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Journal of Photochemistry & Photobiology A: Chemistry

journal homepage: www.elsevier.com/locate/jphotochem

Determination of the crystallinity of TiO2 photocatalysts

Marianna Bellarditaa,⁎, Agatino Di Paolaa, Bartolomeo Megnab, Leonardo Palmisanoa

a “Schiavello-Grillone” Photocatalysis Group, Dipartimento di Energia, Ingegneria dell’informazione, e modelli Matematici (DEIM), University of Palermo, Viale delleScienze, 90128 Palermo, ItalybDipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali (DICAM), University of Palermo, Viale delle Scienze, 90128 Palermo, Italy

A R T I C L E I N F O

Keywords:PhotocatalysisCrystallinityAnataseRutileXRD

A B S T R A C T

This study reports a new simple method to determine the crystallinity of anatase and rutile TiO2 photocatalysts.The crystallinity degree of various anatase and rutile samples was estimated by XRD analysis from the ratioamong the full width at half maximum intensities of the main diffraction peaks of anatase or rutile and the (111)peak of CaF2 as internal standard. The photocatalytic activity of selected powders was tested employing thephotodegradation of 4-nitrophenol in aqueous solution as probe reaction. The results showed that the photo-activity of the investigated samples increased with increasing crystallinity and crystallite size.

1. Introduction

Heterogeneous photocatalysis is an attractive low-temperature andpressure technology for its promising applications in the field of solarenergy, green chemistry and environmental remediation [1–5]. TiO2 isthe most studied photocatalyst due to its low cost, chemical stability,natural abundance, non toxicity and applicative versatility [6]. Underambient conditions, titanium dioxide exists in various polymorphs, ofwhich anatase, rutile, and brookite are the most common. Anatase isgenerally the most efficient photocatalyst to degrade organic and in-organic pollutants both in vapour and in liquid phases [7–9] whereasrutile and brookite are more selective for photocatalytic organicsyntheses [10–12].

The photocatalytic properties of TiO2 are influenced by many phy-sico-chemical variables such as particle size, surface area, pore volume,surface hydroxyls content and crystallinity degree. In particular, a highcrystallinity is considered an important parameter for a high photo-activity [13–15] since the presence of amorphous phase favours therecombination of photoexcited electrons and holes at defects located onthe surface and in the bulk of the particles [16].

Ohtani et al. [16] were the first to evaluate the weight fraction ofcrystalline anatase in amorphous-anatase mixtures by means of differ-ential scanning calorimetry (DSC) and powder X-ray diffraction (XRD)measurements. The results gave essential and important information forthe design and synthesis of highly active semiconductor photocatalysts.Zhang et al. [17] estimated the amorphous content of TiO2 nano-powders prepared by TiCl4 hydrolysis comparing the peak areas of theXRD patterns with those of the anatase or rutile samples obtained by

calcination at 600 °C.Bertoni et al. [18] quantified the crystalline and amorphous con-

tents of porous TiO2 samples by electron energy loss spectroscopy.Recently, the weight percentage of the amorphous phase of variousTiO2 samples was calculated using reference intensity ratios (RIRs) forthe X-ray diffraction patterns measured with crystalline silicon ad-ditives [19]. The quantitative analysis of the amorphous phase contentallowed to determine the impact on the photocatalytic activity of dif-ferent TiO2 samples, before and after crystallization processing.

Tobaldi et al. [20] calculated the full phase composition (bothcrystalline and amorphous content) of the commercial Evonik TiO2 P25sample with the Rietveld-RIR method. The results were in contrast withmany previously reported, stating that P25 did not have any noticeableamount of amorphous phase.

The crystallinity of anatase samples has been qualitatively estimatedby the full width at half maximum (FWHM) [21–23] or the intensity ofthe (main) (101) XRD peak [24–26]. Akurati et al. [27] calculated therelative degree of crystallinity (RDOC) of anatase nanoparticles bytaking into account both FWHM and reflection intensity. The RDOC (Γ)was determined by the ratio of intensity (h) times FWHM (β) of thepowders to a sub-micron commercial anatase powder, assumed asstandard (s):

= ⎡⎣⎢

× ⎤⎦⎥

Γ hβO

Oh β

s

s s (1)

The ratio was normalized by the background signal, taken as offset (O)of the XRD patterns from the 2θ axis.

The fraction of crystalline anatase contained in samples prepared by

https://doi.org/10.1016/j.jphotochem.2018.08.042Received 22 May 2018; Received in revised form 26 August 2018; Accepted 27 August 2018

⁎ Corresponding author.E-mail address: marianna.bellardita@unipa.it (M. Bellardita).

Journal of Photochemistry & Photobiology A: Chemistry 367 (2018) 312–320

Available online 29 August 20181010-6030/ © 2018 Elsevier B.V. All rights reserved.

T

different routes has been often evaluated with respect to the integralintensity of the (101) diffraction peak of selected samples assumed as100% crystalline [28–30].

Recent papers have reported quantitative analyses of the crystal-linity of TiO2 powders [31–40]. In particular, the absolute crystallinityof anatase and rutile was calculated according to a theoretical methodproposed by Jensen et al. [31] while a different approach, based on theXRD patterns of natural crystals, was followed for brookite [36].

The aim of the present study was to develop an experimentalmethod to determine the crystallinity of unknown TiO2 samples byusing natural crystals of anatase and rutile as references. Photocatalyticexperiments were carried out to verify if the photoactivity of variousTiO2 samples could be correlated to their crystallinity calculated withthe new method. The photodegradation of 4-nitrophenol in aqueousmedium was used as probe reaction.

2. Experimental

2.1. Materials and instruments

The natural anatase and rutile samples were fragments of crystalscoming from Minas Gerais (Brazil) and Graves Mountain, Georgia(USA), respectively (Fig. 1). Home-made anatase powder was preparedas follows: 20 g of TiOSO4 (Sigma-Aldrich) were added to 90mL ofdeionized water. After ca. 2 h of continuous stirring at room tempera-ture, a clear solution was obtained. This solution was heated in a closedbottle and aged at 100 °C in an oven for 48 h. The resultant precipitatewas washed many times to eliminate most of the sulfate ions by with-drawing the supernatant liquid and by adding pure water to restore theinitial volume. The solid (named HPA) was recovered using a rotaryevaporator at 60 °C for 6 h. To verify the presence of residual adsorbedwater, HPA sample was subjected to a thermal gravimetric analysis(TGA) by using a Perkin Elmer STA 6000 instrument, in the 25–300 °Crange. The amount of water determined was 1.25 w/w %.

Two commercial available anatase samples, Merck (> 99%) andHombicat UV 100 (approx. 99%), were also used.

Home-made rutile powder was prepared by adding 10mL of TiCl4(Sigma-Aldrich) to 40mL of distilled water at room temperature. Theclear solution obtained after ca. 15min of stirring, was transferred intoa Teflon-lined stainless steel autoclave and heated at 100 °C for 48 h.The resultant solid was washed in water and dried under vacuum at60 °C for 6 h and indicated as HPR.

X-ray diffraction patterns of the powders were recorded at roomtemperature on a PANalytical Empyrean diffractometer equipped witha PIXcel1D (tm) detector using the CuKɑ radiation and a 2θ scan rate of1.28min−1. A Ni filter was mounted at the exit of the X-ray source. The

operating conditions were 40 V and 40mA (maximum power suggestedby the supplier) in order to achieve the highest signal to noise ratio. Nobase line correction or smoothing have been performed. All the calcu-lations were based on the raw data. The base line of every single peakwas assumed as linear.

The crystallite sizes of anatase and rutile were calculated by meansof the Scherrer equation [41]:

=D kλβcosθ (2)

where β is the line broadening ( = −β β βexp2

s2 ) where βexp and βs are the

measured breadths at half-maximum intensity of the XRD peaks of thesample and of a standard, respectively), k is the Scherrer constant(k= 0.9), λ is the radiation wavelength (λ=0.154056 nm) and θ is theangle associated to the (101) or (110) peaks of anatase or rutile. A100% crystalline CaF2 (βs = 0.162°) was used as internal standard.

The crystallinity of the TiO2 samples was evaluated with respect toCaF2. Mixtures of TiO2 and CaF2 with a weight ratio of 50% wereprepared by weighing 0.5 g of TiO2 and 0.5 g of CaF2 (Sigma-Aldrich,99.9%) with an analytical balance (accuracy = ± 0.1mg). The pow-ders were mixed and finely milled in a mortar. The specimens for dif-fractometry were prepared by lightly pressing ca. 100mg of powderinto a Plexiglas holder.

The specific surface areas were determined with a MicromeriticsASAP™ 2020 apparatus by using the five-points BET method. Ramanspectra were obtained by means of a Renishaw inVia RamanMicroscope equipped with a 533 nm laser coupled with a 2400 lines/mm grid. The maximum laser power on the sample was around110mW. The power on the sample was adjusted by means of holo-graphic filters ranging from 1 to 10%.The laser was focused on thesample with a 50x LWD lens that coupled with a 10x magnification lensled to a 2 μm analyzed spot.

Scanning electron microscopy (SEM) observations were performedon the samples after deposition by sputtering of a thin gold layer, usinga FEI Quanta 200 SEM microscope operating in high vacuum at 20 kV.TEM analyses were performed by means a Tecnai G2 transmissionelectron microscope, operating at 200 kV. Samples were prepared bysuspending ca. 1 g L−1 of powder in 2-propanol, treating by ultrasoundsfor 5min, and dropping 2 μL on a Forvar/Carbon 300-mesh Cu grid.

2.2. Photoreactivity experiments

A 0.5 L Pyrex batch reactor of cylindrical shape was used.0.6 mgmL−1 of catalyst were suspended in an aqueous solution con-taining 20mg L−1 of 4-nitrophenol. The light source was a 125W

Fig. 1. Photos of natural (a) anatase and (b) rutile crystals.

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medium pressure Hg lamp (Helios Italquartz, Italy) axially positionedwithin the photoreactor. The lamp emitted across the entire spectrum ofthe ultraviolet light with a maximum emission peak at 366 nm. Thetemperature of the suspension was controlled by circulation of waterthrough a Pyrex thimble surrounding the lamp. The photon flux emittedby the lamp was Φi = 11mW cm−2. O2 was continuously bubbled forca. 0.5 h before switching on the lamp and throughout the occurrenceof the photoreactivity experiments. The decomposition of 4-nitrophenolwas followed by determining the concentration of the substrate atvarious time intervals. Samples of 5mL were withdrawn with a syringe,and the catalyst was separated from the solution by filtration through0.2 μm Teflon membranes (Whatman). The quantitative determinationof 4-nitrophenol was performed by measuring its maximum absorptionat 315 nm. The degradation rate, r0, was calculated from the initialslope of the concentration versus time profiles.

3. Results and discussion

Figs. 2 and 3 show the X-ray diffraction patterns of the natural andcommercial samples. The diffraction data of natural anatase (Fig. 2a)and rutile (Fig. 2b) exactly corresponded to those reported in the JCPDScard Nos. 21–1272 and 21–1276, respectively. All the peaks were at-tributed to anatase or rutile with the exception of a small peak at2θ=26.54° attributable to quartz that likely contaminated the anatasesample during the separation of the crystals. The peaks of anatase orrutile were very sharp and intense, confirming that the natural sampleswere very crystalline. Both the commercial Hombicat UV100 (Fig. 3a)and Merck (Fig. 3b) samples consisted of pure anatase but the peaks ofthe former were quite broad indicating that this material was nano-crystalline or only partly crystalline.

Jensen et al. [31] calculated the absolute crystallinity of anatasepowders by using the theoretical ratio between the area of the (101)diffraction peak of a 100% crystalline anatase sample and the area ofthe (220) peak of 100% crystalline CaF2 in a 50% weight ratio mixture:

=AA

1.25(100% crystalline anatase), 101

(100% crystalline CaF ), 2202 (3)

The crystallinity of an unknown anatase sample was determined bycomparing the ratio between the area of the (101) peak of the partlycrystalline anatase sample and the area of the (220) peak of 100%

crystalline CaF2 to 1.25 [31]:

= × ×Absolute crystallinity of anatase 11.25

AA

100%( anatase), 101

(100% crystalline CaF ), 2202

(4)

A similar method can be applied to determine the absolute crystallinityof rutile powders [31] by using the ratio between the area of the (110)peak of a 100% crystalline rutile sample and the area of the (220) peakof 100% crystalline CaF2:

=AA

0.90crystalline rutile

crystalline CaF

(100% ), 110

(100% ), 2202 (5)

= × ×Absolute crystallinity of rutile 10.90

AA

100%( rutile), 110

(100% crystalline CaF ), 2202

(6)

Wang et al. [35] employed the same method to calculate the crys-tallinity of anatase nanoparticles but, differently, they compared thearea of the (101) peak of anatase with the area of the (111) peak of CaF2rather than with the (220) peak. The corresponding equations were:

=AA

1.31(100% crystalline anatase), 101

(100% crystalline CaF ), 1112 (7)

= × ×Absolute crystallinity of anatase 11.31

AA

100%( anatase), 101

(100% crystalline CaF ), 1112

(8)

Fig. 4 shows the XRD patterns of the mixtures obtained by mixingCaF2 and the commercial anatase TiO2 samples with a 50% weightratio. By comparing the values obtained from the ratios between theareas of the anatase (101) peaks and the areas of the (220) or (111)peaks of CaF2 to 1.25 or 1.31, respectively, it was possible to evaluatethe absolute crystallinity of the anatase powders. The data extractedfrom the XRD patterns are shown in Table 1.

The crystallinity values calculated for the Merck sample werepractically the same even if the peaks of CaF2 selected as referenceswere different. The crystallinity percentage (69%) of the UV100 samplecalculated by using the (220) peak of CaF2 was in agreement with thevalue (67%) reported by Jensen et al. [31] but noticeably far from the

Fig. 2. XRD patterns of the natural samples: (a) anatase, (b) rutile. Fig. 3. XRD patterns of the commercial samples: (a) Hombicat UV 100, (b)Merck.

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value (89%) determined by the comparison with the (111) peak ofCaF2.

As shown in Table 1, both methods proposed for obtaining the ab-solute crystallinity of anatase samples are not applicable to the mineralsample because the areas of the main peaks exhibited by the dif-fractogram of the natural crystals are lower than those of the leastcrystalline commercial samples. Consequently, the crystallinity degreeof the mineral sample would result only 62% or 67%, respectively.

Similar results were obtained by applying the method of Jensenet al. [31] to the natural rutile sample. Table 2 shows the crystallinitydegree calculated by comparing the value obtained from the ratio be-tween the area of the rutile (110) peak and the area of the (220) peak ofCaF2 to 0.90. An unlikely value of 21% was calculated for the crystal-linity of the natural rutile sample.

The low values of absolute crystallinity derived from the XRD pat-terns of highly crystalline natural crystals make unreliable the evalua-tion of the crystallinity of unknown anatase or rutile samples by com-parison of the areas of the main diffraction peaks of anatase or rutilewith the area of the (220) or (111) peak of 100% crystalline CaF2. Thesame difficulty was found by Bellardita et al. [36] to estimate the ab-solute crystallinity of brookite samples.

Inagaki et al. [21–23] found a correlation between the crystallinity

of anatase powders annealed at different temperatures and their pho-tocatalytic performances for the decomposition of methylene blue. Thecrystallinity was qualitatively evaluated from the full width at halfmaximum (FWHM) of the X-ray diffraction lines. In particular, the mostcrystalline anatase samples exhibited the lowest values of FWHM.

A perfect crystal would extend infinitely in all directions but nocrystals are perfect due to their finite size. The FWHM should not de-pend upon the material but the deviation from perfect crystallinityleads to a broadening of the diffusion peaks. The FHWM is related to theinstrumental broadening, crystallite size and non-uniform strain. Thelattice strain arises from grain boundaries, lack of long-range order andpoint defects, that characterize the amorphous part of a sample.Ultimately, the FWHM of a diffraction peak can be used as a practicalparameter to determine the crystallinity of a material.

Fig. 5 shows the XRD patterns of home-prepared anatase samplessynthesized from TiOSO4 and calcined at 300, 450, 600 and 700 °C for2 h. The samples were referred to as HPA, HPA 300 °C, HPA 450 °C,HPA 600 °C, and HPA 700 °C, respectively. The peaks of the home-prepared samples were broad and corresponded to those of anatase. Thepeaks became sharper with increasing temperature and no trace ofdiffraction peaks of rutile was detected. The increased peak intensitieswith temperature is attributable to the crystallization of amorphousTiO2 to anatase crystallites. The values of FWHM continuously de-creased with increasing temperature and crystallinity of the samples.

A quantitative estimation of the crystallinity of partly crystallineanatase samples can be obtained by the XRD patterns using the FWHMof the (101) peak rather than the area as suggested by Jensen et al. [31]It is worth noting that the value of a XRD peak area is more affected byminor mistakes during the XRD powder sample preparation, e.g. aweight ratio between CaF2 and anatase slightly far from 50%, than thefull width at half maximum and moreover the latter shows a higherreproducibility.

The ratio between the FWHM value of the (111) peak of CaF2 andthe FWHM value of the (101) peak of the crystalline natural anatasesample was taken as the reference. The measurements were repeated

Fig. 4. XRD patterns of the commercial anatase samples mixed with CaF2 in a50% wt ratio: (a) Hombicat UV 100, (b) Merck.

Table 1Data extracted from the XRD patterns of the various anatase samples.

Sample × × 100%AanataseACaF

11.25

, 101

2, 220a 1

1.31× × 100%

AanataseACaF

, 101

2, 111b × × 100%

FWHMCaFFWHManatase

11.15

2, 111, 101

c

Merck 81 80 79Hombikat UV 100 69 89 15Natural anatase 62 67 100HPA 36 39 21HPA 300 °C 33 35 22HPA 450 °C 36 39 23HPA 600 °C 43 46 29HPA 700 °C 46 50 42

a Crystallinity of anatase calculated according to Jensen et al. [31].b Crystallinity of anatase calculated according to Wang et al. [35].c Crystallinity of anatase calculated according to the present work. Deviation± 1.

Table 2Data extracted from the XRD patterns of the various rutile samples.

Sample 10.90

× × 100%A rutileACaF( ), 110

2, 220a × × 100%

FWHMCaFFWHMrutile

10.98

2, 111, 110

b

Natural rutile 21 100HPR 8 30HPR 300 °C 16 37HPR 450 °C 24 56HPR 600 °C 21 59HPR 700 °C 46 73

a Crystallinity of rutile calculated according to Jensen et al. [31].b Crystallinity of rutile calculated according to the present work.

Deviation±1.

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three times (the accuracy of each measure was verified by the constancyof the FWHM values for the two samples). The error of each measure-ment was±0.01° and the following value was obtained:

= =FWHM

FWHM0.1620.141

1.15CaF , 111

anatase, 101

2

(9)

In order to reduce the influence of the incident angle of the di-vergent X-ray beam, the (111) peak of CaF2 was selected as referencerather than the (220) peak since the former is near to the (101) peak ofanatase used to calculate the ratio of the FWHM values. By comparingthe values obtained from the ratios between the FWHM values of the(111) peak of CaF2 and the (101) peak of the various samples to 1.15, itwas possible to evaluate the crystallinity of the anatase powders:

= × ×Crystallinity of anatase 11.15

FWHMFWHM

100%CaF , 111

anatase, 101

2

(10)

Table 1 reports the values of the crystallinity degrees of the varioussamples, calculated following the three different methods. The crys-tallinity percentages of the Merck sample were very close to each otherwhereas significant differences were found for Hombikat UV 100. Inparticular, the crystallinity value of Hombikat UV 100 determined byusing the FWHM intensity of the (101) peak was noticeably lower withrespect to the values estimated by using the area of the same peak. It isworth noting that a crystallinity percentage of 16% is much morecompatible with the small broad peaks of the XRD pattern of HombicatUV 100 reported in Fig. 3 rather than the values of 69 or 89%. Thehome-made sample showed a low value of crystallinity that increasedwith the temperature reaching the maximum by annealing at 700 °C.

Fig. 6 shows the Raman spectra of the natural and commercialanatase powders. The spectra of the natural and Merck samples showed5 peaks corresponding to one A1g (515 cm−1), one B1g (395 cm−1) andthree Eg (144, 195, 638 cm−1) active optical modes.42 The frequenciesof the Raman shifts match well with those reported for natural andsynthetic anatase crystals [42–44]. The peaks were significantly moreintense than those of Hombicat UV 100 confirming that this sample was

poorly crystalline as already suggested by the XRD results.Fig. 7 shows a comparison among the Raman spectra of Hombicat

UV 100 and two selected home-prepared anatase samples. The spec-trum of the commercial sample revealed both broadening and shifts ofthe Raman bands with respect to those of the most crystalline HPA700 °C sample. The Raman shifts are attributable to the effects of de-creasing particle size on the force constants and vibrational amplitudesof the nearest neighbor bonds [45]. These findings are in agreementwith the low crystallite size (9 nm) and crystallinity (16%) of HombicatUV 100 estimated from the FWHM value of the (101) X-ray diffractionpeaks.

Fig. 8 shows the XRD patterns of home-prepared rutile samplessynthesized from TiCl4 and calcined at various temperatures for 2 h.The samples were referred to as HPR, HPR 300 °C, HPR 450 °C, HPR600 °C, and HPR 700 °C, respectively. The peaks of the various sampleswere broad and corresponded to those of rutile. The intensities of thepeaks increased with increasing the calcination temperature. Table 2shows the crystallinity degrees obtained by comparing the values ob-tained from the ratio between the area of the rutile (110) peak and thearea of the (220) peak of CaF2 to 0.90.

A more reliable estimation of the crystallinity of the rutile samplescan be obtained by using the FWHM of the (110) rutile peak rather thanthe area. The ratio between the FWHM value of the (111) peak of CaF2and the FWHM value of the (110) peak of the natural rutile was taken asthe reference:

Fig. 5. XRD patterns of the samples obtained by calcination of HPA at differenttemperatures for 2 h: (a) as-prepared, (b) 300 °C, (c) 450 °C, (d) 600 °C, (e)700 °C.

Fig. 6. Raman spectra of: (a) natural anatase, (b) Merck, (c) Hombicat UV 100.Beware of the different intensity scaling of the spectrum of Hombicat UV 100.

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= =FWHMFWHM

0.1620.165

0.98CaF , 111

rutile, 110

2

(11)

The crystallinity of the rutile samples was calculated by comparingthe values obtained from the ratios between the FWHM values of the(111) peak of CaF2 and the (110) peak of the various samples to 0.98:

= × ×Crystallinity of rutile 10.98

FWHMFWHM

100%CaF , 111

rutile, 110

2

(12)

As shown in Table 2, the crystallinity of the home-prepared samplesincreased with the temperature and the percentage of crystallinityreached 73% after 2 h of annealing at 700 °C.

Jensen et al. [31–33] also estimated the crystallinity of P25 TiO2,that is one of the most active commercially available photocatalysts.

P25 consists of both the anatase phase and the rutile phase but nobodyknows the exact crystalline composition that is not provided by thesuppliers (Degussa or Evonik) [46]. The percentages of anatase andrutile are usually obtained by means of the Spurr and Myers equation[47]:

x= 1/[1+ 1.26 (IR/IA)] (13)

where x is the weight fraction of anatase in the powders, while IA and IRcorrespond to the areas of the anatase (101) and rutile (110) XRDpeaks, respectively. The typical reported compositions range between70–80% anatase and 20–30% rutile. [14,48–51]

The pioneering study of Bickley et al. [48] revealed that DegussaP25 is a multiphasic material consisting of an amorphous state, togetherwith the crystalline phases anatase and rutile in the approximate pro-portions 80/20. Jensen et al. [31] reported that P25 consisted of 72.6%anatase, 18.4% rutile and 9% amorphous TiO2 although in a successivepublication [33] the crystalline composition of P25was evaluated to be75.6% anatase and 21.6% rutile. The remaining 2.8% was attributed toan amorphous fraction of the TiO2 or to the uncertainty of the method.

Ohtani et al. [46] evaluated the typical crystalline composition ofP25 to be 78% anatase and 14% rutile. Assuming the remaining 8% partcorresponded to amorphous phase, the anatase–rutile–amorphous ratiowas determined to be 78:14:8. Tobaldi et al. [20] estimated thatP25 was composed of 76.3 wt% anatase, 10.6 wt% rutile and 13.0 wt%amorphous phase. Recently, Lebedev et al. [19] calculated that thecrystalline part of P25 consisted of 86 ± 2% anatase and 14 ± 2%rutile while the amorphous part was 14 ± 1%.

The results described above suggest that P25 is a mixture of anatase,rutile and amorphous TiO2 with different ratios depending on theproduction batch. [46] The description of the exact crystalline com-position of P25 does not make sense since it depends on the samplestored in own laboratory.

The method proposed by Jensen et al. [31] should allow to de-termine the absolute crystallinity of TiO2 powders and consequently toevaluate the amorphous part of the samples. In particular, the percen-tage of amorphous phase should be determined as the complement tothe fraction of crystalline material in the sample. Anyway, it is not clearif the values of crystallinity reported by Jensen et al. [31–33] for P25represent the weight percentages of anatase and rutile estimated by the

Fig. 8. XRD patterns of the samples obtained by calcination of HPR at differenttemperatures for 2 h: (a) as-prepared, (b) 300 °C, (c) 450 °C, (d) 600 °C, (e)700 °C.

Fig. 9. XRD patterns of the commercial P25 samples: (a) Evonik, (b) Degussa.

Fig. 7. Raman spectra of partly crystalline anatase samples: (a) Hombicat UV100, (b) HPA, (c) HPA 700 °C.

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areas of their main peaks or the absolute crystallinities calculated byusing the Eqs. (3) and (5), respectively.

Fig. 9 shows the XRD patterns of two different P25 samples suppliedby Degussa and Evonik, respectively. As far as the Degussa sample, thepercentages of anatase and rutile (85% anatase, 15% rutile) calculatedby means of the Spurr and Myers equation very closely matched withthose (86 ± 2% anatase, 14 ± 2% rutile) reported by Lebedev et al.[19] Instead, the estimated composition of the Evonik sample (93%anatase, 7% rutile) revealed much less rutile.

In Table 3 are shown data extracted from the XRD patterns of Fig. 9.The values of crystallinity of anatase and rutile calculated for the twoP25 samples by following the method of Jensen et al. [31] are verydifferent from each other whereas the crystallinity percentages

evaluated by using the FWHM values are quite similar. It should benoted that the areas of the peaks of anatase and rutile are proportionalto their weight percentages so that using the areas to determine thecrystallinity of each polymorph is misleading when a mixture with aconstant amount of CaF2 (50%) is used. According to the method pro-posed by Jensen et al. [31], the most abundant phase, i.e. anatase, willexhibit always the highest crystallinity. On the contrary, by using themethod proposed in this work, rutile would be the most crystallinephase. These findings suggest that the calculations of the crystallinityshould be applied only to materials consisting of a single phase but notto mixtures of different crystalline phases as P25.

Photoreactivity experiments were carried out to find a correlationbetween photoactivity and structural characteristics of some selectedTiO2 samples. The photoactivity of the powders was investigated byutilizing the degradation of an aqueous solution of 4-nitrophenol as aprobe reaction [52,53]. As shown in Fig. 10, all the samples were activefor the photodegradation of the substrate. The photoactivity decreasedaccording to the following sequence: Degussa P25>Merck>HPA700 °C>HPA>Hombicat UV 100.

Table 4 shows a comparison among the degradation rates of 4-ni-trophenol and some physical features of the samples. The crystallitesizes of the various samples, calculate by means of the widely usedScherrer equation, are in good agreement with the mean dimensions ofthe particles deduced from SEM and TEM images.

The photoactivity of the powders consisting only of anatase de-creased with increasing the surface area but increased with increasingcrystallinity and crystallite size. The scarce performance exhibited bythe Hombicat UV 100 sample is in agreement with the low value ofcrystallinity estimated from the FWHM value of the (101) peak ofanatase rather than from its area.

The superior photoreactivity of Degussa P25 is probably due both tothe low amount of amorphous material and to the contemporaneouspresence of anatase and rutile that may enhance the charge separationby transfer of electrons from the conduction band of anatase to that ofrutile [48] or from rutile to anatase trapping sites lower in energy thanthe anatase conduction band [54,55].

The similar photoreactivity values of Merck and HPA 700 °C are notsurprising because the photocatalytic activity does not depend only on

Table 4Characteristics of various TiO2 samples.

sample phasea surface area(m2 g −1)

crystallite size(nm)

crystallinity%

r0×109b

(mol L−1 s −1)

Merck A 10 110.5 79 38.3Hombikat UV 100 A 316 9.1 15 5.8HPA A 181 12.6 21 9.4HPA 300 °C A 163 12.7 22 7.6HPA 450 °C A 101 13.9 23 17.9HPA 600 °C A 60 17.8 29 24.7HPA 700 °C A 29 27.7 42 36.6Degussa P25 A, R 50 20 (A) 31 (R) – 45.4

a A, anatase; R, rutile.b Initial 4-nitrophenol photodegradation rate.

Fig. 10. Photodegradation of 4-nitrophenol with various TiO2 samples.

Table 3Data extracted from the XRD patterns of the P25 samples.

sample 11.25

× × 100%AanataseACaF

, 101

2, 220a 1

0.90× × 100%

A rutileACaF( ), 110

2, 220b

× × 100%FWHMCaF

FWHManatase1

1.152, 111

, 101c × × 100%

FWHMCaFFWHMrutile

10.98

2, 111, 110

d

Degussa P25 94 18 37 62Evonik P25 74 6 34 65

a Crystallinity of anatase calculated according to Jensen et al. [31].b Crystallinity of rutile calculated according to Jensen et al. [31].c Crystallinity of anatase calculated according to the present work.d Crystallinity of rutile calculated according to the present work.

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the crystallinity but on many other parameters, as specific surface area,hydroxylation degree, surface acid and basic centers amount, zerocharge point and the two samples have been prepared by differentmethods.

4. Conclusions

Crystallinity is believed to be one of the most important propertiesregarding the photocatalytic activity but only few studies have reporteda quantitative measurements of the crystallinity degree of TiO2 samples.Incorrect results were obtained when the methods proposed to de-termine the absolute crystallinity were applied to very crystalline ma-terials as natural crystals of anatase and rutile. This study has shownthat more reliable results can be achieved using a simple quantitativemethod that was successfully employed to determine the crystallinity ofpartly crystalline brookite photocatalysts. The proposed method isbased on the idea that the crystallinity of a sample is strictly related tothe experimental values of FWHM. The method is empirical but pro-vides coherent data that agree with our experimental XRD patterns.Natural anatase and rutile crystals were arbitrarily selected as touch-stones assuming they were 100% crystalline.

Photoreactivity results confirmed the importance of quantitativemeasurements of the crystallinity degree of the various samples. A goodcorrelation between photocatalytic activity and crystallinity was foundwith samples obtained by annealing a home-prepared anatase sample atdifferent temperatures.

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