CHAPTER VI ULTRA-LONG TUNGSTEN OXIDE
6.4 Conclusion
142
6f shows the sensing response of the long WO3 nanowire sensors to a nonanal without and with a bending strain (flat and bending with curvature radius of ~5.75 mm). The sensing response and the repeatability shows negligible changes when compared with the sensing data with the flat form. Figure 6g shows the results of bending curvature radius (strain) dependent sensing responses, implying that the sensor performances do not exhibit any degradations when the curvature radius lower than 2.1mm. Even for bending with a curvature radius as small as 0.5mm (high bending strain), the sensor still remains good sensing properties (RN2/Rgas≈2.15). In addition, bending cycle endurance data is also shown in Figure 6h. There are no significant sensing performance degradations for the long WO3
nanowire based flexible sensor up to 104 cycles (Supplementary Figure S9). These results highlight the superior molecule sensing properties with the excellent mechanical flexibility due to the enhanced textile effects of long WO3 nanowires.
143
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145
6.7 Supporting Information
Part I h-WO3 nanowire growth
Figure S1. Effect of Na2SO4 concentration (CNS) on h-WO3 nanowire morphologies (2.72mM tungsten precursor) when varying concentration of Na2SO4 from 0.5 to 50mM.
All growth experiments are performed at 200 °C for 24 h, initial pH 1.68. (a) SEM images of nanowires grown with 2.72mM tungsten precursor; (b) Tungsten precursor concentration dependence on the nanowire morphology data (including length and radius) and the aspect ratio (insert), which are measured from SEM images of regular arrays; (c) XRD patterns of h-WO3 nanowires grown with different concentration of Na2SO4.
146
Figure S2. Effect of tungsten precursor concentration (CW) on h-WO3 nanowire morphologies (With Na2SO4, 3.5 mM) when varying concentration of tungstic acid sol from 0.65 to 54.4 mM. All growth experiments are performed at 200 °C for 24 h, initial pH 1.68. (a) SEM images of nanowires grown with increasing of CNS; (b) Tungsten precursor concentration dependence on the nanowire morphology data (including length and radius) and the aspect ratio (insert), which are measured from SEM images of regular arrays; (c) XRD patterns of h-WO3 nanowires grown with different concentration of tungsten precursor.
147
Figure S3. Comparison between the present WO3-x nanowires and previous works on the aspect ratio.
Figure S4. pH of the growth solution after addition of different concentration of Na2SO4
at room temperature (Purple) and growth temperature (Orange).
148
Figure S5. Calculated data of pH dependent equilibrium concentrations of ionic species in the growth solutions with 2.72mM tungsten precursor and 3.5mM Na2SO4.
Figure S6. Calculated the populations of two sulfur oxoanions (SO42- and HSO4-) varied with concentration of tungsten precursor and Na2SO4.
149 Part II Computational Details
We used density functional theory (DFT) to compute adsorption energies of SO42- and HSO4- on the h-WO3(100) surface. Different levels of theory were combined via the ONIOM extrapolation scheme, which allowed both to consider hydration effects and to refine results from pure DFT by exact-exchange hybrid DFT calculations. In this study, the ONIOM energy 𝐸ONIOM was defined as 𝐸𝑂𝑁𝐼𝑂𝑀 = 𝐸𝑝𝑒𝑟𝑖𝑜𝑑𝑖𝑐𝐿𝑜𝑤 − 𝐸𝑓𝑖𝑛𝑖𝑡𝑒𝐿𝑜𝑤 + 𝐸𝑓𝑖𝑛𝑖𝑡𝑒𝐻𝑖𝑔ℎ . 𝐸𝑝𝑒𝑟𝑖𝑜𝑑𝑖𝑐𝐿𝑜𝑤 indicates low-level DFT energy of a periodic slab model, while 𝐸𝑓𝑖𝑛𝑖𝑡𝑒𝐿𝑜𝑤 means the energy of a finite-size fragment of the slab model computed at the same level of theory. 𝐸𝑓𝑖𝑛𝑖𝑡𝑒𝐻𝑖𝑔ℎ is high-level DFT energy of the fragment with an implicit solvent model. The slab model for the h-WO3(100) surface consisted of (2×2)-supercell (a=15.0Å, b=7.7Å) with 13 atomic layers and the 20Å vacuum region. The finite-size fragment used to compute 𝐸𝑓𝑖𝑛𝑖𝑡𝑒𝐿𝑜𝑤 and 𝐸𝑓𝑖𝑛𝑖𝑡𝑒𝐻𝑖𝑔ℎ consisted of adsorbate, a fragment of the surface at the adsorption site W2O9, and link hydrogen atoms. The link atoms were placed along each W-O bond on the boundary between the fragment and non-fragment regions. The bond length between a boundary oxygen atom and a link atom was set to be 1.0Å. Moreover, one or two protons were introduced on the surface to maintain cell's charge neutrality.
All the low-level DFT calculations were carried out with Vienna Ab initio Simulation Package (VASP) version 5.4.41-4. RPBE was chosen for the exchange-correlation functional. Core-electrons were treated by the projector-augmented wave (PAW) approach.
We adopted the gamma point approximation and the Gaussian smearing method with a width of 0.05 eV. The cut-off energies were 400 eV for geometry optimizations and 520 eV adsorption energy calculations, respectively. The convergence criterion for geometry optimizations was set to be 0.01 eV/Å. High-level DFT calculations were performed at the B3LYP/def2-TZVPP level of theory using Gaussian 16 program package Revision A. 035. They incorporated hydration effects by the SMD implicit solvation method developed by Truhlar and his coworkers6. Adsorption energy was evaluated by the energy difference between the adsorbed and desorbed states. The initial geometry of the desorbed state was prepared by lifting the adsorbed molecule along the surface normal by 5Å. The geometry of the desorbed slab model was optimized using low-level DFT while fixing the position of sulfur.
150
Part III Nonanal sensing properties of the ultra-long h-WO3 nanowire based flexible sensor device
Figure S7. Nonanal concentration calibration by gas chromatograph mass spectroscopy (GC-MS).
For the bubbling nonanal gas, the detailed calibration method is presented as follows.
Firstly, different mole ratio of nonanal and isopropanol solutions (1-nonanal/isopropanol
= 0.1/1, 1/1, 10/ 1 and 100/1, mM/M) were distributed. Then 10 μL of the mixed solution was injected into the heating chamber of the GC-MS. After the evaporation of the liquid solution, the vapor gas was blown into the chromatographic column by Ar and detected by the mass spectroscopy. Figure S9a shows the GCMS spectra for nonanal measured at m/z=98 with different mole ratio. Here, the integral area of the peak can be utilized to calculate the concentration of the nonanal. Figure 7 plotted the integral area of the peak at m/z=98 that varied with the nonanal concentration. According to the obtained calibration curve, the concentration of the nonanal bubbling gas can be quantitatively measured. To reduce the test error, each mole ratio of nonanal tested for 3 times.
151
Figure S8. Dynamic resistance curves of the flexible WO3 nanowire device to 2.62 ppm nonanal before and after bending.
152
Figure S9. SEM images of the flexible device after bending 10000 cycles.
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C HAPTER VII
O VERALL C ONCLUSION AND
O UTLOOK
157
Metal oxide semiconductor gas sensors are the powerful tools to detect various gas molecules due to their prominent characteristics. The controllable synthesis of nanowires, the understanding of molecule-to-surface interaction and the design of novel sensor devices are all critical issues for determining the final sensor performance. In this thesis, we focus on these points aiming to get a deep understanding of the basic mechanism and accomplish the breakthrough in scientific and technological fields.
The molecular transformation behavior and mechanism of nonanal on single crystalline ZnO nanowire surface were well investigated by using optical spectroscopic (IR pMAIRS) and mass-spectrometric (TPD/GC-MS) techniques, as shown in Chapter IIV. The adsorption and desorption of nonanal itself as well as the aldol condensation and oxidation were all found on the ZnO nanowire surface. The thermal treatment had a significant influence on the surface reaction. The reaction rate could be increased with raising the annealing temperature or reducing annealing gas pressure. These results undoubtedly gave us a clear frame of molecule-to-surface interaction and provided a potential route for optimizing the molecular sensing applications by adjusting the surface activity of sensing material. For further controlling the surface behavior of nonanal, the inactive organic molecule MPA with robust thermal stability was modified on ZnO nanowire surface. As shown in Chapter IV, the aldol condensation reaction of nonanal on ZnO nanowire surface was drastically inhibited after carefully adjusting the MPA concentration. The single ZnO nanowire devices based on various degree of MPA modification were fabricated to explore the correlation between nonanal sensing properties and surface interaction, that the strong binding energy of 2-heptyl-2undecenal molecules induced the long recovery time during nonanal detection. The faster recovery process after MPA modification was observed due to the suppression of aldol condensation reaction. In addition, fabricating cheap and disposable sensor devices could be an alternative method to solve the problem of deteriorating device performance in long-term use. In Chapter V, a paper-based disposable molecular sensor device comprised a cellulose nanofiber paper substrate, ZnO nanowires, and graphite electrodes was fabricated by two-step papermaking and pencil-drawing processes. This paper sensor device presented efficient sensing performance to NO2 and superior merits of mechanical robustness, cut-and-paste use and easy disposal. Besides, ultralong WO3 nanowires were synthesized by hydrothermal method for making flexible sensor device with high mechanical flexibility. As shown in Chapter VI, the length of WO3
158
could be up to millimeter range. We proved that monovalent sulfur oxoanions rather than SO42- substantially enhanced the anisotropic nanowire growth of hexagonal WO3. The sensor device showed excellent property to nonanal molecules and good stability due to the enhanced textile effect.
Although many R&D efforts have been devoted to realizing the real application of metal oxide semiconductor gas sensors, reaching the commercial level still needs a long journey.
Poor selectivity is always an unavoidable problem for metal oxide-based gas sensors. This kind of sensor respond in a similar way toward different oxidizing (or reducing) gas molecules, even though sensitivities may be different depending on the type of molecules.
A variety of methods have been proposed in metal oxide gas sensors to improve the selectivity, such as bulk or surface doping, the application of gas specific prefilters, and the use of E-nose which consists of an array of gas sensors. However, it still can’t satisfy the demand to the complex gas mixture, especially human exhaled breath. The integration of multiple detection devices based on different principles together should be a possible approach to enhance the identification capability. Recently, the rising machine learning technique may also create a potential method to promote the recognition ability of devices, which is worth to further investigation. For realizing next-generation innovation in electronic sensor field, the development of high-quality materials and the novel integration methods should be further explored.
L IST OF P UBLICATIONS
161
LIST OF PUBLICATIONS
Scientific Journals
1. “Rational method of monitoring molecular transformations on metal-oxide nanowire surfaces.”
Wang, C.; Hosomi, T.; Nagashima, K.; Takahashi, T.; Zhang, G.; Kanai, M.; Zeng, H.;
Mizukami, W.; Shiya, N.; Shimoaka, T.; Tamaoka, T.; Yoshida, H.; Takeda, S.; Yasui, T.; Baba, Y.; Aoki, Y.; Terao, J.; Hasegawa, T.; Yanagida, T.
Nano Letters 2019, 19, 2443-2449.
2. “Paper-based disposable molecular sensor constructed from oxide nanowires, cellulose nanofibers, and pencil-drawn electrodes.”
Koga, H.; Nagashima, K.; Huang, Y.; Zhang, G.; Wang, C.; Takahashi, T.; Inoue, A.;
Yan, H.; Kanai, M.; He, Y.; Uetani, K.; Nogi, M.; Yanagida, T.
ACS Applied Materials & Interfaces 2019, 11, 15044-15050.
3. “Inactive molecular modification enhanced molecular discrimination on metal oxide nanowires.”
Wang, C.; Hosomi, T.; Nagashima, K.; Takahashi, T.; Zhang, G.; Yanagida, T.
In submission.
4. “Monovalent sulfur oxoanions enable millimeter-long tungsten oxide nanowire synthesis.”
Zhang, G.; Wang, C.; Mizukami, W.; Hosomi, T.; Nagashima, K.; Nakamura, K.;
Kanai, M.; Yasui, T.; Aoki, Y.; Baba, Y.; Yanagida, T.
In submission.
Proceedings
1. “Effect of UV irradiation on nonanal adsorption on ZnO nanowire array.”
Wang, C.; Zhang, G.; Yanagida, T.
3rd International Exchange and Innovation Conference on Engineering & Sciences
162
(IEICES2017), Kyushu University, Fukuoka, Japan, October 19-20, 2017. pp135-136.
2. “Spectroscopic analysis on single crystalline nanowire surface reveals chemical transformation pathways of aldehyde on ZnO.”
Wang, C.; Hosomi, T.; Nagashima, K., Takahashi, T.; Zhang, G.; Kanai, M.; Zeng, H.;
Yanagida, T.
Cross Straits Symposium on Energy and Environmental Science and Technology (CSS-EEST20), Pusan National University, Pusan, Korea, November 26-28, 2018.
pp233-234.
Conferences
1. “Effect of UV irradiation on nonanal adsorption on ZnO nanowire array.”
Wang, C.; Zhang, G.; Yanagida, T.
3rd International Exchange and Innovation Conference on Engineering & Sciences (IEICES2017), Kyushu University, Fukuoka, Japan, October 19-20, 2017. (Oral) 2. “Impact of thermodynamically defined oxide nanowire surfaces on stability of
molecule-to-surface interaction.”
Wang, C.; Nagashima, K.; Takahashi, T.; Zhang, G.; Nakamura, C.; Kanai, M.;
Yanagida, T.
International Symposium on Materials for Chemistry and Engineering (IMCE 2018), Kyushu University, Fukuoka, Japan, March 16, 2018. (Poster)
3. “Spectroscopic analysis on single crystalline nanowire surface reveals chemical transformation pathways of aldehyde on ZnO.”
Wang, C.; Hosomi, T.; Nagashima, K., Takahashi, T.; Zhang, G.; Kanai, M.; Zeng, H.;
Yanagida, T.
Cross Straits Symposium on Energy and Environmental Science and Technology (CSS-EEST20), Pusan National University, Pusan, Korea, November 26-28, 2018.
(Poster)
4. “Spectroscopic analysis on single crystalline nanowire surface reveals chemical transformation pathways of aldehyde on ZnO.”