Screening tests for the evaluation of nanoparticle titania photocatalysts

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Research ArticleReceived: 8 April 2009 Revised: 18 May 2009 Accepted: 21 May 2009 Published online in Wiley Interscience: 13 July 2009

(www.interscience.wiley.com) DOI 10.1002/jctb.2237

Screening tests for the evaluationof nanoparticle titania photocatalystsKathryn Thompson,a Josephine Goodall,a Suela Kellici,a John A. Mattinson,b

Terry A. Egerton,b∗ Ihtesham Rehmanc and Jawwad A. Darra∗

Abstract

BACKGROUND: Nano-sized titanium dioxide has potential as a photocatalyst, and doped variants may have differentphotocatalytic properties. Nano-titanias with a wide range of dopants and compositions can be prepared using continuoushydrothermal flow synthesis (CHFS), but when many samples are made, a large-scale screening test is required to investigatetheir properties. A range of doped nano-titanias were prepared using a CHFS route, and investigated as powders using a rangeof photocatalytic test methods. These tests included decolourization of methylene blue (in the presence of oxygen), partialoxidation of a simple alcohol (propan-2-ol) and the degradation of aqueous solutions of dichloroacetic acid. The practicality ofthe tests for large-scale screening was considered, and the test results were cross-correlated to see if any of them gave similarranking for activity of the photocatalysts.

RESULTS: Two of the tests, namely DCA degradation and propan-2-ol, gave similar rank ordering for the nanopowders, whilethe MB decolourization results did not suggest a strong correlation with any other test. The addition of metal dopants wasobserved to produce varying results between different dopants and tests.

CONCLUSIONS: Two of the tests, DCA degradation and MB decolourization in visible light, were recommended for further useas screening tests.c© 2009 Society of Chemical Industry

Supporting information may be found in the online version of this article.

Keywords: photocatalysis; titania; methylene blue; visible light; screening

INTRODUCTIONPotential applications of titanium dioxide photocatalysts rangefrom breakdown of organic pollutants to photolytic generation ofhydrogen for fuel use.1 Titania nanoparticles (<100 nm in diam-eter) are of particular interest because of the potential increasein catalytic activity with greater surface area, especially if theparticle size can be tailored for optimum efficiency in the targetedapplication.2 In particular, the long-term stability of titania makes ita promising material for semiconductor photoelectrochemistry.2

However, because only UV photons can excite electrons across the∼3 eV band gap of unmodified titania, only ∼3% of the light in thestandard solar spectrum can be utilized.3 Therefore, there is greatinterest in modifying the band structure of titania, allowing it touse more of the solar spectrum and hence increase its efficiency.4

The addition of dopants to titania can introduce localizedlevels into the band gap. These may act as recombination centresand reduce the photocatalytic activity of the titania. However,Choi, Termin and Hoffman reported that doping 2–4 nm sizedtitania with either Fe3+, Mo5+, Ru3+ , Os3+, Re5+, V4+, or Rh3+at 0.1–0.5 at.%, significantly increased the photoreactivity forboth oxidation and reduction.5 Further, the possibility exists thatphotons with sub-band-gap energy might excite electrons withinthe localized manifolds, and that, following successful transferof the charge carriers to the delocalized bands of the titania,visible-light photocatalysis could result. For example, Zalas andLaniecki6 reported that doping platinized titania with lanthanides

(Gd, Eu, Yb or Ho), increased photocatalytic water splitting, withthe greatest activity at 0.5 mol%. The challenge of exploiting andoptimizing these results necessitates the preparation and testingof a large number of samples, and the task is greatly facilitatedby the ability to synthesize and screen a large number of dopedmaterials – especially if combinations of two or more dopantsare to be evaluated. Characterization and property testing of theproducts can locate potentially useful new materials and help de-velop clearer models of the relation between material propertiesand their composition, structure and production variables.

Among the known fast methods used to make crystallinenanoparticle ceramics,7 continuous hydrothermal flow synthesis

∗ Correspondence to: Terry A. Egerton, School of Chemical Engineering &Advanced Materials, Bedson Building, University of Newcastle Upon Tyne,Newcastle NE1 7RU, UK. E-mail: [email protected]

Jawwad A. Darr, Clean Materials Technology Group, Department of Chemistry,UCL, 20 Gordon St, London WC1H 0AJ, UK. E-mail: [email protected]

a Clean Materials Technology Group, Department of Chemistry, UCL, 20 GordonSt, London WC1H 0AJ, UK

b School of Chemical Engineering & Advanced Materials, Bedson Building,University of Newcastle Upon Tyne, Newcastle NE1 7RU, UK

c Queen Mary, University of London, Mile End Road, London E1 4NS, UK

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www.soci.org K Thompson et al.

(CHFS) routes8 allow production of metal doped nanoceramicsin a single step.9 These, and similar routes potentially allow alarge number of new materials to be produced in a relativelyshort time.10 Ideally, catalyst testing would take place in the sameconditions as the targeted application, but lengthy individualphotocatalytic or other property measurement tests would beimpractical and a potential bottleneck to materials discovery.

Rapid first-screen tests can quickly indicate properties ofinterest.11 Second-screen tests specific to a particular applicationcan then be applied to discriminate between the best materials.The results of a valid first screen test would identify candidates forvarious applications – high activity may indicate a potential newphotocatalyst, while low activity may indicate a material suitablefor use as a UV absorber, for example in sunscreen applications.12

The interpretation of results for immobilized catalysts may becomplicated by factors such as changes in the effectiveness ofillumination13 and/or mass transfer limitations,14 so first-screeningis usually conducted using catalyst suspensions. Methylene blue(MB) decolouration has been used as a convenient test of photocat-alytic activity.15 – 19 As reservations concerning possible maskingeffects of coloured dyes20 and the non-catalytic interactions of MBwith UV light21 have been expressed, other reactions such as partialphoto-oxidation of a simple alcohol (propan-2-ol)22 – 25 or degra-dation of aqueous solutions of an organic acid (dichloroaceticacid),26,27 have also been utilized. The oxidation of propan-2-olhas also been the basis of measures of the photostability of titaniapigments.28 Electrochemical sensing of the chloride generatedduring the photocatalytic degradation of DCA has been used as ascreening test to rank the activity of commercial photocatalysts.29

In this work, three photocatalytic test methods were assessedfor their ability to discriminate between nanoceramic materialsof different activities, and for their potential as high-throughputfirst-screen photocatalyst tests. The use of the same cohort ofnanomaterials in all tests allowed the relationships betweenresults of different test methods to be thoroughly investigated.Preliminary experiments showed that neither propan-2-ol nordichloroacetic acid degradation are significantly photocatalysedby visible radiation.27 However, methylene blue can be dis-coloured by visible light radiation in the presence of TiO2

21 andtherefore its discolouration was measured using two differentlight sources to compare UV and visible light radiation.

Many studies have used the oxidation of phenol30 or substitutedphenols31 to assess the photocatalytic activity of TiO2. We havechosen not to use phenols for three reasons. First, the extrainformation associated with the measurement of hydroquinoneand catechol reaction intermediates is of less relevance inscreening tests than in mechanistic investigations. Second, thereis more opportunity for interference as a result of formation ofoligomeric surface residues from these intermediates. (In separate,preliminary, experiments a pink discolouration was sometimesobserved during phenol degradation, and Okamato and co-workers showed that the rate of phenol degradation decreased ifthe catalyst was left in contact with the phenol solution prior toirradiating the system.) Thirdly, relatively sophisticated equipment,high performance liquid chromatography (HPLC), is needed tofollow the reaction.

EXPERIMENTALMaterialsTitanium(IV) bis(ammonium lactato)dihydroxide [(CH3CH(O)CO2NH4)2Ti(OH)2, 50 wt% in H2O] (TiBALD) solution, zinc nitrate

hexahydrate [Zn(NO3)2.6H2O 98%] strontium nitrate [Sr(NO3)2 ACSReagent], lanthanum nitrate hexahydrate [La(NO3)3.6H2O 99.0%],silver nitrate [Ag(NO3) analytical reagent grade] and hydratedpraseodymium nitrate [Pr(NO3)3.9H2O 99.9%] were supplied bySigma-Aldrich Chemical Company (Dorset, UK). Dichloroaceticacid (99%) and propan-2-ol was supplied by Alfa Aesar (Heysham,UK). Methylene blue (96 + %) was supplied by Acros Organics(Geel, Belgium). Degussa P25 titania (80 : 20 anatase : rutile, parti-cle size ∼21 nm) was obtained from Degussa GmbH (Dusseldorf,Germany). All chemicals were used as obtained. 10 M� de-ionized(DI) water (Millipore, Watford, UK) was used in all reactions.

PREPARATION AND CHARACTERIZATIONA set of five dopants were used to prepare 15 unique doped titaniananopowders using the CHFS system. For a typical reagent solutionthe TiBALD solution was diluted to a concentration of 0.4 mol L−1.Dopant ions were added to 100.0 mL of 0.4 mo L−1 TiBALD suchthat the dopant concentration was 0.5, 2.5 or 5.0 mol% of dopant(i.e. Zn2+, Sr2+, La3+, Ag+ or Pr3+), with respect to the total metalion content. Undoped titania was also produced using a similarprocedure and according to a recently published method by theauthors.32

All samples were made using a CHFS system, whose design isdescribed elsewhere.8 The CHFS system brings metal salt solutionsinto contact with supercritical water (water above its criticaltemperature and pressure, 374 ◦C and 22.0 MPa respectively) ina countercurrent mixer, which produces a continuous stream ofcrystalline metal oxide particles.33 – 35 The system is arranged asshown in Fig. 1, and constructed using 316SS Swagelok 1/8′′

stainless steel tubing and fittings, unless described otherwise. Thesalt solutions and water are pumped using Gilson 305 HPLC pumpsfitted with 25 mL pump heads. Water from pump 1 passes througha 2 kW electrically powered pre-heater, which consists of a 6 mcoil of 1/4′′ stainless steel high pressure tube wound round analuminum central core containing a 1 kW Watlow Firerod heatercartridge (Linby, UK), enclosed by a custom-made 1 kW Watlowheater jacket. The water output from this heater is at a supercriticaltemperature and pressure of 400 ◦C and 24.1 MPa. The pressureof 24.1 MPa is maintained throughout the system by a Tescom(Selmsdorf, Germany) back-pressure regulator (model 26-1762-24). Metal salt solutions from pump 2 combine with in-line dilutionof water from pump 3 at a T-piece mixer, to match the additionof base or other reagents via pump 3 in the CHFS production of

Figure 1. Schematic representation of the three-pump continuous hy-drothermal flow synthesis system used to prepare doped TiO2 nanoparti-cles. Key: P = pump, T = T-piece mixer, H = heater, R = counter-currentreactor, C = cooling, F = filter, B = back-pressure regulator and M =dopant metal ion.

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Screening tests for the evaluation of nanoparticle titania photocatalysts www.soci.org

other materials made by the authors.8,9,36,37 The flow rates for P1,P2 and P3 are 20, 10 and 10 mL min−1, respectively. The metal saltsolutions are brought into contact with the supercritical water atthe in-house built 3/8′′ counter-current mixer,8 producing a streamof nanoparticles. After synthesis, the nanoparticles pass througha vertical water-cooled pipe, constructed of 1/4′′ stainless steeltubing and 97 cm in length, to reduce the temperature. Particlesthen exit the system through the back-pressure regulator.

Samples were collected and centrifuged at 5100 rpm for 3 h.The supernatant was removed and the samples were washed with15 mL of ethanol and centrifuged for a further 2 h. The ethanolwas decanted and the samples washed in 40 mL of DI water twicebefore being freeze-dried. Freeze-drying was performed using aVirtis Advantage freeze-dryer (Gardiner, NY, USA) at ca 10−6 MPafor a period of 12 h.

X-ray powder diffraction (XRD) data were collected on aSiemens D5000 X-ray diffractometer (Munich, Germany) usingCu-Kα radiation (λ = 0.15418 nm). Data were collected over the2θ range 20–80◦ with a step size of 0.02◦ and a time of 2 s perstep. Brunauer–Emmett–Teller (BET) surface area measurementwas performed using a Micromeritics Gemini analyser (Dunstable,UK). The powders were degassed at 100 ◦C in N2 (BOC, UK)for 2 h prior to BET analyses. TEM images were obtained usingan HR-TEM model JEOL (Welwyn Garden City, UK) 4000EX highresolution transmission electron microscope (400 kV acceleratingvoltage). Samples were collected on carbon-coated copper grids(Holey Carbon Film, 300 mesh Cu, Agar Scientific, Essex, UK) afterbeing briefly dispersed ultrasonically in methanol for 3 min. Imageanalysis was performed using Digital Micrograph Gatan software(Abingdon, UK).

MEASUREMENT OF PHOTOACTIVITYPhotodegradation of MBThe photodegradation of methylene blue was carried out in aset of nine parallel reactors under a common light source, asshown in Fig. 2, in a method similar to that previously reportedby the authors.32 The tests were conducted using two differentlight sources, a 400 W mercury discharge lamp (Philips, 400HPLR,Guildford, UK) and two UV fluorescent lamps (Philips PL-L 36 W 09actinic lamps).

The emissions of the two light sources used were measuredusing an Ocean Optics USB4000 spectrophotometer (Duiven,Netherlands). The main emissions are at 365, 405, 436, 546, and577 nm, characteristic of mercury emission in both fluorescent and

Figure 2. Diagram of the apparatus used for the photodegradation of MB,using two different light sources.

discharge lamp types.38 Mercury lamps do not directly emulatethe solar spectrum, but produce wavelengths that fall within it fortest purposes. In the case of the UV lamps, there is an additionalbroad peak of emissions between 330 and 400 nm, which mayactivate materials which absorb in that part of the spectrum.

The UV illumination under the centre of a 4 mm thick frostedglass diffuser (Bow Mirrors, Bow, London) was found to be 3.2 mWcm−2 for the UV lamp, and 2.7 mW cm−2 for the Hg discharge lamp(J-221 UV meter, Ultra Violet Products, Cambridge, UK). The visibleillumination in the same location was found to be 2.4 klx for the UVlamps and 82 klx for the 400 W mercury discharge lamp (HI97500photometric light meter, Hanna Instruments, UK), showing that amuch greater proportion of the output of the Hg discharge lampis in the visible part of the spectrum.

0.010 g of the powder under test was dispersed in 50.0 mLof 0.02 mmol L−1 aqueous MB solution. The suspensions wereconstantly agitated by magnetic stirrer. The sheet of frosted glasswas used as a light diffuser to ensure even illumination of thewells. The apparatus was enclosed in a fan-ventilated dark boxto exclude the influence of external light sources. The reactionmixtures were left to equilibrate for 30 min in the dark beforeswitching on the light source.

The photodegradation process was monitored by a UV-vis spec-trophotometer (Helios Alpha, Thermo Scientific, Loughborough,UK or Perkin Elmer Lambda 25, Beaconsfield, UK) calibrated usingknown concentrations of MB dye. At regular intervals of 1 h, theirradiation was stopped and 5.0 mL aliquots were taken from eachsample and centrifuged for 5 min at 5100 rpm to remove partic-ulates. The UV-vis absorbance of the aliquots was measured, theparticulates re-dispersed in the 5 mL aliquots and returned to thereaction vessels, and irradiation resumed. The total illuminationperiod for both experiments was 5 h.

Photo-oxidation of propan-2-olPropan-2-ol oxidation was measured by monitoring the formationof acetone (propanone) product as has been previously describedby Egerton and Tooley.25 The reactor, shown in Fig. 3, consistedof a 50 mL glass vessel held at 30 ± 2 ◦C. The equipment wasmounted above a UV light source (2× Philips PL-L 36 W 09 actiniclamp, as previously described) with a shutter to block the light asnecessary. A water heat filter was placed between the light sourceand the bottom of the reaction vessel to reduce heating of thereaction mixture.

Figure 3. Diagram of the apparatus used for propan-2-ol oxidation tests.

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0.400 g of catalyst was dispersed in 50.0 mL of propan-2-olin the reaction vessel. The suspension was constantly stirred bya glass stirrer blade, and oxygen was supplied to the vessel’sheadspace at a constant low flow rate. The assembled system wasleft to equilibrate for 15 min in the dark, and then the shutter wasremoved to allow UV illumination from below.

The concentration of the oxidized product, propanone, wasdetermined using a Cambridge GC94 gas chromatograph (GC)with an FID detector. 2.0 mL aliquots of the suspension were takenat 15 min intervals and the particulates were removed using a0.2 µm PTFE syringe filter. Particulates could not be returned tothe reaction vessel, but they were assumed to be evenly distributedin the suspending liquid including the removed aliquots, and thattherefore the concentration of catalyst particles remained the samethroughout the experiment. An internal standard of ethoxyethane(200 µL in 25.0 mL of propan-2-ol, 0.8 vol%) was used to calibratethe results.

Photodegradation of DCAThe photodegradation of dichloroacetic acid (DCA) was carriedout in a glass cylindrical reactor, with a UV light source (Philips PL-L36 W 09 actinic lamp, as previously mentioned) placed in an axialwell in the centre (Fig. 4), in a method similar to that previouslydescribed by Egerton and Mattinson.27

0.400 g of catalyst was dispersed in 250.0 mL of 36 mmol L−1

aqueous DCA solution. The ionic strength of the solution wasadjusted by adding 2.0 mL of 5 mol L−1 NaNO3 solution, tooptimize electrode performance, and 1 mol L−1 NaOH was addeddropwise to the reaction mixture to adjust the pH to 3.0 ± 0.2,since previous experiments have shown that the reaction proceedsbest under these conditions.27 Oxygen gas was supplied to theheadspace of the vessel at a constant flow rate. The reactionmixture was continuously agitated by magnetic stirrer, and leftto equilibrate for 15 min in the dark before the UV light sourcewas fixed in place. The degradation of DCA was followed using achloride ion selective electrode, to monitor the concentration ofchloride ions, at regular intervals over 90 min.

Figure 4. Diagram of the apparatus used for photodegradation of DCAtests.

RESULTSCharacterization of catalystsAll powders showed a diffraction pattern typical of the anataseform of titania (JCPDS pattern no. 21–1272). Ag-doped samplesshowed additional weak peaks associated with metallic silver(JCPDS pattern no. 04–0783). Transmission electron microscopy(TEM) was used to study the particle morphology of the undopedTiO2 powder. Analysis of lattice fringes of an individual nano-crystalshowed an average lattice spacing of ca 0.354 nm (See supportinginformation),32 which was identified as the (101) lattice spacingpreviously reported for anatase TiO2 as 0.356 nm.39 The averagecrystallite size of the doped samples was calculated from thehalf-widths of the XRD peaks using the Scherrer equation,40 andthe average crystallite size for all samples was found to be 4.2 nmwith a standard deviation of 1.3 nm, giving a range of 3.6–5.3 nm.BET surface areas of all the samples were 280 ± 20 m2 g−1 (range237–306 m2 g−1) (See supporting information).

Photodegradation of MB in UV fluorescent lamp illuminationThe photocatalytic decolouration of MB appeared to followpseudo-first-order kinetics, as found in previous studies,41 andresults shown in Fig. 5 are based on the derived first-order rate

Figure 5. Reaction rates for MB degradation in UV light using doped and undoped titania.


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constants. Since MB can decolour in the absence of photocatalysisby titania,21 the extent of direct photochemical reaction wasestimated from control experiments carried out in the sameconditions but without the addition of titania.

For MB degradation under UV light, the control, undopedCHFS titania, had an activity of ∼75% of that of Degussa P25(0.4 × 10−3 s−1). For Zn, Ag and Sr dopants the activities of thedoped titania samples were comparable with that of the control.For La and Pr doping, the activity was significantly less than thecontrol and decreased monotonically with increased doping level.For 5 mol% Pr doping the activity was comparable with the directphotochemical breakdown rate of 7.9 × 10−6 s−1.

Photo-oxidation of propan-2-olPrevious work has shown that no discernable photo-oxidationoccurs for propan-2-ol in the absence of titania, and that the

reaction rates for oxidation can be calculated from the increasein concentration of the oxidation product propanone over time,which is observed to follow zero-order kinetics.25 The results areshown in Fig. 6.

The activity of the undoped control sample (∼0.75 ×10−6 mol dm−3 s−1) was approximately 60% of that of P25(1.3×10−6 mol dm−3 s−1). Doping with Zn, Sr, La, Ag or Pr reducedthe activity to ∼65% of that of undoped titania. In the case of Zn, Srand Ag (no result was available for 2.5 mol% doping Ag sample) thephotoactivity varied little as the dopant level was increased from0.5 to 5% whereas for both La and Pr, the photoactivity halvedwhen the dopant concentration increased from 0.5 to 5%. Thelowest activity was observed with 2.5 and 5.0 mol% of Pr dopant.

Photodegradation of DCAPrevious work has shown that no discernable photodegradationoccurs for DCA in the absence of titania, and that the reaction rates

Figure 6. Reaction rates for propan-2-ol oxidation in UV light using doped and undoped titania. (no result available for 2.5 mol% Ag-titania).

Figure 7. Reaction rates for DCA degradation in UV light using doped and undoped titania.


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for breakdown can be calculated from the increased concentrationof chloride ions over time, using zero-order kinetics as observed.27

For UV degradation of DCA (Fig. 7) the undoped CHFS titaniahad an activity ∼30% of P25 (2.6 × 10−6 mol dm−3 s−1) and againwas the most active of the experimental titania samples. Therate of degradation generally decreased as the extent of dopingincreased from 0.5 to 5.0 mol%. However, for the Sr series of resultsthe highest activity was observed for the 5.0 mol% doped sample.The lowest activity was again observed with 2.5 and 5 mol% Prdoping.

Photodegradation of MB under visible-lamp illuminationMB directly absorbs visible light, predominantly at wavelengthsgreater than 540 nm, and this has been shown to change theapparent activity of photocatalysts.20 The direct photoexcitationof MB molecules may cause it to act as a photosensitizer, alteringthe mechanism of the overall MB degradation reaction.21 However,the reaction appeared once again to follow first-order kinetics, and,accordingly, comparisons are based on the pseudo-first-order rateconstants.

As expected, there was significant photodegradation of MBwithout catalyst (k = 4.5 × 10−5 s−1). For MB, as for propan-2-ol oxidation and DCA degradation, the activity of the undopedcontrol sample (1.9 × 10−4 s−1) was again lower (∼60%) thanthat of P25, 3.3 × 10−4 s−1. However, in this case the 0.5 mol%doped samples generally gave the highest reaction rates. Allexcept La-doped TiO2 had a greater activity than undoped titania,as shown in Fig. 8. Higher dopant levels generally reduced thecatalyst activity. Sr is an exception to this trend and the activity of5.0 mol% Sr-doped titania was comparable with that of P25.

DISCUSSIONTrends in resultsThis paper focuses on the choice of a suitable high throughput testmethod, rather than the discovery of new catalysts. The samplesexemplify the products of a typical production sequence andtheir importance is to provide the range of activities that maybe expected in a practical investigation, rather than an optimizedcatalyst.

The CHFS-synthesized undoped titania nanoparticles had theanatase structure. Crystallinity is usually considered a desirableproperty in potential photocatalysts,7 and although high dopantlevels may reduce crystallinity,42 all the doped nanoparticlestested here had retained the anatase crystalline form. Further,the crystal sizes were comparable with each other, 3.6 to 5.3 nm,and with other types of commercially available ultra-fine titaniapowders.43 Consequently, the nanoparticles used for these testsare judged to be sufficiently similar to each other and to widelytested materials to minimize major artefacts associated with simplephysical differences.31

Doping generally reduced the measured photoactivity belowthe level of the CHFS undoped control. In all four tests Pr dopingtended to give the lowest reaction rates and activity decreased asthe doping levels increased. Zn doping produced broadly similarmid-to-high range reaction rates. For these dopants, there is littledistinction between tests. La-doped samples gave higher resultsin the DCA and propan-2-ol tests, though lower than that ofundoped CHFS titania, and relatively low results in both the MBmethods. Sr-doping was similar to Zn when monitored by eitherDCA, propan-2-ol or MB-UV. The very high MB-visible activity of5.0 mol% Sr (comparable with that of Degussa P25) may indicatea significant contribution from direct UV absorption by adsorbeddye. The 5.0 mol% Ag sample was more active than any of theother CHFS samples in the MB-UV test.

Comparison of methodsAll of the test methods gave adequately high reaction rateswhen used to evaluate the widely used Degussa P25 titaniaphotocatalyst. However, an acceptable method should also givea high degree of discrimination between different samples andgive results that are related rationally to the results obtained usingother test reactions. Both of these aspects are now examined.

Figure 9 shows the results for the undoped CHFS titania, andfor the most and least active CHFS doped-samples, normalized toa value of 1 for Degussa P25 for each method. In each case thelowest activity was found in a CHFS sample doped with 5.0 or2.5 mol% Pr.

For MB decolouration under UV radiation the highest and lowestdecolouration rates appear to vary by a factor of 20, whereas

Figure 8. Reaction rates for MB degradation in visible light using doped and undoped titania.


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Figure 9. Reaction rates for undoped CHFS titania and selected doped samples, normalized to a reaction rate of 1.0 for Degussa P25 in each method.

under visible radiation they appear to vary by a factor of 4.5(this factor changes to 10 when the significant contribution ofdirect photochemical decolouration is subtracted from all results).For DCA, the highest and lowest degradation rates vary by afactor of 7 whereas for propan-2-ol oxidation the highest rateis 2.5 times the lowest. This suggests that propan-2-ol oxidationprovides the lowest degree of discrimination between the differentphotocatalysts in these experiments.

The degree of correlation between the results obtained bythe different methods has been examined in two ways. First, thewidely used Pearson correlation coefficient R2 was derived froman x-y plot of the corresponding pairs of results (See supportinginformation). As our focus was to evaluate the R2 values from aset of related samples, such as might be produced in a series ofCHFS experiments, the results for P25, which often are obviousoutliers, were not initially included in this comparison. However,the corresponding R2 values taking into account the P25 resultsare also included, in brackets, in Table 1. Although the two setsof values are different, the general trends are similar. Second,the value of the Spearman rank-correlation coefficient – a non-parametric measure of correlation – was evaluated for pairs oftests. The advantage of this method is that it compares theranking of the individual sets of results without the need to makeassumptions about the populations from which the results come.44

All the results are listed in Table 1.From inspection of Table 1 it is immediately obvious that the

highest degree of correlation occurs for the comparison of DCA andpropan-2-ol results. This is consistent with the good correlationbetween these methods previously reported for a study of astandard titania coated with increasing amounts of inert oxide.45

The correlation of MB UV-decolouration rates with MB visible-decolouration is poor (especially the R2 value) and this may indicatethat although the UV-induced decolouration is caused by charge-carrier excitation across the titania band-gap, the visible lightdecolouration has a significant contribution from a mechanism ofa different type:46

MB + hν → MB∗ (1)

MB∗ + TiO2 → MB+ + TiO2(e) (2)

TiO2(e) + O2 → TiO2 + O2•− (3)

O2•− + e + 2H+ → H2O2 (4)

H2O2 + e → •OH + OH− (5)

MB•+ + O2 (or O2− or •OH) → intermediates →

degraded dye products (6)

Table 1. Spearman and Pearson correlations between pairs ofmeasurements

Comparison

Spearmanrank-correlation

coefficient,ρ

Pearson correlationcoefficient calculated by

Excel, R2 (valuesincluding P25)

DCA v Propan-2-ol 0.88 0.826 (0.926)

MB vis v MB UV 0.64 0.522 (0.578)

DCA v MB UV 0.50 0.171 (0.303)

Propan-2-ol v MB UV 0.35 0.216 (0.386)

DCA v MB vis 0.36 0.060 (0.155)

Propan-2-ol v MB vis 0.17 0.068 (0.180)

A dependence on a different mechanism is also implied by thepoor correlation of the MB-visible results with those for DCA andpropan-2-ol. In summary, the MB-visible results appear to measurea process which differs from that monitored in the other tests. Thehigh degree of correlation between DCA and propan-2-ol rankingsuggests that little is to be gained by making both measurements,instead of just one, to screen the photoactivity of large numbersof CHFS products as required. By contrast, the MB-visible resultsmay well be sensitive to changes that are not picked up by the UVdegradation experiments.

Practicality of methods for high-throughput analysisThe propan-2-ol method uses gas–liquid chromatography (GLC)to process samples singly. Even if an auto-sampler is fitted tothe GLC, the manual sample-preparation steps make this methodunsuitable for large-scale screening with multiple data points.By contrast, the DCA method uses a chloride electrode inconstant contact with the reaction solution. Reaction progresscould be monitored by data-logger, involving no interventionfor measurements once the test equipment is set up. Therefore,DCA may be more appropriate than propan-2-ol for large-scalescreening of photocatalysts. However, DCA degradation probablyproceeds by direct hole-transfer to an adsorbed DCA anion.27

Because of this, DCA results are more likely to be sensitive to thesurface area of the catalyst.

The MB methods described above can irradiate nine samplessimultaneously, but the time for large-scale screening wasincreased because, as for samples of oxidized propan-2-ol, each5 mL sample had to be treated to remove particulates, being


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centrifuged prior to UV-vis measurement. The rates of MBdecolouration by UV radiation showed a weak correlation with allof the other three sets of results. However, by using a light sourcewith much less UV than visible radiation, the potential visible-lightactivity could be monitored, as was shown by the 5.0 mol% Sr-titania. In this way, changes in the reaction mechanism can behighlighted.

CONCLUSIONSThe addition of metal dopants produced variable results. Generallythe addition of dopants either significantly reduced photoactivity,as in the case of Pr doping, or produced no clear trend inactivity, as in Zn doping. In some cases an effect of doping onphotocatalytic activity was confined to a specific test, suggestingthat the influence of dopant depends on the particular reactionmechanism.

The DCA degradation test, in this case, is an attractive methodfor screening a large number of catalysts because its resultscorrelate well with those of propan-2-ol oxidation, span anadequate range of activities and the method has the potentialfor easy automation. The dye-degradation methods could be usedto make simultaneous measurements on a number of samplesand gave the greatest range of activities. For both methods,comparison of photocatalytic activity will be more meaningful ifthe catalysts have similar surface area, as is the case with thesamples used in this study. The ranking of different catalystsby UV-decolouration of MB did not correlate well with DCAdegradation, propan-2-ol oxidation, or visible decolouration of MB.Nonetheless, decolouration of MB by visible light can potentiallygive information to which the other methods are insensitive,although due allowance must be made for direct photochemicaldecolouration of MB.

We conclude that if only two tests can be used for screening,DCA degradation, and MB decolouration by visible light meritcareful consideration.

ACKNOWLEDGEMENTSEPSRC is thanked for funding the ‘High Throughput NanomaterialsDiscovery’ project for developing better photocatalysts [EPSRCGrant Reference: EP/D038499/1] (JAD, SK, KT) and ProfessorD. Cockayne, Drs L. Karlsson, J. Hutchinson and C. Hetheringtonat Oxford Department of Materials are thanked for assistance andaccess to the HR-TEM instrument (under the EPSRC access schemegrant reference: EP/F01919X/1). Sun Chemicals and ICI Uniqema(now Croda) are thanked for industrial CASE awards (to JG andJAM, respectively).

Supporting informationSupporting information may be found in the online version of thisarticle.

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Screening tests for the evaluation of nanoparticle titania photocatalysts