Role of Oxygen Vacancies in the Electrical Properties of WO 3 − x Nano / Microrods with Identical Morphology

Tungsten oxide (WO3−x) crystalline nano/microrods with identical morphology but different contents of oxygen vacancies were prepared by thermally evaporating fixed amount of WO3 powder in reductive atmosphere from different amounts of S power at 1150C in a vacuum tube furnace, in which both sources were loaded in separate ceramic boat.With increasing amount of S powder, a series of tungsten oxides, WO3, WO2.90, W19O55 (WO2.89), andW18O49 (WO2.72), could be obtained. And devices were fabricated by screen-printing the obtainedWO3−x crystals on ceramic substrates withAg-Pd interdigital electrodes.With increasing content of oxygen vacancies, the devices fabricated withWO3−x crystals present a negative to positive resistance response to relative humidity. Under dry atmosphere, for the devices with increasing x, the strong response to light changed from short to long wavelength; under light irradiation, the conducting ability of the devices was enhanced, due to the more efficient separation and transportation of the photogenerated carriers; and under simulated solar irradiation, the photocurrent intensity of the W18O49 device was roughly 8 times, about 500 times, and even 1000 times larger than that of the W19O55, WO2.90, and WO3 one, respectively. With the versatile optoelectrochemical properties, the obtained WO3−x crystals have the great potential to prepare various humidity sensors and optoelectrical devices.


Introduction
As an important strategic nonferrous metal, the highly efficient utilization and high value-added processing of tungsten resources are of great significance.Among the various forms of tungsten materials, tungsten oxides are important semiconductors, which can be used in solar-driven photocatalysis [1][2][3], electrochromism [4], and photochromism [5].In addition, tungsten oxides are highly sensitive to many kinds of gas, which are considered as one of the most promising gas sensing materials for the detection of NH 3 , H 2 S, NO x , O 3 , H 2 , and so on [6][7][8][9].
However, due to the multiple chemical valences and complicated structure of tungsten oxides, their morphology and composition are difficult to control independently.In literatures [10,11], it was reported that WO 3−x crystals with a variety of oxygen-deficient stoichiometries like WO 2.72 , WO 2.8 , and WO 2.83 are stable under the normal condition, but their morphology might be diversified.Thus, the role of oxygen vacancy alone in the electrical properties of tungsten oxides has still been controversial [12,13].Moreover, because tungsten oxides are versatile materials, it would be applied in diversified conditions, such as in dry or humid atmosphere, under light irradiation, or in the dark.Therefore, it is of much significance to investigate the electrical properties of tungsten oxides with varied contents of oxygen vacancies without influencing by the morphology under different environmental conditions.
Here we propose a simple, fast, environmentally friendly, and energetically efficient thermal evaporation method for the preparation of tungsten oxides (WO 3−x ) crystalline nano/microrods (NMRs) in large scale with only WO 3 and S 2 Journal of Nanomaterials powders as the starting materials.By elaborately controlling the processing parameters, a series of tungsten oxides (WO 3 , WO 2.90 , W 19 O 55 , and W 18 O 49 ) of identical morphology but with different contents of oxygen vacancies were obtained.And devices were fabricated simply by screen-printing the obtained WO 3−x crystals onto ceramic substrates with Ag-Pd interdigital electrodes.The electrical properties of the devices are systematically investigated under different conditions.The measured results reveal that the obtained WO 3−x crystals have the great potential to prepare various humidity sensors and optoelectrical devices.

Experimental Section
2.1.Materials Preparation.The applied WO 3 (Sinopharm Group Co. Ltd.) and elemental S (Aladdin Industrial Corporation) were commercially bought raw powders of analytical grade and directly used without further purification.The proposed WO 3−x crystalline NMRs were synthesized in an experimental vacuum, horizontal alumina tube furnace (SJG-16, China) by using a high-temperature thermal evaporation process [11].
In a typical process, an alumina boat with 1 g WO 3 powder was placed at the heating center of the furnace, while another one with different amounts of S powder (0.1-0.5 g) was located on the upstream of carrier gas flow at 10 cm away from the WO 3 powder.Before heating, the alumina tube was pumped out and flushed with high-purity carrier gas (here N 2 , 99.99 vol.%) repeatedly for several times.Then, the furnace was heated up to 1150 ∘ C in 1.5 h from room temperature, dwelling at this temperature for 1 h.After heating, the furnace was cooled down naturally to room temperature simply by shutting down the electricity of the heating system of the furnace.During the whole heating process, the N 2 gas flow inside the alumina tube was maintained at 200 sccm.Finally, products with a color from light-green to purple-red could be obtained in the alumina boat originally loaded with WO 3 raw powder.

Materials
Characterization.X-ray diffraction (XRD, Bruker D8 diffractometer, Germany) analysis was conducted in continuous mode with Cu K radiation ( = 1.5418Å).The XRD scanning range was from 10 to 60 ∘ and the speed was 6 ∘ /min.X-ray photoelectron spectroscopy (XPS, Thermo escalab 250Xi, ThermoFisher Scientific, America; nonmonochromated Mg K radiation, photon energy 1253.6 eV) was used to investigate the chemical state of the elements in the samples, and the spectrometer was calibrated by the binding energy of C1s line (285 eV).Field emission scanning electron microscopy (FE-SEM, Hitachi SU8020, Japan) was used to examine the morphology of the obtained samples.The specific surface area of the obtained samples was measured by the Brunauer-Emmett-Teller (BET) N 2 adsorption method using a Micromeritics ASAP2020 surface analyzer (China National Pharmaceutical Group Corp., Shanghai, China).Electron spinning resonance (ESR, JEOL JES-FA200, Japan) was carried out at room temperature with a microwave frequency of 9.44 GHz to investigate the unpaired electron and defect in the samples.And diphenylpicrylhydrazyl was used for the  value calibration.The electrochemical impedance spectroscopy (EIS) was performed on an electrochemical workstation (CHI 660D, Chenghua Instrument Company, China) in a frequency from 100 kHz to 0.1 Hz with an open circuit potential at an AC perturbation of 0.1 V.

2.3.
Fabrication and Measurement of Devices.0.1 g of each product was first ground and then mixed with 1 mL of absolute ethanol to form a paste.After that, 0.1 mL of the prepared pastes was spinning-coated by a coater (KW-4 A, China) at a rotational speed of 1000 rpm for 20 s onto an alumina ceramic substrate (Company Elitetech, China) with a size of 6 mm in length, 3 mm in width, and 0.5 mm in thickness, where the screen had already been printed with two Ag-Pd interdigital electrodes of five fingers with a distance of 0.15 mm.Finally, a device was fabricated by drying it at room temperature for 1 h.
The characteristic I-V curves in different levels of relative humidity were measured with an operating voltage from 0 to +3 V and an operating frequency of 1000 Hz.The controlled relative humidity was obtained by using saturated salt solution method as reported in [14,15].The applied six different saturated salt solutions in this work were prepared by LiCl, MgCl 2 , Mg(NO 3 ) 2 , NaCl, KCl, and KNO 3 , which could provide a relative humidity at 25 ∘ C of approximately 11%, 33%, 54%, 75%, 85%, and 95%, respectively.Before measurement, each device was first soaked at 25 ∘ C in an atmosphere of different levels of relative humidity in the six chambers with different salt solutions till it reached the adsorption-desorption equilibrium for water.
The characteristic I-V curves of the devices were also recorded in dry atmosphere under the irradiation with light of different wavelengths (400 and 600 nm) and simulated solar irradiation (SSI), respectively.The SSI was provided by a 300 W xenon lamp (PLS-SXE300, Beijing Perfect Light Co. Ltd., China), and the light of different wavelengths was realized by adding different optical filters.During the tests, the I-V curves of the devices were first measured in the dark, and then the measurements were carried out under different lights after 5 min of irradiation.In addition, the photocurrent measurement was performed at a voltage of 1 V under SSI.

Results and Discussion
3.1.Composition, Structure, and Morphology of WO 3−x Nanomaterials. Figure 1(a) shows the XRD patterns of typical products fabricated by thermal evaporation of 1 g WO 3 powder and different amounts of S powders at 1150 ∘ C. When the applied amount of S was 0.1 g, the obtained sample was still of pure monoclinic WO 3 phase (JCPDS no.43-1035).When it increased to 0.15 g, a pure monoclinic phase of WO 2.90 would be acquired.Its XRD peaks can perfectly match with those in the standard JCPDS card no.36-0102.When 0.2 g of S was applied, the resultant sample was composed of a pure monoclinic phase of W 19 O 55 (JCPDS no.45-0167).However, when the applied amount of S was between 0.25 and 0.3 g, a mixture of W 4 O 11 and W 5 O 14 would be produced (please see Figure S1 in Supplementary Materials).As the applied amount of S increased to 0.5 g, a pure monoclinic phase of Binding energy (eV) W 18 O 49 (JCPDS no.05-0393) was attained.According to our previous studies [11], if the applied amount of S was further increased (e.g., up to 1 g), WS 2 phase would appear in the samples, because at such high temperature the excessive S vapor would take all the O atoms away from some of the tungsten oxides, finally resulting in WS 2 phase.Therefore, in the present work, in order to obtain pure WO 3−x crystals with different contents of oxygen vacancies, the applied amount of S was selected between 0.1 and 0.5 g.The surface chemical state of the obtained samples was examined by XPS analysis with the high-resolution spectra of W4f (please also check Figure S2).The obtained W4f patterns and the fitted results after a Shirley background subtraction are shown in Figure 1(b).From this figure, it was revealed that the W4f core-level spectrum could be fitted into two doublets, in association with two different oxidation states of W atoms.The peak couples (solid blue curve) at 35.55 and 37.64 eV (with a binding energy gap of 2.09 V and peak area ratio of about 4 : 3) correspond to W4f 7/2 and W4f 5/2 of W 6+ , respectively.Meanwhile, the peak couples (dash dot dark yellow) have binding energies of W4f 7/2 = 33.75eV and W4f 5/2 = 35.22eV (with a binding energy gap of 2.1 V and peak area ratio of roughly 4 : 3), which agree well with W 4+ state [16].From this figure, it can be seen that the WO 2.90 , WO 2.89 , and WO 2.72 samples all contain the two oxidation states of tungsten (W 4+ and W 6+ ), but with increasing , the proportion of W 4+ increases and the mean valence of their W ions decreases from 5.61 to 5.36 (see Table S1).In addition, the WO 3 sample was completely oxidized, containing only W 6+ atoms.And all these results are well consistent with those observed from XRD analysis.
The ESR spectra of the obtained samples are displayed in Figure 2. From this figure, it could be seen that the ESR spectra of the oxygen-deficient WO 3−x crystals exhibit a sharp signal at  = 2.28, while that of WO 3 presents no obvious signals.The signal of the present oxygen-deficient WO 3−x compounds could be attributed to oxygen vacancies, because it presents a similar  value with those of the already reported metal oxides, in which the peak assigned to oxygen vacancies always has a  factor of about 2.01 [10,17,18].Furthermore, in metal oxides, most of the excessive electrons are localized at the oxygen vacancies sites, and, usually, one oxygen vacancy bounds one extra electron; and the sharp  signal of metal oxides would become stronger with increasing concentration of oxygen vacancies [17,19].Here in this work, the shift of the  value for the present oxygen-deficient WO 3−x compounds to a higher value might be owing to the presence of two charge centers (two unpaired electrons) trapped in the oxygen vacancy [10,18]; and, with increasing , the sharp  signal gets stronger and stronger, indicating that there are more oxygen vacancies in the obtained WO 3−x samples.But there is no signal in the ESR spectrum of WO 3 , implying that the sample of WO 3 was completely oxidized.And this result is well consistent with those from XPS analysis.
The morphology of the obtained samples prepared by thermal evaporation of 1 g WO 3 powder and 0.1-0.5 g of S powders at 1150 ∘ C was examined by SEM.Typical SEM images of the above-mentioned samples are displayed in Figure 3.For more information, please check Figure S3.SEM examination revealed that, under the present conditions, all the obtained samples had identical morphology, presenting a microstructure of many NMRs.The BET surface areas of the samples were almost the same (please check Figure S4).With a rough evaluation, the diameter of the NMRs was in the range of 100 nm to 5 m, and the length of them was about 30 to 50 m.According to our previous study [11], the growth of the present tungsten oxide nanorods was controlled by a vapor-solid mechanism.

Electrical Properties.
EIS is a powerful tool to study electrochemical behavior of a semiconductor, especially the phenomena of charge transfer [20][21][22].The EIS data are displayed in Figure 4.The inset shows an equivalent circuit for the devices.  is the external resistance,  1 is the constant phase element (CPE), and  1 is the charge transfer resistance across the interface.It is seen that the Nyquist plots of the as-synthesized WO 3 and WO 2.90 both present a complete semicircle, while those of W 19 O 55 and W 18 O 49 only show a trend for a semicircle, possibly because, with increasing , the semiconductivity of WO 3−x decreased and the metallicity increased.But with the increase of oxygen vacancies, the arc radius of the EIS Nyquist plots presents a sharp decrease, indicating a decrease in the solid state interface layer resistance and the charge transfer resistance on the surface [3].This result also suggests that, with the increase of oxygen vacancies, the charge carriers transfer efficiency of WO 3−x increased.
To evaluate the kinetics of charge transfer of the devices fabricated with the obtained WO 3−x crystals under different levels of relative humidity, the complex impedance characteristics of the humidity devices were investigated.The EIS spectra are shown in Figures 5(a)-5(d).The EIS data of the devices fabricated with WO 3 are presented in Figure 5(a).As can be seen from this figure, at low relative humidity (11-33%), a semicircle was observed in the complex plot, indicating a "non-Debye" behavior in the materials, due to a kind of polarization [14].Moreover, the impedance is extremely high, because the dominant electron conduction is difficult due to the least content of oxygen vacancies in the present WO 3 among all the WO 3−x crystals.In addition, although the ionic conduction (here also called electrolytic conduction) existed mainly due to the hopping of H + on the material surface [15], the conducting film was discontiguous at low humidity.With the increase of relative humidity (54-95%), plots with a semicircle at high frequency followed with a straight line at low frequency were recorded.That phenomenon was caused by the H 3 O + and H + traveling via the exchange of the Grotthuss chain mechanism [15].The line represents Warburg impedance, owing to the diffusion of the electroactive species at the electrodes [14].Figure 5(b) shows the EIS data of the devices fabricated with WO 2.90 .Similarly with WO 3 devices, it can be clearly seen that a semicircle was observed in the complex impedance plots at low relative humidity (11-33%).But at high relative humidity (54-95%), it is quite clear that the straight line appeared at low frequency would present at a higher relative humidity than that of WO 3 , due to the fact that the content of oxygen vacancy in WO 2.90 is much higher than that in WO 3 .Moreover, because there are even more oxygen vacancies in W 19 O 55 and W 18 O 49 , they exhibit unique impedance plots as shown in Figures 5(c) and 5(d).At low humidity, only a small amount of water molecules would be adsorbed.Because the coverage of the water molecules adsorbed on the device surface is not continuous, the electrolytic conduction is difficult.The conduction mode is mainly electron conduction due to the vast oxygen vacancies in W 19 O 55 and W 18 O 49 .With increasing relative humidity, a large amount of water molecules would be absorbed by oxygen vacancies, which will result in a continuous electrolytic film.Then the electron conduction becomes less dominant, while the contribution of electrolytic conduction will increase.However, due to the largest amount of oxygen vacancies, the device prepared with W 18 O 49 may be still dominated by electron conduction at a fairly higher relative humidity.The equivalent circuits of the devices are shown in Figure 5(e).  is the external resistance,  1 and  2 are the constant phase elements (CPE),  1 is the charge transfer resistance across the interface, and  2 is the impedance at the electrode/device film interface.For the devices,  1 ≪  2 at low RH, and the impedance change of the devices is mostly determined by  1 .At high RH, the magnitudes of  1 and  2 are the same and the impedance change of the devices is determined by both  1 and  2 .The characteristic I-V curves of the devices fabricated with the obtained WO 3−x crystals were first measured at different levels of relative humidity, and the results are shown in Figure 6.From Figure 6(a), it can be seen that, for the device fabricated with WO 3 , with the increase of relative humidity, the slope of the curve increased gradually.For the WO 2.90 and W 19 O 55 devices (see Figures 6(b) and 6(c)), however, with increasing humidity, the slope of the curves first decreased at low relative humidity (11-54%) and then increased at high relative humidity (75%-95%).Specifically, for the WO 2.90 device, the slope of the curve under a relative humidity of 75% is greater than that under a relative humidity of 11% (see Figure 6(b)), and for the W 19 O 55 device, the slopes of the curve under a relative humidity of 95% are smaller than that under a relative humidity of 54% (see Figure 6(c)).And with the increase of the relative humidity, the slope of the curve of the W 18 O 49 device decreased gradually (see Figure 6(d)).Because the reciprocal of the slope of I-V curve represents the measured resistance, so from Figure 6, it can be deduced that, with the increase of relative humidity, the resistance of the device fabricated with WO 3 decreases gradually; those of the WO 2.90 and W 19 O 55 devices increase first and then decrease; and that of the W 18 O 49 device increases gradually.The response and recovery characteristic of the sensors fabricated with the obtained tungsten oxide crystals is shown in Figure S5.In a word, with more oxygen vacancies, the devices fabricated with WO 3−x crystals present a negative to positive resistance response to relative humidity, showing the great potential for preparing varied humidity sensors.
A possible qualitative mechanism to explain the humidity sensing properties of the obtained tungsten oxide crystals can be proposed hereafter.For the device fabricated with WO 3 , because there is no oxygen vacancy in WO 3 crystal,  (Ω) with the increase of humidity more water molecules would be adsorbed to the surface of the WO 3 crystal.As a result, the electrical conductivity would increase, because the waterrelated electrolytic conduction mainly functionalizes as a surface mechanism [23].In literature, it was reported that, oxygen vacancies would bound electrons as free carriers [24,25], and thus they could introduce donor levels between the conduction and valence bands [10,26], which would  result in increased conductivity for semiconducting oxides.For the WO 2.90 and W 19 O 55 devices, the oxygen vacancies induced conduction mechanism would compete with the water-related surface mechanism, which might even become dominating in the transport process at low humidity.With increasing relative humidity, water molecules would be absorbed by oxygen vacancies, which will result in a continuous electrolytic film on the crystal surface.Then the electron conduction becomes less dominant, while the contribution of electrolytic conduction will increase, causing the resistance of the WO 2.90 and W 19 O 55 devices to increase first and then decrease.But compared with WO 2.90 device, because there are more oxygen vacancies in W 19 O 55 device, the electrolytic conduction would be dominated in higher relative humidity.However, due to the largest amount of oxygen vacancies, the device prepared with W 18 O 49 may still be dominated by electron conduction at a fairly higher relative humidity.So, with the relative humidity increased from 11% to 95%, the resistance of W 18 O 49 device continued to increase.
To examine the electrical properties of the devices fabricated with the obtained WO 3−x crystals in diversified environments, the characteristic I-V curves were also measured in the dark and under SSI.Generally, the characteristic I-V In the dark Under SSI 0 800 1600 2400 Time (s) curves of one-dimensional nanomaterials are linear [27,28], rectifying [29][30][31][32], asymmetric [29,33], or symmetric [27,34,35].For the present devices, normally, the contact of metal electrodes with the WO 3−x crystals would be either a Schottky barrier or an Ohmic contact.As is seen from Figure 7, all the I-V curves of the devices fabricated with the obtained WO 3−x crystals are symmetric, indicating the existence of Schottky barriers between the metal electrodes and WO 3−x crystals.Therefore, the WO 3−x crystals synthesized in this work are semiconducting.With the increase of , the content of oxygen vacancies increases and thus the conductivity becomes stronger, because oxygen vacancies act as shallow donors for metal oxides [36].Moreover, in literature, it was reported that the lifetime and yield of the photogenerated electrons and the holes in WO 3 and similar materials would increase, which can be proven by transient absorption spectroscopy [37,38].In this work, under SSI, it was found that the conductivity of all the devices was improved and, with the increase in , the added value of the conductivity increased.In our previous work [11], it was revealed that the band-gap of the WO 3−x crystals decreased with the increase of .As a result, the absorption range of the spectrum would be expanded, leading to an enhanced absorption efficiency of light by the WO 3−x crystals, and thus more photoelectrons would be produced during illumination, resulting in the higher photocurrents and increased conductivity.In addition, through analyzing the resultant photocurrent-time (I-t) profiles displayed in the insets of Figure 7, it was revealed that, under SSI, the produced photocurrents are reproducible and stable during three light-on/off cycles.Under SSI, the electrons and holes separated and gradually increased to form photocurrents.But when the light was shut off, electrons would no longer be produced and might recombine with the original holes.Thus, the photocurrents will gradually decrease.Comparatively, the photocurrent intensity of the W 18 O 49 device was roughly 8 times, about 500 times, and even 1000 times larger than that of the W 19 O 55 , WO 2.90 , and WO 3 one, respectively.For the WO 3−x crystals with more oxygen vacancies, the dramatic increase in photocurrent response for the light-on/off could mainly be ascribed to the enhanced absorption efficiency of light by the WO 3−x crystals under illumination and possibly their faster separation and transportation of photogenerated e − on the surface of the working electrodes [36,39].This result is consistent with that of the UV-vis measurement in our previous work [11].
Due to the different contents of oxygen vacancies in the present WO 3−x crystals, their band-gaps are different, which will respond to different wavelengths of light.Thus, the characteristic I-V curves of the devices fabricated with the obtained WO 3−x crystals under the irradiation of light with different wavelengths (400 and 600 nm) were investigated.The results are illustrated in Figure 8.For comparison, those measured in the dark are also presented.As can be seen from this figure, all the characteristic I-V curves measured in the dark and under the light irradiation are symmetric, indicating the existence of Schottky barriers between the metal electrodes and WO 3−x crystals.Under the irradiation of light with different wavelengths (400 and 600 nm), it was found that the conductivity of the devices was enhanced, mainly due to the improved separation of electrons and holes in the WO 3−x crystals generated under illumination.Moreover, it can be seen from Figures 8(a) and 8(b) that, for the WO 3 and WO 2.90 devices, with a shorter wavelength of light, under irradiation, the conductivity became stronger.However, for the W 19 O 55 and W 18 O 49 devices, with a longer wavelength of light, under irradiation, the conductivity got better (see Figures 8(c) and 8(d)).These phenomena can be attributed to the fact that WO 3 and WO 2.90 have larger bandgap than W 19 O 55 and W 18 O 49 .In summary, with increasing , the devices present their strong response to the light of short to long wavelength, showing that the as-synthesized WO 3−x crystals have great potential for preparing various optoelectrical devices.

Conclusions
By changing the ratio of WO 3 to S, a series of WO 3−x crystalline NMRs of identical morphology but with different contents of oxygen vacancies were successfully prepared through thermally evaporating WO 3 in reductive S atmosphere from different amounts of S powder loaded separately in a vacuum furnace.With the obtained WO 3−x crystals, devices were fabricated simply by screen-printing the obtained WO 3−x crystals onto ceramic substrates with Ag-Pd interdigital electrodes.With increasing content of oxygen vacancies, the prepared devices present a negative to positive resistance response to relative humidity.Under dry atmosphere, with increasing , the devices present their strong response to the light of short to long wavelength.With more oxygen vacancies, under light irradiation, the conducting ability of the devices was enhanced, due to the more efficient separation and transportation of the photogenerated carriers.Under simulated solar irradiation, the photocurrent intensity of the W 18 O 49 device was roughly 8 times, about 500 times, and even 1000 times larger than that of the W 19 O 55 , WO 2.90 , and WO 3 one, respectively.All these results reveal that the obtained WO 3−x crystals have the great potential for preparing various humidity sensors and optoelectrical devices.

Figure 1 :
Figure 1: XRD (a) and XPS (b) patterns of the obtained samples.The samples were prepared by thermal evaporation of 1 g WO 3 powder and different amounts of S powders (0.1-0.5 g) at 1150 ∘ C separately.The results indicate that pure WO 3 , WO 2.90 , W 19 O 55 , and W 18 O 49 crystals could be obtained, when the applied amount of S was 0.1, 0.15, 0.2, and 0.5 g of S, respectively.

Figure 3 :
Figure 3: Typical SEM images of the samples prepared by thermal evaporation of 1 g WO 3 powder and different amounts of S powders at 1150 ∘ C separately: (a) 0.1, (b) 0.15, (c) 0.2, and (d) 0.5 g.

Figure 4 :
Figure 4: Electrochemical impedance spectra of the devices fabricated with the obtained WO 3−x crystals with different contents of oxygen vacancies.The inset shows an equivalent circuit for the devices.

Figure 5 :
Figure 5: Electrochemical impedance spectra of the devices fabricated with the obtained WO 3−x crystals in different levels of relative humidity: (a) WO 3 , (b) WO 2.90 , (c) W 19 O 55 , and (d) W 18 O 49 ; and the equivalent circuits of the devices (e).

Figure 6 :
Figure 6: Characteristic I-V curves of the devices fabricated with the obtained WO 3−x crystals in different levels of relative humidity: (a) WO 3 , (b) WO 2.90 , (c) W 19 O 55 , and (d) W 18 O 49 .

Figure 7 :
Figure 7: Characteristic I-V curves of the devices fabricated with the obtained WO 3−x crystals under different light atmospheres, in which the insets display their photocurrent responses: (a) WO 3 , (b) WO 2.90 , (c) W 19 O 55 , and (d) W 18 O 49 .

Figure 8 :
Figure 8: Characteristic I-V curves of the devices fabricated with the obtained WO 3−x crystals under irradiation of light with different wavelengths (400 and 600 nm): (a) WO 3 , (b) WO 2.90 , (c) W 19 O 55 , and (d) W 18 O 49 .