Suspension Synthesis of Surfactant-Free Cuprous Oxide Quantum Dots

Suspension methods were used to synthesize surfactant-free Cu 2 O quantum dots (Cu 2 O-QDs) in high precursor concentrations using sodium hypophosphite as a reducing agent. Transmission electron microscopy (TEM) observations indicated that a large amount of Cu 2 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 Cu 2 O-QDs. Noticeably, the Cu 2 O-QDs exhibited high and stable catalytic activity for the reduction of rhodamine B.


Introduction
Cuprous oxide (Cu 2 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, Cu 2 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 Cu 2 O has been previously reported by several groups [1,2,4,[8][9][10].Normally, a template (soft or hard) or capping agent is used to control the growth direction and dimensions of Cu 2 O in the solution-based route [11][12][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 Cu 2 O which excludes templates or capping agents.Pan and colleagues reported that both cubic and octahedral Cu 2 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 Cu 2 O nanocrystals, through reducing newly prepared Cu(OH) 2 using hydrazine hydrate or sodium ascorbate [15].In both methods, the Cu 2 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 Cu 2 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 Cu 2 O-QDs.
In this report, we develop a surfactant (or polymer) free room temperature suspension approach for the synthesis of Cu 2 O-QDs with high precursor concentrations.It is shown that Cu 2 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 Cu 2 O-QDs, which can be applicable to large-scale production of Cu 2 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 Cu 2 O-QDs is applied to test the catalytic activity of Cu 2 O-QDs.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.

Experiments
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] [Cu(OS(CH 3 ) 2 ) 6 ]SO 4 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(CH 3 ) 2 ) 6 ]SO 4 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(CH 3 ) 2 ) 6 ]SO 4 or the sodium hypophosphite.

Catalytic Oxidation Experiment.
The catalytic oxidation of RhB (2.5 × 10 −5 mol/L) was conducted using Cu 2 O (2.5 g/L) and H 2 O 2 (2.5 mmol/L) at 50 ∘ C. At given time intervals, the decoloration efficiency of the RhB was determined using a UV-vis spectrometer.

Characterization X-Ray Diffraction (XRD).
The crystal structure of Cu 2 O-QDs was characterized using a Switzerland Thermo ARL XTRA Xray powder diffraction system using a Cu K  radiation source at 35 kV with a scan rate of 0.05 ∘ /s in the 2 range of 10-80 ∘ .
Fourier Transform Infrared Spectroscopy (FTIR).The Cu 2 O-QDs powders were spread on KBr pellets individually and dried under an infrared lamp.The FTIR analysis was measured using a Nicolet 5700 FTIR spectrometer.

Transmission Electron Microscopy (TEM).
The Cu 2 O-QDs were suspended in ethanol and dropped on an ultra-thin carbon coated copper grid and dried under infrared lamp for 5 minutes.Images were acquired using a JSM-2100 transmission electron microscopy (TEM, JEOL, Japan) at an accelerating voltage of 200 kV.

Ultraviolet-Visible Spectroscopy (UV-Vis
).The spectra were collected using a UV-3010 spectrophotometer across a wavelength range of 450-625 nm.
Copper, carbon, sulfur, and oxygen atomic content and their bonds on the surface of nanoparticles were obtained from the XPS spectra.XPS spectra were acquired using a PHIl600 X hemispherical analyzer equipped with an Al Ka (1486.6 eV) X-ray source.The morphology of the products was characterized by TEM (Figure 3).Cu 2 O-QDs were shown to have an average diameter of 7 nm when the precursor quantity was 4 g/L (seeing Figure 3(a)).No obvious change took place in the morphology and size of Cu 2 O-QDs when the precursor quantity was increased to 8 or 12 g/L (shown in Figure 3(b) or Figure 3(c)).These results revealed that the precursor quantity had little effect on morphology and size of the samples.Several bands could be distinguished in the region of 4000-400 cm −1 , a detailed assignment of which had been reported in a previous study.Briefly, the intense band of FT-IR spectra of DMSO at 1028 cm −1 was attributed to the S=O bond vibration stretching modes and the peak located at 1308 cm −1 was due to the symmetric vibrations of −CH 3 groups.The bands at 1399 and 1432 cm −1 were assigned to the asymmetric vibrations of −CH 3 groups, and the peak split attributed to the vibration coupling effects.The bands at 2909 and 2992 cm −1 were characteristic of the −CH 3 bond vibrations in the region of 4000-2500 cm −1 .

The FT-IR Spectra of Cu
Correspondingly, the intense band of the FTIR spectra of Cu 2 O-QDs at 1096 cm −1 was attributed to the S=O bond vibration stretching modes.The bands at 1374 and 1461 cm −1 were assigned to the symmetric and asymmetric vibrations of −CH 3 groups, respectively.The bands at 2847 and 2917 cm −1 were characteristic of the −CH 3 bond vibrations in the region of 4000-2500 cm −1 .The peak at 628 cm −1 corresponded to the Cu-O bond (due to optically active lattice vibration in the oxide) [20].As these data show, DMSO was adsorbed on the surface of Cu 2 O-QDs.The peak at 1067 cm −1 was assigned to the S=O group which interacted with the Cu 2 O surface via the oxygen atom of DMSO.
The different peak positions of S=O and methyl in the FT-IR spectra of Cu 2 O-QDs and DMSO are mainly due to the following reasons: (1) the FT-IR spectra of Cu 2 O-QDs were tested under solid conditions compared to the dimethyl sulfoxide under liquid conditions which resulted in the FTIR peak positions of S=O and methyl of Cu 2 O-QDs being shifted to a high wave number; (2) the force constant of the S=O increases with the characteristic frequency of the S=O shifting to the high wavenumber displacement due to the inductive effect of the Cu + [21].

The XPS Spectra of Cu
2 O-QDs.The surface properties of the nanoparticles were studied by XPS analysis.Figure 5 shows the complete XPS spectra from 0 to 1350 eV for the sample.The XPS spectra from a wide scan showed that the surface of the Cu 2 O-QDs consisted of copper, sulfur, carbon, and oxygen elements.
The Cu 2p 3/2 core level was used to investigate the Cu 2 O-QDs surface.Figure 6(a) showed the main and satellite peaks of Cu 2p 1/2 and Cu 2p 3/2 in the samples.Cupric oxide (CuO) and cuprous oxide (Cu 2 O) are the two semiconducting phases of copper oxide.In Figure 6(a), the spectrum displayed a Cu 2p 3/2 peak at 931.88 eV with a shoulder at 934.48 eV, which corresponded to Cu and/or Cu 2 O and CuO, respectively.A shake-up satellite peak at 943.48 eV and some peaks above 950 eV were detected arising from spinorbit coupling [22,23].Cu and Cu 2 O cannot be resolved due to their binding energies being very close, differing by only 0.1 eV [24].It was not easy to distinguish the binding energy of Cu 2p between Cu and Cu 2 O, but they could be distinguished from the position of their corresponding LMM-2 auger transition in the XPS spectra at 568 eV and 570 eV for Cu and Cu 2 O, respectively [25].In Figure 6(b), a peak is observed at close to 570 eV in the XPS spectra of the sample.According to the above descriptions, the peak at 931.88 eV relates to Cu 2 O.The relative intensities of the shakeup satellites from these levels were indicative of the presence of CuO at the surface [26].The fact that XRD did not show evidence of CuO phase whilst XPS indicated the surface presence of Cu 2+ ions suggests that CuO was present only on the surface of the Cu 2 O-QDs d forms a thin amorphous outer shell [27].
Figure 7 shows the S 2p (Figure 7(a)), C 1s (Figure 7(b)), and O 1s XPS spectra.Figure 7(a) shows the S 2p peak at 167.88 eV which is attributed to S=O bonds [28,29].In Figure 7(b), the main peak at 284.28 eV of C 1s was attributed to C-H bonds [24].From the position of the peak at 530.88 eV in the O 1s spectra in Figure 7(c), this assigned the O 1s spectral to the Cu-O bonds [30].XPS characterization indicated that the surface of the Cu 2 O-QDs was mainly Cu 2 O phase with CuO and DMSO covering a thin layer.[31][32][33].The catalytic activity of Cu 2 O-QDs is evidenced by the oxidation experiments of the model compound RhB.Degradation of RhB was RhB was calculated from the absorption in  max = 555 nm.As shown in Figure 8, the main absorption bands decreased with increasing reaction time which was consistent with the  gradual colour change of the solution from red to colourless.The RhB was reduced to less than 4% after 240 minutes, indicating that Cu 2 O-QDs possessed remarkable reductive ability.

The Catalytic Activity of Cu
The phase composition of Cu 2 O-QDs after ten cycles of catalysis was further investigated by XRD (Figure 9).Results showed that the phase composition of the samples still matched to Cu 2 O (PDF#05-0667).The height of each diffraction peak had decreased indicating that the Cu 2 O crystal structure was destroyed partly with the degree of crystallinity decreasing after repeated catalysis.

A Plausible Mechanism of the Reaction.
If polymers or surfactants are absent in the reaction system, the nanoparticles would grow at the expense of the smaller particles due to Ostwald ripening.As the prepared Cu 2 O-QDs were small in size and narrow in size distribution, this indicated that DMSO played a central role in controlling the morphology of Cu 2 O-QDs.
It is known that the smaller the particle size, the larger the surface energy of the particle.The initial cuprous oxide crystal nuclei were small and had a high surface energy during the crystals growth stage.The DMSO adsorbed on the surface of the cuprous oxide particles via the oxygen atom of the S=O group reduced the surface energy, which was verified by the FT-IR of Cu 2 O-QDs.The greater steric resistance of DMSO made the cuprous oxide surfaces incompletely passivated, so the similar cuprous oxide particles grew up via Ostwald ripening.When the cuprous oxide particles reached a certain size, the surface energy became low as the surfaces were completely passivated by DMSO.This made the surface energy too small to drive the particles towards increased size causing the particles to remain small.NaH 2 PO 2 was used as the reduction in the overall reaction system.The fundamental reaction can be simplified as in [34] [Cu (OS (CH 3 )

Conclusions
We report a novel, inexpensive, room temperature, high precursor concentration suspension method for the synthesis of surfactant-free Cu 2 O-QDs.Experimental results showed that DMSO played a surface passivation role in the formation of Cu 2 O-QDs.In this reaction system, varying the amount of precursors had little impact on the size of the product.
2 O-QDs and DMSO.The FT-IR spectra of Cu 2 O-QDs and DMSO are presented in Figure 4.
The surface of the Cu 2 O-QDs was mainly Cu 2 O phase with CuO and DMSO covering a thin layer.The Cu 2 O-QDs samples had a high catalytic activity of H 2 O 2 for the reduction of RhB.The results in this paper provide evidence which support the suspension synthesis method for Cu 2 O-QDs, broadening the scope of Cu 2 O-QDs synthesis routes which may potentially be applied to the synthesis of other oxide materials.