Colloidal Counterpart of the TiO2-Supported V2O5 System: A Case

Article pubs.acs.org/JPCC

Colloidal Counterpart of the TiO2‑Supported V2O5 System: A Case Study of Oxide-on-Oxide Deposition by Wet Chemical Techniques. Synthesis, Vanadium Speciation, and Gas-Sensing Enhancement Mauro Epifani,*,† Raül Díaz,‡ Carmen Force,§ Elisabetta Comini,∥ Teresa Andreu,⊥ Reza R. Zamani,⊥,# Jordi Arbiol,#,▽ Pietro Siciliano,† Guido Faglia,∥ and Joan R. Morante⊥,○ †

Istituto per la Microelettronica e i Microsistemi, IMM-CNR, Via Monteroni, 73100 Lecce, Italy Electrochemical Processes Unit, IMDEA Energy Institute, Avda. Ramón de la Sagra, 3 28935 Móstoles, Spain § NMR Unit, Centro de Apoyo Tecnológico, Universidad Rey Juan Carlos, c/Tulipán, s/n, 28933 Móstoles, Spain ∥ SENSOR Lab, Department of Information Engineering, Brescia University and CNR-INO, Via Valotti 9, 25133 Brescia, Italy ⊥ Catalonia Institute for Energy Research, IREC, c/Jardins de les Dones de Negre 1, 08930 Sant Adria del Besos, Barcelona, Spain # Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, Spain ▽ Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys, 23, 08010 Barcelona, CAT, Spain ○ M2E-IN2UB-XaRMAE, Departament d’Electrònica, Universitat de Barcelona, c/Martí i Franquès 1, 08028 Barcelona, Spain ‡

S Supporting Information *

ABSTRACT: TiO2 anatase nanocrystals were surface modified by deposition of V(V) species. The starting amorphous TiO2 nanoparticles were prepared by hydrolytic processing of TiCl4-derived solutions. A V-containing solution, prepared from methanolysis of VCl4, was added to the TiO2 suspension before a solvothermal crystallization step in oleic acid. The resulting materials were characterized by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared, Raman, and magic angle spinning solid-state 51V nuclear magnetic resonance spectroscopy (MAS NMR). It was shown that in the as-prepared nanocrystals V was deposited onto the surface, forming Ti−O−V bonds. After heat treatment at 400 °C, TEM/electron energy loss spectroscopy and MAS NMR showed that V was partially inserted in the anatase lattice, while the surface was covered with a denser V−O−V network. After heating at 500 °C, V2O5 phase separation occurred, further evidenced by thermal analyses. The 400 °C nanocrystals had a mean size of about 5 nm, proving the successful synthesis of the colloidal counterpart of the well-known TiO2−V2O5 catalytic system. Hence, and also due to the complete elimination of organic residuals, this sample was used for processing chemoresistive devices. Ethanol was used as a test gas, and the results showed the beneficial effect of the V surface modification of anatase, with a response improvement up to almost 2 orders of magnitude with respect to pure TiO2. Moreover, simple comparison of the temperature dependence of the response clearly evidenced the catalytic effect of V addition. catalysis field, where the reaction of the target gas with the sensing layer is followed by a change of the resistance of the catalyst. Hence, it is reasonable to search for hints in catalysis field about how to improve chemoresistive sensors. A wellknown catalyst system is the titania-supported vanadium pentoxide.6 In particular, it is known as a promoter of oxidation reactions of many organic compounds,7 which makes it an ideal candidate for detection of volatile organic compounds. Our question was whether it is possible to transfer this material architecture to the processing of gas-sensing devices. The

1. INTRODUCTION The modification of metal oxides by additives or dopants is well-known for improving the performances of resulting chemoresistive gas sensors.1 Common additives are transition metals, dispersed as ions in the oxide structure,2 or noble metals,3 usually present as nanoparticles dispersed among the oxide grains. Additives may induce electronic sensitization, based on the modification of the electronic properties of the host oxide,4 or spillover.5 In the latter, the noble metal reacts with the gaseous analyte molecules to produce more reactive species that subsequently “spill” over the surface of the surrounding oxide host. This mechanism is well-known in catalysis. The relationship with catalysis is not surprising since chemoresistive gas sensing is an application of heterogeneous © 2013 American Chemical Society

Received: July 2, 2013 Revised: September 15, 2013 Published: September 16, 2013 20697

dx.doi.org/10.1021/jp406518w | J. Phys. Chem. C 2013, 117, 20697−20705

The Journal of Physical Chemistry C

Article

precursor.12 In detail, 10 mL of MeOH was slowly added to 0.75 g of VCl4 in a glovebox (Braun, LabStar) with H2O < 1 ppm. A green solution was obtained, with evolution of green smoke. After the smoke evolution ceased, the solution was taken out of the glovebox, and water was added dropwise. The H2O:V molar ratio was 16. After a few days, a bright blue solution was obtained, which is stable over years. X-ray diffraction (XRD) measurements were performed on a Panalytical diffractometer working with the Cu Kα radiation (λ = 1.5406 Å) using a Bragg−Brentano geometry. The XRD results were analyzed by Rietveld refinement, using the Fullprof software. High-resolution transmission electron microscopy (HRTEM) analyses of the powders were carried out with a field-emission gun microscope Jeol 2010F, working at 200 kV and with a point-to-point resolution of 0.19 nm. Electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) mode was also performed for chemical analyses at the nanoscale13 in the same microscope with an embedded Gatan Image Filter (GIF2001). Fourier transform infrared (FTIR) measurements were carried out with a Nicolet 6700 spectrometer in diffuse reflectance setup, after dispersing the sample powders in KBr. Raman spectroscopy was performed by means of a Jasco NRS-5100 spectrometer with a green laser in a micro-Raman configuration with 100× objective and with a laser power of 10 mW. The magic angle spinning nuclear magnetic resonance (MAS NMR) 51V spectra were recorded at room temperature on a Varian Infinity 400 spectrometer at 9.4 T, using 4 mm zircon rotors and spinning speeds between 8 and 12 kHz. The Larmor frequency for 51V was 105.24 MHz. A single pulse was the sequence used in all the cases, with 3.5 μs as π/2 pulse excitations. The interval between successive accumulations, 1 s, was chosen to avoid saturation effects. The number of accumulations, 66 000, was selected to obtain a correct signalto-noise ratio (S/N = 20). The chemical shift was determined with NH4VO3 as the external reference (−571.5 ppm) and VOCl3 as the internal reference (0 ppm). Thermal analyses were carried out in a thermal balance model SDT Q-600 from TA Instruments under air flow of 100 mL/min and thermal ramp of 10 °C/min. The gas-sensing tests were carried out using a standard configuration for resistive sensor measurement, with Ptinterdigitated electrodes and a Pt-resistive-type heater printed onto an alumina substrate. Sensors were fabricated by depositing a paste made by mixing the prepared powder with 1,2-propanediol onto interdigitated electrodes. Before measurements, the sensors were kept at a temperature of 400 °C, provided by the sensor heaters, to decompose the organic residuals and stabilize the electrical signal. For this aim, and for sensing tests, the sensor devices were placed in a sealed chamber with a constant flux of 0.3 L/min of synthetic air into which the desired amount of test gases was mixed. The sensor response was defined as Ggas − G0/G0, where G0 is the sensor baseline electrical conductance in synthetic air and Ggas indicates the sensor electrical conductance after exposure to the target gas. The gas tested in the present work was ethanol, in concentrations ranging from 50 to 500 ppm.

leading idea was that the surface vanadium oxide species could favor the electronic exchange between the gaseous analyte and the titania support. This is different from the promoting mechanisms previously mentioned since now the aim is the presence of a whole surface-modifying layer. Of course, the preparation of titania-supported vanadia is deeply explored by many different approaches, mainly based on impregnation procedures.8 Our aim was to enhance the material architecture. In fact, vanadia surface modification of nanosized supports would be even more appealing, given the well-established potential of nanocrystalline oxides in boosting the performance of chemoresistive sensors. Hence what is required is the development of a sort of “core−shell” nanocrystal structure, or of a monolayer deposition. This requirement naturally implies working with colloidal systems, where each titania nanocrystal can be independently functionalized. While this configuration is very well-known for II−VI semiconductors or other nonoxide systems, its extension to colloidal oxide nanocrystals is still quite exotic, and the very few available examples are mainly related to vapor deposition processed nanostructures. 9 Matching the chemistry of the two oxide components can be a remarkable problem. In general, rapid self-polymerization of each oxide precursor is observed, with low probability to form heterobonds. Moreover, the capping ligands used for preparing colloidal dispersion of the core component may further hinder the deposition of the other oxide. These may be possible reasons for the scarcely explored synthesis of such systems. We begun investigating the titania−vanadia nanocomposites in recent work, and we previously established the beneficial surface modification by vanadium addition. Nevertheless, we shortly focused onto the as-prepared materials for supercapacitor applications,10 and the problem of thermal stability for use in harsh environments was not investigated. Above all, the exact placement of vanadium and the nature and actual demonstration of the surface modification were also neglected. In the present work we will show the successful deposition of vanadium oxide onto the surface of colloidal titania nanocrystals. The actual presence of a surface layer of vanadium oxide, together with core doping of the titania support, was ensured by detailed cross-characterization techniques. The detailed proof of colloidal modification of oxide nanocrystals by another oxide species is the synthesis breakthrough presented in the paper. Moreover, we will show that the surface deposition of another active oxide is very effective for overall improvement of the sensing properties. Ethanol sensing will be a simple example of such properties.

2. EXPERIMENTAL SECTION Pure, amorphous TiO2 nanoparticles were synthesized by a sol−gel process. They were crystallized by solvothermal treatment in oleic acid. In detail,11 0.75 g of TiCl4 was reacted with 10 mL of methanol (MeOH), followed by water addition, with a H2O:Ti molar ratio of 16. Then 2 mL of the resulting solution was injected into 10 mL of n-dodecylamine at room temperature, and the resulting slurry was heated at 100 °C for 1 h, followed by extraction of the white precipitate and purification with acetone. The following solvothermal crystallization step was carried out for 2 h at 250 °C after dispersing the extracted TiO2 nanoparticles in 10 mL of oleic acid. For preparing TiO2 −V2 O5 materials, 0.5 mL of vanadium chloromethoxide solution was added to the titania suspension just before the solvothermal treatment. The vanadium chloromethoxide was prepared in an analogous way to the Ti

3. RESULTS AND DISCUSSION The pure V chloroalkoxide used in the present work is extremely stable against inorganic cross-linking, even after 20698



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Moreover, the extent of Δ, which is the separation between the carboxylate stretching bands, is 110 cm−1 and indicates the presence of chelating bonds.14a,c Such a chelating configuration indicates less crowded ligands on the nanocrystal surface, hence favoring the reaction with vanadium precursor species. In the TiO2−V2O5 sample, the carbonyl band at 1709 cm−1 is still present after repeated sample washings. This result indicates that in the presence of vanadium oleic acid does not simply form a monolayer, pointing to different surface composition of the nanocrystals with respect to pure TiO2. Structural characterization was first carried out by XRD. The patterns are shown in Figure 2, where only the characteristic reflections

hydrolysis with large water excess. For this reason the resulting solutions are stable over years. In our previous work,12 the presence in solution of the vanadyl (VO) groups was indicated as the main reason for the precursor stability. Extended networks between V chloroalkoxide molecules were hardly obtained. On the other hand, during the solvothermal treatment with the amorphous titania suspension, hetero crosslinking may occur between Ti−OH species on the surface of preformed titania nanoparticles and single, hydrolyzed V chloroalkoxide molecules. Hence, the surface of the titania support would act as a support for anchoring single V precursor species. If sufficiently dense, these surface V species may further co-condensate to form surface layers of V oxide. The high temperatures and high pressures involved in solvothermal treatment may enhance the kinetics of such processes. For establishing whether the actual structure of the product had achieved the initial aim, a multitechnique approach was used. Our strategy was to get indirect evidence about the surface structure, by comparing the results of the various techniques and investigating whether they were pointing toward a unique conclusion. The as-prepared samples were first analyzed more in detail with respect to our early work, where chemical analysis previously ensured V presence, with 7 wt % concentration.10 FTIR spectroscopy was used to evidence possible changes due to the presence of vanadium. The results are shown in Figure 1.

Figure 2. XRD patterns measured on the indicated as-prepared samples, together with the Rietveld calculated profiles and the residual plot.

of the TiO2 tetragonal (anatase) crystallographic phase could be seen. The data were analyzed by the Rietveld method, and no differences emerged. The a and c parameters were 3.79 and 9.50 Å and 3.80 and 9.49 Å for pure TiO2 and TiO2−V2O5, respectively. Similar results were obtained from the HRTEM investigations. An example is shown in Figure 3, related to the TiO2−V2O5 sample. Characteristic rod-like structures were observed in both TiO2 and TiO2−V2O5, together with more spherical particles. The structural parameters were in agreement with those of the anatase crystallographic phase, without

Figure 1. FTIR spectra of TiO2 and TiO2−V2O5 samples and of pure oleic acid.

The signal of pure oleic acid is characterized by an intense carbonyl band at 1709 cm−1, indicating the presence of oleic acid dimers. C−O stretching is observed at 1285 cm−1 and O− H in-plane and out-of-plane bending at 1464 and 939 cm−1, respectively. In the pure TiO2 curve, the carbonyl band disappears, and peaks at 1520 and 1410 cm−1 appear, which are typical of asymmetric and symmetric carboxylate stretching, respectively, and show that a surface Ti oleate complex was formed.14

Figure 3. HRTEM image of the as-prepared TiO2−V2O5 sample is shown (left) and related power spectrum (FFT, right). Notice the perfect anatase crystallization of the nanorods (shown in the FFT). 20699



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notable differences between the two samples, confirming the XRD results. The data presented until now showed that there was no structural modification of the anatase host in the as-prepared samples. The possibility of amorphous, separate vanadium oxide species could not be excluded by this early analysis. We used the following strategy to further assess this crucial point. In previous work, we observed that V2O5 was crystallized after heating at only 350 °C.12 The vanadium precursor was the same that we have used in the present work. Hence, if in the asprepared TiO2−V2O5 material some amorphous vanadium oxides species were present, not detected by both XRD and TEM, heat treatment at temperature starting from 350 °C would reveal them. In this way, information could also be gained about the overall thermal stability of the nanocomposites at typical sensor operating temperatures. Figure 4 Figure 5. Raman spectra of TiO2−V2O5 samples heat-treated at the indicated temperatures. The low-frequency region is not shown for clarity and only contains the Eg mode of anatase. Stars in the 500 °C spectra indicate the V2O5 bands.

The presence of such groups inside the anatase structure is also unlikely since it would require extensive reduction of Ti(IV), which is very unfavorable in the heat-treatment conditions. From these data, we conclude about the presence of surface vanadyls up to 400 °C. Close inspection of the region around 1000 cm−1 provides further insight about surface vanadium species (see Figure S1, Supporting Information, for enlarged region). In the 400 °C series, a weak satellite band is observed at about 1010 cm−1. At 500 °C, this band is a very weak shoulder of the 990 cm−1 vanadyl signal, while the 1030 cm−1 band is still present despite it now being very weak. From the XRD and Raman data, the transition from 400 to 500 °C induced phase separation of vanadia. Hence, we attribute the 1010 cm−1 band at 400 °C to the surface Ti−O−V bond. When the samples were heated at 500 °C, much of the surface vanadyls were expelled with the growing V2O5 phase, resulting in the just described band changes. The 1010 cm−1 band is not reported in other Raman studies of the classical TiO2−V2O5 system. We believe that its observation was made possible by enhanced importance of the surface with respect to the bulk of the material in our nanocrystalline species: even after heating at 400 °C the nanocrystal size is about 5 nm (see TEM results in Figure 7). The presence of the V(V) monolayer on the titania support is a common finding in catalysis studies, but all the investigated systems were prepared by the classical impregnation method of preformed titania powders. As concerns V surface coordination, it has been recently shown that any umbrella structure should be excluded,16 favoring trigonal geometry with the vanadyl bond outward the surface. A dynamic view of the vanadia phase separation was obtained by thermal analysis. The results are shown in Figure 6A. The curves have a simple structure, mainly characterized by an intense exothermic peak at about 250 °C, obviously related to the elimination of oleic acid, and an associated mass loss lasting up to about 400 °C. At higher temperatures, the curve enlargement shown in the inset evidences two additional exothermic peaks associated with mass gain. In agreement with the previous XRD and Raman results, these phenomena were attributed to the segregation of the V2O5 phase and the related oxygen uptake from the surrounding atmosphere. The exothermic feature is associated with the entropy decrease

Figure 4. XRD patterns of the TiO2−V2O5 samples heat-treated at the indicated temperatures.

shows the XRD patterns of the samples heat-treated up to 500 °C. After 400 °C, the pattern showed only the anatase reflections, similarly to the as-prepared sample. The peak width did not indicate appreciable grain growth. Only after heating at 500 °C, the peak of the V2O5 phase appeared, together with the anatase peaks. The remarkably enhanced crystallization temperature of V2O5 with respect to its pure precursor was a clear indication that vanadium oxide was not present as amorphous, separate species in the as-prepared sample. More detailed confirmation of this hypothesis was obtained from Raman spectroscopy, which is more sensitive than XRD in revealing low concentrations of crystalline species. The results are shown in Figure 5. In agreement with XRD results, V2O5 phase separation occurs only after heating at 500 °C. In this case, the Raman spectra contain all the typical V2O5 bands.15 Micro Raman mapping showed that this phenomenon uniformly affected the sample. In the 400 °C sample, not only the V2O5 bands were absent but also the shape of the anatase bands ruled out even the presence of small phase separations. In the 400 °C spectra, an additional band is located at about 1030 cm−1. In the 500 °C series, a band is located at 990 cm−1. Both bands do not belong to the anatase structure. The latter is just the vanadyl (VO) stretching band of V2O5. The 400 °C band at 1030 cm−1 has a similar sharp shape and can only be attributed to VO stretching, but its position and the absence of other V2O5 bands suggest the presence of vanadyl groups with a different coordination with respect to that in V2O5. 20700



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Figure 6. DSC/TG curves measured on dried TiO2−V2O5 nanocrystals (A) and pure TiO2 (B) in air atmosphere. The insets detail the region above 300 °C.

in Figure 8, the indicated nanocrystal contains both Ti and V. The relative chemical composition as measured by EELS is: Ti = 30−36%, V = 7−10%, O = 50−60%. Additional analysis is included in the Supporting Information, indicating similar results. EELS relative composition analysis showed that there was a slight variation of chemical composition while moving from the center of the nanocrystal to the borders; i.e., percentage of V inside was approximately 2% higher than the borders. Due to the overlap of the edges in EELS spectra, there is a high uncertainty in the final calculated composition result (over 5%), and V and O contents could have been underestimated (since in the EELS maps Ti and V signals are in the same region). Nevertheless, the results indicated once again that there were no separate V2O5 structures or pure TiO2 host. Moreover, now we know that V implies not only surface modification but also overall doping of the anatase host. The previous TEM results indicate complex evolution of the vanadium distribution in the titania host induced by the heat treatment. More detailed information could be obtained by NMR spectroscopy, probing the local environment of the nucleus. The aim of this investigation was to obtain more clear information about the sites occupied by vanadium and how they are influenced by the heat treatment. In fact, the vanadium topology influences the electronic properties of the material. Moreover, our colloidal synthesis procedure could be more carefully compared with other TiO2−V2O5 impregnation approaches, which in the past have been extensively studied by NMR spectroscopy. Due to the presence of strong electric quadrupolar interaction and significant chemical shift anisotropy (CSA), 51 V (I = 7/2) is an NMR nucleus with a complicated spectra interpretation. These two effects produce a very broad solid NMR spectrum, and hence it is difficult to determine the intensity of central transitions which are distributed over numerous spinning side bands. Nevertheless, the intensive investigation carried out in the past by MAS NMR techniques on the TiO2−V2O5 catalyst provides a solid base for a more straightforward understanding of the spectra.17 This allowed detailed identification of the local symmetry of vanadium species, which was of interest in the present work. The 51V MAS NMR spectra for samples treated at 400 and 500 °C, together with the related chemical shifts, are reported in Figure 9. In both figures the arrows show 51V signals. Data from asprepared samples show a family of spinning sidebands with isotropic chemical shift placed at −530 and −587 ppm.10 The

corresponding to the phase separation. The same measurements were carried out on pure TiO2. The related curves, presented in Figure 6B, show the same intense exothermic feature in the lower-temperature region. Instead, the hightemperature part of the thermogravimetric curve only displays very weak mass loss, contrarily to TiO2−V2O5. Even the DSC signal does not show any exothermic peak. The different behavior of the two samples in the high-temperature region is in full agreement with the interpretation of Figure 6A. For further elucidating the vanadium placement into the titania host, HRTEM and STEM/EELS investigations were undertaken on the 400 °C sample. From the previous investigations, it turned out to be the candidate material for device processing, due to the successful surface deposition of vanadia, the lack of phase separations, and the completion of organics elimination. An image of the sample is shown in Figure 7, while more images are presented in the Supporting Information. The nanocrystals were agglomerated due to the elimination of the oleic acid capping.

Figure 7. HRTEM image of the TiO2−V2O5 sample heat-treated at 400 °C; higher magnification of the squared region and the related power spectrum.

Nevertheless, it was still possible to evaluate their size, which was between 3 and 8 nm, with a mean value of about 5 nm. As anticipated above, the heat treatment does not result in an appreciable increase of the grain size. Only anatase nanocrystals were detected, without any V2O5 structures. The nanocrystals were analyzed by means of spectrum imaging EELS to see the position of Ti, V, and O elements in their structure. As shown 20701



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Figure 8. Annular dark-field (ADF) STEM image of the 400 °C TiO2−V2O5 sample. EELS chemical mapping was obtained for each element (see the right side).

about V placement. The incorporation equation for V(V) in the anatase host can be written, in Kroger−Vink notation 5TiO2

⁗ 2V2O5 ⎯⎯⎯⎯⎯→ 4V •Ti + 10OO + Vac Ti 5TiO2

⁗ 2V2O5 ⎯⎯⎯⎯⎯→ 4V 5i • + 10OO + 5Vac Ti

for substitutional and interstitial species, respectively. In the equations, OO is a regular oxygen site, and the superscripts • and ′ indicate positive and negative charges, respectively. With respect to the usual conventions for Kroger−Vink notation, we have used the symbol “Vac” for vacancies, to avoid confusion with the vanadium symbol. It appears that interstitial V is extremely unlike, taking into account the high charge of the compensating Ti vacancies and the close packing of the anatase lattice. We can also exclude V(IV) species on the basis of XPS results (see Supporting Information) and electrical data considerations. Hence, into the anatase lattice, we may have substitutional V(V) in octahedral sites. Comparing NMR with TEM and Raman results, we conclude that the as-prepared TiO2−V2O5 samples were composed of anatase nanocrystals whose surface was covered with V5+ species with tetrahedral coordination, bonded to the surface through mainly single Ti− O−V bonds (otherwise the V symmetry would have been octahedral). After heating at 400 °C, both tetrahedral and octahedral V5+ species were present, but the first species feature two different signals. This result shows that complex surface reorganization took place. The signal of octahedral V can be due to: (a) Reorganization of the surface layer resulting in octahedral environment (related to the trigonal geometry of V in V2O521) due to multiple bonding with surface Ti species and condensation to form a denser V−O−V monolayer. This process was made possible by elimination of oleic acid, making available new surface Ti sites and reaction between neighboring V species. (b) Migration of V5+ ions inside anatase occurred, where, as we have seen above, only octahedral sites are favorable. After heating at 500 °C, NMR shows that one species of tetrahedral sites (that at −585 ppm) has completely disappeared due to continuing reorganization of surface V species. This most probably reflects further condensation of V species, which is not surprising since they are continuously being expelled to the surface to give V2O5 phase segregation (the −620 ppm resonance). The results of the previous sections show the achievement of a colloidal version of the TiO2−V2O5 system, which results in the absence of phase separation and remarkable limitation of the grain growth even after heating at 400 °C, as clearly shown by the results shown until now. This was just the fulfillment of

Figure 9. 51V MAS NMR spectra recorded at 8 kHz on 400 and 500 °C TiO2−V2O5 samples.

second is in typical position for V5+ in tetrahedral environment,17a while the former is more ambiguous. Luca et al.17e attributed a similar signal to tetrahedral V5+ too, but in the fundamental work by Eckert and Wachs similar shifts were typically observed in model compounds with distorted octahedral symmetry. Moreover, the latter authors experimentally detected signals at −510 and −550 ppm in TiO2−V2O5 samples, attributed to the octahedral and tetrahedral V5+ environment, respectively. Our −530 ppm signal lies just in the middle of such an interval. Due to our signal broadening, the simultaneous presence of V with tetrahedral and octahedral site symmetry was then suggested. After heating at 400 °C, three signals placed at −330, −587, and −822 ppm were observed. The signal at −587 ppm is still associated to the distorted tetrahedral vanadium environment. The new resonances indicate two new local environments of vanadium nuclei. The −330 ppm resonance resembles the perpendicular component of the chemical shift tensor of V2O5 and other compounds with distorted V5+ octahedral symmetry.17e The −822 ppm signal is more seldom reported but resembles that of V5+ in such compounds as NbVO5, TaVO5,18 and ZrV2O7.19 In these compounds, V is present in VO4 tetrahedral units.20 After heating at 500 °C, the main features are different positions of the octahedral and tetrahedral V5+ signals and a new resonance at −620 ppm, which is just the signal of V2O5 in agreement with the observed phase segregation. With the help of the NMR data, we can now fully describe the structural and chemical evolution of the samples through the entire thermal history. We first add important consideration 20702



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Figure 10. Left: Conductance variation for TiO2 and TiO2−V2O5 sensors as a function of ethanol square concentration pulses at an operating temperature of 200 °C and 40% RH. Right: The related ethanol calibration curves.

Figure 11. Temperature dependence for various ethanol concentrations for TiO2 and TiO2−V2O5 sensors.

the initial synthesis aim, so the following step was to test the actual sensing properties of the resulting material, to which the following section will be devoted. We further stress that the 400 °C materials were used in the device processing since they were: (a) organic free, as shown by previous thermal analysis; (b) homogeneous, since we have seen that there is no V2O5 separation; (c) stabilized, which means that they were heat-treated at a temperature higher than the typical operating temperatures that may be required by ethanol sensing (up to 350 °C, typically). Hence no structural

transformations may occur during the tests. Figure 10 shows the dynamic signal and the response of the sensors exposed to pulses of different ethanol concentration obtained at an operating temperature of 200 °C. The conductance curves in the left plot show remarkable differences between the two materials. First we observe the base electrical current values, i.e., the values at time t = 0, in the absence of ethanol. Pure titania is characterized by much lower conductance. A break in the vertical axis was necessary for visualizing both curves. This 20703


The Journal of Physical Chemistry C result clearly indicates n type doping of the titania lattice by substitutional V(V). In these conditions, the lowest conductance of pure TiO2 should rapidly and largely benefit from the reaction with a reducing gas like ethanol, capable of injecting charges into the semiconducting oxide. Instead, we see that the TiO2−V2O5 sensor signal displayed much larger current variation upon ethanol injection. This result is further evidenced in the calibration curves also shown in Figure 10, in the right plot. Hence TiO2−V2O5 is characterized by enhanced surface reactivity in the operating conditions. The response obviously increased with the gas concentration, and for the TiO2−V2O5 sensor it is almost 2 orders of magnitude higher than for pure titania for 500 ppm ethanol concentration. The sensing results showed that by introducing V in the anatase structure it was possible to obtain larger responses at lower operating temperatures, which is just the aim of surface modifications. However, it is interesting to observe what happened by enlarging the range of operating conditions. Figure 11 shows the temperature dependence of the response for various ethanol concentrations. In all cases it can be strikingly seen that the largest response was obtained by the TiO2−V2O5 sensors at the lowest operating temperature, confirming the effectiveness of V addition to anatase nanocrystals. However, it can also be seen that the two kinds of sensors have systematically opposite behavior, with the TiO2−V2O5 response decreasing with increasing operating temperature, contrarily to pure TiO2. The results clearly point to different reaction mechanisms in the two materials. In TiO2−V2O5, more effective ethanol oxidation took place at lower temperatures, while in pure titania a reaction with higher activation energy was operative.

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REFERENCES

(1) (a) Korotcenkov, G. Gas Response Control through Structural and Chemical Modification of Metal Oxide Films: State of the Art and Approaches. Sens. Actuators, B: Chem. 2005, 107, 209−232. (b) Rumyantseva, M. N.; Safonova, O. V.; Boulova, M. N.; Ryabova, L. I.; Gas’kov, A. M. Dopants in Nanocrystalline Tin Dioxide. Russ. Chem. Bull. 2003, 52, 1217−1238. (2) (a) Arbiol, J.; Cerda, J.; Dezanneau, G.; Cirera, A.; Peiro, F.; Cornet, A.; Morante, J. R. Effects of Nb Doping on the TiO2 Anataseto-Rutile Phase Transition. J. Appl. Phys. 2002, 92, 853−861. (b) Ruiz, A. M.; Dezanneau, G.; Arbiol, J.; Cornet, A.; Morante, J. R. Insights into the Structural and Chemical Modifications of Nb Additive on TiO2 Nanoparticles. Chem. Mater. 2004, 16, 862−871. (3) (a) Arbiol, J.; Cirera, A.; Peiro, F.; Cornet, A.; Morante, J. R.; Delgado, J. J.; Calvino, J. J. Optimization of Tin Dioxide Nanosticks Faceting for the Improvement of Palladium Nanocluster Epitaxy. Appl. Phys. Lett. 2002, 80, 329−331. (b) Cabot, A.; Arbiol, J.; Morante, J. R.; Weimar, U.; Barsan, N.; Gopel, W. Analysis of the Noble Metal Catalytic Additives Introduced by Impregnation of as Obtained SnO2 Sol-Gel Nanocrystals for Gas Sensors. Sens. Actuators, B: Chem. 2000, 70, 87−100. (c) Epifani, M.; Arbiol, J.; Pellicer, E.; Comini, E.; Siciliano, P.; Faglia, G.; Morante, J. R. Synthesis and Gas-Sensing Properties of Pd-Doped SnO2 Nanocrystals. A Case Study of a General Methodology for Doping Metal Oxide Nanocrystals. Cryst. Growth Des. 2008, 8, 1774−1778. (d) Epifani, M.; Andreu, T.; Zamani, R.; Arbiol, J.; Comini, E.; Siciliano, P.; Faglia, G.; Morante, J. R. Pt Doping Triggers Growth of TiO2 Nanorods: Nanocomposite Synthesis and Gas-Sensing Properties. CrystEngComm 2012, 14, 3882−3887. (4) (a) Dobrokhotov, V.; McIlroy, D. N.; Norton, M. G.; Abuzir, A.; Yeh, W. J.; Stevenson, I.; Pouy, R.; Bochenek, J.; Cartwright, M.; Wang, L.; et al. Principles and Mechanisms of Gas Sensing by GaN Nanowires Functionalized with Gold Nanoparticles. J. Appl. Phys. 2006, 99, 104302. (b) Kolmakov, A.; Klenov, D. O.; Lilach, Y.; Stemmer, S.; Moskovits, M. Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles. Nano Lett. 2005, 5, 667−673. (5) (a) Conner, W. C.; Falconer, J. L. Spillover in Heterogeneous Catalysis. Chem. Rev. 1995, 95, 759−788. (b) Heiz, U.; Bullock, E. L. Fundamental Aspects of Catalysis on Supported Metal Clusters. J. Mater. Chem. 2004, 14, 564−577. (c) Pajonk, G. M. Spillover Effects in Catalysis. Heterogeneous Chem. Rev. 1994, 1, 329−338. (d) Pajonk, G. M. Contribution of Spillover Effects to Heterogeneous Catalysis. Appl. Catal., A 2000, 202, 157−169. (6) (a) Haber, J. Fifty Years of My Romance with Vanadium Oxide Catalysts. Catal. Today 2009, 142, 100−113. (b) Weckhuysen, B. M.; Keller, D. E. Chemistry, Spectroscopy and the Role of Supported Vanadium Oxides in Heterogeneous Catalysis. Catal. Today 2003, 78, 25−46. (7) (a) Bruckner, A.; Baerns, M. Selective Gas-Phase Oxidation of Polycyclic Aromatic Hydrocarbons on Vanadium Oxide-Based Catalysts. Appl. Catal., A 1997, 157, 311−334. (b) GrzybowskaSwierkosz, B. Vanadia-Titania Catalysts for Oxidation of O-Xylene and Other Hydrocarbons. Appl. Catal., A 1997, 157, 263−310. (8) (a) Bentrup, U.; Bruckner, A.; Rudinger, C.; Eberle, H. J. Elucidating Structure and Function of Active Sites in VOx/TiO2

ASSOCIATED CONTENT

S Supporting Information *

Further Raman, XPS spectra, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

Authors acknowledge CSIC/CNR project 2010IT0001 (SYNCAMON). This work was partially supported by the SOLAR project DM19447 and the Spanish Government projects Consolider Ingenio 2010 CSD2009 00013 IMAGINE and CSD2009 00050 MULTICAT. J.A. acknowledges the funding from the Spanish MICINN project MAT2010-15138 (COPEON) and Generalitat de Catalunya 2009 SGR 770. We thank Giovanni Battista Pace for help with sample preparation and FTIR measurements.

4. CONCLUSIONS In this paper it has been shown that the presence of capping ligands does not prevent the surface modification of metal oxide nanocrystals, and it was possible to prepare a colloidal counterpart of the TiO2−V2O5 system. Colloidal synthesis allowed keeping the grain size fully in the nanometer range even after heating at 400 °C, which was necessary for material purification and stabilization. This result was enabled by the suitable hydrolytic chemistry of the surface oxide precursor. Homogeneous V surface coverage of the anatase nanocrystals influenced the following V distribution after the heat treatment, affecting also the bulk of the anatase nanocrystals. This peculiar material modification resulted in enhanced catalytic properties of the final material, evidenced in the sample ethanol tests carried out with the nanocomposites. The synthesis procedure is technically simple and readily available for generalization, but in each case the final material architecture will be dictated by the hydrolysis rate of the surface oxide precursor.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 20704



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Influence of Surface Impurities on the Vanadium(V) Coordination. Colloids Surf. 1990, 45, 347−359. (c) Lapina, O. B.; Mastikhin, V. M.; Shubin, A. A.; Krasilnikov, V. N.; Zamaraev, K. I. 51V Solid-State NMR-Studies of Vanadia Based Catalysts. Prog. Nucl. Magn. Reson. Spectrosc. 1992, 24, 457−525. (d) Fernandez, C.; Guelton, M.; Fernandez, C.; Bodart, P.; Guelton, M.; Rigole, M.; Lefebvre, F. Characterization of V2O5/TiO2 Catalysts by Solid-State NMR of 51V. Catal. Today 1994, 20, 77−86. (e) Luca, V.; Thomson, S.; Howe, R. F. Spectroscopic Investigation of Vanadium Speciation in VanadiumDoped Nanocrystalline Anatase. J. Chem. Soc., Faraday Trans. 1997, 93, 2195−2202. (f) Shubin, A. A.; Lapina, O. B.; Bondareva, V. M. Characterisation of Strongly Bonded V(V) Species in VOx/TiO2 Catalyst by Static and MAS Solid-State 51V NMR Spectroscopy. Chem. Phys. Lett. 1999, 302, 341−346. (g) Nielsen, U. G.; Topsoe, N. Y.; Brorson, M.; Skibsted, J.; Jakobsen, H. J. The Complete 51V MAS NMR Spectrum of Surface Vanadia Nanoparticles on Anatase (TiO2): Vanadia Surface Structure of a DeNO(x) Catalyst. J. Am. Chem. Soc. 2004, 126, 4926−4933. (18) Skibsted, J.; Jacobsen, C. J. H.; Jakobsen, H. J. 51V Chemical Shielding and Quadrupole Coupling in Ortho- and Metavanadates from 51V MAS NMR Spectroscopy. Inorg. Chem. 1998, 37, 3083− 3092. (19) Park, E. H.; Lee, M. H.; Sohn, J. R. Solid-State 51V NMR and Infrared Spectroscopic Study of Vanadium Oxide Supported on TiO2ZrO2. Bull. Korean Chem. Soc. 2000, 21, 913−918. (20) (a) Chahboun, H.; Groult, D.; Raveau, B. TaVO5, a Novel Derivative of the Series of Monophosphate Tungsten Bronzes (PO2)4 (WO3)2m. Mater. Res. Bull. 1988, 23, 805−812. (b) Evans, J. S. O.; Hanson, J. C.; Sleight, A. W. Room-Temperature Superstructure of ZrV2O7. Acta Crystallogr., Sect. B: Struct. Sci. 1998, 54, 705−713. (21) Haber, J.; Witko, M.; Tokarz, R. Vanadium Pentoxide I. Structures and Properties. Appl. Catal., A 1997, 157, 3−22.

Catalysts During Oxyhydrative Scission of 1-Butene by in Situ and Operando Spectroscopy. Appl. Catal., A 2004, 269, 237−248. (b) Bond, G. C.; Tahir, S. F. Vanadium-Oxide Monolayer Catalysts - Preparation, Characterization and Catalytic Activity. Appl. Catal. 1991, 71, 1−31. (c) Chiker, F.; Nogier, J. P.; Bonardet, J. L. SubMonolayer V2O5-Anatase TiO2 and Eurocat Catalysts: Ir, Raman and XPS Characterisation of VOx Dispersion. Catal. Today 2003, 78, 139− 147. (d) He, Y. B.; Rui, Z. B.; Ji, H. B. In Situ Drifts Study on the Catalytic Oxidation of Toluene over V2O5/TiO2 under Mild Conditions. Catal. Commun. 2011, 14, 77−81. (e) Debecker, D. P.; Bouchmella, K.; Delaigle, R.; Eloy, P.; Poleunis, C.; Bertrand, P.; Gaigneaux, E. M.; Mutin, P. H. One-Step Non-Hydrolytic Sol-Gel Preparation of Efficient V2O5-TiO2 Catalysts for VOC Total Oxidation. Appl. Catal., B 2010, 94, 38−45. (f) Dias, C. R.; Portela, M. F.; Bond, G. C. Oxidation of O-Xylene to Phthalic Anhydride over V2O5/TiO2 Catalysts.1. Influence of Catalyst Composition, Preparation Method and Operating Conditions on Conversion and Product Selectivities. J. Catal. 1995, 157, 344−352. (g) Oliveri, G.; Ramis, G.; Busca, G.; Escribano, V. S. Thermal-Stability of Vanadia-Titania Catalysts. J. Mater. Chem. 1993, 3, 1239−1249. (9) Zamani, R.; Fiz, R.; Pan, J.; Fischer, T.; Mathur, S.; Morante, J. R.; Arbiol, J. Oxide-Oxide Nanojunctions in Coaxial SnO2/TiO2, SnO2/V2O3 and SnO2/(Ti0.5V0.5)2O3 Nanowire Heterostructures. CrystEngComm 2013, 15, 4532. (10) Epifani, M.; Chávez-Capilla, T.; Andreu, T.; Arbiol, J.; Palma, J.; Morante, J. R.; Díaz, R. Surface Modification of Metal Oxide Nanocrystals for Improved Supercapacitors. Energy Environ. Sci. 2012, 5, 7555. (11) Epifani, M.; Comini, E.; Faglia, G.; Arbiol, J.; Andreu, T.; Pace, G.; Siciliano, P.; Morante, J. R. Two Step, Hydrolytic-Solvothermal Synthesis of Redispersible Titania Nanocrystals and Their Gas-Sensing Properties. J. Sol-Gel Sci. Technol. 2011, 60, 254−259. (12) Epifani, M.; Andreu, T.; Magana, C. R.; Arbiol, J.; Siciliano, P.; D’Arienzo, M.; Scotti, R.; Morazzoni, F.; Morante, J. R. The Chloroalkoxide Route to Transition Metal Oxides. Synthesis of V2O5 Thin Films and Powders from a Vanadium Chloromethoxide. Chem. Mater. 2009, 21, 1618−1626. (13) (a) Ibanez, M.; Cadavid, D.; Zamani, R.; Garcia-Castello, N.; Izquierdo-Roca, V.; Li, W. H.; Fairbrother, A.; Prades, J. D.; Shavel, A.; Arbiol, J.; et al. Composition Control and Thermoelectric Properties of Quaternary Chalcogenide Nanocrystals: The Case of Stannite Cu2CdSnSe4. Chem. Mater. 2012, 24, 562−570. (b) Schuster, F.; Furtmayr, F.; Zamani, R.; Magen, C.; Morante, J. R.; Arbiol, J.; Garrido, J. A.; Stutzmann, M. Self-Assembled GaN Nanowires on Diamond. Nano Lett. 2012, 12, 2199−2204. (c) Shavel, A.; Arbiol, J.; Cabot, A. Synthesis of Quaternary Chalcogenide Nanocrystals: Stannite Cu2ZnxSnySe1+x+2y. J. Am. Chem. Soc. 2010, 132, 4514. (14) (a) Lee, D. H.; Condrate, R. A. Ftir Spectral Characterization of Thin Film Coatings of Oleic Acid on Glasses: I. Coatings on Glasses from Ethyl Alcohol. J. Mater. Sci. 1999, 34, 139−146. (b) Young, A. G.; Al-Salim, N.; Green, D. P.; McQuillan, A. J. Attenuated Total Reflection Infrared Studies of Oleate and Trioctylphosphine Oxide Ligand Adsorption and Exchange Reactions on CdS Quantum Dot Films. Langmuir 2008, 24, 3841−3849. (c) Yu, W. W.; Wang, Y. A.; Peng, X. G. Formation and Stability of Size-, Shape-, and StructureControlled CdTe Nanocrystals: Ligand Effects on Monomers and Nanocrystals. Chem. Mater. 2003, 15, 4300−4308. (15) Gilson, T. R.; Bizri, O. F.; Cheetham, N. Single-Crystal Raman and Infrared Spectra of Vanadium(V) Oxide. J. Chem. Soc., Dalton Trans. 1973, 3, 291−294. (16) Molinari, J. E.; Wachs, I. E. Presence of Surface Vanadium Peroxo-Oxo Umbrella Structures in Supported Vanadium Oxide Catalysts: Fact or Fiction? J. Am. Chem. Soc. 2010, 132, 12559−12561. (17) (a) Eckert, H.; Wachs, I. E. Solid-State 51V NMR Structural Studies on Supported Vanadium(V) Oxide Catalysts - VanadiumOxide Surface-Layers on Alumina and Titania Supports. J. Phys. Chem. 1989, 93, 6796−6805. (b) Eckert, H.; Deo, G.; Wachs, I. E.; Hirt, A. M. Solid State 51V NMR Structural Studies of Vanadium(V) Oxide Catalysts Supported on Titania(Anatase) and Titania(Rutile). The 20705


Colloidal Counterpart of the TiO2-Supported V2O5 System: A Case

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