Thermal annealing effect on nitrogen-doped TiO2 thin films grown by high power impulse magnetron sputtering plasma power source

Accepted Manuscript Thermal annealing effect on nitrogen-doped TiO2 thin films grown by high power impulse magnetron sputtering plasma power source

C. Stegemann, R.S. Moraes, D.A. Duarte, M. Massi PII: DOI: Reference:

S0040-6090(17)30053-6 doi: 10.1016/j.tsf.2017.01.043 TSF 35755

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

26 April 2016 14 January 2017 21 January 2017

Please cite this article as: C. Stegemann, R.S. Moraes, D.A. Duarte, M. Massi , Thermal annealing effect on nitrogen-doped TiO2 thin films grown by high power impulse magnetron sputtering plasma power source. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tsf(2017), doi: 10.1016/ j.tsf.2017.01.043

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Thermal annealing effect on nitrogen-doped TiO2 thin films grown by high power impulse magnetron sputtering plasma power source

C. Stegemann1,a, R. S. Moraes1,b, D. A. Duarte2,c, M. Massi1,3,d

Technological Institute of Aeronautics, Plasmas and Processes Laboratory, São José

dos Campos, SP, Brazil. 2

Federal University of Santa Catarina, Technological Centre of Joinville, Department

of Mobility Engineering, Joinville, SC, Brazil

E-mail: [email protected], telephone +55 12 39475942.

(Corresponding author)

E-mail: [email protected]

E-mail: [email protected]

PT

d

AC

Abstract

CE

c

E-mail: [email protected]

ED

b

MA

a

Mackenzie Presbyterian University, São Paulo, SP, Brazil

NU

3

SC RI PT

1

The work reports plasma assisted growth of nitrogen-doped titanium dioxide (NTiO2) thin films using high power impulse magnetron sputtering (HiPIMS) power source and effect of post-deposition thermal annealing. The films were deposited at low pressure. The binding energies of elements of interest, the energy gap, crystallinity and morphology of the films were analyzed before and after annealing. The results showed an increase in binding energies, a fact attributed to enhanced

ACCEPTED MANUSCRIPT oxidation, after the annealing process. Only nitrogen doped samples grown by HiPIMS exhibited the presence of substitutional nitrogen. The energy gap of the films was found to decrease after doping and annealing. Improvement in crystallinity with a small shift in the crystalline peaks indicating decrease in lattice parameter, which could be seen by surface smoothing, was observed after thermal annealing. As a result

SC RI PT

of the growth of films by HiPIMS, nitrogen doping and post-deposition thermal annealing, improvements in the properties of the films which have relevance for photocatalytic and energy applications were observed.

NU

Keywords: N-doped TiO2, high power impulse magnetron sputtering, binding

MA

energy, surface oxidation.

ED

1. Introduction

Titanium dioxide (TiO2) is one of the most used semiconductors in several

PT

technological applications due its high corrosion resistance, photocatalytic activity,

CE

long-term stability, nontoxicity and wide band gap [1]. For electric power generation, this structure is commonly used as anchoring material in the so-called third generation

AC

solar cells, also named dye-sensitized solar cells (DSSC) [2], in which recent publications show overall conversion efficiencies above 15% for the new perovskitesensitized solar cells [3]. Other applications include reduction of hydrogen in photoelectrochemical solar cells, sterilization of environments, cancer treatment, decomposition of organic compounds, and fabrication of thin layers with high dielectric constant for microelectronic devices [4–7].

ACCEPTED MANUSCRIPT Regarding to the application of titanium dioxide for power generation some electronic structure modifications are necessary, for example, amplify the energy absorption range of the solar spectrum since the absorption of radiation in spectral regions near to ultraviolet is greater than other regions [8]. Among several alternatives, creation of oxygen vacancies and doping are the most used [9,10]. Both

SC RI PT

procedures incorporate mid-gap states in the film lattice that absorb radiation from visible and near infrared spectra [11]. Oxygen vacancies induce energy states between 0.75 and 1.18 eV below CBM (conduction band minimum) due to the incorporation of active Ti3+ states [12], and it may be generated by annealing in vacuum, thermal

NU

treatment under reduced atmospheres, and bombardment with electron beam, gamma ray or other kind of energetic particles [10]. Some doping elements create electronic

MA

states above VBM (valence band maximum) or below/inside CBM. Among the various doping elements, nitrogen was found to be one of the most efficient for TiO2

ED

due to the narrowing of the band-gap caused by incorporation of N2p states above VBM [9].

PT

Several methods are used for deposition of N-doped TiO2 [9,11–17] among

CE

which the reactive sputtering is one of the most efficient on account of its possibility for controlling a wide range of properties in thin film technology [18]. However,

AC

studies conducted in the past show that excessive concentration of nitrogen by conventional sputtering sources leads to amorphous film, even after thermal annealing [11,19]. Thus, several deposition methods with heated substrates, annealing with lasers and alternative sputtering sources have been suggested to improve the film crystallinity [16,20–22]. In this paper, we have demonstrated the capability of the reactive HiPIMS for deposition of crystalline N-doped TiO2 thin films with high concentration of nitrogen particles in the film lattice.

ACCEPTED MANUSCRIPT

2. Material and Methods

Films were deposited on FTO glass (glass covered with a thin transparent conductive coating layer of tin oxide doped with fluorine with resistivity of 7

SC RI PT

ohm/square - Solaronix TCO22-7). The depositions were performed using a commercial HiPIMS power source (Solvix HiP3) which work as in DCMS (direct current magnetron sputtering) mode, without substrate heating, by sputtering from a 4 inches metallic titanium target (Kurt J. Lesker Company, purity 99.9%) and gases

NU

argon (Ar), oxygen (O2) and nitrogen (N2) . The deposition parameters are given in table 1. Films were analyzed before and after thermal annealing at 400 °C for 2 hours

MA

under argon atmosphere at 0.76 Torr (~ 100 Pa).

The XPS (X-ray photoelectron spectroscopy) measures were obtained on

ED

VSW HA-100 spherical analyzer and unmonochrometed AlKα radiation (hν = 1486.6 eV). The high-resolution spectra were measured with constant analyzer pass energies

PT

of 44eV, which produce a full width at half-maximum (FWHM) line width of 1.8 eV

CE

for the Au (4f7/2) line on 1.5 10-8 Torr of pressure. The optical parameters, like transmittance and reflectance, were measured by spectrophotometry (JASCO V570),

AC

and the band-gap was calculated by the Tauc extrapolation method [23] for indirect semiconductors using the transmittance and reflectance curves. The crystallographic phases were obtained from GIXRD (grazing incidence X-ray diffraction) patterns taken at room temperature in a Shimadzu XRD 6000 goniometer using copper target (CuKα radiation 1.5418Å), from 10° to 80° 2θ values, 0.02°/s scanning speed, 40 kV voltage, 30 mA current and 0.39° grazing incidence angle. In addition, changes in surface morphology of the films were analyzed by AFM (atomic force microscopy)

ACCEPTED MANUSCRIPT using Shimadzu SPM 9500J3 equipment, and the wetting contact angle of the samples (12 hours after the deposition of the films and stored protected from light) was measured using a Ramé-Hard Model 500 goniometer with 6 μl water drop. The analyses were performed by the software provided. The goniometer software provided measurements of contact angles with their respective error bar. The distances between

SC RI PT

peaks and valleys, grain size and average roughness RA were obtained from AFM images.

NU

3. Results and Discussion

MA

3.1. X-ray photoelectron spectroscopy (XPS)

All XPS graphics are in the same range of intensity and is possible to compare

ED

them with each other, although their measurement units are arbitrary. The XPS results are shown in Figs. 1-4 for annealed and as-deposited samples. Figure 1 shows the

PT

Ti2p core levels. Spectra were decomposed in three contributions: Ti4+2p1/2, Ti4+2p3/2

CE

and Ti3+2p1/2 [17,24,25]. The first two bands are assigned to TiO2 and the last one to Ti2O3 [10,26]. The peak positions, shown in table 2, indicate that the thermal

AC

annealing shifted the binding energy to higher values, which was caused by surface oxidation. The incorporation of nitrogen also shifted the binding energy to higher values. This effect was previously reported in ref. [13] and was ascribed to the O–Ti– N bonding in the film lattice. The relative concentration of oxygen was increased from 64.3 to 65.7% for NTiO2 and from 65.8 to 67.1% for TiO2 after thermal annealing. These values were calculated from the integrated peak area according to described in ref. [27] taking into

ACCEPTED MANUSCRIPT account the Ti2p, O1s and N1s core levels. The atomic sensitive factors were taken from ref. [28]. The increase in oxygen concentration after the annealing process is due to the occupation of available sites on the film surface by oxygen atoms. The high electronegativity of oxygen attracts electrons (or electron density) towards itself [29] which in turn increases the binding energy because of the higher potential energy

SC RI PT

between the species involved [30].

Spectra for O1s core levels are shown in Fig. 2. Results indicate the presence of two peaks around 529 and 531 eV, shown in table 3, in which the first one is assigned to Ti–O bonding in the TiO2 lattice (Lattice) [13], and another one to

NU

interstitial bonding such as H2O [12] or nitrogen-based compounds (Surface) [13]. All peaks were also shifted to higher binding energies after thermal annealing.

MA

The most interesting results are shown in the N1s core levels depicted in Fig. 3. The spectra for the non-doped TiO2 indicate a peak around to 400 eV that is

ED

assigned to interstitial nitrogen (γ-N) [31,32]. The spectra for the N-TiO2 indicate four contributions; two related to interstitial nitrogen (γ-N), one related to substitutional

PT

nitrogen in the TiO2 lattice (β-N) and another one ascribed for the nitrogen in the TiN

CE

(titanium nitrite) lattice [11]. The positions of each one is shown in table 4. It was observed that the peaks related to γ-N decreased after thermal annealing, mainly the

AC

peak around 402 eV. This effect is caused by desorption [31]. Past studies assigned the peak of titanium nitrite by XPS around 396 eV to nitrogen in the TiN lattice [31]. Similar studies about titanium oxynitride found that the O–Ti–N bonding was identified by a peak around 397.5 and 397.6 eV [33]. This increase in the binding energy upon incorporation of oxygen atoms into TiN is also correlated with the higher electro-negativity of the oxygen in comparison to nitrogen [11]. Therefore, for the as-deposited films, the peaks located at 395.36 and 397.79 eV

ACCEPTED MANUSCRIPT are assigned to TiN and O–Ti–N, respectively. After thermal annealing, the surface oxidation also shifted the binding energies to higher values. In order to study the ability of HiPIMS discharge technique to synthesize substitutional nitrogen doped TiO2 films, synthesis of N-doped TiO2 films were carried out using reactive DCMS technique under same experimental conditions and

SC RI PT

investigated by XPS. Figure 4 shows the N1s core level for the annealed and asdeposited samples. Results indicate the absence of the peak assigned to the substitutional nitrogen for all samples.

This observation is quite interesting and may be explained by the enhanced

NU

production of atomic oxygen in the HiPIMS discharge [34]. Due to the high nitrogen flow rate defined in the experimental setup, TiN layers are firstly grown on the

MA

substrate. Then, oxygen oxidizes the film surface and gives rise to O–Ti–N bonds. The incorporation coefficient, known as sticking coefficient [35], in this case is

ED

actually a combination of atomic oxygen and molecular oxygen and there is no consensus yet in literature between experiments or models concerning the sticking

PT

coefficient of a reactive gas during a sputtering deposition process [36].

CE

Depla et al. found that decrease in sticking coefficient is due to the decrease in target to substrate distance for TiN deposition, that was associated with increase of

AC

the desorption effect by energy flux variation in the process [37]. Moreover, high difference in electro-negativity of the metal and the reactive gas during a plasma deposition elevate the sticking coefficient [36]. For many cases it was observed that the sticking coefficient for atomic oxygen is higher than molecular oxygen. Kubart et al. has found that the sticking coefficient for O2 is about 0.3 whereas that of O is about 1 [38]. The HiPIMS discharge is characterized by higher degree of ionization than DCMS. Hence it is plausible to assume that the higher amount of atomic oxygen

ACCEPTED MANUSCRIPT in the HiPIMS discharge increases the oxidation processes on the film surface because of the higher sticking coefficient of atomic oxygen compared to molecular oxygen.

SC RI PT

3.2. Optical spectrophotometry

Figure 5 shows the Tauc plot. The extrapolation of data back to the horizontal axis was done by application of first-order derivatives. The Tauc extrapolation method is not suitable for determining the band gaps of crystalline semiconductors

NU

because it was developed for amorphous solids, although, it is a useful method for qualitative studies. In graphs h is the Planck’s constant,

MA

absorption coefficient.

frequency of vibration and α

Results indicate that the incorporation of nitrogen in the TiO2 lattice decreased

ED

the energy gap from 3.45 to 3.40 eV (see Fig. 1(a) and Fig. 1(b)), and this decreasing is assigned to the N2p states added above the valence band [9]. The energy gaps of

PT

the post-annealed TiO2 and N-TiO2 decreased to 3.41 and 3.27 eV, respectively (see

CE

Fig. 1(c) and Fig. 1(d)). The reduction in band gap from 3.40 to 3.27 eV for N-TiO2 after thermal annealing may be attributed to the oxidation of the remaining nitride

AC

layers (increase in intensity of peak assigned to β-N, Fig. 3d).

3.3. X-ray diffraction (XRD), atomic force microscopy (AFM), and goniometry.

The XRD spectra for N-TiO2 are shown in Fig. 6. The as-deposited sample shows a tiny peak around 24.96o that is assigned to the anatase phase with (101)

ACCEPTED MANUSCRIPT orientation (anatase single crystal) according to JCPDS (Joint Committee on Powder Diffraction Standards) Card N° 21-1272. After thermal annealing this signal increases and is shifted to 25.4o. Studies conducted in ref. [39] show that the oxidation decreases the lattice parameters, so that, the shift towards to higher Bragg angles is expected.

SC RI PT

From GIXRD data, we can also investigate the crystallite size D from Scherrer’s equation: D = Kλ/βcos(θ), where K = 0.89, λ the X-ray wavelength, β is the FWHM and θ the Bragg angle [40], which crystallite size D is given in nanometers. The FWHM could not be measured accurately, so only a qualitative analysis can be

NU

done from the behavior of the curves. It was observed that the peaks are more intense in the annealed film, indicating an increase in FWHM. Since the crystallite size is

smaller in the annealed films.

MA

inversely proportional to FWHM from Scherrer's equation, the crystallite size is

ED

This prediction was confirmed by AFM images (5 µm x 5 µm) for the asdeposited and annealed N-TiO2, which is shown in Fig. 7. The results of surface

PT

analysis are listed in Table 5, in which is possible to observe that the film surface has

CE

a distribution of smaller grains and lower roughness after thermal annealing. These results corroborate with goniometry results, which indicates an increase in contact

AC

angle of the samples after thermal annealing. In the literature the crystalline improvement effect of TiO2 films after annealing is already known, in addition, after thermal treatment there are changes in the surface which directly affect parameters as increase in grain size, roughness, the amount of defects and as a result, wettability rises [41]. Increased roughness of the films associated with the increased number of defects, and these in turn are associated

ACCEPTED MANUSCRIPT with creation of oxygen vacancies [42]. This effect promotes the absorption of OH groups increasing the surface wettability [43]. The TiO2 films exhibit hydrophobic behavior when they are not exposed to any radiation [32], such that it is possible to establish a standard contact angle of 82° between a drop of water and an anatase polycrystalline thin film [44]. Due to energy

SC RI PT

absorption from the light spectrum the films present photoinduced hydrophilicity [41– 43], which in turn is related to the increase of photocatalytic properties [45]. Although photocatalytic characteristics have not been investigated directly, optical results indicated that the films of this work showed opposite surface behavior than expected

deposited by the technique HiPIMS [46].

NU

and this may be associated with characteristic of high compression of the films

MA

In general the suitable morphology which includes low contact angle and high surface area contributes to the photocatalytic reaction by supplying abundant active

ED

points and favors applications as production of hydrogen from water splitting under sunlight [47]. However, the advantages of HiPIMS deposition and heat treatment are:

PT

increase in oxidation and crystallinity, incorporation of substitutional nitrogen and

AC

photo-anodes.

CE

hence decrease in energy gap, which favours the application of films as solar cell

4. Conclusions

Titanium dioxide films were grown by HiPIMS on FTO, without substrate heating, subjected a post-deposition thermal annealing treatment at 400 °C for 2 hours under argon atmosphere. For both worked conditions, TiO2 and N-doped TiO2, the thermal annealing shifted the binding energy to higher values, which was caused by

ACCEPTED MANUSCRIPT surface oxidation. Moreover, TiO2 films present only interstitial nitrogen whereas NTiO2 films exhibit also substitutional nitrogen and titanium nitride. After annealing NTiO2 films shown an increase in the binding energy for all peaks observed, which is owing to oxygen incorporation into TiN films because oxygen is more electronegative than nitrogen. DCMS power source grown films exhibit a resembling behavior to

SC RI PT

HiPIMS in terms of XPS analysis, however, was not found substitutional nitrogen. This result corroborates with studies that demonstrate the HiPIMS discharge enhances considerably the production of atomic oxygen, which in turn, elevates surface oxidation of the film. The nitrogen incorporation in films decreased energy gap

NU

values, a fact that was intensified after thermal annealing as a result of samples oxidation, crystallinity increase and smoothing of superficial parameters, like grain

MA

size and average roughness reduction, which consequently increase contact angle.

ED

5. Acknowledgments

PT

The financial support of FAPESP – Grant 2011/50773-0, CNPq - Grant

CE

555.686/2010-8, and CAPES are strongly acknowledged. Authors also would like to thank to Dr. Walter Miyakawa from Institute of Advanced Studies (IEAv) by AFM

AC

and optical spectrophotometry measurements, Dr. Richard Landers from Campinas State University (UNICAMP) by XPS measurements, and Dr. Rodrigo Pessoa from University of Vale of Paraiba (UNIVAP) by XRD measurements.

6. References

[1]

U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (2003)

ACCEPTED MANUSCRIPT 53–229. doi:10.1016/S0167-5729(02)00100-0. [2]

B. O’REGAN, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature. 353 (1991) 737.

[3]

J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Grätzel, Sequential deposition as a route to high-performance

SC RI PT

perovskite-sensitized solar cells., Nature 499 (2013) 316–9. doi:10.1038/nature12340. [4]

R.S. Pessoa, M. a. Fraga, L. V. Santos, M. Massi, H.S. Maciel, Nanostructured thin films based on TiO2 and/or SiC for use in photoelectrochemical cells: A

NU

review of the material characteristics, synthesis and recent applications, Mater. Sci. Semicond. Process. 29 (2014) 56–68. doi:10.1016/j.mssp.2014.05.053. A. Fujishima, T.N. Rao, D. A. Tryk, Titanium dioxide photocatalysis, J.

MA

[5]

Photochem. Photobiol. C Photochem. Rev. 1 (2000) 1–21. doi:10.1016/S1389-

[6]

ED

5567(00)00002-2.

A. Fujishima, T.N. Rao, D. A. Tryk, TiO2 photocatalysts and diamond

PT

electrodes, Electrochim. Acta. 45 (2000) 4683–4690. doi:10.1016/S0013-

[7]

CE

4686(00)00620-4.

N. Xu, X. Xiao, H. Yang, E. Yu, Q. Zhang, Enhanced dielectric constant and

AC

suppressed dielectric loss of ternary composites based on Ag-P (VDF-HFP) matrix and TiO2 nanowires, Ceram. Int. 42 (2016) 12475–12481. doi:10.1016/j.ceramint.2016.05.031. [8]

S.H. Moon, S.J. Park, S.H. Kim, M.W. Lee, J. Han, J.Y. Kim, H. Kim, Y.J. Hwang, D. K. Lee, B.K. Min, Monolithic DSSC/CIGS tandem solar cell fabricated by a solution process., Sci. Rep. 5 (2015) 8970. doi:10.1038/srep08970.

ACCEPTED MANUSCRIPT [9]

R. Asahi, T. Morikawa, T. Ohwaki, Y. Taga, Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides, Science 293 (2001) 269–271. doi:10.1126/science.1061051.

[10] L. Bin Xiong, J. L. Li, B. Yang, Y. Yu, Ti3+ in the surface of titanium dioxide: Generation, properties and photocatalytic application, J. Nanomater. 2012

SC RI PT

(2012). doi:10.1155/2012/831524.

[11] D.A. Duarte, M. Massi, A.S. da S. Sobrinho, Development of Dye-Sensitized Solar Cells with Sputtered N-Doped TiO2 Thin Films : From Modeling the Growth Mechanism of the Films to Fabrication of the Solar Cells, Int. J.

NU

Photoenergy. 2014 (2014) 13. doi:10.1155/2014/839757.

[12] I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara, K. Takeuchi, Role

MA

of oxygen vacancy in the plasma-treated TiO2 photocatalyst with visible light activity for NO removal, J. Mol. Catal. A Chem. 161 (2000) 205–212.

ED

doi:10.1016/S1381-1169(00)00362-9.

[13] G. He, L.D. Zhang, G.H. Li, M. Liu, X.J. Wang, Structure, composition and

PT

evolution of dispersive optical constants of sputtered TiO2 thin films: effects of

CE

nitrogen doping, J. Phys. D. Appl. Phys. 41 (2008) 45304. doi:10.1088/00223727/41/4/045304.

AC

[14] H. Irie, Y. Watanabe, K. Hashimoto, Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO2-xNx Powders, J. Phys. Chem. B. 107 (2003) 5483–5486. doi:10.1021/jp030133h. [15] B. Kosowska, S. Mozia, A.W. Morawski, B. Grzmil, M. Janus, K. Kałucki, The preparation of TiO2–nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis, Sol. Energy Mater. Sol. Cells. 88 (2005) 269–280. doi:10.1016/j.solmat.2004.11.001.

ACCEPTED MANUSCRIPT [16] V. Stranak, M. Quaas, R. Bogdanowicz, H. Steffen, H. Wulff, Z. Hubicka, M. Tichy, R. Hippler, Effect of nitrogen doping on TiOxNy thin film formation at reactive high-power pulsed magnetron sputtering, J. Phys. D. Appl. Phys. 43 (2010) 285203. doi:10.1088/0022-3727/43/28/285203. [17] N.C. Raut, T. Mathews, S. Rajagopalan, R.V.S. Rao, S. Dash, A.K. Tyagi,

SC RI PT

Secondary ion mass spectrometry and X-ray photoelectron spectroscopy studies on TiO2 and nitrogen doped TiO2 thin films, Solid State Commun. 151 (2011) 245–249. doi:10.1016/j.ssc.2010.11.017.

[18] D.L. Smith, Thin-Film Deposition: Principles and Practice, 1st ed., McGraw-

NU

Hill, New York, 1995.

[19] D. Heřman, J. Šícha, J. Musil, Magnetron sputtering of TiOxNy films, Vacuum.

MA

81 (2006) 285–290. doi:10.1016/j.vacuum.2006.04.004. [20] Q. Li, J.K. Shang, Heavily nitrogen-doped titanium oxide thin films by reactive

ED

sputtering and excimer laser annealing, J. Am. Ceram. Soc. 93 (2010) 3039– 3042. doi:10.1111/j.1551-2916.2010.04012.x.

PT

[21] M. C. Wang, H. J. Lin, C. H. Wang, H. C. Wu, Effects of annealing

CE

temperature on the photocatalytic activity of N-doped TiO2 thin films, Ceram. Int. 38 (2012) 195–200. doi:10.1016/j.ceramint.2011.05.160.

AC

[22] N.C. Raut, T. Mathews, P. Chandramohan, M.P. Srinivasan, S. Dash, A.K. Tyagi, Effect of temperature on the growth of TiO2 thin films synthesized by spray pyrolysis: Structural, compositional and optical properties, Mater. Res. Bull. 46 (2011) 2057–2063. doi:10.1016/j.materresbull.2011.06.043. [23] T. Toyoda, H. Nakanishi, Fundamental absorption edge in the semiconductor CdInGaS4 at high temperatures, J. Phys. D. Appl. Phys. 18 (1985) 747–751. http://iopscience.iop.org/0022-3727/18/4/019.

ACCEPTED MANUSCRIPT [24] R.P. Antony, T. Mathews, S. Dash, A.K. Tyagi, B. Raj, X-ray photoelectron spectroscopic studies of anodically synthesized self aligned TiO2 nanotube arrays and the effect of electrochemical parameters on tube morphology, Mater. Chem. Phys. 132 (2012) 957–966. doi:10.1016/j.matchemphys.2011.12.041.

SC RI PT

[25] R. an in s, H. Tang, H. Berger, F. Gozzo, G. Margaritondo, F.

,

Electronic structure of anatase TiO2 oxide, J. Appl. Phys. 75 (1994) 2945. doi:10.1063/1.356190.

[26] S. Hashimoto, A. Tanaka, A. Murata, T. Sakurada, Formulation for XPS

NU

spectral change of oxides by ion bombardment as a function of sputtering time, Surf. Sci. 556 (2004) 22–32. doi:10.1016/j.susc.2004.03.002.

MA

[27] C.J. Powell, A. Jablonski, Progress in quantitative surface analysis by X-ray photoelectron spectroscopy: Current status and perspectives, J. Electron

ED

Spectros. Relat. Phenomena. 178 (2010) 331–346. doi:10.1016/j.elspec.2009.05.004.

PT

[28] C.D. Wagner, Sensitivity factors for XPS analysis of surface atoms, J. Electron

CE

Spectros. Relat. Phenomena. 32 (1983) 99–102. doi:10.1016/03682048(83)85087-7.

AC

[29] Z. Lu, X. Jiang, B. Zhou, X. Wu, L. Lu, Study of effect annealing temperature on the structure, morphology and photocatalytic activity of Si doped TiO2 thin films deposited by electron beam evaporation, Appl. Surf. Sci. 257 (2011) 10715–10720. doi:10.1016/j.apsusc.2011.07.085. [30] X. Chen, C. Burda, Photoelectron spectroscopic investigation of nitrogendoped titania nanoparticles, J. Phys. Chem. B. 108 (2004) 15446–15449. doi:10.1021/jp054495w.

ACCEPTED MANUSCRIPT [31] N.C. Saha, H.G. Tompkins, Titanium nitride oxidation chemistry: An x-ray photoelectron spectroscopy study, J. Appl. Phys. 72 (1992) 3072. doi:10.1063/1.351465. [32] M. Sahoo, T. Mathews, R.P. Antony, D.N. Krishna, S. Dash, A.K. Tyagi, Physico-chemical processes and kinetics of sunlight-induced hydrophobic -

SC RI PT

superhydrophilic switching of transparent N-doped TiO2 thin films, ACS Appl. Mater. Interfaces. 5 (2013) 3967–3974. doi:10.1021/am400785x. [33] M.V. Kuznetsov, J.F. Zhuravlev, V.A. Gubanov, XPS analysis of adsorption of oxygen molecules on the surface of Ti and TiNx films in vacuum, J. Electron

NU

Spectros. Relat. Phenomena. 58 (1992) 169–176. doi:10.1016/03682048(92)80016-2.

MA

[34] S. Konstantinidis, R. Snyders, Reactive ionized physical vapor deposition of thin films, Eur. Phys. J. Appl. Phys. 56 (2011) 24002.

ED

doi:10.1051/epjap/2011110199.

[35] N. Nafarizal, S. Kikuchi, K. Sasaki, Sticking probabilities of Cu, Zn, Sn, and S

PT

atoms in magnetron sputtering plasmas employing a Cu2ZnSnS4 stoichiometric

CE

target, Vacuum. 121 (2015) 26–31. doi:10.1016/j.vacuum.2015.07.019. [36] W.P. Leroy, S. Mahieu, R. Persoons, D. Depla, Quantification of the

AC

incorporation coefficient of a reactive gas on a metallic film during magnetron sputtering: The method and results, Thin Solid Films. 518 (2009) 1527–1531. doi:10.1016/j.tsf.2009.07.190. [37] S. Mahieu, K. Van Aeken, D. Depla, D. Smeets, a Vantomme, Dependence of the sticking coefficient of sputtered atoms on the target–substrate distance, J. Phys. D. Appl. Phys. 41 (2008) 152005. doi:10.1088/0022-3727/41/15/152005. [38] T. Kubart, O. Kappertz, T. Nyberg, S. Berg, Dynamic behaviour of the reactive

ACCEPTED MANUSCRIPT sputtering process, Thin Solid Films. 515 (2006) 421–424. doi:10.1016/j.tsf.2005.12.250. [39] R. Kobayashi, Y. Moriya, M. Imamura, K. Oosawa, K. Oh-Ishi, Relation between oxygen concentration in AlN lattice and thermal conductivity of AlN ceramics sintered with various sintering additives, J. Ceram. Soc. Japan. 119

SC RI PT

(2011) 291–294. doi:10.2109/jcersj2.119.291.

[40] W. Guo, Y. Shen, G. Boschloo, A. Hagfeldt, T. Ma, Influence of nitrogen dopants on N-doped TiO2 electrodes and their applications in dye-sensitized solar cells, Electrochim. Acta. 56 (2011) 4611–4617.

NU

doi:10.1016/j.electacta.2011.02.091.

[41] S.S. Pradhan, S. Sahoo, S.K. Pradhan, Influence of annealing temperature on

MA

the structural, mechanical and wetting property of TiO2 films deposited by RF magnetron sputtering, Thin Solid Films. 518 (2010) 6904–6908.

ED

doi:10.1016/j.tsf.2010.07.036.

[42] D.D. Irala, H.S. Maciel, D. a. Duarte, M. Massi, A.S. Da Silva Sobrinho, M.

PT

Pavanello, C. Claeys, J. Martino, Influence of the Nitrogen Concentration on

CE

the Photoinduced Hydrophilicity of N-doped Titanium Dioxide Thin Films Deposited by Plasma Sputtering, (2010) 109–115. doi:10.1149/1.3474148.

AC

[43] R. Wang, N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, Studies of Surface Wettability Conversion on TiO2 Single-Crystal Surfaces, J. Phys. Chem. B. 103 (1999) 2188–2194. doi:10.1021/jp983386x. [44] A. Borras, A.R. González-Elipe, Wetting properties of polycrystalline TiO2 surfaces: A scaling approach to the roughness factors, Langmuir. 26 (2010) 15875–15882. doi:10.1021/la101975e. [45] M. Ratova, G.T. West, P.J. Kelly, Optimisation of HiPIMS photocatalytic

ACCEPTED MANUSCRIPT titania coatings for low temperature deposition, Surf. Coatings Technol. 250 (2014) 7–13. doi:10.1016/j.surfcoat.2014.02.020. [46] K. Sarakinos, J. Alami, S. Konstantinidis, High power pulsed magnetron sputtering: A review on scientific and engineering state of the art, Surf. Coatings Technol. 204 (2010) 1661–1684. doi:10.1016/j.surfcoat.2009.11.013.

SC RI PT

[47] Z. Zhu, C.T. Kao, B. H. Tang, W. C. Chang, R. J. Wu, Efficient hydrogen production by photocatalytic water-splitting using Pt-doped TiO2 hollow spheres under visible light, Ceram. Int. 42 (2016) 6749–6754.

AC

CE

PT

ED

MA

NU

doi:10.1016/j.ceramint.2016.01.047.

ACCEPTED MANUSCRIPT Figure Captions

Figure 1. Ti2p core levels of the XPS spectra for films deposited by HiPIMS. Figure 2. O1s core levels of the XPS spectra for films deposited by HiPIMS. Figure 3. N1s core levels of the XPS spectra for films deposited by HiPIMS. On the

SC RI PT

image is possible to identify the binding energies assigned to interstitial nitrogen (γN), substitutional nitrogen (β-N) and one ascribed for the nitrogen in the TiN lattice. Figure 4. N1s core levels of the XPS spectra for films deposited by DCMS. On the image is possible to identify the binding energies assigned to interstitial nitrogen (γ-

NU

N) and one ascribed for the nitrogen in the TiN lattice.

Figure 5. Tauc plots of the films deposited by HiPIMS.

MA

Figure 6. XRD spectra of N-TiO2 deposited by HiPIMS. Dotted lines represent the position of some peaks that indicate anatase structure (A) in different planes, in

ED

brackets.

Figure 7. AFM images of (a) as-deposited and (b) annealed N-TiO2 deposited by

AC

CE

PT

HiPIMS.

ACCEPTED MANUSCRIPT Table Captions

Table 1. Deposition parameters. Table 2. Positions of the peaks of Ti2p core levels of the XPS spectra for films deposited by HiPIMS, shown in Fig. 1.

SC RI PT

Table 3. Positions of the peaks of O1s core levels of the XPS spectra for films deposited by HiPIMS, shown in Fig. 2.

Table 4. Positions of the peaks of N1s core levels of the XPS spectra for films deposited by HiPIMS, shown in Fig. 3.

NU

Table 5. Results of surface analysis performed by AFM and goniometry of as-

AC

CE

PT

ED

MA

deposited and annealed N-TiO2 deposited by HiPIMS.

ACCEPTED MANUSCRIPT Table 1

300 W 120 Hz 150 ms 344 – 360 V 464 - 470 V 0.8 – 0.9 A 64 -74 A (peak) 110 mm 120 min 200 nm 10.4 sccm of Ar 3.5 sccm of O2 10.4 sccm of Ar 0.6 sccm of O2 10.0 sccm of N2 7.0 mTorr (0.93 Pa) 5.5 mTorr (0.74 Pa) of Ar 1.5 mTorr (0.20 Pa) of O2 4.1 mTorr (0.54 Pa) of Ar 0.2 mTorr (0.02 Pa) of O2 2.7 mTorr (0.37 Pa) of N2

MA

Working pressure Partial pressure for TiO2

AC

CE

PT

ED

Partial pressure for NTiO2

HiPIMS

SC RI PT

N-TiO2 flow rates

DCMS

NU

Parameter Plasma power Frequency Acting time Cathode voltage Current Target-substrate distance Deposition time Average thickness TiO2 flow rates

ACCEPTED MANUSCRIPT Table 2 Band

AC

CE

PT

ED

MA

NU

SC RI PT

Ti4+2p1/2 Ti4+2p3/2 Ti3+2p1/2

Position (eV) As-deposited Annealed TiO2 N-TiO2 TiO2 N-TiO2 463.95 464.28 464.34 464.45 458.16 458.54 458.52 458.68 459.50 460.57 460.16 460.64

ACCEPTED MANUSCRIPT Table 3 Position (eV) As-deposited Annealed TiO2 N-TiO2 TiO2 N-TiO2 Ti-O (Lattice) 529.51 529.84 529.89 530.06 Surface 531.09 531.40 531.96 531.82

AC

CE

PT

ED

MA

NU

SC RI PT

Band

ACCEPTED MANUSCRIPT Table 4 Band TiN β-N

AC

CE

PT

ED

MA

NU

SC RI PT

γ-N

Position (eV) As-deposited Annealed TiO2 N-TiO2 TiO2 N-TiO2 395.36 395.79 397.79 398.74 399.25 400.19 399.35 399.61 401.45 402.29

ACCEPTED MANUSCRIPT Table 5 As-deposited 0.28 ± 0.06 26.6 201.18 78.5 ± 2

Annealed 0.23 ±0.04 25.4 173.90 101.7 ±3

AC

CE

PT

ED

MA

NU

SC RI PT

Surface parameters Average grain size (m) Average roughness RA (nm) Maximum peak-valley distance (nm) Contact angle (deg)

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

SC RI PT

Figure 1

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

SC RI PT

Figure 2

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

SC RI PT

Figure 3

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

SC RI PT

Figure 4

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

SC RI PT

Figure 5

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

SC RI PT

Figure 6

ACCEPTED MANUSCRIPT Figure 7

(a)

(b) Grain size: 0.23 μm

Grain size: 0.28 μm

201.18 [nm]

0.00

SC RI PT

5.00 x 5.00 um

2.00 um

AC

CE

PT

ED

MA

NU

2.00 um

173.90 [nm]

5.00 x 5.00 um

0.00

ACCEPTED MANUSCRIPT Highlights HiPIMS discharge favors the incorporation of substitutional nitrogen compared to DCMS.

SC RI PT

Binding energy increases due to oxidation promoted by thermal annealing.

AC

CE

PT

ED

MA

NU

The presence of substitutional nitrogen decreases the energy gap.