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Sub-10 nm Mixed Ti/Ta Oxide Nanoporous Films with Visible Light Photocatalytic Activity for Water Treatment Andrea Merenda, Lingxue Kong, Narges Fahim, Abu Sadek, Edwin Lawrence Harrop Mayes, Adrian Hawley, Bo Zhu, Stephen Gray, and Ludovic F Dumée ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02350 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Applied Nano Materials

Sub-10 nm Mixed Ti/Ta Oxide Nanoporous Films with Visible Light Photocatalytic Activity for Water Treatment

Andrea Merenda1,*, Lingxue Kong1, Narges Fahim1, Abu Sadek2, Edwin LH Mayes3, Adrian Hawley4, Bo Zhu5, Stephen R. Gray5 and Ludovic F. Dumée1*

1

Deakin University, Geelong, Institute for Frontier Materials, Waurn Ponds 3216,

Victoria, Australia 2

Melbourne Center for Nanofabrication (MCN) Institute of Biotechnology, 151

Wellington Road, Clayton 3168, Victoria, Australia 3

RMIT Microscopy and Microanalysis Facility, RMIT University, RMIT University

GPO Box 2476, Melbourne, 3001, Victoria, Australia 4 Australian 5

Synchrotron, ANSTO Clayton 3168, Victoria, Australia

Institute for Sustainable Industries & Liveable Cities, VU Research, Victoria

University, PO Box 14428, Melbourne 8001 Victoria, Australia

*

Corresponding author:

Ludovic F. Dumée ([email protected]) Andrea Merenda ([email protected])

1 ACS Paragon Plus Environment

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Abstract In the present work, anodic Ti/Ta mixed oxides nanotubes are prepared for the first time with sub-10 nm surface pore size and tube inner diameter. The morphological changes induced by the introduction of Ta into the Ti metal matrix are investigated, leading to remarkable geometrical variations dependant on the Ta loading. The UV-light activation necessary to trigger the electron transfer in TiO2 limit the range of applications and the shift in light absorption towards the visible range represents a significant challenge. Here, the band-gap of the as created nanotube thin film arrays are calculated and the results, showing the presence of a minimum in the band-gap, correlated to the presence of Ti and Ta suboxides and Ta loading. The potential of the thin films as an advanced material for photocatalytic water treatment is tested against pure TiO2 and an enhancement in visible light absorption as well an almost threefold increase in degradation kinetics under pure visible light irradiation are demonstrated.

Keywords: sub-10 nm nanotubes, doped titania, oxygen vacancies, band-gap engineering, catalytic thin films


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Introduction Over the last few decades, TiO2 has surged as an attractive material for photocatalytic applications in water treatment domain due to its intrinsic activity under UV irradiation.1-2 Nanostructured TiO2 materials including nano-particles, sheets or fibres offer exceptional properties that can be beneficial in various fields where a high surface area to volume ratio is required. TiO2 nano-architectures were efficiently implemented in applications such as dye-sensitized solar cells, sensors, membrane separation and photo or electrocatalytic degradation of organic pollutants.3-5 In these cases, the investigation of surface and interface properties plays an essential role to provide and eventually enhance the expected efficiency.6-8 Properties such as well-defined and homogeneous surface morphology, high specific surface area and facilitated electronic transfers are required to improve the catalytic efficiency in photo and electrocatalysis.7,

9

To address these requirements, titanium dioxide nanotubes

(TiNTs) fabricated by electrochemical anodization have been studied and their morphological and geometrical aspects, such as porosity, tube density, length and even interfacial grain boundaries deeply investigated.10 TiNTs with pore size distributions from 12 to 350 nm, wall thickness from 5 to 34 nm and tube length from the nanoscale to microns have been processed and shown to be promising platforms for nano and micro fluidics applications.11 The major challenge limiting TiO2 catalytic activity is its band gap as UV-light irradiation is required to trigger the electron-holes generation in the semi-conductor.7, 12

A promising way to modify the optical properties and shift the light activation

consists of doping the TiO2 lattice with a foreign atom, such as a non-metal element13 or a transition metal.5, 14 Commercial Ti alloys can represent a valid option due to the pre-existent microstructure containing one or more foreign doping elements, avoiding



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expensive, time-consuming post-treatment chemistry.15-16 Transition metals such as V, Nb and Ta have been widely used to fabricate Ti alloys because of their stability and easy incorporation into the Ti lattice.17-19 Among these candidates, Ta emerges as a valuable option for its properties as the capability of enhancing the corrosion resistance20-21 and effect on reducing the TiO2 band gap.19, 22 Although commercial TiTa alloys, typically fabricated by arc-melting, are commercially available, limited control over chemical or microstructural properties is provided, warranting further thermal or chemical treatments to achieve the desired properties.23 Conversely, fabrication routes such as physical vapour deposition (PVD) sputtering can offer a flexible and versatile solution to create a customized alloy with controlled surface morphology and chemical composition.24-26 PVD sputtering has been beneficial to fabricate thin films with enhanced electronic and optical properties,27 nevertheless no reports are available on PVD sputtering of Ti alloy to investigate the properties of TiNTs upon electrochemical anodization. Furthermore, little or no studies are available on the impact of Ta content on the anodization process of Ti-Ta alloys and final geometrical and morphological peculiarities. In this work, the fabrication of sub-10 nm nanoporous Ti/Ta mixed oxides substrates by electrochemical anodization with excellent catalytic activity upon irradiation with visible light is reported. The addition of Ta and its final content into the Ti matrix, the surface oxidation state of the two metals as well as alloy morphology tailored by the PVD deposition conditions, are correlated to the electrochemical process to evaluate the final surface morphology upon anodization and optical properties of the substrate. These parameters are critical for an efficient, cost-effective application in advanced oxidation processes by ruling out UV-light activation and extending potentially to other applications where the use of sun-light as radiation source is


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desirable, such as solar cells, H2 generation by photocatalytic water splitting, CO2 photo-conversion for fuel production and state-of-the-art battery technology.28-31



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Experimental

Materials The 4-inch Si (100) wafers were diced into 1.8 cm x 2 cm pieces using an automated dicing saw (Disco DAD321) at Melbourne Center for Nanofabrication (MCN), Melbourne, Australia. The Ti and Ta sputter targets were acquired from Ezzi Vision with 99.99% purity. Ethylene glycol (>99.8%), ammonium fluoride (>99.99%) ethanol (>99.5%), acetone (>99.5%) were purchased from Sigma-Aldrich, Milli-Q water was purchased from Merck Millipore, and used without further purifications. Methylene blue (>99.5%) for the catalytic tests was purchased from Sigma-Aldrich.

Physical Vapour Deposition of Ti/Ta alloy The deposition of Ti/Ta alloy was performed on an INTLVAC Nanochrome I AC/DC sputtering rig at the MCN. Two Ti targets were mounted on the AC magnetrons, while a Ta target was positioned on the DC one. Prior to the deposition, the chamber was cleaned with a vacuum cleaner to reduce the risk of cross-contamination from metal residues and 2-min warm up was performed on the targets to remove the top layer of the surface. Ultimately, the chamber was pumped down to 5 × 10-6 T before proceeding with the deposition. The applied power on the AC Ti targets was fixed at 300 W while a series of deposition power was considered on the DC Ta, spanning from 0 to 250 W. The deposition took place at a pressure of 2 × 10-3 ± 0.1 T, with Ar at a nominal flow of 20 sccm as a carrier gas. The duration was set to 60 min for each deposition set up.

Electrochemical anodization The anodization experiments were carried out in a home-made two-electrode


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electrochemical cell with a piece of 1 cm2 Pt foil as the cathode. Both the electrodes had an area of the same size, nominally 1 cm × 1 cm exposed to the solution. The electrochemical cell was consisted of a Teflon beaker containing the etching solution, with two electrodes immersed and kept at a fixed distance of 2 cm. No stirring was applied to the system, while the exposed area of both the electrodes was 2 cm × 2 cm. A DC power supply (BK Precision, 9124, LabVIEW software) was used to control and datalog the experimental current and voltage, which was fixed for each experiment at 20 V. The electrolyte contained 0.3 wt% NH4F, 2 % v/v Milli-Q Water, and ethylene glycol 98 % v/v. The experiments were conducted at room temperature of 25 °C. After anodization, the specimens were cleaned in ethanol and dried in a furnace at 100 °C for 1 h. The samples were subsequently thermally annealed at 450 °C for 1 h, with a ramp of 2 °C /min, in a GSL 1100X tubular furnace (MTI Corporation) with Ar set as carrier gas.

Catalytic degradation of organic dye The catalytic performance was initiated with an OmniCure S2000 UV lamp. Three different light filters (OmniCure) with a respective wavelength of 320-480 nm, 320-390 nm, and 400-500 nm were applied on the system. For each filter, the irradiance was calculated by an OmniCure R2000 Radiometer and fixed at 180 mW/cm2 for the broad filter and 80 mW/cm2 for the visible light filter, respectively. The catalytic substrates were dipped into a 15 ml Methylene Blue aqueous solution by following established protocols19, 32 and the distance between the catalytic substrates and the lamp iris was fixed at 10 cm. Prior to the experiment, the catalytic system was stored in the dark for 30 min in order to reach the absorption-desorption equilibrium. The ultraviolet



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visible (UV-Visible) absorbance spectra of Methylene Blue aqueous solution were obtained from USB-2000 ultraviolet visible spectrometer (Ocean Optics, United States). The Methylene Blue absorbance peak at the wavelength of 665 nm under UV-Vis was evaluated at fixed time intervals by sampling the aqueous solution. The schematic of the fabrication process for the TiO2-Ta nanotubes is presented in Figure 1.

Materials characterization Scanning electron micrographs were acquired on a JEOL JSM 7800F SEM and Zeiss Supra 55VP FEG SEM at an accelerating voltage of 5 kV and a working distance of 5 mm. The detectors used for the analysis were a Zeiss Secondary Electron detector and InLens for higher surface resolution. The samples were mounted on aluminium stubs with carbon tape and silver paste to improve the grounding and avoid undesirable charging. Carbon coating was applied prior to the analysis to furtherly improved the conductivity. A 2-nm carbon deposition was performed on a Leica ACE600, subsequently the samples were immediately mounted on the Supra 55VP to avoid any coating loss. Graphic analysis of the acquired micrographs was conducted on ImageJ software, the statistics concerning the surface morphology and pore size was calculated on a sample size of at least 80 pores. Further analysis on the nanotubes was carried out with the Gimp graphic software. Small Angle X-ray Scattering experiments were carried out at the Australian Synchrotron, operated by ANSTO. The scattering patterns were analysed on the Scatterbrain 2.10 software developed in house by the SAXS scientists following procedures previously described.33-34 The detector is consisted of a Pilatus 1M with a beam energy of 16 keV. A


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camera length of 7.236 m was used for this study. The resulting wavelength was 0.774901 Å. The samples were mounted on static plates and analysed with a beam size of 150 μm × 150 μm and 1 s acquisition time. The analysis of the scattering patterns in the q-I plane was conducted on Igor Pro 6.37 software equipped with the IRENA package. Modelling II was used to conduct the knee fitting at given intervals. Further details are provided in the Supplementary Materials section, Equation S1 and Figure S6.33 AFM analysis was carried out on a Bruker Dimension Icon Scanning Probe Microscope (SPM) connected to a NanoScope V Controller. The AFM mapping was performed in tapping mode. The AFM probes (Ted Pella, TAP300-G-10, silicon AFM probes, no coating) presented resonance frequency and force constant of 300 kHz (±100 kHz) and 40 N/m, respectively. The AFM surface topography mapping was carried out on three different spots for each sample, with a scan rate of 0.5 Hz, scan size variable from 500 nm to 1 µm and 5 µm, and 512 samples/lines. The amplitude setpoint was fixed at 351 mV. Data analysis was performed on a Nanoscope 8.4 software. XRD measurements were conducted using X’Pert Pro MRD XL (PANalytical), with Cu Kα anode (1.5405 Å), operating at 40 kV and 30 mA. Prior to the analysis, the beam size was set to 3 mm × 3 mm and a Z-axis calibration was performed to single out the surface of the sample. The measurements were performed with a 2θ varying from 8 to 80 °2θ while ψ and χ rotation angles were fixed at 0, a step size of 0.02 °2θ, and a time per step of 0.5 s. Data was acquired on the X’Pert Data Collector software and X’Pert HighScore was used for background removal, peak identification and classification based on the ICDD PDF2 Library available. The X-ray Photoelectron Spectroscopy (XPS) was carried out at the RMIT Microscopy and Microanalysis Research Facility (RMMF), using a K-Alpha X-ray photoelectron spectrometer (Thermo Fisher Scientific). An Al Kα (1486.6 eV) X-ray



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source was used as the excitation source, and the anode was maintained at 250 W, 10 kV, and 27 mA at a chamber pressure of 5 × 10−8 ± 0.1 Pa with an oval beam spot size of 400 μm × 400 μm. The survey spectra were acquired on 10 scans and a pass energy of 100 eV, while the high-resolution elemental scans were conducted on 10 scans and a pass energy of 20 eV. For each sample, three different spots were measured for statistical relevance. In a typical setup, the X-ray source was set at 45 μm from the sample surface while the angle between the analyser and the sample surface is 90. The depth profiling XPS analysis was performed with an Ar+ ion gun operating at fixed excitation energy source of 1000 eV and etching time of 80 s, resulting in a nominal etching of 0.12 nm/s calculated on Ta2O5 reference. For this analysis, seven consecutive etching were carried out and the XPS survey and elemental analysis were performed for each level. Shirley backgrounds were introduced and quantitative analysis was carried out based on the Relative Sensitivity Factors (RSF) for each element in accordance with the CasaXPS library. The spectra were then deconvoluted using the CasaXPS software and the high resolution peaks fitted with a mixed GaussianLorentzian function (GL30), as previously reported.35 The asymmetric shape of Ti (0) in the Ti 2p spectra is fitted with a LA(1.1,5,7).36 Prior to the computational analysis, the C peaks were centered at 284.5 eV. The HR spectra analysis was conducted according to the references for each element, whilst peak positions and FWHM are fixed with a maximum deviation of ±0.1 eV. The UV-Visible diffuse reflectance spectroscopy was conducted on a LAMBDA 1050 UV-Vis spectrophotometer (Perkin Elmer, United States) equipped with a 100 mm Diffuse Reflectance and Transmission Integrating Sphere accessory (Perkin Elmer). In order to evaluate only the diffuse reflectance along the analysis, the reflectance port was removed in order to take off the contribution of the specular 10 ACS Paragon Plus Environment

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reflection from the total reflectance. The reflection was calibrated on a Spectralon standard, and the calculated %R is referred to this standard. The data was acquired with a UV WinLab software at a fixed scan speed of 485.33 nm/s, and the Kubelka-Munk function was subsequently determined, resulting in the final calculation of the tauc plot.



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Results and Discussion The mixed Ti-Ta films were formed by metal sputtering and the TiNTs and their alloys generated by anodization. The morphology and microstructure of both the thin films and the anodized porous substrates were evaluated (Figure2.A). The as-sputtered films consist of grains growing as columnar-like structures with quasi-geometrical shape and sharp edges, in good accordance with a previous study.25 The dependency of the grain morphology and texture shown in Figure 2.B indicates that as the applied power on the metal targets increases, the grains tend to become larger, more strained and elongated, with a maximum of 7,500 nm2 for Ti Ta 100 W corresponding to almost a fourfold increase from 1940 nm2for pure Ti. Along with the increase in surface area, the grain Feret reaches a maximum at 250 W at 171 nm, three times higher than pure Ti, confirming the relationship between applied power and texture reported in previous studies.24,

37

Further information on the surface texture is presented in Figure S1,

Figure S2 and summarized in Table S1, confirming that the nano-texture presents a smooth surface, with sub-10 nm mean roughness. Upon electrochemical anodization, the substrates present a nano-porous surface layer partially covering the nanotube arrays grown below. The SEM micrographs shown in Figure 2.A reveal that the surface morphology presents nanopores in the sub50 nm scale across the entire series. The Ta sputtering power is firstly correlated to the surface pore size as reported in Figure 2.C. There is one distinct transition at low Ta sputtering power of 20 W, where the average geometric pore size drops quickly from 33.5 nm for pure TiO2 TiNTs, to 20 nm a reduction of almost 40%. It can be therefore assumed that a slight amount of Ta introduced into the original TiTa film can induce a severe variation into the surface morphology, thus generating nanopores of remarkable smaller size. As the sputtering power increases, the pore size is further reduced to 12 12 ACS Paragon Plus Environment

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nm at 100 W down to a minimum of 8.9 nm at 250 W Ta power input. Interestingly, the pores seem to be primarily located at the grain boundaries and are more visible for the samples formed at higher Ta deposition power such as 150 W or more (Figure 2.A and Figure S3). These results indicate that the chemical composition on the surface of TiTa alloy is likely to play a role as the sputtering power influences the Ta deposition rate. The presence of nanotubes below the surface of top-layer can be observed in the insets in Figure 2.A and in Figure S4, through the fractures present across the matrix upon thermal annealing, typically related to stress-release and crystallization.38 The presence of a post-anodization layer is consistent with the literature available on nanotube growth and anodization.23, 39 However, in the present case, the techniques normally applied to overcome this issue and eliminate the top-layer, such as a 2-step anodization, could not be carried out, considering that the anodized substrates consist of a deposited thin film. The nanotube morphology varies considerably across the series of samples, depending on the Ta sputtering power. For a pure TiO2 film, a larger porous structure can be revealed from SEM images (inserts in Figure 2.A). In this case, as shown in Figure 2.D, the inner and outer diameters are 30.4 nm and 62.9, respectively although the geometrical features of the nanotubes are not as well defined as the rest of the series. Nevertheless, as Ta power increases, the nanotube morphology dramatically changes and the tubular shape is developed. At low Ta sputtering power, such as 10 W or 20 W, the tubes have a relatively large outer diameter (OD) and thin wall thickness (WT), standing at 45.2 nm and 11 nm, respectively as shown in Figure 2.E. As the Ta sputtering power increases, the pore aperture decreases and the wall thickness becomes progressively thicker. There are two transitions with the pore change (Figure 2.D). The first transition can be evidenced between 20 and 50 W, with tube diameter decreasing 13 ACS Paragon Plus Environment


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from 45.3 nm to 41.8 nm in outer diameter and average wall thickness increasing from 10.9 nm to 13.7 nm. The second transition towards a steady state occurs between 150 and 250 W of Ta deposition power. In this case, a third tube morphology is revealed, with a minimum tube outer diameter of 38.3 nm for a Ta deposition power of 150 W and a maximum wall thickness of 16 nm for a deposition power of 200 W, respectively. This result demonstrates a different trend for tube morphology other than what was found in previous reports on Ti-Ta alloy nanotubes, where the average diameter corresponding to a low at% Ta content, such as 0.4at%, of a higher loading such as a 25 wt% Ta, was varying between 70 and 150 nm.22-23 This evidence indicates that other properties, such as microstructure and surface morphology, have a remarkable impact on predicting and optimising the geometrical characteristics of the anodized nanotubes other than merely the chemical composition of the alloy. In order to confirm the morphological analysis conducted on the SEM micrographs, the morphology was further investigated by the Small Angle X-ray Scattering (SAXS) scattering intensity (Figure S5). The knees observed along the raw 2-D SAXS patterns were assigned to specific scattering features as in previous studies,34 such as outer diameter, wall thickness, and inner diameter distributions. The results are combined in Figure 2.D and Figure 2.E, confirming the overall trend observed from the SEM micrographs on the variation of outer diameter and wall thickness. Specifically, these two parameters present a relative error between 1% and 20% and between 2% and 21%, for the outer diameter and wall thickness, respectively, as summarized in Table S2. The largest variation of SAXS results from the SEM analysis is assigned to the pure TiO2 sample. This result may be likely due to the poorer geometrical definition of the nanotube morphology than the TiO2 with Ta loading, as in Figure 2.A. Consequently, the evaluation of the SEM micrographs is affected by a larger margin of 14 ACS Paragon Plus Environment

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error. The variation of geometrical features across the series analysed on both SEM micrographs and SAXS scattering is however very similar as they both follow the same trend in terms of decrease of OD and increase of WT, hence confirming that the surface and tube morphology can be varied upon the increase of sputtering power. The chemical composition of the substrate prior and post anodization was analysed by XPS depth-profiling. The thin films reveal a homogeneous bulk content of metals with a progressive decrease of Ti and increase of Ta as the sputtering power applied on the Ta target increases from 10 W to 250 W, (Figure 3.A, Figure 3.C and Figure S.6). Similarly, Figure 3.C and Figure 3.D reveal a slightly different composition for both sputtered and anodized TiTa from the top layer to the core material with a transition corresponding to the anodization top layer of approximately 20 nm in thickness, consistent with previous reports on CVD sputtered thin films.39-40 The presence of organic contamination and fluorides is also shown, confirming that the organic electrolyte is partially absorbed or trapped in the pores upon the electrochemical treatment.41 The depth profiling analysis for both Ti-Ta sputtered alloys and anodized Ti/Ta mixed oxide nanotubes is summarized in Table S3 and Table S4. The calculation of the elemental content confirms that as the Ta sputtering power in the Ti-Ta alloy increases, the Ta loading in the Ti/Ta mixed oxide nanotubes increases accordingly up to a maximum of 23 at% Ta for the core of the Ti/Ta mixed oxide nanotubes corresponding to a 250 W Ta sputtering power. Three different chemical composition bands can be observed, the first corresponding to a sputtering power from 10 to 20 W with a Ta/Ti ratio below 0.1. The second band is for 50 and 100 W Ta sputtering power, with a Ta/Ti ratio between 0.57 and 0.88 and the third group attributed to 150, 200 and 250 W Ta deposition power, with a Ta at% loading varying 15 ACS Paragon Plus Environment


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from 20.5 to 23 at% and a maximum Ta/Ti ratio of 2.77. Interestingly, these three groups corresponding to three significantly different compositions comply with the different surface texture, morphology and tube dimension observed, confirming that these characteristics are strictly dependant on the Ti-Ta alloy composition and can be controlled by tuning the Ta sputtering power, as summarized in Table S4. Microstructural crystallization was induced on the substrates by annealing to improve the catalytic performance, as amorphous TiO2 is known to be a less efficient photocatalytic material. The microstructure of the as-deposited films was investigated by XRD analysis (Figure S7). The deposited pure Ti film presents only one peak at 69.19°, which can be assigned to the Si (400) substrate. However, the β-titanium film spectra from the literature (ICDD: 04-003-7297) presents the Ti (211) at slightly higher intensity, at 69.6°. The amorphous nature of the sputtered pure Ti can be ascribed to the deposition rate and the presence of impurities, as described in previous studies. 24, 42 . As for the as-deposited Ti-Ta films, the presence of (110) crystal plane from β-titanium at 38.62° is revealed whilst (400) planes from Si are clearly visible only on Ti-Ta deposited at 10 W Ta power, while the peak intensity progressively decreases as the Ta deposition power increases. The presence of β-Ti phase in Ti-Ta alloys is in good agreement with the literature, confirming the role of Ta as β-Ti stabilizer.43 Less significant peaks of low intensity assigned to β-Ti (200) planes at 82.6° are visible from Ti-Ta 20 W to Ti-Ta 200 W. The partially crystallized nature of the deposited films is therefore confirmed, which is consistent with previous reports.24 Nonetheless, there is no clear evidence of crystallization of Ta. The variation of the microstructure upon thermal treatment of the anodized samples can be observed in Figure S8, where Si (400) peaks previously discussed were


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removed. Crystal planes typically attributed to TiO2 anatase (ICDD: 04-06-9240) can be shown in the XRD micrographs across the series, typically at 25.4° (101). Other peaks with intensity weaker than (101) attributed to anatase can be observed at 48.1°, 54.1° and 62.8°, corresponding to the crystal planes (200), (105) and (204), respectively.44 However, these peaks only appear for TiO2 – 0.5 at% Ta and do not occur for the rest of the series. The first two elements of the series, corresponding to Ti-Ta 10 W and Ti-Ta 20 W having a Ta loading of 0.2 at% and 0.5 at% respectively, reveal further crystal planes. For a 0.2 at% Ta loading, the presence of tantalum oxide (111) (ICDD: 04-003-6375) can be observed as a broad peak appears at 35°, while the peak at 36.6 (ICDD: 00-025-0922) can be attributed to Ta2O5 (1 11 1). Conversely, TiO2 – 0.5 at% Ta shows no sign of these specific crystal planes whilst the peak occurring at 28.7 can be assigned to Ta2O5 (200). Interestingly, the presence of rutile TiO2 (ICDD: 00-021-1272) for TiO2 – 0.5 at% Ta can be evidenced from the observation of peaks appearing at 27.4°, 54.3° and 56.6°, corresponding to (110), (211) and (220) crystal planes, respectively. The limited crystallinity concerning Ta2O5 is in good agreement with previous reports, where partial crystallisation could be obtained below 500 °C 45 or up to 600 °C.46 Partial crystallization of Ta2O5 can be observed from Figure S9, with the presence of (200) planes as well as (221) at 32.7° and (200) plane from TaO suboxide at 31.8° (ICDD: 04-007-0866). Further insights into the variation in microstructure across the series of the anodized samples at different Ta sputtering power and at% content can be gained by observing the shifting of the peak occurred at 38.04° for annealed pure TiO2. This peak is constantly shifting towards lower angles down to 37.7° and 36.7° for a Ta loading of 22.5 at% and 23 at%, respectively. However, its position does not match the previously described (110) crystal plane from β-Ti occurring at 38.62° and it can be therefore 17 ACS Paragon Plus Environment


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attributed to (004) crystal plane for anatase (ICDD: 04-006-9240). Nonetheless, the fact that this peak is progressively broadening across the series may suggest that other crystal planes have been formes, such as (201) and (211) of Ta2O5 (ICDD: 00-025-0922) at 37.06° and 37.17°, respectively. The different microstructure revealed across the series can affect the charge carrier mechanism upon light activation and, together with other properties, accounts for the catalytic performance of TiO2-based materials. The optical properties of the annealed substrates were assessed to evaluate the band gap of the Ti-Ta films. The Diffuse Reflectance Spectroscopy (DRS) of the substrates was calculated 47 and the the Kubelka-Munk (F(K-M)) function as expressed in Equation S2 was used as an approximation of the thin film absorption.48-49Further discussion on the band-gap calculation is introduced in the Supplementary Materials section . The acquired patterns of Kubelka-Munk function are presented in Figure 4.A. The F(K-M) presents an oscillatory trend that is typical in thin films that are typically affected by the interference phenomenon.45, 50-51 It is assumed that the constructive and destructive interferences are generated by the difference in refractive index between the film and the support when the beam goes through the different layers,52 consisting in this case of a 10-20 nm porous top layer, an array of nanotubes with geometrical parameters depending on the sputtering power, and the Si substrate. The analysis of the F(K-M) reveals a maximum absorption edge between 400 and 500 nm corresponding to a Ta loading of 17.4 at%, conversely as the content of Ta increases further, this edge becomes less pronounced and disappears at a Ta loading of 23 at%. The semi-conductor band-gap was subsequently calculated via a Tauc plot, shown in Figure S10, according to the Equation S3 and Equation S4 and the results, attributed to a direct band-gap transition, are shown in Figure 4.B. The band-gap value of 3.44 eV calculated for pure TiO2 is in good agreement with value reported in 18 ACS Paragon Plus Environment

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literature, assigning a direct band-gap value for TiO2 nanotubes spanning from 3.1 to 3.87 eV.25, 50, 53 On the contrary, pure Ta2O5 presents a higher band-gap than TiO2, standing at 3.9 eV, and it has therefore no visible light activity.54 In this case, the minimum band-gap of 2.91 eV is registered for a Ta loading of 17.4 at%, representing a 15% decrease from pure TiO2. Conversely, the highest Ta loading (23 at%) corresponds to the highest band-gap value, 3.31 eV. Interestingly, the analysis of Figure 4.B reveals that the reduction of band gap is not linearly dependent on the Ta loading, hence there is an optimal Ti-Ta alloy composition at which the visible light absorption will be enhanced. The variation of band-gap across the series and the presence of a minimum can be ascribed not only to the Ta loading, but also to the specific surface morphology of each sample. The impact of the TiO2 substrate on the optical properties has therefore been investigated. It has been reported that an increase in tube diameter from 30 nm to 90 nm leads to a decrease in direct band gap from 3.31 eV to 3.2 eV.55 Another study investigated the impact of tube wall thickness on the band gap, registering a decrease of Eg from 4.03 to 3.64 eV when the wall thickness increased from 2 to 30 nm.56 This is particularly evident when the wall thickness is below 10 nm where a steep increase in band gap takes place, due to the quantum confinement of the electrons as the linear dimension approaches the Bohr radius of TiO2, which has been estimated to be between 1 and 10 nm.57 In this study, however, the nanotubes are only partially observable while the surface morphology varies with the Ta loading in the matrix. In terms of band-gap modification, it has also been reported that the introduction of Ta would induce Ta 5d states near the minimum of the conduction band in TiO250 Different morphologies, crystallisation and synthesis protocols can also affect the final band gap of Ta-doped TiO2 58. It can be therefore assumed that the non-linear variation of band-gap and the



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presence of a band gap minimum can be ascribed to a complex combination of phenomena, including the variation of morphology as noted in Figure 2 as well the presence of additional charge carriers due to the introduction of Ta and generation of defects in the TiO2 structure. Nevertheless, the band-gap increases after reaching a minimum, in agreement with previous reports 50, which can be likely attributed to the increase of charge recombination centres due to an excess of doping and the formation of Ta2O5 which has a higher band-gap than TiO2.59 The specific composition in terms of Ti and Ta oxides was consequently investigated to determine the presence of Ti or Ta suboxides with potential favourable catalytic impact. Substrates such as pure TiO2, TiO2 – 0.5 at% Ta, TiO2 – 17.4 at% Ta and TiO2 – 22.5 at% Ta, representing different surface chemistry and morphology, were selected to carry out the catalytic tests. Prior to the evaluation of the catalytic performance, the chemical composition was assessed in detail with High Resolution XPS (Figure 5.A and Figure S11). The Ti 2p spectra were evaluated. The orbital split related to Ti(IV) 3/2p and 1/2p is clearly shown at 458.8 eV (FWHM = 1.2 eV ) and 464.5 eV (FWHM = 2.2 eV ), respectively, in good agreement with observation reported in the literature.36, 60 Two more oxidation states are however present, as Ti3+, Ti2+ and Ti(0), classified in Table S5. A higher concentration of Ti3+ species, up to 15.7 and 14.5 for TiO2 – 0.5 at% Ta and TiO2 – 17.4 at% Ta, can be observed in Figure 5.A, attributed to the higher integrated area associated to peak position and FWHM, 457.8 eV and 2.2 eV, respectively, for Ti3+ 2p 3/2, as shown in Table S5. Interestingly, the presence of Ta at high concentrations as 17.4 at% and 22.5 at% can be highlighted on the Ti 2p spectra as the Ta 4p 1/2 peak overlaps strongly with Ti 2p 1/2.61 The variation of Ti3+ species across the series of Ta/Ti ratio for the samples analysed is shown in Figure 5.B. It can be observed that the introduction of a low Ta, specifically 0.5 at% 20 ACS Paragon Plus Environment

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corresponding to a 0.05 Ta/Ti ratio, induces a spike of Ti3+ species from 6.1 for pure TiO2 to 15.7, while a further increase of Ta does not lead to a higher concentration of suboxides. The presence of a higher content of Ti suboxides should be taken into account for applications in catalysis as it can be favourable towards the charge transfer and hinder the recombination of charge carries, as holes and electrons,62-63 although these species are highly unstable and their generation and control is particularly challenging.64-65 The O1s spectra for the series are consequently analysed to evaluate the metal-O bonds and the presence of vacancies. The main peak, located at 530 eV (FWHM = 1.1 eV) as shown in Figure 5.A and Figure S11 is representative of the Ti-O bond for pure TiO2. Nonetheless for the substrates containing Ta, this peak shifts towards 530.4 eV and a FWHM= 1.4 eV. This increase in binding energy can be attributed to the presence of Ta-O bonds, hence a different chemical environment, as this chemical bond typically occurs at 530.5 eV.66 Additionally, the presence of Ti-OH is also revealed by the peak 531.9 eV as this functionality is normally assigned to binding energies 1.5-1.8 eV higher than Ti-O.67-68 This peak, however, can also be assigned, together with the peak at 533 eV, to organic R-CO functionalities.69 Interestingly, the presence of oxygen vacancies is confirmed by the presence of a peak located at 531.2 eV (FWHM = 2.2 eV),63-64 complying with the Ti 2p spectra and suggesting that the species are active on surface. The significant increase in O vacancies in the spectra in Figure 5.D for a Ta loading of 22.5 at% can be explained by the intensification of Ta-O bonds due to a higher Ta loading into the matrix. It is likely that this spike in integrated area accounts for a different metal-O bond ratio, consistent with the increase in Ta/Ti ratio from 0.88 to 1.92 reported in Table S4 and shown in Figure 5.D, for a Ta loading of 17.4 at% and 22.5 at%, respectively, rather than representing only oxygen vacancies.



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The analysis of the Ta 4f HR spectra reveals two main Ta 4f 7/2 and 4f 5/2 peaks, attributed to Ta2O5, at 26.8 eV and 28.7 eV respectively, with an energy separation of 1.9 eV, consistent with the literature.70-71 The FWHM for Ta 4f 7/2 is 1.2 eV, however the presence of a higher content of suboxides at 24.8 eV or Ta (0) at 21 eV

50, 72-73

for TiO2 – 0.5 at% Ta and TiO2 – 22.5 at% Ta (Table S6), determines an

increase of FWHM to 1.6 eV. The significantly high concentration of suboxides for the TiO2 – 0.5 at% Ta, calculated at 30.3 %, as shown in Figure 5.C, confirms that for low concentration of Ta, up to 0.5 at%, some evidence of crystalline Ta-oxides can be found (Figure S8). Interestingly, the concentration of suboxides species drops dramatically, from 30.3 % to 2.6%, as the Ta/Ti ratio increases, similarly to the Ti3+ content. This result can be attributed to the formation of more passive Ta2O5 as the Ta at% increases, in good agreement with previous studies conducted on Ti-Ta alloys

74.

It can be

concluded that the presence at the same time of Ti and Ta suboxides, as well as O vacancies, leads to a complex surface chemistry that, together with the assessment of the surface morphology, influences the final band-gap and the solid-liquid interface where the catalytic mineralization reactions take place. To gain more insight into the catalytic performance of the substrates influenced by the surface chemistry and morphology investigated thus far, the degradation of methylene blue (MB), typically selected as a reference dye for benchmark studies,75 was carried out. First, the degradation with a 320-480 nm filter, applied on the light source, was investigated. To prevent the photolysis of methylene blue, no wavelength above 500 nm was applied.76 Error bars corresponding to a 3% standard deviation were added to reflect the presence of an experimental error on the acquired data points, calculated on three different TiO2 and TiO2-Ta samples at fixed conditions.


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The maximum conversion achievable by the benchmark TiO2 nanotubes was ~85% of the original MB concentration, after 7 h of treatment (Figure 6.A). The Ta doped nanotubes showed a complete mineralization of the compound, with the optimum performance obtained for the 17.4 at%. loading, in less than 6 h. However, as the Ta loading increases further, the degradation kinetics becomes slower and the curves tend towards the pure electrochemical anodized Ta2O5 calculated as a reference, , showing a 40% degradation rate over the same time span of 6 h. This result suggests that the catalytic performance is s strictly dependant on the Ta loading, with a specific concentration individuated as the most efficient across the series, whilst past this point any further addition of Ta into the matrix is detrimental. As previously highlighted, this effect can be ascribed to the generation of less photoresponsive Ta2O5 in the TiO2-Ta nanotubes microstructure, resulting in a loss of catalytic efficiency. The degradation kinetics are, however, significantly slower than those for TiO2 nanoparticles that are in the range of 10-180 min.19, 77-78 The slower degradation kinetics can be attributed to a significantly lower specific surface area, if compared to TiO2 powders, presenting therefore fewer active sites available for the catalytic reactions.78 In this case, the magnitude of the degradation kinetics of methylene blue is in the order of hours, complying with studies on surface-decorated TiO2 nanotubes or doped TiO2 substrates.79 It can be noticed, however, in the current study, the anodized Ti-Ta alloy outperforms similar substrates decorated with noble metals such as Ag or Au. TiO2 nanotubes decorated with the aforementioned nanoparticles by metal sputtering can degrade up to 50% and 75% of methylene blue, respectively, after 250 min exposure at higher irradiance

32

whilst a 17.4 Ta at% loading upon the same time exposure can

mineralize up to 85% of MB. Another study on Ag-decorated TiO2 nanotubes demonstrated an average 70% MB degradation rate in 3 h under pure UV light at 365



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nm wavelength, comparable to the 70% degradation rate obtained on the bestperforming Ti-Ta anodized alloy mentioned above.80 Similar degradation rate upon 4 h of treatment was found for C-modified TiO2 nanotubes, prepared on a commercial Ti foil by the deposition of carbamide, resulting in a minimum bandgap of 2.11 eV.81 The photo-catalytic kinetics constants were calculated from the degradation profiles. Typically, the degradation of organic compounds in heterogeneous catalysis follows the Langmuir-Hinselwood model with a pseudo-first-order kinetics, as shown in Equation S5 and Equation S6.82 The studies conducted on dyes typically follow this law, even though the experimental set-ups may vary from one case to another including immobilized particles, suspended particles as Degussa P25 for TiO2.77 The calculated pseudo first-order and zero-order kinetics constants with Equation S7 are summarized in Table S7. It can be noticed that the zero-order kinetics, in this case, comply with most of the experimental results from the Ti-Ta anodized alloy series. According to the literature in heterogeneous photocatalysis, this kinetic model is typical of reactions where the surface available for the catalytic reaction is saturated very quickly and mass diffusion becomes the controlling stage.76, 83 This assumption is in good agreement with the results of this study and the potential issue, represented by the reduction of specific surface area available for the catalytic reaction due to the presence of the surface top layer that has already been addressed. The variation of zero order kinetics is shown in Figure 6.B. The fastest zero order kinetics can be attributed to a 17.4 at% Ta concentration, resulting in a kinetic constant of 3.31 mmol/min which represents a 40 % increase from pure TiO2, standing at 2.35 mmol/min. Conversely, low kinetics can be evinced for a 22.5 at% and 23 at% loading of Ta, respectively, leading to a -k of 1.69 mmol/min and 1.32 mmol/min. These results are consistent with the light absorption registered and band-gaps tested, whilst the highest Ta loading 24 ACS Paragon Plus Environment

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resulted in the least efficient optical performance of the series. It is worth noticing that all the kinetic constants, both for zero order and pseudo first-order, are close in terms of R2 although the zero-kinetic model better fits the experimental data. The calculation and theoretical modelling of a kinetic study is however out of the scope of the current study. The catalytic activity under pure visible light was investigated for 4 substrates, nominally TiO2, TiO2 – 0.5 at% Ta, TiO2 – 17.4 at% Ta and TiO2 – 22.5 at% Ta. These substrates were selected to cover the three different morphological structures revealed in the first part of this study. Figure 7 compares the activity of these substrates under the 320-480 nm light filter, as previously discussed, then under pure UV light and pure visible light, at 320-390 nm and 400-500 nm, respectively. It can be observed that TiO2 – 17.4 at% Ta reported in Figure 7.C is the most efficient photocatalytic substrate under pure visible light, as predicted by the UV-vis absorbance in that range reported in Figure 4. The sample with highest light absorbance can achieve almost 50 % degradation of methylene blue over 6 h. Conversely, pure TiO2 can degrade only 26% of the original dye loading. The fact that pure TiO2 shows some activity in the low-visible range canbe attributed to the presence of Ti3+ species as shown in Figure 5, having an impact on the catalytic performance. However, these species are present across all the series. The other two substrates, having a 0.5 at% Ta and 22.5 at% Ta loading, lead to a mineralization of 41% and 39%, respectively. The ratio between the zero-order kinetic constant of the Ta-modified TiO2 nanotubes and pure TiO2 nanotubes is presented in Figure 7.E. All the Ti-Ta anodized alloys present higher k under pure visible light than TiO2, with almost a threefold increase corresponding to a 17.4 at% Ta loading. Conversely, the k calculated for pure UV light experiments is slightly lower than pure TiO2, with a maximum 14% drop attributed to the sample with minimum band-gap and 25 ACS Paragon Plus Environment


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highest visible light activity, TiO2 – 17.4 at% Ta. It can also be noticed that all the substrates seem to have a higher degradation rate within the first hour of treatment, whilst the curves tend towards a steady state for longer durations. This is consistent with a reaction that is controlled by the mass transport and therefore by the absorption/desorption of molecules from the saturated active sites. Enhanced kinetic constants have been reported for anodized Ti alloys to forming a nanotubular structure, including a few metals, such as W, Mo, Cu, Nb, W. 84 Table S8 presents a comparison of different metal-doped TiO2 nanotubes on the degradation of organic dyes. It can be noticed that W doping resulted in the highest kinetic constants at low and high concentrations,85 whilst for Mo and Ta, as in this study, the catalytic performance varied upon the metal loading and surface morphology. 86Further analysis is presented in the Supplementary Material section. The faster degradation rate within the first hour of treatment can also be presented in the recyclability study carried out on the best performing substrate, TiO2 – 17.4 at% Ta. (Figure 7.F). Although the kinetic constant decreases from 1.79 mmol/min to 1.15 mmol/min upon three catalytic tests under visible light, corresponding to a loss of 35% in kinetic activity, the degradation of MB only decreases slightly, from 49% to 42%. This result is attributed to a different degradation trend that can be clearly evinced from Figure S12, as some catalytic sites can be partially deactivated, resulting in a loss of activityupon different cycles of treatment. The loss of activity on TiO2 nanotubes represents a well known limitation in heterogeneous photocatalysis. 87 The loss of activity can be ascribed to the consumption of available lattice O, responsible for the formation of oxidating agents, whereas carbon-rich byproducts can adsorb on the catalyst surface and deactivate the catalytic sites.

88The

surface functionalities, including a combination of TiO2, Ta2O5 and a mixture of Ti , 26 ACS Paragon Plus Environment

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Ta suboxides and oxygen vacancies as shown on the XPS analysis in Figure 5, were likely altered upon the photocatalytic tests, resulting in a loss of activity. The complex surface chemistry evidenced can however sustain the degradation of the dye by offering different kinetic pathways, indicating that some active sites are more resilient towards deactivation than others. The identification and quantification of all the active sites generated the Ti/Ta mixed oxide nanotubes is beyond the scope of the present study. In this scenario, it can be assumed that the catalytic performance of the anodized Ti-Ta substrates does not depend strictly on the optical properties, although the most active substrate under visible irradiation corresponds to the lowest band gap. The contribution of the morphology, influencing the surface area and the active sites available, thus the turnover of dye molecules absorbed on surface prior to the degradation cannot be discarded as it can be demonstrated that the performance of the catalyst slightly decreases. The presence of multiple charge carriers evidenced by the surface chemistry analysis is likely having an impact on the catalytic degradation described and on the different degradation trends observed on the recyclability study, contributing to complex reaction pathway where multiple species and sites are involved. Conclusions In summary, we have fabricated mixed Ti-Ta oxide nanotubes with sub-10 nm pore size and tuneable chemical composition by a simple process based on Ti-Ta alloy thin film sputtering and electrochemical anodization. The dependence of the surface morphology and nanotube geometrical features was investigated and correlated to the Ta magnetron deposition power. The presence of Ti and Ta suboxides in the TiO2-Ta matrix generated upon electrochemical anodization was investigated across the series of Ta loading, whilst a specific composition corresponding to a 17.4 at% Ta resulted in a band-gap minimum. The impact of the induced surface morphology and chemistry on 27 ACS Paragon Plus Environment


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the photocatalytic efficiency of the mixed Ti-Ta oxide nanotubes was evaluated. The catalytic tests indicated that the sample showing the minimum band gap presented the highest degradation rate under pure visible light than pure TiO2 with almost a threefold increase in zero-order kinetic constant. The study highlighted the potential of sputtered Ti alloy nanotubes as catalytic substrate with controlled morphology and band gap engineering, offering promising results for further development of the nanotube properties based on their geometry and optical properties. The exploitation of the optical properties of the nanotubes based on the generation of Ti and Ta suboxides species is envisaged, leading to potential applications in the advanced oxidation processes field and photonics.


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Conflict of interests There are no conflicts to declare. Supporting Information Figure S1. As-deposited Ti-Ta thin films at different Ta sputtering power; Figure S2. AFM micrographs of as-deposited Ti-Ta films at different sputtering power; Table S1. 3D surface texture parameters of as-deposited Ti-Ta films; Figure S3. Anodized Ti-Ta alloys at different Ta sputtering power; Figure S4. Inset of nanotubes emerging from surface fractures or cracks. Series based on Ta deposition power; Figure S5. SAXS scattering curves in the 1-I plane of anodized Ti-Ta alloys and corresponding SAXS patterns for different Ta deposition power; Equation S1: SAXS modelling; Figure S6. Schematic of scattering features assigned to the nanotubes; Table S2. Combined SEM and SAXS analysis and identification of scattering figures; Figure S7. Elemental composition of as-deposited Ti-Ta films and anodized films calculated by XPS analysis at different Ta sputtering power; Table S3. XPS analysis of anodized Ti-Ta alloy core level at different Ta deposition power; Table S4. XPS depth profiling elemental composition for Ti, Ta and Ta/Ti ratio corresponding to each sputtering deposition condition; Figure S8. XRD analysis of Ti-Ta alloy thin films as-deposited at different Ta deposition power; Figure S9. XRD analysis of anodized Ti-Ta alloy upon annealing treatment; Equations S2-S4: Kubelka Munk function and Tauc Plot; Figure S10. Tauc plot calculations; Table S5. XPS HR spectra of Ti 2p; Table S6. XPS HR spectra of Ta 4f; Figure S11. XPS HR spectra of Ti 2p, O1s and Ta 4f for TiO2 and TiO2 – 22.5 at% Ta; Equations S5-S7: Langmuir-Hinselwood model; Table S7. Kinetic constants for zero order and apparent first order reactions calculated for series of anodized TiO2-Ta; Figure S12. Cyclability analysis on TiO2 – 17.4 at% Ta sample on degradation of methylene blue. 29 ACS Paragon Plus Environment


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Acknowledgment This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). Mr Andrea Merenda would like to acknowledge the Australian Research Council (ARC) for funding the Linkage LP140100374 project and Dr. Ludovic Dumée acknowledges the ARC for his DECRA DE180100130 fellowship. Mr Andrea Merenda and Dr. Ludovic Dumée would also like to acknowledge ARC Research Hub for Energyefficient Separation IH170100009. The authors acknowledge the facilities, and the scientific and technical assistance, of the RMIT Microscopy & Microanalysis Facility, at RMIT University a linked laboratory of the Australian Microscopy & Microanalysis Research Facility. The authors also acknowledge the Australian Synchrotron, SAXS/WAXS beamline, for funding the proposal M12173. The authors would like to acknowledge Prof. Peter Hodgson for mentoring and support to the study. The authors would like also to thank the support from Dr Kallista Sears, CSIRO Clayton Manufactory, for the UV-vis experimental tests. Deakin University’s Advanced Characterisation Facility is acknowledged for use of the JEOL JSM 7800F SEM and Zeiss Supra 55VP FEG SEM and assistance from Dr Andrew Sullivan.


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Figures

Figure 1. Schematic of the fabrication process for TiO2-Ta nanotubes



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Figure 2. SEM micrographs of as-deposited Ti-Ta thin films and anodized films (A) at different Ta sputtering powers: 0 W, 20 W, 100 W and 200 W. Grain surface and Feret analysis of the Ti Ta alloy (B), surface porosity and pore size calculation of the anodized films (C), nanotube geometrical parameter as Outer Diameter (OD), Inner Diameter + Wall Thickness (ID+WT) (D), Wall Thickness (E), SEM and SAXS combined evaluation.


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Figure 3. Elemental composition of as-deposited Ti-Ta films and anodized films calculated by XPS analysis at different Ta sputtering power: ▬ 10 W, ▬ 20 W, ▬ 50W, ▬ 100 W, ▬ 150 W, ▬ 200 W, ▬ 250 W, representing Ta at% and Ta/Ti ratio, (A) and (C), (B) and (D) respectively.



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Figure 4. Kubelka-Munk function (A) for series of anodized TiTa films at different Ta at% loading, and (B) calculated band-gap from Tauc plot equation.


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Figure 5. (A): XPS HR spectra of Ti 2p, O1s and Ta 4f for TiO2 – 0.5 at% Ta and TiO2 – 17.4 at% Ta. Ti HR spectra: ▬ Ti 2p Scan, ▬ Ti (IV), ▬ Ti (III), ▬ Ti (II), ▬ Ti (0) W, ▬ Ta 4p 1/2 ▬ Background, ▬ Fitting. Ta HR spectra: ▬ Ti 2p Scan, ▬ Ta (V), ▬ Ta (0), ▬ Ta suboxide, ▬ Ta suboxide (2), ▬ Ti (0) W, ▬ Background, ▬ Fitting. O HR spectra: ▬ O 1s Scan, ▬ Ti-O ▬ Ti-OH (organic), ▬ O-organic, ▬ Background, ▬ Fitting. Vvariation of Ti 3+ species, Ta suboxides species and O vacancies at different Ta/Ti ratio, (B), (C) and (D) respectively.



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Figure 6. Degradation of methylene blue for different Ti-Ta anodized substrates at different Ta % at loading (A): ▬ TiO2, ▬ TiO2 – 0.2 at% Ta, ▬ TiO2 – 0.5 at% Ta,, ▬ TiO2 – 13 at% Ta, ▬ TiO2 – 17.4 at% Ta, ▬ TiO2 – 20.5 at% Ta, ▬ TiO2 – 22.5 at% Ta,. ▬ TiO2 – 23 at% Ta, ▬ Ta2O5, ▬ blank (MB photolysis) 320-480 nm filter was applied on the light source. Calculation of 0 order kinetic constant across the series of Ta content (B)


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Figure 7. Catalytic degradation of methylene blue at different light filters for: TiO2 (A), TiO2 – 0.5 at% Ta (B), TiO2 – 17.4 at% Ta (C) and TiO2 – 22.5 at% Ta (D), kinetic constant of TiO2 – X Ta over kinetic constant of pure TiO2 at different light filters (E) and cyclability test (F).



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