Flame-made visible light active TiO2:Cr photocatalysts: Correlation between structural, optical and photocatalytic properties

Catalysis Today 209 (2013) 47–53

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Flame-made visible light active TiO2 :Cr photocatalysts: Correlation between structural, optical and photocatalytic properties Katarzyna A. Michalow a,∗ , Eugenio H. Otal b , Dariusz Burnat a , Giuseppino Fortunato c , Hermann Emerich d , Davide Ferri b , Andre Heel e , Thomas Graule a,∗ a

Laboratory for High Performance Ceramics, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland Laboratory for Solid State Chemistry and Catalysis, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland c Laboratory for Advanced Fibers, Empa Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland d Swiss-Norwegian Beamlines, ESRF BP 220F-38043 Grenoble Cedex, France e Marketing, Knowledge & Technology Transfer, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland b

a r t i c l e

i n f o

Article history: Received 30 June 2012 Received in revised form 12 September 2012 Accepted 9 October 2012 Available online 14 November 2012 Keywords: Visible light Flame spray synthesis Formaldehyde Photocatalysis XANES Chromium doping

a b s t r a c t TiO2 :Cr nanoparticles with dopant concentration from 0.1 to 10 at.% were synthesized by a liquid-fed one-step flame spray synthesis. All investigated nanopowders were synthesized with a specific surface area of about 60 m2 g−1 . Undoped flame-made TiO2 consisted mainly of anatase. The increase of Cr content in TiO2 :Cr nanopowder was accompanied by a decrease of the anatase phase and a consequent increase of the rutile polymorph. The comparison of the XANES spectra of samples with different doping level of Cr showed that Cr is incorporated in the structure. The presence and the concentration of Cr significantly affected the optical properties of TiO2 and caused a red-shift of the fundamental absorption edge. Photocatalytic performance of TiO2 for the gas phase formaldehyde decomposition under visible light irradiation was enhanced by Cr doping and reached its maximum at 3 at.% Cr. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, human beings spend more than 80% of their time in an indoor environment. Therefore, the sick building syndrome, which is caused by nitrogen oxides and volatile organic compounds (VOCs), is a serious global problem. Formaldehyde (FA) is classified as a VOC and is a very common pollutant and irritant in the indoor air due to its frequent presence in adhesives, resins, textiles and customer products as well as an intermediate oxidation product of other organic contaminants [1–3]. TiO2 -based photocatalysis is a potential way to purify indoor air in an efficient way [1–4]. However, TiO2 is activated only by UV irradiation due to its wide band gap (3.2 eV). The indoor UV light irradiation is poor and cannot be harvested for an efficient photocatalytic abatement of VOCs. Therefore, modification of TiO2 to improve its efficiency and allow excitation by lower energy irradiation is necessary. One of the successful modification methods is doping of the TiO2 lattice with other elements. Transition metal ions like Cr3+ [5–7], W6+ [8–10] or Nb5+ [11] were introduced into

TiO2 to generate localized narrow bands in the forbidden band and thus to improve light absorption and photoactivity of TiO2 in the visible range. It is known that acceptor type of doping like Cr3+ has a tremendous effect on the optical properties of TiO2 [7,12]. However, there are controversial results concerning the influence of Cr on photocatalytic activity of the host material [5,13,14]. A comparison of the reported results is difficult because synthesis conditions have a substantial impact on the material performance and varying photocatalytic experiments are applied to investigate photoactivity [5,7,9,13,14]. In this paper, we have synthesized TiO2 :Cr nanoparticles using a well controllable synthesis process, the flame spray synthesis (FSS). Structure–activity relationships have been derived based on the characterization of the structural and optical properties of the samples by XRD, UV–vis and XANES, and on their photocatalytic activity for formaldehyde decomposition in the gas phase.

2. Experimental 2.1. Synthesis of TiO2 :Cr

∗ Corresponding authors. E-mail addresses: [email protected] (K.A. Michalow), [email protected] (T. Graule). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.10.007

TiO2 :Cr nanopowders with a dopant concentration in the range of 0.1–10 at.% were prepared by flame spray synthesis (FSS) using

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titanium diisopropoxide bis(acetylacetonate), 75% in isopropanol (TiC16 H28 O, ABCR) as a Ti precursor and 0.15 mol dm−3 solution of chromium (III) nitrate (Cr(NO3 )3 ·9H2 O; ≥97%, Fluka) in absolute ethanol (C2 H5 OH, 99%, Sigma Aldrich) as a Cr precursor. Details of the flame spray setup are reported elsewhere [8,10]. The required composition of the produced particles was obtained by adjusting the precursor mixture. The total precursor concentration in the flame was 2.67 × 10−4 mol s−1 . The total flow rate of the precursor mixture was in the range of 0.133–0.284 cm3 s−1 . The precursor solution was fed by a syringe pump and was atomized by a gas-assisted external mixing nozzle by oxygen (583 cm3 s−1 ). This combustible aerosol was ignited by six acetylene-oxygen flamelets (C2 H2 , 217 cm3 s−1 ; O2 , 283 cm3 s−1 ). The produced particles were collected on glass fibre filters (GF/A 150, Whatman) using vacuum pumps. All flame-made nanopowders were post-treated under ambient air at 400 ◦ C for 60 min to remove possible organic precursor/solvent-related surface contamination [9]. 2.2. Characterization of the photocatalysts The specific surface area (SSA) of as-prepared powders was determined from a 5-point N2 adsorption isotherm obtained from Brunauer–Emmett–Teller (BET) measurements using a BeckmanCoulter SA3100. Prior to analysis, powder samples were dried for 120 min at 180 ◦ C in synthetic air. Structure, phase composition and crystallite size of the flamemade nanopowders were studied by X-ray powder diffraction (XRPD) using a PANalytical X’Pert PRO –2 scan system equipped ˚ with a Johansson monochromator (Cu-K␣1 radiation, 1.5406 A) and a X’Celerator linear detector. The diffraction patterns were scanned from 10◦ to 90◦ (2) with an angular step interval of 0.0167◦ . The phase content was determined by Rietveld refinement (X’Pert HighScorePlus, Panalytical, Netherlands). Determination of fitting parameters was based on the Newton–Raphson least square method. The background of all XRD diffraction patterns was approximated by fourth-order polynomial function and the peak shape was assumed to be pseudo-Voigt type. The profile fitting used for the purpose of this work included most important parameters such as scale factors, the global instrumental parameters, the lattice parameters, preferred orientation, site occupancy and Caglioti half-width parameters. Structural information of anatase and rutile were obtained from ICSD CIF files (rutile, ICSD 16636; anatase, ICSD 202242). The mean crystallite size was obtained by the Scherrer equation [15,16]. XAFS measurements were performed at the Swiss Norwegian Beam Line (SNBL-BM01B) of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The spectra were obtained at room temperature at the Cr K-edge (5989 eV) in the transmission mode without further dilution. Cr metal foil was used as reference for energy calibration. Cr2 O3 , K2 CrO4 and Cr foil were used as standards for oxidation state estimation. Data treatment was performed with ATHENA codes [17]. Spectrophotometric measurements were performed with a Shimadzu 3600 double beam spectrophotometer, equipped with integrating sphere and used to measure the spectral dependence of total and diffuse reflectance in the wavelength range from 250 to 900 nm. Spectralon (LabSphere) was used as a reflectance reference (Rref ). Powder samples were measured in 2.5 mm thick quartz cell (Quartz Suprasil, Hellma). The thickness of powder samples fulfilled the assumption of the Kubelka–Munk function, which was applied to convert diffuse reflectance spectra into the equivalent absorption coefficient [18,19]: ˛∼

K() (1 − R∞ )2 , = 2R∞ S()

(1)

where K and S are the absorption and scattering coefficients for the measured sample and the reflectance R∞ = R/Rref . If the weak dependence of the scattering coefficient S on the wavelength is taken into account, K/S can be assumed to be proportional to the absorption within the narrow range, which contains the fundamental absorption edge. 2.3. Photocatalytic experiment The evaluation of photocatalytic activity was based on the modified quick screening method reported by Ritter et al. [4]. The scheme of the photocatalytic experiment is shown in Fig. 1. 10 mg of powder were placed in 20 ml headspace vial. For each powder composition three out of six identical samples were exposed to irradiation and the other three samples were kept in darkness. Samples were preconditioned for 24 h at room temperature in a desiccator (filled with saturated calcium chloride solution) with 32% of relative humidity. 0.2 ␮l of an aqueous solution with 33.6% formaldehyde and additional 10–15% of methanol as a stabilizer were placed on the neck of the headspace vial and immediately closed with a cap. The concentration of the water solution of formaldehyde (FA) was determined iodometrically. Three samples of each type of powder were placed in the middle of the light exposure cabinet equipped with five 15 W Master TL-D Super 80 colour 865 bulbs and a UV cutoff filter (spectra given as inset in Fig. 5). The intensity of light was set to 1 mW cm−2 . Further three samples were placed in a drying chamber at 28 ◦ C for the experiment in darkness. Both experiments were running for 2 h (Fig. 1a). Empty vials were used as reference samples. After exposure, 5 ml of the reagent solution was injected through the septum to each headspace vial and the samples were vigorously shaken. The reagent solution was prepared in a volumetric flask and contained 150 g of ammonium acetate, 2 ml of acetyl acetone and 3 ml acetic acid and was filled up to 1000 ml by deionized water. Afterwards, samples were placed for 10 min in a water bath with 60 ◦ C and then cooled down to room temperature. 10 ml of butanol were added through the septum to each vial and vigorously shaken for 30 s. The headspace vials were opened and 5 ml of the butanol phase were transferred to a centrifuge tube and centrifuged at 4000 rpm for 8 min. The clear butanol phase was transferred to the photometric cuvette and the extinction maximum was measured by a spectrophotometer at 412 nm (Fig. 1b). The calibration curve of FA concentration vs. extinction was prepared according to Ritter et al. [4]. 3. Results and discussion The X-ray diffraction patterns of flame-made TiO2 and TiO2 :Cr nanopowders with a dopant concentration in the range of 0.1–10 at.% are shown in Fig. 2. In all cases, anatase and rutile, two titania polymorphs were observed. The increase of the Cr content promoted the anatase to rutile transformation. This tendency is clearly indicated by the continuously growing rutile peak at 27.47◦ at the expenses of the anatase reflection at 25.29◦ [20,21]. A quantification of the phase composition on the base of measured intensities is related with a considerable error for nanosized materials. Both, instrumental and specimen factors cause peak broadening [22]. Therefore, phases were quantified on base of a Rietveld refinement [23,24]. The phase quantification data are listed in Table 1. The rutile content increased from 11.8 wt.% for undoped TiO2 to 81.5 wt.% for TiO2 :10 at.% Cr. The change of anatase-to-rutile ratio with the increasing Cr concentration indicates that Cr favours rutile formation, the most thermodynamically stable phase of TiO2 [25]. This tendency has been also observed for TiO2 with other trivalent doping elements (V, Cr, Ni and Mn) [26]. Cr3+ as an acceptor dopant promotes the anatase-to-rutile

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Fig. 1. The scheme of the photocatalytic experiment (a) sample preparation, exposure to irradiation and to darkness; (b) successive steps of sample treatment after exposure to evaluate FA concentration, where PC indicates photocatalyst nanopowder, FA – formaldehyde; RS – reagent solution; E – extinction.

transformation thanks to the formation of oxygen vacancies according to the following Kröger–Vink notation [27]: Cr2 O3 → 2Cr Ti + 3Oo + Vo ••

(2)

High mobility of oxygen vacancies allow ions to rearrange and therefore to accelerate the anatase to rutile transformation and stabilize the rutile structure [5,28–31]. It has to be mentioned, that even with 10 at.% Cr no chromium related phases, e.g. chromium oxide or chromium titanate were detected by XRD. Therefore, substitution of Ti4+ by Cr3+ is probable, also when their ionic radii in octahedral coordination are taken into account (Ti4+ , 60.5 pm; Cr3+ , 61.5 pm) [32]. The solubility limit of Cr in TiO2 is not clearly defined so far, especially because the reports concern separate anatase or rutile polymorphs and not a mixture of them, as in

the present study. Somiya et al. [33] reported the coexistence of TiO2 rutile–Cr2 O3 solid solution below 14 wt.% Cr2 O3 . According to Gibb and Anderson [34], a solid solution was found below 5 mol % of CrO1.5 in rutile while above 15 mol % of CrO1.5 crystallographic shears planes were formed. Venezia et al. [35] studied a co-precipitated anatase powder doped with Cr3+ up to 5 at.% and no presence of any chromium oxide related phase was detected. However, the solubility limit was claimed to be at 1.4 at.% based on the observation of unit cell expansion. In spite of insignificant changes, the solubility limit was determined on base of the stabilization of the unit cell volume. The high solubility of Cr in flame-made TiO2 :Cr was already reported by some of the authors [7]. Only anatase and rutile phases were observed up to 15 at.% Cr. Also after calcination at 800 ◦ C no other phases were observed for TiO2 :10 at.% Cr [12].

Table 1 Summary of data obtained by BET and XRD measurements of flame-made TiO2 and TiO2 :Cr nanopowders. Sample name

SSA/m2 g−1

dBET /nm

Rutile wt. %

dXRD, A /nm

dXRD, R /nm

TiO2 TiO2 :0.1 at.% Cr TiO2 :1 at.% Cr TiO2 :2 at.% Cr TiO2 :3 at.% Cr TiO2 :5 at.% Cr TiO2 :10 at.% Cr

59.4 59.4 62.6 66.3 65.9 64.9 67.7

25.7 25.7 24.2 22.7 22.4 22.4 21.8

11.8 13.4 18.2 30.2 49.9 69.7 81.4

27.7 25.8 25.8 28.1 36.5 46.0 60.3

18.1 17.4 16.7 15.6 15.0 11.6 9.3

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Fig. 2. Comparison of XRD patterns of (a) flame-made TiO2 and TiO2 :Cr nanopowders, where 0.1 Cr–10 Cr indicates the corresponding concentration expressed as at.% Cr in TiO2 , grey indicates enlarged area (b), which is shown in 3D View. As a reference anatase (ICSD 202242) and rutile (ICSD 16636) are plotted.

Flame-made TiO2 and TiO2 :Cr nanopowders displayed similar specific surface area (Table 1), which resulted from the same molar concentration of the precursor fed into the flame. The slight increase of the specific surface area with higher chromium loading is probably related to changes of the phase composition and the crystallite size of anatase and rutile. The average particles size dBET was determined from the surface area measurements using Eq. (3) and it was assumed that particles are spherically shaped and non-porous: dBET =

6000 (VA A + VR R )SSA

(3)

where VA , VR are the volume fractions of anatase (A) and rutile (R) as determined from the XRD patterns and A , R are the densities of anatase and rutile (A = 3.89 g cm−3 ; R = 4.25 g cm−3 ). For all flame-made powder the specific surface area was on a comparable level, resulting in similar particle diameter dBET . On the contrary, a different behaviour was obtained when the crystallite size obtained from the XRD measurements is considered (Table 1). In agreement with our earlier report [36] larger crystallite sizes were observed for anatase, which is in contrast to observations made for commercial TiO2 -P25. The anatase crystallite size was nearly constant up to 2 at.% Cr but it increased significantly with higher Cr content. The rutile crystallite size decreased progressively with increasing Cr amount. This phenomenon was also observed by Peng et al. [37] for sol–gel dip-coated Cr-doped TiO2 thin films. They related it to the presence of Cr–O–Ti, which prevents the coalescence of the neighbouring grains thus suppressing the crystal growth during heat-treatment. In FSS, all components of the liquid precursor are homogeneously dispersed and mixed within the dispersed aerosol droplets. In this state, chromium randomly distributed in the reaction space acts as “seed” for the formation of the rutile fraction in the thermodynamically preferred and dominating homogeneous particle condensation process. Therefore, a higher Cr concentration in the flame will lead to a higher amount of rutile crystallites, but with

a smaller crystallite diameter. The noticeable discrepancy between the particle size value, expressed by dBET , and the crystallite size, expressed by dXRD , A and dXRD , R , is related to the different sensitivity of the measurements techniques and the different definition of the size. dBET is an average value based on SSA measurements (Eq. (3)). The XRD measurement provides separate information about the average crystallite size of each phase present in the material sample [15,16]. X-ray absorption near edge structure (XANES) spectra of TiO2 :Cr nanopowders at the Cr K-edge are similar (Fig. 3), indicating that similar Cr species are present. Since the reference spectrum of Cr2 O3 shows some differences compared to those of the TiO2 :Cr samples, it is likely that Cr is not segregated in the form of a sesquioxide. The pre-edge region of XANES spectrum is sensitive to changes in the symmetry surrounding the absorbing atom. The presence of a pre-peak is associated with the loss of an inversion centre in the environment of the absorbing atom [38,39]. The increase of the pre-peak intensity with the decrease of Cr content can be related to the lattice distortion induced by doping [40]. Comparison of the E0 shift of different Cr-containing samples with the reference spectra showed that the Cr oxidation state is between 3+ and 4+, independently of the Cr content. Similar conclusions were derived from XPS results in an earlier report on TiO2 :Cr, synthesized by FSS but from different type of precursors and different process parameters [7]. The influence of the dopant content on the host matrix determined by XANES and on the fraction of rutile determined by XRD is tentatively associated with the incorporation of Cr in the TiO2 host matrix. The colour of undoped TiO2 made by FSS was white what is reflected in the absorption spectrum (Fig. 4) as a lack of absorption in the range of 400–850 nm. The colour changed to bright yellow for TiO2 :0.1 at.% Cr and with further increase of the Cr content the colour intensified to dark yellow-brownish for TiO2 :10 at.% Cr. The colour change is also reflected in the absorption coefficient (K/S) calculated from the Kubelka–Munk equation (Eq. (1), Fig. 4a). The

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Fig. 3. (a) Cr K-edge of Cr2 O3 and TiO2 :Cr nanopowders, where 2 Cr–10 Cr indicates the corresponding concentration expressed as at.% Cr in TiO2 , (b) a zoom of pre-edge region (indicated in (a)) in grey).

fundamental absorption edge was calculated from the linear fitting of the Tauc plot with the assumption of indirect transition (Fig. 4b): K()  × hv = ˛0 (hv − Eg ) , S()

(4)

where ˛0 is a material constant (105 –106 cm−1 eV−1 for typical oxide semiconductors), Eg is the band gap and the power coefficient  can be: 1/2, 3/2, 2, 3 depending on the type of transition: direct allowed, direct forbidden, indirect allowed, indirect forbidden. Eq. (4) is only valid over the strong absorption region and was originally used for amorphous semiconductors [41]. As can be seen in the inset in Fig. 4b the band gap is shifted from about 3.0 eV for undoped TiO2 to about 1.5 eV for TiO2 :10 at.% Cr. It has to be mentioned that the Tauc method, where the absorption coefficient is calculated from a Kubelka–Munk function, provides only a rough value for the band gap, which in this case is an average value of all optical transitions appearing in the material [36]. As can be seen in Fig. 4b, it is difficult to choose and estimate a proper linear range in Tauc plots. However, it is clear from both, absorption spectra and band gap calculations, that the red-shift is a function of the increasing Cr content. An additional absorption peak appears between 630 and 830 nm with a maximum at about 710 nm (1.75 eV) (Fig. 4a, inset). The peak in TiO2 :10 at.% Cr is embedded in the overall increased absorption in this region. The shift of the fundamental absorption edge towards longer wavelengths and the presence of an additional absorption peak can be related to defects in the electronic structure of TiO2 induced by Cr doping according to Eq. (2) [6,7,12–14,27]. Serpone and Kuznetsov [42,43] showed that irrespective of the doping type the light absorption in the visible range can be deconvoluted into the three contributions at 2.9–3.0 eV, 2.4–2.6 eV and 1.7–2.1 eV, all being associated to colour centres and oxygen vacancies. The reported values are in the good agreement with our data. The photocatalytic activity of flame-made TiO2 and TiO2 :Cr nanopowders was evaluated by measuring formaldehyde (FA) photodecomposition in the gas phase. Due to improved visible light absorption in the visible range (Fig. 4), the photocatalysts were subjected to visible light irradiation as shown in the inset in Fig. 5. The applied irradiation simulated indoor conditions. A UV cut-off filter (400 nm) was used to ensure that only wavelengths above 400 nm will act as an excitation source. Fig. 5 shows that TiO2 -P25 and similarly TiO2 -FSS (marked as 0 at.% Cr in Fig. 5) are not active under these conditions, thus confirming the role of the UV cut-off

filter. Interestingly, TiO2 :Cr nanopowders expressed both, activity in darkness and under irradiation. The decrease of FA concentration in darkness followed the same trend as under irradiation. This behaviour can be explained by an enhanced catalytic activity and/or improved surface adsorption, driven by the increasing Cr content followed by modification of surface properties. Removal of 35% of FA in darkness was also observed by Stevens et al. [44], who suggested a small extent of activity or FA adsorption on TiO2 . However, in our case removal of FA by undoped TiO2 was negligible, probably due to the applied high FA concentration. The difference of the FA concentration in the darkness and under visible irradiation in presence of TiO2 :Cr shows clearly that FA is decomposed photocatalytically. However, one has to take in to account that there is a superposition of both catalytic and photocatalytic activity. The overall activity increased with increasing Cr content and reaches the maximum for TiO2 :3 at.% Cr. Higher loading of Cr caused a decrease of both, photocatalytic and catalytic performance and TiO2 :10 at.% Cr was not active. Precise specification and definition of the optimum dopant loading with respect to optimal photocatalytic performance strongly depends on the intrinsic photocatalyst properties as well as on the type and conditions of the photocatalytic experiment. Lam et al. [45] reported that Cr ion implanted TiO2 thin films showed improved visible light activity using gas phase FA as a model compound. The optimum Cr content was 2 × 1016 ions cm−2 (2.37 at.%). The loss of FA degradation with higher Cr loading was associated with the construction of the reactor and not with the intrinsic properties of the material. Lam et al. [45] did not provide any information about the performance in the darkness. Choi et al. [5] investigated the effect of 13 different metal ions on the optical properties and photocatalytic activity under visible light irradiation. Among them, Cr next to Pt, was the only metal ion that extended light absorption towards visible range and enhanced photocatalytic activity in this range. Cr doping lowered the anatase to rutile transformation temperature, thus increasing the rutile content. Enhanced photocatalytic activity was assigned to altered charge-carrier recombination and interfacial charge-transfer. On the other hand, Herrmann [13,14] showed that Cr has a negative effect on the photocatalytic performance under both, UV and vis light irradiation in spite of improved visible light absorption. Reported TiO2 nanopowders doped with 0.56 at.% Cr and 0.85 at.% Cr were obtained by flame synthesis in a oxhydric flame. The synthesis method is similar to the one used for

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commercial production of TiO2 -P25 and therefore this material was used as a reference. Surprisingly, Herrmann did not observe rutile formation in spite of Cr3+ doping and presence of Cl− in the synthesis routine. Reported nanopowders consisted of pure anatase. Decreased photocatalytic performance induced by Cr doping is explained by enhanced electron–hole recombination and reduced charge separation. In the most recent publication, Herrmann [13] extend his theory of negative Cr effect on the charge carrier separation and claimed that every cationic doping has a detrimental effect on the photocatalytic activity. Based on the presented data it can be stated that Cr has a significant influence on all investigated properties of the host TiO2 . For TiO2 :Cr nanoparticles the following dependencies were observed with an increasing Cr content: (i) decrease of anatase-to-rutile ratio, (ii) decrease of the rutile crystallite size, (iii) increase of visible light absorption and finally, (iv) enhanced catalytic and photocatalytic activity. The optimal Cr content with respect to catalytic and photocatalytic performance was at 3 at.%. This can be explained by an improved visible light absorption and modified photocatalyst surface properties as well as optimal structural properties (50 wt.% of anatase and rutile), which may allow an improved charge carrier separation as it takes place in TiO2 -P25 under UV irradiation [46]. The decrease of the photocatalytic performance for higher Cr loading can be explained by an improved charge carrier recombination due to a high defect concentration, small rutile crystallite size and low anatase content. 4. Conclusions

Fig. 4. Comparison of (a) the absorption coefficient (K/S) of flame made TiO2 and TiO2 :Cr nanopowders, the grey region zoomed as an inset; (b) Tauc plots with indicated linear fittings, inset shows calculated band gap energy vs. Cr concentration. 0.1 Cr–10 Cr indicates the corresponding concentration expressed as at.% Cr in TiO2 .

TiO2 :Cr nanopowders with chromium concentration in the range 0–10 at.% with comparable specific surface area were synthesized by flame spray synthesis. All nanopowders, irrespective of dopant level consisted of anatase and rutile. The ratio changed with an increasing Cr load. TiO2 -FSS comprised 11.8 at.% of rutile, which increased to 81.4 at.% in TiO2 :10 at.% Cr. Cr was incorporated in the structure and its oxidation state was between 3+ and 4+. Catalytic activity or adsorption properties towards formaldehyde removal increased with a higher Cr content in TiO2 . Significant improvement of light absorption in the visible range was followed by an enhanced photocatalytic activity towards decomposition of gaseous formaldehyde. The maximum performance was observed for TiO2 :3 at.% Cr due to its optimal optical and structural properties: improved visible light absorption and a phase composition of 50 wt.% anatase and 50 wt.% of rutile, which may allow a better charge carrier separation, similar to TiO2 -P25. Acknowledgements The authors would like to thank CTI Project (10598.1 PFNMNM), Keller AG Kreativweberei, Christian Eschler AG and Tersuisse Multifils SA for financial support, as well as to Elisabeth Michel and Felix Reifler for their support with photocatalytic experiments. XAFS experiments were performed on the Swiss-Norwegian Beam Line (SNBL-BM01B) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. We are grateful to Hermann Emerich at ESRF for providing assistance during the measurements (proposal #01-01-861). References

Fig. 5. Formaldehyde (FA) degradation vs. Cr concentration in darkness and under vis irradiation compared to TiO2 -P25, the upper boarder of grey stripe indicates the blank in darkness and lower border represents blank under irradiation. The applied type of irradiation overlapped with the UV cut-off filter in inset.

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