Au@Cu₂O core–shell nanoparticles as chemiresistors for gas sensor applications: effect of potential barrier modulation on the sensing performance

Abstract

Au@Cu₂O core–shell nanoparticles (NPs) were synthesized by a solution method at room temperature and applied for gas sensor applications. Transmission electron microscopy (TEM) images showed the formation of Au@Cu₂O core–shell NPs, where 12–15 nm Au NPs were covered with 60–30 nm Cu₂O shell layers. The surface plasmon resonance (SPR) peak of Au NPs was red-shifted (520–598 nm) after Cu₂O shell formation. The response of Au@Cu₂O core–shell NPs was higher than that of bare Cu₂O NPs to CO at different temperatures and concentrations. Similarly, the response of Au@Cu₂O core–shell NPs was higher than that of bare Cu₂O NPs for NO₂ gas at low temperature. The improved performance of Au@Cu₂O core–shell NPs was attributed to the pronounced electronic sensitization, high thermal stability and low screening effect of Au NPs.

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Introduction

Heterostructured nanomaterials with chemically distinct components induced by heterointerfaces have led to new applications in the field of catalysis, photovoltaic devices, sensors, and so on.^1–3 Recently, enormous efforts have been devoted to fabricating heterostructured materials, heterojunction nanowires and arrays.^4–6 Heterojunction nanomaterials with multi-layer structures (which have multi-barrier structures) lead to accumulation of carriers and control of their transport properties which play an important role in modern device physics and have various important applications.^7,8 Also, much attention has been focused on creating high performance gas sensors in recent decades, due to the growing awareness about environmental pollution and safety issues. Among various gas sensors, metal oxide semiconductors (MOS) are commercially used for gas monitoring. Resistance changes in metal oxide based gas sensors are used extensively to monitor toxic gases, including CO, hydrocarbons, NOx and hydrogen.^9–16 Many metal oxide gas sensors do not exhibit gas sensitivity at room temperature. As a result, gas sensors often require a heating device to achieve the sensitivity. Unfortunately, the high temperature is harmful and is a drawback to the durability of sensors and their performances.¹⁷ It is well known that the performance of gas sensors is gauged by their sensitivity, selectivity, response, recovery time, stability, and reproducibility.^18–20 Rapid response characteristics with adequate sensitivity, in particular, are required for real-time monitoring of toxic gases and preventing possible disasters from such harmful gases. To meet the issue, it is very important to make efforts in the design and engineering of novel materials to achieve the desired performance. There have been efforts to solve the sensor operating temperature limitation²¹via nanostructured materials with ultra-high surface-to-volume ratios, element doping, and surface modification with noble metals.^22–25 However, noble metal NPs are not stable against heating, which results in a loss of catalytic activity, a serious issue in many applications including gas sensors. The catalytic activity loss is caused by the increased mobility of the metal NPs on the support at higher temperatures.²⁶ It results in either a shunting layer or an active membrane filter effectively obstructing the penetration of the targeting gas into the surface of the gas sensing matrix. In addition, noble metal NPs can be poisoned by many chemicals that contain sulfur (H₂S, SO₂, and thiols) or phosphorus.²⁷ Nevertheless, the production of gas sensors operating at room temperature with high sensitivity and selectivity with low power consumption remains a challenging task. Therefore, the developments of new sensors to overcome such issues are required.

Heterostructures with metal cores and semiconductor shells become the strongest candidates as gas sensors due to their controllable chemical and thermal stabilities within the shell and charge transfer between metal core and semiconductor shells.^28–30 Recently, in our lab, the same strategy was applied to Au@SnO₂ core–shell nanostructures, which enhanced the sensitivity to CO gas as compared to bare SnO₂ at low operating temperature.²⁸ Also, a similar strategy was used in Au@TiO₂ core–shell nanostructures for high temperature CO sensing.²⁹ The improved response in metal/n-type MOS is mainly explained by the increase in depletion layer formation due to chemical as well as electronic sensitization of noble metals. Since the response (R_s) of n-type MOS is examined either by R_a/R_g or (R_a − R_g)/R_a for reducing gases, the high R_a value for metal/metal oxide improves the response. Although, the response increases with metal NPs in n-type MOS, the high baseline resistance results in an increase in power consumption for their practical applications. However, such a problem or drawback can be solved using p-type MOS. The transfer of electrons from p-type MOS to metal NPs will increase the hole mobility, thereby decreasing their resistance. Furthermore, in p-type MOS, the response (R_s) is measured either by R_g/R_a or (R_g − R_a)/R_a for reducing gases, and therefore the low R_a value for metal/p-type MOS improves the response.

Among various metal oxides, cuprous oxide (Cu₂O), an important p-type semiconducting material with a unique optical and electrical property, is an appealing candidate for solar energy conversion, sensors, photo-catalytic degradation, and coherent propagation of excitons.^31–34 The p-type Cu₂O has been widely chosen as a sensing material in recent years.³⁵ In this work, Au@Cu₂O core–shell NPs have been synthesized and used for the detection of CO and NO₂ gases at various temperatures and concentrations and their sensing mechanisms are discussed in detail. The use of Au@Cu₂O core–shell NPs towards CO and NO₂ gas detection has not been reported before to our knowledge. Also, the advantage of Au@Cu₂O core–shell NPs over bare Cu₂O NPs is examined and the possible role of Au NPs in sensing mechanisms is discussed. The results demonstrate that Au@Cu₂O core–shell NPs are better sensing materials as compared to bare Cu₂O NPs.

Experimental

Material synthesis

The Au NPs and Au@Cu₂O core–shell NPs were synthesized by using a simple solution process reported elsewhere.^28,36 In brief, for synthesis of Au NPs, HAuCl₄ solution (250 mL, 1 mM) was heated with mild stirring until boiling. A solution of tri-sodium citrate (25 mL, 34 mM) was then added to the solution with rapid stirring. The resulting solution was kept at 97 °C for 15 min with constant stirring. The resulting solution was finally allowed to cool down at room temperature.

For the synthesis of Au@Cu₂O core–shell NPs, 1.0 g of PVP was dissolved in 50 mL of 0.01 M Cu(NO₃)₂ aqueous solution under constant magnetic stirring and the solution was kept for 10 min to dissolve the PVP completely. Afterwards, 1 mL of fresh prepared Au colloid solution was added, followed by the immediate introduction of 20 μL of N₂H₄·3H₂O solution (80 wt%, Sigma-Aldrich) into the reaction mixture. After completion of the reaction, Au@Cu₂O core–shell NPs were washed with water and anhydrous ethanol and re-dispersed in ethanol and finally stored in a refrigerator at 4 °C for further investigation. Bare Cu₂O NPs were also synthesized by a similar method without Au NPs.

Material characterization

The transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were taken with a JEM-2010 JEOL instrument with a tungsten filament, using an accelerating voltage of 200 kV. The crystallographic structures of the solid samples were determined with a D/Max 2005 Rigaku X-ray diffractometer equipped with graphite monochromatized high-intensity Cu Kα radiation (λ = 1.54187 Å). The UV-visible spectra were recorded on a UV-visible spectrometer (UV-2550, Shimadzu).

Gas sensing measurements

The gas sensors were fabricated with the as-synthesized Au@Cu₂O core–shell NPs as sensing materials. To measure the sensor response of the Au@Cu₂O core–shell NPs for CO and NO₂ gases, a sensor device was prepared as follows;²⁸ the synthesized Au@Cu₂O colloid was dropped onto the cleaned Al₂O₃ board (15 mm × 15 mm) with interdigitated platinum electrodes (10 mm × 10 mm) and the colloid was dried on a hot plate. The Au@Cu₂O core–shell NPs loaded device was heated treated at 300 °C in a muffle furnace for 5 h. The change in resistance of the device due to the presence of CO and NO₂ gases was measured using a high resistance meter (Agilent 34970A). The device was examined in the temperature range of 25–250 °C at various concentrations of CO (100–1000 ppm) and NO₂ (5–100 ppm) in a temperature-controlled environment. The background gas was N₂, and the dry air was mixed with 10.5% of oxygen. The gas flow rate (100 cm³ min⁻¹) over the sensor device was switched between the sample gas and dry air to record the sensor response to electrical resistance.

The sensor response (R_s) was calculated using R_g/R_a for CO gas and R_a/R_g for NO₂ gas. Here, R_a is the resistance in dry air and R_g is the resistance in the probe gas.

Results and discussion

Fig. 1 displays the formation of Au@Cu₂O core–shell NPs with Au NPs as a core and Cu₂O as a shell. The Au NPs are well dispersed, spherical in shape and their size ranges from 12–15 nm as shown in Fig. 1a. The formation of heterogeneous Au@Cu₂O core–shell NPs is shown in Fig. 1b and c. The overall size of Au@Cu₂O core–shell NPs is 120–80 nm, where the size of the Au NP cores is about 12–15 nm and the Cu₂O shell thickness is about 60–30 nm. It was apparent from the image that each core–shell NP only contained a single Au NP core. In order to obtain more detailed growth information on the shell formation of the Au@Cu₂O core–shell NPs, HRTEM was applied and the results are shown in Fig. 1d. The local crystalline lattices at the hetero-interface of the Au and Cu₂O shells revealed that the well-defined interface and continuity between metallic Au NPs and Cu₂O shells are indicative of the strong interaction between exposed Au and Cu₂O crystals.³⁶ However, without Au NPs, solid spherical shaped Cu₂O NPs are formed with size ranges from 150–200 nm (see Fig. S1 in the ESI†). X-ray diffraction (XRD) patterns of Au@Cu₂O core–shell NPs exhibit that the Cu₂O shells (JCPDS no. 03-065-3288) have formed on the surface of the face-centered-cubic (fcc) metallic Au nanocrystals (JCPDS no. 98-000-0230) without amorphous or additional phases (see Fig. S2 in the ESI†). The XRD peaks from Cu₂O were significantly broadened largely due to the small size of the primary Cu₂O nanocrystallites in the shells.

Fig. 1 TEM images of Au NPs (a), Au@Cu₂O core–shell NPs (b and c) and HRTEM of the selected area of single Au@Cu₂O core–shell NPs (d).

The formation of Au@Cu₂O core–shell NPs is further characterized by UV-visible spectroscopy. The SPR peaks of the Au NPs is red-shifted in Au@Cu₂O core–shell nanostructures, caused by the larger refractive index of Cu₂O encapsulating the Au cores.³⁶ As shown in Fig. 2, the Au NP absorption peak is recorded at 520 nm, which is red-shifted to 598 nm after Cu₂O shell formation. The weak SPR response is attributed to the thick Cu₂O shells for individual core–shell Au@Cu₂O NPs. The Cu₂O characteristic absorption peak for Au@Cu₂O core–shell NPs is recorded at 458 nm.

Fig. 2 UV-visible spectra of Au and Au@Cu₂O core–shell NPs.

To demonstrate the potential application of the as-prepared Au@Cu₂O core–shell NPs in a gas sensor, we examined the detection of CO and NO₂ gases based on the present Au@Cu₂O core–shell NPs. Also, the advantage of using Au NPs in Au@Cu₂O core–shell NPs is examined by comparing their response with that of bare Cu₂O NPs. Fig. 3 presents the sensing performance toward CO based on the two types of Cu₂O spheres, that is, Au@Cu₂O core–shell NPs and Cu₂O NPs. It shows that the resistance increases after the introduction of CO gas and reaches a saturation stage. When the supply of CO gas is stopped, the resistance starts to decrease again and returns to its original value. This typically shows the p-type semiconducting behavior. It can be seen that the baseline resistance of Au@Cu₂O core–shell NPs is very low as compared to Cu₂O NPs. The sensitivity of metal oxides depends on the extent of adsorption and desorption, whereas the response and recovery time depend on the speed of adsorption and desorption processes. Therefore, Au@Cu₂O core–shell NPs show better sensing capability than bare Cu₂O NPs. The response of Au@Cu₂O core–shell NPs is higher than that of Cu₂O NPs. For example, the response of Cu₂O and Au@Cu₂O core–shell NPs for 1000 ppm of CO gas is 3.35 and 5.67, respectively. For both devices, the response increases with increasing gas concentration. Furthermore, reproducibility is also an important factor, which is checked by repeating the response for 1000 ppm three times. It can be seen from Fig. 3 that Au@Cu₂O core–shell NPs exhibit better reproducibility than bare Cu₂O NPs. The value for response and recovery times is also measured. For example the response time (T_90%) for Au@Cu₂O core–shell and bare Cu₂O NPs is calculated as 3.37 and 5.12 min, respectively, for 1000 ppm of CO gas from Fig. 3. Similarly, the recovery time (T_90%) of Au@Cu₂O core–shell NPs is calculated as 6.45 min from Fig. 3, whereas bare Cu₂O has a very long recovery time. The values indicate that the response and recovery times of Au@Cu₂O core–shell NPs are better than those of bare Cu₂O NPs. Thus, the sensor fabricated from Au@Cu₂O core–shell NPs exhibits a detection limit of 100 ppm, a broader dynamic linear range from 100 to 1000 ppm, higher sensitivity, shorter response time, and recovery time for CO sensing, compared with those obtained by bare Cu₂O NPs.

Fig. 3 Changes in electrical resistance toward different concentrations of CO at a working temperature of 200 °C obtained at bare Cu₂O and Au@Cu₂O core–shell NPs.

The effect of testing temperature on the sensing behavior of both devices is also examined and illustrated in Fig. 4. It can be seen that the response increased with increasing temperature for bare Cu₂O NPs. However, for Au@Cu₂O core–shell NPs the response increases up to 200 °C and then slightly decreased at 250 °C. Au@Cu₂O core–shell NPs show the maximum response of 5.67 at 200 °C, whereas bare Cu₂O shows a response of 4.35 at 250 °C for 1000 ppm of CO gas. Meanwhile, no response curve is observed for bare Cu₂O NPs below 200 °C, whereas Au@Cu₂O core–shell NPs show a 3.09 response at 150 °C for 1000 ppm of CO gas. Furthermore, the sensitivity (slope between ‘R_g/R_a’ and ‘concentration’) of these two samples can be compared by comparing the slope of a calibration graph in Fig. 4a and b. The sensor having an Au@Cu₂O core–shell layer exhibits a higher sensitivity and better dynamics behavior towards CO than that with bare Cu₂O NP layers. Thus, the above results demonstrate that the introduction of Au NPs as a core in the Cu₂O system improves its sensing performance in terms of sensitivity, response time, recovery time and lower operating temperature.

Fig. 4 Plot of response versus CO concentration, and working temperature obtained at (a) Au@Cu₂O core–shell NPs and (b) bare Cu₂O NPs.

Also, the sensing properties of both devices are examined for oxidizing gas, such as NO₂, to investigate the role of Au NPs on the sensing performance of Cu₂O NPs. The dynamic response of bare Cu₂O and Au@Cu₂O core–shell NP sensors for room temperature detection with increasing concentration of NO₂ gas (5–100 ppm) is displayed in Fig. 5a. The plot reveals that the resistance decreases after the introduction of NO₂ gas and reaches saturation. Once the NO₂ feed is stopped, the resistance begins to increase again and returns to its original value. Interestingly, the baseline resistance of Au@Cu₂O core–shell NPs is very high as compared to that of bare Cu₂O NPs. During the exposure of NO₂ the response of both devices increases with increasing gas concentration. The maximum response is 3.23 for Au@Cu₂O core–shell NPs, whereas it is 1.28 for bare Cu₂O NPs for 100 ppm of NO₂ gas. This is the first demonstration of NO₂ gas sensing by Au@Cu₂O core–shell NPs to our knowledge, where a low detection limit has been achieved at room temperature. Previously, Deng et al. studied on NO/NO₂ sensing by Cu₂O nanostructures and found room temperature sensitivity after addition of reduced graphene.³⁷

Fig. 5 Changes in electrical resistance of bare Cu₂O NPs and Au@Cu₂O core–shell NPs toward different concentrations of NO₂ at (a) room temperature and (b) 250 °C.

In addition, the temperature dependent sensing property is investigated for core–shell NPs. The baseline resistance of Au@Cu₂O core–shell NPs is very high as compared to that of bare Cu₂O NPs at low temperatures and decreases with increasing temperature as shown in Fig. 5 (also see the ESI, Fig. S3†). Finally, the baseline resistance of Au@Cu₂O core–shell NPs becomes lower than that of bare Cu₂O NPs above 50 °C. Although, the baseline resistance decreases for both devices but the decrease for Au@Cu₂O core–shell NPs is much sharper. More interestingly, the response curve is opposite for both devices at 250 °C as displayed in Fig. 5b. The responses of these devices are plotted against gas concentration at different temperatures and are presented in Fig. 6. The sensitivity of these two samples can be compared by comparing the slope of a calibration graph in Fig. 6a and b. The sensor having Au@Cu₂O core–shell layers exhibits a higher sensitivity to NO₂ than that with bare Cu₂O NP layers at low operating temperatures. It can be seen that the response decreases with increasing temperature for Au@Cu₂O core–shell NPs, whereas the sensor response increases for bare Cu₂O NPs. For example, the response decreases from 3.23 to 1.47 at 25 °C to 250 °C for Au@Cu₂O core–shell NPs. Also, it did not show any response at 200 °C. Meanwhile, the response at 150 °C is slightly higher than that at 250 °C, but the response curve is not stable, and therefore it could be ignored (see the ESI, Fig. S3†). However, the response of Cu₂O NPs increases from 1.28 to 2.14 for 100 ppm of NO₂ gas, when the temperature is increased from 25 °C to 200 °C. In addition, when the temperature reaches above 200 °C the response slightly decreases to 1.75. However, the response of Au@Cu₂O core–shell NPs is higher than that of bare Cu₂O NPs below 200 °C and lower above 150 °C at every concentration of NO₂ gas. Meanwhile, the difference in the response curve of both devices suggests that they behave like p-type semiconductors below 200 °C and n-type semiconductors above 200 °C for NO₂ gas.

Fig. 6 Plot of response versus NO₂ concentration, and the working temperature obtained at (a) Au@Cu₂O core–shell NPs and (b) bare Cu₂O NPs.

The operating principles of p-type transition metal oxide gas sensors are based on the change of sensor conductivity by controlling mobile charge carriers. When the Cu₂O sensor is exposed to air, oxygen molecules adsorb on the surface of the materials to form O₂⁻, O⁻, and O²⁻ ions by capturing electrons from the conduction band. The transfer of electrons to oxygen species generates holes, and therefore the resistance decreases.

When the Cu₂O sensor is exposed to CO environments, the adsorbed CO gas then reacts with the chemisorbed oxygen anions at the surface; the reaction can be described as follows:

2CO_(gas) + 2O⁻_(ads) → 2CO_2(gas) + 2e⁻ (1)

The gas removes chemisorbed oxygen anions and is oxidized. The electrons from the red–ox reactions between CO and adsorbed surface oxygen are injected into the conduction band of Cu₂O, and the recombination between the electrons and holes results in a lower carrier concentration. Subsequently, the resistance of Cu₂O increases. The significant variation in resistance can be attributed to the special structural advantages of the Au@Cu₂O core–shell NPs. The low baseline resistance of Au@Cu₂O compared to that of bare Cu₂O NPs can be explained on the basis of electronic and chemical sensitization of Au NPs.^10,28 The electronic sensitization of Au NPs results in the transfer of electrons from the conduction band of Cu₂O to Au NPs, because the Au work function (5.1 eV) is larger than that of Cu₂O (4.84 eV). Thus, the energy band bends higher at the Au–Cu₂O interface, which results in greater electron withdrawal and the increase in hole mobility.³⁸ In chemical mechanisms, Au NPs catalytically activate the dissociation of molecular oxygen, whose atomic products then adsorb onto the Cu₂O support resulting in a greater degree of electron withdrawal from the Cu₂O than for the bare Cu₂O, which again results in the increase in hole mobility. Therefore, the resistance of Cu₂O decreases with the introduction of Au NPs. A schematic illustration of charge transfer in the Au–Cu₂O system is shown in Fig. 7. Since the response is measured by R_g/R_a for CO gas, the lowering of the R_a for Au@Cu₂O core–shell NPs results in an increase in response.

Fig. 7 Energy band diagram and conduction of current in Au@Cu₂O core–shell NPs.

On the other hand, when the Cu₂O sensors are exposed to NO₂ gas, which is a better oxidizing agent than oxygen, it adsorbs directly on the surface of Cu₂O NPs.³⁹

NO_2(gas) + e⁻ → NO₂⁻_(ads) (2)

The above reactions result in the decrease in concentration of electrons on the surface of Cu₂O and promotes the hole conductivity in the Cu₂O device, which results in a decrease in resistance of the material. This change in resistivity can be used for the detection of NO₂. However, this phenomenon occurs only at low temperatures (below 200 °C) while at high temperatures (250 °C) the response transient is opposite and behaves like an n-type semiconductor. This change in response transient can not be explained on the basis of transition of p-type Cu₂O NPs to n-type because the same sample shows a typical p-type semiconductor sensing property for CO gas at these temperatures. Therefore, this change in response transient is possibly related to the NO₂ gas chemistry. It is well known that above 150 °C, NO₂ decomposes with the release of oxygen via an endothermic process.⁴⁰

2NO_2(g) → 2NO_(g) + O_2(g) (3)

In the case of NO, it reacts with adsorbed oxygen species and forms NO₂ gas.⁴¹

NO_(g) + O²⁻_(ads) → NO_2(g) + 2e⁻ (4)

The electrons from the red–ox reactions between NO and adsorbed surface oxygen are injected into the conduction band of Cu₂O, and the recombination between the electrons and holes results in a lower carrier concentration. Subsequently, the resistance of Cu₂O increases. However, still more work is needed to completely understand such behaviors.

Again, the response of Au@Cu₂O is higher than bare Cu₂O NPs at low temperature and it can be explained on the basis of baseline resistance. For example, at room temperature, the sensor shows a typical p-type semiconductor sensing behavior and the response is measured by R_a/R_g. Since R_a of Au@Cu₂O is higher than bare Cu₂O NPs, its response is also higher. A further increase in temperature leads to a decrease in response for Au@Cu₂O, whereas an increase in bare Cu₂O as shown in Fig. 8a. However, the response of Au@Cu₂O is higher than that of bare Cu₂O NPs up to 100 °C. From 150 °C to 250 °C, the response transients are not stable, therefore it is difficult to distinguish which sensor device has better sensing performance (see the ESI, Fig. S3†). Again above 200 °C the response transient of Au@Cu₂O is better than bare Cu₂O NPs. Furthermore, it can be seen from Fig. 8b that the baseline resistance decreases with an increase in temperature, but this decrease in Au@Cu₂O is very sharp and possibly this is the reason why the response decreases with increasing temperature.

Fig. 8 Temperature dependent (a) response for 100 ppm NO₂ gas and (b) baseline resistance of bare Cu₂O and Au@Cu₂O core–shell NPs.

The decrease in baseline resistance with temperatures is interesting and needs to be explained. The baseline resistance depends on the height of the potential energy barrier to carrier transport between neighboring grains in the metal oxide, which is a function of temperature. It is examined by the manner in which carrier transport occurs. In metal oxide sensors, the carrier migrates from one grain to another, which is a temperature-dependent property.⁴² If we consider a migration transport path with resistance R_m, then R_m increases exponentially according to eqn (5)

R_m = R_mo exp(eVs/RT) (5)

where, Vs is the double potential energy barrier height, RT is the thermal energy, and R_mo is the resistance at Vs = 0. Thus, the resistance decreases with increasing temperature.

Apart from temperature, the potential energy barrier is also a function of atmosphere oxygen partial pressure and dopant concentration;

where, N_t is the surface density of adsorbed oxygen ions (O₂⁻, O⁻), ε_r and ε_o are the permittivities of the semiconductor, and N_d is the volumetric density of the electron donors. Each of these parameters influences the energy barrier, the conductivity, and thus the sensitivity. In n-type metal oxides, the baseline resistance depends on the formation of a depletion layer due to dissociation and adsorption of oxygen molecules and electron transport. The resistance is gradually increased with the O₂⁻_(ads) concentration due to conductivity lowering near the n-type metal oxide surface. The O₂⁻_(ads) chemisorption process is accelerated as the temperature is raised, leading to a more obvious current decay at high temperatures. However, Cu₂O is a p-type metal oxide in which conductivity increases with oxygen chemisorption. It means that the electron withdrawal from the conduction band of Cu₂O by oxygen molecules leads to increment in the hole mobility. Thus the baseline resistance decreases with increasing temperature. Therefore, the baseline resistance is a cumulative effect of these two factors. However, the high base line resistance of Au@Cu₂O core–shell compared to Cu₂O NPs at low temperature is unknown and needs further investigation. At this stage, it may be possible that electrons are the majority carriers and transfer of electrons from Cu₂O to Au NPs results in the increase in resistance.⁴³

Conclusions

Au@Cu₂O core–shell NPs were synthesized and used for the detection of CO and NO₂ gases at various temperatures and concentrations. The sensing mechanism was discussed. The Au@Cu₂O core–shell NPs were made of 12–15 nm Au NP cores were covered with 60–30 nm Cu₂O shell layers. The SPR peak of Au NPs was red-shifted (520–598 nm) after Cu₂O shell formation due to the high refractive index of Cu₂O NPs. The response of Cu₂O NPs to CO was increased with the addition of Au NPs at high temperatures, whereas the response to NO₂ was found to be higher at lower temperatures. The response curve to NO₂ gas changed above 200 °C due to the dissociation of NO₂ into NO and O₂. The better performance of Au@Cu₂O core–shell NPs was mainly attributed to the pronounced electronic sensitization, high thermal stability and low screening effect of Au NPs.

Acknowledgements

This work was supported by the research fund from Chonbuk National University in 2012, BK21 plus program from Ministry of Education and Human-Resource Development and National Research Foundation (NRF) grant funded by the Korea government (MEST) (NRF-2010-0019626, 2012R1A2A2A01006787).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: TEM images of Cu₂O, XRD profile of Au@Cu₂O core–shell NPs and resistance change of materials at various temperatures and concentrations of NO₂ gas. See DOI: 10.1039/c3nr04118b

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