.8b00295 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials VB, respectively, of ZnTe. However, these photogenerated e− and h+ have a tendency to recombine in ZnTe; as a result, a low photocatalytic effect is observed. When ZnTe nanoparticles are grafted onto the RGO nanomats and form the RGO-ZnTe photocatalyst, the photogenerated electrons at CB of ZnTe can easily transfer to the RGO nanomat leaving a hole at the back in the VB of the ZnTe due to favorable matching of energy levels.49 Here, a huge number of well-coupled interfaces among the ZnTe particles and RGO nanomats offer an outstanding matrix for quick and effective transportation of photoinduced e− from the CB of attached ZnTe. As an outcome, RGO plays the crucial role of the electron carrier by capturing and transporting the photogenerated electrons rapidly and efficiently from the ZnTe sensitizer resulting in significant lowering of the charge recombination rate. Due to the swift mobility through the RGO nanosheet pathway, an electron can easily be transferred to the active sites of oxygen reduction. These electrons may reduce the adsorbed O2 presence on the surface of RGO-ZnTe or dissolved in water, to the superoxide radical anion O2•−. These O2•− could degrade the TC. O2•− may also create hydroxyl radicals (OH•) by successive reactions, and the photoinduced holes may react with OH− or H2O, oxidizing them into OH•; these OH• species have a mild effect on the degradation of TC. Our results depict that the photogenerated h+ and O2•− have the major contribution toward the degradation of the TC in an aqueous medium. In the composite, RGO has multifunctional activity toward the enchantment of photocatalytic performance. First, the 2Dcanvas of the RGO sheet could facilitate the uniform dispersion of ZnTe nanoparticles. At the same time, the presence of RGO can efficiently prevent the aggregation of ZnTe nanoparticles. Second, the wrinkled surface presence in RGO (as can be seen in TEM images) could offer large contact interface among ZnTe and the RGO nanosheets. Third, an enhancement of photon absorption in the range 200−900 nm is achieved after loading RGO to ZnTe, which makes this composite a promising material for the solar-light-responsive catalytic application. Last, full quenching of PL intensity of ZnTe after attachment on RGO indicates an additional pathway for charge transfer at the time of interactions between excited ZnTe and RGO sheets.50 In addition to that, RGO has a mild photocatalytic effect. On the other hand, ZnTe plays the key role for exciton generation after absorption of visible light. The highly crystalline ZnTe (as shown from the HRTEM) offers a better exciton diffusion length which subsequently increases photocurrent generation and consequently the photocatalytic efficiency. Therefore, the exceptional synergetic outcome among RGO and ZnTe significantly improves the photocatalytic performance of RGO-ZnTe composites. A schematic presentation of the enhancement of photocatalytic activity of the RGO-ZnTe photocatalysts is illustrated in Figure 6. The recycle test shows that the degradation efficiency of RGO-ZnTe does not alter noticeably even after its recycled use for five times and has been shown in Figure S11A, in Supporting Information. To examine the stability of the RGO-ZnTe composite as a photocatalyst, XRD was performed after five cycles of TC degradation and is presented in Figure S11B, in Supporting Information. It is observed from the comparative XRD patterns (with Figure 1A) that the crystalline structure and the phase of RGO-ZnTe photocatalyst are not changed noticeably after five
Figure 6. Mechanism of reactive species generation and photocatalytic degradation mechanism of TC in the presence of RGO− ZnTe photocatalyst under solar-light illumination.
degradation reaction cycles of TC which also confirms the stability of the RGO-ZnTe photocatalyst.
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CONCLUSIONS Novel photocatalysts based on the RGO-ZnTe composite were synthesized via a one-pot, single-step solvothermal method. Here, well-spread attachment of a ZnTe nanosphere onto a 2D graphene sheet with many wrinkles has been seen. An intimate interfacial contact between ZnTe and RGO is clearly observed, facilitating a better charge transfer during the embodiment. PL intensity is quenched fully after attachment on RGO, which indicates an additional pathway for charge transfer at the time of the interactions between excited ZnTe and RGO sheets. The RGO-ZnTe photocatalyst demonstrated superior photocatalytic activity compared to its controlled counterparts for TC degradation under visible-light illumination. A synergy among the RGO mat and ZnTe catalysts was anticipated to account for its improved photocatalytic performance. The photocatalytic decomposition kinetics was also investigated, and pseudo-first-order reaction kinetics is observed. Our study reveals that holes have a dominating role toward the degradation; in addition to that, O2•− radicals also have some effect on the degradation of TC by RGO-ZnTe photocatalyst.
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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/acsanm.8b00295. Additional characterization results, including TGA, SEM, TEM, UV−vis, and photodegradation details (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (S.G.). ORCID
Surajit Ghosh: 0000-0001-6604-8857 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Council of Scientific & Industrial Research (CSIR), New Delhi, India, via Grant 03(1392)/16/EMR-II. We are also thankful to the Department of Science and Technology (DST) and University Grants F
DOI: 10.1021/acsanm.8b00295 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials
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Commission (UGC), New Delhi, India, for providing infrastructural support and special assistance to the Department of Physics & Technophysics, Vidyasagar University, via the FIST and SAP program, respectively. We expand our thanks to the Department of Physics and Meteorology, IIT Kharagpur, for providing the DST-FIST funded XPS facility.
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DOI: 10.1021/acsanm.8b00295 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsanm.8b00295 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
