NITROGEN AND COPPER DOPED SOLAR LIGHT ACTIVE TIO2 PHOTOCATALYSTS FOR WATER DECONTAMINATION
A new category of photocatalytic coating that can degrade chemical as well as bacterial contaminants under visible sunlight wavelengths was generated by the deposition of a stable photocatalytic TiO2 film over the internal lumen of glass bottles by means of a sol–gel technique. Such a coating was produced either in doped or undoped form. This coating used copper and/ or nitrogen for producing visible light-active TiO2 films that were normalized or annealed at 600 ◦C and were characterized by Raman and X-ray photoelectron spectroscopy. The degradation of methylene blue was seen to accelerate with doped and undoped TiO2 films in the presence of natural sunlight. On the other hand, the bacterial inactivation (of Escherichia coli and Enterococcus faecalis) was seen to accelerate copper-doped TiO2 films in the presence of natural sunlight.
This report presents a summary of the results and findings of the aforementioned experiment. The paper also successfully explains the significance of the results found in the wider context of the subject.
One of the major causes for diarrheal illness is contaminated drinking water that greatly contributes to the bacterial disease in most in developing countries. Over two million deaths annually are recorded, mainly among children below five years of age. Other severe diseases caused due to contaminated water can be particularly seen among immune-compromised individuals. This calls for the availability of affordable technologies that can efficiently reduce or discard the risk of water related harmful diseases in developing as well as developed countries, specifically within settings that have failed traditional drinking water distribution systems installed that cannot eradicate waterborne pathogens. An advanced and state-of-the-art technology that has been applied on a large scale globally is solar water disinfection, or SODIS, which makes use of natural sunlight for inactivating microorganisms.
A new innovation of this technology involves the enhancement of microbial inactivation and degradation of chemical contaminants present in drinking water.
“Titanium dioxide (TiO2) is a naturally-occurring mineral with optical and electronic properties that make it suitable for a range of applications in the areas of photovoltaics, sensors and photocatalysis”
Undoped TiO2 photocatalysts, which has been the topic of interest for several researchers developing literature, depict comparatively high degree of photo-reactivity and self-sterilization attributes when exposed to UV or ultraviolet light. Nonetheless, the enhancement of photocatalysts that have high quantum yields in the presence of visible light (>400 nm) is needed for the purpose of harnessing the remainder of the solar spectrum for effectively degrading chemical contaminants and inactive pathogens.
There have been numerous research studies demonstrating the capability of TiO2 in order to speed up the inactivation of microorganisms mediated by natural light and the deceleration of organic and inorganic contaminants.
Results of research conducted by means of suspended titania photocatalysts within water have distinctively showcased higher effectiveness in contrast to results of studies that used heterogenous photocatalysts immobilized on substrates. The main reason for this is the mass transfer constraints that decrease the efficacy of immobilized TiO2 photocatalysts within bulk solution. Even so, suspended catalysts are associated with logical issues as they need to be removed from solution before the treated water becomes usable. Outcomes of recent studies have depicted that different kinds of dopants are capable of modifying the band gap of titamium dioxide photocatalysts which shift the photocatalytic activity of the resulting materials within the visible range. Whereas surface-modified and nanocomposite titania formulations demonstrate similar activities. Recently published reports suggested that the co-doping of non-metal and metal elements may lead to an increase in visible light photocatalytic performance.
The current study attempts to measure the power of thin films of copper- and nitrogen-doped TiO2 to raise the natural light-mediated degradation of organic elements and indicator microorganisms within contaminated water. Earlier research studies have confirmed the applications of TiO2 species to the correction of chemical and microbial contaminants, and have characterized the activities of titania compounds/ particles doped with copper and other transition metals.
It has been the first attempt for the authors of the research paper to characterize the ability of a self-bound copper- or nitrogen-doped titania thin film for degrading microbial and chemical contaminants in water.
This preparation used coated glass microscope slides to investigate whether or not well-adhered thin films of anatase-phase titanium dioxide could be produced. Well-adhered coating was detected even before the heat-treatment. The film was annealed at 450 ◦C to further improve the adhesion (BS EN ISO 2409:2007). The use of Raman spectroscopy confirmed the presence and phase of the TiO2 thin film on glass slides. The arrangement clearly indicated four bands located at 141, 394, 517 and 637 cm−1, property of the anatase crystalline phase of TiO2. Furthermore, XPS studies were conducted to determine if Cu and N were successfully doped into the TiO2 thin films. Both un-doped and doped powders and thin films on glass were characterized, and seen to be similar excluding the additional peaks in the film due to the presence of calcium (Ca2p), silicon (Si2p), and chloride (Cl2p) in the glass slide. Every sample showed the characteristic peaks of titanium (Ti2p), carbon (C1s), and oxygen (O1s). Cu in the (+2) oxidation state (Cu2p3/2), was easily seen in all Cu containing samples, indicating that Cu was successfully doped into the photocatalyst. However, due to minute amounts of nitrogen observed, determining whether or not it’s a dopant or as an adventitious contaminant, was impossible. In contrast to uncoated containers, the coated bottles generated increased degradation of the dye. Post six hours, total decolouration was seen in the coated bottles, while visible colour stayed in the uncoated ones. Further, it can be confirmed that a dye degradation rate in the coated containers was little more than twice of that observed in the uncoated bottles. Importantly, the doped titania coated containers indicated no increase in MB degradation as against undoped titania.
A faster and consistent decolouration of methylene blue was found in the presence of titania films with the broadly described photocatalytic properties of TiO2. Methylene blue can be degraded by three different ways, under solar light irradiation, a) photo-bleaching or photolysis 2) photosensitization, and 3) photocatalysis. Additionally, without a photocatalyst, methylene blue can undergo photolytic degradation. In addition, irradiated methylene blue creates an excited state and can transfer electrons to molecular oxygen which produces superoxide ions, contributing to the degradation of the dye. With semiconductor photocatalyst, photosensitized degradation takes place when excited electrons from irradiated dye molecules shift to the conduction band of titania photocatalysts which degrade the dye, after reacting with oxygen to create superoxide anions. Generally, the photosensitization reaction does not depend on the dopants or band gap of the material used, since there is direct reaction that occurs at the conduction band. In short, the absorption of a photon with energy more than the band gap of the semiconductor excites a valence band electron to the conduction band (e-CB), generating a positive hole in the valence band (h+VB).
Positive holes generated by light are trapped by surface adsorbed H2 O, which is oxidized by h+VB, which produces H+ and •OH radicals. Superoxide anions and hydroxyl radicals generated by the photocatalytic process oxidize most of the organic compounds till the entire mineralization is accomplished. Oxygen has two singlets excited states above the triplet ground ones. Moreover, the study confirmed that the photocatalytic inactivation of E. coli did not produce hydroxyl radicals. The visible light irradiation-generated hole within the mid-gap levels (induced by doping) was observed to lack in redox potential for oxidizing organic molecules like methylene blue. The bacterial decontamination was chiefly caused by the generation of singlet oxygen, a less oxidative, reactive oxidation species. According to Hermann and colleagues, the decomposition of methylene blue results in the conversion of organic carbon into the formation of gaseous CO2 and that of sulphur and nitrogen heteroatoms into inorganic ions.
It was further found that the enhanced inactivation of E. coli and Enterococcus faecalis under the influence of Copper-doped titania was caused by the visible light activity of this photocatalyst, by the anti-microbial properties of Cu at exposed surfaces, or by other characteristics of the material. Interestingly, while 1% Cu accelerated bacterial inactivation, 1% Cu + 3.5% N did not, possibly due to the fact that 1% Cu-doped catalyst depicted a rise in visible light absorption in as against the CO-doped 1% Cu/3.5% N-doped catalyst. Hence, this clearly points out that the co-doped catalysts in this research were not optimized, and that nitrogen atoms in the lattice might have behaved as recombination points for photogenerated electrons and holes. Similarly, the findings of Song et al also observed reduced photocatalytic action (dye degradation) whilst doping levels were below or above the optimal ratio.
As Cu-doped TiO2 films enhanced E. coli inactivation when UV wavelengths were absent, involving visible-light activity, Nitrogen-doped films either did not demonstrate such activity or this activity was outbalanced by other impacts that decreased recorded bacterial inactivation rates.
The speedy inactivation of E. coli and Enterococcus faecalis under un-doped and doped titania-coated beads could be because of the light absorption occurring on the surface of the catalyst in contact with the media, unlike uncoated bottles. Another factor that contributes to this observation is the increase in surface-area-to-volume ratio of the glass bead substrate proportional to coated bottles, and to the accordingly shorter mean distance between illuminated photocatalytic surfaces and target microorganisms. The observation that Cu/N-doped TiO2 accelerated inactivation when coated on glass beads but not on the internal surface of glass bottles may also indicate that any reactive species produced at Cu/N-doped photocatalytic surfaces can only diffuse shorter distances, and that bacterial inactivation by these microorganisms might therefore be transport-limited. H2O2 and O2•− are crucial reactive oxygen species that may probably having a mean diffusion distances on this length scale. Moreover, bacterial cells can also bind to catalytic surfaces, amplifying the results of short-lived radical species. The observation that Cu/N-doped titanium-coated beads enhanced E. coli inactivation without UV wavelengths, which was not found in the same photocatalyst coated on the inside of a glass bottle. This indicated that this coating may have substantial visible light activity with the ability to inactivate microorganisms at short diffusion distances. Without mixing or under static conditions, microorganisms tend to settle to the bottom of the bottles. The inactivation process, in the glass beads, can be favoured if this settlement would occur over more TiO2 surface. Nevertheless, the researchers have previously explored this kind of microbial settling effect in solar disinfection bottles but have failed to find any supporting. This might be due to the convective mixing created inside the liquid on exposure.
“In comparison, the enhanced inactivation of bacteria in 250-ml bottles coated with Cu-doped titania depict a greater diffusion distance of one or more reactive species produced by this photocatalyst”. Based on the above quoted observation and the efficacy of bottles coated with titania doped only with 1% Cur, further studies of glass beads and other media coated with this photocatalyst are justified.
“Standard borosilicate glass is opaque to UVB but has higher UVA transmittance than PET plastic”. The lower UVA transmit-tance of PET is balanced by the thinner wall thickness in the plastic bottles balances, in a way that the glass and PET bottles have similar UVA transmittance. Due to low baseline rate of methylene blue degradation by sunlight, the photocatalytic degradation of this compound was recorded observed and measured with reasonable accuracy throughout these trials. The high baseline inactivation rates of bacterial indicators however under solar UV light makes it even harder to study minor enhancement effects because of due to photocatalytic coatings, since these coatings might screen germicidal wavelengths whilst simultaneously damaging bacteria through photo-catalytic mechanisms. Therefore, coatings that showed fewer enhancements of bacterial inactivation rates may have some antimicrobial effectiveness that was not assessed in the current study. Also, it is important to note that outcomes of anti-bacterial studies and photocatalysis research do not support each other. Additionally, Ryu et al.’s work proved that photocatalytic activities are correlated only in terms of the degradation of structurally related compounds. Therefore, the Cu/N- and Cu-doped titania thin films examined in this trial seem to be the most promising candidates for future study and water remediation applications.
It can be concluded that robust, transparent titania thin films were deposited by a sol–gel technique. Methylene blue degraded more rapidly by the bottles coated with these films when annealed at 600◦C than uncoated bottles in field trials. Nonetheless, N-doped titania photocatalytic coatings did not show higher MB degradation rates than undoped titania. On the contrary, N- and Cu-doped photocatalyst-coated bottles appeared to show enhanced bacterial photovinactivation compared to undoped titania. These effects seemed to stay even in the absence of UV wavelengths. Cu- and Cu/N-doped titania thin films demonstrated great possibility for the degradation of chemical and microbiological contaminants when natural sunlight or visible light was present in these trials. The latter only proved efficient when coated on 3-mm glass beads, whereas the former showed increased acceleration in the inactivation of indicator bacteria when a glass bottle was coated from inside. Such thin film VLAT photocatalysts can be applied to the remediation and treatment of ground water, drinking water, and wastewater, as well as in the fields of clinical and emergency systems, as a further exploration of the study.
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