Suspension Synthesis of Surfactant-Free Cuprous Oxide Quantum Dots

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2015, Article ID 825021, 8 pages http://dx.doi.org/10.1155/2015/825021 Research Article Suspension Synthesis of Surfactant-Free Cuprous Oxide Quantum Dots Dongzhi Lai,1,2 Tao Liu,1 Xinyun Gu,1 Ying Chen,1 Jin Niu,1 Lingmin Yi,1,2 and Wenxing Chen1,2 1 Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China 2 National Engineering Lab of Textile Fiber Materials and Processing Technology, Zhejiang Sci-Tech University, Hangzhou 310018, China Correspondence should be addressed to Dongzhi Lai; and Wenxing Chen; Received 20 October 2014; Revised 17 January 2015; Accepted 21 January 2015 Academic Editor: Shou-Yi Kuo Copyright © 2015 Dongzhi Lai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Suspension methods were used to synthesize surfactant-free Cu2 O quantum dots (Cu2 O-QDs) in high precursor concentrations using sodium hypophosphite as a reducing agent. Transmission electron microscopy (TEM) observations indicated that a large amount of Cu2 O-QDs were synthesized with diameters ranging from 7 to 10 nm. We propose a mechanism where DMSO acts as a surface passivation agent, explaining the possible formation of Cu2 O-QDs. Noticeably, the Cu2 O-QDs exhibited high and stable catalytic activity for the reduction of rhodamine B. 1. Introduction Cuprous oxide (Cu2 O) is an important transition metal oxide with potential applications in a range of technological fields. Being a p-type oxide semiconductor with a direct band gap of 2.17 eV, Cu2 O has been found in wide ranging applications including photon-activated water splitting [1], solar cells [2], gas sensing [3], low-temperature CO oxidation [4], negativeelectrode materials [5], solar energy conversion [6], and photodegradation of organic pollutants [7]. The synthesis of Cu2 O has been previously reported by several groups [1, 2, 4, 8–10]. Normally, a template (soft or hard) or capping agent is used to control the growth direction and dimensions of Cu2 O in the solution-based route [11– 13]; however, the template or capping agents may have an undesirable role in the final applications. It is therefore highly beneficial to develop an alternative method to produce Cu2 O which excludes templates or capping agents. Pan and colleagues reported that both cubic and octahedral Cu2 O nano- and microcrystals can be selectively synthesized by a simple wet chemical and capping-agent-free reduction route at room temperature [14]. Xu et al. reported a facile room temperature surfactant-free solution chemical route to fabricate Cu2 O nanocrystals, through reducing newly prepared Cu(OH)2 using hydrazine hydrate or sodium ascorbate [15]. In both methods, the Cu2 O nanoparticle sizes are between 200 and 500 nm showing the larger particles size weakened the nanoparticle size effect. On the other hand, the precursor concentrations in literature [15] are low, at only 0.0032 mol/L which reduces the productivity of the particles and whilst increasing production costs. Singhal et al. [16] reported a surfactant-free solvothermal approach for the preparation of Cu2 O nanocrystals involving the reaction of copper (II) acetylacetonate in acetone with a reaction temperature that reaches up to 140∘ C, resulting in higher production costs. It is therefore desirable to design a low cost, room-temperature, convenient, high precursor concentration, and simple approach to produce surfactantfree Cu2 O-QDs. In this report, we develop a surfactant (or polymer) free room temperature suspension approach for the synthesis of Cu2 O-QDs with high precursor concentrations. It is shown 2 Journal of Nanomaterials that Cu2 O-QDs can be successfully fabricated through reducing copper sulfate pentahydrate using sodium hypophosphite in the dimethyl sulfoxide. Furthermore, the precursor quantity has little effect on the particle size and morphology of the Cu2 O-QDs, which can be applicable to large-scale production of Cu2 O-QDs. Rhodamine B (RhB) is one of the most commonly used xanthenes dyes in the textile industry due to its good stability. In this study, degradation of RhB by Cu2 O-QDs is applied to test the catalytic activity of Cu2 OQDs. 2. Experiments 2.1. Materials. All chemicals used were of analytical grade and commercially obtained without further purification. Copper sulfate pentahydrate (CuSO4 ⋅5H2 O, Hangzhou Gaojing Fine Chemical Co. Ltd.) acted as the precursor for the formation of Cu2 O-QDs and sodium hypophosphite (NaH2 PO2 , Taicang Meida Reagent Co., Ltd.) acted as a reductant. Dimethyl sulfoxide (DMSO, Tianjin Yongda Chemical Reagent Co., Ltd.) was used as the solvent. 2.2. Synthesis 2.2.1. Suspension Phase Synthesis. 4, 8, or 12 g of copper sulfate pentahydrate and 20 g of sodium hypophosphite were added to 1 L of DMSO. The mixture was vigorously stirred for 3 hours at 25∘ C. The yellow/green coloured mixture turned drab gradually with the addition of 1.5 mL of H2 SO4 . The nanoparticles produced were then washed three times in deionized water followed by washing in ethanol, then centrifuged, and vacuum-dried at 40∘ C overnight. DMSO is a good ligand for Cu(II) [17, 18]. In the reaction system, DMSO complexed with Cu(II) and to form hexakis (dimethylsulfoxide) copper(II) complex as in [19] CuSO4 ⋅ 5H2 O + 6OS (CH3 )2 (1) 󳨀→ [Cu (OS (CH3 )2 )6 ] SO4 + 5H2 O [Cu(OS(CH3 )2 )6 ]SO4 was suspended in DMSO solution. This was confirmed by the addition of the copper sulfate pentahydrate into DMSO solution which becomes grass green under vigorous stirring (Figure 1(a)). However, when stirring was stopped, the [Cu(OS(CH3 )2 )6 ]SO4 solid particles settled to the bottom of the vessel after 24 hours and the DMSO solution became transparent (Figure 1(b)). Similarly, sodium hypophosphite was insoluble in DMSO; therefore the reaction occurred on the surface of the [Cu(OS(CH3 )2 )6 ]SO4 or the sodium hypophosphite. 2.3. Catalytic Oxidation Experiment. The catalytic oxidation of RhB (2.5 × 10−5 mol/L) was conducted using Cu2 O (2.5 g/L) and H2 O2 (2.5 mmol/L) at 50∘ C. At given time intervals, the decoloration efficiency of the RhB was determined using a UV-vis spectrometer. (a) (b) Figure 1: Copper sulfate pentahydrate in DMSO ((a) under stirring, (b) standing for 24 h). 2.4. Characterization X-Ray Diffraction (XRD). The crystal structure of Cu2 O-QDs was characterized using a Switzerland Thermo ARL XTRA Xray powder diffraction system using a Cu K𝛼 radiation source at 35 kV with a scan rate of 0.05∘ /s in the 2𝜃 range of 10–80∘ . Fourier Transform Infrared Spectroscopy (FTIR). The Cu2 OQDs powders were spread on KBr pellets individually and dried under an infrared lamp. The FTIR analysis was measured using a Nicolet 5700 FTIR spe (...truncated)