CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201300922

Photocatalysis with Chromium-Doped TiO2 : Bulk and Surface Doping Samy Ould-Chikh,*[a] Olivier Proux,[b] Pavel Afanasiev,[c] Lhoussain Khrouz,[d] Mohamed N. Hedhili,[e] Dalaver H. Anjum,[e] Moussab Harb,[a] Christophe Geantet,[c] JeanMarie Basset,[a] and Eric Puzenat[c] The photocatalytic properties of TiO2 modified by chromium are usually found to depend strongly on the preparation method. To clarify this problem, two series of chromium-doped titania with a chromium content of up to 1.56 wt % have been prepared under hydrothermal conditions: the first series (Cr:TiO2) is intended to dope the bulk of TiO2, whereas the second series (Cr/TiO2) is intended to load the surface of TiO2 with Cr. The catalytic properties have been compared in the photocatalytic oxidation of formic acid. Characterization data

provides evidence that in the Cr/TiO2 catalysts chromium is located on the surface of TiO2 as amorphous CrOOH clusters. In contrast, in the Cr:TiO2 series, chromium is mostly dissolved in the titania lattice, although a minor part is still present on the surface. Photocatalytic tests show that both series of chromium-doped titania demonstrate visible-light-driven photo-oxidation activity. Surface-doped Cr/TiO2 solids appear to be more efficient photocatalysts than the bulk-doped Cr:TiO2 counterparts.

Introduction TiO2 photocatalysts have attracted extensive interest owing to their great advantages in the complete mineralization of organic pollutants in wastewater and contaminated air. Common TiO2 polymorphs are known to be active only in UV light range due to their large band gap (Eg = 3.0 to 3.4 eV) for heliophotocatalysis (use of sunlight) because only around 4 % of the solar light energy reaching Earth is in this range. Thus, one of the greatest challenges for TiO2-based photocatalysis is extension of the photoresponse to the visible range with a high quantum yield over the whole light absorption range. During the last decade, many research teams have studied chemically and physically synthesized anion-doped titania (N, S, F, P, …), presenting visible absorption, and their resultant photocatalytic activities in the ultraviolet and visible ranges.[1, 2] However, cat[a] Dr. S. Ould-Chikh, Dr. M. Harb, Prof. J.-M. Basset KAUST Catalysis Center King Abdullah University of Science and Technology Thuwal (Saudi Arabia) E-mail: [email protected] [b] Dr. O. Proux BM30B/CRG-FAME beamline ESRF 38043 Grenoble cedex 9 (France) [c] Dr. P. Afanasiev, Dr. C. Geantet, Dr. E. Puzenat Institut de Recherches sur la Catalyse et l’Environnement de Lyon Universit Lyon 1, 2 av A. Einstein 69626 Villeurbanne (France) [d] Dr. L. Khrouz Chemistry laboratory, ENS Lyon 46 alle d’Italie 69364 Lyon cedex 7 (France) [e] Dr. M. N. Hedhili, Dr. D. H. Anjum Imaging and Characterization Core Lab King Abdullah University of Science and Technology Thuwal (Saudi Arabia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201300922.

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ionic doping by transition metals remains a field worth investigating. Numerous contradictory results published so far imply that rationalization is still needed. Moreover, photocorrosion issues that hamper the utilization of anion-doped titanias do not represent a problem for cationic doping. For chromium-modified TiO2, most of the work is intended to insert chromium into the lattice of TiO2 by using different methods: sol–gel synthesis,[3] hydrothermal/coprecipitation synthesis,[4] flame pyrolysis,[5] solid-state synthesis,[6] physical vapor deposition (PVD),[7] and ion implantation.[8] A complex picture emerges, in which the performance of the chromiumdoped materials are highly varied. As stated by Anpo and Takeuchi, most of the “chemical methods” give similar optical properties for chromium-modified TiO2 that consist of the appearance of one or two new bands in the l = 400–450 and 600– 800 nm regions.[8] On the other hand, “physical methods” (e.g., ion implantation), cause a redshift of the absorption edge of TiO2, which, so far, seems to yield to the best photocatalytic performances both in ultraviolet and visible regions. Although new optical absorption bands correspond to the introduction of energy levels within the band gap of TiO2, band gap narrowing reflects a hybridization of chromium orbitals with titanium and oxygen orbitals. This demonstrates that the position and oxidation state of chromium ions in bulk TiO2 is one key factor to achieve interesting photocatalytic properties. Another reason for the dispersion of the results might arise from the possibility of loading the titania surface with oxide, oxyhydroxide, or grafted inorganic polymer species rather than successfully producing doped titania. It is known that transition metals such as chromium, tungsten, niobium, vanadium, an molybdenum spread easily on the surface of oxides.[9, 10] For chromium, this property is used for a variety of catalytic reacChemSusChem 0000, 00, 1 – 12

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tions (oxidative dehydrogenation, hydrogenation, aromatization, denitrification (DeNOx), polymerization of olefins).[11] Concerning photocatalysis, only a few reports mention the deposition of chromium species on the TiO2 surface. The photocatalytic activities seem to depend on the sintering temperature: above 300 8C, UV activity is decreased and visible activity is negligible,[12–14] whereas below 300 8C the stability of UV activity is not reported, to the best of our knowledge. The highest photonic efficiency reached 1.9 % in the visible region.[15–17] Chromium oxidation state and the extent of oligo-/polymerization of chromium oxyhydroxide on the titania surface are suspected to set the photocatalytic activity. Thus, some of the interesting photocatalytic performances may be attributed to grafted chromium moieties or to a mixture of chromium oxide or oxyhydroxide with titania. In addition, the crystallographic position of chromium upon insertion into the titania unit cell remains to be determined. This uncertainty related to the characterization of materials might partially explain the contradictory photocatalytic results that have been reported. The purpose of this study is to contribute to the understanding of the photocatalytic behavior of chromium-modified TiO2 thanks to the extensive characterization of two sets of materials, namely, chromium-doped titania (Cr:TiO2) and chromium supported on the surface of titania (Cr/TiO2).

Results and Discussion The chromium-doped titania batch (Cr:TiO2) has an orange– yellow color, whereas the chromium-loaded titania (Cr/TiO2) has a greenish color that increases in intensity with the amount of chromium chloride used in the synthesis. Visual aspects of the two sets of powdered materials are shown in Figure S1 in the Supporting Information. Elemental analysis indicates good fixation of the chromium content initially present in the synthetic mixtures for all materials (Table 1). Cr:TiO2 and Cr/TiO2 are both mesoporous (type IV isotherms). The total porous volume is about 0.28 mL g1 for Cr:TiO2, whereas it inTable 1. Characterization of the photocatalysts. Sample

Cr:TiO2

Cr/TiO2

creases from 0.32 to 0.36 mL g1 with the initial chromium content in Cr/TiO2. The theoretical specific surface areas calculated from the crystallite size (SXRD) are in close agreement with those measured by N2 physisorption (SBET) for Cr:TiO2 (Table 1), whereas, for Cr/TiO2, the measured SBET values are notably larger. Raman spectra of Cr/TiO2 and Cr:TiO2 are given in Figure 1. Phonons of the anatase structure are clearly visible for all samples:[18] n˜ (Eg) = 144 cm1, n˜ (B1g) = 396 cm1, n˜ (A1g+B1g) = 516 cm1, n˜ (Eg) = 637 cm1, n˜ (B1g) = 792 cm1 (overtone). The presence of brookite as an impurity is detected thanks to its most intense phonons: n˜ (A1g) = 246 cm1, n˜ (B3g) = 288 cm1, n˜ (B1g) = 321 cm1, n˜ (B2g) = 363 cm1.[19, 20] XRD patterns also show the characteristic reflections of anatase (95–96 wt %) with some brookite impurity (4–5 wt %). The average anatase crystallite size is measured in the 24–27 nm range for Cr:TiO2, whereas it remains constant and equal to 30 nm for Cr/TiO2 (Table 1). No bulk chromium oxide or oxyhydroxide were detected by either Raman scattering or XRD. The synthetic conditions used for Cr/TiO2 were reproduced in the absence of titania particles. Small crystallites (5 nm, SBET = 296 m2 g1) of chromium oxyhydroxide (grimaldiite-a-CrOOH) were obtained (Figure S2 in the Supporting Information). Bright-field images obtained by high-resolution transmission electron microscopy (HRTEM) performed on 1.56 wt % chromium content shows a great difference between the two synthetic methods (Figure 2). Both show highly crystalline titania particles, but the particle edges look much rougher for Cr/TiO2, which suggests that some amorphous matter surrounds the titania particles (Figure 2 b). The surface of the titania particles is highly loaded with chromium, as evidenced by EFTEM (Figure 2 d) and scanning transmission electron microscopy– energy-dispersive X-ray spectroscopy (STEM-EDS) studies (Figure S3 in the Supporting Information). The existence of amorphous clusters may explain the discrepancies observed between the specific surface areas calculated from the crystallite sizes and N2 physisorption isotherms for Cr/TiO2, although the chromium coverage remains lower than that of monolayer coverage (6.6 Cr nm2 on the titania surface).[21]

Cr content [wt %] theor.[a] exp.[b]

d[c] [nm]

SXRD [m2 g1]

SBET [m2 g1]

q[d] [at nm2]

Vp[e] [mL g1]

0 0.06 0.12 0.20 0.59 1.17 1.56 0 0.06 0.12 0.20 0.59 1.17 1.56

27 26 26 24 26 25 24 30 30 30 30 30 30 30

59 60 60 66 61 63 65 52 52 52 52 52 52 52

58 60 63 68 64 71 62 47 73 76 73 72 85 80

– – – – – – – 0.0 0.1 0.2 0.3 0.9 1.6 2.3

0.22 0.27 0.28 0.28 0.27 0.30 0.28 0.30 0.32 0.32 0.33 0.34 0.37 0.36

– 0.06 0.12 0.19 0.57 1.14 1.47 – 0.07 0.13 0.23 0.66 1.34 1.62

[a] Initial Cr content in the synthesis. [b] Final Cr content measured by inductively coupled plasma (ICP). [c] Coherent domain. [d] Chromium coverage on the titania surface. [e] Porous volume.

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Oxidation state and local environment of chromium Chromium K-edge X-ray absorption near-edge structure (XANES) signals for the reference compounds and the synthesized materials are shown in Figure 3. By comparing five model compounds in which Cr is trivalent and octahedrally coordinated to oxygen ligands, Farges concluded that the assessment of the chromium valence based on the position of the edge jump region was not straightforward ChemSusChem 0000, 00, 1 – 12

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Figure 1. Raman spectra of Cr:TiO2 (top) and Cr/TiO2 (bottom) according to the chromium content. The insets show the evolution of the most intense Eg anatase phonon.

Figure 3. XANES spectra at the Cr K edge of reference CrO2 and Cr2O3 and as-synthesized a-CrOOH, Cr/TiO2, and Cr:TiO2 with 1.56 wt % chromium content. The inset shows a magnification of the energy range for the pre-edge. Figure 2. Bright-field HRTEM images of titania particles: 1.56 wt % chromium content for a) Cr:TiO2 and b) Cr/TiO2. In energy-filtered transmission electron microscopy (EFTEM) images of the same areas, chromium appears in green and titanium is in red: 1.56 wt % chromium content for c) Cr:TiO2, d) Cr/TiO2.

because the main jump energy could vary up to 2.6 eV.[22] Hence, analysis of the pre-edge region at around 5992 eV seems to be more informative. Indeed, the position and intensity of the localized transitions observed at the K pre-edge are known to be characteristic of the chromium oxidation state and the geometry of its environment.[22] For the tri- (Cr2O3, aCrOOH) and tetravalent states (CrO2), doublet pre-peaks are observed and are especially well resolved if chromium is trivalent. The origin of this doublet is usually related to the presence of quadrupolar 1s!3d transitions that arise from hybridi 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

zation of the Cr 3d levels in the presence of surrounding oxygen ligands.[23–25] The local symmetry around CrIII in aCrOOH and Cr2O3 is C3v, which implies that the Cr site is not centrosymmetric, and therefore, Cr 3d–4p mixing is expected to contribute to the overall intensity of the pre-peaks.[26, 27] The pre-edge of Cr/TiO2 is almost identical to that of synthesized a-CrOOH with regard to the positions (centroids: 5990.6, 5993.4 eV) and intensities of the pre-peaks; thus suggesting a very similar local environment and oxidation state. This justifies the choice of using the crystallographic data of a-CrOOH as an initial structure to fit the extended X-ray absorption fine structure (EXAFS) spectrum of Cr/TiO2.[28] In addition, the Cr/ TiO2 sample was also X-ray beam sensitive, because a relatively intense pre-peak centered at 5993.4 eV, which was also obChemSusChem 0000, 00, 1 – 12

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CHEMSUSCHEM FULL PAPERS served in pure crystalline CrO3, completely vanished during acquisition of the second spectrum. This is attributed to the Xray photoreduction of hexavalent to trivalent chromium on the surface of Cr/TiO2. The initial presence of hexavalent chromium was also confirmed by X-ray photoelectron spectroscopy (XPS; see the Supporting Information) The pre-edge region of Cr:TiO2 shows an additional pre-peak located at 5995.8 eV, which has not been observed, as far as we know, in any pure chromium oxide materials. Insertion of chromium within the lattice of TiO2 can occur in many different ways, although the substitution of titanium by chromium is the most obvious doping mode to consider. Indeed, the close Shannon ionic radii of TiIV and CrIII in the octahedral field (60.5 and 61.5 pm) are likely to favor simple substitution.[29] To ascribe the origin of the third pre-peak, calculations with FDMNES code were performed for a cluster of anatase TiO2 in

Figure 4. Pre-edge XANES spectra at the Cr K edge of Cr:TiO2 (red) and calculated spectra with the FDMNES code by using an anatase cluster size of 7.2  with one chromium atom substituting for one titanium atom.

www.chemsuschem.org which a titanium atom was substituted for a chromium atom (central atom for the calculation). By comparing the experimental and theoretical normalized spectra, good agreement was obtained for the three pre-peaks positions, whereas the intensities appeared slightly higher in our calculations (Figure 4). The inclusion of quadrupolar transitions in the calculation did not drastically change the intensities of the pre-peaks consistent with the local D2d symmetry (e.g., non-centrosymmetric). Because the theoretical intensities are higher than those of the experimental spectrum, partial structural relaxation around chromium towards a symmetry closer to octahedral may explain this difference. Thus, the experimental spectrum probably reflects a mixed p–d density of state at an absorbing chromium atom that substitutes a titanium atom in the anatase lattice. The Cr K-edge k2-weighted EXAFS signal for the reference compounds and the synthesized materials, as well as the corresponding Fourier transforms, are shown in Figure 5. Table S1 in the Supporting Information lists the results of the fits performed on the k2c(k) EXAFS data. The first neighbor contribution was fitted with a total of six oxygen atoms at (1.96–1.97  0.01)  for all compounds, except CrO2, which exhibited shorter CrO distances because of the chromium tetravalent state (2 CrO paths of (1.87  0.01)  and 4 CrO paths of (1.91  0.01) ). Values obtained in transmission mode for the reference compounds were in complete agreement with those deduced from the crystallographic structure. Hence, the presence of trivalent chromium, whether on the surface or inserted within the titania lattice, is confirmed. For Cr:TiO2, even though the local symmetry of the titanium position is D2d, it was not possible to fit the EXAFS data reliably by using two different types of oxygen atom because strong correlations between the fitted distances and the Debye–Waller factors were found. The best fit was obtained upon using six equivalent oxygen atoms with a CrO distance of (1.96  0.01)  and a low Debye–Waller factor (s2 = (0.0025  0.0004) 2) compared with the reference compound simulations, that is, a narrow bond

Figure 5. EXAFS k2c(k) functions (top) and Fourier transforms (bottom) for reference CrO2 and Cr2O3 and as-synthesized a-CrOOH, Cr/TiO2, and Cr:TiO2 with 1.56 wt % chromium content. The solid lines are the experimental data, whereas the dashed lines are the fit results with appropriate k and R ranges.

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Figure 6. Comparison of the ESR signals of Cr/TiO2 and Cr:TiO2 (top), and a representation of the Cr:TiO2 ESR signal as a superposition of two components (bottom).

length distribution. Meanwhile, the results derived from the Rietveld refinement (Table S1 in the Supporting Information) give, on average, two MO distances of 1.98  and four MO distances of 1.94 . Thus, the EXAFS fit suggests a radial structural relaxation and possible damping of the tetragonal distortion around chromium. The second peak (second coordination sphere) observed in the Fourier transform (Figure 6 b) between 2.1 and 3.0  is due solely to the contribution of CrCr scattering paths, except with Cr:TiO2, for which it is was not possible to distinguish Cr and Ti because of their close atomic number (the simulation presented herein is that using Ti as the second neighbor). Using a similar preparation method for Cr/TiO2, Irie et al. proposed that amorphous-like Cr2O3 clusters were deposited on the TiO2 surface, although they were not able to analyze quantitatively the CrCr bond length because of low signal intensity.[16] In the present case, the CrCr distance in Cr/TiO2 is (3.00  0.02) , which is much closer to the distance obtained for a-CrOOH ((2.99  0.01) ) rather than that for Cr2O3 (2 Cr Cr scattering paths: (2.64  0.02) and (2.90  0.01) )). Calculated coordination numbers for Cr/TiO2 and Cr:TiO2 are 2.5  1.3 and 3.3  1.0, respectively. However, those were not completely reliable because it was also possible to compute an equivalent fit by fixing the numbers of neighboring atoms to its maximum theoretical value (e.g., 6 Cr for Cr/TiO2 and 4 Ti for Cr:TiO2), while increasing the Debye–Waller factor (accounts for structural disorder and thermal vibration). The possibility of paramagnetic species of CrIII and CrV was determined by ESR on Cr/TiO2 and Cr:TiO2. Several ESR studies on the chromium—titanium oxide systems have been published and the paramagnetic species are well documented.[30, 31] The ESR signals of the two different preparations, Cr/TiO2 and Cr:TiO2 (1.56 wt %), have comparable intensity and are centered at the same g factor near 1.98 (Figure 6). Double integration and comparison with the strong pitch intensity gives the values of spin concentrations of 1.3  1016 and 1.7  1016 spins cm1 for Cr/TiO2 and Cr:TiO2, respectively; this sug 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

gests that the majority of chromium species in both samples are paramagnetic. However, the shape of the signal is different for differently prepared samples. The signal of Cr/TiO2 is broad (line width 90 mT) and can be identified as a b signal, which was earlier attributed to amorphous Cr2O3 clusters smeared over the support surface.[32] The signal of Cr:TiO2 is much narrower and seems to be composed of two isotropic signals centered at the same value of g = 1.98 (Figure 6). One component has the same line width as the b signal of Cr/TiO2 and probably corresponds to the same type of surface Cr2O3 clusters. The second component is much narrower (20 mT). It can be attributed to the CrIII d3 ions dissolved in the titania lattice. The g factor is close to two because a high (quasi-octahedral) symmetry crystalline field does not split the Kramers doublets, beside a small S3 term. A symmetric signal of isolated, dissolved CrIII with a g factor of 1.98 was observed in high-symmetry crystalline environments, for example, in magnesia.[33] The tetragonal distortion present in the anatase environment might be relaxed or might provide small additional components to the main signal.[30, 31] In any event, small components at g = 2.23 and 1.76 observed previously could not be reliably detected in our case because they should be obscured by the broad b signal of surface clusters.[30, 31] The formation of some CrV species cannot be totally excluded on the basis of our ESR study, but its contribution cannot be significant because a narrow axial signal should be observed otherwise. In summary, the X-ray absorption spectroscopy (XAS)–ESR results are interpreted, in the case of Cr/TiO2, by the presence of an amorphous chromium oxyhydroxide displaying structural similarities to a-CrOOH (CrIII in quasi-octahedral field). For Cr:TiO2, XAS-ESR results are consistent with CrIII cations dissolved in titania lattice. Such insertion might occur with a significant change to the environment around introduced chromium atoms compared with regular anatase sites (D2d !Oh). The creation of titanium vacancies in the vicinity of central Cr atom might explain the apparent decrease in N[CrTi] relative to the N[TiTi] of TiO2 anatase. Otherwise, structural disorder may also ChemSusChem 0000, 00, 1 – 12

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CHEMSUSCHEM FULL PAPERS account for the observed increase in the Debye–Waller value, as induced by an increase in the width of the CrTi bond length distribution. In Cr:TiO2, apart from dissolved species, some CrIII is also present at the surface, as revealed by ESR.

Long-range order and charge compensation Incorporation of chromium within the titania lattice is characterized by shifts of the Bragg peaks to higher and lower angles, depending on the Miller indexes of the anatase reflections (Figure S4 in the Supporting Information), as observed for Cr:TiO2. Cell parameters and associated cell volumes were computed from Rietveld refinement (Table S2 in the Supporting Information). Two linear trends are observed as the chromium content is increased (Figure 7): the square base of the unit

www.chemsuschem.org Concerning charge compensation, XANES calculations showed that the insertion of CrIII within the lattice of TiO2 occurred through the substitution of titanium atoms. Because charge neutrality must be respected, a hole may be created when chromium is trivalent. With the Krçger–Vink notation, it is written as Equation (1):[34] TiO

0

Cr2 O3 þ 2TixTi ƒƒ! ƒ2ƒ 2CrTi þ 2hþ þ 3OxO

ð1Þ

The Mott–Schottky plots indicates that the n-type character of TiO2 is retained for the highest chromium content (Figure S6 in the Supporting Information), excluding the latter type of charge compensation. Thus, the created holes may rather be compensated for by additional oxygen vacancies [Eq. (2)]: TiO

0

Cr2 O3 þ 2TixTi ƒƒ! ƒ2ƒ 2CrTi þ VO þ 3OxO

Figure 7. Structural evolution of the anatase unit cell with increasing amounts of chromium in the synthesis of Cr:TiO2.

cell expands, while its height shrinks. Overall, increasing the amount of chromium leads to a larger anatase cell volume increase. For Cr/TiO2, none of these observations were true, that is, the cell parameters and cell volume remained roughly constant. Raman scattering also provides insights into structural modification when chromium is inserted into the titania lattice. Upon comparing anatase phonons with an increasing chromium content, differences are only observed in the case of Cr:TiO2. The most intense Eg mode (144 cm1) is highlighted in the insets of Figure 1 as an illustration: peak broadening and a shift towards higher wavenumbers are observed for Cr:TiO2, whereas no modification occurs for Cr/TiO2. In addition, a strong feature in the 750 cm1 region, the intensity of which increases with chromium content for Cr:TiO2, remains difficult to attribute on the basis of published results. The intensity of this band was found to vanish when laser excitation at l = 785 nm was used instead of that at l = 532 nm (Figure S5 in the Supporting Information). Because the l = 532 nm excitation line falls in the range of the optical absorption of chromium species (see below optical properties), the band at 750 cm1 is tentatively attributed to a local n(CrIIIO) vibration in the TiO2 lattice and is possibly subjected to a Raman resonance effect.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ð2Þ

The oxygen site occupation factor remains constant and equal to unity regardless of the chromium amount (Figure S8 in the Supporting Information). Either the presence of oxygen vacancies cannot be detected by XRD or chromium doping does not occur with the parallel creation of oxygen vacancies. If there is any consequent amount of oxygen vacancies, EXAFS experiments have at least demonstrated that they do not exist in the first coordination sphere of chromium. Interestingly, the site occupation factor of titanium exhibits variation as a function of chromium content. Without chromium doping, it is initially equal to 0.95, which suggests pre-existing titanium vacancies. When chromium content is increased, a considerable decrease in structural factors is observed at low angles, which is treated during the refinement as a reduction of the titanium occupation factor down to 0.90 (Figure S7 in the Supporting Information). The latter results is also consistent with N[CrTi] calculated previously by EXAFS (Table S2 in the Supporting Information). Hence, charge compensation involving titanium vacancies is proposed [Eq. (3)]: TiO

0

0000

2CrOOHðgÞ þ 2H2 O þ 2TixTi ƒƒ! ƒ2ƒ 2CrTi þ VTi þ 6OHi

ð3Þ

Those titanium vacancies are supposedly stabilized by hydroxyl groups and could stem from condensation defects remaining in the crystals after the crystallization step. Moreover, titanium vacancies in pristine TiO2 shortened the c parameter and lengthened the a parameter by releasing strong TiTi repulsions across the shared edges of consecutive octahedra.[35] This is consistent with the cell parameter evolution observed when the chromium content is increased in Cr:TiO2 (Figure 7). Surface chemistry Numerous studies have proved that Raman spectroscopy can provide detailed structural information on supported chromium oxide with a formal oxidation state of CrVI.[36–38] Considering Cr/TiO2 samples, a broad band is observed in the 800– 1000 cm1 wavenumber region only for samples with a chromium content higher than 0.59 wt % (Figure 1). According to ChemSusChem 0000, 00, 1 – 12

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Figure 8. UV/Vis absorption spectra converted by the Kubelka–Munk function of a) Cr:TiO2 and b) Cr/TiO2 at different chromium contents.

Hardcastle and Wachs,[37] the 800–950 cm1 region encompasses the antisymmetric stretching of the CrOCr vibration and the symmetric stretching of the CrO3 terminal unit. Bands in the 950–1000 cm1 region are characteristic of symmetric and antisymmetric stretching vibrations of the CrO2 moiety in the tri- or tetramer.[36] Trivalent chromium is usually difficult to characterize because of the forbidden ligand-field transitions in the visible region, which cause a decrease in the Raman intensity.[39] However, Yang et al. recently studied the Raman spectrum of a-CrOOH and attributed two intense bands at 823 and 630 cm1 to ns(CrIIIO) vibrations.[40] Low-intensity bands were also observed in the 985–889 cm1 region, but this wavenumber range corresponded to a bond length that was too short and a bond order that was too large to be solely due to CrIIIO vibrational modes.[41] Those vibrations were then assumed to be n(CrVIO) vibrations or mixed CrIII/CrVIO vibrational modes, following a proposition by Maslar et al.[42] The observed Raman shifts of our samples are similarly attributed to the presence of chromium oxyhydroxide clusters with a mixed oxidation state, which are deposited on the surface of titania. For Cr:TiO2, two bands at 995 and 1005 cm1 were observed for a chromium content higher than 0.59 wt % (Figure 1). The first band is attributed to the vibration of a Cr=O bond on the surface of titania. The second band corresponds to the same vibration, but under dehydrated conditions caused by laser heating. Indeed, dehydration shortens the terminal Cr=O bond due to the absence of hydrogen bonding, and thus, generally shifts this vibration to above 1000 cm1.[21] The presence of Cr=O bonds indicates that some CrIII cations on the surface are oxidized to a higher oxidation state. As mentioned above, switching the laser irradiation wavelength from l = 532 to 785 nm decreases the intensity of the broad peaks at 750 cm1 previously attributed to trivalent chromium inserted in the TiO2 lattice. With l = 785 nm irradiation, a weak and broad absorption located at 820 cm1 could then be resolved and was attributed to CrIIIO vibrations on the surface of TiO2 (Figure S6 in the Supporting Information). Thus, in agreement with ESR  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

results, the preparation method applied for Cr:TiO2 fails to introduce all trivalent chromium cations into the TiO2 lattice. Structure versus optoelectronic properties For Cr/TiO2, three transitions, the intensities of which increase with chromium loading, are found in the UV/Vis range at l = 390, 440, and 622 nm (Figure 8). These bands are very similar to those observed for pure a-CrOOH (Figure S8 in the Supporting Information). The spin-allowed quartet–quartet transitions in d3 transition-metal cations are known to give rise to broad absorption bands. These are easily assigned to 4A2g !4T2g(t22e), 4 A2g !4T1g(t22e), and 4A2g !4T1g(t2e2) in an octahedral field, according to a d3 Tanabe–Sugano diagram.[43] The 4A2g !4T1g(t2e2) transition occurs in UV region and is concealed by the strong charge-transfer transition of TiO2. The band at l = 390 nm is then attributed to a charge-transfer transition O!Cr, and the bands at l = 440 and 622 nm are attributed to 4A2g !4T1g and 4 A2g !4T2g transitions, respectively. The Racah parameter is equal to 661 cm1, which is an acceptable value when compared with different materials in which CrIII appears in octahedral coordination.[44] The crystal field parameter is 1.99 eV and the ligand-field stabilization energy is equal to 230 kJ mol1, which is also in accordance with octahedral field stabilization.[45] With Cr:TiO2, a broad feature at low wavelength is easily decomposed with two bands centered at l = 703 and 739 nm (inset of Figure 8). The clear splitting of the low-energy transition is difficult to interpret. From octahedral to tetragonal symmetry, ground-state A2g(Oh) transforms into 4B1g, whereas 4 T2g(Oh) splits into (4B2g, 4Eg).[46] A strong tetragonal distortion could explain why two transitions are observed in this energy region if the local symmetry (D2d) around the chromium center remains. Nevertheless, previous EXAFS and ESR results have suggested possible relaxation around chromium, which, in this case, discards the previous explanation. Also, a simple estimation of the crystalline field parameter by using one of those ChemSusChem 0000, 00, 1 – 12

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CHEMSUSCHEM FULL PAPERS two bands gives either 1.68 or 1.76 eV. In both cases, this is much lower than that of Cr/TiO2 despite the quasi-identical CrO bond length (1.96 vs. (1.97  0.01) ). Recent studies from Gaudry et al. and Juhin et al. have extensively studied the relationships between the optical properties of trivalent chromium inserted in various oxides and its local environment.[27, 47, 48] The main conclusion from their extensive work is that not only the CrO bond length influences the crystal field parameter, but also the CrO bond covalence and electron density on the surrounding oxygen atoms. Of course, those properties are expected to be different when trivalent chromium is inserted within the lattice of anatase or considered in pure chromium oxide/oxyhydroxide. Additionally, the possibility of charge transfer between CrIII and TiIV were also mentioned earlier by Borgarello et al. to attribute the broad absorption at l = 450 nm.[4] The latter absorption may overlap with the 4 A2g !4T1g transition and the charge-transfer transition O!Cr. An unequivocal assignment of the observed absorption bands requires DFT calculations that include different types of defects to compute the optical properties of Cr:TiO2. Those calculations are ongoing and will be reported later.

Photocatalytic oxidation of formic acid Photocatalytic oxidation of formic acid in the presence of O2 by using UV/Vis irradiation shows a decrease in the reaction rate for both preparation methods as the chromium content is increased (rate from > 100 to  20 mmol h1; Figure S9 in the Supporting Information). The decrease in photocatalytic activity is explained by considering the action spectra (Figure 9) in the UV range of Cr/TiO2 and Cr:TiO2 (Cr content = 0.2 wt %). For l = 330 and 370 nm, the photonic efficiencies are less than those of pristine TiO2 (  2.5–3 % vs. 8 %). Thus, electron–hole pairs created thanks to titania interband transitions are recombined faster in the presence of chromium, regardless of whether it is on the surface or inserted into the titania lattice. In addition, the decrease in activity is higher when chromium is dissolved the in titania lattice.

www.chemsuschem.org In contrast, if photocatalytic oxidation is carried out in the visible region (l > 420 nm), a maximum rate reaching 0.5 mmol h1 is observed for 0.2 wt % chromium in Cr:TiO2 and not for Cr/TiO2 (Figure S9 in the Supporting Information). In the latter case, the rate remains constant for a chromium content larger than 0.6 wt %. A value that is two times higher (1.1 mmol h1) is reached when chromium is supported on titania instead of inserted in the lattice. The photocatalytic rates in the visible region remain very low compared with the UV/Vis region. In summary, a twofold rate increase in the visible region is reached when chromium is supported on titania instead of inserted inside the lattice, but for both materials the photocatalytic rates in the visible region remain very low compared with those in the UV/Vis region. Let us examine now the structure/ activity relationship in the two different materials in view of the battery of physical tests carried out in both samples.

Cr/TiO2 To exclude the oxidation of formic acid by CrVI, the test was repeated five times with the same samples. 271 mmol of formic acid were oxidized, largely exceeding the 6 mmol of stoichiometric threshold. There is also the possibility that a small amount of a-CrOOH in Cr/TiO2 exhibits considerable photocatalytic activity, even though a-CrOOH has never been reported as a photocatalyst. To verify this hypothesis, the photocatalytic oxidation of formic acid was performed by using as-synthesized a-CrOOH with visible-light irradiation (l > 420 nm) and different suspension loadings (from 0.026 1 g L1). Formic acid degradation was observed at a higher rate as the suspension loading increased (0.05 up to 1.0 mmol h1). Then, a suspension that was a mechanical mixture of titania and a-CrOOH was used to check the degradation rate of formic acid under the same conditions. The rate was ten times lower (0.1 vs. 1.1 mmol h1) when the mechanical mixture of TiO2 and aCrOOH was applied, compared with Cr/TiO2 at an equivalent chromium content (e.g., 1.56 wt %). Thus, a clear synergistic effect is achieved when amorphous chromium oxyhydroxide is

Figure 9. Overlap of the action spectra with UV/Vis absorbance for Cr:TiO2 (left) and Cr/TiO2 (right) with 0.2 wt % chromium content.

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CHEMSUSCHEM FULL PAPERS supported on the surface of titania. A closer look at the action spectrum of Cr/TiO2 in the visible region (Figure 9) shows that only wavelengths from l = 420 to 560 nm are active. Irie et al. proposed that the origin of the photocatalytic activity arose from charge transfer from t2g levels of trivalent chromium to the conduction band of titania.[16, 17] A control experiment with chromium supported on a silica surface did not exhibit any photocatalytic activity; this indicates that a suitable crystalline semiconductor is indeed needed. Irie et al. also proposed that the wavelength of the charge-transfer transition might have corresponded to the shoulder at l = 390 nm in the UV/Vis absorption spectrum of Cr/TiO2.[16, 17] The latter attribution seems unlikely because an identical absorption band at the same position is observed in the absorption spectrum of pure a-CrOOH (Figure S10 in the Supporting Information). Thus, the latter band was previously attributed to the O!Cr charge-transfer transition. Furthermore, the photocatalytic activity extends up to l = 560 nm, which indicates that the 4A2g !4T1g transition is the first step in the photocatalytic mechanism. Then, charge transfer from the 4T1g term to the conduction band of titania is proposed as a second step. Upon electron transfer, trivalent chromium is oxidized to the tetravalent state and may allow the further oxidation of formic acid. Simultaneously, electrons in the titania conduction band are expected to reduce O2. Besides, it is not clear at the moment if the dramatic decrease in activity in the UV region is caused by chromium oxyhydroxide playing the role of a surface recombination center or hindering the reduction of O2 on the titania surface. Cr:TiO2 The action spectrum of Cr:TiO2 (Figure 9) shows that photonic efficiencies are negligible beyond l = 600 nm and are barely higher than bare TiO2 between l = 400 and 600 nm. Compared with Cr/TiO2, the photonic efficiencies are half those in the l = 400–600 nm range. The origin of photocatalytic activities in the visible region is not straightforward to explain. If the major species is considered, namely, trivalent chromium within the TiO2 lattice, the most probable electronic transition that could be at stake is also charge transfer from chromium t2g levels to the conduction band of titania. Nevertheless, ESR and Raman scattering results suggested that a minor amount of chromium was also deposited on the TiO2 surface as a chromium oxyhydroxide. Previous photocatalytic results with Cr/TiO2 have also demonstrated that the latter appears to be photocatalytic and exhibits high photonic efficiencies in the same wavelength range. It means that chromium located in the TiO2 lattice is not necessarily the species from which the visible photocatalytic activities originate, although it is present as the major species. Furthermore, the decrease in the activity in the UV range has been classically interpreted by the role of chromium as a recombination center in TiO2.[49] We propose an alternative explanation based on our previously presented structural characterization. XRD patterns of Cr:TiO2 with increasing Cr content exhibit a strong decrease in the intensity of the reflections at low angle. Whether or not this is attributed to the presence titanium or oxygen vacancies, it is indicative of a loss of the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org overall anatase crystallinity. The decrease in the photocatalytic activity in the UV region may then be attributed to the lower quality of bulk titania once doped with chromium.

Conclusions We described the synthesis and in-depth characterization of two types of photocatalysts based on chromium modification of titania. The first type of modification consisted of supporting amorphous chromium oxyhydroxide clusters on the surface of titania (Cr/TiO2). Chromium species in Cr/TiO2 displayed some structural similarities to the a-CrOOH crystalline phase and also similar optical properties (d–d transitions and chargetransfer transitions). Chromium was mostly trivalent, although a minor amount of hexavalent chromium was also present. The second type of modification involved the substitution of titanium for chromium in the anatase lattice (Cr:TiO2). Chromium was present in trivalent state and charge neutrality was provided by either oxygen vacancies or condensation defects that might have remained after hydrothermal synthesis (e.g., 0000 VTi þ 6OHi ). Although the first coordination sphere of chromium appeared to be similar to Cr/TiO2 (octahedral environment and close CrO bond lengths), the optical properties of Cr:TiO2 were clearly different. A minor amount of chromium was also found in the form of amorphous oxyhydroxide on the surface of titania. Photocatalytic oxidation of formic acid proved that both types of photocatalysts were active in the visible region. The strategy involving the surface modification of titania was more effective. Unfortunately, both types of modification led to deactivation in the UV region due to the creation of new electron–hole recombination centers.

Experimental Section Photocatalyst preparation Chromium-doped titania (Cr:TiO2): A solution of TiOCl2 ([Ti] = 0.1 mol L1) was precipitated with the sudden addition of concentrated ammonia (aiming at pH 9). To introduce chromium within the TiO2 lattice, variable amounts of CrCl3·6 H2O were added before starting the precipitation. Maximum chromium content was selected to remain within the solubility limits of Cr in the TiO2 lattice.[50] The initial weight ratios m(Cr)/m(TiO2) were 0, 0.06, 0.12, 0.20, 0.59, 1.17, and 1.56 wt %. The suspensions were placed in 200 mL Teflonlined autoclaves at 180 8C for 48 h. After hydrothermal treatment, samples were thoroughly washed with ultrapure water and dried at 120 8C. Chromium-loaded titania (Cr/TiO2): A solution of TiOCl2 ([Ti] = 0.5 mol L1) was precipitated with concentrated ammonia until pH 9. The suspension was autoclaved at 180 8C for 48 h to crystallize TiO2. The resulting TiO2 suspension was then divided into seven parts to which variable amounts of CrCl3·6 H2O were added. The weight ratios were also 0, 0.06, 0.12, 0.20, 0.59, 1.17, and 1.56 wt %. The suspensions were then placed in 200 mL Teflonlined autoclaves at 180 8C for 48 h. After hydrothermal treatment, samples were thoroughly washed with ultrapure water and dried at 120 8C. a-CrOOH: A solution of CrCl3·6 H2O ([Cr] = 0.10 mol L1) was precipitated with concentrated ammonia until pH 9 was reached. The susChemSusChem 0000, 00, 1 – 12

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CHEMSUSCHEM FULL PAPERS pension was autoclaved at 180 8C for 48 h. After hydrothermal treatment, the solid was thoroughly washed with ultrapure water and dried at 120 8C.

Characterization of the photocatalysts N2 adsorption/desorption isotherms were acquired on a Micromeritics ASAP2420 device. The samples were degassed for 2 h at 120 8C before nitrogen adsorption. Surface areas of the samples were analyzed by the multipoint BET analysis method, and the pore volume was estimated at P/P0 = 0.99. Inductively coupled plasma–atomic emission spectroscopy (ICP-AES; Activa–Horiba Jobin Yvon) was performed to quantify the absolute amount of titanium and chromium. Diffuse reflectance UV/Vis spectra were measured with an AvaSpec-2048 fiber optic spectrometer equipped with a 2048 pixel CCD detector array. BaSO4 was used as a reference. Impedance spectroscopy measurements were performed by using a 16-channel, research-grade potentiostat system (VMP3) from BioLogic Science Instruments in a conventional three-electrode single electrochemical cell. Raman experiments were achieved by using a UV/Vis/near-IR LabRam HR spectrometer (Horiba–Jobin Yvon). The exciting line at l = 532 and 785 nm delivered by two solid-state diode pumped lasers (Sacher Lasertechnik and Torus) were focused by using a 50  objective and the diffused light was dispersed with a 1800 grooves/mm diffraction grating, leading to spectral resolution of at least 0.5 cm 1. Extreme care was taken to choose a working laser power (68 mW) for which the heating effect was negligible. XRD patterns were collected by using a Bruker D8 Advanced A25 diffractometer in Bragg–Brentano geometry equipped with a copper tube. The XRD data were analyzed by the Rietveld method by using the fundamental parameters approach contained within the software TOPAS V4.2 (Bruker-AXS).[51] More details are given in the Supporting Information. XAS experiments were performed on the CRG-FAME beamline (BM30B) at the European Synchrotron Radiation Facility in Grenoble. Spectra were recorded either in fluorescence (photocatalysts) or in transmission (references) modes at the Cr K edge. XAS data were analyzed by using the HORAE package; a graphical interface to the AUTOBK and IFEFFIT code.[52] XANES and EXAFS spectra were obtained after performing standard procedures for pre-edge subtraction, normalization, polynomial removal, and wave vector conversion. The amplitude factor (S02) was fitted to the EXAFS spectrum obtained for the Cr2O3 reference compounds; the crystallographic structures of the compounds were well known.[53] S02 was equal to 0.80; a value close to that obtained by Juhin et al. (0.81).[48] Quantitative XANES analyses were performed with the FDMNES code by using the multiple scattering theory, in which the muffin-tin approximation was made to describe the shape of the potential, and self-consistent calculations with LDA + U.[54, 55] The Hubbard parameter, U, for chromium was set to 3.0 eV, following DFT calculations by Yang et al.[56] X-band (9.4 GHz) EPR spectra were recorded at 119 K on a Bruker ESP 500E spectrometer by using a standard rectangular (4102ST) EPR cavity (Bruker ER4131VT). A microwave power of 1.6 mW and a modulation amplitude of 1 G were used. A quantitative study was also performed with 1 cm of powder sample in ESR quartz tube (3 mm). A strong pitch was used as a spin reference (3  1015 spins cm1). Different powers of microwave were used to test the saturation of ESR signals. The signal amplitude was proportional to the square root of microwave power, which meant the ESR intensity remained proportional to the quantity of spins. Quantitative  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org analysis was performed by fitting the experimental EPR spectra by using the simulation software EasySpin (Matlab toolbox).[57] Imaging of samples was performed on a Titan G2 80–300 kV TEM from FEI (FEI Company, Hillsboro, OR) equipped with a 4k  4k charge coupled device (CCD) camera of model US4000 and a postcolumn energy filter model GIF Tridiem (Gatan Inc., Pleasanton, CA). The energy filter was utilized in EFTEM mode to highlight the distribution of Ti and Cr in the samples. The Ti L edge of energy 456 eV and Cr L edge of energy 575 eV were selected for the EFTEM maps. Furthermore, each elemental map was created by using a so-called three-window method.[58] EDX mapping was performed on a Titan G2 80–200 kV S/TEM equipped with SuperX EDX detectors. EDX maps were recorded by using a beam current of 0.4 nA and a dwell time of 25 ms/pixel.

Photocatalytic tests A 100 mL photoreactor with an optical window at the bottom of the reactor, equipped with a circulating water cell to remove IR radiation, was used for photocatalytic oxidation of formic acid (50 ppm aqueous solution). The volume of the suspension was 50 mL with 50 mg of photocatalyst. Adsorption of formic acid was performed for 1 h to reach the stationary state prior to irradiation. Quantitative analysis of formic acid concentration was performed by liquid-phase chromatography (Agilent) by using a cation-exchange chromatography column (Sarasep CAR-H 300  7.8 mm), a solution of H2SO4 (5  103 m) as the mobile phase, and a photodiode array detector adjusted at l = 210 nm. A 300 W xenon lamp (Tokina) was positioned under the reactor and glass filters were placed between the water cell to provide irradiation. Photon flux was measured with an AvaSpec-2048 fiber optic spectrometer equipped with a 2048 pixel CCD detector. Irradiance for UV/Vis, Vis irradiation, and band pass filtered irradiations (used for action spectra) are given in Figure S10 in the Supporting Information. For the action spectra, the integrated intensities of the irradiations were all adjusted to 3.7  108 einstein s1. Photonic efficiency was calculated as the ratio of the rate of formic acid oxidation to the flux of incident photons. Because some residual UV photons could not be completely filtered, pure titania was also tested by using the same wavelengths to obtain a blank action spectrum.

Acknowledgements We gratefully acknowledge the King Abdullah University of Science and Technology for support of this research through the CADENCED project. The analytical service of IRCELYON is acknowledged for elemental analysis. We acknowledge the KAUST Imaging and Characterization core lab, and scientific and technical assistance of Drs. Rachid Sougrat (TEM), Yang Yang (Raman), and Bei Zhang (SQUID). Yves Joly (Institut Neel) is kindly acknowledged for fruitful discussions related to XANES and his assistance with using the FDMNES code. Anna Carlsson is thanked for STEM characterizations at FEI Company (Netherlands). Keywords: chromium · doping · photochemistry · surface chemistry · titanium [1] T. Sano, E. Puzenat, C. Guillard, C. Geantet, S. Matsuzawa, J. Mol. Catal. A 2008, 284, 127 – 133. [2] A. Kachina, E. Puzenat, S. Ould-Chikh, C. Geantet, P. Delichere, P. Afanasiev, Chem. Mater. 2012, 24, 636 – 642.

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FULL PAPERS S. Ould-Chikh,* O. Proux, P. Afanasiev, L. Khrouz, M. N. Hedhili, D. H. Anjum, M. Harb, C. Geantet, J.-M. Basset, E. Puzenat && – && Photocatalysis with Chromium-Doped TiO2 : Bulk and Surface Doping

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It’s classified! The photocatalytic properties of TiO2 modified by chromium depend strongly on the preparation method. To clarify this problem, two types of modified titania are discussed: one with CrIII doped in the bulk and one with CrOOH clusters on the TiO2 surface (see picture). Both series show visiblelight-driven photo-oxidation activity. However, surface modification appears to be a more efficient strategy.

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