Tantalum- and Silver-Doped Titanium Dioxide Nanosheets Film

Dec 19, 2016 - Department of Mechanical Engineering, Indian Institute of Technology, BHU, Varanasi-221005, India. ABSTRACT: Surface modification via ...
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Tantalum- and Silver-Doped Titanium Dioxide Nanosheets Film: Influence on Interfacial Bonding Structure and Hardness of the Surface System Jalal Azadmanjiri,*,† James Wang,*,† Christopher C. Berndt,†,‡ Ajay Kapoor,† De Ming Zhu,† Andrew S. M. Ang,† and Vijay K. Srivastava§ †

School of Engineering, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia ‡ Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York 11794, United States § Department of Mechanical Engineering, Indian Institute of Technology, BHU, Varanasi-221005, India ABSTRACT: Surface modification via advanced technologies to manufacture durable coatings for increasing mechanical, chemical, and biological efficacy is an active research area for biomedical industries. The surface and coating/substrate interface play important roles regarding the biological performance. Precise knowledge concerning the interfacial layer in heterostructures is not well understood because of a lack of in situ characterization. Here, we investigate interfacial bonding structures by an electronic model in (i) tantalum-coated titanium alloy (Ta/Tialloy) and (ii) tantalum−silver-coated titanium alloy (TaAg/Tialloy) bilayer interfaces. The tantalum and tantalum−silver thin films were grown onto Tialloy substrates in a high-vacuum system by magnetron sputtering. We use X-ray photoelectron spectroscopy to probe the electronic structure of the so-formed Ta/Tialloy and TaAg/Tialloy bilayer interfaces. The results demonstrate that the thin-film/substrate interfaces exhibit different bonding characteristics. The TaAg/ Tialloy exhibits two semiconductor phases in the interfacial layer with a band gap that is larger than that for the Ta/Tialloy. The large band gap semiconductors that evolve at the interface of the TaAg/Tialloy, as well as the elements with high electronegativity in the overlayer, increase the satellite peak intensity in the interface layer and generate interface polarization. There is an associated increase in the interfacial bonding strength by ionic bonding formation that is determined by measuring the hardness of this material system. itania (TiO2) has attracted much attention in many fields of science and technology and is extensively used in photocatalysts,1−3 water splitting,4,5 dielectrics,6,7 energy applications,7,8 and bioapplications.9 Hence, nanostructured TiO2 materials with different morphologies such as zero-, one-, and two-dimensional structures have been produced using various techniques. TiO2 exists mainly in three crystalline structures: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic). Among the different crystal structures, rutile is the most stable bulk phase. Surface functionalization of TiO2 has a positive efficacy in many biomedical applications such as bone scaffolds, implants, drug delivery systems, and so on. For instance, the apatite formation rate could be accelerated with nano-TiO2 scaffolds, increasing osteoblast adhesion, proliferation, and differentiation.10 Therefore, coating developments for dental and orthopedic implants to enhance their long-term mechanical stability (i.e., wear resistance), chemical inertness (i.e., corrosion resistance), and biological performance (i.e., osseointegration, bioactivity, antibacterial properties) is an active research area within dental

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© XXXX American Chemical Society

and orthopedic industries.11,12 The challenge of understanding bonding strength at interfaces and the hardness of thin film− substrate systems arises from the richness of the atomic coordination at the interface between the thin film and substrate. Recent investigations indicate that tantalum (Ta) has superior corrosion resistant and bioactivity that is higher than that of the other biocompatible elements such as titanium (Ti), zirconium (Zr), and niobium (Nb).13,14 However, the high bulk density (16.6 g/cm3)15 and high cost of Ta make it unattractive for direct use in a bulk form for clinical applications. On one hand, a nano- or microlayer of Ta on the surface of a Ti implant can improve both bioactivity and corrosion resistance. On the other hand, the addition of antibacterial ions such as silver (Ag+) or gold (Au+) alleviates serious microbial Received: Revised: Accepted: Published: A

September 15, 2016 November 8, 2016 December 19, 2016 December 19, 2016 DOI: 10.1021/acs.iecr.6b03557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

Industrial & Engineering Chemistry Research Table 1. XPS Elemental Composition for the Different Surfaces element detection(atom %) sample

Ti 2p

Al 2p

V 2p

O 1s

Ta 4d

uncoated Tialloy Ta/Tialloy TaAg/Tialloy

25.3 ± 0.5

5.1 ± 0.9

2.2 ± 0.2

63.9 ± 1.9 63.8 ± 1.4 60.9 ± 1.8

36.2 ± 1.3 28.2 ± 0.7

infection.16,17 Therefore, the antibacterial capability of tantalum-coated titanium alloy (Ta/Tialloy) can be significantly enhanced by doping antibacterial agent ions (i.e., Ag+) into Ta to fabricate tantalum−silver (TaAg) thin films onto Ti, that is, to form a tantalum−silver-coated titanium alloy (TaAg/Tialloy) material system. Although Ta ions (Ta4+ or Ta5+) and Ag+ have high bioactivity, corrosion resistivity, and broad-spectrum antibacterial properties, cytotoxicity has been observed because of the fast release of Ta ion and Ag+ debris, which is a significant drawback.18 Therefore, strengthening interfacial bonding and enhancing thin-film hardness plays a decisive role in improving the wear- and scratch-resistance of the thin films and hence decreasing the release of Ta and Ag debris from implants. In this work, a conceptually simple electrostatic polarization approach toward an electronic model description19 investigated the interfacial bonding structures between Ta or TaAg thin films and Ti alloy (i.e., Tialloy, Ti-6Al-4 V ELI) substrates, denoted as Ta/Tialloy and TaAg/Tialloy, respectively. The fundamental problem concerns identifying the electronic structure of the conjugated thin film on a metal surface. Thus, X-ray photoelectron spectroscopy (XPS) was employed to analyze the interfaces of Ta/Tialloy and TaAg/Tialloy systems with an emphasis on the satellite peak feature of the Ti 2p photoelectron line. The satellite peaks were collected from photoemission interaction of the X-ray with valence electrons of the sample. These peaks are influenced strongly by valence electrons and electron density distributions.19 Hence, the satellite peak intensity is a direct measure of the interface valence electron density in the interfacial structure. The satellite peak intensity is related directly to the interfacial bond strength and surface hardness.19 Study of the satellite peaks with XPS allows probing of the chemical and electronic structures of interfaces. To achieve this goal, 45 nm Ta or TaAg thin films were deposited by sputtering Ta (>99.99 wt % purity) or cosputtering Ta and Ag (>99.99 wt % purity) targets onto polished Tialloy discs (8 mm in diameter and 2 mm thick) in argon gas (99.999% purity) of 4 mTorr, at a power of 150 kW and room temperature to fabricate Ta/Tialloy and TaAg/Tialloy thin films. The base pressure of the chamber was below 5 × 10−8 Torr, and the distance between the sputtering targets and substrate was 20 cm. The thickness of the thin film was modulated by controlling the deposition time. The Ti alloy substrates were degreased prior to deposition by sonication in methanol, isopropanol, ethanol, and acetone. The substrates were then washed with Milli-Q water and dried in a stream of nitrogen gas (99.99% purity) before being mounted onto a rotating sample holder and introduced into a high-vacuum sputtering chamber. This procedure minimized any contamination. The atomic percentage (atom %) of the elements in uncoated Tialloy, Ta/ Tialloy, and TaAg/Tialloy samples with 45 nm coating are presented in Table 1; the values are obtained using XPS characterization.

Ag 3d

trace (C 1s) 3.5 ± 0.4

10.9 ± 0.6

X-ray diffraction (XRD) patterns of the uncoated Tialloy, Ta/ Tialloy, and TaAg/Tialloy samples and atomic force microscopy (AFM) images from the boundary of the coated and uncoated TaAg/Tialloy sample along with profiles (taken from line scans on the AFM image) are shown in Figure 1. The XRD pattern of

Figure 1. (a) XRD pattern of uncoated Tialloy (a1), Ta/Tialloy (a2), and TaAg/Tialloy (a3) samples; (b) AFM image from the boundary of the coated and uncoated TaAg/Tialloy sample, as well as profiles taken from line scans on the AFM image.

the uncoated Tialloy (Figure 1, trace a1) indicates that the intense peaks belong to α-Ti. Evolution of crystalline growth of the thin-film components can be observed in the XRD patterns of Ta/Tialloy (Figure 1, trace a2) and TaAg/Tialloy (Figure 1, trace a3). Oxygen can be readily adsorbed onto the surface as soon as the samples are exposed to the ambient environment because Tialloy has a high affinity for oxygen.9,20 The main phases in the sputtered samples are TiO2 (rutile), TaO2 (triclinic), and Ag2O. Thus, a range of interfaces from insulating or semiconducting to metal overlayers (Ta and Ag) on Tialloy result. The AFM profiles, Figure 1b, reveal an approximate thickness of 45 nm for the TaAg thin film. Surface morphology has a significant effect on the hardness of the coating/substrate system. Therefore, surface morphology and roughness characterizations were performed on the surface of uncoated Tialloy, Ta/Tialloy, and TaAg/Tialloy samples using field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), and a 3D profilometer. Figure 2 shows FESEM and EDS data of the samples. The Ra and Rq roughness parameters are presented in Table 2. It can be observed that the average roughness decreased when the samples were coated with Ta and TaAg. It is noted that there is some statistical overlap among these measurements. The strength and hardness of the Ta and TaAg thin films will be related to the electron model and electron valence at the B

DOI: 10.1021/acs.iecr.6b03557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

Industrial & Engineering Chemistry Research

Figure 2. FESEM images and EDS mapping of the (a) uncoated Tialloy, (b) Ta/Tialloy, and (c) TaAg/Tialloy samples.

Table 2. Roughness Parameters (Ra and Rq) for the Uncoated Tialloy, Ta/Tialloy, and TaAg/Tialloy, Obtained via 3D Profilometer sample

average roughness (nm), Ra

rms (nm), Rq

uncoated Tialloy Ta/Tialloy TaAg/Tialloy

18 ± 5 14 ± 3 14 ± 3

23 ± 5 19 ± 3 18 ± 3

The Ti 2p peaks at 472.2 eV on the three samples correspond to satellite peaks (Figure 3a). The intensity of the satellite peak was enhanced after a thin layer of Ta or TaAg was coated onto the Tialloy surface. The intensity values of the satellite peak for uncoated Tialloy, Ta/ Tialloy, and TaAg/Tialloy samples are 5800, 6300, and 7200 (arbitrary units), respectively. The variation in the observed satellite peak intensity may well reflect the intrinsic difference in the valence electron density at the interface between the thin film and the substrate of the samples.19 Panels i and ii of Figure 3 illustrate the normal and AR-XPS experiments, respectively. The influence of the emission angle on the XPS spectra acquired from different depths is shown in the figures. XPS data from the interface of the thin film and substrate to the surface of the thin film may be obtained by increasing the emission angle size. The chemical species can be distinguished with respect to depth by comparing spectra collected at a range of angles. The presence of Ti 2p peaks in XPS spectra of Ta/Tialloy and TaAg/Tialloy samples in normal mode XPS experiment (Figure 3a) shows that the thickness of Ta and TaAg coatings are thin enough such that the Ti 2p can be evaluated. To reveal whether

interface, which can be determined by the satellite peaks of the XPS spectra. This assessment was performed for several monolayers of pristine Ta and TaAg deposited onto the Tialloy. The thicknesses of the Ta and TaAg overlayers were sufficiently thin to ensure electron transparency that facilitated probing of the interfaces. The interfacial chemical structure of the samples was determined by performing normal and in situ angleresolved XPS (AR-XPS) experiments. In situ AR-XPS provides quantitative and chemical state information from the top surface as well as differentiating signals emitted from subsurface locations and the substrate.21,22 Figure 3a shows highresolution XPS spectra of Ti 2p and the corresponding satellite features of uncoated Tialloy, Ta/Tialloy, and TaAg/Tialloy samples. C

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Figure 3. (a) XPS spectra of Ti 2p and corresponding satellite peaks for uncoated Tialloy, Ta/Tialloy, and TaAg/Tialloy; (b, c) in situ AR-XPS spectra of Ti 2p and Ta 4f for Ta/Tialloy; (d−f) in situ AR-XPS spectra of Ti 2p, Ta 4f, and Ag 3d for TaAg/Tialloy. The inset schematics illustrate the (i) normal and (ii) AR-XPS experiments.

the thin interface of a metal−nonmetal couple. Electrons could flow from the valence band of the metal with lower electronegativity toward the higher nonmetal, thus generating an electrostatic polarization that would strengthen the interface by formation of ionic bonding. The formation of interfacial electrostatic polarization in Ta/Tialloy and TaAg/Tialloy is attributed to the different electronegativities between the overlayer elements and the substrate (O = 3.44 eV, Ta = 1.5 eV, Ag = 1.93 eV, and Ti = 1.54 eV). On the other hand, the presence of an insulating or semiconducting material with a band gap (Eg) at the interface leads to interfacial charge accumulation that is evidenced by the corresponding changes in the measured satellite peak intensities.19 The XRD results show that three semiconductorsTiO2 (rutile), TaO2 (triclinic), and Ag2Oformed at the Ta/Tialloy and TaAg/Tialloy interfaces. The formation of TiO2, TaO2, and Ag2O semiconductors with Eg of 3.03,23 1.0,24 and 1.45,25 respectively, is direct evidence of the charge accumulation in the TaAg/Tialloy interface being

the Ti 2p peaks shown in XPS spectra of the samples in Figure 3a are emitted from the interface between the thin film and substrate, in situ AR-XPS was performed at electron emission angles of 15°, 30°, and 45° (Figure 3b−f). The in situ AR-XPS spectra show that the oxide and elemental components on a sample change in relative intensity with respect to the emission angle. It can be observed (Figure 3b−f) that as electron emission angle size increases, the relative intensities of Ti 2p, Ta 4f, and Ag 3d decrease, so that the relative intensities of these elements are negligible in the TaAg/Tialloy sample measured at 45°. The maximum intensities on all elements were noticed in normal mode (θ = 0). Reduction of Ti 2p3/2 metal with an increase of the electron emission angle (Figure 3b,d) (i) confirms that the Ti metal exists in the substrate and (ii) indicates that Ti oxide forms at the interface of the bilayers for both coated samples. The existence of a nonmetal element (i.e., oxygen) near the surface of a metal (i.e., titanium) results in a negative polarization due to the different electronegativities.19 Consider D

DOI: 10.1021/acs.iecr.6b03557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 4. Schematic demonstration of the interfacial valence band structure and polarization at (a) Ta/Tialloy and (b) TaAg/Tialloy interfaces.

indentation loading−unloading curves of the three samples. Ten indentation tests were executed on uncoated specimens and each sample with the 45 nm overlayer coating. The hardnesses of the uncoated Tialloy, Ta/Tialloy, and TaAg/Tialloy samples are 4.85 ± 0.5, 5.94 ± 0.4, and 7.46 ± 0.8 GPa, respectively. Using the Oliver−Pharr method,26 the reduced modulus of the uncoated Tialloy, Ta/Tialloy, and TaAg/Tialloy samples are 130 ± 16, 129 ± 12, and 136 ± 11 GPa, respectively. It can be noted that the indentation depth of the indenter is about 50% of the coating thickness. Thus, the parent metal hardness would be contributing to the thin-film hardness. However, regardless of this technical issue, the composite hardness of the film and substrate can be used to discriminate between these surface modification techniques. The experimental results are consistent with the XPS data for the heterostructure samples. The observed interfacial polarization demonstrated by the XPS spectra, with fractionally negatively charged semiconductors at the interface and fractionally positively charged at the substrate and overlayer, indicates an enhanced bond coordination around Ti atoms at the interface. Increasing the quantity of bond coordination and likely interstitial and substitutional diffusions of oxygen and Ag with atomic radius (ra) of 60 and 172 pm, respectively, less than Ti (ra = 176 pm) and Ta (ra = 220 pm), tends to create dense and stiffer coatings. Such coatings restrict the movement of interfacial dislocations and hence increase the hardness of the surface system. Further investigations concerning the interfacial bond structure can be explored in the atomic scale via rigorous quantum mechanical effects. In addition, further characterization by means of X-ray scattering and scanning transmission electron microscopy is of interest in such interfaces but is not within the scope of this report. In summary, interfacial bonding structures of sputtered Ta and TaAg thin films on Ti alloy surfaces have been discussed in terms of an electronic model and investigated further using XPS, XRD, and nanoindentation. It was revealed that the satellite peak intensity has a direct relationship to the interfacial bonding strength and hardness of the surface system. The formation of semiconductor phases with a larger band gap at the TaAg/Tialloy interface significantly improves the bonding strength of the thin film to the substrate. The fabricated large band gap phases in the interface and the presence of elements with high electronegativity in the overlayer promote the accumulation of electrons. Interfacial polarization is generated within the interfacial structure, and consequently there is an enhancement in the interfacial bonding strength and hardness

higher than that for the Ta/Tialloy; uncoated Tialloy exhibits the weakest peak intensity among the three samples. Figure 4 depicts the interfacial band structure of the Ta/ Tialloy (Figure 4a) and TaAg/Tialloy (Figure 4b) heterostructures. A semiconductor phase with a large band gap at the interface implies that a significant population of free electrons may accumulate in this region. The accumulation of free electrons may arise because the generated semiconductors could serve as a barrier to block electrons from moving freely across the interface and resulting in interfacial charge accumulation, thus increasing the polarization. Because the fabricated semiconductor phases in the interfacial structure of TaAg/Tialloy exhibit a larger band gap than those in Ta/Tialloy, further free electrons could accumulate in the interfacial layer of TaAg/Tialloy, therefore presenting a relatively larger polarization. The formation of larger ionic bonding structures enhances the interfacial bonding at the TaAg/Tialloy interface. The incorporation of Ag into Ta may contribute to this improved interfacial bonding and thus to higher strength and hardness of the thin films. Conversely, if a lower band gap exists at the Ta/Tialloy interface, then the electrons will donate to the substrate, leading to the formation of a weaker polarization and interfacial bonding, which implies lower interface strength and hardness. Nanoindentation was performed to investigate the interface strengthening mechanism that was proposed on the basis of the XPS results. Figure 5 shows the hardness values and

Figure 5. Hardness values and loading−unloading curves of uncoated Tialloy, Ta/Tialloy, and TaAg/Tialloy samples (10 tests were executed). E

DOI: 10.1021/acs.iecr.6b03557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research of the thin films. This fundamental understanding relates the physics of surfaces and interfaces to their resulting physical properties.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +61 3 9214 8657. Fax: +61 3 9214 8264. E-mail: [email protected]. *Tel.: +61 3 9214 8657. Fax: +61 3 9214 8264. E-mail: [email protected]. ORCID

Jalal Azadmanjiri: 0000-0002-9757-5892 Andrew S. M. Ang: 0000-0001-7664-9971 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Australia-India Strategic Research Fund (AISRF) through grant ST060048. REFERENCES

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DOI: 10.1021/acs.iecr.6b03557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX