Annealing Effect of Glancing Angle Electron Beam Deposited TiO2/In2O3 Nanowires Array on Surface Wettability

TiO2/In2O3 nanowire (NW) array are prepared using catalyst free glancing angle deposition technique. The wettability of TiO2/In2O3 NW surface are tuned and controlled by the annealing treatment without altering the surface with additional chemical coating. The phase change, surface roughness, change in static and dynamic contact angles due to the heat treatment are studied. Moreover, the surface properties such as frictional force and work of adhesion are calculated for all the samples. The samples annealed at 600 °C shows nearly superhydrophilic with static water contact angle of 12°, frictional force of 85.00748 µN and work of adhesion of 142.3721 mN/m. The surface of TiO2/In2O3 NW is controlled to attain desired water contact angles and sliding angles, which is paramount for designing practical application in self-cleaning, electronic and biomedical fields.


Results and Discussions
Figure 1(a) shows the XRD analysis of TiO 2 -In 2 O 3 NWs deposited on Si (100) substrates as a function of temperature (400-800 °C) and Fig. 1(b) shows the size-strain graph versus as deposited and annealed samples calculated from the Eq. (1) 20 given below: Where D stands for crystallite size, λ stands for wavelength of incident light, k is constant (=0.9), θ is bragg angle, β is the full width half maximum of peaks. The as-deposited TiO 2 -In 2 O 3 NWs shows two peaks representing planes (211) and (440) of cubic phase for In 2 O 3 while no peak can be seen for TiO 2 due to its amorphous nature which agrees with the result reported using e-beam evaporator for deposition 21 . As annealing temperature is increased from 400 °C to 600 °C, In 2 O 3 exhibited more extra peaks. Increment in the diffraction peak intensities can be observed with the increase in annealing temperature. The peak intensities increment suggest better crystallinity of the samples. There is shift in XRD spectra peak with planes (211) from samples annealed at 600 °C towards lower angle compared to samples annealed at 400 °C. This is due to lattice expansion 22 . TiO 2 in TiO 2 -In 2 O 3 NWs is amorphous upto 400 °C and starts to crystallize when annealed at 600 °C and exhibits anatase phase with planes (101) and (004) orientations due to its high stability 23 . Similar result was observed for TiO 2 annealed above 500 °C 24 . Moreover, the XRD peak intensities of In 2 O 3 in TiO 2 -In 2 O 3 NWs increased with the increase in annealing temperature. As the annealing temperature is increased, crystallinity is improved. As a result, strain is reduced till 600 °C. However, for samples annealed at 800 °C, the In 2 O 3 crystallinity is decreased as peak intensities is decreased compared with samples annealed at 600 °C, where the grain stops growing or exhibits amorphous nature above 600 °C as reported 14,15 . Further, TiO 2 exhibits mixed anatase-rutile phase diffraction peaks at 800 °C. Yan et al. also reported on the appearance of crystallization transformation from anatase to rutile phase above 600 °C 25 . Ma et al. reported on collapse of TiO 2 nanotube arrays structure grown at 800 °C with Ti support in oxygen owing to the mechanical stress and grain growth arising from the 'feeding effect' 26 . During the phase transformation from anatase to rutile, the TiO 2 nanowires may distort or disrupt the lattice of TiO 2 . This phase transformation led to break the two Ti-O bonds in the anatase, to rearrange Ti-O in octahedral to form rutile phase 27 . Of all the annealed samples, the sample annealed at 600 °C showed better crystallinity and lesser strain. Lesser strain implies decrease in lattice imperfections 28 . Figure 2(a) shows the schematics of vertically aligned TiO 2 /In 2 O 3 NW array sample and Fig. 2(b) depicts the typical FESEM cross section image of the TiO 2 /In 2 O 3 NW sample annealed at 600 °C in which the top section is In 2 O 3 and bottom being TiO 2 . TiO 2 NWs is deposited beneath the In 2 O 3 NW to obtain a separated TiO 2 /In 2 O 3 NWs because In 2 O 3 has been reported to form interconnected column beyond Zone I structure even under extreme shadowing effect condition due to sufficient diffusion of surface adatoms 29 . In this zone I, the homologous growth temperature parameter (θ = T S /T m ) is less than 0.2 where T s and T m are the substrate temperature and melting point of the deposited material, respectively with little surface diffusion of adatoms take place and the film microstructure and texture are controlled by shadowing 30 . Figure 2a shows that NWs are aligned closely, uniformly and vertically. It can also be seen that some NWs are undergrown due to competitive growth arising from shadowing effect using the GLAD technique. The GLAD technique follows the principle of self-shadowing and vapor fluxes cancellation during azimuthal rotation of substrate to obtain perpendicular aligned growth of NW 31 . Here, with the arrival of vapor fluxes, the taller NW overshadowed the shorter neighboring NWs and as a result, the shorter NWs remain as undergrown NWs. Figure 2(c) shows the TEM image of annealed TiO 2 /In 2 O 3 NW sample at 600 °C. In the junction, color contrast can be seen, which confirms the formation of TiO 2 /In 2 O 3 NW. From the Fig. 2(c), the NW has top and bottom diameter of ~50 nm and ~25 nm respectively. Undergrown region is also seen in the TEM image due to shadowing effect, which plays the main vital role in the formation of NWs. Figure 3 shows the AFM images of the as-deposited and annealed (400-800 °C) TiO 2 -In 2 O 3 NW respectively. The images show recrystallisation due to annealing. With the increase in annealing temperature, the grains initiate to cluster and agglomerate. From the AFM analysis, root mean square (RMS) of the roughness was found to be 10.806 nm (as deposited), 3.867 nm (400 °C), 3.26 nm (600 °C) and 3.54 nm (800 °C). The decrease in the roughness with the increase in annealing temperature is due to the increase in grain size and also the grain agglomeration, which reduces the gaps between the NWs. However, at 800 °C the roughness of TiO 2 -In 2 O 3 NW increases. This is due to the phase transformation from anatase to rutile; the TiO 2 NWs may disrupt the lattice of TiO 2 and thus increases the roughness of the heterostructure 26,27 .
The room temperature PL readings of the as-deposited and annealed samples were recorded by exciting the samples at 250 nm wavelength. Figure 4 highlights the PL emission intensity from the as-deposited and annealed TiO 2 -In 2 O 3 NW. From Fig. 4 it can be understood that PL intensity decreases as we increase the annealing temperature upto 600 °C and then the emission intensity decrease with further increase in annealing temperature. There is irregularity in the variation of PL intensity with annealing temperature. It has been reported that annealing treatment reduces the number of surface states 32 . On this basis, the PL intensity should shrink upon annealing. However, there can be migration of defects present within the grains onto the grain surface during high temperature annealing treatment. This would raise the amount of surface states, thus enabling the PL emission to enhance to some extent. At 800 °C annealing temperature, oxygen vacancy is created in TiO 2 and may be the oxygen from In 2 O 3 might diffuse into TiO 2 creating oxygen vacancy in In 2 O 3 . As a result, surface state is enhanced and a small shoulder PL emission is introduced at 402 nm (3.08 eV) and 465 nm (2.6 eV) related to oxygen vacancy in In 2 O 3 and TiO 2 respectively in TiO 2 -In 2 O 3 NW samples annealed at 800 °C 33,34 . Moreover, the main-band related PL emission at 340 nm for In 2 O 3 and 360 nm for TiO 2 as obtained in our previous work for TiO 2 -In 2 O 3 NW 35 is shifted towards shorter wavelength with increase in annealing temperature. This blue-shift cannot be related to quantum confinement 36 but can be attributed to the widening of energy bandgap with increase in the grain size, which is a function of annealing temperature. Further annealing at 800 °C, the main-bandgap PL emission should be red-shifted due to the formation of lower energy TiO 2 rutile phase, but the PL emission band is blue-shifted. This may be due to the formation of amorphous Titanium silicate where the Ti ions are diffused out toward the  www.nature.com/scientificreports www.nature.com/scientificreports/ interface during high temperature annealing which cause the blue shift of main band gap emission 24 . It can be concluded that the grain size and annealing treatment can affect the PL emission. Taking these effects into consideration, there is possibility for PL emission to vary irregularly with the annealing treatment as shown in Fig. 4. Among the various annealed samples, samples annealed at 600 °C exhibits reduced PL emission indicating lesser defects.
Room temperature static water contact angle (θ WCA ) measurement was performed on the as-deposited and annealed samples. Figure 5(a) depicts the θ WCA values and water dropping profile of the as-deposited and annealed samples taken during the measurement.
The as-deposited samples exhibits a higher θ WCA of 140° compared to annealed samples. As reported by the author 35 , as-deposited NWs structure are porous due to gap between consecutive NWs. The under-grown NWs arising due to shadowing effect creates more air gaps between the NWs and the air trapped in the gap reduces the fractional coverage at solid-liquid interface which decreases the van der waal forces at the surface. The roughness of AFM is more in as deposited samples. As a result, the θ WCA value of as-deposited sample is higher due to the air trapped between TiO 2 -In 2 O 3 NWs. Increase in surface roughness enhances the water contact angle which is in consistent with the Cassie and Baxter 37 . Moreover, as seen from the PL analysis, the higher emission from oxygen vacancies in as-deposited TiO 2 -In 2 O 3 NWs might be one of the reasons for showing higher contact angle. After annealing, the samples θ WCA is reduced compared to as-deposited sample and started showing nearly superhydrophilic nature. This is because after annealing the grain size of the TiO 2 -In 2 O 3 NWs increases, the NWs starts to cluster and agglomerate which in return reduces the air gap. Since the annealing is carried out in an ambient oxygen environment, this reduces the oxygen vacancies and the oxygen trapped by the surface and hence, θ WCA reduce with the increase in annealing temperature. However, due to the change in phase of TiO 2 from anatase to rutile at 800 °C, there is lattice distortion which increases the surface roughness of heterostructure and results in trapping of more air, thus enhancing the θ WCA to 70° for TiO 2 -In 2 O 3 NWs. It can be seen that water contact angle for all the samples is maintained when observed for 10 min as seen in Fig. 5(b).
It is also necessary to characterize the surface wetting behavior using dynamic water contact angle measurement. The advancing contact angle (θ ACA ) as well as receding contact angle (θ RCA ) are analyzed through adding and then withdrawing the liquid volume from water droplet 38 . Here, the θ ACA and θ RCA are the angles obtained by liquid expansion and contraction respectively and the resulting contact angle hysteresis (θ H ) are given in Table 1.
The θ H depends on the surface roughness and droplet adhesion to the surface. The value of θ H increased with the increase in annealing temperature due to increase in interaction of surface with water droplet. The as-deposited TiO 2 -In 2 O 3 NW showed the least difference in the hysteresis. The hysteresis increased with the increase in annealing temperature due to decrease in θ WCA . The next parameter, the sliding angle is measured. It is the angle of the droplet at which the droplet starts to slide when the substrate tilts. When the frictional force is low, the water droplet tends to slide effortlessly from the surface. The required maximum frictional force to displace liquid on surface can be found from the following Eq. (2) 39 : where m is the mass of the water, g is the acceleration due to gravity and α is the sliding angle of the droplet. Another main parameter for wetting of surface is the surface adhesion. It is the attraction of molecules between the surfaces in contact. Work of adhesion (W) between the water droplets and the surfaces of as-deposited and annealed samples are calculated respectively using young-dupre's formula as given in Eq.(3) 40 .
Where, γ w represents the surface tension of water. The as-deposited samples slide the water droplet when the surface was tilted at an angle of 20°. Accordingly, the force that is needed to slide the droplet of water from surface was found to be 33.60399 µN. However, the same droplet was not sliding until the tilt angle of the surface was 85° for samples annealed at 600 °C, which is due to the strong force of 97.66022 µN needed to slide the droplet from the surface. The sliding angle increased with the increase in annealing temperature because of the decrease in water contact angle. The air trapped between the nanowires is reduced due to agglomeration, grain size is increased and surface roughness is minimized. Moreover, hydrophobic surface materials acquire lower work of adhesion. The as-deposited TiO 2 -In 2 O 3 NW showed lower work of adhesion on comparison with annealed samples as oxygen adsorption tendency is higher at oxygen vacancy sites on the surface in as-deposited TiO 2 -In 2 O 3 NWs. The as-deposited TiO 2 -In 2 O 3 NW showed work of adhesion of 16.54105 mN/m, which increased to 142.3721 mN/m for samples annealed at 600 °C. This increase is owing to the increase in contact angle hysteresis. Tuning the oxygen vacancies and structure can change the surface roughness and hence the θ WCA . The structure having higher θ WCA can be used in self-cleaning surfaces whereas lower θ WCA can be used in biomedical applications. The surface has been modified for tuning the surface wettability using heat treatment without any chemical surface modification which may be harmful. Experimental detail. TiO 2 /In 2 O 3 coaxial NWs array is deposited on Si<100 > p-type substrate inside ebeam evaporator (BC 300, HHV India) incorporating GLAD technique. Cleaned substrate cut into 1 cm × 1 cm is placed inside the e-beam chamber with base pressure and deposition rate maintained at 6 × 10 −6 mbar and 0.5 Ås −1 respectively. During deposition, the substrate holder is aligned at 85° with respect to the source and also azimuthal rotation of the substrate is maintained at rate of 20 rpm. TiO 2 NW has been deposited using GLAD technique on Si. Likewise, In 2 O 3 NW was further evaporated over TiO 2 NW to obtain TiO 2 /In 2 O 3 NW. The film thickness was monitored by digital thickness monitor present inside the chamber. To study the annealing effect on the heterostructure NW, the as deposited samples is annealed at various temperatures ranging from 400 °C to 800 °C. Annealing has been executed using muffle furnace, maintaining the same time duration for 1 hr and 6 °C/ min heating and cooling ramp rate.
The annealed TiO 2 /In 2 O 3 coaxial NW morphology was studied using FESEM (SUPRA 55VP, Gemini Column) and transmission electron microscope (TEM). The structural properties of the as-deposited and annealed samples were analyzed using x-ray diffraction (XRD) Cu Kα radiation (RigaKu smart lab guidance). Optical property was studied using F-7000 fluorescence spectrophotometer for photoluminescence (PL) measurement under excitation wavelength of 250 nm. Atomic force microscope (AFM) (Nanosurf, Model: Easyscan 2 AFM) was used to calculate surface roughness of the samples. Contact angle measurement was carried out using sessile drop method with contact angle meter (DMS-401, Kyowa Interface Science CO LTD, Japan).

Conclusion
Perpendicularly aligned coaxial TiO 2 /In 2 O 3 NW array are prepared on Si substrate using catalyst free glancing angle deposition technique within electron-beam evaporator. Wettability of TiO 2 /In 2 O 3 NW surface is tuned using heat treatment in which the surface properties such as phase changes, surface roughness, changes in static and dynamic contact angles are studied. The samples annealed at 600 °C shows nearly superhydrophilic with static