Candle-Soot Derived Photoactive and Superamphiphobic Fractal

Oct 24, 2016 - Carbon soot is one of the oldest materials known for its hydrophobic properties, robustness, and availability, making it an ideal mater...
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Candle-Soot Derived Photoactive and Superamphiphobic Fractal Titania Electrode Ahmad Esmaielzadeh Kandjani,† Ylias M. Sabri,*,† Matthew R. Field,‡ Victoria E. Coyle,† Rynhardt Smith,† and Suresh K. Bhargava*,† †

Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, GPO Box 2476 V, Melbourne, Victoria 3001, Australia ‡ RMIT Microscopy and Microanalysis Facility (RMMF), RMIT University, Melbourne, Victoria 3001, Australia S Supporting Information *

ABSTRACT: Carbon soot is one of the oldest materials known for its hydrophobic properties, robustness, and availability, making it an ideal material for use in various applications. The drawbacks, however, are the loose structural binding between constructing carbon nanoparticles and the amorphous nature of soot itself. In this paper, we present a facile chemical vapor deposition (CVD) method that maintains the soot template structural integrity and enables its modification into a highly photoactive, self-cleaning titania fractal network. The results show that the small air pockets available on the surface combined with the salinization process produces a TiO2 fractal network with superamphiphobic properties. Given the high surface area of the fractal network structure and titania’s well-known photocatalytic activity, the designed surfaces were assessed for their photocatalytic decoloration activities. The results showed that the soot template derived TiO2 films can offer enormous potential in many different applications where self-cleaning and/or high surface area and photoactive properties are required.

1. INTRODUCTION The ability to synthesize multifunctional materials with tailored properties has been at the forefront of breakthroughs observed in nanotechnology development in recent years. Furthermore, the physical properties like morphological modification and packing of nanoparticles in thin film fabrication as well as structural properties such as modification of the electronic band structure and the control in crystal phase can allow for tuning their performance for a specific application.1−6 Among the numerous semiconductor materials available, TiO2 nanostructure based surfaces have been extensively used to develop technologies for many applications including water purification (through photo/electrocatalysis),7−9 solar cells,10−12 lithiumion batteries,13−15 water splitting,16−20 and self-cleaning surfaces.21,22 Therefore, when considering the development of materials for these applications, it is important to develop substrates that encompass high surface area, excellent structural durability, and simple manufacturing processes to enable largescale production.13,23 Among these applications, the selfcleaning surfaces have attracted much attention as they have revolutionized many different fields of study such as antimist, anti-icing, antibacterial, oil/water separation, microdroplet manipulation, and corrosion resistance applications.24−30The self-cleaning properties of TiO2 arise as a result of the reduction in the surface tension due to the physio/chemical surface interactions when in contact with water as well as the photocatalytic degradation of the residual organic compounds adjacent to the surface upon activation of the material.22 © 2016 American Chemical Society

There are also many self-cleaning surfaces (i.e., lotus leaves, bird’s feathers, insect’s wings, and body parts, etc.) that exist in nature where microscopic and wettability studies of such natural superhydrophobic structures have revealed their physical and chemical aspects that give them such properties.31,32 These properties arise from their surface morphological designs where structures with specific dimensions are patterned throughout the substrate and are responsible for the trapping of air into small air pockets between the substrate and the liquid (Cassie’s state).33−35 One of the earliest discovered hydrophobic materials is carbon soot that evolves from fire due to the incomplete combustion of carbohydrate fuels.36 The nanostructured fractal morphology of the carbon soot layer deposited on a substrate usually consists of small carbon based particles (generally >60% carbon content depending on the fire source) ranging from 1 to 100 nm.37,38 Stacking multiple layers of these nanoparticles develops into fractal type structure networks with hydrophobic properties due to the particular spacings between the structures that allow air to be trapped and halt water diffusion upon the deposition of a water droplet.36,38−40 Although the soot nanostructures have been known for centuries, their application has been limited to chemical absorption and fluorescence as they are formed as loosely packed nanoparticles.41,42 The soot layer itself consists Received: August 23, 2016 Revised: September 28, 2016 Published: October 24, 2016 7919

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Chemistry of Materials of loosely stacked nanoparticles that form a dark film formation on a substrate. The structure can disassemble through the application of small friction or movements such as the rolling of small water droplets which can rip the carbon soot particles from the structure during its movements.36 Recently, a method by which such disassembling can be avoided was introduced by Xu Deng et al. where they coated the soot structure with a silica layer via chemical vapor deposition.43 As the deposition was carried out at conditions involving atmospheric pressure and room temperature, the coating of silica on soot nanoparticles did not alter the morphology of the soot layer. This resulted in the physical structure to trap air in the fractal voids being retained in the silica structure thereby demonstrating the fabrication of robust and durable superhydrophobic surfaces following the functionalizing of the surface with self-assembly monolayer (SAM) organic structures.34,43,44 Interestingly, due to the structural durability and the morphology of the soot template, these surfaces not only showed superhydrophobic properties but also exhibited a promising oil repelling property making them real superamphiphobic structures. This property has increased the number of surface functionalities, making them useful in many applications including the formation of single polymer bead microsphere,45 macro-crystal,46 hydrophobic nozzles,25 microdroplet manipulator,24 and the development of artificial lungs for oxygenating blood.47 Although the soot templated silica surfaces have proved to exhibit enhanced performance in many applications, the number of functionalities is thought to multiply given that silica is replaced by semiconducting materials. To date, such attempts have remained a challenge; however, there have been many efforts to develop simple, durable, and reproducible semiconducting electrodes that encompass high surface areas. This is important when considering that most semiconductor electrode applications involve the interaction of the surface of the electrode with its surrounding media. Therefore, a large surface area and homogeneous spread of the structures would ensure a relatively higher number of electrons transferring over the entire electrode surface. Many studies are reporting the fabrication of semiconducting substrates such as TiO2 using different methods, some of which include block copolymer templating, monodisperse TiO2 based material stacking, and one-dimensional TiO2 nanoarrays.10,13,48−50 Although high surface areas and seamless structures can be achieved, these methods need multistage, and complicated synthesis approaches thereby making them unsuitable for industrial and large scale productions. Furthermore, these methods usually result in amorphous TiO2 structures that need calcination under high temperatures in order to crystallize the amorphous material, thereby taking advantage of the semiconducting properties of TiO2. However, such heat treatments can have significant drawbacks as they can result in the enlargement of the structures and change the topology of the final surface.50 To overcome these drawbacks, it is essential to employ a templating method that can withstand high temperatures without altering the material’s surface morphology while forming reproducible semiconducting material morphologies allowing for an extended list of applications that can be explored.50 Inspired by the high surface area and extraordinary water/oil repelling properties of the fractal structure of candle soot, here we introduce a simple approach for uniform deposition of TiO2 shells on the soot layers to form high surface area electrodes with photoactive properties. The modified chemical vapor

deposition (CVD) method used for TiO2 fractal structure formation in this study allows for the uniform synthesis of amorphous TiO2 structures on soot materials in less than 30 min. As soot structures are stable up to temperatures of 600 °C, the fractal structure of TiO2 was retained through the 65 °C CVD process. This process allows for the formation of a semiconducting layer of TiO2 instead of the nonconductive SiO2, allowing for a wealth of functionalities to be added to the photoactive electrode with tunable surface area enhancement and water/oil-repelling properties.

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RESULTS AND DISCUSSION The schematics shown in Figure 1 depict the synthesis procedure for fabricating fractal TiO2 surfaces. Briefly, the

Figure 1. Schematic procedure of depositing TiO2 based fractal layer.

soot layer is deposited from a candle onto the substrate. Thereon, a TiO2 layer was evaporated through the CVD process (10 sccm of humidity introduced into a 65 °C reaction vessel) and onto the soot structure forming soot/TiO2 core/ shell structures. Annealing was then performed at 650 °C to remove the soot template with the resultant structure being fractal TiO2 structures formed on the substrate. SEM images of the fabricated substrates are shown in Figure 2 and Figure S3. The soot layer consists of small carbon based nanoparticles with sizes ranging from 1 to 100 nm. The SEM images demonstrate that the structures are formed uniformly throughout the surface of the substrate. The EDS analysis was carried out on aluminum foil (EDS analysis of Al-foil is shown in Figure S4) whereby the samples were prepared by depositing soot and titania/soot on aluminum foil that was fixed on a glass slide. The EDS results for soot (Figure 2a3) showed that the soot is mainly formed from carbon, oxygen, and some silicon contamination.36 The deposition of titania onto the soot layer proved to retain the original structure of the soot template intact (i.e., ST20 before annealing shown in Figure 2b1 and b2). The Ti peaks observed when undergoing EDS analysis from various spots on the substrate (e.g., shown in Figure 2b3) prove the formation of titania across the surface of the substrate. Following the annealing of the soot/titania sample at 650 °C, the soot template had decomposed off from the material; however, the fractal structure of soot was maintained, and a hollow titania layer had formed on the substrate. Although the morphology of the TiO2 structures formed was similar to those reported recently for SiO2 fractal structure deposition,43 the latter was deposited over a 24 h process (as opposed to 10−30 min process in this study) and did not possess semiconducting properties as is otherwise the case for the TiO2 structures developed in this study. An analysis of the EDS data for the same sample showed that the ratio of the constituents (obtained from the relative peak intensities between the different elements) changed following the annealing process. A drastic reduction in the carbon and silica peak intensities and an increase in the oxygen and titanium 7920

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Figure 2. SEM images and EDS spectra of (a) soot layer; (b) soot/TiO2 core/shell (ST20 before calcination); (c) ST20 and (d) side view image of ST20 deposited on silica substrates. The notation 1 is for low and 2 for high magnification SEM images; 3 is for EDS spectrum of the same material deposited on aluminum foil.

peak intensities were observed for the ST20 sample thereby confirming the removal of the soot template from the material. The additional advantage of the proposed method is the uniform layer of the semiconducting material coating that is formed on flat substrates. As shown in Figure 2d1 and d2, the fractal layer that is formed through 30 s of soot deposition followed by 20 min of titania deposition before calcination at 650 °C forms an ∼8 μm thick layer of seamless TiO2 fractal structure over the entirety of the substrate. The SEM images of other soot based titania fractal samples (i.e., ST5, ST10, and ST30) are presented in the Supporting Information Figure S3a−c. The results showed that fractal structures were formed in all samples. In the case of shorter times of titania deposition (i.e., 5 min (ST5)), a dense layer of titania is not formed on all the soot fractal structure template available on the surface which leads to the soot from these areas being removed during the calcination process. On the other hand, a relatively higher time of deposition (i.e., 30 min (ST30)) lead to the individual fractal branches connecting to each other, forming a dense and thick network of titania that is undesirable due to such structures tending to have less surface area and hydrophobicity. Finally, when no soot layer was present, a thin flat layer of titania consisting of small titania particles was formed on the glass substrate as shown in Figure S3d. To understand better the TiO2 layer formation process during calcination, pure soot and soot/titania core/shell materials were scratched from the glass substrates and TGA analysis was performed (the results of which are shown in Figure 3a). The results indicated that the formed candle soot is stable in the structure up to a temperature of 600 °C before it starts to decompose at higher temperatures.38 Interestingly, unlike soot, an initial drop appears up to 110 °C when titania is deposited on the soot structure. This weight loss at low temperatures indicates that the soot/titania core/shell structure had changed from a hydrophobic to more hydrophilic structure (due to the presence of TiO2) thus having adsorbed some water molecules from the surrounding air and onto the surface. This is not the case with soot due to the physical structure and chemical content resulting in its hydrophobic nature and lack of water sorption. By increasing the temperature to ∼580 °C, a second weight drop in the core/shell structure starts to appear and is attributed to the soot degradation process. The soot/

Figure 3. (a) TGA analysis of soot and soot/TiO2 (ST20); (b) XRD patterns of soot, ST20 before calcination, and ST20 after calcination deposited on glass substrates.

titania structure is observed to lose around 70% of its weight during the calcination process, with the remaining being the fractal TiO2 shell that has formed after decomposition of the soot template. Figure 3b shows the XRD structure of soot, soot/titania (ST20 before calcination), and TiO2 fractal structures (ST20) that are deposited on glass substrates. The XRD patterns show that the soot layer and soot/titania are both amorphous. Following the calcination process at 650 °C, the formation of the anatase phase (JCPDS File No. 21-1272) was observed without any indication of other crystalline phases in the structure. The HT-XRD studies (Figure S5) related to ST20 deposited on the silicon substrate clearly shows that after 1 h of calcination at 500 °C, the TiO2 shells start to change from an amorphous to a crystalline structure. As the removal of the soot is important for better optical properties and higher surface area aspects, the calcination process was carried out at 650 °C to ensure the complete removal of the soot template. The structure morphology and formation of the developed materials were studied before and after calcination using TEM and HR-TEM analysis. Figure 4 shows the TEM images of the soot deposited directly on TEM grid (Figure 4a), ST20 before calcination deposited directly on TEM grid (Figure 4b), and ST20 after calcination prepared by scratching from the substrate and followed by sonication in ethanol (Figure 4c). The discrete soot particles with sizes ranging from 1 to 100 nm are connected and surrounded by a shell of TiO2 layer (Figure 7921

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Figure 4. TEM images of (a) soot deposited on TEM grid, (b) ST20 before calcination where titania covers multiple soot carbon particles, and (c) ST20 after calcination followed by scratching and sonication in ethanol; HR-TEM images of TiO2 shells (d) before and (e) after calcination. The inset micrographs are SAED patterns of the respective HR-TEM areas shown in each figure; (f) STEM-HAADF and respective EELS mapping for carbon (green) and titanium (red) of soot/TiO2 core/shells.

morphology (see Figure S3) that provide the water/oil repelling properties associated with these structures. XPS analyses were carried out to determine the possible contamination effect or doping effect that may have taken part in the ST20 structure after the calcination process. The XPS survey spectra presented in Figure 5a shows the formed TiO2

4a,b). During the TiO2 CVD process, a layer of the amorphous TiO2 covers loosely connected carbon soot particles and trap the carbon soot inside. Figure 4d shows the HR-TEM image of the deposited TiO2 layer before calcination where no ordered orientation of the atoms can be seen in the structure, indicating that the formed system is amorphous in nature. The inset image representing the selected area electron diffraction (SAED) pattern of the sample shows no diffraction pattern, thereby confirming the nature of the titania deposited through the modified CVD method. The calcination process resulted in the soot template decomposing and leaving the structure (Figure 4c) while simultaneously converting the TiO2 particles into the crystalline form. To confirm the latter, the soot and soot/TiO2 samples were synthesized directly onto the TEM copper grids. The calcined soot/TiO2 (ST samples) could not be formed directly on the TEM Cu grids due to their lack of temperature stability at 650 °C and were first formed on glass substrates, scratched, and sonicated in ethanol before being drop cast on the TEM grids. Although this process destroyed the fractal structure of TiO2, the study of the collected TiO2 shell nanoparticles with TEM confirmed their crystalline nature. The HR-TEM of ST20 shown in Figure 4e demonstrates the formation of crystalline structures of calcined TiO2 and the lattice resolved TEM image with d-spacing of 0.32 nm is an indication of the (110) anatase phase.20 The SAED pattern shown in the inset confirmed the formation of anatase crystalline phase of TiO2 shells with the related (101), (004), (002), and (211) rings shown relating to the anatase phase. Energy electron loss spectroscopy (EELS) was performed to gain a better understanding of the uniformity of the TiO2 layer formed on the soot nanoparticles. The EELS map referring to the ST20 sample before the calcination process is shown in Figure 4f as well as in the Supporting Information, Figure S6. Two distinctive layers of carbon and titanium can be observed to a have formed in the structure. Titanium EDS mapping shows a uniform coverage of TiO2 on discrete carbon particles. It is important to obtain an optimum thickness of TiO2 in order to maintain the original fractal structures of the carbon soot template. That is, a layer too thin fails to retain the template structure as discussed above; however, an increased deposition time relates to a thicker TiO2 shell as well as the filling of the fractal structure cavities. Both of these cases are undesirable as they would result in a reduced surface area and absence of

Figure 5. XPS spectra (a) broad energy survey, (b) high-resolution Ti 2p, and (c) O 1s core level spectra; (d) valence band spectra of ST20 and TT20 (after calcination).

layer with no impurities detected. The carbon peak (C 1s) shown is attributed to the adventitious carbon absorbed on the structure. The high-resolution XPS spectra for Ti 2p and O 1s binding energies are provided in Figure 5b,c. The binding energy of 458.7 eV is related to the formation of Ti4+ species in the TiO2 structure.51 Two separate binding energies of 529.8 and 531.7 eV in O 1s spectrum are related to the formation of Ti−O and surface hydroxyl groups on the titania structure.51 This data was typical for all sample types (ST and TT) where the data is shown in the Supporting Information, Figure S7. 7922

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Figure 6. (a) Deposition time dependent thicknesses of TiO2 films on silica substrate (TT5, TT10, TT20, and TT30) using the modified CVD process, (b) UV−vis transmittance spectra of thin film TiO2 films (TT samples), (c) UV−vis transmittance spectra of soot template TiO2 (ST samples), and (d) static contact angle test of ST10 (1) before, and after silane treatment toward (2) water and (3) hexadecane (5 μL droplet).

superamphiphobic properties renders the develop materials as excellent candidates for self-cleaning applications.28,52 The fact that these superamphiphobic surfaces can also be hydrophilic (without salinization process) while having a seamless 8 μm TiO2 layer with high surface area and semiconducting properties makes them very ideal electrodes for photoelectrochemical applications. The samples were first assessed for their photocatalytic degradation property toward RhB as a representative dye for azo dye pollutants.53 Figure 7a,b shows the degradation process of the dye under UV without and with the presence of ST20 as the photocatalyst, respectively. The degradation of the dye without any photocatalyst is negligible; however, the presence of ST20 in the solution results in almost 80% of the dye being removed during a 2 h UV illumination period. Photocatalytic decoloration activity calculations also revealed that the ST samples have higher photocatalytic activities than the thin film (TT samples) counterparts (Figure 7c). Furthermore, the ST30 and ST20 were found to have the highest photocatalytic activities among all samples tested. The higher photocatalytic activity of these materials can be due to two related phenomena. First, the surface area is increased resulting in a higher density of free radicals formed during photoexcitation54 and second, the higher surface area and therefore available TiO2 content results in relatively more UV photons being absorbed in the structure.54 To understand better these photoelectron processes that occur during the photocatalytic experiments, the ST samples were employed for photoelectrochemical (PEC) tests. The linear sweep spectra of the samples from −0.2 to +1.0 V vs Ag/AgCl are shown in Figure 7d. The data demonstrates that a thicker and relatively denser TiO2 structure on the soot template (through increased deposition time) results in relatively higher photocurrent densities. This is the reason that a higher photocatalytic activity is observed from the ST30 sample among all other samples; however, the enhancement is only slightly higher than the ST20 sample. This indicates that there is a limit to which deposition time and thus TiO2 thickness can be increased for enhanced photocatalytic

The valence analysis revealed that there is no possible carbon or silicon doping that has occurred, which is evidenced by the similar valence band values of ∼2.4 eV for both TT20 and ST20 samples. The deposition rate of TiO2 formed during the modified CVD process was determined by analyzing the TT samples (without soot template) under the surface profilometer, which is presented in Figure 6a. The thickness of the TiO2 layer increased from 61.8 to 206.3 nm when undergoing 5 to 30 min of deposition. Although the thickness of the samples had increased with CVD time, they showed similar optical behavior with more than 70% transparency at wavelengths more than 300 nm, as presented in Figure 6b. The band gap of these materials were also similar at ∼3.3 eV. However, the UV−vis transmittance spectra in the case of the ST samples followed different profiles with increasing deposition time (Figure 6c) which is due to the deposited TiO2 layer being formed on the ∼8 μm soot template rather than a flat glass substrate. The fact that the ST5 absorbance profile is similar to TT5 is due to the thin nature of the TiO2 film formed in both cases thereby forming transparent surfaces with relatively low scattering following the calcination process. The use of higher CVD deposition times to form thicker layers of TiO2 fractal structures results in the drastic drop in the transparency of the samples. This is not only due to the formation of a dense TiO2 fractal layer that is ∼8 μm in thickness but also due to increased light scattering when compared to flat thin films of TiO2. The ST10 sample was selected for contact angle studies due to its good transparency and uniform TiO2 coverage, maintaining the soot template structure. When ST10 was treated with the silane molecule, the hydrophilic surface of TiO2 (contact angle of 20° as shown in Figure 6d1) transformed into a superamphiphobic layer (contact angle > 150°) with contact angles of 165° and 154° for water and hexadecane, respectively. The superamphiphobic property arisen from the TiO2 maintaining the soot template fractal structure following CVD and annealing processes. The semiconducting property of TiO2, high transparency, and 7923

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CONCLUSIONS

In this research, we have shown a simple method to fabricate high surface area TiO2 thin film surfaces with tunable hydrophilic/superamphiphobic and photoelectrocatalytic properties. The results showed that using candle soot as a structural template can produce fractal networks of 1−100 nm carbon based nanoparticle assemblies which can be coated with titania via a modified chemical vapor deposition method. SEM, TEM, and HR-TEM analysis showed a uniform formation of TiO2 fractal structures with high purity and stability (confirmed by XRD, XPS, and TGA studies) after the removal of the soot template. The fractal structure provides the superamphiphobicity where >150° of contact angle for both water and oil droplets were observed. Given the structure is composed of interconnected fractal network structures of semiconducting material, it can be used for a variety of applications desiring high surface area and excellent stability such as photocatalytic substrates and as electrodes for lithium-ion batteries, solar cells, and photoelectrochemical water splitting. In this paper, the photocatalytic activity of the soot derived TiO2 fractal network was explored by using Rhodamin B degradation to represent organic pollutants followed by in-depth photoelectrochemical characterisations. The results showed that the soot templated TiO2 surfaces (30 min CVD dubbed as ST30) showed ∼3 times higher photocatalytic activity when compared to its thin film control (TT30) counterpart.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Substrates. All chemicals were purchased from Sigma-Aldrich and used as received. Milli-Q water (18.2 MΩ· cm−1) was utilized for all synthesis and testing procedures. All substrates including glass slides, quartz, silicon wafers (1 cm × 1 cm), and fluorine-doped tin oxide (FTO, 8 Ω/sq, Dysol) were washed with acetone and ethanol before being dried under nitrogen before use. 2.2. Synthesis of Fractal TiO2 Structures. A layer of candle soot was deposited on the substrate over a 30 s period with a distance of 2 mm away from the center of the candle flame. The candle used for soot deposition was an unscented household white candle. The CVD synthesis of TiO2 layers was carried out in an in-house built simple glassware setup as shown in Figure S1. A 500 μL volume of titanium(IV) isopropoxide (TTIP) was added to a 100 mL round bottle flask, and the mixture was kept at 65 °C under a nitrogen atmosphere in a water bath. The substrate was placed over the TTIP solution with the side containing the soot layer facing down. This was necessary to avoid large particle formation and deposition that occur due to condensation above the substrate level within the flask. The types of undesired TiO2 surfaces that form on the substrate side facing up are shown in Figure S2. The water vapor necessary for TTIP conversion to TiO2 was carried from a dreschel bottle and into the reaction flask through N2 carrier gas where the water content was introduced with a constant flow rate of 10 sccm (mL/min) using a mass flow controller. The reaction was carried out for 5, 10, 20, and 30 min at 65 °C. At the end of the reaction period, the water vapor stream was closed, and pure N2 gas was purged (at 2000 sccm) into the reaction flask in order to remove all vapor content and halt the reaction. The reaction vessel was then cooled down to room temperature, and the substrate was removed from the chamber for further analysis. In addition to the samples with soot layers (abbreviated with ST), TiO2 layers were also deposited on plain substrates without any soot layer (abbreviated with TT) and used as reference samples. The samples were named as TT5, TT10, TT20, and TT30 for thin film reference substrates and ST5, ST10, ST20, and ST30 for soot templated substrates with the numbers representing the 5, 10, 20, and 30 min deposition times; respectively. To transform the deposited TiO2 into a crystalline phase TiO2 and simultaneously remove the underlying soot structure, the synthesized substrates were

Figure 7. UV−vis absorbance of RhB in the (a) absence and (b) presence of ST20 photocatalyst; (c) photocatalytic activity of all samples when measuring the photodegradation of RhB; (d) linear sweeps voltammogram of ST samples in the dark (blue color) and under UV illumination (red color); (e) transient photocurrent density under chopped illuminations of UV at 0.5 V potential vs Ag/AgCl reference electrode involving 10 s intervals between on/off switching; (f) EIS spectra with and without UV illumination under a DC potential of 0.5 V.

activity. The transient photocurrent measurements also showed that the ST30 and ST20 have the highest current density (Figure 7e). The measurements of three consecutive on/off cycles show that the current density is stable, and thus the fabricated material has excellent stability and repeatable response.17,55 The resistance of semiconductor and electrolyte can be explored using EIS studies at constant voltage and under both light and dark conditions. The EIS measurements were based on Nyquist plots, and the results are shown in Figure 7f where the imaginary and real parts of the impedance are plotted against each other. All samples showed similar behavior that is due to the similar materials and fractal structures within each sample resulting in similar resistance between the electrolyte and the electrode.55 7924

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Chemistry of Materials calcined at 650 °C for a 1 h period in air. The superamphiphobic property was introduced by transferring the fabricated samples into a vacuum desiccator that housed a small open beaker containing 200 μL of semifluorinated silane. The desiccator was closed and kept for 3 h while under vacuum in order to increase the vapor pressure of the silane complex. After the salinization process, the open beaker was removed, and a vacuum applied once again for 1 h to remove the loosely adhered silane residues from the surface. 2.3. Characterization. For morphological studies scanning and transmission electron microscopy (SEM and TEM) techniques were utilized. SEM studies were performed using an FEI Verios XHR-SEM instrument that was operated at an accelerating voltage of 5 kV and a beam current of 50 pA. The SEM was equipped with an energydispersive X-ray spectroscopy (EDS) detector (Oxford Instruments XMaxN-80T). All SEM studies were carried out on glass substrates whereas the EDS characterizations were conducted by employing aluminum foil as the substrate in order to avoid any overlaps from the Si peak associated with glass. TEM and high resolution-TEM (HRTEM) analyses were performed using a JEOL-1010 and JEOL-2100F with acceleration voltages of 100 and 200 kV, respectively. HR-TEM images were obtained while operating in TEM mode using a Gatan Orius SC1000 CCD camera. Electron energy loss spectroscopy (EELS) mapping was collected using a Gatan Imaging Filter (GIF) Tridium using a 2 nm step size. All HR-TEM samples were deposited directly on carbon-free Cu grids; however, the calcined TiO2 sample was scratched from the glass substrate, suspended in water through a sonication technique, and drop cast on holey carbon grids. The crystalline structures of the as-prepared samples were analyzed using a Bruker D8 Discover microdiffraction system with a Cu Kα radiation source (40 kV, 40 mA) and a general area detector diffraction system (GADDS) as well as a D8 Advance diffraction system that can undergo XRD analysis at high temperatures ranging from 30 to 700 °C (HTXRD). The GADDS based XRD analysis was carried out at room temperature on glass substrates whereas HT-XRD analysis was conducted on a silicon wafer from 20° to 60° with 0.02° step size. X-ray photoemission spectroscopy (XPS) analyses were used to study chemical bonding and high-resolution valence-band studies using a Thermo K-Alpha XPS instrument equipped with an Al Kα monochromated X-ray radiation source. The samples prepared on FTO substrates were used for XPS analysis. The flood gun was turned on for charge neutralization; however, it was turned off for experiments where the data was used for material valence band analysis. The XPS alignment was checked with internal Au standard before analysis. All spectra (except valence-band) were background corrected using the Shirley algorithm, and all binding energies (BE) were aligned considering adventitious C 1s has a BE of 285 eV. Thermogravimetric analysis (TGA) was used (PerkinElmer TGA-7) to study the temperature required to remove the soot template from the structure. The tests were performed by increasing the temperature from 35 to 850 °C using a heating rate of 10 °C/min in air and observing the resulting mass loss from. Optical properties of the prepared samples, as well as photocatalysis decolouration studies, were carried out using a UV−vis spectrophotometer (Varian Cary 60) operated at a stepsize of 2 nm. For UV−vis studies, the CVD was performed on one side of optically polished quartz substrates whereas a masking tape was used for the back side to avoid the formation of an unwanted TiO2 layer. The thickness of the evaporated TiO2 layer on the reference samples (TT5, TT10, TT20, and TT30) was measured using a surface profiler (Tencor P-16+). For thickness measurements, half of the silicon substrate was covered with a photoresist before CVD was carried out on the samples. After the CVD process, the photoresist and the TiO2 layer on it were removed with a lift-off process that involved immersing the substrate in acetone followed by a thorough wash and dry in order to form the step-edge for thickness measurements. Sessile drop method was used for contact angle and wettability studies at room temperature with water and hexadecane employing contact-angle system (OCA20, Dataphysics Co., Germany). The contact angle measurements were carried out on at least three different points of the substrates to ensure the consistency of the measurements. All electrochemical measurements were performed

using a three electrode system (Vertex potentiostat/galvanostat; MIUM technologies) with Ag/AgCl as a reference electrode, Pt wire as a counter electrode, and FTO substrate (∼1 cm2 of CVD material immersed in solution) as the working electrode. 2.4. Photocatalysis and Photoelectrochemistry Measurements. The photocatalysis activity of the samples was evaluated using Rhodamine B (RhB) as an organic pollutant at 25 °C. The material coatings were performed on one side of glass slides (2 cm × 2 cm) whereas the back side was masked with Kapton tape to avoid the formation of TiO2 thin film on the back of the glass substrate. Photocatalysis experiments were carried out using 5 mL of RhB solution (7 mM) using UV LED lights with a wavelength of 370 nm and 60 W/m2 output power (Edmon Optics). The dye concentration was calculated with a calibration curve considering UV−vis absorbance of RhB at 552 nm. Each sample was kept in RhB solution for 30 min in dark to allow absorption of the dye onto the surface of the catalyst. The degree of photodegradation activity of the prepared samples was calculated as ((C0 − C)/C0) × 100, wherein C0 and C correspond to dye concentrations derived from the calibration curve before and after irradiation, respectively. Photoelectrochemical (PEC) analysis was carried out in a quartz chamber using the same LED (370 nm, 60 W/m2) as that used for photocatalysis. An aqueous solution of 1.0 M NaOH was used as the supporting electrolyte. The voltammetric measurements were carried out in the potential range of −0.2 to +1.0 V (vs Ag/AgCl) with a scan rate of 10 mV/s. Electrochemical impedance spectroscopy (EIS) was measured in 0.5 V potential with frequencies from 10 to 105 Hz, with an amplitude of 5 mV around the tested potential.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03537. Schematic figure for synthesis setup, supporting SEM images, EDS spectrum for Al foil, HT-XRD patterns, EELS mapping details, XPS spectra for ST20 and TT20 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Y.M.S. E-mail: [email protected]. *S.K.B. E-mail: [email protected]. Phone: +61 3 99252330. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge RMIT Microscopy and Microanalysis Facility (RMMF) for the help received from their technical staff and for allowing the use of their comprehensive facilities and services.

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REFERENCES

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