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Biomolecule-mediated CdS-TiO2-reduced graphene oxide ternary nanocomposites for efficient visible light-driven photocatalysis† Soumen Dutta,a Ramakrishna Sahoo,a Chaiti Ray,a Sougata Sarkar,a Jayasmita Jana,a Yuichi Negishib and Tarasankar Pal*a We report an environmentally friendly synthetic strategy to fabricate reduced graphene oxide (rGO)based ternary nanocomposites, in which glutathione (GSH) acts both as a reducing agent for graphene oxide and sulfur donor for CdS synthesis under modified hydrothermal (MHT) conditions. The report becomes interesting as pH variation evolves two distinctly different semiconducting nanocrystals of anatase/rutile TiO2 and hexagonal yellow/cubic red CdS, and their packaging makes them suitable photocatalysts for dye degradation. Herein, a titanium peroxo compound, obtained from commercial TiO2, is hydrolyzed to TiO2 nanostructures without any additives. The yellow colored CdS-TiO2-rGO (YCTG), one of the ternary photocatalysts, shows maximum efficiency compared to the corresponding red ternary CdS-TiO2-rGO or binary photocatalysts (CdS-rGO, TiO2-rGO and CdS-TiO2) for dye degradation under visible light irradiation. Systematic characterizations reveal that TiO2 presents at the interface of rGO and CdS in YCTG and thus makes a barrier that inhibits the direct interaction between rGO and CdS. This leads to a relatively higher bandgap value for CdS in YCTG (2.15 eV vs. 2.04 eV for CdS-rGO) but with better photocatalytic activity simply by diminishing the possibility of the charge-recombination process.
Received 9th September 2014, Accepted 17th October 2014
In the present situation, rGO in the YCTG also supports faster dye degradation through higher dye adsorption and rapid internal electron transfer (CdS→TiO2→rGO) in the YCTG nanocomposite. Thus, a
DOI: 10.1039/c4dt02749c
simple aqueous phase and a greener synthetic procedure results in a low-cost, highly effective visible light-responsive material for environmental application.
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Introduction During the past few years, a special attraction has been generated among the scientific communities towards a unique twodimensional carbon allotrope known as graphene due to its multifunctional activities, as well as its outstanding physical and chemical properties.1 This material encompasses a monolayer of single-atomic thick, sp2 hybridized carbon atoms with extremely high mechanical strength, remarkable thermal and electrical conductivity, and ultra-high surface area, which makes this substrate an ideal candidate for exploring its possible applications in various fields.2 Solution-based synthetic strategies for graphene proceed through graphene oxide (GO), a highly oxidized and exfoliated product of pristine graphite,
a Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India. E-mail: [email protected] b Department of Applied Chemistry, Tokyo University of Science, Tokyo-1628601, Japan † Electronic supplementary information (ESI) available: Digital images, XRD, EDAX, elemental mapping analysis, UV-vis graphs. See DOI: 10.1039/c4dt02749c
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which is then reduced to form reduced graphene oxide (rGO). The incorporation of various nanoparticles or polymer matrices into rGO creates plentiful avenues for interesting applications such as photocatalysis,3a detection of toxic ions,3b catalysis,3c removal of water pollutants,3d lithium-ion batteries,3e fuel cells,3f supercapacitors,3g and so forth. Currently, the synthesis of semiconductor nanoparticles and their application in photocatalysis4 or photovoltaic devices5 have been investigated quite extensively. Photocatalysis is a promising area of research, which surrounds the photoinduced H2 production6 as well as the degradation of various water pollutants4 in a low-cost, well-organized process. Photo irradiation leads to the generation of multiple charge carriers (i.e. holes and electrons) out of a semiconductor, and these carriers are responsible for its improved performance. However, the charge pairs have a strong tendency to recombine easily. This problem of recombination needs to be overcome in order to achieve the maximum outcome from a respective semiconductor. This process can be prevented by a support, which affords a continuous electron relay or fast electron transport from a specified semiconductor.7 Various approaches
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have been followed to inhibit the recombination process, such as transition metal doping,8a noble metal disposition,8b,c the assistance of an additional semiconductor,8d heterojunction fabrication,4b or supported by any conducting material (graphene,8e carbon nanotube8f or conducting polymer8g), etc. Cadmium sulfide (CdS) is an n-type semiconductor with bandgap value of ∼2.40 eV, which explains its intriguing visible light-driven photocatalytic activity. However, the photogenerated charge carriers recombine easily due to their narrow bandgap energy, and the holes in the valance band of CdS are highly active to oxidize CdS, which creates a problem regarding its efficiency and stability as photocatalyst.9a CdS nanoparticles are frequently embedded with other matrices, such as wide bandgap semiconductors,9b noble metals,9c support3a or sacrificial inorganic electron donors9d in order to increase their photocatalytic efficiency. In that context, the coupling of titanium dioxide (TiO2) with CdS8d,g,9e is widely studied where their interface facilitates fast electron transfer, and also the noble metal deposited CdS-TiO2 nanocomposites9f turn out to be an active photocatalyst with a lower charge recombination process. The use of expensive noble metals in the above mentioned tri-component nanocomposites explain its limitation as a potentially useful material on a large scale. Thus, the development of CdS-TiO2-based nanocomposites without any noble metal nanoparticles for highly active visible light responsive catalytic activity is highly advantageous. Reduced graphene oxide, an economically suitable material with fast electron transfer capacity and high adsorption power can be an ideal substitution for noble metal in the production of CdS-TiO2 based ternary photocatalyst. There are reports on CdS-TiO2-rGO, a tri-component nanocomposite with some interesting applications such as dye degradation,10a selective organic transformation10b,c or for the study of electron transportation10d under photoexcitation. In these cases, either commercial TiO210a,d or a multi-step synthetic procedure10b,c were used for the large scale production of the hybrid material. Additionally, the use of dimethyl sulfoxide (DMSO)10b,d as a source of sulfide under solvothermal conditions may not be an attractive choice due to its toxicity at higher concentrations.11 In the present work, we have followed a one step, biomolecule-assisted synthetic procedure for the previously mentioned tri-component nanocomposites, in which glutathione (GSH) acts as a reducing agent12 as well as a sulfur source for CdS synthesis. Moreover, in situ generated TiO2 nanoflowers from the hydrolysis of titanium peroxo compounds under modified hydrothermal (MHT) conditions claims its novel synthetic procedure for CdS-TiO2-rGO nanocomposites with efficient visible light-driven photocatalytic activity for dye degradation.
Experimental Materials All reagents used were of AR grade and used as received without any further purification. Graphite powder, glutathione
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(GSH) was purchased from Sigma-Aldrich. Potassium permanganate, sodium nitrate, hydrogen peroxide, sulfuric acid, hydrochloric acid, titanium oxide (TiO2), sodium hydroxide and ethanol were received from Merck, India. Cadmium chloride (CdCl2) was purchased from Fisher Scientific, India. Sodium hydroxide, potassium hydrogen sulfate, cation exchange resin Seralite SRC-120, rhodamine B and crystal violet were bought from SRL, India. Before the start of the experiment, all glasswares were carefully cleaned using aquaregia and subsequently rinsed with a copious amount of double distilled water and dried well prior to use. Double distilled water was used during the entire experimental process. Synthesis of red CdS-TiO2-rGO (RCTG) nanocomposite First, graphene oxide (GO) was prepared from graphite powder according to the Hummers method13 and titanium peroxo compound was produced following our reported procedure.14 36 mg of GO was then sonicated in 36 mL distilled water for 2 h for complete dispersion, followed by the addition of 60 mL of aqueous titanium peroxo solution (having optical density 1.0). The mixture was stirred for 30 min and then 54 mg of solid cadmium chloride was introduced separately into the aforementioned suspension under stirring conditions. The resultant mixture was stirred for 1 h at room temperature to make a complete exchange of both the cations (Ti4+ and Cd2+) into the GO moiety through the negative functionalities present in the GO matrix. Next, 180 mg of glutathione (GSH) was mixed into the stirred homogeneous mixture and stirred for another 15 minutes. The entire mixture was taken in a screw-capped test tube to perform the modified hydrothermal (MHT)14 process at 180 °C for 30 h. Synthesis of yellow CdS-TiO2-rGO (YCTG) nanocomposite The aqueous titanium peroxo solution was highly acidic as this solution was prepared in 5% H2SO4 solution. In this case, 5 M sodium hydroxide (NaOH) was freshly prepared and added dropwise into the aqueous titanium peroxo solution (60 mL) until the pH value of the mixture reached 5.0 with a noticeable change from orange color to almost colorless aqueous titanium peroxo solution (see Fig. S1, ESI†). This colorless solution was then used as a precursor for TiO2. For the synthesis of yellow TiO2-CdS-rGO, the same procedure has been followed as indicated in the previous RCTG synthetic protocol. Synthesis of CdS-rGO (CG) and TiO2-rGO (TG) nanocomposite For both the cases, the adopted synthetic procedures were exactly the same as indicated for red TiO2-CdS-rGO nanocomposite synthesis. For the CdS-rGO (CG) nanocomposite, 60 mL distilled water was added instead of aqueous titanium peroxo solution, whereas solid cadmium chloride was not employed for the TiO2-rGO (TG) nanocomposite. After the completion of all reactions, the products were centrifuged at 7000 rpm and washed thoroughly several times with distilled water to remove excess ions or unbound CdS or TiO2 from the nanocomposite material. Finally, the products
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were washed with ethanol and dried in air prior to their characterizations and applications.
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Instrumentation The XRD patterns of the nanocomposites were recorded on a Philips PW-1729 X-ray diffractometer (40 kV, 20 mA) with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 5°–75° at a scanning rate of 0.5° min−1, and the XRD data were analyzed by using JCPDS software. FESEM and compositional analyses of the synthesized samples were done with a supra, Carl Zeiss Pvt. Ltd. Instrument, and its attached energy dispersive X-ray microanalyzer (OXFORD ISI 300 EDAX) for a dispersed solid sample after drop-casting it on microscopic glass slides. Transmission electron microscopic (TEM) analyses of the samples were carried out on a Hitachi H-9000 NAR transmission electron microscope, operating at 100 kV. Detailed elemental analysis was done by X-ray photoelectron spectroscopy (XPS) with a VG Scientific ESCALAB MK II spectrometer (UK) equipped with a Mg Kα excitation source (1253.6 eV) and a five-channeltron detection system. Raman spectra were recorded for the solid samples by using a fiber-coupled micro-Raman spectrometer (Horiba Jobin Yvon Technology) equipped with 488 nm (2.55 eV) of 5 mW air cooled Ar+ laser as the excitation light source, a spectrometer (model TRIAX550, JY), optical microscope (Model BX 41, Olympus, Japan), and a Peltier-cooled CCD detector (Horiba Scientific, France). The optical properties of the samples were analyzed from their corresponding reflectance spectra measured in diffuse reflectance spectra (DRS) mode with a Cary model 5000 UV-vis-NIR spectrophotometer. All of the absorption spectra for dye degradation were recorded by using a SPECTRASCAN UV 2600 spectrophotometer (Chemito, India) for an aqueous solution in a 1 cm quartz cuvette. Nitrogen adsorption–desorption isotherms were measured by using a Quantachrome Autosorb automated gas adsorption system at 77 K. Photocatalytic activity study To compare the photocatalytic activities of various as-synthesized nanocomposites, crystal violet (CV) and rhodamine B (RB) were used as model dye molecules to study their visible light-driven photodegradation process. First, 10 mL aqueous dye solution (5 × 10−5 M for CV and 2.5 × 10−5 M for RB) was mixed with as-prepared 4 mg of catalyst and stirred under darkness for 30 minutes to establish the adsorption–desorption equilibrium. The entire mixture was then kept in front of a visible-light source (W bulb; 100 W) under stirring condition. The distance between the light source and reaction mixture was 35 cm. At various time intervals, 3 mL of the suspension was centrifuged, and the absorbance data was recorded for the supernatant using UV-vis spectrophotometer.
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favor the attachment of metal ions through electrostatic attraction. Herein, the sequential addition of Ti4+ and Cd2+ ions onto the GO sheets helps to grow into their respective composites one after the other under MHT conditions in the presence of glutathione (GSH). Previously, the reducing property of GSH for GO reduction has been addressed by Jeong et al.,12 which is also applicable in our case, but the additional function of GSH as a sulfur donor under MHT condition15 concludes the bi-functional activities of GSH in our reaction protocol. A titanium peroxo compound has been used as a source of TiO2 nanostructures in very limited cases.14,16 In our study, a titanium peroxo compound was prepared from commercial TiO2, which makes the process economical, and the advantage of using this precursor is its easy hydrolysis, detectable by the naked eye (orange color to colorless) under MHT to generate TiO2 nanostructures even without any hydrolyzing agent as found in the present case. As the solution of titanium peroxo compound is acidic one, it creates a problem regarding the fabrication of yellow CdS, rather it leads to a reddish CdS product17 during ternary nanocomposite synthesis. However, a titanium peroxo compound at a slightly higher pH medium (∼5.0, maintained by dropwise addition of aqueous NaOH solution) confers our desired product, yellow CdS deposited on TiO2 attached rGO sheets. Thus, we produced four possible rGO based nanocomposites: (i) yellow CdS-TiO2-rGO (YCTG) at higher pH, (ii) red CdS-TiO2-rGO (RCTG) at lower pH, (iii) CdS-rGO (CG), and (iv) TiO2-rGO (TG) by using bi-functional glutathione under MHT conditions. Fig. 1 represents the digital images of various synthesized rGO based nanocomposites (also see Fig. S1, ESI† for the digital images of GO dispersion and titanium peroxo compound solutions). To investigate the phase purity of our as-synthesized samples, we performed the XRD analysis. Fig. 2 describes the typical XRD patterns of various nanocomposites, i.e. YCTG, RCTG, CG and TG. For both YCTG and CG, the diffraction patterns correspond to the CdS match with the standard JCPDS file no. 772306, which suggests the hexagonal crystal systems for CdS and the crystal planes are indicated in Fig. 2a and c. Anatase TiO2 nanoparticles (JCPDS file no. 861157) are identified to be present in YCTG, but two main diffraction peaks for (101) and (200) planes merged with the intense peaks of CdS at 2θ = 25.3 and 48.0, respectively. This may be due to their presence in very low quantities, compared with CdS. In the XRD pattern of RCTG and TG, the anchored TiO2 nanoparticles are in the rutile phase with the tetragonal crystal system (JCPDS file no. 860147), and the XRD patterns of the CdS present in RCTG are in good agreement with the standard
Results and discussion Graphene oxide (GO) is known to consist of various oxygen functionalities such as hydroxyl, epoxy, carboxyl, etc., which
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Fig. 1
Digital images of (a) YCTG, (b) RCTG, (c) CG, and (d) TG.
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Fig. 3 Fig. 2
FESEM images of (a) YCTG, (b) RCTG, (c) CG, and (d) TG.
Powder XRD pattern of (a) YCTG, (b) RCTG, (c) CG, and (d) TG.
JCPDS No. 751546, which indicates their cubic crystal system. The main XRD peak at 2θ = 10.12 for GO (Fig. S2†) vanishes completely during the production of the aforementioned nanocomposites, which rationalizes the reduction of GO under our synthesis protocol. In YCTG and CG, the XRD pattern for rGO could not be recognized as the peak position for rGO and crystalline yellow CdS appearing in the same domain. Highly crystalline yellow CdS peaks are so intense that these peaks mask the expected rGO signatures. In the RCTG and TG cases, the peak corresponding to rGO has been easily detected as the peak positions for synthesized rutile TiO2 and red CdS, appearing in distinctly different domains that make distinctive rGO peaks. Herein, it is interesting to note that the pH of the starting solution plays an important role for generating CdS with varying colors and crystal structures.17,18 Generally, phase transformation of TiO2 nanocrystals from rutile to anatase requires high temperature,14 but in our case, we have easily obtained rutile or anatase TiO2 by simple pH control of the titanium peroxo solution. Thus, pH variation evolves two distinctly different ternary composites bearing not only anatase and rutile TiO2 nanostructures from exactly the same metal salt concentrations in solutions, but also yellow and red CdS nanocrystals. In order to understand the morphologies of various as-synthesized nanocomposites, we have performed FESEM analysis (Fig. 3). In YCTG, the reduced graphene oxide sheets are completely immobilized with the spherical CdS nanostructures, thus TiO2 nanostructures are not identified separately (Fig. 3a). Elemental area mapping analysis suggests the presence of various elements such as Ti, Cd, S, etc., and the Ti element is found in quite uniform distribution over the entire matrix (Fig. 4) but in low amounts, which is also concluded from the XRD analysis. In the EDAX analysis, Cd and S are present in quite the same atomic amount, which again supports the presence of CdS nanoparticles in the nanocomposite system. The red product, i.e. RCTG, shows flower-like TiO2
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Fig. 4 Elemental area mapping and EDAX analysis of yellow-CdS-TiO2rGO (YCTG) nanocomposite. The inset shows the atomic percentage of various elements.
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nanostructure with bigger size polyhedral CdS distributed on rGO sheets (Fig. 3b, S3, S4†). It is worth mentioning that both the size and the content of CdS particles differ in YCTG and RCTG samples. The CdS content in YCTG is ∼5%, which also bears the size ∼200–300 nm in the nanocomposite, whereas ∼2% CdS is present in RCTG with a large size of ∼2–3 µm. This variation in size and CdS content distinctively offers different photocatalytic activities, which will be discussed later. Again, spherical CdS and flower-like hierarchical TiO2 nanostructures are found to be attached on rGO sheets in CdSrGO (CG) and TiO2-rGO (TG) nanocomposites, respectively, as described in Fig. 3c, d, S5 and S6.† TEM analysis of YCTG in Fig. 5a clearly demonstrates highly decorated, wrinkled, reduced graphene oxide sheets by smaller size CdS and TiO2, whereas another view (Fig. 5b) suggests the aggregation of CdS in certain cases. The higher magnification view, i.e. HRTEM image, shows two well-resolved fringes of 0.32 nm and 0.368 nm, which correspond to (101) crystal plane of CdS and (101) the crystal plane of TiO2 nanoparticles, respectively (Fig. 5c). A selected area electron diffraction (SAED) pattern of YCTG exhibits clear diffraction spots, which originate from the crystalline nature of the nanocomposite, and the respective planes of both hexagonal CdS and anatase TiO2 are indicated in Fig. 5d. To investigate the effect of rGO for the production of visible light-active nanocomposites, we have synthesized CdS-TiO2 nanocomposites (CT) following the same procedure for YCTG, but in the absence of GO, and TEM analysis reveals its highly aggregated nature (Fig. S7d†). X-ray photoelectron spectroscopy (XPS) was further used to investigate the detailed elemental compositions of our desired yellow-colored ternary nanocomposites, i.e. YCTG. The widescan XPS spectrum in Fig. 6a clearly illustrates the peaks of C
Fig. 5 (a, b) TEM images, (c) HRTEM image and (d) SAED pattern of YCTG.
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Fig. 6 (a) Wide scan XPS spectra of YCTG, (b) narrow scan C 1s XPS spectra of GO, (c) narrow scan C 1s XPS spectra of YCTG, and (d) comparative Raman spectra of GO and YCTG.
1s, O 2p, Ti 2p, Cd 3d and S 2p. Two clear doublet peaks at 405.4 and 412.1 eV correspond to Cd 3d5/2 and Cd 3d3/2 electronic states, respectively, whereas two small peaks at around 162 and 460 eV suggest the presence of S 2p and Ti 2p, respectively, in YCTG. Moreover, the degree of graphene oxide reduction can be predicted from the C 1s spectra of GO and YCTG, as compared in Fig. 6b, c. The C 1s XPS spectrum of GO can be deconvoluted in three major contributing peaks corresponding to various functional groups such as graphitic sp2 C (∼284.9 eV), C–O (∼286.5 eV) and CvO (∼288.1 eV).3b It is very important to note that all of the oxygen-containing functional groups have experienced a decrease in intensity, as well as the peak corresponding to sp2 C appears as a primary carbon moiety during the transformation from GO to YCTG. The slight change in peak positions after GO reduction has been previously reported.19 The reduction process is also confirmed from their comparative Raman spectra as the intensity ratio of the two bands (i.e. ID/IG) clearly increases in the nanocomposite, compared with GO (Fig. 6d).3c An increase in ID/IG value indicates the generation of sp2 hybridized conjugated carbon domains during GO reduction.20 Furthermore, the ternary nanocomposite exhibits three intense peaks at 148, 291 and 591 cm−1. The peak at 148 cm−1 confirms the anatase phase of TiO2 present in the nanocomposites, and the peaks at 296 and 596 cm−1 signify the longitudinal optical (LO) phonons and its first overtone for CdS nanoparticles, respectively. The optical properties of the synthesized nanocomposites are analyzed by UV-vis diffuse reflectance spectroscopy (DRS), and the corresponding bandgap energies have been calculated from the plot of (αEp)2 versus Ep (α and Ep stands for absorption coefficient and photon energy, respectively).4b To study the effect of rGO on visible light absorption, we have compared the DRS result with its rGO-free nanocomposite, i.e. CdS-TiO2 (CT), which has been synthesized in a similar synthetic procedure but without the addition of GO (TEM and digital
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Fig. 7 (a) Comparative UV-vis diffuse reflectance absorption spectra (DRS) and (b) plots of (αEp)2 vs. photon energy (Ep) for bandgap calculation of various nanocomposites.
images of CT are shown in Fig. S7d, S8, ESI†). This CT nanocomposite shows an absorption edge at ∼550 nm with narrower bandgap energy of 2.33 eV, compared with pure CdS in the bulk state (2.40 eV) as depicted in Fig. 7. It is clearly observed that the incorporation of reduced graphene oxide results in the redshifting of absorption edges related to CdS with broader and enhanced absorption in the visible light region (500–800 nm) in both the cases of YCTG and CG. This phenomenon can be explained in terms of the hybridization of CdS nanoparticles with the carbon-based material herein, i.e. rGO, in which a possible chemical attachment of CdS on the specific sites of rGO plays an important role3a,10b to show much lower bandgap energies for YCTG (2.15 eV) and CG (2.04 eV). In YCTG, in situ formed TiO2 nanostructure inhibits the effective chemical interactions between CdS and rGO. This has a bearing with the addition of TiO2 at the first instance to GO suspension, followed by CdCl2 and GSH additions. This sequential addition results in the lowering of the bandgap energy for YCTG than that of CG. This result also supports that TiO2 nanoparticles are present at the interface of the in situ produced CdS and rGO, which has been previously concluded by other techniques such as XRD, EDAX, etc. Herein, the synthesized TiO2 nanoparticles are first deposited on rGO sheets followed by CdCl2 addition, and Cd2+ then evolves CdS nanoparticles. Thus, CdS, which covers the entire surface of the nanocomposite, and for this covering TEM and FESEM techniques fail to isolate the information of TiO2 nanostructure separately. Additionally, both TG and RCTG show the absorption peak in the UV region (∼385 nm) corresponding to TiO2 nanoparticles with an enhanced visible-light absorption property as it is hybridized with rGO component. There is a hump in the UV-vis absorption spectra of RCTG at 550 nm, which can be attributed to red colored CdS nanoparticles present in the composites. In short, the modification of semiconductors (CdS and TiO2) with rGO influences their extended lightabsorption capability, which also contributes to their visible light-driven photocatalytic activities. Organic dye molecules are highly toxic and their uncontrolled disposal from various industries into water creates a huge concern for aquatic species. Various reports are found related to their removal,3c degradation4 or even reduction21 to assure environmental abatement. In our study, the photocatalytic activities of the synthesized nanocomposites were
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Fig. 8 Time-dependent UV-vis spectra of (a) rhodamine B and (b) crystal violet degradation in the presence of YCTG nanocomposites under visible light irradiation. Comparative (c) rhodamine B and (d) crystal violet degradation efficiencies by five nanocomposites.
examined through a dye degradation process where rhodamine B (RB) and crystal violet (CV), a key component in the textile and printing industries, respectively, were used as model dye molecules for photocatalysis under visible light irradiation (λ > 420 nm). Time-dependent degradation processes of two dye molecules in the presence of various nanocomposites are analyzed through UV-vis absorption spectroscopy, and the results are described in Fig. 8a,b, S9 and S10.† It is worth mentioning that both RB and CV do not degrade under visible light irradiation without the addition of a photocatalyst (Fig. S11, ESI†). The degradation efficiencies of each photocatalyst were compared from the relative concentration change with respect to time by monitoring the absorbance peak of the respective dyes (λRB = 552 nm and λCV = 580 nm). It can be pointed out that in both the cases (i.e. RB and CV), maximum photocatalytic activity has been observed with the YCTG nanocomposite. Fig. 8c and d clearly suggest that the presence of rGO and TiO2 in a ternary nanocomposite, i.e. YCTG, exploit to show superior photocatalytic activity (85 and 90% decomposition for RB and CV, respectively), compared with two other CdS-based binary nanocomposites, i.e. CT and CG with only 60–70% and 73–75% respective degradation efficiencies at the same time scale. It is also important to note that introduction of yellow CdS into the nanocomposite definitely results in a better photocatalyst than the TiO2-based photocatalyst (i.e. TG), although red CdS in RCTG fails to improve the degradation efficiency of dyes significantly, compared with TG. Photocatalysis is a surface controlled process, i.e. a material with a high surface area will provide more surface active sites for the adsorption of dye molecules causing enhanced efficiency.6e,22a In order to correlate the photocatalytic activity of the synthesized composites with their active surface areas, we have performed N2 adsorption–desorption isotherm studies (Fig. S12†) and the specific Brunauer–Emmett–Teller (BET)
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surface areas are 77.3, 74.09, 67.3, 64.7 and 35.8 m2 g−1 for YCTG, CG, TG, RCTG and CT, respectively. In an acidic medium ( pH ∼ 1), the GO sheets are assembled together due to the protonation of the carboxylate groups, which may be the reason for the lower BET surface areas of TG and RCTG, compared with YCTG.22b The photocatalytic performances of all the materials follow the order YCTG > CG > CT > RCTG > TG, which is in quite good agreement with the BET surface area, except for CT. In CT, the presence of highly visible light active material, yellow CdS plays the most important role for photocatalysis, which was absent in both TG and RCTG. The lower photocatalytic activity of RCTG can be explained in terms of low CdS content and larger sized CdS particles in RCTG, compared with YCTG. In order to investigate the actual reason behind the poor photocatalytic activity of RCTG, we performed the same photocatalytic experiment with a catalyst dose 2.5 times higher in order to maintain the same CdS amount (i.e. 5% with respect to atomic percentage) as is present in YCTG under normal conditions. Herein, the photocatalytic performance under visible light irradiation has improved slightly after the proposition showed higher dye adsorption under darkness as expected from the higher dose of rGO content in the reaction mixture (Fig. S13, ESI†). However, the efficiency with RCTG is not attained as is true for YCTG case with the same amount of CdS content. This concludes that larger sized red CdS with lower visible light-driven photocatalytic activity slows down the dye degradation process in the case of RCTG. The photocatalytic dye-degradation process proceeds through excitation, transportation and degradation pathways. Herein, two main mechanisms can be put forward for dye degradation, one governed by dye sensitization and the other by CdS excitation.9b In the case of self-sensitized dye degradation, the photoinduced electrons from the dyes under visible light irradiation are transferred to rGO through the conduction band (CB) of CdS and TiO2, as suggested by their potential energy values.10a,b On the other hand, visible light excitation of CdS nanostructures generate holes in the valance band and electrons in the conduction band, which easily transfer to the conduction band of TiO2 due to favorable potential energy, and finally, to rGO. Now the transported electrons in both the cases react with the dissolved oxygen in water to produce a reactive oxidizing agent in the form of oxygen radical anion O2•− which is responsible for the oxidative dye degradation under visible light irradiation.23 Scheme 1 demonstrates the generation of a photoinduced charge pair from CdS and their fast electron transportation through TiO2 and rGO for the generation of the activated species O2•−. It is important to note that a dye sensitization mechanism can even be fruitful in the presence of TiO2-rGO nanocomposites under a favorable visible light source (λ > 420 nm), but herein, the catalytic activity of this composite is very low, which substantiates that the major degradation pathway proceeds through the excitation of yellow CdS. Herein, the importance of rGO in the nanocomposites can be understood from two of its fundamental properties: highly conducting and increased surface areas. The superior conduc-
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Scheme 1 Schematic illustration of the charge separation and photogenerated electron transportation in yellow CdS-TiO2-rGO ternary composite under visible light illumination.
tivity of rGO removes the photogenerated electrons rapidly, and as a result, the hole–electron recombination process, a problem to show improved photocatalytic performance for a low bandgap semiconductor such as CdS, becomes slowed in YCTG nanocomposites. Additionally, the captured dye molecules in rGO-based composites decompose quite easily by their photoinduced charge carriers as previously reported.24 Our synthesized rGO-free binary CdS-based nanocomposite (CT) fails to show comparable photocatalytic performance similar to YCTG, which concludes the essentiality of rGO to show maximum photocatalytic activity. It is worthwhile that associated TiO2 at the interface of rGO and CdS again boost up the hole–electron separation to perform YCTG as an excellent photocatalyst, although its visible light absorption capability is lower than TiO2 free binary nanocomposite, i.e. CG, as suggested from DRS analysis. In short, the synergistic effect from TiO2 and rGO suppresses the recombination probability of photoexcited charge carriers in CdS to show highest visible light-driven photocatalytic activities among the other. In RCTG, the larger sized red CdS (∼2–3 μm) are loosely bound to the rGO sheets in haphazard way and then TiO2 nanoflowers are attached to the surfaces. However, in YCTG, the smaller sized yellow CdS (∼200 nm) are uniformly deposited onto the TiO2 decorated rGO sheets. In addition, CdS content in YCTG has been observed to be much higher than in RCTG. As a result, YCTG has produced superior charge separation under visible light irradiation than that by RCTG, which has been extended by the interfacial charge transfer by the conduction band of TiO2 nanostructures and finally through highly conducting rGO in YCTG. Thus, YCTG becomes a better photocatalyst than RCTG.
Conclusions In summary, a biomolecule-mediated, one-step MHT process has been reported to construct four distinctly different
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reduced graphene oxide-based nanocomposites with visible light-active photocatalytic properties. The dual role of glutathione has been addressed to synthesize the nanocomposites in an eco-friendly process. A titanium peroxo compound from commercial bulk TiO2 was used as a source for TiO2 nanostructures, and differently controlled acidic conditions of this solution play crucial roles in the formation of two different types of CdS (cubic red and hexagonal yellow) nanoparticles under similar hydrothermal conditions. The visible lightdriven photocatalytic performance of the yellow colored ternary nanocomposite has been found to be the highest among the other binary or red colored ternary nanocomposites, which is due to the outstanding visible light harvesting property of hexagonal yellow CdS with faster photogenerated electron relay in the nanocomposite system. Herein, the combined effects from both TiO2 and rGO results in increased surface area, rapid charge transfer and lower photoinduced hole–electron recombination rates in yellow CdS-TiO2-rGO ternary nanocomposites, which show excellent photocatalytic performance from CdS sensitized dye degradation process under visible light irradiation.
Acknowledgements The authors are thankful to CSIR, New Delhi, India, and IIT Kharagpur for financial and instrumental assistance. The authors are thankful to Prof. Kumar Biradha, IIT Kharagpur, for providing the facility for the BET surface area measurements.
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Biomolecule-mediated CdS-TiO2-reduced graphene oxide ternary nanocomposites for efficient visible light-driven photocatalysis† Soumen Dutta,a Ramakrishna Sahoo,a Chaiti Ray,a Sougata Sarkar,a Jayasmita Jana,a Yuichi Negishib and Tarasankar Pal*a We report an environmentally friendly synthetic strategy to fabricate reduced graphene oxide (rGO)based ternary nanocomposites, in which glutathione (GSH) acts both as a reducing agent for graphene oxide and sulfur donor for CdS synthesis under modified hydrothermal (MHT) conditions. The report becomes interesting as pH variation evolves two distinctly different semiconducting nanocrystals of anatase/rutile TiO2 and hexagonal yellow/cubic red CdS, and their packaging makes them suitable photocatalysts for dye degradation. Herein, a titanium peroxo compound, obtained from commercial TiO2, is hydrolyzed to TiO2 nanostructures without any additives. The yellow colored CdS-TiO2-rGO (YCTG), one of the ternary photocatalysts, shows maximum efficiency compared to the corresponding red ternary CdS-TiO2-rGO or binary photocatalysts (CdS-rGO, TiO2-rGO and CdS-TiO2) for dye degradation under visible light irradiation. Systematic characterizations reveal that TiO2 presents at the interface of rGO and CdS in YCTG and thus makes a barrier that inhibits the direct interaction between rGO and CdS. This leads to a relatively higher bandgap value for CdS in YCTG (2.15 eV vs. 2.04 eV for CdS-rGO) but with better photocatalytic activity simply by diminishing the possibility of the charge-recombination process.
Received 9th September 2014, Accepted 17th October 2014
In the present situation, rGO in the YCTG also supports faster dye degradation through higher dye adsorption and rapid internal electron transfer (CdS→TiO2→rGO) in the YCTG nanocomposite. Thus, a
DOI: 10.1039/c4dt02749c
simple aqueous phase and a greener synthetic procedure results in a low-cost, highly effective visible light-responsive material for environmental application.
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Introduction During the past few years, a special attraction has been generated among the scientific communities towards a unique twodimensional carbon allotrope known as graphene due to its multifunctional activities, as well as its outstanding physical and chemical properties.1 This material encompasses a monolayer of single-atomic thick, sp2 hybridized carbon atoms with extremely high mechanical strength, remarkable thermal and electrical conductivity, and ultra-high surface area, which makes this substrate an ideal candidate for exploring its possible applications in various fields.2 Solution-based synthetic strategies for graphene proceed through graphene oxide (GO), a highly oxidized and exfoliated product of pristine graphite,
a Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India. E-mail: [email protected] b Department of Applied Chemistry, Tokyo University of Science, Tokyo-1628601, Japan † Electronic supplementary information (ESI) available: Digital images, XRD, EDAX, elemental mapping analysis, UV-vis graphs. See DOI: 10.1039/c4dt02749c
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which is then reduced to form reduced graphene oxide (rGO). The incorporation of various nanoparticles or polymer matrices into rGO creates plentiful avenues for interesting applications such as photocatalysis,3a detection of toxic ions,3b catalysis,3c removal of water pollutants,3d lithium-ion batteries,3e fuel cells,3f supercapacitors,3g and so forth. Currently, the synthesis of semiconductor nanoparticles and their application in photocatalysis4 or photovoltaic devices5 have been investigated quite extensively. Photocatalysis is a promising area of research, which surrounds the photoinduced H2 production6 as well as the degradation of various water pollutants4 in a low-cost, well-organized process. Photo irradiation leads to the generation of multiple charge carriers (i.e. holes and electrons) out of a semiconductor, and these carriers are responsible for its improved performance. However, the charge pairs have a strong tendency to recombine easily. This problem of recombination needs to be overcome in order to achieve the maximum outcome from a respective semiconductor. This process can be prevented by a support, which affords a continuous electron relay or fast electron transport from a specified semiconductor.7 Various approaches
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have been followed to inhibit the recombination process, such as transition metal doping,8a noble metal disposition,8b,c the assistance of an additional semiconductor,8d heterojunction fabrication,4b or supported by any conducting material (graphene,8e carbon nanotube8f or conducting polymer8g), etc. Cadmium sulfide (CdS) is an n-type semiconductor with bandgap value of ∼2.40 eV, which explains its intriguing visible light-driven photocatalytic activity. However, the photogenerated charge carriers recombine easily due to their narrow bandgap energy, and the holes in the valance band of CdS are highly active to oxidize CdS, which creates a problem regarding its efficiency and stability as photocatalyst.9a CdS nanoparticles are frequently embedded with other matrices, such as wide bandgap semiconductors,9b noble metals,9c support3a or sacrificial inorganic electron donors9d in order to increase their photocatalytic efficiency. In that context, the coupling of titanium dioxide (TiO2) with CdS8d,g,9e is widely studied where their interface facilitates fast electron transfer, and also the noble metal deposited CdS-TiO2 nanocomposites9f turn out to be an active photocatalyst with a lower charge recombination process. The use of expensive noble metals in the above mentioned tri-component nanocomposites explain its limitation as a potentially useful material on a large scale. Thus, the development of CdS-TiO2-based nanocomposites without any noble metal nanoparticles for highly active visible light responsive catalytic activity is highly advantageous. Reduced graphene oxide, an economically suitable material with fast electron transfer capacity and high adsorption power can be an ideal substitution for noble metal in the production of CdS-TiO2 based ternary photocatalyst. There are reports on CdS-TiO2-rGO, a tri-component nanocomposite with some interesting applications such as dye degradation,10a selective organic transformation10b,c or for the study of electron transportation10d under photoexcitation. In these cases, either commercial TiO210a,d or a multi-step synthetic procedure10b,c were used for the large scale production of the hybrid material. Additionally, the use of dimethyl sulfoxide (DMSO)10b,d as a source of sulfide under solvothermal conditions may not be an attractive choice due to its toxicity at higher concentrations.11 In the present work, we have followed a one step, biomolecule-assisted synthetic procedure for the previously mentioned tri-component nanocomposites, in which glutathione (GSH) acts as a reducing agent12 as well as a sulfur source for CdS synthesis. Moreover, in situ generated TiO2 nanoflowers from the hydrolysis of titanium peroxo compounds under modified hydrothermal (MHT) conditions claims its novel synthetic procedure for CdS-TiO2-rGO nanocomposites with efficient visible light-driven photocatalytic activity for dye degradation.
Experimental Materials All reagents used were of AR grade and used as received without any further purification. Graphite powder, glutathione
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(GSH) was purchased from Sigma-Aldrich. Potassium permanganate, sodium nitrate, hydrogen peroxide, sulfuric acid, hydrochloric acid, titanium oxide (TiO2), sodium hydroxide and ethanol were received from Merck, India. Cadmium chloride (CdCl2) was purchased from Fisher Scientific, India. Sodium hydroxide, potassium hydrogen sulfate, cation exchange resin Seralite SRC-120, rhodamine B and crystal violet were bought from SRL, India. Before the start of the experiment, all glasswares were carefully cleaned using aquaregia and subsequently rinsed with a copious amount of double distilled water and dried well prior to use. Double distilled water was used during the entire experimental process. Synthesis of red CdS-TiO2-rGO (RCTG) nanocomposite First, graphene oxide (GO) was prepared from graphite powder according to the Hummers method13 and titanium peroxo compound was produced following our reported procedure.14 36 mg of GO was then sonicated in 36 mL distilled water for 2 h for complete dispersion, followed by the addition of 60 mL of aqueous titanium peroxo solution (having optical density 1.0). The mixture was stirred for 30 min and then 54 mg of solid cadmium chloride was introduced separately into the aforementioned suspension under stirring conditions. The resultant mixture was stirred for 1 h at room temperature to make a complete exchange of both the cations (Ti4+ and Cd2+) into the GO moiety through the negative functionalities present in the GO matrix. Next, 180 mg of glutathione (GSH) was mixed into the stirred homogeneous mixture and stirred for another 15 minutes. The entire mixture was taken in a screw-capped test tube to perform the modified hydrothermal (MHT)14 process at 180 °C for 30 h. Synthesis of yellow CdS-TiO2-rGO (YCTG) nanocomposite The aqueous titanium peroxo solution was highly acidic as this solution was prepared in 5% H2SO4 solution. In this case, 5 M sodium hydroxide (NaOH) was freshly prepared and added dropwise into the aqueous titanium peroxo solution (60 mL) until the pH value of the mixture reached 5.0 with a noticeable change from orange color to almost colorless aqueous titanium peroxo solution (see Fig. S1, ESI†). This colorless solution was then used as a precursor for TiO2. For the synthesis of yellow TiO2-CdS-rGO, the same procedure has been followed as indicated in the previous RCTG synthetic protocol. Synthesis of CdS-rGO (CG) and TiO2-rGO (TG) nanocomposite For both the cases, the adopted synthetic procedures were exactly the same as indicated for red TiO2-CdS-rGO nanocomposite synthesis. For the CdS-rGO (CG) nanocomposite, 60 mL distilled water was added instead of aqueous titanium peroxo solution, whereas solid cadmium chloride was not employed for the TiO2-rGO (TG) nanocomposite. After the completion of all reactions, the products were centrifuged at 7000 rpm and washed thoroughly several times with distilled water to remove excess ions or unbound CdS or TiO2 from the nanocomposite material. Finally, the products
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were washed with ethanol and dried in air prior to their characterizations and applications.
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Instrumentation The XRD patterns of the nanocomposites were recorded on a Philips PW-1729 X-ray diffractometer (40 kV, 20 mA) with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 5°–75° at a scanning rate of 0.5° min−1, and the XRD data were analyzed by using JCPDS software. FESEM and compositional analyses of the synthesized samples were done with a supra, Carl Zeiss Pvt. Ltd. Instrument, and its attached energy dispersive X-ray microanalyzer (OXFORD ISI 300 EDAX) for a dispersed solid sample after drop-casting it on microscopic glass slides. Transmission electron microscopic (TEM) analyses of the samples were carried out on a Hitachi H-9000 NAR transmission electron microscope, operating at 100 kV. Detailed elemental analysis was done by X-ray photoelectron spectroscopy (XPS) with a VG Scientific ESCALAB MK II spectrometer (UK) equipped with a Mg Kα excitation source (1253.6 eV) and a five-channeltron detection system. Raman spectra were recorded for the solid samples by using a fiber-coupled micro-Raman spectrometer (Horiba Jobin Yvon Technology) equipped with 488 nm (2.55 eV) of 5 mW air cooled Ar+ laser as the excitation light source, a spectrometer (model TRIAX550, JY), optical microscope (Model BX 41, Olympus, Japan), and a Peltier-cooled CCD detector (Horiba Scientific, France). The optical properties of the samples were analyzed from their corresponding reflectance spectra measured in diffuse reflectance spectra (DRS) mode with a Cary model 5000 UV-vis-NIR spectrophotometer. All of the absorption spectra for dye degradation were recorded by using a SPECTRASCAN UV 2600 spectrophotometer (Chemito, India) for an aqueous solution in a 1 cm quartz cuvette. Nitrogen adsorption–desorption isotherms were measured by using a Quantachrome Autosorb automated gas adsorption system at 77 K. Photocatalytic activity study To compare the photocatalytic activities of various as-synthesized nanocomposites, crystal violet (CV) and rhodamine B (RB) were used as model dye molecules to study their visible light-driven photodegradation process. First, 10 mL aqueous dye solution (5 × 10−5 M for CV and 2.5 × 10−5 M for RB) was mixed with as-prepared 4 mg of catalyst and stirred under darkness for 30 minutes to establish the adsorption–desorption equilibrium. The entire mixture was then kept in front of a visible-light source (W bulb; 100 W) under stirring condition. The distance between the light source and reaction mixture was 35 cm. At various time intervals, 3 mL of the suspension was centrifuged, and the absorbance data was recorded for the supernatant using UV-vis spectrophotometer.
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favor the attachment of metal ions through electrostatic attraction. Herein, the sequential addition of Ti4+ and Cd2+ ions onto the GO sheets helps to grow into their respective composites one after the other under MHT conditions in the presence of glutathione (GSH). Previously, the reducing property of GSH for GO reduction has been addressed by Jeong et al.,12 which is also applicable in our case, but the additional function of GSH as a sulfur donor under MHT condition15 concludes the bi-functional activities of GSH in our reaction protocol. A titanium peroxo compound has been used as a source of TiO2 nanostructures in very limited cases.14,16 In our study, a titanium peroxo compound was prepared from commercial TiO2, which makes the process economical, and the advantage of using this precursor is its easy hydrolysis, detectable by the naked eye (orange color to colorless) under MHT to generate TiO2 nanostructures even without any hydrolyzing agent as found in the present case. As the solution of titanium peroxo compound is acidic one, it creates a problem regarding the fabrication of yellow CdS, rather it leads to a reddish CdS product17 during ternary nanocomposite synthesis. However, a titanium peroxo compound at a slightly higher pH medium (∼5.0, maintained by dropwise addition of aqueous NaOH solution) confers our desired product, yellow CdS deposited on TiO2 attached rGO sheets. Thus, we produced four possible rGO based nanocomposites: (i) yellow CdS-TiO2-rGO (YCTG) at higher pH, (ii) red CdS-TiO2-rGO (RCTG) at lower pH, (iii) CdS-rGO (CG), and (iv) TiO2-rGO (TG) by using bi-functional glutathione under MHT conditions. Fig. 1 represents the digital images of various synthesized rGO based nanocomposites (also see Fig. S1, ESI† for the digital images of GO dispersion and titanium peroxo compound solutions). To investigate the phase purity of our as-synthesized samples, we performed the XRD analysis. Fig. 2 describes the typical XRD patterns of various nanocomposites, i.e. YCTG, RCTG, CG and TG. For both YCTG and CG, the diffraction patterns correspond to the CdS match with the standard JCPDS file no. 772306, which suggests the hexagonal crystal systems for CdS and the crystal planes are indicated in Fig. 2a and c. Anatase TiO2 nanoparticles (JCPDS file no. 861157) are identified to be present in YCTG, but two main diffraction peaks for (101) and (200) planes merged with the intense peaks of CdS at 2θ = 25.3 and 48.0, respectively. This may be due to their presence in very low quantities, compared with CdS. In the XRD pattern of RCTG and TG, the anchored TiO2 nanoparticles are in the rutile phase with the tetragonal crystal system (JCPDS file no. 860147), and the XRD patterns of the CdS present in RCTG are in good agreement with the standard
Results and discussion Graphene oxide (GO) is known to consist of various oxygen functionalities such as hydroxyl, epoxy, carboxyl, etc., which
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Fig. 1
Digital images of (a) YCTG, (b) RCTG, (c) CG, and (d) TG.
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Fig. 3 Fig. 2
FESEM images of (a) YCTG, (b) RCTG, (c) CG, and (d) TG.
Powder XRD pattern of (a) YCTG, (b) RCTG, (c) CG, and (d) TG.
JCPDS No. 751546, which indicates their cubic crystal system. The main XRD peak at 2θ = 10.12 for GO (Fig. S2†) vanishes completely during the production of the aforementioned nanocomposites, which rationalizes the reduction of GO under our synthesis protocol. In YCTG and CG, the XRD pattern for rGO could not be recognized as the peak position for rGO and crystalline yellow CdS appearing in the same domain. Highly crystalline yellow CdS peaks are so intense that these peaks mask the expected rGO signatures. In the RCTG and TG cases, the peak corresponding to rGO has been easily detected as the peak positions for synthesized rutile TiO2 and red CdS, appearing in distinctly different domains that make distinctive rGO peaks. Herein, it is interesting to note that the pH of the starting solution plays an important role for generating CdS with varying colors and crystal structures.17,18 Generally, phase transformation of TiO2 nanocrystals from rutile to anatase requires high temperature,14 but in our case, we have easily obtained rutile or anatase TiO2 by simple pH control of the titanium peroxo solution. Thus, pH variation evolves two distinctly different ternary composites bearing not only anatase and rutile TiO2 nanostructures from exactly the same metal salt concentrations in solutions, but also yellow and red CdS nanocrystals. In order to understand the morphologies of various as-synthesized nanocomposites, we have performed FESEM analysis (Fig. 3). In YCTG, the reduced graphene oxide sheets are completely immobilized with the spherical CdS nanostructures, thus TiO2 nanostructures are not identified separately (Fig. 3a). Elemental area mapping analysis suggests the presence of various elements such as Ti, Cd, S, etc., and the Ti element is found in quite uniform distribution over the entire matrix (Fig. 4) but in low amounts, which is also concluded from the XRD analysis. In the EDAX analysis, Cd and S are present in quite the same atomic amount, which again supports the presence of CdS nanoparticles in the nanocomposite system. The red product, i.e. RCTG, shows flower-like TiO2
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Fig. 4 Elemental area mapping and EDAX analysis of yellow-CdS-TiO2rGO (YCTG) nanocomposite. The inset shows the atomic percentage of various elements.
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nanostructure with bigger size polyhedral CdS distributed on rGO sheets (Fig. 3b, S3, S4†). It is worth mentioning that both the size and the content of CdS particles differ in YCTG and RCTG samples. The CdS content in YCTG is ∼5%, which also bears the size ∼200–300 nm in the nanocomposite, whereas ∼2% CdS is present in RCTG with a large size of ∼2–3 µm. This variation in size and CdS content distinctively offers different photocatalytic activities, which will be discussed later. Again, spherical CdS and flower-like hierarchical TiO2 nanostructures are found to be attached on rGO sheets in CdSrGO (CG) and TiO2-rGO (TG) nanocomposites, respectively, as described in Fig. 3c, d, S5 and S6.† TEM analysis of YCTG in Fig. 5a clearly demonstrates highly decorated, wrinkled, reduced graphene oxide sheets by smaller size CdS and TiO2, whereas another view (Fig. 5b) suggests the aggregation of CdS in certain cases. The higher magnification view, i.e. HRTEM image, shows two well-resolved fringes of 0.32 nm and 0.368 nm, which correspond to (101) crystal plane of CdS and (101) the crystal plane of TiO2 nanoparticles, respectively (Fig. 5c). A selected area electron diffraction (SAED) pattern of YCTG exhibits clear diffraction spots, which originate from the crystalline nature of the nanocomposite, and the respective planes of both hexagonal CdS and anatase TiO2 are indicated in Fig. 5d. To investigate the effect of rGO for the production of visible light-active nanocomposites, we have synthesized CdS-TiO2 nanocomposites (CT) following the same procedure for YCTG, but in the absence of GO, and TEM analysis reveals its highly aggregated nature (Fig. S7d†). X-ray photoelectron spectroscopy (XPS) was further used to investigate the detailed elemental compositions of our desired yellow-colored ternary nanocomposites, i.e. YCTG. The widescan XPS spectrum in Fig. 6a clearly illustrates the peaks of C
Fig. 5 (a, b) TEM images, (c) HRTEM image and (d) SAED pattern of YCTG.
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Fig. 6 (a) Wide scan XPS spectra of YCTG, (b) narrow scan C 1s XPS spectra of GO, (c) narrow scan C 1s XPS spectra of YCTG, and (d) comparative Raman spectra of GO and YCTG.
1s, O 2p, Ti 2p, Cd 3d and S 2p. Two clear doublet peaks at 405.4 and 412.1 eV correspond to Cd 3d5/2 and Cd 3d3/2 electronic states, respectively, whereas two small peaks at around 162 and 460 eV suggest the presence of S 2p and Ti 2p, respectively, in YCTG. Moreover, the degree of graphene oxide reduction can be predicted from the C 1s spectra of GO and YCTG, as compared in Fig. 6b, c. The C 1s XPS spectrum of GO can be deconvoluted in three major contributing peaks corresponding to various functional groups such as graphitic sp2 C (∼284.9 eV), C–O (∼286.5 eV) and CvO (∼288.1 eV).3b It is very important to note that all of the oxygen-containing functional groups have experienced a decrease in intensity, as well as the peak corresponding to sp2 C appears as a primary carbon moiety during the transformation from GO to YCTG. The slight change in peak positions after GO reduction has been previously reported.19 The reduction process is also confirmed from their comparative Raman spectra as the intensity ratio of the two bands (i.e. ID/IG) clearly increases in the nanocomposite, compared with GO (Fig. 6d).3c An increase in ID/IG value indicates the generation of sp2 hybridized conjugated carbon domains during GO reduction.20 Furthermore, the ternary nanocomposite exhibits three intense peaks at 148, 291 and 591 cm−1. The peak at 148 cm−1 confirms the anatase phase of TiO2 present in the nanocomposites, and the peaks at 296 and 596 cm−1 signify the longitudinal optical (LO) phonons and its first overtone for CdS nanoparticles, respectively. The optical properties of the synthesized nanocomposites are analyzed by UV-vis diffuse reflectance spectroscopy (DRS), and the corresponding bandgap energies have been calculated from the plot of (αEp)2 versus Ep (α and Ep stands for absorption coefficient and photon energy, respectively).4b To study the effect of rGO on visible light absorption, we have compared the DRS result with its rGO-free nanocomposite, i.e. CdS-TiO2 (CT), which has been synthesized in a similar synthetic procedure but without the addition of GO (TEM and digital
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Fig. 7 (a) Comparative UV-vis diffuse reflectance absorption spectra (DRS) and (b) plots of (αEp)2 vs. photon energy (Ep) for bandgap calculation of various nanocomposites.
images of CT are shown in Fig. S7d, S8, ESI†). This CT nanocomposite shows an absorption edge at ∼550 nm with narrower bandgap energy of 2.33 eV, compared with pure CdS in the bulk state (2.40 eV) as depicted in Fig. 7. It is clearly observed that the incorporation of reduced graphene oxide results in the redshifting of absorption edges related to CdS with broader and enhanced absorption in the visible light region (500–800 nm) in both the cases of YCTG and CG. This phenomenon can be explained in terms of the hybridization of CdS nanoparticles with the carbon-based material herein, i.e. rGO, in which a possible chemical attachment of CdS on the specific sites of rGO plays an important role3a,10b to show much lower bandgap energies for YCTG (2.15 eV) and CG (2.04 eV). In YCTG, in situ formed TiO2 nanostructure inhibits the effective chemical interactions between CdS and rGO. This has a bearing with the addition of TiO2 at the first instance to GO suspension, followed by CdCl2 and GSH additions. This sequential addition results in the lowering of the bandgap energy for YCTG than that of CG. This result also supports that TiO2 nanoparticles are present at the interface of the in situ produced CdS and rGO, which has been previously concluded by other techniques such as XRD, EDAX, etc. Herein, the synthesized TiO2 nanoparticles are first deposited on rGO sheets followed by CdCl2 addition, and Cd2+ then evolves CdS nanoparticles. Thus, CdS, which covers the entire surface of the nanocomposite, and for this covering TEM and FESEM techniques fail to isolate the information of TiO2 nanostructure separately. Additionally, both TG and RCTG show the absorption peak in the UV region (∼385 nm) corresponding to TiO2 nanoparticles with an enhanced visible-light absorption property as it is hybridized with rGO component. There is a hump in the UV-vis absorption spectra of RCTG at 550 nm, which can be attributed to red colored CdS nanoparticles present in the composites. In short, the modification of semiconductors (CdS and TiO2) with rGO influences their extended lightabsorption capability, which also contributes to their visible light-driven photocatalytic activities. Organic dye molecules are highly toxic and their uncontrolled disposal from various industries into water creates a huge concern for aquatic species. Various reports are found related to their removal,3c degradation4 or even reduction21 to assure environmental abatement. In our study, the photocatalytic activities of the synthesized nanocomposites were
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Fig. 8 Time-dependent UV-vis spectra of (a) rhodamine B and (b) crystal violet degradation in the presence of YCTG nanocomposites under visible light irradiation. Comparative (c) rhodamine B and (d) crystal violet degradation efficiencies by five nanocomposites.
examined through a dye degradation process where rhodamine B (RB) and crystal violet (CV), a key component in the textile and printing industries, respectively, were used as model dye molecules for photocatalysis under visible light irradiation (λ > 420 nm). Time-dependent degradation processes of two dye molecules in the presence of various nanocomposites are analyzed through UV-vis absorption spectroscopy, and the results are described in Fig. 8a,b, S9 and S10.† It is worth mentioning that both RB and CV do not degrade under visible light irradiation without the addition of a photocatalyst (Fig. S11, ESI†). The degradation efficiencies of each photocatalyst were compared from the relative concentration change with respect to time by monitoring the absorbance peak of the respective dyes (λRB = 552 nm and λCV = 580 nm). It can be pointed out that in both the cases (i.e. RB and CV), maximum photocatalytic activity has been observed with the YCTG nanocomposite. Fig. 8c and d clearly suggest that the presence of rGO and TiO2 in a ternary nanocomposite, i.e. YCTG, exploit to show superior photocatalytic activity (85 and 90% decomposition for RB and CV, respectively), compared with two other CdS-based binary nanocomposites, i.e. CT and CG with only 60–70% and 73–75% respective degradation efficiencies at the same time scale. It is also important to note that introduction of yellow CdS into the nanocomposite definitely results in a better photocatalyst than the TiO2-based photocatalyst (i.e. TG), although red CdS in RCTG fails to improve the degradation efficiency of dyes significantly, compared with TG. Photocatalysis is a surface controlled process, i.e. a material with a high surface area will provide more surface active sites for the adsorption of dye molecules causing enhanced efficiency.6e,22a In order to correlate the photocatalytic activity of the synthesized composites with their active surface areas, we have performed N2 adsorption–desorption isotherm studies (Fig. S12†) and the specific Brunauer–Emmett–Teller (BET)
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surface areas are 77.3, 74.09, 67.3, 64.7 and 35.8 m2 g−1 for YCTG, CG, TG, RCTG and CT, respectively. In an acidic medium ( pH ∼ 1), the GO sheets are assembled together due to the protonation of the carboxylate groups, which may be the reason for the lower BET surface areas of TG and RCTG, compared with YCTG.22b The photocatalytic performances of all the materials follow the order YCTG > CG > CT > RCTG > TG, which is in quite good agreement with the BET surface area, except for CT. In CT, the presence of highly visible light active material, yellow CdS plays the most important role for photocatalysis, which was absent in both TG and RCTG. The lower photocatalytic activity of RCTG can be explained in terms of low CdS content and larger sized CdS particles in RCTG, compared with YCTG. In order to investigate the actual reason behind the poor photocatalytic activity of RCTG, we performed the same photocatalytic experiment with a catalyst dose 2.5 times higher in order to maintain the same CdS amount (i.e. 5% with respect to atomic percentage) as is present in YCTG under normal conditions. Herein, the photocatalytic performance under visible light irradiation has improved slightly after the proposition showed higher dye adsorption under darkness as expected from the higher dose of rGO content in the reaction mixture (Fig. S13, ESI†). However, the efficiency with RCTG is not attained as is true for YCTG case with the same amount of CdS content. This concludes that larger sized red CdS with lower visible light-driven photocatalytic activity slows down the dye degradation process in the case of RCTG. The photocatalytic dye-degradation process proceeds through excitation, transportation and degradation pathways. Herein, two main mechanisms can be put forward for dye degradation, one governed by dye sensitization and the other by CdS excitation.9b In the case of self-sensitized dye degradation, the photoinduced electrons from the dyes under visible light irradiation are transferred to rGO through the conduction band (CB) of CdS and TiO2, as suggested by their potential energy values.10a,b On the other hand, visible light excitation of CdS nanostructures generate holes in the valance band and electrons in the conduction band, which easily transfer to the conduction band of TiO2 due to favorable potential energy, and finally, to rGO. Now the transported electrons in both the cases react with the dissolved oxygen in water to produce a reactive oxidizing agent in the form of oxygen radical anion O2•− which is responsible for the oxidative dye degradation under visible light irradiation.23 Scheme 1 demonstrates the generation of a photoinduced charge pair from CdS and their fast electron transportation through TiO2 and rGO for the generation of the activated species O2•−. It is important to note that a dye sensitization mechanism can even be fruitful in the presence of TiO2-rGO nanocomposites under a favorable visible light source (λ > 420 nm), but herein, the catalytic activity of this composite is very low, which substantiates that the major degradation pathway proceeds through the excitation of yellow CdS. Herein, the importance of rGO in the nanocomposites can be understood from two of its fundamental properties: highly conducting and increased surface areas. The superior conduc-
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Scheme 1 Schematic illustration of the charge separation and photogenerated electron transportation in yellow CdS-TiO2-rGO ternary composite under visible light illumination.
tivity of rGO removes the photogenerated electrons rapidly, and as a result, the hole–electron recombination process, a problem to show improved photocatalytic performance for a low bandgap semiconductor such as CdS, becomes slowed in YCTG nanocomposites. Additionally, the captured dye molecules in rGO-based composites decompose quite easily by their photoinduced charge carriers as previously reported.24 Our synthesized rGO-free binary CdS-based nanocomposite (CT) fails to show comparable photocatalytic performance similar to YCTG, which concludes the essentiality of rGO to show maximum photocatalytic activity. It is worthwhile that associated TiO2 at the interface of rGO and CdS again boost up the hole–electron separation to perform YCTG as an excellent photocatalyst, although its visible light absorption capability is lower than TiO2 free binary nanocomposite, i.e. CG, as suggested from DRS analysis. In short, the synergistic effect from TiO2 and rGO suppresses the recombination probability of photoexcited charge carriers in CdS to show highest visible light-driven photocatalytic activities among the other. In RCTG, the larger sized red CdS (∼2–3 μm) are loosely bound to the rGO sheets in haphazard way and then TiO2 nanoflowers are attached to the surfaces. However, in YCTG, the smaller sized yellow CdS (∼200 nm) are uniformly deposited onto the TiO2 decorated rGO sheets. In addition, CdS content in YCTG has been observed to be much higher than in RCTG. As a result, YCTG has produced superior charge separation under visible light irradiation than that by RCTG, which has been extended by the interfacial charge transfer by the conduction band of TiO2 nanostructures and finally through highly conducting rGO in YCTG. Thus, YCTG becomes a better photocatalyst than RCTG.
Conclusions In summary, a biomolecule-mediated, one-step MHT process has been reported to construct four distinctly different
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reduced graphene oxide-based nanocomposites with visible light-active photocatalytic properties. The dual role of glutathione has been addressed to synthesize the nanocomposites in an eco-friendly process. A titanium peroxo compound from commercial bulk TiO2 was used as a source for TiO2 nanostructures, and differently controlled acidic conditions of this solution play crucial roles in the formation of two different types of CdS (cubic red and hexagonal yellow) nanoparticles under similar hydrothermal conditions. The visible lightdriven photocatalytic performance of the yellow colored ternary nanocomposite has been found to be the highest among the other binary or red colored ternary nanocomposites, which is due to the outstanding visible light harvesting property of hexagonal yellow CdS with faster photogenerated electron relay in the nanocomposite system. Herein, the combined effects from both TiO2 and rGO results in increased surface area, rapid charge transfer and lower photoinduced hole–electron recombination rates in yellow CdS-TiO2-rGO ternary nanocomposites, which show excellent photocatalytic performance from CdS sensitized dye degradation process under visible light irradiation.
Acknowledgements The authors are thankful to CSIR, New Delhi, India, and IIT Kharagpur for financial and instrumental assistance. The authors are thankful to Prof. Kumar Biradha, IIT Kharagpur, for providing the facility for the BET surface area measurements.
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