PbO-Modified TiO2 Thin Films: A Route to Visible Light

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PbO modified TiO2 Thin Films; A Route to Visible Light Photocatalysts Davinder Singh Bhachu, Sanjayan Sathasivam, Claire Jane Carmalt, and Ivan Paul Parkin Langmuir, Just Accepted Manuscript • Publication Date (Web): 19 Dec 2013 Downloaded from http://pubs.acs.org on December 24, 2013

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PbO modified TiO2 Thin Films; A Route to Visible Light Photocatalysts Davinder S. Bhachu,1 Sanjayan Sathasivam,1,2 Claire J. Carmalt1 and Ivan P. Parkin*1 *Corresponding author 1

Materials Chemistry Centre, Department of Chemistry, University College London

20 Gordon Street, London WC1H 0AJ, UK Fax: (+44) 20-7679-7463 E-mail: [email protected] 2

Bio Nano Consulting Ltd

338 Euston Road, London, UK, Fax: (+44) 20-7396-1056 E-mail: [email protected]

Abstract

PbO clusters were deposited onto polycrystalline titanium dioxide (anatase) films on glass substrates by aerosol assisted chemical vapour deposition (AACVD). The asdeposited PbO/TiO2 films were then tested for visible light photocatalysis. This was monitored by the photodegradation of stearic acid under visible light conditions. PbO/TiO2 composite films were able to degrade stearic acid at a rate of 2.28 x 1015 molecules cm-2 h-1, two orders of magnitude greater than what has previously been reported. The PbO/TiO2 composite film demonstrated UVA degradation of resazurin redox dye, with the formal quantum yield (FQY) and formal quantum efficiency (FQE) exceeding that of a TiO2 film grown under the same conditions and Pilkington ActivTM, a commercially available self-cleaning glass. This work correlates with computational studies that predicted PbO nanoclusters on TiO2 form active visible light photocatalysts through new electronic states, through PbO/TiO2 interfacial bonds resulting in new electronic states above the valence band maximum in TiO2 shifting the valence band upwards as well as more efficient electron/hole separation with hole localisation on PbO particles and electron on the TiO2 surface. Keywords: Titanium Dioxide, lead oxide, visible light photocatalysis, thin films

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Introduction Titanium dioxide (TiO2) is an important non-stoichiometric semiconductor that exhibits outstanding photoelectrochemical (PEC) properties as demonstrated by Fujishima and Honda. 1–4 They showed that TiO2 could be used for the conversion of solar energy to chemical energy, by essentially using TiO2 as a photoelectrode for water splitting. As well as the PEC properties of anatase, TiO2 also finds applications in photoinduced hydrophilicity, utilised for self-cleaning applications (PilkingtonNSG Activ), which allows for greater wetting of a surface as well as the formation of electron/hole pairs for dirt degradation.2,3,5,6 The band gap of TiO2 however absorbs in the UV range, 3.0 eV for rutile and 3.2 eV for anatase. There have been many attempts to engineer the band gap of TiO2 such that light is absorbed in the visible. It has recently been reported by Scanlon and coworkers through computational/X-ray photoemission studies that photogenerated conduction electrons in rutile/anatase mixtures flow from rutile to anatase that may help in material fabrication.4 The most common approach to altering the band gap towards to the visible are by doping the TiO2 structure with metal cations and/or nonmetal anions at Ti and O sites respectively.7–10 Nitrogen doping of TiO2 is a common route to visible light photocatalysis but controlling the nature of the dopant (i.e. nitrogen doping on oxygen sites or interstitial sites) still remains a challenge.11–21 Heterostructures/layered materials have garnered attention of late as alternatives to doped TiO2 materials in order to modify the band gap into the visible.22 Some examples of layered materials that have shown visible light photocatalysis are AgI/BiO and BiVO4/WO3.23,24 Less exotic materials but equally fascinating systems such as FeOx, NiOx and SnO supported on TiO2 have also been explored both experimentally and computationally.21,25,26 Computational studies have very recently shown that PbO modified TiO2 introduced new states above the valence band of TiO2 resulting in a shift of the band onset into the visible. This arrangement results in hole localisation on the PbO and electron localisation on the TiO2 surface.27 PbO2 modification of TiO2 also results in visible light photocatalysis via new states just below the conduction band of TiO2, resulting in hole localisation on the TiO2 and electron localisation on the PbO2.27


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Motivated by these findings, we deposited PbO films on polycrystalline TiO2/glass substrates by aerosol assisted chemical vapour deposition (AACVD). We showed that the PbO/TiO2 composite films do indeed show enhanced visible light photocatalysis.

Experimental General Procedure Pristine TiO2 films were deposited from [Ti(OiPr)4] (0.5 g, 1.8 mmol) dissolved in ethyl acetate (25 ml). The resulting solution was stirred for 30 minutes. The precursors were placed in a glass bubbler and an aerosol mist was created using a piezoelectric device. The mist was then carried towards a cold walled CVD reactor loaded with SiO2 pre-coated (ca. 50 nm thick SiO2 barrier layer) standard float glass (NSG) 15 cm × 4 cm × 0.3 cm. A top plate was suspended 0.5 cm above the glass substrate to ensure a laminar flow. The precursor flow was kept at 1.0 l.min-1. The substrate temperature was kept at 450 °C. The deposition time was 30 minutes. After the deposition the bubblers were closed and the substrates were cooled under a flow of nitrogen. Titanium dioxide depositions were carried out under nitrogen (99.99% from BOC). At the end of the deposition the nitrogen flow through the aerosol was diverted and only nitrogen passed over the substrate. The glass substrate was allowed to cool with the graphite block to less than 100 °C before it was removed.

PbO was then deposited on pristine TiO2 polycrystalline films by dissolving [Pb(C2H3O2)2.3H2O] (0.8 g, 2 mmol) in methanol. The solution was stirred 30 minutes and then atomised. The deposition conditions were as above, however the nature of the carrier gas (N2 or air) influenced the nature of the PbO films. PbO layers deposited using air as the carrier gas resulted in phase pure litharge films. PbO films deposited with N2 as the carrier gas resulted in metallic lead that was then annealed in air (post deposition) at the reaction temperature. This route produced PbO films with a mixture of litharge/massicot PbO phases. The phase pure litharge PbO/TiO2 films were used for functional testing.


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Film Analysis Powder X-ray diffraction (PXRD) was used to analyse the samples in a modified Bruker-Axs D8 diffractometer with parallel beam optics equipped with a PSD LynxEye silicon strip detector to collect diffracted X-ray photons. This instrument uses a Cu source for X-ray generation with CuKα1 and CuKα2 radiation of wavelengths 1.54056 Å and 1.54439 Å respectively, emitted with an intensity ratio of 2:1, a voltage of 40 kV and current of 30 mA. The incident beam angle was kept at 1° and the angular range of the patterns collected was 10° < 2θ < 66° with a step size of 0.05° counted at 0.5s/sep.

Scanning Electron Microscopy (SEM) was performed to determine surface morphology and film thickness using a JEOL JSM-6301F Field Emission SEM at an accelerating voltage of 5 keV.

X-ray photoemission spectroscopy (XPS) was performed using a Thermo Scientific K-alpha photoelectron spectrometer using monochromatic Al-Kα radiation. Survey scans were collected in the range 0–1100 eV (binding energy) at a pass energy of 160 eV. Higher resolution scans were recorded for the principal peaks of Ti (2p), Pb (4f), O (1s), C (1s) and Si (2p) at a pass energy of 50 eV. Peak positions were calibrated to carbon and plotted using the CasaXPS software.

UV/Visible/near IR spectra were taken using a Perkin Elmer Fourier transform Lambda 950 UV/Vis spectrometer over a wavelength range of 320 nm to 650 nm in transmission mode. The transmission spectra were taken against an air background.

Functional Testing The UV (365 nm) light activated photocatalytic activity of the films was tested using resazurin based ‘intelligent ink’. Prior to the photocatalysis study, the samples were washed with distilled water, rinsed in isopropanol and irradiated for 30 minutes with UVA (254 nm) light to clean and activate the surface. The photocatalysis test involved spray coating the samples with an even coating (ca. 10 µm) of the resazurin based ‘intelligent ink’ (with modifications to the original recipe used by Mills et al.).


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The intelligent ink consisted of resazurin (40 mg) redox dye in an aqueous solution (40 mL) with glycerol (3 g) and hydroxyl-ethyl cellulose (0.45 g). The mixture was aged for 24 hours in a fridge (2-5 oC) and thoroughly mixed before use. The intelligent ink coating on the films was sufficiently thick to minimise differences in the adsorption properties of resazurin between TiO2 and PbO, thus enabling a valid comparison of both (TiO2/PbO and pristine TiO2) films.

UVA light (flux = 3.76 x 1014 photons cm-2 s-1) was used to induce the photoreduction of the resazurin redox dye on the surface of the films and a UV-visible spectrometer was employed to monitor the degradation of the dye. Formal quantum efficiency (FQE) was calculated by dividing the rate of dye molecules destroyed cm-2 s-1 by the photon flux (flux = 3.76 x 1014 photons cm-2 s-1). The formal quantum yield (FQY) was calculated by dividing the rate of dye molecules destroyed cm-2 s-1 by the number of photons absorbed cm-2 s-1 by the film. The photon flux and photon absorption for each film was determined using a UVX radiometer with a detector for 365 nm radiation attached.

The self-cleaning properties of the thin films were assessed using via the photodestruction of stearic acid. To measure the photo-oxidation of a stearic acid overlayer, duplicate samples were housed in a dark drawer for 72 h prior to being attached to an IR sample holder consisting of an aluminum sheet with a circular hole in the middle. The stearic acid over-layer was applied from a saturated solution of stearic acid in chloroform and dipcoated onto PbO/TiO2 layered film, PbO on glass, Pilkington Activ and blank glass. The samples were then returned to a dark draw for > 72 h prior to the initial reading at 0 h. The reason for this was to have a standard starting point for all the samples. FTIR spectra were obtained between 2800 and 3000 cm−1 using a Perkin Elmer Spectrum RX1 FTIR spectrometer. Measurements were taken at 2 h intervals with the samples irradiated using a filtered white light source with two sheets of Optivex™ film as a light filters. This film has been designed for use as a UV shield to preserve works of art and is deposited on a 3 mm thick piece of Borosilicate glass. The filter was positioned 1 cm above the samples and completely filled the box to the edges. The setup was such that there was no chance of any light coming from anywhere and


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arriving at the samples not having passed through the filter. The filter cuts off all radiation below 400 nm. The concentration of the stearic acid on the surface was assessed using IR absorption spectroscopy.

Stearic

acid

absorbs

at

−1

2958 cm−1 (C–H

Stretch

CH3),

−1

2923 cm (symmetric C–H stretch CH2), and 2853 cm (asymmetric C–H stretch CH2). The peaks were then integrated to give an approximate concentration of stearic acid on the surface. 1 A cm−1 in the integrated area between 2800 and 3000 cm−1corresponds to approximately 9.7 × 1015 molecules cm−2. The rate of decay can then be measured by the decrease in concentration over time. The data is given in terms of the raw IR data plotted to show the decrease in integrated area.

Results and Discussion PbO clusters were deposited on TiO2 films via AACVD. The TiO2 film was deposited on silica glass substrates from the CVD decomposition of [Ti(OiPr)4].28,29 PbO clusters were deposited on top of the polycrystalline pristine TiO2 coated silica glass substrates from the decomposition of [Pb(C2H3O2)2.3H2O]. Individual TiO2 and PbO films were also deposited on silica glass substrates for comparison and to show their respective inactivity at degrading stearic acid under visible light conditions. It should be noted that no PbO2 phase was observed after photocatalytic degradation of stearic acid indicating the photostability of PbO/TiO2 composite film as monitored by XRD. PbO is also harmful if swallowed or ingested.

Figure 1 shows the PXRD patterns of PbO/TiO2 composite film, individual TiO2 and PbO films on glass as well as standard TiO2 (anatase)/PbO (litharge)/PbO (massicot) reflections from simulated patterns. The TiO2 film deposited on glass resulted in polycrystalline phase pure anatase, with a high degree of preferred orientation in the (211) plane at ~ 55° (2θ). PbO/TiO2 composite films resulted in the lead component being present as only litharge PbO with a large degree of texture for the (001) and (002) planes of litharge on TiO2. Low-index surfaces of anatase TiO2 have been studied computationally but the (211) plane of anatase TiO2 coupled to the (001)/(002) planes of PbO could be of importance for enhanced visible light photocatalytic activity and has not previously been observed in the literature. It was also noted that for PbO/TiO2 composite films that reflections resulting from the


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underlying anatase film were reduced in intensity. This is most likely due to the absorbing nature of the PbO layer. The PbO layer can be thought of as a beam stop, absorbing the incoming X-ray beam and the outgoing diffracted beam reducing the intensity of the underlying TiO2 layer. This absorption problem has been seen before with TiO2 modified PbO2 electrodes where the majority of the intensity arises from the PbO2 substrate however showing reflections from both PbO2 and TiO2 as seen in our work also.30 PbO films deposited on glass resulted in a mixed phase of litharge and massicot whereas PbO deposition on anatase had a directing effect forming only the litharge phase of PbO.

Figure 1 X-ray diffraction pattern of PbO/TiO2 composite film and TiO2 film on glass deposited by AACVD. The PbO/TiO2 composite film shows only phase pure litharge PbO and weak reflections from the underlying anatase film were observed. This is due to the absorbing nature of the PbO layer. The PbO layer essentially acts as a beam stop, absorbing the incoming X-ray beam and the outgoing diffracted beam reducing the intensity of the underlying TiO 2 layer.

The microstructure of the films was probed using SEM. Figure 2 shows the SEM image of PbO film deposited on silica-coated glass. It can be seen that the PbO particles are on the order of nm as well as larger micron-sized agglomerates.


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Figure 2 top down SEM image of PbO film on glass.

Figure 3(a) shows the top down SEM image of TiO2 deposited on silica-coated glass displaying dense and compact pyramidal particles in the nm regime. PbO/TiO2 composite films deposited on silica-coated glass are shown in figure 3(b). There appears to be a change in the microstructure of the TiO2 film after PbO deposition. Figure 3(b) clearly shows the underlying TiO2 film as well as PbO particles on top of the TiO2 surface ranging from nm to µm. It can be seen that the PbO particles do not form a dense layer that would be detrimental to the visible photocatalytic activity of this system. In order to confirm this XPS was performed on the TiO2/PbO composite.


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Figure 3 (a) top down SEM image of TiO2 film on glass and (b) top down SEM image of PbO/TiO2 composite film on glass.

X-ray photoemission studies were carried out to determine the chemical environment of titanium and lead states in the composite films as well as comparing to PbO and PbO2 powders. Figure 4 shows the XPS for PbO/TiO2 composite films and PbO/PbO2 powders for reference. Figure 4(a) clearly shows that the binding energy for Pb in PbO/TiO2 composite films corresponds to Pb(II) 4f7/2 and 4f5/2 transitions with a binding energy of 137.6 eV and 142.5 eV respectively. Figure 4(b) shows the binding energy for titanium in PbO/TiO2 composite films corresponding to the Ti(IV) 2p3/2 and 2p1/2 transitions with a BE of 457.8 eV and 463.7 eV respectively. This shows that there is a combination of Pb/Ti states at the surface. Figure 4(c) and (d) also show the binding energies for PbO and PbO2 powders respectively. This clearly shows that there are no Pb(IV) states in the PbO/TiO2 composite films. Due to the metallic nature of PbO2, plasmon peaks are seen in figure 4(d) for the PbO2 standard that are absent from the PbO/TiO2 composite films, as well as core peaks from 4f transitions.31


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Figure 4 XPS spectra of (a) PbO/TiO2 composite film showing the Pb(II) 4f7/2 and 4f5/2 transitions with a BE of 137.6 eV and 142.5 eV respectively, (b) PbO/TiO2 layered film showing the Ti(IV) 2p3/2 and 2p1/2 transitions with a BE of 458 eV and 463.9 eV respectively, (c) PbO powder showing Pb(II) 4f7/2 and 4f5/2 transitions with a BE of 137.7 eV and 142.6 eV respectively and (d) PbO2 powder showing Pb(IV) 4f7/2 and 4f5/2 transitions with a BE of 136.7 eV and 141.5 eV respectively and the associated plasmon (metallic nature of PbO2) peaks at 137.3 eV and 142.2 eV.

Figure 5 shows a schematic for the VB and CB alignment for PbO/TiO2 composite as predicted by Nolan et al.27 PbO particles supported on TiO2 modify the top of the TiO2 valence band such that PbO states form the top of the conduction band and TiO2 form the bottom of the conduction band. This arrangement results in enhanced electron/hole separation upon photoexcitation.


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Figure 5 Energy band schematic for PbO/TiO2 composite film showing the localisation of photogenerated e- and h+. Photogenerated e- are localised on the TiO2 surface as the photogenerated h+ are localised on the PbO particles.

In order to confirm this band alignment, valence band XPS (VB-XPS) was performed on the PbO/TiO2 composite film, PbO film on glass and anatase TiO2 powder as shown in figure 6. The width of the valence band extends from 1 to 10 eV for the PbO/TiO2 composite film with a new spectral feature in the valence band at ~ 1 and 9 eV in comparison to pure TiO2, most likely due to the introduction of Pb states on the anatase surface. The width of the valence band for the anatase TiO2 powder extends from 2 to 9 eV as expected.31 It can clearly be seen from figure 6 that the top of the valence band rises with respect to the Fermi level for the PbO/TiO2 composite film due to the formation of interfacial bonds between the TiO2 and PbO surfaces as predicted by computational work.27 In comparison to anatase TiO2 powder and PbO on glass it can be seen that there is a clear difference between the spectral features of anatase TiO2 powder, PbO film on glass and PbO/TiO2 composite film. It is clear that it is not just a linear superposition of anatase TiO2/PbO spectra. The magnitude of the change in the top of the valence band is about 1 eV for PbO particles deposited on TiO2.


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Figure 6 VB-XPS spectrum of PbO/TiO2 composite film, PbO film on glass and anatase TiO2 powder. Note the shift in the top of the valence band (to higher binding energy) for a pure anatase TiO2 powder and a PbO/TiO2 composite film with a new spectral feature in the valence band at ~ 1 for the PbO/TiO2 composite film due to the introduction of Pb states on the anatase surface.

The absorption spectra for TiO2 thin film on glass, PbO thin film on glass and PbO/TiO2 composite film on glass is shown in figure 7. The absorption spectra for the PbO/TiO2 composite film appear to show a marked band gap narrowing compared to plain TiO2 and PbO films deposited on glass as expected. This compliments the valence band XPS shown in figure 6 highlighting the nature of Pb states above the TiO2 valence band resulting to a red shift in the absorption. Tada et al.22 also observed this type of band gap narrowing for FeOx modified TiO2.


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Figure 7 UV/Vis absorption spectra of TiO2 film on glass, PbO film on glass and PbO/TiO2 composite film on glass.

Functional Testing The photocatalytic activities of the PbO/TiO2 layered film and the pristine TiO2 film (grown under the same conditions) under UV light was studied by spraying a ca. 10 µm coating of resazurin ‘intelligent ink’ that consisted of Resazurin dye, hydroxyethyl cellulose, glycerol and distilled water.2 The photoreduction reaction of resazurin (royal blue) to resorufin (pink) was induced by UVA radiation (flux = 3.76 x 1014 photons cm-2 s-1) and followed using UV-visible absorption spectroscopy. The formal quantum efficiencies (FQE) and formal quantum yields (FQY) were also calculated.

The PbO/TiO2 composite film was able to reduce resazurin to resorufin at a rate of 7.3 x 1011 d. molecules cm-2 s-1, which is higher than the rate of 5.3 x 1011 observed for pristine TiO2. The formal quantum efficiency (FQE), the measure of the number of dye molecules reduced per incident photon and the formal quantum yield, the measure of the number of dye molecules reduced per absorbed photon were calculated using the dye reduction rates and the photon flux/photon absorption.5 The FQE of the


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PbO/TiO2 layered film was 1.9 x 10-3 molecules/incident photon and the FQY was 5.6 x 10-3 molecules/absorbed photon (Figure 8). These were better than the FQE and FQY, 1.39 x 10-3 molecules/incident photon and 1.57 x 10-3 molecules/absorbed photon respectively, determined for the pristine TiO2 sample (Figure 8). Compared to Pilkington NSG ActivTM (a photocatalytic active film that is often used as a standard) the FQE and FQY of the PbO/TiO2 layered film were an order of magnitude better.5 The superior UVA induced photocatalytic activity of the PbO modified TiO2 sample over the pristine TiO2 film and Pilkington NSG ActivTM was most likely due to the localisation of holes on PbO and electrons on the TiO2 as determined by computational studies carried out by M. Nolan et. al.27 This configuration reduces charge recombination that is detrimental to photocatalytic activity.

Figure 8 Graph comparing the formal quantum efficiency and yield of the photodegradation of Resazurin redox dye on PbO/TiO2 layered film, pristine TiO2 and Pilkington NSG ActivTM selfcleaning glass.

The visible light photocatalytic activity of the PbO/TiO2 layered film was also studied using the stearic acid test.32 The test involved the dip coating of a thin layer of stearic acid from a chloroform solution onto the surface of the PbO/TiO2 layered film and the


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monitoring of its visible light induced degradation using IR absorption spectroscopy. The photocatalytic degradation of the stearic acid on the surface was induced by a white light filtered by two OptivexTM sheets to prevent any UV light leakage.

Figure 9 shows the change in intensity of the stearic acid C-H IR stretches with time for the PbO/TiO2 composite film. A degradation rate of 2.28 x 1015 molecules cm-2 h-1 (for 70% destruction) was observed as monitored by the changes in stearic acid due to photocatalytic destruction.33 The rate observed for our films was better than previous literature reports for other visible light active films using the same test set up. The previous best films oxidise stearic acid at a rate of 6.25 x 1013 molecules cm-2 h-1 under OptivexTM filtered white light. This rate is two orders of magnitude lower to that observed for the PbO/TiO2 modified thin film. The PbO/TiO2 film also had a better stearic acid destruction rate than that observed for nitrogen doped TiO2 sol gel films (1.70 x 1014 molecules cm-2 h-1) that was tested under unfiltered white light.19 Other unfiltered white light induced stearic acid degradation tests show rates of 1.4 x 1014 molecules cm-2 h-1 for nitrogen doped TiO2 samples grown via atmospheric pressure (AP) CVD and 1.8 x 1016 for sulfur doped TiO2 films again grown by APCVD.

The visible light activity observed for the PbO/TiO2 layered film is due to the reduction of the TiO2 bandgap arising from the presence of lead states above the TiO2 valence band.27 The high visible light photocatalytic activity is attributed to the same reasons as the high UVA light photoactivity, primarily due to efficient photoinduced charge separation with the holes localising on the PbO and the electrons on the TiO2. These photoinduced charge carriers proceed to oxidise stearic acid through the formation of hydroxy radicals.34 Stearic acid degradation under visible light conditions was also performed on PbO, TiO2 and blank glass that showed no visible light photocatalysis (indicating that the PbO/TiO2 composite film is indeed critical for this enhanced visible light photocatalysis).


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Figure 9 Raw data showing the photo-oxidation of stearic acid molecules on the surface of PbO/TiO2 layered films over 70 h using a white light source typically found in UK hospitals and two sheets of Optivex™ coated glass.

The stearic acid on the surface of the PbO/TiO2 layered film is not completely degraded via visible light irradiation, roughly 30% of the stearic acid still remains on the film (Figure 9).

Figure 10 Raw data showing the intensity of the IR peaks corresponding to stearic acid pre and post washing with chloroform. Washing with chloroform was unable to remove the stearic acid form the PbO/TiO2 film, where as on Pilkington ActivTM, the stearic acid is almost completely removed.


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As shown by the IR spectra in Figure 10, there is little change in stearic acid concentration even after washing the PbO/TiO2 layered film with chloroform. This indicates that there is a strong affinity of stearic acid to adhere on PbO especially in comparison to TiO2.35-37 The strong adhesion prevents moisture adsorbing onto the surface of the PbO and being oxidised to hydroxy radicals that are the main species that take part in the oxidation of organic matter such as stearic acid. The direct oxidation of stearic acid by holes is still possible but this is an inefficient and slow, especially in PbO where the charge carrier mobility of holes is low (Hall mobility of 1.8 cm2 V-1 s-1 under light illumination) compared to TiO2.

Conclusion

This paper shows that a layered PbO/TiO2 thin film grown via AACVD on glass substrates from lead acetate and titanium isopropoxide at 450 oC, with ethyl acetate (for the TiO2 layer) and methanol (for the PbO layer). It also shows that this layered film is an excellent photocatalyst, active under visible light conditions.

The initial visible light induced photocatalytic activity of the PbO/TiO2 layered film compared to previous systems. The PbO/TiO2 film was able to degrade stearic acid at a rate of 2.28 x 1015 molecules cm-2 h-1, orders of magnitude better than what has been seen before. The layered film was also shown to be UVA active as demonstrated by the degradation of resazurin redox dye, the FQY and FQE are better compared to a TiO2 film grown under the same conditions and Pilkington ActivTM, worlds leading self-cleaning glass.


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PbO-Modified TiO2 Thin Films: A Route to Visible Light

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