Magnetic Separable Visible-Light Active Photocatalysts

During the recent years, water and air pollutions are mainly caused in the fields such as industry, urban effluents, and transportation in worldwide over major cities. Serious environmental pollution, and related issues and accelerated global warming can be attributed due to the rapid consumption of fossil fuels, the increasing population, and the rapid development of the economy. Thus, the development of innovative and renewable environmental remediation materials is becoming increasingly one.

Recently, many researchers are focusing on “A noble metal/ MoS₂ nanocomposites (NCs)” which can induce local surface Plasmon resonance (LSPR) for activating the photoelectrocatalysis of hydrogen and enhancing the light emission or absorption of MoS₂. Moreover, the LSPR can generate surface-enhanced Raman scattering (SERS), which can be used in many applications, particularly it is used in chemical sensing and biological applications.

Over the last two decades, magnetic materials have emerged as a potential alternative to facilitate catalyst isolation in heterogeneously catalyzed liquid-phase reactions. Semiconductor photocatalysis has been highlighted as a promising technology for the treatment of water laden with organic, inorganic, and microbial pollutants. However, these semiconductor photocatalysts are applied in powdered form, which makes separation and recycling after treatment extremely difficult. This not only leads to loss of the photocatalyst, but also to secondary pollution by the photocatalyst particles. Therefore, highly efficient, easily separable and more stable catalysts photocatalysts required for related applications.

Figure 1: Magnetic separable photocatalytic process [1].

Recently, an area of intense research has allow for the easy separation of the photocatalyst from the treated water using an external magnetic field via the design of various magnetic nanoparticles like magnetite, maghemite, ferrites, etc. into the photocatalyst matrix.

Currently, several research groups have begun to focus on magnetically separable photocatalysts for waste water treatment, by demonstrating the value of the special properties of magnetic materials.

As a pristine material, haematite is a narrow band gap semiconductor (2–2.2 eV) with a good visible light response, low-cost, ferromagnetic behaviour, good chemical stability, high resistance to corrosion, abundantly available, and environmentally friendly. On its own, haematite shows poor photocatalytic activity, due to the high recombination rate of the charge carriers and poor conductivity.

On the other hand, careful control of the magnetic nanoparticle, especially loading haematite, maghemite or magnetite, is very important in ensuring a balance between the detrimental effect of recombination and good magnetic response that occurs on the surface of the NCs. Otherwise, silica is used as a barrier between the magnetic core and the other semiconductor coupled with haematite, maghemite or magnetite. This prevents the electron charge transfer between the coupled materials, which results to photodissolution and recombination.

The incorporation of the ferrites in various photocatalysts will provide more beneficial properties than the good magnetic response; they contribute towards visible light absorption, charge separation, and the photocatalytic performance of the photocatalytic nanocomposite. Therefore, the ferrites are the most attractive nanoparticles compared to haematite, maghemite, and magnetite, in terms of their photocatalytic performance and multifaceted contribution in the magnetic nanocomposites. However, a systematic, comparative study of the photocatalytic performance of the ferrites, haematite, magnetite, and maghemite is seldom reported. Hence, this makes it difficult to map a way forward in terms of exploiting the best materials for photocatalytic applications.

However, there are still some challenges that possibly hinder the practical exploitation of these visible-light active magnetic separable photocatalytic materials. The most obvious issue relates to the rate of the reaction (kinetics); most of the photocatalytic decomposition process are slow (may take several hours) despite the improvements in visible light absorption of the photocatalyst, and this presents a massive challenge for practical applications. Further, the work still needs to be done for developing the synthesis routes that will ensure uniform shape and size of the magnetic particles. Also their uniform distribution within the nanocomposite matrix to be ensured to ensure good magnetic response and efficient recovery [2].

Prof. Ming Gao and his team from Jilin Normal University, China have reported a new magnetically separable photocatalyst of MoS₂/Fe₂O₃NCs. Rationally designed MoS₂/Fe₂O₃ NCs can serve as a reusable SERS substrate for detection and easily reclaimed photocatalyst, because of Fe₂O₃ has a narrow bandgap, high chemical resistance, and high corrosion resistance. The recovery and economical reuse of MoS₂/Fe₂O₃ NCs Photocatalysts are easily achieved by adding an external magnetic field [3]. The enhanced photocatalytic activity and SERS activity of this reported photocatalysts were attributed to the efficient separation and transfer of electron–hole pairs by the Z-scheme heterojunction system. Hence, an efficient multifunctional catalysts, MoS₂/ Fe₂O₃ NCs were employed to replace the metal catalysts for the organic matter removal from the environment and water. However, it will also pave the way for SERS applications for introducing new methods for chemical and medical detection and for environmental monitoring.

Zhao et al. from Donghua University, China has developed a nanostructured α-Fe₂O₃-AgBr nonwoven cloth via a simple electrospinning-calcination method and its visible light photocatalytic behaviour studies. The α-Fe₂O₃-AgBr nonwoven cloth showed superior activity over α-Fe₂O₃ and AgBr clothes individually with good efficiency. This newly designed catalyst α-Fe₂O₃-AgBr nonwoven cloth will resulted to a significant red-shift in the formation of heterojunctions, which facilitates the charge separation, thereby improving the overall photocatalytic activity. In addition, the cloth was easy to handle and recycle during photocatalysis by simple dipping and removal or by using an external magnet [4].

Our SNB team have mainly emphasize this new research article to enrich our viewer’s knowledge about the area of intense research for the easy separation of the photocatalyst from the treated water using an external magnetic field via various magnetic nanoparticles (like magnetite, maghemite, ferrites, etc.) into the photocatalyst matrix. Further, the higher photocatalytic activity of Fe₂O₃–AgBr nonwoven cloth should result from the synergic effects between Fe₂O₃ and AgBr due to the broadening of photo absorption and the energy level matching were been found. Importantly, Fe₂O₃–AgBr nonwoven cloth can be easily transferred and/or recycled by the dipping/pulling method and/or external magnetic field, which has excellent photocatalytic stability during recycling tests. Some of the other magnetic seperable nano-photocatalysts, are pure Fe₂O₃, Fe₂O₃–Bi₂WO₆, Ag–AgI/Fe₃O₄@SiO₂ (X = Cl, Br, or I), etc. These magnetic photocatalytic nanocomposites can be easily recycled by external magnetic field in the laboratory, but it is still inconvenient to recycle them in practical application (such as degrading organic pollutants in lake and/or river).

In addition, the photocatalytic performances of these photocatalysts are still unsatisfied for the practical applications.Therefore, this work provides some insight into the design and development of novel, efficient and easily recyclable macro scale nonwoven cloths, for future practical photocatalytic application (for example, especiallyin degrading organic pollutants in polluted rivers).

References

M. J. Jacinto et al., J Sol-Gel Sci Tech., 2020, 96, 1.
Gcina Mamba and Ajay MishraCatalysts., 2016, 6, 79.
Hu et al. Microsystems &Nanoengg., 2020, 6, 111.
Zhao et al. RSC Adv., 2015, 5, 10951.

Dr. A. S. Ganeshraja

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