Article pubs.acs.org/JPCC
Interaction between Quantum Dots of CdTe and Reduced Graphene Oxide: Investigation through Cyclic Voltammetry and Spectroscopy Ganesh B. Markad,†,‡ Shateesh Battu,† Sudhir Kapoor,‡ and Santosh K. Haram*,† †
Department of Chemistry, University of Pune, Pune 411 007, India Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
‡
S Supporting Information *
ABSTRACT: Cyclic voltammetry has been used to investigate the interaction between reduced graphene oxide (r-GO) and CdTe quantum dots (Q-CdTe). For that, the composite of Q-CdTe with r-GO (r-GO-CdTe) was prepared by carrying out the reduction of graphene oxide and the synthesis of Q-CdTe simultaneously, in a single bath. r-GO-CdTe was characterized by UV−visible, steady state fluorescence, time-resolved fluorescence, X-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM). Cyclic voltammetry was employed to determine the quasi-particle gap and band edge parameters of Q-CdTe and r-GO-CdTe. The blue shifts in the quasi-particle gap of r-GO-CdTe have been attributed to the strong interaction of graphene with CdTe. These interactions were further verified by time-resolved fluorescence and Raman spectroscopy which suggested strong electronic coupling between Qdots and graphene. Fe3O4, and CoFe2O4.14−16 Shifts in the D and G bands have been attributed to the charge transfer among graphene and the semiconductor Q-dots. These interactions have been further underlined by change in magnetization of Q-dots.15 The interaction of graphene with electron donating or accepting groups is known to shift in the Fermi level of the system.17 On a similar ground, an interaction of graphene with Q-dots would impact the band structure parameters, namely, conduction (LUMO) and valence band edges (HOMO) of Qdots. Thus, graphene gives yet another handle in modulating band structure parameters of Q-dots which would be very valuable in development of junction based devices. Changes in HOMO and LUMO and qasi-particle gap (εqp gap) of Q-dots as function of size have been studied extensively by scanning tunneling spectroscopy (STS)18,19 and photoelectron spectroscopy (PES).20,21 These techniques however involved tedious instrumentation and thus less suitable for the routine analysis. Recently, work reported by us and other groups demonstrated that equally reliable data can be generated through cyclic voltammetry (CV) measurements on the dispersions of Qdots.22−25 The main advantage of CV is the measurements are performed in solution phase at room temperature and ambient pressure. Moreover, band structure parameters estimated by CV are based on the charge transfer across semiconductor− electrolyte interface in the situation which is more close to the actual performance of devices. CVs have been extensively used to determine band structure parameters of semiconductor Qdots as a function of size, shape, and composition.22−25 To our knowledge, there is no report which describes the similar
1. INTRODUCTION In recent years, composites of semiconductor quantum dots (Q-dots) with graphene based materials have been studied extensively due to their encouraging performance in the fields of light-emitting diodes,1,2 photocatalytic water splitting,3,4 and in the next generation quantum dot based solar cells (QDSCs).5−7 In the semiconductor−graphene composites, the efficient charge separation is observed due to the ability of graphene to accept the electrons and disperse them through a conducting path at the nanosize regime.7 Thus, the development of graphene/Q-dots based devices would be pivoted on the fundamental understanding of interactions among them. These interactions are known to impact the rate of energy and charge transfer between them and have been quantified by time-resolved spectroscopy techniques.8 In this regard, from the excited state deactivation life times, Kamat et al. have estimated rate constants for energy and electron transfer in case of CdSe−graphene oxide composite. These values were found to be 5.5 × 108 and 6.7 × 108 s−1, respectively. They further reported 150% improvement in the photocurrent response in case of CdSe−graphene oxide based solar cells.8 There are many other reports which have verified an increase in photoresponse by incorporating the graphene in semiconductor quantum dots.9−11 Energy transfer from the individual photo excited Q-CdSe to graphene have been studied by Brus et al. by measuring the fluorescence quenching of a Q-dot in the graphene composite.12 The femtosecond transient absorption spectroscopy study on Q-CdTe−graphene composite brought out that the interaction of graphene with QCdTe prolongs the recombination lifetime in CdTe− graphene.13 Raman spectroscopy also has been used to quantify the interactions of graphene with Q-dots of CdSe, ZnO, TiO2, © 2013 American Chemical Society
Received: July 6, 2013 Revised: August 25, 2013 Published: September 11, 2013 20944
dx.doi.org/10.1021/jp406679s | J. Phys. Chem. C 2013, 117, 20944−20950
The Journal of Physical Chemistry C
Article
added. A portion of 0.5 mL of thioglycerol was then added slowly in above solution. The pH of the solution was adjusted to 11.0 by dropwise addition of 2.0 M NaOH with a constant stirring. The freshly prepared NaHTe solution was rapidly added to the cadmium precursor solution at room temperature under vigorous stirring. The solution was stirred continuously at room temperature for 10−20 min which resulted into orange colored solution. This solution was refluxed for 10 h. Product was precipitated by using acetone as an antisolvent and purified by repeated centrifugation and decantation. The final product was again redispersed in water and treated with acetone, centrifuged, and decanted. This procedure was repeated several times to ensured that all of the precursors are washed away. It was then dried under vacuum and stored in desiccator for further use. For the comparison, a physical mixture of r-GO and Q-CdTe (10:90 wt %) was prepared by simply mixing the dispersion of r-GO with Q-CdTe under sonication.15 The product were isolated by centrifugation and stored in desiccator for subsequent use. 2.5. Materials Characterization. Freshly prepared aqueous solutions of r-GO-CdTe and Q-CdTe (0.2 mg/mL) were used for UV−vis, steady-state, and time-resolved-fluorescence measurements. UV−visible spectra were recorded using an Agilent 8453 diode array single beam spectrophotometer. Steady state photoluminescence (PL) spectra were recorded with the help of a Shimadzu RF-5301PC spectrofluorimeter. Time-resolved fluorescence were recorded using Horiba Jobin Yvon TCSPC, and time-resolved fluorescence data were fitted using Dos 6 software. FTIR spectra were recorded using a Shimadzu FTIR-8400 spectrophotometer, having the attenuated total reflection (ATR) attachment. Powder X-ray diffractograms (XRD) were recorded on the dried product using a Bruker, D8-Advance, X-ray diffractometer (Cukα, 40 kV and 40 mA). Transmission electron microscopic (TEM) images were recorded on the samples using a Technai G2 20 V TWIN transmission electron microscope (20−200 kV), and Raman spectra of powder samples were recorded using Jobin Yvon LabRam HR800 having a laser wavelength of 514.5 nm/30 mW power and acquisition time of 30 s with two averaging. 2.6. Cyclic Voltammetry Measurements. The electrochemical measurements were performed with the help of a Metrohm potentiostat/galvanostat (model Autolab PGSTAT 100) workstation. A commercial Au-disk (CHI Instruments, USA, 2-mm diameter), homemade Hg/HgO/sat. Ca(OH)2, and Pt-wire loop were used as working, reference, and counter electrodes, respectively. Prior to use, the working electrode was polished over 0.5 μm alumina powder, rinsed with copious amounts of Milli-Q water, and potentiodynamically cycled in 0.5 M H2SO4 between 1.2 and 0.00 V (scan rate of 1 V s−1), until characteristic H2/O2 adsorption/desorption peaks reported for the clean Au surface were observed.30 All of the cyclic voltammetry experiments were started with first recording the controlled measurements in blank solution consisted of a buffer of pH 9.2 (0.1 M Na2SO4 and 0.02 M Na2B4O7).31 After this measurement, the dispersion of Q-CdTe or r-GO-CdTe was injected (final concentration 1 mg/mL) in the cell, and further experiments were performed.
investigation of graphene−semiconductor composite by the voltammetry methods. Therefore it is of our great interest to prepare Q-dot-graphene composite and estimate their band structure parameters through CV measurements. In present study we have chosen the reduced graphene-oxide (r-GO) and Q-CdTe interface because of their direct application in quantum dot based solar cells (QDSCs). For that, the composite of r-GO with Q-CdTe (here after r-GOCdTe) were prepared by single pot synthesis. Such a strategy leads to the formation of composite, in which Q-CdTe is believed to be incorporated in the graphene matrix whose properties are different than that of simple physical mixture of Q-CdTe and r-GO. Cyclic voltammetry have been performed on the dispersions of these composites, which allowed us to determine εqp gap as well as band edge parameters (HOMO and LUMO) in the presence of r-GO. The results were compared with a physical mixture of r-GO and Q-CdTe.
2. EXPERIMENTAL SECTION 2.1. Materials. Tellurium powder, sodium hydroxide, sodium borohydride (Sigma Aldrich), cadmium chloride, thioglycerol (TG), acetone, disodium tetraborate decahydrate, and sodium sulfate anhydrous (S. D. Fine) were used, without further purification. All of the solutions were prepared in Millipore water (conductivity
