Synthesis and optoelectronic properties of reduced graphene oxide/InP quantum dot hybrids

Abstract

Graphene/quantum dot (QD) hybrids have recently emerged as a new class of functional materials due to the enhancement of exciton dissociation and electron transport between graphene and QDs, giving potential applications in photocatalysts, photodetectors and photovoltaics. Herein, we report a simple approach to synthesize reduced graphene oxide (rGO)–indium phosphide (InP) QD hybrids, wherein the growth of InP QDs on graphene nanosheets and the reduction of GO occur simultaneously. The electron microscopy indicated that uniform InP QDs with size of 3.4–5.5 nm were well distributed on the rGO nanosheets. The charge transfer between rGO and InP QDs was analysed using Raman and PL spectra, confirming that the hybrids enable efficient separation of the photo-induced charges. The optoelectronic properties of the rGO/InP QD hybrids have also been investigated. Our results suggest that the hybrids exhibit a sensitive photoelectric response under blue light irradiation.

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The superior electrical conductivity and single-atom-thick 2D features of graphene make it an excellent electron-transport carrier in graphene/semiconductor hybrids.^1–3 In the past few years, graphene/semiconductor hybrids have captured considerable attention owing to the enhancement of exciton dissociation and electron transport, enabling potential applications in new photocatalysts, photodetectors, dye-sensitized solar cells and H₂ production from water splitting.^4–10 An et al.⁴ found that the rGO-1%/CdS nanorod nanocomposites possessed excellent photocatalytic properties under visible light for the degradation of methyl orange with a rate constant of 1.76 × 10⁻² min⁻¹, which is about three times higher than that of pure CdS nanorods. Bera et al.⁵ demonstrated that the catalytic activity of the rGO/CdS nanosheet nanocomposites was found to be ∼2.5 times than that of pure CdS nanosheets. Cao et al.⁶ directly synthesized rGO/CdS hybrids by the solvothermal reduction of graphene oxide (GO) and demonstrated ultrafast electron transfer process from the excited CdS quantum dots (QDs) to rGO nanosheets. Lin et al.⁷ reported that the rGO/CdSe hybrids possess a dramatically enhanced photoresponse with very fast response time compared with that of the pure CdSe nanoparticle, the bare rGO, and the physically mixed rGO/CdSe NPs. Ye et al.⁹ synthesized the rGO/CdS nanocomposites using a hydrothermal method, which showed a noble metal-free photocatalytic H₂ production rate of 4.8 times as high as that of pure CdS under visible-light irradiation.

Although the abovementioned rGO/(II–VI) semiconductor hybrids have been widely used as promising visible-light optoelectronic nanomaterials, the intrinsic toxicity of cadmium requires people to develop cadmium-free alternatives which have similar optical and electrical properties. Indium phosphide (InP) with direct band gap of 1.35 eV is considered as the most promising alternative and its band gap spans the entire visible-light range. InP QDs have been used as a replacement for Cd-based QDs in optoelectronic applications, such as organic solar cells, quantum dot sensitized solar cells.^11,12 Nevertheless, there have been no reports on the synthesis and optoelectronic properties of the graphene/InP QD hybrids. The main challenge is the controlled synthesis of InP QDs and the simultaneous reduction of GO in soluble solvent.^13–15 Although the QDs can be decorated on the rGO nanosheet through functionalization of rGO,¹⁶ the direct growth of InP QDs on rGO nanosheets has considered as the best solution, which will be benefit for the exciton dissociation and electron transport between graphene and QDs.

Herein, we firstly carried out one-step synthesis of rGO/InP QD hybrids using a hot-injection method, wherein the growth of InP QDs on rGO nanosheets and the reduction of GO occur simultaneously. The photoelectric properties of the as-synthesized rGO/InP QD hybrids have also been investigated.

All the chemicals used in our experiments were of analytical reagent grade and directly used without further purification. Before the hybrid synthesis, the 6 mg GO prepared by Hummers method was added to 20 mL isopropanol and 5 mL oleylamine (OLA), the mixture was sonicated continuously until GO was homogeneously dispersed in the solution, followed by centrifugal separation, functionalized GO (fGO) was redispersed in 5 mL OLA for the hybrid synthesis. In a typical experiment, 1 mmol indium chloride (InCl₃·4H₂O), 5 mL the fGO–OLA solution were added into a three-necked flask (100 mL) at room temperature, then, the mixture was heated to 140 °C and kept for 60 min under vacuum pumping and N₂ bubbling. After the fGO–InCl₃–OLA solution was heated to 220 °C, 0.18 mL tris(dimethylamino)phosphine (P(N(CH₃)₂)₃)/0.5 mL OLA was injected and kept at for 30 min with vigorous stirring under N₂ atmosphere. After cooling, the final product were finally purified by repeated centrifugation and washed in the chloroform/ethanol solution. The as-synthesized rGO/InP QD hybrids were redispersed in chloroform for further characterization.

The optoelectronic properties were evaluated by measuring I–V curves of the hybrids on a SiO₂ substrate between two Au electrodes under and without blue light irradiation (405 nm, 80 mW cm⁻²) using Agilent 4156C.

Before the synthesis of rGO/InP QD hybrids, the functionalization of GO using OLA molecules enable GO nanosheets to disperse homogeneously in the In³⁺–OLA solution, where the GO nanosheets acted as a supporter for the growth of InP QDs and the GO was also reduced to rGO by OLA molecule.³ The morphology of the rGO/InP QD hybrids was demonstrated by transmission electron microscopy (TEM, JEM-2100, JEOL, Japan) to directly observe the structure of the InP QDs decorated rGO nanosheets. Fig. 1a shows a typical TEM image of the as-synthesized rGO/InP QD hybrids. It can be seen that the individual InP QDs are distributed uniformly on the surface of rGO nanosheets without apparent aggregation. From the high-resolution TEM (HRTEM) image in Fig. 1b, the interplanar distance of the InP QDs was measured to be about 0.29 nm, which corresponds to the (200) planes of InP QDs.¹¹ The sizes of the InP QDs were determined by measuring the diameters of the QDs on rGO nanosheets in the TEM images, as shown in Fig. 1c. It is found that the sizes are mostly distributed in the range of 3.4–5.5 nm, with an average value of 4.8 nm. Oxford INCA energy-dispersive X-ray spectroscopy (EDS) was used to examine the chemical composition of the hybrids as shown in Fig. 1d, one can see that the hybrids compose of C, In and P, corresponding to the rGO and InP QDs. The appearance of oxygen peak can be attributed to the partial reduction of the GO.

Fig. 1 Characterizations of the as-synthesized rGO/InP QD hybrids. (a) TEM and (b) HRTEM images of the rGO/InP QD hybrids. (c) Size histogram of the InP QDs. (d) EDS spectrum of the rGO/InP QD hybrids.

The X-ray photoelectron spectra (XPS) were acquired using a Japan Kratos Axis UltraDLD spectrometer with a monochromatic Al Kα source (1486.6 eV). Fig. 2a shows a typical high-resolution XPS spectrum of In 3d peaks for the rGO/InP QD hybrids. It can be seen clearly that two peaks at 444.4 eV and 451.9 eV, corresponding to In 3d_5/2 and In 3d_3/2, respectively. Compare with that of pure InP QDs,¹⁷ two peaks were shifted to 0.3–0.4 eV higher energy values, confirming the modification of the chemical surroundings of the In³⁺ ions in the rGO/InP QD hybrids compared with pure InP QDs.¹¹ The P_2p region (Fig. S1†) shows two peaks at 128.4 and 133 eV, corresponding to P from InP QDs and oxidized P species, respectively.

Fig. 2 (a) High-resolution XPS spectrum of In 3d peaks for the rGO/InP QD hybrids. (b) Raman spectra of the rGO/InP QD hybrids and rGO. (c) UV-vis absorption and (d) PL spectra of the rGO/InP QD hybrids and InP QDs.

In order to characterize the rGO/InP QD hybrids and pure rGO, Raman spectroscopy was performed using a confocal Raman microscope (inVia Reflex, RENISHAW, England) with 514.5 nm wavelength, as shown in Fig. 2b. For pure rGO, prominent D-band and G-band peaks can be observed at around 1354 cm⁻¹ and 1580 cm⁻¹, respectively. After the InP QD growth on the surface of rGO nanosheets, one can see two major Raman peaks at about 1354 cm⁻¹ (D-band) and 1590 cm⁻¹ (G-band). The blue-shifted position of the G-band peak relative to that of pure rGO is indicative of charge transfer between rGO and InP QDs,¹⁸ which is consistent with the XPS result. Two weak peaks located at about 306.6, 339 cm⁻¹ correspond to transverse optical (TO) phonon and the longitudinal optical (LO) phonon of InP QDs.^19,20

The optical properties of the rGO/InP QD hybrids were collected with a UV-vis-NIR spectrophotometer (Lambda 950, PerkinElmer, USA). Fig. 2c shows typical absorption spectra of the rGO/InP QD hybrids as well as that of pure InP QDs, both absorption spectra show a peak at about 592 nm, by comparing the absorption of pure InP QDs and rGO/InP QD hybrids, one can find that the absorption in the visible zone is relatively enhanced after the addition of rGO, which could be mainly due to the carbon doping effect.

To further characterize the optical properties and charge transfer of the rGO/InP QD hybrids, the photoluminescence (PL) spectra of the rGO/InP QD hybrids and the pure InP QDs were also using fluorescence spectrophotometer (F-4600, Hitachi, Japan), excited at 380 nm. Fig. 2d shows typical PL spectra of the hybrids and pure InP QDs. A dominant emission peak at 597 nm can be clearly observed for the pure InP QDs, while the PL intensity of the rGO/InP QD hybrids is too weak to be identified. This PL quenching behaviour has been attributed to a photo-induced charge transfer from the InP QDs to the rGO sheets,^3,16,21 confirming that the rGO nanosheets provide a novel electron-transfer path between InP QDs and graphene.

Excellent photo-induced carrier transfer from the InP QDs to the rGO nanosheets enable a remarkable photoelectric response for the photodetectors based on the rGO/InP QD hybrids. Therefore, we fabricated a prototype photodetector device by drop casting a dispersion of the rGO/InP QD hybrids in chloroform onto a SiO₂ substrate between two Au electrodes with a spacing of 1 μm, as shown in Fig. 3a and b. The comparative I–V characteristics of the device, which was illuminated with blue light irradiation (405 nm, 80 mW cm⁻²) and under dark conditions, are shown in Fig. 3c. Both current curves are nearly linear, indicating good physical contact between the hybrids and the Au electrodes. Compared with that of the device in the dark, the current is dramatically increased by over 150% when the light is turned on. This increase of current under light irradiation is always observed for different hybrid films with different thicknesses, demonstrating the excellent photosensitivity of the rGO/InP QD hybrids. The photoelectric response is much stronger than that of pure InP QDs (see Fig. S2†), further confirming high exciton dissociation and effective electron transport from InP QDs to the rGO nanosheets.

Fig. 3 (a) Schematic illustration of the hybrid-based prototype photodetector. (b) A representative SEM image the rGO/InP QD hybrids integrated between two Au electrodes. (c) Typical I–V curves with (red) and without (black) blue light irradiation. (d) The current response versus time under intermittent blue light irradiation with a voltage of 1 V.

The repeatability of the photoelectric response of the rGO/InP QD hybrids was also estimated. Fig. 3d shows the typical time-dependent photocurrent generated using blue light irradiation for four cycles. It can be seen clearly that the photocurrent increases drastically after the light is turned on, exhibiting a fast photoelectric response. Although the reduction rate of current is relative slow after turning off light, the photocurrent still exhibits good repeatability over repeated on–off cycles of light irradiation. Combined with the G-band blueshift and PL quenching of the rGO/InP QD hybrids, we believe that this drastic photoresponse can be attributed to the efficient separation of the photo-induced charges and the fast transfer of electrons from the InP QDs to the rGO nanosheets.

In summary, the rGO/InP QD hybrids have been firstly synthesized using a simple hot-injection method, in which the growth of InP QDs on graphene sheets and the reduction of GO occur simultaneously. The morphology characterizations indicate that uniform InP QDs with size of 3.4–5.5 nm were well distributed on the rGO nanosheets. The charge transfer between rGO and InP QDs was analysed using Raman and PL spectra, confirming that the hybrids enable efficient separation of the photo-induced charges for the hybrids. The optoelectronic properties of the rGO/InP QD hybrids have also been investigated. Our results show that the hybrids exhibit a sensitive photoelectric response under blue light irradiation. The current is dramatically increased by over 150% when the light is turned on.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51402190), Shanghai Natural Science Foundation (No. 13ZR1456600). We also acknowledge the analysis support from the Center for Advanced Electronic Materials and Devices and the Instrumental Analysis Center of SJTU.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22858e

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