As a renewable energy source, solar energy is an attractive option to disinfect water through the method of Solar Drinking Water Disinfection (SODIS), which is a widely accepted method, particularly in developing countries that lack distribution systems for drinking water. Solar energy can be used directly to inactive bacteria, as photons from the ultraviolet B (UVB) or ultraviolet A (UVA) portion of sunlight interact with nucleic acids or other macromolecules to cause inactivation, which is time-consuming. Solar energy also can be used for indirect disinfection, as photons from the ultraviolet or visible portion of sunlight convert into electrons by catalyst to produce reactive oxygen species (ROS) for disinfection. This heterogeneous water disinfection with sunlight can avoid secondary pollution and other shortcomings of homogeneous system. However, at present, its speed and efficiency are far from those of homogeneous processes, which can effectively attack bacteria though molecular/ion scale collision, such as chlorination. Here, we successfully address this issues of heterogeneous disinfection by developing discrete nanoflakes of (Al₂O₃@v-MoS₂)/Cu/Fe₃O₄ in which vertically aligned 2-D nanofingerprint MoS₂ is hybridized with Cu metal and magnetic Fe₃O₄ particles.

Three exciting features are integrated into such a novel structure of (Al₂O₃@v-MoS₂)/Cu/Fe₃O₄: bifacial vertically aligned MoS₂ grown on the top and bottom surfaces of light-transparent Al₂O₃ nanoflakes that can absorb not only UV light but also almost the full spectrum of visible light, and both sides can operate simultaneously; a Cu-MoS₂ junction that enhances charge separation for the efficient generation of reactive oxygen species (ROS); and magnetic Fe₃O₄ nanoparticles that have magnetic separation capability and conveniently regenerate after disinfection. (Fig. 1) This system exhibits outstanding water disinfection with thorough inactivation of over 5.7 log10 CFU ml^-1 E. coli within 1 minute in real sunlight and with facile separation and reuse.

Fig. 1 Solar water disinfection test of recyclable (Al₂O₃@v-MoS₂)/Cu/Fe₃O₄. a Schematic representation of photocatalytic mechanism and cyclic utilization for water disinfection. b Comparison of the disinfection performance of (Al₂O₃@v-MoS₂)/Cu and (Al₂O₃@v-MoS₂)/Cu/Fe₃O₄ under different light sources (UV-filtered light and real sunlight). In real sunlight, a thorough disinfection with (Al₂O₃@v-MoS₂)/Cu/Fe₃O₄ is completed in 60 seconds (sec-kill). (The point at 0 CFU/mL indicate that all the E. coli cells are dead. Experimental conditions: T = 25℃, [pH]0 = 6.5, the initial concentrations of E. coli are 10^5.74 CFU/mL, the data are presented as mean ± s.d. (n = 3).). c Cyclic system proving the stability of (Al₂O₃@v-MoS₂)/Cu/Fe₃O₄. d SEM images of solution samples (E. coli) separated at low speed before and after disinfection. After disinfection, the cell membrane was seriously damaged as shown in red circles.

The reasons why our photocatalytic system can disinfect in water so fast are based on the following three points. Firstly, the discrete bi-facial nanoflakes with vertically aligned 2-D nanofingerprint MoS₂ can provide abundant active edges for ROS production (Fig. 1a). Secondly, our photocatalyst can absorb most part of sunlight (the cutoff of UV-Vis absorption at around 750 nm, Fig. 2a), which enhances the utilization rate of solar energy from ~7% to ~57%. Last but most important, our structure designed photocatalyst can reach an optimized band gap (Fig. 2e), which can perfectly produce all kinds of ROS (Fig. 2b-d).   

Fig. 2 Key characterization of our photocatalyst to explain its ultra fast water disinfection. a UV-Vis absorption spectra of Al₂O₃@v-MoS₂. The cut-off wavelength of Al₂O₃@v-MoS₂ is approximately 750 nm, whereas Al₂O₃ has no absorption (P = 1.5 × 10⁻³ mg l⁻¹). b-d The signals of superoxide radical, hydroxyl radical and singlet oxygen in EPR spectrum, respectively. (1 sun; UV-filtered; 10 mg Al₂O₃@v-MoS₂/Cu in 50 ml water; 25^oC). e Band gap energy, valance band maximum, and conduction band minimum positions of several semiconductors for water disinfection, including TiO₂, ZnS, CdSe, g-C₃N₄, BiVO₄, bulk MoS₂, and red phosphorus, in comparison to that of Al₂O₃@v-MoS₂. The potentials in the absolute energy scale and in normal hydrogen electrode scale were also indicated.

 In this study, we explored the ultrafast heterogeneous disinfection system by means of nanostructure predesign, experimental synthesis and performance verification. Our study shows that the selection of photocatalysts with the most suitable band gap width and as many active sites as possible is the key to achieve ultrafast heterogeneous water disinfection. The band gap of the catalyst should be narrow enough to absorb a large amount of the visible energy of sunlight, but wide enough to produce as many kinds of ROS as possible. At the same time, the vertical aligned 2-D fingerprint structure of MoS₂ also provides sufficient amount of active edges. We believe that our catalyst design represents state-of-the-art photodisinfection and will inspire more innovations in this exciting interdisciplinary field.

More details can be found in our paper " Solar-driven Efficient Heterogeneous Subminute Water Disinfection Nanosystem Assembled with Fingerprint MoS₂" published in Nature Water.