Chukin, A. V.

Kamalov, R. V.

Kozhevina, A. V.

Martemyanov, N. A.

Vokhmintsev, A. S.

Weinstein, I. A.

Journal of Physics Conference Series

Institutional repository of Ural Federal University named after the first President of Russia B.N.Yeltsin

1Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd1234567890Saint Petersburg OPEN 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 917 (2017) 062031  doi :10.1088/1742-6596/917/6/062031      Optical absorption edge parameters of zirconium dioxide nanotubular structures A V Kozhevina1, A S Vokhmintsev1, R V Kamalov1, N A Martemyanov1, A V Chukin1 and I A Weinstein1,2 1Ural Federal University, NANOTECH Center, Ekaterinburg,620002, Russia 2 Institute of Solid State Chemistry, Russian Academy of Sciences, Yekaterinburg 620990, Russia  Abstract. Nanotubular layer of zirconia with an thickness of 10  1 m and an outer/inner diameters of nanotubes of 80/60  5 nm was synthesized by anodization in the electrolyte based on ethylene glycol with small amount of water and ammonia fluoride. It is shown that the obtained samples before and after annealing in air at 400 °С consist of 90 % and 79 % tetragonal, 10 % and 21 % monoclinic phases, respectively. Analysis of diffuse reflectance spectra showed that the thermal treatment leads to a decrease in concentration of surface Zr3+-centers. Under the assumption of direct allowed transitions, the bandgap width was estimated to be Eg = 5.41  0.01 eV and 5.66  0.01 eV for as-grown and annealed samples, respectively. 1. Introduction Self-organized nanotubular structures of metal oxides obtained by anodizing are promising matrices for gas sensors, solar cells, photochemical and photoelectrochemical cells [1, 2]. It is known [3] that ZrO2 with the energy gap of Eg = 5 – 5.8 eV [1, 4, 5] has one the largest photocatalytic activity under UV exposure (254 nm) in the series of transition metal oxides. However, the photocatalytic efficiency of ZrO2 under sunlight drops considerably in comparison, for example, to TiO2 (Eg = 3.1 to 3.2 eV [6]) that limits its practical use. Electrochemical oxidation of Zr is mainly carried out in the potentiostatic mode at a voltage of  120 V in both inorganic aqueous media [7] - sulfates [8, 9], phosphates [1] and their mixtures [10, 11], and organic electrolytes based on ethylene glycol [9, 12], glycerol [13], mixtures of glycerol and ethylene glycol [14], glycerol and formamide [14 – 16]. However, it remains unclear how the anodizing conditions have an effect on the defectiveness of the synthesized ZrO2 nanotubular layers and, consequently, on photocatalytic properties of zirconia. At the same time, it is known that self-organized highly ordered nanotubular structures of TiO2, obtained in organic electrolytes based on ethylene glycol with fluoride ions and subjected to thermal treatment, have more efficient electron transport and, as a result, exhibit higher photocatalytic activity [17]. During the excitation of the samples by photons with energy of h < Eg the main role in electron-hole pairs generation is played by impurities and/or intrinsic defects forming a system of allowed energy levels within the forbidden gap and distort the optical absorption edge [18, 19 – 21]. The surface states with uncompensated chemical bonds of metal atoms (for example, Zr3+-ions) and oxygen vacancies in a different charge state (F-type centers) are the predominant defects for ZrO2 nanostructures. For this reason, a thermal treatment of samples under oxidizing conditions can reduce the concentration of intrinsic defects influencing the optical absorption edge. 21234567890Saint Petersburg OPEN 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 917 (2017) 062031  doi :10.1088/1742-6596/917/6/062031      In despite of the considerable interest in zirconium dioxide in various structural and phase states [18, 12, 22 – 27], a knowledge about the absorption characteristics of anodized ZrO2 in the region of the optical absorption edge [1, 10] is insufficient. As an example, the energies Eg = 4.8 [1] and 4.13 eV [10] corresponding to ZrO2 nanotubes fabricated by anodization process are significantly different from the energies corresponding to nanopowders and thin films obtained by other methods [1, 4, 5]. Thus, the aim of this work was to study structural and optical features of the zirconia layers synthesized by anodization in an electrolyte based on ethylene glycol with fluorine ions using scanning electron microscopy, X-ray diffraction and diffuse reflection techniques. 2. Samples and experimental techniques 2.1 Synthesis of ZrO2 nanotubular layers Before the anodizing a 300 μm thick zirconium foil (99.6%) was treated with acetone, etched in a solution containing HF/HNO3/H2O (1:6:20), washed with distilled water and dried in air. The electrochemical oxidation was carried out in a two-electrode cell with a thermostatic control. The cathode was a stainless steel plate. The anodization was conducted at a constant voltage of 50 V and a temperature of 20 °C for 9 hours in the solution of ethylene glycol containing 5 wt. % H2O and 0.5 wt. % NH4F. After anodization the samples was washed and dried in air. The thermal treatment of synthesized structures was performed in air at 400 °C for 1 h. Thus, as-grown (S1) and annealed (S2) samples of anodized zirconia were obtained. 2.2 Characterization The surface morphology of the anodized ZrO2 was studied using scanning electron microscopy (SEM) by CarlZeiss SigmaVP. Structural characterization of as-grown and annealed samples was performed on PANalytical X’Pert PRO MPD diffractometer with a copper anode using the Rietveld method. The scanning step was fixed at 0.05. 2.3 Diffuse reflection spectroscopy An investigation of the absorption properties at room temperature was carried out using a Shimadzu UV-2450 two-beam spectrometer and an ISR-2200 integrating sphere. Scanning was conducted in the range of 190-850 nm with a spectral slit width of 2 nm at a rate of 90 nm/min. 3. Results 3.1 SEM characterization The SEM images of the surface and cleavage of sample S1 is shown in Figure 1. It can be note that the synthesized oxide layer consists of highly ordered nanotubes with an external 80  5 nm and an internal 60  5 nm diameters. Some tubes (Figure 1a) are closed, presumably by a barrier layer of metal oxide [6] or remnants of a loose precipitated zirconium hydroxide [8]. The layer thickness of nanotubular ZrO2 is 10  1 m.  31234567890Saint Petersburg OPEN 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 917 (2017) 062031  doi :10.1088/1742-6596/917/6/062031        a b Figure 1. SEM images of surface (а) and lateral view (b) of anodized ZrO2 layer. 3.2 XRD analysis Figure 2 shows the XDR patterns of as-grown and annealed ZrO2 samples. It can be seen that synthesized oxide layers have a crystalline structure. These spectra show characteristic peaks of both tetragonal (t) (scattering angles 2θ = 30.2, 50.2, 60.2, 74.5, 85.2), and monoclinic (m) (at 2θ = 24.1, 28.2, 41.4, 55.9, 74.7, 83.1) phases. The figure also presents the Zr peaks caused by the metal substrate.  20 30 40 50 60 70 80 90Zrtt ttmtmmmttIntensity, a.u.S1 tmmm mm mZr ZrZrZrZrZrZrmtS2 Figure 2. XRD patterns for as-grown (S1) and annealed (S2) samples. 3.3 Diffuse reflectance spectra Figure 3 demonstrates the diffuse reflectance spectra R(). It is seen that spectra have a sharp decline at  ≤ 250 nm due to the optical absorption edge. In the  = 250 – 850 nm region, the R coefficient for the S2 sample is 5 to 15 % higher than for S1. This fact indicates the concentration decrease of the optically active color centers.  41234567890Saint Petersburg OPEN 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 917 (2017) 062031  doi :10.1088/1742-6596/917/6/062031      200 400 600 8000306090R, %, nm0246FKM, a.u.1122 Figure 3. Diffuse reflectance spectra (1, 2 curves) and the Kubelka-Munk function (1, 2 curves) of S1 (1, 1 curves) and S2 (2, 2 curves) samples. 4. Discussion A quantitative evaluation of the phase composition of the two-component system was carried out by a direct method. The experimental intensities of the main peaks for t- (30.2°) and m-ZrO2 (28.2°) were compared with the literature data. The 90 and 79 % of t-ZrO2, 10 and 21 % of m-ZrO2 were detected in samples S1 and S2, respectively. The obtained experimental data are in satisfactory agreement with studies of phase transformations in the low-temperature sintering process (200 – 350 °C) during the crystallization of an amorphous ZrO2 nanopowder performed by decomposition of zirconium carbonate [28]. The authors of this work report that the crystallization proceeds with the formation of predominantly t- with the addition of m-ZrO2 (15 – 20 %) with an average grain size of 36 ± 6 nm. It is also known that for macrosamples of ZrO2 the phase transition of the monoclinic structure to the tetragonal one takes place at a temperature of 1170 °С [29]. In our case, the dominance of the tetragonal symmetry even in the as-grown samples and after annealing at 400 °C of ZrO2 may exhibit the presence of structural features < 30 nm. This is consistent with the work data since the thickness of the zirconia nanotubes walls, according to SEM study, is near 20 nm, see 3.1. For the analysis of the diffuse-reflectance spectra the Kubelka-Munk function was used, which is proportional to the optical absorption coefficient of the material:  212KMRFR      (1) where R is the diffuse reflection of the material measured with respect to the absolutely white body. The calculated dependences of FKM() are shown in figure 3, curves 1 and 2. Within the  = 250 – 350 nm a shoulder is observed for the samples with clearer presence for the S1 structure. According to [18], the absorption band with a maximum at  = 280 nm is due to Zr3+ centers. In this connection, it can be assumed that annealing in the air atmosphere leads to the oxidation of surface defects and, as a consequence, to bonds compensation and decrease in the concentration of Zr3+ centers. The Tauc plot was used to determine the bandgap width [30] using: 51234567890Saint Petersburg OPEN 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 917 (2017) 062031  doi :10.1088/1742-6596/917/6/062031         1 nKM gh F A h E    ,    (2) where n is a constant depending on the type of electronic transition in the material; A is a proportionality coefficient. Analysis of the optical absorption spectra using expression (2) under the assumption of direct allowed transitions (n = 1/2) is shown in figure 4. Estimations of Eg for S1 and S2 samples were 5.41  0.01 and 5.66  0.01 eV, respectively. Obtained results are in satisfactory agreement with independent study of crystalline (Eg = 5.65 – 5.74 eV) and amorphous (Eg = 5.65 eV) zirconia films on Si-substrate obtained by magnetron sputtering [4] and atomic-layer deposition [5], respectively. However, for crystalline films of anodized ZrO2, lower values of Eg = 4.8 [1] and 4.13 eV [10] are reported. This fact indicates a significant influence of the samples synthesis conditions on the characteristics of nanostructural ZrO2 optical absorption edge.  5 60.00.51.01.55 6Eg=5.66 eV(hFKM)2, a.u.h, eV experiment linear fitEg=5.41 eVS1 S2 Figure 4. Tauc plot of optical absorption edge spectra for as-grown (S1) and annealed (S2) samples.  5. Conclusion In the present research of nanotubular ZrO2 samples synthesized by the anodic oxidation method was carried out. The morphology of the anodized oxide was studied by scanning electron microscopy. The thickness of the nanotubular layer was 10  1 m, the outer diameter of the nanotubes was 80  5 nm, the inner diameter was 60  5 nm. XRD analysis showed that as-grown samples and after 400 °C annealing consist of a mixture of tetragonal and monoclinic phases. The analysis of diffuse reflection spectra was performed. The bandgap width of Eg = 5.41  0.01 and 5.66  0.01 eV for nanotubular ZrO2 samples before and after thermal treatment was determined under the assumption of direct allowed transitions using the Tauc plot. It was concluded that annealing of the investigated samples in the air atmosphere leads to compensation of break bonds of surface defects and a decrease in the concentration of Zr3+ centers. 61234567890Saint Petersburg OPEN 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 917 (2017) 062031  doi :10.1088/1742-6596/917/6/062031      Acknowledgments The work was supported in part by Act 211 Government of the Russian Federation, contract № 02.A03.21.0006. References [1] Trivinho-Strixino F, Guimarães F E G and Pereira E C 2008 Chem. Phys. Lett 461 82 [2] Kamalov R, Vokhmintsev A, Dorosheva I, Kravets N, Weinstein I 2016 Adv. Sci. Lett 22 688 [3] Karunakaran C and Senthilvelan S 2005 J. Mol. Catal. A-Chem 233 1 [4] Zhao S, Ma F, Xu K W and Liang H F 2008 J. Alloy. Compd 453 453 [5] Puthenkovilakam R and Chang J P 2004 Appl. Phys. Lett 84 1353 [6] Roy P, Berger S and Schmuki P 2011 Angew. Chem.-Int. Edit 50 2904 [7] Lee W-J and Smyrl W H 2008 Curr. Appl. Phys 8 818  [8] Tsuchiya H, Macak J M, Sieber I and Schmuki P 2005 Small 1 722  [9] Tsuchiya H, Akaki T, Nakata J, Terada D, Tsuji N, Koizumi Y, Minamino Y, Schmuki P and Fujimoto S 2009 Electrochim. Acta 54 5155 [10] Jiang W, He J, Zhong J, Lu J, Yuan S and Liang B 2014 Appl. Surf. Sci 307 407 [11] Hahn R, Berger S and Schmuki P 2010 J Solid State Electrochem 14 285 [12] Shin Y and Lee S 2009 Nanotechnology 20 105301 [13] Muratore F, Baron-Wiecheć A, Hashimoto T, Skeldon P and Thompson G E 2010 Electrochem. Commun 12 1727 [14] Berger S, Faltenbacher J, Bauer S and Schmuki P 2008 Phys. Status Solidi-Rapid Res. Lett 2 102 [15] Fang D, Luo Z, Liu S, Zeng T, Liu L, Xu J, Bai Z and Xu W 2013 Opt. Mater 35 1461 [16] Zhao J, Wang X, Xu R, Meng F, Guo L and Li Y 2008 Mater. Lett 62 4428 [17] Kahnt A, Oelsner C, Werner F, Guldi D M, Albu S P, Kirchgeorg R, Lee K and Schmuki P 2013 Appl. Phys. Lett 102 233109 [18] Emeline A, Kataeva G V, Litke A S, Rudakova A V, Ryabchuk V K and Serpone N 1998 Langmuir 14 5011 [19] Ilin D O, Vokhmintsev A S and Weinstein I A. 2016 AIP Conf. Proc 1767 020028 [20] Savchenko S S, Vokhmintsev A S and Weinstein I A 2017 Opt. Mater. Express 7 354 [21] Savchenko S S, Vokhmintsev A S and Weinstein I A 2017 Tech. Phys. Lett 43 297 [22] Liang J, Deng Z, Jiang X, Li F and Li Y 2002 Inorg. Chem. Commun 41 3602 [23] Singh A K and Nakate U T 2014 The Scientiﬁc World Journal 2014 349457 [24] Lucovsky G 2007 J. Mol. Struct 838 187 [25] Rivera T, Olvera L, Azorín J, Sosa R, Barrera M, Soto A M and Furetta C 2006 Radiat. Eff. Defects Solids 161 91 [26] Clament Sagaya Selvam N, Manikandan A, John Kennedy L and Judith Vijaya J 2013 J. Colloid Interface Sci 389 91 [27] Nikiforov S V, Kortov V S, Savushkin D L, Vokhmintsev A S, Weinstein I A 2017 Rad. Meas in press DOI: 10.1016/j.radmeas.2017.03.020 [28] Gabelkov S V, Tarasov R V, Poltavcev N S, Logvinkov D S i Mironova A G 2004 Voprosy atomnoj nauki i tekhniki 3 116 (in russian) [29] Ackermann R J, Garg S P and Rauh E G 1977 J. Am. Ceram. Soc 60 341 [30] Tauc J, Grigorovici R and Vancu A 1966, Phys. Status Solidi B-Basic Solid State Phys. 15 627 

Optical absorption edge parameters of zirconium dioxide nanotubular structures

A V Kozhevina

A S Vokhmintsev

R V Kamalov

N A Martemyanov

A V Chukin

I A Weinstein

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Optical absorption edge parameters of zirconium dioxide nanotubular structures

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Institutional repository of Ural Federal University named after the first President of Russia B.N.Yeltsin

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