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Heat Treatment Effect on Crystalline Structure and Photoelectrochemical Properties of Anodic TiO Nanotube Arrays Formed in Ethylene Glycol and Glycerol Based Electrolytes 2

Magdalena Jarosz, Karolina Syrek, Joanna Kapusta-Ko#odziej, Justyna Mech, Kamilla Malek, Katarzyna El#bieta Hnida, Tomasz #ojewski, Marian Jaskula, and Grzegorz Dariusz Sulka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08403 • Publication Date (Web): 05 Oct 2015 Downloaded from http://pubs.acs.org on October 5, 2015

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The Journal of Physical Chemistry

Heat Treatment Effect on Crystalline Structure and Photoelectrochemical Properties of Anodic TiO2 Nanotube Arrays Formed in Ethylene Glycol and Glycerol Based Electrolytes

Magdalena Jarosz1, Karolina Syrek1, Joanna Kapusta-Kołodziej1, Justyna Mech2, Kamilla Małek3,4, Katarzyna Hnida1,5, Tomasz Łojewski3, Marian Jaskuła1,Grzegorz D. Sulka1*

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Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry Jagiellonian University in Krakow, Ingardena 3, 30060 Krakow, Poland

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AGH University of Science and Technology, Faculty of Non-Ferrous Metals, Division of Physical Chemistry and Electrochemistry, Al. Mickiewicza 30, 30-059 Krakow, Poland 3

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Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30060 Krakow, Poland

Jagiellonian Centre for Experimental Therapeutics, Jagiellonian University, Bobrzyńskiego 14, 30348 Krakow, Poland 5

AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, A. Mickiewicza 30, Krakow, Poland

* Corresponding author. E-mail: [email protected]

Department of Physical Chemistry & Electrochemistry, Faculty of Chemistry Jagiellonian University in Krakow Ingardena 3, 30060 Krakow, Poland Tel: +48 12 663 22 66 Fax: +48 12 634 05 15 1

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Abstract The effect of annealing temperature on crystal structure of anodic titanium dioxide (ATO) layers prepared via anodization in the ethylene glycol and glycerol based electrolytes was studied. Then samples were annealed in air at the temperatures ranging from 400 to 1000 °C. The XRD measurements proved that a gradual phase changes from anatase to rutile occurs with increasing annealing temperature. The anatase-to-rutile transformation occurs between 500 and 600 °C. The changes in the average crystallite sizes of anatase and rutile occurring during heat treatment of ATO layers were correlated with the mechanism of rutile phase nucleation. It was found also that the transition to the rutile phase in the samples formed in the ethylene glycol based electrolyte is considerably retarded and takes place at higher annealing temperatures due to the higher content of the embedded fluoride ions. The photoelectrochemical performance of ATO layers were studied under pulsed UV illumination. Photocurrent vs. incident light wavelength and applied potential plots were recorded. The highest photocurrents were observed for the samples annealed at 400 °C, regardless the electrolyte. It was demonstrated that the decrease in photocurrent values is related with the decreasing amount of the anatase phase in ATO samples. The enhanced photocurrent response was observed for ATO layers decorated with Ag nanoparticles. The highest photoconversion efficiencies, determined by incident photon-to-current efficiency (IPCE) calculations, were observed for the wavelength of 350 nm.

Keywords: titanium dioxide; anodization;;; annealing temperature, photoactivity of TiO2, Ag nanoparticles;

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1. Introduction Titanium dioxide, TiO2, especially in a nanoporous or nanotubular form, has attracted substantial attention due to its unique properties. On account of its geometry and high surface to volume ratio, anodic TiO2 possesses extraordinary chemical, electrical and optical properties that enable to use it in various applications, e.g. dye-sensitized solar cells1,2, photocatalysis3,4 and gas sensors5. In particular, nanotubular TiO2 has been used as the efficient photocatalyst for hydrogen generation6,7, water splitting8,9 and electrocatalytic oxidation of methanol10,11. The area of application of TiO2 is still expanding, covering also such fields as biosensors12,13 and biocompatible bone grafting materials14,15. The synthesis of nanostructured titania has witnessed a kind of an explosive growth in recent years. Among different methods used to synthesize nanoporous/nanotubular titania, such as sol-gel16, electrodeposition17 and hydrothermal method18, anodic oxidation is a facile, low cost and efficient method for acquiring well-defined nanostructures19-22. The geometric parameters of the anodized titanium dioxide (ATO) are determined by the electrolyte composition20,21-25, applied potential26 and anodizing temperature27. The quality of the nanopore/nanotube arrangement determines potential applications of ATO layers. Another feature that may influence the field of utilization of nanoporous/nanotubular titania, especially its photocatalytic performance, is the crystalline structure of TiO2. Except amorphous, titanium dioxide can occur in three different crystallographic phases: rutile, anatase, and brookite28. As-prepared nanoporous anodic TiO2 layers are amorphous and their properties towards e.g., photocatalysis are not sufficient enough29. Therefore, anatase and rutile phases are much more desired. Although rutile is the stable phase, the metastable anatase seems to be more efficient in applications, such as solar cells30-33 and photocatalysis34,35. Anatase phase was proved to be the most efficient photoactive material when it comes to air or water

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purification and wastewater treatment36. On the other hand, rutile is typically photoinactive in decomposition of organic compounds, (e.g. 4-chlorophenol), however, oxidation of cyanide ions proceeds more efficiently when rutile is used as photocatalyst37. Frequently, a mixture of anatase and rutile, as a more catalytically active material than pure anatase, is used for various photocatalytic applications35,38. It has been widely reported that TiO2 mixed-phases catalyst containing 70 – 75 % of anatase is more active than pure anatase39,40. There are few models explaining this synergistic effect between anatase and rutile in the mixed-phase catalyst. The enhanced activity of the multiphase catalyst can be attributed to (i) electron transfer from photoactivated anatase to rutile41, (ii) electron transfer from the rutile conduction band to anatase trapping sites42, and (iii) electron transfer from photoactivated anatase to rutile under UV irradiation (below 380 nm) since anatase has a more negative conduction band energy than rutile, or photoexcited electron transfer between upward shifted conduction band in rutile (as a result of accumulation of photoexcited electrons during light irradiation with wavelength greater than 380 nm) and anatase conduction band43. Therefore, from perspective of the efficient and successful photocatalysis it is important to possess the mixed-phase TiO2 catalyst with an appropriate anatase content. The crystalline phases of TiO2 can be obtained by annealing amorphous ATO layers at elevated temperatures. According to the different sources, the anatase phase starts to appear between 230 and 280 °C28. The anatase–rutile phase transition occurs at a wide range of temperatures: 400 – 1000 °C, and there is not defined a distinct transformation temperature44,45. This phase conversion is time-dependent and it is influenced by several factors, for example, particles size and shape, surface area, volume of the sample, annealing atmosphere, heating rate and the presence of impurities45-47. The composition of anodizing electrolyte, especially the presence of different ionic species, like , or may also influence the speed of the phase transition48,49. Nevertheless, an impact of impurity ions on

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the oxide layer properties is not so explicit. According to Okazaki and Shinoda, a fluoride ion modified titania surface decreases the photocatalytic activity of TiO250. On the contrary, Hattori et al. reported that photoactivity of titania can be enhanced by doping the titania surface with the F- ions51. The latter was verified by Yu et al., who showed that the TiO2 samples doped with fluoride ions exhibited the strongest absorption in the UV-visible range49. In addition, the photocatalytic properties of TiO2 can be enhanced by combining TiO2 with another semiconductor or metal, which can produce a so-called charge separation effect that increases the lifetime of the photogenerated excitons52-54. In particular, silver nanoparticles seems to enhance photocatalytic properties of TiO2 because they effectively suppress the electron–hole recombination52. It was found that a maximum enhancement of photocatalytic performance of Ag nanoparticle-modified TiO2 occurs at an optimal Ag content of about 0.24 wt.%53,55. Despite the fact that annealing of TiO2 was broadly represented in the literature, some aspects of the mechanism of anatase to rutile transition are still not clear. In this work, we present an attempt to explain the influence of the annealing temperature on the crystalline structure and photoelectrochemical properties of anodic titania nanotube arrays formed in the ethylene glycol and glycerol based electrolytes. In addition, photoelectrochemical properties were studied for ATO samples decorated with Ag nanoparticles.

2. Experimental The Ti foil (99.5% in purity, 0.25 mm thick) was pre-cut in coupons (1 cm × 2 cm). After degreasing in acetone and ethanol, the samples were polished both electrochemically and chemically, rinsed with distilled water and ethanol, and then dried in the air. The anodic titanium dioxide (ATO) layers were prepared by a three-step anodization process in electrolytes based on ethylene glycol or glycerol containing NH4F (0.38 wt.%) and H2O (1.79

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wt.%) at cell potentials ranging from 30 to 60 V19,27. The process was performed at 20 °C in a two-electrode cell, where the Ti sample was used as an anode and the Ti plate as a cathode. The first and second anodizing steps were carried out for 3 h. The oxide films formed during the first and second step of anodization were removed by mechanical detachment. Before the third anodization, the electrochemical cell was filled with the fresh electrolyte, and the sample was anodized in the ethylene glycol and glycerol based solution for 10 min and 60 min, respectively. For example, the thickness of oxide layers formed at 40 V in the ethylene glycol and glycerol based electrolyte was 2.09 and 0.60 µm, respectively. Afterwards, the sample was annealed in air at temperatures ranging from 400 to 1000 °C for 2 h using a muffle furnace (model FCF 5SHM Z, Czylok). The heating rate was 2 °C min-1. All samples were weighed before and after the annealing process. Prior to weighing, samples were kept in a desiccator for 2 h. Silver nanoparticles were deposited into nanopores by soaking TiO2 samples (asreceived and annealed) in the mixture of silver nitrate (0.01 M) and sodium citrate (0.01 M) and then sodium borohydride was added as a reducing agent. Samples were kept in ultrasonic bath to eliminate air bubbles from the pores. The structural and morphological characterizations of ATO layers were performed using a field emission scanning electron microscope (FE-SEM, Hitachi S-4700). The detection of crystalline phases were performed using X-ray diffractometer Rigaku Mini Flex II with monochromatic Cu Kα radiation (λ = 1.5418 Å, U = 30 kV, I = 15 mA, 2θ: 20 - 60° and a step size 0.5 ° min-1). Grazing incident X-ray diffraction measurements were performed on a Panalytical Empyrean diffractometer with Co Kα radiation (λ = 1,78892 Å, ω = 1 - 5º with step size 1º, 2θ: 20 - 60°). The reflectance spectra within the wavelength range of 300– 700 nm were measured by using an ultraviolet–visible spectrophotometer (Avantes, AvaSpec-

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ULS2048) equipped with a xenon lamp (Avantes, AvaLight-XE) and integrating sphere (Avantes, AvaSphere-30). Raman spectra were collected by using a Raman microspectrometer WITec Alpha 300, equipped with an air cooled solid state laser operating at 532 nm, a 600 grooves/nm grating, and a CCD detector. A microscope was coupled with a laser and a spectrograph via a single mode optical fiber with a diameter of 50 µm. All samples were illuminated with the output laser power ca. 3.5 mW by using a 20× air objective. A Raman spectrum of each sample, ATO layers, anatase nanopowder (Alfa Aesar,

Heat Treatment Effect on Crystalline Structure and ... - ACS Publications

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