Synthesis of TiO₂/SiO₂-B₂O₃ Ternary Nanocomposites: Influence of Interfacial Properties on their Photocatalytic Activities with High Resolution Mass Spectrometry Monitoring

Abstract

An investigation on unusual interface properties of unprecedented ternary composites, formed by the inclusion of assorted proportions of B₂O₃ into TiO₂/SiO₂ structure, is conducted herein. The influences of B₂O₃ content and calcination temperature were evaluated. The precursor TiO₂/SiO₂ material was synthesized via a simple sol-gel procedure that was followed by B₂O₃ inclusion via maceration and calcination. The materials were fully characterized and their photocatalytic performance to degrade the Indigo Carmine dye investigated. The material prepared with a B/Ti molar ratio of 1 and at calcination temperature of 350 °C (B_1-350) showed the best performance, with a superior photocatalytic activity than that of commercial TiO₂. The presence of B₂O₃-TiO₂-SiO₂ interfaces in the structure of such material was of critical importance in producing a material with these attractive features. Finally, high resolution mass spectrometry monitoring allowed for the characterization of the main degradation products formed under these conditions.

Keywords:
B₂O₃-TiO₂-SiO₂ ternary nanocomposites; structural defects; photocatalytic activity; degradation of Indigo Carmine; by-products characterization; high resolution mass spectrometry

Introduction

In recent years, several investigations ^1,2 have been dedicated to the improvement of the photocatalytic performance of TiO₂. This is due to its appealing properties, such as strong oxidizing ability for the decomposition of organic pollutants, remarkable chemical stability and low cost/toxicity. ³ However, some drawbacks, such as its relatively large energy band gap (3.2 and 3.0 eV for anatase and rutile, respectively) and its low quantum yield, prevent its further application in photocatalytic processes. ⁴ Hence, a variety of strategies has been developed in an attempt to produce modified materials with a superior photocatalytic proficiency. ⁵

In this context, the incorporation of other oxides, such as SiO₂, ZrO₂, WO₃ and Al₂O₃, into the TiO₂ structure has been widely studied. ^6-10 It is well-known that the addition of SiO₂ to TiO₂ improves the specific surface area (SSA) of the modified material, thus, increasing the adsorption rate of organic pollutants. ¹¹ In our previous work, ¹² it was shown that the presence of SiO₂ inhibits the growth of the anatase crystals during TiO₂ calcination, retarding the transformation of anatase to rutile phase. Guan et al.¹³ suggested that the addition of SiO₂ can increase the concentration of hydroxyl groups on the TiO₂ surface, resulting in samples with improved photocatalytic and hydrophilic properties. All of these factors can lead to an increase in the number of active sites, as well as a better efficiency for the separation of photogenerated charges, thus enhancing the photocatalytic activity, as proven by several studies. ^14,15

B₂O₃ is another oxide that has been incorporated into the TiO₂ structure to produce a material with superior performance. This possibility has been reported, however, in just few studies. ^16-18 To the best of our knowledge, a detailed study regarding the role of the B₂O₃-TiO₂ interfaces on the superior photocatalytic activity of this type of material is unprecedented.

Taking into account these remarks, the synthesis of TiO₂/SiO₂-B₂O₃ ternary nanocomposites is reported herein. For this purpose, we evaluated the influence of two key variables during the nanocomposite preparation: the boron content and the calcination temperature. These materials were synthesized by sol-gel route followed by hydrothermal and calcination treatments, and characterized by nitrogen sorption tests, X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The performance of all synthesized TiO₂/SiO₂-B₂O₃ nanocomposites towards the photodegradation of a prototype pollutant (Indigo Carmine dye) in an aqueous solution and under irradiation, provided by a commercial fluorescent lamp, was measured by UV-Vis spectrometry. Chemical structures of the by-products formed under these conditions were proposed based on the data obtained via a direct monitoring by electrospray ionization high-resolution mass spectrometry (ESI-HRMS). The mineralization rate was also estimated by measuring the oxygen consumption. Indigo Carmine was chosen as a prototype substrate because it is a synthetic dye widely used as a coloring agent or additive in industry. Moreover, it is quite harmful to aquatic life and often used as a model pollutant in photocatalytic studies due to its advantageous characteristics, such as ease of access, low cost and easy degradation monitoring by UV-Vis spectrophotometry. The photocatalytic activity of the best material was then compared with those provided by two types of commercially available TiO₂: TiO₂ (99% anatase) and TiO₂ P25 (80% anatase, 20% rutile).

Experimental

Synthesis

The Pluronic^® F127 triblock copolymer [(EO)₁₀₆(PO)₇₀(EO)₁₀₆, M: 12600] was used as a structural directing agent. Titanium(IV) isopropoxide (TIP), tetraethoxysilane (TEOS) and boric acid (H₃BO₃) were used as precursors for TiO₂, SiO₂ and boron, respectively. Sulfuric acid (H₂SO₄) and 2,4-pentanedione (acac) were employed as acidifying and complexing agents, respectively. All reagents were purchased from Sigma-Aldrich and used as received without additional treatment. To conduct the comparative photocatalytic tests, the following commercial materials were evaluated: TiO₂ (99% anatase) from Sigma-Aldrich, and TiO₂ P25 (80% anatase, 20% rutile) from Evonik.

Initially, 1.64 g of the F127 copolymer was dissolved in 100 mL of deionized water (at 40 °C) containing H₂SO₄ sufficient to maintain the pH value at approximately 0.65. After complete the copolymer dissolution, 4.1 mmol of TEOS were added to this solution, which was kept under constant stirring. To this solution, it was added a mixture of TIP (41.4 mmol) and acac (41.4 mmol) under vigorous stirring. The final solution was then maintained at 55 °C for 2 h without stirring. After several minutes, a light yellow colloidal suspension was formed. The hydrothermal treatment was conducted by leaving the as-prepared suspension at 110 °C for 6 h. The resulting solid (consisting of TiO₂/SiO₂ nanocomposite particles) was filtered and washed with water and then dried overnight in an oven at 60 °C.

To obtain the TiO₂/SiO₂-B₂O₃ nanocomposites, the TiO₂/SiO₂ particles were ground with different amounts of H₃BO₃ and calcined at different temperatures (350, 450 or 550 ºC). The nomenclature for the synthesized materials was B_x-y, where x is the molar ratio B/Ti (R_B) and y is the calcination temperature. Therefore, the calcined samples at 450 °C, B_0-450, B_0.125-450, B_0.25-450, B_0.5-450, B_1-450, B_2-450 and B_4-450, were prepared using R_B of 0, 0.125, 0.25, 0.5, 1, 2 and 4, respectively. On the other hand, the B_1-350, B_1-450 and B_1-550 samples were prepared using R_B = 1 and calcination temperatures of 350, 450 and 550 °C, respectively. Commercial TiO₂ were named anatase (TiO₂ Sigma-Aldrich) and P25 (TiO₂ Evonik). The SiO₂/TiO₂ molar ratio was kept constant at 0.1 for each nanocomposite, whereas the B/Ti molar ratio (R_B) was varied from 0 to 4.

Characterizations

Nitrogen sorption tests were performed in an Autosorb-1C apparatus. Samples used in these analyses were previously degassed at 130 °C for up to 48 h under vacuum. SSA and pore size distribution were assessed by the multipoint BET (Brunauer, Emmett and Teller) and NLDFT (non-local density functional theory) methods, respectively. XRD analysis was carried out in a Philips-PANalytical PW17-10 diffractometer using Cu Kα radiation and operating at 40 kV and 40 mA. XRD patterns were collected in the 2θ range of 10 to 90°, using a scan velocity of 0.06 degree min^-1. The identification of the crystalline phases was performed using the JCPDS (International Centre for Diffraction Data^®) file numbers 21-1272 and 30-0199 for anatase and sassolite (H₃BO₃), respectively. The TiO₂ crystallite size was assessed using the Scherrer equation and considering XRD peaks at 25.3° for anatase. Diffuse reflectance spectra were obtained in a UV-Vis-NIR instrument (Hitachi model U-3501) with the diffuse reflectance accessory equipped with an integrating sphere. BaSO₄ was used as reference material and for sample dilutions. The gap values were calculated using the Kubelka-Munk function. Samples for the FTIR spectra were prepared as pellets with KBr and examined in a FTIR/FIR spectrometer (PerkinElmer model Frontier). The spectra were recorded from 4000 to 350 cm^-1, with resolution of 4 cm^-1 and acquisition of 128 scans. SEM images were obtained on a FEI QUANTA 3D field emission gun scanning electron microscope (FEG-SEM). Samples were sputtercoated with a carbon layer of about 5 nm thickness before the SEM analysis.

Photocatalytic tests

The photocatalytic activities of the TiO₂/SiO₂-B₂O₃ nanocomposites were estimated by measuring the rate of degradation of the Indigo Carmine dye in aqueous suspension. The degradation tests were preceded by adsorption tests in the absence of light. Photolysis, hydrolysis and tests with pure B₂O₃ were also performed as controls. All photocatalytic tests were performed in triplicate in order to evaluate reproducibility. The reactor used for the photocatalytic tests consisted of a wooden box coated with aluminum foil. The dimensions of the reactor were 55 cm height, 85 cm length and 30 cm depth (Figure S1, in the Supplementary Information (SI) section, shows the design of the reactor). In a typical run, 30 mg of the ternary nanocomposite were added to 250 mL of Indigo Carmine solution (10 mg L^-1). This suspension was kept under constant magnetic stirring and submitted to irradiation by a commercial fluorescent lamp (power of 32 W, Taschibra model TKS 32-1 NJY). The emission spectrum of this lamp is provided in Figure S2 (in the SI section). Aliquots of 5 mL were collected at the following times: 0, 5, 15, 30, 45, 60, 90 and 120 min. The aliquots were centrifuged at 4000 rpm for 10 min in a model Centribio 80-2B centrifuge. The supernatant was collected and filtered through a disposable filter unit for syringes (Millex^®, PVDF, Ø = 0.45 µm). The Indigo Carmine concentration in each aliquot was estimated using a UV-Vis spectrophotometer (Varian, model Cary 50 Conc) working at 610 nm (λ_max for Indigo Carmine). Full scan UV-Vis spectra (from 300 to 700 nm) were also recorded for some samples. The apparent reaction rate constant (k_app) for each material was calculated during the first 60 min of reaction and by considering a pseudo-first order model. The pH value of the solution, monitored by a pHmeter (HANNA model HI98128), remained roughly constant (approximately 4.5) during the experiment. Chemical structures of the by-products were proposed based on the data obtained via direct infusion ESI-HRMS analyzes. The analyses were performed on a hybrid mass spectrometry system. The mass spectrometer (IT-TOF, Shimadzu Corporation, Kyoto, Japan) provides high sensitivity and accuracy with a resolving power over 10,000. The mass spectrometer was equipped with an electrospray ionization (ESI) source operating in the negative (-3.5 kV) mode and with a nebulizer gas (N₂) at a flow rate of 1.5 L min^-1. The interface and CDL (curved dessolvation line) were operated at constant temperature of 200 °C. A mass-to-charge (m/z) range of 100-500 was recorded for each aliquot. The samples (10 µL) were directly infused into the ESI source via an autosampler. Mineralization rates were estimated according to the oxygen consumption method that makes use of potassium permanganate to oxidize the organic material in solution. A detailed description about this procedure is provided in the directive NBR 10739/1989. ¹⁹ Chemical oxygen demand (COD) and total organic carbon (TOC) methods could not be applied herein due to the low concentrations of Indigo Carmine employed.

Results and Discussion

B/Ti molar ratio (R_B)

Figure 1 shows the XRD patterns obtained for the produced materials, i.e., B_0-450, B_0.125-450, B_0.25-450, B_0.5-450, B_1-450, B_2-450 and B_4-450. As noted, all samples showed anatase as the crystal phase, whereas no peaks associated with the brookite or rutile phases were found. The peaks at 14.58 and 27.94° (2θ) confirm the formation of the boron trioxide (B₂O₃) crystalline phase.

Figure 1
XRD patterns for the B_0-450 to B_4-450 materials. The letters 'A' and 'B' indicate peaks attributed to the anatase crystalline phase and sassolite (B₂O₃), respectively.

Table 1 shows the physicochemical properties of the synthesized and commercial materials. Compared with B_0-450, the TiO₂/SiO₂-B₂O₃ nanocomposites (B_0.125-450, B_0.25-450, B_0.5-450, B_1-450, B_2-450 and B_4-450) display no significant changes in the crystallite size, confirming the results presented in Figure 1. The presence of SiO₂ and the formation of B₂O₃ phase may obstruct the nucleation of anatase nanoparticles, allowing only a slight increase in the average crystallite size. As seen in Table 1, the gap value oscillated between 3.32 and 3.29 eV for the B_0-450 to B_4-450 materials. This finding indicates that the B₂O₃ incorporation has no significant influence on this parameter. The data displayed in Table 1 for the B_0-450 to B_4-450 materials also indicate that an increase in R_B causes a decrease in SSA and in the average pore size. That is probably because the B₂O₃ phase deposited on the nanocomposite surface blocks the pores of the material leading to the reduction in the values of both parameters.

Table 1
Physicochemical properties of the synthesized (B_0-450, B_0.125-450, B_0.25-450, B_0.5-450, B_1-450, B_2-450, B_4-450, B_1-350 and B_1-550) and commercial materials (TiO₂-anatase and TiO₂-P25)

Sample	SSA / (m2 g-1)	Pore size / nm	Pore volume / (cm3 g-1)	Crystallite size / nm	Gap / eV	kapp / min-1	R2
B0-450	159.00	5.90	0.2065	14.46	3.32	0.0027	0.9934
B0.125-450	115.90	5.90	0.1736	14.46	3.31	0.0059	0.9998
B0.25-450	105.70	5.30	0.1642	14.14	3.30	0.0088	0.9859
B0.5-450	56.97	5.30	0.0984	14.26	3.32	0.0107	0.9900
B1-450	9.33	5.30	0.0182	14.60	3.31	0.0164	0.9901
B2-450	0.67	4.18	0.0020	14.66	3.31	0.0093	0.9898
B4-450	0.86	1.69	0.0012	17.88	3.29	0.0030	0.9863
B1-350	7.66	5.90	0.0105	12.05	3.30	0.0227	0.9916
B1-550	6.64	6.81	0.0158	24.09	3.29	0.0042	0.9935
Hydrolysis	-	-	-	-	-	0.0000008	0.9949
Photolysis	-	-	-	-	-	0.0003	0.9866
B2O3	-	-	-	-	-	0.00009	0.9898
TiO2-anatase	11a	-	-	68.08a	3.29	0.0049	0.9975
TiO2-P25	50 ± 15a	-	-	21a	3.29	0.0113	0.9929

• ^a
  Data provided by the manufacturer. Gap: energy band gap; R²: determination coefficient.

Figure 2 shows the FTIR analysis of the B_0-450, B_0.125-450, B_0.25-450, B_1-450 and B_4-450 materials. The bands at 3400 and 1630 cm^-1 are attributed to vibrations of surface water and O-H stretching. ²⁰ The band at approximately 3220 cm^-1 can be attributed to B-O bond vibrations. ²⁰ In fact, the intensity of this band increases by increasing R_B. The bands between 2260 and 2510 cm^-1 are related to B-H vibrations. ²¹ The bands at 1440 and 1200 cm^-1 are associated with the B-O bond in boroxol rings. ^20,22,23 At 1125 and 1050 cm^-1, a Si-O-Si vibration can be observed. ^16,24 In this region (1000 to 1200 cm^-1), C-O vibrational modes may also appear due to organic waste from the syntheses. The broad band between 900 and 400 cm^-1 is associated with the vibrational modes of Ti-O in TiO₂. ^23,24 Finally, the increasing of R_B causes the appearance of bands at 810 and 540 cm^-1 (B-O deformation) and 644 cm^-1 (stretching B-B). ²⁵ Note that the appearance of these bands decreases the intensity of the TiO₂ band, corroborating to the proposal of a thick B₂O₃ layer deposited on the TiO₂/SiO₂ nanocomposites particles. SEM micrographs of B_0-450, B_1-450 and B_4-450 are displayed in Figure S3 (in the SI section). In these images, changes in the morphology of the particles are clearly observed.

Figure 2
FTIR spectra for the B_0-450, B_0.125-450, B_0.25-450, B_1-450 and B_4-450 materials.

Figure 3 shows the results of the tests dealing with the photocatalytic activities of the TiO₂/SiO₂-B₂O₃ materials. The k_app and R² (determination coefficient) values of each material are shown in Table 1. The adsorption-desorption equilibrium was reached after 30 min. The activity increases with R_B until it reaches a maximum in R_B = 1 (B_1-450). Taking into account that this sample does not have the highest SSA (Table 1), these results therefore seem to indicate that the presence of an interfacial B₂O₃ phase is essential to produce materials with enhanced photocatalytic activities. For the material B_1-450, the B₂O₃ layer, which is deposited on the surface of the nanoparticles and is part of an interface with the other two oxides, helps to improve the degradation rate due to the formation of traps for the photogenerated charges. ⁴ However, for the materials with R_B > 1 (B_2-450 and B_4-450), these traps drastically increase and act as centers of charge recombination, which disfavors the photocatalytic process. ²² Furthermore, for these materials (B_2-450 and B_4-450), the thicker B₂O₃ layer probably hampers the radiation to reach the TiO₂ surface. ¹⁶ The difference in activity of these materials, as well as the results obtained by mass spectrometry (discussed following in this paper), confirms that the dye did not act as a sensitizer agent for the photocatalysts. The B_1-450 sample was therefore chosen as the prototype material to be further evaluated, as following described in this article.

Figure 3
Photocatalytic activities of the degradation of the Indigo Carmine dye in aqueous solution for the TiO₂/SiO₂-B₂O₃ materials.

Figure 4
XRD patterns for the B_1-350, B_1-450 and B_1-550 materials. The letters 'A' and 'B' indicate peaks attributed to the anatase crystalline phase and sassolite (B₂O₃), respectively.

Effect of calcination temperature

Figure 4 shows the XRD patterns recorded for the B_1-350, B_1-450 and B_1-550 samples. As the calcination temperature increases, narrow peaks emerge indicating a higher crystallinity, which can be confirmed by the increase in the average crystallite size (Table 1). Moreover, the rutile phase is not detected, even at 550 °C, corroborating the previous assumption that SiO₂ and B₂O₃ can suppress diffusion between the anatase particles, thus, limiting anatase to rutile transformation. It was verified that the calcination temperature has no evident effect in the gap value. However, the B_1-350 and B_1-550 samples showed higher average pore size and lower SSA than B_1-450. Hence, the calcination at 350 °C is probably not sufficient to remove the organic residues that can block the smaller pores and yields a material with a higher average pore size and a smaller SSA. At higher temperatures (550 °C), however, the formation of larger nanoparticles results in the closing of the smaller pores, thus, leading to an increase of the average pore size and a reduction of SSA. The FTIR analysis of the B_1-350, B_1-450 and B_1-550 samples (not shown) presents the same profile as that shown in Figure 2 for B_1-450. The only exception was the reduction in the intensities of the bands at approximately 3400 and 1630 cm^-1 from B_1-350 to B_1-550 that indicates the loss of water at higher temperatures. ²⁰

The results for the photocatalytic activities of B_1-350, B_1-450 and B_1-550 samples is displayed in Table 1 (k_app and R²) and Figure 3. These data indicate that higher calcination temperatures cause a decrease in the degradation rate. After 90 min, the B_1-350 and B_1-450 samples showed degradation rates of almost 100%, while the B_1-550 sample showed approximately 40% degradation. Higher temperatures improve the average crystallite size, which increases not only the distance that photogenerated charges must overcome to reach the surface but also the charge recombination rate. ²⁶ The reduction of SSA at higher temperatures also minimizes the availability of active sites. Furthermore, full scan absorption spectra (not shown) of the reaction aliquots (from 300 to 700 nm) indicate no formation of by-products that absorb in the visible region. Therefore, the absorbance at 610 nm (the λ_max of Indigo Carmine) was a reliable measurement to determine the concentration of the remaining dye in solution.

Analyzes by direct infusion electrospray ionization high resolution mass spectrometry (ESI-HRMS) was conducted aiming at detecting at least the most abundant by-products, probably formed under these conditions. ESI-HRMS is a key technique for the identification of by-products resulting from the degradation of water contaminants, and therefore has played an important role in elucidating degradation pathways. ²⁷ Figure 5 shows the mass spectra (negative mode) of aliquots collected at times 0 and 120 min for the three photocatalytic systems (B_1-350, B_1-450 and B_1-550). Note that the mass spectra for such photocatalytic systems showed the same profile, the only difference being the relative intensity of each ion. In the mass spectrum of the initial aliquots, the following ions that refer to Indigo Carmine are clearly noticed (Table 2 and Figure 5): m/z 209.9863 (doubly-deprotonated Indigo Carmine: [C₁₆H₈N₂O₈S₂]^2- or [IC]^2-), and m/z 442.9626 (mono-deprotonated Indigo Carmine: [C₁₆H₉N₂O₈S₂]^- or [IC + H]^-).

Table 2
High-resolution mass spectrometry data and molecular formula inferred for the by products generated during the photodegradation of Indigo Carmine dye in water by the three photocatalytic systems: B_1-350, B_1-450 and B_1-550

Anion	Molecular formulae	m/z (theoretical)	m/z (experimental)	Error / ppm	DBE
Anions derived from Indigo Carmine dye (IC)
[IC]2-	C16H8N2O8S2	209.9867	209.9863	-1.90	13.0
[IC + H]-	C16H9N2O8S2	442.9625	442.9626	0.23	13.0
[(IC)2 + H]3-	C32H17N4O16S4	287.6453	287.6452	-0.35	25.0
[(IC)3 + 2H]4-	C48H26N6O24S6	326.4746	326.4745	-0.31	37.0
By-products (anionic form)
[1]-	C8H5NO5S	225.9816	225.9810	-2.66	7.0
[2]-	C8H5NO6S	241.9765	241.9773	3.31	7.0
[3]-	C7H6NO5S	215.9972	215.9979	3.24	5.0
[4]-	C8H4NO7S	257.9714	257.9743	11.24	7.0
[5]-	C6H3O6S	202.9656	202.9651	-2.46	5.0

• DBE: double-bond equivalence.

Figure 5
High resolution mass spectrometry (HRMS) recorded for aliquots collected after 0 and 120 min of exposure to the photocatalytic systems: B_1-350, B_1-450 and B_1-550. The molecular formulae attributed to some by-products are indicated in Table 2.

Non-covalent species derived directly from Indigo Carmine are also detectable in this mass spectrum (Table 2 and Figure 5): [(IC)₂ + H]^3- of m/z 287.6452 and [(IC)₃ + 2H]^4- of m/z 326.4745. After 120 min of exposure to the photocatalytic systems, these anions are no longer detected only in the mass spectrum of the aliquot collected from the B_1-350 system. These results confirm therefore that the B_1-350 photocatalyst possesses indeed a superior efficiency to degrade Indigo Carmine in comparison to the other systems (B_1-450 and B_1-550). The emergence of extra anions (not detectable in the original solutions) in the mass spectra of the aliquots collected after an exposure time of 120 min is attributed to the formation of by-products (Figure 5). Table 2 displays a list of these anions and their respective molecular formula. The errors between the theoretical and experimental m/z values were smaller than 11.24 ppm and then support the proposed molecular formula. All these by-products originate from an initial oxidation of the Indigo Carmine molecule followed by successive oxidation/ring hydroxylations. Note, as expected, that the mass spectrum of the B_1-350 aliquot exhibit these by-products at higher intensities than in the mass spectra of the other systems. Finally, based on these experimental data as well as on previous studies reported in the literature, a route for the degradation of Indigo Carmine in aqueous medium by the three photocatalytic systems could therefore be proposed, as displayed in Figure 6. ^27,28 Due to its superior photocatalytic efficiency, the B_1-350 sample was selected to accomplish comparative tests with commercial TiO₂.

Figure 6
Proposed route for the photodegradation of Indigo Carmine dye in water as induced by the three photocatalysts: B_1-350, B_1-450 and B_1-550.

Photocatalytic efficiency: comparative tests

To conduct the comparative tests, the commercial materials (anatase and P25) were evaluated. Table 1 and Figure 7 summarize the results of the photocatalytic activities of these materials as well as B_0-450, B_1-350 and controls tests. Note that the control experiments did not produce any significant degradation of the Indigo Carmine dye. The B_1-350 material proved to be more effective than both commercial samples, with a k_app that is twice that of P25. The B_1-350 sample has a lower SSA than the other materials, proving that the B₂O₃-TiO₂-SiO₂ interfaces (by reasons previously stated) probably explains this superior photocatalytic activity.

Figure 7
Photocatalytic activities of the degradation of the Indigo Carmine dye in aqueous solution for hydrolysis, photolysis, B₂O₃, B_0-450, B_1-350 and commercial TiO₂ materials.

Finally, data on oxygen consumption indicated that that only the B_0.5-450, B_1-450, B_2-450, B_1-350, anatase and P25 systems were able to cause mineralization, but at low rates. Moreover, the systems comprised by P25 (the most widely used photocatalyst) and B_1-350 yielded the best results, i.e., rates of about 11.5%.

Conclusions

TiO₂/SiO₂ nanocomposites were obtained in a single step and at low temperature using a synthetic procedure that employs water as the solvent. B₂O₃ inclusion allowed the attainment of materials with improved photocatalytic activities, as verified in degradation tests with a common organic pollutant (the Indigo Carmine dye). SiO₂ and B₂O₃ contributed to stabilize the anatase phase, even when a calcination at 550 °C was applied. The optimized material was obtained with a B/Ti ratio (R_B) of 1 and by employing a calcination temperature of 350 °C. This material showed a performance superior to those of commercial TiO₂ materials in causing the depletion of Indigo Carmine. Although the surface area is of fundamental importance for photocatalytic processes, this work demonstrated that the interactions that occur at the material interfaces are also critical. Hence, the B₂O₃-TiO₂-SiO₂ interfaces seem to act as sites of charge separation up to a maximum content of B₂O₃. After this maximum, the B₂O₃ layer causes the opposite effect. The reactions were monitored by direct infusion electrospray ionization high resolution mass spectrometry which revealed that Indigo Carmine is efficiently degraded and that the main by-products are continuously formed by successive oxidations/hydroxylations. It is noteworthy that in all of the degradation experiments a common fluorescent lamp was used as the light source. Fluorescent lamps are easily accessible, of low cost and consume less power than the lamps commonly used in similar photocatalytic experiments. These results therefore reveal that this TiO₂/SiO₂-B₂O₃ nanocomposite can be used without the need of high energy light sources and has clear potential to be employed for the treatment of real effluents contaminated with organic matter.

Acknowledgments

The authors thank the financial support from CAPES and CNPq/FAPEMIG (agreement recorded in SICONV: 793988/2013). We kindly acknowledge the Microscopy Center at the Universidade Federal de Minas Gerais for the execution of the SEM tests and Prof Vicente Buono for the attainment of the XRD data. R. L. G. and B. S. O. are also grateful to the sponsorship from the Pró-Reitoria de Pesquisa of Universidade Federal de Minas Gerais.

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br as PDF file.

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Publication Dates

• Publication in this collection
  Oct 2017

History

• Received
  11 Nov 2016
• Accepted
  06 Mar 2017