Au-deposited porous single-crystalline ZnO nanoplates for gas sensing detection of total volatile organic compounds

Abstract

The indoors volatile organic compounds (VOCs), which can be classified into seven groups as oxy hydrocarbons, halogenated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons, terpenes, esters and aldehydes, have been brought to the forefront because millions of people are suffering from indoor air pollution. In this study, well-dispersive Au nanoparticles were deposited on porous single-crystalline ZnO nanoplates (Au@ZnO) via photodeposition free from additives for the gas sensing detection of the seven-group VOCs. The structure and morphology of Au@ZnO nanoplates were characterized by TEM, HRTEM, mapping and XRD. Comparing to pure ZnO nanoplates, the Au@ZnO sensor has prominent enhanced performance at 360 °C with high sensitivity (about 2–9 times for each group of VOCs), fast response/recovery time (less than 30/14 s) and stable repeatability (relative standard deviation less than 0.039) in the gas sensing tests. The fluctuation range of relative humidity is less than 80% and the contribution ratios of oxy hydrocarbons, halogenated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons, terpenes, esters and aldehydes were 0.1678, 0.0989, 0.0826, 0.0739, 0.2396, 0.1887, 0.1495, respectively. Finally, the possible sensing mechanism was discussed based on these results.

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1 Introduction

Volatile organic compounds (VOCs), which can cause several kinds of diseases, including allergies, emphysema, asthma, and cancer, are very harmful for human beings.^1,2 More than ten kinds of VOCs were listed as priority pollutants due to their carcinogenicity, teratogenicity and mutagenicity.³ The “sick house syndrome” caused by indoor VOCs has become a world-wide issue, because most people spend more than 80% of their time indoors.^4,5 According to the WHO's report in 2011, over 200 million in developing countries were caused by interior space pollution each year.⁶ The indoor VOCs are usually include seven groups which can be classified into oxy hydrocarbons, halogenated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons, terpenes, esters and aldehydes.⁷

Various on-site techniques to detect VOCs have been proposed, such as spectrum monitoring system (e.g. FTIR^8,9), Surface Plasmon Resonance (SPR) photodiode detector,¹⁰ gravimetric chemical sensor,¹¹ catalytic combustion sensor,⁷ metal oxide semiconductors gas monitor equipment,^12–15 photoionization detector¹⁶ and so on.^17,18 Among these sensors, metal-oxide semiconductor sensor, which works based on the change of conductivity caused by the temperature, humidity and concentration of target gases, is suitable for VOCs detection.¹⁹ Various metal-oxide semiconductors, such as SnO₂, Co₃O₄, ZnO and so on, have been applied for gas sensors due to their novel morphologies and structures of metal-oxide semiconductor.^20–27 SnO₂, a typical n-types semiconductor, which was prepared as nanobelts, nanotubes, nanowires, nanospheres and nanorods, has been introduced to build gas-sensor for advanced ethanol, NO₂ and formaldehyde sensors.^28–34 Co₃O₄ is a common p-types semiconductor, which has also been reported as effective gas sensors of acetone, benzene, and ethanol for its structures of nanorods, hollow microspheres, and nanocubes to ethanol, formaldehyde and toluene.^12,35–37

ZnO, as an n-type semiconductor with a direct band gap of 3.37 eV and common sensor material, has been widely explored.²¹ Various ZnO morphologies, such as nanowires, nanobelts and nanoflowers, fabricating by various methods such as metal-catalyzing growth, thermal evaporation and hydrothermal synthesis, have been studied as gases detection for alcohol and so on.^38–42 Porous ZnO nanoplates, a hierarchical-structured materials with high surface areas and porosity which can provide more active sites and enhanced reactivity, has been extensively applied as gas sensors for the detection of VOCs.^43–45 Noble metal deposition onto metal oxide nanostructures has also been introduced for further improvement of the gas sensor performance.^19,46–48 Au, as a common and effective catalyst, has been used to promote the gas sensor properties of various ZnO nanostructured materials for ethanol and acetone detection, via one-pot hydrothermal method, solvothermal method or chemical reduction method.^49–54

In this report, high dispersed Au nanoparticles were deposited on porous ZnO single-crystalline nanoplates via photoreduction free from additives in the products, showing excellent sensor performance for detecting seven groups of total volatile organic compounds. The contribution ratios of each group in the VOCs gas tests were explored and the gas-sensing properties of the Au@ZnO sensor were studied systematically.

2 Experimental

2.1 Materials

All the reagents were analytically pure and used without further purification. Urea (CO(NH₂)₂), zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), and chloroauric acid (HAuCl₄·4H₂O) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).

2.2 Characterization techniques

A Bruker D8 advanced X-ray powder diffractometer with graphite monochromatized Cu Kα radiation (λ = 0.15418 Å) at room temperature was performed to obtain the X-ray diffraction (XRD) patterns of the structure of the Au@ZnO nanoplates. The detailed morphologies and microstructures were observed by high-resolution transmission electron microscopy (HRTEM, Japan, JEM-2100, operated at 200 kV). The elemental analysis were investigated by the selected area electron diffraction (SAED) and Energy Dispersive X-Ray Spectroscopy (XEDS), using an X-ray micro-analyzer embedded in the JEM-2100 microscope. Gas sensing properties were measured using a static system controlled by a computer (HW-30A, Hanwei Electronics Co. Ltd.) under a humidity of 30%.

2.3 Synthesis of porous ZnO nanoplates

In a typical experiment, 15 mL Zn(CH₃COO)₂ solution (0.2 mol L⁻¹) was added into 15 mL urea solution (0.4 mol L⁻¹) followed by ultrasonic dispersing for 10 min. Then the mixture was sealed into a Teflon-lined stainless-steel autoclave with a capacity of about 50 mL and heated at 120 °C for 2 h. After cooled down to room temperature naturally, the resulting precipitate was centrifuged and washed three times, and dried at 60 °C in air. Finally, porous ZnO nanoplates were obtained after annealing the precursors at 400 °C for 2 h in air.

2.4 Photodeposition of Au@ZnO

Au-doped ZnO (Au@ZnO) nanoplates were prepared by photodeposition. ZnO (0.2 g) was suspended in 20 mL of solution containing 0.02457, 0.07372 and 0.1229 mmol HAuCl₄ to obtain different content Au@ZnO sensors. The slurry was illuminated for 5 h with 500 W mercury lamps under magnetic stirring. Finally, the products was separated by centrifugation, washed for 3 times, and labelled as 1% Au@ZnO, 3% Au@ZnO, 5% Au@ZnO, respectively.

2.5 Gas sensing measurements

To fabricate a gas sensor, the ZnO and Au@ZnO powders were mixed in ethanol to obtain paste respectively, followed by being printed onto alumina tubes with a Ni–Cr heating wire placed inside. And then, the tubes were aged at 450 °C for 120 h to improve the stability and repeatability. Gas sensing properties were measured using a static system controlled by a computer (HW-30A, Hanwei Electronics Co. Ltd.) under a humidity of 30%. The operating temperature could be adjusted precisely through the current flow across the Ni–Cr heater. Sensor response (S) was defined as S = R_a/R_g, where the R_a and R_g were resistance in air and target gas, respectively.

3 Results and discussion

3.1 Structure and morphology

The structures of the obtained materials were analyzed by X-ray powder diffractometer (XRD). As shown in Fig. 1, the XRD patterns indicates that all the peaks of the pure porous ZnO nanoplates can be indexed to the Joint Committee for Powder Diffraction Standards (JCPDS, file no. 36-1451). No characteristic peaks of other impurities were observed, indicating the high purity of the obtained ZnO nanoplates. A comparison among the curves of the pure porous ZnO nanoplates and Au@ZnO (1% Au@ZnO, 3% Au@ZnO and 5% Au@ZnO) materials suggested that the XRD patterns of Au@ZnO materials are similar to that of ZnO, indicating that the formation of Au particles in the deposition process has no influence on the crystal structure of ZnO. All the peaks of the three curves of Au@ZnO materials, (111), (200), (220), and (311), can be indexed to the JCPDS of file no. 04-0784. With the increasing of Au content, the peaks of (111), (200), (220), and (311) shape and become strong, indicating that more metallic Au nanoparticles formed in the Au@ZnO heterostructures. The (111) peak of Au can also correspond to the lattice fringes of Fig. 2(b₂–d₂).

Fig. 1 XRD patterns of (a) pure porous ZnO nanoplates, (b) 1% Au@ZnO, (c) 3% Au@ZnO and (d) 5% Au@ZnO.

Fig. 2 (a), (b), (c) and (d) represent TEM images of the pure porous ZnO nanoplates, 1% Au@ZnO, 3% Au@ZnO and 5% Au@ZnO, respectively. (a₁) represents SEAD images of the pure porous ZnO nanoplates; (b₁), (c₁) and (d₁) represent particle size distribution of 1% Au@ZnO, 3% Au@ZnO and 5% Au@ZnO, respectively. (a₂), (b₂), (c₂) and (d₂) represent HRTEM images of the pure porous ZnO nanoplates, 1% Au@ZnO, 3% Au@ZnO and 5% Au@ZnO, respectively.

The products were then imaged by transmission electron microscopy (TEM). The Fig. 2(a) reveals the random porous structures in the nanoplates of the pure ZnO. Compared to the pure ZnO, high dispersed addition particles on porous ZnO nanoplates were displayed in the Fig. 2(b–d). Obviously, the particles are small and scarce in Fig. 2(b). With the increasing amount of additions, the amount and size of the Au particles on porous ZnO nanoplates increase. The locally crystalline nature of the porous ZnO nanoplates was analysed by selected-area electron diffraction (SAED). The SAED patterns can be indexed to the [1−10] zone axis of wurtzite single-crystalline ZnO in Fig. 2(a₁). As shown in Fig. 2(b₁–d₁) the 1% Au@ZnO, 3% Au@ZnO and 5% Au@ZnO have an average size of 3.88 nm, 9.13 nm and 7.54 nm, respectively. It may indicate that the Au particles are preferred to randomly deposit on the porous ZnO single crystalline nanoplates with limited particle size.

Further detailed structure analysis of the products, presenting in the Fig. 2(a₂–d₂), were achieved through high resolution transmission electron microscopy (HRTEM). As shown in Fig. 2(a₂), fringes separated by 0.281 nm are consistent with the (110) lattice spacings of wurtzite single-crystalline ZnO. Those fringes separated by 0.236 nm in Fig. 2(b₂–d₂) could be matched to the (111) lattice spacings of cubic Au. As shown in Fig. S1,† the particles observed in TEM were verified to be Au through elemental mapping. The results of elemental analysis is shown in Table S1,† verifying the existence of gold. Therefore the heterostructure can be considered as the deposition of Au particles on porous ZnO nanoplates.

3.2 Gas-sensing properties

The operating temperature of the sensors composed by the obtained materials were studied by using representative VOCs as target gases at 50 ppm. The temperature gradient was set as 100 °C, ranging from 60–460 °C, to ensure that all the optimum temperatures of VOCs can be covered. The VOCs can be classified into 7 groups and the representative VOCs were chosen as follows (Table 1).

Table 1 Classification and representative gases of VOCs

Classification of VOCs	Representative gases
Oxy hydrocarbons	Methanol
Halogenated hydrocarbons	Chlorobenzene
Aromatic hydrocarbons	Benzene
Aliphatic hydrocarbons	n-Heptane
Terpenes	Isoprene
Esters	Ethyl acetate
Aldehydes	Formaldehyde

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As shown in Fig. 3, at a relative low temperature of 60 °C, all the sensors display weak response to VOCs. The responses of the four sensors then increase with the growing operating temperature before 260 °C. Afterwards, the response of the four sensors exhibit different tendencies. Fig. 3(a) shows the responses of the pure ZnO nanoplates sensors. The highest responses of chlorobenzene, n-heptane, and benzene appear at 460 °C, those of diethyl ether and methanol appear at 260 °C, and those of the rest appear at 360 °C. Comparing to the pure ZnO sensor, the optimum temperature of the Au@ZnO sensors have some changes. According to the Fig. 3(b–d), most of the target gases show highest responses at 360 °C. Other representative gases (diethyl ether, acetone, trichloro ethylene, toluene, o-xylene, n-decane, butyl acetate, hexaldehyde) are also test to obtain the optimum temperature. The optimum temperatures of the four sensors to the VOCs were displayed in Table S2.† The fluctuations of optimum temperature may be caused by the large gaps of operating temperature. And 360 °C was defined as the optimum operating temperature of the four sensors and all the following tests were performed at 360 °C.

Fig. 3 Responses of sensors (a) pure ZnO, (b) 1% Au@ZnO, (c) 3% Au@ZnO, (d) 5% Au@ZnO, exposed to 50 ppm representative gases of VOCs (methanol, chlorobenzene, benzene, n-heptane, isoprene, ethyl acetate, formaldehyde) at different operating temperatures.

As shown in Fig. 4, all the Au@ZnO sensors display higher responses than the pure ZnO nanoplates sensor, indicating the sensitization of Au nanoparticles.⁵⁵ The 1% Au@ZnO sensor shows a higher response than the pure ZnO nanoplates sensor, but lower response than 3% Au@ZnO sensor and 5% Au@ZnO sensor except for acetone. The 3% Au@ZnO sensor has the highest responses to diethyl ether, acetone, chlorobenzene, trichloro ethylene, isoprene, ethyl acetate, butyl acetate, formaldehyde and hexaldehyde. And the 5% Au@ZnO sensor has the highest responses to methanol, benzene, toluene, o-xylene, n-heptane and n-decane. Moreover, it can be seen that the 5% Au@ZnO sensor has a better performance in the detection of benzene series and aliphatic compounds, which are always difficult to test by ZnO-based sensors. When the target gas was acetone, formaldehyde, trichloro ethylene, diethyl ether, ethyl acetate, or chlorobenzene, the 5% Au@ZnO sensor also exhibits higher response values than 1% Au@ZnO. The responses of 5% Au@ZnO sensor are 2–9 times to the seven groups VOCs than that of the pure ZnO sensor. Thus, the 5% Au@ZnO sensor was chosen as the optimal sensor, and the following tests were contrasted between the pure ZnO nanoplates sensor and 5% Au@ZnO sensor.

Fig. 4 Responses of pure ZnO nanoplates and Au@ZnO sensors exposed to 50 ppm VOCs (methanol, diethyl ether, acetone, chlorobenzene, trichloro ethylene, benzene, toluene, o-xylene, n-heptane, n-decane, isoprene, ethyl acetate, butyl acetate, formaldehyde, hexaldehyde) at 360 °C.

Fig. 5 shows the response of the 5% Au@ZnO sensor toward representative VOCs at 360 °C and the concentrations of representative VOCs range from 1–500 ppm. The response increase rapidly below 200 ppm, and then increase slowly with further increase of VOCs concentration, which indicate that the sensor tended to more or less saturated.⁵⁶ To explore the response dependence on the VOCs concentration, all the curves are linearly fitted as shown in Fig. S2.† It can be seen that the responses of acetone, methanol, trichloro ethylene, diethyl ether, ethyl acetate, benzene and toluene increase in proportion to the increasing gas concentration. And the responses of chlorobenzene, o-xylene, n-heptane and formaldehyde increase relatively slow. Furthermore, the responses of most representative VOCs showed a linear relationship with VOCs concentration, which is highly desirable for the calculation of the contribution ratio of each group in the VOCs tests.

Fig. 5 Responses of 5% Au@ZnO sensor versus the representative VOCs concentration at 360 °C.

Fig. S3† displays typical dynamic response curves of 5% Au@ZnO sensor towards the representative VOCs with increasing concentration at 360 °C. The curves show a stepped rise as the increase of the VOCs concentration. The shapes of curves may be influenced by the adsorption rate of target gases and oxygen in air.⁵⁷ The response and recovery time are also significant parameters for gas sensor. The response time is defined as the time necessary for the resistance variation to reach 90% of the equilibrium value after the injection of target gases and the recovery time is defined as the time required to return to 10% above the original resistance in air after the test gas has been released.⁵⁸ The response time and recovery time of 5% Au@ZnO sensor for representative gases at 100 ppm were listed in Table S3.† It reveals that 5% Au@ZnO sensor has fast response/recovery times (compared with other sensor materials shown in Table S4†) for the rapid detection. The gas response performance of the 5% Au@ZnO is comparable with the previous reports (as shown in Table S5†).

The effect of relative humidity (RH) on the 5% Au@ZnO sensor performance was also explored. As shown in Fig. 6(a), the sensor performances of different target gases were influenced by relative humidity differently. With the rise of RH, the responses of isoprene, ethyl acetate and n-heptane increase up to a maximum value at the RH of 35% (n-heptane at 50%) and then decrease. With the rise of RH, the responses of chlorobenzene, benzene and formaldehyde increase up to the maximum value at the RH of 35%. Then the responses decrease to the minimum value at the RH of 50% and rise up again. Compared to other target gases, the response of methanol displays a special tendency, which decreases with the rise of RH at first, and then increase. The fluctuation ranges of the seven groups were 0.023, 0.090, 0.058, 0.163, 0.031, 0.185 and 0.053, respectively. The fluctuation of RH on the sensor performance could be tentatively attributed to the adsorption competition between the target gases and H₂O molecules on the surface of 5% Au@ZnO sensor.⁵⁹

Fig. 6 (a) Gas response of 5% Au@ZnO sensor to 100 ppm VOCs in different relative humidity at 360 °C. The response is defined as the ratio of sensor resistance in humid air (R_ah) to that in humid air with 50 ppm VOCs (R_gh); (b) stability of 5% Au@ZnO sensor towards benzene, formaldehyde and n-heptane at 360 °C for a month, the inset shows the reproducibility at 50 ppm.

The stability of 5% Au@ZnO sensor to benzene, formaldehyde and n-heptane at 360 °C in a month was shown in Fig. 6(b), revealing a good stability of the 5% Au@ZnO sensor which is highly required for practical application. The relative standard deviation is kept in 0.039. The inset figures shows the reproducibility of 5% Au@ZnO sensor, which indicates that the sensor remains its original response value.

The 5% Au@ZnO sensor shows an excellent sensing property for the seven groups of VOCs. Generally, the chemically stable gases, such as benzene, chlorobenzene and benzene own low response value. The contribution ratio (T) was developed to explore the contribution of each group in the VOCs test. As shown in Fig. S2,† most of the gas response can be fitted a straight line. Thus, the contribution ratio can be defined as follow:⁶⁰

where T_j represents the contribution ratio of each group of the representative gases, C_j is the concentration of each group, and S_j denotes the sensor response magnitude to 50 ppm of each group.

The values of S_j are shown in Table S6.† The C_jS_j of each group was equal to the average of the product of concentration and response of each group. For example, the C_j of oxy hydrocarbons was equal to the average of methanol, diethyl ether and acetone concentrations, and the S_j of oxy hydrocarbons was equal to the average of methanol, diethyl ether and acetone response magnitudes. The C_jS_j of oxy hydrocarbons was equal to the average of the product of C_j and S_j.

As shown in Fig. 7, the terpenes give the largest contribution to the response of 5% Au@ZnO sensor because of the high response of isoprene. The esters, aldehydes and oxy hydrocarbons also had considerable contribution ratios. The halogenated hydrocarbons, aromatic hydrocarbons and aliphatic hydrocarbons display just a little lower contribution than others. The contribution ratios of oxy hydrocarbons, halogenated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons, terpenes, esters and aldehydes were 0.1678, 0.0989, 0.0826, 0.0739, 0.2396, 0.1887 and 0.1495, respectively. The contribution ratio can help us to understand the interrelation between the properties of target gases in different environments and the response values.

Fig. 7 Mean contribution ratio (T) of each group 360 °C.

3.3 Gas-sensing mechanism

Many previous studies reported that the specific surface area may have influence on the gas sensing performance.^61,62 The surface area and pore size of porous ZnO nanoplates and 5% Au@ZnO were tested to explore the reasons of the improvements of the gas sensor performance of the Au-deposited ZnO nanoplates. As calculated by the Brunauer–Emmett–Teller (BET) specific surface area from nitrogen adsorption–desorption isotherm in Fig. S4,† the BET surface area of the pure ZnO nanoplates and the 5% Au@ZnO nanoplates are 33.56 m² g⁻¹ and 28.85 m² g⁻¹, respectively. The little difference in surface area suggests that higher response value of the 5% Au@ZnO originates from the deposition of gold nanoparticles.

The mechanism of ZnO-based gas sensor is mainly described as the change of resistance. ZnO is a characteristic n-type semiconductor and the conductivity of ZnO is effected by the surface depletion layer.¹⁹ The porous-nanoplates structure shows high specific surface area and mobility of electrons which is advantageous for the sensing performance. The gas-sensing mechanism of ZnO-based sensor can be defined as an adsorption–oxidation–desorption process. When the ZnO-based gas sensor was exposed in the air, the oxygen molecules are trapped electrons from the conduction band of ZnO and formed oxygen species (O₂⁻, O⁻ and O²⁻), resulting the form of an electron depletion layer. After target gases injected into the test chamber, the target gas molecules were adsorbed onto the working material surface, causing the change of conductivity, and then react with the oxygen species (O₂⁻, O⁻ and O²⁻) as follows:

Target gas + nO⁻ → aCO₂ + bH₂O + ne⁻

And then the resistance of the sensor decreased.

Compared to the pure ZnO nanoplates sensor, the gas sensing property of Au@ZnO sensor increases, which can be explained as the three reasons below. Firstly, more oxygen molecules were adsorbed onto the sensor surface due to the deposition of Au nanoparticles and its spill-over effect. Thus, more electrons were trapped, generating more oxygen species and active sites.⁵⁵ Secondly, around the Au nanoparticles, the generation of depletion region advances the modulation of Schottky barriers, making it easier for the reaction of oxygen species and target gases.⁶³ Thirdly, Au is also a common catalyst for the oxidation of reducing gases, which can expedite the surface reactions due to the strong metal-support interactions between gold nanoparticles and ZnO nanoplates, exhibiting an excellent sensor performance.⁶⁴

4 Conclusion

In this study, well-dispersive Au nanoparticles were deposited on porous single-crystalline ZnO nanoplates via photodeposition free from additives for sensing seven groups of VOCs. Compared to pure ZnO nanoplates, the Au@ZnO sensor displays preferable performance in the gas sensing tests owing to the formation of Au@ZnO hybrids, generating more oxygen species and active sites for adsorption of target gases. The optimum temperature of Au@ZnO sensor were determined as 360 °C. The optimal content was 5% Au@ZnO sensor, which displays high sensitivity (2–9 times for each group), fast response/recovery time (less than 30/14 s) and stable repeatability (relative standard deviation is less than 0.039). The fluctuation ranges of the seven groups causing by relative humidity were kept in 0.2. The contribution ratios of oxy hydrocarbons, halogenated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons, terpenes, esters and aldehydes were 0.1678, 0.0989, 0.0826, 0.0739, 0.2396, 0.1887 and 0.1495, respectively, which can be used to understand the interrelation between the properties of target gases in different environments and the response values.

Acknowledgements

This work has been financially supported by National Basic Research Program of China (973 Program 2013CB934301) and the National Natural Science Foundation of China (NSFC21377068 and 21575077).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05941d

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