nanomaterials Article
Modification of the Surface Topography and Composition of Ultrafine and Coarse Grained Titanium by Chemical Etching Denis V. Nazarov *, Elena G. Zemtsova, Alexandr Yu. Solokhin, Ruslan Z. Valiev and Vladimir M. Smirnov Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia ;
[email protected] (E.G.Z.);
[email protected] (A.Y.S.);
[email protected] (R.Z.V.);
[email protected] (V.M.S.) * Correspondence:
[email protected]; Tel.: +7-812-428-4033 Academic Editor: Thomas Nann Received: 27 October 2016; Accepted: 9 December 2016; Published: 13 January 2017
Abstract: In this study, we present the detailed investigation of the influence of the etching medium (acidic or basic Piranha solutions) and the etching time on the morphology and surface relief of ultrafine grained (UFG) and coarse grained (CG) titanium. The surface relief and morphology have been studied by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), and the spectral ellipsometry. The composition of the samples has been determined by X-ray fluorescence analysis (XRF) and X-ray Photoelectron Spectroscopy (XPS). Significant difference in the etching behavior of UFG and CG titanium has been found. UFG titanium exhibits higher etching activity independently of the etching medium. Formed structures possess higher homogeneity. The variation of the etching medium and time leads to micro-, nano-, or hierarchical micro/nanostructures on the surface. Significant difference has been found between surface composition for UFG titanium etched in basic and acidic Piranha solution. Based on the experimental data, the possible reasons and mechanisms are considered for the formation of nano- and microstructures. The prospects of etched UFG titanium as the material for implants are discussed. Keywords: chemical etching; UFG titanium; surface; roughness; titanium implants
1. Introduction Titanium and its alloys have a unique combination of mechanical properties (hardness, strength, low density, and relatively low Young modulus) and excellent biocompatibility [1,2]. This allows it to be widely used as the most suitable material for orthopedic and dental implants [1–3]. The alloys provide enhanced mechanical properties of the material, however, they are potentially dangerous due to possible release of allergens and toxic elements [4]. The most suitable alternative is to use pure titanium in the ultrafine grained (UFG) forms, i.e., with the grain size between tens and hundreds nanometers [5–7]. UFG metals (particularly, titanium) are expected to have more implant-suitable mechanical properties (high fatigue strength, tensile strength, and low Young modulus value) as compared to coarse grained (CG) analogs [5,7,8]. Therefore, UFG-based implants provide better reliability and durability. In addition to the mechanical properties, UFG structure can promote adhesion, spreading, proliferation, differentiation of bone tissue cells, and also accelerated tissue mineralization [9], which eventually promotes the implant’s engraftment. In turn, acceleration of the implant’s engraftment is the most important and the most complicated task in the development of the new generation of implants [1,2]. Nevertheless, according to the data reviewed in [9], the enhancement of the biomedical properties of UFG materials compared to CG analogs is not valuable. This is not sufficient for the meaningful Nanomaterials 2017, 7, 15; doi:10.3390/nano7010015
www.mdpi.com/journal/nanomaterials
Nanomaterials 2017, 7, 15 Nanomaterials 2017, 7, 15
2 of 15 2 of 14
Nevertheless, according to the datareliability, reviewed and in [9], the enhancement of the biomedical acceleration of the implant’s engraftment, improvement of biocompatibility. In this properties of UFG materials comparedis to CG analogs is not valuable. This is not sufficient the of regard, additional surface modification required. Necessary surface relief is developed byfor means meaningful acceleration of the implant’s engraftment, reliability, and improvement of electrochemical anodization [10,11], sand blasting [12,13], and chemical etching [12–15]. Alternatively, biocompatibility. In this regard, additional surface modification is required. Necessary surface relief bioactive coatings are deposited by means of chemical vapor deposition (CVD), physical vapor is developed bysol-gel, means of electrochemical [10,11], sand blasting [12,13], and chemical deposition (PVD), ionic implantationanodization [13]. etching [12–15]. Alternatively, bioactive coatings are deposited by means of chemical vapor Among the above-mentioned methods, chemical etching is currently the most interesting one deposition (CVD), physical vapor deposition (PVD), sol-gel, ionic implantation [13]. due to its simplicity combined with wide possibilities to variate both relief and composition of the Among the above-mentioned methods, chemical etching is currently the most interesting one surface [14,15]. At the moment, numerous experimental studies are known on the etching of CG due to its simplicity combined with wide possibilities to variate both relief and composition of the titanium its alloys various etching media [13–16]. studies Oppositely, despite prospects surfaceand [14,15]. At theinmoment, numerous experimental are known onthe thewide etching of CG of UFG titanium and great scientific interest in it, the UFG titanium etching is still not studied enough. titanium and its alloys in various etching media [13–16]. Oppositely, despite the wide prospects of There aretitanium some works dedicated to UFG titanium corrosion [17–20] that suggest significant differences UFG and great scientific interest in it, the UFG titanium etching is still not studied enough. between character, anddedicated mechanism of corrosion UFG and[17–20] CG titanium. One can expect the There rate, are some works to UFG titaniumofcorrosion that suggest significant differences also in the case of etching. So, we demonstrated earlier using mass loss analysis differences between rate, character, and mechanism of corrosion of UFG and CG titanium. One[21] canthat differences alsotitanium in the case etching.solutions So, we demonstrated earlier usingthan massfor loss analysis the expect rate ofthe etching of UFG in of Piranha is significantly higher CG titanium. [21] that the were rate ofdescribed etching ofin UFG in Piranha solutions is higher than for CG as Similar results the titanium work [22], where solutions ofsignificantly HF and HF/HNO were used 3 titanium. Similar results were described in the work [22], where solutions of HF and HF/HNO 3 were etching media. used etching media. Theas current study is devoted to the more detailed investigation of the etching features of UFG and The current study is devoted to the more detailed investigation of the etching features of UFG CG titanium in acidic (H2 SO 4 /H2 O2 ) and basic (NH4 OH/H2 O2 ) Piranha solutions. Namely, changes and CG titanium in acidic (H2SO4/H2O2) and basic (NH4OH/H2O2) Piranha solutions. Namely, of relief and surface composition during etching are described; optimal conditions of UFG titanium changes of relief and surface composition during etching are described; optimal conditions of UFG etching are specified in order to make the material most suitable for new generation implants. titanium etching are specified in order to make the material most suitable for new generation implants. 2. Results Results of the Surface 2.1. 2.Morphology 2.1.HMorphology of the Surface 2.1.1. 2 SO4 /H2 O2 Etching
The H surface of etched UFG and CG titanium has been studied by scanning electron 2.1.1. 2SO4/Hmorphology 2O2 Etching microscopy (SEM). Micrographs with magnification from 300 to 600,000 have been used for morphology The surface morphology of etched UFG and CG titanium has been studied by scanning electron analysis both on(SEM). the nano(600,000–100,000 ×) and microscale (10,000–300 ×). have been used for microscopy Micrographs with magnification from 300 to 600,000 UFG titanium etching O2 during 5 and 15 min does not(10,000–300×). influence on morphology, 2 SO 4 /H2(600,000–100,000×) morphology analysis bothinonHthe nanoand microscale but our previous results [21] suggest significant mass loss of the sample. Therefore, can conclude UFG titanium etching in H2SO4/H2O2 during 5 and 15 min does not influence onwe morphology, thatbut within this time interval, layer by layer etching with diffusion control takes place. More prolonged our previous results [21] suggest significant mass loss of the sample. Therefore, we can conclude treatment in thethis etching on thecontrol nanoscale 1 and 2). that within time medium interval, leads layer to by“sponge-like” layer etching structure with diffusion takes(Figures place. More treatment in theonly etching medium leadsof to structure the of nanoscale Thisprolonged change can be explained by the difference the“sponge-like” etching rate of variouson spots the surface. (Figures and 2). Thistakes change canwith be explained only byWhen the difference etching rate of various In this case, 1the etching place kinetic control. the timeofofthe etching increases to 2 h, the spots of the surface. In this case, the etching takes place with kinetic control. When the time of etching “sponge-like” structure becomes more homogeneous; after more prolonged treatment the structure is increases 2 h, the “sponge-like” structurecontrast becomesbecomes more homogeneous; after more prolonged densified. It to is worth noting that the image’s worse. It can be caused by material treatment the structure is densified. It is worth noting that the image’s contrast becomes worse. It can oxidation during long-term etching. be caused by material oxidation during long-term etching.
Figure 1. Characteristic scanning electron microscopy (SEM) images of ultrafine grained-ultrafine grained (UFG) (a) and coarse grained-coarse grained (CG) (b) titanium etched in H2 SO4 /H2 O2 solutions during 2 h (magnification—400,000×). Red lines mark inhomogeneous areas.
Nanomaterials 2017, 7, 15
3 of 14
Nanomaterials 7, 15 Figure 2017, 1. Characteristic scanning electron microscopy (SEM) images of ultrafine grained-ultrafine3 of 14
grained Nanomaterials 2017,(UFG) 7, 15 (a) and coarse grained-coarse grained (CG) (b) titanium etched in H2SO4/H2O2
Figure 1. during Characteristic scanning electron microscopy of ultrafine grained-ultrafine solutions 2 h (magnification—400,000×). Red lines(SEM) mark images inhomogeneous areas. grained (UFG) (a) and coarse grained-coarse grained (CG) (b) titanium etched in H2SO4/H2O2 solutions during 2 h (magnification—400,000×). Red lines mark inhomogeneous areas.
3 of 15
Figure CharacteristicSEM SEMimages images of UFG 2SO 4/H O2 2solutions during 5 (a), 15 Figure 2. 2. Characteristic UFG titanium titaniumetched etchedininHH O2 solutions during 5 (a), 2 SO 42/H min (b); and 1 (c), 2 (d), 6 (e) and 24 h (f) (magnification 200,000×—main picture, 10,000×—insets). 15 min (b); and 1 (c), 2 (d), 6 (e) and 24 h (f) (magnification 200,000×—main picture, 10,000×—insets). Figure 2. Characteristic SEM images of UFG titanium etched in H2SO4/H2O2 solutions during 5 (a), 15 min (b); andof 1 (c), (d), 6 (e) and h (f) (magnification picture, 10,000×—insets). In the case CG2titanium, the24 surface morphology200,000×—main changes become visible already after 15 min
In the case of CG titanium, the surface morphology changes become visible already after 15 min etching (Figure 3). “Sponge-like” structure is locally appeared after 1 h (Figure 3—in orange circles), etchingIn(Figure 3).of“Sponge-like” structure locally appeared 1 h (Figure 3—in orange the case CG titanium, the surface is morphology changesafter become visible already after 15circles), min but fully covers the sample surface only after 2 h treatment. After 24 h etching, sponge morphology butetching fully covers the sample surfacestructure only after 2 h treatment. After 24 h(Figure etching, sponge morphology (Figure 3). “Sponge-like” is locally appeared after 1 h 3—in orange circles), varies depending on the location on the surface (Figure 3—differing regions are separated by orange varies depending the location on the surface 3—differing regions areof separated by orange but fully covers on thethis sample surface only afterstructure 2 (Figure h treatment. After h etching, morphology lines). Supposedly, change relates to the of CG Ti (i.e.,24the presencesponge the micron-sized varies depending this on the location on the surface (Figureof3—differing regions are separated by orange lines). Supposedly, change relates to the structure CG Ti (i.e., the presence of the micron-sized grains and their boundaries). lines). Supposedly, this change relates to the structure of CG Ti (i.e., the presence of the micron-sized grains and their boundaries). grains and their boundaries).
Figure 3. Characteristic SEM images of СG titanium etched in H2SO4/H2O2 solutions during 5 (a), 15 min (b); and 1 (c), 2 (d), 6 (e) and 24 h (f) (magnification 200,000×—main picture, 10,000×—insets). Figure Characteristic SEM images 2SO 4/H/H during 5 (a), 515(a), Figure 3. 3. Characteristic SEM images of СG CG titanium titaniumetched etchedininHH O2 solutions during 2 SO 4 2O2 2solutions Orange lines mark various etching areas. (d), 66 (e) (e) and and 24 24 hh (f) (f) (magnification (magnification 200,000 200,000×—main 15 min min (b); (b); and and 11 (c), (c), 22 (d), ×—mainpicture, picture,10,000×—insets). 10,000×—insets). Orange lines mark various etchingareas. areas. Orange lines mark various etching SEM micrographs of smaller magnification (Figures 2 and 3 insets) do not demonstrate
significant morphology changes in the microscale while etching in the acidic Piranha solution except SEMdiameters micrographs ofµ smaller magnification (Figures 2 and after 3 insets) do not demonstrate pitsSEM with m. Such pits are typical for the samples anddemonstrate 24 h treatment UFG micrographsofof1–3 smaller magnification (Figures 2 and 3 insets) do6not significant significant morphology changes in the microscale while etching in the acidic Piranha solution except titanium and after 24 h treatment CG titanium. Moreover, the amount of pits is greater on the UFG morphology changes of in 1–3 the µmicroscale while etchingfor inthe thesamples acidic Piranha solution except pits with pits with diameters m. Such pits are typical after 6 and 24 h treatment UFG diameters of 1–3 µm. Such pits are typical for the samples after 6 and 24 h treatment UFG titanium and titanium and after 24 h treatment CG titanium. Moreover, the amount of pits is greater on the UFG after 24 h treatment CG titanium. Moreover, the amount of pits is greater on the UFG titanium surface. Analysis of the large series of SEM microphotographs with different magnifications revealed more significant inhomogeneity of nano- and microstructures formed in etched CG titanium compared to
Nanomaterials 2017, 7, 15 Nanomaterials 2017, 7, 15
4 of4 14 of 15
titanium surface. Analysis of the large series of SEM microphotographs with different magnifications revealed more significant inhomogeneity of nano- andand microstructures formed in etched CG titanium UFG titanium. The examples of typical homogeneous inhomogeneous surfaces of UFG and CG Ti UFG 1. titanium. The examples of typical homogeneous and inhomogeneous surfaces of arecompared shown into Figure UFG and CG Ti are shown in Figure 1. 2.1.2. NH4 OH/H2 O2 Etching 2.1.2. NH4OH/H2O2 Etching Etching of UFG titanium in NH4 OH/H2 O2 solution leads to the significant mass loss [21] and of UFG titanium 4OH/H2O 2 solution the significant mass loss [21]sample and surface Etching morphology variation in in theNH nanoscale even on theleads earlytoetching stages (Figure 4). The surface morphology variation in the nanoscale even on the early etching stages (Figure 4). The sample after a 5 min etching has the developed relief, however, it is not homogenous on the whole sample after aSome 5 minsurface etchingspots has the relief, however, it isspots not homogenous on thelocated whole sample surface. aredeveloped “mesh-like” whereas other consist of densely particles surface. Some surface spots are “mesh-like” whereas other spots consist of densely located particles with diameters of tens of nanometers. 15 min etching gives the network structure. More prolonged with diameters of tens of nanometers. 15 min etching gives the network structure. More prolonged etching leads to the formation of shortened “ridges”—such structure can be called “coral-like” [23]. etching leads to the formation of shortened “ridges”—such structure can be called “coral-like” [23].
Figure 4. Characteristic SEM imagesofofUFG UFGand andCG СGtitanium titaniumetched etchedin in H H2SO SO4/H 2O2 solutions during Figure 4. Characteristic SEM images 2 4 /H2 O2 solutions during 5 (a), 15 min (b); and 1 (c), 2 (d), 6 (e) and 24 h (f) (magnifications 200,000× and 10,000×). 5 (a), 15 min (b); and 1 (c), 2 (d), 6 (e) and 24 h (f) (magnifications 200,000× and 10,000×).
Nanomaterials 2017, 7, 15
5 of 15
Nanomaterials 2017, 7, 15 Etching of CG titanium
5 of 14 is similar to that of UFG titanium. However, network elements appeared only after 15 min treatment and “coral-like” structure is found after 2 h etching. Etching of CG titanium is similar to that of UFG titanium. However, network elements appeared Less magnified SEM images also demonstrate significant morphology change in microscale on only after 15 min treatment and “coral-like” structure is found after 2 h etching. the very first etching stages. The most valuable changes for both titanium types are found after 2 h Less magnified SEM images also demonstrate significant morphology change in microscale on treatment: micron-sized pits become evident. However, for CG titanium, the pits are elongated and the very first etching stages. The most valuable changes for both titanium types are found after 2 h their number is much less. Besides, “lamellar” formations can be found on CG titanium surface, treatment: micron-sized pits become evident. However, for CG titanium, the pits are elongated and especially for theissample etched for 6 h. It is worth noting that sample after 6 h of surface, etching is their number much less. Besides, “lamellar” formations canthe beUFG found on CG titanium also slightly for different by morphology size.sample after 6 h of etching is especially the sample etched for 6as h. “ridges” It is worthbecome noting larger that theinUFG
also slightly different by morphology as “ridges” become larger in size. 2.2. Relief of the Surface 2.2. Relief of the Surface 2.2.1. H2 SO4 /H2 O2 Etching 2.2.1. 2SO4/H2O 2 Etching TheHsurface relief of the etched UFG and CG titanium has been studied by AFM. The surface topographies were measured theUFG scales 50 titanium × 50, 30has × 30, 10studied × 10 (Figure 5) and × 1 µm. The surface relief of the with etched andofCG been by AFM. The 1surface The task of quantitative characterization of of relief complicated [24,25], so1 we calculated topographies were measured with the scales 50 ×is50,rather 30 × 30, 10 × 10 (Figure 5) and ×1µ m. The thetask parameters of the arithmetic mean (R ) and root mean square (RMS) roughness, maximal of quantitative characterization of arelief is rather complicated [24,25], so we calculatedheight the amplitudes (Rmax ), and specific surface area (Ssurf ). These provide the most full quantitative parameters of the arithmetic mean (R a) and root meanparameters square (RMS) roughness, maximal height description the surface relief, because Ra and RMS work very the wellmost for robust amplitudesof (R max), and specific surface the areavalues (Ssurf). of These parameters provide full characterization of overallofroughness etched isotropic surfaces, is very sensitive to noise, quantitative description the surfaceof relief, because the values of RR a and RMS work very well for max robustand characterization overall roughness of etched isotropic surfaces, Rmax is very defects, spikes; and Sof shows the degree of development of the surface [24]. sensitive to noise, surf defects, and spikes; and Ssurf shows the degree of development of the surface [24].
Figure Atomic force microscopy(AFM) (AFM)surface surfacetopographies topographiesofofthe theUFG UFGand andCG CGtitanium titanium etched etched in Figure 5. 5. Atomic force microscopy in H 2SO4/H2O2. Nonetched UFG-Ti (a), UFG-Ti etched 15 min (b), 1 h (c), nonetched CG-Ti (d), CG-Ti H2 SO4 /H2 O2 . Nonetched UFG-Ti (a), UFG-Ti etched 15 min (b), 1 h (c), nonetched CG-Ti (d), CG-Ti etched 15 min (e), 1 h (f), UFG-Ti etched 2 h (g), 6 h (h), 24 h (i), CG-Ti etched 2 h (j), 6 h (k), 24 h (l). etched 15 min (e), 1 h (f), UFG-Ti etched 2 h (g), 6 h (h), 24 h (i), CG-Ti etched 2 h (j), 6 h (k), 24 h (l).
Nanomaterials 2017, 7, 15
Nanomaterials 2017, 7, 15 Nanomaterials 2017, 7, 15 parameters Calculated relief
6 of 15
6 of 14
6 of 14 for the scans 1 × 1 and 10 × 10 µm are given in the Tables S1–S4 and also represented graphically in Figures 6 and 7. The relief parameters (roughness—Figure 6a, specific Calculated Calculated relief relief parameters parameters for for the the scans scans 11 × × 11 and and 10 10 × × 10 10 µ µm m are are given given in in the the Tables Tables S1–S4 S1–S4 and and surface area—Figure 7a, heightinamplitudes—Figure 7b) slowly increase both in nano (1 ×6a, 1 µm scans) also represented graphically Figures 6 and 7. The relief parameters (roughness—Figure also represented graphically in Figures 6 and 7. The relief parameters (roughness—Figure 6a, specific specific andsurface in micro scale (10 ×7a, 10 µm scans) while etching timeslowly increased fromboth 5 min to 2 h. The changes are surface area—Figure area—Figure 7a, height height amplitudes—Figure amplitudes—Figure 7b) 7b) slowly increase increase both in in nano nano (1 (1 × × 11 µ µm m scans) scans) notand monotonic. In the micro scale, a jump in roughness is observed for etching times between 2 and and in in micro micro scale scale (10 (10 × × 10 10 µ µm m scans) scans) while while etching etching time time increased increased from from 55 min min to to 22 h. h. The The changes changes are are 6 h.not In this interval, the values R , RMS, R , and S for 10 × 10 µm scans are changed fromand 3.50, monotonic. In micro scale, a jump in roughness is observed for etching times between a max surf not monotonic. In the micro scale, a jump in roughness is observed for etching times between 22 and 66 h. In this interval, the values R a, RMS, R max, and S surf for 10 × 10 µ m scans are changed from 3.50, 4.93, 142, and 1.014 to 21.5, 31.9, 474, and 1.072 for UFG titanium. Analogous values for CG titanium h. In this interval, the values Ra, RMS, Rmax, and Ssurf for 10 × 10 µ m scans are changed from 3.50, 4.93, and to 31.9, and 1.072 for Analogous values for increase from 5, 1.014 7.1, 83, and to 17.1, and titanium. 1.058. A less pronounced jump is 4.93, 142, 142, and 1.014 to 21.5, 21.5, 1.005 31.9, 474, 474, and 24.4, 1.072343, for UFG UFG titanium. Analogous valuesparameter for CG CG titanium titanium increase from 5, 7.1, 83, and 1.005 to 17.1, 24.4, 343, and 1.058. A less pronounced parameter jump is observed also for 1 × 1 µm scans. The analysis of 3D surface topographies (Figure 5) together increase from 5, 7.1, 83, and 1.005 to 17.1, 24.4, 343, and 1.058. A less pronounced parameter jumpwith is observed also for 1 × 1 µ m scans. The analysis of 3D surface topographies (Figure 5) together with SEM images (Figures 2 and 3) suggests that this jump is caused both by micron-sized pits and observed also for 1 × 1 µ m scans. The analysis of 3D surface topographies (Figure 5) together withby SEM ((Fig ure ss 22surface and nanoscale changes the morphology. SEM images images Figin ure and 3) 3) suggests suggests that that this this jump jump is is caused caused both both by by micron-sized micron-sized pits pits and and by by nanoscale changes in the surface morphology. nanoscale changes in the surface morphology.
Figure Roughnessvalues values as a functionofofthe the etchingtime time forsolutions solutions H22SO SO4/H 2—(a) and Figure 6. 6. Roughness /H22O and 2O 2 —(a) Figure 6. Roughness valuesasasa afunction function of theetching etching timefor for solutions H H2SO44/H O 2—(a) and 4OH/HO 2O2—(b). —(b). NHNH OH/H 4 4OH/H 2 2O 22—(b). NH
Figure 7. Specific surface area (a) and height amplitudes (b) as a functions of the etching time. Figure Specificsurface surfacearea area(a) (a)and andheight height amplitudes amplitudes (b) Figure 7. 7. Specific (b) as as aafunctions functionsofofthe theetching etchingtime. time.
Values Values of of roughness, roughness, height height amplitude, amplitude, and and surface surface area area of of UFG UFG titanium titanium are are meaningfully meaningfully higher than the analogous values for CG titanium. This fact confirms higher of Values of roughness, height amplitude, and surface area of UFG titanium are meaningfully higher than the analogous values for CG titanium. This fact confirms higher activity activityhigher of nanostructured material while etching. than the analogous values for CG titanium. This fact confirms higher activity of nanostructured nanostructured material while etching. material while etching. 2.2.2. 2.2.2. NH NH44OH/H OH/H22O O22 Etching Etching 2.2.2. NH OH/H O 4 2NH 2 4Etching Etching in Etching in NH4OH/H OH/H22O O22 solution solution leads leads to to significant significant increase increase of of topography topography parameters parameters even even on the etching stages. However, these are non-monotonically increasing in NH solution leads to significant increase of topography with parameters even 4 OH/H 2 O2However, onEtching the earliest earliest etching stages. these values values are changed changed non-monotonically with increasing etching time. Between 5 and 15 min, a sharp increase of R a, RMS, and Rmax values is observed for UFG on etching the earliest stages. theseincrease values are non-monotonically with increasing time.etching Between 5 and However, 15 min, a sharp of Rchanged a, RMS, and Rmax values is observed for UFG titanium only (Figures 6b 7b). due modification surface etching time. Between 5 and 15 sharpis of Rqualitative Rmax valuesof observed titanium only (Figures 6b and andmin, 7b).aThis This is increase due to to the the qualitative modification ofis the the surfacefor a , RMS, and morphology. The values R a, RMS, and Rmax are significantly increased also in the etching time interval UFG titanium only (Figures 6b and 7b). is due to the qualitative of the surface morphology. The values Ra, RMS, and RmaxThis are significantly increased also modification in the etching time interval between 1 and 2 h both for UFG and CG titanium. In the same time interval, a sharp increase of S between 1 and h bothRfor UFG and and R CG In the same time interval, sharp increase Ssurf surf morphology. The 2values are significantly increased also in athe etching timeof interval a , RMS, maxtitanium. from to 35% is for UFG titanium (Figure 7a). AFM surface topographies for from 15.4% 15.4% is observed observed forand UFG titanium (Figure 7a).same AFM 3D 3D surface topographies for UFG UFG of between 1 andto235% h both for UFG CG titanium. In the time interval, a sharp increase and CG titanium (Figure 8) the formation of at times of or and CG 15.4% titanium (Figure 8) demonstrate demonstrate thetitanium formation(Figure of pits pits 7a). at etching etching times of 22 h htopographies or more. more. More Morefor Ssurf from to 35% is observed for UFG AFM 3D surface prolonged etching times (6 and 24 h) lead to a decrease of specific surface area of UFG titanium. prolonged etching times (6 and 24 h) lead to a decrease of specific surface area of UFG titanium. UFG and CG titanium (Figure 8) demonstrate the formation of pits at etching times of 2 h or more. Maximal value for CG titanium (29.9%) is reached only after 6 h etching. It is worth noting that Maximal S Ssurf surf value for CG titanium (29.9%) is reached only after 6 h etching. It is worth noting that
Nanomaterials 2017, 7, 15
7 of 15
More prolonged etching times (6 and 24 h) lead to a decrease of specific surface area of UFG titanium. Maximal Ssurf value for CG titanium (29.9%) is reached only after 6 h etching. It is worth noting that Nanomaterials 2017, 7, 15 7 of 14 according to AFM topographies (Figure 8) and SEM images (Figure 4), this sample differs from others by lamellar structure. according to AFM topographies (Figure 8) and SEM images (Figure 4), this sample differs from others by lamellar structure.
Figure 8. AFM surface topographies the UFGand andCG CGtitanium titanium etched 4OH/H2O2. Nonetched Figure 8. AFM surface topographies of of the UFG etchedininNH NH 4 OH/H2 O2 . Nonetched UFG-Ti (a), UFG-Ti etched 15 min (b), 1 h (c), nonetched CG-Ti (d), CG-Ti etched 15 min (e), 1 h (f), UFG-Ti (a), UFG-Ti etched 15 min (b), 1 h (c), nonetched CG-Ti (d), CG-Ti etched 15 min (e), 1 h (f), UFG-Ti etched 2 h (g), 6 h (h), 24 h (i), CG-Ti etched 2 h (j), 6 h (k), 24 h (l). UFG-Ti etched 2 h (g), 6 h (h), 24 h (i), CG-Ti etched 2 h (j), 6 h (k), 24 h (l).
It is also important that ammonia Piranha solution etching for UFG titanium is quicker than for (analogously to sulfuricPiranha acid Piranha solution) [21],for and relief parameters are higher. ItCG is titanium also important that ammonia solution etching UFG titanium is quicker than for For NH4OH/H 2O2, the difference is sufficiently higher than for H 2SO4and /H2Orelief 2. CG titanium (analogously to sulfuric acid Piranha solution) [21], parameters are higher. For NH4 OH/H2 O2 , the difference is sufficiently higher than for H2 SO4 /H2 O2 . 2.3. Composition of the Surface
2.3. Composition of the Surface The study of the composition of the etched and non-etched CG and UFG titanium by XRF have demonstrated that impurity content of the material was Fe—0.22–0.25, O—0.10–0.24, Cu—0.09–0.15
The study of the composition of the etched and non-etched CG and UFG titanium by XRF have (wt %). The study of the samples surface composition by X-ray Photoelectron Spectroscopy (XPS) demonstrated that impurity content of the material was Fe—0.22–0.25, O—0.10–0.24, Cu—0.09–0.15 (wt %). showed the presence of Ti, O, and C. The carbon contamination is seemingly caused by adventitious The study of the samples surface Photoelectron Spectroscopyof(XPS) showed the atmospheric hydrocarbon on composition the surface ofby theX-ray sample. No other contaminants the samples’ presence of Ti,were O, and C. The carbon contamination is seemingly caused by adventitious atmospheric surfaces found. hydrocarbon on the to surface of the sample. No other contaminants the surfaces wereare found. According high resolution XPS spectra of Ti 2p (Figure 9),of the Tisamples’ 2p3/2 and Ti 2p1/2 peaks 4+ [26]. 3+ or 2+ are According to high resolution XPS of Tiand 2p can (Figure 9), the Tito2pTi and Ti 2p peaks located at 459.2 eV and 464.9 eV for spectra all samples be attributed No Ti Ti 3/2 1/2 4+ 3+ shoulders at lower binding energy are detected, suggesting that all samples have a stoichiometric located at 459.2 eV and 464.9 eV for all samples and can be attributed to Ti [26]. No Ti or Ti2+ TiO2 surface. Ti energy peak (453.9 is quitesuggesting evident in the of initial This peak TiO shoulders at lowerMetallic binding are eV) detected, thatspectrum all samples havetitanium. a stoichiometric 2 is still found in sample etched forquite 5 minevident in H2SO4in /Hthe 2O2. However, the peak disappears when the surface. Metallic Tithe peak (453.9 eV) is spectrum of initial titanium. This peak is time increases to 15 min and above (Figure 9a, inset). Since XPS allows to study only surface layer still found in the sample etched for 5 min in H2 SO4 /H2 O2 . However, the peak disappears when the (several nm) of the material, peak Ti is disappeared, probably due to the oxidation. Titanium time increases to 15 min and above (Figure 9a, inset). Since XPS allows to study only surface layer oxidation while acidic Piranha etching is confirmed by the ellipsometry data. Initial TiO2 layer (several nm) of the material, peak Ti is disappeared, probably due to the oxidation. Titanium oxidation
Nanomaterials 2017, 7, 15 Nanomaterials 2017, 7, 15 Nanomaterials 2017, 7, 15
8 of 15 8 of 14 8 of 14
while acidic Piranha etching is confirmed by the ellipsometry data. Initial TiO2 layer thickness is thickness at 6thickness nm and this thickness grows to 45–50 whentime the etching time increases estimated atis6 estimated nm and this grows to 45–50 nm when thenm etching increases from 0 to 24 h. thickness is estimated at 6 nm and this thickness grows to 45–50 nm when the etching time increases from 0 to 24 h. from 0 to 24 h.
Figure XPS high resolution Ti spectra 2p spectra of nanotitanium etched (a) acidic Piranha and (b)Piranha. basic Figure 9. 9. XPS high resolution Ti 2p of nanotitanium etched in (a)in acidic Piranha and (b) basic Figure 9. XPS high resolution Ti 2p spectra of nanotitanium etched in (a) acidic Piranha and (b) basic Piranha. Piranha.
The result is different while using the NH4 OH/H2 O2 etching medium. On the early etching The result is different while using the NH4OH/H2O2 etching medium. On the early etching stages stages (5 min) Ti peak disappears, but the later this peak2Ostarts to grow, its intensity reaches maximum The different while NH the early etching stages (5 min) Tiresult peakisdisappears, but using later this peak4OH/H starts to2 etching grow, itsmedium. intensityOn reaches maximum at 2 h at (5 2 hmin) etching. According to AFM and SEM data, this time is characterized by the formation peak disappears, but SEM later data, this peak startsistocharacterized grow, its intensity maximum at 2 h of etching.Ti According to AFM and this time by the reaches formation of micron-size micron-size pits. Unfortunately, it SEM is impossible confirm variations of oxide layer thickness by means etching. According toitAFM and data, thisto time is characterized bylayer the formation micron-size pits. Unfortunately, is impossible to confirm variations of oxide thicknessofby means of of pits. ellipsometry. This is due to the significant roughness and light absorption by the samples’ surfaces. Unfortunately, it is to impossible to confirm variations of absorption oxide layerbythickness by means of ellipsometry. This is due the significant roughness and light the samples’ surfaces. The plausible ellipsometric results for the TiO layer thickness have been obtained only for the samples ellipsometry. This is due to the significant roughness and light absorption by the samples’ surfaces. 2 The plausible ellipsometric results for the TiO2 layer thickness have been obtained only for the etched for 5etched and ellipsometric 15 The thicknesses were estimated as 15–25 nm. The plausible results for thethicknesses TiO 2 layer thickness haveasbeen obtained only for the samples formin. 5 and 15 layer min. The layer were estimated 15–25 nm. samples etched for 5Piranha and 15 min. The Ti layer thicknesses estimated 15–25 nm. Both initial treated showwere an intensive intensive peakfrom fromTi–O Ti–O bonds Both initialand and Piranha treated Ti samples samples show an OOas 1s1s peak bonds at at Both initial and Piranha treated Ti samples show an intensive O 1s peak from Ti–O bonds at 530.5 eV (Figure 10) and a second peak at higher energy which can be attributed to the contributions 530.5 eV (Figure 10) and a second peak at higher energy which can be attributed to the contributions of 530.5 eVH (Figure 10) and aspecies second peak atintensity higheris energy which can be attributed to the contributions –OH and surface species [26]. ItsIts intensity changed afterchemical chemical treatment of –OH and 2O surface [26]. isnot notstrongly strongly changed after treatment in in 2 OH of –OH and H 2 O surface species [26]. Its intensity is not strongly changed after chemical treatment in H2H SO OH/H2O are characterized meaningful variation 2SO 4/H 2O In turn, turn, the the samples samples of NH4OH/H 2 series are characterized byby meaningful variation 4 /H 2O 22.. In 2O 2 series 2SO4/H2O2. In turn, the samples of NH4OH/H2O2 series are characterized by meaningful variation of H relative intensities is increased. increased.However, However,no nodistinct distinct correlations of relative intensitiesofofthe thepeaks peakswhen whenetching etching time is correlations of relative intensities of the peaks when etching time is increased. However, nothat distinct correlations between intensities etching times were found. Relative intensities suggest that the samples between intensities andand etching times were found. Relative intensities suggest the samples etched between intensities and etching times were found. Relative intensities suggest that the samples in ammonia Piranha solution contain more functional thanetched ones etched in acidic in etched ammonia Piranha solution contain more functional groups groups than ones in acidic Piranha etched ammoniaHowever, Piranha this solution contain more groups ones etched insurface acidic Piranhain solutions. difference also be to the variation of thesurface specific solutions. However, this difference can also becan due to functional thedue variation of thethan specific area values Piranha solutions. However, this difference can also be due to the variation of the specific surface area values and morphologies. and morphologies. area values and morphologies.
Figure 10. XPS high resolution O 1s spectra of nanotitanium etched in (a) acidic Piranha and (b) basic Figure XPS high O 1s of nanotitanium etched etched in (a) acidic Piranha (b) basic Figure 10.10. XPS highresolution resolution O spectra 1s spectra of nanotitanium in (a) acidicand Piranha and Piranha. Piranha. (b) basic Piranha.
3. Discussion 3. Discussion 3. Discussion 3.1. Etching of UFG and CG Titanium Etching UFGand andCG CGTitanium Titanium 3.1.3.1. Etching of of UFG The processes that accompany the chemical etching are rather complicated. They involve The processes that accompany the passivation. chemical etching are rather complicated. They medium involve material removal, oxidation, and the surface These processes depend on etching The processes that accompany chemical etching are rather complicated. They involve material material removal, oxidation, and surface passivation. These processes depend on etching medium and etching conditions (duration, concentration, temperature). on the conditions, removal, oxidation, and surface passivation. These and processes depend Depending on etching medium and etching and etching conditions (duration, concentration, and temperature). Depending on the conditions,
Nanomaterials 2017, 7, 15
9 of 15
conditions (duration, concentration, and temperature). Depending on the conditions, etching obeys either diffusion or kinetic control. In the case of diffusion control, the processes are controlled by the rates of reagent supply and reaction product removal from the surface area. In this case, the etching is performed layer by layer; surface roughness is smoothed. The rate limiting stage for kinetic control reactions is the reaction of the local areas on the surface. In the case of surface inhomogeneity and meaningful difference of reaction ability of local areas, the significant change in relief is possible. Data of this combined with data on the mass loss at the etching [21] suggest that etching by Piranha solutions can follow both abovementioned mechanisms. For H2 SO4 /H2 O2 solution, within minutes or tens of minutes, diffusion control is realized. This leads to slow layer-by-layer etching and smoothing of surface defects. With more prolonged treatment (several h or more) etching can perform with kinetic control that leads to the formation of micron-sized pits. In ammonia Piranha solution, etching is performed with kinetic control in the first minutes. This leads to rapid modification of the surface relief. In a general case, switching of the etching mechanism can be due to the variation of conditions (e.g., concentration, temperature, and mixing). In our work, the temperature remained unchanged, mixing has not been applied. Reagents’ concentrations are really decreased, but for this variation, switching from kinetic to diffusion control is expected. However, we observe either no changes (NH4 OH/H2 O2 ), or reverse effect (H2 SO4 /H2 O2 ). Therefore, the changes in the etching mechanism are caused by the features and composition of the surface, but do not depend on changes in the process conditions. It is important to note that sample surfaces were mechanically treated (polished) before etching. Therefore, surface structure was modified. We assume that the surface structure of polished titanium is homogenous and isotropic. Such surface is etched uniformly and layer-by-layer. The difference in the etching rate for the different spots of the surface becomes visible only after removal of the surface layer and structured material layers appear on the surface. Thus, in the case of etching in sulfuric acid Piranha, removal of the surface layer is slow and we can observe the transition from diffusion to the kinetic control. In the case of etching in the ammonia Piranha, the surface layer is removed very quickly and we cannot observe changes in the etching mechanism. From the thermodynamic principle, UFG materials have lower dissolution potential and therefore a higher tendency of dissolution in aggressive media than coarse-grained counterparts because the former has high density of grain boundaries and higher internal energy. Really, our results suggest higher activity of UFG titanium compared to CG analogs. The values of topography parameters (Ra , RMS, Rmax , and Ssurf ), that characterize development and roughness of the surface, are higher for UFG titanium. This is valid for both types of etching media. At present, there is not enough information about the chemical etching of UFG titanium. Most known publications are dedicated to etching of only CG titanium [13–16]. Moreover, most of them are exceptionally experimental. There are no discussions about possible reasons and mechanisms of the formation of one or another relief and surface composition. Therefore, there is a great interest in the works that studied the corrosion (electrochemical etching) of titanium. Some of them are dedicated to the comparison of corrosion resistance of UFG and CG titanium [17]. The data of various authors are contradictory. So, Garbacz et al. [27] suggest that CG titanium is more corrosion resistant compared to UFG analogs. In turn, other authors [18,19,28] report that UFG titanium has a lower tendency to corrode than CG one. It is worth noting that UFG titanium studied in the above-mentioned works was produced using different technologies: hydrostatic extrusion [27], equal channel angular pressing [19,28], and high ratio differential rolling [18]. The difference in the techniques of UFG titanium production led to the difference in the grain size and microstructures. These factors can significantly influence on the chemical and electrochemical stability of the material. Besides, there was a difference in the testing procedures. Garbacz et al. [27] and Hoseini et al. [28] used NaCl-based electrolyte whereas Kim et al. [18] and Balyanov et al. [19] were used HCl and H2 SO4 solutions. It should be emphasized, that UFG materials not only have smaller grain size, but also specific structure on the grain boundaries. Therefore, the main factors determining the corrosion rate of UFG
Nanomaterials 2017, 7, 15
10 of 15
Nanomaterials 2017, 7, 15are not only grain size, but also volume of grain boundaries, value of internal 10 of 14 and CG materials stress, and the value of electrode potential [17]. Crystal orientation also has a significant impact on and the value of electrode potential [17]. Crystal orientation also has a significant impact on etch rate etch rate and topography of surface. Matykina et al. [20] and Hoseini et al. [28] demonstrated that and topography of surface. Matykina et al. [20] and Hoseini et al. [28] demonstrated that crystallographic texture can significantly influence the corrosion rate of UFG and CG Ti. Moreover, it is crystallographic texture can significantly influence the corrosion rate of UFG and CG Ti. Moreover, shown in [28] that texture can play greater role than grain size. In the current work, we did not study it is shown in [28] that texture can play greater role than grain size. In the current work, we did not crystallographic orientation of the samples in detail, however, we can suppose that while etching in study crystallographic orientation of the samples in detail, however, we can suppose that while H2 SO4 /H2 O2 for a short time the factor of crystallographic texture should not play significant role. etching in H2SO4/H2O2 for a short time the factor of crystallographic texture should not play This is because of slow etching and low rate of surface layer removal (small mass loss). At longer significant role. This is because of slow etching and low rate of surface layer removal (small mass treatment times we can see (Figure 11) that material grains start to appear and the crystallographic loss). At longer treatment times we can see (Figure 11) that material grains start to appear and the texture factor can be more important. While NH4 OH/H2 O2 etching, surface layer is removed very crystallographic texture factor can be more important. While NH4OH/H2O2 etching, surface layer is quickly and the crystallographic texture factor can play the crucial role. Due to significant influence removed very quickly and the crystallographic texture factor can play the crucial role. Due to of the electrode factor, electrochemical etching differs from purely chemical etching. Nevertheless, significant influence of the electrode factor, electrochemical etching differs from purely chemical the influence of internal stress, grain size, and number of grain boundaries is very important for etching. Nevertheless, the influence of internal stress, grain size, and number of grain boundaries is chemical etching. very important for chemical etching.
Figure Characteristic SEM O 2 SO 4 /H Figure 11. 11. Characteristic SEM images imagesof ofCG CGtitanium titanium(a) (a)and andUFG UFGtitanium titanium(b) (b)etched etchedininHH 2SO 4/H22O22 solution ×). solution during during 24 24 h h (magnification—3000 (magnification—3000×).
etch mechanisms mechanisms and and growth growth rates rates of of titanium titanium in in acidic/basic acidic/basic medium is The difference in etch in in chemistry. Chemistry of Tiofetching in Piranha solutions is rather mainly determined determinedby bya adifference difference chemistry. Chemistry Ti etching in Piranha solutions is complicated. There may be amay variety chemical Ti and Piranha solution components rather complicated. There be of a variety ofreactions chemicalwhere reactions where Ti and Piranha solution are involved are (oxidation to oxides TiO Ti2O3, TiO TiO,2 ,complex formation information the presence of components involvedleading (oxidation leading to2, oxides Ti2 O3 , TiO, complex in the hydrogenof peroxide—Ti(OH) 2O2, hydrogen peroxide/ammonia—Ti(OH) 2 O 2 , formation of low presence hydrogen peroxide—Ti(OH) O , hydrogen peroxide/ammonia—Ti(OH) O , formation of 2 2 2 2 stability sulfates Ti(SO 4)2, TiOSO 4 etc.) [29,30]. In order to discuss the the mechanisms from thethe point of low stability sulfates Ti(SO [29,30]. In order to discuss mechanisms from point 4 )2 , TiOSO 4 etc.) view of chemical processes during etching in detail, one needs to perform a series of additional timeof view of chemical processes during etching in detail, one needs to perform a series of additional consuming and thorough experiments usingusing chemical analysis techniques. time-consuming and thorough experiments chemical analysis techniques. It is worth noting that etching can lead not only to material removal, but also to passivation due to surface oxidation. According According to XPS and ellipsometry, passivation really takes place while using acidic Piranha solution. Therefore, Therefore, etching, etching, depending depending on on the the conditions, conditions, should should lead lead either to the removal of grain boundaries and appearance of the material material grains grains (Figure (Figure 11), 11), or or to totheir theirpassivation. passivation. (“sponge”, “coral”, “coral”, Such etching mechanisms can really explain a number of produced nanostructures nanostructures (“sponge”, “mesh”, and and “ridges”). “ridges”). Quantitative difference of morphology of these structures for UFG and CG “mesh”, difference in in the the structure structure and and density density of of grain grain boundaries. boundaries. titanium is caused by the difference Another feature featureofoftreated treated samples at prolonged etching is the presence of the size of samples at prolonged etching is the presence of pitsofofpits the size of several several microns. Micrographs thatappearance their appearance is not related to material grainand sizegrain and microns. Micrographs suggestsuggest that their is not related to material grain size grain boundaries. Probably, pit formation can be explained by theofpresence and boundaries. Probably, pit formation can be explained by the presence structure of andstructure composition composition defects. It is known that SPD treatment that was used to produce UFG titanium samples defects. It is known that SPD treatment that was used to produce UFG titanium samples leads to leads to redistribution of chemical inhomogeneity into scale. a finerTherefore, scale. Therefore, homogeneity redistribution of chemical inhomogeneity into a finer higher higher homogeneity of the of the structures while etching UFG titanium (especially, micron-sized is caused by a structures formedformed while etching UFG titanium (especially, micron-sized pits) ispits) caused by a more more homogenous distribution of impurities. homogenous distribution of impurities. 3.2. Prospects for the Use of Materials in Medicine
Nanomaterials 2017, 7, 15
11 of 15
3.2. Prospects for the Use of Materials in Medicine It was established that basic and acidic Piranha solutions lead, depending on the etching conditions, to the structures of various relief and morphology. The structure size can lie either in micro or in nanoscale. Based on the numerous in vitro and in vivo studies, the researchers agree that the developed relief both on a micro- and nanoscale is necessary for the most successful osteointegration of the implant [24,31,32]. Micron-sized relief, especially the presence of micropores, significantly enhances the adhesion of bone tissue cells [24,33]. The presence of specific nanorelief is very important for enhancement of the circulation and for acceleration of biomolecules (proteins, nutrients) adsorption [24,34], and it can potentially lead to antibacterial effects [35,36]. Besides, from a biomechanical viewpoint, the expanded surface area of the implant surface, which is in contact with the surrounding bone tissue, increases the friction coefficient and the kinetic friction during implant insertion. Ultimately, the increased kinetic friction naturally provides higher implant primary stability [24]. Summarizing aforementioned, the most prominent pathway is surface etching in NH4 OH/H2 O2 for 2 h, because in this case the samples have the maximal specific surface area, and also developed relief both in nano-ridges and microscale pits. The surfaces of samples etched in H2 SO4 /H2 O2 for long periods (from 6 to 24 h) are also characterized by micro- and nanorelief. However, their specific surface area is not so large. Moreover, we can propose that due to higher activity while etching and, correspondingly, due to higher specific surface area, UFG titanium has an advantage over CG analogs. Surface composition is also very important for a successful medical implant. The most valuable parameter is the presence of a rather thick layer of titanium oxide that prevents biocorrosion and metallic Ti diffusion. Another important parameter is the presence of a large amount of hydroxyl groups on the surface that provide hydrophilic properties of the material [1,2]. This causes faster adsorption of proteins and influences cell activity [37]. From this viewpoint, the advantages of ammonia etchant are not so evident. Despite the fact that etching in NH4 OH/H2 O2 leads to increasing concentration of surface functional groups and the total amount of these groups is much higher than on the surface of untreated titanium or acidic Piranha etched titanium, oxide layer thickness remains almost unchanged. Besides, experimental data suggest that surface species composition is not sufficiently proved; and the surface can contain not only hydroxyl groups but also some groups of another nature. Nevertheless, the disadvantages of the surface composition produced by chemical etching can be compensated by additional surface modification [12,13,23,38]. 4. Materials and Methods 4.1. Sample Preparation Nanotitanium samples were prepared in Limited Liability Company “Nanomet”, Ufa, Russia, from titanium Grade 4. Titanium rods of 1 m length were subjected to Equal-Channel Angular Pressing by ECAP-Conform processing. Detailed description of ECAP-Conform processing technique can be found in [6]. Temperature of processing was 400 ◦ C. Number of passes was five. The value of total accumulated true strain was equal to 3.5. After ECAP-Conform processing, the billets were subjected to drawing at 200 ◦ C resulting in the production of rods with a diameter of 6 mm. The average grain size of nanotitanium was ~50–100 nm according to Rietveld XRD processing. Rods were treated by machining before etching. Firstly, nanotitanium rods were cut into discs having thickness of 2–3 mm by the Buehler IsoMet 1000 machine (Buehler, Lake Bluff, IL, USA). Then, these discs were ground and polished by a semiautomatic Buehler MiniMet 1000 machine (Buehler, Lake Bluff, IL, USA) to mirror-like surface (roughness less than 10 nm) using 600, 800, and 1200 grit sandpapers and silicon dioxide nanoparticles suspension (20 nm). Prior to etching, the samples were cleaned repeatedly with acetone and deionized water in an ultrasonic bath for 15 min and subsequently dried in a desiccator. Finally, the samples were dropped into a Pyrex glass container with basic (NH4 OH/H2 O2 ) or acidic (H2 SO4 /H2 O2 ) Piranha solution at 20 ◦ C. Temperature was maintained by thermostat Elmi
Nanomaterials 2017, 7, 15
12 of 15
TW-2.03. Piranha solutions were prepared by 50% v/v ammonium hydroxide (NH4 OH; Vecton, Saint-Petersburg, Russia), 36 N sulfuric acid (H2 SO4 ; Vecton, Saint-Petersburg, Russia), and 30% aqueous hydrogen peroxide (H2 O2 ; Vecton, Saint-Petersburg, Russia). The ratio of reactants was 7/3; exposure times were 5, 15 min, and 1, 2, 6, and 24 h for both types of etchants. Immediately after etching, the samples were taken out of the etchant and thoroughly washed in distilled water using an ultrasonic bath. 4.2. Samples Characterization The topography of the samples surfaces was studied using a Solver P47 Pro (NT-MDT, Moscow, Russia) probe microscope in the tapping mode via atomic force microscopy (AFM). The measurements were conducted in ambient air with scan areas of 50 × 50, 30 × 30, 10 × 10, and 1 × 1 µm2 . A total of five or six random positions on the sample surface were measured. Four parameters including the average mean value of surface roughness (Ra ), root mean square roughness (RMS), surface area difference (the percentage increase of 3D surface area over 2D surface area.), and vertical range were calculated by the associated Gwyddion 2.37 software. AFM surface parameters were calculated for all scans and the average values were obtained. Untreated and treated samples were imagined with scanning electron microscope Zeiss Merlin operated at 10–15 kV at the “Nanotechnology” Interdisciplinary Resource Center SPbSU. Microscope spatial resolution was around 1 nm and magnification up to 600,000×. In-lens, SE and SE2 regimes were used. A total of two or three random positions on the sample surface were scanned. Chemical composition of the samples before and after etching was study by energy dispersive X-ray fluorescence spectrometer EDX Series 800 HS (Shimadzu, Kyoto, Japan). Chemical composition of the samples surface was study by X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectra were registered with a “Thermo Fisher Scientific Escalab 250Xi” spectrometer (Thermo Fisher, Waltham, MA, USA) at the Resource Center of “Physical Methods of Surface Investigation” SPbSU. The samples were excited by Al Kα (1486.7 eV) X-rays in a base pressure of 7 × 10−8 Pa. High resolution spectra were automatically charge compensated by setting the binding energy of C 1s carbon line to 284.8 eV [39]. The thicknesses of oxide layers on the surface of the samples were estimated by spectral ellipsometry (350–1000 nm) method using Ellips-1891 SAG instrument (CNT, Novosibirsk, Russia). Accuracy of the film thickness determination was 0.3 nm in a thickness range of 1–100 nm 5. Conclusions In this study, we investigated in detail the features of the etching of ultrafine grain (UFG) and coarse grain (CG) titanium in Piranha solutions (NH4 OH/H2 O2 and H2 SO4 /H2 O2 ). Using AFM and SEM methods, it was found that the variation of the etching medium and time leads to various micro-, nano-, and hierarchical micro/nano-structures on the UFG or CG titanium surface. AFM results suggest that ammonia Piranha solution (in contrast to the acidic one) provides more significant roughness and value of specific surface area even at small etching times. H2 SO4 /H2 O2 Piranha solution gives a meaningful increase of relief parameter only after 2 h etching. Based on XPS and ellipsometry data, we suggest that the difference is caused by a less distinct oxidation (passivation) process in the basic Piranha solution compared to the acidic one. UFG titanium surface composition is different after etching in NH4 OH/H2 O2 and H2 SO4 /H2 O2 solutions. SEM and AFM data showed that UFG titanium is more actively etched compared to CG titanium independently of the etching medium. The difference is both qualitative (variation of surface morphology and homogeneity) and quantitative (roughness and specific surface area). The causes of these differences can be either variations of the materials structures (grain size, amount, and the structures of grain boundaries) or impurities distribution.
Nanomaterials 2017, 7, 15
13 of 15
We proposed that UFG titanium samples etched in NH4 OH/H2 O2 for 2 h and in H2 SO4 /H2 O2 for 24 h can be very prominent materials for dental and orthopedic implants due to well-developed surface and presence of hierarchal micro/nano structures on the surface. Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/7/1/15/s1. Acknowledgments: The authors would like to acknowledge the Russian Ministry of Science and Education for financial support through megagrant No. 14.B25.31.0017. This work was also supported in part by a grants from the St-Petersburg state university, No. 12.37.433.2015 and No. 6.37.204.2016. This research was conducted using the equipment of the resource centers of the Research Park of the St. Petersburg State University “Innovative Technologies of Composite Nanomaterials”, “Center for Physical Methods of Surface Investigation”, “Center for X-ray Diffraction Studies”, and “Nanotechnology Interdisciplinary Center”. Author Contributions: Alexandr Yu. Solokhin performed chemical etching, Denis V. Nazarov performed material characteristics measurement and data treatment (AFM, XPS, and SEM), Elena G. Zemtsova supervised the project and edited the manuscript, Ruslan Z. Valiev provided the samples of nanostructured titanium for etching and edited the manuscript, Vladimir M. Smirnov conceived the work flow and experiments and prepared the initial draft of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3. 4.
5. 6. 7.
8.
9.
10. 11. 12. 13. 14.
Geetha, M. Ti based biomaterials, the ultimate choice for orthopaedic implants—A review. Prog. Mater. Sci. 2009, 54, 397–425. [CrossRef] Li, Y.; Yang, C.; Zhao, H.; Qu, S.; Li, X.; Li, Y. New developments of Ti-based alloys for biomedical applications. Materials 2014, 7, 1709–1800. [CrossRef] Brunette, D.M.; Tengvall, P.; Textor, M.; Thomsen, P. Titanium in Medicine; Springer: Berlin/Heidelberg, Germany; New York, NY, USA, 2001; pp. 1–1019. Matusiewicz, H. Potential release of in vivo trace metals from metallic medical implants in the human body: From ions to nanoparticles—A systematic analytical review. Acta Biomater. 2014, 10, 2379–2403. [CrossRef] [PubMed] Valiev, R.Z.; Zhilyaev, A.P.; Langdon, T.G. Bulk Nanostructured Materials: Fundamentals and Applications; John Wiley &. Sons, Inc.: Hoboken, NJ, USA, 2014; pp. 1–440. Valiev, R.Z.; Langdon, T.G. Principles of equal-channel angular pressing as a processing tool for grain refinement. Prog. Mater. Sci. 2006, 51, 881–981. [CrossRef] Mishnaevsky, L.; Levashov, E.; Valiev, R.Z.; Segurado, J.; Sabirov, I.; Enikeev, N.; Prokoshkin, S.; Solov’yov, A.V.; Korotitskiy, A.; Gutmanas, E.; et al. Nanostructured titanium-based materials for medical implants: Modeling and development. Mater. Sci. Eng. R. Rep. 2014, 81, 1–19. [CrossRef] Lowe, T.C.; Valiev, R.Z. Frontiers for Bulk Nanostructured Metals in Biomedical Applications. In Advanced Biomaterials and Biodevices; Tiwari, A., Nordin, A.N., Eds.; JohnWiley & Sons, Inc.: Hoboken, NJ, USA, 2014; pp. 1–52. Bagherifard, S.; Ghelichi, R.; Khademhosseini, A.; Guagliano, M. Cell response to nanocrystallized metallic substrates obtained through severe plastic deformation. ACS Appl. Mater. Interfaces 2014, 6, 7963–7985. [CrossRef] [PubMed] Yang, B.; Uchida, M.; Kim, H.M.; Zhang, X.; Kukubo, T. Preparation of bioactive titanium metal via anodic oxidation treatment. Biomaterials 2004, 25, 1003–1010. [CrossRef] Roy, P.; Berger, S.; Schmuki, P. TiO2 nanotubes: Synthesis and applications. Angew. Chem. Int. Ed. 2011, 50, 2904–2939. [CrossRef] [PubMed] Bauer, S.; Schmuki, P.; von der Mark, K.; Park, J. Engineering biocompatible implant surfaces Part I: Materials and surfaces. Prog. Mater. Sci. 2013, 58, 261–326. [CrossRef] Liu, X.; Chu, P.K.; Ding, C. Surface modification of titanium, titanium alloys, and related materials for biomedical applications. Mater. Sci. Eng. R 2004, 47, 49–121. [CrossRef] Vetrone, F.; Variola, F.; De Oliveira, P.T.; Zalzal, S.F.; Yi, J.-H.; Sam, J.; Bombonato-Prado, K.F.; Sarkissian, A.; Perepichka, D.F.; Wuest, J.D.; et al. Nanoscale oxidative patterning of metallic surfaces to modulate cell activity and fate. Nano Lett. 2009, 9, 659–665. [CrossRef] [PubMed]
Nanomaterials 2017, 7, 15
15.
16. 17. 18. 19.
20.
21.
22.
23.
24. 25. 26. 27. 28.
29. 30. 31. 32.
33. 34. 35. 36.
14 of 15
Variola, F.; Brunski, J.B.; Orsini, G.; Tambasco de Oliveira, P.; Wazen, R.; Nanci, A. Nanoscale surface modifications of medically relevant metals: State-of-the art and perspectives. Nanoscale 2011, 3, 335–353. [CrossRef] [PubMed] Richert, L.; Vetrone, F.; Yi, J.-H.; Zalzal, S.F.; Wuest, J.D.; Rosei, F.; Nanci, A. Surface Nanopatterning to Control Cell Growth. Adv. Mater. 2008, 20, 1488–1492. [CrossRef] Miyamoto, H. Corrosion of ultrafine grained materials by severe plastic deformation, an overview. Mater. Trans. 2016, 57, 559–572. [CrossRef] Kim, H.S.; Yoo, S.J.; Ahn, J.W.; Kim, D.H.; Kim, W.J. Ultrafine grained titanium sheets with high strength and high corrosion resistance. Mater. Sci. Eng. A 2011, 528, 8479–8485. [CrossRef] Balyanov, A.; Kutnyakova, J.; Amirkhanova, N.A.; Stolyarov, V.V.; Valiev, R.Z.; Liao, X.Z.; Zhao, Y.H.; Jiang, Y.B.; Xu, H.F.; Lowe, T.C.; et al. Corrosion resistance of ultra fine-grained Ti. Scr. Mater. 2004, 51, 225–229. [CrossRef] Matykina, E.; Arrabal, R.; Valiev, R.Z.; Molina-Aldareguia, J.M.; Belov, P.; Sabirov, I. Electrochemical anisotropy of nanostructured titanium for biomedical implants. Electrochim. Acta. 2015, 176, 1221–1232. [CrossRef] Nazarov, D.V.; Zemtsova, E.G.; Valiev, R.Z.; Smirnov, V.M. Specific features of etching of ultrafine and coarse-grained titanium in base and acid solutions of hydrogen peroxide. Russ. J. Appl. Chem. 2016, 89, 284–286. [CrossRef] D'jkonov, G.S.; Stenina, E.V.; Sviridova, E.V.; Salimgareeva, G.C.; Semenova, I.P.; Zemtsova, E.G.; Valiev, R.Z. Regulation of the surface microrelief of coarse-grained and ultrafine-grained titanium by etching method. Mater. Phys. Mech. 2014, 21, 259–265. Nazarov, D.V.; Zemtsova, E.G.; Valiev, R.Z.; Smirnov, V.M. Formation of micro- and nanostructures on the nanotitanium surface by chemical etching and deposition of titania films by atomic layer deposition (ALD). Materials 2015, 8, 8366–8377. [CrossRef] Wennerberg, A.; Albrektsson, T.; Jimbo, R. Implant Surfaces and Their Biological and Clinical Impact; Springer: Heidelberg, Germany; New York, NY, USA, 2015; pp. 1–182. Wennerberg, A.; Albrektsson, T. Suggested guidelines for the topographic evaluation of implant surfaces. Int. J. Oral Maxillofac. Implant. 2000, 15, 331–344. Martin, H.J.; Schulz, K.H.; Walters, K.B. Piranha treated titanium compared to passivated titanium as characterized by XPS. Surf. Sci. Spectra 2008, 15, 23–30. [CrossRef] Garbacz, H.; Pisarek, M.; Kurzydłowski, K.J. Corrosion resistance of nanostructured titanium. Biomol. Eng. 2007, 24, 559–563. [CrossRef] [PubMed] Hoseini, M.; Shahryari, A.; Omanovic, S.; Szpunar, J.A. Comparative effect of grain size and texture on the corrosion behavior of commercially pure titanium processed by equal channel angular pressing. Corros. Sci. 2009, 51, 3064–3067. [CrossRef] Cotton, F.A.; Wilkinson, G. Advanced Inorganic Chemistry; John Willey & Sons, Inc.: London, UK; New York, NY, USA, 1972; pp. 807–818. Mühlebach, J.; Müller, K.; Schwarzenbach, G. The peroxo complexes of titanium. Inorg. Chem. 1970, 9, 2381–2390. Jiang, N.; Zhu, S.; Li, J.; Zhang, L.; Liao, Y.; Hu, J. Development of a novel biomimetic micro/nanohierarchical interface for enhancement of osseointegration. RSC Adv. 2016, 6, 49954–49965. [CrossRef] Li, B.E.; Li, Y.; Min, Y.; Hao, J.Z.; Liang, C.Y.; Li, H.P.; Wang, G.C.; Liu, S.M.; Wang, H.S. Synergistic effects of hierarchical hybrid micro/nanostructures on the biological properties of titanium orthopaedic implants. RSC Adv. 2015, 5, 49552–49558. [CrossRef] Jones, F.H. Teeth and bones: Applications of surface science to dental materials and related biomaterials. Surf. Sci. Rep. 2001, 42, 75–205. [CrossRef] Lord, M.S.; Foss, M.; Besenbacher, F. Influence of nanoscale surface topography on protein adsorption and cellular response. Nano Today 2010, 5, 66–78. [CrossRef] Seddiki, O.; Harnagea, C.; Levesque, L.; Mantovani, D.; Rosei, F. Evidence of antibacterial activity on titanium surfaces through nanotextures. Appl. Surf. Sci. 2014, 308, 275–284. [CrossRef] Variola, F.; Zalzal, S.F.; Leduc, A.; Barbeau, J.; Nanci, A. Oxidative nanopatterning of titanium generates mesoporous surfaces with antimicrobial properties. Int. J. Nanomed. 2014, 9, 2319–2325. [CrossRef] [PubMed]
Nanomaterials 2017, 7, 15
37.
38. 39.
15 of 15
Lu, X.; Wang, Y.; Yang, X.; Zhang, Q.; Zhao, Z.; Weng, L.-T.; Leng, Y. Spectroscopic analysis of titanium surface functional groups under various surface modification and their behaviors in vitro and in vivo. J. Biomed. Mater. Res. Part A 2008, 84, 523–534. [CrossRef] [PubMed] Rautray, T.R.; Narayanan, R.; Kim, K.-H. Ion implantation of titanium based biomaterials. Prog. Mater. Sci. 2011, 56, 1137–1177. [CrossRef] Moulder, J.F.; Stickle, W.F.; Sobol, P.E.; Bomben, K.D. Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data, 2nd ed.; Physical Electronics, Inc.: Eden Prairie, MN, USA, 1995; pp. 1–261. © 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
