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titanium oxynitride bilayer films for hydrogen generation and solar cells applications
Materials Science in Semiconductor Processing 105 (2020) 104704
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Optical, water splitting and wettability of titanium nitride/titanium oxynitride bilayer films for hydrogen generation and solar cells applications
T
S.H. Mohameda,b,∗, Huaping Zhaoa, Henry Romanusc, F.M. El-Hossaryb, M. Abo EL-Kassemb, M.A. Awadb, Mohamed Rabiaa,d, Yong Leia a
Institute of Physics and Macro- and Nanotechnologies MacroNano® (IMN & ZIK), Ilmenau University of Technology, 98693 Ilmenau, Germany Physics Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt c Zentrum FürMikro- und Nanotechnologien, TechnischeUniversitätIlmenau, Gustav-Kirchhoff-Strasse 7, 98693 Ilmenau, Germany d Nanophotonics and Applications Lab, Chemistry Department, Faculty of Science, Beni-Suef University, Beni-Suef 62514, Egypt b
A R T I C LE I N FO
A B S T R A C T
Keywords: TiN/TiOxNy bilayer films Optical properties Photoelectrochemical water splitting wettability
TiN/TiOxNy bilayer films with various TiN thicknesses were prepared using direct current (d.c.) reactive magnetron sputtering. The ratio of N/O in the bilayer films increased with increasing TiN top layer thickness. The observed crystalline peaks for the bilayer films, for different TiN layer thicknesses, were indexed to anatase TiO2 with tetragonal structure. Only 0.31° peak shift, to lower 2θ angles was detected. FESEM indicated that the morphology of TiOxNy single layer is consisted of elongated nanocrystals with diameters in the range of 27.9–46.8 nm and the diameters of the nanocrystallites changed slightly with increasing the thickness of TiN layer. The transmittance, absorbance, reflectance spectra and the optical constants of the bilayer films were presented. The photoelectrochemical performance for water splitting of TiN/TiOxNy bilayer films was studied by examination both the continuous and chopped photocurrent density under light illumination. The photocurrent density increases with increasing TiN top layer up to a thickness of 9 nm, after which a reduction in photocurrent is observed. The wettability of TiN/TiOxNy bilayer films could be transformed by UV illumination from hydrophobic to hydrophilic.
1. Introduction Titanium oxynitrides (TiOxNy) are belonging to transition metal oxynitride family which displays a wide range of performances and shows good optical, mechanical, electrical, photocatalytic and electrochemical properties [1–6]. Specially, TiOxNy have been extensively studied as optical functional coatings for commercial glass architecture applications and as environmental friendly materials with excellent in photocatalytic applications [6]. On the other hand titanium nitride (TiN) is important functional material and it has numerous attractive properties which depend on the preparation conditions and methods. TiN shows high hardness, low electrical conductivity, excellent thermal conductivity, good chemical stability, high corrosion resistance and high visible light absorption [7–9]. Therefore thin films of TiN have been planned for numerous applications; including pollution degradation [10], nitrite detection [11], resonators [12], heterojunction photovoltaic [13], fuel cells [14], supercapacitors [15], water splitting [16], dye-sensitized solar cells [17] and superconductors [18]. Due to its nontoxicity, chemical stability and widespread
∗
availability, titanium oxide (TiO2) has extensively proposed as photoanode for photoelectrochemical (PEC) water splitting that can transform water into chemical fuels, H2 and O2, under solar energy illumination. Nevertheless, TiO2 is naturally characterized by a wide band gap energy (> 3.0 eV) that capture small portion of visible light. This wide band gap energy limits the theoretical solar to hydrogen conversion efficiency to 2.2%, which is much less than 10% of the conversion efficiency requested for commercial use [1]. Recently, TiOxNy photoanodes have arose as an attractive candidates for PEC water oxidation, because of the narrower band gap that extends to visible light absorption as well as the suitability of band positions that being well straddle for the water reduction/oxidation potential [1,19]. The materials' wettability plays a significant role in technology. For example, the self-cleaning engineering artificial surfaces can be used as solar panel coatings because they assist in the elimination of the deposited dirt and dust there by enhance their light transmittance and their energy conversion efficiency. These surfaces can be generallyclassified into two types: (i) hydrophobic surfaces and (ii) hydrophilic surfaces [20,21]. In hydrophobic surfaces, the drops of water roll on the
Corresponding author. Physics Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt. E-mail address: [email protected] (S.H. Mohamed).
https://doi.org/10.1016/j.mssp.2019.104704 Received 12 April 2019; Received in revised form 14 August 2019; Accepted 28 August 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
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surface and carry the dirt from the surface. In case of hydrophilic surfaces, the drops of water spread on the surface and form a thin layer of water. After that the dirt is washed away during spreading process. There are many reports on wetting properties of TiO2 films, but the wetting performance of TiOxNy and TiN has been scarcely explored [22]. Also, the use of TiN/TiOxNy bilayer films for PEC water splitting has not seen in the literatures. Therefore TiN/TiOxNy bilayer films with various TiN thicknesses are prepared by d.c. reactive magnetron sputtering. Their structural, optical, PEC water splitting performance and wettability are examined.
Table 1 Chemical composition atomic (%), average Grain size via FESEM and optical band gap (Eg). dTiN (nm)
0 3 6 9 12 15 TiN
2. Experimental details TiN/TiOxNy films were prepared using dc reactive magnetron sputtering from the same metallic Ti target. The preparation of the bilayer films was achieved sequentially under the same vacuum from the same Ti target and without opening the sputtering chamber in between the deposition of the various layers. Sputtering process was carried out after reaching a base pressure of 2.8 × 10−7 mbar. The deposition of TiOxNy layer was carried out using Ar, N2 and O2 gases for flow rates of 25, 70 and 5 sccm, respectively, while the deposition of TiN layer was carried out using Ar and N2 with 25 and 75 sccm, respectively. In both cases the working pressure was 1.3 × 10−2 mbar and the sputtering power was adjusted to be 100 W. Sputtering was achieved on well cleaned Si(100), microscopic glass and fluorine doped tin oxide coated glass, FTO, (~10 Ω/sq) substrates. The substrates were maintained at 200 °C and the target to substrate distance was maintained at 5 cm. Before depositing each layer, the surface of the target was cleaned using Ar atmosphere sputtering for 10 min. The thickness of the TiOxNy bottom layer was fixed to 298 nm and the thicknesses of the TiN top layers were 0, 3, 6, 9, 12 and 15 nm. For phase identification, X-ray diffraction was carried out using Bruker/Siemens D5000 diffractometer. The patterns were achieved for samples prepared on glass substrates using grazing incidence scan with angle of 3°. Before measurements the machine was pre-calibrated using LaB6 standard powder. The film thickness determined by Veeco Dektak 150 profilometer. Field emission scanning microscopy (FESEM) model Hitachi S4800 was used to examine the surface morphology of the samples. EDAX analysis was achieved on a unit attached to the FESEM to determine the chemical composition. Both FESEM and EDAX were carried out for samples prepared on Si(100) substrates. Varian Cary 5000 UV–Vis–NIR spectrophotometer was employed to measure the transmittance (T), reflectance (R) and absorbance (Abs) for samples prepared on glass substrates. Photoelectrochemical (PEC) measurements were carried out onto samples prepared on FTO substrates using a three electrodes electrochemical cell with a BioLogic Potentiostat. The counter and the reference electrodes through the PEC measurements Pt mesh and Ag/ AgCl (in saturated KCl) electrodes, respectively. The used electrolyte was 0.1 M of NaOH (Ph = 13). The used light source was Oriel solar simulator, 300 W Xe lamp, AM 1.5 global filter which was previously calibrated to 1 sun via Si photodiode model 818, Newport. The photoinduced hydrophilicity was estimated by determining the contact angles between TiN/TiOxNy surface and pure water after UV illumination by PHONIX-300 contact angle meter. The measurements were done on samples prepared on glass substrates. The illumination was done using UV light lamp, model Germicidal UV-C, G13T8, Japan, of wavelength 254 nm for different periods of time.
Average Grain size via FESEM (nm)
40.4 34.2 37.1 36.4 33.7 41.9 –
Chemical composition atomic (%) N
O
Ti
8.72 9.14 9.63 11.53 14.19 14.72 48.65
62.75 63.3 64.01 58.81 55.4 54.48 15.03
28.53 27.56 26.36 29.66 30.41 30.8 36.32
Eg (eV)
3.29 3.25 3.22 3.14 2.96 2.82 -
quantify the chemical composition, such as limitation to concentrations is in the order of 0.1% and the used sampled volume from the silicon substrate is larger compared to that used from thin film, it can give good guide for the change in N/O ratio with increasing TiN top layer since the TiOxNy layer thickness is constant. Examples of EDAX spectra are shown in Fig. 1a–d. The peaks heights in the EDAX spectra are related to the concentrations of the elements. In addition to N, O and Ti peaks the EDAX spectra of the single TiOxNy layer contains Si and C peaks which may be attributed to the influence of the Si substrate and the contamination by hydrocarbons, respectively. The obtained nitrogen to oxygen ratio for the single TiOxNy layer film is very small comparing to the used flows ratio. Also, the titanium nitride single layer (Fig. 1d), where the flow of oxygen is zero, contains a pronounced amount of oxygen. This is attributed to: (i) the higher reactivity of oxygen than nitrogen with the metallic titanium [4,23], (ii) the presence of native oxide layer, SiO2, which usually grows naturally on top of the silicon wafers (with few nanometer in thickness) and (iii) the residual oxygen in the growth chamber. The ratio of N/O in the TiN/ TiOxNy bilayer films increases with increasing TiN top layer thickness. Fig. 2 shows XRD patterns of TiN/TiOxNy with various TiN thicknesses. All the observed peaks of the uncovered TiOxNy film can be indexed to anatase TiO2 with a body-centered tetragonal structure [JCPDS card no.: 01-089-4921]. Only 0.31° peak shift, to lower 2θ angles, was observed and no additional peaks related to titanium nitride or titanium oxynitride phases are observed. This peak shift is possibly ascribed to the substitution of some oxygen atoms by nitrogen atoms in the anatase TiO2. The nitrogen atoms can either occupy the positions of oxygen atoms in the anatase TiO2 crystal lattice or located at the grain boundaries and, thereby create some additional stresses which in turn lead to peak shift [5,24,25]. The experimental film height misalignment may also contribute slightly to the peak shift [25]. It is worth mentioning that although TiOxNy is deposited, diffraction peaks of TiO2 are obtained. This is because for titanium oxynitrides prepared with even very small amount of oxygen, the formation of TiO2 phase is favored since it is thermodynamically stable phase [4,26,27]. Also the higher reactivity of oxygen towards titanium which favors the formation of Ti–O bonds over Ti–N bonds can facilitate the formation of TiO2 phase [26–28]. The addition of TiN layer on top of the TiOxNy film does not create any additional phases; however it reduces the intensity of the anatase TiO2 peaks as the TiN thicknesses increases. The effect of TiN top layer on the morphology growth of TiN/ TiOxNy bilayer films is established in Fig. 3. The morphology of TiOxNy single layer (Fig. 3a) is consisted of elongated nanocrystals with various diameters ranging from 27.9 to 46.8 nm. As the TiN layer is deposited on top of TiOxNy, the morphology does not change extensively (Fig. 3b–d). With further increase of the TiN layer thickness up to 15 nm, the morphology of the nanocrystallites becomes slightly bigger. The obtained nanocrystallites are summarized in Table 1.
3. Results and discussion 3.1. Chemical composition, structural and morphological analysis The evaluated chemical composition of TiN/TiOxNy bilayer films with various TiN thicknesses, identified by EDAX, is summarized in Table 1. Although the EDAX technique has certain limitations to 2
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Fig. 1. EDAX spectra of TiN/TiOxNybilayer films.
3.2. Optical properties Fig. 4 shows spectral transmission, reflection and absorbance of TiN/TiOxNy bilayer films. The TiOxNy film has higher transmittance (Fig. 4a) with clear interference fringes and an abrupt decrease at the onset of absorption. This indicates a good quality of the film and lower level of defects. The addition of 3 nm TiN top layer film leads to an overall decrease in transmission. With further increase in TiN layer thickness, the overall transmission decreased to very low values. A shift in onset of absorption towards longer wavelengths is also observed with TiN layer thickness. The reflection of TiN/TiOxNy bilayer films are decreased also with increasing TiN top layer (Fig. 4b). It has been previously reported that the sub-stoichiometric titanium nitride films have much lower reflection than stoichiometric films [29–31]. At the same time, the measured absorbance (Fig. 4c) increased with increasing TiN layer thickness. This trend has also been observed by Li et al. [7]. for TiN/TiO2 bilayer films and they ascribed it to the narrower optical band gap of TiN comparing to TiO2 [32] and the possible plasmonic effect given by TiN [33,34]. The absorption coefficient (α) was calculated via the measured spectral reflection and transmission, and the total bilayer film thickness (d) from the equation:
α=
1 1−R ⎞ ln ⎛ d ⎝ T ⎠
(1)
The energy band gap (Eg) was estimated from Tauc's formula by assuming indirect allowed transition: 1
(αhν ) 2 = β (hν − Eg )
(2)
where ν is the frequency of the incident light and β is a constant. The Eg 1 values for TiN/TiOxNy bilayer films are evaluated from (αhν ) 2 versus hν 1
plots as shown in Fig. 5, by extending the tangent line to (αhν ) 2 = 0 , and are listed in Table 1. Generally, the Eg decreases from 3.27 to 2.85 eV with increasing TiN thickness from 0 to 15 nm, however the decrease is slow for thinner TiN thicknesses variation and it becomes sharper at thicker TiN thicknesses. The Eg value determined for TiOxNy single layer film agrees well with the value of titanium oxynitride films
Fig. 2. XRD patterns of TiN/TiOxNybilayer films with various TiN thicknesses.
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Fig. 3. FESEM images of TiN/TiOxNy bilayer films with various TiN thicknesses of 0 nm (a), 3 nm (b), 6 nm (c), 9 nm (d), 12 nm (f) and 15 nm (g). The scale bar of all the images is 100 nm.
bilayer films. The effective refractive indexes of all the TiN/TiOxNy bilayer films follow the normal dispersion. The n values decrease with increasing TiN layer thickness. TiOxNy single layer film has very low k values, close to zero, in most of the spectrum which designate low optical losses due to absorption. The strident increase in k values at short wavelengths (below 500 nm) is attributed to the fundamental absorption through the band gap. With increasing TiN layer thickness, a remarkable increase in k values is observed which indicates the association of defect and absorbing centers whose amount increase with increasing TiN layer thickness.
containing approximately, the same amount of nitrogen content [4,19,35]. The effective refractive index (n) and effective extinction coefficient (k) of TiN/TiOxNy bilayer films were calculated using the search technique based on minimizing (ΔR)2 and (ΔT)2 simultaneously [36–38] where
R calc (n, k , d, λ ) − R exp 2 , Tcalc (n, k , d, λ ) − Texp
2
(3)
Texp and Rexp are the experimentally determined values of the T and R, respectively, while Tcalc and Rcalc are the calculated values of the T and R, respectively, using Murmann's exact equations [36–38]. Fig. 6a and b shows variations of n and k with wavelength for TiN/TiOxNy 4
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Fig. 4. Spectral transmission (a), reflection (b) and absorbance (c) of TiN/ TiOxNy bilayer films.
Fig. 6. The refractive index (a) and extinction coefficient (b) variations with wavelength for TiN/TiOxNy bilayer films.
interface. The bilayer films display onset potential around −0.60 VAg/ above which the photocurrent density increased with increasing voltage. This increase is ascribed to the obviously improved charge separation and enhanced transfer rate under the cascading band arrangement and indicates that the charge transfer and recombination happen concurrently [19]. The photocurrent density increases with increasing TiN top layer until a thickness of 9 nm after which a reduction in photocurrent is observed. The higher photocurrent density for the films having TiN layer on top, comparing to the single TiOxNy film is due to the lower optical band gap values, the extra light absorption, the better electrical conductivity, and the better efficient photo-electron-hole separation [7,39–44]. It is worth mentioning that, while the optical absorbance, average grain size and the smallest band gap energy are for the bilayer films with TiN thickness greater than 9 nm, the best photo electrochemical performance is recorded for 9 nm TiN thickness. This may explained as follows. It is believed that the TiN layer thickness is a decisive parameter which determined the efficiency of the bilayer films. Actually, the presence of TiN layer on top of TiOxNy layer can lead to the formation of heterojunction, by TiOxNy and TiN, with proper electronic structures [3], affect the charge space region and narrow the optical band gap of the bilayer films [7]. Thereby provide more visible light absorption and reduce electron–hole recombination [3,7,42] which result in increasing photocurrent density. This increase in photocurrent is continued with increasing TiN layer thickness to an optimal thickness of 9 nm. The reduction in photocurrent density at TiN thicknesses greater than 9 nm may be related to the more occurrences of oxygen vacancies in TiN thick films which cause more charge recombination. Li et al. [7] reported that the higher TiN layer, in the bilayer film can cause more negative flat-band value. For water splitting, the more negative flatAgCl,
1
Fig. 5. (αhν ) 2 versus hν plots for TiN/TiOxNy bilayer films.
3.3. Photoelectrochemical performance for water splitting Fig. 7a shows the electrochemical potentiodynamic scans (J-V) for TiN/TiOxNy bilayer films deposited on FTO glass. The photocurrent density (J) measures the photo induced reactions (the oxidation power) of the photo-generated holes that reach the bilayer-film/electrolyte
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Fig. 7. The photocurrent density versus potential for TiN/TiOxNy bilayer films deposited on FTO glass under light illumination (a) and under the chopped light illumination (b).
Fig. 8. Variations of water contact angle on TiN/TiOxNy bilayers films with different UV exposure times (a) and ln(θ) versus t plots as designated by the exponential function of equation (3) (b).
band value provides larger driving force to trigger the H2O redox. Torres et al. [45] examined the photoelectrochemical performance of N-doped TiO2 in aqueous electrolyte. They found that N-doped electrodes exhibited a significant increase in photocurrent under visible light irradiation, comparing with un-doped TiO2 electrode. However the N-doping could create high density of electron trapping states, thus optimization of N doping is needed. From the above argument it is inferred that the better water splitting of our bilayer films is due to the mixture of TiN and TiO2 properties [46]. Fig. 7b presents the photocurrent density versus potential under the chopped light illumination which is measured further to study the kinetics of photoresponse of TiN/TiOxNy bilayer films. The abrupt rise and decay of photocurrent density with rectangular shapes during the on/off light illumination reveals the fast charge transport/transfer rate between the bilayer films. Moreover, it is significant that the photocurrent density generated in the chopped curves certainly coincides with that of the continuous illumination curves shown Fig. 7a.
nitrogen into TiO2, to form TiOxNy or N doped TiO2, resulted in better wettability due to the increase of absorption and surface roughness [28,47,48]. The decrease in contact angle after UV illumination can be fitted with an exponential function (Fig. 8b) in the form:
θ = θ0 exp(−kUV t )
(4)
This behavior is similar to that reported for titania particles prepared via hydrolyzing titanium tetraethoxide [49]. The rate constant (kUV) values are 0.051, 0.042, 0.054, 0.057, 0.054, 0.051 min−1. 4. Conclusion TiN/TiOxNy bilayer films with various TiN thicknesses were prepared using reactive dc magnetron sputtering. XRD analysis revealed the formation of anatase TiO2 with a body-centered tetragonal for all the bilayer films. Enlarged nanocrystallites that averaged to 40.4 nm and increased to 41.9 nm with increasing the TiN layer thickness to 15 nm were observed from SEM images. The optical transmission and reflection were found to decrease with increasing TiN layer thickness while the absorbance increased. The optical band gap and refractive index decreased with increasing TiN layer. The photocurrent density increased with increasing TiN top layer up to a thickness of 9 nm. The TiN/TiOxNy bilayer photoanode with TiN thickness of 9 nm is the best photoanode for water splitting performance. Therefore, the addition of TiN layer on TiOxNy nanocrystallites could improve the overall PEC performance. This indicated that the optimal combination of these two materials to make heterojunction could encourage the synergetic effects to PEC performance. The wettability of TiN/TiOxNy bilayer films
3.4. Wettability Fig. 8a shows the variation of contact angle for TiN/TiOxNy bilayer films with the UV illumination time. Before illumination the measured contact angles are 83, 91.8, 92.6, 80.9, 84.5 and 90° for samples with 0, 3, 6, 9, 12 and 15 nm TiN top layer thicknesses. With UV illumination for 10–30 min, the contact angle decreased sharply and the bilayer samples become hydrophilic. With further increase in the illumination time, the contact angles decrease slowly to reach zero after UV illumination of 60 min. It is observed that the TiN layer thickness does not have much effect on the wettability of TiN/TiOxNy bilayer films. It has been previously reported that the incorporation of small amount of 6
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increased with increasing UV illumination time.
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Optical, water splitting and wettability of titanium nitride/titanium oxynitride bilayer films for hydrogen generation and solar cells applications
T
S.H. Mohameda,b,∗, Huaping Zhaoa, Henry Romanusc, F.M. El-Hossaryb, M. Abo EL-Kassemb, M.A. Awadb, Mohamed Rabiaa,d, Yong Leia a
Institute of Physics and Macro- and Nanotechnologies MacroNano® (IMN & ZIK), Ilmenau University of Technology, 98693 Ilmenau, Germany Physics Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt c Zentrum FürMikro- und Nanotechnologien, TechnischeUniversitätIlmenau, Gustav-Kirchhoff-Strasse 7, 98693 Ilmenau, Germany d Nanophotonics and Applications Lab, Chemistry Department, Faculty of Science, Beni-Suef University, Beni-Suef 62514, Egypt b
A R T I C LE I N FO
A B S T R A C T
Keywords: TiN/TiOxNy bilayer films Optical properties Photoelectrochemical water splitting wettability
TiN/TiOxNy bilayer films with various TiN thicknesses were prepared using direct current (d.c.) reactive magnetron sputtering. The ratio of N/O in the bilayer films increased with increasing TiN top layer thickness. The observed crystalline peaks for the bilayer films, for different TiN layer thicknesses, were indexed to anatase TiO2 with tetragonal structure. Only 0.31° peak shift, to lower 2θ angles was detected. FESEM indicated that the morphology of TiOxNy single layer is consisted of elongated nanocrystals with diameters in the range of 27.9–46.8 nm and the diameters of the nanocrystallites changed slightly with increasing the thickness of TiN layer. The transmittance, absorbance, reflectance spectra and the optical constants of the bilayer films were presented. The photoelectrochemical performance for water splitting of TiN/TiOxNy bilayer films was studied by examination both the continuous and chopped photocurrent density under light illumination. The photocurrent density increases with increasing TiN top layer up to a thickness of 9 nm, after which a reduction in photocurrent is observed. The wettability of TiN/TiOxNy bilayer films could be transformed by UV illumination from hydrophobic to hydrophilic.
1. Introduction Titanium oxynitrides (TiOxNy) are belonging to transition metal oxynitride family which displays a wide range of performances and shows good optical, mechanical, electrical, photocatalytic and electrochemical properties [1–6]. Specially, TiOxNy have been extensively studied as optical functional coatings for commercial glass architecture applications and as environmental friendly materials with excellent in photocatalytic applications [6]. On the other hand titanium nitride (TiN) is important functional material and it has numerous attractive properties which depend on the preparation conditions and methods. TiN shows high hardness, low electrical conductivity, excellent thermal conductivity, good chemical stability, high corrosion resistance and high visible light absorption [7–9]. Therefore thin films of TiN have been planned for numerous applications; including pollution degradation [10], nitrite detection [11], resonators [12], heterojunction photovoltaic [13], fuel cells [14], supercapacitors [15], water splitting [16], dye-sensitized solar cells [17] and superconductors [18]. Due to its nontoxicity, chemical stability and widespread
∗
availability, titanium oxide (TiO2) has extensively proposed as photoanode for photoelectrochemical (PEC) water splitting that can transform water into chemical fuels, H2 and O2, under solar energy illumination. Nevertheless, TiO2 is naturally characterized by a wide band gap energy (> 3.0 eV) that capture small portion of visible light. This wide band gap energy limits the theoretical solar to hydrogen conversion efficiency to 2.2%, which is much less than 10% of the conversion efficiency requested for commercial use [1]. Recently, TiOxNy photoanodes have arose as an attractive candidates for PEC water oxidation, because of the narrower band gap that extends to visible light absorption as well as the suitability of band positions that being well straddle for the water reduction/oxidation potential [1,19]. The materials' wettability plays a significant role in technology. For example, the self-cleaning engineering artificial surfaces can be used as solar panel coatings because they assist in the elimination of the deposited dirt and dust there by enhance their light transmittance and their energy conversion efficiency. These surfaces can be generallyclassified into two types: (i) hydrophobic surfaces and (ii) hydrophilic surfaces [20,21]. In hydrophobic surfaces, the drops of water roll on the
Corresponding author. Physics Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt. E-mail address: [email protected] (S.H. Mohamed).
https://doi.org/10.1016/j.mssp.2019.104704 Received 12 April 2019; Received in revised form 14 August 2019; Accepted 28 August 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
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surface and carry the dirt from the surface. In case of hydrophilic surfaces, the drops of water spread on the surface and form a thin layer of water. After that the dirt is washed away during spreading process. There are many reports on wetting properties of TiO2 films, but the wetting performance of TiOxNy and TiN has been scarcely explored [22]. Also, the use of TiN/TiOxNy bilayer films for PEC water splitting has not seen in the literatures. Therefore TiN/TiOxNy bilayer films with various TiN thicknesses are prepared by d.c. reactive magnetron sputtering. Their structural, optical, PEC water splitting performance and wettability are examined.
Table 1 Chemical composition atomic (%), average Grain size via FESEM and optical band gap (Eg). dTiN (nm)
0 3 6 9 12 15 TiN
2. Experimental details TiN/TiOxNy films were prepared using dc reactive magnetron sputtering from the same metallic Ti target. The preparation of the bilayer films was achieved sequentially under the same vacuum from the same Ti target and without opening the sputtering chamber in between the deposition of the various layers. Sputtering process was carried out after reaching a base pressure of 2.8 × 10−7 mbar. The deposition of TiOxNy layer was carried out using Ar, N2 and O2 gases for flow rates of 25, 70 and 5 sccm, respectively, while the deposition of TiN layer was carried out using Ar and N2 with 25 and 75 sccm, respectively. In both cases the working pressure was 1.3 × 10−2 mbar and the sputtering power was adjusted to be 100 W. Sputtering was achieved on well cleaned Si(100), microscopic glass and fluorine doped tin oxide coated glass, FTO, (~10 Ω/sq) substrates. The substrates were maintained at 200 °C and the target to substrate distance was maintained at 5 cm. Before depositing each layer, the surface of the target was cleaned using Ar atmosphere sputtering for 10 min. The thickness of the TiOxNy bottom layer was fixed to 298 nm and the thicknesses of the TiN top layers were 0, 3, 6, 9, 12 and 15 nm. For phase identification, X-ray diffraction was carried out using Bruker/Siemens D5000 diffractometer. The patterns were achieved for samples prepared on glass substrates using grazing incidence scan with angle of 3°. Before measurements the machine was pre-calibrated using LaB6 standard powder. The film thickness determined by Veeco Dektak 150 profilometer. Field emission scanning microscopy (FESEM) model Hitachi S4800 was used to examine the surface morphology of the samples. EDAX analysis was achieved on a unit attached to the FESEM to determine the chemical composition. Both FESEM and EDAX were carried out for samples prepared on Si(100) substrates. Varian Cary 5000 UV–Vis–NIR spectrophotometer was employed to measure the transmittance (T), reflectance (R) and absorbance (Abs) for samples prepared on glass substrates. Photoelectrochemical (PEC) measurements were carried out onto samples prepared on FTO substrates using a three electrodes electrochemical cell with a BioLogic Potentiostat. The counter and the reference electrodes through the PEC measurements Pt mesh and Ag/ AgCl (in saturated KCl) electrodes, respectively. The used electrolyte was 0.1 M of NaOH (Ph = 13). The used light source was Oriel solar simulator, 300 W Xe lamp, AM 1.5 global filter which was previously calibrated to 1 sun via Si photodiode model 818, Newport. The photoinduced hydrophilicity was estimated by determining the contact angles between TiN/TiOxNy surface and pure water after UV illumination by PHONIX-300 contact angle meter. The measurements were done on samples prepared on glass substrates. The illumination was done using UV light lamp, model Germicidal UV-C, G13T8, Japan, of wavelength 254 nm for different periods of time.
Average Grain size via FESEM (nm)
40.4 34.2 37.1 36.4 33.7 41.9 –
Chemical composition atomic (%) N
O
Ti
8.72 9.14 9.63 11.53 14.19 14.72 48.65
62.75 63.3 64.01 58.81 55.4 54.48 15.03
28.53 27.56 26.36 29.66 30.41 30.8 36.32
Eg (eV)
3.29 3.25 3.22 3.14 2.96 2.82 -
quantify the chemical composition, such as limitation to concentrations is in the order of 0.1% and the used sampled volume from the silicon substrate is larger compared to that used from thin film, it can give good guide for the change in N/O ratio with increasing TiN top layer since the TiOxNy layer thickness is constant. Examples of EDAX spectra are shown in Fig. 1a–d. The peaks heights in the EDAX spectra are related to the concentrations of the elements. In addition to N, O and Ti peaks the EDAX spectra of the single TiOxNy layer contains Si and C peaks which may be attributed to the influence of the Si substrate and the contamination by hydrocarbons, respectively. The obtained nitrogen to oxygen ratio for the single TiOxNy layer film is very small comparing to the used flows ratio. Also, the titanium nitride single layer (Fig. 1d), where the flow of oxygen is zero, contains a pronounced amount of oxygen. This is attributed to: (i) the higher reactivity of oxygen than nitrogen with the metallic titanium [4,23], (ii) the presence of native oxide layer, SiO2, which usually grows naturally on top of the silicon wafers (with few nanometer in thickness) and (iii) the residual oxygen in the growth chamber. The ratio of N/O in the TiN/ TiOxNy bilayer films increases with increasing TiN top layer thickness. Fig. 2 shows XRD patterns of TiN/TiOxNy with various TiN thicknesses. All the observed peaks of the uncovered TiOxNy film can be indexed to anatase TiO2 with a body-centered tetragonal structure [JCPDS card no.: 01-089-4921]. Only 0.31° peak shift, to lower 2θ angles, was observed and no additional peaks related to titanium nitride or titanium oxynitride phases are observed. This peak shift is possibly ascribed to the substitution of some oxygen atoms by nitrogen atoms in the anatase TiO2. The nitrogen atoms can either occupy the positions of oxygen atoms in the anatase TiO2 crystal lattice or located at the grain boundaries and, thereby create some additional stresses which in turn lead to peak shift [5,24,25]. The experimental film height misalignment may also contribute slightly to the peak shift [25]. It is worth mentioning that although TiOxNy is deposited, diffraction peaks of TiO2 are obtained. This is because for titanium oxynitrides prepared with even very small amount of oxygen, the formation of TiO2 phase is favored since it is thermodynamically stable phase [4,26,27]. Also the higher reactivity of oxygen towards titanium which favors the formation of Ti–O bonds over Ti–N bonds can facilitate the formation of TiO2 phase [26–28]. The addition of TiN layer on top of the TiOxNy film does not create any additional phases; however it reduces the intensity of the anatase TiO2 peaks as the TiN thicknesses increases. The effect of TiN top layer on the morphology growth of TiN/ TiOxNy bilayer films is established in Fig. 3. The morphology of TiOxNy single layer (Fig. 3a) is consisted of elongated nanocrystals with various diameters ranging from 27.9 to 46.8 nm. As the TiN layer is deposited on top of TiOxNy, the morphology does not change extensively (Fig. 3b–d). With further increase of the TiN layer thickness up to 15 nm, the morphology of the nanocrystallites becomes slightly bigger. The obtained nanocrystallites are summarized in Table 1.
3. Results and discussion 3.1. Chemical composition, structural and morphological analysis The evaluated chemical composition of TiN/TiOxNy bilayer films with various TiN thicknesses, identified by EDAX, is summarized in Table 1. Although the EDAX technique has certain limitations to 2
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Fig. 1. EDAX spectra of TiN/TiOxNybilayer films.
3.2. Optical properties Fig. 4 shows spectral transmission, reflection and absorbance of TiN/TiOxNy bilayer films. The TiOxNy film has higher transmittance (Fig. 4a) with clear interference fringes and an abrupt decrease at the onset of absorption. This indicates a good quality of the film and lower level of defects. The addition of 3 nm TiN top layer film leads to an overall decrease in transmission. With further increase in TiN layer thickness, the overall transmission decreased to very low values. A shift in onset of absorption towards longer wavelengths is also observed with TiN layer thickness. The reflection of TiN/TiOxNy bilayer films are decreased also with increasing TiN top layer (Fig. 4b). It has been previously reported that the sub-stoichiometric titanium nitride films have much lower reflection than stoichiometric films [29–31]. At the same time, the measured absorbance (Fig. 4c) increased with increasing TiN layer thickness. This trend has also been observed by Li et al. [7]. for TiN/TiO2 bilayer films and they ascribed it to the narrower optical band gap of TiN comparing to TiO2 [32] and the possible plasmonic effect given by TiN [33,34]. The absorption coefficient (α) was calculated via the measured spectral reflection and transmission, and the total bilayer film thickness (d) from the equation:
α=
1 1−R ⎞ ln ⎛ d ⎝ T ⎠
(1)
The energy band gap (Eg) was estimated from Tauc's formula by assuming indirect allowed transition: 1
(αhν ) 2 = β (hν − Eg )
(2)
where ν is the frequency of the incident light and β is a constant. The Eg 1 values for TiN/TiOxNy bilayer films are evaluated from (αhν ) 2 versus hν 1
plots as shown in Fig. 5, by extending the tangent line to (αhν ) 2 = 0 , and are listed in Table 1. Generally, the Eg decreases from 3.27 to 2.85 eV with increasing TiN thickness from 0 to 15 nm, however the decrease is slow for thinner TiN thicknesses variation and it becomes sharper at thicker TiN thicknesses. The Eg value determined for TiOxNy single layer film agrees well with the value of titanium oxynitride films
Fig. 2. XRD patterns of TiN/TiOxNybilayer films with various TiN thicknesses.
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Fig. 3. FESEM images of TiN/TiOxNy bilayer films with various TiN thicknesses of 0 nm (a), 3 nm (b), 6 nm (c), 9 nm (d), 12 nm (f) and 15 nm (g). The scale bar of all the images is 100 nm.
bilayer films. The effective refractive indexes of all the TiN/TiOxNy bilayer films follow the normal dispersion. The n values decrease with increasing TiN layer thickness. TiOxNy single layer film has very low k values, close to zero, in most of the spectrum which designate low optical losses due to absorption. The strident increase in k values at short wavelengths (below 500 nm) is attributed to the fundamental absorption through the band gap. With increasing TiN layer thickness, a remarkable increase in k values is observed which indicates the association of defect and absorbing centers whose amount increase with increasing TiN layer thickness.
containing approximately, the same amount of nitrogen content [4,19,35]. The effective refractive index (n) and effective extinction coefficient (k) of TiN/TiOxNy bilayer films were calculated using the search technique based on minimizing (ΔR)2 and (ΔT)2 simultaneously [36–38] where
R calc (n, k , d, λ ) − R exp 2 , Tcalc (n, k , d, λ ) − Texp
2
(3)
Texp and Rexp are the experimentally determined values of the T and R, respectively, while Tcalc and Rcalc are the calculated values of the T and R, respectively, using Murmann's exact equations [36–38]. Fig. 6a and b shows variations of n and k with wavelength for TiN/TiOxNy 4
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Fig. 4. Spectral transmission (a), reflection (b) and absorbance (c) of TiN/ TiOxNy bilayer films.
Fig. 6. The refractive index (a) and extinction coefficient (b) variations with wavelength for TiN/TiOxNy bilayer films.
interface. The bilayer films display onset potential around −0.60 VAg/ above which the photocurrent density increased with increasing voltage. This increase is ascribed to the obviously improved charge separation and enhanced transfer rate under the cascading band arrangement and indicates that the charge transfer and recombination happen concurrently [19]. The photocurrent density increases with increasing TiN top layer until a thickness of 9 nm after which a reduction in photocurrent is observed. The higher photocurrent density for the films having TiN layer on top, comparing to the single TiOxNy film is due to the lower optical band gap values, the extra light absorption, the better electrical conductivity, and the better efficient photo-electron-hole separation [7,39–44]. It is worth mentioning that, while the optical absorbance, average grain size and the smallest band gap energy are for the bilayer films with TiN thickness greater than 9 nm, the best photo electrochemical performance is recorded for 9 nm TiN thickness. This may explained as follows. It is believed that the TiN layer thickness is a decisive parameter which determined the efficiency of the bilayer films. Actually, the presence of TiN layer on top of TiOxNy layer can lead to the formation of heterojunction, by TiOxNy and TiN, with proper electronic structures [3], affect the charge space region and narrow the optical band gap of the bilayer films [7]. Thereby provide more visible light absorption and reduce electron–hole recombination [3,7,42] which result in increasing photocurrent density. This increase in photocurrent is continued with increasing TiN layer thickness to an optimal thickness of 9 nm. The reduction in photocurrent density at TiN thicknesses greater than 9 nm may be related to the more occurrences of oxygen vacancies in TiN thick films which cause more charge recombination. Li et al. [7] reported that the higher TiN layer, in the bilayer film can cause more negative flat-band value. For water splitting, the more negative flatAgCl,
1
Fig. 5. (αhν ) 2 versus hν plots for TiN/TiOxNy bilayer films.
3.3. Photoelectrochemical performance for water splitting Fig. 7a shows the electrochemical potentiodynamic scans (J-V) for TiN/TiOxNy bilayer films deposited on FTO glass. The photocurrent density (J) measures the photo induced reactions (the oxidation power) of the photo-generated holes that reach the bilayer-film/electrolyte
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Fig. 7. The photocurrent density versus potential for TiN/TiOxNy bilayer films deposited on FTO glass under light illumination (a) and under the chopped light illumination (b).
Fig. 8. Variations of water contact angle on TiN/TiOxNy bilayers films with different UV exposure times (a) and ln(θ) versus t plots as designated by the exponential function of equation (3) (b).
band value provides larger driving force to trigger the H2O redox. Torres et al. [45] examined the photoelectrochemical performance of N-doped TiO2 in aqueous electrolyte. They found that N-doped electrodes exhibited a significant increase in photocurrent under visible light irradiation, comparing with un-doped TiO2 electrode. However the N-doping could create high density of electron trapping states, thus optimization of N doping is needed. From the above argument it is inferred that the better water splitting of our bilayer films is due to the mixture of TiN and TiO2 properties [46]. Fig. 7b presents the photocurrent density versus potential under the chopped light illumination which is measured further to study the kinetics of photoresponse of TiN/TiOxNy bilayer films. The abrupt rise and decay of photocurrent density with rectangular shapes during the on/off light illumination reveals the fast charge transport/transfer rate between the bilayer films. Moreover, it is significant that the photocurrent density generated in the chopped curves certainly coincides with that of the continuous illumination curves shown Fig. 7a.
nitrogen into TiO2, to form TiOxNy or N doped TiO2, resulted in better wettability due to the increase of absorption and surface roughness [28,47,48]. The decrease in contact angle after UV illumination can be fitted with an exponential function (Fig. 8b) in the form:
θ = θ0 exp(−kUV t )
(4)
This behavior is similar to that reported for titania particles prepared via hydrolyzing titanium tetraethoxide [49]. The rate constant (kUV) values are 0.051, 0.042, 0.054, 0.057, 0.054, 0.051 min−1. 4. Conclusion TiN/TiOxNy bilayer films with various TiN thicknesses were prepared using reactive dc magnetron sputtering. XRD analysis revealed the formation of anatase TiO2 with a body-centered tetragonal for all the bilayer films. Enlarged nanocrystallites that averaged to 40.4 nm and increased to 41.9 nm with increasing the TiN layer thickness to 15 nm were observed from SEM images. The optical transmission and reflection were found to decrease with increasing TiN layer thickness while the absorbance increased. The optical band gap and refractive index decreased with increasing TiN layer. The photocurrent density increased with increasing TiN top layer up to a thickness of 9 nm. The TiN/TiOxNy bilayer photoanode with TiN thickness of 9 nm is the best photoanode for water splitting performance. Therefore, the addition of TiN layer on TiOxNy nanocrystallites could improve the overall PEC performance. This indicated that the optimal combination of these two materials to make heterojunction could encourage the synergetic effects to PEC performance. The wettability of TiN/TiOxNy bilayer films
3.4. Wettability Fig. 8a shows the variation of contact angle for TiN/TiOxNy bilayer films with the UV illumination time. Before illumination the measured contact angles are 83, 91.8, 92.6, 80.9, 84.5 and 90° for samples with 0, 3, 6, 9, 12 and 15 nm TiN top layer thicknesses. With UV illumination for 10–30 min, the contact angle decreased sharply and the bilayer samples become hydrophilic. With further increase in the illumination time, the contact angles decrease slowly to reach zero after UV illumination of 60 min. It is observed that the TiN layer thickness does not have much effect on the wettability of TiN/TiOxNy bilayer films. It has been previously reported that the incorporation of small amount of 6
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increased with increasing UV illumination time.
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