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DOI: 10.1039/C4NR07559E

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ARTICLE

Received 00th January 2012, Accepted 00th January 2012

Zhenyi Zhang,a Jindou Huang,a Bin Dong,*a Qing Yuan,a Yangyang Hea and Otto S. Wolfbeisb

DOI: 10.1039/x0xx00000x

We develop a novel kind of branched heterostructures by hydrothermal growth of the ZnSnO 3 nanostructures on TiO 2 electrospun nanofibers, and demonstrate its enhanced www.rsc.org/ sensing performance on humidity through sequential tailoring ZnSnO 3 nanostructures inspired by the cactus. Combining with the first-principles calculations, it is deduced that the adsorbed water molecules can be increased on the ZnSnO 3/TiO 2 heterojunction surface by reducing the surface potential barrier. Meanwhile, the bioinspired ZnSnO 3 nanoneedles, as the branches on the heterostructures, can further boost their adsorption abilities for water molecules via a water-collection process. The adsorbed water molecules on the tips of ZnSnO 3 nanoneedles are very easy to desorb at a lower humidity environment due to the small area of the tips (1.5∼2.5 nm). Thus, the optimal ZnSnO 3/TiO 2 heterostructure exhibits the response and recovery times of ∼2.5 s and ∼3 s, respectively. Its good sensitivity may enable it to detect tiny fluctuations in moisture and relative humidity that is surrounding any high-precision instrumentation. through rebuilding the Fermi level in heterojunction.[4] On the Introduction other hand, the secondary nanostructures of hierarchical Efficient and accurate detection of environmental moisture heterostructure may possess tailorable feature. For example, its through a reliable humidity sensor is able to promote the morphology or structure may be manipulated so to improve its development of healthcare, industrial production, electronic properties. Thus, the sequential mediation of secondary equipment, and even aerospace technology.[1] One-dimensional nanostructures which is able to seek the optimally surface (1D) semiconductor nanomaterials which own the advantages wettability during the fabrication process has become a critical of large surface area, high surface to volume ratio, as well as point. unique physicochemical properties have been focused on the Inspired by the properties of desert living matter, such as corresponding design and construction toward highly sensitive beetles, cacti, and spider, several artificial structures including humidity nanosensors.[2] In order to achieve high sensitivity, the conical-shaped wire, beaded fibers, films, and so forth, have sensing layer assembled by 1D semiconductor nanomaterials been recently developed to mimic fog-collection.[6] These should be nanoporous to enable water molecules to penetrate bioinspired nanostructures with excellent surface wettability and diffuse throughout the entire layer. Large aspect ratios are can not only condense the fog into small water drops, but also also expected so to boost the proton transport directionally.[3] guide the collected water drops to directional transport.[6a-c] If From this point of view, semiconductor nanofibers used for bioinspired nanostructures such as branch are incorporated into sensing layer of chemical nanosensors could be successfully the electrospun backbone to form a 1D branched obtained by electrospinning technique. However, the ionized heterostructure, the humidity sensing performance (e.g., oxygen species (O-, O2-, etc.) absorbed on the semiconductor response time, recovery time, reproducibility, stability, and surface due to relative energetic position with respect to the linearity) will be promoted by the synergy among the structural Fermi level, often occupy the surface-adsorbed sites for water property of electrospun nanofibers, the heterojunction effect of molecules that is therefore hardly to bridge more water semiconductors, and the surface wettability of bioinspired channels for proton transport in the humidity environment.[4] nanostructures. This would affect the characteristics of semiconductor-based Herein, we report on a successful attempt to fabricate the humidity nanosensors to a certain degree. 1D branched hierarchical heterostructures by hydrothermal On one hand, controllable growth of secondary growth of ZnSnO3 nanostructures on TiO2 electrospun semiconductor nanostructures on the electrospun nanofibers, and demonstrate its enhanced sensing performance semiconductor nanofibers to construct 1D branched hierarchical on humidity through the optimization of ZnSnO3 nanostructures heterostructures may circumvent the drawbacks.[5] For example, in the heterostructures inspired by cactus. The optimal the surface potential barrier that reflects the amount of adsorbed ZnSnO3/TiO2 heterostructure with its good stability exhibits oxygen species for semiconductors can be effectively adjusted response and recovery times of ∼2.5 s and ∼3 s, respectively.

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Cite this: DOI: 10.1039/x0xx00000x

Rational Tailoring of ZnSnO3/TiO2 Heterojunctions with Bioinspired Surface Wettability for HighPerformance Humidity Nanosensors†

Nanoscale ARTICLE

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Journal Name DOI: 10.1039/C4NR07559E

This comparable to the corresponding values of alkali metal salt-based humidity sensing nanomaterials that usually show short response-recovery times, but poor stability due to the dissolution of highly soluble alkali metal salts.[7]

Figure 1 SEM images of (A) TiO2 nanofibers; (B) ZnSnO3 nanoparticles/TiO2 nanofiber heterojunction; insets are the corresponding TEM images; (C) High-resolution TEM of the interface region between ZnSnO3 nanoparticles and TiO2 nanofibers.

ZnSnO3 and TiO2 are the materials of choice for the branch and backbone in the semiconductor heterostructures, respectively. ZnSnO3 is a newly emerging class of humidity-sensitive material while TiO2 as a traditional substrate material whose steady chemical structure can be easily processed into 1D nanofibers through a facile electrospinning technique.[8] The rhombohedral crystal structure of ZnSnO3 exhibits a low lattice mismatch with tetragonal anatase TiO2. In order to investigate the heterojunction effect that could adjust the amount of ionized oxygen absorbed on the semiconductor surface, at first, we assembled the ZnSnO3 nanoparticles on the TiO2 nanofibers backbones through a glucose-assisted hydrothermal process in which the glucose served as the template and/or ligament to guide the ZnSnO3 nanostructure with tailorable feature. Figure 1A shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the aselectrospun TiO2 nanofibers with diameters of 200 to 300 nm and lengths of several micrometers. It can be seen that the surfaces of the nanofibers are relatively smooth without secondary nanostructures. After hydrothermal treatment, these nanofibers surfaces become rough and are densely covered with of secondary ZnSnO3 nanoparticles with diameters of 30 to 50 nm (Figure 1B). The corresponding energy-dispersive X-ray (EDX) spectra suggest the ZnSnO3/TiO2 heterostructures to be composed of Zn, Sn, Ti, and O elements (Figure S1). This is in good agreement with the results of X-ray photoelectron spectroscopy (XPS) (Figure S2). The high resolution (HR)-TEM image, shown in Figure 1C, indicates the simultaneous presence of crystalline ZnSnO3 (110) lattice fringes (d=0.263 nm) and TiO2 (101) lattice features (d=0.352 nm) in the ZnSnO3/TiO2 heterostructures. Furthermore, the crystallographic structure and phase purity of the ZnSnO3/TiO2 heterostructures were investigated by the Xray diffraction (XRD) patterns (Figure S3), which further confirm the co-existence of rhombohedral ZnSnO3 (JCPDS, no. 28-1486) and tetragonal anatase TiO2 (JCPDS, no. 21-1272) in the heterostructures. Work functions for the ZnSnO3 (110) and TiO2 (101) surfaces were calculated by aligning the Fermi level relative to the vacuum energy level, and are shown in Figures 2A and B. The work function of TiO2 [1.49-(-7.131)≈8.62 eV] is higher than that of ZnSnO3 [1.58-(-6.4086)≈7.99 eV]. This result indicates that a positive shift in the Fermi level of the TiO2 and a negative shift in the Femi energy level of the ZnSnO3 can be expected. When the new Fermi level is formed in the heterojunction, the surface potential barrier of ZnSnO3

2 | J. Name., 2012, 00, 1-3

Figure 2 Electrostatic potentials and the corresponding geometry structure models for (A) TiO2 (101) surface and (B) ZnSnO3 (110) surface; (C) Schematic diagram showing the energy band structure and electron transfers in the ZnSnO3/TiO2 heterojunction; (D) The dependence of impedance on the RH for the nanosensors, and (E) Response and recovery characteristic curves based on the nanosensors: (a) TiO2 nanofibers, (b) ZnSnO3 nanoparticles/TiO2 nanofiber heterojunction, and (c) ZnSnO3 nanoparticles; (F) Response and recovery characteristics of the nanosensors under the different light irradiation (325±10 nm or 550 nm±10): (a, c) ZnSnO3 nanoparticles/TiO2 nanofiber heterojunction; (b, d) TiO2 nanofibers.

nanostructure grown on the TiO2 nanofibers surface will be reduced because of interfacial electron transfer. Thus, the ionized oxygen species absorbed on the ZnSnO3 surface can be decreased after introducing the TiO2 nanofibers, as illustrated in Figure 2C. The decrease of ionized oxygen species may induce the increase of water-adsorbed sites on the ZnSnO3 surface, which can boost the formation of more water channels, thereby promoting the proton transport. To evaluate the heterojunction effect on the enhancement of humidity sensing performance, we tested the dependence of impedance on relative humidity (RH) for the nanosensor devices produced by the ZnSnO3 nanoparticles, TiO2 nanofibers, and ZnSnO3/TiO2 heterostructures, respectively, under the AC voltage of 1V and a frequency of 100 Hz. It can be seen that when shifting the RH from the 11% to 95%, the impedance change of ZnSnO3 nanoparticles (∼102 Ω) are inferior to that of TiO2 nanofibers (∼102.5 Ω) because of the higher Fermi level of ZnSnO3 nanoparticles. However, after coupling the ZnSnO3 nanoparticles with the TiO2 nanofibers, the corresponding impedance change reaches nearly three orders of magnitude (107 ∼ 104 Ω). This value is obviously superior to that obtained by the ZnSnO3 nanoparticles, and even better than the corresponding value from the TiO2 nanofibers (Figure 2D). The response and recovery characteristic curves based on the TiO2 nanofibers and ZnSnO3 nanoparticles/TiO2 nanofiber heterojunction are given in Figure 2E, in which the responserecovery time is defined as the time taken by the sensor to achieve 90% of the total change in impedance upon varying the RH between 11% and 95%, and vice versa. The results indicate that the response time of heterostructures-based nanosensor is enhanced over 2.5 and 4.0 times as compared to these of TiO2 nanofibers and ZnSnO3 nanoparticles-based nanosensors (Figure S4), respectively. These results imply that, besides the heterojunction effect, the 1D branched structures also influence on the improvement of humidity sensing characteristics.

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Results and discussion

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ARTICLE DOI: 10.1039/C4NR07559E

Figure 4 (A) The dependence of impedance on the RH for the branched heterostructures with the ZnSnO3 nanoparticles, nanorods, and nanoneedles, as the secondary nanostructures, respectively; (B) Response and recovery characteristic curves based on the nanosensors containing (a) ZnSnO3 nanorods/TiO2 nanofiber heterojunction,and (b) ZnSnO3 nanoneedle/TiO2 nanofiber heterojunction; (C) hysteresis characteristics of the ZnSnO3 nanoneedle/TiO2 nanofiber heterojunction based nanosensor. Figure 3 (A) Schematic illustration of the formation process of ZnSnO3 nanostructures hydrothermally grown on TiO2 nanofibers by using glucose as the template and/or ligament; SEM images of (B) ZnSnO3 nanorods/TiO2 nanofiber heterojunction; (C) ZnSnO3 nanoneedles/TiO2 nanofiber heterojunction; the insets show the corresponding TEM images; (D) TEM image and the corresponding elemental mapping images of an individual ZnSnO3 nanoneedle/TiO2 nanofiber heterojunction, HRTEM image and the corresponding SAED pattern of the nanoneedles; (E) HRTEM image and the corresponding FFT pattern of the interface region between the ZnSnO3 nanoneedle and TiO2 nanofiber.

In order to better analyze the electron transfer in the heterojunction interface, monochromatic light with wavelength of 325±10 nm and 550±10 nm was employed to irradiate the humidity nanosensors at the 95% RH during the sensing tests. As can be seen in Figure 2F, the impedances of nanosensors significantly increased under irradiation at 325±10 nm while there is little change on the impedances under irradiation at 550±10 nm. These results suggest that the water channels on the material surfaces can be cut off through the reaction between the photogenerated charge carriers and surfaceadsorbed water molecules, resulting in the obstruction of proton transport. On the other hand, this process is also a strong evidence to prove that the protons are the dominant carriers responsible for the electrical conductivity through tunneling the adjacent water molecule by hydrogen bonding.[1d] It should be noted that, after irradiation at 325±10 nm for 25 s, the impedance of heterostructures obviously is higher than that of TiO2, further indicating that the efficient interfacial charge transfer could occur on the heterojunction, therefore improving the generation of charge carriers. If the surface potential barrier determined by the Fermi level of semiconductor is the only factor that influences the sensing characteristics of the heterojunction, the impedance change should be lower than that of TiO2 nanofibers during the testing because of their higher Fermi level. However, the result is opposite as illustrated in Figure 2D, which implies that the humidity sensing performance can be further optimized through rational tailoring of 1D-branched heterostructures. To corroboration of above hypothesis, we attempted to design the ZnSnO3 nanostructures on the TiO2 nanofibers surface through optimizing the hydrothermal reaction conditions. Inspired by the structure of desert cactus that can harvest and transport the water from fog, the ZnSnO3 grown on the TiO2 nanofibers should possess a 1D nanostructure.[6a-c] Fortunately, glucose (possessing one aldehyde group and five hydroxyls groups) can be used as the template and/or ligament to guide the ZnSnO3 nanostructure on the TiO2 nanofibers.[9] In our case, upon dispersal of glucose in the TiO2 nanofiber suspensions, the aldehyde groups are able to bind the TiO2 surface through coordination interactions between the carbonyl oxygen (C=O) and Ti sites because of their high activities.[9a]

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When adding the Zn2+, Sn4+, and OH- for the subsequent hydrothermal process, the other hydroxy groups on the catenulate structure of glucose will absorb these metal ions and induce them to grow into 1D nanostructure. Respective reaction equations are shown in Figure 3A. Furthermore, the polarization effect between different glucose molecules may make these catenulate molecules interlace with each other so to form another kind of multi-molecule template, which leads the products to interweave each other. Thus, after increasing the glucose contents during the hydrothermal process, the ZnSnO3 nanostructure on the TiO2 nanofibers changes to the nanorod with the aspect ratio (the ratio of length to diameter) of ∼9.6 (Figure 3B). On further addition of glucose, the tips of ZnSnO3 nanorods shift to conical shapes with the tip diameter of 0.5∼1.5 nm (Figure 3C and Figure S5), because some lattice planes with high surface energies might disappear during the growth of crystals.[10] Moreover, the average aspect ratio of conical-shaped nanorods (so-called nanoneedles) reaches a value of ∼28, which is more than 3 times larger than that of nanorods obtained by low content of glucose. The HRTEM image of an individual ZnSnO3 nanoneedle on the TiO2 nanofiber shows the crystal lattice with interplanar distance of 0.263 nm, which is consistent with the d-spacing of the (110) plane of the ZnSnO3. Combining with the corresponding selected area electron diffraction (SAED) pattern reveal that the single-crystalline nature of nanoneedle is grown along the [001] direction.[10b] The TEM elemental mapping from the individual ZnSnO3/TiO2 branched heterostructure (Figure 3D) is conducted to clearly identify the spatial distributions of Ti, O, Zn, and Sn elements. The results unambiguously confirm the TiO2 nanofiber surface coated by ZnSnO3 to form the 1D branched heterostructure. The crystallographic relationship between the ZnSnO3 nanoneedle and TiO2 nanofiber was also investigated in more detail by HRTEM imaging of their interface. As can be seen in Figure 3E, the interplanar distances of 0.263 and 0.352 nm are in good agreement with the d-spacings of the (110) and (101) planes of ZnSnO3 and TiO2, respectively. The intersection angle between the normal direction of the (110)ZnSnO3 plane and the (101)TiO2 plane is about 50º, which is similar with the corresponding intersection angle in the heterojunction interface of ZnSnO3 nanoparticles/TiO2 nanofibers (as shown in Figure 1C). Moreover, the lattice distortions in the connecting region are negligible, because the interfacial lattice mismatch between the (101)TiO2 and the (110)ZnSnO3 plane is relatively low [(0.3520.343)/0.352≈2.5%] (Figure S6). The low lattice mismatch at the interface of ZnSnO3/TiO2 heterostructures can lead to a less defective and abrupt interface, which is beneficial to the electron transfer in the heterojunction interface, thereby

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Figure 5 (A) Response and recovery times for the (1) TiO2 nanofibers and branched heterostructures with the ZnSnO3 (2) nanoparticles, (3) nanorods, and (4) nanoneedles, as the secondary nanostructures, respectively; (B) Schematic diagram showing the water adsorption and desorption process on the different branched heterostructure surfaces; TEM images of the individual ZnSnO3 (C) nanoneedle and (D) nanorod.

effectively lowering down the surface potential barrier of ZnSnO3 nanostructures by rebuilding the Fermi level. As expected, after optimizing the ZnSnO3 into 1D nanostructure, the humidity sensing performance of heterostructures are obviously promoted, as observed in Figure 4A. In particular, when constructing the ZnSnO3 nanoneedles on the TiO2 nanofibers surface, the heterostructures-based nanosensor displays the excellent linearity with the impedance spanning nearly four orders of magnitude (107 ∼ 103 Ω). Moreover, the response and recovery times of this nanosensor are reduced to 2.5 s and 3 s (Figure 4B), which are about 3.6 and 8.3 times shorter than the corresponding values for the pure TiO2 nanofibers. Interestingly, these values seem to be faster than the corresponding values of traditional alkali metal salts-based humidity sensing nanomaterials.[11,2b,4b,4c] The narrow hysteresis loop with the maximum humidity hysteresis of 3.5% RH (shown in Figure 4C) indicates that the nanosensor also possesses good reliability. More data on the humidity sensing principles are given in Figure S7-S9. Thus, we have successfully demonstrated that the humidity sensing behavior of the heterojunction-based nanosensor can be further promoted through rational tailoring of secondary nanostructures. Based on the above results, we conclude that the water adsorption and desorption behaviors of heterojunction-based nanosensor with good surface wettability may be also dependent on and/or related to the ZnSnO3 nanostructures on the TiO2 nanofibers surface, therefore influencing on the response-recovery time that is the key parameter to evaluate the performance of humidity sensors in practical applications. Figure 5A reveals that by shifting the morphology of ZnSnO3 from zero-dimension to 1D on the TiO2 nanofibers, the response-recovery time of nanosensor is much shortened. This can be attributed to the high surface area and large aspect ratio of the 1D nanostructure. It can boost the water adsorption process and also guide the proton transport directionally in a high RH environment.[2] Water molecules adsorbed on the tip of 1D ZnSnO3, such as nanorods and nanoneedles, will then be desorbed rapidly at a low RH environment by combining the strong diffusion effect of water molecules with the small surface area of the tip, leading to cut off the water channels for proton transfers (Figure 5B). However, it is important to note that the heterostructure with the ZnSnO3 nanoneedles as the secondary nanostructures

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Journal Name DOI: 10.1039/C4NR07559E

Figure 6 (A) FT-IR spectra of (a) TiO2 nanofibers and branched heterostructures with the ZnSnO3 (b) nanoparticles, (c) nanorods, and (d) nanoneedles, as the secondary nanostructures, respectively; (B) N2 adsorption and desorption isotherms for (a) TiO2 nanofibers and (b) the ZnSnO3 nanoneedle/TiO2 nanofiber heterojunction. The inset shows the corresponding distribution of pore diameter measured by the Barrett-JoynerHalenda (BJH) method.

exhibits a faster response-recovery time than the ZnSnO3 nanorod-based heterostructure. This phenomenon can be explained by means of bionics and mechanics. As known, a cactus can easily harvest water from the fog or dew in an arid environment by using the conical shaped spines distributed on its surface. This is a result of the capillary force induced by the diameter gradient of spine and causing the transport of collected water from the tip to the root along the spine.[6b, 6c] Thus, the surface wettability of cactus is strongly dependent on the microstructure of 1D spine. According to the literature,[6c] the capillary force suffered by the quasi-spherical water drop on a conical shaped spine can be described below:       where P denotes the Laplace pressure inside a water drop, z the  position of a water drop on the spine, the Laplace pressure  gradient along the spine which represents the capillary force,  the surface tension of the water, θ the apex angle of a conical shaped spine, r the mean radius of the spine at the place where the water drop is located, and R0 the mean radius of the water drop. It can be concluded that when  is zero, the corresponding capillary force is zero. That is to say, a conical shaped 1D structure is the important prerequisite to cause the capillary force, thereby enhancing the surface wettability. The above equation also indicates that the capillary force will increase with the decrease in the radius of 1D nanostructure and with an increase in the apex angle. Thus, a conically shaped 1D structure with small radius and large variation in its cross section possesses high surface wettability. At last, the capillary force should also increase with the decrease in the volume of water drop, implying that a small water drop would move faster than a large one on a conical shaped 1D structure. Interestingly, the spines of cactus also have a branch-like hierarchical structure on which many microbarbs with the conical shape are grown, which is very similar to the structure of ZnSnO3/TiO2 heterojunction obtained in our work. In comparison to the millimeter-scale spines, the radius of microbarbs is more narrow and only several micrometers long. Even though, the water drop condensed on the tip of the microbarbs can be also driven to the root by the capillary force. Thus, we believe that the water-collection process may occur on the 1D ZnSnO3-based heterostructures, leading to promoting the response time of the nanosensor through the enhanced surface wettability. As compared to the ZnSnO3 nanorod (Figure 5C), the ZnSnO3 nanoneedle (Figure 5D) has the larger apex angle and smaller radius. This may

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Nanoscale Accepted Manuscript

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ARTICLE

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Journal Name result in the better surface wettability, as analysed by equation (1). That is why the ZnSnO3 nanoneedle-based heterojunction exhibits the fastest response time in our work. Besides, the area surface of the tip for the ZnSnO3 nanoneedle is more than 6 times smaller than that for the ZnSnO3 nanorod, implying that the water channels is more easily to cut off on the tip of ZnSnO3 nanoneedle at a low RH environment. Thus, the recovery time of ZnSnO3 nanoneedle-based heterojunction nanosensor is 4 times shorter than that of ZnSnO3 rod-based heterojunction nanosensor. To further corroborate the high surface wettability of ZnSnO3 nanoneedles grown on the TiO2 nanofibers, Fourier transform infrared (FT-IR) spectroscopy was carried out. As can be seen from Figure 6A, besides the vibration bands relating to the inorganic materials, such as O-Ti-O, O-Zn-O, and O-Sn-O, appeared before 800 cm-1, some bands attributed to the O-H vibrations can be also found at wavenumbers centered around 3730, 3340, and 1655 cm-1. The band peaking at 3730 cm-1 originates from the basic terminal hydroxyl (OH)t on the material surface, which is relatively stable at the temperature below 350 ℃.[12] Moreover, the bands peaking at 3340 and 1655 cm-1 are ascribed to surface-adsorbed water molecules of material, which are strong determined by the RH environment surrounding the material.[12] By comparing the FTIR results, it can be deduced that the peak intensities of (OH)t are nearly the same for all the samples. However, in the case of (OH)w vibrations, the peak intensity order for the ZnSnO3/TiO2 heterstructures is as follows: Nanoneedle coating > nanorod coating > nanoparticle coating > bare TiO2. This further indicates the high surface wettability of ZnSnO3 nanoneedles/TiO2 nanofibers heterojunction.

ARTICLE DOI: 10.1039/C4NR07559E Additionally, the nanoporous structures with high specific surface areas are also beneficial for the enhancement of humidity sensing performance for the nanosensor, which cannot be neglected. As shown in Figure 6B, the BET specific surface area of a ZnSnO3 nanoneedle-based heterojunction (29.2 m2/g) is more than 2 times higher than that of pure TiO2 nanofibers (12.4 m2/g). The corresponding pore size distribution curves obtained by the Barrett-Joyner-Halenda method indicate the presence of large pores with a size of around 10∼40 nm, probably caused by the intertwined TiO2 electrospun nanofibers. Nanopores with the small size of ∼4 nm also are being formed after assembly of ZnSnO3 nanoneedles on TiO2 nanofibers. These small sized pores may originate from the tiny spaces between the intertwined ZnSnO3 nanoneedles. The higher specific surface area with the smaller sized nanopores can provide more active sites for water adsorption or desorption, resulting in the faster response-recovery time for the nanosensor. Overall, it is rational to believe that the humidity nanosensors with optimized sensitivity can be designed through tailoring the semiconductor heterojunction with low surface potential barrier and high surface wettability. In addition to the stipulations of sensitivity, linearity, resolution, and hysteresis, a practically useful sensor also is expected to be stable over time (both on storage and if operated) and to be reusable. Stability was tested by storing the ZnSnO3 nanoneedle/TiO2 nanofiber heterojunction in air for 90 days, followed by measuring the impedances at various RH environments. Almost no changes in the impedances were found, this proving the good reliability of our nanosensor (Figure 7 A). Notably, as observed in Figure 7B and C, it should be pointed out that the ultra-sensitive humidity performance of the optimal heterojunction-based nanosensor enables the potential applications on sensing the breathing and whistled tunes through monitoring the moisture fluctuation driven by the user. This interesting phenomenon may enable the detection of tiny fluctuations in moisture and RH that is surrounding any high-precision instrumentation.

Conclusions

Figure 7 (A) The stability of ZnSnO3 nanoneedles/TiO2 nanofiber heterojunction. The voltage and the frequency are 1V and 100 Hz, respectively. ZnSnO3 nanoneedles/TiO2 nanofiber heterojunction-based nanosensor response to (B) breathing and (C) whistling.

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In summary, we have developed a high-performance humidity nanosensor based on one-dimensional branched heterostructures fabricated via rational tailoring of ZnSnO3 nanostructures on TiO2 electrospun nanofibers. The high quality of heterojunction interface between the ZnSnO3 (110) plane and the TiO2 (101) plane induce an effective electron transfer to lower down the surface potential barrier of heterostructures, thereby enhancing their surface active-sites for water adsorptions. The optimal nanostructure for ZnSnO3 designed as the nanoneedles was inspired by the shape of conical spines or microbarbs of cactus. It is shown that this design improves the efficiency of water collection and the transport of water caused by capillary forces originating from the Laplace pressure. Moreover, the narrow surface area for the tips of ZnSnO3 nanoneedles can result in water desorption on them and within short time. Thus, by taking these advantages, the optimal ZnSnO3/TiO2 heterostructure nanosensor exhibited an ultra-fast response-recovery time, good linearity, low hysteresis, and high stability. This humidity sensing performance seems to be better than the traditional alkali metal salts-based humidity sensing nanomaterials. It is expected that our present work provides a useful platform for the well-design of high-performance humidity sensors based on heterostructural

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Nanoscale Accepted Manuscript

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semiconductor nanomaterials, such as CeO2/ZnO, SnO2/TiO2, In2O3/TiO2 branched heterostructures, and so forth.

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Acknowledgements This work is supported by the 973 Program (Grant No. 2012CB626801), the National Science Foundation of China (Gr ant Nos. 51402038, 11474046, and 11274057), Program for New Century Excellent Talents in University (NCET-13-0702), Scientific Research Foundation for Doctor of Liaoning Province (Grant No. 20141118), Educational Committee Foundation of Liaoning Province (Grant No. L2014547), Science and Technology Project of Liaoning Province (Grant N o. 2012222009), Program for Liaoning Excellent Talents in Uni versity (LNET) (Grant No. LJQ2012112), Fundamental Resear ch Funds for the Central Universities (Grant No. DC12010117), and Science and Technique Foundation of Dalian (Grant No. 2013A14GX040).

Notes and references a

Key Laboratory of New Energy and Rare Earth Resource Utilization of State Ethnic Affairs Commission, Dalian Nationalities University, 18 Liaohe West Road, Dalian 116600, P. R. China. E-mail: [email protected]; Tel. 8641187556959 b Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, D-93040 Regensburg, Germany. † Electronic Supplementary Information (ESI) available: [experimental section and Figure S1-S9]. See DOI: 10.1039/b000000x/ 1

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TiO₂ heterojunctions with bioinspired surface wettability for high-performance humidity nanosensors.

We developed a novel kind of branched heterostructure by hydrothermal growth of ZnSnO3 nanostructures on TiO2 electrospun nanofibers, and demonstrated...
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