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Applied Surface Science

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Corrosion behavior of titanium silicide surface with hydrogen peroxide:Formation of sub-μm TiOx- based spheres, nanocomposite TiOx/SiOx phases,and mesoporous TiOx/SiOx network

V. Jandováa,b, R. Fajgarb, J. Kupčíkb,c, J. Polaa,b,⁎, K. Soukupb, P. Mikysekd,e, T. Křeneka,T. Kováříka, T. Stichf, D. Dochevafa Research Centre of New Technologies, University of West Bohemia, Univerzitní 8, 30614 Plzeň, Czech Republicb Institute of Chemical Process Fundamentals of the Czech Academy of Sciences, Rozvojová 135, 16502 Prague 6, Czech Republicc Institute of Inorganic Chemistry of Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republicd Institute of Geology of the Czech Academy of Sciences, Rozvojová 269, 16500 Prague 6, Czech Republice Institute of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republicf Clinic and Policlinic for Trauma Surgery, University Hospital of Regensburg, Franz-Josef-Strauss-Allee 11, 93053 Regensburg, Germany

A R T I C L E I N F O

Keywords:Titanium silicideHydrogen peroxideSiOx microphasesTiOx and SiOx nanophasesMesoporous and microporous TiOx-basedparticles

A B S T R A C T

So far unexplored corrosion of titanium silicide (Ti5Si3) surface with acidified hydrogen peroxide is of interestdue to its potential use in improving osseointegration of titanium implants coated by titanium silicides. Detailedexamination of corrosion products by FTIR, Raman and XP spectroscopy, electron microscopy, XRD, BET andlight scattering techniques allows recognition of hydrated nanocomposite TiOx/SiOx (x≤ 2) phases composed ofsegregated amorphous SiOx species and TiOx–based networks containing Ti-O-Si and -O-O- bonds. The ap-pearance of the TiOx networks depends on the extent of peroxidation. A less progressed peroxidation yields sub-μm-sized TiOx-based spheres which upon annealing develop anatase nanograins withstanding 800° C. A moreprogressed peroxidation produces larger mesoporous TiOx-based bodies which disintegrate upon sonication intomicrometer-sized entities. The proposed mechanism of surface corrosion is based on the complementary use ofanalytical techniques. The one-step production of bioactive (hydrated TiOx and SiOx) species deserves to beexplored in osseointegration studies of slightly corroded Ti5Si3-coated titanium implants.

1. Introduction

Titanium and titanium alloys are attractive materials for biomedicalapplication, but require surface modification to improve osseointegra-tion with bone tissue. This can be accomplished through plenty ofphysical (e.g. plasma, ion-beam and laser) and chemical (e.g. sol–gel,anodic, or microwave) approaches involving hydroxyapatite in-corporation [1] and wet oxidation of titanium surface [2]. The latterapproach is extensively documented in the literature and enables facileformation of bioactive titanium hydrogel by chemical treatments ofclean titanium surface in acidic [2,3] alkaline [2,4], hydrogen peroxide[5,6] and acidic hydrogen peroxide [7,8] solutions. Other bioactiveentities inducing hydroxyapatite nucleation and assisting new boneformation and attachment are various forms of silicon-containing spe-cies like silica gel [9-11], Si-containing hydroxyapatite [12,13] calciumtitanium silicate (CaTiSiO5) [14], Ti-5Si alloy [15] and titanium silicide

(Ti5Si3) [16].The effect of Ti5Si3 has been so far little studied and would deserve

more effort in order to gain its better understanding through inter-pretation of chemical changes of this silicide in aqueous or salinephases. In fact, chemical treatments of Ti5Si3 or TiSi alloys have beenexamined only with alkaline [15] or acidic [16,17] aqueous solutionsand the enhancement of early bonding of Ti5Si3-coated titanium im-plants to bone has been hitherto confirmed only for the Ti5Si3 coat pre-treated by soaking in HCl solution [16].

It is known that the acid pre-treated Ti-Si alloys with 2–11Si wt.%exhibit high corrosion resistance and form a passive layer composed ofSiO2/TiO2 oxides which contain TiIII, Ti II species and possibly Si–O–Tibonds [17]. Such corroded surface therefore differs from surfaces de-veloped by acid treatments of pure titanium which are rich in Ti-OHgroups [18] or rich in phosphate anions [19] when immersed in si-mulated body fluid (SBF) (e.g. [20,21]). It is also known that the alkali-

https://doi.org/10.1016/j.apsusc.2020.147133Received 14 April 2020; Received in revised form 25 June 2020; Accepted 30 June 2020

⁎ Corresponding author.E-mail address: pola@icpf.cas.cz (J. Pola).

Applied Surface Science 529 (2020) 147133

Available online 06 July 20200169-4332/ © 2020 Elsevier B.V. All rights reserved.

T

treated Ti5Si surface develops [15] a porous network structure of so-dium titanate hydrogel and after annealing a blend of sodium titanate,anatase and rutile, but it does not enhance hydroxyapatite formationwhen immersed into SBF.

Here we report on so far unexplored corrosion of small Ti5Si3 par-ticles with acidic hydrogen peroxide solution. Such process can producea dual system of bioactive Si oxidic and Ti oxidic species which areanalogous to bioactive titanium hydrogel [5-8] and silica gel [9-11] andcan enhance osseointegration to bone. The examination of the structureand composition of corrosion products formed in this one-step processleading to two different but both bioactive (hydrated TiOx and SiOx)species thus appears promising for applications in the synthesis ofbioconductive coats and particularly for our ongoing studies of os-seointegration capability of corroded Ti5Si3 coats.

2. Experimental

2.1. Corrosion of Ti5Si3 particles with acidic hydrogen peroxide solution.

The black particles of titanium silicide (0.20 g, Ti5Si3, Alfa Aesar,99.5%, −325 mesh) were loaded into 25 ml of 30 wt% H2O2 solutionsacidified with 2 ml of 0.5 M HCl, which were heated for 2 h at 100° C orstirred for two days at room temperature by a magnetic bar. The sametreatments were also conducted with 0.40 g of Ti5Si3 and quadrupleamounts of the HCl-acidified H2O2 solutions in order to increase con-version of Ti5Si3 corrosion and get sufficient amounts of corrodedproducts for their characterization. The treated particles suspended inthe aqueous phase were centrifuged, washed with deionized (DI) waterand then dispersed by a short intense stirring in DI water in a tubularvessel. Those having white, pale and grey colour were separated as anupper light and a middle pale zone from heavier grey particles whichdescended fast to the bottom of the vessel. The separated white, paleand grey particles were then dried in air and submitted for analyses assuch or after annealing at 400, 800 and 1000° C for 2 h in a quartz tubein a Thermolyne furnace while exposed to a vacuum of 3 Pa to preventoxidation.

2.2. Analyses of pristine and HCl/H2O2-corroded Ti5Si3 particles

The structure of the pristine and white, pale and grey particles wasanalyzed by using FT-infrared, X-ray photoelectron, UV–Vis and Ramanspectroscopy, scanning and transmission electron microscopy, X-raydiffraction analysis, Brunauer–Emmett–Teller and dynamic light scat-tering methods.

The infrared spectra of the particles were measured as FTIR ab-sorption spectra on KBr substrate on a Nicolet Impact spectrometer andthe Raman spectra were recorded using a dispersive Raman NicoletAlmega XR instrument (excitation wavelength 473 nm, power 10 mW).UV–vis spectra of the yellow solutions formed during the HCl/H2O2

treatment of Ti5Si3 particles and Ti sheet (for comparison) were re-corded on a Shimadzu UV-1601 spectrophotometer.

The X-Ray photoelectron spectra (XPS) were obtained by KratosAmicus/Esca 3400 with working pressure better then 5.0 10-7 Pa, usingpolychromatic Mg X-Ray source (Mg Kα, 1253.4 eV). Spectra weretaken over Ti 2p, Si 2p and O 1 s valence bands. Samples were re-peatedly sputtered with Ar+ ions at 1000 V with current of 10 µA for90 s. The overlapping spectral features were resolved into individualcomponents using the damped non-linear least squares method and thelines of Gaussian-Lorentzian shape. Prior to fitting, the Shirley back-ground was subtracted, when needed. The XPS depth profiling, per-formed as a stepwise 50 nm sputtering with Ar+ ions on pristine andHCl/H2O2-treated Ti5Si3 particles was applied to assess the atomic Si/Tiratios and the populations of different valence Ti and Si constituents onca. 7 mm2 areas of different layers of these specimens.

The scanning electron microscopy (SEM) images were obtained witha SEM Tescan Indusem (Bruker Quantax) microscope equipped with an

EDAX detector. The Transmission electron microscopy (TEM) mea-surements on the samples scraped from the deposited films were per-formed on a JEOL JEM 3010TEM microscope equipped with an EDSdetector (INCA/Oxford) and CCD Gatan (Digital Micrograph software).The samples were dispersed in isopropanol, treated in ultrasound and adrop of very dilute suspension was placed on a holey-carbon coated Cu-grid and allowed to dry by evaporation at ambient temperature.Electron diffraction patterns were evaluated using JCPDS PDF-4 data-base [22] and the Process Diffraction software package [23].

X-ray particles diffraction investigation was performed with theBruker D8 Discover diffractometer equipped with a silicon-strip linearLynxEye detector and a focusing germanium primary monochromatorof Johansson type providing CuKα1 radiation (λ=1.54056 Å). Data formineral identification were collected in the 2ϴ range of 5–70° with astep size of 0.014° and a counting time of 1.2 s at each step, and de-tector angular opening of 2.935°. The phase identification was per-formed with Diffrac.Eva software v4.5.0 (Bruker AXS GmbH, Karlsruhe,Germany; 2010–2018).

The BET surface area and total pore volume were respectivelyevaluated from the nitrogen physical adsorption–desorption isothermsmeasured at 77 K and the nitrogen adsorption isotherm at P/P0 = 0,99.

The specific mesopore surface and specific micropore volume wereassessed by t-plot Harkins and Jury method based on standard iso-therm.

The size distribution and zeta potential of 50 mg of particles in50 ml water was measured on a Litesizer™ 500 DLS-based instrument(Anton Paar) and the water suspension was shaken or dispersed usingan ultrasonic homogeniser Sonopuls 3200 (Bandelin) equipped with atitanium horn operating for 1 h at a frequency of 20 Hz and an energyconversion of 100 kJ.

3. Results and discussion

3.1. Surface composition of pristine Ti5Si3 particles

Titanium silicides (TiSi2 or Ti5Si3) with atomic volumes comparableto those of close-packed unit cells of refractory transition metals exhibithigh binding energies [24] and their surface is covered by a ca. 4 nmthick film of native oxide composed of TiO2, TiO and SiO2 in the givenratios 25: 1: 15 [25] without being known whether this film is a se-quence of layers or an intimate mixture of oxides. (For comparison, theincipient air-formed oxide layer on titanium (composed of outmostportion of TiO2 and inner portion of TiO and Ti2O3, [26]) and air- [27]or ultrapure water- [27,28] grown oxide layer on silicon (with differentproperties and solubility in water) [28] are respectively 8 nm and1–1.5 nm in thickness.)

The surface layer properties in Ti5Si3 specimens can, however, alterdue to slight variation in alloy composition (small amounts of silicon-rich Ti5Si4 or TiSi2 phases) and alloy synthesis methods employed indifferent studies. In view of this fact, thorough analyses of the structureof each pristine Ti5Si3 samples are required in order to avoid possiblediscrepancies in future research of behaviour of Ti5Si3 in liquid media.Reaching this objective, XPS, XRD and SEM characterizations of surfaceof the pristine Ti5Si3 specimens are required.

As for the pristine particles, the Ti 2p and Si 2p XP spectra of thetopmost layer (Fig. 1) confirm the existence of the native oxide layer[25]. The Ti 2p spectrum shows two narrow bands typical for Ti4+

bonded to oxygen and the Ti 2p3/2 band centred at 458.5 eV, which istypical for TiO2. The Si 2p spectrum has a major band centred at102.0 eV (typical for Si-O bonds) and much weaker band at 98.0 eV ofSi in Ti silicide. The TiO2 oxide band almost disappears after short Arion sputtering and the spectra of unveiled surface show, apart from theminor peaks of Ti2+, Ti3+ and Ti4+ states, the prevalence of peaks of Tiat 453.9 eV and of Si 2p at 98.2 eV related to the alloying elements. TheXPS-derived Si/Ti atomic ratios of layers in depths< 500 nm vary inthe range 0.93–0.85 and are notably higher than 0.6, which is obviously

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due to some content of equally diffracting Ti5Si4 and TiSi2 in the outerlayers of the pristine sample (as later observed by XRD). Indeed, the Si/Ti ratios assessed from the SEM (Fig. 2) and EDX analysis on 20 –400 μm2 areas are 0.54–0.57 and confirm that the composition in ca.1 μm depth is quite close to the Ti5Si3 stoichiometry.

3.2. Corrosion of Ti5Si3 particles with acidified H2O2

The chemical treatment of black Ti5Si3 particles with the acidifiedH2O2 solution at 100° C and at room temperature leads to yellow col-oration of the initially transparent aqueous phase and different col-oration of the particles.

The corrosion with the low amount of acidified H2O2 solution at100° C yields grey particles, while the treatment at room temperatureyields grey particles along with tiny amounts of white particles floating

on the surface of the aqueous phase. The corrosion with the highamount of the acidified H2O2 solution at 100° C allows formation ofpale (lighter) particles accompanied with low amounts of grey particles.It can be therefore reasoned that grey particles are intermediates fromthose pale particles develop through further reaction steps with H2O2.

The UV–Vis spectra of the yellow aqueous phase formed at roomtemperature and at 100° C are the same as that of a yellow solutionproduced by the same HCl/H2O2 treatment of metallic titanium sheet(Fig. 3), which implies that the coloration is due to heterogeneouscatalytic decomposition of H2O2 on titanium centers and Ti-H2O2 redoxreactions yielding [29-31] intermediary Ti-peroxy radicals, Ti(IV) andTi(III) oxides, and soluble Ti-peroxy complexes. Such complexes areknown to be formed by H2O2 chemisorption to TiO2 [32] and they showhigh absorption in UV region near 250 nm and lower absorption around420 nm when heated above 100° C due to formation of hydrous

Fig. 1. Ti 2p (a,b) and Si 2p (c,d) XP spectra of Ti5Si3 particles as such (a, c) and as Ar ion-sputtered (b, d).

Fig. 2. SEM and mapping analysis of pristine Ti5Si3 particles.

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titanium gel TiOOH matrix (aqua gel) [33]. The bands observed at414 nm and 402 nm (Fig. 3) therefore relate to Ti-peroxy complexesformed from titanium and Ti5Si3. The absorption tail above 500 nm(Fig. 3 b) suggests the presence of solid or soluble silicon-containingforms incorporated in the Ti-peroxy complex.

The Si and Ti content in the yellow aqueous phase assessed by EDX-SEM and XPS-analyses of the solid residue remaining on Al substrateafter evaporation of the yellow liquid phase reveals about 60 timesmore Si than Ti. The Si 2p binding energy (103 eV) of the residuecorresponds to Si-OH and the EDX-derived atomic O/Si ratios (3.0–3.6)indicate that the dissolved species is a hydrated oxygenated form ofsilicon, presumably hydrated silica. These findings therefore reveal thatSi is amply released from Ti5Si3 particles into the HCl/H2O2 solution,while Ti remains as an insoluble species on the surface of white, paleand grey particles.

3.3. Surface composition of corroded Ti5Si3 particles

3.3.1. XP spectraThe XPS sputter profile examinations of grey particles show that the

Si/Ti atomic ratios of layers in depths< 500 nm vary in the range0.49–0.52 and are somewhat lower than 0.6, which indicates that thedisposal to aqueous phase for silicon is greater than for titanium. The Ti2p and Si 2p spectra of grey particles show the presence of oxidizedtitanium and silicon in the topmost layer (Fig. 4). The Ti 2p spectrumcontains a major peak centred at 458.5 eV (typical for TiO2) along withpeaks at lower binding energies characteristic for Ti3+ (456.9 eV) andTi2+ (455.0 eV) states of Ti suboxides. These sub-oxides are usuallyobserved as minor peaks after sputtering of titanium dioxide but in oursample they occur at high (ca.35%) concentration. As to the Si 2pspectra, silicon is present as SiO2 and in Si-OH species (103.5 eV and101.7 eV respectively). After prolonged Ar ion sputtering, prevailingforms in the Ti 2p spectrum are titanium sub-oxides (60%) and those inthe Si 2p spectrum are silicon oxides along with an Si-OH form.

3.3.2. SEM-EDX analysisThe comparison of the SEM images of grey (Fig. 5) and pristine

Ti5Si3 particles (Fig. 2) reveals that the former particles have theirsurface cracked into 1 μm-thick plates and accommodate scarce or as-sembled half-micron-sized spheres. Neither the developed thin platesnor the attached spheres along with still smaller protrusions arisingfrom the treated surface change their appearance after high-tempera-ture annealing under vacuum. The particles are smaller and larger than44 μm, which implies that the pristine Ti5Si3 particles (-325 mesh) werenot only HCl/H2O2-etched, but also driven to merge into larger bodiesafter etching. The stoichiometries for a number of 10–200 μm-sizedparticles reveal oxidized Ti-rich (TiSi 0.10-0.35O4.30-4.70) and oxidized Si-rich (SiTi0.49-0.88O3.91-4.79) spots which are apparently affected by dif-ferent amounts of sub-μm-sized spheres deposited on the measuredlocations. These patterns of EDX elemental mappings showing hetero-geneous distribution of oxidized Ti-rich and Si-rich spots are also ob-served at high temperatures.

The SEM images of the pale particles (Fig. 6) produced with the high

Fig. 3. UV–Vis spectra of yellow solutions obtained by HCl/H2O2 treatment oftitanium sheet (a) and Ti5Si3 particles (b) carried out at 100° C. (The identicalspectra observed with the room temperature treatments are omitted.) (For in-terpretation of the references to colour in this figure legend, the reader is re-ferred to the web version of this article.)

Fig. 4. Ti 2p (a,b) and Si 2p (c,d) XP spectra of grey particles before (a, c) and after (b, d) Ar ion-sputtering.

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amount of HCl/H2O2 solution show only particles smaller than 20 μmand imply that pristine particles were etched to smaller sizes. Fur-thermore, the half-μm-sized spheres are not developed, but a porousnetwork is formed instead. The EDX elemental analysis and elementalmapping of a number of μm-sized locations confirm heterogeneousdistribution of micron-sized spots with an excess of Ti over Si (stoi-chiometries TiSi0.10-0.24O3.29-4.54) and an excess of Si over Ti (SiTi0.35-0.94O4.10-4,30). These two different sets of stoichiometries resemble thoseobserved for grey particles.

3.3.3. FTIR spectraThe FTIR spectra of grey particles formed at room temperature and

obtained at 100° C differ from those of these particles annealed at 400

and 800° C (Fig. 7). The former show absorption bands at 690 cm−1,904 cm−1, 1080 cm−1 (with a shoulder descending behind1200 cm−1), 1400 cm−1, 1626 cm−1 and a broad absorption centred at3300 cm−1, whereas the latter show only the 1080 cm−1 band with theshoulder and absorption between 1000 cm−1 and 500 cm−1. We notethat the band at 690 and 898 cm−1 were observed in yellow Ti aerogels(formed from Ti peroxo-complex by hydro-treatment and solvent ex-change [30,33]) and the latter band can be assigned [34] as a free orcomplex stretch of -O-O- bond. The observed 904 cm−1 band is as-signable to the peroxo-bond, but may also correspond to Ti-O-Si stretch,since absorption at 910–960 cm−1 is widely accepted [35-37] as thecharacteristic vibration of Ti-O-Si bonds. It is seen that this band dis-appears at 400° C. As for the bands at 1080, 1626, 1400 and

Fig. 5. SEM images and EDX mapping of grey particles before (a) and after annealing to 400° C (b), 800° C (c) and 1000° C (d).

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3300 cm−1, they are, in the given order, assignable to asymmetric ν(Si-O-Si) stretching vibration of the Si-O skeleton in silica [38], –O–Hbending vibration and –OH stretching vibration of physisorbed and/orchemisorbed water. The large shoulder of the 1080 cm−1 band is anindication of some scattering of the normal incident IR radiation withina porous framework (see below).

The FTIR spectrum of pale particles (Fig. 8) shows bands at1621 cm−1 (-O–H bending vibration), 1060–1080 cm−1 (asymmetricν(Si-O-Si) stretching vibration), 894 and 685 cm−1 (vibrations of Ti-O-O bonds) and 474 cm−1 (Si-O bending mode) but not the band at904 cm−1 (Ti-O-Si stretch) which is characteristic of grey particles.

The FTIR spectra of white particles (Fig. 9) exert major bands

Fig. 6. SEM images (a-c) and elemental mapping (d) of pale particles.

Fig. 7. FTIR spectra of grey particles formed at room temperature (a) and 100°C (b) and further annealed to 400 °C (c) and 800 °C (d). Fig. 8. FTIR spectrum of pale particles.

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located at 504, 628, 1148 and 1205 cm−1 and a very weak band at1625 cm−1 of molecularly physisorbed water. The former two bandsare assignable to a vibrational mode of Ti–O bonds in TiO2 lattice [39],while the other two are a signature of a silica in a highly porous en-vironment. It is known [40,41] that the transverse-optical (TO) vibra-tional modes observed in normal-incidence absorption spectra of siliconoxides correspond to stretching, bending, and rocking motions of the Si-O-Si skeleton and are usually found near 1020–1080 cm−1, 800 cm−1

and 460 cm−1. However, the strongest TO stretching band can alsohave a high wavenumber shoulder associated with an intense

longitudinal-optic (LO) band in oblique-incidence absorption spectra[38]. Such shoulders are observed in thin sol–gel silica films and theirintensity increases with films porosity due to scattering of the normalincident IR radiation within the pores and the subsequent activation ofthe LO modes [42].

3.3.4. Raman spectraThe Raman spectra of grey particles formed at room temperature

and 100° C and those annealed at 400 and 800° C reveal differentscattering patterns (Fig. 10). The particles treated at room temperatureand 100° C exhibit a high-intensity band at 700 cm−1 along with bandsat 504 (or 504 and 454) and 282 cm−1 assignable [43] to Ti2O3; a weakband at 892 cm−1 is assignable [34] to free or complexed –O–O– vi-brations. The particles annealed at 400 and 800° C show bands at 151(or 148), 396, 518 and 635 (or 638) cm−1 assignable to anatase [44].Further annealing to 1000° C results in the formation of rutile as in-dicated by bands at 247, 440 and 609 cm−1. The spectra of the greyparticles annealed at 400° C (Fig. 10 b) and 800° C (Fig. 10 c) slightlydiffer. The more intense bands observed after heating to the highertemperature relate to anatase and the less intense and broader bands at635 and 151 cm−1 observed at the lower temperature are obviouslyrelated to the onset of anatase formation.

It is known that the persistence of anatase at 800° C is not commonfor bulk samples but typical for nano-sized particles. Fine micron-sizedparticles of high purity normally show phase transformation around600–1000° C, but the transition temperature is affected [44] by dopantsand admixtures. We therefore presume that anatase→rutile transitionwith the heated grey particles is retarded by SiOx nanophases and canbe due to Si4+ substitutionally entering the anatase lattice or inhibitingdiffusion by reducing anatase inter particle contacts as a grainboundary phase [45].

Fig. 9. FTIR spectrum of white particles formed at room temperature.

Fig. 10. Raman spectra of grey particles formed at room temperature (a),100 °C (b) and annealed to 400 °C (c), 800 °C (d) and 1000 °C (e).

Fig. 11. Raman spectra of white low-density particles formed by Ti5Si3 treat-ment with HCl/H2O2 at room temperature.

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It is therefore possible that the band at 700 cm−1 relates to anamorphous precursor of anatase which can be a blend of an amorphousTiOx network (involving Ti2O3) and minor amounts of SiOx species asidentified with the FTIR spectra. However, the assignment of this bandto some specific structure is not straightforward, despite that such bandhas been observed with titanium silicalite [46,47] or with Ti-peroxy gel

formed at surface of Ti metal by action of hydrogen peroxide [48]. Anassignment to a vibration of crystalline [46] or amorphous [47] hex-acoordinated Ti–O–Ti linkages of titanium silicalite is unlikely, becausethese compounds show scattering at 700 cm−1 only with UV- and notvisible Raman spectra. Neither tentative assignment to metatitanic acidH2TiO3, which in fact has [49] a bifurcated band at 660 and 730 cm−1,is acceptable.

The Raman spectrum of pale particles shows scattering bands ob-served for grey particles and is therefore not shown. The spectrum ofwhite particles (Fig. 11) reveals bands at 286, 384, 516, 691, 731 and906 cm−1. The three main bands are located at wavenumbers which arevery close to those of bands observed in Raman spectra of grey particles(Fig. 10 a, b) and indicate TiOx network (involving Ti2O3) and free orcomplexed –O–O– vibrations. We admit that Si-O-Si bonds are notRaman active and that the band at 516 cm−1 may also reflect a smallcontribution of crystalline silicon in amorphous silica matrix [50]. Thebifurcated band at 691 and 731 cm−1 somewhat resembles band ofmetatitanic acid [48].

3.3.5. High-resolution TEM analysisDetails on the nano-structure of grey, pale and white particles were

obtained from TEM images, selected area electron diffraction (SAED)and energy dispersive x-ray spectroscopy (EDS).

As for grey particles, the EDS-derived compositions on different nm-sized areas (Fig. 12) reveal a variety of nanophases (TiO0.75-1.57Si0.01and SiO0.53-2.00 Ti 0.01-0.08) corresponding to (i) SiOx (x ≤ 2) structureswith tiny contents of Ti and (ii) TiOx (x < 2) structures with tinyamounts of Si (e.g. Table 1). The SiOx nanophases identified on outmostflat layers of treated Ti5Si3 particles (locations 3,6,7) prove oxidation ofsilicon, whereas the TiOx nanophases (locations 1,2,4,5) resemblingcabbage-like nanospheres cover the outmost flat layers and are ex-clusive products of titanium oxidation. The SAED found for nano-cab-bages is consistent with the cubic TiO (Pdf 652900), whereas SAED ofTiOx structures with tiny amounts of Si and that of SiOx structures withtiny amounts of Ti are corresponding to amorphous entities.

The annealing of grey particles to 400° C confirms no change inHRTEM image of nano-cabbages on SiOx outerlayer, but shows acomplete amorphization of the sample. Further heating to 800° C af-fords anatase (mostly ca. 20 nm-sized) particles which are dispersed orinterconnected but not merged to larger bodies. This nano-dimensionalconstraint is obviously caused by the presence of another (SiOx) phasewhich prevents these nanoparticles from coalescing in the course oftheir birth from the amorphous anatase precursor.

As for white particles (Fig. 13), they show morphology of agglom-erated nanoobjects and diffuse continuous rings indicating the dom-inance of amorphous phase. A number of EDS stoichiometries (TiO1.00-

1.61Si0.02-0.04 and SiO 1.05-2.00Ti0.01-0.15) reveal (Table 2) that theseparticles are mostly composed of adjacent SiOx and TiOx nanosizedentities. A diffuse rings pattern is tentatively assigned to titanium acidH2Ti2O5·H2O

As for pale particles, HRTEM images, ED and EDS-derived compo-sitions of different nm-sized areas (Fig. 14, Table 3) show amorphousregions manifested by hazy ring diffraction patterns, nanophases cor-responding to binary TiOx structures and Ti/O/Si structures rich in Siand also small flakes which form larger bodies composed of disorderednano-plates having porous regions (Fig. 15 c). Similar flakes havingbeen observed for grey particles (Fig. 13c) make us believe that theseentities are primary products of the corrosions with low and highamounts of HCl/H2O2 solutions.

3.3.6. BET, XRD and size distribution analysesMore understanding of the structure of pale and grey particles is

gained from their texture analyses (Table 4, Fig. 15) showing highvalues of the BET surface area (SBET), the mesopore surface area (Smeso)and the total pore volume (Vtotal) but very low values of the microporevolume. The lower SBET, Smeso and Vtotal values for grey particles, as

Fig. 12. HRTEM analysis of grey particles obtained at 100° C (EDS-analyzedlocations of cabbage-like (a) and cabbage/flat (b) morphologies, enlargedcabbage morphology (c) and SAED (d) assigned to TiO; HRTEM analysis ofparticles annealed to 400° C (SAED and morphologies, e) and at 800° C (mor-phology, EDS, image of nanograins (f) and SAED (g) assigned to anatase).

Table 1EDS-derived stochiometries at different nanosized locations of grey particles.

Location Stoichiometry Location Stoichiometry

1 TiO1.39Si0.01 5 TiO0.75

2 TiOSi0.01 6 SiO2Ti0.013 SiO2Ti0.02 7 SiO1.45 Ti0.024 TiO1.57 Si0.01 * SiO0.53Ti 0.08

* SiO1.73Ti 0.13 * SiO1.45Ti 0.02

*other locations

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compared to those of pale particles, are due to the non-porous (un-reacted) Ti5Si3 interior of these particles. The pore size distribution ofpale particles shows abundant mesopores with radii up to 50 nm andalso macropores with larger radii. The wide pore size distribution seenwith both pale and grey particles is likely linked to the coexistence ofnanoparticles with different morphologies formed in the course of thedevelopment of SiOx and TiOx species.

The size distribution of pale particles suspended in water was ex-plored by the dynamic light scattering (Fig. 16) and shows a peak ataround 4 μm after suspension shaken, a peak at 890 nm after 1 h so-nication, and a peak at 260 nm after 24 h sonication (not shown). Thesedimensions are much smaller than 20 µm as measured by the SEManalysis and we assume that these particles grow to larger bodies uponevaporation of water. However, upon no sonication, the smaller parti-cles agglomerate to 4 μm particles. Repetitive measurements confirmedthat the processes of spontaneous agglomeration and ultrasound in-duced disintegration are reversible. We suggest that the initial size ofpale particle is 170 nm, since only such particles along with minor1.2 μm particles were observed upon centrifugation of the suspensionsonicated for 24 h. Zeta potential (- 15 mV) of pale particles in deio-nized water reveals their negative surface charge and hence their in-cipient instability which agrees with their agglomerating behaviour.

Such value is often observed with TiO2 nanoparticles at pH higher thantheir pHpzc and ascribed to TiO2 participation in the acid-base (par-ticle + H2O → particle-OH- + H+) interaction.

These findings can be reconciled with the X-ray diffraction analysesof grey, white and pale particles which reveal (Fig. 17) only diffusepeaks at 2θ = 12.298° and 18.044°. We assume that the diffuse peakobserved with all these particles can be assigned to a micropores-con-taining mesoporous [51-54] Ti/Si/O framework formed by self-as-sembly of amorphous TiOx- and SiOx-based nanoparticles. The XRDanalyses also confirm formation of anatase upon annealing grey parti-cles to 800° C and an anatase → rutile transformation at 1000° C(Fig. 18).

3.4. Corrosion paths on the surface of Ti5Si3 particles inferred from theobserved products

The above data enable us to clarify reactions which take place onthe surface of the Ti5Si3 particles in the acidified H2O2 liquid phase. Thetitanium silicide must first alter the nature of its native oxide layer(composed [25] of TiO2, TiO and SiO2) via (i) formation and dissolutionof titanium peroxo complex [32] and (ii) dissolution of non-reactivesilica into [55,56] monosilicic acid. This acid is known to be in equi-librium with silica gel and colloidal silica which can reversibly in-corporate H2O2 [57-59]. These reactions should take place not onlywith the superficial TiO2 layer but also with TiO layers beneath theTiO2 layer. Reactions with both titanium and silicon centres after ex-posing titanium silicide surface take place next and are specified below.

The reaction with titanium is a heterogeneous catalytic decom-position of H2O2, which produces soluble intermediates and complexesvia Ti-H2O2 redox reactions and may involve HO2

–, HO2. and HO.

transients [29,31]. The reaction with silicon (examined with hydrogen-terminated Si surfaces) is [60-62] a slow insertion of oxygen atoms tothe back bonds of the topmost silicon atoms. We speculate that anotheroxidation mechanism of silicon can involve reaction of the Ti-adjacentSi centre with highly reactive OH. radical transiently produced in Ti-

Fig. 13. Typical SAED (a, d, e) and morphology patterns with EDS locations (b,c) of white particles.

Table 2Some EDS-derived stoichiometries at different nanosized locations of whiteparticles.

Location Stoichiometry Location Stoichiometry

1 TiO1.37Si0.04 8 SiO1.05 Ti0.082 TiOSi0.02 9 SiO1.05Ti0.153 TiO1.58Si0.06 10 SiO2.0Ti 0.04

4 SiO1.53 Ti0.06 11 TiO1.45Si0.045 SiO1.73Ti 0.13 12 SiO1.96Ti 0.04

6 SiO1.32Ti0.03 13 SiO2.0Ti 0.12

7 SiO2Ti0.01 14 TiO1.61Si0.04

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catalyzed H2O2 decomposition to H2O and O2.

4. The specific features of corrosion of the exposed Ti5Si3 surface.

The identification of the μm-sized and nano-sized SiOx (x ≤ 2)

structures and that of the TiOx (x < 2) structures prove early nano-scale segregation of Ti- and Si-oxidized products. This early pathsprecede the partial release of Si into the HCl/H2O2 solution and theformation of insoluble (isolated or assembled) porous sub-μm sizedTiOx-based cabbage-like spheres, larger porous TiOx-based networks

Fig. 14. Pale particles: typical SAED (a), morphology of initial (b, c) and developed (d, e) particle with EDS-locations.

Table 3EDS-derived stoichiometries at different nanosized locations of pale particles.

Location Stoichiometry Location Stoichiometry

1 TiO2.18 3 SiO0.13Ti0.022 TiO3.12Si0.26 4 TiO0.40

another location TiO2.77

Fig. 15. Pore-size distribution from adsorption branch of nitrogen sorption isotherm for pale (a) and grey (b) particles.

Table 4Texture characteristics of corroded and pristine Ti5Si3 particles.

Sample SBET (m2/g) Smeso (m2/g) Vtotal (mm3/g Vmicro (mm3/g

pale particles 150 75 174 39grey particles 118 39 75 39Ti5Si3 particles 0.5 0.4 1.8 0.8

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and nano- up to micron-sized SiOx patches on Ti5Si3 surface. The na-nosized blend of TiOx network and SiOx species is the intermediaryamorphous precursor of the crystalline anatase grains formed at theelevated temperatures. The major paths of the corrosion process(Fig. 19) are summarized below.

They involve

(i) precipitation of nano-TiOx particles agglomerating to larger TiOx-based networks and/or into spherical particles to minimize theirsurface area,

(ii) dissolution of SiOx and SiO2 nanoproducts giving rise to mono-silicic acid establishing [56,57] solubility equilibrium with silicagel and colloidal silica,

(iii) incorporation of hydrated SiOx species into porous TiOx-basedspheres and TiOx-based networks and

(iv) agglomerization of nano SiOx species to larger bodies which be-come accommodated on Ti5Si3 surface.

The reasons for such conclusions are to be re-emphasized in moredetail for corrosion with low and high amounts of HCl/H2O2 solution.

5. Corrosion with low amount of HCl/H2O2 solution - grey andwhite particles

The LO modes of Si-O vibration in oblique-incidence infrared ab-sorption spectra are in line with the TiOx-based porous structures whichwere also confirmed by BET and XRD analyses. The shoulder of theinfrared 1080 cm−1 band (indicating scattering of the normal incidentIR radiation on SiO2 within a porous framework) supports the view thatanatase grains observed on the surface of the heated grey particles areformed from the TiOx-based spheres whose porous network involvesTiO, Ti2O3 and Ti-OO-Ti segments (proved by EDS, Raman and IRspectra) and incorporates tiny amounts of adsorbed SiO2 species. Thedepletion at 400° C of the infrared band at 904 cm−1 (ascribed to theperoxo and/or Ti-O-Si bonds) is brought about by oxidation of TiOx

structures to anatase and by cleavage of the Ti-O-Si bonds, the latterinducing segregation of SiOx and TiOx moieties.

6. Corrosion with high amount of HCl/H2O2 solution – Paleparticles

Under these conditions, porous bodies larger than sub-μm sizedTiOx-based cabbage-like spheres were identified by TEM and theirstoichiometry on nanosized scale reveals binary Ti/O phases and Si-richSi/Ti/O phases. These larger bodies therefore show heterogeneousdistribution of micron-sized spots of two different sets of stoichiome-tries (excess of Ti over Si and excess of Si over Ti) and resemble theSEM-EDX-derived composition of grey particles produced with lowamount of acidified H2O2. Also the Raman spectra match those of greyparticles and so do the IR spectra which differ only due to the absenceof the band at 904 cm−1 (Ti-O-Si stretch) observed with grey particles.These similarities reveal that both pale and grey particles originatefrom the same initial structure composed of disordered nano-plates(Fig. 12c and 14b) and that the development of the larger porous bodiesreflects a growth of these initial structures in the HCl/H2O2 solutionthrough agglomeration of nanosized particles. Such growth is obviouslyinhibited under conditions of limited supply of H2O2 when the initialstructures can only grow to small cabbage-like spheres.

Approaching the end of this section we remark that the develop-ment of the nanosized grains of anatase and rutile upon heating the

Fig. 16. Size distribution analysis of pale particles in initial shaken suspension(green curve), 1 h sonicated suspension (ocher curve) and 24 h sonicated andcentrifuged suspension (red curve). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of thisarticle.)

Fig. 17. XRD pattern of pristine sample (a) with diffraction lines of Ti5Si3 and Ti5Si4 (marked by black dots), compared with patterns of grey particles obtained atroom temperature (b) and pale (c) and white (d) particles. Assignment: titanium silicide Ti5Si3, Pdf# 29–1362; titanium silicide Ti5Si4, Pdf# 23–1079.

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corroded Ti5Si3 particles to temperatures higher than 400° C can beaccompanied by diffusion-controlled 2 SiOx → Si + SiO2x dis-proportionation and reactions between TiO and SiOx entities [63,64]leading to TiO2 and SiOx-1, which are induced by reducing action[65,66] of TiO. We also conclude that the reported corrosion process israther peculiar. It does produce a passive TiOx/SiO2 coat but does notcease in excess of the HCl/H2O2 solution, which is due to SiO2 being notstrongly attached to the treated Ti5Si3 surface but partly dissolving inthe liquid phase and thereby exposing TiOx-enriched Ti5Si3 surfacetowards extended corrosion by next amounts of acidified H2O2.

The reported results thus reveal the details on corrosion of Ti5Si3 inthe acidic H2O2 solution and will be useful in our next studies of os-seointegration behaviour of HCl/H2O2-treated Ti5Si3 coat. They alsoadd to the earlier examinations of titanium silicides exposed to harshenvironments (Table 5).

7. Conclusions

Corrosion of titanium silicide with acidified hydrogen peroxide re-sults in segregation of nanosized amorphous SiOx (x ≤ 2) and TiOx

(x < 2) species and subsequent partial dissolution of SiOx species andprecipitation of nano-TiOx particles which agglomerate to larger porousbodies and incorporate nanosizied SiOx entities.

The appearance of the agglomerated TiOx bodies depends on theprogress of the corrosion. Less progressed corrosion yields sub-μm-sizedTiOx-based spheres, whereas more progressed corrosion gives rise tolarger mesoporous TiOx-based bodies which disintegrate upon sonica-tion into micrometer-sized entities.

The crystalline structure of inner layers of corroded Ti5Si3 particlesbecomes amorphous at 400° C.

The Ti-O-Si and -O-O- bonds identified in both porous spheres andlarger porous bodies decompose upon heating and allow formation ofanatase nanograins withstanding 800° C. These nanograins developfrom a TiOx network which incorporates SiOx species.

The one-step production of the two different but both bioactive(hydrated TiOx and SiOx) species appears promising for application insynthesis of bioconductive coats and will be used in our osseointegra-tion studies of slightly corroded Ti5Si3-coated titanium surfaces, whichare in progress.

Fig. 18. XRD pattern of pristine sample (Ti5Si3and Ti5Si4 marked by black dots), compared withpatterns of grey particles obtained at 100 °C andsubsequently annealed to 400 °C , 800 °C, and1000 °C. Assignment: titanium silicide Ti5Si3,Pdf# 29–1362; titanium silicide Ti5Si4, Pdf#23–1079; anatase, syn Ti0.72O2 Pdf# 21–1272;rutile, syn Ti0.992O2 Pdf#86–0148.

Fig. 19. Scheme of Ti5Si3 corrosion.

Table 5Corrosion of titanium silicides.

Corrosion conditions Titanium silicides Ti-Si performance Ref.

Alkaline hydrothermal treatment Ti5-Si alloy Porous surface of Na/Ti hydrogel, Na titanate, precipitation of a-TiO2 layer,bioactivity studies

15

Acidic treatment Ti5Si3 In vivo studies, bone bonding behaviour 16Acidic electrochemical and autoclave treatment Ti-Si alloys − 2, 4, 6, 8.5, 11Si wt.

%Accelerated corrosion, formation of passive TiO2/SiO2 films 17

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CRediT authorship contribution statement

V. Jandová: Formal analysis, Data curation. R. Fajgar: Formalanalysis, Data curation. J. Kupčík: Formal analysis, Data curation. J.Pola: Investigation, Conceptualization, Methodology. K. Soukup:Formal analysis, Data curation. P. Mikysek: Formal analysis, Datacuration. T. Křenek: Project administration, Investigation. T. Kovářík:Data curation, Investigation. T. Stich: Formal analysis, Data curation.D. Docheva: Funding acquisition, Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgements

This work was supported by the project n. 201 ”Advanced porousbiomaterials functionalized with stem cells for enhancing of osseoin-tegration of implants: MATEGRA” realised within the frame of theProgram IN-TERREG V-A: Cross-border cooperation between the CzechRepublic and the Federal State of Germany Bavaria, Aim EuropeanCross-border cooperation 2014-2020. The XRD analyses were con-ducted under institutional support from the Institute of Geology of theCzech Academy of Sciences (RVO67985831).

Data availability statement

The raw/processed data required to reproduce these findings cannotbe shared at this time due to legal or ethical reasons.

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