N.Petkova_2011.pdf

Journal of Non-Crystalline Solids 357 (2011) 1547–1551 Contents lists available at ScienceDirect Journal of No...

Journal of Non-Crystalline Solids 357 (2011) 1547–1551 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l Preparation and optical properties of transparent zirconia sol–gel materials Nina Petkova a, Stephen Dlugocz b, Stoyan Gutzov a,⁎ a University of Sofia “St. Kliment Ohridski”, Faculty of Chemistry, Department of Physical Chemistry, James Bourchier Boulevard 1, 1164 Sofia, Bulgaria b Institute of Physical and Theoretical Chemistry, Braunschweig University of Technology, Braunschweig, Germany a r t i c l e i n f o a b s t r a c t Article history: The optical properties of zirconia sol–gel glasses, prepared from zirconium butoxide with organic modifying Received 25 May 2010 agents using different ways of preparation are described. The gels obtained are characterized with UV/Vis/NIR Received in revised form 4 December 2010 reflectance and transmission spectroscopy between 220 nm and 2600 nm, X-ray diffraction, SEM microscopy Available online 13 January 2011 and IR spectroscopy. The optical band gap of the amorphous gels depends on the presence of complex forming modifying agents such as acetic acid and acetylacetone and varies between 4.84 eV and 2.97 eV, respectively. Keywords: Complex formation between zirconium and acetylacetone starts in the sol and is characterized by a strong Optical properties; Zirconia; peak in the UV region at about 285 nm which changes during gelation. It is shown that samples, prepared Sol–gel; without protection agents (acetic acid or acetylacetone) display a typical granulated microstructure while UV/Vis/NIR spectroscopy gels, obtained by addition of organic additives display a flat and compact surface. © 2010 Elsevier B.V. All rights reserved. 1. Introduction 2. Experimental details The sol–gel technology is a low temperature method for the Three series of samples based on zirconium butoxide as the preparation of different oxide materials like SiO2, ZrO2, Al2O3 or SnO2 starting precursor were carried out at room temperature. The from liquid precursors. The low working temperatures allow doping preparation conditions are summarized in Fig. 1. The syntheses of the sol–gel glasses with organic dyes and d- and f-ion complexes. In were performed with Zr(OC4H9)4 (80% solution in 1-buthanol this way new hybrid materials with interesting optical, electrical and “Aldrich”); secondary buthanol (99%, “Fluka”), ethanol (99%, mechanical properties are designed [1–9]. Zirconia (ZrO2) is a suitable “Riedel-de Haën”); hydrochloric acid (37%, “Riedel-de Haën”), nitric matrix for optical materials and coatings due to its high chemical and acid (65%, “Fluka”), acetylacetone (99%, “Sigma Aldrich”) and acetic photochemical stability and its significantly lower phonon energy acid (99.8% “Fluka”). compared with SiO2 and Al2O3 [2–5]. The sol–gel chemistry of zirconia is complicated because of the low chemical stability of zirconium Scheme A The hydrolysis of the starting zirconium precursor was alkoxides against water [10,11]. A useful method for protection of carried out without protection and resulted in the zirconium alkoxides against hydrolysis, leading to the formation of formation of white precipitates. Reaction times were in transparent gels is the addition of complex forming organic the range of a few seconds. The process is similar to the substances [8,10,11]. precipitation of hydrated zirconia with ammonia solu- Despite numerous investigations the relation preparation–struc- tions. The molar ratio of the reagents was nZr(OC4H9)4: ture–optical properties of zirconia sol–gel materials containing nC4H9OH: nH2O = 1:1:4, respectively. The zirconia content chelate modifying agents are still not understood. On the other of the gels determined by weight analysis was about hand, it is important to analyze the role of complex forming organic 32%. molecules for coloration purposes and as band gap modifiers. In this Scheme B Acetylacetone (AcAc) was used as a chelate ligand which study reproducible preparation methods for amorphous zirconia resulted in the formation of the already described yellow– hybrid sol–gel materials are described related to their optical brown complexes between acetylacetone and zirconium properties in the ultra violet/visible/near infrared (UV/Vis/NIR) [12,13]. The molar ratio of the reagents was: nZr:nButOH: region. nAcAc:nH2O = 1:1:c:4, respectively, where “c” is the ratio nAcAc/nZr. Samples with c = 0.12; 0.3; 0.35; 0.45 and 0.6 were prepared. In some cases the overall kinetics of gel formation ⁎ Corresponding author. Tel.: + 359 2 8161 281; fax: + 359 29625438. was modified by using HCl, which increased the gelation E-mail addresses: [email protected] (N. Petkova), time. The zirconia content of the gels determined by weight [email protected] (S. Dlugocz), [email protected]fia.bg (S. Gutzov). analysis was about 58%. 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.12.012 1548 N. Petkova et al. / Journal of Non-Crystalline Solids 357 (2011) 1547–1551 Zr(OC4H9)4 Zr(OC4H9)4 C2H5OH C4H9OH C4H9OH CH3COOH HNO3 Zirconia sol Zirconia sol 1 Solution H2O CH3COCH2COCH3 Zr(OC4H9)4 White precipitate Zirconia sol 2 Zirconia sol H2O H2O Yellow gel Transparent gel Fig. 1. Preparation schemes used for the sol–gel synthesis of zirconia gels. Left scheme — gels prepared without protection, middle scheme — gels prepared with acetylacetone as protection agent, and right scheme — gels prepared with acetic acid. Scheme C A mixture of nitric acid/acetic acid (AA) was used to control 3. Experimental results the hydrolysis time (thyd) and gelation time (tgel). The method allowed the preparation of homogeneous, trans- In Fig. 2 X-ray diagrams of zirconia materials prepared by parent, colorless gels with a gelation time tgel = 2–3 min. schemes A, B and C are presented. The results prove that all the The mol ratio between the reagents was nZr(OC4H9)4: prepared gels are X-ray amorphous. There are indications, however, nC2H5OH:nCH3COOH:nHNO3:nH2O = 1:12:1:0.47:4, respectively. for the presence of tetragonal zirconia nanocrystals in the gel matrix. It is known that acetic acid leads to the formation of Fig. 2 shows that the formation of tetragonal zirconia nanocrystals complexes and that nitric acid causes additional stabiliza- seems to be preferred in the case of fast precipitation without a tion of the sol due to an increase of repulsion forces between protection agent. the zirconia particles. The zirconia content of the gels The X-ray analysis results are supported by scanning electron was about 31%. microscopy (SEM) observations. From the SEM-photographs differ- ences in the morphology of the materials synthesized by different Room temperature UV/Vis and NIR reflectance spectra of pow- methods are evident (Fig. 3). It is seen that samples, prepared dered species were recordered on a PerkinElmer Lambda 900 without protection display a typical granule microstructure. spectrophotometer with a specular reflectance unit (“Praying Man- Transparent gels, obtained by addition of organic additives have a tis”, Harric Scientific) using KCl as a standard between 200 and smooth surface, which can be useful for the preparation of zirconia 3300 nm. From the diffuse reflectance R(%) the Kubelka–Munk coatings. Surface roughness decreases when acetic acid is used as a function F(R) was calculated : modifying agent. Diffuse reflectance spectra of the synthesized materials in the K ð1−RÞ2 UV/Vis region are shown in Fig. 4. The spectra obtained indicate that F ðRÞ = = : ð1Þ the optical properties of zirconia sol–gel materials strongly depend S 2R on the preparation method. The M3 sample, obtained by rapid hydrolysis of the precursor is transparent up to 230 nm in contrast Here, K is the absorption coefficient, S indicates the scattering to samples, synthesized by the acetylacetone (AcAc) complex coefficient and R represents the diffuse reflectance. The absorption method. The gels prepared using AcAc are yellow–brown colored peaks were mathematically treated as overlapping Gaussian curves to because of the presence of n → π* and π → π* intraligand electronic obtain the integrated diffuse reflectance Fint(R), peak maxima xc and transitions [16,17]. Samples obtained by acetic acid/nitric acid half widths ω. All R2-factors in this procedure were in the range of 0.999. The solution chemistry of our samples was investigated with a Thermo-Spectrotronic Unicam UV500 UV/Vis spectrophotometer. From the UV/Vis spectra the oscillator strengths f of the electronic transitions were calculated : −9 A int ð ṽÞ f = 4:32⋅10 ⋅ : ð2Þ c ⋅d Here, A int ð ṽÞ is the integrated absorbance, c — the concentration in mol/l, and d — the solution thickness in cm. XRD investigations were performed on a Philips PW 1050 standard powder diffractometer and SEM observations were made on a standard scanning electron microscope JEOL 5510. IR spectra of the gels were measured using a standard Bomem Michelson 100 spectrometer having a resolution of 2 cm− 1. The zirconia contents of the gels were obtained by weight analysis (heating 6 h at 900 °C and slow cooling down to room temperatures). IR spectra of the Fig. 2. X-ray diagrams of the gels prepared. 1() — gels obtained with acetylacetone as samples were recordered on a Bomem Michelson 100 unit with a a protection agent; 2 (○) — gels prepared with acetic acid as a protection agent; and resolution of 2 cm− 1. 3 (▲) — gels formed without protection.  N. Petkova et al. / Journal of Non-Crystalline Solids 357 (2011) 1547–1551 1549 Fig. 4. UV/Vis reflectance spectra of the zirconia sol–gel materials investigated: 1(dash-dot) — gels without protection of the precursor; 2 (○) — gels with acetic acid as protective ligand; 3, 4, and 5 (lines) — gels with acetylacetone as a complex forming agent; and 6 (dash) — reference spectrum of KCl. In Fig. 5 the absorption spectrum of a sol containing zirconium butoxide, acetylacetone and water is compared to the reflectance spectrum of a gel containing Zr(IV)–AcAc complexes with nAcAc/ nZr = 0.3. The absorption spectra of the zirconium butoxide precursor, buthanol and acetylacetone are given for comparison. In Fig. 6 the NIR absorption spectra of gels prepared by the three preparation schemes A, B and C are shown. The sample M3 displays peaks at about λmax = 1900 nm (5263 cm− 1) and λmax = 2100 nm (4762 cm− 1) due to OH and H2O combination vibrations. In the spectra of materials prepared with protection (acetylacetone and acetic acid) weak peaks at around λmax = 1200 nm (8333 cm− 1), λ max = 1400 nm (7142 cm − 1 ), λ max = 1700 nm (5882 cm − 1 ), λ max = 1920 nm (5208 cm − 1 ), λ max = 2300 nm (4347 cm − 1 ), λmax = 2500 nm (4000 cm− 1), and λmax = 2600 nm (3846 cm− 1) are observed. The features can be related to overtones and combination vibrations of organic functional group such as C–H, C=O; COOH. It is evident from the NIR diffuse reflectance spectra that all the samples are transparent up to 2600 nm. The Fig. 3. SEM pictures of zirconia gels prepared using different sol–gel schemes. Notations: a) gels prepared with acetylacetone protection, b) gels prepared with acetic acid protection, and c) fast precipitation. addition are transparent in the visible region; they possess a weak absorption shoulder at 285 nm due to n → π* transitions. A possible way for the formation of complexes between zirconium and acetic acid is through the hydroxyl oxygen atoms from the carboxyl group. At wavelengths about 220 nm the O2− → Zr4+ charge transfer transition (CTT) is evident for all the samples. The spectra of AcAc-containing samples are characterized by two peaks: a strong one denoted as peak 1 in the UV region at λmax = 295 nm and a weaker transition denoted as peak 2 in the visible region, located Fig. 5. Comparison between absorption and reflectance spectra: 1(line) — butanol; between 450 and 550 nm. It is evident, that the complex formation 2 (dash-dot) — acetylacetone; 3 (dash) — zirconium butoxide c = 2.79·10− 4 mol/l; between AcAc and Zr(IV) seems to be similar to the case of Ti(IV)– 4 (line) — Zr(IV)–AcAc complex c = 2.79·10− 4 mol/l; 5 (○) — diffuse reflectance acetylacetone complexes discussed in. spectrum of the Zr(IV)–AcAc complex containing silica gels. 1550 N. Petkova et al. / Journal of Non-Crystalline Solids 357 (2011) 1547–1551 Fig. 6. NIR absorption spectra of hybrid zirconia sol–gel materials: 1(line) — gels with acetic acid as protection; 2 (dash-doth) — gels with acetylacetone as a protection agent Fig. 7. Relationship between the integrated intensities Fint(R) calculated from the with nAcAc/nZr = 0.3; 3 (dash) — gels without protection; and 4 (line) — reference reflectance spectra and the molar ratio nAcAc/nZr. Notations: peak 1 (), and peak spectrum of KCl. 2 (○). The R-factor of the straight line is 0.91. broad and weak, Fint(R) of peak 2 is about 0.1% from that of peak 1. integrated intensities of the NIR peaks detected are in the order of Fint(R) of peak 1 is the overall intensity of the three features at 0.1% of the intensities in the UV/Vis region. about 35,000 cm− 1 (286 nm), 31,000 cm− 1 (322 nm) and 28,000 The IR spectra of samples, containing AcAc are characterized by (357 nm) cm− 1. strong peaks between 1276 cm− 1 and 1594 cm− 1, which proves formation of a bidentate chelate complex between zirconia and acetylacetone, in agreement with results reported in [16,20]. IR 4.2. Solution chemistry: NIR spectra spectra of samples, prepared with acetic acid are characterized with peaks between 1215 cm− 1 and 1723 cm− 1 related to complexes From the UV/Vis absorption spectra of the investigated solutions between acetic acid and zirconium. In all the IR spectra well known the following observations can be drawn: peaks associated with Zr–O lattice vibrations are detected in agreement with experimental results published in [6,16,17,20]. 1. The chelate complex formation between zirconium and acetylace- tone starts during sol formation and is characterized by a strong peak in the UV region at about λmax = 285 nm. 4. Discussion 2. The gel formation leads to a red shift of the absorption maximum of the Zr(IV)–AcAc complex from λmax = 285 nm to λmax = 295 nm 4.1. UV/Vis spectra of gels which indicates interactions between the complex and the host matrix [16,18]. The results from the Gaussian analysis of the spectra, prepared 3. The absorption of the Zr(IV)–AcAc complex even in the solution is using the acetylacetone method are summarized in Table 1. Peak 1 strong ( = 2400 l/mol cm at λmax = 285 nm). is splitted into three features at about 31,000 cm− 1, 35,000 cm− 1 and 28,000 cm− 1. The different relative intensities of the three The Gaussian deconvolution of the spectrum of the Zr–AcAc peaks seem to depend on solution chemistry and gel formation complex in the solution (R2 = 0.999) shows the presence of four conditions, which lead to different coordination geometries in the peaks at xc1 = 28,531 cm− 1, xc2 = 31,793 cm− 1, xc3 = 33,032 cm− 1 hybrid gels. and xc4 = 35,908 cm− 1. The oscillator strengths of the transi- The integrated absorbance Fint(R) of peak 1 and peak 2 increases tions are f 1 = 1.03·10 − 3 , f 2 = 3.56·10 − 3 , f 3 = 8.16·10 − 3 and with the increase of the ratio nAcAc/nZr at equal preparation f4 = 36.14·10− 3, respectively. It is evident that the UV/Vis solution conditions. In Fig. 7 a linear Fint(R) dependence with a R-factor = spectra of the Zr–AcAc complex in the solution changes as a result of 0.91 of peak 1 (285 nm) vs. nAcAc/nZr is demonstrated. Peak 2 is their introduction in the zirconia gel. Table 1 Results of the Gaussian deconvolution of peak 1 of samples prepared using acetylacetone compared to the preparation conditions. Samples M15 and M16 are prepared using HCl (nHCl:nZr = 0.003). The ratio nZr:nH2O of sample M16 is 8. The maximal error of xc is about 30 cm− 1. The R2-factors are in a range of 0.999. Sample nAcAc/nZr Xc1 Fint(R)1 Xc2 Fint(R)2 Xc3 Fint(R)3 – cm− 1 – cm− 1 – cm− 1 – M13 0.12 31,091 5849 ± 570 34,243 41,310 ± 519 27,995 985 ± 132 M15* 0.12 30,962 2319 ± 135 34,029 27,049 ± 778 27,867 3699 ± 422 M9 0.3 31,353 5930 ± 899 33,683 42,244 ± 1023 27,773 9525 ± 436 M16* 0.35 30,866 2522 ± 295 34,671 47,704 ± 269 27,658 1944 ± 202 M11 0.45 30,721 2538 ± 297 34,598 56,925 ± 409 29,611 8059 ± 875 M10 0.6 30,629 3264 ± 263 34,510 49,717 ± 308 28,071 3591 ± 395  N. Petkova et al. / Journal of Non-Crystalline Solids 357 (2011) 1547–1551 1551 5. Conclusions Both optical properties and morphology of zirconia hybrid sol– gel materials strongly depend on the addition of modifying agents like acetylacetone and acetic acid. Acetylacetone leads to the formation of yellow–brown complexes with high absorption, and acetic acid/nitric acid addition results in transparent, colorless gels. The optical band gap (4.84 ± 0.29 eV) of zirconia gels obtained without protection agents is close to that of yttrium doped zirconia single crystals. Acetic acid/nitric acid and acetylacetone addition decrease the band gap down to 4.41 ± 0.185 eV and 2.97 ± 0.45 eV, respectively, due to complex formation with zirconium. Detailed UV/Vis/NIR analysis turns out to be a useful method for under- standing the physical nature of processes, including complex forming modifying agents in sol–gel chemistry. Acknowledgments Fig. 8. Experimental UV/Vis data on different prepared gels give straight lines in coordinates corresponding to the Tauc equation. 1() — gels with acetylacetone as a Support from the project DTK 02/26 of the Bulgarian National protection agent, Eg = 2.97 ± 0.45 eV, R-factor = 0.97; 2 () — gels with acetic acid as a Science Fund (BNSF) is gratefully appreciated. The structure investi- protection agent, Eg = 4.41 ± 0.185 eV, R-factor = 0.99; and 3 (○) — fast precipitation, gations were sponsored by the project UNION DO-02-82/2008 of the Eg = 4.84 ± 0.29 eV, R-factor = 0.99. BNSF. References The first three peaks (xc1, xc2, and xc3) coincide with that in the gels, while the peak xc4 appears only in the sol. The coordination R. Reisfeld, Optical Materials 16 (2001) 1. geometry of the Zr–AcAc in gels and solution needs additional R. Reisfeld, M. Zelner, A. Patra, Journal of Alloys and Compounds 300–301 (2000) 147. calculations, which will be published in a next contribution. The K. Kuratani, M. Mizuhata, A. Kajinami, S. Deki, Journal of Alloys and Compounds decrease of the absorption features qualitatively proves that the site 408–412 (2006) 711. symmetry of zirconium ions increases as a result of the gelation of Z.W. Quan, L.S. Wang, J. Lin, Materials Research Bulletin 40 (2005) 810. H.-Q. Liu, L.-L. Wang, S.-G. Chen, B.-S. Zou, Journal of Alloys and Compounds 448 the sol. (2008) 336. S. Gutzov, J. Ponahlo, C.L. Lengauer, A. Beran, Journal of the American Ceramic 4.3. Band gap calculations Society 77 (1994) 1649. M. Bredol, S. Gutzov Optical Materials 20 (2002) 233. P. Comez-Romero, C. Sanchez, Functional Hybrid Materials, Wiley-VCH, London, From the spectra in the UV region, the optical band gaps for 2004. different prepared gel matrixes are calculated (Fig. 8). The optical S. Gutzov, M. Bredol, J. Mat. Sci. Letters 41 (2006) 1835. band gap can be obtained from the absorption spectra using the Tauc N.Y. Turova, E.P. Turevskaya, V.G. Kessler, M.I. Yanovskaya, The Chemistry of Metal Alkoxides, Kluwer, New York, 2002. equation [20–23]: J.C. Brinker, G.W. Scherer, The Physics and Chemistry of Sol–Gel Processing,   Academic Press, INC, London, 1990. 1=β ð AhνÞ = C hν−Eg : ð3Þ X. Changrong, C. Huaqiang, W. Hong, Y. Pinghua, M. Guangyao, P. Dingkun, Journal of Membrane Science 162 (1999) 181. D. Hoebbel, T. Reinert, H. Schmidt, Journal of Sol–Gel Science and Technology 10 Here, A represents the experimental absorbance, h the Plank (1997) 115. C. Wu, L. Cheng, Journal of Membrane Science 167 (2000) 253. constant, ν is the photon frequency, β = 2, C is a constant and Eg the W. Schmidt, Optische Spektroskopie, Wiley-VCH, Weinheim, 2000. optical band gap. Q. Li, X. Zhong, J. Hu, W. Kang, Progress in Organic Coatings 63 (2008) 222. Our results show that the optical band gaps depend on the use of G.V. Loukova, W. Huhn, V.P. Vasiliev, V. Smirnov, The Journal of Physical Chemistry. A 111 (2007) 4117. complex forming organic additives. The preparation with acetic acid G. Ahmed, I. Petkov, S. Gutzov, Journal of Inclusion Phenomena and Macrocyclic and acetylacetone protection decreases the optical band gap in Chemistry 64 (2009) 143. comparison with materials prepared by rapid hydrolysis. The value of M. Hesse, H. Meier, B. Zeeh, Spektroskopische Methoden in der organischen Chemie, Georg Thieme Verlag, Stuttgart, 1991. the optical bang gap of fast precipitad materials (4.84 eV) is in good K. Izumi, M. Murakami, T. Deguchi, A. Morita, N. Tahge, T. Minami, J. Am. Cer. Soc agreement with the results known for single crystal ZrO2–Y2O3 72 (1989) 1465. [21,22]. It is evident, that complex formation between Zr and acetic S. Gutzov, S. Berendts, M. Lerch, Ch. Geffert, A. Börger, K.D. Becker, PCCP 11 (2009) acid or acetylacetone decreases the band gap depending on doping, 636. S. Gutzov, A. Börger, K.D. Becker, PCCP 9 (2007) 491. similar to the case of polycrystalline oxynitrides, which has been S.R. Elliott, The Physics and Chemistry of Solids, Wiley, New York, 1998. successfully applied for coloration purposes. M. Jansen, H.P. Letschert, Nature 404 (2000) 980.