Communication pubs.acs.org/JACS
Enhanced Photocatalytic Reaction at Air−Liquid−Solid Joint Interfaces Xia Sheng,† Zhen Liu,† Ruosha Zeng,† Liping Chen,† Xinjian Feng,*,† and Lei Jiang‡ †
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People’s Republic of China ‡ School of Chemistry and Environment, Beihang University, Beijing 100191, People’s Republic of China S Supporting Information *
reaction interface is dependent upon its mass transfer through the water phase. The low concentration and slow diffusion rate of oxygen in water limits the removal of photogenerated electrons. As a result, high light intensities do not correspond to high photocatalytic reaction rates.19−21 Herein, in contrast to previous approaches, we address this limitation by developing a novel triphase photocatalytic system, see Figure 1, where photocatalysts are immobilized on the
ABSTRACT: Semiconductor photocatalysis has long been considered as a promising approach for water pollution remediation. However, limited by the recombination of electrons and holes, low kinetics of photocatalysts and slow reaction rate impede large-scale applications. Herein, we addressed this limitation by developing a triphase photocatalytic system in which a photocatalytic reaction is carried out at air−liquid−solid joint interfaces. Such a triphase system allows the rapid delivery of oxygen, a natural electron scavenger, from air to the reaction interface. This enables the efficient removal of photogenerated electrons from the photocatalyst surface and minimization of electron−hole recombination even at high light intensities, thereby resulting in an approximate 10-fold enhancement in the photocatalytic reaction rate as compared to a conventional liquid/solid diphase system. The triphase system appears an enabling platform for understanding and maximizing photocatalyst kinetics, aiding in the application of semiconductor photocatalysis.
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ater pollution has become a global environmental problem, with well over 1 billion people lacking access to safe drinking water.1,2 Therefore, rapid water remediation methods are urgently required. Semiconductor photocatalysis is a promising approach for water purification as light-excited charge carriers can be used to decompose a wide range of organic pollutants.3−8 The recombination of electrons (e) and holes (h) is in competition with this photocatalytic degradation process, which is generally recognized as a key factor limiting the kinetics of photocatalysts and reaction rate.9 To overcome this limitation, a wide variety of approaches have been proposed, such as the use of metal/semiconductor hybrids.10−15 However, these efforts generally focus on the design of photocatalysts and only enable charges temporally separated within the nanoscale photocatalysts spatially. For the charges to be efficiently separated and subsequently utilized, an effective approach is to remove them from the photocatalyst surface by suitable and sufficient acceptors. Compared to photocatalysts, the reaction interface that dominates the solid/liquid contact and mass transfer is also crucial to the photocatalytic process, but has received limited attention. Oxygen is an effective natural electron scavenger in semiconductor photocatalysis.16−19 However, in traditional diphase photocatalytic systems the availability of oxygen at the © 2017 American Chemical Society
Figure 1. (a) Schematic illustration of the triphase photocatalytic system. (b) Enlarged view of the solid−liquid−air triphase reaction interface. Panels (c) and (f) are FE-SEM images of the porous carbon fiber substrate coated with anatase TiO2 nanoparticles at low and high magnifications, respectively. Panels (d) and (e) are, respectively, photographs of water droplet that placed on the substrate with and without TiO2 immobilization, which indicates that water can wet the top photocatalyst layer but cannot penetrate into the underneath porous superhydrophobic substrate.
surface of a superhydrophobic porous substrate and photocatalytic reaction is carried out at air−liquid−solid joint interfaces. Such a triphase interface architecture allows the rapid delivery of oxygen from air to the reaction interface, thereby minimizing electron−hole recombination even at high light intensities. This results in higher rates of photocatalytic activity. Superhydrophobic substrates have received rapidly increasing interest since the 1990s for their use in diverse Received: July 10, 2017 Published: August 30, 2017 12402
DOI: 10.1021/jacs.7b07187 J. Am. Chem. Soc. 2017, 139, 12402−12405
Communication
Journal of the American Chemical Society
Figure 2. (a) Variation of the SA concentrations as a function of illumination time under different UV light intensities, wavelength of 367 ± 5 nm, for the triphase (left) and diphase (right) systems. (b) Calibrated plots of the degradation rate versus UV light intensity. (c and d) Illustration depicting the behavior of photogenerated charge carriers in a diphase system under low (c) and high (d) light intensities. (e) Suggested behavior of the photogenerated charge carriers in a triphase system under high-intensity light. (f) Effect of the UV light intensity on the apparent quantum yield (AQY) of the triphase and diphase systems.
scientific and technological applications,22−34 such as drug release,27 microdroplet transport,28 crystals assembly,29 enzymatic reaction30,31 and electrochemical deposition.32 Superhydrophobic substrates can trap atmosphere-connected air pockets upon contact with a liquid, thereby resulting in the formation of an interface where air, liquid and solid phases coexist at the micro/nanoscale level.22−24 Oxygen is rapidly obtained from the air rather than by diffusion through the liquid phase, as is common in conventional diphase photocatalytic systems. The rapid supply of oxygen efficiently removes photogenerated electrons from the surface of the photocatalyst, as presented in Figure 1b, thereby resulting in the effective separation of e−h pairs and the generation of oxidative reactive oxygen species (ROS) such as O2•− and OH•. Moreover, a sufficient interface oxygen supply enables the efficient execution of photocatalytic reactions even in the presence of high charge densities, i.e., at high light-intensities, thereby significantly enhancing the photocatalyst kinetics and pollution degradation rate. To demonstrate the triphase photocatalytic system, we immobilized anatase-phase TiO2 nanoparticles with an average particle size of 25 nm (Figure S1, Supporting Information) onto a superhydrophobic polytetrafluoroethylene-treated carbon fiber substrate. The superhydrophobic carbon fiber substrate has a porous structure, as presented in the bottom of Figure 1c, and has a water contact angle (CA) of 155 ± 2° (Figure 1e). Following TiO2 immobilization (see top of Figure 1c), the surface becomes hydrophilic with a CA of 30 ± 2° (Figure 1d). In this case, water can wet the photocatalyst layer but cannot penetrate further into the porous substrate, thereby resulting in ambient-connected air trapping and the formation of a solid−liquid−air triphase reaction interface. Figure 1f is a scanning electron microscopy (SEM) image of the TiO2 nanoparticle−nanostructured carbon fiber interface. For control experiments, a diphase photocatalytic system was fabricated by immobilizing TiO2 nanoparticles on ground glass substrates. Salicylic acid (SA) was chosen as a model pollutant to establish system performance as it is frequently used
in industrial processes but is toxic to aquatic life. The photocatalytic tests were conducted in a quartz cell containing an aqueous solution of SA (80 mg L−1). The portion of substrate immobilized with TiO2 was immersed in the solution, while the portion (free of TiO2) was exposed to air (Figure S2). Before testing, a dark absorption step was carried out for 30 min. Photocatalytic reactions were performed using ultraviolet (UV) light centered at a wavelength of 367 ± 5 nm (Figure S3). Photocatalytic performance was first evaluated at different light intensities. Using the triphase system, as presented in Figure 2a, the degradation rate (R, −dC/dt) of SA steadily increased with increasing light intensity (I). Though the rate of the diphase system also increased with I, its increase was much slower than that of the triphase system. For example, to achieve a comparable triphase system R value at a light intensity of 5 mW cm−2, a 10fold higher intensity is required for the diphase system. A control experiment was conducted using the superhydrophobic substrate without TiO2 immobilization (dark line in Figure 2a); no SA degradation was observed. Figure 2b presents the calibration plot of R versus I; for the triphase system (red line), R increased linearly with I up to a value of 12 mW cm−2, which is 8-fold higher than that of the diphase system (∼1.5 mW cm−2). The difference in performance of these two systems can be attributed to their different reaction interface architectures. Generally, the relationship between R and I can be expressed using the Langmuir−Hinshelwood kinetic model:35 R = kf (O2 )f (SA)I θ
With f (O2 ) = K O2CO2/(1 + K O2CO2) f (SA) = KSACSA /(1 + KSACSA )
CO2 and CSA are the concentrations of oxygen and SA at the reaction interface, respectively; KO2 and KSA are the adsorption constant of oxygen and SA, respectively; k is the reaction constant. As presented in Figure 2a and Figure S4, at each light 12403
DOI: 10.1021/jacs.7b07187 J. Am. Chem. Soc. 2017, 139, 12402−12405
Communication
Journal of the American Chemical Society intensity the degradation ratio of SA increased linearly with time during the initial 2 h, suggesting that the photocatalytic reactions follow zero-order kinetics, i.e., KSACSA ≫ 1. Thus, the influence of f(SA) on R is negligible and the overall SA degradation kinetics are mainly f(O2)- and I-dependent. In such a case, a higher CO2 will generate a higher f(O2) and faster R; if a constant interface CO2 can be maintained, a linear increase in R can be achieved with I. When the diphase system is under low illumination intensities, oxygen consumption and supply can be balanced, thus a constant interface CO2 can be maintained (Figure 2c) resulting in a linear relationship between R and I (R ∝ I), see Figure 2b (black line), wherein the photocatalytic process is determined by surface reactions. A further increase in the light intensity (I > 1.5 mW cm−2) results in the formation of an interfacial oxygen depletion layer due to the slow supply of oxygen from the liquid (Figure 2d), a decrease in CO2 (f(O2)), and a nonlinear increase of R with I (R ∝ Iθ, 0
