In this work, we report on the photocatalytic properties of β-FeOOH/TiO2 heterojunction material for the inactivation of Escherischia coli. XRD, HRTEM, EELS, ELNEFS were used to characterize the as-prepared material. A log reduction of the initial bacterial population was achieved after 45 min of irradiation in the presence of 0.1 mL of hydrogen peroxide. The enhanced photocatalytic activity was due to the effective charge transfer between Ti4+, Fe3+, and O2+ as shown from the EELS analysis of the heterojunction structure. The role of various reactive species formed due to the photocatalytic reaction was also investigated. Presence of •OH radicals in the bulk solution was the key factor in the photocatalytic inactivation of E. coli.
- heterojunction structure
- microbial inactivation
- E. coli
- charge transfer
Risk associated with waterborne diseases such as typhoid, hepatitis A and E, polio, diarrhoea, and cholera are increasing in developing countries due to shortages of clean and safe drinking water . Various chemical and physical treatment processes have been used to disinfect drinking water. Chlorination is a widely used method to disinfect water. However, chlorination can be problematic as it reacts with naturally occurring organic compounds in water to produce carcinogenic by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs) . Ozonation is another method that is used to disinfect water . On the other hand, ozonation is an energy intensive technique which can be proved to be costly in many developing countries. Physical separation process like thermal destruction at elevated temperatures and membrane filtration of pathogens is effective but not economically feasible. Water disinfection by UV radiation can also be utilized. Conversely, the use of direct and intensive UV radiation possesses health hazards. Hence, its application is limited within the special medical and laboratory purposes only [3, 4]. Therefore, new techniques are required to control the spread of microorganisms in water due to the complications related to different water disinfection processes . Photocatalysis is a promising method that can be used to disinfect water cheaply, as the energy required to activate photocatalytic reaction can be obtained freely from the sun. In the presence of light, electron (
Since the discovery of water splitting by Fujishima and Honda using TiO2, there has been an explosion of various studies on TiO2 as a photocatalyst . TiO2 is generally considered as nontoxic, photocorrosion resistant, and inexpensive photocatalyst . One of the biggest drawback of TiO2 as photocatalyst is that it can only utilise photon from near UV light range to generate electron (
The efficacy of the VLD photocatalyst depends on the efficient separation of charge carriers. Formation of heterojunction between TiO2 and other oxide materials have has been proven to be an effective way of enhancing the efficiency of photocatalysis process under visible light [6, 9, 10, 15]. Besides the band potential matching of the semiconductors, ability to conduct (
2. Materials and Methods
Analytical grade reagents, without any further purifications, were used in all preparation methods. FeCl3.6H2O, NH4OH, EtOH, hydrogen peroxide (30% V/V), isopropanol, sodium oxalate, Cr(VI), and Degussa-P25 were purchased from Sigma Aldrich South Africa.
2.1. Catalyst preparation
Nanorod-shaped β-FeOOH particles were synthesised according to the previously reported method [17, 18]. Typical synthesis process consists of adding certain amount of FeCl3.6H2O in a solution of equal amount of water and EtOH (V/V). The final pH of the solution was kept at ~2. 200 mL of the solution was placed in a Teflon lined autoclave. The autoclave was slowly heated to 100°C and was kept at that temperature for 2 h; then followed by washing and centrifuging of the precipitated sample once the autoclave cooled down naturally. The sample were dried in a desiccator.
The method of heterojunction formation between TiO2 and P25 was adopted from previously reported study [6, 19, 20]. Maleic acid was used as an organic binder to form a chemically bonded interface between TiO2 and β-FeOOH. The role of maleic acid was, to anchor the TiO2 on the surface of β-FeOOH through its dicarboxylic functional group . In a typical synthesis procedure, an appropriate amount of β-FeOOH nanorods were dispersed in 30 mL of EtOH. In a separate beaker, a gram of P25 powder was dispersed in 30 ml of EtOH. Maleic acid of 0.1 M concentration (10 mL) was added to the β-FeOOH suspension. Both solutions were stirred for 4 h. After 4 h, the TiO2 suspension was added to the β-FeOOH suspension and stirred for 12 h. The mixture was washed and centrifuged several times. The samples were dried in an oven at 60°C. The dried samples were calcined at 300°C. The calcined samples were further treated by UV irradiation for 4 h.
E. coli cell preparation
Single colony of
2.3. Photocatalytic inactivation of
The photocatalytic inactivation of
2.4. Characterization of the material
A Phillips PW 3830/40 Generator with Cu-Kα radiation was used to determine the X-ray diffraction patterns of the heterojunction structure for phase identification purposes. High resolution transmission electron microscopy (HR-TEM) was performed using a Tecnai F20 FEG-TEM, equipped with a Gatan Image Filter (GIF2001) for morphology evaluation and also electron energy loss spectroscopy (EELS) and energy-filtered TEM (EFTEM) analysis. Plural scattering and the contribution from low low-energy plasmon losses were eliminated by applying a power law background -shape to each spectrum.
3. Results and discussion
3.1. β-FeOOH/TiO2 heterojunction characterization
Degusa P25, a mixture of anatase and rutile, was used as TiO2 material. Hence, both anatase (JCPDS card no: 71–1167) and rutile (JCPDS card no: 75–1748) peaks could be observed from the XRD patterns of TiO2 (Data not shown). Figure 1a presents TEM Micrographs of TiO2. Selected area electron diffraction pattern (Figure 1b) shows that the TiO2 is predominantly anatase as the first four diffracting planes indicates. High-resolution TEM micrograph viewed along the  direction (Inset Figure 1a), allowed for direct measurement of the lattice constants and found to be value of
Anatase and rutile both have Ti4+ valency as well as a tetragonal crystal structure. However, rutile contains six atoms per unit cell whereas anatase has twelve. In either polymorph, each Ti atom is coordinated to six O atoms, whereas each O atom is coordinated to three Ti atoms, thereby forming a perfect TiO6 octahedron. These octahedra are slightly slanted in both anatase and rutile, with two Ti-O distances being 0.1 Å greater in length compared to the other four bond lengths. This is accompanied by a deviation from 90° of the O-Ti-O angles. Subsequently, the local point-group symmetry is lowered around the Ti atom from Oh to D2h in rutile and D2d in anatase. Anatase and rutile differ as a result of secondary coordination during which the TiO6 octahedra are joined together by sharing two edges in rutile and four in anatase [22, 23]. A study of the molecular orbital (MO) energy level diagram of rutile  shows that the two lowest unoccupied orbitals separate into a threefold t2g and twofold eg orbital, commonly known as the crystal-field orbitals. In ideal Oh symmetry, they consist of two distinct levels; however, with the lowering of the symmetry in rutile, t2g orbital splits into b3g, ag, and b2g sub-orbitals, whereas the eg orbital morphs into b1g and ag sub-levels. Tight-binding theory  shows that the electronic band structure for rutile exhibits L2,3 ionization edges in the conduction band, which are basically t2g and eg levels, their maxima separated by about 4.5 eV; for anatase, the L2 and L3 bands are slightly closer at 3.8 eV. From Figure 2a, it can be seen that the L2,3 obtained during electron energy loss maxima is separated by 5.17 eV (465.31 and 470.48 eV), with the crystal-field splitting of the L2 and L3 lineshapes into the respective t2g and eg sub-bands clearly visible, as indicated by the arrows in the inset. As determined from the SAED pattern of Figure 1b, the P25 particles are predominantly anatase; however, the separation of the L3 and L2 ionization edges of 5.17 eV suggests the presence of rutile.
Figure 2b shows the L2,3 ionization edge of the β-FeOOH nanorods. In all iron-oxides, this lineshape is characterized by Fe 2p → 3d and 4s transitions. However, a lower probability of transition to the s orbitals exist, and hence, the Fe L2,3 edges are comprised of mainly excitation to Fe 3d orbitals. This spin–orbit interaction separates the Fe L3 (2p3/2) and L2 (2p1/2) edges by about 13 eV. The unoccupied states in the 3d bands of the Fe atom are related to the white-line intensities. EELS of β-FeOOH shows an ionization edge onset at 708 eV, peaking at 716.58 eV for L3, and 729.63 eV for the L2 edge. A deconvolution of the L3 peak shows the characteristic crystal field splitting into the t2g and eg bands. This is shown in the inset of Figure 2b. The L3/L2 ratio of 2.29 is recorded for these nanostructures.
The EELS spectrum of the composite material is shown in Figure 2c and compared to that of the P25 and as-synthesized β-FeOOH nanorods samples. A study of the L3,2 ionization edge of Ti and Fe in Figure 2c shows a lack of evidence of the crystal-field splitting in the Ti L3,2 lineshape, which is accompanied by a rearrangement of the L3/L2 ratio of Fe to 1.65. Investigation of the O K edge shows a similar decrease in the splitting. This suggested the presence of charge transfer/bonding between the Ti4+, Fe3+, and O2+ ions during the composite synthesis.
3.2. Evaluation of photocatalytic inactivation of
Figure 3a presents the photocatalytic activity of the heterojunction structure under visible light. A common microorganism; i.e. that is
3.3. Photocatalytic inactivation mechanism of
In our previous study , we have showed the existence of an inter semiconductor
As discussed previously, in the absence of the hydrogen peroxide, no significant reduction in bacterial population was observed. This was due to the rapid recombination of
β-FeOOH/TiO2 heterojunction prepared via organic linker mediated route was used for photocatalytic inactivation of
This work was supported by National Research Foundation of South Africa (Grant No: 88220).
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