synthesis, structure and optical properties …dl.uctm.edu/journal/node/j2015-4/15_albena...

Radka Gegova, Reni Iordanova, Albena Bachvarova-Nedelcheva, Yanko Dimitriev

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Journal of Chemical Technology and Metallurgy, 50, 4, 2015, 449-458

SYNTHESIS, STRUCTURE AND OPTICAL PROPERTIES OF TiO2/TeO2/MnOm (M = Zn, B) GELS: A COMPARISON

Radka Gegova1, Reni Iordanova1, Albena Bachvarova-Nedelcheva1, Yanko Dimitriev2

1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev”, bl. 11, 1113 Sofia, Bulgaria E-mail: [email protected] University of Chemical Technology and Metallurgy, 8 Kl. Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected]

ABSTRACT

We report in this study the preparation of gels of a nominal composition 50TiO2-25TeO2-25MnOm (M = Zn, B) and our attempt to verify the oxide components behaviour using Ti butoxide, Te (VI) acid, Zn acetate and boric acid as precursors. The phase and structural transformations of the prepared gels upon heat treatment in the temperature range of 200 oC - 700oC are thoroughly studied. The powdered samples still contain organics at 200oC as verified by XRD. Composites containing an amorphous phase along with different crystalline phases (elementary Te, α-TeO2, TiO2 (anatase), ZnTeO3 and TiTe3O8) are formed with temperature increase (above 300oC). The IR results obtained show that the amorphous phases (300 oC - 400oC) of the composite materials consist of TiO6, TeO4, ZnO4, BO3 and BO4 units in different ratio depending on the composi-tion. The UV-Vis spectra of the gels prepared exhibit a red shift of the cut-off compared to pure Ti butoxide gel behavior.

Keywords: sol-gel, phase transformations, UV-Vis spectra, structure.

Received 15 January 2015Accepted 06 April 2015

INTRODUCTION

It is well known that the sol-gel technology is a widespread method for synthesis of glasses, ceramics, powders, thin films, fibers, and membranes and its major peculiarities and possibilities are discussed in some ex-cellent reviews [1 - 5]. Sol-gel derived materials attract the scientists’ attention due to such advantages as good homogeneities, ease of the composition control, lower melting or sintering temperature and better control of the microstructure. It is of scientific and technical inter-est to study the structural evolution in the course of the various stages of sol-gel transition and glass or ceramics production as well as the factors which determine the structure and properties of the materials obtained. Up to now, mainly silicate, titanate, zirconate and niobate gels

are studied [2, 6 - 9]. Irrespective of the fact that B2O3 is a well known classical glass former, the sol-gel method is not very often used to synthesize amorphous borate materials. Various gel-derived borosilicate glasses of a more homogeneous network compared to that obtained along the traditional melting route are prepared [10 - 13]. It is worth adding that until now boro-titanate glasses are not obtained from melts. It is reported that TiO2-B2O3

catalysts photoactivity is improved by boron content in-crease [14, 15]. Glasses are obtained in the binary system of TeO2-B2O3 containing less than 25 mol % of B2O3 due to the presence of a wide region of stable and metastable phase separation [16-19]. The binary TiO2-TeO2 system is of special interest and sol-gel preparation of powders and films is reported in several papers irrespective of the high hydrolysis rate of Te (VI) alkoxides [20 - 22].

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On the other hand, glasses are obtained in the same system when TeO2 content is higher than 75 % [23 - 25]. The structural investigations show that TiO2 inhibits Te polyhedra structural changes and maintains a continuous glass network [23, 25].

ZnO is an intermediate oxide which was preferred in the past years in preparing transparent glass-ceramics by the conventional melt-quenching method. The ma-terials obtained combine the unique properties of ZnO as well as its good network formation ability. A large number of investigations report sol-gel preparation of composite materials on the ground of the binary TiO2-ZnO system. Numerous investigations are devoted to the coupled semiconductor TiO2/ZnO photocatalyst aiming to improve its photodegradation efficiency [26, 27]. The glass forming tendency as well as different structural and optical properties of the binary ZnO-TeO2 system is investigated [28 - 38] with the application of IR, Raman, EXAFS.

We may conclude, considering the literature data pointed above, that melt quenched glasses of high TeO2 content (above 60 mol %) and microheterogeneous structures formation is observed in a wide concentration range [16, 39] of the ternary TiO2-TeO2-B2O3 system. The thermal, optical and structural properties of glasses in the ternary TiO2-TeO2-ZnO system are studied by Kabalci et al. [40] and N. Ghribi et al. [41]. Recently, we defined the gel formation regions in these systems and published the results obtained [25, 42].

The present paper reports comparative studies on the phase and structural evolution upon heating (200 oC - 700oC) of two gels of a nominal composition 50TiO2-25TeO2-25MnOm (mol %, M = Zn, B). A comparison between the gel formation ability of the classical network former B2O3 and that of the intermediate oxide ZnO is also attempted.

ExPERIMENTAL

Gelling, drying and heat treatment The scheme of sol-gel synthesis of TiO2/TeO2/MnOm

(M= Zn, B) powders is presented in Fig.1. It is worth noting that the use of more stable and less expensive boric and telluric (VI) acids compared to their alkoxides is found suitable for the preparation procedure [21, 43, 44]. Zn acetate and Zn nitrate are among the various Zn precursors used in the sol-gel technology but the usage

of Zn acetate provides more advantages than inorganic Zn nitrate [45] according to the literature data collected.

A combination of Te(VI) acid (Aldrich) along with Ti butoxide (Fluka AG), Zn acetate (Merck), boric acid (H3BO3) precursors dissolved in ethylene glycol (C2H6O2) (99 % Aldrich) were used. The initial solutions were subjected to intensive stirring ranging from 5 min to 30 min at room temperature aiming to achieve complete dissolution. The sol-gel hydrolysis reaction proceeded with the participation of absorbed atmospheric moisture only (no water was added to the precursor’s solutions). The pH values measured referred to 4 - 5 depending on composition. The gelation time for the investigated compositions varied from 1 min to 5 min, while the aging was performed in air for several days to complete the process. The gels obtained were subjected to the stepwise heating from 150oC to 700oC for one hour exposure time in air. The heat treatment at 200oC was performed to hydrolyze all OR groups still present, while the further temperature increase (300oC-700oC) was applied to determine the phase and structural transformations. Two representative gels of a nominal composition of 50TiO2.25TeO2.25ZnO (mol %, sample 1) and 50TiO2.25TeO2.25B2O3 (mol %, sample 2) were subjected to detailed phase and structural analysis (Figs. 4 and 7, Table 1).

Samples characterizationPowder XRD patterns were registered at room

temperature with a Bruker D8 Advance diffractometer using Cu-Kα radiation. The morphology of the samples was examined by scanning electron microscopy using a JEOL JSM 6390 electron microscope (Japan) equipped with ultrahigh resolution scanning system (ASID-3D). The optical absorption spectra of the powdered samples were recorded in the wavelength range of 200 nm–1000 nm by a UV-VIS diffused reflectance Spectrophotometer “Evolution 300” using the magnesium oxide reflectance standard as a baseline. The absorption edge and the optical band gap were determined following Dharma et al. instructions [46]. The band gap energies (Eg) of the samples were calculated by the Planck’s equation:

. 1240g

h cEλ λ

= =

where Eg is the band gap energy (eV), h is the Planck’s constant, c is the light velocity (m/s), while λ is the


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wavelength (nm). The infrared spectra were registered in the range of 1600cm-1 - 400cm-1 using the KBr pellet technique on a Nicolet-320 FTIR spectrometer with 64 scans and a resolution of ±1 cm-1.

RESULTS AND DISCUSSION

Phase transformationsThe gel formation regions in both systems [25, 42]

are determined at room temperature. Transparent and monolithic gels are obtained applying the scheme shown in Fig. 1. It is noteworthy that the gel formation region in the TiO2-TeO2-ZnO system is expanded toward ZnO corner in comparison to that in TiO2-TeO2-B2O3 (Fig. 2). Probably, the Zn acetate promotes in higher degree the hydrolysis and polycondensation processes compared to the system with H3BO3 presence. Moreover, there is data that the acetate groups can act as coordinating or bridging ligands, which is an important premise for bet-ter molecular association [47]. The images of the ternary gels (1 and 2) which are under consideration in this paper are given in Fig. 3. It is evident that the gel containing ZnO is more transparent than that containing B2O3. The XRD patterns of the gels subjected to heat treatment in the temperature range of 200oC -700oC are shown in Fig. 4. Those obtained below 200oC are complicated due to the presence of organics and they are not con-sidered in this paper. It is seen that composite materials

containing an amorphous phase and different crystalline phases: metallic Te (JCPDS 78-2312), α-TeO2 (JCPDS 42-1365), anatase (JCPDS 78-2486), rutile (JCPDS 21-1276), ZnTeO3 (JCPDS 44-0240), and TiTe3O8 (JCPDS 50-0250) are formed in the temperature range 200oC - 500oC. The amorphous phase is dominant up to 400ºC but decreases gradually with temperature increase and its amount is negligible at ca 500 ºC. It is evident that organic constituents still exist at 200oC in both samples (Fig. 4). Presence of elementary tellurium in sample 2 (50TiO2.25TeO2.25B2O3) is detected between 200oC and 400oC, while it is fully oxidized at ca 500oC. Probably, B2O3 retards tellurium oxidation. For a comparison, elementary tellurium is found in a narrow temperature range (~ 300oC) in sample 1 (50TiO2.25TeO2.25ZnO). Anatase is observed in both samples at 400oC and a small amount is converted to rutile at this temperature in sample 1 (50TiO2.25TeO2.25ZnO). Obviously, the presence of ZnO enhances the crystallization of rutile at lower temperatures (400oC). It appears at 7000C in the XRD patterns of sample 2 (Fig. 4). The average crystallite size of TiO2 (anatase) in the powdered sample 1 (calculated with the application of the Sherrer’s equa-tion) is ca 60 nm (Fig. 4) at 400oC. Crystalline phases (anatase, rutile, TiTe3O8 and ZnTeO3) only are found (Fig. 4) in the powders prepared at temperature values higher than 500oC.

SEM observations of the gels treated at 400oC (sam-

Fig. 1. Scheme for the sol-gel synthesis.


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ple 1) and 500oC (sample 2) are performed (Fig. 5). It is seen that the morphology results from crashing of the monolithic gels during the heating. Both samples are characterized by a strong tendency to agglomeration with aggregates average size greater than 10 mm. The microprobe analysis shows no presence of carbon in both samples, which in fact indicates that the organic con-stituents are completely removed at these temperatures. The phase morphology of sample 2 is more interesting. Bright needle- and sphere-like particles rich in TeO2 are observed on the sample surface (Fig. 5, sample 2f).

Optical properties and structural transformationsThe diffuse reflectance absorption spectra of the

investigated gels (aged at room temperature) as well as of pure Ti butoxide gel are illustrated in Fig. 6. The observed absorption edges and calculated optical band gap values are pointed out in Table 1. It is evident that the ternary gels containing TeO2, ZnO and B2O3 exhibit higher absorption in the UV region in comparison to pure Ti butoxide gel. The appearance of two absorption bands below and above 300 nm, the so called charge transfer bands, is another peculiarity of the UV-Vis spectra [25, 42]. Isolated TiO4 groups with absorption band in the region of 200 nm-260 nm are the main building units in the unhydrolyzed Ti butoxide. They are transformed into TiO6 units as a result of the polymerization processes and show absorption above 300 nm [48]. The more intensive absorption peak observed for both gel compositions (samples 1 and 2) at ca 300 nm - 310 nm is associated with an increase of the degree of Ti atoms polymerization when compared to that in pure Ti butoxide gel. Our data is in accordance with that obtained by Klein et al. [48] for TiO2/SiO2 gels. It has to be noted that the broader absorption peak at ca 300 nm in the UV-Vis spectra of 50TiO2.25TeO2.25ZnO (sample 1) is assigned to the exciton absorption peak of bulk ZnO, which normally exists in the 300 nm-360 nm spectral region (Eg ~ 3.24 Fig. 3. Images of the investigated gels.

Fig. 2. Gel formation regions in the: a) TiO2-TeO2-ZnO and b) TiO2-TeO2-B2O3 systems.

Table 1. Observed cut-off and calculated band gap (Eg) values. No Gel compositions Cut-off, nm Eg, eV

Sample 1 50TiO2.25TeO2.25ZnO 493.55 2.51

Sample 2 50TiO2.25TeO2.25B2O3 506.70 2.45

Sample 3 Ti (IV) n-butoxide_gel 445.82 2.78


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Fig. 4. XRD patterns of the investigated compositions.


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eV) [49]. The overlapping of the exciton absorption peak and the charge transfer bands hampers to obtain more precise information about ZnO effect on Ti polymeriza-tion ability. The absorption edge of gels 1 and 2 refers to 493.5 nm and 506.7 nm, respectively. Obviously, TeO2, ZnO and B2O3 shift the absorption edge of pure Ti

Fig. 5. SEM images of sample 1 (50TiO2.25TeO2.25ZnO) heat treated at 400oC (a, b, c) and sample 2 (50TiO2.25TeO2. 25B2O3) heat treated at 500oC (d, e, f).

Fig. 6. UV-Vis spectra of the investigated ternary gels.

Fig. 7. IR spectra of the investigated samples in the temperature range 200-7000C: a) sample 1, b) sample 2.

butoxide gel (445.82 eV) towards a higher wavelength (red shift).

IR spectroscopy is used to monitor the phase trans-formations in the gels subjected to heat treatment (in the temperature range of 200oC - 700oC) (Fig. 7). The assign-ments of the vibrational bands of the separate structural units are made on the basis of the well known spectral data for the precursors (Ti (IV) n-butoxide, H6TeO6, Zn acetate and boric acid (Fig. 8) and the crystalline phases existing in the TiO2-TeO2, TiO2-ZnO, TeO2-ZnO systems [25, 50, 51]. The IR spectra of samples heated to 200oC show an absorption band in the region of 1120 cm-1-1040 cm-1 which is due to Ti-O-C stretching vibrations and another one at 890 cm-1-880 cm-1 which is assigned


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to C-O or C-H stretching vibrations of the residual sol-vent ethylene glycol [52]. The band at 620 cm-1 with a shoulder at 640 cm-1 can be related to the overlapping of the vibrations of TiO6, TiO4 and TeO4 units [25, 42, 53, 54]. The lower intensity of both bands in the spectrum of 50TiO2.25TeO2.25ZnO (sample 1) indicates a higher degree of hydrolysis and condensation processes prob-ably as a result of ZnO presence. Thus, we can suggest that at lower temperatures (200oC) the amorphous phases consist both of organic and inorganic building units. The bands located in the range of 1500 cm-1- 1300 cm-1 are as-signed to the bending vibrations of CH3 and CH2 organic groups. They are not visible above 300oC and which is why the spectra recorded at higher temperature values (300oC - 700oC) are mainly characterized by bands below 900 cm-1. The latter are typical for inorganic units. They are broadened up to 500oC but their intensity stays low. This is a peculiarity attributed to disordered systems. The IR and XRD data discussed above provide to sug-gest that the amorphous phases in the temperature range 300oC - 500oC consist only of inorganic building units TeO4, TiO6 and ZnO4 whose absorption bands are in the range 700 cm-1 - 400 cm-1. It is difficult to distinguish the vibrations of these groups due to their strong overlapping [53 - 60]. The characteristic vibrations of BO3 and BO4

groups are not well defined in the IR spectra. It is a well known fact [61 - 63] that B-O stretching for trigonal BO3 units is situated in the spectral region of 1500 cm-1-1200 cm-1, that for tetrahedral BO4 units is observed at 1200 cm-1-850 cm-1, while the bending vibrations of various borate segments are outlined in the range of 800 cm-1-600 cm-1. These bands are not detected in our spectra. Only a weak band centered at ca 970 cm-1, typical for BO4 units, is observed in the spectrum of sample 2 at 500oC. The formation of BO3 units in the amorphous phase can not be excluded. Their presence is proved in compositions of higher B2O3 content [25]. The IR spectra of samples heated above 500oC show vibrations characteristic for the inorganic structural units building the crystalline phases (TiO2 - anatase, rutile, TiTe3O8 and ZnTeO3). In fact they verify the phase transitions observed by XRD (Fig. 4).

CONCLUSIONS

It is found that the gel formation region in the ternary TiO2-TeO2-ZnO system is expanded toward ZnO corner in comparison to that in TiO2-TeO2-B2O3. According to the XRD results referring to samples heated up to 300oC the composites consist mainly of an organic–inorganic amorphous phase which is completely transformed into

Fig. 8. IR spectra of the used precursors: a) H3BO3 and H6TeO6; b) Zn acetate and Ti butoxide.


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inorganic one above 300oC. The simultaneous presence of Te, Zn and B in the ternary gels causes the red-shifting of the cut-off in comparison to pure Ti butoxide gel behavior. The UV-Vis results show that the ternary gels exhibit a higher degree of condensation compared to that of pure Ti butoxide gel. It is suggested, on the ground of the IR results of samples treated at 200oC, that ZnO presence leads to a greater extent of hydrolysis and condensation processes compared to that in presence of B2O3. The latter is a classical network former but obvi-ously, under the experimental conditions provided and the combination of precursors used in this study, ZnO behaves as a better amorphous network former. More ex-periments are required to explain this interesting result.

Acknowledgements

The study was performed with financial support of The Ministry of Education and Science of Bulgaria, Op-erational Program “Human Resources Development”, co-financed by the European Social Fund of the Euro-pean Union, contracts: BGO51PO001-3.3.06-0050. The authors are also grateful to the financial support of The Ministry of Education and Science of Bulgaria, Contract No ДНТС/India 01/6, 2013: Bulgarian-Indian Inter-governmental Programme of Cooperation in Sci-ence and Technology.

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