Preparation, Characterization, and Mechanistic Understanding of Pd-Decorated ZnO Nanowires for Ethanol Sensing

1 MicroNano System Research Center, College of Information Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China 2 Key Lab of Advanced Transducers and Intelligent Control System of the Ministry of Education, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China 3 College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China


Introduction
Semiconducting metal oxides are widely used in environmental monitoring, industrial safety monitoring and medical diagnosis [1][2][3].Among different types of metal oxides, zinc oxide (ZnO) is of great interest because of its versatile suitability for a large variety of applications.For example, it can be used in gas sensors [4], solar cells [5], field-effect transistors [6], photodetectors [7], and photocatalysts [8].In particular, ZnO can be used in toxic [2] and combustible [9] gas sensors.
Comparing to the bulk material, micro-and nanostructured ZnO can provide a much improved performance for device applications [10].Recently, efforts have been devoted to the realization of gas sensor using ZnO thin films [11], nanoparticles [12], and nanowires [13].Among them, ZnO-NWs are regarded as the most promising candidate for high sensitivity gas sensors for their higher specific surface area and larger length-to-diameter ratio [14].It has also been shown that surface decoration of ZnO with noble metal elements such as Pd, Ag, Pt, and Au can largely improve the sensitivity of the device because of the modification of the energy band gap and the increase of the carrier concentration [15][16][17][18][19]. Pd, for example, is one of the most versatile catalytic materials for its excellent oxidation capability of converting hydrocarbons at lower temperature [20].Hsueh et al. reported that the response of Pd-ZnO-NWs can be increased from 1.56 to 2.60 when exposed to 500 ppm ethanol at 230 ∘ C, with a response of a few seconds and a recovery time of more than 100 seconds, respectively [21].Roy et al. also investigated the Pd-sensitized ZnO nanorods for ethanol sensing, the results show that the response magnitude, response and recovery time are 94%, 14 s and 70 s, respectively, when exposed to 1530 ppm ethanol (in air) at an optimum operating temperature of 200 ∘ C [22].Note that despite the significant improvement in sensitivity, there are still large challenge for improvement of the recovery time and the detection limit of these ZnO ethanol sensors.
In this work, we studied the sensing performance of ZnO-NWs with or without Pd decoration.Vertical ZnO-NWs with an average diameter of 80 nm and a length of 5 m were prepared by hydrothermal growth.To investigate the detection sensitivities of ZnO-NWs and Pd-ZnO-NWs, Pd nanoparticles with various concentrations were deposited on the surface of ZnO-NWs.The results show that ZnO-NWs and Pd-ZnO-NWs can have the highest responses and gas selectivity to ethanol at different operating temperatures.Comparing to the ZnO-NWs sensor, Pd-ZnO-NWs (1.3 at.%) sensor showed a much improved sensitivity and responserecovery time to ethanol.

Experiment
2.1.Preparation of ZnO-NWs.Vertically aligned ZnO-NWs arrays were grown on glass substrate by two-step hydrothermal method as previously reported [25].Firstly, 0.2 mM ZnAc (Zn(CH 3 CO 2 ) 2 , 99.99%, Sigma-Aldrich) was dissolved in 40 mL of ethanol (C 2 H 5 OH, 99.8%, Guangfu) solution to form a seed solution, which was uniformly dropped on the substrate.Afterwards, the glass sheet was put into an ethanol solution for rinsing for 10 seconds, and the sample was placed on a hot plate for 30 minutes at 300 ∘ C. The above process was repeated several times to form a seed layer.Secondly, a growth solution was prepared by dissolving 10 mM zinc nitrate (Zn(NO 3 ) 2 ⋅6H 2 O, 99.0%, Sigma-Aldrich) and 5 mM HMTA (C 6 H 12 N 4 , 99.5%, Sigma-Aldrich) in 400 mL deionized water with magnetical stirring for 30 minutes and then gradually adding 18.8 mL of ammonia (NH 4 OH, 28.0%, Sigma-Aldrich) until the end of preparation.Next, the glass sheet with the pretreated ZnO seed layer facing downward was immersed in the growth solution for 8 hours at 90 ∘ C. Finally, the glass sheet was washed with deionized water and dried in nitrogen.
To obtain Pd-ZnO-NWs, the ZnO-NWs were dipped in an ethanol solution of 9.6 mM palladium chloride (PdCl 2 , 99.9%, Sigma-Aldrich) for 8 s and then dried using an air gun.The cycle was repeated several times.The Pd-ZnO-NWs were annealed at 500 ∘ C for 1 hour and then cooled down to room temperature.To improve the stability and the repeatability of the gas sensors, samples of ZnO-NWs and Pd-ZnO-NWs were aged at 200 ∘ C for 10 days.Finally, a conductive silver paste was uniformly coated on the sample to form electrodes to ensure the electrical contact between the nanowires and probes.

Characterization Methods.
The morphology of ZnO-NWs and Pd-ZnO-NWs was observed using scanning electron microscope (SEM, Hitachi S-3400N), equipped with Xray energy dispersive spectroscope (EDS, Bruker).The crystal structure and components of samples were characterized by X-ray diffraction technique (XRD, DRIGC-Y 2000A) with Cu K radiation ( = 0.15406 nm) at 30 kV.
The gas sensing properties of the fabricated sensors were tested using the intelligent analysis system (CGS-1TP, Beijing Elite Tech Co., Ltd, China).Figure 1 shows the schematic of the gas sensitivity analysis system.The gas sensor is fixed on the heated platform until the resistance of the sensor is stable.Then the target gas is injected into the testing chamber (with a testing volume of 18 L) by a microinjector, and two rotating fans in the testing chamber make the gas mixed homogeneously.After the resistance of the sensor reaches a new stable value, the testing chamber is opened to refill air.The response and recovery times are defined as the time needed for 90% of total resistance change after the sensor exposed to the testing gas and to the air, respectively.The target gas at different concentrations was prepared by a static state method.The whole process of gas dilution could be approximately considered as an isobaric process, resulting in the following relationship between the volume of injected liquid, and the concentration of target gas [26]: where  liquid is the volume of injected liquid,   is the volume of testing chamber,  gas is the concentration of target gas,  is the molecular weight of liquid,  is the density of liquid, and  is the purity of liquid.
Considering the n-type semiconductor characteristic of ZnO, the response value of gas sensor to reductive gas can be defined as  =   /  , where   and   are the resistance of sensor in air and in target gas, respectively.To investigate the crystalline structures of ZnO-NWs and Pd-ZnO-NWs, XRD spectra of the samples are obtained as shown in Figure 2(f).The diffraction peaks in the XRD patterns are the same as the standard card (JCPDS 36-1451) with a wurtzite structure of ZnO by indexing.A strong and sharp peak located at (002) crystal plane of the prepared ZnO-NWs reveals their high quality crystallinity and axis orientation [27].No diffraction peaks of Pd could be observed, however, in the XRD pattern of Pd-ZnO-NWs, due probably to the fact that the Pd content is much lower than that of Zn or O in Pd-ZnO-NWs [28,29].

Gas Sensing Properties.
In order to confirm a good gas selectivity of the sensors, we have performed sensing studies of ZnO-NWs and Pd-ZnO-NWs for various gases such as CH 4 , CO, CH 3 OH, CH 3 COCH 3 , and C 2 H 5 OH (Figure 3).The concentration of all the analytes is 200 ppm.Figure 3(a) shows that the ZnO-NWs sensor does not respond to CH 4 and CO gases in the whole range of testing temperatures.On the other hand, the same sensor shows the detectable signal with CH 3 OH, CH 3 COCH 3 , and C 2 H 5 OH gases, indicating also a maximum response at a temperature of 425 ∘ C. The maximum sensitivities to CH 3 COCH 3 and CH 3 OH are found to be 1.68 and 1.30, respectively, which are much lower compared to that of C 2 H 5 OH (∼2.69).The gas selectivity Temperature ( ∘ C) Pd-ZnO-NWs@CH 4 Pd-ZnO-NWs@CO Pd-ZnO-NWs@CH 3 OH Pd-ZnO-NWs@CH 3 COCH 3 Pd-ZnO-NWs@C 2 H 5 OH  To evaluate the gas selectivity of sensors, we define a selectivity factor (Q) as  =   /  [28], where   and   are the responses of the sensors to 200 ppm ethanol and 200 ppm of other gases in this work, respectively.The bigger the value of  of a sensor, the higher the selectivity to the ethanol gas.From the results listed in Table 1, we can find that the two types of sensors have a better selectivity to ethanol at the ambient of CO or CH 4 .Clearly, the sensing selectivity has been increased after Pd decoration.
To investigate the sensing properties of ZnO-NWs and Pd-ZnO-NWs for ethanol gas, the ZnO-NWs grown on the substrate were dipped in an ethanol solution of 9.6 mM palladium chloride and this process was repeated 3, 5, and 7 times, respectively.We refer to these samples as Pd-ZnO-NWs-3, Pd-ZnO-NWs-5, and Pd-ZnO-NWs-7, representing 0.5%, 1.3%, and 3.1% atomic percentage of Pd on the surface of ZnO-NWs. Figure 4 shows the response value and responserecovery time of the ZnO-NWs and Pd-ZnO-NWs- ( = 3, 5 and 7) sensors exposed to 200 ppm ethanol at their own optimum operating temperature of 425 ∘ C and 325 ∘ C. When the Pd concentration on the surface of samples is increased from 0.5 to 3.1 at.%, Pd-ZnO-NWs-5 shows an improved sensor performance in terms of response value (4.23) and response-recovery times (9 s and 9 s), which are better than those of ZnO-NWs and Pd-ZnO-NWs-3 as well as those of previously reported data (Table 2).A further increase of the Pd concentration leads to the increase of the recovery time and the deterioration of the sensitivity.Figure 5 shows the dynamic responses of ZnO-NWs and Pd-ZnO-NWs-5 exposed to different concentrations of ethanol at their optimum temperature, showing clearly a fast and repeatable response-recovery performance for ethanol.When the concentration of ethanol changes from 1 to 200 ppm, the response of Pd-ZnO-NWs-5 is higher than that of ZnO-NWs.When the concentration is larger than 500 ppm, however, the ZnO-NWs and Pd-ZnO-NWs-5 sensors show almost the same response.Remarkably, both sensors can detect ethanol of a concentration as low as 1 ppm, but Pd-ZnO-NWs-5 sensor shows a better performance (insert of Figure 5) than the ZnO-NWs, with a response and recovery time of 12 s and 10 s, respectively.
Figure 6(a) shows the response curves of two sensors at their optimum temperature as a function of ethanol concentration in the range of 1 to 800 ppm.Clearly, the response of Pd-ZnO-NWs-5 sensor increases faster than that of ZnO-NWs sensor.The Pd-ZnO-NWs-5 sensor also has a better linearity than that of ZnO-NWs up to 100 ppm, indicating that Pd-ZnO-NWs-5 sensor is more favorable to detect low concentration ethanol.When the concentration of ethanol is increased to 500 ppm, the responses of two sensors increase slowly, due presumably to the saturation of both sensors.

Mechanistic Understanding of Ethanol Sensing.
It is well known that chemisorbed oxygen plays a key role in electrical transmission of semiconducting metal oxides [30,31], and the sensitivity of the gas sensor depends on the interaction between chemisorbed oxygen and sensing materials.As reactive oxygen, chemisorbed oxygen can be divided into O 2 − , O 2− , and O − .Empirically, the response of the sensor can be expressed as follows [31]: where  is the response of sensor,  gas is the concentration of target gas,  and  are constants,  is a value represents the species of oxygen ion on the surface of ZnO-NWs [32].It was reported that for  of 0.5 the chemisorbed oxygen ion on the surface is O 2− and for  of 1, the chemisorbed oxygen ion is O − [31].Figure 6(b) shows the linear relationship between the sensor response and the ethanol concentration.The slopes of the two lines (0.61 and 0.52, resp.) are close to 0.5, which means that the dominated species of the oxygen ion on ZnO surface are O 2− ions.These oxygen ions can trap electrons from ZnO and form a space-charge region which makes the resistance of sensor increase.When the reducing gas (such as ethanol) is injected into the chamber, the ionic oxygen reacts with ethanol and produces CO 2 and H 2 O and then releases a large number of electrons.These released electrons will go back to the conduction band, leading to the decrease of metal oxide resistance.The reaction process can be described as follows [33]: O Our data have shown that the additive Pd can improve the gas sensing properties of the ZnO sensors to ethanol.At high temperature, oxygen molecules can weakly bond to the catalytic atoms Pd [34], and the oxygen atom is produced in a complex dissociation reaction (4) [35].This oxygen atom finally becomes negatively charged oxygen ion by trapping an electron from the surface of ZnO (5).
Consider the following: Meanwhile, because of the PdO attached on the surface of ZnO, a heterojunction at the interface between ZnO (ntype semiconductor) and PdO (p-type semiconductor) will be formed as shown in Figure 7(a).The heterojunction leads to the band bending in the depletion layers [36].The electrons transfer from n-type ZnO to p-type PdO, whereas the holes transfer from PdO to ZnO until the system reaches the equilibrium state.These charge transfers will lead to a wider depletion layer and a greater resistance.As the reaction processing, the electrons will pass through the depletion layers of heterojunction under the certain high temperature, which makes the depletion layer at the PdO/ZnO interface once again become thin, facilitating the larger amount of electron transport between the PdO and ZnO, and as a result, the response-recovery times and optimum operating temperature of the sensor decrease significantly compared to its undecorated counterpart [22,24].
Simultaneously, ethanol molecule can also combine with the hole in PdO ( 6) and produce the intermediates, which will react with the O 2− on the n-type ZnO (7) and release more electrons.Such catalytic dissociation of ethanol on Pd nanoparticles lowers the activation energy, enhances the reaction velocity, and improves the performance of the sensor as a whole.The schematic model of the ZnO-NWs and Pd-ZnO-NWs sensors when exposed to ethanol gas is shown in Figure 7

Conclusion
ZnO-NWs were prepared by hydrothermal growth, showing high quality crystallinity and -axis orientation.They were then used for gas sensing with or without Pd-decoration.
Our results showed that the ZnO-NWs and Pd-ZnO-NWs were more selective to ethanol in the presence of CH 4 , CO, CH 3 OH, and CH 3 COCH 3 .Comparing to the as-grown ZnO-NWs, Pd-ZnO-NWs showed a higher sensitivity, faster response, higher selectivity, and lower operating temperature.Typically, the response of Pd-ZnO-NWs was 57% higher and the optimum operating temperature was 100 ∘ C lower than those of ZnO-NWs.When exposed to 200 ppm ethanol, the response and recovery time (both 9 s) of Pd-ZnO-NWs-

Figure 1 :
Figure 1: Schematic illustration of a set-up for gas sensitivity analyses.The insert shows a typical device configuration of ZnO-NWs sensor.

3. 1 .Figure 2 :
Figure 2: (a) Top-view and (b) side-view SEM images of ZnO-NWs grown on the substrate, (c) magnified top-view SEM images of ZnO-NWs and (d) Pd-ZnO-NWs (the inserted image shows the nanoparticles of Pd on ZnO-NWs), (e) EDS spectrum of Pd-ZnO-NWs, and (f) the XRD spectra of ZnO-NWs and Pd-ZnO-NWs.

Figure 3 :
Figure 3: Response of (a) ZnO-NWs and (b) Pd-ZnO-NWs to different gases of 200 ppm concentration at various operating temperatures.

Figure 5 :
Figure 5: Dynamic responses of ZnO-NWs and Pd-ZnO-NWs-5 sensors to ethanol in the detection range from 1 to 800 ppm.The inserted image shows the partial magnified response curve of sensors exposed to 1 ppm ethanol.

Figure 6 :
Figure 6: (a) Response of ZnO-NWs and Pd-ZnO-NWs-5 versus ethanol concentrations (the inserted image shows the partial magnified response curve of the two sensors exposed to 1-100 ppm ethanol), (b) linear relationship between log(Response-1) and log  gas .

Figure 7 :
Figure 7: (a) Diagram of energy band structure for p-type PdO/ntype ZnO heterojunction, (b) schematic model of the ZnO-NWs and Pd-ZnO-NWs sensors exposed to ethanol gas.

Table 1 :
Selectivity factor () of the sensors fabricated by ZnO-NWs and Pd-ZnO-NWs.( = S ethanol /S other , where S ethanol and S other are the responses of the sensors to 200 ppm ethanol and other gases, resp.).The results show that the sensor is sensitive to all testing gases after Pd was decorated, and the maximum responses to different gases appear at the same temperature of 325 ∘ C, which is lower than that of ZnO-NWs.Now, the maximum sensitivities to C 2 H 5 OH, CH 3 COCH 3 , CH 3 OH, CH 4 , and CO are 4.23, 2.79, 2.13, 1.47, and 1.4, respectively.