Au functionalized ZnO nanowire gas sensor for detection of benzene and toluence. One novel sensing hybridmaterial of Au nanoparticles (Au NPs)functionalized ZnO nanowires (AuZnO NWs) was successfully synthesized by a twostage solution process. First, ZnO NWs were fabricated via a lowtemperature onepot hydrothermal method with SDSN introduced as structuredirecting agent. Afterward, the asprepared ZnO NWs were used as supports to load Au NPs with small sizes via precipitating HAuCl4 aqueous solution by ammonia. The obtained samples were characterized by means of XRD, SEM, TEM and EDX. Both pristine and AuZnO NWs were practically applied as gas sensors to compare the effect of Au NPs on the sensing performances, and the obtained results demonstrated that after functionalization by catalytic Au NPs, the hybrid sensor exhibited not only faster response and recovery speeds but also higher response to benzene and toluene than the pristine ZnO sensor at 340 oC, especially showed high selectivity and longterm stability to low concentration toluene, which is rarely reported with this method, indicating its original sensor application in detecting benzene and toluene. To interpret the enhanced gas sensing mechanism, the strong spillover effect of the Au NPs and the increased Schottky barriers caused by the electronic interaction between Au NPs and ZnO NW support are believed to contribute to the improved sensor performance.
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Trang 2Au-functionalized ZnO nanowire gas sensor for detection of
benzene and toluene†
Liwei Wang,a Shurong Wang*,a Mijuan Xu,ab Xiaojing Hu,a Hongxin Zhang,aYanshuang Wanga and Weiping Huanga
Chemistry, Nankai University, Tianjin, 300071, China
Trang 3Abstract
One novel sensing hybrid-material of Au nanoparticles (Au NPs)-functionalized ZnO nanowires (Au/ZnO NWs) was successfully synthesized by a two-stage solution process First, ZnO NWs were fabricated via a low-temperature one-pot hydrothermal method with SDSN introduced as structure-directing agent Afterward, the as-prepared ZnO NWs were used as supports to load Au NPs with small sizes via precipitating HAuCl4 aqueous solution by ammonia The obtained samples were characterized by means of XRD, SEM, TEM and EDX Both pristine and Au/ZnO NWs were practically applied as gas sensors to compare the effect of Au NPs on the sensing performances, and the obtained results demonstrated that after functionalization by catalytic Au NPs, the hybrid sensor exhibited not only faster response and recovery speeds but also higher response to benzene and toluene than the pristine ZnO sensor at 340 oC, especially showed high selectivity and long-term stability to low concentration toluene, which is rarely reported with this method, indicating its original sensor application in detecting benzene and toluene To interpret the enhanced gas sensing mechanism, the strong spillover effect of the Au NPs and the increased Schottky barriers caused by the electronic interaction between Au NPs and ZnO NW support are believed to contribute to the improved sensor performance
Trang 4TOC/Abstract Art
ZnO nanowires
1 2 3 4 5 6 7 8 9 10
0 500 1000 1500 2000 0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
gas out
0 500 1000 1500 2000 0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
gas out
Trang 51 Introduction
With the development of science and technology, large numbers of organic hazardous air pollutants (HAPs)1 have been produced with serious endangerment Among the toxic volatile organic compounds (VOCs),2-5 aromatic hydrocarbon, such as benzene and toluene, may reasonably be carcinogenic, mutagenic and exhibit other adverse health effects.6 Therefore, the task to detect benzene and toluene as fast and accurate
as possible, esp in very low concentration to alarm people the extent of the out-door and in-door inhalation noxious pollutions is of great importance and very imperative
Many conventional technologies applied to determine VOCs accompany various shortcomings, e.g for gas chromatography-mass spectrometry (GC-MS), the disadvantages may be that the subsequent analytical procedures are very complex and time-consuming, and the equipment is expensive, complicated, bulky and energy-consuming.7-9 Currently, the gas sensors are widely adopted to meet these requirements, because they facilitate the gas detection, possess high sensitivity to certain target gases and obtain the real-time detection result Furthermore, the equipment handles easily but is low-cost So the appearance of gas sensors is doomed
to create novel avenues for VOCs detection.7, 10
As the central part of a gas sensor, the sensing materials always favor various metal oxide semiconductor (MOS) nanomaterials, among which ZnO, an n-type semiconductor, is one of the most promising multifunctional materials and has been extensively used as a promising gas sensor candidate due to its suitability to doping, non-toxicity, high electrochemical stability and low cost.11-14 Moreover, one
Trang 6dimensional (1D) ZnO nanowires (NWs) have received the most attention, due to the large surface-to-volume ratio, lower tendency to agglomerate and unique electron transportation properties, which are favorable for gases to diffuse rapidly and effectively through the devices, incurring more surface atoms have chances to participate in the surface reactions.15-18 In recent years, extensive studies have been taken concerning further enhancing the sensor performances Hybrid nanomaterials combine two or more compositions with multiple functions that are not available from the respective component, and thus have gained more attention for various applications in chemical sensor, catalysts, optics, biomedicine and so on.19,20Moreover, the doping of noble metal nanoparticles (NPs) onto MOS is more popular, since the strong spillover effect of the noble metals (Au, Ag, Pt, Pd, etc.) with unique electronic and catalytic properties and the synergic electronic interaction with the MOS might enhance the surface depletion layers, thus promoting the sensing performance.21-29 Many reports have been focused on the assembly of noble metal
onto MOS applied in gas sensors For example, Chai et al reported that the ZnO
microwire sensor functionalized by Pd showed reliable natural gas response at room temperature.22 Liao et al found that incorporation of Pt nanocrystals on CeO2nanowire could significantly increase the sensor response.23 The previous studies also demonstrated that the ZnO nanostructures functionalized by Au NPs presented enhanced gas sensing performances.24,25 Despite the success of the above work, these synthetic processes are high-cost, not facile, toxic or time-consuming Besides, to the best of our knowledge, there are few detailed studies on the gas sensing behaviour of
Trang 7Au NPs-functionalized ZnO NWs Especially, it is still scarce about reports on the Au/ZnO NW-based sensor for the detection of hazardous and carcinogenic benzene or toluene with good selectivity and high sensitivity.
Herein, we present a facile two-stage solution process to successfully fabricate the
Au NPs-functionalized ZnO NWs The Au NPs with small sizes are anchored onto the surface of ZnO NWs via precipitating HAuCl4 aqueous solution by ammonia This green, nontoxic and cost-effective procedure is general to fabricate other noble metal doped MOS hybrids To demonstrate the practical gas sensing applications, the gas sensing performances of the sensor based on the as-fabricated Au/ZnO NWs have been systematically investigated As expected, the hybrid sensor displays enhanced gas sensing performances including response/recovery speed, sensitivity and selectivity in detecting benzene and toluene in comparison with the pristine ZnO NW sensor Besides, the long-term stability of the sensor has been obtained by testing 1 ppm toluene after 5 months, which indicates its great potential for practical application To explain the enhanced gas sensing properties of the Au NPs-functionalized ZnO NW sensor, the gas sensing mechanism is discussed
2 Experimental 2.1 Materials
Chemical reagents (analytical grade) such as Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), sodium carbonate (Na2CO3), sodium dodecyl sulfonate (SDSN), chloroauric acid hydrated (HAuCl4·4H2O) and ammonia were purchased from Tianjin
Trang 8Guangfu Fine Chemical Research Institute and used as received without further purification Distilled water and absolute ethanol was used throughout the experiment
2.2 Preparation of Au/ZnO NWs
This includes a two-stage solution process First, the fabrication process of ZnO NWs was described elsewhere31 but with some modifications, which is presented in the
ESI†, the section of “Preparation of ZnO NWs” Secondly, Au/ZnO NWs were
synthesized by adding 0.04 g of the prepared ZnO NWs into 10 mL of deionized water under stirring and then ultrasonic treated for 10 min, then 0.812 mL of 0.01 mol/L HAuCl4 aqueous solution was introduced into the system, followed by adding the diluted ammonia solution till the pH reached 9 After stirring for 0.5 h, the precipitate was centrifuged and washed alternately with deionized water and ethanol till the pH was 7, and then dried at 80 oC overnight
2.4 Gas sensor fabrication and test
The detailed fabrication of the gas sensor and processes of gas tests can be seen in our previous works,11-13,24,25,32 which are shown in the ESI†, the section of “Gas sensor
fabrication and test” (inserted with Fig S1) The sensor response is defined as the
Trang 9ratio Ra/Rg, where Ra and Rg are the electrical resistance of the sensor in air and in the tested reductive gas, respectively.13,16,24,29 The response time is defined as the time interval over which resistance attain a fixed percentage (90%) of final value when sensor is exposed to specific concentration of the gas The recovery time is defined as the time interval over which sensor resistance reduces to 10% of the saturation value when the sensor is exposed to specific gas concentration and then is exposed to clean air
3 Results and discussion 3.1 Characterization
The crystal phase of the pristine ZnO NWs and Au/ZnO NWs was characterized by XRD Fig 1a shows the XRD patterns of the pristine ZnO NWs and Au/ZnO NWs, and it is clear that the main reflection peaks in the pattern are in well agreement with the standard data of wurtzite structure of ZnO (JCPDS: 36-1451) with lattice parameters a=3.25 Å and c=5.207Å.25 But in the XRD pattern of Au/ZnO NWs, apart from the characteristic reflection peaks from ZnO, two weak reflection peaks at 2θ=38.2° and 64.6° are currently indistinguishable from the signal noise, which may also be caused by the low content of Au crystals or the small crystal sizes After zooming the two sections (2θ from 35° to 39°, and from 62° to 66°) in a larger scale shown in Fig 1b and c, the two weak peaks can be clearly observed, which can be respectively ascribed to Au (111) and Au (220) planes of face-centered cubic (fcc) Au (JCPDS: 04-0784), respectively This well confirms the presence of Au species Fig 2
Trang 10shows the low-magnification (a) and high-magnification (b) FESEM images of the as-synthesized pristine ZnO products, which indicates that the samples were composed of wire-like nanostructures The length of ZnO NWs is up to 16 µm and the diameter is about 50–120 nm, and the smooth surface of the ZnO NWs can also be seen from the magnified image in Fig 2b
Further structural characterizations of the samples were performed by TEM analysis Fig 3a and b clearly show the details of an individual wire-like ZnO nanostructure with smooth surface Fig 3c displays the HRTEM image from a certain ZnO nanowire, where the crystal lattice fringes are clearly observed and average distance between the adjacent lattice planes is 0.26 nm, corresponding to the (0002) plane lattice distance of hexagonal-structured ZnO, which proves that the prepared ZnO NWs grew along [0001] direction The TEM images with different magnification
of the Au/ZnO NWs are shown in Fig 3d and e, and from the images, a high density
of Au NPs with small sizes can be seen clearly and uniformly anchored on the surface
of ZnO NW, because they present as the small black dots contrast against the support
But the irrelevant existence of some individual large Au bulks marked by white arrows in Fig 3d and e is probably due to the aggregation of some small Au NPs that act as initial seeds.19 Supportingly, Fig 3f displays the size distribution histogram for the ca 55 Au NPs in Fig 3e, revealing a particle size range mainly between 2-10 nm with an average particle size of around 6 nm, and such fine size and high disperse of
Au NPs might interpret why the diffraction peaks of Au NPs are weak in the XRD pattern (Fig 1a) EDS analysis (shown in the ESI†, Fig S2) was carried out from Fig
Trang 113e to confirm the surface compositions of the hybrid and the result displays the existence of Au, Zn and O elements, confirming the successful assembly of Au on ZnO NWs In addition, the EDS test reveals that the Au loading content in the hybrid sample is 6.57 wt.%, which is higher than the theoretical value of 3.85 wt.% This is possibly caused by such factors: during the doping process, ultrasonic treatment of the as-synthesized ZnO NWs would generate some super fine ZnO particles, and then the inevitable loss of these fine particles occurs during the several washing processes after doping So the reduction of ZnO support increases the loading content of Au NPs
Based on the above results, the possible formation processes for Au/ZnO NWs described in Fig 4 can be expressed as the following two steps:
1 For pristine ZnO NWs under hydrothermal condition:31,33
2 For Au/ZnO NWs under aqueous solution and then oven drying at 80 oC:32
In step 1, according to Eq 1 and 2, CO32−can hydrolyze to form OH−, and then
OH− reacts with Zn2+ to produce Zn(OH)2 precipitation (Eq 3) In the hydrothermal process, the above reactions can be accelerated, and more and more Zn(OH)2 growth units are generated and dehydrate into ZnO nuclei simultaneously (Eq 4) Afterwards, these ZnO nuclei can oriented grow under the structure-directing action of surfactant
Trang 12SDSN, which can reduce the surface tension of solution, lower the energy needed to form a new phase, and further control the growth rates of different crystalline faces of ZnO crystals by kinetically incorporating these ZnO nuclei along the c-axis of ZnO crystal lattice through suitable adsorption and desorption, and finally resulting in the formation of ZnO NWs.34 In step 2, the formation of the bonding between Au and ZnO support is not merely a simple ammonia-precipitation process but also based on the direct anionic exchange (DAE) theory of the gold species with the OH groups on the support in different pH solutions.35 The as-prepared ZnO NWs are pre-dispersed
in water by ultrasonic treating for 10 min to enhance its hydrophility, then after adding the HAuCl4 to the solution, AuCl4− species are adsorbed onto ZnO surface depending on the strong interparticle forces like electrostatic interactions and van der Waals to form an intergral combination.30 As ammonia is added dropwise, the change
of pH leads to increasing the OH groups quantitatively, which promotes the hydrolysis of AuCl4− via the anionic exchange between the OH− on the support and
Cl− ligands bonding to gold species to form the various different gold complexes, such
as [AuCl3(OH)]−, [AuCl2(OH)2]− and [AuCl(OH)3]− And all the species have strong interactions with ZnO support.35 Finally Au(OH)3 precipitates on the ZnO surfaces at
pH about 9, and then decompose into pink thermal-unstable Au2O3 and finally Au NPs are generated after drying to obtain Au/ZnO NW products
In all, the achievements are two points: firstly, these small size Au NPs present a uniform dispersion throughout the ZnO supports to form the so-called metal–support interaction;36-39 secondly, these fine Au NPs can dissociate oxygen molecules due to
Trang 13the well-known spillover effect.38-42 Thus the Au NPs could serve as active sites to promote sensing reactions between ZnO and target molecules which should be very favorable for gas sensing application
3.2 Gas sensing performance
Although various noble metal–metal oxide hybrid nanostructures have been investigated for gas sensing test applications,23-29,32,40 there has been rare report on the aromatic hydrocarbon detection, esp for the kind of Au/ZnO NW based sensor So it should be a great contribution and progress to the detection of benzene and toluene due to the fabrication of the special Au/ZnO NW based sensor
Fig 5 compares the dynamic sensing response–recovery curves of the pristine ZnO and Au/ZnO based sensors to toluene (a) and benzene (b) at the working temperature
of 340 oC, with different gas concentrations in the sequence of 1, 5, 10, 50, 100, 200 and 500 ppm It shows clearly that the response amplitudes of the two sensors exhibit prominent increases with increasing gas concentrations, and obviously the response amplitudes of Au/ZnO based sensor to all gas concentrations are much higher than those of pristine ZnO based one Furthermore, as shown in Fig 5, the output voltage undergoes a drastic ascent when gases are injected in and is quickly restored to its initial value after the gases are out, which indicates both sensors possess fast response and recovery properties
Response and recovery times are very important parameters for gas sensor application A small value of response time means a good sensor and a small recovery time means that the sensor can be used over and over again As shown in Fig 5, the
Trang 14as-fabricated hybrid Au/ZnO NW sensor exhibits faster response and recovery speeds than the pristine ZnO NW sensor via 1 to 500 ppm toluene and benzene Table 1 analyses the response and recovery times of the Au/ZnO NWs sensor to toluene and benzene from Fig 5, as can be seen from which that all the response times via toluene are shorter than via benzene, but all the recovery times are relatively longer Such result confirms a contradictory phenomenon for gas sensors that when the tested gases are easily adsorbed on the sensor, the desorption after reaction often relatively more difficult, as can be seen in other reports.19,30,43 Take 10 ppm toluene and benzene for example, the response/rescovery time is 50/35 s and 70/27 s, respectively, which is short enough for its practical application
The relations between sensor response and gas concentration are shown in Fig 6
Obviously, the hybrid sensor exhibits higher responses than those of a pristine ZnO sensor, e.g for 100 ppm toluene, the responses of hybrid and pristine sensors are 8.632 and 6.433, respectively, and the former is as 1.34 times as the latter Similarly, for benzene, the responses are 5.712 and 4.057, respectively, and the former is as 1.4 times as the latter, indicating its improved and novel toluene-sensing performance that should be related to the doping of Au NPs Besides, the responses of hybrid sensor to toluene are higher than those to benzene, e.g for 10 ppm under 340 oC, the responses are 6.275 and 3.635, respectively, and such response is higher than the other reported literatures,44-46 Furthermore, the hybrid sensor are more sensitive in lower toluene concentrations, as a sharp and nearly linear increase below 100 ppm in Fig 6a can be seen It is possibly that when the gas concentrations are very high, e.g above 100 ppm,
Trang 15the adsorption of gas molecules on the sensing surface reaches saturated and will on the contrary reduce the sensitivity, so the response changes gently To get more details of the hybrid sensing properties, the responses for toluene in the range of 1–100 ppm are measured in Fig 7 From Fig 7, an obvious change can be seen from
1 ppm to 30 ppm which presents a linear increase of the responses However, a close comparison from 30 ppm to 100 ppm reveals that the responses increase with a small amplitude Based on the results, the sensor’s effective and sensitive detection limit can be estimated about 30 ppm
In order to investigate the selectivity of the Au/ZnO sensor, the gas sensing performance of 10 ppm other gases (acetone, chlorobenzene, formaldehyde, chloroform, ether and ammonia) were also measured, and the results are summarized along with benzene and toluene and shown in Fig 8 It is noted that compared with all the other gases, both hybrid and pristine ZnO sensors exhibits the highest responses to toluene with the values up to 6.275 and 4.199 respectively, while the responses of hybrid sensor to all tested gases are significantly larger than those of pristine ZnO sensor, indicating that the sensing ability of ZnO has been effectively improved by Au modifying Besides, the Au/ZnO sensor exhibits higher response to toluene than benzene, which is probably because toluene molecule has a methyl and is more active than benzene, thus it may be easily selectively adsorbed on the sensor Moreover, high selectivity property of the hybrid sensor also get approved by comparing lower concentration of toluene with a slightly higher concentration of other VOCs, e.g the response of 10 ppm toluene is 1.5 times as that of 30 ppm ether, and 1.8 times as that
Trang 16of 30 ppm chloroform So all the above results prove that the Au/ZnO NWs has a potential for developing toluene sensor
The Long Term Stability (LTS) is another important parameter to characterize the gas sensor To investigate the LTS of Au/ZnO NW sensor, the gas sensing performance is evaluated once again after five months Fig 9 illustrates the dynamic response-recovery curves of Au/ZnO NW sensor to 1 ppm toluene for five circles at
340 °C after five months It can be clearly seen that the sensor still shows high sensitivity, fast response/recovery characteristic and good reproducibility after five months, confirming the good LTS of the prepared Au/ZnO NW sensor
3.3 Sensing mechanism
Fig 10 presents the schematic illustration for the sensing mechanism of the Au/ZnO
NW sensor ZnO, as a typical n-type MOS, belongs to the surface-controlled conductivity type to detect gases When ZnO NW sensor is exposed to air, O2 in air can be adsorbed on the surface of ZnO NWs and then extract electrons from the conduction band of ZnO to form adsorbed oxygen negative ions species Oδ− (O2 , O−and O2−), which can produce a depletion layer on the surface of ZnO NWs and result
in a decrease in the conductivity or an increase in sensor resistance.11-13,22,47,48 When the ZnO NW sensor is exposed to tested reductive gases, such as toluene or benzene vapors, the surface Oδ− can react with the reductive gas molecules and release electrons back to the conduction band of ZnO NWs As a result, the depletion layer becomes thin and the sensor resistance increases The above obtained gas sensing measure results have shown that the gas sensing performances of ZnO NWs have
Trang 17been significantly improved by Au NPs functionalization, which can be interpreted by the spillover effect and catalytic effect of Au NPs on the surface sensing reaction As shown in Fig 10, oxygen molecules in air can be easily adsorbed on the Au NPs and
be dissociated into oxygen species, due to the spillover effect of noble metals,32,37,40,41which will then be transported and distributed onto the surface of ZnO NWs The result is favorable to accelerate the reactions happening at the interface between Au and ZnO support
As shown in the TEM picture (Fig 4e), the small Au NPs with an average size of
ca 6 nm are highly dispersed on ZnO NWs, which is very beneficial for oxygen spillover, and thus facilitate the sensing reaction between the surface Oδ− and target gas On the other hand, there exists a catalytic synergy effect36,42,49 between Au and ZnO phases In this effect, the Au NPs play the role of active sites for gas sensing reactions, as well as an excellent medium to supply nanochannels for electron transfer
in the sensing process In a word, the significantly improved gas sensing performances of Au/ZnO NW sensor indicates its promising sensor application in detecting toluene
4 Conclusions
In summary, a novel Au nanoparticles-functionalized ZnO nanowires (Au/ZnO NWs ) has been successfully synthesized by a two-stage solution process XRD results indicated that ZnO with hexagonal wurtzite structure could be generated through a simple hydrothermal treatment without calcination SEM and TEM analyses revealed