Tian
Lv
a,
Likun
Pan
*a,
Xinjuan
Liu
a,
Ting
Lu
a,
Guang
Zhu
a,
Zhuo
Sun
a and
Chang Q.
Sun
b
aEngineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Department of Physics, East China Normal University, Shanghai, China. E-mail: lkpan@phy.ecnu.edu.cn; Fax: +86 21 62234321; Tel: +86 21 62234132
bSchool of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
First published on 9th December 2011
One-step synthesis of CdS–TiO2–chemically reduced graphene oxide (RGO) composites was carried out using microwave-assisted reduction of graphite oxide in CdS precursor solution with TiO2 suspension. The photocatalytic performance of CdS–TiO2–RGO composites in degradation of methyl orange was examined. Results show that the RGO addition could enhance the photocatalytic performance of CdS–TiO2 composites with maximum degradation efficiency of 99.5% under visible light irradiation as compared with the pure TiO2 (43%) and CdS–TiO2 (79.9%) composites due to the increase of specific surface area for more adsorbed MO and the reduction of electron–hole pair recombination with the introduction of RGO.
Unfortunately, the quick recombination of photo-generated charge carriers still exists in the CdS–TiO2 composite and the recombination significantly decreases its photocatalytic performance. Therefore, conductive nanomaterials with a work function lower than the conduction band of TiO2, such as carbon nanotubes (CNTs)13,14 and Pt,15–17 have been incorporated into the CdS–TiO2 composite to form hybrid materials to solve this problem. In the photocatalysis process, these nanomaterials can act as an excellent electron-acceptor/transport material to effectively facilitate the migration of photo-induced electrons and hinder the charge recombination in electron-transfer processes due to the electronic interaction between the CdS–TiO2 composite and them, which enhances the photocatalytic performance.
Recently, graphene, a rising star in the carbon family, has generated increasing interest both in fundamental science and potential applications due to its good conductivity, high electron mobility, superior chemical stability and large surface-to-volume ratio.18–24 Similar to CNTs, graphene can also act as an excellent electron-acceptor/transport material.25 Efforts have been made to combine TiO2 and graphene to obtain hybrid materials with superior photocatalytic performance.26–31 Li et al.32 reported the incorporation of chemically reduced graphene oxide (RGO) in TiO2 to form TiO2–graphene composites by visible-light photocatalytic reduction of graphite oxide (GO). However, the study on the incorporation of RGO into CdS–TiO2 composites for photocatalysis has not yet been reported. Thus, the exploration of the CdS–TiO2–RGO hybrid is necessary to gain the understanding of the individual contribution from RGO, CdS and TiO2 to form the basis for photocatalysis design and application.
In this work, we report the one-step synthesis of CdS–TiO2–RGO composites using microwave-assisted reduction of GO in CdS precursor solution with TiO2 suspension. Microwave irradiation heats the reactant to a high temperature in a short time by transferring energy selectively to microwave absorbing polar solvents. Thus it can facilitate mass production in a short time span with little energy consumption33,34 and form an intimate contact between the components.35–37 Such a microwave-assisted reaction is hardly employed to synthesize CdS–TiO2–RGO composites though it has been used successfully to fabricate RGO,36–40 CdS41–43 or CdS–TiO2,44 respectively. The photocatalytic efficiency of the as-synthesized CdS–TiO2–RGO composite in degradation of methyl orange (MO) under visible light irradiation is much improved compared with the pure TiO2 and CdS–TiO2.
Microwave-assisted synthesis of CdS–TiO2–RGO composites is a one-step process. RGO is synthesized by direct microwave assisted reduction of GO suspension. Due to the distribution of carboxylic acid groups on GO, TiO2 nanoparticles dispersed on the carbon support are found to accumulate along the wrinkles and edge of RGO.26 Meanwhile, positively charged Cd2+ ions form complexes with TiO2 nanoparticles and S2− from Na2S can move quickly to react with Cd2+ producing uniform CdS nuclei under microwave irradiation. The CdS nuclei grow, crystallize, and stabilize on TiO2 under the microwave irradiation, leading to a good contact between CdS and TiO2.37,47
The surface morphology, structure and composition of the samples were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800), high-resolution transmission electron microscopy (HRTEM, JEOL-2010), energy dispersive X-ray spectroscopy (EDS, JEM-2100) and X-ray diffraction spectroscopy (XRD, Holland PANalytical PRO PW3040/60) with Cu Kα radiation (V = 30 kV, I = 25 mA), respectively. The Brunauer–Emmett–Teller (BET) specific surface areas of the samples were evaluated on the basis of nitrogen adsorption isotherms measured at 77 K using a BELSORP-max nitrogen adsorption apparatus (Japan Inc.). All the samples were degassed at 150 °C before nitrogen adsorption measurements. The BET surface area was determined using the adsorption data in the relative pressure (P/P0) range of 0.05–0.35. The UV-vis absorption spectra and diffuse reflectance spectra with BaSO4 as a standard were recorded using a Hitachi U-3900 UV-vis spectrophotometer.
The photocatalytic performance of the as-prepared samples was evaluated through the experiment of photocatalytic degradation of MO under visible light irradiation. 150 mg CdS–TiO2–RGO powders were dispersed into 100 ml MO aqueous solution (10 mg l−1). The mixed suspensions were continuously stirred for 0.5 h in the dark to reach an adsorption–desorption equilibrium. Under the ambient conditions and stirring, the mixed suspensions were exposed to visible light irradiation produced by a 400 W metal halogen lamp with a cut off filter (λ > 400 nm). The irradiation intensity was approximately 8.7 mW cm−2 measured using a chromameter (CS-100A). At certain time intervals, 2 ml of the mixed suspensions were extracted and centrifugated to remove the photocatalyst. The degradation process was monitored by measuring the absorption of MO in the filtrate at 463 nm using a UV-vis absorption spectrometer.
![]() | ||
Fig. 1 FESEM images of (a) RGO, (b) CTG-0 and (c) CTG-4; (d) low-magnification and (e) high-magnification HRTEM images of CTG-4; (f) EDS spectrum of CTG-4. |
The XRD patterns of CdS, RGO, TiO2, CTG-0 and CTG-4 are shown in Fig. 2. The main peaks located at 26.6°, 43.8°, and 51.8° correspond to the (111), (220) and (311) planes of CdS (JCPDS#80-0019). RGO sheets exhibit a (002) diffraction peak at 26° and a (100) peak at 44.5°.49 Compared with the XRD pattern of pure TiO2 in which the peaks correspond to anatase and rutile phases of TiO2 (JCPDS#21-1272), three main peaks of CdS appear in the XRD profiles of CTG-0 and CTG-4. This finding further confirms the existence of CdS on TiO2. The signals corresponding to CdS indicate that CdS crystallites are mainly present as a cubic structure. No typical diffraction peaks of carbon species are observed, which may be due to the low amount and relatively low diffraction intensity of RGO.50
![]() | ||
Fig. 2 XRD spectra of CdS, RGO, TiO2, CTG-0 and CTG-4 with notations of a—anatase TiO2, r—rutile TiO2, c—cubic CdS. |
Fig. 3 shows the UV-vis absorption spectra of TiO2, CTG-0, CTG-1, CTG-2, CTG-3, CTG-4 and CTG-5. It can be seen that the RGO shows an absorption peak at ∼240 nm, which is generally regarded as the excitation of π-plasmon of graphitic structure.51,52 The absorption peak near 350 nm is dominated by TiO2. The addition of CdS results in the appearance of an absorption peak near 500 nm. The increase of the absorbance with the increase of RGO content in the composite, also observed by others,26–28 is suggested to be due to the absorption contribution from RGO, the increase of surface electric charge of the oxides and the modification of the fundamental process of electron–hole pair formation during irradiation.53,54 Furthermore, the diffuse reflectance spectra were also used to approximate the optical absorbance precisely through the Kubelka–Munk function:55K/S = F(R) = (1 − R)2/2R and the similar result as UV-vis absorption spectra was revealed (see Fig. S1, ESI†).
![]() | ||
Fig. 3 UV-vis absorption spectra of TiO2, CTG-0, CTG-1, CTG-2, CTG-3, CTG-4 and CTG-5. |
The photocatalytic degradation of MO under visible light irradiation was conducted to evaluate the photocatalytic performance of TiO2, CTG-0, CTG-1, CTG-2, CTG-3, CTG-4 and CTG-5, as shown in Fig. 4a. It is observed that the concentration of MO is hardly reduced under visible light irradiation in the absence of the photocatalyst. The CdS–TiO2 composite (CTG-0) exhibits much higher photocatalytic degradation efficiency (79.9%) than the pure TiO2 (43%) because the incorporation of CdS extends the absorption to the visible light range. When the RGO is introduced into the composite, the degradation efficiency is increased to 82.1%, 93.4%, and 98.5% for CTG-1, CTG-2 and CTG-3 and reaches the maximum value of 99.5% for CTG-4. The enhancement of the photocatalytic performance should be ascribed to the increase of the amount of adsorbed MO in the presence of RGO with giant two-dimensional planar structure in the composite,26 as indicated by BET measurement, and the stepwise structure of energy levels constructed in the dye–CdS–TiO2–RGO composite,31 as shown in Fig. 5. Two photoinduced degradation paths, one initiated by CdS excitation and another from the self-sensitized destruction of the dyes,56,57 are at work simultaneously under visible light: (i) considering the potential of the conduction band of TiO2 (−4.2 eV), CdS (−3.98 eV) vs. vacuum and the work function of RGO (−4.4 eV),58–61 photo-induced electrons easily transfer from photoexcited dye (MO*) to RGO via the conduction bands of CdS and TiO2, which react with adsorbed oxidants, usually O2, to produce reactive oxygen radicals (O2−). The increase of the amount of adsorbed MO can cause its more efficient self-sensitized degradation; (ii) the photoexcited electrons at the CdS conduction band also easily transfer to RGO via the TiO2 conduction band, which could efficiently separate the photo-induced electrons and hinder the charge recombination in the processes of electron-transfer, thus enhance the photocatalytic performance. However, when the RGO content is further increased above its optimum value, the photocatalytic performance deteriorates. This deterioration should be due to the following possible reasons: (i) the excessive RGO can act as a kind of recombination center instead of providing an electron pathway;31,61,62 (ii) RGO simply filters light and decreases the number of charge carriers in the composite to be photogenerated.
![]() | ||
Fig. 4 Photocatalytic degradation of MO by (a) TiO2, CTG-0, CTG-1, CTG-2, CTG-3, CTG-4 and CTG-5, (b) TiO2, TG-1, TG-2 and TG-3, (c) CdS, CG-1, CG-2 and CG-3 under visible light irradiation. |
![]() | ||
Fig. 5 Schematic diagram of energy levels of MO, CdS, TiO2 and RGO. CB, VB, and Φ are conduction band, valence band and work function. |
Because both CdS excitation and self-sensitized degradation of dyes contribute to the photocatalytic performance in this work, and self-sensitized destruction of the dyes still exists if only using wide-band gap TiO2 under visible light irradiation,63 the photocatalytic activities of TiO2–RGO in degradation of MO under visible light irradiation are investigated in order to discriminate the contribution of CdS from self-sensitized degradation of dyes. The degradation efficiencies, as shown in Fig. 4b, for TG-1, TG-2 and TG-3 are 53%, 63% and 57%, respectively. Therefore, the large contribution of CdS can be obviously observed from the enhanced degradation efficiencies of CdS–TiO2–RGO composites as compared with TiO2–RGO composites.
Fig. 4c shows the degradation results of MO under visible light irradiation in the presence of pure CdS and CdS–RGO composites. The degradation efficiencies for pure CdS, CG-1, CG-2 and CG-3 are 30%, 43%, 48% and 40%, respectively. Although CdS can absorb the visible-light radiant energy,64 CdS nanoparticles are easily aggregated in aqueous solution during the preparation process, which accordingly results in their rather lower efficiency.65 As is expected, an enhanced photocatalysis of the CdS–RGO composite is observed, which is attributed to migration of photo-induced electrons from the CdS conduction band to RGO. However, compared to CdS–TiO2–RGO composites, affinity between CdS–RGO and dye may decrease, thus affecting the photooxidation reaction rate.66
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy00452f |
This journal is © The Royal Society of Chemistry 2012 |