Accepted Manuscript Solar Photocatalytic Disinfection of Water using Titanium Dioxide Graphene Composites P. Fernández-Ibá ñez, M.I. Polo-López, S. Malato, S. Wadhwa, J.W.J. Hamilton, P.S.M. Dunlop, R. D’Sa, E. Magee, K. O’Shea, D.D. Dionysiou, J.A. Byrne PII: DOI: Reference:
S1385-8947(14)00838-9 http://dx.doi.org/10.1016/j.cej.2014.06.089 CEJ 12333
To appear in:
Chemical Engineering Journal
Please cite this article as: P. Fernández-Ibá ñez, M.I. Polo-López, S. Malato, S. Wadhwa, J.W.J. Hamilton, P.S.M. Dunlop, R. D’Sa, E. Magee, K. O’Shea, D.D. Dionysiou, J.A. Byrne, Solar Photocatalytic Disinfection of Water using Titanium Dioxide Graphene Composites, Chemical Engineering Journal (2014), doi: http://dx.doi.org/ 10.1016/j.cej.2014.06.089
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Solar Photocatalytic Disinfection of Water using Titanium Dioxide Graphene Composites P Fernández-Ibáñeza, MI Polo-Lópeza, S. Malatoa, S. Wadhwab, JWJ Hamiltonb, P.S.M. Dunlopb, R D’Sa, E Mageeb, K. O’Sheac, DD Dionysioud, JA Byrneb
a. Plataforma Solar de Almería – CIEMAT, PO Box 22, 04200 Tabernas, Almería, Spain. (
[email protected];
[email protected];
[email protected]) (P. Fernández-Ibanez), b. NIBEC, University of Ulster, BT370QB, United Kingdom: (
[email protected];
[email protected];
[email protected]) c. Department of Chemistry and Biochemistry, Florida International University, University Park, Miami, FL 3319, USA. (
[email protected]) d. Environmental Engineering and Science Program, School of Energy, Environmental, Biological, and Medical Engineering, University of Cincinnati, Cincinnati, OH 452210012, USA (
[email protected])
Abstract Interest has grown in the modification of titanium dioxide with graphene to improve the photocatalytic behaviour. In this work, titanium dioxide – reduced graphene oxide (TiO2RGO) composites were synthesised by the photocatalytic reduction of exfoliated graphene oxide (GO) by TiO2 (Evonik P25) under UV irradiation in the presence of methanol as a hole acceptor. The composite materials were characterised using high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Raman and XPS analysis provided evidence that GO was converted to RGO by photocatalytic reduction. The TiO2-RGO composites were compared to TiO2 in suspension reactors for the disinfection of water contaminated with Escherichia coli and Fusarium solani spores under real sunlight. Very rapid water disinfection was observed with both E. coli and F. solani spores. An enhancement in the rate of inactivation of E. coli was observed with the TiO2-RGO composite compared to P25 alone. The rate of inactivation of F. solani spores was similar for both the TiO2-RGO and P25. When the major part of the solar UVA was cut-off (λ > 380 nm) using a methacrylate screen, there was a marked increase in the time required for inactivation of E. coli with P25 but no change in the inactivation rate for the TiO2-RGO. There is evidence of singlet oxygen production with visible light excitation of the TiO2-RGO composites which would lead to E. coli inactivation.
Key words: Photocatalysis, water disinfection, titanium dioxide, graphene, singlet oxygen, solar
1.0 Introduction The availability of safe drinking water is a high priority issue for human existence and quality of life. It is estimated that around 780 million people are without access to an improved drinking water source and many more are forced to rely on sources that are microbiologically unsafe [1]. Furthermore, water recycling and reuse is becoming very important in several regions throughout the world and a sustainable method for the disinfection of water for reuse is needed. Irradiating water with solar light can improve the safety of the water by reducing the pathogen loading. This process is referred to as solar disinfection or SODIS [2,3,4]. One potential method for enhancing the efficiency of solar disinfection is by the addition of a photocatalyst, such as titanium dioxide (TiO2) [2]. Heterogeneous photocatalysis utilises light along with a semiconductor to produce reactive oxygen species (ROS) which can inactivate bacteria and degrade a wide range of chemical contaminants in water [5]. UV excitation of TiO2 results in the formation of charge carriers, conduction band electrons (e-CB) and valence band holes (h+VB). The conduction band has a negative electrochemical reduction potential relative to normal hydrogen electrode (NHE) and is able to reduce dissolved oxygen to form superoxide radical anions, hydroperoxyl radicals, and through subsequent reduction reactions, hydrogen peroxide and hydroxyl radicals. . The valence band hole has a very positive electrochemical reduction potential and is able to oxidise water to form hydroxyl radicals. These reactive oxygen species can attack and inactivate microorganisms in water [6]. TiO2 has a wide band gap (anatase Ebg = 3.2 eV or λ < 387 nm) and requires UV excitation. Only ca. 5% of the solar radiation at sea level is in the UV domain. Different approaches have been utilised to improve the solar efficiency for TiO2 including metal doping (or metal loading of the surface) [7] and non-metal doping [8]. An interesting approach to improving photocatalytic efficiency is to create TiO2 composite materials with graphene.
The preparation of graphene, a one-atom-thick sheet of sp2-bonded carbon atoms in a hexagonal two-dimensional lattice, was first reported in 2004 [9].
The properties of
graphene include very high surface area (2600 m2/g), ballistic electronic conduction, exceptional thermal conduction, and good chemical and thermal stability. The utilization of graphene as a two-dimensional support to anchor catalyst nanoparticles (NPs) and facilitate electron transport opens up new possibilities for designing the next generation catalysts. Graphene oxide (GO) is most commonly prepared by the oxidation of graphitic carbon using Hummers’ method [10] along with an exfoliating step, e.g. high intensity ultrasound. The GO can then be reduced back towards single or multilayer graphene using thermal or chemical reduction [11]. As complete removal of oxygen functionalities may not be achieved in the reduction stage, the reduced form is referred to as reduced graphene oxide (RGO). In 2008 Williams, Seger and Kamat reported that GO undergoes photocatalytic reduction to RGO as it accepts electrons from UV-irradiated TiO2 suspensions (in the presence of a hole scavenger) [12]. The direct interaction between TiO2 particles and graphene sheets hinders the collapse/re-stacking of exfoliated sheets of graphene.
Therefore, the photocatalytic
reduction of GO by TiO2 presents a clean and safe route to the formation of TiO2-RGO composites. There are a number of potential advantages for utilising TiO2-RGO composites for photocatalysis. For example, conduction band electrons may be rapidly transferred to the RGO therefore inhibiting charge carrier recombination, behaving similar to metal clusters on the surface of the photocatalyst [13]. Graphene (RGO) may act as an electrocatalyst for the oxygen reduction reaction [14, 15]. RGO should present a high surface area for adsorption of pollutants, possibly yielding a synergistic effect for pollutant removal. In addition, visible light activity might be enhanced due to the GO/RGO absorbing UV/Vis and producing reactive oxygen species. UV or visible irradiation of GO can lead to the expulsion of oxygen as reactive oxygen species [16,17]. Graphene based photocatalytic composites have been
reviewed by An and Yu in 2011 [18]. Others have previously reported on the formation of superoxide and singlet oxygen through visible light excitation of fullerenes [19]. Akhavan and GhaderiIt reported that such TiO2-RGO nano-composites could improve the efficiency for the killing of E. coli bacteria under solar irradiation [20]. They reported that the optical absorption was not significantly different following the deposition of the RGO and they used the ‘so-called’ anti-bacterial drop test in their experiments. They suggested that the enhanced solar activity was due to the reduced graphene oxide platelets acting as electron sinks, accepting conduction band electrons from the UV excited TiO2 and effectively decreasing the rate of recombination of charge carriers. In 2011, Liu et al. reported on simple two-phase assembling method to produce graphene oxide–TiO2 nanorod composites [21]. After combining with graphene oxide (GO), the TiO2-GO composites showed higher photocatalytic activities than that of TiO2 nanorods alone for the inactivation of E. coli under solar simulated light.
The enhanced effect of graphene oxide nano-sheets on the
photocatalytic properties of TiO2 was attributed to a thin two dimensional sheet support, a large surface area and much increased adsorption capacity, and the strong electron transfer ability of the thermally reduced graphene oxide in the composite.
In 2013, Gao et al.
reported on the inactivation of E. coli under visible light irradiation of TiO2-GO composites [22]. The GO was produced using Hummers’ method and the TiO2-GO composites were formed by reacting partially hydrolysed TiCl 3 with sonicated GO. Antibacterial tests were carried out by dispersing the TiO2-GO composite directly to an agar plate seeded with E. coli followed by illumination from an indoor light (400-700 nm).They reported an increase in the percentage inactivation for the TiO2-GO composites as compared to TiO2 alone. Although TiO2-RGO and TiO2-GO composites have been previously reported to improve the inactivation efficiency for E. coli, none of the previously published work has utilised a common protocol for the assessment of water disinfection and none have compared the efficiency against a well known material e.g. Evonik Aeroxide P25.
In this work we modified P25 to form TiO2-RGO composites using the photocatalytic reduction of exfoliated GO in the presence of a hole scavenger. The materials were then tested for the disinfection of water under real sun conditions using two different microorganisms, using P25 as a test standard. We also investigated the contribution of visible light activity to the disinfection mechanism.
2.0 Materials and Methods 2.1 Synthesis of TiO2-RGO Graphene oxide (GO) (Nanoinnova) and TiO2 P25 particles (2% w/v) were suspended separately each in 100 mL absolute methanol and sonicated using a tip sonication for 10 min. The two suspensions were mixed to obtain 5% w/v GO concentration with respect to TiO2. The suspension was ultrasonicated for about 1 h to obtain grey-blue suspension. The suspension obtained was irradiated with UV-B light source for 6 h with constant stirring with oxygen free nitrogen (OFN) sparging (flow rate 200 mL min-1). The product was dried off in air to obtain a dry powder. 2.2 Characterisation of the TiO2-RGO The TiO2-RGO/GO composites were characterised by HR-TEM using a Joel 2100F with a 200 kV field emission source. Raman spectra were recorded using an ISA instruments Labram 300 spectrometer with 514 nm argon and 633 nm helium neon laser sources. Chemical composition analysis was undertaken by X-ray photoelectron spectroscopy (XPS) with a Kratos Axis Ultra employing an Al Kα source. The binding energy of the samples were calibrated relative to the C 1s peak at 284 eV. Linear background subtraction and Gaussian-Lorentzian curves were used for peak fitting. 2.3 Solar photocatalytic disinfection 2.3.1
Bacterial strain enumeration and quantification
E. coli K-12 (ATCC 23631) was used as the model bacterium for assessing the disinfection kinetics under real sun conditions. It was inoculated from stocks in Luria broth nutrient medium (Miller’s LB Broth, Sigma–Aldrich, USA) and incubated at 37 ºC by constant agitation in a rotator shaker under aerobic conditions. Bacteria were collected after 20 h of incubation which corresponds to the initial bacterial stationary phase, yielding a concentration of 109 CFU/mL. E. coli suspensions were harvested by centrifugation at 800 x g for 10 min. Finally, the bacterial pellet was re-suspended in phosphate buffer saline (PBS) and diluted in the reactor to the desired initial concentration (106 CFU/mL). The samples taken during the experiments were enumerated using the standard plated count method through serial 10-fold dilutions in PBS and volumes of 20 µL were plated in triplicate on Luria agar Petri dishes (Sigma–Aldrich, USA). When very low concentrations of E. coli were expected to be found in samples, 500 µL-samples were spread over a plate to decrease the detection limit down to 2 CFU/mL. Colonies were counted after incubation of 24 h at 37 ºC. 2.3.2
Fungal strain enumeration and quantification
F. solani (CECT 20232) was used as a fungal spore model for characterizing inactivation kinetics of the solar photocatalytic process. This fungus was chosen because it is a common phytopathogen in soil and the water distribution system affecting crops and it has been demonstrated to be highly resistant to photocatalytic treatment [23]. The same strain and enumeration–quantification methods have been described elsewhere [23,24]. Fungal colonies were transferred to a lab prepared sporulation agar made of 5 mg/L of bacteriological agar (Cultimed, spain) and 15 mg/L of potassium chloride (Panreac, Spain) in Petri dishes, and kept at 25 ºC for 15–30 days exposed to UV-C radiation (Mercury lamp, 40 W). The characteristics spores so-called “microconidia” formed by F. solani are generated in response to this stress conditions. Microconidia were recovered by washing the plates with autoclaved distilled water. Spore concentration expressed like Colony Forming Units per mililiter (CFU/mL) was determined by direct counting with a Neubauer plate (Brand, Germany) using a phase contrast microscope (Nikon, Japan) and diluted in 250mL-solar
vessel reactors to the desired initial spore concentration (∼103 CFU/mL). The concentration
of fungal spores in water was measured using the plate counting technique. Samples (50– 250–500 µL) were plated out on acidified malt agar containing 20 mg/L of bacteriological agar (Cultimed, Spain) and 20 mg/L of malt extract (Cultimed, Spain) with an addition of 0.25 g/L of citric acid (Sigma–Aldrich, USA). The detection limit (DL) of these quantification methods is 2 CFU/mL. Fungal colonies were counted after 48 h of incubation at 28 ºC in dark. 2.4
Experimental procedure for solar photocatalytic disinfection
All experiments were carried out in 250-mL DURAN-glass (Schott) batch vessel reactors in triplicate under natural solar radiation at Plataforma Solar de Almería (PSA, Spain). Reactors were magnetically stirred at 100 rpm, and exposed to sunlight for 5 h in sunny days. Glass covers were used instead of plastic lids, to allow the solar radiation entering in the bottle reactor from all directions. The total irradiated volume was 0.1 L in each bottle. Distilled water was used for all experiments, with conductivity 435 nm). Samples (0.2 mL) were removed from the reactor at 2 min intervals and the fluorescent intensity was measured with excitation and emission at 504 and 525 nm respectively on a Tecan Genios FL Fluorometer.
3. Results and discussion 3.1 Materials characterisation In the preparation stage, it was found that much better exfoliation of the GO was obtained using an ultrasonic horn. FIGURE 2 & FIGURE 3 Once the GO was mixed with the TiO2 a high level of aggregation was observed (Figure 2). Following photocatalytic reduction under UV irradiation (in the presence of methanol as a hole scavenger) it was observed that the composites were less aggregated, possibly due to the TiO2 preventing restacking of the RGO (figure 3).
Raman spectroscopy is widely used to study the crystal structures of carbonaceous materials. The D peak in the GO spectrum around 1325 cm-1 is due to –OH and epoxy groups while G peak around 1571 cm-1 is attributed to C—C and C=C bonds. The ratio of intensity of D (ID) and G (IG) peaks in carbon structures indicates disorder in the structure. If ID/IG > 1, the structure is more disordered. The Raman spectra of the GO and TiO2-RGO are shown in Figure 4. FIGURE 4
The D peak intensity increased marginally (relative to the G peak) following the photocatalytic reduction of GO to RGO (Figure 4). The small change in the ID/IG ratio is indicative of limited structural change in the material upon reduction [25]. The G peak observed at 1595.78 cm-1 for the GO sample was shifted to 1570.95 cm-1 following reduction to RGO. Bell et al. reported that a blue shift in the G band of GO relative to that of graphite (after oxidation of graphite to GO) was due to increased numbers of isolated C=C bonds which resonate at higher frequencies than the G band of graphite [26].
In addition an
asymmetric G peak was observed for the TiO2-RGO following [27]. The photo-reduction of GO to RGO also showed an increase in the 2D and 3S peaks associated with graphitisation [25].
The Raman analysis is evidence of photocatalytic reduction of GO to RGO. A
symmetric 2D peak should be observed around 2600 cm-1 for single layer graphene. This peak becomes asymmetric and broadens for few layered and multi-layered graphene [28,29]. For the as-received GO, a very weak, broad asymmetric peak was observed in the 2D peak position, indicative of multilayer graphene.
The 2D peak in the photocatalytically
reduced GO showed a strong symmetrical peak at ~2660 cm-1. The high level of symmetry and short Raman shift observed for the 2D peak is consistent with the presence of single layer graphene.
Figure 5 shows the XPS spectra of the GO and TiO2-RGO.
There was a noticeable
decrease in the C-O peak in the C1s region for the TiO2-RGO sample confirming the reduction of GO to RGO [11]. Table 1 gives the C1s components and the ratio of C-C to C-O and C=O bonds. The C-C /C-O and C-C/C=O ratios were much higher for TiO2-RGO sample in comparison to the GO. In order to determine if the RGO would be re-oxidised when used in photocatalytic experiments as TiO2-RGO composites, the TiO2-RGO was analysed by XPS following photocatalytic disinfection experiments. Figure 6 shows the XPS analysis before and after use in photocatalytic experiments. Contrary to the expected re-oxidation of RGO by reactive oxygen species produced during photocatalysis, there was a decrease in the C-O region indicating further reduction of the RGO. This may be due to expulsion of oxygen groups under UV-Vis irradiation which has been reported previously [16]. FIGURE 5 & FIGURE 6 TABLE 1 3.2 Solar photocatalytic disinfection 3.2.1 Solar inactivation of E. coli
Figure 7 shows the inactivation of E. coli in distilled water with TiO2-RGO at 0, 10, 20, 50, 100, 300 and 500 mg/L of catalyst concentration under natural sunlight for 2 hours of solar exposure. In all cases, the addition of TiO2-RGO accelerates the bacterial inactivation. When the amount of catalyst increased from 0 to 500 mg/L, the solar UV doses required to achieve the detection limit (DL) decreased 123 to 11 kJ/m2, respectively. The best inactivation rate was obtained with 500 mg/L of TiO2-RGO, which led to a complete inactivation of E. coli from 106 CFU/mL to DL within 10 minutes of solar photocatalytic treatment. The typical shoulder of photocatalytic disinfection and solar disinfection was observed in the inactivation curves obtained for low catalyst concentrations (0, 10 and 20 mg/L); while a progressive reduction until complete disappearance of the shoulder is observed when the catalyst concentration increased from 50 to 500 mg/L. The water temperature ranged from 22 to 39 ºC during the experimental time. This thermal profile has no significant effects in terms of loses of E. coli viable cells, since temperature was below the optimal growth temperature (37 – 44 ºC) in all cases. Therefore, bacterial concentration loses due to thermal inactivation was not considered to be important for these experiments. FIGURE 7 Table 2 shows the initial and final values of pH and dissolved oxygen measured (DO) in the reactors. The variation in pH during the course of the experiments was not significant. Regarding the DO, there was ~ 0.5 mg/L decrease in all cases but this was not enough to affect the disinfection efficiency. As a control, the viability of E. coli in the presence of TiO2-RGO particles was evaluated in dark. E. coli was exposure for 5 h to 20 mg/L of TiO2-RGO at 25 ºC. No significant reduction in the E. coli viability was observed (data not shown), showing a non-toxic effect of TiO2RGO at the concentrations studied. TABLE 2
The disinfection activity of TiO2-RGO was compared with unmodified P25 (Figure 7 inset). The optimal loading for P25 in this reactor was 500 mg/L for the inactivation of E. coli. P25 gave complete inactivation of bacteria (from 4x106 to DL=2 CFU/mL) with a solar UVA dose of 18.4 kJ/m2 (15 min of solar treatment); while 500 mg/L TiO2-RGO required only 12.2 kJ/m2 (10 min of solar treatment) to reach the DL. This result showed that TiO2-RGO lead to a faster than TiO2-P25 for E. coli, although the difference between both catalysts is not marked due to the rapid disinfection rates observed for both materials. 3.2.2 Solar inactivation of F. solani spores Figure 8 shows the efficiency of TiO2-RGO at 0, 10, 35, 50, 100, 300 and 500 mg/L for the inactivation of F. solani spores in water. The catalyst enhances the inactivation kinetics as compared to solar disinfection alone, where 336.2 kJ/m2 of solar UV-A dose was required to achieve the DL. On the contrary to what was observed for E. coli (Figure 7), the efficiency of the process increased as the TiO2-RGO concentration decreased for F. solani. The best F. solani inactivation result was observed for 10 mg/L, requiring 42.1 kJ/m2 of solar UV-A dose in 30 min of solar exposure. For 300 and 500 mg/L of TiO2-RGO, similar inactivation curves were observed where a decrease in concentration from 103 to 2 CFU/mL within 90 min was observed (solar UV-A dose of 145.8 kJ/m2). Water temperature ranged from 22 to 40 ºC during the solar photocatalytic experiments. Fusarium spores are unaffected by these temperatures as described before [23]. Therefore, thermal killing was not considered as a significant effect in these experiments. Table 3 shows the initial and final values of pH and dissolved oxygen in the different vessel reactors. No significant reductions were observed between initial and final pH, where the difference was ~ 0.2, except for 10 and 35 mg/L were this difference increase to 0.4 and 0.7, respectively. As expected, the DO decreased during the photocatalytic treatments, with the decrease in DO being greater for higher catalyst loadings, i.e., 300 and 500 mg/L, from 8.5 mg/L to 5.9 mg/L. FIGURE 8
TABLE 3 The rate of spore inactivation for TiO2-RGO and P25 is compared in the inset of Figure 8. The optimal loading for P25 in the solar the inactivation of F. solani spores in this reactor configuration was previously determined to be 35 mg/L [24]. The results show that inactivation rate is the same for both the TiO2-RGO (10 mg/L) and P25 (35 mg/L); the detection limit was achieved in 30 minutes with a solar UV-A dose of 42.1 kJ/m2.
Given the
resistance of F. solani spores to solar disinfection, this is a most promising result for photocatalytic inactivation under real sun. If we compare E. coli and F. solani results, it is clear that E. coli is more susceptible than F. solani spores to the photocatalytic treatment, which has been observed previously using either TiO2 photocatalysis or photo-Fenton reaction in different reactor configurations and
water conditions [C. Sichel, M. de Cara, J. Tello, J. Blanco, P. Fernández-Ibáñez, App. Cat.
B: Environ. 74 (2007a) 152-160. I. García-Fernández, M.I. Polo-López, I. Oller, P.
Fernández-Ibáñez, App. Catal. B: Environ. 121-122 (2012) 20-29.6]. This difference is attributed to the resistant structure and composition of Fusarium spores. Contrary to E. coli cells (vegetative forms), fungal spore walls are rigid structures composed of polymeric sugars, proteins and glycoproteins. Additionally, spores wall contain also an outer xylan
layer [I. García-Fernández et al., 2012]. This multifunctional structure confers these
structures a high resistance against different stress factors [F. Nielsen, Fungal Genetics and
Biology 39 (2003) 103–117]. Previous experimental studies on solar photocatalysis with TiO2 also demonstrated that UV dose required for removal of different types of microorganism
depends on cell structure [C. Sichel, M. de Cara, J. Tello, J. Blanco, P. Fernández-Ibáñez,
App. Cat. B: Environ. 74 (2007) 152-160].
3.3.3 E. coli inactivation efficiency of TiO2-RGO with UV cut-off filter (λ >380 nm)
The visible light activity of TiO2-RGO was experimentally tested using a methacrylate cover as UV-A filter.
The methacrylate cuts off UV below 380 nm and allows a number of
simultaneous experiments to be undertaken with real sunlight. Photocatalytic experiments were done in several vessel reactors containing 500 mg/L of catalyst concentration (TiO2RGO and TiO2-P25) and E. coli (106 CFU/mL initial concentration) and exposed to direct sunlight under the methacrylate cover. Figure 9 shows the inactivation results obtained under both solar conditions. The detection limit was achieved in all cases. For TiO2-RGO, DL was achieved at the same time, with and without the methacrylate cover, requiring 17 kJ/m2 of solar UV-A dose in 10 minutes of photocatalytic treatment. The solar UVA irradiance with and without the methacrylate screen are shown in figure 1. With the methacrylate screen, the UVA irradiance was decreased by 70% from an average 25 W/m2 to 7 W/m2 as compared to the direct solar UVA. The fact that TiO2-RGO activity with and without the filter is the same for E. coli inactivation means that TiO2-RGO has a predominant activity at wavelengths greater than 380 nm and can utilise the near-visible/visible region of the solar spectrum. P25 is predominantly anatase requiring wavelengths less than 387 nm and, therefore, there was a marked reduction in the photocatalytic disinfection efficiency for P25 with the methacrylate screen, as compared to without i.e., 40.7 kJ/m2 was required to reduce the E. coli concentration to the detection limit under the methacrylate cover, while under natural sunlight 13 kJ/m2 of solar UV-A dose was necessary. FIGURE 9 Both TiO2-RGO composites and GO have been reported to be capable of visible light photocatalysis although the mechanism is not clear [30]. Visible light photocatalytic disinfection with non-metal doped titania has previously been reported to be mediated through singlet oxygen production [31]. Others have previously reported on the formation of superoxide and singlet oxygen through visible light excitation of fullerenes [19].
To
determine if singlet oxygenation was playing a role in the disinfection mechanism, we measured singlet oxygen formation using a fluorescent probe under visible light excitation (λ
> 435 nm) in laboratory experiments. This probe has been used previously by others [19]. Figure 10 shows a greater increase in fluorescence at λem= 525 nm for the TiO2-RGO composite under visible only irradiation which is evidence of singlet oxygen formation. More research is needed to understand the contribution of singlet oxygen for disinfection with TiO2-RGO composites under solar UV-Vis irradiation.
Perhaps this can explain the
difference in the disinfection kinetics for E. coli and F. solani spores where the latter would be more resistant to singlet oxygenation. Given that F. solani produces naphthazarin toxins a class of toxins shown to be active in the production of singlet oxygen [32] F. solani could be expected to be more resistant to singlet oxygen than E. coli. FIGURE 10 It has been reported that the membrane of the cell in microorganisms is the likely target for the 1O2 reactions which cause inactivation. Schafer et al studied the use of visible light activated Rose Bengal for the production of singlet oxygen and the disinfection of water [33]. They found that singlet oxygen could inactivate the vegetative cell of E. coli (Gram-negative) and Deinococcus radiodurans (Gram-positive); however, the spores of Bacillus subtilis, with an entirely different structure of the cell envelope, were not affected by the singlet oxygen over the same period of exposure. The resistance of spores to attack by singlet oxygen would explain that no difference was observed in the rate of inactivation for Fusarium solani spores when the P25 was modified with RGO. In this case the band-gap excitation of P25 yields hydroxyl radical, which is a stronger biocidal agent than singlet oxygen.
5.0 Conclusions TiO2-RGO composites were formed via the photocatalytic reduction of exfoliated GO by UV irradiation in the presence of methanol as a hole scavenger.
The composites were
characterised using HRTEM, XPS and Raman spectroscopy. It was confirmed by XPS and Raman analysis that GO is reduced to RGO in the photocatalytic treatment to form the
composite. Following use in photocatalytic experiments, no evidence was found for the reoxidation of the RGO. The TiO2-RGO composites were compared to P25 alone for the disinfection of water contaminated with Escherichia coli and Fusarium solani spores under real sunlight. Very rapid water disinfection was observed with both E. coli and F.solani spores. An enhancement in the rate of inactivation of E. coli was observed with the TiO2RGO composite as compared to P25 alone. The rate of inactivation of F. solani spores was similar for both the TiO2-RGO and P25. When the major part of the solar UVA was cut-off (λ > 380 nm) using a methacrylate screen, there was a marked increase in the time required for inactivation of E. coli with P25 but no change in the inactivation rate for the TiO2-RGO. There is evidence of singlet oxygen production with visible light excitation of the TiO2-RGO composites which would lead to E. coli inactivation.
Acknowledgements The authors wish to acknowledge financial support from the US National Science Foundation and the Department of Employment and Learning Northern Ireland for funding under the US-Ireland Collaborative Partnership Initiative (NSF-CBET 1300 Award 1033317). The financial funds by the European Commission (SFERA - Solar Facilities for the European Research Area project’.contract no. 228296) are also acknowledged.
Figure captions Figure 1. a) Solar methacrylate cover at PSA facilities during the performance of a photocatalytic experiment. b) Transmission of solar radiation of borosilicate glass and methacrylate cover. Figure 2. TiO2-GO aggregate (before photocatalytic reduction)Figure 3. TiO2-RGO after photocatalytic reduction Figure 4. Raman spectra of GO and TiO2-RGO Figure 5. C1s region spectra of (a) GO (b) TiO2-RGO Figure 6. XPS of C1s region before (a) and after (b) photocatalytic experiments under UVVis irradiation Figure 7. E. coli inactivation at several TiO2-RGO concentrations: () 0 mg/L; () 10 mg/L; () 20 mg/L; () 50 mg/L; () 100 mg/L; () 300 mg/L; () 500 mg/L. DL (detection limit) = 2 CFU/mL. Control test in the presence of 100mg/L TiO2-RGO in the dark ( ). Inset shows the TiO2-RGO () versus P25 () efficiency on the E. coli inactivation at 500 mg/L of catalyst concentration. DL (detection limit) = 2 CFU/mL. Figure 8. F. solani spores inactivation at several TiO2-RGO concentrations: () 0 mg/L; () 10 mg/L; () 35 mg/L; () 50 mg/L; () 100 mg/L; () 300 mg/L; () 500 mg/L. DL (detection limit) = 2 CFU/mL. Figure insert shows the efficiency of TiO2-RGO () with 10 mg/L versus TiO2-P25 () with 35 mg/L on the F. solani spores inactivation. DL (detection limit) = 2 CFU/mL. Figure 9. Efficiency of TiO2-RGO () and P25 () on the E. coli inactivation at concentrations 500 mg/L under natural sunlight; and TiO2-RGO () and TiO2-P25 () under a methacrylate cover. DL (detection limit) = 2 CFU/mL. The data series (),(), and () are overlapping. Figure 10. Singlet oxygen production with TiO2-RGO and RGO under visible light irradiation (435 nm cut off filter). () 10 mg/L RGO; ()light control (no catalyst); () 10 mg/L P25; () 10 mg/L TiO2-RGO
Tables Table 1. C 1s components of GO and TiO2-RGO Sample
C-C (eV)
C-O (eV)
C=O (eV)
C-C/C-O
C-C/C=O
GO
284.64 ± 0.01
286.76 ± 0.06
288.69 ± 0.07
0.94
3.05
TiO2-RGO
284.81 ± 0.11
286.40 ± 0.21
288.59 ± 0.11
6.02
9.38
Table 2. Initial and final pH and Dissolved Oxygen (DO) measured during E. coli inactivation with the different TiO2-RGO loadings investigated. TiO2-RGO load
Initial pH
Final pH
Initial DO
Final DO
0 mg/L
5.5
6.6
7.0
5.9
10 mg/L
5.3
6.1
6.6
6.5
20 mg/L
5.9
5.9
6.5
6.0
50 mg/L
5.5
5.6
6.5
6.2
100 mg/L
5.3
5.4
7.9
7.5
300 mg/L
5.2
5.4
7.9
6.0
500 mg/L
4.85
5.0
8.2
7.7
Table 3. initial and final pH and Dissolved Oxygen (DO) measured trough the F. solani spores inactivation experiments with all TiO2-RGO catalyst concentration tested. TiO2-RGO load
Initial pH
Final pH
Initial DO
Final DO
0 mg/L
6.1
5.9
8.4
7.6
10 mg/L
5.7
5.3
8.5
7.5
35 mg/L
6.0
5.3
8.3
7.8
50 mg/L
6.3
6.1
8.5
7.7
100 mg/L
6.0
5.8
8.3
7.3
300 mg/L
6.1
5.9
8.4
5.9
500 mg/L
6.1
5.9
8.4
5.9
Figure 1
90
UVA 320-400 nm
100
UVB 280-320 nm
a.
methacrylate screen
80
% Transmittance
70 60 50 40 30 20 10
200
300
400
500
wavelength (nm)
b.
600
700
800
Figure 2
Figure 3
Figure 4.
D peak G peak
2D Peak 3S Peak
Figure 5
C=O
C--O
C--C
a.
C=O
b.
C--O
C--C
Figure 6 C=O
C--O C--C
a.
C=O
a.
b.
C--O C--C
Figure 7
Exposure time (min) 0
15
30
45
60
75
90
105
Exposure time (min)
6
10
0
2
4
6
8
10 12 14 16 18 20 TiO2-RGO versus TiO2-P25
6
10
500 mg/L
5
E. coli (CFU/mL)
10
E. coli (CFU/mL)
120
4
10
3
10
5
10
4
10
3
10
2
10
1
10
DL = 2 CFU/mL 0
10
0
5
10
15
20 2
Solar UV-Dose (kJ/m ) 2
10
1
10
DL 0
10
0
20
40
60
80
100
120 2
Solar UV-Dose (kJ/m )
140
160
25
Figure 8
Exposure time (min) 0
30
60
90
120
150
180
Exposure time (min)
3
0
10
F. solani (CFU/mL)
10
F. solani (CFU/mL)
210
2
10
5
10
15
20
25
30
TiO2-RGO versus TiO2-P25 10
2
10
1
10
0
DL = 2 CFU/mL
0
10
20
30
40 2
Solar UV Dose (kJ/m )
10
DL 0
10
50
100
150
200
40
3
1
0
35
250 2
Solar UV-Dose (kJ/m )
300
350
50
Figure 9
Local Time (min) 0
5
10
15
20
25
30
35 40
6
10
E. coli (CFU/mL)
UV-A irradiance sunlight
30 25
4
10
20
3
10
UV-A irradiance under methacrylate screen
2
10
12 9 6
1
10
3
DL = 2 CFU/mL 0
10
0
10
20
30
40 2
Solar UV-Dose (kJ/m )
0 50
2
10
UV-A irradiance (W/m )
35
5
Figure 10
Fluorescence (counts at 525 nm)
16000
Control (water) 10 mg/L RGO 10 mg/L TiO2-RGO
14000 12000 10000 8000 6000 4000 2000
0
5
10
Time (min)
15
20
References
1. WHO/UNICEF, Progress on Sanitation and Drinking-water: 2012 Update, World Health Organization and UNICEF, 2012.
2
J.A. Byrne, P.A. Fernandez-Ibanez, P.S.M. Dunlop, D.M.A. Alrousan, and J.W.J. Hamilton,
Photocatalytic Enhancement for Solar Disinfection of Water: A Review, International Journal of Photoenergy 2011, (2011) Article ID 798051.
3
M.I. Polo-López, P. Fernández-Ibanez, E. Ubomba-Jaswab, C. Navntoft, I. García-
Fernández, P.S.M. Dunlop, M. Schmid, J.A. Byrne, K.G. McGuigan, Elimination of water pathogens with solar radiation using an automated sequential batch CPC reactor, Journal of Hazardous Materials 196 (2011) 16– 21.
4
T.F. Clasen, L. Haller, Water Quality Interventions to Prevent Diarrhoea: Cost
and Cost-Effectiveness, WHO, 2008.
5
S. Malato, P. Fernandez-Ibanez, M.I. Maldonado, J. Blanco and W. Gernjak,
Decontamination and disinfection of water by solar photocatalysis: recent overview and trends, Catalysis Today 147 (2009) 1–59.
6
D.M.A. Alrousan, P.S.M. Dunlop, T.A. McMurray, J.A Byrne, Photocatalytic inactivation of
E. coli in surface water using immobilised nanoparticle TiO2 films, Water Research 43 (2009) 47 –54.
7
J.W.J. Hamilton, J.A. Byrne, C. McCullagh, and P.S.M. Dunlop, Electrochemical
Investigation of Doped Titanium Dioxide, International Journal of Photoenergy, 2008 (2008) Article ID 631597. 8
M. Pelaez, N.T. Nolan, S.C. Pillai , M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop,
J.W.J. Hamilton, J.A. Byrne, K. O’Shea, M.H. Entezari, D.D. Dionysiou, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Applied Catalysis B: Environmental 125 (2012) 331– 349.
9
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I.V.
Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films. Science 306 (2004) 666–669.
10
W.S. Hummers, R.E Offeman, Preparation of Graphitic oxide, Journal of American
Chemistry Society 80 (1958), 1339-1339.
11
A. Ganguly, S. Sharma, P. Papakonstantinou, and J. Hamilton, Probing the Thermal
Deoxygenation
of
Graphene
Oxide
Using
High-Resolution
In
Situ
X-ray-Based
Spectroscopies, Journal of Physical Chemistry C 115 (2011) 17009–17019.
12
G. Williams, B. Seger, P.V. Kamat, TiO2-Graphene Nanocomposites.UV-Assisted
Photocatalytic Reduction of Graphene Oxide, American Chemical Society Nano 2 (2008) 1487–1491.
13
I.V. Lightcap, T.H. Kosel, and P.V. Kamat, Anchoring Semiconductor and Metal
Nanoparticles on a Two-Dimensional Catalyst Mat. Storing and Shuttling Electrons with Reduced Graphene Oxide, Nano Letters 10 (2010) 577-583.
14
S. Guo and S. Sun, FePt Nanoparticles Assembled on Graphene as Enhanced Catalyst
for Oxygen Reduction Reaction, Journal of the American Chemical Society 134 (2012) 2492−2495.
15
L. Yang, Y. Zhao, S. Chen, Q. Wu, X. Wang and Z. Hu, A mini review on carbon-based
metal-free electrocatalysts for oxygen reduction reaction, Chinese Journal of Catalysis 34 (2013) 1986–1991.
16
Y.H. Ding, P. Zhang, Q. Zhuo, H.M. Ren, Z.M. Yang and Y. Jiang, A green approach to
the synthesis of reduced graphene oxide nanosheets under UV Irradiation, Nanotechnology 22 (2011) 215601 (5pp).
17
H. Li, W. Zhang, L. Zou, L. Pan. and Z. Sun, Synthesis of TiO2–graphene composites via
visible-light photocatalytic reduction of graphene oxide, Journal of Material Research 26 (2011) 970-973.
18 X. An, J.C. Yu, Graphene based photocatalytic composites, RSC Advances, 1 (2011) 1426–1434
19
L. Brunet, D.Y. Lyon, E.M. Hotze, P.J. Alvarez, M.R. Wiesner, Comparative Photoactivity
and Antibacterial Properties of C60 Fullerenes and Titanium Dioxide Nanoparticles, Environmental Science and Technology 43 (2009) 4355–4360.
20
O. Akhavan and E. Ghaderi, Photocatalytic Reduction of Graphene Oxide Nanosheets on
TiO2 Thin Film for Photoinactivation of Bacteria in Solar Light Irradiation, Journal of Physical Chemistry C 113 (2009) 20214–20220.
21
J. Liu, L. Liu, H. Bai, Y. Wang and D. D. Sun, Gram-scale production of graphene oxide–
TiO2 nanorod composites: Towards high-activity photocatalytic materials, Applied Catalysis B 106 (2011) 76– 82.
22
B. Cao, S. Cao, P. Dong, J. Gao and J. Wang, High antibacterial activity of ultrafine
TiO2/graphene sheets nanocomposites under visible light irradiation, Materials Letters 93 (2013) 349–352.
23
M.I. Polo-López, P. Fernández-Ibáñez, I. García-Fernández, I. Oller, I. Salgado-Tránsito
and C. Sichel, Resistance of Fusarium sp spores to solar TiO2 photocatalysis: influence of spore type and water (scaling-up results). Journal of Chemical Technology and Biotechnology 85 (2010) 1038–1048.
24
P. Fernández-Ibáñez, C. Sichel, M.I. Polo-López, M. de Cara-García, J.C. Tello.
Photocatalytic disinfection of natural well water contaminated by Fusarium solani using TiO2 slurry in solar CPC photo-reactors, Catalysis Today 144 (2009) 62–68. 25
I. K. Moon, J. Lee, R.S. Ruoff
and
H. Lee “Reduced graphene oxide by chemical
graphitization” Nature Communications 1 (2010) 73-78. 26
N.J. Bell, Y.H. Ng, A. Du, H. Coster, S.C. Smith, and R. Amal, Understanding the
Enhancement in Photoelectrochemical Properties of Photocatalytically Prepared TiO2Reduced Graphene Oxide Composite, Journal of Physical Chemistry 115 (2011) 6004-6009. 27
T.N. Lambert, C.A. Chavez, B. Hernandez-Sanchez, P. Lu, N.S. Bell, A. Ambrosini, T.
Friedman, T.J. Boyle, D.R. Wheeler and D.L. Huber, Synthesis and Characterization of Titania-Graphene Nanocomposites Journal of Physical Chemistry C, 113 (2009) 1981219823.
28
A. C. Ferrari J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F.
Mauri, S. Piscanec, Da Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, The Raman Fingerprint
of
Graphene,
Physical
Review
Letters,
97
(2006)
187401,
DOI:
http://dx.doi.org/10.1103/PhysRevLett.97.187401
29 I. Calizo, A. A. Balandin, W. Bao, F. Miao, C. N. Lau, Temperature dependence of the Raman spectra of graphene and graphene multilayers, Nano Letters, 7 (2007) 2645-2649
30
L. Guardia, S. Villar-Rodil, J.I. Paredes, R. Rozada, A. Martınez-Alonso, J.M.D. Tascon,
UV light exposure of aqueous graphene oxide suspensions to promote their direct reduction, formation of graphene–metal nanoparticle hybrids and dye degradation, Carbon 50 (2012) 1014 –1024.
31
J.A. Rengifo-Herrera, K. Pierzchała, A. Sienkiewicz, L. Forro, J. Kiwi and C. Pulgarin,
Abatement of organics and Escherichia coli by N, S co-doped TiO2 under UV and visible light. Implications of the formation of singlet oxygen (1O2) under visible light, Applied Catalysis B, 88 (2009) 398–406.
32
I. Heiser, W. Oswald and E.F. Elstner “The formation of reactive oxygen species by fungal
and bacterial phytotoxins” Plant Physiology and Biochemistry 36, (1998) 10, 703–713.
33
M. Schafer, C. Schmitz, R. Facius, G. Horneck, B. Milow, K.H. Funken and J. Ortner,
Systematic study of parameters influencing the action of Rose Bengal with visible light on bacterial cells: Comparison between the biological effect and singlet-oxygen production, Photochem. Photobiol. 71(2005)14–523.
Highlights
TiO2-RGO composites formed by photocatalytic reduction of graphene oxide TiO2-RGO composites used for photocatalytic disinfection of water under real sun Rapid solar photocatalytic inactivation of E. coli and Fusarium solani spores was observed The TiO2-RGO composites form singlet oxygen under visible irradiation Evidence of enhanced disinfection of E. coli under solar irradiation
