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Complex ZnO-TiO core-shell flower-like architectures with enhanced photocatalytic performance and superhydrophilicity without UV irradiation Evangelia Vasilaki, Maria Vamvakaki, and Nikos Katsarakis Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01619 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018
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Complex ZnO-TiO2 core-shell flower-like architectures with enhanced photocatalytic performance and superhydrophilicity without UV irradiation Evangelia Vasilaki,a,b,c* Maria Vamvakakic,d and Nikos Katsarakisb,c a
Department of Chemistry, University of Crete 710 03 Heraklion, Crete, Greece. Center of Materials Technology and Photonics, School of Applied Technology, Technological Educational Institute of Crete, 710 04 Heraklion, Crete, Greece. c Institute of Electronic Structure and Laser, Foundation for Research & Technology-Hellas, 711 10 Heraklion, Crete, Greece. d Department of Materials Science and Technology, University of Crete, 710 03 Heraklion, Crete, Greece.
b
KEYWORDS: Photocatalysis; titanium dioxide; zinc oxide; core-shell flowers; complex architectures; hollow TiO2 flowers;. ABSTRACT ZnO-TiO2 core-shell photocatalysts of a complex flower-like architecture were synthesized, using a well-controlled sol-gel coating reaction of pre-synthesized ZnO flower-like structures. The samples were characterized by X-ray diffraction, field emission scanning and transmission electron microscopy, energy-dispersive x-ray spectroscopy, diffuse reflectance UV-Vis and attenuated total reflectance-fourier transform infrared spectroscopy, nitrogen adsorptiondesorption measurements, as well as photoluminescence measurements. Well-defined, core-shell flowers with a wurtzite ZnO core and anatase TiO2 shells, with variable shell thickness, were acquired by appropriately adjusting the ZnO/Ti precursor mass feed ratio in the reaction. Moreover, hollow TiO2 flowers, that retained their morphology intact, following the etching of the ZnO core in an acidic solution, were obtained. The photocatalytic performance of the coreshell and hollow semiconductors was evaluated via the decoloration of a methylene blue dye solution under UV-Vis irradiation. The core-shell flowers exhibited a higher decoloration rate, when compared to bare ZnO flowers, TiO2 particles and hollow TiO2 flowers, and a photoactivity which was depended on the TiO2 shell thickness. This was attributed to the efficient separation of the photogenerated holes and electrons at the ZnO-TiO2 interface. Moreover, the most photoactive core-shell catalyst exhibited excellent reusability and stability for at least three photocatalytic cycles and excellent superhydrophilicity without UV irradiation, due to the increased roughness of the flower-like structures.
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INTRODUCTION Rapid technological advances over the last decades have resulted in the introduction of various contaminants into the environment, for which the conventional treatment methods have been proven ineffective. Therefore, the demand for more efficient purification methods is higher than ever and the scientific interest in the field of heterogeneous photocatalysis is continuously increasing. In addition, materials that simultaneously exhibit high photocatalytic performance and superhydrophilic properties are widely desirable for self-cleaning applications ranging from fabrics and paints to cement, roof tiles and solar panels.1-9 Contaminants on these surfaces can be either decomposed, following light activation, or washed away by water, which in turn reduces their maintenance costs. The most extensively studied photocatalysts so far, are the wide band gap semiconductors, TiO2 (Eg~3.2 eV for anatase),10 and ZnO (Eg~3.37 eV).11 However, their photocatalytic performance is suppressed by a number of drawbacks, including a high degree of recombination of photogenerated electrons and holes, high aggregation tendency and, in the case of ZnO, susceptibility to photochemical corrosion.12 In order to address these issues the rational engineering of TiO2 and ZnO structures with desired properties has been proposed.13,14 Different approaches can be adopted towards this direction, such as mesoporous materials and hierarchical structures.15-17 Another promising alternative is considered the synthesis of core-shell structures18. The basic principle behind the core-shell architecture is that the favorable energy difference between the two semiconductors will facilitate the separation of the electrons and holes, thus decreasing their recombination and enhancing the overall performance of the composite photocatalyst.19-21 Furthermore, the coating of ZnO with a TiO2 layer has been reported to improve its chemical stability against photocorrosion.22 So far, the scientific interest has focused on surface immobilized core-shell rods, which are synthesized using expensive and/or complex methods, such as atomic layer deposition (ALD) and chemical vapour deposition CVD.23-25 These techniques provide a high level of control over important structural characteristics, such as the shell thickness, but are considered disadvantageous due to their high cost, complexity and low growth rates.26 Other simpler and more cost-efficient approaches have been also employed for the synthesis of the shell, among which are the chemical routes (sol-gel, hydrothermal methods). However, these methods possess certain disadvantages, such as low control over the morphology of the final structures, the need for multiple successive deposition cycles to obtain a targeted thickness, inhomogeneous and nonuniform contact between the two semiconductors and side nucleation in the bulk of the solution in parallel to the surface of the structures. 22,26-29 It is well-known though, that the properties of the core-shell semiconductors are strongly affected by the morphology of the ZnO structure, the amount of deposited TiO2 and the accessibility of the pollutant to the ZnO/TiO2 heterojunction.27 Therefore, control over the kinetics of the shell synthesis and the morphology of the final structures is of great importance for their practical applications and requires further investigation. Interestingly, ZnO-TiO2 core-shell photocatalysts in a powder form are scarce in the literature and, to the best of our knowledge, only spherical core-shell particles, which often result in
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agglomerated structures, have been investigated.30-34 However, ZnO is a semiconductor which exhibits the richest range of morphologies and therefore properties.35-39 Control over the morphology of ZnO is thus essential in order to attain more complex core-shell architectures, synthesized by simple, efficient and controllable methods in solution, and explore their use in various fields. Moreover, the surface wettability of metal oxides has been switched from (super)hydrophobic to (super)hydrophilic, upon UV irradiation.40-42 However, the indispensability of UV irradiation in order to tune the wettability limits the range of possible applications of these materials and increases the overall costs. In addition, the photoinduced hydrophilicity of TiO2 is not permanent, and hydrophobicity is quickly restored upon keeping the catalyst in the dark, affecting adversely its self-cleaning efficiency. In this study, the facile and controllable synthesis of ZnO-TiO2 core-shell flower-like 3D structures in solution and their photocatalytic activity towards the decoloration of an aqueous solution of methylene blue dye under UV-Vis light irradiation was systematically investigated. To the best of our knowledge, the synthesis of such complex ZnO-TiO2 core-shell flower-like architectures has not been reported in the literature so far. Additionally, the effect of the titania shell thickness on the morphology and the photoactivity of the synthesized semiconductors was examined. Furthermore, our synthetic approach allowed us to access hollow TiO2 flowers, following the removal of the ZnO core via etching in an acidic solution at room temperature. Therefore, the significance of the core-shell architecture was evaluated by comparing the photocatalytic performance of the hollow TiO2 flowers to that of the ZnO-TiO2 core-shell flower-like structures. Finally, the wettability of the core-shell flowers, when deposited on a glass substrate, was investigated in the absence of irradiation, to evaluate their self-cleaning properties.
EXPERIMENTAL SECTION Materials Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 99%) and hydrochloric acid (HCl, 37 wt%) were supplied by Sigma Aldrich, while methylene blue (MB, 82%) and titanium isopropoxide (Ti[OCH(CH3)2]4, TIP, 97%) were received from Aldrich. Sodium hydroxide (NaOH) pellets were obtained from Panreac and all reagents were used as received. Ethanol (98%) was received from Honeywell and was dried overnight over CaH2 prior to use. Finally, Milli-Q water, obtained from a Millipore apparatus, with a resistivity of 18.2 MΩ cm at 298 K, was used for the preparation of all samples. Synthesis of the ZnO flower-like structures The growth of the ZnO flower-like structures was performed via a hydrothermal method, using aqueous solutions of Zn(CH3COO)2·2H2O and NaOH as the precursors.43 In a typical synthesis, 40 mL of a 0.035 M Zn(CH3COO)2·2H2O and 0.055 M NaOH aqueous solution was transferred
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to a 100 mL Pyrex bottle sealed with an autoclavable cap and was hydrothermally treated in an oven, thermostated at 70 °C, for 24 h. The synthesized catalyst was isolated via vacuum filtration, purified via extensive washing with H2O until pH 7 was reached and was finally dried under vacuum. Synthesis of ZnO-TiO2 core-shell and hollow TiO2 flower-like structures To prepare the ZnO-TiO2 core-shell flowers with different shell thickness, in the following denoted as ZT, a modified protocol reported by Li et al. was followed.44 Specifically, different mass loadings of the synthesized ZnO flowers (Table 1) were dispersed in 35 mL ethanol by ultrasonication for 1 hr. In a second step, the ZnO dispersion was transferred in an ice bath and 0.25 mL Ti[OCH(CH3)2]4 in 5 mL of ethanol were rapidly injected into the suspension under vigorous stirring. The suspension was stirred for 20 min, before immersion in a preheated water bath at 80 °C. Subsequently, a 10 mL mixture of water in ethanol (50 mL/L) was added dropwisely and the reaction was allowed to proceed for 2 hr at 350 rpm/min. The as-prepared ZnO-TiO2 core-shell catalyst was collected by vacuum filtration, washed three times with ethanol, and then dried in a vacuum oven overnight. Finally, the crystallization of the amorphous titania shell was induced by annealing the photocatalysts in a tube furnace at 500 °C for 2 hr, at a heating and cooling rate of ~5 °C/min, under air. Table 1. ZnO mass loadings, TiO2 shell thickness and calculated optical band gap values, Eg, for bare ZnO, TiO2 and the ZnO-TiO2 core-shell structures. Sample ZnO mass (gr) TiO2 shell Optical band gap Eg thickness (nm) (eV) ZnO
-
-
3.30
TiO2
-
-
3.16
ZT1
0.125
35 ± 3
3.04
ZT2
0.1
59 ± 7
3.08
ZT3
0.05
85 ± 8
3.11
ZT4
0.035
Side nucleation and agglomeration
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Next, hollow TiO2 flowers were acquired following the removal of the ZnO core from the coreshell structures, by etching in an acidic solution. The annealed ZnO-TiO2 flower-like architectures were stirred slowly in 1 M HCl for ~10 min and were then collected and rinsed with water till the supernatant reached pH 7.
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For comparison, bare TiO2 particles, in the absence of the ZnO core, were also synthesized following the aforementioned protocol. Characterization X-ray diffraction (XRD) patterns were collected on a PANalytical Xpert Pro X-ray diffractometer, using Cu Kα radiation (45 kv and 40 mA). Photoluminescence (PL) spectra were recorded on a Jobin-Yvon/Horiba Fluoromax-P scanning spectrofluorometer equipped with a 150 W xenon lamp. All measurements were conducted at room temperature, using a 0.3 mg/mL aqueous dispersion of the powder samples, at an excitation wavelength of 325 nm. Diffuse reflectance UV-Vis spectra were obtained in the wavelength range of 300-900 nm, on a Shimadzu UV-2401 PC spectrophotometer using an ISR-240A integrating sphere. BaSO4 powder was used as a 100% reflectance standard and measurements were conducted by coating the powder sample on top of the BaSO4 base material. Attenuated total reflectance-fourier transform infrared (ATR-FTIR) spectra were obtained using a Thermo Scientific Nicolet 6700 spectrometer, while nitrogen adsorption-desorption isotherms were measured on a Quantachrome NOVA 3200e sorption analyzer at −196 °C. All samples were outgassed at 80 °C under vacuum (
