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Thermal treatment effects on the material and gas-sensing properties of room-temperature tungsten oxide nanorod sensors
Sensors and Actuators B 137 (2009) 297–304
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Thermal treatment effects on the material and gas-sensing properties of room-temperature tungsten oxide nanorod sensors Yong Shin Kim ∗ Department of Applied Chemistry, Hanyang University, Ansan 426-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 5 September 2008 Received in revised form 7 November 2008 Accepted 27 November 2008 Available online 9 December 2008 Keywords: Gas sensor Tungsten oxide nanorod Metal oxide semiconductor Thermal treatment
a b s t r a c t Room-temperature tungsten oxide sensors were prepared by using a solution containing singlecrystalline and monodispersed WO2.72 nanorods with an average 75 nm length and 4 nm diameter. Thermal treatment-dependent gas-sensing characteristics of the sensors were examined for achieving a sensor with good performance. They were explained and discussed with their material properties probed by SEM, XRD, XPS and Raman spectroscopy. Optimized thermal treatment was found to be an annealing process at around 400 ◦ C under the flow condition of inert N2 or Ar gas. This treatment leads to the partial oxidation of nonstoichiometric W5+ states into the fully oxidative W6+ without any noticeable change in morphology or crystalline structure. These changes in material properties result in a great improvement in detection and recovery times with only a slight sacrifice of detection response. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide semiconductors (MOS) have been utilized as gassensing active materials for half a century [1,2]. One of the most promising solid-state MOS chemosensors is a tungsten oxide-based gas sensor. Several studies have proved that this sensor could be used for the detection of nitrogen oxide (NO and NO2 ), ammonia vapors, hydrogen sulfide, and hydrocarbons [3–7]. In the last few years, the nanostructures of tungsten oxides have been found to be more effective sensing materials due to their high surface-tovolume ratio and small grain size [8–12]. They have demonstrated novel sensing properties such as high sensitivity, fast response time, and low operation temperature. These properties are unattainable by using classical MOS materials consisting of submicrometer-sized polycrystalline particles. MOS sensors usually operate in the temperature range of 200–500 ◦ C. This operation temperature results in high electrical power consumption, which limits the use of MOS sensor as a sensing element in battery-powered portable devices. To overcome such a drawback, there have been many investigations of novel one-dimensional (1-D) MOS nanostructure sensors able to operate at room temperature. These include carbon nanotubes, SnO2 nanowires or nanobelts, In2 O3 nanowires, and WO2.72 nanorods [11–16]. Among these 1-D MOS, tungsten oxide are regarded as an encouraging material for achieving low operation temperature
∗ Tel.: +82 31 400 5507; fax: +82 31 400 3949. E-mail address: [email protected]. 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.11.037
since tungsten oxide has lower intergrain energy barrier than SnO2 or TiO2 [17,18]. In this work, we prepared tungsten oxide films by drop-coating single-crystalline, size-controlled WO2.72 nanorod solution, and then investigated the dependence of their material and gas-sensing properties on thermal treatment conditions. The thermal treatments were performed with the variation of annealing temperature in the range of 200–700 ◦ C under the ambient conditions of nitrogen, Ar, oxygen or dry air. Our previous works reported that WO2.72 nanorod sensors had highly sensitive detection capability even at room temperature for various reducing and oxidizing compounds [11,12]. The purpose of this study is to find thermal treatment conditions that can optimize gas-sensing performance in a tungsten oxide nanorod system. In addition, thermal treatment-dependent gas-sensing characteristics are explained and discussed in terms of observed material properties. 2. Experimental Tungsten oxide nanorods with an average 4 nm diameter and 75 nm length were synthesized in massive quantity by the colloidbased synthetic approach [19]. Diluted HCl solution was added into the colloid solution to precipitate WO2.72 nanorods stabilized by amine-based surfactants. The resultant precipitates were separated by a centrifuge, and further purified by dissolving in toluene and consecutive centrifuging three times. Tungsten oxide films were deposited by casting isopropyl alcohol solution of WO2.72 nanorods. The isopropyl alcohol solution was severely agitated to disperse the nanorods homogenously just before the deposition process. As-
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deposited blue films were dried at 100 ◦ C under vacuum for more than 10 h to remove residual organic compounds. Additional thermal treatments were performed for 3 h at a temperature in the range of 200–700 ◦ C under the flow conditions of N2 , O2 , Ar or dry air. Tungsten oxide films prepared on Si were utilized to characterize their material properties through various methods of analysis. Thickness was measured by using a mechanical profiler (Alpha-Step 500, KLA-Tencor), and surface microstructures were measured by scanning electron microscopy (SEM; Sirion, FEI). X-ray diffraction (XRD; D/MaxRC, Rigaku) having a Cu K␣ anode and Raman spectra excited at 514 nm were utilized for probing a crystalline structure. In addition, atomic constituents and chemical bonding states were evaluated through X-ray photoelectron spectroscopy (XPS; SCALAB 200R, VG Scientific). Curve fittings of XPS spectra were performed with the freeware program of XPSPEAK 4.1 using the sum of Gaussian and Lorentzian functions as the model functions. During fitting, the spin-orbit splitting energy between W 4f7/2 and W 4f5/2 was fixed at 2.1 eV. The fitting was started with the initial W 4f7/2 binding energies of 35.5 and 34.3 eV for the W6+ and W5+ oxidation states, respectively [20]. Tungsten oxide sensors were fabricated by using Au-patterned glass substrates. The sensor substrate was prepared by the consecutive e-beam depositions of 5 nm Cr and 100 nm Au layers through a shadow metal mask. It possesses two interdigitated Au electrodes with a sensing area of 3 mm × 10 mm and an electrode gap of 300 m. Sensing characteristics were measured by using the flow injection system as previously described [21]. They were carried out by placing a gas sensor in a detection chamber and blowing analyte vapors over it with a flow rate of 1000 ml/min. Analyte concentrations were regulated with the relative flow ratio between the carrier dry air and diluted analyte vapor. The sensing measurements were performed upon exposure to the four different
analytes of ethanol, hexane, benzene, and ammonia. Measured analog signals were delivered to the digital interface (NI, DAQ 6062E) connected to a laptop PC through a shielded cable. Data acquisition was performed by the user software programmed under LabVIEW (NI) environments with a typical sampling rate of 5 Hz. 3. Results and discussion 3.1. Thermal treatment effects on the material properties As-deposited tungsten oxide films were measured to have a thickness of 3–7 m by the mechanical profiler. Their surface morphology observed by SEM displayed a porous appearance resulting from randomly arranged agglomerates, favorably formed by parallel alignment of high anisotropic WO2.72 nanorods [11]. This appearance is almost identical to the SEM image of a N2 -annealed sample at 500 ◦ C (see Fig. 1A), suggesting no noticeable change in surface morphology. In fact, we were not able to observe any microstructural change in SEM images of samples annealed at below 500 ◦ C in ambient N2 conditions. The surface appearance however begins to change significantly for N2 -annealed samples at above 550 ◦ C (see Fig. 1B–D). In spite of the similarity in overall size, the surface appearance of a sample annealed at 550 ◦ C exhibits that tungsten oxide nanorod agglomerates are no longer basic constitutional components. Instead, new larger, collapsed crystalline particles have appeared. They seem to form through the recrystallization of nanorod aggregates. As the anneal temperature increases, the surface morphology reveals more dramatic changes. Above 600 ◦ C, the newly born crystals must undergo severe arrangement, resulting in a favorable columnar crystal growth along the vertical direction. Moreover crystal and void sizes become larger when the anneal temperature increases from 600 to 700 ◦ C. The average surface crystal size of N2 -annealed sample at 700 ◦ C finds
Fig. 1. Typical surface SEM images of tungsten oxide nanorod films annealed under the ambient nitrogen conditions at the four different temperatures of (A) 500 ◦ C, (B) 550 ◦ C, (C) 600 ◦ C and (D) 700 ◦ C.
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Fig. 3. X-ray deflection spectra of the same tungsten oxide samples displayed in Fig. 1. They were annealed under the ambient nitrogen condition at the four different temperatures of (A) 500 ◦ C, (B) 550 ◦ C, (C) 600 ◦ C and (D) 700 ◦ C. The Miller indices are determined from the monoclinic structure (JCPDS no. 05-0363), and the peak indicated with an asterisk results from a used Si substrate. Fig. 2. Typical surface SEM images of tungsten oxide nanorod films annealed under the ambient oxygen conditions at the two different temperatures of (A) 400 ◦ C and (B) 500 ◦ C.
to be around 100 nm, which is two times larger than that of 600 ◦ C. Similar changes in surface morphology were also examined under the thermal treatments with different ambient conditions of dry air, O2 or Ar. An identical result was observed in the Ar-annealing condition: the onset of recrystallization is around 550 ◦ C and the columnar crystals grow larger when an anneal temperature reaches above 600 ◦ C. On the other hand, dry air and O2 anneal treatments result in a different onset temperature. Fig. 2A and B shows surface SEM images of O2 -annealed WO2.72 films at 400 and 500 ◦ C, respectively. They distinctly demonstrate that crystal growth takes place at a temperature between 400 and 500 ◦ C. Since the average crystal size of a 500 ◦ C O2 -annealing sample is observed to be larger than that of 600 ◦ C N2 -annealed, oxygen environments must encourage the coalescence processes among the aggregated nanorods. An identical enhancement in crystal growth was also observed for airannealed samples. The dependence of an onset temperature on the ambient environments suggests that oxygen molecules can act as a crystallization enhancer. Due to the nonstoichiometric nature of WO2.72 nanorods, oxygen infiltration might easily initiate the oxidative recrystallization into the most stable WO3 state. Even under the flow condition of inert N2 or Ar, the oxidation process seems to proceed at higher temperature due to a small number of residual oxygen-containing compounds within a furnace. Fig. 3 shows the XRD patterns of the N2 -annealed samples shown in Fig. 1. The deflection pattern of 500 ◦ C sample shown in Fig. 3A displays no discernable change from the as-deposited films, which reported broad background peaks due to the small nano-sized dimension of nanorods and the very weak (0 1 0) monoclinic peak at the diffraction angle of 23.3◦ assigned to the growth direction [12]. The asterisk-designated peak at around 33◦ comes from a used Si substrate. However, there are many new peaks for the samples annealed at over 550 ◦ C, indicating the forma-
tion of new crystalline phases. These peaks have higher intensity and narrower FWHM as temperature increases from 550 to 700 ◦ C. It is well coincident with previous observations in SEM that the higher temperature-annealed sample shows a larger crystal structure. Stoichiometric WO3 crystals have known to have monoclinic or triclinic structures at room temperature [22]. The most intensive three diffraction peaks at the angle of 22–25◦ correspond to the pseudo-cubic reflections originating from the slight distortion of the ideal cubic {1 0 0} lattice planes. These three reflections cannot be used solely to determine whether the crystalline structure is triclinic or monoclinic, since their position and relative intensity is very similar. On the other hand, the intensity distribution of the diffraction peaks in the range of 32–35◦ can provide a clue to distinguish between triclinic and monoclinic structures [23]. Our intensity distributions were found to agree better with that of monoclinic WO3 , so that the newborn peaks were assigned based on monoclinic structure (JCPDS no. 05-0363). The temperature-dependent crystalline changes were also investigated for the samples annealed under different ambient conditions. All observed XRD peaks were well interpreted with the monoclinic WO3 structure even though there is a little difference in preferential crystal structure. They exhibited an identical tendency as observed in SEM: N2 and Ar anneal treatments displayed the same temperature dependence while the dry air and O2 environments induced the recrystallization at the lower onset temperature of less than 500 ◦ C. Raman shift spectra are shown in Fig. 4A and B for the films annealed under ambient N2 and O2 environments, respectively. Each consists of two spectra obtained at two different anneal temperatures, which correspond to before and after the onset of an extensive crystal growth. The two spectra obtained at 400 ◦ C observed to be similar to that of as-deposited sample [12]. All the spectra display four distinct bands at 270, 327, 712, and 807 cm−1 except for the lattice mode band below 200 cm−1 and a small instrumental artifact around 520 cm−1 . The four bands fall very
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Fig. 4. Raman shift spectra of tungsten oxide films annealed at two different temperatures under the ambient conditions of (A) nitrogen and (B) oxygen. They were observed with an Ar-ion laser excitation at 514 nm.
close to the wavenumbers of the strongest modes of monoclinic tungsten oxide. The two bands at 270 and 327 cm−1 had been assigned to O W O bending modes of the bridging oxide, while the 712 and 807 cm−1 bands are the corresponding stretching modes [24,25]. Moreover, the band intensities of the samples annealed at higher temperature increase by more than one order in magnitude compared with those at lower temperature. Consequently, these results give further evidence that the monoclinic crystals are grown and become larger as the anneal temperature goes higher. Main constituent elements of as-deposited films were observed to be tungsten and oxygen atoms from wide-scan XPS measurements, except for additional minor peaks resulting from carbon and Si elements. The appearance of Si 2s and 2p peaks can be explained with photoelectrons ejected from a Si substrate due to the highly porous nature of nanorod films. Small amount of carbon impurities were confirmed to exist within the film probably due to the incorporation of carbon atoms originated from carbon-containing chemicals used in the synthesis processes, together with a considerable amount of surface-adsorbed carbon moieties. However, there was no other discernable impurity except for the carbon atoms. Narrow-scan XPS measurements were performed to quantify the amount of carbon impurities in the binding energy range of 240–295 eV. Fig. 5 shows spectra obtained from the samples with different thermal treatments: (A) no annealing, (B) N2 annealing at 400 ◦ C, (C) N2 annealing at 600 ◦ C, (D) O2 annealing at 300 ◦ C and (E) O2 annealing at 500 ◦ C. In the case of N2 annealing treatment, the intensity of C 1s peak begins to decrease considerably at 600 ◦ C under the condition of maintaining the W 4d peak intensities (see Fig. 5A–C). It implies that carbon contaminants are not removed noticeably for the N2 400 ◦ C annealing while they decrease by about one third of as-deposited films for the 600 ◦ C annealing. A similar decrease in carbon content was also observed in the O2 annealing treatments: the C 1s intensity decreases with ascending O2 annealing temperature (see Fig. 5D and E). Judging from the slight decrease in C 1s intensity even for the sample annealed at
Fig. 5. XPS spectra of tungsten oxide samples in the binding energy range of 240–295 eV. They have undergone different thermal treatments: (A) no annealing, (B) N2 annealing at 400 ◦ C, (C) N2 annealing at 600 ◦ C, (D) O2 annealing at 300 ◦ C and (E) O2 annealing at 500 ◦ C.
300 ◦ C, carbon-containing impurities could be favorably eliminated in ambient O2 conditions, probably through oxidative reaction pathways. The W 4f XPS spectrum of an as-deposited blue WO2.72 film was previously reported to decompose into two components resulting from W5+ and W6+ oxidation states [12]. Since the nanorod has crystallographic shear planes relative to a ReO3 -type structure, the cations will have the W5+ formal oxidation state at shear plane boundaries in which a WO6 octahedron has three corner-sharing and two edge-sharing octahedral neighbors. Each component consists of W 4f7/2 and W 4f5/2 doublet peaks with the spin-orbit splitting energy of 2.1 eV. Fig. 6A and B shows W 4f XPS spectra of N2 annealed samples at the two temperatures of 400 and 600 ◦ C, respectively. The two dashed lines correspond to best-fitted curves for the W6+ and W5+ oxidation states while the solid line displays a sum spectrum of the two components. The sum spectrum displays fairly good agreement with the raw data displayed by open circles. In addition, the W 4f XPS spectra obtained from 300 and 500 ◦ C O2 annealed samples are shown in Fig. 7A and B, respectively. These spectra are also well interpreted with the two components corresponding to W6+ and W5+ oxidation states. Table 1 summarizes the parameters used in the deconvolution of W 4f XPS peaks, i.e. the binding energy of the W 4f7/2 , FWHM and relative contribution of the two oxidation states. All the samples exhibit identical binding energies of around 35.7 and 34.2 eV for the W6+ and W5+ oxidation states, respectively, which are well consistent with the values in the literatures [20,26,27]. This observation suggests that chemical bonding characteristics of the two oxidation states do not alter noticeably according to the thermal treatments. However, the relative W5+ population steadily decreases as N2 annealing temperature increases: 19% for the as-deposited sample, 12% at 400 ◦ C, and 6% at 600 ◦ C. It must be attributed to the oxidation of partially-reduced W5+ states into fully oxidized stable W6+ states. This interpretation is consistent with the SEM and XRD results indicating the extensive monoclinic WO3 crystal growth at above 600 ◦ C. It is
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Table 1 Parameters used in the deconvolution of W 4f XPS peaks into the two possible oxidation state components. Annealing conditions
W6+ component a
b
None N2 , 400 ◦ C N2 , 600 ◦ C O2 , 300 ◦ C O2 , 500 ◦ C a b
W5+ component a
BE (eV)
FWHM (eV)
Population (%)
BEa (eV)
FWHMa (eV)
Population (%)
35.7 35.7 35.7 35.8 35.7
1.9 1.7 1.6 1.9 1.6
81 88 94 85 94
34.2 34.3 34.2 34.3 34.3
1.4 1.5 1.2 1.4 1.4
19 12 6 15 6
BE and FWHM are abbreviations for binding energy and full width at half maximum of the main W 4f7/2 peak, respectively. These results were obtained from our previous work [12].
worth mentioning that the oxidation significantly occurs for the 400 ◦ C N2 -annealing sample with the same surface morphology of as-deposited films. It suggests that the nanorod elements first undergo partial oxidation at relatively low temperature and then agglomerate to form a larger crystal above the onset temperature. Furthermore, an identical trend is observed for the O2 -annealing samples: the relative W5+ populations are 15% at 300 ◦ C and 6% at 500 ◦ C. Judging from the relative W5+ populations, the oxidation process can be activated more easily in the O2 -containing ambient conditions.
3.2. Thermal treatment effects on the gas-sensing properties
Fig. 6. Narrow-scan W 4f XPS spectra of tungsten oxide samples N2 -annealed at the two different temperatures of (A) 400 ◦ C and (B) 600 ◦ C. The open circle symbols show raw data, and the solid line corresponds to a sum spectrum of the two dashed lines which are best-fitted curves for the W6+ and W5+ oxidation states.
Fig. 7. Narrow-scan W 4f XPS spectra of tungsten oxide samples O2 -annealed at the two different temperatures of (A) 300 ◦ C and (B) 500 ◦ C. The open circle symbols show raw data, and the solid line corresponds to a sum spectrum of the two dashed lines which are best-fitted curves for the W6+ and W5+ oxidation states.
Gas-sensing measurements were performed for WO2.72 nanorod sensors annealed at different temperatures under ambient N2 conditions. Fig. 8 shows annealing temperature-dependent sensor response profiles at room temperature as a function of a detection time. The sensors were exposed to air-diluted ethanol vapors of 1000 ppm with the concentration variation of step function. The lowest plot in Fig. 8 displays the expected time profile of ethanol concentration for the sequence of 5 min injection and 20 min recovery period. In our detection chamber, the elapsed times of filling and removing ethanol vapors were observed to be less than 10 s from
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Fig. 8. Time-dependent response profiles of WO2.72 nanorod sensors having the different N2 anneal temperatures in the range of 300–700 ◦ C. No annealed sample is also given for comparison. All sensors were measured under the condition of flowinjecting air-diluted ethanol vapors of 1000 ppm as presented in the lowest curve. The vertical arrow corresponds to the magnitude of 10 in sensor response.
separated measurements using a fast response sensor. The length of the vertical arrow corresponds to the magnitude of 10 in sensor response, which was defined by a relative percentage change of a sensor resistance R with respect to a stabilized initial value R0 , i.e. response = 100 × (R–R0 )/R0 = 100 × R/R0 . The response profiles exhibit an increase change in resistance upon exposure to the reducing ethanol vapors, corresponding to p-type response. This phenomenon was previously explained with the competition adsorption between ambient molecular oxygen and analyte vapors on the surface of WO2.72 nanorods instead of the conventional mechanism observed at above 200 ◦ C in MOS sensor, i.e. the reaction between ionosorbed oxygen moieties and analytes [11]. The adsorbed analyte molecules may act as active scattering centers, thus suppressing the electrical conductance of free electron carriers in n-type tungsten oxide system. This scattering results in the resistance increase for both oxidizing and reducing analytes. The response of a no-annealed sample steadily increases for the ethanol injection period and eventually reaches to a maximum response of 9.1. It slowly decreases into the initial position for a following recovery time of 20 min. The recovery time of no-annealed sensor was observed to be around 30 min which is too long to use as a practical MOS gas sensor. Such a long recovery time was previously observed in other MOS sensor systems operated at ambient temperature [13–15]. It might be attributed to the low operation temperature leading to slow desorption rate of pre-adsorbed moieties. As an annealing temperature increases until 500 ◦ C, the response profile becomes closer to a square shape, indicating that the sensor has fast response and short recovery time. In the case of a 400 ◦ C annealed sensor, the response abruptly increases to the 90% level of a maximum value within a time of less than 1 min, and the recovery time becomes two times shorter compared with the no-annealed sample. However, there is a slight decrease in maximum response: a maximum response of the 400 ◦ C annealed sensor
corresponds to 85% of the no-annealed one. As a result, N2 anneal treatment below 500 ◦ C leads to a great improvement in response and recovery times with a slight sacrifice of response. This behavior might be explained in the light of the observed material characteristics as stated below. The material properties of N2 -annealed films at below 500 ◦ C have been confirmed to be identical to those of as-deposited samples except for the population difference in W6+ and W5+ oxidation states. Therefore, the temperature-dependent response variation might result from the difference in sensing ability of the two oxidation states. Fully oxidized WO3 is the most stable tungsten oxide compound so that the W6+ state may be more inert to the sensing-related surface reactions than the W5+ state. The slight decrease in response with the increase in the anneal temperature can be interpreted as the local oxidation of more reactive W5+ states to W6+ as probed by XPS. Furthermore, the W5+ states have a stronger interaction with analyte molecules so that the long recovery time of as-deposited sensor can be also understood with the high W5+ population. Consequently, N2 thermal treatment of WO2.72 nanorod films below 500 ◦ C results in the partial oxidation of nonstoichiometric W5+ states without any noticeable change in morphology or crystalline structure, which gives a chance to modulate systematically gas-sensing characteristics such as response magnitude and detection time by means of careful control of the relative population ratio between W5+ and W6+ states. The sensing characteristics become suddenly worse as the N2 annealing temperature increases beyond 600 ◦ C at which point the nanorods begin to coalesce and develop large, columnar WO3 crystals. Considering that porous film made of small crystalline nanorods with a high aspect ratio are favorable for achieving a high sensitive MOS sensor, the deterioration in sensing capability can be explained by the formation of larger crystals with a columnar crack. Response profiles were also evaluated for the sensors annealed in ambient O2 conditions at a temperature range of 300–600 ◦ C. On the whole, O2 -annealed sensors had exhibited bad sensing properties. The most serious problem was their weak operation durability: the O2 -annealed sensors easily broke down in spite of material properties similar to N2 -annealed analogues. This propensity was also observed for the sensors annealed in ambient air conditions. The thermal treatment under the O2 -containing ambience therefore gives adverse effects on gas-sensing characteristics. Even though there has been no direct clue to explain this effect, it seems to be ascribed to delicate differences in microstructures induced by O2 thermal treatment. Additional measurements were performed at room temperature upon exposure to other volatile vapors for evaluating the sensing characteristics of our nanorod sensors in terms of sensitivity and selectivity. Fig. 9 shows time-dependent sensor response profiles of 400 and 600 ◦ C N2 -annealing samples upon exposure to the three different analytes of 1000 ppm hexane, 1000 ppm benzene, and 10 ppm NH3 . The bottom square pulses display the time-dependent profiles of supplying air-diluted analyte vapors. The middle profiles were obtained from the sensor annealed at 400 ◦ C while the uppers correspond to the sample annealed at 600 ◦ C. The upper curves were shifted upward with respect to the middle ones in order to distinguish easily between them. The scale in sensor response is, however, identical for the two cases and the vertical arrow size matches up to the magnitude of 2. The relative detection magnitudes in sensor response exhibit great differences for the three different analytes: the 400 ◦ C sensor is possible to achieve feasible detection of hexane and benzene while the 600 ◦ C sensor can detect NH3 vapors more sensitively. In fact, the detection responses of the 400 ◦ C sensor are fond to be 1.2 for hexane and 2.1 for benzene. These values are much larger compared with the 600 ◦ C response of <0.1. On the other hand the NH3 detection response of 600 ◦ C sensor is about six times larger
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Acknowledgement This work was supported by the research funds of KOSEF (R012008-000-20460-0) and SMBA (S708002511 and S6070381211).
References
Fig. 9. Time-dependent response profiles of WO2.72 nanorod sensors for the three different analyte exposures of 1000 ppm hexane, 1000 ppm benzene, and 10 ppm NH3 . The top profiles are sensor responses of the 600 ◦ C N2 -annealing sample while the middles correspond to those of the 400 ◦ C N2 -annealing. The bottom square pulses exhibit the time profiles of supplying three different analytes. The vertical arrow corresponds to the magnitude of 2 in sensor response.
than that of 400 ◦ C. As a result these response variation according to anneal temperatures implies that carefully controlled thermal treatments could provide a chance to improve detection selectivity which is regarded as obstacles in a tungsten oxide nanorod system for achieving a good performance sensor operated at room temperature. 4. Conclusion We have investigated the dependence of gas-sensing characteristics on thermal treatment conditions in a tungsten oxide nanorod system that demonstrated the facile detection of various analytes at ambient temperature. As the annealing temperature increases, the surface morphology of as-deposited films consisting of randomly arranged WO2.72 nanorod agglomerates were observed to develop larger WO3 particles with a columnar monoclinic crystal structures, probably through a recrystallization process of nanorod aggregates. The onset temperatures for the recrystallization were found to be around 550 ◦ C for the inert N2 or Ar annealing conditions and 450 ◦ C for the O2 -containing ambient, which suggests that oxygen molecules act as a crystallization enhancer. Since such a crystallization process leads to large crystals and many cracks between grain boundaries, the thermal treatments above the onset temperature were found to deteriorate the detection capability of WO2.72 sensors. In addition, thermal treatments under the O2 -containing active environments were found to result in bad reproducibility in sensor response compared with the inert N2 conditions. As a result, the recommendable thermal treatment conditions for WO2.72 sensors were an annealing temperature of 300–500 ◦ C under inert N2 or Ar ambient. The annealing temperature within the range could be utilized as a parameter to regulate a relative population ratio between W5+ and W6+ states without any noticeable change in morphology or crystalline structure. We can have an opportunity to optimize gas-sensing properties such as detection selectivity and a recovery time through the careful control of relative populations between the two oxidation states.
[1] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram. 7 (2001) 143–167. [2] Y. Shimizu, M. Egashira, Basic aspects and challenges of semiconductor gas sensors, MRS Bull. 24 (1999) 18–24. [3] L.G. Teoh, I.M. Hung, J. Shieh, W.H. Lai, M.H. Hon, High sensitivity semiconductor NO2 gas sensor based on mesoporous WO3 thin film, Electrochem. Solid-State Lett. 6 (2003) G108–G111. [4] C. Cantalini, H.T. Sun, M. Faccio, M. Pelino, S. Santucci, L. Lozzi, M. Passacantando, NO2 sensitivity of WO3 thin film obtained by high vacuum thermal evaporation, Sens. Actuators B-Chem. 31 (1996) 81–87. [5] E. Llobet, G. Molas, P. Molinàs, J. Calderer, X. Vilanova, J. Brezmes, J.E. Sueiras, X. Correig, Fabrication of highly selective tungsten oxide ammonia sensors, J. Electrochem. Soc. 147 (2000) 776–779. [6] C. Bittencourt, R. Landers, E. Llobet, G. Molas, X. Correig, M.A.P. Silva, J.E. Sueiras, J. Calderer, Effects of oxygen partial pressure and annealing temperature on the formation of sputtered tungsten oxide films, J. Electrochem. Soc. 149 (2002) H81–H86. [7] R. Ionescu, A. Hoel, C.G. Granqvist, E. Llobet, P. Heszler, Low-level detection of ethanol and H2 S with temperature-modulated WO3 nanoparticle gas sensors, Sens. Actuators B-Chem. 104 (2005) 132–139. [8] J. Polleux, A. Gurlo, N. Barsan, U. Weimar, M. Antonietti, M. Niederberger, Template-free synthesis and assembly of single-crystalline tungsten oxide nanowires and their gas-sensing properties, Angew. Chem. Int. Ed. 45 (2006) 261–265. [9] C.S. Rout, M. Hegde, C.N.R. Rao, H2 S sensors based on tungsten oxide nanostructures, Sens. Actuators B-Chem. 128 (2008) 488–493. [10] C. Balázsi, L. Wang, E.O. Zayim, I.M. Szilágyi, K. Sedlacková, J. Pfeifer, A.L. Tóth, P.-I. Gouma, Nanosize hexagonal tungsten oxide for gas sensing applications, J. Eur. Ceram. Soc. 28 (2008) 913–917. [11] Y.S. Kim, S.-C. Ha, K. Kim, H. Yang, S.-Y. Choi, Y.T. Kim, J.T. Park, C.H. Lee, J. Choi, J. Paek, K. Lee, Room temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film, Appl. Phys. Lett. 86 (2005) 213105. [12] Y. S. Kim, K. Lee, Material and gas-sensing properties of tungsten oxide nanorod thin-films, J. Nanosci. Nanotechnol. doi:10.1166/jnn.2008.SE32. [13] J. Li, Y. Lu, Q. Ye, M. Cinke, J. Han, M. Meyyappan, Carbon nanotube sensors for gas and organic vapor detection, Nano Lett. 3 (2003) 929–933. [14] M. Law, H. Kind, B. Messer, F. Kim, P. Yang, Photochemical sensing of NO2 with SnO2 nanoribbon nanosensors at room temperature, Angew. Chem. Int. Ed. 41 (2002) 2405–2408. [15] Y. Wang, X. Jiang, Y. Xia, A solution-phase precursor route to polycrystalline SnO2 nanowires that can be used for gas sensing under ambient conditions, J. Am. Chem. Soc. 125 (2003) 16176–16178. [16] D. Zhang, Z. Liu, C. Li, T. Tang, X. Liu, S. Han, B. Lei, C. Zhou, Detection of NO2 down to ppb levels using individual and multiple In2 O3 nanowire devices, Nano Lett. 4 (2004) 1919–1924. [17] C. Malagù, V. Guidi, M.C. Carotta, G. Martinelli, Unpinning of Fermi level in nanocrystalline semiconductors, Appl. Phys. Lett. 84 (2004) 4158–4160. [18] C. Malagù, M.C. Carotta, S. Gherardi, V. Guidi, B. Vendemiati, G. Martinelli, AC measurements and modeling of WO3 thick film gas sensors, Sens. Actuators B-Chem. 108 (2005) 70–74. [19] K. Lee, W.S. Seo, J.T. Park, Synthesis and optical properties of colloidal tungsten oxide nanorods, J. Am. Chem. Soc. 125 (2003) 3408–3409. [20] D. Barreca, S. Bozza, G. Carta, G. Rossetto, E. Tondello, P. Zanella, Structural and Morphological analyses of tungsten oxide nanophasic thin films obtained by MOCVD, Surf. Sci. 532 (2003) 439–443. [21] Y.S. Kim, S.-C. Ha, H. Yang, Y.T. Kim, Gas sensor measurement system capable of sampling volatile organic compounds (VOCs) in wide concentration range, Sens. Actuators B-Chem. 122 (2007) 211–218. [22] P. Woodward, A. Sleight, T. Vogt, Structure refinement of triclinic tungsten trioxide, J. Phys. Chem. Solids 56 (1995) 1305–1315. [23] E. Cazzanelli, C. Vinegoni, G. Mariotto, A. Kuzmin, J. Purans, Low temperature polymorphism in tungsten trioxide powders and its dependence on mechanical treatments, J. Solid State Chem. 143 (1999) 24–32. [24] M.F. Daniel, B. Desbat, J.C. Lassegues, B. Gerand, M. Figlarz, Infrared, Raman study of WO3 tungsten trioxides and WO3 ·xH2 O tungsten trioxide hydrates, J. Solid State Chem. 67 (1987) 235–247. [25] C. Santato, M. Odziemkowski, M. Ulmann, J. Augustynski, Crystallographically oriented mesoporous WO3 films: synthesis, characterization, and applications, J. Am. Chem. Soc. 123 (2001) 10639–10649. [26] http://www.lasurface.com. [27] S.C. Moulzolf, S. Ding, R.J. Lad, Stoichiometry and microstructure effects on tungsten oxide chemiresistive films, Sens. Actuators B-Chem. 77 (2001) 375–382.
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Biography Yong Shin Kim received a PhD in chemistry from Korea Advanced Institute of Science and Technology (KAIST) in 1997. After his degree, he had worked as a senior research member at Electronics and Telecommunications Research Institute in the
field of developing flat panel displays, i.e. thin-film electroluminescent devices and miniaturized electronic nose system. Since 2007, he has been employed in Hanyang University. Now his research activities are focused on the development of miniaturized, smart chemical sensor systems and novel nanomaterials for the sensor applications.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Thermal treatment effects on the material and gas-sensing properties of room-temperature tungsten oxide nanorod sensors Yong Shin Kim ∗ Department of Applied Chemistry, Hanyang University, Ansan 426-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 5 September 2008 Received in revised form 7 November 2008 Accepted 27 November 2008 Available online 9 December 2008 Keywords: Gas sensor Tungsten oxide nanorod Metal oxide semiconductor Thermal treatment
a b s t r a c t Room-temperature tungsten oxide sensors were prepared by using a solution containing singlecrystalline and monodispersed WO2.72 nanorods with an average 75 nm length and 4 nm diameter. Thermal treatment-dependent gas-sensing characteristics of the sensors were examined for achieving a sensor with good performance. They were explained and discussed with their material properties probed by SEM, XRD, XPS and Raman spectroscopy. Optimized thermal treatment was found to be an annealing process at around 400 ◦ C under the flow condition of inert N2 or Ar gas. This treatment leads to the partial oxidation of nonstoichiometric W5+ states into the fully oxidative W6+ without any noticeable change in morphology or crystalline structure. These changes in material properties result in a great improvement in detection and recovery times with only a slight sacrifice of detection response. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide semiconductors (MOS) have been utilized as gassensing active materials for half a century [1,2]. One of the most promising solid-state MOS chemosensors is a tungsten oxide-based gas sensor. Several studies have proved that this sensor could be used for the detection of nitrogen oxide (NO and NO2 ), ammonia vapors, hydrogen sulfide, and hydrocarbons [3–7]. In the last few years, the nanostructures of tungsten oxides have been found to be more effective sensing materials due to their high surface-tovolume ratio and small grain size [8–12]. They have demonstrated novel sensing properties such as high sensitivity, fast response time, and low operation temperature. These properties are unattainable by using classical MOS materials consisting of submicrometer-sized polycrystalline particles. MOS sensors usually operate in the temperature range of 200–500 ◦ C. This operation temperature results in high electrical power consumption, which limits the use of MOS sensor as a sensing element in battery-powered portable devices. To overcome such a drawback, there have been many investigations of novel one-dimensional (1-D) MOS nanostructure sensors able to operate at room temperature. These include carbon nanotubes, SnO2 nanowires or nanobelts, In2 O3 nanowires, and WO2.72 nanorods [11–16]. Among these 1-D MOS, tungsten oxide are regarded as an encouraging material for achieving low operation temperature
∗ Tel.: +82 31 400 5507; fax: +82 31 400 3949. E-mail address: [email protected]. 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.11.037
since tungsten oxide has lower intergrain energy barrier than SnO2 or TiO2 [17,18]. In this work, we prepared tungsten oxide films by drop-coating single-crystalline, size-controlled WO2.72 nanorod solution, and then investigated the dependence of their material and gas-sensing properties on thermal treatment conditions. The thermal treatments were performed with the variation of annealing temperature in the range of 200–700 ◦ C under the ambient conditions of nitrogen, Ar, oxygen or dry air. Our previous works reported that WO2.72 nanorod sensors had highly sensitive detection capability even at room temperature for various reducing and oxidizing compounds [11,12]. The purpose of this study is to find thermal treatment conditions that can optimize gas-sensing performance in a tungsten oxide nanorod system. In addition, thermal treatment-dependent gas-sensing characteristics are explained and discussed in terms of observed material properties. 2. Experimental Tungsten oxide nanorods with an average 4 nm diameter and 75 nm length were synthesized in massive quantity by the colloidbased synthetic approach [19]. Diluted HCl solution was added into the colloid solution to precipitate WO2.72 nanorods stabilized by amine-based surfactants. The resultant precipitates were separated by a centrifuge, and further purified by dissolving in toluene and consecutive centrifuging three times. Tungsten oxide films were deposited by casting isopropyl alcohol solution of WO2.72 nanorods. The isopropyl alcohol solution was severely agitated to disperse the nanorods homogenously just before the deposition process. As-
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deposited blue films were dried at 100 ◦ C under vacuum for more than 10 h to remove residual organic compounds. Additional thermal treatments were performed for 3 h at a temperature in the range of 200–700 ◦ C under the flow conditions of N2 , O2 , Ar or dry air. Tungsten oxide films prepared on Si were utilized to characterize their material properties through various methods of analysis. Thickness was measured by using a mechanical profiler (Alpha-Step 500, KLA-Tencor), and surface microstructures were measured by scanning electron microscopy (SEM; Sirion, FEI). X-ray diffraction (XRD; D/MaxRC, Rigaku) having a Cu K␣ anode and Raman spectra excited at 514 nm were utilized for probing a crystalline structure. In addition, atomic constituents and chemical bonding states were evaluated through X-ray photoelectron spectroscopy (XPS; SCALAB 200R, VG Scientific). Curve fittings of XPS spectra were performed with the freeware program of XPSPEAK 4.1 using the sum of Gaussian and Lorentzian functions as the model functions. During fitting, the spin-orbit splitting energy between W 4f7/2 and W 4f5/2 was fixed at 2.1 eV. The fitting was started with the initial W 4f7/2 binding energies of 35.5 and 34.3 eV for the W6+ and W5+ oxidation states, respectively [20]. Tungsten oxide sensors were fabricated by using Au-patterned glass substrates. The sensor substrate was prepared by the consecutive e-beam depositions of 5 nm Cr and 100 nm Au layers through a shadow metal mask. It possesses two interdigitated Au electrodes with a sensing area of 3 mm × 10 mm and an electrode gap of 300 m. Sensing characteristics were measured by using the flow injection system as previously described [21]. They were carried out by placing a gas sensor in a detection chamber and blowing analyte vapors over it with a flow rate of 1000 ml/min. Analyte concentrations were regulated with the relative flow ratio between the carrier dry air and diluted analyte vapor. The sensing measurements were performed upon exposure to the four different
analytes of ethanol, hexane, benzene, and ammonia. Measured analog signals were delivered to the digital interface (NI, DAQ 6062E) connected to a laptop PC through a shielded cable. Data acquisition was performed by the user software programmed under LabVIEW (NI) environments with a typical sampling rate of 5 Hz. 3. Results and discussion 3.1. Thermal treatment effects on the material properties As-deposited tungsten oxide films were measured to have a thickness of 3–7 m by the mechanical profiler. Their surface morphology observed by SEM displayed a porous appearance resulting from randomly arranged agglomerates, favorably formed by parallel alignment of high anisotropic WO2.72 nanorods [11]. This appearance is almost identical to the SEM image of a N2 -annealed sample at 500 ◦ C (see Fig. 1A), suggesting no noticeable change in surface morphology. In fact, we were not able to observe any microstructural change in SEM images of samples annealed at below 500 ◦ C in ambient N2 conditions. The surface appearance however begins to change significantly for N2 -annealed samples at above 550 ◦ C (see Fig. 1B–D). In spite of the similarity in overall size, the surface appearance of a sample annealed at 550 ◦ C exhibits that tungsten oxide nanorod agglomerates are no longer basic constitutional components. Instead, new larger, collapsed crystalline particles have appeared. They seem to form through the recrystallization of nanorod aggregates. As the anneal temperature increases, the surface morphology reveals more dramatic changes. Above 600 ◦ C, the newly born crystals must undergo severe arrangement, resulting in a favorable columnar crystal growth along the vertical direction. Moreover crystal and void sizes become larger when the anneal temperature increases from 600 to 700 ◦ C. The average surface crystal size of N2 -annealed sample at 700 ◦ C finds
Fig. 1. Typical surface SEM images of tungsten oxide nanorod films annealed under the ambient nitrogen conditions at the four different temperatures of (A) 500 ◦ C, (B) 550 ◦ C, (C) 600 ◦ C and (D) 700 ◦ C.
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Fig. 3. X-ray deflection spectra of the same tungsten oxide samples displayed in Fig. 1. They were annealed under the ambient nitrogen condition at the four different temperatures of (A) 500 ◦ C, (B) 550 ◦ C, (C) 600 ◦ C and (D) 700 ◦ C. The Miller indices are determined from the monoclinic structure (JCPDS no. 05-0363), and the peak indicated with an asterisk results from a used Si substrate. Fig. 2. Typical surface SEM images of tungsten oxide nanorod films annealed under the ambient oxygen conditions at the two different temperatures of (A) 400 ◦ C and (B) 500 ◦ C.
to be around 100 nm, which is two times larger than that of 600 ◦ C. Similar changes in surface morphology were also examined under the thermal treatments with different ambient conditions of dry air, O2 or Ar. An identical result was observed in the Ar-annealing condition: the onset of recrystallization is around 550 ◦ C and the columnar crystals grow larger when an anneal temperature reaches above 600 ◦ C. On the other hand, dry air and O2 anneal treatments result in a different onset temperature. Fig. 2A and B shows surface SEM images of O2 -annealed WO2.72 films at 400 and 500 ◦ C, respectively. They distinctly demonstrate that crystal growth takes place at a temperature between 400 and 500 ◦ C. Since the average crystal size of a 500 ◦ C O2 -annealing sample is observed to be larger than that of 600 ◦ C N2 -annealed, oxygen environments must encourage the coalescence processes among the aggregated nanorods. An identical enhancement in crystal growth was also observed for airannealed samples. The dependence of an onset temperature on the ambient environments suggests that oxygen molecules can act as a crystallization enhancer. Due to the nonstoichiometric nature of WO2.72 nanorods, oxygen infiltration might easily initiate the oxidative recrystallization into the most stable WO3 state. Even under the flow condition of inert N2 or Ar, the oxidation process seems to proceed at higher temperature due to a small number of residual oxygen-containing compounds within a furnace. Fig. 3 shows the XRD patterns of the N2 -annealed samples shown in Fig. 1. The deflection pattern of 500 ◦ C sample shown in Fig. 3A displays no discernable change from the as-deposited films, which reported broad background peaks due to the small nano-sized dimension of nanorods and the very weak (0 1 0) monoclinic peak at the diffraction angle of 23.3◦ assigned to the growth direction [12]. The asterisk-designated peak at around 33◦ comes from a used Si substrate. However, there are many new peaks for the samples annealed at over 550 ◦ C, indicating the forma-
tion of new crystalline phases. These peaks have higher intensity and narrower FWHM as temperature increases from 550 to 700 ◦ C. It is well coincident with previous observations in SEM that the higher temperature-annealed sample shows a larger crystal structure. Stoichiometric WO3 crystals have known to have monoclinic or triclinic structures at room temperature [22]. The most intensive three diffraction peaks at the angle of 22–25◦ correspond to the pseudo-cubic reflections originating from the slight distortion of the ideal cubic {1 0 0} lattice planes. These three reflections cannot be used solely to determine whether the crystalline structure is triclinic or monoclinic, since their position and relative intensity is very similar. On the other hand, the intensity distribution of the diffraction peaks in the range of 32–35◦ can provide a clue to distinguish between triclinic and monoclinic structures [23]. Our intensity distributions were found to agree better with that of monoclinic WO3 , so that the newborn peaks were assigned based on monoclinic structure (JCPDS no. 05-0363). The temperature-dependent crystalline changes were also investigated for the samples annealed under different ambient conditions. All observed XRD peaks were well interpreted with the monoclinic WO3 structure even though there is a little difference in preferential crystal structure. They exhibited an identical tendency as observed in SEM: N2 and Ar anneal treatments displayed the same temperature dependence while the dry air and O2 environments induced the recrystallization at the lower onset temperature of less than 500 ◦ C. Raman shift spectra are shown in Fig. 4A and B for the films annealed under ambient N2 and O2 environments, respectively. Each consists of two spectra obtained at two different anneal temperatures, which correspond to before and after the onset of an extensive crystal growth. The two spectra obtained at 400 ◦ C observed to be similar to that of as-deposited sample [12]. All the spectra display four distinct bands at 270, 327, 712, and 807 cm−1 except for the lattice mode band below 200 cm−1 and a small instrumental artifact around 520 cm−1 . The four bands fall very
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Fig. 4. Raman shift spectra of tungsten oxide films annealed at two different temperatures under the ambient conditions of (A) nitrogen and (B) oxygen. They were observed with an Ar-ion laser excitation at 514 nm.
close to the wavenumbers of the strongest modes of monoclinic tungsten oxide. The two bands at 270 and 327 cm−1 had been assigned to O W O bending modes of the bridging oxide, while the 712 and 807 cm−1 bands are the corresponding stretching modes [24,25]. Moreover, the band intensities of the samples annealed at higher temperature increase by more than one order in magnitude compared with those at lower temperature. Consequently, these results give further evidence that the monoclinic crystals are grown and become larger as the anneal temperature goes higher. Main constituent elements of as-deposited films were observed to be tungsten and oxygen atoms from wide-scan XPS measurements, except for additional minor peaks resulting from carbon and Si elements. The appearance of Si 2s and 2p peaks can be explained with photoelectrons ejected from a Si substrate due to the highly porous nature of nanorod films. Small amount of carbon impurities were confirmed to exist within the film probably due to the incorporation of carbon atoms originated from carbon-containing chemicals used in the synthesis processes, together with a considerable amount of surface-adsorbed carbon moieties. However, there was no other discernable impurity except for the carbon atoms. Narrow-scan XPS measurements were performed to quantify the amount of carbon impurities in the binding energy range of 240–295 eV. Fig. 5 shows spectra obtained from the samples with different thermal treatments: (A) no annealing, (B) N2 annealing at 400 ◦ C, (C) N2 annealing at 600 ◦ C, (D) O2 annealing at 300 ◦ C and (E) O2 annealing at 500 ◦ C. In the case of N2 annealing treatment, the intensity of C 1s peak begins to decrease considerably at 600 ◦ C under the condition of maintaining the W 4d peak intensities (see Fig. 5A–C). It implies that carbon contaminants are not removed noticeably for the N2 400 ◦ C annealing while they decrease by about one third of as-deposited films for the 600 ◦ C annealing. A similar decrease in carbon content was also observed in the O2 annealing treatments: the C 1s intensity decreases with ascending O2 annealing temperature (see Fig. 5D and E). Judging from the slight decrease in C 1s intensity even for the sample annealed at
Fig. 5. XPS spectra of tungsten oxide samples in the binding energy range of 240–295 eV. They have undergone different thermal treatments: (A) no annealing, (B) N2 annealing at 400 ◦ C, (C) N2 annealing at 600 ◦ C, (D) O2 annealing at 300 ◦ C and (E) O2 annealing at 500 ◦ C.
300 ◦ C, carbon-containing impurities could be favorably eliminated in ambient O2 conditions, probably through oxidative reaction pathways. The W 4f XPS spectrum of an as-deposited blue WO2.72 film was previously reported to decompose into two components resulting from W5+ and W6+ oxidation states [12]. Since the nanorod has crystallographic shear planes relative to a ReO3 -type structure, the cations will have the W5+ formal oxidation state at shear plane boundaries in which a WO6 octahedron has three corner-sharing and two edge-sharing octahedral neighbors. Each component consists of W 4f7/2 and W 4f5/2 doublet peaks with the spin-orbit splitting energy of 2.1 eV. Fig. 6A and B shows W 4f XPS spectra of N2 annealed samples at the two temperatures of 400 and 600 ◦ C, respectively. The two dashed lines correspond to best-fitted curves for the W6+ and W5+ oxidation states while the solid line displays a sum spectrum of the two components. The sum spectrum displays fairly good agreement with the raw data displayed by open circles. In addition, the W 4f XPS spectra obtained from 300 and 500 ◦ C O2 annealed samples are shown in Fig. 7A and B, respectively. These spectra are also well interpreted with the two components corresponding to W6+ and W5+ oxidation states. Table 1 summarizes the parameters used in the deconvolution of W 4f XPS peaks, i.e. the binding energy of the W 4f7/2 , FWHM and relative contribution of the two oxidation states. All the samples exhibit identical binding energies of around 35.7 and 34.2 eV for the W6+ and W5+ oxidation states, respectively, which are well consistent with the values in the literatures [20,26,27]. This observation suggests that chemical bonding characteristics of the two oxidation states do not alter noticeably according to the thermal treatments. However, the relative W5+ population steadily decreases as N2 annealing temperature increases: 19% for the as-deposited sample, 12% at 400 ◦ C, and 6% at 600 ◦ C. It must be attributed to the oxidation of partially-reduced W5+ states into fully oxidized stable W6+ states. This interpretation is consistent with the SEM and XRD results indicating the extensive monoclinic WO3 crystal growth at above 600 ◦ C. It is
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Table 1 Parameters used in the deconvolution of W 4f XPS peaks into the two possible oxidation state components. Annealing conditions
W6+ component a
b
None N2 , 400 ◦ C N2 , 600 ◦ C O2 , 300 ◦ C O2 , 500 ◦ C a b
W5+ component a
BE (eV)
FWHM (eV)
Population (%)
BEa (eV)
FWHMa (eV)
Population (%)
35.7 35.7 35.7 35.8 35.7
1.9 1.7 1.6 1.9 1.6
81 88 94 85 94
34.2 34.3 34.2 34.3 34.3
1.4 1.5 1.2 1.4 1.4
19 12 6 15 6
BE and FWHM are abbreviations for binding energy and full width at half maximum of the main W 4f7/2 peak, respectively. These results were obtained from our previous work [12].
worth mentioning that the oxidation significantly occurs for the 400 ◦ C N2 -annealing sample with the same surface morphology of as-deposited films. It suggests that the nanorod elements first undergo partial oxidation at relatively low temperature and then agglomerate to form a larger crystal above the onset temperature. Furthermore, an identical trend is observed for the O2 -annealing samples: the relative W5+ populations are 15% at 300 ◦ C and 6% at 500 ◦ C. Judging from the relative W5+ populations, the oxidation process can be activated more easily in the O2 -containing ambient conditions.
3.2. Thermal treatment effects on the gas-sensing properties
Fig. 6. Narrow-scan W 4f XPS spectra of tungsten oxide samples N2 -annealed at the two different temperatures of (A) 400 ◦ C and (B) 600 ◦ C. The open circle symbols show raw data, and the solid line corresponds to a sum spectrum of the two dashed lines which are best-fitted curves for the W6+ and W5+ oxidation states.
Fig. 7. Narrow-scan W 4f XPS spectra of tungsten oxide samples O2 -annealed at the two different temperatures of (A) 300 ◦ C and (B) 500 ◦ C. The open circle symbols show raw data, and the solid line corresponds to a sum spectrum of the two dashed lines which are best-fitted curves for the W6+ and W5+ oxidation states.
Gas-sensing measurements were performed for WO2.72 nanorod sensors annealed at different temperatures under ambient N2 conditions. Fig. 8 shows annealing temperature-dependent sensor response profiles at room temperature as a function of a detection time. The sensors were exposed to air-diluted ethanol vapors of 1000 ppm with the concentration variation of step function. The lowest plot in Fig. 8 displays the expected time profile of ethanol concentration for the sequence of 5 min injection and 20 min recovery period. In our detection chamber, the elapsed times of filling and removing ethanol vapors were observed to be less than 10 s from
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Fig. 8. Time-dependent response profiles of WO2.72 nanorod sensors having the different N2 anneal temperatures in the range of 300–700 ◦ C. No annealed sample is also given for comparison. All sensors were measured under the condition of flowinjecting air-diluted ethanol vapors of 1000 ppm as presented in the lowest curve. The vertical arrow corresponds to the magnitude of 10 in sensor response.
separated measurements using a fast response sensor. The length of the vertical arrow corresponds to the magnitude of 10 in sensor response, which was defined by a relative percentage change of a sensor resistance R with respect to a stabilized initial value R0 , i.e. response = 100 × (R–R0 )/R0 = 100 × R/R0 . The response profiles exhibit an increase change in resistance upon exposure to the reducing ethanol vapors, corresponding to p-type response. This phenomenon was previously explained with the competition adsorption between ambient molecular oxygen and analyte vapors on the surface of WO2.72 nanorods instead of the conventional mechanism observed at above 200 ◦ C in MOS sensor, i.e. the reaction between ionosorbed oxygen moieties and analytes [11]. The adsorbed analyte molecules may act as active scattering centers, thus suppressing the electrical conductance of free electron carriers in n-type tungsten oxide system. This scattering results in the resistance increase for both oxidizing and reducing analytes. The response of a no-annealed sample steadily increases for the ethanol injection period and eventually reaches to a maximum response of 9.1. It slowly decreases into the initial position for a following recovery time of 20 min. The recovery time of no-annealed sensor was observed to be around 30 min which is too long to use as a practical MOS gas sensor. Such a long recovery time was previously observed in other MOS sensor systems operated at ambient temperature [13–15]. It might be attributed to the low operation temperature leading to slow desorption rate of pre-adsorbed moieties. As an annealing temperature increases until 500 ◦ C, the response profile becomes closer to a square shape, indicating that the sensor has fast response and short recovery time. In the case of a 400 ◦ C annealed sensor, the response abruptly increases to the 90% level of a maximum value within a time of less than 1 min, and the recovery time becomes two times shorter compared with the no-annealed sample. However, there is a slight decrease in maximum response: a maximum response of the 400 ◦ C annealed sensor
corresponds to 85% of the no-annealed one. As a result, N2 anneal treatment below 500 ◦ C leads to a great improvement in response and recovery times with a slight sacrifice of response. This behavior might be explained in the light of the observed material characteristics as stated below. The material properties of N2 -annealed films at below 500 ◦ C have been confirmed to be identical to those of as-deposited samples except for the population difference in W6+ and W5+ oxidation states. Therefore, the temperature-dependent response variation might result from the difference in sensing ability of the two oxidation states. Fully oxidized WO3 is the most stable tungsten oxide compound so that the W6+ state may be more inert to the sensing-related surface reactions than the W5+ state. The slight decrease in response with the increase in the anneal temperature can be interpreted as the local oxidation of more reactive W5+ states to W6+ as probed by XPS. Furthermore, the W5+ states have a stronger interaction with analyte molecules so that the long recovery time of as-deposited sensor can be also understood with the high W5+ population. Consequently, N2 thermal treatment of WO2.72 nanorod films below 500 ◦ C results in the partial oxidation of nonstoichiometric W5+ states without any noticeable change in morphology or crystalline structure, which gives a chance to modulate systematically gas-sensing characteristics such as response magnitude and detection time by means of careful control of the relative population ratio between W5+ and W6+ states. The sensing characteristics become suddenly worse as the N2 annealing temperature increases beyond 600 ◦ C at which point the nanorods begin to coalesce and develop large, columnar WO3 crystals. Considering that porous film made of small crystalline nanorods with a high aspect ratio are favorable for achieving a high sensitive MOS sensor, the deterioration in sensing capability can be explained by the formation of larger crystals with a columnar crack. Response profiles were also evaluated for the sensors annealed in ambient O2 conditions at a temperature range of 300–600 ◦ C. On the whole, O2 -annealed sensors had exhibited bad sensing properties. The most serious problem was their weak operation durability: the O2 -annealed sensors easily broke down in spite of material properties similar to N2 -annealed analogues. This propensity was also observed for the sensors annealed in ambient air conditions. The thermal treatment under the O2 -containing ambience therefore gives adverse effects on gas-sensing characteristics. Even though there has been no direct clue to explain this effect, it seems to be ascribed to delicate differences in microstructures induced by O2 thermal treatment. Additional measurements were performed at room temperature upon exposure to other volatile vapors for evaluating the sensing characteristics of our nanorod sensors in terms of sensitivity and selectivity. Fig. 9 shows time-dependent sensor response profiles of 400 and 600 ◦ C N2 -annealing samples upon exposure to the three different analytes of 1000 ppm hexane, 1000 ppm benzene, and 10 ppm NH3 . The bottom square pulses display the time-dependent profiles of supplying air-diluted analyte vapors. The middle profiles were obtained from the sensor annealed at 400 ◦ C while the uppers correspond to the sample annealed at 600 ◦ C. The upper curves were shifted upward with respect to the middle ones in order to distinguish easily between them. The scale in sensor response is, however, identical for the two cases and the vertical arrow size matches up to the magnitude of 2. The relative detection magnitudes in sensor response exhibit great differences for the three different analytes: the 400 ◦ C sensor is possible to achieve feasible detection of hexane and benzene while the 600 ◦ C sensor can detect NH3 vapors more sensitively. In fact, the detection responses of the 400 ◦ C sensor are fond to be 1.2 for hexane and 2.1 for benzene. These values are much larger compared with the 600 ◦ C response of <0.1. On the other hand the NH3 detection response of 600 ◦ C sensor is about six times larger
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Acknowledgement This work was supported by the research funds of KOSEF (R012008-000-20460-0) and SMBA (S708002511 and S6070381211).
References
Fig. 9. Time-dependent response profiles of WO2.72 nanorod sensors for the three different analyte exposures of 1000 ppm hexane, 1000 ppm benzene, and 10 ppm NH3 . The top profiles are sensor responses of the 600 ◦ C N2 -annealing sample while the middles correspond to those of the 400 ◦ C N2 -annealing. The bottom square pulses exhibit the time profiles of supplying three different analytes. The vertical arrow corresponds to the magnitude of 2 in sensor response.
than that of 400 ◦ C. As a result these response variation according to anneal temperatures implies that carefully controlled thermal treatments could provide a chance to improve detection selectivity which is regarded as obstacles in a tungsten oxide nanorod system for achieving a good performance sensor operated at room temperature. 4. Conclusion We have investigated the dependence of gas-sensing characteristics on thermal treatment conditions in a tungsten oxide nanorod system that demonstrated the facile detection of various analytes at ambient temperature. As the annealing temperature increases, the surface morphology of as-deposited films consisting of randomly arranged WO2.72 nanorod agglomerates were observed to develop larger WO3 particles with a columnar monoclinic crystal structures, probably through a recrystallization process of nanorod aggregates. The onset temperatures for the recrystallization were found to be around 550 ◦ C for the inert N2 or Ar annealing conditions and 450 ◦ C for the O2 -containing ambient, which suggests that oxygen molecules act as a crystallization enhancer. Since such a crystallization process leads to large crystals and many cracks between grain boundaries, the thermal treatments above the onset temperature were found to deteriorate the detection capability of WO2.72 sensors. In addition, thermal treatments under the O2 -containing active environments were found to result in bad reproducibility in sensor response compared with the inert N2 conditions. As a result, the recommendable thermal treatment conditions for WO2.72 sensors were an annealing temperature of 300–500 ◦ C under inert N2 or Ar ambient. The annealing temperature within the range could be utilized as a parameter to regulate a relative population ratio between W5+ and W6+ states without any noticeable change in morphology or crystalline structure. We can have an opportunity to optimize gas-sensing properties such as detection selectivity and a recovery time through the careful control of relative populations between the two oxidation states.
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Biography Yong Shin Kim received a PhD in chemistry from Korea Advanced Institute of Science and Technology (KAIST) in 1997. After his degree, he had worked as a senior research member at Electronics and Telecommunications Research Institute in the
field of developing flat panel displays, i.e. thin-film electroluminescent devices and miniaturized electronic nose system. Since 2007, he has been employed in Hanyang University. Now his research activities are focused on the development of miniaturized, smart chemical sensor systems and novel nanomaterials for the sensor applications.