In situ growth of Au nanoparticles on the surfaces of Cu₂O nanocubes for chemical sensors with enhanced performance

Abstract

An in situ strategy has been developed to directly grow Au nanoparticles (NPs) on the surfaces of Cu₂O nanocubes by exposing Cu₂O nanocubes to diluted HAuCl₄ solution at room temperature. The sensing performances of the Cu₂O nanocubes toward ethanol and acetone improved significantly after the growth of the Au NPs.

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Fabrication of nanostructure-based gas sensors with high sensitivity, selectivity, stability and improved response and recovery time is one of the most important branches of nanotechnology.^1,2 The bottom-up approach, a commonly used strategy to synthesize nanomaterials, offers us a powerful tool to control the physical properties of the products, such as size, shape and composition, subsequently allowing for the development of low-cost, fast-response and sensitive gas sensors.^3–5 In most cases, metal oxides, which have a receptor and transducer function, are used as gas-sensing materials as their electrical resistances will change correspondingly upon exposure to oxidizing or reducing gases. In the sensing process, specific interaction between a metal oxide surface and target molecules is required and the materials should have further ability to transfer the interaction-induced-alterations from the surfaces into a macroscopically accessible signal, typically, a change in the electrical resistance. In principle, engineering sensing materials’ structures can enhance the properties of gas sensors, such as increasing the sensitivity and generating a faster response and recovery time.^6–8 As a result, highly porous and hollow structures have been synthesized via various methods in order to improve gas diffusion and mass transport in the sensing process.^9–12 Another potential way to improve sensor performance is to enhance the catalytic activity as their conductivity response is determined by the catalytic efficiency of the reactions between detected gas molecules and gas-sensing materials taking place on the surfaces.¹³ Noble metal NPs, such as Au, Ag, Pt and Pd, are highly-effective oxidation catalysts that make them of use as active materials to improve the reactions occurring on gas-sensor surfaces.^14–17 Thus, various techniques, including impregnation, sol–gel, sputtering and thermal evaporation, have been exploited for introducing noble metal additives onto/into oxide semiconductors. After the functionalization, different gas sensors with improved behaviors were obtained.

Cu₂O, a p-type semiconductor with a direct band gap of about 2.17 eV, has emerged as a star material in recent years owing to its distinguished properties and great potential in gas sensing,^18–21 catalysis,²² solar energy conversion,²³ biosensing,²⁴ and electrode materials.²⁵ The possibility of using Cu₂O nanostructures as gas-sensor materials has been demonstrated,^18–21 but in principle pure Cu₂O is not an ideal gas-sensing material in terms of its physical characteristics. On one hand, the operating mechanism for a transition-metal-oxide is based on the change in resistance which is determined by the mobility of charge carriers. The mobility of positive holes in p-type metal oxides is usually much lower than that of conduction electrons in n-type semiconductors, for example, the mobility of Cu₂O is about 10 cm² V⁻¹ s,²⁶ while the mobilities are about 160 and 200 cm² V⁻¹ s for SnO₂ and ZnO, respectively. On the other hand, regardless of p-type or n-type semiconductor sensor, ionosorption of oxygen and the formation of negatively charged ions, such as O⁻, O₂⁻ and O²⁻, are necessary pre-procedures in gas sensing. Obviously, the quantity of absorbed oxygen anions plays a vital role in determining the performance of a sensor as they will directly react with target analyte, releasing electrons back to the conduction band and leading to a significant decrease in resistance for n-type semiconductors and a sharp increase in resistance for p-type gas-sensing materials. For n-type semiconductors, electron conduction is dominant, that is, plenty of electrons are available for ionization of absorbed oxygen; however, for p-type semiconductors, positive holes are the major charge carriers and the surface concentrations of electrons are much lower than those in n-type oxides.²⁷ As a result, less attention has been paid to the fabrication of gas sensors based on p-type metal oxides. However, if other advantages of n-type oxides such as low cost and non-toxicity are taken into account, the sophisticated design and synthesis of n-type oxide nanostructures or n-type oxide-based nanocomposites for gas sensors with improved sensing performance is still of great interest. Herein, we report a facile in situ method for the homogeneous growth of Au NPs on the surfaces of Cu₂O nanocubes by exposing Cu₂O nanocubes to diluted chloroauric acid directly. In comparison with sputtering, impregnation and other surface modification techniques, in situ growth methods have exhibited several features in the uniform loading of Au NPs: (1) avoiding introduction of impurities as no extra reducing agents or surfactants are required; (2) operations are performed under mild conditions.^28,29 Gas-sensing studies indicate that the sensitivity of Cu₂O nanocubes toward ethanol and acetone is significantly improved after the decoration of Au NPs. This enhanced performance may derive from deeper, continuous electron-depleted layers formed on the surfaces of Cu₂O nanocubes. These layers may be caused by the homogeneous distribution of Au NPs and small size of the Cu₂O nanocubes.¹⁶

Cu₂O nanostructures were first synthesized by a surfactant-free method (ESI†). TEM measurements (Fig. 1a) show that the as-prepared nanostructures have cubic morphology with an average width of about 45 nm, and the edges of the nanocubes are clear and smooth. The light yellow colloid solution gradually changed to black, forming CuO nanostructures at room temperature when the products were dispersed in distilled water under the atmosphere, indicating that the as-prepared Cu₂O nanocubes have quite a strong reducing capacity as they can be easily oxidized by oxygen. The reduction capacity of the Cu₂O nanocubes may be employed to reduce noble metal ions without the assistance of additional reductive reagents; the freshly produced noble metal atoms might deposit on the surfaces of the Cu₂O nanocubes and act as seeds to further grow NPs in order to reduce surface energy in the reaction system. As a result, Cu₂O–metal nanocomposites may be produced. As expected, when a Cu₂O nanocube colloid solution was injected into HAuCl₄ at room temperature, the color of the mixture changed from light yellow to black immediately. Fig. 1b shows a typical TEM image of Cu₂O nanocubes after incubation with Au precursor for 5 s. It is clear that a great number of NPs with diameters of about 5 nm were homogeneously formed on the surfaces of Cu₂O nanocubes and no separated NPs were observed. Because of the low metal loading and/or tiny metal particle size, no other characteristic diffractions of Au(0) (Fig. S1†) were detected except the diffraction pattern from cubic Cu₂O (JCPDS file no. 05-0667), which is consistent with previous reports.^28,29 The regular fringe distance (Fig. 1c) in the nanocubes at 0.21 nm is in good agreement with the d value of (200) planes of Cu₂O, while the other fringe spacings with 0.20 and 0.23 nm of the NPs deposited on Cu₂O nanocubes match well with the d values of the (200) and (111) planes of Au(0), respectively. The EDS analysis (Fig. 1d) of the nanocomposites also illustrates the existence of Au as well as C, O, Cu elements in the product. The content of Au loaded on the surface of Cu₂O is about 2.15 atom%. Both measurements show that the smaller NPs formed on the surfaces of Cu₂O nanocubes may be Au NPs.

Fig. 1 TEM images of Cu₂O nanocubes before (a) and after (b) exposure to HAuCl₄ precursor; HRTEM image (c); and EDS analysis (d) of as-prepared Cu₂O–Au nanocomposites.

In order to confirm the composition of the smaller NPs formed on the surfaces of the Cu₂O nanocubes, the XPS technique was exploited. The overview XPS spectrum (Fig. 2a) displays Au 4f, Cu 2p, C 1s and O 1s peaks. The peaks (Fig. 2b) located at 83.9 and 87.6 eV are assigned to the spin–orbit split components of the Au 4f level, which matches well with the values of the zero oxidation state of metallic Au,³⁰ confirming that the newly produced NPs are Au nanocrystals. The content of Au on the surface of the Cu₂O nanocubes obtained here is about 2.50 atom%, which is consistent with the EDS analysis. The fine spectra of the Cu 2p_3/2 and Cu 2p_1/2 binding energies (Fig. 2c) are located at 932.48 and 952.37 eV, arising from Cu₂O in the surface region. Note that the peaks ascribed to Cu²⁺ states, such as CuO and Cu(OH)₂, were not detected obviously here. The O 1s spectrum in Fig. 2d can be resolved into two peaks. The high binding energies component at 530.1 eV indicates the formation of Cu₂O phase, and the small component at 531.1 eV is ascribable to hydroxyl groups and molecular water adsorbed on the surface,³¹ respectively. The C 1s peak at 284.7 eV is due to adventitious carbon present in the experiment. These results demonstrate that the surface state of the Cu₂O nanocubes has not changed significantly after the in situ growth of Au nanocrystals on their surfaces. The reaction between Cu₂O nanocubes and Au precursor is a spontaneous redox process as a result of a large difference in the reduction potential between the Cu²⁺/Cu₂O (0.203 V vs. SHE) pair and the AuCl₄⁻/Au pair (0.99 V).³² Specifically, during the reduction of metal ions, a small portion of Cu(I) on the nanocubic surfaces may be oxidized to Cu(II) which will release from the solids as Cu²⁺ in the presence of H⁺ provided by HAuCl₄.

Fig. 2 XPS spectra of as-synthesized Cu₂O–Au nanocomposites: (a) overview XPS spectrum of the products; (b) Au 4f spectrum; (c) Cu 2p spectrum and (d) O 1s spectrum.

To investigate the gas sensing properties of as-prepared Cu₂O–Au nanocomposites, we placed a sample in a sealed chamber and measured the output voltage in air and in the presence of ethanol with concentrations from 10 to 400 ppm. The results are shown in Fig. 3a at a working temperature of 210 °C. For comparison, the sensing performances of pure Cu₂O nanocubes were also investigated under the same conditions. As expected, when ethanol gas was injected into the chamber, electrical resistances for both Cu₂O–Au nanocomposites and pure Cu₂O nanocubes were significantly increased. The on and off characteristics of both sensors were observed when ethanol gas was injected into and taken out of the gas chamber. The response amplitudes of Cu₂O–Au nanocomposites toward each concentration of ethanol were higher than those of pure Cu₂O nanocubes, especially in high gas concentrations. For example, the sensitivity of the Au-decorated sensor (Fig. 3b) toward 400 ppm ethanol is estimated to be 4.34, which is much higher than that of pure Cu₂O nanocubes (1.88) under the same conditions. The average response times of Cu₂O nanocubes before and after Au deposition are less than 6 s, implying that the growth of Au NPs on Cu₂O cube surfaces has no obvious effect on the response time to ethanol. This may be attributed to the small size of Cu₂O nanocubes, that is, a large number of negatively charged oxygen ions are presented at the surface for interaction with ethanol at the working temperature. Note that the as the Cu₂O nanocubes were synthesized through a surfactant-free method, a less negative effect on oxygen ionosorption will take place as a result of limited molecular species on the surfaces of the Cu₂O nanocubes. Compared with the recovery time of the Cu₂O nanocube-based sensor, the Au-decorated one did not require obviously more time to recover its initial resistance in the equilibrium with respect to re-absorbed surface oxygen species. This can be explained by the fact that the difference in their work functions (4.84 and 5.10 eV for Cu₂O and Au, respectively) is not large enough to cause in-depth electron transfer between the sensing layer and the target molecules in this system. This point is confirmed by XPS analysis. The binding energy of Au 4f_7/2 does not show an obvious shift in comparison to that of bulk Au, which is different from the results obtained from ZnO–Au nanocomposites.¹⁴ When acetone (Fig. 3c and 3d) was employed as the target gas, similar results were obtained due to their analogous sensing mechanisms.³³ It is important to note that the detection limit of Cu₂O–Au nanocomposites for acetone is down to 1 ppm with excellent sensitivity, which is comparable to that of n-type oxide sensors.³⁴

Fig. 3 Real-time response and sensitivity curves of sensors based on pure Cu₂O nanocubes (black curves/dots) and Cu₂O–Au nanocomposites (red curves/dots) upon exposure to different concentrations of ethanol (a, b) and acetone (c, d) at a working temperature of 210 °C.

The enhanced sensing performance of the Au-decorated sensor can be explained by the formation of deeper, continuous electron-depleted layers on the surfaces of Cu₂O nanocubes. Firstly, the small diameter of the Au NPs favors a so-called spillover effect, which can enhance the sensitivity of the gas sensor. In fact, the spillover effect is the process where the newly produced oxygen atoms will migrate onto the Cu₂O nanocubes forming oxygen ions, such as O⁻, O₂⁻ and O²⁻, with the assistance of Au NPs. A homogeneous distribution of Au NPs implies that dissociation of oxygen would be improved significantly on the surfaces of the Cu₂O nanocubes. As a result, a deeper depletion layer is formed compared to the pure Cu₂O nanocubes. Furthermore, the small size of the Cu₂O nanocubes facilitates the formation of depletion layers with larger width. According to a former report,¹⁷ the change in the width of depletion layer (ΔR) can be deduced as:

ΔR = (2εε₀Δv/qN_A)^1/2

where ε is the dielectric constant of Cu₂O, ε₀ the permittivity of free space, Δv the change in the potential induced by Au NPs, q the electronic charge, and N_A the carrier concentration of Cu₂O nanocubes. The formula implies that the width of the depletion layer is inversely proportional to N_A^1/2, which means that this parameter will be much larger for Cu₂O nanocubes with a smaller size as a result of low carrier concentration. With the combination of Au NPs with homogeneous distribution and small size Cu₂O nanocubes, deeper, continuous electron-depleted layers are produced (Fig. 4). When Cu₂O–Au nanocomposites were exposed to ethanol or acetone, the layer would decrease or disappear rapidly, resulting in a dramatic change in the conductivity of semiconductor materials.

Fig. 4 Schematic illustration for gas sensitization of Cu₂O nanocubes modified with Au NPs.

In conclusion, Au NPs with a narrow size distribution have been successfully grown on the surfaces of Cu₂O nanocubes via a spontaneous redox reaction. The driving force for the reaction is the difference in reduction potentials between Cu²⁺/Cu₂O and AuCl₄⁻/Au pairs. The enhanced sensing performance of the Au-decorated sensor has been observed, which may be attributed to the effect caused by homogeneous distribution of Au NPs on nanosized Cu₂O cubes. This method may be extended to the deposition of other noble metal or alloy NPs on the surface of Cu₂O nanostructures in order to further improve the sensing behavior of pure Cu₂O nanostructures.

Acknowledgements

This work has been supported by the National Natural Science Foundation (Nos. 21001006, 21171007 and 21105001) of P. R. China.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental materials and procedures, XRD patterns of pure Cu₂O and Cu₂O–Au nanocomposites. See DOI: 10.1039/c2ra20789c

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