Ceramics International 42 (2016) 15767–15772

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Enhanced reactivity of peroxo-modified surface of titanium dioxidenanoparticles used to synthesize ultrafine bismuth titanate powders atlower temperatures

P. Francatto a, F.N. Souza Neto a, A.E. Nogueira b, A.M. Kubo a, L.S. Ribeiro a, L.P. Gonçalves a,L.F. Gorup a, E.R. Leite a, E.R. Camargo a,n

a LIEC – Interdisciplinary Laboratory of Electrochemistry and Ceramics, Department of Chemistry, UFSCar – Federal University of São Carlos, Rod. WashingtonLuis km 235, São Carlos, SP 13565-905, Brazilb LNNA – National Laboratory of Nanotechnology for Agrobusiness, EMBRAPA – Brazilian Agricultural Research Corporation, Rua XV de Novembro 1452, SãoCarlos, SP 13560-970, Brazil

a r t i c l e i n f o

Article history:Received 6 June 2016Received in revised form1 July 2016Accepted 7 July 2016Available online 11 July 2016

Keywords:NanopowdersWet-chemical synthesisReactive powderHydrogen peroxide

x.doi.org/10.1016/j.ceramint.2016.07.03942/& 2016 Elsevier Ltd and Techna Group S.r

esponding author.ail address: camargo@ufscar.br (E.R. Camargo)

a b s t r a c t

Bismuth titanate with sillenite structure (Bi12TiO20) was prepared at lower temperatures and shortertimes using a modified oxidant peroxide method (OPM). Bi12TiO20 was synthesized utilizing commercialBi2O3 and reactive titanium dioxide nanoparticles having peroxo-modified surfaces. Rather than de-pending on particle size, the reaction mechanism is related to the highly exothermic decomposition ofperoxo groups, regardless the titanium source used, which locally releases a large amount of energy thatcan accelerate the reaction, similar to self-propagating high temperature routes (SHS).

& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

1. Introduction

There is a growing need for faster and more efficient methodsfor synthesizing technologically important materials. In the case ofnanomaterials, their physical and chemical properties are depen-dent on their composition and morphology [1], which makes thesynthetic route a critical decision. In this context, wet-chemicalroutes are superior to traditional solid-state reactions [2–4]. Un-fortunately, these wet methods are tedious, time consuming andhave low batch yields. On the other hand, the synthesis of multi-component oxides through the simple heating of a mixture ofoxides or carbonates require additional treatment at high tem-peratures for long periods to overcome kinetic barriers [5,6],which usually results in large particles of irregular shape, andsecondary phases in some instances [7–9]. To solve this problem,new approaches have been proposed that combine the robustnessof solid-state reactions with the chemical design of wet-chemicalroutes, with special attention being paid to self-propagating hightemperature routes (SHS) [6,10–14], partial oxalate methods [15],and the oxidant peroxide method (OPM) [4,16].

Bismuth titanates with different composition and structures

.l. All rights reserved.

.

have attracted considerable interest due their excellent optical,electrical and catalytic properties [17]. For instance, Bi4Ti3O12 haspotential use in photocatalysis [18] and is a promising ferroelectricmaterial for high-temperature applications [19], while nanos-tructured Bi12TiO20 shows superior photocatalytic performance[20–22].

In this study, we prepared pure Bi12TiO20 nanoparticles (re-ferred to as BT) as a model compound to evaluate this new OPMapproach, which begins with the synthesis of reactive nano-particles of titanium dioxide modified with peroxo groups on thesurface using titanium isopropoxide or titanium metal as pre-cursor. In a second step, these reactive nanoparticles were mixedwith commercial bismuth oxide, resulting in single-phase BT atlower temperatures and shorter reaction times than conventionalsolid-state reactions.

2. Materials and method

2.1. Synthesis of reactive titanium dioxide

Titanium dioxide nanoparticles with peroxo-modified surfaceswere synthesized from a yellow gel obtained by heating solutionsof peroxo complexes of titanium. In a typical procedure, 250 mg oftitanium metal (98% Aldrich, USA) or 5 mL of titanium

Fig. 1. (a) XRD patterns of TiO2-Met, TiO2-Iso and TiO2-Red reactive precursors, in which crystalline structure and particle size were maintained even after removing theperoxo groups from the surface using H2. (b) TEM image 5 nm TiO2-Iso crystalline particles, and (c) XPS profiles of O1s region of TiO2-Met and TiO2-Red showing theelimination of peroxo groups by H2 treatment.

Fig. 2. XRD patterns of three different mixtures of Bi2O3 and titanium precursors(TiO2-Met, TiO2-Red and TiO2-Com) calcined at 600° C for 1 h. Note the absence ofany secondary phases in the TiO2-Met pattern and the presence of unreacted Bi2O3

in the TiO2-Com and TiO2-Red patterns. Arrows in the pattern of TiO2-Com indicatethe presence of Bi12TiO20.

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isopropoxide (97% Aldrich, USA) were added to 100 mL of anaqueous solution of hydrogen peroxide (60 mL, 30% Synth, Brazil)and ammonia (40 mL, 28% Synth Brazil). This mixture was left inan ice-water cooling bath until complete dissolution of the

titanium precursors (i.e. several hours for the metal and a fewminutes for the isopropoxide), which resulted in a transparentyellow aqueous solution of the soluble peroxytitanate ion [Ti(OH)3O2]� [4,16,20]. In order to obtain titanium dioxide nano-particles covered by peroxo groups, the solutions were heated to80 °C until a yellow gel formed, and was held at this temperaturefor several hours to form yellow powders of TiO2-Met (from tita-nium metal) or TiO2-Iso (from titanium isopropoxide). The controlmaterial was obtained by heating TiO2-Met at 250 °C for 30 minunder H2, which resulted in a white powder of titanium dioxide(TiO2-Red) with reduced surfaces free of peroxo groups, but withthe same morphological and structural properties of the originalyellow nanoparticles covered with peroxo groups.

2.2. Synthesis of bismuth titanate

Analytical grade Bi2O3 (99.99% Aldrich, USA), commercial TiO2

anatase (99% Alfa Aesar; TiO2-Com) and the as-prepared TiO2-Met,TiO2-Iso, and TiO2-Red were used to prepare bismuth titanate witha selenite phase structure (Bi12TiO20) through a solid-state reac-tion. Appropriate amounts of the constituent oxides were weighedand mixed in a conventional ball-mill for 24 h using small zirconiaspheres and ethanol as milling media. The slurries were dried at60 °C and sieved through 120-mesh sieves. The mixtures werecalcinated at different temperatures for 1 h at a heating rate of10 °C min�1 in closed alumina boats.

Fig. 3. XRD patterns of BT synthesized with (a) TiO2-Met and (b) TiO2-Iso reactive titanium dioxide calcined at temperatures varying from 500 to 800 °C for 1 h. Both series ofXRD patterns show crystalline Bi12TiO20 powders when calcined at 600 °C or higher. Samples calcined at 500 °C show unreacted Bi2O3 and of Bi4Ti3O7 (see SupportingInformation for phase assignments).

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2.3. Characterization

All powders were characterized at room temperature by X-raydiffraction (XRD) using Cu Kα radiation (Rigaku D/MAX 200 with arotary anode operating at 150 kV and 40 mA) in the 2θ range from15° to 75°, with a step scan of 0.02°. Powder crystalline structurewas characterized by transmission electron microscopy (TEM-FEI/PHILIPS CM120) and by scanning electron microscopy (SEM-FEG,ZEISS model-SUPRA 35). XPS spectra were collected in a XPS VGMicrotech ESCA3000 (MgKα and AlKα radiations) operating at3�10�10 mbar of pressure. Binding energies were corrected forthe charging effect, assuming a constant binding energy for theadventitious O1s peak. Thermogravimetric analysis (TGA) werecarried out with a NETZSCH TG 209F1 and differential scatteringcalorimetry (DSC) measurements were done using a NETZSCH DSC204. All analysis were collected from 50 to 400 °C using a heatingrate of 10 °C min�1 and under nitrogen atmosphere.

3. Results and discussion

The key idea behind OPM is the substitution of hydrogen per-oxide by soluble peroxo complexes, which react with lead or bis-muth ions to form a stoichiometric amorphous precipitate thatcrystallizes in a desired oxide, free of any typical contaminants[4,16]. Peroxo complexes of titanium are commonly prepared byreacting titanium metal with hydrogen peroxide and ammonia [4].

Although this technique has successfully yielded a series of im-portant technological compounds [23–27], titanium metal is anexpensive and low reactive chemical that is an obstacle for large-scale OPM syntheses. For example, while approximately 12 h in anice bath is necessary to dissolve no more than 1 g of titaniummetal, a much larger amount of titanium peroxo complexes couldbe obtained at room temperature by diluting titanium isoprop-oxide in an aqueous solution of hydrogen peroxide and ammonia[28–31]. For this reason, there are ongoing efforts to find betteralternatives to titanium metal.

Titanium isopropoxide was chosen as a starting reagent in or-der to understand the role played by these reactive nanoparticlesduring the synthesis of complex oxides, and to minimize reagentcosts. Unlike most titanium compounds, titanium isopropoxide isrelatively safe and inexpensive, halide-free, and its residual alcoholis easily removed [28–31].

Solutions of titanium peroxo complexes are relatively unstableand spontaneously form yellow gels that are used to synthesizewhite nanoparticles of titanium dioxide after hydrothermal pro-cessing [28–31]. However, unlike previous studies, we preservedthe peroxo groups on the surface of the TiO2 nanoparticles,keeping the gel at 80 °C for a few hours under ambient pressure toobtain a fine yellow powder. For comparison, Fig. 1a shows theXRD patterns of two yellow reactive TiO2 powders synthesizedfrom different titanium sources. One of them (TiO2-Met) was ob-tained by means of traditional OPM using titanium metal, whilethe second (TiO2-Iso) was obtained using titanium isopropoxide.

Fig. 4. (a) DSC curves of TiO2-Iso and TiO2-Met samples. The exothermic decom-position of peroxo group occurs between 200 and 250 °C. In this temperaturerange, there is major weight loss rate as seen on the (b)–(c) TG and dTG curves.

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Both powders present a mixture of phases, mainly anatase andrutile, which can be observed by the shoulder at left side of thebroad diffraction peak at approximately 27°. However, it is inter-esting that the amount of each phase seems to be the same, re-gardless the reagent used. TEM image (Fig. 1b) confirmed crys-talline nanoparticles of approximately 5 nm in the sample syn-thesized with titanium isopropoxide.

A recurring question concerning the effect of particle sizeemerges during discussions about nanoparticle reactivity. To this

Fig. 5. Particle formation schematics, comparing (a) the traditional solid-state reactioncomposition releases enough energy to accelerate the reaction at lower temperatures.

end, we exposed a small amount of TiO2-Met to H2 for 30 min at250 °C in order to remove the peroxo groups from the surface ofnanoparticles. This yielded a white powder (TiO2-Red) with a XRDpattern identical to the patterns of our yellow powders (TiO2-Metand TiO2-Iso), indicating that their structure, morphology andparticle size were successfully preserved. XPS spectra collectedbefore and after H2 treatment (Fig. 1c) confirmed the eliminationof peroxo groups from the particle surfaces.

Fig. 1 We evaluated the reactivity of titanium dioxide nano-particles to form Bi12TiO20 by solid-state reactions using threedifferent precursors, (i) yellow TiO2-Met with peroxo-modifiedsurfaces, (ii) TiO2-Red with reduced surfaces, and (iii) commercialmicrometric titania (TiO2-Com) with anatase structure. Fig. 2shows XRD patterns after reacting these precursors with Bi2O3 at600 °C for 1 h. There are evident differences between the patternof pure BT (PDF 34-0097) obtained using reactive TiO2-Met na-noparticles with peroxo groups and the other two patterns ob-tained under the same conditions with white micrometric TiO2-Com or nanometric TiO2-Red nanoparticles. These two patternsshow unreacted monoclinic Bi2O3 (PDF 71-465) as the main phasewith Bi12TiO20 as a secondary phase (most intense peaks aremarked with arrows).

Fu and Ozoe [32] reported that BT synthesis using commercialBi2O3 and TiO2 is diffusion-controlled, and concluded that pureBi12TiO20 cannot be obtained by this method at temperatures be-low 780 °C, even after 800 h of reactive TiO2-Met (Fig. 2) at a re-latively low annealing temperature of 600 °C for 1 h, suggestingthat the peroxide surface may play an important role in the re-action mechanism.

Fig. 2 In principle, the surface of oxide nanoparticles are muchmore complex than metals due the presence of voluminous oxy-gen centers [33]. These surfaces are rich in defects or imperfec-tions that lead to charge redistribution [34]. Evidently, the pre-sence of peroxo groups bonded to the surface of TiO2 leads toenhanced reactivity relative to typical white TiO2.

To compare the chemical reactivity of both yellow titaniumdioxides (TiO2-Met and TiO2-Iso) covered with peroxo groups, wecalcined their mixtures with commercial bismuth oxide at differ-ent temperatures with the aim to synthesize Bi12TiO20 whilekeeping all other conditions fixed (Fig. 3). Both series of XRDpatterns showed crystalline Bi12TiO20 powders when calcined at600 °C or higher. The patterns were quite similar, including

with (b) reactions involving peroxo-modified surfaces. The local exothermic de-

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relative normalized intensities, indicating no differences in thechemical reactivity of yellow titanium dioxide powders preparedfrom titanium metal (TiO2-Met) or titanium isopropoxide (TiO2-Iso). Similar results could likely be achieved using other titaniumsources like chlorides or sulfates. Diffraction peaks were indexedaccording to the sillenite phase of bismuth titanate (PDF 34-0097)except for the samples calcined at 500 °C, which showed extrapeaks related to unreacted monoclinic Bi2O3 (PDF 71-465) and thepresence of a Bi4Ti3O7 (PDF 35-0795) metastable phase (see Sup-porting Information).

Much attention has been paid to the synthesis of white TiO2

colloids using hydrogen peroxide to control their structure andmorphology [28,29,31,35–38], but only a few of these have ex-amined the influence of the peroxo groups on the reactivity of TiO2

nanoparticles. Despite the lack of experimental data regarding thistype of nanoparticles in solid-state reactions, some studies aboutthe chemical bond between peroxo ligands and titanium can shedlights on this problem. Peroxides can increase the lability of che-mical bonds near central titanium [17], which in turn decreasesthe energy barrier during the solid-state reaction, favouring thediffusion of atoms at lower temperatures and shorter times.

Fig. 3 Some energy-efficient methods employed to synthesizeinorganic ceramics and composites, known as self-propagatinghigh temperature routes (SHS), utilize the energy released fromexothermic transformations to accelerate reaction in the solidstate [6,10–14,39]. In the case of TiO2-Iso and TiO2-Met nano-particles, peroxo groups bonded to their surface decompose exo-thermically [30], which means that they can act as an energysource similar to other chemicals in SHS reactions.

Fig. 4a shows differential scanning calorimetry (DSC) curves ofTiO2-Iso and TiO2-Met samples with peaks related to the exo-thermic decomposition of peroxo groups between 200–250 °C,which coincide with the maximum weight loss rates at 235 °C and265 °C in thermogravimetric (TG) curves of TiO2-Iso and TiO2-Met,respectively (Fig. 4b). Anatase and brookite phases are metastableand transform to stable rutile at temperatures above 350 °C[40,41], much higher than the decomposition of peroxide groupsand should not affect the thermal profile in the temperature rangeshown in Fig. 4.

Figs. 4 and 5 illustrates the differences between the conven-tional solid-state reaction and a route based on surfaces activatedby peroxo groups. Conventional white TiO2 requires a largeamount of energy from an external source to overcome the kineticbarrier, which is quite high in solid-state reactions, while a smallamount of energy is needed to start the decomposition of peroxogroups bonded to yellow TiO2. Due to short time needed to dis-perse the energy released during this exothermic decomposition,which is a good approximation of an adiabatic transformation [42],there is enough local energy to active the nanoparticle surfaces toreact faster and at lower temperatures than in conventional solid-state reactions. The SEM images on the right of Fig. 5 show BTparticles after treatment at 600 °C for 1 h obtained by traditionalsolid-state reaction (above) and using TiO2 nanoparticles with theperoxo-modified surfaces (bellow) that confirm this difference inreactivity.

Fig. 5 This concept of a reactive surface activated by peroxogroups can be extended to other valve metal oxides such astungsten, niobium or vanadium [43]. These oxides also form per-oxo-modified surfaces and are present in the composition of sev-eral important technological materials. In addition to the evidentenergetic advantage of performing such reactions at milder con-ditions, their syntheses by this new approach can preserve theirsize scale, morphology and sinterability, which have a positiveimpact on the performance and quality of resulting devices. It isalso important to mention that this new method allows the pre-paration of large amounts of material using the same apparatus

employed in traditional solid-state reactions, which can be easilyadopted by the industrial sector to produce high-quality oxides oncommercial scales.

4. Conclusions

Peroxo groups on the surface of titanium dioxide (TiO2-Iso orTiO2-Met) create a gentle environment to promote the formationof crystalline Bi12TiO20 powders at lower temperatures and shortertimes regardless the titanium source used through a mechanismsimilar to self-propagating high temperature routes. The reactiveproperties of peroxided titanium dioxide seem to be related to thehighly exothermic decomposition of peroxo groups, which locallyreleases a large amount of energy that accelerates the reaction.This new OPM approach is a promising way to obtain a hugenumber of commercial and technological titanates of complexstructures, using low calcination temperatures, and this approachcan also be used with others elements, mainly other metals thatform chemical bonds with peroxo groups such as niobium, tung-sten, and zirconium.

Acknowledgments

This study was supported by FAPESP (Grants 2015/139580-3,2014/09014-7 and 2013/07296-2), CNPq and CAPES.

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.ceramint.2016.07.039.

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