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Inorganics 2014, 2(2), 264-277; doi:10.3390/inorganics2020264
Abstract: An efficient microwave-assisted synthesis of TiO2:(B) nanorods, using titanium tetraisopropoxide (TTIP), benzyl alcohol as the solvent, together with boric acid and oleic acid as the additive reagents, has been developed. Chemical modification of TTIP by oleic acid was demonstrated as a rational strategy to tune the shape of TiO2 nanocrystals toward nanorod formation. The differently-shaped TiO2:(B) nanocrystals were characterized in detail by transmission electron microscopy (TEM), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and nitrogen absorption-desorption. Oleic acid coordinated on the nanocrystal surface was removed by the reduction of its carboxyl group, and the photocatalytic activity of bare TiO2 nanocrystals, under visible light irradiation, was also evaluated. The synthesized TiO2 anatase nanorods exhibited a good photoactivity and completely degraded Rhodamine B solution within three hours.
Titanium dioxide (TiO2) is a well-studied material, used in a wide variety of applications, such as photoelectrode in dye-sensitized solar cells (DSSCs), in photochromic devices, in lithium batteries and as photocatalysts [1,2,3,4,5].
TiO2 combines good electrical properties with high catalytic activity and excellent stability in many solvents over a wide pH range. A promising strategy to affect the crystallographic and optical characteristics of the final material involves the doping of TiO2 with non-metals (N, C, S, B) [6,7,8,9,10,11].
In particular there is a growing interest in boron-doped TiO2 materials, with low costs and easiness of processing as the key issues for planning advantageous productions .
Different preparation methods were explored to obtain B-modified nanoparticles that are prepared by hydrolysis of a titanium precursor in the presence of a boron-source. The typical procedures often include anaerobic conditions, different reaction steps that require many hours or days, the presence of water to hydrolyze the titanium precursor and, then, a final calcination treatment at high temperatures (400–800 °C) for some hours in order to obtain crystalline anatase [7,13] and amorphous phase composition  of titanium dioxide.
Despite the development of many synthetic protocols at high yield, the massive production of titania nanoparticles represents still an issue. For the abovementioned reasons, it is interesting to explore new convenient synthetic routes to finely tune the properties of boron-modified titanium dioxide, TiO2:(B), using simple one-step methods. In our previous work, a simple synthesis was applied to prepare B-modified nanocrystals using titanium tetra-isopropoxide (TTIP) together with boric acid and benzyl alcohol as the reaction solvent .
Usually, with the use of microwaves as the energy source to trigger chemical reactions, it is possible to obtain nanoparticles with controlled composition, cristallinity and morphology, with an improvement in the kinetics of reaction of two or three orders of magnitude [19,20,21,22,23].
Moreover, microwave-assisted synthesis allows one to obtain high purity titania crystalline phases at relatively low temperature, with a significant reduction of the reaction time and temperature of post-synthesis treatments. This technique offers several advantages, compared to other synthetic wet processes, as the efficient energy transfer with rapid processing of materials, leading to a clean and low-cost approach for the production of highly crystalline anatase TiO2 nanocrystals in a high yield [24,25].
Here, we report a new, simple and rapid synthetic route to produce boron-modified anatase TiO2 nanocrystals. The microwave-assisted synthesis took place in a single and fast step, leading to good control of the boron level and without the requirement of a post-synthetic calcination treatment of the nanocrystals. The present method constitutes a convenient compromise between the non-aqueous synthetic approach and the microwave-solvothermal method to synthesize anatase TiO2 nanorods. We gained also good control of particle size, shape (nanorods), crystallinity (100% anatase phase) and surface properties, with the use of TTIP as the precursor, oleic acid as the additive reagent and benzyl alcohol as the solvent, at a temperature of 210 °C with a reaction time of 45 min.
The use of benzyl alcohol was essential, as the microwave-assisted synthesis requires a polar solvent, necessary to facilitate the heating of the system. The addition of oleic acid in the reaction mixture led to a fast esterification with the simultaneous release of water, which favored the nanorod formation.
2. Results and Discussion
TEM images (Figure 1, Table 1) revealed rod-shaped nanocrystals. TEM analysis evidenced that the sample TOB1 (Figure 1a), consisted of the longest nanorods with an estimated average length of 27 nm (statistics on a hundred particles). The diameters were comparable in all the samples and were of about 3 nm. The nanoparticles size, estimated by TEM images, showed a clear dependency on the precursors nominal amounts employed in the reaction, both boric acid (BA) and oleic acid (OA), as reported in Table 1. The nanorods elongation gradually decreased as the amount of oleic acid increased for the 1:1 TTIP:BA nominal ratio, while it gradually increased as the amount of oleic acid increased for the 1:2 TTIP:BA nominal ratio. It should have been expected that increasing amounts of BA would have simply determined a decrease of the rod size, according to the limiting effect on nanocrystal growth demonstrated in a previous work, but here, the OA additions presented the ability to influence the nanocrystal growth with different trends for the two tested nominal BA ratios .
It can be seen that the increase in BA nominal quantity caused only a limited reduction in the nanorod size. For a deeper understanding of these trends, inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis was performed (Table 1), which allowed us to determine the actual ratio of B:Ti in the whole nanoparticles volume, either on the crystal surface or inside the lattice. A general increase in the B:Ti ratio was detected with the nominal 1:2 respect to the 1:1 TTIP:BA ratios. Therefore, it was observed, for the 1:1 TTIP: BA ratio, that an increase of added OA favored an increase in the amount of incorporated boron, with nanorod length reductions. For the 1:2 TTIP:BA ratio, the trend was opposite and implied a boron reduction with increasing OA content, with the consequent increase in nanorod length. Aspect ratios followed these trends, while rod diameters had almost the same value for all the samples (Table 1). This careful analysis confirms that the increased effective boron amount is a limiting factor in the nanocrystal size, as found in our previous work . Moreover, OA not only tuned the morphology, driving the nanorod formation, but also influenced the effective amount of boron in the nanocrystals.
|Sample||TTIP:BA a||TTIP:OA a||B:Ti b||Mean Rod Length (nm) ± σ c||Rod Diameter (nm) ± Δd d||Aspect Ratio ± Δ e|
|TOB1||1:1||1:6||0.0108||27.0 ± 8.6||3.5 ± 0.6||7.7 ± 0.5|
|TOB2||1:1||1:8||0.0240||18.75 ± 9.5||3.3 ± 0.4||5.7 ± 0.6|
|TOB3||1:1||1:12||0.0245||13.6 ± 6.5||3.7 ± 0.5||3.6 ± 0.6|
|TOB4||1:2||1:6||0.1138||14.2 ± 7.2||3.6 ± 0.6||3.9 ± 0.7|
|TOB5||1:2||1:8||0.0987||16.5 ± 9.2||3.3 ± 0.4||5.0 ± 0.7|
|TOB6||1:2||1:12||0.0641||22.7 ± 10.6||3.1 ± 0.8||7.3 ± 0.7|
a Molar ratio. The amount of TTIP was fixed (3.36 mmol); b B:Ti measured molar ratios, obtained by ICP-AES measurements; c mean value ± standard deviation, σ; statistics on a hundred particles; d estimated mean value of diameter ± error of the measurement, Δd; e calculated mean aspect ratio ± the calculated error, Δ.
XRD was carried out to investigate the crystalline structure of all the TiO2:(B) nanoparticles, and the corresponding patterns are shown in Figure 2.
The black profiles correspond to the experimental raw data, the red curves to the Rietveld fitted profiles and the blue markers to the anatase diffraction peaks positions. The XRD patterns showed also a clear inhomogeneous peak broadening, which was ascribed to crystalline domain anisotropy. This effect was analyzed with a whole-profile Rietveld-based fitting program, as described in the Experimental Section, which allowed us to determine the anatase cell parameter and crystallite apparent domain size along representative reflections, as summarized in Table 2. For anatase, the elongation direction was expected to be along the [00l] crystallographic axis, so that the (004) and the (200) reflections mainly describe the length and the width of the nanocrystalline shape, respectively. The ratios between apparent size at (004) and at (200) reflections were also calculated and shown in Table 2. The apparent size ratios (a.s.r.) of nanocrystalline domains presented a little increase with the amount of OA, but were slightly smaller in the samples prepared with the nominal 1:2 TTIP:BA ratio. The quality of the obtained fits was checked by means of a goodness-of-fit statistical indicator (GoF). GoF values of <3–4 were found and considered to be satisfactory.
|Sample||Apparent Size Ratio||Ti a||O a||004 b||200 b||Eg(1) c||Eg(3) c|
a Occupancy (theoretical values are one for Ti and two for O); b apparent size along the indicated directions (Å); c Raman shifts (cm−1).
2.3. Micro Raman
Micro Raman spectra were collected on TiO2:(B) samples and are reported in Figure 3.
In the range of 200–800 cm−1, all of the samples displayed the bands, Eg(2) (at ~197 cm−1), B1g(1) (at ~400 cm−1), A1g and B1g(2) (at ~520 cm−1) and Eg(3) (at ~640 cm−1).
The Eg(1) band, very sensitive to the size of nanocrystalline domains and to the presence of oleate ligands , was detected around 150 cm−1 (Figure 3b), corresponding to the 144 cm−1 of bulk anatase . As already mentioned, the small differences in peak wavenumbers of the Eg(1) seems poorly related to the boron content in anatase. The other five abovementioned normal modes denoted also small shifts with respect to anatase bulk values, as evidenced in nanocrystals .
Qualitatively, a blue shift from a 144 cm−1 value of bulk anatase to about 150 cm−1 is due to nanocrystal domains decreasing in size (Table 2), as commonly observed in the literature [27,28]; in detail, for the 1:1 TTIP:BA ratio, the blue shift increased as the amount of OA did; TEM aspect ratios increased; and the XRD apparent size ratio decreased (thus, the size of nanocrystalline domains decreased). In the case of 1:2 TTIP: BA samples, a maximum can be observed in correspondence of a 1:8 TTIP:OA ratio. Values of the Raman shift of two bands, sensitive to domain size and to surfactant coating , are reported in Table 2. The blue shifts of the Eg(1) and Eg(3) Raman bands resulted in being poorly sensitive to the boron amount in the anatase structure, following roughly a trend with the amount of added OA.
The surface presence of OA coating on boron-modified samples was evidenced in IR spectra and shown in Figure 4.
In the OA-capped TiO2, a weak, but definite, band at 3008 cm−1 was present, due to the olefinic C–H stretching. All the TiO2:(B) samples exhibited the intense antisymmetric and symmetric C–H stretching vibrations (at 2933 and 2857 cm−1, respectively) of the –CH2– groups in the hydrocarbon moiety. The shoulder at 2960 cm−1 could be associated with the asymmetric stretching of the terminal –CH3 group of the alkyl chain .
The COO− antisymmetric and symmetric stretching vibrations (two characteristic bands centered at 1526 and 1435 cm−1, respectively) of carboxylate anions were also detected, and their frequency difference (~91 cm−1) indicated a chelating bidentate binding mode onto the TiO2 surface .
The surface areas were checked by Brunauer–Emmett–Teller (BET) characterization. The N2 adsorption-desorption isotherms of the as-prepared TiO2 nanocrystals are reported in Figure 5. The relative isotherms of the samples were of Type IV, characteristic of mesoporous materials (Figure 5a) .
The BET specific surface areas followed are 140.3 (TOB1), 112.9 (TOB2), 41.1 (TOB3), 112.8 (TOB4), 116.7 (TOB5) and 60.2 (TOB6) m2/g, respectively. Thus, in the first group, where the ratio TTIP:BA is 1:1, an area decrease is observed with increasing TTIP:OA. In the second group, where TTIP:BA is 1:2, there is a similar trend for the first two samples and a decrease for the last one (with increasing TTIP:OA). Boron did not affect the area measurements as OA did.
The pore size distribution curves calculated according to the Barrett–Joyner–Halenda (BJH) approach of the as-prepared TiO2 nanocrystals are reported in Figure 5b.
The photocatalytic performances of the TiO2:(B) samples were tested by the degradation of Rhodamine B (RhB) solution under visible-light irradiation. Since BET results showed that the TOB1, TOB2 and TOB3 samples were characterized by evident decreasing areas, we focused the photodegradation measurements on these samples. Before the photocatalytic measurements, the TiO2 nanorods were treated under ozone flow in order to remove the surface-linked organic molecules (oleic acid), as was proven by FTIR analysis. Such a treatment allowed for obtaining a wide catalytic surface area. Blank experiments carried out on an RhB solution not containing TiO2 proved the negligible degradation of the dye upon direct visible light. In addition, RhB/TiO2 solutions kept in dark did not show any appreciable absorbance change over time.
The results of photocatalytic experiments are shown in Figure 6, where commercial TiO2 Degussa P25 was used as the reference.
The concentrations of RhB, measured from relative absorbance intensities at 550 nm at reaction times t and t0 = 0, were denoted as C and C0, respectively.
The reaction kinetics of RhB degradation catalyzed by the different samples is reported in Figure 6, where the relationships between ln(C/C0) and their irradiation time are shown. According to numerous literature works, the influence of the initial concentration of the solute on the photocatalytic degradation rate of the most organic compounds is described by pseudo-first-order kinetics .
The photocatalytic degradation plots of the experimental data fit well with pseudo-first-order reactions, and the apparent rate constants, k, were calculated from the slope of the fitting straight lines (Figure 6). Among all the samples, TOB1 presented the best activity and the highest surface area, as determined by BET. In fact, under visible light irradiation, RhB was completely degraded within three hours. Instead, TOB3 photocatalyst exhibited the lowest activity and surface area, even with respect to commercial P25.
The photocatalyst TOB1 exhibited the highest rate constant (k = 0.017 ± 0.001 min−1), which was approximately one and half times higher than the TiO2 Degussa P25.
The possible photo-degradation of RhB may include four main processes: N-de-ethylation, chromophore cleavage, ring-opening and mineralization. RhB molecules may link on the surface of the photocatalyst through diethylamino or carboxylic groups. Under visible light irradiation and in the presence of a photocatalyst, RhB (and subsequently, its degradation products) can absorb photons. The excited molecules thus transfer the LUMO electrons to the conductive band of TiO2, where the conductive electrons can react with pre-adsorbed oxygen molecules to produce reactive oxygen species, such as O2•−, •OOH and •OH. These reactive oxygen species mainly attack the auxochromic groups and induce N-de-ethylation of the alkyl amine group . It is also postulated that photogenerated holes (h+) can degrade suspended un-adsorbed RhB molecules and the N-de-ethylated products that are released in solution .
In Figure 7, we report the typical variation of the absorption of RhB in the presence of the photocatalyst TOB1.
The peak position at 550 nm gradually decreased till its complete disappearance in less than three hours, suggesting that the fused aromatic ring structures and dye chromophores were destroyed, as highlighted by the progressive de-coloration of solutions. Thereafter, the hypsochromic (blue) shift observed was attributed to partial N-de-ethylation via visible light-assisted photocatalysis.
3. Experimental Section
All analytical-grade reagents were purchased from Sigma-Aldrich (Milan, Italy) and used without further purification. In a typical preparation, TTIP (1 mL, 3.36 mmol) was dispersed in benzyl alcohol (10 mL, 96.4 mmol), under stirring, in a Teflon® vessel. Boric and oleic acids were added to the reaction mixture, in different amounts, as additive reagents. Then, the reaction vessel was sealed and maintained at 210 °C for 45 min. The resulting milky suspension was centrifuged; the white precipitate was collected, washed twice with methanol and, finally, dried overnight at room temperature.
Different TTIP:OA ratios (1:6, 1:8, 1:12) and TTIP:BA molar ratios (1:1, 1:2) were used, and the samples were respectively labelled as TOB1, TOB2, TOB3, TOB4, TOB5 and TOB6 (Table 1).
3.2. Removal of Capping Layer from the TiO2 Samples
Oleic acid coordinated on the TiO2 nanocrystals was removed by reducing its carboxylic group. Dried TiO2 samples were placed in a spherical glass container with cylindrical necks, and they were treated at room temperature with an ozone flow for 1 h . The ozone generator, Ozonis (Sepra s.r.l., Cesano Maderno, Italy), used in our experiments ensured a stable gas production of 250 mg/h.
3.3. Samples Characterization
Morphological characterizations of the TiO2:(B) nanopowders were performed using transmission electron microscopy (TEM). Images were obtained by using a Jeol Jem 1011 microscope (Jeol, Tokyo, Japan) operating at an accelerating voltage of 100 kV. The samples were prepared by dropping titania nanoparticle diluted solutions in ethanol onto 400-mesh carbon-coated copper grids and evaporating the solvent.
Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was performed with a Vista AX ICP-AES (Varian Inc., Palo Alto, CA, USA, from 2010 Agilent Technologies, Santa Clara, CA, USA) instrument. The samples were digested using a mixture of HNO3 and HF 1:4, and ICP-AES measurements were accomplished after calibration by aqueous standards of Ti and B, with dilution ranges selected according to the expected concentrations of the elements of interest.
Powder X-ray diffraction (XRD) for the phase analysis of the nanocrystals was performed with a Rigaku (Tokyo, Japan) RINT2500 diffractometer, using Cu Kα monochromatic radiation (λ = 1.5418 Å). The XRD patterns were recorded in the range of 2θ = 20°–90° by coupled θ/2θ scanning mode, using 2θ increments of 0.03°. The XRD patterns were analyzed by using a whole-profile Rietveld-based fitting program (FULLPROF) , according to the following procedure:
(1) The instrumental resolution function (IRF) was evaluated by fitting the XRD pattern of a LaB6 NIST (National Institute of Standards and Technology) standard recorded under the same experimental conditions used for measuring the samples. The IRF data file was provided separately to the program in order to allow subsequent refinement of the XRD patterns of the samples.
(2) The phase composition of the samples was determined by fitting the XRD patterns with the crystal structure models of tetragonal TiO2 nanoanatase (Inorganic Crystal Structure Database # 92363; space group: I41/amd; cell parameters: a = b = 3.77 Å and c = 9.43 Å; α = β = γ = 90°). The weight percentage accuracy of the estimation was determined to be 5%–7% w/w;
(3) The inhomogeneous peak broadening of the anatase reflections was described by a phenomenological model based on a modified Scherrer formula:
Raman spectra were acquired with a inVia Raman microscope (Renishaw plc, Wotton-under-Edge, Gloucestershire, UK), edge filtered working at a 633 nm excitation wavelength; powders of the prepared samples were placed on a glass slide, and spectra were collected at room temperature. The power on the samples was around 0.8 mW, under a 50× microscope objective, giving a spot diameter of 2 μm, and a resolution of 4 cm−1 was used.
Fourier transform infrared spectroscopy (FTIR) measurements in the 4000–400 cm−1 spectral range were carried out using a FT/IR-6300 type A spectrophotometer (JASCO Inc, Easton, MD, USA) with the diffuse reflectance method.
The nitrogen adsorption-desorption isotherms at 77 K by BET characterization were recorded with a ASiQwin system (Quantachrome, Boynton Beach, FL, USA).
3.4. Photocatalytic Activity Measurement
The photocatalytic activities of the TiO2 samples were evaluated by the degradation of RhB in aqueous solutions.
The photocatalyst (0.05 g) was suspended in 10 mL of water. The solution was continuously stirred for about 15 min and then added to a 50 mL RhB aqueous solution of 5 mg/L; the temperature of the RhB solution, stirred by a dynamoelectric stirrer in an open reactor, was about 25 °C.
The solution was kept in dark for 30 min before irradiation, stirred to establish an adsorption desorption equilibrium and, then, exposed to a 150-W LOT Oriel -LS0106 tungsten halogen lamp LOT-QuantumDesign group, Darmstadt, Germany) with a 420-nm-cutoff filter, removing the UV radiation. The distance between the light source and the bottom of the solution was about 15 cm.
The concentration of RhB was monitored by colorimetry with a Cary 5000 UV-Vis spectrometer (Agilent Technologies, Santa Clara, CA, USA).
In this research work, TiO2:(B) nanocrystals were prepared by a single and fast reaction step (a reaction time of about 45 min) by a microwave-assisted method that allows one to control the doping level, using TTIP as the precursor, BA as the boron source, OA as the capping reagent and benzyl alcohol as the solvent.
The preparation process showed that oleic acid was driving towards anatase nanorod formation, thus controlling the nanocrystal morphology. Moreover, its amount was active in controlling the effective boron quantity in the nanorods, in the investigated concentration range. The nanorod size and aspect size ratios were controlled both by the nominal added boron and by oleic acid.
The visible light photocatalytic activity was tested, and a sensible improvement with respect to commercial P25 was observed for some of the samples. Their activity was directly correlatable with the increasing BET surface areas, decreasing the XRD aspect size ratios, a smaller size of nanocrystalline domains (lower Raman shift of Eg(1) band), higher TEM aspect ratios, more than the increasing effective boron concentration in the nanorods, for the same nominal boron amount.
This research was supported by PON (National Operative Program) 254/Ric. Strengthening of a “Human and Environmental Health Research Centre” Cod. PONa3_00334, by PON n. 84/Ric. Nanomaterials for Sustainable Construction (NAMASTE) Cod. PON04a3_00107—by PO FESR (Operative Program, European Found Regional Development) 2007–2013 Nano/Micro Structure for Building materials and other productive sectors (NAMISTE) Cod. 2Y6DME5, by “Networks of Public Research Laboratories: Cod. 20, Regional Laboratory of Synthesis and Characterization of New Materials for Organic and Nanostructured Electronics, Photonics and Advanced Technologies” and by the FIRB 2009/2010 project “Integrated network for Nano Medicine (RINAME)”—RBAP114AMK_006.
Claudia Carlucci performed the synthesis, FT-IR and BET; Barbara Federica Scremin performed Raman and TEM analysis; Teresa Sibillano and Cinzia Giannini performed XRD experiments; Emanuela Filippo acquired TEM images, and analyzed photocatalytic activity measurements; Patrizia Perulli and Agostina Lina Capodilupo performed capping layer removal, UV-Vis spectroscopy; Giuseppina Anna Corrente performed ICP-AES; Giuseppe Ciccarella supervised the research. The results were discussed by all the authors. Claudia Carlucci, Barbara Federica Scremin, Cinzia Giannini, Emanuela Filippo wrote the paper.
Conflicts of Interest
The authors declare no conflict of interest.
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