Fabrication of Pd Doped WO 3 Nanofiber as Hydrogen Sensor

Pd doped WO3 fibers were synthesized by electro-spinning. The sol gel method was employed to prepare peroxopolytungstic acid (P-PTA). Palladium chloride and Polyvinyl pyrrolidone (PVP) was dissolved in the sol Pd:WO3 = 10% molar ratio. The prepared sol was loaded into a syringe connected to a high voltage of 18.3 kV and electrospun fibers were collected on the alumina substrates. Scanning electron microscope (SEM), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were used to analyze the crystal structure and chemical composition of the fibers after heat treatment at 500 °C. Resistance-sensing measurements exhibited a sensitivity of about 30 at 500 ppm hydrogen in air, and the response and recovery times were about 20 and 30 s, respectively, at 300 °C. Hydrogen gas sensing mechanism of the sensor was also studied.


Introduction
Hydrogen burns cleanly without releasing pollutants or greenhouse gases and therefore, in recent years, more attention has been paid to hydrogen as an excellent candidate for fossil fuel replacement in OPEN ACCESS household and transportation applications.However, hydrogen is a highly flammable fuel with a wide combustion range of 4%-75%.Besides, it has a large diffusion coefficient of 0.61 cm 2 /s in air.Therefore, detection and leakage control of this gas is a challenging subject.Today, development of a hydrogen sensor with negligible power consumption and high stability, sensitivity and fast response is desired.
It is known that gas absorption depends on the surface area, which is affected by the preparation method.Increasing the surface to volume ratio to enhance gas sensing properties is currently under development by introducing different nanostructures such as nanowires, nanorods, nanobelts and nanofibers.Nanofibers have high density of surface sites that make them excellent candidates for gas sensing applications [32].
Among different fabrication methods, electrospinning is a simple and cost effective method for preparing one dimensional materials.Recently, several researches have reported fabrication of metal oxide nanofibers [33][34][35][36][37][38][39] in which metal oxide nanoparticles can be added to a polymer matrix to make nanocomposite fibers.Heat treatment removes polymer and leaves metal oxide material with fiber shape.
Metal oxide fibers could be synthesized by combining sol-gel and electrospinning methods.Adding catalysts to the solution (sol) produces nanofibers with improved catalytic properties for selective gas detection at lower temperatures.Polymer matrix can be decomposed through heat treatment and leaves metal oxide fiber with lengths from several hundred nanometers to few micrometers.
Preparing one dimensional WO 3 fibers by sol-gel method was first reported by Lu et al. [40].Others used pure WO 3 fiber for detecting ammonia, NO 2 and CO gasses [41,42].In our previous work [29], the sensitivity of both pure and Pd-modified WO 3 thin films was measured towards hydrogen as a function of operating temperatures in the range of 30 °C to 350 °C.A systematic increase in sensitivity was observed for the modified films in the whole temperature range.The sensitivity increased due to a reduction of the activation energy between the WO 3 surface and the hydrogen gas in the presence of Pd (PdO).It was seen that the sensitivity of Pd:W = 10% films have considerable amounts at room temperature and the working temperature (the temperature which has maximum sensitivity) decreased by increasing the Pd concentrations.
Since high response and recovery times were observed at low working temperatures.Given the results of our previous experiments on Pd modified WO 3 films, we decided to study structure effect on modification of sensing behavior.Therefore, a combination of electrospinning and the sol gel method was studied to compare the result of the sensing behavior of Pd:WO 3 thin film with the electrospun Pd doped WO 3 nanofiber samples with the same ratio (Pd:W = 10%).In this work, we prepared WO 3 nanoparticles by the sol-gel method and then Pd and PVP were added to the sol.To remove polymer and make WO 3 nanoparticles with fiber shape, samples were annealed in air at 500 °C for 1 h.The Pd doped WO 3 nanofibers showed fast response time and high sensitivity toward hydrogen gas at low temperature.

Experimental Section
In our sol-gel route, Peroxopolytungstic acid (P-PTA) sol was prepared according to the Kudo route [43].Five grams of tungsten wire and 20 mL of H 2 O 2 (30%) was stirred at room temperature for 48 h until the whole tungsten wire was dissolved.Unreacted H 2 O 2 was removed by using platinum net.Then 20 mL ethanol was added and the solution was heated at 80 °C which resulted in a sol color change from clear to orange.
For the activation process we used Pd catalyst.Since in our previous report, we obtained best sensitivity for Pd:W = 10% molar ratio [29], we chose this ratio for adding PdCl 2 salt (Merk) directly to the P-PTA sol.
To fabricate fibers, 1.0 g polyvinyl pyrrolidone (PVP) (Aldrich) was added into the P-PTA sol and was stirred for one day.The resulting solution was loaded into a plastic syringe and its needle was connected to a high-voltage DC supply up to 30 kV.A voltage of 18.3 kV was applied between the needle and grounded target.Al 2 O 3 substrates were placed on a piece of flat aluminum with 20 cm distance from the tip of the needle to collect fibers.The prepared samples were annealed in air at 500 °C for 1 h.
X-ray powder diffraction (XRD) analysis was recorded by a Philips X'pert instrument operating with CuKα radiation (λ = 1.54178Ǻ) at 40 kV/40 mA.A Philips XL30 model Scanning electron microscope (SEM) was used to obtain the SEM images.For the X-ray photoelectron spectroscopy (XPS) experiment, an Al anode X-ray source was employed with a concentric hemispherical analyzer (Specs model EA10 plus) to analyze the energy of the emitted photoelectrons.Gas sensing properties of the as-fabricated sensor were measured using a static test system.A data logger was used to collect the data and the whole system was automatically controlled by computer.
For measuring the electrical resistance, Au/Ti comb-like interdigitated electrodes with 150 nm thickness were evaporated on the sample surface through a mask.The samples were placed in a small stainless steel chamber with several electrical feed-through, gas inlet and gas outlet.For measuring sensitivity, a constant dc voltage of 5 V was applied to the circuit; including the sensor in series with a constant resistor.Voltage variation across the resistor was measured.During the measurements, the fibers were warmed by a heater located on the back of the substrates.We used dry air as the reference gas and hydrogen as the target gas.Sensitivity was defined as S = (Vrg -Vra)/Vra, where Vrg and Vra are the sensor voltage in the presence and absence of hydrogen, respectively.The measurement setup consists of mass flow controllers and a data acquisition system for recording resistance changes.We define the response time as the time taken for the sensor's resistance to undergo a 90% variation with respect to its equilibrium value and recovery time as the time taken to achieve 60% variation to reach its initial value in air after the removal of H 2 .The gas sensing performance was tested at hydrogen concentration of 500 ppm in the range of 30 °C to 300 °C.

Structure Analysis
SEM images of as-spun PVP composite fibers and the sintered fibers at 500 °C for 1 h are displayed in Figure 1a,b, respectively.Figure 1a shows the used Aluminum foil surface and some PVP composite fibers with an average diameter of about 500 nm.The heating process compresses the fibers and reduces their average thickness to about 150 nm while keeping their morphology as is shown in Figure 1b.Heat treatment at 500 °C removes the polymer and leaves metal oxide material with fiber shapes.SEM images show continuous fibers with lengths of several tens micrometers.By electrospinning, a huge number of fibers electrospun over the substrate and form as a film.To observe better by SEM, we made a sample with low density fibers (Figure 1a,b).Figure 1c shows another sample with high density of fibers (sample for gas sensing experiments).Both high fibrous surface area as well as high porosity increase effective sensitive surface area.As presented in Figure 2, heat treatment at 500 °C resulted in the formation of crystals of both monoclinic WO 3 and oxidized Pd.The evidence of PdO after annealing can be observed by XPS analysis.Data in Figure 3a indicates that the surface of the annealed Pd doped WO 3 nanofiber is composed of tungsten, oxygen, carbon contamination, and small amounts of palladium.Figure 3b, shows the peaks related to palladium oxide formation after the annealing process.

Hydrogen Sensing Measurements
Hydrogen sensing properties of the Pd-modified WO 3 samples were investigated while they were exposed to 500 ppm H 2 at different operating temperatures.Figure 4 shows the changes in the voltage of a series resistor vs. time (transient response).The used electrical circuit is shown in the inset of  We measured the sensitivity of the samples as a function of operating temperature in the range of 75 °C to 300 °C.As shown in Figure 5, the sensitivity is noticeable at a temperature of about 75 °C and it is enhanced by increasing the operating temperature to 300 °C.This high sensitivity may be attributed to the reduction of the activation energy between the WO 3 surface and hydrogen gas in the presence of Pd (PdO).Response and recovery times of the Pd-modified WO 3 fiber for 500 ppm hydrogen are presented in Figure 6.It presents a decaying behavior with considerable fast response and recovery times at high temperatures (300 °C).At low working temperature the response time is higher due to slow desorption of the formed water molecules on the surface especially at temperatures below 100 °C.Our results demonstrated faster response and recovery times in the order of 160-190 s for low temperature sensing which is so considerable compare to 60 min recovery time for Pd:WO 3 = 10% in the form of thin film [29].It can be attributed to the facilitate occupancy and desorption of H 2 molecules at grain boundaries on the large surface area of nanofibers in a porous network.However, the obtained results of the response magnitudes and the working temperature are not as good as our previous work on thin film sensors.

Hydrogen Sensing Mechanism
The WO 3 fibers have been made of small grains and grain boundary barriers are existed between neighboring grains located in a fiber structure.This structure has an increased surface area as well as providing electronic transmission channels for electron transferring between WO 3 particulates.Since WO 3 nanofibers in this study are continuous with WO 3 grains, we expect sensing mechanism of the single WO 3 fiber is principally similar to that of metal oxide films.So, grain size has important effect on sensing performances [44][45][46].However, despite fibers being continuous media, there are a great number of fiber to fiber connections, where electron current passing through these connection potential barriers has an important role in total current passing through the fibers' film.
In addition, regarding our previous work [29], presence of Pd makes a systematic increase in the sensitivity in the whole temperature ranges.This can be attributed to the effect of Pd catalyst in performing electronic sensitization mechanism and reducing the activation energy between the WO 3 surface and the hydrogen gas [47].As shown here in Figure 7a,b, the O 2 molecules chemisorb and dissociate on the surface of the WO 3 grains.Then they trap and extract electrons from the conduction band of n-type semiconductor (WO 3 ) due to their high electron affinity.A depletion or space-charge region develops on the surface of the grain.This increases the potential barrier height between neighboring grains and also widens it, which is called energy band bending as shown in Figure 7a.This makes electron transferring between grains difficult which decreases the conductance of the fiber.By exposing fibers to hydrogen gas, reaction with the adsorbed oxygen molecule takes place.This reinjects captured electrons to the depletion region and the reverse effect will take place (Figure 7b).

Conclusions
Pd doped WO 3 fibers were synthesized by combining electrospinning and sol-gel methods and were characterized by SEM, XRD and XPS methods.The sensor fabricated from these fibers exhibited high sensitivity and rapid response/recovery to hydrogen at 300 °C.The highest sensitivity was about 30 when the sensor was exposed to 500 ppm, and the response and recovery times were about 20 and 30 s, respectively.The WO 3 fiber sensors showed fast response times toward hydrogen even at low operating temperatures.These results demonstrate that Pd doped WO 3 fibers can be used as the sensing material for fabricating high performance hydrogen sensors.Besides, hydrogen gas sensing mechanism of the sensor was studied and a model proposed.

Figure 3 .
Figure 3.The X-ray photoelectron spectroscopy (XPS) patterns of the Pd doped WO 3 nanofiber that was heat treated at 500 °C, (a) Peak survey and (b) Deconvolution in the Pd 3d region.

Figure 4 .
It presents fast response and recovery times of about 20 and 30 s, respectively.

Figure 4 .
Figure 4. Normalized voltage variation across the series resistor as output signal

Figure 5 .
Figure 5.The sensitivity of the Pd-modified WO 3 nanofiber towards 500 ppm hydrogen as a function of operating temperature.

Figure 6 .
Figure 6.The response and recovery times of the Pd-modified WO 3 nanofiber towards 500 ppm hydrogen as a function of operating temperature.

Figure 7 .
Figure 7. Hydrogen sensing mechanism of WO 3 neighboring grains (a) before and (b) after hydrogen exposure.