Role of Oxygen Vacancy on the Hydrophobic Behavior of TiO2

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Role of Oxygen Vacancy on the Hydrophobic Behavior of TiO Nanorods on Chemically Etched Si Pyramids 2

Chetan Prakash Saini, Arabinda Barman, Dip Das, Biswarup Satpati, Satya Ranjan Bhattacharyya, Dinakar Kanjilal, Alexey N. Ponomaryov, Sergei Zvyagin, and Aloke Kanjilal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08991 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 14, 2016

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The Journal of Physical Chemistry

Role of Oxygen Vacancy on the Hydrophobic Behavior of TiO2 Nanorods on Chemically Etched Si Pyramids C. P. Saini,† A. Barman,† D. Das,† B. Satpati,‡ S. R. Bhattacharyya,‡ D. Kanjilal,§ A. Ponomaryov,‼ S. Zvyagin, ‼ and A. Kanjilal*,† †

Department of Physics, School of Natural Sciences, Shiv Nadar University, NH-91, Tehsil Dadri, Gautam Buddha Nagar, Uttar Pradesh 201 314, India ‡

Surface Physics and Material Science Division, Saha Institute of Nuclear Physics,1/AF Bidhannagar, Kolkata 700 064, India

§

Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India

‼

Dresden High Magnetic Field Laboratory (Hld), Helmholtz-Zentrum Dresden-Rossendorf (Hzdr), Po Box 51 01 19, 01314 Dresden, Germany

Address correspondence to: [email protected] ABSTRACT Oxygen vacancy (OV) controlled hydrophobicity of self-assembled TiO2 nanorods (NRs) on chemically etched Si pyramids is investigated by irradiating with 50 keV Ar+-ions at room temperature. Apparent contact angle (CA) is found to increase from 122° to 141° up to a fluence of 1×1015 ions/cm2, followed by a gradual reduction to 130° at 1×1017 ions/cm2. However, the drop in apparent CA is found to be associated with the decrease in fractional surface area via transformation of NRs to an amorphous layer above 1×1015 ions/cm2, though it is still higher than that of as-grown one. Detailed X-ray photoelectron spectroscopy and electron paramagnetic resonance measurements suggest that the control of hydrophobic behavior is related to the suppression of surface free energy via migration of OVs into the voids in TiOx layers.

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INTRODUCTION Understanding the wetting characteristics of liquid on solid surfaces is not only important for fundamental interaction process, but also for many practical applications such as impermeable textiles, sensors, anti-icing, self-cleaning, drug delivery, solar panels, etc.1 This is particularly important for water droplets on solid surfaces where the wetting level can be classified in three categories in terms of apparent contact angle (CA) at the liquid/solid interfaces, such as: (i) hydrophilic (CA < 90o), (ii) hydrophobic (CA ≥ 90o), and superhydrophobic (CA ≥ 150o)2. In addition to high apparent CA, the surface can only be considered as superhydrophobic if it gives a low CA hysteresis along with high potential barrier between the Cassie and Wenzel wetting states.3-6 Although the superhydrophobic phenomenon has commonly been addressed in the light of surface roughness either by simple Cassie-Baxter or Wenzel model, in conjunction with surface chemical composition,2 recent experimental and theoretical analyses, however, suggest that these models alone cannot explain the wettability of complex surface structures.6-7 For instance, Feng et al. mimicked the “rose petal structure” by artificially designing the hierechial surfaces with high apparent CA, but having high adhesive force with water that does not allow to roll off by turning the sample surface upside down.8 Exploring these phenomena on metal oxides has attracted special attention as they widely serve as anti-reflective, photocatalysts, detectors and transparent electrodes for optoelectronic and energy applications.9-12 Even though the wetting property of metal oxides can be modified by introducing surface roughness at nanoscale regime in the form of nanorods and/or nanoparticles,13 or by coating with low surface energy materials, its origin is however not straightforward owing to the involvement of local stoichiometry14 in the presence of oxygen vacancies (OVs) at surfaces.14-16 It was shown that the increase in OV can play a decisive role in controlling the surface wettability based on the electronic structure of a


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material.9 For example, it was found that rare earth oxide materials exhibit hydrophobicity due to shielding of partially-filled f-shells by the presence of outer s and p orbitals,17 whereas Schaub et al. have shown that OVs on rutile TiO2 surfaces can act as active sites for dissociation of water molecule via transfer of one proton to neighbor oxygen (O) atom and in turn it makes the surface hydrophilic.15 Moreover, Sun et al. demonstrated that Ar+-ion irradiation or vacuum annealing effect can lead to the formation of OV-related defects on ZnO and TiO2 surfaces, and make these systems hydrophobic to hydrophilic.18 In contrary, In2O3 and ZnO nanowires under oxygen plasma treatment and also by annealing in O2 atmosphere can transform them hydrophilic from hydrophobic through O enrichment on the surfaces.19-22 These contradictory results therefore suggest that the underlying mechanism behind the wetting behavior of metal oxide surfaces needs further investigation for in-depth understanding of the impact of surface defects as a function of morphology for practical application. Nontoxic, wide bandgap TiO2 has been considered to be a key model system for many experimental and theoretical studies (Ref. 12 and references therein) and also to be an appropriate candidate for investigating the correlation between OV and wettability23 due to its chemical stability and low production cost. Ion beam irradiation is an unique tool in this respect due to its ability to produce and control the distribution of vacancies in a predetermined depth by appropriate choice of ion species, energy and fluence (i.e. ions/cm2).24 In this article, we demostrate how OVs near the surface play a crucial role in tuning hydrophobicity of TiO2 nanorods (NRs) on chemically textured Si wafers. In particular, we report how 50 keV Ar+-ion irradiation at room temperature (RT) can increase the apparent CA of TiO2 NRs with increasing ion fluence up to 1×1015 ions/cm2 owing to controlled evolution of surface OVs, followed by a decrease above this fluence due to a sudden transformation of NRs to



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an amorphous layer. Finally, we show the loss of a large number of singly-charged OV (V0+) through evolution of voids in the sub-oxide layers at an ion fluence of 1×1017 ions/cm2.

EXPERIMENTAL PROCEDURES To reveal the phenomenon, ultrasonically cleaned pieces of 500 µm thick p-type Si(100) wafers (area 1×1 cm2) were initially chemically etched for developing self-assembled pyramids (described in details in Ref. 10). About 70 nm thick TiO2 layer was deposited on textured Si at RT by radio-frequency (RF) magnetron sputtering with a 100 W power supply. Following this, asgrown TiO2 layers were irradiated at RT by 50 keV Ar+-ions at an incidence angle of 45o from the Si facets with a beam current of 1 µA and fluences in the range of (0.05-10)×1016 ions/cm2 (see Fig. 1). Detailed deposition process and irradiation technique were reported elsewhere.12 For better presentation, as-grown TiO2 is referred as S0 while samples irradiated with ion fluences of 5×1014, 1×1015, 1×1016 and 1×1017 ions/cm2 are called as S1, S2, S3 and S4, respectively.

Figure 1. Schematic representation of 50 keV Ar+-ion irradiation of conformally grown TiO2 layer on Si pyramids at room temperature.


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The microstructure of TiO2 layers on textured Si surfaces before and after ion beam exposure were investigated by 300 keV FEI Tecnai G2 S-Twin transmission electron microscopy (TEM) in cross-sectional geometry while the thickness of the TiO2 films was estimated by stylus profilometer with a resolution of 0.5 nm (Bruker, DektakXT). In addition, surface chemical analyses were studied by Mg-Kα (hν ≈ 1253 eV) radiation in a X-ray photoelectron spectroscopy (XPS) system integrated with a hemispherical analyzer of radius ~150 mm (VSW Ltd., UK). Calibration of binding energy (BE) scale was made using the gold Fermi edge. Moreover, the hydrophobic property before and after ion irradiation was investigated by static and dynamic CA (i.e. CA hysteresis) measurements (Krüss GmbH, DSA-25) where the error of measured apparent CA was found to be within ± 2°.13 Here, CA hysteresis was determined by measuring the difference between the advancing (i.e. expansion of droplet by pumping in of water to its maximum) and receding (i.e. contraction of droplet by pumping out of water to its minimum) CAs. In addition, Electron Paramagnetic Resonance (EPR) experiments were carried out at the Dresden High Magnetic Field Laboratory (Hochfeld-Magnetlabor Dresden, HLD), HelmholtzZentrum Dresden-Rossendorf (HZDR) with the help of an X-band “Bruker Elexsys E500” EPR spectrometer operated at a frequency range of 9.35 to 9.40 GHz and equipped with “Oxford Instruments” helium-4 flow-type cryostat with a lowest accessible temperature of ∼2 K. For measurements, the samples were mounted to a quartz rod with an outer diameter of 4 mm. The field was applied perpendicular to the Si substrate. The data were acquired at a microwave power of 1 mW and 5 Oe field modulation amplitude. RESULT AND DISCUSSION Typical cross-sectional TEM (XTEM) image of S0 is displayed in Fig. 2(a) showing a conformal growth of TiO2 on Si pyramids. High-resolution TEM (HRTEM) image of the dashed5 ACS Paragon Plus Environment


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rectangular region in Fig. 2(a), however, depicts the formation of dense, randomly oriented TiO2 NRs on Si facets [Fig. 2(b)]. Clearly, the micro-scale Si pyramids are not only providing a suitable platform to grow TiO2 NRs, but also for making dual scale nano-micro structures for improving hydrophobicity.13 In recent studies, we showed that these crystalline NRs are mainly dominated by anatase phase, where the average length and width have been found to be ∼65 and 15 nm, respectively.12 Interestingly, the surface color has also been changed with increasing fluence from 5×1014 to 1×1017 ions/cm2 (see Fig. S1), though no significant change was recorded in TiO2 NRs up to a fluence of 1×1015 ions/cm2. On further increase in fluence up to 1×1016 ions/cm2, all these NRs were transformed to an amorphous TiOx layer as shown in Fig. 2(c), and this phenomenon is more clear from the HRTEM image [Fig. 2(d)]. However, a large number of voids in the valley regions (shown by dashed circles) were noticed at a fluence of 1×1017 ions/cm2 [see Fig. 2(e)] where the voids in the TiOx matrix are visible in the HRTEM image [Fig. 2(f)]. Close inspection also reveals the formation of an amorphous region in Si substrate below the TiOx layer [Fig. 2(f)]. This is due to the reduction of TiOx thickness by ion beam sputtering of atoms,


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Figure 2. (a) XTEM image of S0 displays the conformal growth of TiO2 layer on one of Si pyramids, where (b) HRTEM image from the yellow dashed rectangular region in (a) shows the formation of TiO2 NRs. However, (c) XTEM image of S3 reflects a clear transformation to an amorphous TiOx layer; this is evident from the HRTEM image (d) as the one taken from the dashed rectangular region in (c). (e) XTEM image of S4 depicts the evolution of voids, mostly at the valley region of Si pyramids (as indicated by the dashed circles), while (f) corresponds to the HRTEM image, evidencing the formation of voids in TiOx layer, in conjunction with a clear transformation of underneath crystalline Si to an amorphous layer up to a depth of ~30 nm.

specifically O at higher fluences as demonstrated in Ref. 25. The formation of a large number of vacancies in the TiO2 layer has further been established by Stopping and Range of Ions in Material (SRIM) calculations.12 Based on these simulated results, it seems that the voids are most likely formed by the accumulation of ion beam induced vacancies in TiOx, while the increase in size of voids as a function of ion fluence can be attributed to the migration of newly created vacancies towards the existing ones; details are given in Ref. 12. Since the ion induced sputtering loss of O is dominated over Ti,25 the formation of a Ti-rich zone near the surface is expected. In order to understand this, the change in composition across TiOx layer has also been examined by Energy Dispersive X-ray spectroscopy (EDX) as the one documented for S4 in Fig. S2, showing the increase in O concentration deep inside the layer with a composition of TiO1.1 near the TiOx/Si interfaces. It is therefore evident that the surface becomes more Ti-rich in the presence of a large number of OVs. To understand the role of geometric architecture and also the participation of near surface OVs in improving hydrophobicity, detailed CA measurements have systematically been carried out as a function of ion fluence for 2 µl water droplet, as exhibited in Fig. 3. Ion fluence mediated gradual modification of water droplet can be revealed from Fig. 3(a), whereas the 7 ACS Paragon Plus Environment


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measured apparent CA of the corresponding samples is exhibited in Fig. 3(b) within an error bar of ± 1°. As discerned, the apparent CA for S0 is of ~122°, indicating the hydrophobic in nature. Since the apparent CA of bare Si pyramid is ~98° (Ref. 13), the observed enhancement in apparent CA in S0 is most probably associated with the growth of randomly oriented TiO2 NRs on pyramidal Si leading to the development of a dual-scale nano-micro structures.26 Although dual-scale structures reduce the effective solid fractional surface area via incorporation of solidair composite surfaces,27 however, the large CA hysteresis of about 20° within an error of ± 2° (supplementary information given in Video-S1 and Video-S2) indicates the presence of adhesive force between the textured surfaces and impregnated water droplet [schematically shown in the inset of Fig. 3(b)]. This phenomenon can be discussed in the framework of Cassie impregnating state where the high apparent CA along with a large adhesive force take part simultaneously.6, 8 Interestingly, the apparent CA is increased significantly to 136° for S1 till ~141° at a critical fluence of 1×1015 ions/cm2 (S2), followed by a gradual decrease to 133° for S3 and 130°

Figure 3. (a) Snapshots of 2 µl water droplets on S0, S1, S2 and S4 surfaces for measuring respective CAs where each scale bar represents 0.5 mm. Corresponding CA profile with increasing ion fluence is shown in (b). The arrow represents the schematic view of the water droplet on as-grown sample (S0). 8 ACS Paragon Plus Environment

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for S4. Based on recent morphological analyses,12 the measured increase in apparent CA up to ~141° can be attributed to the ion beam modification of surface free energy due to gradual evolution of OVs at surfaces22 through transformation of TiO2 to TiOx NRs via dominant sputtering of O.25 This is consistent with our XPS and EPR results as will be discussed in the following. Although the OV evolves more with increase in ion fluence to 1×1016 ions/cm2 (S3), the decrease in apparent CA can be assigned to the reduction of the effective surface area via transformation of NRs to an amorphous TiOx layer [Fig. 2]. Moreover, further decrease in apparent CA to 130° at the highest ion fluence (S4) is most likely due to the loss of surface OVs by diffusion to the underlying bigger voids [see Fig. 2]. Indeed, the voids behave like sinks for OVs. In any case, no significant change has been observed in the measured CA hysteresis after ion bombardment. This trend is also consistent with both apparent CA and hysteresis on planar TiO2 surfaces, though the overall drop of observed CA can be attributed to the decrease in surface roughness (Fig. S3). As the surface chemical composition plays an important role in controlling OVs and so the influence on CA, all the samples have been examined by XPS. It should be noted here that in stoichiometric TiO2 surfaces, the dissociation of water molecules is highly probable as the hydrogen of water molecules feels strong electrostatic attraction to the surface O atoms, and eventually forms two hydroxyl (OH) groups.16 The increase in OH on TiO2 surface enhances the relative wettability and therefore the reduction of apparent CA in S0. However, the energetic ion bombardment with increasing fluence is expected to weaken the bond strength between Ti and lattice O atoms, and in turn leads to the removal of O from TiO2 due to lower surface binding energy.25 As a result, the liberation of O atoms is expected to promote the evolution of surface



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OVs, where the lack of O atoms on TiO2 surfaces could not attract the water molecules. This can be examined by XPS as it is known to be a powerful tool to follow the surface chemistry as a function of ion fluence.12 One can also track the O 1s region to have better insight. Typical XPS spectra at O 1s region before and after ion irradiation are shown in Fig. 4. From these spectra, it appears that the O 1s peak is asymmetric in nature, even in S0, indicating the existence of at least two chemical states of O. Based on various operating condition, O 1s peak has commonly been deconvoluted in literature considering the involvement of species like lattice O atoms, OH, adsorbed H2O, and so on.16 Since in the present study, the XPS spectra have been recorded under ultrahigh vacuum (UHV) condition, the possibility of detecting physically absorbed H2O on TiO2 surfaces can be neglected. The recorded XPS spectra were deconvoluted by conventional fitting procedure using Voigt function (30% Lorentzian and 70% Gaussian) after Shirley-type background subtraction25 in CASA-XPS where the fitting details are summarized in Table 1. Careful analysis revealed the appearance of peaks at the BEs of 531.2 eV and 532.8 eV: The former one can be attributed to the lattice O of TiO2 and Ti2O3, while the later one is associated with the mixed contribution from surface OH groups, generated by the dissociation of water molecules.16 In fact, the amount of surface OH is most possibly connected with the available O on the TiO2 surfaces. Since the O concentration on stoichiometric surface of S0 will be the maximum, the 532 eV peak intensity is expected to be governed by the availability of surface OH groups. We should note here that the TiO2 NRs in S0 are mainly dominated by the anatase phase (A-TiO2) where the grains are preferentially oriented along the {101} direction.12 In this system, as the OVs experience a low energy barrier to diffuse from surface to bulk region, they are not energetically stable at surfaces.27 Comparing the extracted values in Table 1, one can


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Figure 4. Typical high-resolution XPS spectra near the O-1s region before (S0), and after ion beam irradiation with a fluences of 1×1016 ions/cm2 (S3) and 1×1017 ions/cm2 (S4). The experimental data are shown by open circles, whereas fitting results are presented by thick grey curves. The background subtraction curves are shown in magenta, while the deconvoluted curves for Ti-O-Ti and Ti-OH are highlighted by red and blue colors.

see that the 532 eV peak intensity is the lowest for S2 (~30 %) for having minimum OH concentration, indicating the contribution of surface OVs will dominate on Ti-rich NRs However, the continuous rise in 532 eV peak intensity with increasing ion fluence above 1×1015 ions/cm2 (S2) can give the information not only about the surface OVs, but also the ones residing below the surface up to a 5-10 nm range. Under this circumstance, the drop of apparent CA in S3 [Fig. 3] can be attributed to the loss of effective surface area due to the formation of amorphous TiOx layer (discussed in Fig. 2). Further decrease in apparent CA in case of S4 [Fig. 3] can be assigned



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to the reduction of surface OVs due to the migration to the underlying voids for minimizing energy.

Table 1: XPS fitting parameters showing the compositions and corresponding binding energies of TiO2 O1s peaks before and after Ar+-ion irradiation.

Sample

Composition

Binding energy (eV) ± 0.2 eV Relative percentage

Ti-O-Ti

531.2

68.9

Ti-OH

532.8

31.1

Ti-O-Ti

531.4

69.8

Ti-OH

532.6

30.2

Ti-O-Ti

531.2

61.8

Ti-OH

532.7

38.2

Ti-O-Ti

531.2

60.3

Ti-OH

532.7

39.7

S0

S2

S3

S4

In order to evaluate the OV-related defects, low temperature EPR spectroscopy has been carried out in a systematic way as this measurements are highly sensitive to paramagnetic species containing unpaired electrons.28 The recorded spectra are shown in Fig. 5(a) (the intensity of each spectrum was normalized taking into account the sample size and TiOx layer thicknesses). As shown in Fig. 5(a), the EPR signals give a g value of 2.003, confirming the presence of +1 charge state of OV (V0+) in all the ion beam irradiated samples, 28 though a tiny EPR signal has also been detected in S0. The development of V0+-related shallow doners can be attributed to the displacement of the Ti and O atoms of TiO2 from their regular lattice sites29 which increases 12 ACS Paragon Plus Environment

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further under the exposure of energetic ions. Moreover, the probability of the formation of V0+ will be higher than that of Ti interstitials due to lower BE of O than the Ti atoms,25 in conjunction with +2 charge state.29 Close inspection of Fig. 5(a), however, reveals a clear decrease in linewidth of the EPR signal with increasing ion fluence (Fig. 5(b)). We believe that the decrease

Figure 5. (a) Low temperature (2 K) EPR spectra of S0, S2, S3 and S4. (b) The EPR linewidth for different samples.

in EPR linewidth is most likely associated with the reduction of heterogeneous distribution of V0+ centers by suppressing strain via evolution of voids in the TiOx matrix (Fig. 2), and so the effect on g tensor.30-31 This as a consequence decreases the paramagnetic V0+ centers at surfaces due to the diffusion into the voids. Hence, it leads to an increase in hydrophobicity through reduction of surface free energy in the presence of limited OVs (discussed above).22



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Conclusions In conclusion, we report the effect of 50 keV Ar+-ion induced modification of surface morphology and chemical structure, and consequently the concentration of oxygen vacancy near the surface on the hydrophobic behavior of self-assembled TiO2 nanorods on chemically etched Si pyramids. Although as-grown samples with nano-micro dual-scale surfaces showed a apparent contact angle of ~122°, a systematic increment in contact angle up to 141° was observed at a fluence of 1×1015 ions/cm2. This increase in apparent contact angle was found to be correlated with the reduction of solid fractional surface area due to complete transformation of nanorods to an amorphous TiOx layer. Further increase in ion fluence up to 1×1017 ions/cm2 led to a gradual decrease in contact angle to 130°. This was attributed to the rise in surface free energy via loss of oxygen vacancy near the surface during the evolution of voids in TiOx matrix. Detailed EPR studies confirmed the participation of single-charged oxygen vacancies (V0+) in controlling the above hydrophobic behavior. However, in any case no significant change was found in the measured contact angle hysteresis with respect to the one in as-grown sample, and it was found to be about 20° (within an error of ± 2°). In fact, the existence of a large adhesive force along with observed modification in wetting behavior of TiO2 nanorods on Si pyramids under ion beam irradiation was discussed in the framework of Cassie impregnating state in the presence of surface oxygen vacancies.

Acknowledgements


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The authors, especially AK would like to acknowledge the financial support received from Shiv Nadar University, and also the Alexander von Humboldt Foundation for purchasing contact angle measurement system. The help received from the scientists at Inter-University Accelerator Centre, and the support of the HLD at HZDR, the member of the European Magnetic Field Laboratory (EMFL) is highly acknowledged.

Supporting information Defect induced color changes in TiO2 layer (Figure S1), STEM-HAADF image of S4 sample (Figure S2). The ion beam induced variation of apparent contact angle of TiO2 layers on planar Si surfaces is shown in Figure S3. The videos taken duering the measurement of advancing and receding contact angles are presented in Video-S1 and Video-S2, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.



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TOC Graphic


Role of Oxygen Vacancy on the Hydrophobic Behavior of TiO2

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