Synthesis of TiO2 Nanoparticles with Narrow Size Distribution and

Jun 17, 2010 - Coyoacán, 04510, México D.F., México. ReceiVed: March 9, 2010; ReVised Manuscript ReceiVed: May 28, 2010. A new synthesis method of ...
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J. Phys. Chem. C 2010, 114, 11381–11389

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Synthesis of TiO2 Nanoparticles with Narrow Size Distribution and Their Evaluation in the Photocatalytic Oxidative Degradation of Bis(4-nitrophenyl) Phosphate Inti Zumeta,† David Dı´az,*,† and Patricia Santiago‡ Facultad de Quı´mica and Instituto de Fı´sica, UniVersidad Nacional Auto´noma de Me´xico, Coyoaca´n, 04510, Me´xico D.F., Me´xico ReceiVed: March 9, 2010; ReVised Manuscript ReceiVed: May 28, 2010

A new synthesis method of anatase titanium dioxide (TiO2) nanoparticles (NPs) of an average size of 2.1 ( 0.3 nm is reported. This is a fast, inexpensive, one-pot, and one-step method. It takes place under normal reaction conditions in dimethyl sulfoxide (DMSO) colloidal dispersions. The electronic absorption spectrum of these TiO2 colloidal particles shows an absorption band edge blue shift with respect to the macrocrystal due to the quantum confinement effect. After TiO2 NPs precipitation, the isolated powder changes its crystalline phase with the annealing process, up to 500 °C anatase is the unique phase, and at 1000 °C the precipitate is totally rutile. UV irradiated aqueous TiO2 NPs suspensions play an active role in the degradation of the bis-4-nitrophenyl phosphate (BNPP), as well as of its primary decomposition products, the 4-nitrophenyl phosphate (4-NPP) and the 4-nitrophenol (4-NP). As far as we now, the complete BNPP and 4-NPP photocatalytic or just catalytic degradation, using inorganic nanoclusters, has not been reported. Commercial TiO2 powder (Degussa P25) exhibits the highest photocatalytic activity in the BNPP degradation process; however, the activity of the synthesized TiO2 NPs is of the same order of magnitude as that of Degussa’s P25. Introduction Titanium dioxide nanoparticles constitute a very important class of material that has been widely studied and applied in photovoltaic cells,1 gas sensors,2 pigments or coatings,3 selfcleaning surfaces,4,5 water photolysis for hydrogen production,6,7 environmental remediation,8,9 and so on. Physical and chemical properties of TiO2 NPs (that determine their application potentialities) are greatly influenced by their morphology, size distribution, crystalline phase structure, and the final electronic states present at the surface of materials.10 Also, particle aggregation, specific surface area, pore size distribution, bulk and surface hydroxyl groups, and impurities can play an important role in specific applications.11 All of the abovementioned characteristics are determined by the synthesis procedure. That is why the preparation and postsynthesis treatments are crucial in determining TiO2 NPs properties, and several general methods have been developed. Among them one can find pulsed-laser ablation,10 hydrolytic sol-gel process,12 nonhydrolytic sol-gel process,13 metalorganic chemical vapor deposition,14 hydrothermal methods,11 solvothermal methods,15 and others.16 Some authors propose synthesis and processing routes that lead to crystalline nanoparticles of size under 5 nm,10,13,16 but most of the mentioned methods require either multiple steps, temperatures other than room temperature, relatively long reaction times (several minutes or hours), complicated equipments, controlled atmosphere, postsynthesis treatments, and some expensive raw materials such as dendrimers or alkoxides. The latter hydrolyze quickly and spontaneously, which could be inconvenient to the synthesis procedure. It is therefore desirable to find simpler methods than those mentioned * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (52) 55 56223813. † Facultad de Quı´mica. ‡ Instituto de Fı´sica.

for the synthesis of small TiO2 NPs with a narrow size distribution and good crystallinity. In particular, obtaining highly crystalline TiO2 NPs smaller than 3 nm with an almost monodispersed distribution remains as one of the greatest challenges. Details of the mentioned synthesis pathways are summarized in Table S1 in the Supporting Information. On the other hand, the Bohr’s radius reported for TiO2 is in the range of 0.8-1.5 nm.17,18 When the particle radius is comparable with this parameter there is a chance that the nanostructured titanium oxide exhibits quantum confinement effects. Therefore, due to the relatively small value of the Bohr’s radius of TiO2, and the intrinsic difficulties to synthesize crystalline TiO2 NPs smaller than 3 nm, it is hard to observe quantum confinement effects in such particles. Besides, the size distribution of NPs must be very narrow to observe the quantum size effect by electronic absorption measurements, since larger particles absorb light at larger wavelengths interfering with the visibility of the absorption edge of smaller particles. The published explanation of the blue shift observed in TiO2 NPs electronic absorption spectra has been somewhat controversial.16,17,19,20 Serpone et al. reported that no size quantization effects are shown in anatase particles larger than 2.1 nm.20 They concluded that the blue shifts in the absorption spectra observed by Choi et al.,19 attributed to quantum size effect in 2-4 nm TiO2 NPs, are in fact direct transitions in an otherwise indirect band gap semiconductor. Monticone et al. found that the band gap energy remains almost unchanged when the particle size is under 1.5 nm;17 this behavior was explained by the considerable structural size effect that causes curvatures changes at the band minimum resulting in effective mass variations. Satoh et al. obtained TiO2 NPs of size between 1.47 and 2.43 nm and with a very narrow size distribution (0.16-0.29 nm).16 They developed an expression like the Brus equation (but assuming an apparent effective mass, an apparent dielectric constant, and a finite depth well model) to theoretically describe the observed quantum size

10.1021/jp1021399  2010 American Chemical Society Published on Web 06/17/2010

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effects. The authors in ref 16 also point to the importance of sharp size distribution for the distinctive observation of quantum size effects in TiO2 NPs and also demonstrate that the direct nature of the electronic transition in TiO2 particles (1-2 nm size range) stated by Serpone and co-workers20 is unacceptable. On the other hand, TiO2 is a semiconductor that has been successfully used in the degradation of some organophosphorus pesticides in aqueous solutions,21-24 even in natural waters.25 Most of these compounds are very toxic and can contaminate water, food, and fish, or in a general way can damage natural environments and human health. The photooxidative degradation process of pesticides can transform them into substances that are nontoxic and safe for human health.24 When using TiO2 in aqueous media, photodegradation processes are mainly based on the generation of the very reactive hydroxyl radicals (OH•) that are able to react with most organic compounds; however, other radicals can participate and play important roles. The OH• radicals can even promote the mineralization of the organic compounds.24 For example, fenitrothion,21 Methyl Parathion,22,23 Monocrotophos,23 and Dichlorvos Phorate23 are very toxic organophosphorus pesticides which can be degraded by TiO2. Bis-4-nitrophenyl phosphate (BNPP) is an organophosphate molecule than can be used as a model to study TiO2 photocatalytic activity avoiding the manipulation of more toxic organophosphate species. The stability of this molecule is relatively high; its t1/2 is 2000 years in water at 20 °C and 53 years in water at 50 °C.26 The hydrolytic cleavage of this molecule, assisted by a coordination compound catalyst, to yield 4-nitrophenyl phosphate (4-NPP) and 4-nitrophenol (4-NP) is well-known.27-33 The BNPP cleavage can be easily monitored by electronic absorption spectroscopy since the released 4-nitrophenolate anions (4-NP-) display a characteristic band whose maximum is at 400 nm. Consequently, reported in this work is a new and simple synthesis method of TiO2 NPs smaller than 3 nm. We have avoided the use of dendrimers and titanium alkoxides. The synthesis takes place under normal reaction conditions. Dimethyl sulfoxide (DMSO) was used as solvent and TiI4 salt as precursor, and a solution of NaOH is added to produce the reaction under vigorous magnetic stirring. The synthesized particles showed photocatalytic activity in the BNPP decomposition process. UV electronic absorption spectroscopy, diffuse reflectance spectroscopy, Raman light scattering spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used for the TiO2 NPs characterization. Experimental Section Chemical reagents used in this work were as follows: titanium(IV) iodide (TiI4; 99.99%, Strem Chemical), dimethyl sulfoxide (DMSO; 99.9%, high-performance liquid chromatography (HPLC) grade, J. T. Baker), sodium hydroxide (NaOH; 98.15%, J. T. Baker), titanium dioxide (P25, Degussa Corporation, Akron OH), ultrapure water (18 MΩ cm-1, obtained from an Easy-pure compact Ultra, Barnstead, deionization system), bis-4-nitrophenyl) phosphate (99%, Sigma-Aldrich), 4-nitrophenyl phosphate disodium salt hexahydrate (99%, SigmaAldrich), and 4-nitrophenol (99%, Sigma-Aldrich). TiO2 Synthesis. The colloidal TiO2 suspension is synthesized by dissolving 14.17 mg of TiI4 in 24 mL of DMSO under constant magnetic stirring at ambient temperature, and then 1 mL of 0.1 M NaOH aqueous solution is added to the DMSO solution. The final concentration was 10-3 M for TiI4 and 4 × 10-3 M for NaOH. The reaction occurs almost instantaneously. The reaction that takes place is:

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TiI4 + 4NaOH f TiO2 + 2H2O + 4 NaI The suspensions were centrifuged at 2500 rpm for 20 min and washed with a water/acetone mixture (3:1 volume ratio) at least 5 times. The resulting powder was dried in air and resuspended in 20 mL of water. In some cases the dried powder was used for the material characterization. BNPP Degradation Studies. The photocatalytic degradation studies were carried out with use of UV radiation, by means of two different lamps: A Mineralight UVG-D68, with a power of 25 W and a spectral maximum at 255 nm, and a Black-Ray B 100AP, with a power of 100 W and a maximum at 365 nm. To evaluate the photocatalytic activity, TiO2 NPs were suspended in water solutions of BNPP, 4-NPP, or 4-NP and UV irradiated in a 1 cm path quartz cell. The irradiation time, power, and wavelength were varied conveniently according to the experiment objective, and are specified in the text. Instrumentation. Electronic absorption spectra (in the UV-visible region) were obtained in an Ocean Optic CHEM200 fiber optic spectrometer. UV-vis diffuse reflectance spectra were recorded on a Cary-5E (Varian) spectrophotometer. Highresolution transmission electron microscopy (HR-TEM) was carried out in a JEOL FASTEM 2010 analytical microscope, operating at 400 kV. In the experiment, a drop of the TiO2 dispersion is deposited onto a 200 mesh copper grid coated with carbon/collodion layers. Fourier transforms of HRTEM micrographs were obtained with the Digital Micrograph GATAN v-3.7.0 software. X-ray diffraction (X-RD) patterns were recorded on a Siemens D5000 equipment, using Cu KR radiation (20 mA, 40 kV, λ ) 1.5418 Å). Raman spectra were recorded with an Almega XR Dispersive Raman spectrometer. The laser light was focused on the sample, on an area of 1 µm × 1 µm, using an Olympus microscope (BX51) with an Olympus ×50 objective (NA 0.80). The microscope also helps collecting the scattered light. A charge-coupled device (CCD) detector, thermoelectrically cooled at -50 °C, was used in this experiment. The Raman spectra were accumulated over 80 s with a resolution of 4 cm-1. The excitation source was the radiation (50 mW) coming from a Nd:YVO4 laser (532 nm, frequencydoubled). The annealing process was carried out in a Barnstead Thermolyne 1300 furnace, under aerobic conditions. Results and Discussion TEM Analysis of the Synthesized TiO2 Powder. As no peaks were observed in XRD patterns of the prepared TiO2 NPs, a low-range crystalline order is expected in the NPs structure. The TEM image in Figure 1a shows a zone of low aggregation of the synthesized TiO2 NPs. It can be noted (in Figure 1b) that the particles have a sphere-like shape with an average particle size of 2.1 nm and a relatively narrow standard deviation (SD) of 0.3 nm (Figure 1b). In this case, the corresponding particle radius is smaller than the Bohr’s radius of TiO2, therefore the obtained TiO2 NPs are in a quantum confinement regime. It is important to note that the sharp SD (14% of the mean size) favors a possible observation of a blue shift in the absorption spectrum. The very small size of the NPs and the contrasting characteristics of the material made it difficult to obtain good quality images of isolated particles to determine their interplanar distances and obtain their crystalline structure. Small and isolated NPs are relatively light and their movement due to the interaction with the electron beam makes the focusing process hard. Nevertheless, it was possible to get an HR-TEM micrograph of one isolated TiO2 nanoparticle, Figure 1c. The

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Figure 1. (a) TEM image of the synthesized TiO2 NPs. (b) Histogram from the image in panel a. (c) HR-TEM micrograph of an isolated particle and the corresponding fast Fourier transform (FFT) image. (d) HR-TEM micrograph of a zone of high aggregation of TiO2 NPs and the FFT images of different regions.

interplanar distance obtained from the fast Fourier reduced ´ This distance can either transform in Figure 1c was 2.06 Å. belong to the (120) family planes of rutile structure or (113) of anatase. Therefore, from this result it is not possible to assign the phase structure. Then, micrographs of superimposed and crowded NPs were used to study the crystalline phase composition of the NPs by Fourier transform processing (Figure 1d). Interplanar distances of 3.516, 2.961, 2.676, 2.430, and 2.332 ´ corresponding to (101), (102), (110), (103), and (112) planes, Å, respectively, were calculated for the anatase unit cell with CaRine software version 3.1. These distances, except for 2.961 ´ are clearly different from those of the rutile planes. It is Å, important to mention that most of the calculated interplanar distances for anatase and rutile are very close to each other,

which makes the assignment difficult. It is possible then to be sure of the presence of the anatase phase, while that of rutile cannot be discarded. As is known, anatase is the most thermodynamically stable phase for nanostructured TiO2.11,34-36 For a more detailed discussion concerning Figure 1, specifically a comparison between particles exhibited in the micrograph 1d (with apparent larger size clusters) and micrograph 1a, see the Supporting Information. Raman Spectroscopy and X-RD Analysis of the Synthesized TiO2. The Raman spectra of different TiO2 powder samples, previously annealed for 5 h, at different temperatures, under aerobic reaction conditions, were analyzed (Figure 2a,b). All the spectra were obtained at room temperature. As can be seen, the samples underwent a crystalline phase evolution when

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Figure 2. Raman spectra of the TiO2 NPs previously annealed at different temperatures: (a) up to 650 °C and (b) from 500 to 1000 °C. (c) X-RD patterns of the synthesized TiO2 powders annealed; the letters A and R denote the anatase and rutile peak positions, respectively.

the temperature is increased from room temperature to 1000 °C. The first crystalline phase observed was anatase, then a mixture of anatase and rutile, and finally only rutile. An analogous phase transformation, as a function of temperature, in TiO2 nanostructures has been observed by other authors.12,37-41 The phase transformation temperatures typically depend on the synthesis pathways, the nature of the precursors, the starting particle size, and the annealing exposures and conditions. For a detailed discussion of panels a and bo of Figure 2, see the Supporting Information. The temperature induced crystalline phase evolution, of the synthesized TiO2 NPs, was also studied by X-RD (Figure 2c) in order to complement the Raman spectroscopy results. The X-ray patterns of annealed NPs, at 300 °C or lower temperatures, did not show any diffraction peak, probably due to the low range crystalline order. The annealed sample at 500 °C showed the characteristic peaks of anatase crystalline lattice and this is the only phase in this powder sample under the sensitivity limits of this technique. This is caused by the growth of small particles of anatase structure; this crystalline phase was detected in the as-prepared NPs by HR-TEM. The pattern of the annealed sample, at 800 °C, shows diffraction peaks of anatase and rutile phases, where the rutile crystallites are being formed from those of anatase. At 1000 °C the only detected phase was rutile. These results are in agreement with those obtained from Raman spectroscopy. Diffuse Reflectance Spectra of Synthesized TiO2 Powders. The absorbance spectra of several powders of the synthesized TiO2 NPs were obtained by the diffuse reflectance technique. The first TiO2 powder sample (TiO2-1) was prepared as previously described in the Experimental Section. The two other TiO2 NPs samples were prepared at 5 (TiO2-5) and 100 times (TiO2-100) the concentrations used for TiO2-1. No peaks were observed in X-RD patterns of the obtained TiO2 NPs powders

in any of the cases, indicating a very low range crystalline order. The corresponding absorbance spectra and the Tauc’s plots of TiO2-1, TiO2-5, and TiO2-100 powders are shown in Figure 3. It is observed in Figure 3a that the absorbance values are clearly higher, for wavelengths shorter than 300 nm, when the precursor’s concentration is increased 5 times. However, the absorption spectra of TiO2-5 and TiO2-100, in the wavelength range from 250 to 350 nm, are very similar. At the same time, the spectra of TiO2-5 and TiO2-100 samples are slightly redshifted with respect to that of TiO2-1. This effect might be occurring due to the increase in NPs size when increasing the precursor’s concentration. According to the observed variation in the absorption spectrum profile, a small difference in size (and the range of crystalline order) must be expected between TiO2-5 and TiO2-100 samples. The gap energy values (Eg) obtained from Tauc plots (Figure 3b) are in all cases larger than that of the macrocrystal anatase phase (Eg ) 3.2 eV).42 This trend can be explained on the basis of the quantum confinement effect43 provoked by the small particle size (reported above for TiO2-1). The larger Eg value of the TiO2-1 powder sample is related to the smaller size. Photodegradation of BNPP, 4-NPP, and 4-NP by UV Irradiated TiO2 Powder Degussa P25. As mentioned, the photocatalytic activity of TiO2 has been proved toward insecticides and other organophosphorus compounds.21-24 However, as far as we know, the photocatalytic activity of TiO2 in BNPP decomposition has not been determined. One can find many reports about different catalysts based on Cu(II),27 Zn(II),28 Mn(III),29 Tb(III),30 Ni(II),31 Co(II),31 Y(III),32 and La(III)33 organometallic compounds that participate in the hydrolytic cleavage of BNPP into 4-NPP and 4-NP, or their anions. The ability of the nitrophenolate anion (4-NP-) to absorb radiation at 400 nm has been widely used to monitor the BNPP decomposition by releasing 4-NP-.27-33 This absorption band

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Figure 3. (a) The absorption spectra and (b) Tauc’s plots of TiO2-1 (from a 10-3 M TiI4 and 4 × 10-3 M NaOH solution), TiO2-5 (from a 5 × 10-3 M TiI4 and 20 × 10-3 M NaOH solution), and TiO2-100 (from a 10-1 M TiI4 and 4 × 10-1 M NaOH solution). The numbers in parentheses in panel b indicate the calculated gap energy of the samples.

Figure 4. Absorption spectra of 10-4 M BNPP, 5 × 10-5 M 4-NPP, and 5 × 10-5 M 4-NP aqueous solutions (a) at pH 7 and (b) pH 14.

is clearly separated from those of 4-NPP and BNPP. In basic media 4-NP is easily deprotonated to yield 4-NP- and develops a typical yellow color. The absorption spectra of 10-4 M BNPP, 5 × 10-5 M 4-NPP, and 5 × 10-5 M 4-NP aqueous solutions, at pH 7 and 14, are shown in Figure 4. As can be seen in Figure 4, there was no significant change in band positions corresponding to BNPP and 4-NPP on varying the pH from 7 to 14; on the other hand, the band corresponding to 4-NP shifts from 317 to 400 nm and the intensity is increased when it is deprotonated at pH 14. To test the efficacy of TiO2 on the cleavage of BNPP in aqueous solutions, the TiO2 Degussa P25 powder was used in the first experiment. A 2 mg sample of TiO2 Degussa P25 was homogeneously suspended into 20 mL of a 10-4 M BNPP aqueous solution for a final loading of 0.1 g/L. All the samples were initially at pH 7. The BNPP solution and the TiO2 suspensions were irradiated at 365 nm with an intensity of 25 mW/cm2, in a quartz cell of 1 cm pathway. After irradiating, the suspension was centrifuged and the supernatant (S-Degussa) was collected to be analyzed. This procedure minimizes the scattered light component on the absorbance measurements. The pH of all the samples was 7 after the irradiation. The pH of the centrifuged samples pH was increased up to 14 with use of a 1 M NaOH aqueous solution. Additionally, aqueous solutions of BNPP (6 × 10-5 M), 4-NPP (1.36 × 10-5 M), and 4-NP (1.36 × 10-5 M) for reference mixture A, and BNPP (5.68 × 10-5 M), 4-NPP (1.68 × 10-5 M), and 4-NP (1.68 × 10-5 M) for reference mixture B, were prepared and analyzed for comparison. In Figure 5a the absorption spectra of the irradiated and nonirradiated BNPP solutions and in Figure 5b the spectra of the S-Degussa, for different exposure times, together with those of the irradiated BNPP solution and reference mixtures A and B, are shown. In Figure 5a, the absorption spectrum of the BNPP solution did not change significantly after irradiating for 1 h. The

radiation wavelength (365 nm) is not strongly absorbed by BNPP (see Figure 4), therefore this organophosphate compound was not photochemically degraded. In contrast, Figure 5b shows that the spectra of S-Degussa samples have a lower absorbance, around 290 nm, than the irradiated BNPP solution. This can be explained in terms of a decrease of the concentration of BNPP in the samples containing S-Degussa due to its partial photocatalytic degradation; the sample irradiated for 2 h showed larger decomposition. The spectra of S-Degussa samples display a new band at 400 nm, indicative of the presence of 4-NP-, which is a degradation product of BNPP, as observed with other catalysts.27-33 The absorbance at 400 nm of the sample increases after 2 h of irradiation. The formation of the 4-NPP degradation product of BNPP has also been observed,27-33 therefore we expected to obtain this compound during the photocatalytic degradation experiments. The good coincidence between the spectra of S-Degussa 1 h and that of solution A, and the spectra of S-Degussa 2 h and solution B, supports this hypothesis. At the same time, the calibration curves might help to estimate the concentrations of BNPP, 4-NPP, and 4-NP in the solution. The agreement of the calibration curves A and B with the spectra of S-Degussa 1 h and S-Degussa 2 h is better for wavelengths larger than 290 nm, where 4-NPP and 4-NP strongly absorb. It can be verified, in the calibrated solutions composition, that the concentrations of 4-NPP and 4-NP do not coincide with the “consumed” BNPP (the cleavage of one molecule of BNPP is supposed to generate one molecule of 4-NPP and one of 4-NP). That points to the ability of TiO2 to also degrade the products 4-NPP and 4-NP. The total mineralization of 4-NP in aqueous solution, photocatalyzed by TiO2 particles, to yield CO2, H2O, and HNO3 has been learned, where the generation of the strongly oxidizing OH• radical greatly contributes to this process in aerated aqueous solutions.43,44 The photooxidative degradation of 4-NP in aqueous solution, using H2O2 and UV radiation, yields the same

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Figure 5. (a) Absorption spectra of the irradiated (1 h) and nonirradiated 10-4 M BNPP solutions. (b) Spectra of the supernatant of irradiated and centrifuged Degussa P25 suspension (S-Degussa) compared to the irradiated solution (symbolized curves). Absorption spectra of reference aqueous solutions (continuous line) containing BNPP (6 × 10-5 M), 4-NPP (1.36 × 10-5 M), and 4-NP (1.36 × 10-5 M) for solution A, and BNPP (5.68 × 10-5 M), 4-NPP (1.68 × 10-5 M), and 4-NP (1.68 × 10-5 M) for solution B are also included in part b. (c) Spectra of irradiated and not irradiated 10-4 M 4-NP solutions and of the corresponding supernatant of irradiated and centrifuged Degussa P25 suspension (S1-Degussa). (d) Absorption spectra of irradiated and not irradiated 10-4 M 4-NPP and the corresponding supernatant of irradiated and centrifuged Degussa P25 suspension (S2-Degussa).

products.45 Formation of OH• from UV irradiated H2O2, again, is crucial in this oxidative degradation process.46 To our best knowledge, the photocatalytic degradation of 4-NPP by UV irradiated TiO2 has not been learned. However, the hydrolytic decomposition of 4-NPP, catalyzed by Cu(II) and La(III) complexes, to 4-NP- and H2PO4- is well-known.33,46 To test the ability of TiO2 on the photocatalytic degradation of 4-NP and 4-NPP, the TiO2 Degussa P25 powder was homogeneously suspended in 10-4 M solutions of 4-NP and 4-NPP independently. The TiO2 loading in both suspensions was 0.1 g/L. Again, the wavelength used was 365 nm and the intensity was 25 mW/cm2. After the irradiation, the suspension of TiO2 Degussa P25, in 4-NP and 4-NPP solutions, were centrifuged and the supernatants (S1-Degussa for 4-NP and S2-Degussa for 4-NPP) were collected to be analyzed. The pH of all samples was 7 before and after the irradiation. To obtain the absorbance measurements, the pH was taken to 14 with a 1 M NaOH aqueous solution. Figure 5 shows the absorption spectra of irradiated and nonirradiated 10-4 M 4-NP and S1-Degussa samples (part c), as well as irradiated and nonirradiated 10-4 M 4-NPP and S2-Degussa samples (part d). The spectra of the irradiated solutions of 4-NP and 4-NPP are fairly similar to those of the nonirradiated samples; therefore there is no significant degradation upon irradiation of the samples. As can be seen in Figure 5c, the S1-Degussa sample has lower absorbance at 400 nm than that of the 4-NP solution, indicative of a lower concentration of 4-NP on the TiO2-irradiated sample. The absorbance of S1-Degussa is larger than that of the 4-NP solution in the wavelengths range between 250 and 310 nm, which could be due to the presence of compounds preceding the total mineralization process. These compounds could be nitrocatechols, phenol family substances, and ring-opening products.44-46 In Figure 5d it is observed that the spectrum corresponding to S2-Degussa has a shoulder around 400 nm that corresponds to absorption of 4-NP- anions, with 4-NP is a degradation product. As proved, 4-NP can be degraded in UV

irradiated TiO2 aqueous suspensions. This means that the known mechanisms suggested for 4-NP degradation44 can explain the degradation process of 4-NPP, after the initial decomposition to 4-NP. The presence of other degradation intermediates could explain the almost constant dependence of the absorbance, for wavelengths lower than 310 nm, in the S2-Degussa sample. Photodegradation of BNPP by the UV Irradiated Synthesized TiO2 Nanoparticles. As observed in Figure 3, absorbance of the synthesized TiO2 NPs is relatively low at 365 nm, which is the excitation wavelength used in the preceding experiments. To ensure a more efficient light absorption, for these experiments, the samples were irradiated at 255 nm. The light intensity was approximately 2 mW/cm2. The samples were irradiated in a 1 cm pathway quartz cell. The experiment was carried out in the same way as the one previously described. Once again, after the irradiation, suspensions were centrifuged and the supernatants were collected to be analyzed. They were named as follows: S-TiO2-1 for the supernatant of TiO2-1 NPs suspension, S-TiO2-5 for the supernatant of NPs TiO2-5 NPs suspension, and S-TiO2-100 for the supernatant of NPs TiO2100 NPs suspension. A Degussa P25 sample was included for comparison (S3-Degussa). The TiO2 loading was 0.2 g/L in all cases. To approximately evaluate the photocatalytic activity of the TiO2 samples, reference aqueous solutions were prepared: reference mixture C, containing 7.68 × 10-5 M of BNPP and 0.75 × 10-5 M of 4-NPP and 4-NP; reference mixture D, containing 7.28 × 10-5 M of BNPP and 1.15 × 10-5 M of 4-NPP and 4-NP; and reference mixture E, containing 5.42 × 10-5 M of BNPP and 0.95 × 10-5 M of 4-NPP and 4-NP. Figure 6a shows the absorption spectra of nonirradiated and irradiated 10-4 M BNPP aqueous solutions. As can be seen, no significant changes were observed. In Figure 6b, the absorption spectra of S-TiO2-1, S-TiO2-5, S-TiO2-100, S3-Degussa, and reference solutions C, D, and E are shown. The spectra of all the supernatants from the synthesized TiO2 samples show a band at 400 nm due to the presence of 4-NP-

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Figure 6. (a) Absorption spectra of nonirradiated and irradiated (1 h) 10-4 M BNPP aqueous solutions. (b) Spectra of supernatant (symbolized curves) from the suspension of NPs synthesized as described in the Experimental Section (S-TiO2-1), supernatant from suspension of the NPs synthesized at 5 times the concentration of the precursors (S-TiO2-5), supernatant from suspension of the NPs synthesized at 100 times the concentration of the precursors S-TiO2-100, and (for comparison) supernatant from the suspension of TiO2 NPs Degussa P25 (S3-Degussa). Absorption spectra of reference aqueous solutions (continuous lines) containing BNPP (7.68 × 10-5 M), 4-NPP (0.75 × 10-5 M), and 4-NP (0.75 × 10-5 M) for solution C; BNPP (7.28 × 10-5 M), 4-NPP (1.15 × 10-5 M), and 4-NP (1.15 × 10-5 M) for solution D; and BNPP (5.42 × 10-5 M), 4-NPP (0.95 × 10-5 M), and 4-NP (0.95 × 10-5 M) are also included in part b. The irradiation wavelength was 255 nm with an intensity of 2 mW/cm2 and the exposure time was 1 h.

anions, as shown in Figure 6b. Therefore, the synthesized TiO2 NPs can promote the degradation of BNPP, as is learned for other catalysts.27-33 We observed that the absorbance at 400 nm is only slightly increased from sample S-TiO2-1 to S-TiO25. This fact and the good fitting with the spectra of the reference mixtures C and D, respectively, suggest that the degradation of BNPP is slightly larger for the latter sample, probably because of a larger absorption of the radiation and a larger light scattering. The spectra of S-TiO2-5 and S-TiO2-100 do not show significant differences. These TiO2 NPs have the same Eg values (therefore, the average particle size must be very similar), and absorbance is very similar, which explains the observed results in Figure 6b. The spectrum of the S3-Degussa sample has a lower absorbance at 290 nm, indicative of a larger decomposition of BNPP. The absorbance at 400 nm of the S3-Degussa sample lies between that of the synthesized NPs, in spite of the fact that the formation of 4-NP in the S3-Degussa sample is expected to be higher. This can be explained provided that further degradation of 4-NP is taking place in the S3-Degussa sample, as was shown above. The good agreement of the absorbance of the reference mixture E to that of the S3-Degussa sample supports that conclusion. From the fitting of the reference solutions, and taking into account that not all of the decomposition reaction subproducts are present in those solutions, it can be concluded that the photocatalytic activity of synthesized TiO2 NPs is lower (23.2-27.2% of the initial BNPP sample is degraded in 1 h), but of the same order of magnitude, as that of Degussa P25 (45.8% of the initial BNPP sample is degraded in 1 h). TiO2 Degussa P25 powder has a composition of about 70-80% anatase (Eg ) 3.2 eV) and 30-20% rutile (Eg ) 3.0 eV) with an average particle size of 20-30 nm, and higher absorption coefficient than many other commercial TiO2 NPS (Aldrich; Merck; Hombikat; Fisher; Fluka).47 So, if particles are too small the powder has a high specific surface area, but Eg might increase, decreasing the absorption wavelength range. At the same time, the larger the specific surface of the system the larger the number of surface recombination centers. So, there must be an optimal size to increase the photocatalytic efficiencies. This size was not reached by the synthesized TiO2 NPs; studies in this direction are in progress. It is also convenient to optimize the TiO2/BNPP loading and the radiation light intensity to increase the process efficiency. Suggested Degradation Mechanism. It is important to take into account that when electron-hole pairs are created in TiO2

in contact with an aqueous medium, very strong reactive species can be created. That is, the photogenerated electron can reduce molecular oxygen to create superoxide O2-• anion radicals, and photogenerated holes can oxidize molecular H2O or OH- anions to create OH• radicals, which can promote the oxidation and eventually the mineralization of organic compounds.24 The direct oxidation of the organic compounds by the valence band holes can also play an important role.24 However, OH• radicals are indicated as the actual oxidants in the degradation of aerated aqueous solutions of 2- and 4-nitrophenol.46 Taking into account our results and those presented by other groups,24,44-46 a possible mechanism of BNPP degradation is proposed in Scheme 1. In Scheme 1, in primary events from a to g, the strong oxidizing OH• radicals are generated as indicated in previous investigations.24,45 In the second part of Scheme 1, numbers are only used to identify the steps and not necessarily to indicate a strict order or sequence. In step 1 BNPP is hydrolyzed to 4-NPP and 4-NP by the active radicals generated in the UV irradiated TiO2 suspension or directly oxidized by holes. In the second step, 4-NP is degraded as previously described.44,45 In step 3, the OH• radical reacts with 4-NPP causing the nitro group to release the NO2-• radical. The attack of OH• yielding to the substitution of the nitro group by OH has been reported.44,45 In step 4, the direct attack of the OH• radicals to the phosphate may cause the formation of hydroquinone, releasing H2PO4anions. The oxidizing attack of the OH• radicals to the P-O bonds, resulting in the formation of the corresponding phenol, has been observed for other organophosphate compounds.24 In step 5, 4-phosphate phenol reacts with OH• radicals, by electrophilic addition, analogously to what has been learned44,45 for nitro phenols. In step 6, 4-hydroquinone reacts with OH• radicals, also by electrophilic addition, to form other phenolic substances.44,45 The following steps are all based on subsequent attacks of OH• radicals to the nitro and phosphate groups or electrophilic additions to the ring structure. Subsequent reactions of the OH• radicals with intermediates, which correspond to further degraded products, lead to benzene ring cleavage and formation of oxygenated aliphatic compounds and final mineralization.45 Direct electrophilic additions to the ring structures and/or direct attacks to the nitro group, by OH• radicals, in BNPP were omitted in this scheme for simplicity. These attacks are also possible, and the subsequent reactions of the products with OH• radicals would also lead to simpler degradation products.

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SCHEME 1: Suggested Mechanism of BNPP Degradation in UV Irradiated Aqueous TiO2 Suspensiona

a It is important to emphasize that in this case, for these elemental reaction steps the stoichiometric coefficients are not displayed and reactive intermediates are not shown.

Concluding Remarks The proposed synthesis method yields anatase TiO2 NPs with average size of 2.1 nm having a relatively narrow size distribution and standard deviation of 0.3 nm. The small particle radius combined with the narrow size distribution allowed the observation of a blue shift in the absorption spectrum derived from quantum confinement effect. The method is very simple,

and works in short times and under normal reaction conditions. The synthesized particles change the crystalline phase with the annealing process: at 500 °C anatase remains as the only present phase and at 1000 °C the powder is totally rutile. UV irradiated TiO2 aqueous suspensions can degrade, in short times, the BNPP and the primary products 4-NPP and 4-NP are also subsequently degraded. Results with the synthesized TiO2 NPs in photocata-

Synthesis of TiO2 Nanoparticles lytic degradation of BNPP showed that, under the tested conditions, by increasing the particle size the photocatalytic activity is slightly increased. However, all the synthesized TiO2 NPs did not reach the optimal size consequently; the experimental conditions have to be changed to increase the degradation efficiency. Acknowledgment. The authors want to thank DGAPAUNAM for supporting this research (INI00907 PAPIIT project). I.Z. thanks DGAPA-UNAM for the postdoctoral award. The authors also thank M.Sc. G. Osorio for preparing one sample used in this work and Dr. Roberto Sato for helping with Raman spectroscopy measurements. Also, the authors thank Dr. Silvia Castillo, Dr. Ame´rica Va´zquez, and Dr. Rita Patakfalvi for kindly revising this paper. Supporting Information Available: Table of some representative methods for TiO2 NPs synthesis; representative size distribution of anatase TiO2 nanoparticles and a discussion on the size of aggregated particles in Figure 1d with respect to those observed in Figure 1a; discussion about the crystalline phase evolution of the synthesized TiO2 studied by Raman spectroscopy; and photodegradation of BNPP assisted by TiO2 powder Degussa P25 irradiated with natural solar light. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Gra¨tzel, M. Inorg.Chem. 2005, 44, 6841. (2) Wang, G.; Wang, Q.; Lu, W.; Li, J. J. Phys. Chem. B. 2006, 110, 22029. (3) Ojama¨e, L.; Aulin, C.; Pedersen, H.; Ka¨ll, P. J. Colloid Interface Sci. 2006, 296, 71. (4) Arami, H.; Mazloumi, M.; Khalifehzadeh, R.; Sadrnezhaad, S. K. Mater. Lett. 2007, 61, 4559. (5) Charlene, J. W. Ng.; Gao, H.; Yang-Tan, T. T. Nanotechnology 2008, 19, 445604. (6) Brudnik, A.; Gorzkowska-Sobaœ, A.; Pamuła, E.; Radecka, M.; Zakrzewska, K. J. Power Sources 2007, 173, 774. (7) Bae, E.; Choi, W. J. Phys. Chem. B 2006, 110, 14792. ¨ sterlund, L.; Sˇtengl, V.; Mattsson, A.; Bakardjieva, S.; Andersson, (8) O P. O.; Oplusˇtil, F. Appl. Catal., B 2009, 88, 194. (9) Zhao, X.; Quan, X.; Chen, S.; Zhao, H.; Liu, Y. J. EnViron. Sci. 2007, 19, 358. (10) Liang, C.; Shimizu, Y.; Sasaki, T.; Koshizaki, N. Appl. Phys. A: Mater. Sci. Process 2005, 80, 819. (11) Testino, A.; Bellobono, I. R.; Buscaglia, V.; Canevali, C.; D’Arienzo, M.; Polizzi, S.; Scotti, R.; Morazzoni, F. J. Am. Chem. Soc. 2007, 129, 3564. (12) Bessekhouad, Y.; Robert, D.; Weber, J. V. Int. J. Photoenergy 2003, 5, 153. (13) Niederberger, M.; Garnweitner, G.; Krumeich, F.; Nesper, R.; Co¨lfen, H.; Antonietti, M. Chem. Mater. 2004, 16, 1202. (14) Li, W.; Shah, S. I.; Huang, C. P.; Jung, O.; Ni, C. Mater. Sci. Eng., B 2002, 96, 247.

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