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Characterization of photochemically grown Pd loaded WO3 thin films and its evaluation as ammonia gas sensor
Journal of Alloys and Compounds 825 (2020) 154166
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Characterization of photochemically grown Pd loaded WO3 thin films and its evaluation as ammonia gas sensor C. Castillo a, G. Cabello b, B. Chornik c, Y. Huentupil d, G.E. Buono-Core a, * lica de Valparaíso, Valparaíso, Chile Instituto de Química, Pontificia Universidad Cato sicas, Universidad del Bío-Bío, Campus Fernando May Chilla n, Chile Departamento de Ciencias Ba c ticas, Universidad de Chile, Casilla 487-3, Santiago, 8370415, Chile Departamento de Física, Facultad de Ciencias Físicas y Matema d n, Concepcio n, Chile Facultad de Ciencias Químicas, Universidad de Concepcio a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 August 2019 Received in revised form 1 February 2020 Accepted 3 February 2020 Available online 4 February 2020
Pd-loaded tungsten oxide thin films have been successfully fabricated by direct UV irradiation of bis(bdiketonate)dioxotungsten(VI) and Pd(II) precursor complexes spin-coated on Si(100) substrates. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to analyze the crystal structure and the chemical composition of the films before and after annealing at 500 C. The results of XRD and AFM analysis showed that the as-photodeposited films are amorphous whereas thermally treated films present a rougher morphology. Post-annealing of the films in air at 500 C transforms the oxides to a monoclinic WO3 phase. Annealed 10% Pd/WO3 films exhibited an excellent response towards 50 ppm ammonia gas at an operating temperature of 300 C. The Pd-loaded sensors presented higher sensitivity and quicker response-recovery rates than unloaded WO3 films. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Ammonia is a toxic gas which is considered a hazard to human health. It is a major air contaminant produced from agricultural practices, industrial emissions and refrigerators [1]. At high concentrations (300 ppm) may cause immediate burning of the eyes, nose, throat and respiratory tract and can result in blindness, lung damage or death. The lower limit of human ammonia perception by smell is around 50 ppm [2], but still at that low level ammonia can be irritating to the respiratory system, skin and eyes [3,4]. There have been many efforts in the search of new sensors to detect low concentrations of NH3 such as electrochemical sensors [5], optical sensors [6,7] and SAW (surface acoustic wave) sensors [8,9]. In recent years there has been much progress in the use of semiconducting metal oxides for gas sensing and new materials combinations have been investigated to improve sensor performance [10]. Tungsten oxide (WO3) is a wide band gap (Eg ~ 3.3 eV) semiconductor and has been used in many applications such as surface acoustic wave (SAW) devices [11], ultrasonic transducer arrays [12], chemisorption gas sensors [13] and optical waveguides [14]. It has been shown that a WO3 sensor is very sensitive to
* Corresponding author. E-mail address: [email protected] (G.E. Buono-Core). https://doi.org/10.1016/j.jallcom.2020.154166 0925-8388/© 2020 Elsevier B.V. All rights reserved.
oxidizing gases such as ozone and nitrogen dioxide [15] but in general exhibits low response to reducing gases such as hydrogen or ammonia. However, when WO3 film sensors are functionalized with noble metals, such as Pt and Pd, there is a decrease in operation temperature and a significant enhancement of sensitivity to different gases, especially to hydrogen [16e21]. A variety of deposition techniques have been used for the preparation of WO3 thin films, e.g., wet-chemical process [22], spray pyrolysis [23e25], chemical precipitation [26], r.f. and d.c. magnetron sputtering [27e31], chemical vapor deposition [32e34], sol-gel methods [35e38], photoassisted CVD [39] and pulsed laser deposition (PLD) [40e42]. For the last few years we have been using a simple photochemical method for the deposition of metal oxide thin films [43e45] named PMOD (photochemical metal-organic deposition), which can be carried out at ambient temperature, from simple precursor compounds. This method, which involves the irradiation of a coordination complex with ultraviolet light, has been successfully used by our group, for the deposition of a variety of metal oxide thin films. For example, thin films of Mo [43], Zn [44], W [45] and Th [46] oxides have been photodeposited by this method. Very thin films of metallic materials or metallic oxides can be deposited by this simple method, on substrates which are not affected by the UV light. Photolysis of the spin coated precursor complexes thin
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films result in the photoextrusion of the ligands leaving the inorganic products on the surface. This method was developed as a photolithographic deposition method to deposit metal oxide materials, without the use of photoresists, in ambient conditions, requiring no thermal or high vacuum processing. The versatility of this methodology has allowed its use in the deposition of a wide range of materials and has attracted much attention because the experimental procedure is easy to handle, does not require sophisticated implementation and does not demand greater energetic cost. In this study, we report on the preparation and characterization of Pd loaded WO3 thin films by a direct photochemical deposition using tungsten and palladium b-diketonate complexes as source materials. The potential use of these films as gas sensor devices for NH3 detection is also reported.
reduced to one-half and the solution stored in the freezer for 24 h. A small amount of anhydrous diethyl ether was added and the pale yellow precipitate collected, washed with four 10.0-mL portions of diethyl ether, and dried over MgSO4. Yield: 36.0%; m.p. 219-221 C; IR (KBr, cm1): 1592, 1555 (vs C]O), 1489 (vs C]C), 962 (vs W]O), 875 (vs W]O); 1H NMR (400 MHz, CDCl3, d ppm): 2.21 (s, 3H), 6.19 (s, 1H), 7.44 (t, 2H), 7.53 (t, 1H), 7.88 (2H); 13C NMR (100.6 MHz, CDCl3, ppm): 25.84, 96.68, 126.98, 128.59, 132.26, 134.86, 183.32, 193.77; UVeVis (in EtOH) lmax (log ε): 249 (3.87), 310 (4.30). Anal. Calc. for C20H18O6W: C, 44.63; H, 3.37. Found: C, 44.71; H, 3.29%. 2.3. Synthesis of b-diketonate Pd(II) complex
2. Experimental
For the synthesis of the bis(1-phenyl-1,3-butanodionate)Pd(II) complex, a method reported by W. Lin et al. was used [47]. To an aqueous solution of NaOH (250 mg in 10 mL) is added 1benzoylacetone (2 mmol) under constant stirring. After the addition of 1 mmol of PdCl2, the mixture is stirred for 24 h at room temperature. The crude product is filtered, dried under vacuum and purified by passing through a column packed with silica gel. The solvent (CH2Cl2) is evaporated at room temperature until crystals are obtained (m.p.: 139e140 C). FT-IR data (film) nCO 1585(s); 1545 (m), 1514(s) cm1; UV data l (log ε) in CH2Cl2: 354 nm (4.21), 258 nm (4.55), 232 nm (4.47); Elem. Anal. for C20H18O4Pd cal. C: 56.02; H: 4.23; found: C: 56.04; H: 4.10.
2.1. General procedure
2.4. Preparation of amorphous thin films
The FT-IR spectra were obtained with 2 cm1 resolution on a Perkin Elmer Model Spectrum One FT-IR spectrophotometer. UV spectra were obtained on a Hewlett-Packard 8452-A diode array spectrophotometer. X-ray diffraction patterns were obtained using a D5000 X-ray diffractometer. The X-ray source was Cu 40 kV/ 30 mA. X-ray photoelectron spectra (XPS) were recorded on an XPSeAuger Perkin Elmer electron spectrometer Model PHI 1257 which included an ultra high vacuum chamber, a hemispherical electron energy analyzer and an Xeray source providing unfiltered Ka radiation from its Al anode (hn ¼ 1486.6 eV). The pressure of the main spectrometer chamber during data acquisition was maintained at ca 107 Pa. The binding energy (BE) scale was calibrated by using the peak of adventitious carbon, setting it to 284.8 eV. The accuracy of the BE scale was ±0.1 eV. Atomic Force Microscopy was performed on a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) in contact mode. Film thickness was determined using a Leica DMLB optical microscope with a Michelson interference attachment. The substrates for deposition of films were borosilicate glass microslides (Fischer, 2 2 cm) and p-type silicon(100) wafers (1 1 cm) obtained from WaferNet, San Diego, CA. Prior to use the wafers were cleaned successively with ether, methylene chloride, ethanol, aqueous HF (50:1) for 30 s and finally with deionized water. They were dried in an oven at 110 C and stored in glass containers. The solid state photolysis was carried out at room temperature by placing the substrates under a UVS-38 254 nm lamp equipped with two 8W tubes, in an air atmosphere. Progress of the reactions was monitored by determining the FT-IR spectra at different time intervals, following the decrease in IR absorption of the complexes.
The thin films of the precursor complexes were prepared by the following procedure: A silicon chip was placed on a spin coater. A portion (0.1 ml) of a solution of the diketonate complexes in CH2Cl2 was dispensed onto the silicon chip and rotated at a speed of 1500 rpm. The motor was then stopped and a thin film of the complex remained on the chip. The quality of the films was examined by optical microscopy (500x magnification) and in some cases by SEM.
2.2. Synthesis of bis-dioxo-b-diketonate W(VI) complex Bis (1-phenyl-1,3-butanedionato)dioxotungsten(VI) (WO2(bac)2) was prepared by a modification of the method of Nikolovski [45]. A solution of 0.011 mol of WO2C12 and 0.195 mol of 1-benzoylacetone in 50.0 mL of dry toluene was refluxed for 12 h. The volume was
2.5. Photolysis of complexes as films on Si (100) surfaces All photolysis experiments were done following the same procedure. Here is the description of a typical experiment. A film of the diketonate complexes was deposited on p-type Si(100) by spincoating from a CH2Cl2 solution. The spin coater was rotated at 1500 rpm for 60 s. This resulted in the formation of a smooth, uniform coating on the chip. The precursor complex film was placed under a UV apparatus equipped with two 8 W lamps emitting at 254 nm under aerated conditions. The power of the UV light at the sample was 3.34 mJ/cm2 and the temperature of the substrate was 36 C. The progress of the reaction was monitored by FT-IR spectroscopy at different time intervals. After the FT-IR spectrum showed no evidence of the starting material, and prior to analysis, the films were rinsed several times with acetone to remove any organic products remaining on the surface. 2.6. Evaluation of gas-sensing properties Sensing tests were carried out in a home-made gas-flow setup. The total flow rate of the gases was kept constant at 1000 sccm using a 167 MKS mass flow controller and dry synthetic air was used as the reference gas. The gas mixture was passed through a tube furnace (Lindberg/Blue), which was heated at a programmed rate. The as-deposited films on Si(100) were located in a 24-pins ceramics platform to permit the electrical connections. This platform was placed inside the tube furnace with quartz tube (1 in. diameter and 24 in. length) and was electrically connected to
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outside leads using gold wires. Tests were carried out for 50 ppm NH3 provided from a certified bottle (BOC). Sensor behavior was studied at various temperatures between 200 and 400 C. Minimum three tests were made at each sensor operating temperature. At each temperature, the sensor was equilibrated till a steady base line resistance in air was attained. The resistance of the sensor was measured using a Keithley 2000 multimeter interfaced to a PC for acquisition, storage and data analysis. The analyte gas was injected into the test chamber through an injection port and the resistance was measured as a function of time until a constant resistance was reached. The chamber was then purged with air for about 2 min, before the next experiment was carried out. 3. Results and discussion 3.1. Characterization of tungsten oxide thin films In order to obtain 10% Pd loaded WO3 films, precursor films were prepared by dissolving bis(1-phenyl-1,3-butanedionato)dioxo tungsten(VI) with 10% of bis(1-phenyl-1,3-butanedionato)Pd(II) with respect to tungsten(VI) complex in CH2Cl2 and spin-coating the mixture on a silicon(100) substrate. After examination of the films by optical microscopy (500x magnification), they were irradiated under a UV light (254 nm) for 24 h under air atmosphere. Fig. 1 shows the FT-IR spectra of the precursor complexes deposited on Si(100) which was used to monitor the reaction throughout the photochemical process. The FT-IR spectral changes associated with the solid state photolysis shows at time zero (t ¼ 0) an intense band located at around 1500 cm1 associated to the antisymmetric and symmetric carbonyl group (C]O). The intensity of these bands associated with the b-diketonate complexes gradually decreases and after 50 min (t ¼ 50) of UV irradiation only a minimal intensity of bands attributed to by-products from the photo-degradation of the b-diketonate complexes are observed. The loss of starting material was clearly evident after a 24 h irradiation period, and no detectable absorptions associated with the diketone ligand were observed in the FT-IR spectrum (see Fig. 1). The chemical composition of the palladium-tungsten oxide films was investigated by XPS. The wide scan XPS spectrum, in the binding energy range of 0e1000 eV, of as-deposited and annealed
Fig. 1. FT-IR spectral changes associated with the irradiation for a) 0 min, b) 25 min and c) 50 min of WO2(bac)2, bis (1-phenyl-1,3-butanedionato)dioxo tungsten(VI) and 10% of bis(1-phenyl-1,3-butanedionato)Pd(II) film on Si(100).
Fig. 2. XPS survey scan of a 10% Pd-doped WO3 thin film obtained by irradiation of bis(1-phenyl-1,3-butanedionato)dioxo tungsten(VI) and 10% of bis(1-phenyl-1,3butanedionato)Pd(II) film on Si(100).
10% Pd/WO3 films photodeposited on a Si(100) surface, show that the main constituent elements were tungsten, oxygen and Pd, plus additional minor peaks resulting from C and Si (Fig. 2). The appearance of Si 2s and 2p signals can be attributed to photoelectrons ejected from the Si substrate due to the highly porous nature of the films. The carbon detected on the surface of the photodeposited films is probably the result of contamination rather than an inefficient photolysis. After 60 s Arþ sputtering no carbon was detected on the film surface. The most representative signals which show the formation of WO3 correspond to W4f7/2 located at 35 eV and W4f5/2 located at 37 eV. High resolution spectra for W 4f, Pd 4f and O 1s photoelectron lines for annealed Pd/WO3 thin film surface were recorded (Fig. 3). The W 4f core level spectrum exhibited two peaks located at 37.6 and 35.5 eV which were assigned to W 4f5/2 and W 4f7/2 spin orbit respectively (Fig. 3a). This is in agreement with those found in the literature for Wþ6 in WO3 stoichiometric films [48]. Fig. 3a also shows a relatively small fraction of Wþ4 (<10%) which indicates the presence of a partially reduced state. The XPS core level spectrum of Pd 3d5/2 (Fig. 3b) centered at 337.4 eV revealed that Pd exists mainly in the form of Pdþ2 (337.12 eV) with some contributions from Pd(0) (335.77 eV) and Pdþ4 (337.99). The XPS pattern for O 1s (Fig. 3c) is fitted with two peaks: A lower energy signal with a BE of 530.6 eV assigned to the oxygen atoms that form the strong W]O bonds in the oxide [49] and a higher energy signal (532.7 eV), corresponding to oxygen in water molecules bound in the film structure or adsorbed on the sample surface [50,51]. A satellite peak at 534.3 eV is also observed in the spectrum. The phase composition of the as-deposited films and the effect of the thermal treatment on the film crystallinity, was examined by XRD technique. The XRD pattern of a 10%Pd/WO3 film as-deposited and annealed at 500 C for 2 h is shown in Fig. 4. As-deposited Pd/WO3 films have amorphous structure exhibiting no XRD peaks. After annealing at 500 C for 2 h the XRD pattern reveals the characteristic peaks at 2q value of 23.12 , 23.59 and 24.38 which are indexed to the diffractions (002), (020) and (200) planes of monoclinic WO3, respectively (JCPDS Card N 89-4476). It
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a
Fig. 4. XRD pattern of a photodeposited 10%Pd/WO3 film (250 nm thickness): a) asdeposited and b) annealed in air at 500 C for 2 h.
3.2. NH3 gas-sensing properties The NH3 gas-sensing properties of pure WO3 and Pd-modified WO3 films were studied by measuring the film resistance in air (Rair) and in the presence of air containing NH3 (RNH3). For reducing gases such as NH3, the gas response S is defined as (DR/Rair)x100%, where DR¼Rair eRNH3. The gas response of both pure WO3 and Pd-doped WO3 thin films was measured towards 50 ppm NH3 as a function of operating temperature in the ranges of 200 Ce400 C. As shown in Fig. 5, maximum gas response for undoped WO3 film occurs at 350 C, while for a 10% mol Pd-loaded WO3 film the optimum temperature is 300 C. Moreover, the loaded sensors exhibit a much higher response than the unloaded one. For Pd loaded sensors, an increment of the working temperature to 350 C causes a sharp decrease in gas response. It is observed that the 10% mol Pd-loaded WO3 gas sensor shows typical n-type sensing characteristics (a bell-shape
Fig. 3. High resolution XPS spectra of a 10%PdWO3 film: (a) W 4f, (b) Pd 3d5/2, (c) O 1s.
is known that WO3 changes the phase from amorphous to crystalline at around 300 C [52]. A weak and broad diffraction peak at 2q 42.3 that can be assigned to PdO is also present in the XRD spectrum.
Fig. 5. Sensor response of annealed unloaded and 10% mol Pd loaded WO3 films to 50 ppm NH3 vs. operating temperature.
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curve)dthe response curve increases, at first, until reaching a maximum, and then decreases with the increase of the working temperature which means that the gas sensor reacts most effectively at a particular temperature. In our case, the optimal working temperature is 300 C and its corresponding response is 43.6 for 50 ppm of ammonia. This is probably because at higher temperatures the desorption processes of NH3 from the surface of the oxide becomes a determinant factor and consequently the changes of conductivity are diminished (Fig. 5). Other factors such as response time and recovery time were used to evaluate the behavior of pure WO3 and Pd-loaded WO3 films as semiconductor sensors. Some parameters such as the response time and recovery time define the efficiency of the device exposed to a particular gas. Fig. 6 displays an example of a dynamic response-recovery curve of the sensor based on 10% mol Pd-loaded WO3 to 50 ppm NH3 at 300 C. As can be seen in Fig. 6, the NH3 gas showed a reducing effect, leading to a decrease in the electrical resistance as in most of the n-type metal oxide semiconductors. The response and recovery times decreases with increase of operating temperature. This is probably due to the fast adsorption and desorption of the ammonia gas molecules on the surface of the sensor at higher temperature. At 300 C, the response and recovery times for 10% mol Pd-loaded sensor are 10 and 20 s respectively, whereas for the unloaded WO3 sensor are 25 and 54 s respectively. It is evident from the response and recovery times that the 10% mol Pd-loaded WO3 sensor enhance the gas response significantly towards ammonia and at the same time demonstrate the catalytic activity of Pd. Thermal treatment of Pd/WO3 films should cause some morphological changes in the surface of the oxides. This is further demonstrated by the XRD analysis which shows that as-deposited films are amorphous while annealed films show a crystalline phase for WO3. As indicated by an AFM examination, the asdeposited film (Fig. 7a) showed a non-uniform rough surface with a root-mean-square (rms) roughness of 87.8 nm and a maximum height, Rmax, of 528 nm. On the other hand, the annealed Pd/WO3 film showed a more uniform and rougher surface with small grains ranging in size from 8 to 10 nm and a rms roughness of 99.8 nm and a Rmax of 566 nm (Fig. 7b). It has been demonstrated that an increase in roughness favors the interaction or diffusion of
Fig. 6. Dynamic sensor response of 300 C-annealed 10% mol Pd loaded-WO3 to 50 ppm NH3 at 300 C working temperature.
5
Fig. 7a. AFM micrography of an as-photodeposited 10%Pd/WO3 film (250 nm thickness) (image size 7 7 mm with z-scale of 1000 nm).
the gas in the surface of the oxide, and consequently with this, the gas response of these devices increase [49,50]. At the same time, this new disposition morphology is also responsible for the stability of the device that extends to a greater operation temperature range. It is possible that this new disposition can retard or bear the changes of phase when the as-deposited doped film is submitted to high temperatures. A comparison of the gas sensing performance of various gas sensors to ammonia are shown in Table 1. As the table shows, the overall performance of the 10%Pd/WO3 gas sensor is at a satisfactory level, indicating that this sensor could be an excellent ammonia gas sensing material.
Fig. 7b. AFM micrography of a photodeposited 10%Pd/WO3 film (250 nm thickness) annealed in air at 500 C for 2 h (image size 10 10 mm with z-scale of 1500 nm).
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Table 1 Gas sensing performance of various oxide based sensors toward ammonia gas. Sensor
T ( C)
C (ppm)
(Ra/Rg)
Refs.
10% Pd-WO3 1.0% Pt-WO3 SnO2 2.0% V-WO3 WO3 nanofiber
300 125 200 700 200
50 200 300 500 100
8.6 13.6 6 14 5.5
This work [53] [54] [55] [56]
4. Conclusions WO3 and Pd/WO3 thin films have been successfully prepared by direct UV irradiation of amorphous films of b-diketonate complexes on Si(100) substrates. The surface characterization using Atomic Force Microscopy (AFM) revealed that the microstructure of the films was significantly affected by loading with noble metal. Pdloaded tungsten oxide films have a much rougher surface than unloaded tungsten oxide films, with rms roughness values of 99.8 nm for 10% Pd/WO3. It is well known that the sensing response of the sensor improves with increasing roughness of the film, because the number of active adsorption sites for oxygen on the sensor surface is increased. It was found that Pd-loaded WO3 films enhance the response significantly towards 50 ppm of ammonia at optimum operating temperature of 300 C. Apart from the outstanding response, the Pd-loaded WO3 also present good response-recovery properties. The results showed that the uniform distribution of the additive over the surface is another factor to consider since offers a greater specific surface area and facilitates a better interaction with the gaseous species. The results of continual short-term tests conducted for 15 days, using NH3 concentration of 50 ppm, show that 10% Pd/WO3 exhibits good stability at 300 C when used as an ammonia sensor. Preliminary results showed that the response was nearly unchanged upon testing repeatedly, thus indicating excellent stability and repeatability of the 10% Pd/WO3-based sensor. These results demonstrate that these photochemically produced Pd-loaded WO3 thin films are good ammonia gas sensing material for chemical sensors. Declaration of competing interest No conflict of interest.
CRediT authorship contribution statement C. Castillo: Visualization, Investigation. G. Cabello: Supervision. B. Chornik: Validation, Formal analysis. Y. Huentupil: Validation. G.E. Buono-Core: Project administration, Writing - review & editing.
Acknowledgments This research was supported by FONDECYT, Chile (Project No. lica de Valparaíso (Project 1080225) and Pontificia Universidad Cato D.I. No. 125.727/08).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154166.
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Characterization of photochemically grown Pd loaded WO3 thin films and its evaluation as ammonia gas sensor C. Castillo a, G. Cabello b, B. Chornik c, Y. Huentupil d, G.E. Buono-Core a, * lica de Valparaíso, Valparaíso, Chile Instituto de Química, Pontificia Universidad Cato sicas, Universidad del Bío-Bío, Campus Fernando May Chilla n, Chile Departamento de Ciencias Ba c ticas, Universidad de Chile, Casilla 487-3, Santiago, 8370415, Chile Departamento de Física, Facultad de Ciencias Físicas y Matema d n, Concepcio n, Chile Facultad de Ciencias Químicas, Universidad de Concepcio a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 August 2019 Received in revised form 1 February 2020 Accepted 3 February 2020 Available online 4 February 2020
Pd-loaded tungsten oxide thin films have been successfully fabricated by direct UV irradiation of bis(bdiketonate)dioxotungsten(VI) and Pd(II) precursor complexes spin-coated on Si(100) substrates. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to analyze the crystal structure and the chemical composition of the films before and after annealing at 500 C. The results of XRD and AFM analysis showed that the as-photodeposited films are amorphous whereas thermally treated films present a rougher morphology. Post-annealing of the films in air at 500 C transforms the oxides to a monoclinic WO3 phase. Annealed 10% Pd/WO3 films exhibited an excellent response towards 50 ppm ammonia gas at an operating temperature of 300 C. The Pd-loaded sensors presented higher sensitivity and quicker response-recovery rates than unloaded WO3 films. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Ammonia is a toxic gas which is considered a hazard to human health. It is a major air contaminant produced from agricultural practices, industrial emissions and refrigerators [1]. At high concentrations (300 ppm) may cause immediate burning of the eyes, nose, throat and respiratory tract and can result in blindness, lung damage or death. The lower limit of human ammonia perception by smell is around 50 ppm [2], but still at that low level ammonia can be irritating to the respiratory system, skin and eyes [3,4]. There have been many efforts in the search of new sensors to detect low concentrations of NH3 such as electrochemical sensors [5], optical sensors [6,7] and SAW (surface acoustic wave) sensors [8,9]. In recent years there has been much progress in the use of semiconducting metal oxides for gas sensing and new materials combinations have been investigated to improve sensor performance [10]. Tungsten oxide (WO3) is a wide band gap (Eg ~ 3.3 eV) semiconductor and has been used in many applications such as surface acoustic wave (SAW) devices [11], ultrasonic transducer arrays [12], chemisorption gas sensors [13] and optical waveguides [14]. It has been shown that a WO3 sensor is very sensitive to
* Corresponding author. E-mail address: [email protected] (G.E. Buono-Core). https://doi.org/10.1016/j.jallcom.2020.154166 0925-8388/© 2020 Elsevier B.V. All rights reserved.
oxidizing gases such as ozone and nitrogen dioxide [15] but in general exhibits low response to reducing gases such as hydrogen or ammonia. However, when WO3 film sensors are functionalized with noble metals, such as Pt and Pd, there is a decrease in operation temperature and a significant enhancement of sensitivity to different gases, especially to hydrogen [16e21]. A variety of deposition techniques have been used for the preparation of WO3 thin films, e.g., wet-chemical process [22], spray pyrolysis [23e25], chemical precipitation [26], r.f. and d.c. magnetron sputtering [27e31], chemical vapor deposition [32e34], sol-gel methods [35e38], photoassisted CVD [39] and pulsed laser deposition (PLD) [40e42]. For the last few years we have been using a simple photochemical method for the deposition of metal oxide thin films [43e45] named PMOD (photochemical metal-organic deposition), which can be carried out at ambient temperature, from simple precursor compounds. This method, which involves the irradiation of a coordination complex with ultraviolet light, has been successfully used by our group, for the deposition of a variety of metal oxide thin films. For example, thin films of Mo [43], Zn [44], W [45] and Th [46] oxides have been photodeposited by this method. Very thin films of metallic materials or metallic oxides can be deposited by this simple method, on substrates which are not affected by the UV light. Photolysis of the spin coated precursor complexes thin
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films result in the photoextrusion of the ligands leaving the inorganic products on the surface. This method was developed as a photolithographic deposition method to deposit metal oxide materials, without the use of photoresists, in ambient conditions, requiring no thermal or high vacuum processing. The versatility of this methodology has allowed its use in the deposition of a wide range of materials and has attracted much attention because the experimental procedure is easy to handle, does not require sophisticated implementation and does not demand greater energetic cost. In this study, we report on the preparation and characterization of Pd loaded WO3 thin films by a direct photochemical deposition using tungsten and palladium b-diketonate complexes as source materials. The potential use of these films as gas sensor devices for NH3 detection is also reported.
reduced to one-half and the solution stored in the freezer for 24 h. A small amount of anhydrous diethyl ether was added and the pale yellow precipitate collected, washed with four 10.0-mL portions of diethyl ether, and dried over MgSO4. Yield: 36.0%; m.p. 219-221 C; IR (KBr, cm1): 1592, 1555 (vs C]O), 1489 (vs C]C), 962 (vs W]O), 875 (vs W]O); 1H NMR (400 MHz, CDCl3, d ppm): 2.21 (s, 3H), 6.19 (s, 1H), 7.44 (t, 2H), 7.53 (t, 1H), 7.88 (2H); 13C NMR (100.6 MHz, CDCl3, ppm): 25.84, 96.68, 126.98, 128.59, 132.26, 134.86, 183.32, 193.77; UVeVis (in EtOH) lmax (log ε): 249 (3.87), 310 (4.30). Anal. Calc. for C20H18O6W: C, 44.63; H, 3.37. Found: C, 44.71; H, 3.29%. 2.3. Synthesis of b-diketonate Pd(II) complex
2. Experimental
For the synthesis of the bis(1-phenyl-1,3-butanodionate)Pd(II) complex, a method reported by W. Lin et al. was used [47]. To an aqueous solution of NaOH (250 mg in 10 mL) is added 1benzoylacetone (2 mmol) under constant stirring. After the addition of 1 mmol of PdCl2, the mixture is stirred for 24 h at room temperature. The crude product is filtered, dried under vacuum and purified by passing through a column packed with silica gel. The solvent (CH2Cl2) is evaporated at room temperature until crystals are obtained (m.p.: 139e140 C). FT-IR data (film) nCO 1585(s); 1545 (m), 1514(s) cm1; UV data l (log ε) in CH2Cl2: 354 nm (4.21), 258 nm (4.55), 232 nm (4.47); Elem. Anal. for C20H18O4Pd cal. C: 56.02; H: 4.23; found: C: 56.04; H: 4.10.
2.1. General procedure
2.4. Preparation of amorphous thin films
The FT-IR spectra were obtained with 2 cm1 resolution on a Perkin Elmer Model Spectrum One FT-IR spectrophotometer. UV spectra were obtained on a Hewlett-Packard 8452-A diode array spectrophotometer. X-ray diffraction patterns were obtained using a D5000 X-ray diffractometer. The X-ray source was Cu 40 kV/ 30 mA. X-ray photoelectron spectra (XPS) were recorded on an XPSeAuger Perkin Elmer electron spectrometer Model PHI 1257 which included an ultra high vacuum chamber, a hemispherical electron energy analyzer and an Xeray source providing unfiltered Ka radiation from its Al anode (hn ¼ 1486.6 eV). The pressure of the main spectrometer chamber during data acquisition was maintained at ca 107 Pa. The binding energy (BE) scale was calibrated by using the peak of adventitious carbon, setting it to 284.8 eV. The accuracy of the BE scale was ±0.1 eV. Atomic Force Microscopy was performed on a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) in contact mode. Film thickness was determined using a Leica DMLB optical microscope with a Michelson interference attachment. The substrates for deposition of films were borosilicate glass microslides (Fischer, 2 2 cm) and p-type silicon(100) wafers (1 1 cm) obtained from WaferNet, San Diego, CA. Prior to use the wafers were cleaned successively with ether, methylene chloride, ethanol, aqueous HF (50:1) for 30 s and finally with deionized water. They were dried in an oven at 110 C and stored in glass containers. The solid state photolysis was carried out at room temperature by placing the substrates under a UVS-38 254 nm lamp equipped with two 8W tubes, in an air atmosphere. Progress of the reactions was monitored by determining the FT-IR spectra at different time intervals, following the decrease in IR absorption of the complexes.
The thin films of the precursor complexes were prepared by the following procedure: A silicon chip was placed on a spin coater. A portion (0.1 ml) of a solution of the diketonate complexes in CH2Cl2 was dispensed onto the silicon chip and rotated at a speed of 1500 rpm. The motor was then stopped and a thin film of the complex remained on the chip. The quality of the films was examined by optical microscopy (500x magnification) and in some cases by SEM.
2.2. Synthesis of bis-dioxo-b-diketonate W(VI) complex Bis (1-phenyl-1,3-butanedionato)dioxotungsten(VI) (WO2(bac)2) was prepared by a modification of the method of Nikolovski [45]. A solution of 0.011 mol of WO2C12 and 0.195 mol of 1-benzoylacetone in 50.0 mL of dry toluene was refluxed for 12 h. The volume was
2.5. Photolysis of complexes as films on Si (100) surfaces All photolysis experiments were done following the same procedure. Here is the description of a typical experiment. A film of the diketonate complexes was deposited on p-type Si(100) by spincoating from a CH2Cl2 solution. The spin coater was rotated at 1500 rpm for 60 s. This resulted in the formation of a smooth, uniform coating on the chip. The precursor complex film was placed under a UV apparatus equipped with two 8 W lamps emitting at 254 nm under aerated conditions. The power of the UV light at the sample was 3.34 mJ/cm2 and the temperature of the substrate was 36 C. The progress of the reaction was monitored by FT-IR spectroscopy at different time intervals. After the FT-IR spectrum showed no evidence of the starting material, and prior to analysis, the films were rinsed several times with acetone to remove any organic products remaining on the surface. 2.6. Evaluation of gas-sensing properties Sensing tests were carried out in a home-made gas-flow setup. The total flow rate of the gases was kept constant at 1000 sccm using a 167 MKS mass flow controller and dry synthetic air was used as the reference gas. The gas mixture was passed through a tube furnace (Lindberg/Blue), which was heated at a programmed rate. The as-deposited films on Si(100) were located in a 24-pins ceramics platform to permit the electrical connections. This platform was placed inside the tube furnace with quartz tube (1 in. diameter and 24 in. length) and was electrically connected to
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outside leads using gold wires. Tests were carried out for 50 ppm NH3 provided from a certified bottle (BOC). Sensor behavior was studied at various temperatures between 200 and 400 C. Minimum three tests were made at each sensor operating temperature. At each temperature, the sensor was equilibrated till a steady base line resistance in air was attained. The resistance of the sensor was measured using a Keithley 2000 multimeter interfaced to a PC for acquisition, storage and data analysis. The analyte gas was injected into the test chamber through an injection port and the resistance was measured as a function of time until a constant resistance was reached. The chamber was then purged with air for about 2 min, before the next experiment was carried out. 3. Results and discussion 3.1. Characterization of tungsten oxide thin films In order to obtain 10% Pd loaded WO3 films, precursor films were prepared by dissolving bis(1-phenyl-1,3-butanedionato)dioxo tungsten(VI) with 10% of bis(1-phenyl-1,3-butanedionato)Pd(II) with respect to tungsten(VI) complex in CH2Cl2 and spin-coating the mixture on a silicon(100) substrate. After examination of the films by optical microscopy (500x magnification), they were irradiated under a UV light (254 nm) for 24 h under air atmosphere. Fig. 1 shows the FT-IR spectra of the precursor complexes deposited on Si(100) which was used to monitor the reaction throughout the photochemical process. The FT-IR spectral changes associated with the solid state photolysis shows at time zero (t ¼ 0) an intense band located at around 1500 cm1 associated to the antisymmetric and symmetric carbonyl group (C]O). The intensity of these bands associated with the b-diketonate complexes gradually decreases and after 50 min (t ¼ 50) of UV irradiation only a minimal intensity of bands attributed to by-products from the photo-degradation of the b-diketonate complexes are observed. The loss of starting material was clearly evident after a 24 h irradiation period, and no detectable absorptions associated with the diketone ligand were observed in the FT-IR spectrum (see Fig. 1). The chemical composition of the palladium-tungsten oxide films was investigated by XPS. The wide scan XPS spectrum, in the binding energy range of 0e1000 eV, of as-deposited and annealed
Fig. 1. FT-IR spectral changes associated with the irradiation for a) 0 min, b) 25 min and c) 50 min of WO2(bac)2, bis (1-phenyl-1,3-butanedionato)dioxo tungsten(VI) and 10% of bis(1-phenyl-1,3-butanedionato)Pd(II) film on Si(100).
Fig. 2. XPS survey scan of a 10% Pd-doped WO3 thin film obtained by irradiation of bis(1-phenyl-1,3-butanedionato)dioxo tungsten(VI) and 10% of bis(1-phenyl-1,3butanedionato)Pd(II) film on Si(100).
10% Pd/WO3 films photodeposited on a Si(100) surface, show that the main constituent elements were tungsten, oxygen and Pd, plus additional minor peaks resulting from C and Si (Fig. 2). The appearance of Si 2s and 2p signals can be attributed to photoelectrons ejected from the Si substrate due to the highly porous nature of the films. The carbon detected on the surface of the photodeposited films is probably the result of contamination rather than an inefficient photolysis. After 60 s Arþ sputtering no carbon was detected on the film surface. The most representative signals which show the formation of WO3 correspond to W4f7/2 located at 35 eV and W4f5/2 located at 37 eV. High resolution spectra for W 4f, Pd 4f and O 1s photoelectron lines for annealed Pd/WO3 thin film surface were recorded (Fig. 3). The W 4f core level spectrum exhibited two peaks located at 37.6 and 35.5 eV which were assigned to W 4f5/2 and W 4f7/2 spin orbit respectively (Fig. 3a). This is in agreement with those found in the literature for Wþ6 in WO3 stoichiometric films [48]. Fig. 3a also shows a relatively small fraction of Wþ4 (<10%) which indicates the presence of a partially reduced state. The XPS core level spectrum of Pd 3d5/2 (Fig. 3b) centered at 337.4 eV revealed that Pd exists mainly in the form of Pdþ2 (337.12 eV) with some contributions from Pd(0) (335.77 eV) and Pdþ4 (337.99). The XPS pattern for O 1s (Fig. 3c) is fitted with two peaks: A lower energy signal with a BE of 530.6 eV assigned to the oxygen atoms that form the strong W]O bonds in the oxide [49] and a higher energy signal (532.7 eV), corresponding to oxygen in water molecules bound in the film structure or adsorbed on the sample surface [50,51]. A satellite peak at 534.3 eV is also observed in the spectrum. The phase composition of the as-deposited films and the effect of the thermal treatment on the film crystallinity, was examined by XRD technique. The XRD pattern of a 10%Pd/WO3 film as-deposited and annealed at 500 C for 2 h is shown in Fig. 4. As-deposited Pd/WO3 films have amorphous structure exhibiting no XRD peaks. After annealing at 500 C for 2 h the XRD pattern reveals the characteristic peaks at 2q value of 23.12 , 23.59 and 24.38 which are indexed to the diffractions (002), (020) and (200) planes of monoclinic WO3, respectively (JCPDS Card N 89-4476). It
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a
Fig. 4. XRD pattern of a photodeposited 10%Pd/WO3 film (250 nm thickness): a) asdeposited and b) annealed in air at 500 C for 2 h.
3.2. NH3 gas-sensing properties The NH3 gas-sensing properties of pure WO3 and Pd-modified WO3 films were studied by measuring the film resistance in air (Rair) and in the presence of air containing NH3 (RNH3). For reducing gases such as NH3, the gas response S is defined as (DR/Rair)x100%, where DR¼Rair eRNH3. The gas response of both pure WO3 and Pd-doped WO3 thin films was measured towards 50 ppm NH3 as a function of operating temperature in the ranges of 200 Ce400 C. As shown in Fig. 5, maximum gas response for undoped WO3 film occurs at 350 C, while for a 10% mol Pd-loaded WO3 film the optimum temperature is 300 C. Moreover, the loaded sensors exhibit a much higher response than the unloaded one. For Pd loaded sensors, an increment of the working temperature to 350 C causes a sharp decrease in gas response. It is observed that the 10% mol Pd-loaded WO3 gas sensor shows typical n-type sensing characteristics (a bell-shape
Fig. 3. High resolution XPS spectra of a 10%PdWO3 film: (a) W 4f, (b) Pd 3d5/2, (c) O 1s.
is known that WO3 changes the phase from amorphous to crystalline at around 300 C [52]. A weak and broad diffraction peak at 2q 42.3 that can be assigned to PdO is also present in the XRD spectrum.
Fig. 5. Sensor response of annealed unloaded and 10% mol Pd loaded WO3 films to 50 ppm NH3 vs. operating temperature.
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curve)dthe response curve increases, at first, until reaching a maximum, and then decreases with the increase of the working temperature which means that the gas sensor reacts most effectively at a particular temperature. In our case, the optimal working temperature is 300 C and its corresponding response is 43.6 for 50 ppm of ammonia. This is probably because at higher temperatures the desorption processes of NH3 from the surface of the oxide becomes a determinant factor and consequently the changes of conductivity are diminished (Fig. 5). Other factors such as response time and recovery time were used to evaluate the behavior of pure WO3 and Pd-loaded WO3 films as semiconductor sensors. Some parameters such as the response time and recovery time define the efficiency of the device exposed to a particular gas. Fig. 6 displays an example of a dynamic response-recovery curve of the sensor based on 10% mol Pd-loaded WO3 to 50 ppm NH3 at 300 C. As can be seen in Fig. 6, the NH3 gas showed a reducing effect, leading to a decrease in the electrical resistance as in most of the n-type metal oxide semiconductors. The response and recovery times decreases with increase of operating temperature. This is probably due to the fast adsorption and desorption of the ammonia gas molecules on the surface of the sensor at higher temperature. At 300 C, the response and recovery times for 10% mol Pd-loaded sensor are 10 and 20 s respectively, whereas for the unloaded WO3 sensor are 25 and 54 s respectively. It is evident from the response and recovery times that the 10% mol Pd-loaded WO3 sensor enhance the gas response significantly towards ammonia and at the same time demonstrate the catalytic activity of Pd. Thermal treatment of Pd/WO3 films should cause some morphological changes in the surface of the oxides. This is further demonstrated by the XRD analysis which shows that as-deposited films are amorphous while annealed films show a crystalline phase for WO3. As indicated by an AFM examination, the asdeposited film (Fig. 7a) showed a non-uniform rough surface with a root-mean-square (rms) roughness of 87.8 nm and a maximum height, Rmax, of 528 nm. On the other hand, the annealed Pd/WO3 film showed a more uniform and rougher surface with small grains ranging in size from 8 to 10 nm and a rms roughness of 99.8 nm and a Rmax of 566 nm (Fig. 7b). It has been demonstrated that an increase in roughness favors the interaction or diffusion of
Fig. 6. Dynamic sensor response of 300 C-annealed 10% mol Pd loaded-WO3 to 50 ppm NH3 at 300 C working temperature.
5
Fig. 7a. AFM micrography of an as-photodeposited 10%Pd/WO3 film (250 nm thickness) (image size 7 7 mm with z-scale of 1000 nm).
the gas in the surface of the oxide, and consequently with this, the gas response of these devices increase [49,50]. At the same time, this new disposition morphology is also responsible for the stability of the device that extends to a greater operation temperature range. It is possible that this new disposition can retard or bear the changes of phase when the as-deposited doped film is submitted to high temperatures. A comparison of the gas sensing performance of various gas sensors to ammonia are shown in Table 1. As the table shows, the overall performance of the 10%Pd/WO3 gas sensor is at a satisfactory level, indicating that this sensor could be an excellent ammonia gas sensing material.
Fig. 7b. AFM micrography of a photodeposited 10%Pd/WO3 film (250 nm thickness) annealed in air at 500 C for 2 h (image size 10 10 mm with z-scale of 1500 nm).
6
C. Castillo et al. / Journal of Alloys and Compounds 825 (2020) 154166
Table 1 Gas sensing performance of various oxide based sensors toward ammonia gas. Sensor
T ( C)
C (ppm)
(Ra/Rg)
Refs.
10% Pd-WO3 1.0% Pt-WO3 SnO2 2.0% V-WO3 WO3 nanofiber
300 125 200 700 200
50 200 300 500 100
8.6 13.6 6 14 5.5
This work [53] [54] [55] [56]
4. Conclusions WO3 and Pd/WO3 thin films have been successfully prepared by direct UV irradiation of amorphous films of b-diketonate complexes on Si(100) substrates. The surface characterization using Atomic Force Microscopy (AFM) revealed that the microstructure of the films was significantly affected by loading with noble metal. Pdloaded tungsten oxide films have a much rougher surface than unloaded tungsten oxide films, with rms roughness values of 99.8 nm for 10% Pd/WO3. It is well known that the sensing response of the sensor improves with increasing roughness of the film, because the number of active adsorption sites for oxygen on the sensor surface is increased. It was found that Pd-loaded WO3 films enhance the response significantly towards 50 ppm of ammonia at optimum operating temperature of 300 C. Apart from the outstanding response, the Pd-loaded WO3 also present good response-recovery properties. The results showed that the uniform distribution of the additive over the surface is another factor to consider since offers a greater specific surface area and facilitates a better interaction with the gaseous species. The results of continual short-term tests conducted for 15 days, using NH3 concentration of 50 ppm, show that 10% Pd/WO3 exhibits good stability at 300 C when used as an ammonia sensor. Preliminary results showed that the response was nearly unchanged upon testing repeatedly, thus indicating excellent stability and repeatability of the 10% Pd/WO3-based sensor. These results demonstrate that these photochemically produced Pd-loaded WO3 thin films are good ammonia gas sensing material for chemical sensors. Declaration of competing interest No conflict of interest.
CRediT authorship contribution statement C. Castillo: Visualization, Investigation. G. Cabello: Supervision. B. Chornik: Validation, Formal analysis. Y. Huentupil: Validation. G.E. Buono-Core: Project administration, Writing - review & editing.
Acknowledgments This research was supported by FONDECYT, Chile (Project No. lica de Valparaíso (Project 1080225) and Pontificia Universidad Cato D.I. No. 125.727/08).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154166.
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