Membrane surface modification with TiO2–graphene oxide for enhanced photocatalytic performance
Introduction
Membrane processes are among the most effective means to remove various water contaminants that have significant adverse health and/or environmental effects [1]. A dilemma, however, exists for membrane-based water-contaminant separation: reliance mainly on size exclusion may attain a stable, high rejection rate but the associated energy demand can be enormous [2], [3]. For this reason, it is very desirable to enable additional contaminant removal mechanisms in membrane processes so that contaminants can be adequately rejected from water without unduly decreasing membrane pore sizes (and consequently incurring considerable energy cost). Because photocatalysts can initiate reactions to decompose organic contaminants under ultraviolet (UV) or even sunlight irradiation without consuming chemicals or producing chemical wastes, photocatalytic reactions are considered a sustainable approach to the removal of a variety of environmental pollutants [4]. Therefore, integration of photocatalysis with membrane processes may significantly improve the membrane separation performance [5]. Furthermore, because only less than 7% of sunlight is UV while 47% is visible light [6], synthesis of photocatalytic membranes that are active under not only UV but also visible light irradiations is key to more efficient use of solar energy as a sustainable energy source.
Titanium dioxide (TiO2) is the most commonly used photocatalyst in water treatment due to its high removal efficiency, cost-effectiveness, chemical stability, and low toxicity [4], [7], [8]. TiO2 is able to completely decompose organic pollutants (e.g., organic dyes, toxic micropollutants, oils) into carbon dioxide and water [7], [8]. Incorporation of TiO2 nanoparticles into a water filtration membrane has been found to enhance its flux, contaminant removal, and fouling resistance [9], [10], [11], [12], [13]. The application of photocatalytic membranes made with TiO2 alone, however, has been severely limited due to their very low photoactivity under sunlight [14]. This is because TiO2 has relatively high band-gap energy (3.2 eV) and thus the electron-hole pairs responsible for photocatalysis can only be activated by UV light, which has a wavelength of less than 387 nm and carries energy higher than 3.2 eV [14]. As a result, visible light, which has a longer wavelength (and thus a lower energy level), is unable to initiate any photocatalytic activity of TiO2.
With its exceptional electron-transferring property, the emerging graphene oxide (GO) is considered an ideal nanomaterial to expand the light-response range of TiO2 [15], [16], [17], [18], [19]. GO can work as an electron acceptor/transporter for TiO2 nanoparticles and thus can significantly improve the lifetime of electron-hole pairs [20]. Therefore, the TiO2–GO nanocomposites have a wider light-response range and faster photodegradation kinetics, eventually leading to an improved efficiency of photocatalysis under both UV and visible light [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. Besides, the large surface area of GO (~2630 m2/g) along with its high adsorption capacity may further enhance the photocatalytic efficiency of TiO2–GO by establishing a closer, longer contact between the contaminant and photocatalyst [33]. Potentially acting as a macromolecular photosensitizer, GO may also transform wide-band-gap semiconductors (including TiO2) into visible light photocatalysts [34]. In addition, the unique planar structure of GO nanosheets [35], [36], [37] makes it possible to synthesize TiO2–GO nanocomposite materials via a facile, scalable, and cost-effective layer-by-layer (LbL) technique, compared with other approaches (e.g., dye sensitization [38], metal and/or non-metal doping [7], [8], coupling with other semiconductors [39]) that often involve complex protocols and/or costly materials in expanding the light-response range of TiO2.
TiO2–GO has mostly been used as suspended particles in the solution of batch reactors to remove water contaminants [24], [25], [26], [27], [28], [29]. Although such suspended photocatalysts can ideally contact with contaminants in water and thus achieve the highest possible catalytic efficiency, the burden of separating TiO2–GO particles from the treated water afterwards may considerably increase the complexity, operation, and maintenance cost of the water treatment system. In order to circumvent the recovery of TiO2–GO particles but still retain their excellent photocatalytic properties, TiO2–GO can be immobilized in the matrix or on the surface of a water filtration membrane. It is expected that such an introduction of the photocatalytic process may affect various membrane properties such as hydrophilicity, water permeability, contaminant rejection and photodegradation, and fouling resistance.
So far, the use of TiO2–GO to make photocatalytic membranes for water treatment is still in its infancy. To our best knowledge, there have been only two relevant studies on this topic. In one study, the adsorption capability of the membrane was investigated but without any discussion on its photocatalytic properties [40], while in another study the photocatalysis-enhanced degradation of organic dyes was investigated under UV irradiation only [41]. Note that these studies adopted a relatively complex two-step membrane synthesis procedure that first fabricated the TiO2–GO composite nanoparticles and then assembled them onto a base membrane.
To date, many fundamental questions still remain to be answered regarding the integration of TiO2–GO with membrane processes. For example, it is unknown whether and how TiO2–GO can be advantageously employed to synthesize a sunlight-active photocatalytic membrane for water-contaminant separation. Besides, it is highly desirable to develop a simple yet efficient synthesis approach to advance the photocatalytic membrane technology. In the meantime, basic properties of the TiO2–GO membrane, including the light-response range and photodegradation kinetics, have not been experimentally characterized in any previous studies. Therefore, a systematic study is needed to synthesize and characterize the TiO2–GO membrane as well as to elucidate the underlying photocatalytic mechanisms.
In response to the above research needs, we propose a facile LbL approach to deposit TiO2 and GO on a polysulfone (PSf) base membrane to enable photoactivity under both UV and sunlight. After the deposition phase, stable bonding between GO and TiO2 layers was created by post-treating the membrane with ethanol and UV. We verified the effectiveness of the LbL procedure by quartz crystal microbalance with dissipation (QCM-D) and characterized the properties of the TiO2–GO membrane by various techniques. Finally we tested the performance (i.e., water flux, photodegradation of organic dyes) of the TiO2–GO membrane under different light sources.
Section snippets
Nanoparticles and chemicals
GO nanosheets were synthesized from graphite using the modified Hummers method [42], [43]. The synthesis procedure included oxidizing the graphite in a mixture of KMnO4, H2SO4, and NaNO3, filtering the mixture to collect particulate solids (i.e., graphite oxide), washing the solids in deionized (DI) water through three cycles of re-suspension and centrifugation to remove chemical residues, sonicating the GO particle suspension using an ultrosonicator (S-4000, Misonix, Farmingdale, NY) to
Reaction schemes for surface modification
Fig. 1(b) depicts the reaction schemes for the synthesis of a TiO2–GO photocatalytic membrane. First, TiO2 nanoparticles were attached to the PSf base membrane by the Ti–O bond formed between Ti4+ and sulfonic groups and/or by the hydrogen bond between the hydroxyl groups of TiO2 and sulfonic groups [46]. Next, GO was deposited on the TiO2 layer via Ti–O and/or hydrogen bond between Ti4+ and the carboxyl groups of GO. Finally, GO was partially reduced and bonded to TiO2 by the ethanol–UV
Conclusions
We have shown that a photocatalytic membrane can be conveniently obtained by facile deposition of TiO2 and GO layers on a base membrane. Several excellent properties of the TiO2–GO membrane have been experimentally identified, including the effective photodegradation of organic contaminants under both UV and sunlight, improved membrane flux due to photocatalysis-increased hydrophilicity, and enhanced contaminant removal by photodegradation. Therefore, the TiO2–GO membrane may represent a next
Acknowledgments
This material is based upon work supported by the National Science Foundation (NSF) under Grant nos. CBET-1158601 and CBET-1154572. The work of the first author was supported by the Jiangsu Province Environmental Protection Scientific Research Project (2012) and by the Social Development Sci-tech Programme of Changzhou (CS20090016). These financial supports are gratefully acknowledged. We also acknowledge the facility support of the Maryland NanoCenter and its NispLab. The NispLab is supported
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