Suitable choices of dopant elements and their concentration has enabled coatings to be developed in which the anatase to rutile transformation can be suppressed at temperatures of up to 900 °C31,32, allowing the application of VLA photocatalytic coatings to a wide range of substrates during existing manufacturing processes. Fluorine is an attractive dopant, since it has been found both to confer visible light activity and to stabilise anatase at elevated temperatures, thus fulfilling both roles with a single modification to the synthesis20,31. Addition of F is effective in titania doping as it can minimize the recombination of the electron hole pair due to charge compensation between F- and Ti4+ 33. In addition F-doping is reported to induce the creation of Ti3+ ions and thereby oxygen vacancies33.

X-Ray Diffraction

X-Ray diffraction spectra obtained from powders calcined at 300 °C and 600 °C are shown in Fig. 2, together with the spectrum of an anatase titania reference sample. As can be seen in Fig. 2, calcination at both temperatures results in the formation of a largely crystalline product, with a diffraction pattern characteristic of anatase. The peaks are relatively broad and less well differentiated than those in the reference spectrum, particularly at 300 °C, which we attribute to a combination of incomplete crystallisation at lower temperatures and peak spreading caused by the nanoparticulate nature of the material. Inhibition of titania condensation and crystallisation in fluorine-rich doped systems has already been noted by other workers31.

Figure 2 XRD Spectra of Anatase Reference Sample and Copper Doped Titania Calcined at 300 °C and 600 °C. Full size image

There are no additional peaks that could be attributed to copper oxides and from this we conclude that much of the copper is dissolved in the titania matrix, with any copper oxide being present at concentrations below the detection level of the technique.

Optical Transmission

The optical absorbance spectra of the glass substrate and glass coated with a two layer copper doped coating are shown in Fig. 3, below.

Figure 3 Optical Absorbance Spectra of Uncoated and Coated Glass. Full size image

It can be seen that while the coating reduces the overall transparency of the glass, the absorbance spectrum in the visible range is still essentially flat, so the coated substrates do not have a colour cast. Absorption in the coated materials increases above that of the substrate at wavelengths below approximately 380 nm and rapidly increases at wavelengths below 300 nm, consistent with absorption of blue and particularly of ultraviolet light by the titanium dioxide.

Surface Wetting and Contact Angle Measurements

The results of the contact angle measurements are shown in Table 1, below.

Table 1 Contact Angles of Glass Substrates and Titanium Dioxide Coatings. Full size table

Relatively little difference in contact angle was seen between the two sides of the uncoated glass, though the surface that had been in contact with the tin bath had a marginally higher contact angle (56.46°) than the upper surface that had not been exposed to the tin (52.34°).

The application of the titanium dioxide coating significantly reduced the contact angle to a mean value of 8.51° (superhydrophilic nature).

The reduction in contact angle on the application of the titanium dioxide coating is illustrated by the change in macroscopic wetting behaviour of water droplets applied by pipette as sen in Fig. 4. The droplet placed on the uncoated glass displays modest spreading over the surface, while that on the titanium dioxide layer spreads extensively. This is consistent with results found in the literature and is an important factor in the antimicrobial activity of the coating16. Photocatalytic degradation by titanium dioxide proceeds principally through the formation of oxidative intermediates, produced by holes in the titania valence band oxidising surface water. Thus, it is advantageous for the surface to be strongly hydrophilic, maximising the opportunity for these reactive peroxide and oxygen radicals to form16. The substantial reduction in contact angle also illustrates the importance of obtaining the most uniform coating possible from the first stages of deposition. Since the sol is aqueous and the titania coating is more hydrophilic than the glass substrate, subsequent layers will tend to wet out and adhere preferentially in areas where a partial coating is already present. It is therefore important to ensure that the substrate is uniformly clean and free of contaminants that could increase the water contact angle. Dip coating has been used to prepare the materials in this study, since it ensures complete coverage in a single step.

Microbiological Testing

The results of microbiological testing of films with S. aureus are shown in Table 2, below.

Table 2 Performance of Fluorine Doped and Copper-Fluorine Doped VLA Titanium Dioxide Coating Against S. aureus Culture. Full size table

The microbiological testing results show the efficacy of the copper-doped coating both in light and darkness, with at least a substantial reduction in the bacterial population and at best, reducing the bacterial load below detectable levels.

The distinction between the coatings prepared with and without copper doping illustrates the dramatic improvement in performance resulting from the addition of copper to the formulation. The precise degree to which this improved performance is the result of the toxicity of copper to bacteria and how much is potentially caused by increased photocatalytic activity above the solely fluorine-doped material is the subject of ongoing study, but the persistent effectiveness of the copper-doped coatings in darkness implies that the former mechanism is dominant.

In the present study, the negligible photocatalytic activity of undoped titanium dioxide in the absence of ultraviolet radiation is taken as established. This was previously reported in the literature19,33.

The photocatalytic activity of undoped stoichiometric anatase titania depends on the presence of a significant ultraviolet flux. Previous work by Fisher et al.19 on photocatlytic activity of titania for water decontamination, in an undoped state and with dopants including copper and nitrogen has demonstrated that the undoped material brings no significant reduction in populations of E. coli or Enterococcus faecalis in the absence of ultraviolet radiation19. In that work, it was found that doping with copper increased the activity slightly, but it was acknowledged that this improved activity could have been caused by a combination of VLA photocatalysis and copper ion toxicity.

X-Ray Photoelectron Spectroscopy

Selected energy range scans covering the O1s, Ti2p, Cu2p and F1s regions of the XPS spectra are shown in figures 5, 6, 7 and 8, respectively.

Figure 5 High Resolution Scan of O1s Peak in XPS Spectra. Full size image

Figure 6 High Resolution Scan of Ti2p Peak in XPS Spectra. Full size image

Figure 7 High Resolution Scan of Cu2p Peak in XPS Spectra. Full size image

Figure 8 High Resolution Scan of F1s Peak in XPS Spectra. Full size image

Addition of copper to the sol substantially modifies the binding energies of both the Ti2p and O1s electrons in the materials after heat treatment.

As seen in Fig. 5, a consistent downward shift of 0.5 to 0.7 eV in the O1s binding energy occurs on the addition of copper to the titanium dioxide matrix. This is consistent the oxygen depletion of the titanium dioxide measured in the composition quantification, with a reduction in oxygen content of 1.5 to 1.8 atomic percent in the co-doped material, relative to that doped with fluorine only. Similar shifts in binding energy were reported by Nolan et al.34, on the addition of silver to a titanium dioxide matrix. This is consistent with the binding energy of the Cu2p 3/2 peak in the sample calcined at 300 °C, at 933.74 eV closely matcheing that of CuO, as seen in Fig. 7 35. The absence of any CuO phase detected by XRD suggests incorporation of the copper in the +2 oxidation state into the titanium dioxide matrix, with consequent oxygen depletion of the matrix, relative to stoichiometric titanium dioxide.

The Cu2p 2/3 binding energy in the copper doped sample heat treated at 550 °C displays an upward peak shift of 0.8 eV to 934.54 eV, greater than the expected deviation in measured values in CuO. This is accompanied by an increase in the oxygen content from 58.54% to 64.69% and is consistent with a super-saturation of oxygen relative to that found in CuO, with a consequent increase in the binding energy caused by the more electronegative environment than would be found in CuO.

The Ti2p binding energy displays downward shifts of 0.6 eV and 0.74 eV in the copper-doped material after treatment at 300 °C and 550 °C respectively, accompanied by a peak broadening of 0.5 eV (Fig. 6), Again, this is consistent with oxygen depletion in the titania and the formation of a minor Ti2+ component. Lower Ti2p 3/2 and O s binding energies for copper doped titania are direct measures of the formation of Ti 3+ due to the generation of oxygen18. Such a downward shift indicates the existence of oxygen vacancies and Ti 3+ in the Cu doped samples. The overall composition is thus oxygen depleted with respect to stoichiometric TiO 2 and oxygen super-saturated with respect to stoichiometric CuO.

It is also notable that the fluorine content decreases rapidly with heat treatment temperature, in both compositions, falling by factors of 4.51 and 17.87 in the fluorine doped and copper-fluorine co-doped materials respectively. This is illustrated clearly by the near disappearance of the F1s peak in Fig. 8, following heat treatment at 550 °C. After treatment at 550 °C, it can be expected that the properties of the copper-fluorine co-doped material to be dominated by the copper, rather than fluorine doping. This behaviour matches that observed by Padmanabhan et al.31, in which the fluorine content of titania prepared by the hydrolysis and calcination of a pure titanyl trifluoroacetate was reduced from 0.5 atomic percent after calcination at 550 °C to 0.3 atomic percent after calcination at 700 °C.

The mechanism for the antimicrobial activity of oxide semiconductor catalysts has been established as cell membrane rupture, caused by reactive oxidising species including hydroxyl radicals and peroxides generated by the oxidation of water and oxygen by holes in the valence band that are created during the photonic excitation of electrons to the conduction band (Fig. 9)17.

The superior activity of the copper-doped materials and their continued activity in darkness is the result of a mechanism of a bactericidal action that is independent of that of the photocatalyst, but which has the potential to reinforce it under conditions of illumination.

Copper and its compounds achieve their antimicrobial activity by degrading cell membranes36. Previous studies have shown that rapid accumulation of copper ions in Staphylococcus haemolyticus was observed on both dry and moist metallic copper surfaces, while no significant mutagenic effects were observed, indicating that absorption of copper does not cause lethal DNA damage, but instead kills by fatally compromising the cell membrane, evidence for which is found from the rapid accumulation of copper ions within the cell, on a scale of minutes. This mechanism is further expanded by Lemire et al.37 and Macomber et al.38 who demonstrated that the formation of Cu2+ ions is associated with an accelerated formation of reactive oxygen species and this accelerates the consumption of antioxidants, reducing the capacity of the cell wall to absorb damage from these species and to self-repair.

These reactive species are generated by a Fenton reaction as described in Equation 1, thus39.

Hydrogen peroxide is a transient metabolic by-product of cellular respiration37,39, so this mechanism is active whenever cellular respiration is taking place, but it is noteworthy that in the present study, the dissolved copper in the doped titanium dioxide is in a particularly strongly oxidising environment, as indicated by the XPS data and can thus accelerate the cell wall damage intrinsic to the photocatalytic action.

Similar increases in the bactericidal efficacy of titanium dioxide occurring on the addition of copper have been observed in TiO 2 : Cu films co-sputtered on polyester fabric40,41. In these experiments, solar spectrum and actinic lights with low UV flux were used. High levels of self-cleaning activity were observed, attributed to the increased generation of reactive oxygen species from a combination of hydroxyl radicals generated by Fenton chemistry40 and visible light activated photocatalysis resulting from the presence of intra-band gap energy levels created by the copper doping41. Evidence for the effectiveness of copper in inducing VLA photocatalytic activity comes from the substantial increase in decomposition rate of methylene blue in copper doped films, relative to sputtered undoped titanium dioxide under solar spectrum irradiation, where the UV component is relatively minor, which correlated well with a reduction in the estimated band gap41.

Closely related systems, also comprising co-sputtered TiO 2 : Cu films have demonstrated antibacterial activity on PET fabric42. Evidence for copper-moderated O 2 reduction as a mechanism for bactericidal activity was provided by the increased activity in aerobic, relative to anaerobic conditions.

Taken together, these results support the argument that copper doping of titanium dioxide increases its photocatalytic and antimicrobial activity by a combination of band-gap modification and hydrogen peroxide splitting through Fenton chemistry.