Synthesis of reduced graphene oxide-anatase TiO₂ nanocomposite and its improved photo-induced charge transfer properties

Abstract

The construction of reduced graphene oxide or graphene oxide with semiconductor has gained more and more attention due to its unexpected optoelectronic and electronic properties. The synthesis of reduced graphene oxide (RGO) or graphene oxide-semiconductor nanocomposite with well-dispersed decorated particles is still a challenge now. Herein, we demonstrate a facile method for the synthesis of graphene oxide-amorphous TiO₂ and reduced graphene oxide-anatase TiO₂ nanocomposites with well-dispersed particles. The as-synthesized samples were characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-Vis absorption spectroscopy, Fourier transform infrared spectrometry, and thermogravimetric analysis. The photovoltaic properties of RGO-anatase TiO₂ were also compared with that of similar sized anatase TiO₂ by transient photovoltage technique, and it was interesting to find that the combination of reduced graphene oxide with anatase TiO₂ will significantly increase the photovoltaic response and retard the recombination of electron-hole pairs in the excited anatase TiO₂.

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1. Introduction

Since its discovery in 2004,¹graphene nanosheets, one-atom-thick, two-dimensional, sp²-bonded carbon materials, has gained more and more attention due to its interesting electronic and optoelectronic properties.^2,3 Up to now, many aspects of graphene-based applications, such as sensors,^4–6 transistors,⁷ nanoelectronics⁸ and super capacitors,⁹ have been carried out. To meet this urgent demand, the construction of graphene-semiconductor nanocomposite was assumed to be of significant importance for developing the next generation of photovoltaic devices, owing to its unique optoelectronic properties after the integration of graphene with semiconductor nanoparticles.^10–12 Kamat et al. found that through the interaction of TiO₂ or ZnO nanoparticles with graphene oxide (GO), the excited electrons in the semiconductor nanoparticles will transfer into GO and further partially reduce GO to form reduced graphene oxide.^11,12 However, despite the interesting optoelectronic properties, it is still difficult to synthesize reduced graphene oxide (RGO)-semiconductor nanocomposite with well-dispersed particles, which is expected to have great influence on the performance of the nanocomposite. Only a few works have now been reported on the synthesis of RGO-semiconductor nanocomposite with well-dispersed semiconductor particles.^13,14

Titanium dioxide (TiO₂) as an important wide bandgap semiconductor has been intensively studied due to its promising applications in solar energy conversion and photocatalysis.^15–17 Among the three distinct crystalline phases of TiO₂—anatase, brookite, and rutile—anatase TiO₂ has been proven to possess very high photo-activity and has been widely applied in dye-sensitized solar cells and photodegradation.^18,19 Thus, the effective combination of RGO and anatase TiO₂ nanoparticles should be vital for fabricating electronic devices and other applications.

Herein, we report a facile method to synthesize the GO-amorphous TiO₂ and RGO-anatase TiO₂ nanocomposite, on which the amorphous TiO₂ and anatase TiO₂ nanoparticles are well dispersed. In the synthesis, GO, the starting material of RGO, was first functionalized with poly(acrylic acid) (PAA); and then a very uniform amorphous TiO₂ shell was made to cover GO by controlling the hydrolysis rate of titanium isopropoxide (TTIP); finally, the GO-amorphous TiO₂ was converted into RGO-anatase TiO₂ nanocomposite via a one-step solvothermal treatment, as illustrated in Scheme 1.

Scheme 1 Procedure for synthesis of GO-amorphous TiO₂ and RGO-anatase TiO₂ nanocomposite.

The photovoltaic properties of the as-synthesized RGO-anatase TiO₂ nanocomposite were further investigated by the transient photovoltage (TPV) technique and interesting charge transfer properties were observed.

2. Experimental

All chemicals were used as received and without any further purification.

2.1 Synthesis of GO

Graphite oxide was prepared from natural graphite powder (Alfa Aesar) by a modified Hummers method.^20,21 To obtain GO sheets, GO powder was dispersed into distilled water (0.5 g L⁻¹) followed by ultrasonicating (40 KHz, 100 W) for 1 h to obtain the clear solution under ambient condition.

2.2 Functionalization of GO with PAA

In a typical synthesis, 5.72 mL acetic acid (HAc) (99.5%, Beijing Reagent Co., Ltd) and 0.221 g PAA (MW = ∼1800, Aldrich) was added into 44.28 mL GO aqueous solution (0.5 g L⁻¹); then the solution was stirred for 1 h at 60 °C to complete the functionalization.

2.3 Synthesis of GO-amorphous TiO₂ nanocomposite

The above solution was allowed to cool to room temperature. After that, under vigorous stirring, a titanium precursor composed of 1.65 mL TTIP (97%, Aldrich), 7.21 mL ethylene glycol (EG) (Sinopharm Chemical Reagent Co., Ltd.) and 1.14 mL HAc was slowly added drop by drop.

2.4 Synthesis of RGO-anatase TiO₂ nanocomposite

The GO-amorphous TiO₂ stock solution was transferred into a 100 mL PTFE-lined stainless-steel autoclave and kept under 180 °C for 8 h followed by cooling to ambient temperature naturally. The black precipitate was harvested by centrifuging at 5000 rpm, and washing with ethanol several times to remove the undecorated TiO₂ particles, unreacted reactants and residual EG. Note: the centrifuging speed is very important here to remove the undecorated particles. Finally, the products were kept under vacuum at 50 °C overnight before characterization.

2.5 Characterization

The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) imaging were performed on a TECNAI G²microscope (FEI company). X-ray diffraction patterns were recorded by a D8 ADVANCE (BRUKER) diffractometer with Cu Kα radiation (λ = 1.54056 Å) in the range of 3–80° (2θ). X-ray photoelectron spectroscopy (XPS) measurement was recorded on a Thermo VG Scientific Escalab 250 spectrometer using monochromatized Al Kα excitation. Thermogravimetric analysis (TGA) measurements were carried out on a Perkin-Elmer Pyris Diamond TGA analyzer under air atmosphere from room temperature to 800 °C at a heating rate of 10 °C min⁻¹. Infrared spectra were obtained on a VERTEX 70 Fourier transform infrared (FTIR) spectrometer (Bruker). Raman measurements were performed on a J-Y T64000 Raman spectrometer with 514.5 nm wavelength incident laser light. UV-Vis spectra were obtained on a SHIMADZU UV-3600 UV-VIS-NIR spectrophotometer. AFM was performed on a SPI3800N microscope (Seiko Instruments, Inc.).

The details of the TPV system have been described previously.^22,23 As for the measurement, the sample chamber consisted of an FTO electrode, a 10 mm thick mica spacer as electron isolator (to prevent the photo-induced electrons in the semiconductor from being directly injected to the electrode), and a platinum wire gauze electrode (with a transparency of about 50%). The construction was a sandwich-like structure of FTO electrode/sample/mica/gauze platinum electrode, and the components of the sandwich-like structure were assembled directly by sequence without further treatment. During the measurement, the gauze platinum electrode was connected to the core of a BNC cable, which provided the input signals to the oscilloscope. The samples were excited from platinum wire gauze electrode with a laser radiation pulse (wavelength of 355 nm (0.05 mJ) and pulse width of 5 ns) from a third harmonic Nd : YAG laser (Polaris II, New Wave Research). The intensity of the pulse was regulated with a neutral gray filter and determined with an EM500 single-channel joulemeter (Molectron). The TPV signals were registered with a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix).

3. Result and discussion

The attachment of semiconductor nanoparticles with GO could be realized through the carboxylic acid functional groups.¹² However, due to the residual carboxylic acid functional groups being mainly located on the edge of GO,²⁴ which may not be beneficial for the homogeneous growth of semiconductor nanoparticles on GO, we first functionalized GO with PAA, a polymer that contains plenty of carboxylic acid functional groups, to make TiO₂ homogeneously grow onto GO (Scheme 1). In addition, by comparing with the synthesis without introducing PAA, it was found that the introduction of PAA is beneficial for the fast formation of amorphous TiO₂ on GO. On the other hand, it is well known that titanium alkoxides are very sensitive to H₂O and even the moisture in the air could also cause the decomposition of the titanium precursor, which greatly hinders the homogeneous growth of TiO₂ onto GO in the aqueous solution. Thus, we introduce two reagents, EG and HAc, to co-control the hydrolysis rate of TTIP to achieve the homogeneous growth of TiO₂ shell.^25–27

It can be seen from Fig. 1a that after introducing EG and HAc into the reaction system, the TiO₂ are homogeneously grown on GO and no area that is not decorated with TiO₂ could be observed. X-ray diffraction (XRD) measurement (Fig. 2) shows that after the growth of TiO₂, the 001 reflection of GO (∼10.5°) disappears, indicating the full exfoliation of the graphite oxide, and that the TiO₂ grown on GO is in an amorphous phase. Amorphous TiO₂ normally exhibits very low photoactivity²⁸ and GO possesses lower electronic properties compared with that of RGO. These drawbacks mean that the GO-amorphous TiO₂ nanocomposite should not be useful for applications in developing photovoltaic devices. Therefore, it is necessary to convert GO-amorphous TiO₂ into RGO-anatase TiO₂ nanocomposite.

Fig. 1 TEM image of GO-amorphous TiO₂ nanocomposite (a), TEM (b, c) and HRTEM (d) images of RGO-anatase TiO₂ nanocomposite. Scale bar: 5 nm.

Fig. 2 XRD patterns of graphite oxide, GO-amorphous TiO₂ and RGO-anatase TiO₂ nanocomposite, respectively.

Since the reagent, EG, used to control the hydrolysis rate of titanium precursor also has the ability to reduce GO at high temperature²⁹ and the amorphous TiO₂ could be changed to anatase TiO₂ in the EG/HAc/H₂O reaction system,²⁵ the conversion can be easily achieved via the solvothermal treatment of the as-synthesized GO-amorphous TiO₂ stock solution. After the solvothermal treatment, it can be seen that the color of the solution changed from brown to black (photograph in Scheme 1), and this color change has previously been witnessed through the reduction of graphene oxide.¹²

From Fig. 1b, it is clearly seen that after the solvothermal treatment, unlike the GO-amorphous TiO₂ nanocomposite, relatively well dispersed TiO₂ particles on RGO were observed. Further investigation (Fig. 1c) found that there are undecorated areas of RGO. The reason for this is assumed to be the crystallization process of TiO₂. HRTEM (Fig. 1d) image shows that the TiO₂ particles are well crystallized and the sizes of the particles are 8–10 nm. The well-crystallized structure with lattice spacing of 0.35 nm corresponds to the (101) planes of the anatase TiO₂. XRD (Fig. 2) result confirms the conversion of amorphous TiO₂ to pure anatase TiO₂ (JCPDS No. 21-1272). The average crystallite size of anatase TiO₂ calculated from the Scherrer equation is about 11.1 nm, which is slightly larger than the HRTEM result. AFM image of RGO-anatase TiO₂ (see ESI†) indicates that the as-synthesized nanocomposite has a rough surface and the average thickness is 20–30 nm, which is consistent with the HRTEM result.

TGA curves (Fig. 3) show the weight loss of GO-amorphous TiO₂ and RGO-anatase TiO₂. The total weight loss of GO-amorphous TiO₂ (80.82%) could be ascribed to the loss of the adsorbed H₂O, the decomposition of PAA, the crystallization of amorphous TiO₂ into anatase TiO₂ and the oxidation of GO; while for RGO-anatase TiO₂ (27.56%), it could be attributed to the loss of the adsorbed H₂O, the decomposition of PAA and the oxidation of RGO. From the TGA results, it can be estimated that the content of TiO₂ in the RGO-anatase TiO₂ nanocomposite is 72.44%. UV-Vis results (Fig. 4) shows that after the coating of the amorphous TiO₂ onto GO, the absorbance intensity in the range of 300–350 nm is greatly increased, which is attributed to the strong absorption of amorphous TiO₂. A similar result was also observed for the RGO-anatase TiO₂ nanocomposite.

Fig. 3 TGA curves of GO-amorphous TiO₂ and RGO-anatase TiO₂.

Fig. 4 UV-Vis spectra of GO, GO-amorphous TiO₂ and RGO-anatase TiO₂ aqueous solution.

Fig. 5 is the X-ray photoelectron spectroscopy (XPS) results of graphite oxide, GO-amorphous TiO₂ and RGO-anatase TiO₂. As shown, four peaks located at 284.5, 285.4, 286.8 and 288.5 eV, observed from the C 1s deconvolution spectrum of graphite oxide, correspond to the C–C (aromatic), C–OH, C (epoxy/alkoxy)/C=O (aromatic) and carboxylic groups (O=C–O), respectively. After coating amorphous TiO₂ onto GO, it was unexpectedly found that the C 1s peak of C (epoxy/alkoxy)/C=O (aromatic) disappears even without the reduction of GO, which is assumed to be caused by the high coverage of amorphous TiO₂ on GO. Although the coating of TiO₂ interferes with identifying the reduction of GO through the decrease of C (epoxy/alkoxy)/C=O (aromatic), considerable undecorated areas of the RGO-anatase TiO₂ nanocomposite could be observed after the solvothermal treatment. Thus, the reduction of GO could also be confirmed to some extent by comparing the peak intensity of C (epoxy/alkoxy)/C=O (aromatic) of GO and RGO-anatase TiO₂ nanocomposite.

Fig. 5 C 1s XPS spectra of graphite oxide, GO-amorphous TiO₂ and RGO-anatase TiO₂.

From the FTIR spectra, as shown in Fig. 6, it can be seen that the band at 1046 cm⁻¹ (C–O stretching vibration) of GO disappears for RGO-anatase TiO₂ nanocomposite, which indicates the reduction of this group.¹⁴ For both GO-amorphous TiO₂ and RGO-anatase TiO₂ nanocomposite, the band at 1717 cm⁻¹ (C=O stretching of –COOH groups) indicates the existence of the bonded PAA.

Fig. 6 FTIR spectra of graphite oxide, GO-amorphous TiO₂ and RGO-anatase TiO₂. The solid circle and star denote the Ti–O stretching band associated with the amorphous and anatase TiO₂, respectively.

In addition, the shift of the band corresponding to the Ti–O stretching from 652 cm⁻¹ to 465 cm⁻¹ indicates the phase transformation of TiO₂ from amorphous to anatase.^25,30 After the reduction of GO to RGO, the D/G band ratio in the Raman spectra (see ESI†) was slightly increased, which is the same as the result reported by our group previously.³¹ In addition, there is no obvious difference in the Raman spectra of our RGO-anatase TiO₂ nanocomposite and the standard hydrazine reduced graphene, indicating the reduction of GO into RGO.

Owing to the superior charge transport properties of the RGO,^11,12 the combination of TiO₂ with RGO is expected to create some unique optoelectronic properties. For instance, Liu et al. observed the obvious enhancement of Li-ion insertion/extraction kinetics in TiO₂ for the RGO-TiO₂ nanocomposite.³² To further improve the performance of this novel nanocomposite, a good understanding of the photovoltaic properties of the RGO-anatase TiO₂ is necessary. However, no such works concerning the charge transfer properties of RGO-anatase TiO₂ nanocomposite have been reported to date. Here, we used a transient photovoltage (TPV) technique, a very promising method for the investigation of dynamic properties of the photo-induced charge carriers in semiconductor materials,^{22,23,26,33,34} to study the photovoltaic properties of as-synthesized RGO-anatase TiO₂ nanocomposite. The measurement was performed at atmospheric pressure and room temperature (293 K). For comparison, RGO and anatase TiO₂ nanoparticles were synthesized by a similar method except without introducing TTIP and GO during the synthesis, respectively. From the TEM and XRD results (see ESI†), it can be seen that the size and the crystal phase of anatase TiO₂ have no obvious differences compared with that of RGO-anatase TiO₂ nanocomposite.

Fig. 7 shows the TPV spectra of RGO, anatase TiO₂ and RGO-anatase TiO₂ nanocomposite. As shown, pure RGO exhibits almost no photovoltaic response (the very low response is caused by the apparatus), while for anatase TiO₂, only a very low photovoltaic response (∼0.03 mV) was observed. However, it was interesting to find that after the integration of anatase TiO₂ with RGO, its photovoltaic response significantly increased to 0.16 mV, which is ∼5 times higher than that of anatase TiO₂. This demonstrates that once the anatase TiO₂ is excited, the photo-induced electrons will transfer from anatase TiO₂ into RGO, while the holes are left in anatase TiO₂, which is proved by the positive signal of photovoltaic response. Through this electrons transfer process, the electron-hole pairs generated in the excited anatase TiO₂ could be efficiently separated, thus resulting in the improved photovoltaic response of RGO-anatase TiO₂ nanocomposite. The other characteristic of the photovoltaic response of RGO-anatase nanocomposite compared with anatase TiO₂ is that its mean life time of electron-hole pairs is prolonged from ∼10⁻⁷ s to ∼10⁻⁵ s. This demonstrates that the integration of RGO with anatase TiO₂ will greatly retard the recombination electron-hole pairs in the excited anatase TiO₂. Based on the results above, it can be concluded here that RGO plays dual roles in the RGO-anatase TiO₂ nanocomposite: one is increasing the electron-hole pair separation through the electron injection from conduction band of anatase TiO₂ into RGO, while the other is greatly retarding the recombination of electron-hole pairs in the excited anatase TiO₂.

Fig. 7 TPV spectra of RGO, anatase TiO₂ and RGO-anatase TiO₂ (a) and the dependence of photovoltage magnitude of RGO-anatase TiO₂ on the optical energy of incident laser (b).

It can also be seen from TPV spectra of anatase TiO₂ and RGO-anatase TiO₂ that the decay time of photovoltaic response of RGO-anatase TiO₂ is longer than that of anatase TiO₂, which further proves the result that the integration of RGO with anatase TiO₂ will retard the recombination electron-hole pairs. The decay time is supposed to be the time needed for the whole photo-induced charge separation and recombination process.²⁶ For anatase TiO₂, the time includes the photo-induced electron-hole pair separation, the migration of electrons and holes in and between anatase TiO₂, and the recombination of electrons and holes, while for the RGO-anatase TiO₂ nanocomposite, we suppose its decay time is composed of the separation of electron-hole pairs through the injection of electrons in anatase TiO₂ into RGO, the migration of electrons and holes in RGO and anatase TiO₂, respectively, and the recombination of the electrons and holes at the interface of RGO and anatase TiO₂. It is considered that the process of electron migration in RGO increases the time needed for the recombination of electrons and holes. In addition, we also investigate the dependence of the peak magnitude of photovoltaic response of RGO-anatase TiO₂ nanocomposite on the optical energy of the incident laser. As shown (Fig. 7 (b)), the change of peak magnitude shows a linear characteristic as the optical energy of the incident laser increases from 50 to 150 μJ, while for the higher optical energy (150∼250 μJ), its change shows a nonlinear characteristic. The result demonstrates that the RGO in the nanocomposite exhibits different behaviors for accepting electrons under different energies of the incident light.

4. Conclusions

In summary, a facile method was developed for the synthesis of GO-amorphous TiO₂ and RGO-anatase TiO₂ nanocomposite. Both amorphous and anatase TiO₂ are well dispersed on GO and RGO, respectively. Further TPV studies of RGO-anatase TiO₂ nanocomposite found that the integration of RGO with anatase TiO₂ will significantly increase the photovoltaic response and significantly prolong its mean life time of electron-hole pairs compared with that of anatase TiO₂. The information provided here is expected to be beneficial for developing next-generation RGO-semiconductor based photovoltaic devices.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 20820102037) and the 973 Project (2009CB930100 and 2010CB933600), and Dr Ping Wang is grateful for the partial financial support from China Postdoctoral Science Foundation (No. 20090461047).

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Footnote

† Electronic supplementary information (ESI) available: AFM image of RGO-anatase TiO₂, Raman spectra of GO, GO-amorphous TiO₂, RGO-anatase TiO₂ and hydrazine reduced graphene, TEM image and XRD pattern of anatase TiO₂ nanoparticles. See DOI: 10.1039/c0nr00714e

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