Synthesis of high quality CuO nanoflakes and CuO–Au nanohybrids for superior visible light photocatalytic behavior

Abstract

Well-defined CuO nanoflakes and CuO–Au nanohybrids are successfully obtained by a facile but effective wet chemistry synthesis method. Compared with CuO nanoflakes, the formation of high-quality CuO–Au nanohybrids can improve the visible light absorption efficiency, charge generation and separation efficiency through surface plasmon resonance (SPR), thus yielding remarkably enhanced photoelectrochemical and photocatalytic activities. CuO–Au nanohybrids with an optimized Au nanoparticle loading concentration (10 wt%) and particle size (∼15 nm) present the highest photocurrent density (46 μA cm⁻²) and degradation rate constant (k = 0.64 h⁻¹), which are almost ∼4 times higher than those of the CuO nanoflakes. The high photocatalytic properties and robust synthesis of CuO–Au nanohybrids can expand new material systems for the visible light utilization of solar energy and effective treatment of organic pollutants.

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Introduction

Nanostructured semiconductors, typically TiO₂ and ZnO based nanomaterials, with well-defined morphologies have great potential in the applications of clean energy production and organic pollutants degradation.¹ Nevertheless, TiO₂ and ZnO can only utilize ultraviolet (UV) light (λ < 400 nm) due to their wide band gaps (>3.0 eV). Although many strategies have been developed to expand the utilization of solar energy to the visible light region,² the complicated and expensive procedures of materials preparation are still the major drawback for further broad applications. Therefore, exploration of facile and low-cost materials in visible light-driven photocatalysis has been considered for an effective candidate for practical applications.

Among the metal oxide semiconductors, CuO with a narrow band gap of 1.2–1.7 eV, has exhibited potential in the visible light-driven photocatalytic reaction without any other modification.³ As an important and inexpensive p-type semiconductor metal oxide, CuO with high photosensitivity and physicochemical stability, have been extensively used in various fields, such as energy materials,⁴ electrocatalysis,⁵ photocatalysis,⁶ to name a few. Up to now, quite a few studies regarding synthesis and applications of well-defined CuO nanostructures with different morphologies.⁷ For specific catalytic applications, large specific area of nanostructures is required, such as nanowires,^7a,b nanoribbons,^7f,g and nanosheets.^7k,l However, the high recombination rate of photogenerated h⁺ and e⁻, and relatively poor absorption in the intensest irradiation region (400–600 nm) of sunlight, usually limit the visible light-driven catalytic performance of CuO.

Noble metals (typically Au, Ag, Pd, or Pt) exhibit promising catalytic activities and efficiencies due to their unique geometric and electronic structures.^8a During the catalytic process, different geometric and electronic structures result in specific catalytic pathways.^8b Meanwhile, the size of noble catalysts is highly dependent on the support, which also defines the catalytic performance.^8c Therefore, the hybridization of plasmonic metals (e.g. Au, Ag, Pd or Pt) on the semiconductors to form metal–semiconductor hybrids has been proposed as a promising way to enhance the photocatalytic efficiency.^8d–g The enhancements are mainly interpreted by the surface plasmon resonance (SPR) of metal nanostructures, which can promote the charge separation, electron injection, as well as resonant energy transfer.^8h For example, Kamat and coworkers successfully modified mesoscopic TiO₂ films with Au clusters. The Au_x-SH clusters had limited absorption in the visible region and acted as photosensitizers. Meanwhile, the excited state interactions and delivery of stable photocurrent in metal cluster-sensitized solar cells were discussed.⁸ⁱ Another excellent example was demonstrated by Xie and coworkers, who used thiolated Au nanocrystals as an efficient photosensitizer for TiO₂ nanotube arrays. The incorporation of Au nanocrystals evidently extended the visible light response and promoted the photochemical performance of TiO₂ nanotube arrays.^8j Moreover, the metal size has a significant influence on the SPR effect.⁹ It is reasonably expected that the plasmonic metals with suitable sizes loaded on the CuO nanostructures would not only improve the utilization of visible light, but would also facilitate kinetic separation of photogenerated charges and decrease the recombination rate within the electrode,¹⁰ thus enhancing the photocatalytic performance.

In the present work, we developed a facile but effective wet chemistry method to synthesize the well-defined CuO nanoflakes and CuO–Au nanohybrids (Scheme 1). Three different morphologies of CuO–Au nanohybrids were obtained, namely: CuO–Au-7, CuO–Au-15, and CuO–Au-20 nanohybrids with the similar flake structure of CuO supporter, having the optimum loading amount of Au nanoparticles (NPs) (10 wt%), but different diameters of Au NPs (7, 15 and 20 nm, Fig. S1†). Syntheses details were depicted in ESI,† and the atomic ratio of Au in the CuO–Au nanohybrids was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements (Table S1†). Compared with CuO nanoflakes, the formation of high-quality CuO–Au nanohybrids could improve the visible light absorption efficiency, charge generation and separation efficiency through SPR effect, thus yielding remarkably enhanced photoelectrochemical and photocatalytic activities. CuO–Au nanohybrids with optimized Au NPs loading concentration (10 wt%) and particle size (15 nm) presented the highest photocurrent density (46 μA cm⁻²) and degradation rate constant (k = 0.64 h⁻¹), which were almost 4 times higher than those of CuO nanoflakes. All of the results indicated that our CuO–Au nanohybrids had promising potential application in visible-light driven photocatalytic reaction.

Scheme 1 Schematic illustration for the formation of CuO nanoflakes and CuO–Au nanohybrids (scar bar: 100 nm).

Results and discussion

Fig. 1 shows the powder X-ray diffraction (XRD) patterns of CuO nanoflakes and CuO–Au nanohybrids. Fig. 1a shows the XRD peaks of Cu nanowires, which can be attributed to the cubic phase of Cu (JCPDS: 65-9026, space group: Fm3̄m (225)), with lattice constants of a = b = c = 3.613 Å. Meanwhile, in Fig. 1b, the observable XRD peaks can be readily assigned to the monoclinic phase of CuO (JCPDS: 45-0937, space group: C2/c (15)), with lattice constants of a = 4.69 Å, b = 3.42 Å, c = 5.13 Å, and no diffraction peaks from any other chemical species such as Cu₂O or Cu could be observed. Similarly, the diffraction peaks in Fig. 1c can be indexed to CuO–Au nanohybrids (Au: cubic phase, JCPDS: 04-0784, space group: Fm3̄m (225), lattice constants: a = 4.08 Å, b = 4.08 Å, c = 4.08 Å; CuO: JCPDS: 45-0937). In addition, the broadening of the diffraction peaks suggested the nanocrystalline nature of the samples.

Fig. 1 XRD patterns of (a) Cu nanowires, (b) CuO nanoflakes and (c) CuO–Au nanohybrids.

The morphology of CuO nanoflakes was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM (Fig. 2a) and TEM (Fig. 2b) images indicate that CuO nanoflakes are of flake-like morphology with average size of ∼150 ± 50 nm in length and ∼50 ± 20 nm in width, which was further verified by the high-angle annular dark-field scanning TEM (HAADF-STEM) image (inset of Fig. 2a). The size distribution histogram of the CuO nanoflakes is listed in Fig. S2.† High resolution TEM (HRTEM) of CuO nanoflakes acquired from the black box of Fig. 2b presents that the crystal lattice spacing was calculated to ∼0.23 nm, identical to the distance of (111) plane of monoclinic CuO.

Fig. 2 (a) SEM, HAADF-STEM (inset) and (b) TEM, HRTEM (inset) images of CuO nanoflakes.

The morphologies of CuO–Au nanohybrids were observed by TEM, HAADF-STEM and HRTEM (Fig. 3). Fig. 3a and b show the TEM and HAADF-STEM images of CuO–Au-7 nanohybrids with average size of ∼7 nm Au NPs loaded on CuO nanoflakes. The HRTEM image of CuO–Au-7 nanohybrids is shown in Fig. 3c, the crystal lattice spacing was calculated to ∼0.25 nm, corresponding to the distance of (002) plane of monoclinic CuO, and the (111) plane of Au with crystal lattice spacing of ∼0.23 nm was also observed, suggesting the well-defined crystal structure of CuO–Au-7 nanohybrids. Analogously, the TEM, STEM and HRTEM images of CuO–Au-15 and CuO–Au-20 nanohybrids are shown in Fig. 3d–i, attesting that the CuO nanoflakes acted as substrates to support 15 and 20 nm Au NPs loaded on surface of them. In addition, simulation models of the nanohybrids demonstrate the well-defined structures of CuO–Au nanohybrids (inset of Fig. 3a, d and g). Importantly, digital photos (insets of Fig. 3b, e and h) verify that the as-obtained CuO–Au nanohybrids were readily dispersed and highly stable in polar solvents (e.g. absolute ethanol).

Fig. 3 TEM, HAADF-STEM and HRTEM images of (a–c) CuO–Au-7, (d–f) CuO–Au-15, and (g–i) CuO–Au-20 nanohybrids, respectively. Inset of (a), (d), and (g) represents the corresponding schematic model of CuO–Au nanohybrids; inset of (b), (e), and (h) is the corresponding photograph of samples dispersed in absolute ethanol for more than 1 week.

The surfaces of CuO nanoflakes and CuO–Au nanohybrids were investigated by Fourier transform infrared (FTIR) spectroscopy. The absorption band located at around 3400 cm⁻¹ was attributed to the stretching modes of hydrogen-bond, the 2900 cm⁻¹ and 1300 cm⁻¹ were assigned to the –CH₂– stretching vibrations, 1670 cm⁻¹ was ascribed to the C=O stretching vibrations, and 1070 cm⁻¹ was belonged to the –C–N– stretching vibrations in polyvinyl pyrrolidone (PVP) groups.¹¹ Based on FTIR spectroscopy analysis, it indicated that the retentivity of PVP capping ligands existed on the surfaces of CuO and CuO–Au nanostructures (Fig. S3†).

To study the chemical state of CuO–Au nanohybrids, the detailed X-ray photoelectron spectroscopy (XPS) analysis are shown in Fig. 4. The binding energy signals were corrected by referencing the C 1s binding energy to 284.60 eV. In Fig. 4a, peaks assignable to core levels of C 1s, Cu 2p, O 1s and Au 4f were identified. Fig. 4b shows the XPS signals taken from the Cu 2p region of CuO–Au nanohybrids. Double peaks at 933.60 eV and 953.60 eV were attributed to the core levels of Cu 2p_3/2 and Cu 2p_1/2, respectively, and shakeup satellite peaks at 942.10 eV and 962.20 eV indicated the presence of Cu(II) in CuO–Au nanohybrids.¹² The fitting of O 1s region peak at 531.60 eV hinted that crystal lattice oxygen¹³ in CuO–Au nanohybrids (Fig. 4c). Fig. 4d depicts the XPS spectra taken from the Au 4f_7/2 and Au 4f_5/2 regions of CuO–Au nanohybrids. Observable peaks at 83.50 eV and 87.10 eV were attributed to the core levels of Au 4f_7/2 and Au 4f_5/2, respectively, corresponding to the Au(0) in CuO–Au nanohybrids.^14,15

Fig. 4 (a) XPS survey spectrum of CuO–Au nanohybrids. (b) Cu 2p, (c) O 1s, and (d) Au 4f signals taken from CuO–Au-7 nanohybrids.

The optical properties of CuO nanoflakes and CuO–Au nanohybrids were characterized by ultraviolet-visible (UV-Vis) absorption (Fig. 5). The CuO nanoflakes exhibited absorption across almost the entire visible region. As for CuO–Au nanohybrids, additional absorption peaks at 500–600 nm can be observed, which were due to the surface SPR effect of Au NPs, suggesting that Au NPs acted as sensitizers for trapping the visible photons. The blue shift of absorption peak for CuO–Au-7 nanohybrids was due to the reduced mean free path of electrons for smaller Au NPs.¹⁶ The loading of Au NPs endowed the CuO–Au nanohybrids as promising candidates for efficient visible light-driven photocatalytic and photoelectrochemical applications.

Fig. 5 UV-Vis absorption spectra of CuO nanoflakes and CuO–Au nanohybrids.

Photocatalytic activities of CuO–Au nanohybrids were evaluated by degradation of rhodamine B (RhB) aqueous solution under visible light irradiation. To exclude the PVP effects of photodegradation rate, CuO–Au nanohybrids were annealed (Fig. S3†) before the degradation and the temporal evolution of absorption spectra of RhB solution after successive irradiation of light with CuO nanoflakes and CuO–Au nanohybrids were depicted in Fig. 6a–d and S4,† respectively. Obviously, not any RhB degradation was observed in the blank test.

Fig. 6 UV-Vis absorbance spectra of RhB solution photodegraded under visible light irradiation (a) CuO nanoflakes, (b) CuO–Au-7, (c) CuO–Au-15, and (d) CuO–Au-20 nanohybrids, respectively, (e) photocatalytic degradation and (f) linear fitting of pseudo-first-order kinetics of CuO nanoflakes, CuO–Au-7, CuO–Au-15 and CuO–Au-20 nanohybrids.

As shown in Fig. 6e, CuO–Au nanohybrids exhibited superior visible light photodegradation rate of RhB dye over CuO nanoflakes. Only ∼49.40% RhB was photodegraded by CuO nanoflakes after 4 hours, while over ∼69.70% and ∼83.90% RhB was photodegraded by using CuO–Au-7 and CuO–Au-20 nanohybrids, indicating an enhanced photocatalytic activity. Especially, the CuO–Au-15 nanohybrids showed the highest photocatalytic activity among the measured samples of ∼96.20% of degraded RhB in 4 hours. Fig. 6f shows the fitting curves of −ln (C/C₀) variation with photodegradation time, and a linear relation can be easily fitted for all samples. By adopting pseudo-first-order reaction, photocatalytic decolorization kinetics can be expressed as ln (C/C₀) = −kt, where k is the apparent rate constant, C₀ and C are the initial and after irradiation concentrations of RhB solution, respectively. The k values for RhB degradation were calculated as follows: 0.18 h⁻¹ for CuO nanoflakes, 0.31 h⁻¹ for CuO–Au-7 nanohybrids, 0.64 h⁻¹ for CuO–Au-15 nanohybrids, and 0.46 h⁻¹ for CuO–Au-20 nanohybrids, respectively. Notably, CuO–Au-15 nanohybrids exhibited the highest photodegradation rate, which was 3.56 times than that of CuO nanoflakes. Degradation trends of RhB solution as a function of irradiation time also clearly indicated that CuO–Au nanohybrids had much higher photocatalytic activity compared to CuO nanoflakes. The stable and recyclable performance of the nanohybrids was examined by repeating the photodegradation for 5 times. Fig. 7 presents that the photocatalytic activities of CuO–Au-15 nanohybrids still remained high after 5 cycles, suggesting the firm attachment of Au NPs on CuO and the stability of CuO–Au nanohybrids.

Fig. 7 Recycled photodegradation test of CuO–Au-15 nanohybrids.

The photocurrent is regarded as an important criterion to evaluate the separation efficiency of photogenerated h⁺ and e⁻ pairs and the photocatalytic efficiency of catalysts.¹⁷ The photocurrent responses of CuO nanoflakes and CuO–Au nanohybrids were obtained under visible light illumination for every switch-on and switch-off event (Fig. 8).

Fig. 8 Photocurrent responses of CuO nanoflakes and CuO–Au nanohybrids under visible light illumination.

It can be seen that the photocurrent density increased to a maximum value and then gradually decayed to a steady state. The improved photocurrent density indicated the enhanced separation efficiency of the photoinduced h⁺ and e⁻, which is beneficial for photocatalytic and photoelectrochemical performance.¹⁸ The photocurrent density of CuO nanoflakes was 11 μA cm⁻² after loading of Au NPs, the notably increased photocurrent densities of 23, 46 and 35 μA cm⁻² were obtained, indicating the significant role of Au NPs in charge generation and separation. Specifically, the CuO–Au-15 nanohybrids showed the highest photocurrent, which was 4.18 times higher than that of CuO nanoflakes.

Based on enhanced photocatalytic and photoelectrochemical activities of CuO–Au nanohybrids, a possible mechanism was proposed as shown in Scheme 2. The photocatalytic activity process consists of the following steps: firstly, generation of h⁺ and e⁻ pairs through absorption of adequate photon energy; then charge separation between photogenerated h⁺ and e⁻, and last, their promotion to the surface of hybrid nanostructures to produce highly active oxidative species (AOS) through surface redox reactions. Importantly, the loading of Au NPs improved the absorption of visible light through SPR effect, and consequently increased the yields of photogenerated e⁻ and h⁺ under visible light irradiation. The generated photoinduced e⁻ from conduction band (CB) of CuO (including pathway 1 and 2) were captured by the adsorbed O₂ to form strong reductive species, and therefore degraded RhB into inorganic products. Simultaneously, photoinduced h⁺ also oxidized the organic dye RhB into nontoxic products.

Scheme 2 Schematic representation of the possible mechanism behind the photocatalytic activity of CuO–Au nanohybrids upon degradation of RhB induced by visible light. CB: conduction band; VB: valence band; AOS: active oxidative species; SP: surface plasmon.

The best photocatalytic activity on RhB decomposition and photocurrent response was occurred when the size of the Au NPs was 15 nm. The nanohybrids with smaller Au NPs (size ∼ 7 nm) showed higher number density of the Au NPs loading on the CuO nanoflakes (Fig. 3b), leading to the aggregation of redundant Au NPs, which dually served as recombination centers for photoelectrons and holes.¹¹ For nanohybrids with larger Au NPs (size ∼ 20 nm), the drastically decreased number density of the Au NPs (Fig. 3h) resulted in the low overall efficiency of the Au NPs in producing e⁻ and h⁺ for photocatalysis.¹⁹ Therefore, there should be a balance between the loading number density and particle size of Au NPs under the fixed Au content (10 wt%). Based on our experimental results, we found that the Au NPs in size of ∼15 nm exhibited the best charge separation efficiency.

Conclusions

In summary, we have successfully obtained the well-defined CuO nanoflakes and CuO–Au nanohybrids by a facile but effective wet chemistry method. Subsequently, the visible light photocatalytic and photoelectrochemical activities of CuO nanoflakes and CuO–Au nanohybrids were further evaluated. Compared with CuO nanoflakes (k = 0.18 h⁻¹), CuO–Au nanohybrids presented obviously enhanced visible light activation, especially for CuO–Au-15 nanohybrids (k = 0.64 h⁻¹). The significant enhanced photocatalytic activity should be attributed to the SPR effect of Au NPs, which played important roles in improving visible light absorption efficiency, charge generation and separation efficiency for the nanohybrids. Meanwhile, the optimized Au NPs loading concentration and particle size on the surface of CuO nanoflakes can be easily manipulated. High photocatalytic properties and robust synthesis of CuO–Au nanohybrids could expand new material systems for the visible light utilization of solar energy and efficient treatment of organic pollutants.

Experimental section

Materials

Sodium hydroxide (NaOH, 99.50%, Sigma-Aldrich), ethanediamine (C₂H₈N₂, 99.90%, Sigma-Aldrich), hydrazine (N₂H₄·H₂O, 99.90%, Sigma-Aldrich), copper nitrate (Cu(NO₃)₂, 99.90%, Sigma-Aldrich), polyvinylpyrrolidone (PVP, 99.90%, Sigma-Aldrich), ammonium (NH₃·H₂O, 99.90%, Sigma-Aldrich), chloroauric acid (HAuCl₄·4H₂O, 47.80%, Sinopharm Chemical Reagent Co.), trisodium citrate (Na₃C₆H₅O₇·2H₂O, 99.90%, Tianjin Zhiyuan Chemical Co.), sodium borohydride (NaBH₄, 99.70%, Tianjin Zhiyuan Chemical Co.), L-ascorbic acid (C₆H₈O₆, 99.90%, Tianjin Zhiyuan Chemical Co.), potassium iodide (KI, 99.90%, Tianjin Zhiyuan Chemical Co.), absolute ethanol (C₂H₆O, >99.70%, Guangdong Guanghua Scientific and Technical Co.) were used as received without further purification.

Synthesis of CuO nanoflakes

The synthetic procedure as follows: firstly, 20 mL of H₂O containing 0.10 M of Cu nanowires²⁰ were added into a 100 mL round-bottom flask with vigorous magnetic stirring at room temperature for ∼30 min. Then 5 mL of aqueous solution of 0.07 M Cu(NO₃)₂ and NH₃·H₂O (300 μL, 34 wt%) was added into the solution dropwise one under vigorous stirring. After that, the temperature was elevated to 80 °C and maintained for 4 h. Subsequently, CuO nanoflakes were floated on the top of reacted solution, which was collected by centrifugation at 8000 rpm for 5 min and washed with deionized water and ethanol for three times before drying at 60 °C in a vacuum oven overnight.

Synthesis of Au nanoparticles (Au NPs)

Au NPs with different sizes were prepared by a seeded growth method.²¹ A growth solution of Au was prepared as follows: under vigorous stirring at room temperature, 0.60 mL of H₂O containing 0.01 M NaBH₄ was injected into 43 mL homogeneous solution containing 1.16 × 10⁻⁴ M HAuCl₄ and 5.81 × 10⁻⁴ M trisodium citrate. The resulting solution was stirred for 1 h and maintained for 6 h. 4.80 mL of PVP (5 wt%), 2.40 mL of L-ascorbic acid (0.1 M), 2.40 mL of KI (0.20 M) and 0.60 mL of HAuCl₄ (0.25 M) was added into 24 mL of H₂O one by one under vigorous stirring at room temperature, then a growth solution was obtained. After that, a specific amount of seed solution was injected into the growth solution to generate Au NPs (different volumes of seed solution were injected into the growth solution, resulting in different sizes of Au NPs, such as: 260 mL seed solution for 7 nm Au NPs; 80 mL seed solution for 15 nm Au NPs; 11.4 mL seed solution for 20 nm Au NPs). After 10 min vigorous stirring, Au NPs were centrifuged at 12 000 rpm for 5 min and washed with deionized water for three times, then redispersed in 40 mL of H₂O (concentration of Au: ∼0.73 mg mL⁻¹) (TEM images, Fig. S1†).

Synthesis of CuO–Au nanohybrids

The synthetic procedure was similar to the synthesis of CuO nanoflakes, except: after 80 °C for 4 h, a calculated volume of Au NPs solution was injected under vigorous stirring in a period of 30 min at 80 °C, then CuO–Au nanohybrids were formed.

Instrumentation

Powder X-ray diffraction (XRD) patterns of the samples were recorded on Rigaku D/MAX-RB (Japan) at a scanning rate of 5° min⁻¹ from 30° to 80°, using Cu Kα radiation (λ = 1.5406 Å). Transmission electron microscope (TEM) analyses of the samples were performed with a Hitachi HT-7700 (Japan) transmission electron microscope operating at 100 kV. Scanning electron microscopy (SEM) analyses of the samples were performed with a Carl Zeiss Sigma (Germany) operated at 20 kV. High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) characterizations were performed with a Fei Tecnai G2F20S-Twin (USA) operated at 200 kV. Ultraviolet-visible (UV-Vis) absorption spectra were measured by a JASCO V-570 UV/Vis/NIR spectrometer. Fourier transforms infrared absorption (FTIR) spectra of the samples were carried on NICOLET 6700 FTIR (USA). X-ray photoelectron spectroscopy (XPS) were obtained using an Escalab 250 xi photoelectron spectrometer using Al Kα radiation (15 kV, 225 W, base pressure ≈ 5 × 10⁻¹⁰ Torr). Nitrogen adsorption–desorption isotherms of samples were received on a Micrometrics TriStar 3000 porosimeter and Micrometrics ASAP 2020 microporous characterization at 77 K. The specific surface areas were calculated based on the Brunauer–Emmett–Teller (BET) method. The amount of atomic ratios of Au in the nanocrystals was determined on Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) measurements (HITACHIP-4010, Japan).

Photocatalytic measurement

The photocatalytic activity of CuO nanoflakes and CuO–Au nanohybrids (25 mg) were evaluated by photodegrading rhodamine B (RhB, 5 mg L⁻¹, 50 mL) aqueous solution under the illumination of a 150 W xenon lamp (PLS-SXE300UV, Beijing Perfectlight Co. Ltd.) with a 400 nm cut off filter at ambient temperature. The suspension continuously stirred in the dark for 2 h to ensure the adsorption–desorption equilibrium before irradiation. The concentration of degraded RhB solution was measured at a sequence of time intervals by the UV-Vis spectrophotometer at 554 nm to calculate the degradation of the RhB based on the Beer–Lambert law.

Photoelectrochemical measurement

The photocurrent–time (I–t) measurement was conducted in a typical three-electrode potentiostat system (CuO nanoflakes or CuO–Au nanohybrids as the working electrode, Pt foil as the counter electrode and saturated calomel electrode (SCE) as the reference electrode) on an electrochemistry workstation (CS 350, Wuhan Corrtest Instrument Co. Ltd.) under the irradiation of 150 W xenon lamp with a 400 nm cut off filter. The electrolyte was a 0.5 M Na₂SO₄ aqueous solution. The working electrode was fabricated by spreading CuO or CuO–Au powder (20 mg in 2 mL DMF) on fluorine-doped tin oxide (FTO) glass with an area of 1 × 1 cm², followed by drying at 120 °C in the air.

Acknowledgements

We gratefully acknowledge the financial aid from the start-up funding from Xi'an Jiaotong University, the Fundamental Research Funds for the Central Universities (2015qngz12), the China National Funds for Excellent Young Scientists (grant no. 21522106) and NSFC (grant no. 21371140). We also thank Dr Xinghua Li from Northwest University (China) for the HRTEM characterization. We also thank Prof. Zhiping Zheng from the University of Arizona for his kind help with the data analysis and polish the manuscript.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: TEM images of different size Au nanoparticles; TEM and size-distribution of CuO nanoflakes; FTIR spectra of CuO nanoflakes and CuO–Au nanohybrids; temporal evolution image of CuO nanoflakes and CuO–Au nanohybrids; ICP-AES result of CuO–Au nanohybrids. See DOI: 10.1039/c6ra12281g

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