Chemical Physics Letters 477 (2009) 340–344

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Chemical Physics Letters

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Interaction of molecular oxygen with oxygen vacancies on reduced TiO2:Site specific blocking by probe molecules

Jason Green, Emma Carter, Damien M. Murphy *

School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK

a r t i c l e i n f o

Article history:Received 22 May 2009In final form 1 July 2009Available online 3 July 2009

0009-2614/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cplett.2009.07.002

* Corresponding author. Fax: +44 (0)2920 874030.E-mail address: MurphyDM@cardiff.ac.uk (D.M. M

a b s t r a c t

A heterogeneity of surface sites for superoxide ðO�2 Þ stabilization on thermally reduced TiO2 (P25) havebeen identified by cw EPR. Two main groups of adsorption sites exist; site I due to O�2 adsorbed at oxygenvacancies, ½Vac � � �O�2 �, and sites II–III due to O�2 adsorbed at five co-ordinate Ti4+ centres. Electron transferfrom the precursor Ti3+ centres to O2 (forming O�2 ) can be prevented at each site through selective siteblocking with probe molecules (CO2 and Ar). CO2 preferentially adsorbs at the five co-ordinateTi3þ

non-vacancy centres, while Ar preferentially adsorbs at the Ti3+ centres associated with the oxygen

vacancies ðTi3þvacancyÞ.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Since the pioneering work of Fujishima and Honda on thephotoassisted splitting of water over TiO2 [1] an explosion of inter-est and development in the applications of titania, ranging fromsterilization [2] and solar energy conversion [3] to pollutioncontrol [4] and fuel cells [5], has occurred. In most of these appli-cations a host of possible photochemical, chemical and electro-chemical reactions occur at the surface, the rates of which areoften dependent on a host of other factors that are still poorlyunderstood [6,7].

One of the more active fields of titania research is photocataly-sis. In this application light induced charge separated pairs (elec-trons and holes) are used to drive surface reduction andoxidation reactions as efficiently as possible. The emphasis in re-cent years has therefore focussed on visible light induced photoca-talysis by modification (such as doping with metal and non-metalimpurities, coupling with narrow band-gap semiconductors, andpreparing oxygen deficient materials) or sensitisation (such asanchoring dyes of oxides on the surface) of TiO2 [8].

Regardless of the approach adopted to improve photocatalysis,the activity will depend not only on the charge carrier formation,lifetimes and separation, but also on their utilisation at the inter-face, since certain surface planes are known to be more active thanothers [9,10]. Fundamental to this utilisation step is the electrontransfer (ET) event occurring at the interface, and this too is depen-dent on surface morphology. Quite often the excess electrons gen-erated in the TiO2 semiconductor during photocatalysis can beeasily delocalised through the conduction band, but they can also

ll rights reserved.

urphy).

become readily localised at bulk and surface Ti cations as Ti3+ cen-tres. These Ti3þ

surf centres in particular act as important electron con-duits in photocatalysis, and the efficiency of this ET step may beintimately linked to the co-ordination and location of the Ti3+.For thermally reduced TiO2, an abundance of Ti3þ

surf centres are eas-ily generated in tandem with surface oxygen vacancies. These re-duced surfaces become ideal model systems to investigate therole of localised electron states and oxygen vacancies in the forma-tion and stabilisation of reactive oxygen species of importance tophotocatalysis.

We have recently investigated the nature of various oxygencentred radicals formed over the mixed-phase TiO2 material(Degussa P25). While a series of transient organoperoxy radicalswere identified in the photocatalytic oxidation of organic sub-strates [11–13], stable oxygen centred radicals such as O�2 canbe easily formed and conveniently studied by Electron Paramag-netic Resonance (EPR) [14]. Among the many surface sites avail-able for O�2 stabilization on P25, one site in particular was foundto be unusual in terms of its stability and reactivity compared tothe radicals at other surface sites. This unusual site was assignedto an O�2 radical adsorbed at an oxygen vacancy defect on theanatase surface, labelled ½Vac � � �O�2 � [14]. These O�2 radicals pro-vide an indirect means of interrogating the Ti3þ

surf centres, whichin turn provides information on the important ET sites on theTiO2 surface. In this work we will demonstrate how ET fromthese sites to O2 can be selectively blocked using appropriateprobe molecules (CO2 and Ar). These findings have importantimplications in the field of photocatalysis, revealing not onlythe competitive behaviour of co-adsorbed gases for surface sites,but also how the selective blocking of these key sites may pre-vent important ET events from occurring under reactionconditions.

Fig. 1. X-band cw EPR spectrum of superoxide radicals ðO�2 Þ formed after oxygenexposure to thermally reduced (773 K) P25. Oxygen was admitted at 298 K. Thespectrum was recorded at 120 K. (a) Experimental and (b) simulation.

Table 1Spin Hamiltonian parameters for O�2 radicals adsorbed on thermally reduced TiO2

(P25). The 17OA values are taken from Ref. [14].

Site gxx gyy gzz Axx

(mT)Ayy

(mT)Azz

(mT)% Cont.

I 2.005 2.011 2.019 7.64 <1 <1 43

J. Green et al. / Chemical Physics Letters 477 (2009) 340–344 341

2. Experimental

2.1. Chemicals

The TiO2 sample used in this study was the mixed-phase P25material, supplied by Degussa (�80% anatase, �20% rutile, surfacearea of �50 m2 g�1). The morphology of P25 has been describedelsewhere [15], and it is reported that plate-like particles are pres-ent with an average particle size of ca. 40 nm. TEM images alsoshow that the (0 0 1) and (0 1 0) faces of the anatase phase aremost abundant, with a lower fraction of the (1 1 0) plane [15]. Highpurity O2, CO2 and Ar were supplied by BOC Ltd.

2.2. Sample preparation

Full details of the TiO2 thermal reduction steps have been givenelsewhere [12,14]. Briefly, the TiO2 powder (ca. 10 mg) was slowlyheated (5 h) under dynamic vacuum (10�4 Torr) in an EPR cell upto a temperature of 773 K (for 1 h). This vacuum reduced titania(blue in color) was cooled under vacuum and used for subsequentadsorption studies. This reduced sample was exposed to the probegases (O2, CO2, Ar) at room temperature either under co-adsorptionconditions or sequential adsorption conditions. In the first case (co-adsorption), various ratios of CO2/O2 or Ar/O2 were mixed in a vac-uum manifold and exposed to the sample (ca. 10 min). The gaseswere evacuated at room temperature before EPR measurements.In the latter case (sequential adsorption), CO2 or Ar were first admit-ted to the cell (ca. 10 min) subsequently followed by molecularoxygen (i.e., the CO2 or Ar were not evacuated prior to O2 addition).The oxygen was allowed to react with the surface, and once againthe cell was finally evacuated at room temperature before EPRmeasurements.

2.3. Characterization

The EPR spectra were recorded at low temperatures on a contin-uous wave (cw) X-band Bruker EMX spectrometer operating at100 kHz field modulation, 10 mW microwave power and equippedwith a high sensitivity cavity (ER 4119HS). EPR computer simula-tions were performed using the SIMEPR32 program [16].

II0 2.004 2.011 2.020 7.86 <1 <1 16II 2.004 2.011 2.023 36III 2.001 2.011 2.026 7.97 <1 <1 5

Fig. 2. Perspective view of the (1 0 0) anatase surface schematically illustrating thetwo families of O�2 anions on the P25 surface. (a) Sites II–III based on non-vacancyfive co-ordinate Ti centres. (b) Site I due to ½Vac � � �O�2 �. Atom colours: Ti: blue, O red,O in foreground grey for clarity. Removal of the oxygen atom in (a) leaves thevacancy shown in (b). (For interpretation of the references in colour in this figurelegend, the reader is referred to the web version of this article.)

3. Results and discussion

3:1. Superoxide ðO�2 Þ radicals on P25; stabilization at vacancy andnon-vacancy sites

EPR spectroscopy has been widely used in the past to character-ise the nature of superoxide radicals ðO�2 Þ adsorbed on the TiO2 sur-face and the profile of the EPR spectrum is known to depend on thesurface morphology [17]. We have recently discussed the specificcase of O�2 adsorbed on P25, formed by direct exposure of O2 to athermally reduced sample or by UV irradiation of an oxidised sam-ple under an O2 atmosphere [14]. The salient features of the result-ing spectra, and their assignment to specific surface sites, will bebriefly summarised here in order to facilitate the later discussionsconcerning co-adsorbed CO2/O2 and Ar/O2.

The cw EPR spectrum of O�2 on thermally reduced P25 is shownin Fig. 1a. The corresponding simulation is given in Fig. 1b. As thegzz component of the g tensor for this 13 electron p� diatomic rad-ical is sensitive to slight differences in surface electric fields[18,19], a multitude of gzz peaks therefore reflects the diversity ofdifferent adsorption sites. At least three distinct gzz peaks can beclearly resolved in Fig. 1, labelled sites I–III. Closer analysis of thespectrum by computer simulation reveals an additional minor sitelabelled II0 (Table 1). Further information about the local environ-

ment of adsorbed O�2 anions can be extracted from their 17OA ten-sor [20]. Using 17O enriched molecular oxygen, we identified thedifferent 17OAxx values for the individual species at the principlesites I–III (Table 1). In all cases, both oxygen nuclei possessedequivalent spin densities, implying that all these O�2 anions aresymmetrically adsorbed at sites I–III in a side-on manner. The siteI species were attributed to O�2 stabilised at oxygen vacancies, la-belled ½Vac � � �O�2 �. The remaining sites II–III species were attrib-uted to O�2 anions adsorbed at non-vacancy sites on five co-

342 J. Green et al. / Chemical Physics Letters 477 (2009) 340–344

ordinate Ti4+ centres. Although P25 is a mixed-phase oxide, wedemonstrated that these observed radicals originated from theanatase component of the material [14]. A schematic illustrationof these two distinct sites on the anatase surface is shown in Fig. 2.

The key properties of the ½Vac � � �O�2 � site I were; (1) the radicalsat this site were photolabile, (2) thermally unstable, (3) possessedequivalent oxygen atoms, and (4) displayed higher reactivity withorganic substrates [12,14]. By comparison, the O�2 anions on thefive co-ordinate sites were non-photolabile, were thermally stable,were also bound in a side-on manner, but displayed relatively poorreactivity with organic adsorbates. As discussed in our recent work[14], thermal reduction of TiO2 occurs with loss of O2 leaving a sur-face oxygen vacancy. At least some of the resulting excess elec-trons (from the reaction 2O�surf ! O2ðgÞ þ 2e�Þ are stabilised asTi3þ

surf centres (evidenced by EPR). While the Ti4+ centres in theresulting oxygen vacancy are easily reduced to Ti3þ

vac, neighbouringTi4þ

non-vac centres (adjacent to the vacancy) also appear to be reducedcreating a heterogeneity of surface Ti3+ centres. This heterogeneityis indirectly manifested in the EPR spectrum following the reactionTi3þ þ O2 ! Ti4þ � � �O�2 , where the site I (vacancy) and sites II–III(non-vacancy) species possess distinctively different g/A values(Table 1). The Ti3þ

surf EPR signal is too broad to distinguish betweenspecific Ti3+ co-ordination centres. It should be clearly stated thatthe O�2 anions at the vacancy sites are formed and stabilised atroom temperature (298 K). However, at higher temperatures theEPR signal disappears as the O�2 radicals dissociate presumably tothe lattice O2�anion, thereby filling the vacancy sites [21].

If the physicochemical properties of these two groups of super-oxide radicals are so different, it is reasonable to postulate that ini-tial ET from the different Ti3þ

surf centres (Ti3þvac and Ti3þ

non-vac) may beprevented by blocking the sites using a suitable choice of adsor-bate. In the following we describe precisely how this occurs usingCO2/O2 and Ar/O2.

3.2. CO2/O2 adsorption; specific blocking of non-vacancy sites

The competitive interaction of CO2 and O2 with the reducedTiO2 surface was investigated in different ways (either co-adsorp-tion or sequential adsorption studies). Firstly, the interaction of

Fig. 3. cw EPR spectra of superoxide radicals ðO�2 Þ formed after CO2:O2 (co-adsorbed)pressure. The ratios of CO2:O2 were (a) 0.0:1, (b) 0.006:1, (c) 0.03:1, (d) 0.05:1, (e) 0.06:throughout. Gases were admitted at 298 K, followed by evacuation after 10 min of expo

CO2 with the reduced TiO2 was investigated. As expected nochanges were observed in the EPR spectrum of the Ti3þ

surf centres(no reactivity with CO2) after contact with CO2 (see ESI S1). Evacu-ation of CO2 (at 298 K) and subsequent addition of molecular oxy-gen resulted in the formation of O�2 anions, with a similar spectralprofile to that shown in Fig. 1. In other words, all Ti3þ

surf centres areavailable for O2 reduction, following the facile removal of theweakly adsorbed CO2 by evacuation prior to O2 addition.

However, when CO2/O2 are co-adsorbed onto the reduced sur-face, the profile of the resulting EPR spectra is different (Fig. 3).In particular a notable redistribution of relative peak intensitiesin the gzz region can be seen which is dependent on the CO2 pres-sure (compare Fig. 3a–g). As the CO2 pressure increases (the CO2/O2 ratio increases) the intensity of the gzz peaks arising from sitesII–III (gzz = 2.021–2.026) decreases dramatically, until they are vir-tually absent in Fig. 3g. By comparison, the gzz peak at 2.019, cor-responding to the vacancy site I, remains largely unperturbed. Itshould be noted that at high CO2 pressures (>2 Torr), the overallintegrated intensity of the entire O�2 spectrum begins to steadilydecrease, as expected with excess CO2 on the surface. Neverthelessthe rate of site I loss is substantially less compared to sites II–IIIloss in the low pressure regime (<2 Torr). These trends are moreclearly visualised in Fig. 4A, where the relative signal intensitiesof the O�2 species attributed to sites I and sites II–III are plottedas a function of CO2 pressure. The deconvoluted spectral contribu-tions of site I and sites II–III to the overall O�2 integrated signalintensity were determined by simulation of each spectrum inFig. 3; this enabled the individual contributions from sites I–III tobe extracted and plotted in Fig. 4.

The above experimental findings can be rationalised based onthe idea that selective adsorption of CO2 occurs on the five co-ordi-nate Ti3þ

non-vac surface sites (II–III). If CO2 adsorption (and henceblocking) occurs at these Ti3þ

non-vac sites, ET to molecular oxygen willbe prevented, and so the O�2 species classified as sites II–III will beabsent under these conditions (Fig. 3). By comparison, it is clearthat no blocking or adsorption by CO2 occurs at the Ti3þ

vac site I, sincethe signal at gzz = 2.019 remains largely intact for low CO2 pres-sures (see Figs. 3 and 4A). The EPR evidence therefore demon-strates the clearly divergent perturbations caused by CO2 on the

exposure to thermally reduced (773 K) P25, showing the effects of increasing CO2

1, (f) 0.33:1 and (g) 0.66:1. The total pressure in the cell was maintained at 15 Torrsure, prior to recording the spectrum at 120 K.

Fig. 4. Variation in the relative signal intensities of adsorbed O�2 versus (A) CO2, and(B) Ar pressures, in a CO2:O2 (co-adsorbed) and Ar:O2 (sequential) gas mixture. Therelative intensities of sites I–III were determined by simulation of the spectra. Therelative integrated intensity of the entire superoxide spectrum (including all threecontributing sites) is labelled [all O�2 ].

J. Green et al. / Chemical Physics Letters 477 (2009) 340–344 343

two families of adsorbed O�2 anions on P25; namely no blocking ofthe vacancy (site I) and selective blocking of the non-vacancy (siteII–III) adsorption sites.

A number of studies have appeared describing the use of CO2 asa probe molecule of oxygen defects on vacuum annealed TiO2 sin-gle crystals [22–24]. Henderson for example used TPD to study CO2

adsorption on a vacuum annealed rutile TiO2(1 1 0) surface, anddemonstrated how CO2 preferentially adsorbs nondissociativelyonto the vacancy Ti3+ sites followed by adsorption onto the fiveco-ordinate Ti4+ sites [21]. Upon addition of oxygen, they foundthat oxygen adsorption channels are blocked by the presence ofCO2. This preferential adsorption of CO2 at the oxygen vacancy siteis in contrast to our findings where a reversed behaviour is ob-served (i.e., preferential CO2 adsorption at the five co-ordinateTi3+ sites as opposed to the vacancy). However, it must be notedthat all of the single crystal work [22–24] was performed on a ru-tile (1 1 0) surface rather than the anatase polycrystalline surfaceresponsible for O�2 stabilisation in the current work, and this mayaccount for the differences.

The above experiments with CO2/O2 were also repeated undersequential CO2/O2 adsorption conditions (as opposed to co-adsorp-tion as reported in Fig. 3). In this case, the CO2 was first exposed tothe reduced sample for 10 min, followed by subsequent admissionof oxygen. The resulting EPR spectra (see ESI S2) were identical to

those reported above in Fig. 3, indicating that specific surface siteblocking also occurs under these conditions also.

Finally, it should be noted that variations in the CO2 pressureare not only manifested in the O�2 signal intensity but also in theTi3+ signal. It is not valid to directly compare absolute integratedsignal intensities of [Ti3+] to ½O�2 � for a number of reasons: (1) therelaxation characteristics of both species are very different, (2)the Ti3+ signal is actually a composite of bulk and surface centres,and (3) the high abundance of Ti3þ

surf centres leads to spin broadenedsignals. Nevertheless changes to the [Ti3+] and ½O�2 � abundances canbe gauged by comparing relative intensities. Most of the initial Ti3+

signal on pre-reduced TiO2 is substantially lost upon addition of O2

(see ESI S3). In this case, all available Ti3þsurf centres react leaving

only the Ti3þbulk signal. However, as the CO2 ratio in a co-adsorbed

mixture increases, the amount of residual Ti3þsurf centres also in-

creases (see ESI S3), confirming our earlier results that CO2 simplyblocks certain Ti3þ

surf sites. Removal of CO2 (i.e. during the evacua-tion procedure prior to measurement) should then re-expose thesepreviously blocked Ti3þ

surf centres, and produce more O�2 in the pres-ence of molecular oxygen. To confirm this, a second dose of oxygenwas re-admitted to the EPR cell. The O�2 signal intensity recoveredto the value expected for O�2 in the absence of CO2, and simulta-neously the Ti3þ

surf signal diminished in intensity (see ESI S4). Inother words, the Ti3þ

non-vac surface sites initially blocked by CO2 arefully recovered following CO2 evacuation. The remaining Ti3+ signalobserved following this second dose of oxygen is due to bulk cen-tres, which are unavailable for surface adsorption.

3.3. Ar/O2 adsorption; specific blocking of vacancy sites

A parallel series of experiments, as described above for CO2/O2,were also performed using Ar/O2. Although the interaction of Arwith the TiO2 is expected to be significantly weaker compared toCO2, the Ar atom has an approximately similar dimension com-pared to the oxygen vacancy, and so was considered a suitable can-didate to specifically probe the vacancy sites. It should be notedthat initial exposure of Ar only to the reduced TiO2 surface causedan increase in the Ti3+ EPR signal intensity (by a factor of two; seeESI S5). Presumably, the high charge density of the Ti3+ sites cou-pled with the strongly polarisable electron cloud of Ar, causes apartial quenching of the spin–spin broadening at these sites.

The Ar/O2 mixture was then exposed to the thermally reducedTiO2 surface at room temperature. The excess gas was evacuatedprior to the EPR measurements. Unlike the results obtained withCO2, co-adsorption of argon and oxygen did not lead to any sitespecific blocking (see ESI S6). In other words, regardless of the Arpressure used, the resulting O�2 spectra all had an identical profileto that shown in Fig. 1, implying that all Ti3þ

surf sites are available forreactivity with O2 under these conditions.

We also performed a series of experiments using sequentiallyadsorbed Ar and O2 (Fig. 5). As the Ar pressure was increased, a cor-responding notable change was observed in the gzz region of thespectra (Fig. 5a–h). The intensity of the site I peak at gzz = 2.019selectively decreased compared to the site II–III peaks. This is mostobviously seen by comparing Fig. 5a–h, representing the two ex-treme cases where Ar is absent (Fig. 5a) compared to the situationwith the highest Ar pressure (Fig. 5h).

The relative loss in site I signal intensity compared to sites II–IIIloss can be illustrated more clearly in Fig. 4B. Once again, while theinitial presence of Ar appears to disrupt the overall formation ofO�2 , the loss is more pronounced for site I. Clearly Ar must thereforeblock ET from the Ti3þ

vac sites to oxygen, preventing formation of the½Vac � � �O�2 � site I species. These results are easily understood sinceAr interacts very weakly with the TiO2 surface. In the absence ofcompeting oxygen (i.e. in this sequential experiment), the Ar probeis of a suitable size to sit in the vacancy site, forming a weak inter-

Fig. 5. cw EPR spectra of superoxide radicals ðO�2 Þ formed after Ar:O2 (sequential) exposure to thermally reduced (773 K) P25, showing the effects of increasing Ar pressure.The ratios of Ar:O2 were (a) 0.0:1, (b) 0.006:1, (c) 0.03:1, (d) 0.05:1, (e) 0.06:1, (f) 0.33:1 and (g) 0.66:1. The total pressure in the cell was maintained at 15 Torr throughout.Gases were admitted at 298 K, followed by evacuation after 10 min of exposure, prior to recording the spectrum at 120 K.

344 J. Green et al. / Chemical Physics Letters 477 (2009) 340–344

action with the TiO2 surface, thereby preventing subsequent for-mation of superoxide on addition of molecular oxygen. However,when a mixture of Ar/O2 is exposed to the surface (i.e. in the pre-vious co-adsorbed experiment), there is a greater tendency for theoxygen to chemisorb at the vacancy site I compared to argon,thereby explaining the data in ESI S6. This situation is oppositeto that found earlier with CO2, where it was found that the CO2

molecule was too large to sit in the vacancy site and therefore pref-erentially blocked five co-ordinate sites.

4. Conclusions

Adsorption of molecular oxygen at room temperature onto athermally reduced titania sample (P25) leads to the formation ofa heterogeneity of surface stabilized superoxide radicals ðO�2 Þ. Atleast two groups of stabilization sites were previously identified,and assigned to O�2 adsorbed at oxygen vacancies, labelled½Vac � � �O�2 � or site I, and O�2 adsorbed at other five co-ordinateTi4+ centres, labelled sites II–III. The precursor Ti3þ

surf centres forelectron transfer (ET) and subsequent radical formation, are foundto be specifically blocked using selective probe molecules. Carbondioxide selectively blocks the Ti3þ

non-vac surface sites, preventing ETfrom these five co-ordinates Ti3+ centres; this is evidenced by theabsence of site II–III signals in the EPR spectrum. The Ti3þ

vac sitesare not affected by CO2. On the other hand Argon selectively blocksthe Ti3þ

vac surface sites, preventing ET from these lower co-ordinateTi3+ centres. In this case, the EPR signal of site I ½Vac � � �O�2 � radicalsis absent or significantly diminished in intensity. These results re-veal not only the role of Ti3þ

surf defects in interfacial ET on TiO2, butalso how these defects can be selectively blocked by suitable probemolecules.

Acknowledgement

We would like to thank Dr. D.J. Willock, Cardiff University, forassistance with Fig. 2.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cplett.2009.07.002.

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