Fabrication of anatase/rutile hierarchical nanospheres with enhanced n/p type gas sensing performance at room temperature

Pt activated hierarchical TiO₂ nanospheres were fabricated through post synthetic water vapour hydrothermal treatment. It was interesting to observe that the as-fabricated hierarchical TiO₂ nanospheres at low temperature consisted of both rutile and anatase phases, which showed apparent n-type sensing performance to ethanol and p-type sensing performance for benzene vapour at room temperature.

Room-temperature gas sensing metal oxides have attracted great interest from monitoring the environment, protecting homes, ensuring food quality and controlling chemical processes. As a significant semiconductor gas sensing material, TiO₂ shows the unique advantages of being environmental friendly, having biological compatibility, being inexpensive, and having excellent stability compared with other metal oxides, such as SnO₂,¹ ZnO,² In₂O₃,³ Fe₂O₃,^4,5 and WO₃. As an example, it was reported that the gas-sensing performance of the generally used SnO₂ would significantly decrease in wet ambient conditions, while TiO₂ was well stable. Hence, enormous interest has been focused on the investigation of TiO₂ nanomaterial for the applications of gas sensor.

Generally, anatase TiO₂ is an n-type semiconductor, whereas rutile TiO₂ is a p-type semiconductor, according to theoretical studies and experimental observations.^6–12 Kang et al.¹³ claimed that the conduction band minimum of anatase was found to be about 0.2 eV higher than that of rutile, which indicated that the electron more likely transferred from anatase to rutile. Based on the transient MIR (Middle Infrared Ray) dynamics of anatase-rutile mixed phase TiO₂, Li et al.¹⁴ reported that charge transfer process was confirmed at the anatase-rutile phase junction, and the electron transfer from anatase to rutile was proposed at the interface of anatase/rutile junction in mixed phase TiO₂. It was proposed that the formation of n-type/p-type TiO₂ phase junction was responsible for the improved sensing performance of TiO₂-based sensing materials. The n-type/p-type TiO₂ phase junction was supposed to improve charge transfer and reduce the response times.

In most cases, previous anatase/rutile based gas sensors were sensitive only at elevated temperature. Savage et al. examined the composite mixtures of n-type anatase and p-type rutile for gas sensing, and the composite behaved as a selective sensor towards CO. However, the operation was as high as about 600 °C.¹² The high-temperature operation of the sensors might lead to ignition when detecting flammable or explosive target gas, restricting their wide applications. So sensing performance at room temperature of anatase/rutile nanocomposite based gas sensor was an important parameter for the new generation of gas sensors.

However, Zhang and Banfield¹⁵ indicated that the thermal fluctuation of Ti and O on the surface of TiO₂ particles was not strong enough to form rutile nuclei at lower temperature. In the synthesis of TiO₂, the phase relationships between anatase phase and rutile phase were naturally influenced by temperature and pressure.^14,16,17 When synthesizing nanosized TiO₂ materials the mean particle size was important for the phase relationships: rutile was the thermodynamically favourable phase in bulk materials, but as soon as the particle size was below 10 nm anatase was more stable.¹⁸ On the other hand, as the size of the synthesized particles reached 5 nm, the energies of the surfaces became significantly important in determining the materials' properties.^19–21 Therefore, it was the great challenge to fabricate the anatase/rutile nanocomposites with small particle size at low temperature, which was crucial to promote the sensing property of gas sensors.

Hierarchical nanostructures constructed from nanoparticles contain low density, high structure stability, and excellent diffusion of guest molecules, which open a new view because of their promising applications in advanced nanodevices for the energy, environmental and sensing sectors.^22,23 In recent years, the gas sensors based on hierarchically structured semiconductors showed more significant gas sensing properties. Just recently, Hosseini et al. reported that the aligned ZnO with a hierarchical structure could detect target gas at room temperature.²⁴ It means that the growth of gas sensors based on unique, uniform and stable hierarchical nanostructure could be designed with superior sensing properties, due to their more gas diffusion routes, easy carrier transfer, large specific surface area and more effectively active sites. In this communication we present the post synthetic water vapour hydrothermal treatment in the fabrication of high crystalline anatase/rutile hierarchical nanospheres with small nanoparticles at low temperature. The sensing nanospheres act as n-type sensing materials to ethanol vapour and p-type sensing materials to benzene vapour, and show good stability at room temperature.

Regarding the crystal structure of TiO₂ nanospheres determined from WAXRD (wide-angle XRD), Fig. 1a shows the XRD patterns of hierarchical TiO₂ nanospheres obtained after 72 h psHT (post-synthetic hydrothermal treatment) and different temperature calcination. The narrow and intense peaks at 2θ = 25.5°, 37.9°, and 48.2° are the standard X-ray diffraction peaks of anatase TiO₂, while the peaks at 2θ = 27.6°, 36.1°, 41.2°, 54.3°, 56.6°, 62.8°, and 69.1° demonstrate that the phase structure is rutile TiO₂. The average crystalline grain size (D) calculated from the full width at half-maximum, FWHM, of the (101) reflection line for anatase phase and the (110) reflection line for rutile phase using the Scherrer formula, is 2.3 nm (anatase) and 2.9 nm (rutile) for the psHT treated TiO₂ nanospheres, 2.6 nm (anatase) and 3.6 nm (rutile) for the 200 °C heat-treated nanospheres, 2.9 nm (anatase) and 4.7 nm (rutile) for the 300 °C heat-treated nanospheres, and 3.2 nm (anatase) and 5.9 nm (rutile) for the 400 °C heat-treated nanospheres. However, intensity of peaks of Pt phase has not significantly increase in these patterns. Phase structures of TiO₂ nanospheres are confirmed by Raman spectroscopy, as shown in Fig. S1a.† psHT treated hierarchical nanospheres shows the characteristic Raman bands at 145, 197, 395, and 638 cm⁻¹ of anatase TiO₂, and the characteristic Raman bands at 235, 446, and 611 cm⁻¹ of rutile TiO₂. With the increase of calcination temperature, the Raman results indicate that the nanospheres still obtain anatase-rutile mixed phase with anatase/rutile phase junction. The N₂ adsorption–desorption isotherms and pore size distribution curves of hierarchical anatase/rutile nanospheres are illustrated in Fig. 1b and S1b.† The porosity of Pt decorated hierarchical anatase/rutile nanospheres heat-treated up to 300 °C, 400 °C, and 500 °C were analysed from nitrogen adsorption–desorption isotherms. All samples show similar type-IV isotherms, indicating the presence of well-developed porosity in the nanospheres. The specific surface area of 300 °C, 400 °C, and 500 °C heat-treated nanospheres are 119.8, 54.1, and 47.6 m² g⁻¹, respectively. The 300 °C calcined nanospheres show highest surface areas, high crystallinity, and uniform size distribution. In the following section, further discussion will be focus on the 300 °C calcined nanospheres.

Fig. 1 (a) WAXRD patterns of porous Pt decorated hierarchical nanospheres annealed at 200 °C, 300 °C, 400 °C, and 500 °C; (b) N₂ adsorption/desorption isotherms for Pt-TiO₂ nanospheres annealed at 200 °C, 300 °C, 400 °C, and 500 °C.

To further investigate the morphology and the crystalline structure, the TEM and HRTEM observations were carried out; the corresponding images are shown in Fig. 2. The low-magnification TEM image is depicted in Fig. 2a, from which the hierarchical nanostructure got further confirmed. The relatively uniform hierarchical nanospheres with the size of ca. 200 nm can be observed from the high-magnification TEM image (Fig. 2b). Fig. 2c and d illustrate the TEM image and the corresponding electron diffraction pattern of nanospheres, which are mainly in rutile state (confirmed by the diffraction rings in Fig. 2d). There are also some weak rings presented in the diffraction pattern coming from anatase phase. As shown in Fig. S2,† it is clearly shown that A-TiO₂, and R-TiO₂ are closely connected to each other, forming the well-mixed pairs of heterojunctions. The interplanar distances of 0.32 and 0.35 nm correspond to the lattice spacings of the R-TiO₂ (110) plane and A-TiO₂ (101) plane, respectively. Fig. 2e–f show the TEM elemental mapping of the individual TiO₂ nanosphere. It is obvious that the signal of Pt distributed homogeneously within the hierarchical nanostructure. These results confirm that the obtained nanospheres are constructed by two different phase nanoparticles with hierarchical nanostructure.

Fig. 2 (a–c) TEM images of Pt decorated TiO₂ nanospheres with different magnifications; (d) SAED image; (e–f) elemental mapping of individual nanosphere.

According to the above results, we propose a possible formation mechanism for the TiO₂ hierarchical nanostructures, as illustrated in ESI, Fig. S3.† It is believed that the porous hierarchical TiO₂ nanostructures are formed through the following processes: first, nanonuclei are formed at the original reaction stage. The freshly formed nanonuclei are thermodynamically unstable due to the high surface energy. They tend to aggregate together to minimize the interfacial energy and thus form many agglomerates as the reaction time is up to 12 hours. As the reaction proceeds, new particles deposit continuously on small raised areas of the agglomerates and assemble to form short nanorods along a specific direction at 24 hours; as the reaction go on, each nanorod continues to grow along its axis till the complete nanosphere formed at 72 hours. The formation process can be closely referred to the oriented attachment mechanism.^25,26

The surface composition and the chemical state of the elements in 300 °C calcined hierarchical anatase/rutile sensing nanospheres were investigated by XPS. The high resolution spectra of Ti, O, and Pt are shown in Fig. S4.† As shown in Fig. S4a,† Ti 2p shows two individual symmetric peaks positioned at 458.7 (Ti 2p_3/2) and 464.5 eV (Ti 2p_1/2) with a peak separation of 5.8 eV, suggesting the state of Ti⁴⁺. The high-resolution XPS spectrum of O 1s for the 300 °C heated Pt-decorated hierarchical anatase/rutile nanospheres is shown in Fig. S4b,† one finds that the O 1s consists of two components in different chemical states: O_latt (530.7 eV) and O_x⁻ (531.8 eV). O_x⁻ is attributed to the adsorbed oxygen ions, which have an important role in the gas sensing property.¹ The chemical states of the Pt dopant are disclosed by the Pt 4f XPS spectra. Theoretically, the spectrum of Pt 4f is composed of Pt 4f_7/2 and Pt 4f_5/2 with a separation of 3.4 eV due to the spin–orbit coupling. As shown in Fig. S4c,† Pt(0) can be found in the 300 °C calcined Pt decorated hierarchical anatase/rutile sensing nanospheres. The corresponding BE values of Pt 4f_7/2 and Pt 4f_5/2 are located at 71.1 and 74.5 eV, corresponding to platinum metal.

In this study, the sensor response is defined as S = (R_a − R_g)/R_g or =(R_g − R_a)/R_a, where R_a and R_g are the sensor resistances in air and in the target gas, respectively. This parameter is positive (negative) for n-type (p-type) ethanol (benzene) sensing. Chemiresistive sensors are classified as n or p type depending on whether their conductance increases or decreases when they are exposed to a reducing gas, and vice versa for oxidizing gases, i.e. decreased conductance is expected for n-type sensing. In this context, the 300 °C heated sensing nanospheres exhibit an n-type gas sensing performance to ethanol and an unusual p-type gas sensing response to benzene, as shown in Fig. 3a.²⁷ Each exposure/recovery cycle were carried out for an exposure interval of 60 s followed by a recovery interval of 60 s in dry air. The sensor response clearly tracks the change in the ethanol or benzene concentration. Moreover, the responses are extremely fast, and the sensor can fully recover in short times after ethanol or benzene removal. Specifically, the response times, defined as the time necessary to reach 90% of the maximum response, range between 15 and 20 s and decrease as the ethanol or benzene concentration is increased. The recovery times are in the range of 20–25 s and decreased as the ethanol or benzene concentration decreased. However, it can be seen that the nanospheres show more sensitivity to benzene than ethanol, which may be attributed to different reaction kinetics that benzene could extract Ti-vacancy donated holes through the surface reaction. Taking 125 ppm as an example, the nanospheres exhibits a sensitivity of 4.9 to benzene vapour, which is higher than the value (3.3) of the ethanol vapour. There are also some reports in the literature on the enhanced VOCs sensing performance of TiO₂-based sensing materials, as shown in ESI, Table S1.† Sennik et al. achieved the detection of 5000 ppm VOCs target gas at 200 °C with response value of 3.5 by using anatase/rutile nanotubes as sensing materials.²⁸ Dutta et al. improved the sensing properties, applying p-type nanoporous TiO₂ thin films exposed to low level concentration of VOCs target gas at 75 °C, but the sensitivity was still not effective.²⁹ Mabrook et al. applied new type of TiO₂ films as VOCs sensing materials. The sensing operation temperature was decreased to room temperature and the sensitivity to 150 ppm VOCs target gas was less than 0.1.³⁰ Further improvement of sensing property was owing to the use of GaN nanowire/TiO₂ nanocluster hybrid as VOCs gas sensor, however the response value was still less than 0.3 upon to 200 ppm VOCs target gas.³¹ In this work, the Pt decorated porous hierarchical anatase/rutile nanospheres exhibit significantly enhanced sensing performance to VOCs target gases.

Fig. 3 (a) Response curves of Pt decorated anatase/rutile composite nanospheres sensors measured under various ethanol and benzene gas concentrations; (b) response curves of sensing nanospheres to 125 ppm ethanol, acetone, butanol, isopropanol, toluene, and benzene at room temperature; (c) normalized response of the nanospheres sensors as the function of the concentration of six target gases; (d) response mechanism of nanospheres exposed to reducing gases or air.

The responses of Pt-decorated TiO₂ nanospheres based gas sensors to six target gases with 125 ppm concentration at room temperature were further investigated, and the relevant results are shown in Fig. 3b. The present sensors display quite selectivity to ethanol as n-type gas sensor, especially the 300 °C calcined hierarchical anatase/rutile sensing nanospheres, of which the responses reach 2.1, 1.9, and 1.5 to acetone, butanol, and isopropanol, respectively. At the same time, the nanospheres based gas sensors present excellent selectivity to benzene as p-type gas sensor, of which the responses reach two times than the response to toluene. In contrast, when the nanoporous TiO₂ nanosphere-based sensor without Pt nanoparticles was used, low responsibility was observed. After exposed to 125 ppm ethanol or benzene, the sensor response was not obvious, as shown in ESI, Fig. S5.† Thus, compared to TiO₂ nanospheres without Pt nanoparticles, a Pt-decorated TiO₂ nanospheres-based sensor provided almost 4 times higher sensor response for ethanol or benzene vapour. This result is easily attributed to the presence of metallic Pt, which remarkably enhances the sensing property because of the chemical sensitization mechanism.

For the anatase/rutile composite nanospheres, p–p path and n–n path conduction is presumably comparable with p–p pathway dominated in the air atmosphere. Mabrook M. et al. reported that the benzene molecules on the surface of p type TiO₂ interact with the Ti-vacancy donated holes causing the resistance of the films to increase by injecting electrons into the valence band.³⁰ Therefore, the sensor shows the overall p-type response to benzene, and toluene. Upon to exposure to ethanol, acetone, isopropanol, and butanol, the resistance in the hierarchical nanospheres decrease to minimum value, more electrons are produced in anatase phase with decreasing the Ti-vacancy donated holes concentration in rutile phase, n–n pathway became dominant. For optimal sample (300 °C heated sensing nanospheres), p–p pathway conduction is dominant and the response from n–n path can be partially cancelled out by response from p–p path in the opposite direction, which explains the relative low sensitivity to ethanol. However, when exposed to benzene, the resistance of hierarchical nanospheres increases and reaches the maximum value quickly, the offset effect between n–n path and p–p path is smaller for benzene detection, which makes the sensor maintain the relatively high sensitivity to benzene and consequently results in enhanced selectivity to benzene against ethanol, as shown in Fig. 3c.³² The depletion layer of the p–n heterojunction also plays a significant role in the selectivity improvement. Besides the resistance increase of the forward bias n–p pathway in the benzene vapour, the depletion layer of the p–n heterojunction of reversed-bias p–n act as the electron carrier trap, reducing the concentration of electron and also increases the resistance of the hierarchical nanospheres, as shown in Fig. 3d which results in the enhanced selectivity of anatase/rutile nanospheres to benzene over ethanol as well. In addition, surface modification is proofed to be an effective way to enhance the sensor response. The decoration of Pt can lower the energy barriers of the gas adsorption and gas dissociation, which makes it possible to obtain the enhancement of sensor response to target gas at room temperature.

Conclusions

In summary, Pt decorated porous hierarchical anatase/rutile nanospheres with controlled particle size, high surface area, and excellent crystallinity were synthesized through a facile psHT technique. The gas-sensing performance of the TiO₂ composite nanospheres were tested in detail and the nanostructured morphology could be used to improve the sensing properties evidently, including an enhanced distinguishing ability to VOCs vapour, large detection range and fast response dynamics. Due to its high surface area, proper platinum distribution, and appropriate particle size, the 300 °C calcined TiO₂ sensing nanospheres with hierarchical nanostructure showed the excellent sensitivity to ethanol with n-type sensing property and to benzene with p-type sensing property at room temperature.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10921g

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