pigment and nanofiller photoreactivity database

Pigment and nanofiller photoreactivity database

Stephanie Watson, I-Hsiang Tseng, Tarek Marray,

Bastien Pellegrin, Julien Comte

� ACA and OCCA (outside USA) 2012

Abstract The service life and durability of nanocom-posites containing fillers are affected by photocatalyticproperties of these fillers, particularly narrow band gapmetal oxides (NBMOs) such as titanium dioxide(TiO2). When irradiated with ultraviolet flux, NBMOsproduce electrons and other species that are capable ofcausing rapid degradation of organic materials withwhich they are in contact. Electrons and holes (posi-tively charged species) migrate to the surface and reactwith species to generate various free radicals. Mea-surement science tools for characterizing TiO2 photo-reactivity using electron paramagnetic resonance(EPR) methods have been developed by the Engi-neering Laboratory (EL) at the National Institute ofStandards and Technology (NIST) and a linkagebetween EPR measurements and current industrialmethods has been established. A database of TiO2

photoreactivity values and other data measured via theEPR methods and industrial assays has been compiledand will be accessed through a searchable softwaredatabase in the NIST Standard Reference Databaseprogram—http://www.nist.gov/srd/index.cfm. The data-base provides fundamental photoreactivity data thatcan be used for product selection and developmentpurposes to enable more reliable assessments of end-performance.

Keywords Titanium dioxide, Pigment, Nanoparticle,Photoreactivity, Free radical, EPR

Introduction

Large volumes of pigmentary and nano-TiO2 areconsumed each year as fillers and ultraviolet (UV)absorbers for traditional building materials such aspaints, sealants, and bulk plastics (i.e., laminate flooringor polyvinyl siding).1 These materials have also foundwide-scale application in self-cleaning/disinfecting sur-faces and products with enhanced mechanical proper-ties.2,3 When illuminated with UV radiation, narrowband gap metal oxides (NBMOs) produce electrons andother excited species that are capable of causing rapiddestruction of organic materials with which they comeinto contact.4 NBMOs, specifically TiO2, are complexand supplied in a wide variety of forms (powders,vapor-deposited films, and single crystals), crystalphases, bulk and surface chemistries, and photoreac-tivities.5 Increased interest in nanosized TiO2

( £ 100 nm) has resulted in products with additionalvariations in crystal phases and forms, and has castdoubt on conventional methods of measuring NBMOphotoreactivity, which were intended for particles>250 nm. Furthermore, there is a need to developrapid and reliable measurement science for photoreac-tivity and identify properties that contribute to photo-reactivity. For NBMOs of any particle size, a commonissue shared by product applications containing them isthat their service life and durability are affected, bothnegatively or positively, by the photoreactivity of theNBMO. Therefore, validated measurement tools canbe developed and improvements in material andreductions in life cycle costs can readily be achieved.

NBMO photoreactivity, particularly TiO2, has beenstudied for many years.4–6 However, there has been nocoordinated, systemic, scientifically based approach tolink all of the processes and understand their impact onthe performance of an end-product. Moreover, eachindustry utilizing TiO2 has developed its own test meth-ods for assessing photoreactivity, and the correlations

S. Watson (&), I-Hsiang Tseng, T. Marray,B. Pellegrin, J. ComteEngineering Laboratory, National Institute of Standardsand Technology, Gaithersburg, MD, USAe-mail: [email protected]

J. Coat. Technol. Res., 9 (4) 443–451, 2012

DOI 10.1007/s11998-012-9408-8

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http://www.nist.gov/srd/index.cfm

among these different tests are inadequate.7–10 Mea-surement science tools for characterizing NBMO(TiO2) photoreactivity using electron paramagneticresonance (EPR) methods have been developed byNIST and relationships between EPR measurementsand industrial chemical assays have been established torelate fundamental properties to end-use performance.EPR, which measures unpaired electron species suchas free radicals, was used to elucidate the mechanismsinvolved in the UV response of TiO2.11,12 In thisresearch, EPR analysis was carried out on a series ofTiO2 specimens, ranging from commercial products tostandard reference materials (SRMs). EPR protocolshave been developed to characterize charge carriergeneration mechanisms and kinetics in solid state (drypowder) TiO2 and interfacial charge transfer reactionsvia TiO2 liquid suspensions with spin traps. Correla-tions were established between specific particle char-acteristics (size and size distribution, surface physicaland chemical properties, crystal phase, etc.) and thenature/quantity of free radicals generated was mea-sured. This TiO2 photoreactivity data will be placedinto a searchable software database that is availableas a NIST Standard Reference Database—http://www.nist.gov/srd/index.cfm. This photoreactivity data-base will provide fundamental data on TiO2 photoreac-tivity including the linkage of current chemical assaymethods to fundamental EPR measurements and hence,enable more reliable assessments for product selectionand end-performance.

Experimental procedure

Materials

Commercial TiO2 products were the primary focus ofthis research. A range of pigment surface treatments,crystallinity, and particle sizes was examined as theseare some of the many factors that influence pigmentphotoreactivity.* Table 1 lists the TiO2 pigments andtheir properties. Note that the particle sizes areactually crystallite values reported by manufacturers.For comparison, an in-house prepared TiO2 specimen(sample K) was studied.13

Chemical assays

Methyl viologen assay

A 1.5 g/L suspension of TiO2 with 2.5 9 10�3 mol/L inmethyl viologen dichloride and 3.0 9 10�2 mol/L in

disodium ethylenediaminetetraacetic acid (EDTA),buffered to pH 6.0 with a 0.2 mol/L phosphate buffer,was prepared. In other methyl viologen (MV) exper-iments, a potassium hydrogen phthalate (KHP) bufferwas used as a non-phosphate control buffer. Thesuspension was purged with argon (Ar) in a sealedsparging reactor for 20 min, and then irradiated withUV radiation with a 100 W mercury (Hg) (365 nm)lamp for 2 h. During UV irradiation, the TiO2

suspension was continually mixed with a magnetic stirbar. In an Ar-purged glove bag, 3 mL of the irradiatedsuspension was transferred into a cuvette using asyringe equipped with a 0.2 lm filter using a UV–visible (UV–VIS) spectrometer (Agilent 8453). Theabsorbance of the MV cation radical was measured at602 nm. The standard uncertainty associated withUV–VIS spectroscopy absorbance measurements wastypically ±2%.

Hydrogen peroxide assay with leuco-crystal violet

250 mg of TiO2 pigment was mixed with 25 mL of1 mol/L acetate buffer (pH 4.4) and irradiated withUV radiation with a 100 W Hg lamp (365 nm) for 2 h.During UV irradiation, the TiO2 suspension wascontinually mixed with a magnetic stir bar. Theirradiated pigment suspension was then filtered(0.2 lm) and 8.5 mL of the filtrate was mixed with0.5 mg/mL horseradish peroxidase and 1 mL leuco-crystal violet (LCV) solution (100 mg LCV dye in200 mL of 0.5% by volume hydrochloric acid). Asolution of 0.3% by volume solution of hydrogenperoxide (from 3% volume hydrogen peroxide, USPgrade) was used as a standard for calibration. Theabsorbance of the LCV solution at 596 nm wasimmediately measured using UV–VIS spectroscopy.

Isopropyl alcohol test

TiO2 suspensions in isopropanol (reagent grade) wereprepared from TiO2 dried at 120�C for 2 h and purgedwith compressed air (500 mL/min) for 1 min prior toUV irradiation. Pigment loadings varied and werebased on pigment surface treatment: 1 g for alumina(Al2O3)-coated pigments and 4 g for silica (SiO2)-coated pigments. A jacketed beaker containing theTiO2 suspension was illuminated from the top with UVlight from a 100 W Hg lamp (365 nm). Suspensionswere stirred with a magnetic stir bar to maintain thesuspension of the pigment. After 2 h of UV exposure,the suspension was filtered (0.2 lm) and the acetonecontent of the filtrate was determined by gas chroma-tography with flame ionization detection. A blankmeasurement was also performed using isopropanolalone to establish the initial amount of acetone presentin the isopropanol reagent.

* Certain trade names and company products are mentioned inthe text or identified in an illustration in order to adequatelyspecify the experimental procedure and equipment used. In nocase does such an identification imply recommendation orendorsement by the National Institute of Standards and Tech-nology, nor does it imply that the products are necessarily thebest available for the purpose.


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EPR spectroscopy

Spin trap method

3-Aminoproxyl (AP) was purchased from Aldrichwithout further purification. Stock solutions (500lmol/L) of each spin trap were prepared in deionizedwater and stored at 7�C. Stock preparations (0.2 g/L)of TiO2 suspensions in deionized water were alsoprepared for spin trap studies. For each spin trapexperiment, a new mixture of spin trap stock solutionand TiO2 suspension was prepared to monitor thephoto-produced radicals under UV irradiation atambient temperature. 50 lL aliquots of the spin trap/TiO2 suspension mixture were placed into EPR cap-illary tubes for UV irradiation in the EPR instrumentcavity. A range of TiO2 spin trap molar ratios wasinvestigated.

Solid state method

For solid state EPR studies, TiO2 with different sizes,crystalline forms, and modified surfaces were investi-gated. The same volume of powder for each TiO2

sample was placed into an EPR tube and dried undervacuum (85�C) for at least 8 h to reduce the moisture.The EPR tube was placed in the instrument cavity forin situ UV irradiation.

EPR measurements

All EPR spectra for the spin trap studies wererecorded at ambient conditions with a Bruker ElexsysE500 EPR spectrometer. Most measurements werecarried out using a resonant frequency of 9.38 GHzand a microwave power of 0.6 to 10 mW. The timeconstant, conversion time, sweep time, and signal

receiver gain were adjusted to obtain optimum signalresolution. For some weak-signal experiments, highermicrowave power, receiver gain, and signal-to-noiseratio were applied. Samples of spin trap/TiO2 suspen-sion mixtures were placed in 50 lL capillary tubes thatwere positioned in a conventional EPR tube anddirectly irradiated with a xenon (Xe) arc lamp at500 W. EPR spectra were recorded at regular timeintervals during the UV irradiation period. Severalcontrol experiments were carried out to insure that theobserved signals did not arise from photolysis oroxidation products of the spin traps themselves as wellas to monitor the EPR signal stability in the dark andunder illumination. In addition, the stability of spintrap solutions without TiO2 over time (8 weeks at 7�C)was monitored.

All solid state EPR spectra were recorded at aresonant frequency of 9.3 GHz, microwave power of10 mW, signal receiver gain of 30–70 dB, modulationamplitude of 1.0 9 10�3 T, and modulation frequencyof 100 kHz. The time constant, conversion time, sweeptime, and scan times were adjusted to obtain optimumsignal resolution. Quartz EPR tubes were employed toavoid the background signal during measurement. Aliquid helium (He) cryostat was used in the EPRinstrument to achieve lower sample temperatures(77 K) in the cavity. The samples were directlyirradiated in the cavity with the Xe arc lamp at500 W for in situ measurements.

Results and discussion

Conventional industrial assays

Spectrophotometric assays quantitatively measure theconcentration of holes and electrons generated duringband gap irradiation or of products arising from the

Table 1: TiO2 pigments studied (labeled A through K)

Label Crystallinity Particle size (nm) pH of suspension pH (10 g/100 mL) Surface treatment

A Anatase (70%)rutile (30%)

20 6.6 6.0 None

B Anatase 7–8 4.4 Not available NoneC Anatase 35–40 Not available Not available Al2O3/SiO2 (25%)D Rutile 10–30 Not available 6.5–8.0 Al2O3 (9–14%)

SiO2 (1–7%)E Rutile 250 6.0 7.5 Al2O3 (6%)F Rutile 250 6.7 7.5 Al2O3/SiO2 (12%)G Anatase 300 6.3 Not available NoneH Rutile >400 6.0 Not available NoneI Rutile 30–50 6.3 6.5–8.0 Al2O3 (10–15%)

ZrO2 (2–5%)J Anatase 7 6.2 Not available NoneK Anatase 50 6.7 Not available None

Property values are those provided by the manufacturer, except for the ‘‘pH of suspension’’ value on 1.5 g/L TiO2, which wasmeasured in deionized water


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reactions of holes and electrons with a surroundingmatrix, such as hydrogen peroxide. A number ofchemical assays for assessing the photoreactivity ofTiO2 are documented in the technical and patentliterature and are commonly used in industrial labora-tories.14–16 Some chemical assays involve compoundsthat react directly and stoichiometrically with eitherholes or electrons14–18; while other assays use probecompounds that react stoichiometrically with redoxproducts such as hydrogen peroxide.16,19,20 Variousspectroscopic measurements are then used to measurethe decrease in concentration of the probe compoundor the formation of products resulting from thereaction of the probe compound with the active speciesgenerated from the TiO2.

For this research, three assays were selected thatexemplify the above two categories of assay types. TheMV assay is based on the reaction of MV, an electronacceptor, with conduction band electrons. The MVdi-cation, a colorless compound, is easily reduced to arelatively stable cation radical form, which is blue incolor with strong absorption at 602 nm.17 Upon react-ing with a free electron generated via UV irradiation ofTiO2, a MV cation is reduced to a MV cation radical.The MV cation radical also reacts with holes toregenerate the initial MV di-cation in a competingreaction, thus, EDTA is used to inactivate the photo-generated holes.18 The MV assay was compared to theisopropanol to acetone conversion method (IPA) thatis commonly used in the pigment industry.14,15 Thephoto-oxidation of TiO2 is determined by the amountof acetone generated in isopropanol-pigment slurriesexposed to UV, which is then commonly used to rankpigment photoreactivity.

The amount of hydrogen peroxide produced fromreactions of TiO2 electrons and holes with particlesurface species was measured using a leuco-crystalviolet/horseradish peroxidase (LCV) assay. The con-centration of hydrogen peroxide produced during UVirradiation of aqueous pigment slurries has been usedto quantify the photoreactivity of the pigment.15,18

Horseradish peroxidase catalyzes the oxidation ofLCV dye with the produced hydrogen peroxide andits effectiveness is dependent upon buffer pH andcomposition.19,20 The intensity of the maximum UV–VIS absorption peak of the oxidized LCV dye at596 nm is used to quantify the hydrogen peroxideconcentration.

Figure 1 shows the results of the photoreactivityranking for the various assay methods. Note that thescale for the isopropanol to acetone test was adjusted(data points equally reduced) so that data from allassay methods could fit on one plot. For the MV assay,lower values were observed for surface treated TiO2

pigments. The MV results are indicative of greater netproduction of free electrons by catalytic TiO2 relativeto coated TiO2. Based on the MV results, uncoated,nanosized, anatase TiO2 pigments are more photore-active than rutile TiO2 pigments. The LCV assaymeasures the amount of hydrogen peroxide produced

and a different photoreactivity ranking relative to theMV assay was observed. A 3% by volume hydrogenperoxide control solution was 30% greater in valuecompared to sample A. Pigments without surfacetreatments exhibited a greater peroxide production inthe LCV assay. However, among the uncoated TiO2

pigments, the larger particle size, anatase pigmentsdisplayed the greatest peroxide concentration as com-pared to the nanosized, anatase pigment in the MVassay. By comparison, the IPA test measures conduc-tion band electrons from the TiO2 particles and soshould be closest to the MV assay results. Again,rankings from different assays varied, but anatasepigments were still the most photoreactive as observedin the MV assay. This is a possible indication that theproduct mechanism for each assay provides a slightlydifferent measure of photoreactivity. However, differ-ences observed between each assay could be due to thedifferent assay matrix (buffer composition, pH, andsolvent), which influences nanoparticle to flocculationbehavior and particle cluster size. Larger clusters mayshield inner nanoparticles from UV irradiation andproduce a lower apparent photoreactivity. In a previ-ous study, we showed that the photoreactivity responsefrom TiO2 analyzed in the spectrophotometric assayschanged with experimental conditions, specificallybuffer composition and pH.21 Regardless, the NISTdatabase will contain the assay information obtainedusing the well-defined experimental conditions and theuser should be aware of what process in the photore-activity reaction each assay measures.

0 0.5 1 1.5 2 2.5 3

Photoreactivity value (arbitrary units)

A

B

C

D

E

F

G

H

Pig

men

t spe

cim

en Isopropanol to Acetone

Leuco crystal violet

Methyl viologen

Fig. 1: Photoreactivity comparison for three industrialassay methods. Note that the scale for the isopropanol toacetone test was adjusted (data points equally reduced) sothat data from all assay methods could fit on one plot.Standard uncertainty in the values is ±2%


446

EPR spectroscopy

EPR spectroscopy is capable of detecting short-livedtransient paramagnetic species (molecules with one ormore unpaired electrons), such as free radicals andseveral transition metal ions. In an EPR experiment, asample is exposed to a static magnetic field andbombarded with monochromatic microwave radiationgenerated by a klystron tube or Gunn Diode. Theabsorption of microwave radiation by a paramagneticspecies occurs at the resonance frequency, and an EPRspectrum is obtained by measuring the microwaveabsorption as a function of magnetic field strength.11,12

To extend the lifetimes of the short-lived species,experiments are conducted in liquid helium (He)(4.2 K) or liquid nitrogen (N2) (77 K) atmosphere.

Researchers have documented that trapping ofphotogenerated holes and electrons in the TiO2 latticeand on surface species can be detected using EPRspectroscopy.22–26 Spin trapping with stable free radi-cals such as nitroxide compounds has been used toindirectly detect OH• and HO2

• radicals as well as theformation of the Ti3+ species.25,26 Direct observation offree radical or ionic species on UV irradiated TiO2 hasalso been reported by a number of researchers.27–29

Finally, the reaction of various photogenerated specieswithin organic materials, such as coatings, has alsobeen studied with EPR spectroscopy.30–33 In this studyof TiO2 photoreactivity, EPR was used to study stagesin the photoreactive process.

Spin trap method

EPR detects free radicals formed in a TiO2 system and,as such, provides a measure of photoreactivity. Forshort-lived radical species, a spin trap technique is oftenapplied by mixing an appropriate spin trap or radical

trap in the sample solution. Nitroxyl radicals (or nitrox-ides) are a typical spin trap and have been reported toreact with hydroxyl radicals (which are known to initiateoxidation in most polymers1,4,6) resulting in a stableradical adduct and a loss in the initial nitroxide EPRsignal.34–37 However, the nitroxide compounds can alsoreact with other radicals present in solution, such assuperoxide anion radicals, and cause a similar decreasein the nitroxide EPR signal. Therefore, the decay of theinitial nitroxide concentration indicates a non-selectivereaction of all radicals from the TiO2 suspension andspin traps, but provides a general indication of photo-reactivity of the TiO2 system. A more selective spin trap,5,5-dimethyl-1-pyrroline-N-oxide (DMPO), is widelyused as a spin trap reagent for individual radical species.DMPO is a diamagnetic compound (no EPR signal) but

reacts with other radicals to form paramagnetic spin-

adducts, which produces an EPR signal.37,38 When

DMPO spin-adducts form, a distinctive EPR spectrumcorresponding to a specific radical is produced.

The spin trap technique was used to determine thespecies generated in UV illuminated TiO2 suspensions.The sensitivity of nitroxide radicals to the targetmolecule is related to the chemical structure of thespin trap itself as well as the environment of solu-tion.34,39 In our previous studies, the effect of electro-static interactions for a series of nitroxide spin trapswith TiO2 particles in suspension was investigated.40

Based on these results, AP (pKa » 9 and positivelycharged in aqueous solutions), whose structure isshown in Fig. 2a, was chosen to monitor the generationof radicals during irradiation for our entire series ofTiO2.

EPR spin trap measurements in this research wereconsidered in situ measurements as the various TiO2

suspensions were directly illuminated while in the EPRcavity and the resulting EPR signal was monitored as afunction of UV irradiation time. Because no suspension

0.348 0.349 0.350 0.351 0.352 0.353 0.354–40

–30

–20

–10

0

10

20

30

40

Magnetic field (Tesla)

EPR

sig

nal

3-Amino nitroxide [0.5 mmole/L]

HN

H

N O.

(a)

(b)

Fig. 2: (a) Chemical structures of spin trap, AP = 3-aminoproxyl, used in the EPR spin trap measurements; (b) typical EPRspectrum for AP


447

mixing was possible in these experiments, the concen-tration of TiO2 in the suspension was limited toprevent particle settling and the TiO2 suspensionswere equilibrated to allow for the flocculation processto stabilize. Blank tests were run to determine that theUV source did not significantly affect the EPR signal,that the EPR tubes and deionized water used in thestudy did not result in an EPR signal. Stability of thenitroxide solutions prior to their addition to the TiO2

suspension was also investigated. 500 lmol/L solutionsof nitroxyl spin trap for AP showed no EPR signaldecay for 60 min for both dark conditions and afterUV illumination. Hence, the stability of all spin trapsolutions under UV irradiation was considered accept-able for the measurement period. Any decay in theEPR intensity of the spin traps was considered to resultfrom the TiO2 reactions.

An EPR spectrum of a fresh solution of the APnitroxide spin trap without TiO2 is shown in Fig. 2b.The spectrum shows the three characteristic nitrogenhyperfine lines.34 Quantification of the intensity of theEPR spectrum, which is the first derivative of the EPRabsorption signal, was performed by double integrationof a specific EPR line. The doubly integrated intensityof an unsaturated EPR spectrum is assumed to beproportional to the concentration of free radicals insolution.41 Hence, in this study, reported EPR intensityfor most spin traps is the second-low-field line (middlepeak) after double integration.

EPR intensity decay plots were generated for thenitroxide spin traps to compare the hydroxyl genera-tion and hence the photoreactivity for the TiO2

specimens. A large decrease in the EPR intensity asa function of irradiation time indicates a highlyphotoreactive TiO2 specimen. Reproducibility of thespin trap method was tested. The concentrations ofspin trap and TiO2 in each system were kept constantfor different trials and the EPR spectrum of each TiO2/spin trap system recorded under dark conditions didnot change in intensity for 1 h. The EPR intensity ofthe spin traps decreased significantly with UV irradi-ation and all TiO2 specimens presented reproducibledecay trends. Moreover, various TiO2 pigmentsshowed different effects on spin trap AP consumptionduring UV irradiation. The EPR intensity decay plotsfor the series of TiO2 systems using the AP nitroxidetrap is shown in Fig. 3. The majority of the TiO2

specimens show a decrease in the concentration of thenitroxide trap over UV irradiation time.

Several methods were examined to develop aquantitative measure of filler photoreactivity basedon the EPR spin trap results. Clear trends wereobserved in shapes of the EPR decay curves. Ingeneral, pigmentary sized fillers displayed a lineartrend in the EPR decay curve, while nanosized fillersfollowed a polynomial progression. Due to the diver-sity in curve shape, a simple slope calculation wouldnot accurately describe the filler photoreactivity.Therefore, a method of fitting the area under thecurves was selected. Many mathematical packages

were tested for trend lines fitting in the EPR decaycurves and all packages (i.e., Excel, Origin, and otheronline packages) produced similar results. Values forthe EPR curve areas were then normalized to themaximum area possible for the limits within ourexperiments. The normalized values can then be usedto compare the photoreactivity of the fillers in thespecimen series. Table 2 provides a subset of photore-activity values determined from our fitting method forthe series of TiO2 specimens; a smaller value repre-sents a more photoreactive material. The database willcontain this information.

Solid state method

The EPR technique was also used to directly measurefree radical generation in NBMO materials using solidstate measurements at lower temperature (77 K).

0 5 10 15 20 25 30 35

UV irradiation time (min)

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

inte

nsity

APTiO2 ATiO2 GTiO2 BTiO2 JTiO2 HTiO2 FTiO2 ETiO2 ITiO2 K

Fig. 3: EPR intensity decay plots for TiO2 specimens in theAP spin trap. Standard uncertainty in the intensity values is±2%

Table 2: Spin trap method photoreactivity values for asubset of TiO2 specimens

Sample label EPR curve area Normalized curve value

A 323.9 0.090B 1532.9 0.426C 3628.4 1.008E 3376.4 0.938F 3617.7 1.005G 923.6 0.257J 3216.9 0.894K 1146.3 0.318

Areas calculated under EPR decay curves were normalizedfor specimen comparison. The uncertainty from the pro-cessing of the digitized spectra in the calculated area valuesis ±2%


448

Signal intensity in an EPR spectrum is correlated to theconcentration of unpaired electrons in the sample. Theposition of the EPR signal or g-factor can indicate thetype of unpaired electron; that is, if it is organic orinorganic in nature. The g-factor, g, is defined as

g ¼ hm=lBB0 ð3Þ

where h is Planck’s constant, m is the frequency, lB is theBohr magnetron, and B0 is the external magnetic field.The hyperfine interactions observed by the hyperfinestructure within an EPR spectrum can reveal themolecular structure and environment near theunpaired electron. The linewidth and/or lineshape ofthe EPR spectrum can also provide information aboutthe molecular motion in a sample with the unpairedelectrons.

EPR spectra of the TiO2 series were examined at77 K before and after UV irradiation. The g factors forthe EPR instrument used in this study were calibratedusing a standard reference compound, 2,2-diphenyl-1-picrylhydrazyl (DPPH), which has a known g-factorvalue of 2.00037.24 Figure 4 illustrates the change inthe EPR signal for the TiO2 A. An increase in the EPRintensity at g = 2.014 and g = 1.979 is observed withincreasing UV irradiation time. After the UV source isturned off, the EPR intensity of the peak at g = 2.014decreased more rapidly than the EPR peak atg = 1.979. The EPR peak at g = 2.014 has beenreported to be a component of the hole-centersproduced near the TiO2 surface.42,43 The EPR signallocated at g-factor = 1.979 has been assigned as aspecies related to the electron-trapping sites, whichis usually represented as a Ti3+ group on TiO2

particles.44,45 Figure 4 suggests that electron-trappingsites are more robust and longer lasting than the hole-centers for TiO2 A. Figure 5 shows EPR spectra for aseries of TiO2 specimens after UV irradiation. EachTiO2 specimen has a different EPR spectrum, how-ever, all have EPR peaks in the positions where hole-centers and electron-trapping sites are located. Most ofthe anatase specimens have more complex EPR peaksin the hole-center region. The lineshape and linewidthof the EPR spectra should reveal information aboutthe solid state physics of the photoreactivity for theTiO2 samples and how it differs for the variousmanufacturing processes. For example, the width ofthe hole-center EPR peak is sharper for some TiO2

systems, particularly TiO2 J, than others. This couldindicate possible neighboring nuclei and slower relax-ation times for the electrons in TiO2 J. The electron-trapping sites for TiO2 G are markedly different thanthose observed on the other TiO2 systems. The peak islocated at a higher g-factor, g = 1.988 vs. g = 1.969,indicating a different spin state and coordinationenvironment near the unpaired electron. The widthof the peak is also broader, an indication of rapidlyrelaxing electrons.

Research efforts are underway to provide a moredetailed and precise interpretation of these EPRspectra. This includes fitting the spectra to quantifythe amounts of electrons and hole species generatedunder UV irradiation. Furthermore, experiments in-clude monitoring concentration changes in speciesafter the UV source is removed to understand thelifetimes of species. The database will include the solidstate spectra and species fitting results.

0.325 0.330 0.335 0.340 0.345 0.350 0.355


EPR

sig

nal (

arbi

trar

y un

its)

UV off 48-min

UV off 3-min

10-min UV

2-min UV

before UV

g = 1.979

g = 2.014

Fig. 4: EPR spectra for TiO2 A as a function of UV irradiationtime. Standard uncertainty in peak positions due to spectraresolution is ±1%

0.320 0.325 0.330 0.335 0.340 0.345 0.350


EPR

sig

nal (

arbi

trar

y un

its)

TiO2 H

TiO2 F

TiO2 E

TiO2 A

TiO2 J

TiO2 G

TiO2 K

g = 2.059

g = 2.016

g = 2.012 g = 2.002

g = 1.947g = 1.969

g = 1.988 g = 1.958

x1/50

Fig. 5: EPR spectra for a series of TiO2 specimens after UVirradiation. The EPR spectrum for TiO2 K is reduced by 1/50to achieve the same scale as other TiO2 specimens.Standard uncertainty in peak positions due to spectraresolution is ±1%


449

Photoreactivity database

A database of TiO2 photoreactivity data measured viaindustrial assays and EPR methods, select spin trapsand solid state, is being compiled and will be accessiblethrough a searchable software database in the NISTStandard Reference Database—http://www.nist.gov/srd/index.cfm. The studied series of TiO2 specimenswas obtained from current US pigment manufacturers.NIST requires that all databases contain well-docu-mented numerical data, which is taken from literatureor measured directly, assessed for reliability, andevaluated. To comply with these NIST requirements,the following procedures were employed. All photore-activity data was measured in our laboratories usingthe protocol previously described in this paper. EPRdata was evaluated through a literature review of EPRdata using peer-reviewed journals for instrument cal-ibration, solid state spectra of metal oxides, and spintrap experiments on various systems. EPR experimentswere controlled by following daily EPR instrumentcalibration and well-defined sample preparation andmeasuring replicates of all data for reproducibility andaccuracy. Details of the EPR measurement protocolswill be provided in a future publication. Evaluation ofindustrial assay methods was performed using proto-cols published in patents and peer-reviewed journalsand all experiments employed calibration of instru-ments/equipment (UV–Vis spectrometer and Hg lamp)and well-defined protocols for solution preparation.Replicates of all assay data were also performed forreproducibility and accuracy. The NIST StandardReference Databases are generally free, but sometimesrequire a subscription depending on the extent of data.This photoreactivity database will have free access sothat the information can be utilized for industrialproduct selection and development purposes.

Conclusions

Photoreactivity measurements of TiO2 using industrialassays and EPR spectroscopy were presented. Indus-trial assay methods are most commonly utilized andappear to be useful for measuring and ranking TiO2

pigment photoreactivity, but the reaction mechanisminvolved in an assay must be taken into account toaccurately rank the pigments for photoreactivity.Industrial assay results are also highly susceptible toa large number of variables that historically haveneither been controlled nor measured; the assay matrix(buffer composition and filler concentration) appearsto be able to affect TiO2 particle cluster size, which inturn may influence photoreactivity values. EPRspectroscopy has proven to be a powerful techniquefor measuring small concentrations of free radicals byuse of spin trap methods or directly using low temper-ature, solid state EPR measurements. When optimized,the spin trap method can provide a sensitive, repro-ducible, and repeatable measure for a wide range of

fillers with varying photoreactivity. Solid state EPRmeasurements on fillers have the potential to directlycompare free radical generation and solid state physicsproperties over a range of filler photoreactivity. EPR iscapable of detecting the type of molecular structureand the environment surrounding unpaired electrons.Such information would be useful in characterizing thetype of hole-centers and electron-trapping sites foundin each type of filler.

A database of filler (pigmentary-size and nanopar-ticle TiO2) photoreactivity measured using conven-tional industrial assays and EPR (select spin traps andsolid state) methods is being compiled and will beaccessed through a searchable software database in theNIST Standard Reference Database program—http://www.nist.gov/srd/index.cfm. The results in the data-base will provide critical knowledge to industrial users,including the linkage of current photoreactivity meth-ods (industrial assays) to fundamental photoreactivitymeasurements from EPR methods, and hence, enablemore reliable assessments of end-use performance.

References

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pigment and nanofiller photoreactivity database

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