Enhanced Visible Light Sensitization of N-doped TiO2 Nanotubes

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Enhanced Visible Light Sensitization of N-doped TiO2 Nanotubes Containing Ti-Oxynitride Species Fabricated via Electrochemical Anodization of Titanium Nitride Andrea Merenda, Akshita Rana, Albert Kamal Guirguis, De Ming Zhu, Lingxue Kong, and Ludovic F Dumée J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09762 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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The Journal of Physical Chemistry

Article Enhanced Visible Light Sensitization of N-doped TiO2 Nanotubes Containing TiOxynitride Species Fabricated via Electrochemical Anodization of Titanium Nitride

Authors Andrea Merenda1,*, Akshita Rana1, Albert Kamal Abdou Saad Guirguis 1, De Ming Zhu2, Lingxue Kong1 and Ludovic F. Dumée1*

Affiliations 1

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

Victoria, Australia 2

Faculty of Science, Engineering and Technology, Swinburne University of

Technology, Hawthorn, Victoria 3122, Australia

*

Corresponding authors:

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

1 ACS Paragon Plus Environment

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Abstract The concentration and chemical state of nitrogen represent critical factors to control the band-gap narrowing and the enhancement of visible light harvesting in nitrogen-doped titanium dioxide. In this study, photocatalytic TiO2-N nanoporous structures were fabricated by the electrochemical anodization of titanium nitride sputtered films. Doping was straightforwardly obtained by oxidizing as-sputtered titanium nitride films containing N-metal bonds varying from 7.3 to 18.5 % in the Ti matrix. Severe morphological variations into the as-anodized substrates were registered at different nitrogen concentration and studied by Small-Angle X-ray Scattering. Titanium nitride films with minimum N content of 6.2 at% N led to a quasi-nanotubular geometry, whilst an increase in N concentration up to 23.8 at% determined an inhomogeneous, polydispersed distribution of nanotube apertures. The chemical state of nitrogen in the TiO2 matrix was investigated by X-ray Photoelectron Spectroscopy depth profile analysis and correlated to the photocatalytic performance. The presence of Ti-N and β-Ti substitutional bonds, as well as Ti-oxynitride species was revealed by the analysis of N 1s X-ray Photoelectron Spectroscopy High Resolution spectra. The minimum N content of 4.1 at% in the TiO2-N corresponded to the lowest Ti-oxynitride ratio of 13.5 %. The relative variation of N-metal bonds was correlated to the visible light sensitization and the highest Ti-N/Ti oxynitride ratio of 3.3 was attributed to the lowest band-gap of 2.7 eV and associated to a threefold increase in the degradation of organic dye. Further increase of N doping led to a dramatic drop of Ti-N/Ti oxynitride ratio, from 3.3 to 0.4, which resulted in a loss photocatalytic activity. The impact of the chemical state of nitrogen towards efficient doping of TiO2 nanotubes is demonstrated with a direct correlation to the N loading and a strategy to optimise these factors based on a simple, rapid synthesis from titanium nitride.


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1. Introduction TiO2 nanomaterials have been intensively studied in photocatalytic applications because of their large availability, high chemical resistance, facile design and unique semi-conductor electronic properties.

1-2

Band-gap engineering of TiO2 materials is

however essential to produce a new class of energy-efficient, green photocatalysts with high catalytic performance. 3-5 The intrinsic band gap of the main TiO2 crystalline phases, rutile and anatase, stands at 3 and 3.2 eV, respectively,

6

meaning that only UV light with λ < 387 nm,

accounting for 5% of the solar spectrum, can activate the electron-transfer in crystalline TiO2. 7-9 The band-gap modification of TiO2 is envisioned to increase the harvesting of visible light from sun-light, thus enabling sustainable catalytic applications at low energy cost.

10-11

The separation and migration of charge-carriers is critical to extend

the lifetime of photo-generated holes and electrons and therefore to improve the catalytic performance of materials with engineered band-gap. 12-13 Among the different 1-D and 2-D TiO2 nanomaterials synthesized to date, TiO2 nanotubes (TNTs) have emerged in heterogeneous catalysis because of the high specific surface area and well-ordered surface morphology generated through a simple electrochemical anodization process.14 TNT arrays with well-defined architectures and interconnectivity provide fast electron transfer rate along the quasi-ideal 1-D structure. 15

The reduction of trap sites compared to TiO2 nanoparticles determines also longer

charge lifetime and minimal charge recombination effect.

16-17

Nonetheless,

applications in catalysis are limited by the wide bang-gap and therefore the requirement of UV light irradiation as an activation source.18 The band-gap of TiO2 nanomaterials can be specifically tailored by introducing a doping agent, such as a metal or a nonmetal, into the TiO2 lattice. 19-21 Band-gap engineering can result in a lower shift of the



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Conduction Band (CB), a higher shift of the Valance Band (VB) or in the introduction of additional impurity states. 22 Among the potential non-metal dopants, such as sulphur (S), carbon (C), fluorine (F) and phosphorus (P), N represents the most promising candidate because of the position of its p orbitals if compared to O and Ti in electronic configuration of TiO2. 23-24 Conversely, S possesses a much higher substitutional energy due to its higher ionic radius whereas lower charge carrier lifetime is attributed to C or P doping. The introduction of new energy levels into the TiO2 electronic configuration depends on the chemical state of N bonds, such as substitutional or interstitial.

25

Substitutional N represents the most studied and preferred form of doping as it is expected to lead to better photocatalytic efficiency due to low charge recombination effect. 22, 26 New chemical species resulting from the oxidation of titanium or titanium nitride (TiN), such as Ti-oxynitrides (Ti-NxOy), have gained momentum as potential candidates to enhance further the visible light absorption across TiO2 materials. 27 The reduction of TNTs under ammonia annealing at 600 °C narrowed the band-gap to 2.4 eV or 2.2 eV, attributed to the partial formation of Ti-NxOy in addition to substitutional-N or the complete conversion of substitutional-N to Ti-NxOy, respectively.28-29 Alternatively, Ti-NxOy with enhanced light response were prepared by oxidation of sputtered Ti or TiN films under N2 and O2 atmosphere. 30-31. To date, only one study is available on the fabrication of N-doped TNTs by anodizing TiN films, produced by arc-melting. 32 In this study, 2-3 at% of Ti-NxOy species were induced by the anodization process, resulting in a shift of the light absorption edge to the 400-500 nm range. However, while Ti-NxOy can effectively enhance the light sensitization of TiO2, their role in photocatalytic degradation reactions is still not well understood.

33-34


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Nitridation above 600 °C led to the partial reduction of mesoporous TiO2 into Ti-NxOy and to an increase visible light absorption up to 600 nm.

35

Nonetheless no catalytic

degradation of methylene blue (MB) was observed in the presence of Ti-NxOy, whereas the substrate containing only Ti-N doubled the photocatalytic efficiency of mesoporous TiO2. Increasing N content from 1 to 20.8 at% was found to deteriorate the photocatalytic activity of Ti-NxOy films fabricated by sputtering Ti metal in mixed Ar/O2/N2 atmosphere 31. In this study, an optimal amount of 1.2 at% of N led to a shift in the absorption edge to 520 nm and the fastest degradation rate for MB. The introduction of an optimal amount of N doping is therefore envisaged to attain the highest light sensitization and catalytic performance.36 Authors have however advocated N loading spanning from 0.5 at% to 3.4 at% as ideal concentration to narrow the band gap down to 2.3 eV, whilst higher level of doping were found to hinder the light sensitization. 37-39 It is expected that the addition of Ti-NxOy to substitutional and interstitial N doping may enhance the catalytic performance of TiO2,

40

however the presence of a

synergetic effect between the photoactive N species and the role of N loading on their interaction and recombination have not been explored. Furthermore, the control of the oxidation level during the synthesis of N-doped TiO2 and therefore the formation of TiNxOy intermediates is challenging,41 as the complete conversion of TiN to TiO2 is thermodynamically favoured by the highly exothermic adsorption and dissociation of O2 on TiN. 42 In the present work, N-doped TiO2 nanotubes were fabricated by electrochemical anodization of magnetron sputtered Ti-TiN alloys. The N loading into the TiO2-N substrates was easily controlled by varying the alloy composition upon metal sputtering. XPS depth profile and High Resolution (HR) spectra of the Ti and N



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chemical states provided more insight into the formation and interactions of photoactive N species, such as Ti-N, Ti-O-N and Ti-NxOy, upon the oxidation process. The impact of the concentration of N on the formation of different N chemical states was revealed and associated to the visible light sensitization. The relative ratio of Ti-N/Ti-NxOy species and of elemental N/O was associated to the catalytic performance on the degradation of methylene blue (MB). This study demonstrated the correlation between N doping concentration induced during an oxidation treatment and the chemical state of N on the catalytic efficiency of N-TiO2 TNTs. The use of custom-made titanium nitride allowed for a precise optimisation of the content of N chemical species responsible for the light sensitization, showing promising potential in the heterogeneous photocatalysis domain.


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2. Experimental Methods

2.1 Materials and chemicals 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. Ti and TiN sputter targets (99.99% purity) were purchased from Ezzi Vision. Ethylene glycol (>99.8%), ammonium fluoride (>99.99%) ethanol (>99.5%), Methylene Blue (>99.5%) were purchased from Sigma-Aldrich. Milli-Q water was used without further purifications.

2.2 Physical Vapour Deposition of TiN alloy The deposition of titanium alloys containing titanium nitride (named assputtered Ti-TiN films) was performed on an INTLVAC Nanochrome I AC/DC sputtering rig at the MCN. Two Ti targets were mounted on the AC magnetrons, while the titanium nitride target was positioned on the DC magnetron. The deposition chamber was vacuum-cleaned before each deposition to prevent cross contamination. The chamber was pumped down to 5 × 10-6 T before each deposition. Each target was cleaned by a 5-minute warm up stage prior to the deposition. The applied power on the AC Ti targets was fixed at 300 W while the DC power was varied from 0 to 250 W. 6 DC power were considered, leading to the deposition under 6 different conditions: 300 W AC-0 W DC, 300 W AC-50 W DC, 300 W AC-100 W DC, 300 W AC-150 W DC, 300 W AC-200 W DC, 300 W AC-250 W DC. The Ti-TiN film sputtering conditions are summarized in Table S1 of Supplementary Information. The variation of sputtering power in the manuscript refers therefore exclusively to the titanium nitride target. The deposition was carried out at a pressure of 2 × 10-3 ± 0.1 T. Ar was used as a carrier gas



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at a nominal flow of 20 sccm. The sputtering duration was set to 60 min.

2.3 Electrochemical anodization The anodization experiments were carried out in a custom-built two-electrode electrochemical cell consisting of a Teflon beaker containing the anodization solution. A piece of 1 cm2 Pt foil was used as counter electrode. Both the electrodes had an area of the same size, nominally 1 cm × 1 cm exposed to the solution. The distance between the electrodes was fixed at 2 cm. A DC power supply (BK Precision, 9124, LabVIEW software) was used to register the variation of current and fix the applied voltage at 20 V. The electrolyte contained 0.3 wt% NH4F in a solution of 2 % v/v Milli-Q Water, and ethylene glycol 98 % v/v. The experiments were conducted at room temperature with no stirring applied. After anodization, the anodized substrates were cleaned in ethanol and dried in a furnace at 100 °C for 1 h. The samples were subsequently thermally annealed at 450 °C with a ramp of 2 °C /min and a dwell time of 1 h at 450 °C in GSL 1100X tubular furnace (MTI Corporation). Ar was used as carrier gas.

2.4 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 and 400-500 nm were applied on the system. The irradiance was calculated by an OmniCure R2000 Radiometer and fixed at 180 mW/cm2 for the 320-480 filter and 80 mW/cm2 for the 400-500 nm, respectively. The catalytic substrates were dipped into a 10 ml Methylene Blue aqueous solution by following established protocols.

43-44

The

distance between the catalytic substrates and the lamp iris was fixed at 10 cm. Prior to the experiment, the catalytic cell was stored in dark for 30 min to reach the absorption-


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desorption equilibrium. The UV-Visible absorbance spectra of MB 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 UVVis was evaluated at fixed time intervals by sampling the aqueous solution.

2.5 Material characterization 2.5.1 Surface Morphology Scanning electron micrographs were acquired on a Zeiss Supra 55VP FEG SEM at an accelerating voltage of 5 kV and a working distance of 5 mm. A Zeiss InLens detector was used for the analysis at high surface resolution. The samples were mounted on aluminium stubs with carbon tape and silver paste and carbon coating was applied prior to the analysis to improve the conductivity and minimize charging effect. A 2-nm carbon deposition was performed on a Leica ACE600. Image J software was used for the graphic analysis of the acquired micrographs. Statistical evaluation of pore size and porosity was conducted on a sample size of 80 pores. 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 following procedures previously described. 45-46 The detector consisted of a Pilatus 1M with a beam energy of 16 keV. A camera length of 7.236 m was used for this study with a wavelength of 0.774901 Å. The samples were mounted on static plates, beam size was of 150 μm × 150 μm and 1 s acquisition time was set for each sample. The scattering patterns and the resulting spectra in the q-I plane were analysed on Igor Pro 6.37 software equipped with the IRENA package. Modelling II was used to conduct the knee fitting at given intervals. Details of the analysis are provided in the Supplementary



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Materials section, Equation S1 and Figure S4.45 AFM analysis was carried out on a Bruker Dimension Icon Scanning Probe Microscope (SPM) connected to a NanoScope V Controller. Tapping mode was used for the AFM mapping and imaging. 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 topography mapping was carried out on three different spots for each sample, with a scan rate of 0.5 Hz, and scan size of 1 μm at 512 samples/lines. The amplitude setpoint was fixed at 351 mV. Data analysis was performed on a Nanoscope 8.4 software.

2.5.2 Microstructure evaluation 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. Z-axis calibration was performed prior to each scan to individuate the sample surface. 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 were used. X’Pert HighScore was used for background removal and peak assignment according to the ICDD PDF2 Library.

2.5.3 Surface chemistry and depth profiling XPS analysis was performed with a Kratos AXIS NOVA spectrometer (Kratos Analytical Ltd, Manchester, UK) using a monochromatized AlKa X-ray source (1486.6 eV) operating at a power of 150 W (10mA, 15kV). Survey and High Resolution spectra were acquired at 160 and 20 eV pass energies, with 10 and 20 scans, respectively. A defined area using the 110 um aperture was analysed on each sample. The sputtering


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profiling of the films was carried out by an argon ion beam with a beam energy of 1 keV. The raster size (etched area) was 1.5 mm x 1.5 mm and the sputtered times were: 0s, 90s, 180s and 270s. An etching rate of 0.12 nm/s was calculated from a Ta2O5 reference. 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 Gaussian-Lorentzian functions (GL30), as previously reported. 47-48The asymmetric shape of Ti (0) in the Ti 2p spectra is fitted with a LA (1.1,5,7)47. The N1s spectra were deconvoluted with mixed Gaussian-Lorentzian functions (GL60). Prior to the computational analysis, the C peaks were centred at 284.5 eV. The HR spectra analysis was conducted according to the references for each element, reported in Table S5 and Table S6.

2.5.4 Optical Properties The UV-Visible analysis was conducted on diffuse reflectance spectroscopy mode. The spectra were acquired 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). The reflectance port was removed in order to remove the contribution of the specular reflection from the total reflectance and single out the diffuse reflectance. Calibration was conducted on a Spectralon standard. UV WinLab software was used for data acquisition at a fixed scan speed of 485.33 nm/s. The Kubelka-Munk function was calculated as in Equation S3, and the tauc plot calculated as in Equation S4 and Equation S5.



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3. Results and Discussion The electrochemical anodization of as-sputtered Ti-TiN films was studied to investigate the impact of N concentration to the formation of nanoporous N-doped TiO2 nanotubes with narrowed band-gap. 3.1 Evaluation of morphology The morphological and microstructural properties of both the as-sputtered TiTiN films and the anodized porous substrates were investigated in Figure1A. The assputtered Ti-TiN films present columnar-like structures resulting from the sputtering process, showing sharp edges and quasi-geometrical shape and sharp edges, complying with a previous study. 49 The grain morphology is however dramatically affected by the sputtering deposition power, as shown in Figure 1.A and Figure S1. The grains started to become larger while retaining their quasi-geometrical shape with a maximum of 8,400 nm2 for as-sputtered Ti-TiN film deposited at 50 W, corresponding to more than a fourfold increase from 2,000 nm2 calculated for as-sputtered Ti. As the deposition power increased further, the grain size dramatically dropped to a minimum of 527 nm2 for as-sputtered Ti-TiN deposited at 200 W. It can be observed that a transition occurred between sputtering powers of 100 and 150 W, respectively, evidenced by a severe variation of surface texture across the AFM maps presented in Figure S2 and discussed in Table S2. The transition between large grains of more than 8,000 nm2 and small grains down to 500 nm2 can be ascribed to the increase of sputtering power,50 while the non-linear variation of roughness is in good agreement with the transition within smoothing and roughening processes occurring during the film growth. 51 The electrochemical anodization of the as-sputtered Ti-TiN films induced a sub50 nm nano-porosity in the TiO2-N substrates, with a surface top layer partially


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covering the TiO2 nanotubes as a result of the anodization process.52 The surface morphology of the nanotubes was found to be altered by the power deposition, as can be observed from the SEM micrographs in Figure 1.A and Figure S3. A uniform distribution of pore presenting a geometric average diameter of 34±1.2 nm was calculated for the TiO2 substrate, with a surface porosity of 35%, shown in Figure 1.D. Interestingly, a linear regression of pore size and surface porosity can be revealed for the series of TiO2-N corresponding to a different Ti-TiN film sputtering power. The pore size and surface porosity dropped linearly from 30 nm to 20 nm as the titanium nitride sputtering power increased from 50 to 250 W, respectively. The porosity showed a similar linear trend, varying from 27 to 5 % for TiO2-N obtained from as-sputtered Ti-TiN film deposited at 50 and 250 W, respectively. These results may be attributed to the formation of smaller grains corresponding to a higher density of columnar-like structures. A slight increase of 10 V in applied bias led to a transition from a porous array of coarse columns with 80-100 nm size to a compact, fine distribution of 30-40 nm size.

53

Furthermore, the homogeneous distribution of pores observed from TiO2

substrates up to a sputtering power of 100 W was reduced between 150 and 250 W. High degree of inhomogeneity and morphological disorder compared to as-annealed TiO2 in Figure 1 were observed for an anodized TiN film sputtered at 250 W. It can be assumed that a severe microstructural and chemical alteration of the as-sputtered TiTiN films between 50 and 250 W, as previously described, led to drastically different TiO2-N nanoporous substrates. 54 The correlation between Ti-TiN film sputtering power and TiO2-N morphology and microstructure was assessed by analysing the Small Angle X-ray Scattering (SAXS) patterns across the series of TiO2-N, presented in Figure 2.A. Three main scattering features were identified, assigned to the model of nanotube presented in Figure S4 13 ACS Paragon Plus Environment


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corresponding to the outer diameter, inner diameter, and wall thickness, respectively. The nanotube formed from a sputtered Ti-TiN film, as shown in Figure 2.B, presented rough walls along their length rather than smooth, well-structured cylindrical geometry, likely due to the growth from columnar-like structures.55 The scattering knee well defined between a q range of 0.02 and 0.03 Å-1was attributed to the tube inner diameter. The size of this geometrical feature was calculated from the SAXS model developed following Equation S1 and as previously described. The SEM graphical analysis carried out from Figure 1.A was compared to the SAXS analysis and reported in Figure 2.C. The SEM analysis was in excellent accordance with the SAXS model, with a maximum difference of 18% registered for anodized TiO2, supporting the association of the SAXS knee to the tube inner diameter. Three more knees were observed at 0.009, 0.014 and 0.032 Å-1for anodized TiO2 in Figure 2.A and attributed to scattering features such as tube outer diameter (OD), inner diameter + wall thickness (ID+WT), wall thickness (WT). 56 The SAXS data was used to evaluate variations of morphology and microstructure across the series of anodized as-sputtered Ti-TiN films in response to different sputtering power. The first two SAXS knees presented in Figure 2.A (labelled 1 and 2) showed a shift towards higher q vector, from 0.0097 and 0.014 Å-1 for TiO2N 50 W to 0.013 and 0.016 Å-1 for TiO2-N 100 W. Finally, the first knee moved from 0.014 and to 0.015 Å-1 for TiO2-N 200 W, while the second knee shifted from 0.016 to 0.018 Å-1 . The fourth SAXS knee attributed to WT, occurring at 0.032 Å-1 for asanodized TiO2, could not be revealed from TiO2-N 150 W to TiO2-N 250 W. The shift of knee position was associated with a decrease of ID and OD size as presented in Figure 2.E and in Table S3. This evidence is in good accordance with the loss of uniform distribution of pores and increased morphological disorder reported for 14 ACS Paragon Plus Environment

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anodized titanium nitride films deposited at a sputtering power equal or higher than 150 W. It can also be inferred that the nanotubular configuration observed on as-anodized TiO2 was progressively altered, leading to a poly-dispersion of tube apertures rather than a homogeneous array of TNTs. The broadening of knee 1 and 2, occurring at 0.015 and 0.020 for TiO2-N 250 W, is in good agreement with an increase of poly-dispersion in multi-phase systems. 57 It can be assumed that the chemical composition of the assputtered Ti-TiN film alloy and its microstructure could play a critical role in determining whether nanotubes or nanopores can be fabricated upon electrochemical anodization. 3.2 Microstructure and elemental characterisation The effect of thermal annealing on the fabricated TiO2-N substrates was evaluated to determine the degree of crystallization of amorphous TiO2 produced by electrochemical anodization. (101) planes attributed to TiO2 anatase (ICDD: 04-069240) could be revealed at 25.4 ° across the series of Ti-TiN film sputtering power reported in Figure S5. Interestingly, thermal oxidation of titanium nitride is expected to lead to rutile phase preferentially. 58-59 In this case, however, anatase may have been generated from the sputtered Ti component of the Ti-TiN solid state solution. Further discussion of the TiO2-N microstructure is presented in the Supplementary Material section in reference to Figure S5. The level of doping achievable upon anodization and the chemical state of nitrogen incorporated into the TiO2 matrix was investigated by XPS surface and depthprofile analysis. The chemical composition of both the as-sputtered Ti-TiN films and the TiO2-N nanoporous films were analysed to a depth of 30 nm, corresponding to three cycles of Ar+ etching. The concentration of N was found to increase with the sputtering power as shown in Figure3.A, from a minimum of 3.5 of surface N at% to a maximum 15 ACS Paragon Plus Environment


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of 7.1 at% attributed to a Ti-TiN film sputtering power of 50 and 250 W, respectively. Interestingly, the concentration of Ti and N tended to plateau after 2 etching cycles as shown in Figure S6.A and Figure S6.C. The atomic content of Ti and N evaluated upon the final stage of etching can be associated with the bulk composition of the assputtered Ti-TiN film. The concentration of N in the as sputtered Ti-TiN core showed a quasi-linear variation, from a minimum of 6.2 at% attributed to a sputtering of 50 W, to 32 at% for Ti-TiN film sputtered at 250 W, as reported in Figure 3.A. Conversely, the concentration of Ti in the as-sputtered Ti-TiN film decreased steadily from a maximum of 76.4 at% for the as-sputtered Ti-TiN film deposited at 50 W to 47 at% for as-sputtered Ti-TiN film deposited at 250 W, as shown in Figure S6.C. The chemical composition of the alloy was dramatically affected by the electrochemical anodization, similarly to what previously observed for the morphology. The as-sputtered Ti-TiN alloy deposited at 250 W, corresponding to the maximum loading of 32 at% N, presented only 6.9 at% N in the TiO2-N anodized alloy, resulting in a 78% loss of N. Conversely, the as-sputtered Ti-TiN film deposited at 50 W, containing the lowest loading of N at 6.2 at%, showed a 4.1 at% of N upon anodization, leading to a 34% drop of N. Furthermore, the Ti at% was also sensibly affected by the anodization process, which can be expected from the oxidation and the incorporation of elemental O into the as-sputtered Ti-TiN matrix. As-sputtered Ti-TiN films deposited at 50 W and 250 W presented a 66 and 31 % decrease in Ti at% upon anodization respectively, as presented in Figure 3C, Figure S6.D and summarized in Table S4. Finally, the N/Ti ratio for the as-sputtered Ti-TiN alloy and anodized TiO2N varied remarkably across the series of N concentration in the as-sputtered Ti-TiN matrix, as observed in Figure 3A and Figure 3B. The as-sputtered Ti-TiN films presented a spike in N/Ti ratio with increased N loading, rising from 0.1 to 0.7 for a


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loading of 6.2 and 32 at% N, respectively. Upon anodization, the N/Ti ratio spanned from 0.1 to 0.2, attributed to a loading of 4.1 and 6.9 at% of N, respectively. It can be inferred that the the severe drop of N at% and the plateauing of N/Ti ratio after electrochemical anodization should be attributed the structural and chemical stability of the titanium-titanium nitride solid state solution fabricated by metal sputtering. The deposition of non-equilibrium solid state solutions by PVD sputtering can lead to the formation of highly disordered energy states, which tend to recombine quickly when energy is provided to the system.60-62 In this case, the oxidative environment provided by electrochemical anodization may have thermodinamically favoured the selective removal of N and formation of Ti-O bonds, in good agreement with previous reports. 42, 63

A logarithmic growth model was extrapolated from the comparison of the final N content into the TiO2-N substrates and the original N concentration in the as-sputtered Ti-TiN alloy. According to the growth model reported in Equation S2 and represented in Figure 4A, the quantity of N doping attainable in the final TiO2 matrix was set on a relatively narrow window, from 4.1 at% to 6.9 at% corresponding to a large variation of sputtering power, from 50 W to 250 W. Controlling the amount of N doping into TiO2 nanomaterials is essential to tailor the visible light sensitization . It was postulated that different levels of N loading can have a different impact on the band-gap narrowing and therefore on the photo-response 64-65. Loading of N below 4.2 at% the band gap can be narrowed by mixing N 2p states with O 2p states.

66-67

In the case of the present

study, the electronic configuration of the modified band-gap could be likely described by the second mechanism formulated.



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The chemical configuration of Ti and N atoms and the resulting chemical bonds between the two elements were further assessed to determine the impact on the catalytic activity of the TiO2-N substrates. XPS Depth profile analysis of the as-sputtered Ti-TiN films was carried out to investigate the nature of chemical bonds between Ti and N prior to electrochemical anodization. The Ti 2p XPS depth profile spectra corresponding to different N loading into the as-sputtered Ti-TiN matrix was analysed in Figure S7 and the appearance of different Ti species evaluated. Ti (0), Ti-N, Ti oxynitrides (Ti-NxOy) and Ti (IV) chemical states were attributed to peaks occurring at 453.6, 544.9, 456.9 and 458.8 eV, respectively, as reported in Table S5.

68-70

The

peak attributed to Ti (0) chemical state progressively shifted from 453.6 eV for assputtered Ti film towards 454.3 eV for as-sputtered Ti-TiN film deposited at 250 W. This shift, as well as the progressive increase of the integrated areas corresponding to Ti-N and Ti-NxOy species, revealed the introduction of at least two different Ti-N chemical states. Further insight into the nature of chemical bonding between Ti and N could be gained by evaluating the surface HR spectra of Ti 2p reported in Figure S8. The as-sputtered Ti-TiN film shown in Figure S8.A revealed the presence of different Ti sub-oxide species, such as Ti(II) and Ti (IV), compatible to a partial oxidation of the surface typically registered for sputtered thin films.

71

Furthermore, the concentration

of Ti-N species increased from 6.9 % to 21.3 % as shown in Figure S9 and Table S6, for a N loading of 6.2 and 32 at%, respectively. Nonetheless, the concentration of Ti (0) decreased from 20.4 to 15.6 % as the sputtering power of Ti-TiN films increased from 50 W to 250 W. This trend confirmed that nitride bonds were progressively replacing the Ti (0) chemical state as the as-sputtered Ti-TiN film deposition power increased, hence resulting in a chemical and microstructural recombination of the sputtered films.


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The electrochemical anodization produced a significant alteration to the Ti oxidation spectra, as shown in the XPS depth profile analysis reported in Figure S10. Two peaks, corresponding to the orbital split of Ti (IV), 3/2 and 1/2, can be identified on the surface spectra. Nonetheless, as the etching proceeded, a shoulder at binding energy below 458 eV started to form, corresponding to the integrated area assigned to Ti-NxOy and Ti-N bonds. This shoulder can be partially assigned to a reduction of Ti (IV) operated by the Ar+ bombardment,72 however for higher concentration of N in TiO2-N, such as 6.4 and 6.9 at%, this shoulder became more pronounced and finally appeared as a peak. This evidence confirmed the increase of Ti-NxOy and Ti-N bonds for high loading of N in the TiO2-N substrates. The HR XPS spectra of N 1s were consequently evaluated to investigate the variation of N chemical state prior and upon electrochemical anodization. Four different species were identified in the XPS analysis of as-sputtered Ti-TiN films and summarized in Table S7, attributed to N occurring in Ti oxynitrides (Ti-NxOy) 70, 73-75, Ti-N 31, 51, 74, Ti-O-N 50, 76 and impurities such as NOx 76, at 396.2, 397.3, 399 and 401.6 eV, respectively. The depth profile analysis reported in Figure S11 for the as-sputtered Ti-TiN films revealed the presence of Ti-O-N species on the surface. This result is consistent with previous reports on titanium nitride coatings presenting a partial surface oxidation, which is attributed to Ti bound specifically to NO surface groups 75, 77-78. The HR XPS spectra of N 1s for the as-sputtered Ti-TiN film surface reported in Figure S12 confirmed the presence of surface oxidation involving N and its chemical environment, complying with the Ti 2p surface spectra in Figure S8. Conversely, the as-sputtered Ti-TiN film core showed a high level of purity in terms of Ti-N bonds, as presented in Figure S13. Interestingly, as the N content increased from 6.2 to 32 at%, the % of Ti-N bonds dropped slightly from 82% to 73% while the Ti-NxOy species



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showed a threefold increase, as reported in Figure S14, from 4.2 % to 15.6 %. This result is highlighted by the appeareance of a shoulder on the N 1s spectra at 396.2 eV strarting from a N concentration of 23 at% , clearly visible in Figure S13.D. Upon electrochemical anodization, the surface of the TiO2-N substrates was constituted mostly of Ti-O-N species, as observed from the depth profile analysis in Figure S15 and the HR XPS spectra of N 1s in Figure S16. Nevertheless, a peak at ~ 396.9 eV appeared after only one etching cycle, meaning that the surface oxide layer, compatible with the morphology highlighted in Figure 1, constituted a thin, sub-10 nm layer. Interestingly, the depth profile analysis showed the progressive appearance of peaks attributed to Ti-N and Ti-NxOy. Furthermore, the position of the Ti-N peaks shifted from 397.3 to 396.9 eV, for as-sputtered Ti-TiN films and anodized TiO2-N, respectively, whilst the peak area became broader. This result may be attributed to the generation of atomic β-N states in the TiO2 lattice, assigned to substitutional N-doping, as a result of the oxidation process. 31, 79-80 The variation of integrated areas correlated to the Ti-N and Ti-NxOy peaks was calculated in Figure 4.B. At low N loading, such as 4.2 at%, Ti-N constituted the major costituent of the N chemical state, at 44.5 %. Conversely, a progressive ingrease of Ti-NxOy states appeared as the N loading increased, from 13.3 to 46.4 % for 4.2 and 6.9 N at%, respectively . The variation of the ratio (RN) between Ti-N and Ti-NxOy could be assessed from the observation of Figure 4.D, Figure 4.E and Figure 4.F and Figure S17, referring to a content of N of 4.2, 5.3 and 6.9 at%, respectively. RN varied drastically from 3.3 to 0.4 for a N loading of 4.2 and 6.9 at%, respectively, corresponding to a 90% drop, as shown in Figure 4.C. The elemental N/O ratio (RO) was calculated to assess the incorporation of oxygen into the N-TiO2 matrix. Ro increased from 0.07 to 0.14, as reported in Figure 4.C, for a N content of 4.1 at% and 6.9 at%, in good agreement with the loss of Ti (0) chemical state


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reported in Figure S9 and the increase of available nitride bonds as reaction sites during the anodization. This result can be explained by the tendency of O2 to oxidize the available Ti (0) states and subsequently react with the nitride bonds to form Tioxynitrides, prior to reach a complete conversion to TiO2.

59, 73, 81.

High deposition

power of as-sputtered Ti-TiN films reduced the content of available Ti (0), as showed in Figure S9, therefore O2 can potentially react on a higher extent of nitride bonds to form under-stechiometric intermediate species such as Ti-oxynitrides. Although the presence of β-N can be observed by the Ti-N peak broading and shift towards lower binding energy, it can be assumed that the electrochemical oxidation induced preferentially the formation of Ti-oxynitrides from the Ti-N species and TiO2 from the Ti(0) states. The formation of β-N can be therefore ascribed to diffusion of N generated from the reaction of Ti-N with O towards reactive Ti-O sites in an oxidative environment. 76, 82 3.3 Optical Properties The impact of the N loading and specific chemical state on the optical response of the TiO2-N substrates was consequently evaluated. The light absorption was assessed by Diffuse Reflectance Spectroscopy (DRS)83 spectra analysis of the substrates and the Kubelka-Munk (F(K-M)) function was reported in Figure 5.A and in Equation S3, used as an approximation of the TiO2-N substrate absorption. 84-85 An increase in the absorption edge between 380 and 430 nm was revealed for a N loading of 4.2 at%. The shoulder at the absorption edge started to become less pronounced and progressively leant towards the TiO2 line slope for N loading higher than 5.3 at%. The F(K-M) spectra presented an oscillatory trend typical in thin films affected by the interference phenomenon, attributed to light beam traversing surface layers presenting a difference in refractive index. 86-88 89 The Tauc plot was assessed to



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evaluate the semi-conductor band-gap according to Equation S4 and Equation S5. Asannealed TiO2 presented a band-gap value of 3.4 eV as shown in Figure 5.B, in good agreement with the values reported in literature varying from 3.1 to 3.87 eV. 14, 49, 86 A band-gap minimum of 2.7 eV is registered for a N loading of 4.1 at%, representing a 20% decrease from pure TiO2. Conversely, the highest N loading (6.9 at%) led to the highest band-gap value, 3.2 eV. Interestingly, the trend registered in Figure 5.B complied with the correlation between N loading in N-doped TiO2 and resulting bandgap reported in previous studies. A band-gap minimum is observed for N at% < 3, 38, 90 whilst an increase of N content could not lead to a narrower band-gap.

39

High

concentration of N is also expected to be detrimental to the photoelectron efficiency, as the O vacancies induced to attain the charge neutrality can act as a recombination centre for charge carriers. 22, 91 The photocatalytic efficiency of the N-doped TiO2 substrates was evaluated on MB, used as a reference organic molecule for benchmark studies

92.

First, the

degradation of MB was carried out under a 320-480 nm filter applied on the UV light source. To inhibit the photolysis of MB, no wavelength above 500 nm was applied 93. The benchmark TiO2 nanotubes could degrade only ~80% of the original MB concentration after 6 h of treatment as shown in Figure 6.A. The optimal performance was attributed to TiO2-N substrates containing 4.1 at% of N, which could achieve a ~80% degradation of MB in ~3 h. However, as the concentration of N doping increased further, the degradation kinetics was not improved. Only 60% of MB could be removed by TiO2-N containing 6.9 at% of N in 3 h. As observed for the visible light absorbance and the calculated band-gap, a specific concentration of N could be indicated as the most efficient across the series, whilst past this point any addition of N was not advantageous to the catalytic performance. Furthermore, an increase in N content, from 4.1 to 6.9


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at%, determined a higher concentration of Ti-NxOy and a decrease of the RN ratio to from 3.3 to 0.4 as shown in Figure 4.B and Figure4.C. This result can be associated with the conversion of Ti-N into an excess of Ti-NxOy favoured by high content of N, potentially leading to the formation of oxygen vacancies acting as charge recombination centre.35, 94 The degradation kinetics for TiO2 films are expected to be slower than those for TiO2 nanoparticles, which are in the range of 10-180 min. 44, 95-96 TiO2 nanoparticles typically present higher specific surface area and low bulk charge recombination than TiO2 films when fabricated in an optimal size in the sub-10 nm range.97

96

However,

the fabrication of TNT films allows for fast charge transport rate and better control of crystallinity and surface functionalities than in NPs, leading to fastest degradation kinetics up to 4 times if compared to commercial TiO2 NPs.98-99. In this study, the degradation of MB occurred in a time range of hours, in good accordance with studies on surface-decorated TiO2 nanotubes or doped TiO2 substrates. 43, 100 Specifically, the TiO2-N TNT substrate containing 4.1 at% of N presented similar degradation kinetic under UV light of N-doped TiO2 NPs fabricated by calcination at 400 to 600 °C, resulting in a 35-40% degradation in 30 min. 101 The photo-catalytic kinetic constants were calculated and shown in Table S8. The degradation of organic compounds in heterogeneous catalysis can be modelled according to Langmuir-Hinselwood kinetic theory as shown in Equation S6 and Equation S7.

102

The degradation of MB over TiO2-N substrates was in good

accordance with a pseudo-first order kinetics for almost all the substrates, and it can therefore be correlated to the MB concentration. The severe variation of morphology observed in Figure 1 indicates that the catalytic surface and the surface density of active sites were radically different across the series of doped TiO2-N substrates. The fastest pseudo-first order kinetics could be attributed to a 4.1 at% N concentration, resulting in 23 ACS Paragon Plus Environment


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a kinetic constant of 9.1 10-3 min-1 which represented almost a threefold raise from pure TiO2, standing at 3.4 10-3 min-1. Nonetheless, the kinetic constant dropped as the N concentration in TiO2-N increased, down to 6.9 10-3 min-1. The results were in good accordance with the visible light sensitization reported in Figure 5A and the detrimental effect of high N doping and Ti-NxOy content previously discussed. The activity of the TiO2-N substrates under pure visible light was tested by applying a 400-500 nm filter to the light source. Interestingly, only the substrate presenting a N loading of 4.1 at% showed a significant activity under pure visible light, degrading 50 % of MB in 6 h, as seen in Figure 6.B. As the concentration of N increased, the degradation became slower, down to 17 % and 14 % for as-annealed TiO2 and TiO2-N with 6.9 at% N doping, respectively. The little activity registered for asannealed TiO2 can be attributed to the presence of a small amount of photoactive Ti suboxides, as shown in Figure S18. It can therefore be confirmed that increasing the N loading from 4.1 at%, corresponding to the minimum band-gap of 2.7 eV, was detrimental to the visible light activity and almost inhibited the MB degradation. The kinetic constants were evaluated and reported in Table S9. In this case, a zero order kinetic model fitted adequately the calculations. Zero-order kinetic models are typical of reactions where the surface of active sites is quickly saturated and mass diffusion, rather than the concentration of reactive species, becomes the controlling stage. 93, 103 The fastest degradation rate was assigned to a 4.1 at% N loading, presenting a kinetic constant of 1.9 10-3 min-1 whilst the rate became progressively slower upon the addition of further N doping. Compared to N-doped TiO2 NPs, the kinetic constant was of the same order of N-doped TiO2 fabricated by sol-gel method.

104

Furthermore, TiO2-N

containing 4.1 at% of N presented slight superior kinetic constant than N-doped sputtered TiO2 films presenting N concentration of 1.2 at%.

31

Interestingly, TiO2 24

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nanospheres containing TiO1-xNx could lead to a fastest degradation of MB by nitriding TiO2 nanosphere for at least than 7 h at 700 °C.

33

This result was associated to the

appearance of a broad peak between 396.5 and 398 eV upon 7 h of nitridation treatment, however no quantitative analysis of the N chemical state was carried out to determine the role of the different N species. The impact of N loading on the kinetic constant for both experiments was further investigated. A kinetic ratio, representing the variation of kinetic constant for TiO2-N compared to as-annealed TiO2, was calculated and reported in Figure 6.C. TiO2-N substrate presenting the minimum N doping, 4.1 at%, and the minimum bandgap, 2.7 eV, outperformed the rest of the series, showing a maximum kinetic ratio of 2.7 and 3.3 under UV-visible and pure visible light, respectively. Conversely, the highest N doping, corresponding to 6.9 at% of N, led to a 30% reduction of photocatalytic rate under visible light. Apart from the different surface morphology revealed across the series of TiO2-N substrates, the main factors that could have affected the visible light sensitization and the catalytic activity are the N doping concentration and the N chemical state. It was demonstrated that an increase of N doping led to a strong change of Ti-N/Ti-NxOy ratio, from 3.3 to 0.4 for 4.1 and 6.9 at% loading, respectively. The drastic decrease of this factor can therefore be correlated to the loss of catalytic performance, possibly due to a variation of electronic configuration in the band-gap and the addition of charge recombination centres as previously stated. The efficient removal of MB was evaluated in a recyclability study shown in Figure 6F, carried out on the best performing sample, TiO2-N with 4.1 at% N loading under UV-visible light irradiation. The kinetic constant decreased from 9.1 10-3 min-1 to 6.1 10-3 min-1 upon three catalytic tests under visible light, corresponding to a loss of 25 ACS Paragon Plus Environment


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33% in kinetic activity. Nevertheless, the degradation of MB only decreases slightly, from 75% to 65%, after 3 h of test. This result can be interpreted as some catalytic sites were blocked due to a partial desorption of MB or deactivated upon different cycles of treatment by the absorption of degradation by-products.


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4. Conclusion TiO2-N nanoporous substrates have been fabricated by electrochemical anodization of as-sputtered Ti-TiN films for the first time. The as-fabricated nanoporous substrates presented controllable morphology, strictly dependant on the concentration of N in the thin film. Low concentration of N, such as 6.2 at%, determined a homogeneous, iso-dispersed distribution of nanotubular apertures whereas as the N loading increased, the surface showed a higher degree of disordered and the nanotubes presented a poly-dispersed pore size. Upon anodization, the final quantity of N doping into the TiO2-N substrates spanned from 4.1 to 6.9 at%, showing a logarithmic growth correlated to the original N content in the as-sputtered Ti-TiN film. TiO2-N substrates with 4.1 at% N possessed the lowest band gap of 2.7 eV, attributed to the introduction of Ti-NxOy species and a Ti-N/Ti-NxOy ratio of 3.3. The increased N content was associated to a higher oxidation of Ti-N bonds and resulted in a loss of light absorbance and deterioration of the catalytic performance. The catalytic enhancement was attained for a N concentration of 4.1 at% and a Ti-NxOy content of 13.3 %, enabling the degradation of 80% of MB in 3 h under UV-visible light. The calculation of the kinetic constant showed a threefold increased compared to the benchmark as-annealed TiO2 TNTs.



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Figure 1. SEM micrographs of as-sputtered Ti-TiN films and anodized TiO2-N films (A) at different titanium nitride sputtering powers: 0 W, 50 W, 150 W and 250 W. Grain surface and Feret analysis of the Ti-TiN alloy (B), Root Mean Square of Roughness Sq and Maximum Height Sz calculated from AFM measurements (C), surface porosity and pore size calculation of the anodized films (D).


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Figure 2. SAXS scattering patterns for as-anodized TiO2-N substrates at different titanium nitride sputtering power (A), lateral view of TiO2 nanotubes for as-annealed TiO2 (B), comparison of SEM graphical analysis and SAXS analysis for inner diameter (ID) Comparison of SEM graphical analysis and SAXS modelling for as-annealed TiO2 geometrical features, such as outer diameter (OD), inner diameter (ID), wall thickness (WT) (D). Variation of nanotube geometrical features calculated from SAXS modelling for the series of anodized TiO2-N at different titanium nitride sputtering power (E).



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Figure 3. N at% and N/Ti ratio for surface - - -, core level ▬, surface - - -, core level ▬, respectively, for as-sputtered Ti-TiN films (A) and anodized TiO2-N substrates (B). (C): Ti at% in TiO2-N and N/Ti ratio resulting from different titanium nitride sputtering power, surface - - -, core level ▬, surface - - -, core level ▬, respectively.


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Figure 4. Calculated logarithmic growth model for N doping concentration in anodized TiO2-N substrates (A), variation of ▬ Ti-N and ▬ Ti-NxOy species calculated from HR XPS N 1s spectra of as-sputtered Ti-TiN films (B) and anodized TiO2-N substrates with corresponding Ti-N/Ti-NxOy ratio ▬ (Rn), (C). High Resolution XPS spectra of TiO2-N at different N concentration: 4.1 at%, 5.3 at%, 6.9 at%, (D), (E), (F), respectively. N chemical state was analysed as follows: ▬ Ti-N, ▬ Ti-O-N, ▬ TiNxOy, ▬ NOx.



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Figure 5. Kubelka-Munk function (A) for series of anodized TiO2-N films at different N at% loading: ▬ as-annealed TiO2, ▬ 4.1 at%, ▬ 4.6 at%, ▬ 5.3 at%, ▬ 6.4 at%, ▬ 6.9 at%. (B): calculated band-gap from Tauc plot equation for different N at% loading in anodized TiO2-N substrates with corresponding original titanium nitride sputtering power.


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Figure 6. Degradation of methylene blue for different anodized TiO2-N substrates at different N at% loading under UV-visible light (A) and visible light (B): ▬ as-annealed TiO2, ▬ 4.1 at%, ▬ 4.6 at%, ▬ 5.3 at%, ▬ 6.4 at%, ▬ 6.9 at%. Kinetic ratio evaluation of k TiO2-N/k TiO2 at 320-480 nm light filter and 400-500 nm light filter (C), cyclability test for anodized TiO2-N substrate with 4.1 at% N loading (D).



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5. Supporting Information Table S1, list of as-sputtered Ti-TiN films at different sputtering conditions; Figure S1, SEM micrographs of as-sputtered Ti-TiN films sputtered at different titanium nitride sputtering power; Figure S2, AFM micrographs of as-sputtered Ti-TiN films deposited at different titanium nitride sputtering power; Table S1, calculated area roughness parameters at different titanium nitride sputtering power for as-sputtered TiTiN films; Figure S3, SEM micrographs of anodized TiO2-N films sputtered at different titanium nitride sputtering power; Figure S7, model of TiO2 nanotube considered for SAXS calculations and SAXS modelling parameters; Table S2, fitting of SAXS knee based on formulated SAXS model for series of anodized TiO2-N substrates; Figure S8, XRD analysis of TiO2-N substrates deposited at different Ti-TiN sputtering power; Figure S9, N at% and Ti at% for the as-sputtered Ti-TiN films and anodized TiO2-N substrates calculated from XPS depth profiling; Table S3, variation of N at% and Ti at% for series of as-sputtered Ti-TiN films and anodized TiO2-N substrates; Figure S10, XPS Depth profiling analysis of Ti 2p spectra for as-sputtered Ti-TiN films at different N at%; Table S4, model for Ti 2p XPS High Resolution spectra with peak assignment and relative position and FWHM; Figure S11, high Resolution XPS spectra of Ti 2p for as-sputtered Ti-TiN films at different N concentration; Figure S12, variation of Ti-N bonds and Ti (0) chemical state in Ti 2p spectra for sputtered as-sputtered Ti-TiN films at different N at%; Table S5, variation of Ti chemical state at different as-sputtered Ti-TiN films deposition power; Figure S13, XPS Depth profiling analysis of Ti 2p spectra for anodized TiO2-N films at different N at%; Table S6, fitting of N 1s High Resolution XPS spectra and peak assignment, with relative position and FWHM; Figure S14, XPS depth profile analysis of N 1s for as-sputtered Ti-TiN films at different N at%; Figure S15, high Resolution


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XPS spectra of N 1s corresponding to as-sputtered Ti-TiN film surface at different N at % concentration; Figure S16, High Resolution XPS spectra of N 1s corresponding to as-sputtered Ti-TiN film core at different N at % concentration; Figure S17, variation of Ti-N and Ti-NxOy species calculated from the N 1s spectra for as-sputtered Ti-TiN films; Figure S18, XPS depth profile analysis of N 1s for anodized TiO2-N substrates at different N at%; Figure S19, high Resolution XPS spectra of N 1s corresponding to anodized TiO2-N substrate surface at different N at % concentration; Figure S20, high Resolution XPS spectra of N 1s corresponding to anodized TiO2-N substrate core at different N at % concentration; Table S7, calculation of kinetic constants resulting from methylene blue degradation under 320-480 light filter for anodized TiO2-N substrates; Figure S21, XPS spectra of as-annealed TiO2 substrate; Table S8, Calculation of kinetic constants resulting from methylene blue degradation under 400-500 light filter for anodized TiO2-N substrates.

6. 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 also acknowledges the ARC for his DECRA DE180100130 fellowship. Mr Andrea Merenda and Dr. Ludovic Dumée would also like to acknowledge ARC Research Hub for Energy-efficient Separation IH170100009. The authors also acknowledge the Australian Synchrotron, SAXS/WAXS beamline, for funding the proposal M13027, 2018/1 and Beamline Scientists Dr.Adrian Hawley and Dr. Stephen Mudie for technical support. The authors would like to acknowledge Prof. Peter Hodgson for mentoring and



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support to the study. The authors would also like to acknowledge Dr. François Marie Allioux and Dr. Lachlan Hyde, Swinburne University, for supporting the XPS experimental tests. 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 Zeiss Supra 55VP FEG SEM and assistance from Dr Andrew Sullivan.


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Enhanced Visible Light Sensitization of N-doped TiO2 Nanotubes

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