Solving the problems of water pollution and water shortage is an urgent need for the sustainable development of modern society. Different approaches, including distillation, filtration, and photocatalytic degradation, have been developed for the purification of contaminated water and the generation of clean water. In this study, we explored a new approach that uses solar light for both water purification and clean water generation. A bifunctional membrane consisting of a top layer of TiO2 nanoparticles (NPs), a middle layer of Au NPs, and a bottom layer of anodized aluminum oxide (AAO) was designed and fabricated through multiple filtration processes. Such a design enables both TiO2 NP-based photocatalytic function and Au NP-based solar-driven plasmonic evaporation. With the integration of these two functions into a single membrane, both the purification of contaminated water through photocatalytic degradation and the generation of clean water through evaporation were demonstrated using simulated solar illumination. Such a demonstration should also help open up a new strategy for maximizing solar energy conversion and utilization.
With the accelerated development of modern society and rapid population growth, the consumption of energy increases significantly every year, which in turn leads to severe environmental pollution problems, especially air and water pollution. Overdischarging of contaminants into natural water resources leads to the shortage of clean water globally.(1, 2) It is predicted that by the year 2025, nearly two-thirds of the world’s population will live in water-stressed countries.(3) One way to battle water shortage is to reuse contaminated water through various decontamination processes, such as purification or distillation. Due to the cleanness and sustainability of solar energy, using solar energy for clean water regeneration has attracted significant attention, and tremendous progress has been reported recently.(4-7)
One promising and widely studied route to use solar energy for the purification of water is decontamination through photocatalytic processes. In the photocatalytic decontamination process, a photoactive material absorbs sunlight and creates highly active species, such as H2O2, OH•, O2•–, and O3, which can react with contaminants to produce small nontoxic molecules.(7) Most of the catalysts studied, however, can only be activated by specific ranges of photon wavelengths, while the other wavelengths of the solar spectrum are not efficiently used. For example, titanium dioxide (TiO2), which has received the most attention among all the photocatalytic materials studied in the removal of highly toxic and nonbiodegradable pollutants in water,(8, 9) has a narrow light absorption band in the near-UV region. This narrow absorption band contributes to the low photocatalytic efficiency of TiO2.(10) There have been many studies on the modification of structure and morphology,(11, 12) surface chemistry,(13-15) and composition(16-18) of TiO2 in order to enhance its photocatalytic efficiency. In addition to increasing the range of the absorption through materials engineering of catalysts, expanding the usage of photons in the range outside of the catalytic absorption band will also lead to an increased efficiency of solar energy utilization. Even within the absorption band, some of the absorbed photons are converted into heat,(8) and approaches that take advantage of such heat can also help improve the overall efficiency in solar energy usage.
Another promising route for the use of solar energy for water purification is the solar-driven water evaporation process. In photocatalytic reactions, solar energy is converted into chemical energy and induces the degradation of contaminates, while in solar-driven water evaporation processes the solar energy is converted into thermal energy, which in turn drives efficient water evaporation.(19-21) Solar-driven water evaporation has attracted increased attention lately.(22, 23) In particular, plasmonic photothermal conversion was demonstrated to be a highly efficient approach for solar-driven water evaporation.(24, 25) We recently developed a plasmonic interfacial evaporation system that was inspired by the localized heating effects of biological systems.(26-28) In this bioinspired interfacial evaporation system, a gold nanoparticle (Au NP) film fabricated through either self-assembly or filtration floats at the air–water interface. Under the solar irradiation, the plasmonic Au NP film is heated, and generates a localized “hot-zone” at the air–water interface. This confined heating effect can maximize the energy efficiency of the water evaporation because it avoids the unnecessary heating of the bulk liquid.
Figure 1. Bifunctional TiO2–Au-AAO membrane has three layers: a top TiO2 NP layer, a middle Au NP layer, and a bottom AAO layer. This design integrates both photocatalytic degradation and solar vapor generation functions into a single system for the purification of contaminated water.
Chloroauric acid (HAuCl4, 49–50% Au basis), trisodium citrate dihydrate, hydroxylamine hydrochloride, and Rhodamine B (RhB) were obtained from Aladdin (Shanghai, China). All the materials were analytical grade and used without further purification. Titanium dioxide (TiO2, anatase, ∼25 nm in diameter) was also purchased from Aladdin. Anodic Aluminum Oxide (AAO) membranes were purchased from Lessonnano Technology Co. Ltd. (Guangzhou, China) with a pore size of ∼20 nm. Deionized water was produced by the Millipore Water Purification System (NANO pure, Billerica, MA, U.S.A., 18.2 MΩ).
Fabrication of Bifunctional Membrane
Au NPs were synthesized using the citrate reduction method.(29, 30) First, a 50 mL aqueous solution of 1 mM HAuCl4 was injected into a 100 mL round-bottom flask attached with a condenser and brought to boil under vigorous stirring. Next, a 5 mL solution of 38.8 mM sodium citrate was added into the flask, and the color of the solution immediately changed from pale yellow to reddish-purple. After boiling for 10 min, the solution was cooled down and stirring was continued for an additional 15 min. The as-synthesized colloidal Au NPs were further grown into larger particles through the addition of 1.25 mL of 0.2 M hydroxylamine hydrochloride and 1.5 mL of 1% HAuCl4 at room temperature to the flask. After 35 min of reaction, Au NPs with a diameter of ∼50 nm were obtained, and the particle solution was stored for later use.
To construct the bifunctional TiO2–Au-AAO membrane, first ∼25 mL of as-synthesized 50 nm Au NPs solution was filtered through an AAO membrane and dried in air. Next, a 0.5 mL solution of 25 nm TiO2 NPs (1.0 mg/mL, dispersed in DI water through sonication) was filtered through the Au NP-coated AAO membrane to form a photoactive layer on the top. This composite membrane was dried and stored for the subsequent testing of properties. Two types of control samples that contain either a Au NP-only film or a TiO2 NP-only film on top of the AAO membrane were also prepared using the same filtration method with same amount of corresponding NP solutions.
The morphology and size of the synthesized Au NPs were observed with a transmission electron microscopy (TEM, Tecnai G2 Spirit Biotwin, FEI). The bifunctional TiO2–Au-AAO membrane samples were examined by a field-emission scanning electron microscopy (SEM, FEI Sirion 200, 5 kV). The optical absorption spectra of the samples in the range of 200–800 nm were obtained by an UV–Vis-NIR spectrometer (PerkinElmer, Lambda 750S). The optical absorbance measurement for RhB solution was performed using an Ocean Optics spectrometer (HR2000+CG, Ocean Optics, U.S.A.) attached with two optical fibers (QR400–7-Vis-NIR fiber, Ocean Optics, U.S.A.). A 6W tungsten-halogen light source (HL-2000, Ocean Optics, U.S.A.) was also used for the measurement.
Photocatalytic Activity Test
To evaluate the photocatalytic property of the fabricated bifunctional membrane, an organic dye RhB was used as the model material, and its photodegradation processes by both the bifunctional membrane and the control membrane samples were studied. For a typical test, an aqueous RhB solution (20 mg/L) was prepared and 10 mL of the solution was injected into a 15 mL quartz beaker. The membrane sample was inserted and floated on top of the RhB solution. Before exposure to simulated solar light, the mixtures were kept in the dark at room temperature for 12 h to reach adsorption–desorption equilibrium. A 300 W Xe lamp (Shanghai Bilon Instrument Co., Ltd.) equipped with a Fresnel lens (Shenzhen Salens Technology Co., Ltd.) was used as the light source. After preconditioning, the quartz beaker with RhB solution and membrane sample was placed at the focal position of the Xe lamp and irradiated by the Xe light from the top. The illumination power was maintained at ∼2.1 W during the irradiation, and the absorbance spectra of the sample solution were recorded every 30 min using the Ocean Optics spectrometer. The concentration of RhB was determined by the absorbance at 553 nm. The following equation was used to calculate the degradation degree (De) of RhB:where C0 is the initial concentration of RhB and C is the concentration at the irradiation time of t. Due to the evaporation of water during the photocatalytic testing, the concentration of the RhB was calibrated according to the weight loss of the solution. After finishing each test, the membrane samples were rinsed with DI water thoroughly and dried for the next measurement. The above procedure was repeated at least 3 times for each sample.
Evaporation Performance Measurement
To test the performance of solar-driven water evaporation using the bifunctional membrane, the weight loss of the sample solution was measured every 30 min by an electronic balance with four-decimal precision (FR124CN, Ohaus Instrument, Shanghai, China). The weight loss of the sample solution was plotted as the function of irradiation time. In addition, an infrared (IR) camera (FLIR T620, FLIR Systems, Inc., U.S.A.) was used to monitor the surface temperature of the samples during the experimental process. Before measurement, the temperature of the IR camera was calibrated using a calibrated thermocouple (Model K–H-GGF; Beijing Qiaomu Automation Technology Company, China), and the temperature uncertainty of the IR camera is ∼1.0 °C.
Figure 2. Fabrication of bifunctional membrane. (a) Cross-sectional SEM image of 2-layer structured Au NP/AAO membrane; (b) Cross-sectional SEM image of three-layer structured TiO2 NP/Au NP/AAO membrane; (c) Preparation schematic of bifunctional membrane.
Figure 4. Optical absorption and photodegradation performance of different samples: (a) Absorption spectra; (b) Photocatalytic degradation of RhB; (c) Repeated photocatalytic degradation of RhB for continuous 8 cycles.
The improvement in the photocatalytic property of the TiO2–Au-AAO membrane should be attributed to the following three effects. First, the Au NP film under the TiO2 film helps enhance the light absorption of the TiO2 NPs through the plasmonic enhancement effect.(40-42) Such enhancement leads to the improvement in the efficiency of photoreaction of the nearby TiO2 NPs. Second, the Au NP film should also facilitate electron–hole charge transfer and separation through the formation of Schottky junctions between the Au NPs and TiO2 NPs, which results in the increased photocatalytic activity.(43) Third, the thermal effect caused by plasmonic photothermal conversion of the Au NP film should also play a role in the improvement of photocatalytic activity of the three-layer composite membrane.(44)
Figure 5. Water vapor generation by different samples. (a) Evaporation of different samples under the simulated solar light. (b–e) Surface temperature study of different samples during the testing. (b) TiO2–Au-AAO sample before light illumination; (c) TiO2–Au-AAO sample after 2 h of light illumination; (d) Au-AAO sample after 2 h of light illumination; (e) TiO2-AAO sample after 2 h of light illumination.
In the three-layered bifunctional membrane that was fabricated in this study, each layer has its desired role. The top TiO2 NP layer works as the photocatalytic layer for the degradation of contaminates in water. The middle Au NP layer works as the plasmonic photothermal converter for the water vapor generation and also helps enhance the photocatalytic activity through plasmonic effects.(40-42) As long as the size of the Au NP is larger than the pore size of AAO membrane, the photothermal-enabled evaporation performances are close to each other for films with Au NPs of different sizes (Figure S4) due to the broad band absorption of the films.(26-28) The bottom AAO membrane serves the following three functions: at the fabrication step it works as the filter for the fabrication of bifunctional membrane; later during the water purification step, it serves as the supporting layer to ensure both the mechanical robustness of the composite membrane and the capillary flow of the water through the numerous pores on the membrane. Additionally, the low thermal conductivity of the AAO membrane also makes it a good thermal insulation layer to minimize the heat loss to the lower part of the solution through thermal conduction.
With the three-layer design, this composite membrane works as a bifunctional membrane for different degrees of water purification. After light illumination, the liquid remaining in the beaker is the partially purified water that was cleaned through the photocatalytic reaction by TiO2. The evaporative function of the membrane, on the other hand, generates purified water vapor and thus purified liquid water after the condensation process. Indeed, the absorbance study showed that the liquid collected from the water vapor contains no RhB, while the solution left in the beaker after 2 h of Xe light illumination contains partial purified water with ∼40% of RhB remaining in the solution (Figure 6). Increasing the irradiation time would further purify the water left in the beaker to the degree that is needed for different applications.
Figure 6. Water purification by bifunctional membrane: (a) absorption before and after purification for 2 h; (b) optical images of the water in cuvettes before and after purification for 2 h.
In summary, a three-layer bifunctional composite membrane with photocatalytic function and photothermal driven evaporation function was designed and fabricated for efficient water purification using solar light. Previously, the loss of energy efficiency in the photocatalytic water purification has been the result of two processes: (1) the absorption process with relative narrow absorption band of photocatalysts; and (2) the photothermal conversion with the absorbed photons. The integration of the photothermal driven evaporation function with the photocatalytic function helps expand the range of useful solar light for the clean water generation and also takes advantage of the heat generated by the catalysts for further efficiency enhancement. Such multilayered membrane enables the generation of both pure and partial purified water in a single step. Pure water was generated through the photothermal evaporation process and subsequent condensation, and partially purified water was the result of the photocatalytic degradation process. Compared to the membrane with only a single functionality, the bifunctional design provides a new approach for water purification, and the novel method also opens up an alternative strategy to maximize solar energy conversion and utilization. This reported membrane design could be further improved, for example, by replacing the Au NPs with other low cost photothermal converters, such as carbon NPs, and by utilizing flexible porous substrates instead of the rigid ceramic AAO substrate, to explore practical applications for pure water generation in large scale in the future.
Energy-dispersive spectroscopy (EDS) mapping of the TiO2–Au-AAO membrane; SEMs and performance of membranes with TiO2 NP layer of different thicknesses; evaporation rate of membranes with Au NP (50 and 100 nm in diameter) layer of different thicknesses (PDF)
Y.L. and J.L. contributed to this work equally.
The authors declare no competing financial interest.
This work was supported by National Natural Science Foundation of China (Grant No: 91333115, 51420105009, 51521004, 51403127 and 21401129), Natural Science Foundation of Shanghai (Grant No: 13ZR1421500, 14ZR1423300), and the Zhi-Yuan Endowed fund from Shanghai Jiao Tong University. The authors thank Professor Peng Zhang, Mr. Shipu Li, Mr. Shengtao Yu, Mr. Yao Zhang, Mr. Zhongyong Wang for the help with this project. We also thank the Instrumental Analysis Center of Shanghai Jiao Tong University for the access to SEM and TEM.
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