Article pubs.acs.org/JPCC
Dependences of ZnO Photoinduced Hydrophilic Conversion on Light Intensity and Wavelengths Aida V. Rudakova,*,† Ulyana G. Oparicheva,† Anastasiya E. Grishina,† Maria V. Maevskaya,† Alexei V. Emeline,† and Detlef W. Bahnemann*,†,‡ †
Laboratory “Photoactive Nanocomposite Materials”, Saint-Petersburg State University, Ulyanovskaya str. 1, Peterhof, Saint-Petersburg, 198504 Russia ‡ Institut für Technische Chemie, Photokatalyse und Nanotechnologie, Leibniz Universität Hannover, Callinstrasse 3, 30167 Hannover, Germany S Supporting Information *
ABSTRACT: The present study explored the effect of light intensity and spectral variation of the actinic light on the hydrophilic conversion of the surface of ZnO nanocoatings. The dependence on light intensity indicates that both formation and destruction of the surface hydrophilic states occurs in parallel. The proposed kinetic mechanism corresponds well with experimental dependences. The spectral dependence of the photoinduced hydrophilic conversion of the ZnO surface suggests the important role of electronic photoexcitation of the solid. Similarity between the spectral distributions of the efficiency of photoinduced hydrophilic conversion and the ratio between the surface concentrations of electrons and holes (estimated on the basis of the ratio between quantum yields of oxygen and methane photostimulated adsorption) infers that the reason for the hydrophilicity alteration is the surface charge redistribution caused by actinic light.
■
INTRODUCTION
knowledge the study of such dependences for ZnO has not been reported yet.
■
The wettability of materials is an important functional parameter in various applications.1−3 Self-cleaning properties are dictated not only by photocatalytic activity of photoactive materials but also by their ability to photoinduce hydrophilic/ hydrophobic conversion of the surface [for example, see ref 4]. During the last few decades ZnO has attracted increasing attention due to its potential applications in self-cleaning coating applications. The reversible UV-controlled hydrophilic/ hydrophobic transformation has been found for zinc oxide by many researchers.5−11 They explained this phenomenon by the competition between adsorption and desorption of hydroxyl groups and by the rearrangement of the organic species on the ZnO film surface. In experimental studies of the effect of photoinduced hydrophilicity on the TiO2 surface, it was shown that the wavelength dependence and the dependence on light intensity of conversion efficiency for both water contact angle and surface energy indicates the role of electronic photoexcitation in hydrophilic transformations.12,13 The aim of the present study is to explore the effect of light intensity and spectral composition of actinic light on the efficiency of the photoinduced hydrophilic conversion of the surface of ZnO nanofilms. The results of this study should clarify the mechanism of ZnO photoexcitation that leads to the alteration of the surface hydrophilicity. To the best of our © XXXX American Chemical Society
EXPERIMENTAL SECTION ZnO nanocoatings were formed by a sol−gel dip coating method (KSV Nima dip coater) onto SiO2-coated glasses followed by annealing at 500 °C in the air to remove all organic residuals after synthesis for at least 10 h. Synthesized ZnO films were formed by nanoparticles with crystallite sizes of about 20 nm. The film thickness estimated by electronic microscopy was equal to 60−65 nm. Film surface smoothness was appraised by AFM to be ∼4 nm. No detectable changes of the surface roughness were observed after the performed sets of experiments. XRD data detected the zincite (wurtzite) crystal structure of the ZnO coating. Details of the characterization are presented in the Supporting Information. The water contact angle (Θ) values were measured using the Bioline Theta Lite optical tensiometer. Ultrapure water had a pH value of 5.5. To asure reproducible initial states of the ZnO surfaces for the spectral studies, we used a special procedure developed in our laboratory and described elsewhere.14 Briefly, ZnO nanocoatings were treated at 500 °C for 1 h and cooled to Received: January 12, 2015 Revised: April 13, 2015
A
DOI: 10.1021/acs.jpcc.5b00327 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C ambient temperature under controlled atmosphere. After this treatment, the nanofilm surface was characterized by a water contact angle value of 3.0° (superhydrophilic surface state). At the next step, the samples were wetted in ultrapure water with pH = 5.5 for 40 min followed by drying with an air flow at 80 °C to obtain a hydrated surface, which is characterized by a water contact angle value of 15.5° ± 2°. This state could stay unchanged during approximately 3 h under stationary environmental conditions (temperature +19 °C, relative humidity about 60%) and was used as an initial state for the following set of experiments. The dependence of the efficiency of the hydrophilic transformation on light intensity was measured using a 1000 W Hg−Xe lamp (Oriel Instruments) as a light source with light power range of 0−40 mW·cm−2 at 365 nm. Monochromatic irradiation of the film surface was carried out with the setup consisting of a 1000 W Hg−Xe lamp (Oriel Instruments) and a monochromator MDR-2 (LOMO). Wavelength dependences were measured within the spectral range from 300 to 600 nm with the spectral resolution of monochromatic light being about Δλ = ±5 nm.
Figure 2. Dependence of the initial rate of the water contact angle growth on light intensity.
⎛ dΘ ⎞ aρ ⎜ ⎟ = ⎝ dt ⎠ t ≈ 0 bρ + 1
■
RESULTS Effect of Light Intensity on the Efficiency of the Photoinduced Hydrophilic Conversion. The typical kinetics of the photoinduced water contact angle alteration on a dehydrated ZnO coating surfaces under broadband irradiation are presented in Figure 1. No photoinduced
(1)
where a and b are apparent constants independent of the light intensity. From the kinetics of the photoinduced water contact angle alteration on the ZnO nanofilm surface, the ultimate water contact angle values (achieved at infinite time of irradiation, t → ∞) were found for different light intensities. Figure 3 shows the dependence of the ultimate water contact angle on the light intensity.
Figure 1. Kinetics of photoinduced water contact angle alteration of dehydrated ZnO coating surface under broadband irradiation. Nanocoatings were irradiated by full light of a 150 W Xe-lamp (OSRAM).
Figure 3. Dependence of the ultimate water contact angle (Θinf) on light intensity.
conversion into the superhydrophilic state has been observed. As evident from experimental data, the kinetics of the photoinduced hydrophilic conversion demonstrates a rather complex behavior: there is a rapid rise of the contact angle in the initial time period followed by a fast decay and a slow growth approaching a saturation limit at longer irradiation times. The initial rate of the photoinduced alteration of the water contact angle on the ZnO nanofilm surface was used as a parameter for estimation of the efficiency of the photoinduced alteration of the surface hydrophilicity. Figure 2 demonstrates the dependence of the initial rate of contact angle growth (dΘ/ dt)t≈0 on the light intensity (ρ) of the actinic light. This dependence can be well approximated (R2adj = 0.97596) with eq 1:
It is obvious that at continuous irradiation time the surface is turned into a less hydrophilic state than the initial state. The highest level of photoinduced alteration of surface hydrophilicity is achieved only at light intensities higher than 0.5 mW·cm−2, and the ZnO surface state can be characterized by an ultimate water contact angle Θinf of 23.0° ± 2°. At lower (
