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Phase transformation from rutile to anatase with oxygen ion dose in the TiO2 layer formed on a Ti substrate
Materials Science in Semiconductor Processing 106 (2020) 104776
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Phase transformation from rutile to anatase with oxygen ion dose in the TiO2 layer formed on a Ti substrate A. Medvids a, *, S. Varnagiris b, E. Letko a, D. Milcius b, L. Grase a, c, S. Gaidukovs d, A. Mychko a, A. Pludons c, P. Onufrijevs a, H. Mimura e a
Institute of Technical Physics, Faculty of Materials Science and Applied Chemistry, Riga Technical University, P.Valdena 3/7, Riga, LV-1048, Latvia Center for Hydrogen Energy Technologies, Lithuanian Energy Institute, Kaunas, Lithuania Institute of Silicate Materials, Faculty of Materials Science and Applied Chemistry, Riga Technical University, P.Valdena 3/7, Riga, LV-1048, Latvia d Institute of Polymer Materials, Faculty of Materials Science and Applied Chemistry, Riga Technical University, P.Valdena 3/7, Riga, LV-1048, Latvia e Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, 432-8011, Japan b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase transformation Plasma immersion ion implantation Nanomaterials Oxides Titanium compounds
The oxygen plasma immersion ion implantation (O-PIII) method was used to form a TiO2 layer on a Ti substrate. The content of the anatase polycrystalline phase increased from 5.1% to 22.3% of the total TiO2 amount with a higher Oþ ion dose. The oxygen ion doses were 3.6 � 1016 ions, 8.1 � 1016 ions, 1.26 � 1017 ions, for 1, 2, and 3 h, respectively, with ion energies of E � 0.8 keV. The polycrystalline phases and morphology of the obtained films were characterized by XRD, FESEM, AFM, and Raman spectroscopy. The chemical bond analysis and layerby-layer measurements for TiO2 layer thickness evaluation were performed by XPS. The photocatalytic decomposition reaction constants of a methylene blue solution increased from 3.2 � 10 3 min 1 to 4.2 � 10 3 min 1 under UV-A irradiation and the liquid free surface energy increased from 39.0 mJ/m2 to 49.5 mJ/m2 with a higher O-PIII treatment doses. These results were explained by the partial phase transition of the TiO2 layer from rutile to anatase. Thus, the increase in the Oþ ions implantation dose into the Ti substrate invoked the reduction of material density from Ti (ρ ¼ 4.506 g/cm3) through TiO2 rutile (4.23 g/cm3) to TiO2 anatase (3.78 g/cm3) phase.
1. Introduction Oxygen plasma immersion ion implantation (O-PIII) takes funda mental advantage of ion implantation using the simplest of systems: direct extraction of ions from a plasma and implantation into a surface which is immersed in the plasma or at the boundary of the plasma sheath. Generally, during the O-PIII process, the target is entirely immersed into a plasma sheath by applying a negative voltage to the target. In this case, electrons are repelled near the target surface region and a sheath of positive ions is established. The efficient transfer of ions from the plasma to the target surface initiates a suitable implantation of high-dose low-energy ions. These implanted ions interact with substrate atoms by creating a layer with a different composition (e.g., a TiO2 layer on a Ti substrate in oxygen plasma) [1–3]. PIII is applicable to any material shape due to a plasma’s flexibility to cover a target. Moreover, PIII can be considered as a pure implantation method because the plasma composition can be easily controlled and it
does not require a very high vacuum or a complex process control sys tem [1]. PIII is one of the oldest ion implantation techniques. Recently, it has received an increasing attention for applications in microelectronics, biomedicine, metallurgy, and other surface-related areas [3–6]. Three types of power sources can support the PIII process: radio frequency (RF), pulsed direct current (PDC), and direct current (DC) power sources. RF is the most common due to its ability to cover a relatively high volume in a vacuum chamber. However, at a given power, the implant dose using RF is lower than PDC or DC. PDC prevents plasma depletion and target surface charging during the pulses. It can inhibit overheating of the target. A previous study investigated the formation of the TiO2 rutile phase on Ti substrates using a PDC power source as functions of temperature (265 � C and 550 � C) and dose [7]. The grain size increased with temperature from 10 to 100 nm due to the diffusion of O atoms. A DC power source works very similar as PDC but it lacks a neutralization function. This means that a DC power source is the most
* Corresponding author. E-mail address: [email protected] (A. Medvids). https://doi.org/10.1016/j.mssp.2019.104776 Received 6 August 2019; Received in revised form 8 October 2019; Accepted 10 October 2019 Available online 15 October 2019 1369-8001/© 2019 Published by Elsevier Ltd.
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ions for 2 h, and D3 ¼ 1.26 � 1017 ions for 3 h treatment with ion en ergies E � 0.8 keV). These values were observed by using free available Monte Carlo simulation code named Stopping and Range of Ions in Materials (SRIM). This software is the most spread simulation code which is widely used for researchers in various sputtering and its application areas (simulation of distributed multi energy implanted ions, the damage distribution, ion doses calculation, etc.). SRIM has extensive database on compound target materials and electronic energy loss data. This software is based on TRIM (TRansport of Ions in Matter) code, uses the ZBL (Ziegler-Biersack-Littmark) universal interaction potential and Biersack’s magic formula to solve the scattering integral [20–23].
suitable for thermally stable targets. Normally, DC is used as PIII power source when a higher implanted ion dose is required [1,3,8–10]. Yang et al. formed a thin TiO2 layer (primarily consisting of rutile) on a Ti surface using an oxygen plasma immersion ion implantation (O-PIII) process with an RF plasma source. They showed that the O-PIII treat ment improved cellular adhesion and corrosion resistance of a Ti surface [11]. In another study, Yang et al. demonstrated that this method enhanced the biocompatibility of the Ti surface for better dental implant applications [12]. Meirelles et al. showed that the O-PIII treated Ti provided better bone formation in rabbit tibia femur than primary Ti implants [13]. Other works also reported benefits regarding the morphology, compo sition, and crystalline phase when O-PIII treated Ti metals were used [14–16]. These examples indicated that the O-PIII method is suitable to grow a TiO2 layer from a titanium substrate for applications in medicine or other related areas. It should be mentioned that other ion implantation methods have also been used for TiO2 layer formation on a titanium substrate. For example, C. Hammerl et al. used high-fluence oxygen ion implantation for TiO2 layer formation. They demonstrated the possibility to form a TiO2 layer consisting of α-, β-, δ-TiO, rutile TiO2, and Ti3O5 [17]. Un fortunately, anatase phase formation and control of the anatase/rutile phase ratio, which are very important for photocatalysis, environment purification, etc. remain big challenges using PIII method. Syarif et al. used the pulsed laser deposition method for TiO2 layer formation. They showed that the crystal structure from rutile to anatase can be changed by increasing the oxygen pressure in an Oþ ion atmo sphere [18]. M. Horprathum et al. obtained very similar results [19] using a DC reactive magnetron sputtering method for TiO2 film forma tion at different partial oxygen pressures. They showed that the crys talline structure of TiO2 thin films can be modified by the partial oxygen pressure. A rutile TiO2 thin film was obtained at 4.8 � 10 4 mbar partial pressure, while an anatase TiO2 film formed at 5 � 10 2 mbar. These results showed that the TiO2 crystal structure can be controlled by the partial oxygen pressure or Oþ ion dose. Herein we show that the O-PIII method can control the phase of the TiO2 layer formed on a Ti substrate by varying the Oþ ion dose. More over, the fabricated TiO2 layers exhibit promising characteristics for photocatalysis applications.
2.2. Characterization Surface topography studies as well as the average surface roughness (Ra) of the TiO2 layers were evaluated by using a scanning probe mi croscope (VEECO CP II Scanning Probe Microscope, AFM). All mea surements were performed in the semi-contact mode. The surface morphology was measured with a field emission scanning electron mi croscope (FESEM, FEI Nova NanoSEM 650) using a secondary electron detector. The phase structures of the Ti samples were identified by an XRay diffractometer (Bruker D8, XRD) operating with Cu Kα radiation after the O-PIII treatment. Rietveld analysis was applied using the Topas software in order to refine each spectrum and evaluate the trans formation from the rutile to the anatase phase. The phase composition of the samples was studied using room temperature Raman spectroscopy (RS). Raman spectra were measured by a Renishaw InVia90V727 microRaman spectrometer with a HeNe red laser excitation at λ ¼ 633 nm. The Raman spectra in the spectral range of 100 cm 1 – 1000 cm 1 were measured to determine the phase of formed layers. The thickness of the TiO2 layer was measured using a combination of X-Ray spectroscope (PHI 5000 Versaprobe, XPS) and profilometer (AMBIOS XP-200) tech niques. The sputtering mode (2 kV voltage, 2 � 2 mm beam diameter) of XPS was used to etch the TiO2 layer. The O1s peak was observed during the etching process. It was assumed that the Ti substrate was reached when the O1s peak dropped to a zero intensity and Ti2p peak shifted from oxide to metal phase. In this case, the etching process was per formed through the whole TiO2 layer until the Ti substrate was reached. The thickness of the TiO2 layer was evaluated by measuring the depth of the etched hollow in TiO2 layer.
2. Experimental 2.1. Oxygen plasma immersion ion implantation
2.3. Testing of photocatalytic and wetting properties
Prior to the experiments, a pure titanium plate was cut into samples with dimensions of about 15.0 mm � 20.0 mm � 0.3 mm. These samples were washed in an ultrasonic bath of isopropyl alcohol and then dried under dry air flow. During the PIII process, oxygen gas (99.99% purity) was used. The pressure during O-PIII was kept at 13 Pa while the pri mary pressure was approximately 0.4 Pa in order to avoid plasma contamination. A DC power source was used to generate the oxygen plasma. Ti samples were put on a titanium cathode to apply a negative voltage to the sample. The power during O-PIII was 300 W (~0.6 A). The O-PIII treatment time of Ti samples was 1, 2, or 3 h to provide three different Oþ ion doses (D1 ¼ 3.6 � 1016 ions for 1 h, D2 ¼ 8.1 � 1016
The photocatalytic TiO2 layer properties were evaluated by bleach ing of Methylene Blue (MB) solution (made by Reachem Slovakia s.r.o.) under UV-A irradiation.Each sample was placed in a 30-mm diameter Petri dish. The experiment was started when 3 ml of 20 mg/L aqueous MB solution was syringed above the samples and top of the Petri dish was covered by a 500-μm thick fused silica disc to minimize evaporation of the MB solution. UV-A light was produced using a Thorlabs M365PL1C5 lamp (365-nm nominal wavelength and 43-mm beam diameter). The distance between the UV-A lamp and Ti samples was 100 mm. The lamp irradiation intensity on the surface of Ti sample was 24.5 mW/cm2 (measured using a Thorlabs PM16-401 Power meter).
Fig. 1. SEM views of Ti morphology after the D1 (a), D2 (b) and D3 (c) doses of O-PIII treatment. 2
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Fig. 2. 3D AFM topography images of Ti samples after D1 (a), D2 (b) and D3 (c) doses of O-PIII treatment.
Fig. 3. XPS measurements of D3 sample: O1s (a) and TiO2 (b) fitting curves (top layer of D3 sample) and alteration of O1s (c) and Ti2p (d) peaks with a longer sputtering time; red spectra represent top surface and subsequent spectra’s are recorded alternatively using 20 s sputtering.
The changes in the MB concentration were measured by a UV-VIS spectrophotometer (Jasco V-650). All measurements were repeated in 30 min intervals with 1.5 ml volume. The MB solution was syringed back to the Petri dish with the sample immediately after the spectroscopic analysis (approximately 2 min). The wetting properties of the samples were investigated by the sessile drop method using an Attention Optical Tensiometer. A pair of liquids: polar – water and non-polar – diiodomethane (MI) were chosen to measure the contact angles (CA) of sessile drops with a 0.5-μl volume. The polar and dispersive components γp, γd of the liquid free surface energy γtot were calculated based on Owens and Wendt’s [24,25] method using Wu’s [26] harmonic mean approach.
3. Results and discussion The surface morphology views of the Ti samples after O-PIII treat ment with various doses (D1, D2, and D3) are shown in Fig. 1. The surface smoothness and uniformity were similar for all dosing condi tions. Additionally, defects or cracks were not observed during the morphology analysis. The sample roughness was obtained by the AFM method (Fig. 2) after treatment with various Oþ ion doses. All samples performed very similar roughness values, which ranged from 17.0 nm to 19.1 nm after D3 and D1 doses of O-PIII treatment, respectively. Consequently, the surface morphology and roughness remained relatively stable and did not alternate during the O-PIII treatment. 3
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Table 1 Rutile and anatase phase amount in the layer after different oxygen ion doses. Doses of Oþ
Rutile phase amount, %
Anatase phase amount, %
D1 D2 D3
94.9 88.0 77.7
5.1 12.0 22.3
Fig. 4. XRD data of Ti after implantation of Oþ with doses D1, D2 and D3.
The top layer chemical bond analysis and layer-by-layer measure ments were performed by XPS technique (Fig. 3). First of all, only tita nium, oxygen and carbon elements were observed by analysing elemental composition of D3 sample. This showed that pure TiO2 layers were formed without any impurities. It is important to mention that the same tendencies were observed by analysing D1 and D2 samples. Ti2p spectra are shown in Fig. 3b. Two peaks were observed at binding en ergies of 458.8 eV (Ti2p3/2) and 464.5 eV (Ti2p3/2). According to XPS database [27] and our previous works [28,29], the binding energies and separation between them (Δ ¼ 5.7 eV) confirmed TiO2 formation. However, identification of particular TiO2 phase is almost impossible due to very similar binding energies where the difference between anatase and rutile is only 0.1 eV. The deconvolution of C1s showed three components with the binding energies at approximately 530.0 eV – O). C–O and C– – O bonds are (TiO2), 531.6 eV (C–O) and 532.9 eV (C– attributed to organic compounds. These compounds are attracted to the surface of TiO2 layer when samples were kept in ambient atmosphere. The layer-by-layer measurements were performed in order to un derstand oxygen distribution in TiO2 layer. Fig. 3c and d showed ex amples of layer-by-layer measurements with D3 sample. Results of O1s peak analysis showed that oxygen was observed till approximately 50 nm sputtering depth. The binding energy of these peaks were approximately 530.0 eV, which confirmed existence of TiO2 compound. Nevertheless, O1s peak vanished with the further sputtering cycle. This result confirmed that Oþ ions were incorporated into Ti plate till approximately 50 nm depth. These results were supplemented by Ti2p layer-by-layer analysis. The strong TiO2 peak was observed at the binding energy of approximately 458.8 eV. However, the shift of Ti2p peak to the lower binding energies (approximately 454.3 eV) was observed after five sputtering cycle still approximately 50 nm. This shift showed that TiO2 component was reduced and metal Ti phase was observed. Further analysis showed only metal Ti component. Thickness measurements showed that Oþ ion implantation and TiO2 formation reached a depth of approximately 43 nm, 49 nm and 54 nm after D1, D2 and D3 doses of O-PIII, respectively. The Oþ ions implan tation became partially saturated after 1-h with the D1 dose. Addition ally, the implantation thickness and TiO2 layer formation showed nonlinear dependence tendencies on ions doses. E.J.D.M. Pillaca et al. studied PIII into a silicon substrate using a magnetic mirror geometry [30]. They created additional E � B fields around the target, which increased the ion implantation dose and improved the ion implantation depth, roughness, and wettability. However, implantation depth de pendency on ion doses was non-linear. This nonlinear dependency could be related to the partial saturation of implanted ions. At the beginning of
Fig. 5. Raman spectra of Ti after O-PIII using at doses: D1, D2 and D3. R rutile, A - anatase phases, X – unknown.
O-PIII, ions were implanted into Ti substrate and formed a TiO2 com pound (e.g. in 40 nm depth). These formed TiO2 layers served as a barrier for deeper penetration of implanted Oþ ions. Further implanta tion and TiO2 formation proceeded at a lower depth (e.g. 20 nm). Hence, the Oþ ions implantation depth became partially saturated and nonlinear implantation depth dependence on ion doses was observed. The XRD patterns after O-PIII treatment with different doses are shown in Fig. 4. It was observed that all three graphs have the same components: titanium (hexagonal, P63/mmc, JCPDS No. 071–4632), Ti6O (hexagonal, P31c, JCPDS No. 072–1471), anatase (tetragonal, I41/ amd, JCPDS No. 21–1272), and rutile (tetragonal, P42/mnm, JCPDS No. 086–0148) crystalline phases. The most intensive anatase phases were (004) 37.8� and (101) 25.3� while the rutile phases were (110) 27.4� and (101) 36.2� , respectively. XRD results showed that the main anatase and rutile peaks became more intensive after longer treatment time. This suggested that longer treatment time produced a higher crystallinity in the TiO2 layer. The phase transformation was evaluated by recalculating the ratio between the anatase and rutile phases of each sample. The sample after the D1 dose consisted of 94.9% rutile phase and 5.1% anatase (Table 1). A higher ion dose invoked the TiO2 phase transformations, by increasing the amount of anatase and reducing the rutile TiO2 phase. The calcu lated amount of anatase (rutile) phases were 12.0% (88.0%) and 22.3% (77.7%) after the D2 and D3 dose, respectively. These results showed that the TiO2 phase transformation from rutile to anatase can be controlled by applying different ion doses during the O-PIII process. . The phase transformation with higher Oþ ions doses during O-PIII treatment may be explained by the more intensive anatase phase for mation compared to the rutile phase. Anatase is a metastable TiO2 structure, whereas rutile is a much more stable [25]. Higher oxygen ion doses invoke rapid and more chaotic processes on the top layers of the Ti substrate. These encourage shorter relaxation times of the TiO2 particles with more intensive metastable anatase phase TiO2 formation instead of rutile TiO2 [31–33]. Moreover, oxygen has a smaller density than tita nium. Therefore, higher implantation doses of the Oþ ions into Ti 4
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Table 2 Liquid free surface energy and contact angle of TiO2 samples after the D1, D2 and D3. Doses of Oþ
γtot, mJ/m2
γd, mJ/m2
γp, mJ/m2
CAW, �
CAMI, �
D1 D2 D2
39.0 43.5 49.5
26.1 30.5 34.7
12.9 13.0 14.8
77 74 68
70 60 51
can be trapped in the valence band at such heterojunction. This reduces the possibility of electron-hole recombination, improving the ability to decompose MB solution [40,41]. Moreover, TiO2 layers with a higher crystallinity form may invoke a higher decomposition ratio of the MB solution. Additionally, different microcrystal orientations of the same phase have different photocatalysis characteristics, which may have a positive influence on photocatalysis [42]. One of the most important parameters characterizing the surfaces of materials is the liquid free surface energy [35]. The value of the surface free energy is especially important in this work because it affects the photocatalytic activity. A higher surface free energy value indicates an enhanced adsorption as well as a higher decomposition ratio [34]. A characteristic photo of a water drop measured with the contact angles (CA) made by the sessile drop method on the surface of Ti samples after D3 of the O-PIII treatment is shown in Fig. 7. The summarized results of water contact angle measurements and calculated surface energy are shown in Table 2. The liquids contact angle values CAW and CAMI decreased gradually, while the polar γp and dispersive components γd of the surface free energy γtot increased proportionally with the longer O-PIII treatment time of the Ti surface. The anatase phase corresponds to a higher liquid free surface energy value [34]. The increased surface free energy confirmed the increase of the anatase crystallite concentration in TiO2 film. This hypothesis is supported by the fact that surface morphology remained nearly constant with the higher treatment time. Hence, structural changes explained the increase of liquid free surface energy. Moreover, these results are consistent with the XRD Rietveld analysis, which shows an increase in the anatase phase during O-PIII with higher Oþ ion doses.
Fig. 6. Bleaching of methylene blue solution under UV-A irradiation using Ti samples after the D1, D2 and D3 doses of O-PIII method treatment.
substrate decrease the material density from Ti (ρ ¼ 4.506 g/cm3) through TiO2 rutile (4.23 g/cm3) to TiO2 anatase (3.78 g/cm3) phase. The Raman spectra of three samples after the D1, D2 and D3 doses of O-PIII treatment are shown in Fig. 5. The Raman active modes at 143 cm 1, 238 cm 1, 445 cm 1, 610 cm 1, 829 cm 1 are associated with rutile single crystals [7,34,35]. A very important property of the anatase and rutile phases in Raman spectra is the ratio between the 143 cm 1 band and the 600 cm 1 band. For anatase it is greater than 1, but it is less than 1 for rutile [34]. It was observed that the ratio between mentioned bands is less than 1 after D1 and D2 doses of the O-PIII treatment, suggesting that it may be associated with the rutile phase. However, the Raman spectrum changed after the D3 dose of O-PIII treatment: the observed ratio was greater than 1. Such result can be associated with the anatase phase. In addition to the first order Raman bands, broad struc tures around 143 cm 1, 238 cm 1, 445 cm 1, 610 cm 1 and 829 cm 1 were observed. These can be traced back to either disorder induced scattering or second order processes. The small regular peaks in regions 150 cm 1 – 300 cm 1 and 500 cm 1 – 850 cm 1 are related to the dis order induced scattering or second order processes [36]. The photocatalysis experiment results are shown in Fig. 6. First of all, it should be mentioned that UV-A irradiation had a relatively weak effect on the decomposition ratio of MB solution (first order reaction constant 1 � 10 5 min 1) (Fig. 6 UV-A). Moreover, Ti samples after the O-PIII method treatment without UV-A irradiation (in dark) also showed a negligible MB decomposition ratio, whilethe first order reaction con stants were 2 � 10 4 min 1, 2.5 � 10 4 min 1, 2.4 � 10 4 min 1 after the D1, D2 and D3 doses, respectively. Such negligible decomposition could be observed due to agglomeration and sedimentation processes of the MB solution on the samples surfaces [37–39]. The results using Ti samples after the O-PIII method treatment under UV-A irradiation were completely different. The first order reaction constants using the D1, D2 and D3 doses of O-PIII treated samples were 3.2 � 10 3 min 1, 3.7 � 10 3 min 1, and 4.2 � 10 3 min 1, respectively. The D3 dose of the O-PIII method treated sample invoked the highest decomposition ratio, while sample treated with D1 dose had the lowest. The results of crystallinity analysis showed that the TiO2 layer con sisted of two phases: anatase and rutile. Many researchers demonstrated that the use of two phases as a heterojunction can improve the photo catalytic activity compared to pure TiO2 phases. For example, electrons
4. Conclusions The oxygen plasma immersion ion implantation method was used to form TiO2 layers on a Ti substrate with different Oþ ion doses. The re sults of chemical bond analysis and layer-by-layer measurements showed that the implantation thickness and TiO2 layer formation have non-linear dependency on Oþ ion doses with a partial saturation at approximately 40 nm. The phase transformations with an increased amount of anatase and reduced amount of the rutile TiO2 phase was initiated by a higher ion dose. These observations were explained by the formation of the metastable anatase TiO2 phase due to the shorter relaxation time of TiO2 particles during the O-PIII with higher Oþ ion doses. These phase transformations increased the free surface energy from 39.0 mJ/m2 to 49.5 mJ/m2 as well as the photocatalytic decom position ratio from 3.2 � 10 3 min 1 to 4.2 � 10 3 min 1 under UV-A irradiation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence
Fig. 7. Characteristic water (a) and diidomethane (b) sessile drop on the surface of Ti samples after D3 dose of O-PIII treatment. 5
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the work reported in this paper.
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Acknowledgements This work was carried out under the Cooperative Research Project Program of the Research Institute of Electronics, Shizuoka University. References [1] J.R. Conrad, Handbook of Plasma Immersion Ion Implantation and Deposition, John Wiley & Sons, New-York, 2000. [2] J.R. Conrad, R.A. Dodd, F.J. Worzala, X. Qiu, Plasma source ion implantation: a new, cost-effective, non-line-of-sight technique for ion implantation of materials, Surf. Coat. Technol. 36 (3–4) (1988) 927–937. [3] S. M€ andl, D. Manova, Modification of metals by plasma immersion ion implantation, Surf. Coat. Technol. 365 (2019) 83–93. [4] P.K. Chu, Progress in direct-current plasma immersion ion implantation and recent applications of plasma immersion ion implantation and deposition, Surf. Coat. Technol. 229 (2013) 2–11. [5] W. Jin, P.K. Chu, Surface functionalization of biomaterials by plasma and ion beam, Surf. Coat. Technol. 336 (2018) 2–8. [6] K.W.K. Yeung, et al., Antimicrobial effects of oxygen plasma modified medical grade Ti-6Al-4V alloy, Vacuum 89 (1) (2013) 271–279. [7] S. M€ andl, G. Thorwarth, M. Schreck, Raman study of titanium oxide layers produced with plasma immersion ion implantation, Surf. Coat. Technol. 125 (2000) 84–88. [8] D. Manova, W. Attenberger, S. M€ andl, B. Stritzker, B. Rauschenbach, Evolution of local texture and grain morphology in metal plasma immersion ion implantation &; deposition of TiN, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 22 (6) (2004) 2299–2305. [9] T.E. Sheridan, Effect of target size on dose uniformity in plasma-based ion implantation, J. Appl. Phys. 81 (11) (1997) 7153–7157. [10] K.G. Kostov, J.J. Barroso, M. Ueda, Two dimensional computer simulation of plasma immersion ion implantation, Braz. J. Phys. 34 (4b) (2004) 1689–1695. [11] C.-H. Yang, Y.-T. Wang, W.-F. Tsai, C.-F. Ai, M.-C. Lin, H.-H. Huang, Effect of oxygen plasma immersion ion implantation treatment on corrosion resistance and cell adhesion of titanium surface, Clin. Oral Implant. Res. 22 (12) (2011) 1426–1432. [12] C.-H. Yang, Y.-C. Li, W.-F. Tsai, C.-F. Ai, H.-H. Huang, Oxygen plasma immersion ion implantation treatment enhances the human bone marrow mesenchymal stem cells responses to titanium surface for dental implant application, Clin. Oral Implant. Res. 26 (2) (2015) 166–175. [13] L. Meirelles, et al., A novel technique for tailored surface modification of dental implants - a step wise approach based on plasma immersion ion implantation, Clin. Oral Implant. Res. 24 (4) (2013) 461–467. [14] M. Rinner, J. Gerlach, W. Ensinger, Formation of titanium oxide films on titanium and Ti6Al4V by O2-plasma immersion ion implantation, Surf. Coat. Technol. 132 (2–3) (2000) 111–116. [15] R. Valencia-Alvarado, et al., Oxygen implantation and diffusion in pure titanium by an rf inductively coupled plasma, Vacuum 83 (1) (2009) 264–267. [16] A.E. Mu~ noz-Castro, et al., Ion implantation of oxygen and nitrogen in CpTi, Prog. Org. Coat. 64 (2–3) (2009) 259–263. [17] C. Hammerl, B. Renner, B. Rauschenbach, W. Assmann, Phase formation in titanium after high-fluence oxygen ion implantation, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 148 (1) (1999) 851–857. [18] D.G. Syarif, A. Miyashita, T. Yamaki, T. Sumita, Y. Choi, H. Itoh, Preparation of anatase and rutile thin films by controlling oxygen partial pressure, Appl. Surf. Sci. 193 (1–4) (2002) 287–292.
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Phase transformation from rutile to anatase with oxygen ion dose in the TiO2 layer formed on a Ti substrate A. Medvids a, *, S. Varnagiris b, E. Letko a, D. Milcius b, L. Grase a, c, S. Gaidukovs d, A. Mychko a, A. Pludons c, P. Onufrijevs a, H. Mimura e a
Institute of Technical Physics, Faculty of Materials Science and Applied Chemistry, Riga Technical University, P.Valdena 3/7, Riga, LV-1048, Latvia Center for Hydrogen Energy Technologies, Lithuanian Energy Institute, Kaunas, Lithuania Institute of Silicate Materials, Faculty of Materials Science and Applied Chemistry, Riga Technical University, P.Valdena 3/7, Riga, LV-1048, Latvia d Institute of Polymer Materials, Faculty of Materials Science and Applied Chemistry, Riga Technical University, P.Valdena 3/7, Riga, LV-1048, Latvia e Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, 432-8011, Japan b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase transformation Plasma immersion ion implantation Nanomaterials Oxides Titanium compounds
The oxygen plasma immersion ion implantation (O-PIII) method was used to form a TiO2 layer on a Ti substrate. The content of the anatase polycrystalline phase increased from 5.1% to 22.3% of the total TiO2 amount with a higher Oþ ion dose. The oxygen ion doses were 3.6 � 1016 ions, 8.1 � 1016 ions, 1.26 � 1017 ions, for 1, 2, and 3 h, respectively, with ion energies of E � 0.8 keV. The polycrystalline phases and morphology of the obtained films were characterized by XRD, FESEM, AFM, and Raman spectroscopy. The chemical bond analysis and layerby-layer measurements for TiO2 layer thickness evaluation were performed by XPS. The photocatalytic decomposition reaction constants of a methylene blue solution increased from 3.2 � 10 3 min 1 to 4.2 � 10 3 min 1 under UV-A irradiation and the liquid free surface energy increased from 39.0 mJ/m2 to 49.5 mJ/m2 with a higher O-PIII treatment doses. These results were explained by the partial phase transition of the TiO2 layer from rutile to anatase. Thus, the increase in the Oþ ions implantation dose into the Ti substrate invoked the reduction of material density from Ti (ρ ¼ 4.506 g/cm3) through TiO2 rutile (4.23 g/cm3) to TiO2 anatase (3.78 g/cm3) phase.
1. Introduction Oxygen plasma immersion ion implantation (O-PIII) takes funda mental advantage of ion implantation using the simplest of systems: direct extraction of ions from a plasma and implantation into a surface which is immersed in the plasma or at the boundary of the plasma sheath. Generally, during the O-PIII process, the target is entirely immersed into a plasma sheath by applying a negative voltage to the target. In this case, electrons are repelled near the target surface region and a sheath of positive ions is established. The efficient transfer of ions from the plasma to the target surface initiates a suitable implantation of high-dose low-energy ions. These implanted ions interact with substrate atoms by creating a layer with a different composition (e.g., a TiO2 layer on a Ti substrate in oxygen plasma) [1–3]. PIII is applicable to any material shape due to a plasma’s flexibility to cover a target. Moreover, PIII can be considered as a pure implantation method because the plasma composition can be easily controlled and it
does not require a very high vacuum or a complex process control sys tem [1]. PIII is one of the oldest ion implantation techniques. Recently, it has received an increasing attention for applications in microelectronics, biomedicine, metallurgy, and other surface-related areas [3–6]. Three types of power sources can support the PIII process: radio frequency (RF), pulsed direct current (PDC), and direct current (DC) power sources. RF is the most common due to its ability to cover a relatively high volume in a vacuum chamber. However, at a given power, the implant dose using RF is lower than PDC or DC. PDC prevents plasma depletion and target surface charging during the pulses. It can inhibit overheating of the target. A previous study investigated the formation of the TiO2 rutile phase on Ti substrates using a PDC power source as functions of temperature (265 � C and 550 � C) and dose [7]. The grain size increased with temperature from 10 to 100 nm due to the diffusion of O atoms. A DC power source works very similar as PDC but it lacks a neutralization function. This means that a DC power source is the most
* Corresponding author. E-mail address: [email protected] (A. Medvids). https://doi.org/10.1016/j.mssp.2019.104776 Received 6 August 2019; Received in revised form 8 October 2019; Accepted 10 October 2019 Available online 15 October 2019 1369-8001/© 2019 Published by Elsevier Ltd.
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ions for 2 h, and D3 ¼ 1.26 � 1017 ions for 3 h treatment with ion en ergies E � 0.8 keV). These values were observed by using free available Monte Carlo simulation code named Stopping and Range of Ions in Materials (SRIM). This software is the most spread simulation code which is widely used for researchers in various sputtering and its application areas (simulation of distributed multi energy implanted ions, the damage distribution, ion doses calculation, etc.). SRIM has extensive database on compound target materials and electronic energy loss data. This software is based on TRIM (TRansport of Ions in Matter) code, uses the ZBL (Ziegler-Biersack-Littmark) universal interaction potential and Biersack’s magic formula to solve the scattering integral [20–23].
suitable for thermally stable targets. Normally, DC is used as PIII power source when a higher implanted ion dose is required [1,3,8–10]. Yang et al. formed a thin TiO2 layer (primarily consisting of rutile) on a Ti surface using an oxygen plasma immersion ion implantation (O-PIII) process with an RF plasma source. They showed that the O-PIII treat ment improved cellular adhesion and corrosion resistance of a Ti surface [11]. In another study, Yang et al. demonstrated that this method enhanced the biocompatibility of the Ti surface for better dental implant applications [12]. Meirelles et al. showed that the O-PIII treated Ti provided better bone formation in rabbit tibia femur than primary Ti implants [13]. Other works also reported benefits regarding the morphology, compo sition, and crystalline phase when O-PIII treated Ti metals were used [14–16]. These examples indicated that the O-PIII method is suitable to grow a TiO2 layer from a titanium substrate for applications in medicine or other related areas. It should be mentioned that other ion implantation methods have also been used for TiO2 layer formation on a titanium substrate. For example, C. Hammerl et al. used high-fluence oxygen ion implantation for TiO2 layer formation. They demonstrated the possibility to form a TiO2 layer consisting of α-, β-, δ-TiO, rutile TiO2, and Ti3O5 [17]. Un fortunately, anatase phase formation and control of the anatase/rutile phase ratio, which are very important for photocatalysis, environment purification, etc. remain big challenges using PIII method. Syarif et al. used the pulsed laser deposition method for TiO2 layer formation. They showed that the crystal structure from rutile to anatase can be changed by increasing the oxygen pressure in an Oþ ion atmo sphere [18]. M. Horprathum et al. obtained very similar results [19] using a DC reactive magnetron sputtering method for TiO2 film forma tion at different partial oxygen pressures. They showed that the crys talline structure of TiO2 thin films can be modified by the partial oxygen pressure. A rutile TiO2 thin film was obtained at 4.8 � 10 4 mbar partial pressure, while an anatase TiO2 film formed at 5 � 10 2 mbar. These results showed that the TiO2 crystal structure can be controlled by the partial oxygen pressure or Oþ ion dose. Herein we show that the O-PIII method can control the phase of the TiO2 layer formed on a Ti substrate by varying the Oþ ion dose. More over, the fabricated TiO2 layers exhibit promising characteristics for photocatalysis applications.
2.2. Characterization Surface topography studies as well as the average surface roughness (Ra) of the TiO2 layers were evaluated by using a scanning probe mi croscope (VEECO CP II Scanning Probe Microscope, AFM). All mea surements were performed in the semi-contact mode. The surface morphology was measured with a field emission scanning electron mi croscope (FESEM, FEI Nova NanoSEM 650) using a secondary electron detector. The phase structures of the Ti samples were identified by an XRay diffractometer (Bruker D8, XRD) operating with Cu Kα radiation after the O-PIII treatment. Rietveld analysis was applied using the Topas software in order to refine each spectrum and evaluate the trans formation from the rutile to the anatase phase. The phase composition of the samples was studied using room temperature Raman spectroscopy (RS). Raman spectra were measured by a Renishaw InVia90V727 microRaman spectrometer with a HeNe red laser excitation at λ ¼ 633 nm. The Raman spectra in the spectral range of 100 cm 1 – 1000 cm 1 were measured to determine the phase of formed layers. The thickness of the TiO2 layer was measured using a combination of X-Ray spectroscope (PHI 5000 Versaprobe, XPS) and profilometer (AMBIOS XP-200) tech niques. The sputtering mode (2 kV voltage, 2 � 2 mm beam diameter) of XPS was used to etch the TiO2 layer. The O1s peak was observed during the etching process. It was assumed that the Ti substrate was reached when the O1s peak dropped to a zero intensity and Ti2p peak shifted from oxide to metal phase. In this case, the etching process was per formed through the whole TiO2 layer until the Ti substrate was reached. The thickness of the TiO2 layer was evaluated by measuring the depth of the etched hollow in TiO2 layer.
2. Experimental 2.1. Oxygen plasma immersion ion implantation
2.3. Testing of photocatalytic and wetting properties
Prior to the experiments, a pure titanium plate was cut into samples with dimensions of about 15.0 mm � 20.0 mm � 0.3 mm. These samples were washed in an ultrasonic bath of isopropyl alcohol and then dried under dry air flow. During the PIII process, oxygen gas (99.99% purity) was used. The pressure during O-PIII was kept at 13 Pa while the pri mary pressure was approximately 0.4 Pa in order to avoid plasma contamination. A DC power source was used to generate the oxygen plasma. Ti samples were put on a titanium cathode to apply a negative voltage to the sample. The power during O-PIII was 300 W (~0.6 A). The O-PIII treatment time of Ti samples was 1, 2, or 3 h to provide three different Oþ ion doses (D1 ¼ 3.6 � 1016 ions for 1 h, D2 ¼ 8.1 � 1016
The photocatalytic TiO2 layer properties were evaluated by bleach ing of Methylene Blue (MB) solution (made by Reachem Slovakia s.r.o.) under UV-A irradiation.Each sample was placed in a 30-mm diameter Petri dish. The experiment was started when 3 ml of 20 mg/L aqueous MB solution was syringed above the samples and top of the Petri dish was covered by a 500-μm thick fused silica disc to minimize evaporation of the MB solution. UV-A light was produced using a Thorlabs M365PL1C5 lamp (365-nm nominal wavelength and 43-mm beam diameter). The distance between the UV-A lamp and Ti samples was 100 mm. The lamp irradiation intensity on the surface of Ti sample was 24.5 mW/cm2 (measured using a Thorlabs PM16-401 Power meter).
Fig. 1. SEM views of Ti morphology after the D1 (a), D2 (b) and D3 (c) doses of O-PIII treatment. 2
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Fig. 2. 3D AFM topography images of Ti samples after D1 (a), D2 (b) and D3 (c) doses of O-PIII treatment.
Fig. 3. XPS measurements of D3 sample: O1s (a) and TiO2 (b) fitting curves (top layer of D3 sample) and alteration of O1s (c) and Ti2p (d) peaks with a longer sputtering time; red spectra represent top surface and subsequent spectra’s are recorded alternatively using 20 s sputtering.
The changes in the MB concentration were measured by a UV-VIS spectrophotometer (Jasco V-650). All measurements were repeated in 30 min intervals with 1.5 ml volume. The MB solution was syringed back to the Petri dish with the sample immediately after the spectroscopic analysis (approximately 2 min). The wetting properties of the samples were investigated by the sessile drop method using an Attention Optical Tensiometer. A pair of liquids: polar – water and non-polar – diiodomethane (MI) were chosen to measure the contact angles (CA) of sessile drops with a 0.5-μl volume. The polar and dispersive components γp, γd of the liquid free surface energy γtot were calculated based on Owens and Wendt’s [24,25] method using Wu’s [26] harmonic mean approach.
3. Results and discussion The surface morphology views of the Ti samples after O-PIII treat ment with various doses (D1, D2, and D3) are shown in Fig. 1. The surface smoothness and uniformity were similar for all dosing condi tions. Additionally, defects or cracks were not observed during the morphology analysis. The sample roughness was obtained by the AFM method (Fig. 2) after treatment with various Oþ ion doses. All samples performed very similar roughness values, which ranged from 17.0 nm to 19.1 nm after D3 and D1 doses of O-PIII treatment, respectively. Consequently, the surface morphology and roughness remained relatively stable and did not alternate during the O-PIII treatment. 3
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Table 1 Rutile and anatase phase amount in the layer after different oxygen ion doses. Doses of Oþ
Rutile phase amount, %
Anatase phase amount, %
D1 D2 D3
94.9 88.0 77.7
5.1 12.0 22.3
Fig. 4. XRD data of Ti after implantation of Oþ with doses D1, D2 and D3.
The top layer chemical bond analysis and layer-by-layer measure ments were performed by XPS technique (Fig. 3). First of all, only tita nium, oxygen and carbon elements were observed by analysing elemental composition of D3 sample. This showed that pure TiO2 layers were formed without any impurities. It is important to mention that the same tendencies were observed by analysing D1 and D2 samples. Ti2p spectra are shown in Fig. 3b. Two peaks were observed at binding en ergies of 458.8 eV (Ti2p3/2) and 464.5 eV (Ti2p3/2). According to XPS database [27] and our previous works [28,29], the binding energies and separation between them (Δ ¼ 5.7 eV) confirmed TiO2 formation. However, identification of particular TiO2 phase is almost impossible due to very similar binding energies where the difference between anatase and rutile is only 0.1 eV. The deconvolution of C1s showed three components with the binding energies at approximately 530.0 eV – O). C–O and C– – O bonds are (TiO2), 531.6 eV (C–O) and 532.9 eV (C– attributed to organic compounds. These compounds are attracted to the surface of TiO2 layer when samples were kept in ambient atmosphere. The layer-by-layer measurements were performed in order to un derstand oxygen distribution in TiO2 layer. Fig. 3c and d showed ex amples of layer-by-layer measurements with D3 sample. Results of O1s peak analysis showed that oxygen was observed till approximately 50 nm sputtering depth. The binding energy of these peaks were approximately 530.0 eV, which confirmed existence of TiO2 compound. Nevertheless, O1s peak vanished with the further sputtering cycle. This result confirmed that Oþ ions were incorporated into Ti plate till approximately 50 nm depth. These results were supplemented by Ti2p layer-by-layer analysis. The strong TiO2 peak was observed at the binding energy of approximately 458.8 eV. However, the shift of Ti2p peak to the lower binding energies (approximately 454.3 eV) was observed after five sputtering cycle still approximately 50 nm. This shift showed that TiO2 component was reduced and metal Ti phase was observed. Further analysis showed only metal Ti component. Thickness measurements showed that Oþ ion implantation and TiO2 formation reached a depth of approximately 43 nm, 49 nm and 54 nm after D1, D2 and D3 doses of O-PIII, respectively. The Oþ ions implan tation became partially saturated after 1-h with the D1 dose. Addition ally, the implantation thickness and TiO2 layer formation showed nonlinear dependence tendencies on ions doses. E.J.D.M. Pillaca et al. studied PIII into a silicon substrate using a magnetic mirror geometry [30]. They created additional E � B fields around the target, which increased the ion implantation dose and improved the ion implantation depth, roughness, and wettability. However, implantation depth de pendency on ion doses was non-linear. This nonlinear dependency could be related to the partial saturation of implanted ions. At the beginning of
Fig. 5. Raman spectra of Ti after O-PIII using at doses: D1, D2 and D3. R rutile, A - anatase phases, X – unknown.
O-PIII, ions were implanted into Ti substrate and formed a TiO2 com pound (e.g. in 40 nm depth). These formed TiO2 layers served as a barrier for deeper penetration of implanted Oþ ions. Further implanta tion and TiO2 formation proceeded at a lower depth (e.g. 20 nm). Hence, the Oþ ions implantation depth became partially saturated and nonlinear implantation depth dependence on ion doses was observed. The XRD patterns after O-PIII treatment with different doses are shown in Fig. 4. It was observed that all three graphs have the same components: titanium (hexagonal, P63/mmc, JCPDS No. 071–4632), Ti6O (hexagonal, P31c, JCPDS No. 072–1471), anatase (tetragonal, I41/ amd, JCPDS No. 21–1272), and rutile (tetragonal, P42/mnm, JCPDS No. 086–0148) crystalline phases. The most intensive anatase phases were (004) 37.8� and (101) 25.3� while the rutile phases were (110) 27.4� and (101) 36.2� , respectively. XRD results showed that the main anatase and rutile peaks became more intensive after longer treatment time. This suggested that longer treatment time produced a higher crystallinity in the TiO2 layer. The phase transformation was evaluated by recalculating the ratio between the anatase and rutile phases of each sample. The sample after the D1 dose consisted of 94.9% rutile phase and 5.1% anatase (Table 1). A higher ion dose invoked the TiO2 phase transformations, by increasing the amount of anatase and reducing the rutile TiO2 phase. The calcu lated amount of anatase (rutile) phases were 12.0% (88.0%) and 22.3% (77.7%) after the D2 and D3 dose, respectively. These results showed that the TiO2 phase transformation from rutile to anatase can be controlled by applying different ion doses during the O-PIII process. . The phase transformation with higher Oþ ions doses during O-PIII treatment may be explained by the more intensive anatase phase for mation compared to the rutile phase. Anatase is a metastable TiO2 structure, whereas rutile is a much more stable [25]. Higher oxygen ion doses invoke rapid and more chaotic processes on the top layers of the Ti substrate. These encourage shorter relaxation times of the TiO2 particles with more intensive metastable anatase phase TiO2 formation instead of rutile TiO2 [31–33]. Moreover, oxygen has a smaller density than tita nium. Therefore, higher implantation doses of the Oþ ions into Ti 4
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Table 2 Liquid free surface energy and contact angle of TiO2 samples after the D1, D2 and D3. Doses of Oþ
γtot, mJ/m2
γd, mJ/m2
γp, mJ/m2
CAW, �
CAMI, �
D1 D2 D2
39.0 43.5 49.5
26.1 30.5 34.7
12.9 13.0 14.8
77 74 68
70 60 51
can be trapped in the valence band at such heterojunction. This reduces the possibility of electron-hole recombination, improving the ability to decompose MB solution [40,41]. Moreover, TiO2 layers with a higher crystallinity form may invoke a higher decomposition ratio of the MB solution. Additionally, different microcrystal orientations of the same phase have different photocatalysis characteristics, which may have a positive influence on photocatalysis [42]. One of the most important parameters characterizing the surfaces of materials is the liquid free surface energy [35]. The value of the surface free energy is especially important in this work because it affects the photocatalytic activity. A higher surface free energy value indicates an enhanced adsorption as well as a higher decomposition ratio [34]. A characteristic photo of a water drop measured with the contact angles (CA) made by the sessile drop method on the surface of Ti samples after D3 of the O-PIII treatment is shown in Fig. 7. The summarized results of water contact angle measurements and calculated surface energy are shown in Table 2. The liquids contact angle values CAW and CAMI decreased gradually, while the polar γp and dispersive components γd of the surface free energy γtot increased proportionally with the longer O-PIII treatment time of the Ti surface. The anatase phase corresponds to a higher liquid free surface energy value [34]. The increased surface free energy confirmed the increase of the anatase crystallite concentration in TiO2 film. This hypothesis is supported by the fact that surface morphology remained nearly constant with the higher treatment time. Hence, structural changes explained the increase of liquid free surface energy. Moreover, these results are consistent with the XRD Rietveld analysis, which shows an increase in the anatase phase during O-PIII with higher Oþ ion doses.
Fig. 6. Bleaching of methylene blue solution under UV-A irradiation using Ti samples after the D1, D2 and D3 doses of O-PIII method treatment.
substrate decrease the material density from Ti (ρ ¼ 4.506 g/cm3) through TiO2 rutile (4.23 g/cm3) to TiO2 anatase (3.78 g/cm3) phase. The Raman spectra of three samples after the D1, D2 and D3 doses of O-PIII treatment are shown in Fig. 5. The Raman active modes at 143 cm 1, 238 cm 1, 445 cm 1, 610 cm 1, 829 cm 1 are associated with rutile single crystals [7,34,35]. A very important property of the anatase and rutile phases in Raman spectra is the ratio between the 143 cm 1 band and the 600 cm 1 band. For anatase it is greater than 1, but it is less than 1 for rutile [34]. It was observed that the ratio between mentioned bands is less than 1 after D1 and D2 doses of the O-PIII treatment, suggesting that it may be associated with the rutile phase. However, the Raman spectrum changed after the D3 dose of O-PIII treatment: the observed ratio was greater than 1. Such result can be associated with the anatase phase. In addition to the first order Raman bands, broad struc tures around 143 cm 1, 238 cm 1, 445 cm 1, 610 cm 1 and 829 cm 1 were observed. These can be traced back to either disorder induced scattering or second order processes. The small regular peaks in regions 150 cm 1 – 300 cm 1 and 500 cm 1 – 850 cm 1 are related to the dis order induced scattering or second order processes [36]. The photocatalysis experiment results are shown in Fig. 6. First of all, it should be mentioned that UV-A irradiation had a relatively weak effect on the decomposition ratio of MB solution (first order reaction constant 1 � 10 5 min 1) (Fig. 6 UV-A). Moreover, Ti samples after the O-PIII method treatment without UV-A irradiation (in dark) also showed a negligible MB decomposition ratio, whilethe first order reaction con stants were 2 � 10 4 min 1, 2.5 � 10 4 min 1, 2.4 � 10 4 min 1 after the D1, D2 and D3 doses, respectively. Such negligible decomposition could be observed due to agglomeration and sedimentation processes of the MB solution on the samples surfaces [37–39]. The results using Ti samples after the O-PIII method treatment under UV-A irradiation were completely different. The first order reaction constants using the D1, D2 and D3 doses of O-PIII treated samples were 3.2 � 10 3 min 1, 3.7 � 10 3 min 1, and 4.2 � 10 3 min 1, respectively. The D3 dose of the O-PIII method treated sample invoked the highest decomposition ratio, while sample treated with D1 dose had the lowest. The results of crystallinity analysis showed that the TiO2 layer con sisted of two phases: anatase and rutile. Many researchers demonstrated that the use of two phases as a heterojunction can improve the photo catalytic activity compared to pure TiO2 phases. For example, electrons
4. Conclusions The oxygen plasma immersion ion implantation method was used to form TiO2 layers on a Ti substrate with different Oþ ion doses. The re sults of chemical bond analysis and layer-by-layer measurements showed that the implantation thickness and TiO2 layer formation have non-linear dependency on Oþ ion doses with a partial saturation at approximately 40 nm. The phase transformations with an increased amount of anatase and reduced amount of the rutile TiO2 phase was initiated by a higher ion dose. These observations were explained by the formation of the metastable anatase TiO2 phase due to the shorter relaxation time of TiO2 particles during the O-PIII with higher Oþ ion doses. These phase transformations increased the free surface energy from 39.0 mJ/m2 to 49.5 mJ/m2 as well as the photocatalytic decom position ratio from 3.2 � 10 3 min 1 to 4.2 � 10 3 min 1 under UV-A irradiation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence
Fig. 7. Characteristic water (a) and diidomethane (b) sessile drop on the surface of Ti samples after D3 dose of O-PIII treatment. 5
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the work reported in this paper.
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