Liquid petroleum gas sensing performance enhanced by CuO modification of nanocrystalline ZnO-TiO₂

For obtaining nanocrystalline ZnO-TiO₂ (sample A) with a molar ratio 9:1, analytical grade ZnO and TiO₂ were dispersed in aqueous NaOH solution followed by hydrothermal treatment at 180 °C for 24 h in a Teflon-lined autoclave. Then, it was allowed to cool naturally and the obtained precipitate was isolated from the solution by centrifugation at 5000 rpm for 30 min and subsequently washed with distilled water and ethanol and then dried at 120 °C for 12 h. The same procedure was followed for the synthesis of ZnO-TiO₂ with molar ratios 7:3 (sample B), 1:1 (sample C), 3:7 (sample D) and 1:9 (sample E). The synthesized materials were examined by X-ray diffraction (XRD, X’Pert PRO) and transmission electron microscope (TEM, Techai G2 20).

A thixotropic paste was formulated by mixing the synthesized nanocrystalline ZnO-TiO₂ powder sample with ethyl cellulose and a mixture of organic solvents such as butyl cellulose, butyl carbitol acetate and terpineol. Then the prepared paste was screen printed on a glass substrate. The same procedure was followed for fabrication of thick films of other synthesized material samples. Furthermore, the thick film of ZnO-TiO₂ (with a molar ratio 1:1) was modified by dipping it into 0.1 M, 0.2 M and 0.3 M aqueous solutions of copper chloride (CuCl2) for 30 min. The films were dried in air and fired in a heating furnace at 450 °C for 24 h. They were termed as CuO modified ZnO-TiO₂ thick films. For the measurements of gas sensing properties of all the films, silver electrodes were used for electrical contacts.

Gas-sensing measurements were carried out on a computer-controlled static gas-sensing system. Ni-Cr alloy coil was used for heating and a chromel-alumel thermocouple was used to monitor temperature. Picometer cum voltage source (Keithley 6487) was used to measure the sensor resistance. Test gas was injected into the chamber through an inlet port. The concentration of gas was kept at 286 ppm and a well dried gas with relative humidity 20 % was used to avoid the moisture effect on the sensor response. The sensor response (S) was defined as the ratio of resistance in air (R_a) to that in target gas (R_g) [23]:

S=Ra/Rg$$S = {R_a}/{R_g}$$(1)

Fig. 1 shows X-ray diffraction (XRD) patterns of synthesized ZnO-TiO₂ powder samples. From the XRD patterns, it is seen that the materials are polycrystalline in nature with mixed hexagonal and tetragonal phases. The characteristic peak in the XRD pattern matches the hexagonal ZnO (JCPDS Card No. 00-036-1451) and tetragonal TiO2 (JCPDS Card No. 01-071-1166). Also, the peaks which appeared at 2θ = 29.95°, 36.83°, 56.74°, 62.18° and 70.41° correspond to Zn₂TiO₄ (JCPDS Card No. 01-073-0578). The extra peak that appeared at 2θ = 44.8° is related to surface hydroxyl groups on the ZnO-TiO₂ surface [24]. The average crystallite size (D) was determined by using Debye-Scherrer formula [25]:

Fig. 1

D=0.9λ/βcos⁡θ$$D = 0.9\lambda /\beta \cos \theta $$(2)

where λ is the wavelength of incident beam (1.5406 Å), β is the full width at half maximum (FWHM) of the peak in radian and θ is the diffraction angle. The average crystallite size of samples A to E was calculated from bordering of the diffraction line and found to be in the range of 49 nm to 68 nm.

The morphology of the synthesized material samples was further investigated by transmission electron microscope (TEM). Fig. 2 illustrates TEM images of synthesized ZnO-TiO₂ powder samples with molar ratios (A) 9:1, (B) 7:3, (C)1:1, (D) 3:7 and (E) 1:9. The small amount of agglomerations can be seen in the micrographs. TEM images confirm that the average crystallite size of the synthesized material is in nanometer range.

Fig. 2

The surface morphology and nature of ZnO-TiO₂ thick films with molar ratios (A) 9:1, (B) 7:3, (C) 1:1, (D) 3:7 and (E) 1:9 were analyzed by using scanning electron microscope (SEM, JEOL JSM 6380A). SEM images of ZnO-TiO₂ thick films with molar ratios (A) 9:1, (B) 7:3, (C) 1:1, (D) 3:7 and (E) 1:9 are shown in Fig. 3. From the SEM images it can be seen that the ZnO-TiO₂ thick film (molar ratio 1:1) has a large number of pores which account for maximum porosity and high surface roughness which results in large surface to volume ratio. Moreover, it appears that the film surface is almost crack free. This improves the LPG sensing response of the film.

Fig. 3

The elemental composition of 0.1 M, 0.2 M and 0.3 M CuO modified and unmodified ZnO-TiO₂ thick films (molar ratio 1:1) were analyzed by using an energy dispersive X-ray spectrometer (EDS). From the spectra, it can be seen that major constituent elements of ZnO-TiO₂ thick film are O, Ti, Zn and the major constituent elements of CuO modified ZnO-TiO₂ thick films (molar ratio 1:1) are Cu, O, Ti, Zn. The elemental composition of 0.1 M, 0.2 M and 0.3 M CuO modified and unmodified ZnO-TiO₂ thick films (molar ratio 1:1) is shown in Table 1.

Fig. 4 shows the SEM images of 0.1 M, 0.2 M and 0.3 M CuO modified ZnO-TiO₂ (with a molar ratio 1:1) thick films. CuO dispersion on the surface of the ZnO-TiO₂ thick film is confirmed by comparing the SEM image of ZnO-TiO₂ (with a molar ratio 1:1) thick film with SEM images of 0.1 M, 0.2 M and 0.3 M CuO modified ZnO-TiO₂ (with a molar ratio 1:1) thick films. It indicates that the microstructure of 0.2 M CuO modified film is uniform with an adequate dispersion of CuO.

Fig. 4

Fig. 5 shows I-V characteristics of ZnO-TiO₂ thick films with molar ratios (A) 9:1, (B) 7: 3, (C) 1:1, (D) 3:7 and (E) 1:9 in air ambient at room temperature. The graphs indicate the ohmic nature of the contact between silver electrodes and the film.

Fig. 5

Fig. 6 illustrates the response of ZnO-TiO₂ based thick films with molar ratios (A) 9:1, (B) 7:3, (C) 1:1, (D) 3:7 and (E) 1:9 towards 286 ppm LPG. The films were maintained at different operating temperatures in the range of 30 °C to 225 °C and tested for 286 ppm LPG. From the figure, it can be seen that within the studied temperature range and among the tested compositions, ZnO-TiO₂ based thick film (with a molar ratio 1:1) exhibited maximum response of 498.24 at 185 °C.

Fig. 6

Fig. 7A shows the resistance of ZnO-TiO₂ based thick film (with molar ratio 1:1) at different operating temperatures in air ambient. From the figure it can be seen that the resistance of the film decreases with an increase in operating temperature, indicating a negative temperature coefficient of resistance.

Fig. 7

The Arrhenius plots of 0.1 M, 0.2 M, 0.3 M CuO modified and unmodified ZnO-TiO₂ thick films (with molar ratio 1:1) is shown in Fig. 7B. It is observed that in air ambient, the conductivity of the film goes on increasing with an increase of temperature, showing the semiconducting nature of the film [1]. The activation energy Ea (I) and (II) of 0.1 M, 0.2 M, 0.3 M CuO modified and unmodified ZnO-TiO₂ thick film (with molar ratio 1:1) in air ambient have been calculated by measuring the slope of the best fit straight line of the plot in the temperature range of 30 °C to 130 °C and 175 °C to 225 °C, respectively, and presented in Table 2. The activation energy of CuO modified films is different from that of the unmodified ZnO-TiO₂ thick film. This may due to different adsorption chemistry of CuO modified thick films. The CuO modified film surface adsorbs more oxygen species than unmodified film surface and the number of adsorbed oxygen species would depend on the amount of CuO dispersion on the film surface [21].

The sensing mechanism and the change in electrical transport properties of semiconductor oxide based materials generally involve the adsorption and desorption of oxygen molecules on the surface of the materials and/or the direct reaction of lattice oxygen or interstitial oxygen with test gases [26–31]. When ZnO-TiO₂ thick films are exposed to air, oxygen molecules interact with the film surface to form adsorbed oxygen ions like O2−$O_2^ - $ or O^- or O^2- by capturing electrons from the conduction band and this interaction decreases the concentration of electrons in the conduction band. The processes are as follows [32]:

O2gas↔O2ads$${O_2}\left( {gas} \right) \leftrightarrow {O_2}\left( {ads} \right)$$(3)

O2ads+e−↔O2−ads$${O_2}\left( {ads} \right) + {e^ - } \leftrightarrow O_2^ - \left( {ads} \right)$$(4)

O2−ads+e−↔2O−ads$$O_2^ - \left( {ads} \right) + {e^ - } \leftrightarrow 2{O^ - }\left( {ads} \right)$$(5)

O−ads+e−↔O2−ads$${O^ - }\left( {ads} \right) + {e^ - } \leftrightarrow {O^{2 - }}\left( {ads} \right)$$(6)

When the film is exposed to LPG, the LPG molecules interact with the adsorbed oxygen and hydroxyl species present on the film surface. The hydrocarbons, namely butane (C₄H₁₀) and propane (C₃H₈) present in LPG would be converted to CO₂ and H₂O due to their interaction with oxygen ions and oxygen is evolved in electrically neutral state. The energy released in the decomposition of the molecules would be sufficient to shift the electrons into the conduction band of activated ZnO and TiO2 and this causes a decrease in resistance of the film. The reaction of LPG with adsorbed oxygen can be explained as [33]:

O2ads+e−↔O2−ads$${O_2}\left( {ads} \right) + {e^ - } \leftrightarrow O_2^ - \left( {ads} \right)$$(7)

O2−ads+e−↔2O−ads$$O_2^ - \left( {ads} \right) + {e^ - } \leftrightarrow 2{O^ - }\left( {ads} \right)$$(8)

CnH(2n+2)+2O−(ads)→H2O+CnH2n:O+e−$${C_n}H{(_{2n + 2)}} + 2{O^ - }(ads) \to {H_2}O + {C_n}{H_{2n}}:O + {e^ - }$$(9)

CnH2n:O+O−(ads)→CO2(g)+H2O(g)+e−$${C_n}{H_{2n}}:O + {O^ - }(ads) \to C{O_2}(g) + {H_2}O(g) + {e^ - }$$(10)

Fig. 6 indicates that the ZnO-TiO₂ thick film (with molar ratio 1:1) exhibits the highest response to LPG at 185 °C among all the investigated compositions. The highest sensing response of the film can be attributed to the crack free surface, optimum porosity and the largest surface area available to react with LPG. Also the optimum amount of TiO₂ in the film can account for maximum response, which indicates that there was an increase in the amount of chemisorbed oxygen ions. This is due to the fact that electron induced by Ti⁴⁺ entering into ZnO lattice can form more chemisorbed oxygen on the surface of the film [22]. The amount of TiO2 in the film was optimum; hence, it had high initial resistance in air ambient. When the film is exposed to LPG, LPG molecules interact with the oxygen species available at the film surface. In this interaction, the adsorption of LPG molecules accounts for the consumption of oxygen and this reaction leads to an increase in the sensor response. At temperature 185 °C, the reaction product may get desorbed immediately after its formation providing the opportunity for new LPG species to react effectively with the oxygen chemisorbed at the film surface. Due to this, there would be a significant decrease in resistance and hence, the film shows the highest sensing response to LPG. Above the optimum operating temperature (>185 °C), the amount of oxygen would be less. Therefore, in presence of LPG, the probability of reduction reaction of gas with chemisorbed oxygen would be less which results in the decrease of sensing response of the film. Moreover, the decrease in the response of sample D and sample E to LPG may be attributed to an inhibitory effect of Zn2TiO4 particles: an increase in the proportion of Zn₂TiO₄, with the increase in the content of TiO2 might have resulted in the formation of an insensitive Zn₂TiO₄-ZnO grain boundary and subsequently reduce the number of sensitive ZnO-ZnO and ZnO-TiO₂ grain boundaries.

Fig. 8 illustrates the sensing response of 0.1 M, 0.2 M and 0.3 M CuO modified ZnO-TiO₂ thick films (with molar ratio 1:1) towards 286 ppm of LPG at different operating temperatures. The figure illustrates that the sensing response of the film is a function of CuO content. This would be attributed to the number of CuO dispersed on the surface of the film, forming a number of p-n heterojunctions [21]. The resistance of CuO modified films would be very high in air. When these films are exposed to LPG, LPG molecules interact with the film surface, causing disruption of p-type CuO and n-type ZnO-TiO₂ heterojunctions. The distortion of p-n heterojunctions leads to the decrease in resistance of the film. This would cause an easy flow of electrons [34].

Fig. 8

The 0.2 M CuO modified ZnO-TiO₂ thick film (with molar ratio 1:1) was observed to be the most sensitive among the investigated compositions. This could be attributed to the adequate dispersion of CuO on the surface of the film and optimum number of p-n heterojunctions formed on the surface of the film. In air ambient, the film resistance R_a was observed to be very high (4.99 × 10⁹ Ω) at 185 °C. On exposure to LPG, the p-n heterojunctions would be disrupted and the film resistance Rg would be observed to decrease by three orders of magnitude (3.05 × 10⁶ Ω) at 185 °C. The proportion of p-n heterojunctions in the film is crucial for the resistance of the film and the resistance of p-n heterojunction becomes the key factor to control surface resistance [35].

In 0.1 M CuO modified film, the amount of CuO is less which results in poorer dispersion of CuO than the optimum (0.2 M) on the surface of film. The number of p-n heterojunctions formed on the film surface would be insufficient to drastically change the film resistance and the film would show comparatively lower response.

In 0.3 M CuO modified film, the amount of CuO is higher and thus larger dispersion of CuO on the surface of the film than the optimum (0.2 M). However, only some amount of CuO would be utilized in the formation of p-n heterojunctions and unused CuO would resist the LPG to reach to p-n heterojunction. Due to this, resistance of the film would not change drastically and the film would show comparatively lower response.

In practical applications, response and recovery times of a sensor for a particular gas are important factors. Response and recovery times are defined as the time required for reaching 90 % of the final stable value. Fig. 9 illustrates the response and recovery time of 0.2 M CuO modified ZnO-TiO₂ (with molar ratio 1:1) based thick film to 286 ppm LPG at 185 °C. It indicates that the response and recovery time of the sensor are 30 s and 70 s, respectively. This result may be recommended for practical application of the sensor for LPG detection.

Fig. 9

Fig. 10 demonstrates the stability of 0.2 M CuO modified ZnO-TiO₂ (with molar ratio 1:1) thick film sensor towards 286 ppm LPG at 185 °C. Longterm stability test of the sensor element was conducted by maintaining the sensor element at optimum operating temperature of 185 °C and exposure of the sensor element to 286 ppm LPG was performed after every 30 days for six month. It was observed that there was no noticeable deviation in the sensor response. This was due to the presence of TiO₂ in the film [36].

Fig. 10

The response of 0.2 M CuO modified ZnO-TiO₂ (with molar ratio 1:1) based thick film toward different gases (286 ppm each) like NH₃, CO₂, H₂S and LPG at optimal operating temperature is shown in Fig. 11. The sensor shows an excellent response at 185 °C towards LPG but a negligible response to CO2 and poor response toward NH₃ and H₂S. The excellent sensing response of the sensor element towards LPG resulted from the fact that LPG molecules easily reacted with the sensor element at 185 °C as compared to the other tested gases.

Fig. 11