A Mild One‐Step Process from Graphene Oxide and Cd2+ to a Graphene–CdSe Quantum Dot Nanocomposite with Enhanced Photoelectric Properties

Good connections: A graphene–CdSe quantum dot (QD) nanocomposite is prepared through a one-step hydrothermal method using graphene oxide (GO), Cd(CH3COO)2, Na2SeSO3, and aminoethanethiol (AET). The bifunctional AET acts not only as a covalent linker but also as a reductant to transform GO into graphene. The photoactive graphene–QD nanocomposite exhibits a significantly higher photocurrent compared to the QDs, GO or the graphene substrate under illumination. Graphene, the promising atom-thin two-dimensional (2D) material, has reached enormous attention in recent years due to its specific electrical and chemical properties.1 Up to date, various graphene-based nanocomposite materials with wide applications in hydrogen-storage media, supercapacitors, Li-ions batteries, photocatalysis and photoluminescent devices2 have been reported. Most of these materials were synthesized from graphene oxide (GO), which can be prepared in large quantities from natural graphite powder by strong oxidizing agents.3 The oxidized graphite can be easily exfoliated into single-layer GO sheets, with a plentiful oxygen-containing groups,4 epoxy group and hydroxyl on the basal planes and carboxyl along the edges.5 According to previous reports, these functional groups can react with alkylamine to form “alkylated” graphene or graphene oxide. This functionalization also enhances the stability of layered graphene material.6 Semiconductor quantum dots (QDs), as known, have unique properties, such as quantum effect, size-related light absorption and high quantum yield,7 which make them promising candidates in photovoltaic and photochemical applications. Considering that graphene is a zero-band-gap material8 with a work function of −4.42 V9 (the band gap should be 10–50 meV when the reduction is incomplete10), it can be utilized as an outstanding electron acceptor and a highly conductive substrate. Thus, the connection of QDs and graphene is of great benefit to the separation of the photogenerated charges. It has been reported that QDs can be attached onto graphene or graphene-oxide sheets by direct deposition2f and chemical-linking methods. Cadmium chalcogenide QDs have been deposited directly onto graphene at a relative high temperature (180–200 °C).11 In contrast, the chemical-linking method can be carried out under milder conditions. The as-prepared QDs were linked onto the reduced graphene oxide by π–π staking12 or covalently bonding effects.13 However, multiple steps (preparation of the QDs, reduction of graphene oxide and the attachment process, etc.) were required and the coverage and linking stability of the graphene-QD nanocomposite are expected to be improved. Bifunctional molecules containing -COOH and -SH groups (e.g. thioglycolic acid) have been widely used as linkers to connect QDs and TiO2 in quantum-dot-sensitized solar cells.14 Recently, CdSe QDs have been reported to be linked onto acyl-chloride pre-functionalized single-walled carbon nanotube15 and graphene oxide13 through the bifunctional molecules. Since graphene oxide can be directly functionalized by amine,6 a kind of molecule which contains both amine and thiol groups can be used to potentially link GO (with the amine group) and the QDs (with the thiol group). According to the above concept, bifunctional aminoethanethiol (AET) is chosen as a linker to connect GO and the CdSe QDs herein. The amine group of AET can react with the epoxy group of GO though a nucleophilic reaction;13 moreover, its thiol group can also interact strongly with the CdSe QDs. The fabrication process illustrated in Figure 1 a was carried out in an aqueous solution under mild conditions. In this one-step hydrothermal reaction, the AET acts not only as a bifunctional linker but also as the QD stabilizer and GO reducing agent. Hence, the process starting from GO, Cd(CH2COOH)2 and Na2SeSO3 results in a graphene–QD nanocomposite (abbreviated G–QD), which is constructed with homogeneous and well-crystallized zinc blende CdSe QDs and single-layer graphene. The covalent connection accelerates the photoinduced carrier separation in the CdSe QDs and therefore leads to a photocurrent enhancement for G–QD. a) Scheme of the one-step direct synthesis of the graphene–QD nanocomposite. b) Photographs of water dispersed GO (A), CdSe QDs (B), G-QD (C), a mixture of GO and QDs (D), and G–AET (E). Graphite oxide synthesized using the modified Hummers method was exfoliated by sonication to gain an aqueous GO solution (see Figure 1 b-A). The AFM image (Figure 2 a) shows that the mean thickness of the GO sheet is 1.05 nm, corresponding to the reference single-layer graphene oxide. When the hydrothermal reaction was finished, a homogeneous dark olive suspension was obtained, as shown in Figure 1 b-C. In contrast, the mixture of GO and CdSe QDs (Figure 1 b-D, yellow–brown) exhibits an immiscible appearance. The difference between GO–CdSe QD mixture (Figure 1 b-D) and G–QD nanocomposite (Figure 1 b-C) implies the reduction of GO and the combination of graphene and CdSe QDs during the hydrothermal process. Figure 2 b shows the TEM image of G–QD after 30 min of sonication; the surface of graphene sheet is rough with a large number of QDs on it. The HRTEM image (Figure 2 c) shows clear lattice fringes of the CdSe QDs with a size of 3–4 nm, suggesting the stabilizer role of AET. These QDs can be confirmed as zinc blende CdSe through powder X-ray diffraction (XRD) measurements (Figure 2 d, JCPDS NO. 65–2891), similar to the single CdSe QDs fabricated in the absence of GO (see Figure 1 b-B), in both structure and size (judged by the TEM and HRTEM images shown in Figure S1, Supporting Information). The UV/Vis absorption spectra of GO, QDs and G–QD (Figure S2, Supporting Information) also support the same size of CdSe QDs with or without graphene, according to the equivalent peak position of QDs and G–QD. In addition, no XRD peaks at around 10.4°, which refers to a symbol of GO (002) plane, can be identified for the G–QD sample (Figure 2 d), indicating the reduction of GO in G–QD. Further AFM measurements (see Figure S3 of the Supporting Information) show that the average thickness of a G–QD nanosheet is 7.37 nm. This implies that the graphene nanosheet (1.05 nm in thickness) was covered uniformly by the QDs (3-4 nm in size) on bilateral sides, without serious accumulation. a) AFM image of GO nanosheets with a height profile inset. b) TEM image of a graphene–QD nanosheet. c) HRTEM image of the G–QD nanocomposite in (b). d) XRD patterns of GO, QDs and G–QD powders; the vertical lines are the standard zinc blende CdSe peaks. The covalent connection of CdSe QDs and GO through AET linker can be confirmed by the FTIR and XPS spectra, as shown in Figures 3 and 4. The obvious decrease of the band at 1057 cm−1 (the epoxide band of GO, asymmetric stretching vibration of COC bond) in the FTIR spectrum of G–QD clearly illustrates the ring-opening amination reaction of epoxy groups. The appearance of bands at 1114, 1581 and 2918 cm−1 (representing the vibration of the CN bond, the NH bond and the CH bond of AET, respectively) reveals the formation of an amine bond between AET and the carbon atoms on GO.16 In addition, the appearance of the CN peaks in the XPS N 1 s (399.5 eV)6a and C 1 s spectra (286.1 eV) of G–QD17 also indicates that the amino groups have reacted with the epoxy groups on the plane surface of GO. The disappearance of the -NH2 bands (820 and 732 cm−1) and the -SH band (∼2620 cm−1) for G–QD can be attributed to the connection of AET with GO and CdSe, respectively. Furthermore, in the FTIR spectrum (Figure 3) of single QDs, the strong CN (1115 cm−1), NH (1567 cm−1), -NH2 (820 cm−1 and 732 cm−1) and CH (2923 cm−1) bands are presented, while the SH (∼2620 cm−1) band is absent, supporting that the CdSe QDs are bonded by AET through the thiol groups, and left an alkylamine coat outside. FTIR curves of primary GO, single QDs and the as-synthesized G–QD nanocomposite. a) XPS N1 s spectrum of G–QD; and the XPS C1 s spectra obtained from: GO (b), the G–QD nanocomposite (c), and G–AET (d). The XPS survey spectra (Figure S4 of the Supporting Information) show a C/O ratio improvement from ∼1.8 for GO to 3.0 for G–QD, revealing the reduction of GO in the presence of AET after the hydrothermal reaction. The details of the fitting peaks for the GO and G–QD C 1s spectra are shown in Figures 4 b and 4c, and the relative area data are given in Table S1 of the Supporting Information. For G–QD, the proportion of CO (286.9 eV) is only 4.2 %, much lower than that of GO (39.2 %), while its C sp2 state (284.8 eV) reaches 77.4 %. This evidently demonstrates that the hydroxyl and the epoxy groups are mostly removed from the GO substrate. Another obvious difference between GO and G–QD in the C 1s XPS spectra is the decrease of the OCO peak (289.1 eV)18 in G–QD, which suggests the removal of the carboxyl on GO, correlating well to the FTIR spectra. The FTIR-absorption-band decrease of carboxyl at 1727 and 1230 cm−1 for G–QD, compared to GO, indicates the loss of the carboxyl group. Some control experiments were further carried out to elucidate the reduction of GO to graphene. When the hydrothermal reaction was done only for the GO aqueous solution, no obvious color changes were observed. This fact implies that the reduction of GO does not occur in the absence of AET under the present reaction conditions. However, a great deal of black precipitation was obtained after the hydrothermal reaction of GO and AET in aqueous solution at 80 °C for 10 h (signed as G–AET), as shown in Figure 1 b-E, revealing the reduction of GO. The C 1s XPS spectrum of G-AET in Figure 4 d shows similar characteristics to G–QD (Figure 4 c and Table S1 of the Supporting Information). The ratio variation of the relative intensity of D (edges and disorders in graphene) and G (sp2 carbon atoms) bands in the Raman spectra is usually considered as a signal to identify the reduction of GO. Herein, the Raman spectrum of GQD (see Figure S5 of the Supporting Information) shows an enlarged D/G ratio compared with GO, similar to the result obtained in the hydrazine reduction method.5 This fact also implies the reduction reaction during the hydrothermal process in the presence of AET, and the electronic interaction between CdSe QDs and graphene sheets.19 Based on the above analysis of GO, G–QD and G–AET, it can be proposed that the reduction of GO to graphene should be attributed to the free AET in the solution due to its potential of releasing the reductant H2S. As shown in Figure 5, the CdSe QD sample prepared without GO has a strong emission peak at about 540 nm. In contrast, the emission is hard to be identified for the G–QD nanocomposite. This photoluminescence (PL) quenching can be ascribed to the entire separation of the photogenerated electrons and holes in the G–QDs, owing to the firm covalent connection of CdSe QDs and the graphene sheets. Accordingly, enhanced photoelectric properties of G–QD can be expected. Figure 6 shows the photocurrent density–time curves of reduced graphene oxide (RGO), GO, CdSe QDs and G–QD thin films during the light-on and light-off processes. It clearly shows that the photocurrent density of G–QD (50 μA cm−2) is several times higher than the sum photocurrent density of single QDs (13 μA cm−2) and RGO (2.5 μA cm−2 or GO 0.6 μA cm−2), which is also much better than the previous report (35 μA cm−2) based on a hierarchical ZnO hollow sphere–RGO nanocomposite.2f Similar to the type-II hybrid semiconductor, the energy levels of the CdSe QDs and graphene constitute a steplike structure.20 Therefore, when the G–QD film is irradiated, the photogenerated electrons in the CdSe QDs can be collected by the graphene sheets and transported to the FTO glass, while the holes can be scavenged by the redox couple in the electrolyte. The photoresponse-activity enhancement reveals that the covalently connected graphene–QD nanocomposite realizes an excellent photoelectric transformation, attributed to the efficient charge separation and electron collection of graphene. Photoluminescence spectra of single QDs and the G–QD nanocomposite under 420 nm-light irradiation. Photocurrent density–time (I–t) curves of GO, RGO and the G–QD nanocomposite obtained from photoresponse characterization. In summary, a new synthetic method for the fabrication of a graphene–CdSe QD nanocomposite using bifunctional AET was developed. The bifunctional AET plays three important roles in this process: 1) It controls the formation of the CdSe QDs and modifies the QD surface. 2) It anchors to the CdSe QDs by the thiol group and covalently attaches to epoxy groups on graphene oxide by the amino group. 3) It acts as a reducing agent to remove most of the hydroxyl and carboxyl groups from the graphene-oxide substrate, thus forming a graphene–QD nanocomposite. This graphene–CdSe QD nanocomposite with excellent photogenerated charge separation and transportation properties can be potentially used in photoelectric devices. The present one-step method for the construction of covalently connected graphene–CdSe can be easily extended to other types of graphene–QD nanocomposites, which are under investigation. Graphite oxide was prepared from natural graphite powder by the modified Hummers method.3–4 NaNO3 (0.5 g), H2SO4 (23 mL), and KMnO4 (3 g) were mixed with 0.5 g graphite in an ice bath under strong magnetic stirring. After 120 h of reaction at room temperature, water (46 mL) was added, and the container was kept at 95 °C for 1 h. Then, about 10 mL H2O2 was used to remove the unreacted KMnO4. The oxidized graphite was purified by washing with 1 % HCl and then water/alcohol (1/5, v/v) twice. 1 mg mL−1 graphite-oxide aqueous solution was exfoliated by sonication for 100 min. 10 min of centrifugation at 5000 rpm was carried out to obtain the GO solution. A solution of Na2SeSO3 (5 mL, prepared by refluxing 0.24 g Se powder and 0.5 g Na2SO3 in 15 mL water) was dropped very slowly into a pre-prepared aqueous solution containing 0.154 g AET and 0.532 g Cd(CH3COO)2⋅2H2O and 25 mL water. Then, a GO solution (10 mL, 1 mg mL−1) was injected under stirring. This mixed solution was sealed in a teflon-lined autoclave and reacted in an oven at 80 °C for 10 h. The product was washed using 50 % alcohol–water several times for purification. For comparison, the following experiments at 80 °C for 10 h were also carried out: a) CdSe QDs synthesized in the absence of GO; b) AET solution added into the GO solution; c) GO solution alone. For the preparation of the photoelectrode, the obtained G–QD aqueous suspension was evenly dropped onto the fluorine-doped SnO2 (FTO) glass and dried naturally in air at room temperature. The thicknesses of these thin films was about 1 μm. The graphene-oxide nanosheets were characterized by atomic force microscopy (AFM, Shimadzu SPM-9500 J3). The as-prepared GO, G–QD and CdSe QDs powders were characterized using an Advance X-ray diffractometer (Bruker D8, Cu Kα radiation l=0.15418 nm) and by means of transmission electron microscopy (TEM, JEM2010-HR). The UV/Vis absorption spectra, florescence emission spectra, Fourier transform infrared spectroscopy (FTIR) and Raman absorption spectra were measured with a UV–Vis–NIR spectrophotometer (Shimadzu UV-3150), a spectrofluorophotometer (Shimadzu PL-5301), an FTIR analyzer (Nicolet/Nexus 670) and a laser micro-Raman spectrometer (Renishaw inVia), respectively. The X-ray photoelectron spectroscopic (XPS) element analysis was measured using an X-ray photoelectron spectroscopy/ESCA equipment (Thermo Fisher Scientific, ESCALAB 250, Mono Al Kα source 1486.6 eV). Photoelectrochemical measurements were carried out in a solution containing 0.35 M Na2SO3 and 0.24 M Na2S under irradiation with a 150 W tungsten lamp. The photocurrent of the working electrodes was measured with an electrochemical workstation (Zahner, Zennium). A Pt wire and an Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively. This work was supported by the National Natural Science Foundation of China (20873183, U0934003), the Program for New Century Excellent Talents in University (NCET-11–0533) of the Ministry of Education, the Fundamental Research Funds for the Central Universities, and the Research Fund for the Doctoral Program of Higher Education (20100171110014). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.