1 Fegrel

Cuprous Oxide Synthesis Essay

Abstract

Cupric oxide and cuprous oxide micro-/nanomaterials with well-controlled sizes and morphologies have been synthesized via different crystal growth techniques. Structural and morphological characterizations of these copper oxide micro-/nanomaterials were performed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After that, these copper oxide micro-/nanomaterials were used as catalysts for a typical C-N cross-coupling reaction directly. The catalytic results showed that different copper oxide micro-/nanomaterials had different catalytic activities in C-N cross-coupling reaction. The particle size of cupric oxide and the oxidation state of copper played vital roles in the catalytic process. Cupric oxide with small particle size has the best catalytic activity, while cupric oxide with different morphologies has almost the same yields and cuprous oxide has very poor yields. Further, the possible catalytic mechanism for copper oxide nanomaterials catalyzed cross-coupling reaction was proposed. And the influence of particle size and oxidation state was carefully discussed. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

DOI: 10.1039/C4RA12406E (Paper) RSC Adv., 2014, 4, 62115-62122

Fast and simplified synthesis of cuprous oxide nanoparticles: annealing studies and photocatalytic activity†

Received 15th October 2014 , Accepted 6th November 2014

First published on 6th November 2014


We synthesized highly (111) plane-oriented cuprous oxide (Cu2O) nanoparticles (NPs) by a fast and simplified chemical deposition method using an inorganic process at ambient temperature. The variations of crystal structure parameters, morphology, energy bands, and Brunauer–Emmett–Teller (BET) surface areas were recorded for Cu2O NPs at different annealing temperatures under N2 atmosphere. A phase coexistence of Cu2O and Cu was observed after the Cu2O NPs was annealed at 600 °C for 2 h under N2 atmosphere. The Cu2O NPs show high BET surface areas and a blue shift of absorption edge in comparison with Cu2O microparticles. We observed a superior photocatalytic activity for the degradation of methyl orange (MO) by Cu2O NPs, which reached 100% in 55 min with as high as 7.26 mg min−1 g−1 of decolorization rate. However, Cu2O NPs are more vulnerable to photocorrosion than Cu2O microparticles. The mechanisms of photocatalysis for Cu2O under visible light were also discussed in detail. Hydrogen peroxide and hydroxyl radicals were found responsible for the decolorization of MO.


1. Introduction

Cuprous oxide (Cu2O) nanoparticles (NPs) are widely used as antiseptic, germicide, catalyst and colorant in daily life.1,2 They are also a promising material for photoelectric transition,3,4 lithium ion cell cathode,5 and photon sensing.6 In recent years, Cu2O NPs have been found to be a stable photocatalyst particularly applied to hydrogen generation7,8 and the degradation of organic contamination under visible light (VL).9 Thus, the synthesis of Cu2O NPs is highly interesting not only for the further development of synthetic strategies, but also for the examination of their properties.

Various preparation methods for Cu2O NPs10–15 have been developed. In these methods for forming Cu2O NPs, organic compounds are frequently used as the precursor15 or as surfactant and template for preventing aggregation and controlling size as well as morphology.10–14 However, the use of organics often inhibits the progress of photocatalytic (PC) reactions due to the covering of Cu2O by the surfactant.16 Furthermore, Cu2O NPs with highly ordered nanostructures have attracted much attention in recent years due to their novel properties in photocatalysis and their stability. The (111) plane of Cu2O was found to show superior PC activity,17,18 and Cu2O exposing (111) facets can be used as a stable photocatalyst.19–21 Therefore, the fabrication of Cu2O NPs with preferentially oriented (111) facets by using inorganic precursors has become an important issue.

Recently, we reported the chemical synthesis of Cu2O films with periodic pattern transfer by using a chemical bath deposition technology (at 65 °C).22 In the present work, this deposition technology was improved, and a fast and simplified synthesis of Cu2O NPs with preferentially oriented (111) facets was successfully achieved at ambient temperature without high energy demand, complicated apparatus, or any organic compounds and surfactants. Particularly, we attempted to investigate the effect of heating Cu2O under inert atmosphere on its morphology, optical properties, stability and PC activity, since no study has been carried out on this so far, and heating Cu2O in air could cause a conversion of Cu2O to CuO.23–25 Finally, the behaviors of adsorption, photocatalysis and stability of unannealed and annealed Cu2O were investigated, and the mechanisms of Cu2O for PC degradation of methyl orange (MO) under VL are discussed in detail.

2. Experimental section

2.1 Materials

Copper(II) sulfate (CuSO4·5H2O, 98.0% purity), sodium thiosulfate (Na2S2O3, 99.0% purity), sodium hydroxide (NaOH, 99% purity) and MO (85% purity) were obtained from Sinopharm (China) and used as purchased. 2,3-Bis(2-methoxy-4-nitro-5-sulfophehyl)-2H-tetrazolium-5-carboxanilide (XTT) (>98% purity), terephthalic acid (TA) (98% purity), horseradish peroxidase (POD) (activity: 250–330 units per mg solid), and N,N-diethyl-p-phenylenediamine (DPD) (97% purity) were received from Sigma-Aldrich and used without further purification. The solutions used in this work were prepared with deionized water further purified with a Millipore Milli-Q (Millipore, Bedford, MA, USA) purification system (resistivity ≥ 18.2 MΩ).

2.2 Synthesis of Cu2O NPs

A colorless 500 mL stock solution of the Cu2O NP mixture was prepared by adding 400 mL of 1 M Na2S2O3 to 100 mL of 1 M CuSO4 solution. The same volume (500 mL) of 1 M NaOH solution was placed in another beaker. The mixture was poured into the 1 M NaOH solution with vigorous stirring. The Cu2O powder was obtained by centrifugation and dried. Subsequently, the Cu2O powder was divided into six equal parts, five of which were annealed at 200, 300, 400, 500 and 600 °C in N2 atmosphere with a flow rate of 35 sccm. The unannealed Cu2O powder and those annealed at 200, 300, 400, 500 and 600 °C were named sample 1, 2, 3, 4, 5, and 6, respectively.

2.3 Characterization

The phase purity of all samples was characterized by X-ray powder diffraction (XRD) using an X-ray diffractometer (Y-2000) with Cu Kα radiation (λ = 1.5418 Å). A scan efficiency of 0.1° S−1 was applied to record the powder patterns in the range of 20° ≤ 2θ ≤ 80°. Scanning electron microscopy (SEM) images were obtained on a JEOL SM-6700F microscope operated at 5 kV. X-ray photoelectron spectroscopy (XPS) analyses were performed on a PHI-1600 ESCA spectrometer (USA) using 300 W Mg Kα radiation, and the binding energies were referenced to the C1s line at 285 eV of adventitious carbon. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were carried out with JEOL JEM-100CXII. The Brunauer–Emmett–Teller (BET) surface areas of the samples were determined by a high-speed automated area and pore size analyzer (NOVA 2000e).

2.4 Adsorption and PC activity measurement

The liquid phase degradation of MO was used as a model reaction to evaluate the PC activity of the Cu2O powder. In a typical PC experiment, 0.1 g catalyst was dispersed in 400 mL of 100 mg L−1 MO solution. Prior to irradiation, the suspension was then sonicated for 30 min in the dark to ensure good dispersion of the catalyst solution and the establishment of adsorption/desorption equilibrium of the dye onto the photocatalyst surface. PC reactions were conducted in a cylindrical quartz reactor with water circulation to keep the reaction temperature of 25 °C. The equilibrium dye concentration was used as the initial concentration of the dye solution to calculate degradation efficiency. Subsequently, the suspension was transferred to the reactor. A 300 W xenon lamp (Shanghai Lansheng Ltd., Shanghai) was used as a visible light source. A UV cutoff filter (λ < 400 nm) was used to ensure VL irradiation. The light intensity was measured by a light meter (LI-COR, USA), and the light intensity for the experiments was fixed at 1800 mW cm−2. Aeration was performed by an air pump to ensure a constant supply of oxygen and to accomplish stirring the solution with the photocatalyst during photoreaction. At given irradiation time intervals, a 5 mL liquor was sampled and centrifuged at 4500 rpm for 15 min to remove the photocatalyst. The MO concentration was analyzed by recording variations of the absorption band maximum at 465 nm (defined as At) in the UV-vis spectra of MO by using a UV-vis spectrophotometer (UV-1700, SHIMADZU). The adsorption extent Ra of MO on the photocatalyst was determined by the difference in MO concentration between the original solution (defined as A0) and the filtrate of the MO/photocatalyst suspensions (defined as A0). The adsorption extent Ra of the dye on the photocatalyst was Ra = (A0 − A0)/A0 × 100%. The degradation efficiency (Rd) of MO was calculated by the following equation: Rd = (A0 − At)/A0 × 100%. The PC activity of all Cu2O samples was detected by this method. In addition, the decolorization rates were also calculated according to following equation: Rdec = V0C0/Q0T × 100% (unit: mg min−1 g−1), where Rdec is the decolorization rate, and V0, C0, Q0, and T are the volume of suspension with MO and catalyst, the initial concentration of MO, the amount of catalyst, and the time for PC reaction, respectively. Thus, here, decolorization rate is used to characterize how much dye can be discolored by a given amount of catalyst in a given time. We did this in an attempt to have a rough comparison between the PC activity of as-prepared Cu2O and that reported in publications.

2.5 Measurement of hydroxyl, hydrogen peroxide and superoxide radicals

Hydroxyl radical (˙OH) was detected by a photoluminescence (PL) method using TA as a probe molecule.26 TA readily reacts with ˙OH to produce a highly fluorescent product, 2-hydroxy-TA (TA-˙OH), which was measured by an Infinite M200 fluorescence spectrophotometer (Tecan, Switzerland) at 425 nm emission wavelength and 315 nm excitation wavelength. The experimental procedure is similar to the PC measurement, except that the aqueous MO solution was replaced with 4 × 10−4 M TA and 2 × 10−3 M NaOH.

Superoxide radical (˙O2) was measured by XTT,27 which can be reduced by ˙O2 to form XTT-formazan. The formazan has an absorption spectrum (measured by UV-1700, SHIMADZU) with a peak at 470 nm that can be used to quantify the relative amount of ˙O2 present. The experimental procedure is similar to the PC measurement, except that the MO was replaced with 1 × 10−4 M XTT.

H2O2 was analyzed photometrically by the POD-catalyzed oxidation product of DPD,28,29 which was measured by UV/vis spectrophotometer (UV-1700, SHIMADZU) at 551 nm. The experimental procedure is as follows: (1) 0.3 mL of phosphate buffer (0.5 M Na2HPO4 and NaH2PO4) was added to 2.7 mL of water/sample in cuvette and mixed; (2) 50 μL of DPD reagent and 50 μL of POD reagent were added in rapid succession and mixed for 10 s; (3) the absorption spectrum at 500–620 nm of the solution was taken. Water with the addition of buffer and DPD reagent was used as blank.

3. Results and discussion

3.1 Characterization of Cu2O

Cu2O NPs were synthesized through the reduction of blue copperas (CuSO4·5H2O) by Na2S2O3 as the reducing agent at ambient temperature in an alkaline aqueous solution. A colorless solution of copper thiosulfate complex was formed after an overdose of Na2S2O3 solution was added into the blue copperas solution by the following complexation equilibrium:
2Cu2+ + 4S2O32− → 2[CuS2O3] + [S4O6]2−(1)

In the copper thiosulfate complex solution, free Cu+ ions were formed by the dissociation equilibrium:30

[CuS2O3] → Cu+ + S2O32−(2)

After the copper thiosulfate complex solution was poured into the NaOH solution, a yellow precipitate appeared almost simultaneously, suggesting the generation of Cu2O. The formation of the precipitate could be simply expressed as the reaction of free Cu+ ions with OH ions:

2Cu+ + OH → Cu2O + H+(3)

Fig. 1a shows the XRD spectra obtained in situ for samples 1–5. Five peaks at 2θ = 29.78°, 36.56°, 42.39°, 62.51° and 73.46° could be indexed to the (110), (111), (200), (220) and (311) planes of the Cu2O cubic phase with lattice constant α = 0.4266 nm, which is very close to the values in JCPDS-International Centre for Diffraction Data (PDF, Powder Diffraction File, no. 05-0667, 1996). The XRD spectrum of sample 1 (unannealed Cu2O) is as good as that of sample 2, indicating that heating Cu2O at 200 °C under N2 atmosphere did not change its phase composition. No other peaks suggesting impurities, or significant changes in the peak locations of the XRD spectra, were detected after the heat treatments, indicating that pure Cu2O powders were obtained under current synthetic conditions.


Fig. 1 XRD patterns of (a) Cu2O NPs annealed at 200, 300, 400 and 500 °C; (b) 600 °C in N2 atmosphere for 2 h.

It can also be seen from Fig. 1a that as the annealing temperature increases, so does the intensity of all of the diffraction peaks, but their full width at half maximum (FWHM) decreases. It is well known that the width of the diffraction peak increases with the decreasing size of crystalline grain. The relationship of the diffraction peak FWHM (Δ(2θ)) and the size of crystalline grain (D) can be expressed as the Debye–Scherrer formula: D = 0.89λ/Δ(2θ)cosθ, where λ is the wavelength of the X-ray diffractometer incident ray (Cu Kα, λ = 1.5418 Å). Thus, according to the FWHM of the diffraction peaks of the (111) plane of Cu2O, the average Cu2O crystal sizes for samples 1–5 can be estimated as 7.73 nm, 7.73 nm, 9.39 nm, 21.91 nm, and 26.29 nm, respectively, indicating that the crystallinity of Cu2O increased with increasing annealing temperature. The heat-induced growth of Cu2O crystallite contributes to the increase of crystallinity, because higher ordering in the structure of Cu2O makes the X-ray peak sharper and narrower. The peak at 36.56° was always dominant in all the annealing temperatures, indicating that these Cu2O samples have highly oriented (111) planes. The numerical information of the peak height/area ratio of planes (111) and (200) for samples 1–5 are provided in Table S1.† A gradually increasing peak height/area ratio of the (111) plane indicates the formation of more highly oriented (111) planes with increasing annealing temperature. The formation of Cu2O NPs highly oriented to the (111) plane was also confirmed by the examination of HRTEM and the corresponding selected-area electron diffraction (SED) pattern. Fig. 2a indicates the Cu2O NPs were composed of rod-like and sphere-like nanostructures. One Cu2O nanorod in the square area in Fig. 2a was enlarged and shown in Fig. 2b, in which the non-uniform contrast of the image indicates nanocrystalline particles with diameters of about 5–10 nm in the nanorods. The interplanar spacing of the lattice fringes in the HRTEM image of the Cu2O nanorod (Fig. 2b) is about 0.2412 nm, which corresponds to the (111) plane of the Cu2O NPs.22 The brighter circle corresponds more to the (111) plane than the others in the SED pattern (Fig. 2c) of the whole image (Fig. 2b), also indicating the formation of Cu2O NPs highly oriented to the (111) plane.


Fig. 2 (a) TEM, (b) HRTEM images and (c) selected-area electron diffraction pattern corresponding to the whole-image (b) area for unannealed Cu2O NPs.

Fig. 3 records a representative panoramic view of samples 2–5, from which the variation of morphology of Cu2O with the annealing temperatures is observed. The SEM image of sample 1 is similar to that of sample 2 (not shown here), indicating that the morphology of Cu2O did not change when annealed at 200 °C. The result is in good agreement with our XRD data and the report by Paracchino, et al. that heating Cu2O to 250 °C in N2 atmosphere did not change the morphology of the film.23 However, as the annealing temperature increased, Cu2O NPs with rod-like and sphere-like nanostructures disappeared, and the original Cu2O NPs grew into large architectures with sizes ranging from several hundred nanometers to several micrometers (Fig. 3a–d and a′–d′), suggesting that annealing over 300 °C significantly influences the morphology and size of Cu2O. Interestingly, when Cu2O NPs were annealed at 600 °C, a phase coexistence of Cu2O and Cu was observed due to the appearance of four additional peaks at 2θ = 43.2°, 50.4° and 74.1° corresponding to the (111), (200) and (220) planes of copper (JCPDS card no. 04-0836)31 and the other one at 77.25° corresponding to the (222) plane of Cu2O (Fig. 1b). The heat treatment in N2 atmosphere at 600 °C therefore caused the decomposition of Cu2O (eqn (4)).


Fig. 3 SEM images of Cu2O NPs annealed at (a) 200, (b) 300, (c) 400 and (d) 500 °C and their enlarged images (a′), (b′), (c′), and (d′), respectively.

3.2 Optical property of Cu2O

Fig. 4a shows the optical absorption spectra for samples 2 and 5. To estimate the optical absorption edges for these two samples, (αhν)1/nversus hν curves were plotted (Fig. 4b) for n values (n = 1/2), a direct optical transition.32 Here, α is the optical absorption coefficient calculated from the absorption spectra, and h is the incident photon energy. The band gaps estimated for samples 2 and 5 are 2.15 and 2.10 eV, respectively, which are in agreement with the literature value of 2.1 eV for Cu2O bulk crystal.33 The absorption edge wavelengths for samples 2 and 5 were estimated to be 577 and 590 nm according to their band baps, respectively, indicating a substantial red shift for the absorption of sample 5 in comparison with that of sample 2. The UV-vis diffuse reflectance spectra and their corresponding (αEp)2vs. Ep curves for samples 1, 3 and 4 are given in Fig. S2.† The band gaps of samples 1, 3 and 4 were also calculated by the same method (see Table 1). The studies indicate that the band gaps of Cu2O NPs decrease with increasing annealing temperature. We observed that the colors of samples 2–5 generally changed from dark yellow to orange (see Table 1 and Fig. S1 in ESI†), also suggesting the variation of optical absorption with different annealing temperatures.
Fig. 4 (a) The UV-vis diffuse reflectance spectra for Cu2O NPs annealed at 200 and 500 °C and (b) their corresponding (αEp)2vs. Ep curves, respectively.

Table 1The crystal size, band gap, BET surface area, color, adsorption extent (Ra), and decolorization rates of unannealed Cu2O NPs and Cu2O NPs annealed at 200, 300, 400 and 500 °C



3.3 PC activity of Cu2O

The adsorption extent (Ra) and decolorization ratio, as well as the MO degradation efficiency of the Cu2O samples are given in Table 1 and Fig. 5, respectively. Samples 1 and 2 show a close adsorption extent, decolorization rate and degradation efficiency, indicating that annealing treatment for Cu2O NPs at 200 °C did not significantly affect its adsorption and PC activity towards MO. However, as the annealing temperature increased, these Cu2O samples showed a diminishing adsorption extent and PC activity. The much higher degradation efficiency of samples 1 and 2 compared to other Cu2O samples may be ascribed to their small particle sizes, which can increase their adsorption ability and thereby improve their PC activity. Moreover, the small photocatalyst size can result in more efficient transfer of photogenerated charges to the surface, also contributing to the efficient PC reaction, whereas the aggregation of photocatalyst particles involved a decrease in specific surface area as well as a change in the light scattering property of the particles,34 resulting in the degradation of PC efficiency.35 Moreover, the higher decolorization rates of samples 1 (7.26 mg min−1 g−1) and 2 (6.67 mg min−1 g−1) (see Table 1) than the reported values of 6,36 2.75 (ref. 37) and 0.238 (ref. 12) mg min−1 g−1 further indicate the superior PC activity of Cu2O NPs towards MO.
Fig. 5 Degradation efficiencies of MO for unannealed Cu2O NPs and Cu2O NPs annealed at 200, 300, 400 and 500 °C under VL. 0.1 g catalyst was dispersed in 300 mL of 100 mg L−1 MO solution.

3.4 Stability of Cu2O

The stability of the Cu2O samples was investigated by comparing the structures and valence states of the Cu2O samples before and after PC experiments. Fig. S3† shows XRD patterns of samples 2–5 before and after PC experiments. The XRD pattern of sample 1 is similar to that of sample 2 after PC experiment (not shown here). Two new peaks at 2θ = 38.73° and 48.65°, corresponding to the (111) and (02) planes of tenorite phase CuO (PDF, Powder Diffraction File, no. 02-1040), appeared in the XRD patterns of both samples 2 and 3, indicating that they were partly oxidized into CuO during the PC process. However, no obvious change could be observed for the XRD patterns of samples 4 and 5, indicating that annealing at a higher temperature contributed to a better stability for Cu2O.

XPS spectra were recorded to monitor the valence states of the Cu2O samples before and after PC experiments. Fig. 6a shows a representative XPS spectrum for samples 1–5 before PC experiments. It is hard to simulate the peak, because it is not so broad, and there is no shoulder peak. The binding energy of Cu2p (932.9 eV) is the same as the standard binding energy of Cu2p in Cu2O, indicating that samples 1–5 were prepared as pure Cu2O, which is well in agreement with the XRD data. Fig. 6b and c show the XPS spectra of sample 1 and sample 5 after PC experiments, respectively. The XPS spectra of samples 2, 3 and 4 after PC experiments were similar to those shown in Fig. 6b and c, respectively. All the similar data are not shown here. XPS spectra of both Fig. 6b and c show a broad peak simulated with the Gaussian equation, which leads to two split peaks. According to the cited publication,38 the Cu2p3/2 level of Cu2O is narrow (FWHM = 2.0 ± 0.1 eV) and has a binding energy of 933.05 ± 0.75 eV, while that of CuO is broad (FWHM = 2.85 ± 0.25 eV), with a binding energy of 935.2 ± 0.35 eV. A detailed description of the binding energies, FWHM and relative contents of the different peaks for sample 1 and sample 5 after the PC experiments are listed in Table 2.


Fig. 6 (a) Representative XPS spectrum of samples 1–5 before PC experiments; XPS spectra of (b) sample 1 and (c) sample 5 after PC experiments.

Table 2The binding energies, FWHM, relative contents and area ratio of different peaks of unannealed Cu2O and Cu2O NPs annealed at 500 °C after the PC experiments



It can be seen form Table 2 that both samples 1 and 5, after PC experiments, have two kinds of surface states, Cu+ and Cu2+. The ratio of the area of the peaks denotes the relative amounts of Cu+ to Cu2+ on the surface. The peak area ratio of Cu+ to Cu2+ on the surface for samples 1 and 5 are 0.09 and 10.87, respectively, indicating that the Cu2+ is the main surface state for sample 1 (similar to samples 2 and 3), and Cu+ is the main surface state for sample 5 (similar to sample 4) after PC experiments. The XRD spectra of samples 1–3 before and after the PC experiments also confirm the transformation of Cu2O to CuO. However, XRD spectra of samples 4 and 5 before and after PC experiments could not identify the oxidation of Cu2O to CuO, suggesting that the amount of CuO is too low to be detected by XRD measurement, and only a very thin surface layer of Cu2O was oxidized into CuO for samples 4 and 5 after the PC experiments. The results further confirm that the Cu2O annealed over 400 °C had better stability than that annealed below 400 °C.

3.5 Detection of reactive oxygen species (ROSs)

The generation of ˙OH was detected by the photoluminescence (PL) method. Usually, PL intensity is proportional to the amount of produced ˙OH in solution. A gradual increase in PL intensity of characteristic peaks at about λ = 425 nm was observed (Fig. 7a), indicating that the ˙OH was formed during the PC process. XTT is a widely used ˙O2 probe with a high rate constant for the reaction of ˙O2. Fig. 7b shows no absorption peak at λ = 470 nm, indicating that ˙O2 was not produced.27 H2O2 production was analyzed photometrically. Fig. 7c shows that H2O2 was produced; its concentration increased at first and became stable with prolonged irradiation time due to the decomposition of H2O2 in parallel with its production.39
Fig. 7 (a) Fluorescence emission spectral changes for the filtrate from the suspension of unannealed Cu2O with 4 × 10−4 M TA and 2 × 10−3 M NaOH at different irradiation times (emission at 425 nm, excitation at 315 nm); (b) absorption spectral changes for the filtrate from the suspension with unannealed Cu2O and 1 × 10−4 M XTT at different irradiation times (absorption peak 470 nm); (c) absorption spectra of the DPD/POD reagent after reaction with H2O2 (absorption peak 551 nm).

3.6 Mechanisms of photocatalysis

The general principle of photocatalysis is that when illuminated by light with photoenergy greater than the band gap, the electrons (e) of NPs are promoted across the band gap to the conduction band, which creates a hole (h+) in the valence band. Electrons in the conduction band and holes in the valence band exhibit high reducing and oxidizing power, respectively. The electron can react with molecular oxygen to produce ˙O2 through a reductive process. Although ˙O2 is not a strong oxidant, it has significant PC implications as a precursor for H2O2 and ˙OH.27 The hole can abstract electrons from water and/or hydroxyl ions to generate ˙OH through an oxidative process.27 Thus, h+, ˙OH, H2O2 and ˙O2 are considered to be the major reactive species for PC dye decolorization.40

Fig. 8 shows the energy level diagram of Cu2O as well as the events possibly occurring during the PC process. An examination of the band structure of Cu2O in Fig. 8 leads to the conclusion that the redox potentials for H2, ˙O2, O2 and H2O2 evolution, the oxidation of Cu2O to CuO and the reduction of Cu2O to Cu are all within the band gap, and therefore, all of these processes are possible, in principle. However, not all the reactions are kinetically favorable; for example, the driving force for water oxidation is minimal, while the oxidation of Cu2O is thermodynamically favorable.21 Thus, we have to identify which reactions are available both kinetically and thermodynamically take place according to the experimental results. Then, we can further analyze the roles of the electron, hole, and ROSs for the PC process.


Fig. 8 Energy level diagram of Cu2O and the events possibly occurring during the PC process.

Cu2O is one of the semiconductors with the highest conduction bands. The band gap of Cu2O NPs was determined as 2.02 eV in this work, and the potential of its conduction band is −1.4 V (vs. NHE, the same below).41 When Cu2O NPs are irradiated by VL, electrons are separated from holes (eqn (5)).

Cu2O + hν → Cu2O (e) + Cu2O (h+)(5)

Thus, those photogenerated electrons with high energy in the conduction band of Cu2O, besides recombining with holes (eqn (6)) or reducing Cu2O to Cu (which happens at +0.05 V, eqn (7)),21 are likely to transfer to the surface of Cu2O to react with molecular oxygen and produce ˙O2, H2O2 and ˙OH.

Cu2O (e) + Cu2O (h+) → heat(6)
Cu2O + 2H2O + 2e → 2Cu + 2OH(7)

However, the reduction of Cu2O to Cu by photogenerated electrons was unlikely to occur, according to our XPS data. Photogenerated electrons also were unlikely to react with molecular oxygen to produce ˙O2 (eqn (8)), because ˙O2 was not detected during our PC experiments.

As for the hole, which is produced at about +0.6 V potential, no overpotential is available for the oxidation of water (about +0.57 V, eqn (9)).42 It also cannot oxidize H2O and OH to form H2O2 (+1.763 V) and ˙OH (about +2.8 V), respectively. However, the oxidation of Cu2O by the hole is favorable (+0.19 V, eqn (10)),43 which is supported by our XRD and XPS data before and after the PC experiments.

2H2O → O2 + 4H+ + 4e(9)
Cu2O + 2OH + 2h+ → 2CuO + H2O(10)

Since H2O2 and ˙OH signals were detected in our PC process, and the hole is not their origin, electrons should be responsible for the production of both H2O2 and ˙OH by possible reductive reaction processes. The reductive reaction for the production of H2O2 can proceed as the cited publication44 reported: the photogenerated electron is likely to react with molecular oxygen to produce superoxide anion (˙O2, −0.28 V, eqn (8)),12 then H2O2 through a reductive process. However, no ˙O2 signal was detected in our experiments, so the H2O2 came from the direct reduction of O2 (+0.28 V, eqn (11)), which was more thermodynamically available.2,45 H2O2 can discolor MO with a low degradation rate even in the dark,46 suggesting that H2O2 is involved in the decolorization of MO during the PC process. Moreover, H2O2 is an electron capturer, which can capture photogenerated electrons and result in the evolution of ˙OH according to eqn (12).47,48

O2 + 2H2O + 2e → H2O2 + 2OH(11)
H2O2 + e → ˙OH + OH(12)

Indeed, the process was available because the ˙OH signal was detected in our PC process. ˙OH is a strong and nonselective oxidant45 that can result in the decolorization of MO. So, both H2O2 and ˙OH are responsible for the degradation of MO during the PC process.

4. Conclusions

A fast and simplified synthesis of Cu2O NPs with highly oriented (111) planes can be achieved by a chemical deposition method with an inorganic process at ambient temperature. The annealing of Cu2O NPs shows that the higher the annealing temperature, the more highly oriented (111) planes could be formed with increasing annealing temperature. However, when heating Cu2O at 600 °C for 2 h in N2 atmosphere, a phase transformation occurred, and the coexistence of Cu2O and Cu was observed. The Cu2O NPs show high BET surface areas and a blue shift of absorption edge compared to Cu2O microparticles. The superior PC activity of Cu2O NPs may result from their small particle size, high BET surface areas and highly (111) plane-oriented crystal composition. However, Cu2O NPs are more vulnerable to photocorrosion than Cu2O microparticles. Cu2O annealed at over 400 °C shows better stability than that annealed below 400 °C, which might be due to more highly oriented (111) planes in the Cu2O microparticles than the nanoparticles. H2O2 and ˙OH played a key role for the degradation of MO during the PC process.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 61370223 and 51303048), the Natural Science Foundation of Hubei Province (Grant no. 2014CFB579), Hubei Key Laboratory of Quality Control of Characteristic of Fruits and Vegetables, Hubei Engineering University (Grant no. 2014K11 and SWZ009), and Hubei Provincial Department of Education (Grant no. B2014025).

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Footnote

† Electronic supplementary information (ESI) available: Photographs of samples 2–5, the peak height/area ratio of planes (111) and (200) of samples 1–5, the UV-vis diffuse reflectance spectra for samples 1, 3, 4 and their corresponding (αEp)2vs. Ep
Samples12345
BET surface area (m2 g−1)21.6321.376.1565.5213.011
Crystal size (111) (nm)7.737.739.3921.9126.29
Band gap (eV)2.152.152.122.112.10
ColorDark yellowDark yellowYellowLight orangeOrange
Adsorption extent (Ra)39.1%38.4%11.4%9.6%5.7%
Decolorization rate (Rdec)a7.266.673.272.771.70
Sample15
PeaksCu+Cu2+Cu+Cu2+
Binding energy (eV)933.2935.6932.7934.9
FWHM1.933.01.852.9
Area714279156536944939
Cu+/Cu2+0.0910.87

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