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Journal of Photochemistry and Photobiology A: Chemistry 281 (2014) 27–34

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

jo ur nal homep age: www.elsev ier .com/ locate / jphotochem

-doped TiO2 visible light photocatalyst films via a sol–gel routesing TMEDA as the nitrogen source

ichael J. Powell, Charles W. Dunnill, Ivan P. Parkin ∗

entre for Materials Chemistry, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom

r t i c l e i n f o

rticle history:eceived 9 December 2013eceived in revised form 25 February 2014ccepted 1 March 2014vailable online 12 March 2014

eywords:MEDA-doped TiO2

isible light photocatalysishemical vapour deposition

a b s t r a c t

N-doped TiO2 thin films were successfully prepared using N,N,N′,N′-tetramethylethane-1,2-diamine(TMEDA) via a single step sol–gel synthesis. Characterisation by X-ray diffraction and Raman spec-troscopy confirmed that the anatase crystal type of TiO2 was present in all samples. UV/Vis spectroscopywas used to determine the absorbance and band-gap of the materials. Stearic acid and resazurin dyePhoto-degradation measurements were used to show the enhanced photocatalytic properties of thematerials. Both of these methods are well known for the assessment of photocatalytic films for selfcleaning applications with stearic acid being the preferred model for bacteria. The N-doped films showedsuperhydrophilicity under filtered white light conditions, whereas the undoped films did not show anyphoto-induced superhydrophilicity. The N-doped TiO2 films show interstitial nitrogen content ratherthan substitutional and a substantial red-shift of the band-gap. They were shown to be superior visi-ble light photocatalysts when compared to similar non-doped films. The nature of the chelating ligand,

TMEDA, has a profound effect on the oxidation state of the Ti centres, with low concentrations of TMEDApromoting the formation of Ti3+ centres giving a superior photocatalyst. This method is to the best of ourknowledge the easiest, reliable and most reproducible one step route to achieve N-doped TiO2 films forphotocatalytic applications. These N-doped TiO2 films could have potential applications; as self-cleaningsurfaces in healthcare, water sterilisation, and solar energy harvesting.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

TiO2 is a semi-conductor with a wide variety of applicationsncluding self-cleaning glass [1,2], anti-microbial surfaces [3,4],nti-fogging surfaces [1,5] and water purification treatments [6].heel et al. [7–9] have extensively researched the anti-microbialroperties of TiO2 surfaces and nanoparticles, prepared usinghemical vapour deposition (CVD) processes. Binions et al. havelso reported the production of TiO2 films via CVD processes [10],enerating self-cleaning surfaces with low-emissivity by the incor-oration of both TiO2 and SnO2. TiO2 is biologically and chemically

nert, mechanically robust, cheap and abundant and therefore perfect candidate for wide scale application. Undoped TiO2 isffective for these purposes only using the UV region of the elec-romagnetic spectrum, due to the band gap of 3.0 and 3.2 eV for

utile and anatase respectively [11]. In order for TiO2 to be a visi-le light photocatalyst, it is necessary to reduce the band gap. Thisas been achieved by anion doping for instance using nitrogen

∗ Corresponding author. Tel.: +44 (0) 20 7679 4637; fax: +44 (0) 2076797463.E-mail address: i.p.parkin@ucl.ac.uk (I.P. Parkin).

ttp://dx.doi.org/10.1016/j.jphotochem.2014.03.003010-6030/© 2014 Elsevier B.V. All rights reserved.

[10,12–14], fluorine [15,16], carbon [17,18] and sulphur [19–21].TiO2 has been successfully N-doped by amines [12,22,23], pre-treatment with ammonia [24–26] and post-treatment by ammonia[27]. Nitrogen can be incorporated into the crystal lattice either byinterstitial or substitutional incorporation [28,29]. When incorpo-rated interstitially the nitrogen does not directly replace oxygenwithin the framework, instead the formation of Ti O N occurs,with the nitrogen being incorporated into interstitial sites withinthe crystal lattice, as the O2− is combined with N to form (ON)3−

[28]. This promotes the formation of oxygen vacancies within thelattice. When nitrogen is incorporated substitutionally, nitrogendirectly replaces oxygen in the lattice. For every 2 nitrogen sitesoccupied there must be 1 oxygen vacancy in the lattice due tocharge balancing. Hence nitrogen incorporation promotes oxygenloss [28]. This has been reported by Serpone et al. who suggest thatthe incorporation of nitrogen and other anions into the TiO2 lat-tice increases the oxygen vacancies [30,31]. These oxygen vacanciesthen act as the photocatalytic centres. Yates et al. have shown pre-

viously that the concentration of nitrogen incorporation into thelattice is important, as at high concentrations of nitrogen photo-catalysis is adversely affected [25]. Low concentrations of nitrogenfavour interstitial formation whereas at higher concentrations

28 M.J. Powell et al. / Journal of Photochemistry and P

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ig. 1. Structure of TMEDA showing lone pairs, allowing for bidentate properties ofigand.

ubstitutional formation is favoured. X-ray Photoelectron Spec-roscopy (XPS) is used to determine whether the nitrogen is innterstitial or substitutional positions, with interstitial nitrogenaving a binding energy of 399.9 eV12 and substitutional nitro-en having a binding energy of 396.9 eV [29]. Dunnill et al. haveynthesised TiO2 films containing only interstitial nitrogen andave shown that this leads to improved visible light photocatalysis12,32,33]. Along with photocatalysis, TiO2 shows photoinduceduperhydrophilicity [34–36]. Superhydrophilic surfaces are impor-ant in self-cleaning applications as the primary mechanism foremoving dirt and microbes from a surface is the action of water37]. When water comes in contact with a superhydrophilic surfacet forms a thin layer that sheets easily across the surface, this layer

ashes away any organic matter that has collected. It is importanthat when a photocatalyst, such as TiO2, is doped to reduce the bandap energy, the photo-induced superhydrophilicity is not adverselyffected.

In this paper we describe a reliable and effective methodor incorporating nitrogen via a bidentate chelating ligand thatchieves a significant red-shift of the band-gap towards visibleight. This method allows a lower concentration of nitrogen sourceo be used in comparison to other amine and ammonical sources,ielding a less wasteful process to produce highly effective N-dopediO2 photocatalysts. Dunnill et al. have shown that to produce aood visible light photocatalyst a gentle nitriding source is requiredo achieve interstitial nitrogen with no substitutional [12,28,38].he use of TMEDA provides just such a mild set of reaction condi-ions, as is discussed. This method of incorporating nitrogen into

titania lattice allowing the formation of Ti3+ centres, reasons forhis observation will be discussed.

. Experimental

.1. Sample preparation

A sol–gel method was used to prepare pure TiO2 samples.Acetylacetone (0.025 mol: 2.52 g) was added to butan-1-ol

32 cm3), followed by titanium n-butoxide (0.05 mol: 17.50 g) andeft to stir vigorously for 1 h at room temperature and pressure. Iso-ropanol (0.15 mol: 9.05 g) dissolved in distilled water (3.64 cm3)as added and the solution left to stir for 1 h at room temperature

nd pressure. Finally, acetonitrile (0.04 mol: 1.66 g) was added andhe solution left to stir for 1 h at room temperature and pressure.he solution was then sealed and left to age overnight, at room tem-erature and pressure. This sample will subsequently be referredo as TiO2.

A second sol–gel was created using the method outlined above.MEDA (0.0129 mol: 1.5 g) was added to this sol, Fig. 1, the solutionas then left to stir for 1 h at room temperature and pressure, before

eing sealed and left to age overnight at room temperature andressure. This sample will be subsequently referred to as N1-TiO2.

A final sol–gel was synthesised using the method outlinedbove. To this sol was added TMEDA (0.0258 mol: 3.0 g), the solu-ion was left to stir for an hour at room temperature and pressure

efore being sealed and left to age overnight at room tempera-ure and pressure. This sample will be subsequently referred to as2-TiO2.

hotobiology A: Chemistry 281 (2014) 27–34

Microscope slides (2 cm × 8 cm) were dip-coated into the syn-thesised TiO2 sols, in a 50 cm3 beaker, using a dip-coating machine.The microscope slides were withdrawn with a retraction rate of30 cm min−1. The slides were left to dry at room temperature for10 min, then were taken to a furnace and heated in air to 500 ◦C for18 h (Ramp rate: 10 ◦C min−1). The slides were left to cool naturallyto room temperature inside the furnace.

2.2. Thin film characterisation

X-ray diffraction was performed using a Bruker-Axs D8 (GADDS)diffractometer, utilising a large 2D area detector and a Cu X-raysource, monochromated (K�1 and K�2) fitted with a Gorbel mir-ror. The instrumental setup allowed 34◦ in both � and ω with a0.01◦ resolution and 3–4 mm2 of sample surface illuminated at anyone time. Multiple Debye–Scherrer cones were recorded simulta-neously by the area detector with two sections covering the 65◦

2� range. The Debye–Scherrer cones, once collected were inte-grated along ω to produce standard diffraction patterns of degrees2� against intensity. Scan data was collected for 900 s periods togive sufficiently resolved peaks for indexing. The morphology of theTiO2 films were analysed using a Carl Zeiss XB1540 “Cross Beam”focussed-ion beam scanning electron microscope operated at 5 kV.Samples were prepared for scanning electron microscopy (SEM) bybeing coated in carbon. The Raman spectra were conducted usinga Renishaw InVia Raman microscope between 100 and 1500 cm−1

Raman shift. The UV/Vis spectra transmission measurements wereachieved using a Perkin Elmer �950. XPS was performed using aThermo K� spectrometer with monochromated Al K� radiation,a dual beam charge compensation system and a constant passenergy of 50 eV. Film thickness measurements were determinedusing Filmetrics F20 equipment and secondly by taking reflectancedata from UV/Vis spectroscopy. Both of these methods utilise aSwanepoel method [39] to determine thickness from optical char-acterisations.

2.3. Functional testing

The functional properties of the thin films were assessed usingthe analysis of water contact angles, the conversion of the redoxdye resazurin and the photo-destruction of stearic acid.

Contact angle measurements were made using a FTA1000 sys-tem. Here a 6.8 �l drop or deionised water is dispensed by gravityfrom a gauge 30 needle and photographed side on when in situon the surface of the sample thin films. The contact angle of thewater droplet and the surface is measured by the software fromthis image.

Intelligent ink, formulated as prepared by Mills et al. [40] wascoated onto the samples using an aerosol-spray gun (SIP EmeraldSpray Gun/Halfords Plc.) The ink comprised of 0.45 g of hydroxylethyl cellulose (HEC) polymer, 40 cm3 of distilled water, 3.0 g ofglycerol and 40 mg of the redox dye, resazurin (Rz). The rate ofconversion was measured by imaging the samples using a scannerattached to a computer. This allowed the conversion to be mon-itored as the dye changed colour from blue to red to white as itis reduced. This was a largely qualitative method of assessing thephotocatalytic properties.

For the stearic acid measurements FTIR spectra were obtainedbetween 2800 and 3000 cm−1 using a Perkin Elmer Spectrum RX1FTIR spectrometer. Measurements were taken at 24 h intervalswith the samples irradiated white light. Samples were positioned20 cm below a 2D General electric 28 W Biax compact fluorescent

surements showed that the light intensity at the position of thesamples was recorded at 9500 lx, and is comparable to the lightintensity of a bright pathology lab, and is close to the minimum

M.J. Powell et al. / Journal of Photochemistry and Photobiology A: Chemistry 281 (2014) 27–34 29

and (b) sample N1-TiO2 and (c) sample N2-TiO2.

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Fig. 3. XRD diffraction pattern for samples TiO2 and N1-TiO2 and N2-TiO2 andexpected peak positions for anatase and rutile reflections of TiO2.

Fig. 2. SEM micrographs of: (a) sample TiO2

equirement for an operating theatre, i.e. <10 K Lux. Stearic acidontains many carbon–hydrogen bonds and therefore absorbs at958 cm−1 (C H Stretch CH3), 2923 cm−1 (symmetric C H stretchH2), and 2853 cm−1 (asymmetric C H stretch CH2). The peaks arehen integrated to give an approximate concentration of steariccid on the surface. A 1 cm−2 in the integrated area between800 and 3000 cm−1 corresponds to approximately 9.7 × 1015

olecules cm−2 [41]. The rate of decay can then be monitored ashe decrease in concentration over time.

. Results and discussion

Thin films of undoped TiO2 were synthesised by a sol–gel routehrough dip-coating into a sol that contained Ti(OBun)4. These filmsere then annealed at 500 ◦C for 18 h, this sample will be sub-

equently referred to as TiO2. N-doped samples were preparedy the addition of TMEDA to the sol–gel solution. Two concen-rations of TMEDA sol–gel were prepared, to one sol 0.0129 mol1.5 g) was added, with the thin films being prepared by dip-coatingnd annealing at 500 ◦C for 18 h. This sample will be subsequentlyeferred to as N1-TiO2. To the final sol 0.0258 mol (3.0 g) of TMEDAas added, with the thin films being prepared by dip-coating and

nnealing at 500 ◦C for 18 h. This sample will be subsequentlyeferred to as N2-TiO2.

Fig. 2 shows the typical SEM micrographs for samples TiO2, N1-iO2 and N2-TiO2. All samples were synthesised by dip-coatingnd annealing. The surface morphology of sample TiO2 is uniformith only small pores visible in the structure. Sample N1-TiO2 and2-TiO2 show similar surface morphologies to sample TiO2. This

uggests that the incorporation of TMEDA into the TiO2 sol doesot affect the morphology of the film produced. This is important ashe X-ray diffraction (XRD) indicated that all samples TiO2, N1-TiO2nd N2-TiO2 consisted of the anatase phase, therefore differencesn photoactivity can be attributed to dopants incorporated into thelms.

Fig. 3 shows that the addition of TMEDA does not affect theredominant crystal phase present. This XRD pattern, showedeflections consistent in position with those predicted to thenatase form of TiO2 and gave trace amounts of the rutile phaseeing present. The rutile phase reflections are more apparent in

he undoped sample, TiO2. Samples N1-TiO2 and N2-TiO2 showess intense reflections for the rutile phase. As all samples werennealed for the same length of time, this suggests that the addi-ion of TMEDA favours the formation of the anatase phase. The

Fig. 4. Raman spectra for samples TiO2, N1-TiO2 and N2-TiO2 and expected positionsfor anatase and rutile bands of TiO2.

broad feature apparent between 15 and 35◦ 2� is due to amorphousreflections from the glass substrate that the films were adhered to.

Raman spectra, Fig. 4, show that the inclusion of TMEDA intothe structure does not affect the phase of TiO2 present. Only theanatase phase is observed in the samples as shown by the bands at400, 525 and 634 cm−1 [42]. There are no bands present that canbe attributed to the rutile phase.

The transmission of the films were measured using UV/Vis spec-troscopy, Fig. 5, shows a marked difference in the optical propertiesof the films, with samples N1-TiO2 and N2-TiO2 being red-shifted

in absorbance and having slightly higher absorbance as comparedto sample TiO2. From the transmission data it is possible to drawa Tauc plot [43] and use this information to estimate the band-gappresent for the film.

30 M.J. Powell et al. / Journal of Photochemistry and P

Fig. 5. UV/Vis spectra for samples TiO2, N1-TiO2 and N2-TiO2.

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Fig. 6. Tauc plots for samples TiO2, N1-TiO2 and N2-TiO2.

Tauc plots, Fig. 6, show the band gap for samples TiO2, N1-TiO2nd N2-TiO2. The band-gap energy is reduced by ∼0.1 eV betweeniO2 and N1-TiO2, which is a large shift towards the visible lightegion. Sample N2-TiO2 shows an even larger red-shift with theand gap appearing at ∼3.10 eV. This is attributed to the intro-uction of nitrogen into the crystal structure. The colour of thelms could be described in the form of LAB* coordinates, where* refers to the lightness of the colour/shade, a* and b* togetherive a description of what the colour/shade is. TiO2 was at coordi-ates L* = 98.6, a* = 0.12, b* = 0.23 N1-TiO2 was L* = 86.8, a* = −3.12,* = −8.74 and N2-TiO2 was L* = 89.2, a* = −12.0, b* = 5.83. Theseesults suggest that the undoped form of titania, sample TiO2resents as being colourless, which is the expected result forndoped TiO2. A shift towards the blue colour for sample N1-TiO2,hich is the expected result if Ti3+ centres are being formed within

he crystal structure [44]. Sample N2-TiO2 is shifted towards theellow region of the spectrum, this is the expected result for nitro-en incorporation into the crystal lattice. This result is also anndication of high energy visible light absorption, in the blue region,uggesting improved/enhanced visible light photocatalytic activity.

Film thickness measurements were performed on all samples.iO2, N1-TiO2 and N2-TiO2 had the same retraction rate for for-ation of the thin film (30 cm min−1), it would be assumed that

he film thicknesses would be the same. Sample TiO2 had a filmhickness measure of ∼120 nm, whereas samples N1-TiO2 and N2-iO2 had a film thickness of ∼180 nm. A Swanepoel analysis waslso performed using reflectance data from UV/Vis spectroscopy.he Swanepoel analysis gave a film thickness for sample TiO2 of23.7 nm, standard deviation of 8.570. The Swanepoel analysis gave

film thickness for sample N1-TiO2 of 178.2 nm, with a standardeviation of 5.777. As shown, the information from the film thick-ess as calculated by the two methods matches well. The additionalhickness of sample N1-TiO2 can only be attributed to the chelatingffect of the TMEDA in the sol forming a more viscous sol–gel and a

hicker deposition layer. This is an advantage as it has been shownhat thicker films lead to greater photocatalytic activity [45]. Pre-iously in order to generate thick films from a sol–gel has requiredultiple coats being applied, resulting in films that are highly

hotobiology A: Chemistry 281 (2014) 27–34

susceptible to fracturing. The TMEDA sol would enable fewer layersto be applied to form a similar thickness, and hence less suscepti-bility to fracturing. Having fewer coatings also reduces the cost andtime involved in producing films as each subsequent layer needs tobe annealed independently [46].

XPS data showed a peak at 400 eV, Fig. 7, this is attributed tothe N1s peak. A peak in the XPS of 400.06 eV has been assignedto nitrogen in the interstitial position [12,32,38]. There is no peakapparent at 396.9 eV which is where nitrogen in the substitutionalposition in the lattice would be expected to appear [29]. It hasbeen shown that the incorporation of interstitial nitrogen in to thelattice has beneficial photocatalytic properties at low dopant lev-els [12,32,47]. As there is no peak at 396.9 eV this reinforces theargument that interstitial nitrogen doping is responsible for theimproved photocatalysis of N-doped TiO2. This supports the resultsseen by other research groups for the nature of the incorporatednitrogen into the crystal lattice [12,32,48,49]. XPS data also showedthe presence of reduced Ti3+ existing within the lattice. Althoughthis reduction of Ti4+ to Ti3+ will deactivate some of the photo-catalysis, the functional tests attribute that there are still sufficientphoto-generated electron/hole pairs to enable the photocatalyticoxidation/reduction of surface bound particles. Studies have con-cluded that the presence of nitrogen in the lattice promotes theformation of the paramagnetic Ti3+ species [50–52].

XPS data also showed a peak at 400 eV, Fig. 7, showing that sam-ple N2-TiO2 also shows the incorporation of interstitial nitrogeninto the lattice. Once again there is no peak apparent at 396.9 eVsuggesting that there is no substitutional nitrogen present in thecrystal lattice. For both N-doped samples, the incorporation ofnitrogen was very low, between 0.5 and 2.5% is the maximum incor-poration, this accounts for the relatively high signal to noise ratioin the XPS spectra below.

Depth profiling data was also collected on samples N1-TiO2 andN2-TiO2, Fig. 8 shows the data for N1-TiO2. It can be observedthat the surface of the sample shows high carbon content, but thisrapidly falls away as the sample is etched. This carbon content likelycomes from the TMEDA precursor. The titanium and oxygen con-tent increase as the film is etched, with the final ratio being 1:2Ti:O – the higher concentration of oxygen at the surface can beattributed to surface bound O/OH groups. The nitrogen concentra-tion is 2.27 at.% at the surface, this reduces to 1.87 at.% nitrogen after5 etches. This suggests that the nitrogen content is not just withinthe surface layers of the films, but spread throughout the entirelattice. This is expected from a sol–gel containing a nitrogen pre-cursor. The reduction in nitrogen content suggests that during theannealing process the nitrogen is migrating towards the surfaceof the film. This has been previously observed in other N-dopedTiO2 systems [53]. Sample N2-TiO2 showed a similar result to thatdisplayed for sample N1-TiO2, however the nitrogen content waslower for sample N2-TiO2 compared with N1-TiO2. This is a sur-prising result as sample N2-TiO2 contained a higher concentrationof TMEDA. This could be attributed to the ability to form N2, dur-ing the annealing process there are 2 competing reactions (a) theformation of interstitial nitrogen and oxygen vacancies within thelattice and (b) the formation of N2. The formation of N2 is thermo-dynamically more favourable due to the energy released during theformation of the N triple bond. The limiting factor in this case willbe the concentration of nitrogen, the lower the concentration thelower the rate of N2 formation.

Functional testing showed a marked difference between the 2sets of samples, Contact angle measurements showed a change incontact angle from 24◦ to 22◦ upon irradiation in filtered white

light for 1 h by sample TiO2 however a change of 20 to 2 was seenfor samples N1-TiO2 and N2-TiO2. These results strongly suggestthat under the conditions of visible light for 1 h the sample TiO2 isnot active while samples N1-TiO2 and N2-TiO2 show photoinduced

M.J. Powell et al. / Journal of Photochemistry and Photobiology A: Chemistry 281 (2014) 27–34 31

Fig. 7. XPS peak for the nitrogen 1S indicating the presence of inte

Fig. 8. Atomic % data for sample N1-TiO2 showing the changing concentration ofTi, O, C and N throughout the sample. The oxygen content is shown in blue, thetitanium content is shown in cyan, carbon is shown in red and nitrogen is shown ingreen. (For interpretation of the references to color in this figure legend, the readeri

Fl

s referred to the web version of the article.)

ig. 9. Photographic images of the resazurin redox dye on the surfaces of (a) blank glass,amp generating 9500 lx.

rstitial nitrogen in the lattice in (a) N1-TiO2 and (b) N2-TiO2.

superhydrophilicity with the surface becoming superhydrophilic(� < 10◦).

Resazurin testing, Fig. 9, shows the difference in photocat-alytic properties between sample TiO2 and N1-TiO2, where sampleN1-TiO2 shows a marked improvement over sample TiO2. The tran-sition from blue to pink occurs faster in Fig. 9c than Fig. 9b. Theblank sample (a) shows how the resazurin dye is stable under thelighting conditions and therefore the change in colour is resultantfrom the surface properties of the sample. By the end of the 1sthour there is already considerable conversion of the resazurin onsample N1-TiO2, whereas sample TiO2 shows only a small conver-sion from blue to pink. By the end of the 2nd hour, sample N1-TiO2has converted a substantial amount of the resazurin, with sampleTiO2 also having converted roughly half of the resazurin layer. Inthe latter part of the experiment 4 to 5 h sample TiO2 catches up sothat by the end, both TiO2 samples have comparable effects. This is

due in part to the lack of material for the photocatalysts to degrade.

The stearic acid photodegradation, Fig. 10, was performed underwhite light conditions with the destruction of the stearic acidbeing monitored by FTIR measurements. This gave a much more

(b) TiO2 and (c) N1-TiO2 under white light conditions using a compact fluorescent

32 M.J. Powell et al. / Journal of Photochemistry and Photobiology A: Chemistry 281 (2014) 27–34

Fig. 10. Stearic acid destruction data (a) blank

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Fig. 11. Normalised integrated area for the destruction of stearic acid.

uantitative assessment of the ability of the photocatalysts. SampleiO2 is shown to be an ineffective visible light photocatalyst, withhe rate of stearic acid destruction being low. This is as expectedrom the Tauc plot data where the band-gap energy was estimatedo be 3.17 eV, this energy being in the UV region. Sample N1-TiO2hows a much higher rate of stearic acid destruction over the 188 hhat the measurements were taken. This shows that an alterationf the band-gap energy by ∼0.10 eV has a large effect on the abil-ty to perform photocatalysis under filtered white light conditions.he blank sample showed no destruction over the course of thexperiment. Sample N2-TiO2 shows a similar rate of destruction to1-TiO2, this is a surprising result as with double the concentrationf the TMEDA ligands it would be assumed that there would be areater incorporation of interstitial nitrogen and so a faster rate ofestruction under the conditions tested.

The true potential of the photocatalytic films is shown by theormalised photodestruction, Fig. 11, here the rates of destruc-ion have been normalised against the stearic acid concentration, ashe data was collected by IR studies the initial intensities could beompared. The initial intensities were shown to be similar, whichllowed for comparison, this means that the normalised data hasame validity and so can be compared to measure the effectivenessf the different photocatalytic materials. Sample N2-TiO2 shows theighest degradation of stearic acid across the experiment. Sample

1-TiO2 shows a comparable rate of destruction across the test.

t is interesting to note that although nominally sample N2-TiO2ontained twice the nitrogen content, it does not show twice theate of destruction, this supports the XPS data that suggested there

glass (b) TiO2 (c) N1-TiO2 (d) N2-TiO2.

was not a large difference in nitrogen concentration between sam-ples N1-TiO2 and N2-TiO2. The destruction of stearic acid on theundoped TiO2 surface is attributed to a tiny fraction of UV leakagefrom the lamps employed.

Samples N1-TiO2 and N2-TiO2 show a clear increase inactivity over sample TiO2. Sample TiO2 shows a rate ofdestruction of 4.58 × 1013 molecules destroyed cm−2 h−1, whereassample N1-TiO2 shows a rate of destruction of 1.67 × 1014

molecules destroyed cm−2 h−1. Sample N2-TiO2 shows a rate ofdestruction of 1.31 × 1014 molecules destroyed cm−2 h−1. SampleN1-TiO2 showed a clear improvement of 1.212 × 1014 moleculesdestroyed cm−2 h−1, or an enhancement factor of 3.65 over sam-ple TiO2. Sample N2-TiO2 showed an improvement of 8.52 × 1013

molecules destroyed cm−2 h−1, or an enhancement factor of 2.86over sample TiO2. This suggests that sample N1-TiO2 is the mosteffective photocatalyst under the conditions used; this confirms thedata from XPS studies which suggested that sample N1-TiO2 had ahigher concentration of interstitial nitrogen in the crystal lattice.

The stearic acid and resazurin dye test are good indicators for theability of a surface to act as a self-sterilising surface. The stearic acidin particular is a good model for grease, dirt and bacteria. It is there-fore a reasonable conclusion that these N-doped surfaces wouldoperate well in hospital environments as self sterilising surfaces orindeed water cleaning applications.

In comparison to other reported results, the N-doping of TiO2 viaTMEDA performs well. Dunnill et al. reported on the incorporationof nitrogen via methyl ammonium chloride [47]. The conditionsused for the stearic acid destruction tests were similar to the con-ditions used in this paper, with the intensity of the light being6000 lx. The best performing N-doped TiO2 photocatalyst had a rateof destruction of 1.01 × 1014.

Assuming that a white light source gives a rating of5 × 10−3 W/m2, a conversion of the rates from the two studiescan be achieved. With an intensity of 6000 lx, multiplying by theW/m2 gives a rating of 30 lx per W/m2. The same calculation with9500 lx, the intensity of the white light source for this study, givesa factor of 47.5 lx per W/m2. This allows the rates from the Dun-nill et al. study to be compared. This gives a rate of 1.599 × 1014

molecules destroyed cm−2 h−1 if the study had been repeated under

the 9500 lx. This is slightly lower than the highest rate calculatedfor sample N1-TiO2. This gives evidence that the films generated inthis study are of good quality and will perform at least as well asthose generated via chemical vapour deposition techniques.

M.J. Powell et al. / Journal of Photochemistry and P

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Fig. 12. TMEDA associated with Ti centre.

A further study by Mills et al. reported on the destruction oftearic acid under UV conditions (� = 254 nm) [35]. Under the con-itions tested, the rates of reaction were 6.91 × 1014 moleculesestroyed cm−2 min−1. This is a much higher rate than achievedor the un-doped TiO2 tested here, but unfortunately direct com-arisons cannot be made due to the use of a UV lamp during theseests, which ensured that the energy of the photons present wasignificantly above the band-gap energy of the TiO2.

Nolan et al. have reported on the use of 1,3 diaminopropaneor N-doping TiO2 [23]. The study also found that the incorpora-ion of a chelating ligand led to the interstitial doping of nitrogennto the crystal lattice, as confirmed by XPS studies. This studylso determined that the doping of nitrogen into the crystal lat-ice did not affect the formation of the anatase phase at 500 ◦C,his is in agreement with our results. At 600 ◦C it was found thathe rutile phase was favoured. Un-doped TiO2 still predominantlyhowed the anatase phase at 600 ◦C. This study also showed that thenclusion of a chelating ligand increased the porosity of the sam-les, which is contrary to the results presented here. An increase inorosity would seem unlikely from the method presented in thisaper, as the Ti source is titanium n-butoxide which will form car-on dioxide and water from the decomposition of the butoxide

igands during annealing. TMEDA does not contain more carbonnd hydrogen than butoxide, so it should not increase any pressureuild-up within the film during the annealing process. This pres-ure build-up would likely result in larger pore sizes, this could benfavourable for photocatalytic purposes as the surface area of thelm would be lowered.

An advantage of the method we report here is the ability to use lower concentration of nitrogen atoms in the sol–gel to form aisible light photocatalyst. Diker et al. reported on the use of amineources for the formation of N-doped TiO2 [22], this required themine to be in a 1: 1 ratio with the titania precursor. Oropezat al. reported on the use of ammonia treatment for N-doping TiO2,otentially this required an ammonia to titania precursor ratio of.8:1 [54]. Nolan et al. had a ratio of titania to nitrogen source of:1 [23]. The method outlined in this paper requires an amine toitania precursor ratio of 1:3.88, this means that there is less waste,educing costs of production as well as any potential harmful by-roducts from production. The reason why the TMEDA requires aignificantly lower concentration is believed to be due to the che-ating effect of the ligand, Fig. 12. In solution ligands will displacene another, as the TMEDA is a bidentate ligand it will be moreifficult to displace than a monodentate ligand such as ammonia.herefore there will be a greater proportion of nitrogen in the cor-ect position during the annealing process to allow the formationf an N-doped TiO2 photocatalyst. As annealing is done in air, anyxygen that is lost when the TMEDA displaces the butoxide ligand

an be replaced. This means that the formation of substitutionalitrogen in the crystal is inhibited and so the nitrogen forms in

nterstitial positions within the lattice.

hotobiology A: Chemistry 281 (2014) 27–34 33

Due to the nature of TMEDA, the sol–gel has a tendency tochelate into a jelly-like polymer over the course of a few weeks.This does not appear to affect the results obtained from dip-coatinginto the sol–gel, as when the sol–gel is vigorously stirred with somebutan-1-ol it reforms a less viscous sol that is free-flowing. As theTMEDA concentration remains constant, the interaction betweenthe titanium centres and the nitrogen atoms is maintained. Whensamples are subsequently annealed, the nitrogen is in the closeproximity with the titanium centre and so can form bonds with thetitanium. Once the sol–gel has polymerised and then been stirred,it did not re-polymerise, suggesting that within the sol–gel thereare colloidal particles and any additional bonding between colloidsdoes not readily reoccur.

4. Conclusion

TiO2 can be successfully N-doped via a one-step sol–gel synthe-sis using TMEDA as the nitrogen source. To our knowledge TMEDAprovides the best method for doping of TiO2 with nitrogen currentlyreported in the literature. The bidentate ligand, TMEDA, utilises thechelate effect to form stronger bonds to the Ti centre than wouldbe expected for a monodentate N ligand, leading to the availabil-ity of lower concentrations of the N source and more mild reactionconditions. The inclusion of nitrogen into the lattice occurs by inter-stitial doping as confirmed by XPS analysis. It has been shown thatthe inclusion of nitrogen by interstitial, rather than substitutionaldoping better facilitates the photocatalytic process using stearicacid and methylene blue as models for bacteria and recalcitrantwaste. Good activity with the stearic acid tends to suggest that thephotocatalyst will display good photobiology with bacteriologicalstudies [12,28]. The N-doped TiO2 films do not show significantdifference in morphology from the undoped samples. The N-dopedfilms show visible light photocatalysis and photo-induced superhy-drophilicity, this has been measured by stearic acid destruction andcontact angle measurements. These properties have been relatedto the introduction of nitrogen into the crystal lattice interstitially,which supports the results found by other research groups in thisfield. The N-doped films outperform the un-doped films by a marginof between 2.86 and 3.65, showing a significant increase. The N-doped samples have been shown to have a similar activity to otherreported N-doped systems. This enhancement is believed to be dueto the incorporation of interstitial nitrogen into the crystal lattice,promoting oxygen vacancies and decreasing the band-gap energy,this is in agreement with other research groups. The introductionof TMEDA allows a simple sol–gel synthesis route to enhanced visi-ble light photocatalysts whilst also using less of the nitrogen sourcethan other equivalent sol–gel processes. The chelating nature of theTMEDA ligand assures that the nitrogen is associated with the tita-nium centre during annealing, allowing for the formation of filmscontaining ∼2–3 at.% nitrogen at the surface, with lower concentra-tions within the film due to nitrogen migration during the annealingprocess.

Acknowledgement

C.W.D. would like to thank the Ramsay Memorial Trust for aRamsay Fellowship.

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