Formation and Controlled Growth of Bismuth Titanate Phases into

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Formation and Controlled Growth of Bismuth Titanate Phases into Mesoporous Silica Nanoparticles: an Efficient SelfSealing Nanosystem for UV Filtering in Cosmetic Formulation. Gloria Zaccariello, Michele Back, Marta Zanello, Patrizia Canton, Elti Cattaruzza, Pietro Riello, Alessandro Alimonti, and Alvise Benedetti ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13252 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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ACS Applied Materials & Interfaces

Formation and Controlled Growth of Bismuth Titanate Phases into Mesoporous Silica Nanoparticles: an Efficient Self-Sealing Nanosystem for UV Filtering in Cosmetic Formulation. Gloria Zaccariello, † Michele Back, † Marta Zanello, †Patrizia Canton, †* Elti Cattaruzza, † Pietro Riello, † Alessandro Alimonti§ and Alvise Benedetti†* †

Department of Molecular Sciences and Nanosystems, Università Ca’ Foscari di Venezia, Via

Torino 155/b I-30170, Venezia-Mestre, Italy. §

Istituto Superiore di Sanità, Bioelement and Health Unit, Dept. Environment and Health, Italian

National Institute for Health, Viale Regina Elena 299, 00161, Rome, Italy. KEYWORDS: Bismuth titanate, mesoporous silica nanoparticles, inorganic sunscreen, TiO2, photocatalysis.

ABSTRACT

The application of nanosized inorganic UV filters in cosmetic field is limited by their high photocatalytic properties that could induce the degradation or dangerous transformation of the organic molecules in sunscreen formulations. To overcome this problem and simultaneously

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enlarge the window of filter’s absorption, we propose the growth of bismuth titanates BixTiyOz into Mesoporous Silica Nanoparticles (MSN). We investigated the chemical-physical properties by means of XRPD, TEM, UV-Vis spectroscopy, N2 physisorption, XPS and SF-ICP-MS analysis, while the influence on the environment was evaluated through photocatalytic tests. The growing process of this new nanosystem is discussed underlining the key role of the Bi3+ ion that, acting as a low-melting point agent for the silica framework, led to a self-sealing mechanism. The excellent UV shielding properties combined with a radical suppression of the photocatalytic activity make the proposed nanosystem a perfect candidate for the development of the next generation nanomaterials for sunscreen formulations.

1. INTRODUCTION Ultraviolet (UV) radiation can induce deleterious photodamages on human skin as a consequence of an excessive or unprotected exposure to sunlight.1-5 The main harmful cumulative effects comprise sunburn,2 actinic elastosis and wrinkling, in a process known as photoaging.67

Moreover, an excessive sun exposure can lead to the development of skin cancers.8-9 The rising

awareness about the above-mentioned detrimental effects has meant that sunscreen products became more widespread as a tool to protect skin in our daily life. A typical sunscreen contains two main kinds of UV filters as active ingredients: organic (or chemical) and inorganic (or physical) filters.10 The presence of both types of filters is due to their synergic effect; in this way an almost complete coverage of the UV spectrum is assured. The inorganic components of a sunscreen formulation, mainly titanium dioxide (TiO2) and zinc oxide (ZnO), are present as nanometric size particles; their advantage is to avoid the whitening effect


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when the sun care formulation is applied to the skin.11 ZnO and TiO2 are semiconductors and when they are exposed to UV light they become harmful photocatalysts,12 they degrade the organic components of the cosmetic formulation13-14 and enhance the damage to DNA and cells in in vitro systems.15-17 Different strategies were investigated to limit these drawbacks. Corazzari et al. inactivated TiO2 nanoparticles via carbon-based surface modification, exploring the efficacy of several organic modifiers.18 The proposed treatment suppressed both oxidative and reductive activity of TiO2 but not the release of singlet oxygen and the modified TiO2 powders were still far from commercial use since appropriate coatings were needed to make the powders compatible with the emulsions used in cosmetic preparations. Ukmar et al. adopted a different method by coating TiO2 nanoparticles with a silica layer onto which a stabilizer was strongly bound, demonstrating that also the agglomeration between the TiO2 were avoided.19 Luo et al. modified the surface chemistry of ZnO powder by means of PEGylation reducing the citotoxicity of ZnO nanoparticles, resulting from a decrease in cellular uptake.20 Yabe et al.21 proposed, as an alternative to TiO2 and ZnO, ultrafine particles of Mn+-doped ceria and they showed a decrease of photocatalytic activity of the undoped CeO2 for the air oxidation of castor oil. In recent years, a combination of TiO2/CeO2 or ZnO/CeO2 nanoparticles with low photocatalytic activity has also been developed showing that the photocatalytic activity of these systems for organic material oxidation is smaller than that of titania, ceria, and zinc oxide.22-24 Despite these problems, commercial sunscreens contain modified TiO2 particles as inorganic active phase. An example is Parsol® TX, a commercial UV filter composed by submicron-size TiO2 particles coated by a layer of silica and dimethicone in order to reduce the photocatalytic activity of the material.


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In this paper we report a new strategy to prepare an inorganic UV filter to be used in cosmetic formulation. We are challenging a multifaceted problem: we want to improve the absorption characteristic of semiconductor nanoparticles (320-400 nm) and to isolate the active phases from the environment. To this purpose, the semiconductor is dispersed into the pores of a mesoporous matrix; the isolation of the active phase from the cosmetic formulation is assured through the sealing of the external part of the support. On the basis of their biocompatibility and low cost preparation, we chose Mesoporous Silica Nanoparticles (MSN) as support. The sealing of MSNs external surface is achieved through a thermal treatment. The difficult task is to assure that only the external surface of the MSNs is affected by the thermal treatment while their internal porous structure controls the growth of the active phases. The crucial issue is to find a chemical element that should be able to react with the semiconductor nanoparticles (in our case TiO2) to form a new active phase with wide UV shielding property and to act as low melting agent for SiO2 in order to seal MSNs external surface through a thermal treatment. For this strategy, we selected a bismuth salt. Bismuth ions can react with the TiO2 nanoparticles already formed inside the silica pores to form bismuth-titanate phases. The bismuth-titanate compound family has interesting properties and has been actively exploited in visible-light photocatalysis as well as in the field of microelectronics, electro-optics, and dielectric.25-27 Theoretical and experimental studies were aimed at a deep comprehension of the structural28-30 and photocatalytic31-34 properties of the different BixTiyOz phases. For instance, the perovskite-like Bi4Ti3O12 structure is a well-known candidate for high-temperature device applications due to its high dielectric constant, breakdown strength and anisotropy, and low dielectric dissipation factor.35 The dielectric pyrochlore


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Bi2Ti2O7 phase is used in photocatalysis, from the solar hydrogen production by water splitting26 to the degradation of organic compounds under sunlight irradiation.36 Also its insulating properties, as oxide material in MOS (metal oxide semiconductor) devices, have been evaluated. Furthermore, bismuth has a low melting point and with increasing calcination temperature, bismuth ions can diffuse more and more inside the MSN and a thickness of glassy silica can seal the external part of our MSN. The self-sealing mechanism provides a protective environment for the active phases of the filter.

2. EXPERIMENTAL SECTION 2.1 Materials Tetraethoxysilane (TEOS, 98%), sodium hydroxide (NaOH), cetyltrimethylammonium bromide (CTABr), methanol (MeOH, 99.6%), titanium(IV) isopropoxide (TTIP, 97%), bismuth(III) nitrate pentahydrate ((Bi(NO3)3 ·5H2O), 99.9%), nitric acid (HNO3, 65%), methyl orange (MO – 4-[4-(dimethylamino)phenylazo] benzenesulfonic acid sodium salt), n-hexane (99.6%) and ethanol (EtOH, 99.8%) were purchased from Sigma-Aldrich. The water was distilled before use. For SF-ICP-MS analysis, nitric acid 67% Ultrapure Normatom® was obtained from VWR Prolabo (Leuven, Belgium); hydrofluoric acid (HF, suprapure) and hydrogen peroxide (H2O2, suprapure) from Merck (Darmstadt, Germany); high-purity deionized water from Barnstead EASY-Pure II (Dubuque, IA, USA).

Synthesis of MSN Mesoporous silica nanoparticles were synthesized as described by Ma et al.37 The swelling agent n-hexane (60 mL) was added to a solution of CTABr (0.8 g), NaOH 2M (3 mL) and H2O (400


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mL). The mixture was stirred for 15 min (400 rpm) and then stopped. When the separation between the polar and nonpolar phases occurred, TEOS (4 mL) was dropped and the system was put under vigorous stirring for 5 h. The molar ratio of the reagents of the synthesis was 1TEOS : 0.12CTABr : 1268H2O : 0.32NaOH : 25.1n-hexane. The obtained solution was filtered and the recovered solid was washed several times with MeOH and dried at 60°C overnight. In order to remove the surfactant, the product was calcined at 550°C, with a rate of 2°C min-1 for 5 h. The obtained sample was referred as MSN.

2.2 Synthesis of Bi/Ti-MSN_y The synthesis of the bismuth titanate into the mesopores of silica matrix was conducted by incipient wetness impregnation at room temperature in two steps. In our typical synthetic procedure,38 the first step was carried out adding the TTIP (0.128 mL) dissolved in EtOH (25 mL) to the MSN (0.3 g). The dispersion was kept under magnetic stirring for 24 h, in a nitrogen atmosphere. Then, the solvent was removed by means of a rotary evaporator. The recovered solid was calcined at 550°C, with a rate of 2°C min-1, for 5h and labeled as Ti-MSN. For the second step, to the Ti-MSN sample (0.3 g), bismuth(III) nitrate pentahydrate (0.25 g) dissolved in EtOH/HNO3 (5:1, 25 mL) was added, keeping the dispersion under stirring for 1 h. Then, the solvent was removed by means of a rotary evaporator and the recovered powders were calcined at different temperature, with a rate of 1°C min-1, for 2 h. The samples were referred as Bi/TiMSN_y, where y represents the temperature (°C) of calcination (y = 500, 600, 700, 750, 800).

2.3 Characterization


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The nitrogen adsorption-desorption isotherms were obtained at the liquid nitrogen temperature (77 K) using a Micromeritics ASAP 2010 volumetric adsorption analyser. From the adsorption data, the Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area, while from the adsorption branches of the isotherms, the Barrett-Joyner-Halenda (BJH) model was used to estimate the pore size distribution. X-ray powder diffraction (XRPD) spectra were recorded with a Philips X’Pert powder diffractometer (Bragg-Brentano parafocusing geometry). A nickel-filtered Cu Kα1 radiation (λ = 0.15406 nm) and a step-by-step technique (step of 0.05° 2 h) with collection times of 10s/step were employed. From the XRPD patterns, the quantitative analysis and size distribution of the crystallites were carried out by means of the Rietveld refinement.39 Due to the complexity and the broadening of the patterns, the quantitative analysis was possible only on the samples treated at higher temperatures. Titanium and bismuth concentrations were determined by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS, ThermoFischer, Bremen, Germany). The instrument was equipped with sampler and skimmer cones in Ni, a Meinhard nebulizer, a water-cooled spray chamber (Scott-type) and a guard electrode device. Aliquots of samples (10-20 mg) were added with a mixture of 2 mL of ultrapure HNO3, 0.25 mL of suprapure HF and 1 mL of suprapure H2O2. The samples were then digested at 80±3°C on a hotplate (ModBlock CPI International, California, USA) for 3 hours, transferred into clean polypropylene plastic tubes and filled up to a final volume of 50 mL with high-purity deionized water. The morphological structure of the samples was investigated using a JEOL JEM 3010 transmission electron microscope (TEM) operating at 300 kV. The powder specimens were suspended in isopropyl alcohol and an aliquot of 5 µL was deposited on a copper grid (300


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mesh) coated with holey carbon film. The copper grids were allowed to dry in air. The energy dispersive X-ray spectroscopy (EDX) measurements were carried out with a Oxford Instruments Isis System Series 300. The diffusive reflective UV-vis (DRUV-vis) measurements were collected with a JASCO V570 UV-VIS spectrophotometer equipped with an integrating sphere accessory. Barium sulphate was used as reference. Since the investigated samples are indirect bandgap semiconductors,30 we estimated the bandgap energy of the systems from the Tauc plot40 of the Kubelka-Munk absorption (K-M), plotting (K-M · hν)1/2 versus hν. The powder samples were placed between two quartz slides and the spectra were recorded in 200-800 nm wavelength range. X-ray photoelectron spectroscopy (XPS) surface analyses were performed using a Perkin-Elmer ϕ 5600ci spectrometer using non-monochromatic Al Kα radiation (1486.6 eV) in the 10-7 Pa pressure range. All the binding energy (BE) values are referred to the Fermi level. The correct calibration of the BE scale was verified during the analysis by checking the position of both Au4f7/2 and Cu2p3/2 bands (from pure metal targets), falling at 84.0 eV and 932.6 eV, respectively.41 After a Shirley-type background subtraction, the raw spectra were fitted using a non-linear least-square fitting program adopting Gaussian–Lorentzian peak shapes for all the peaks. Owing to surface charging, samples presented a shift of the bands towards higher BE’s: the charging effect was corrected by using an internal reference, namely after identification of SiO2 presence by means of the silicon “alpha parameter” (binding energy of Si2p band + kinetic energy of SiKLL band). In all samples, the Si alpha parameter lies in between 1712.0-1712.3 eV, confirming the presence of silicon dioxide.41-42 The surface charging, of about 4-5 eV, was then corrected by considering the Si2p centred at 103.3 eV: as a consequence, the O1s band related to SiO2 fell around 532.7 eV, in agreement with the presence of stoichiometric silica.41 The


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uncertainty of all the determined BE’s was lower than 0.2 eV. The atomic composition was evaluated using sensitivity factors as provided by Θ V5.4A software. The relative uncertainty of the calculated atomic fraction of the different elements is lower than 5-10%. The reported atomic fraction data were calculated excluding “adventitious” carbon (originated by environment contamination), anyways present in low amount (few at. %).

2.4 Photocatalytic Experiments

The photocatalytic activity of the synthesized materials was determined by Methyl Orange (MO) degradation under UV-vis irradiation. Experiments were performed at 293K using a 100 mL Pyrex photochemical reactor with a 125 W medium pressure mercury lamp (Model UV13F, Helios Italquartz, Italy). The initial concentration of the target molecule was 7.5 x 10-5 M and the amount of the photocatalyst was fixed at 1.25 g L-1. In order to reach the adsorption equilibrium prior the UV-irradiation, the sample was stirred in the dark for 30 min. After switching on the UV-vis lamp, at each time step, an aliquot of 2 mL of the aqueous suspension was recovered from the reactor and filtered through a 0.45 µm PTFE Millipore disc to remove the catalyst powder. Dye degradation process was monitored by an Agilent 8453 UV-vis spectrometer following the absorbance at the maximum of the UV-vis spectrum of the target molecule (λmax = 465 nm). The MO concentration was estimated using a standard calibration curve. The normalized intensity of the absorption band at 465 nm was plotted as a function of time of irradiation.


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3. RESULTS AND DISCUSSIONS The growth of crystalline nanoparticles embedded into the MSN and the corresponding evolution of the mesoporous structure of the silica nanoparticles were investigated as function of thermal treatments between 500 and 800°C. As reported in our previous work,38 the XRPD patterns of MSN at low angle, are characterized by two well-resolved diffraction peaks, at 2θ = 1.60° ± 0.05° and 2.60° ± 0.05° indexed as (100) and (110) respectively, associated to the ordered hexagonal mesoporous structure, while the TiMSN pattern exhibits reflections characteristic of anatase phase (PDF 21-1272). Figure 1 shows the XRPD patterns of the Bi/Ti-MSN_y samples and the relative crystalline phases formed from 500°C. The patterns show that as the calcination temperature increases, the broad diffraction halo due to the amorphous silica decreases while the crystalline fraction increases. The pattern of the sample treated at 500°C shows that the bismuth ions start to react weakly with the silica matrix, forming a negligible amount of bismuth silicate. In fact, in addition to the amorphous silica matrix contribution, peaks of tetragonal β-Bi2O3 (PDF 010-77-5341) and orthorhombic Bi2SiO5 phase (PDF 01-075-1483) are visible in the XRPD pattern. At 600°C, the amount of bismuth silicate reaches its maximum and a small amount of Bi2Ti2O7 (PDF 01-074-4250) appears, revealing an interaction of bismuth ions with TiO2 nanoparticles. Finally, from 700°C to 800°C, three crystalline phases can be clearly observed: mainly the cubic Bi2Ti2O7 (14.5 wt%) with a small amount of the orthorhombic Bi4Ti3O12 (6 wt%, PDF 010-779500), and a residual content of bismuth silicate Bi2SiO5 (2 wt%) as determined by the Rietveld refinements of the XRPD data. We can observe that even if bismuth silicate is the first crystalline phase to be formed, bismuth titanates are favoured at higher temperatures. This means that in the competitive reactions of


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Bi2O3 with SiO2 and TiO2, bismuth oxide tends to react with titanium dioxide forming BixTiyOz nanoparticles. This conclusion is supported also by the low amount of bismuth silicate phase found in all the samples, in fact, if all the bismuth had reacted with the silica, it would have to form approximately a 30% of bismuth silicate.

Figure 1. XRPD patterns of Bi/Ti-MSN_y samples in the wide-angle region from 15° to 70°(a) and magnification in the 25-35° range showing the formation of Bi-compounds as function of the thermal treatment (b). The analysis of the low angle region (Figure S1) shows that as the calcination temperature increases, the ordered hexagonal mesopores structure of silica matrix is not retained. In particular, an initial decrease of the ordering occurs already at 500°C up to a complete loss of the mesoporous structure at 800°C. This loss can be due either to a high degree of disorder of the pores or to a complete collapse of the silica network. The crystallite sizes, determined by the Scherrer equation, of Bi2Ti2O7, Bi4Ti3O12 and Bi2SiO5 phases for the samples treated at 750°C and 800°C, are reported in Figure 1 by considering the Rietveld refinements XRPD patterns showed in Figure S2. For the samples calcined at lower


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temperatures it was not possible to clearly distinguish the peaks belonging to different phases and, for this reason, XRPD size determination was not feasible. Quantitative determinations of Ti and Bi by SF-ICP-MS measurements were conducted on Bi/TiMSN (not calcined), Bi/Ti-MSN_500 and Bi/Ti-MSN_800. These values are compared with the ones obtained by the XRPD quantitative phase analysis on the Bi/Ti-MSN_800 sample (Figure S2). The ICP and the XRPD results are in good agreement and they show that the Bi and Ti content is lower than the theoretical one, for all the analyzed samples (Table 1). This result suggests that the loss of bismuth is not related to the thermal treatments, as one might have assumed considering the volatility of bismuth at high temperature, but it probably occurs during the impregnation step. The same conclusion can be drawn also in the case of titanium. Moreover, the comparison between the ICP and the XRPD results referred to the percentage of the phases contributions show that, in the Bi/Ti-MSN_800 sample, almost all the Bi2O3 and TiO2 formed crystalline phases and that SiO2 is the main constituent of the residual amorphous phase.


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Table 1. Quantitative determination (wt%) of SiO2, Bi2O3 and TiO2 phase contributions in Bi/TiMSN, Bi/Ti-MSN_500 and Bi/Ti-MSN_800 samples.

Sample

Bi/Ti-MSN (not calcined)

Bi/Ti-MSN_500

Bi/Ti-MSN_800

Crystalline phase

Theoretical

SF-ICP-MS

XRPD

(%)

(%)

(%)

SiO2

66.2

75.6

-

TiO2

7.3

6.6

-

Bi2O3

26.5

17.8

-

SiO2

66.2

72.3

-

TiO2

7.3

6.4

-

Bi2O3

26.5

21.3

-

SiO2

66.2

75.4

78 ± 2

TiO2

7.3

6.6

5±2

Bi2O3

26.5

18.00

17 ± 2

The modification of the pores structure is also confirmed by the nitrogen adsorption-desorption analysis. The isotherms and the corresponding pore size distribution plots are shown in Figure 2a and Figure 2b, respectively, while the textural properties of the samples (specific surface area and specific pore volume) are summarized in Table S2. As expected, the calcined MSN exhibits the typical type IV profile with a H2 hysteresis loop (IUPAC classification) due to the capillary condensation steps at relative pressures p/p0 in the range of 0.3 - 0.6, typical of ordered and mesoporous materials with two-dimensional cylindrical pores. The bare MSN displays a high BET specific surface area (805 m2 g-1) and the distribution of the pore diameter is very narrow


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and centered at 4.2 nm. As shown in Figure 2a, after the MSN impregnation with the TiO2 precursor (Ti-MSN sample), the amount of N2 adsorption slightly decreases due to the formation of TiO2 nanoparticles; nevertheless, the mesoporous silica structure is retained, maintaining a type IV isotherm profile. After the addition of bismuth ions (Bi/Ti-MSN_y series), the mesoporous structure is still kept for the sample treated at the lowest calcination temperature (Bi/Ti-MSN_500). When the temperature raises, a progressive decrease of the specific surface area is observed: compared to Ti-MSN, Bi/Ti-MSN_500 shows a significant reduction of the surface area (413 m2/g) that drastically falls off for Bi/Ti-MSN_800 (36 m2/g). Consequently, also a significant decrease of the pore volume was determined. As reported in Table S2, the decrease occurs already at the lowest temperature (0.5 cm3/g) and drastically lessens for the sample treated at the highest temperature (0.2 cm3/g). Such results could be ascribed both to the formation of nanoparticles inside the silica channels and to the presence of the Bi3+ ions that could have a key role in the closing of the pores as the calcination temperature increases. This second hypothesis is strengthen by the fact that the temperatures of our thermal treatments are above the glass transition temperature (Tg) of the SiO2-Bi2O3-based glasses.43-44 In order to verify the possible key role of the bismuth ions and their interactions with the titanium and silica network, the Ti-MSN sample was also calcined at 800°C. A slight decrease of both the specific surface area (568 m2/g) and the pore volume (0.6 cm3/g) occurs, however the mesoporous structure is maintained and not collapsed (Figure S3).


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Figure 2. N2 adsorption-desorption isotherms (a) and BJH pore size distribution of MSN and Bi/Ti-MSN_y samples (b).

The morphological structure of the MSN and Ti-MSN samples was already discussed in detail in our previous paper,38 showing that the mesoporous silica nanoparticles have a spherical shape with diameters ranging from 100 to 200 nm and an ordered two-hexagonal mesoporous structure with pore diameter of about 5 nm. It was demonstrated that the MSN can be used to control the growth of nanoparticles inside the pores,45 in particular it was shown how the silica pores control the TiO2 nanoparticles growth.38 At the lowest concentration of titania (10%wt of the total composite), the nanoparticles have a size in agreement with the pores diameter. In order to explain the role of the Bi3+ ions added in a Ti-MSN system and the effect of the calcination temperature, TEM analysis on Bi/Ti-MSN_y series were conducted and reported in Figure 3. At 500°C the ordered mesoporous structure is retained in the inner area of the particles, while it is partially lost in the superficial layer, presumably due to the bismuth ions that act as low-melting agent (Figure 3a). At 600°C the mesoporous structure is still distinctly visible in the inner part, but the pore organization is less evident (Figure 3b). On the MSN surface layers the self-sealing process starts to be more


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consistent, with the consequent closure of the channels. With the increasing of the calcination temperature at 700°C, the mesoporous structure is still present but it is not anymore organized in an ordered manner (Figure 3c). At this temperature, bismuth ions are able to strongly interact with the silica matrix leading to the complete closing of the superficial channels during the selfsealing process. This means that thanks to the self-sealing property, the photocatalysts are no longer available to the surrounding environment. Compared to the other samples treated at lower temperatures, the mesoporous structure is completely lost in the specimen calcined at 800°C (Figure 3d). TEM investigations prove that at this temperature the self-sealing process is even more evident and it occurs in a homogeneous manner, assuring the complete closure of the superficial pores. For all the samples, high resolution images show also the presence of small crystallites grown inside the silica matrix (Figure S4a-d). EDX analysis confirm the presence of Si, O, Bi and Ti elements.


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Figure 3. Representative TEM micrographs of Bi/Ti-MSN_500 (a), Bi/Ti-MSN_600 (b), Bi/TiMSN_700 (c) and Bi/Ti-MSN_800 (d) specimens.

With the aim to analyze the first 5-10 nm of the surface of the nanoparticles and to clarify the role of Bi3+ in the self-sealing process, XPS spectra of MSN, Ti-MSN, Bi/Ti-MSN_600 and Bi/Ti-MSN_800 were also performed. The silicon, oxygen, titanium and bismuth atomic fractions are summarized in Table 2 and the binding energy values of the XPS main bands are reported in Table S3.


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Table 2. Si, O, Ti and Bi atomic fraction as obtained by XPS data in MSN, Ti-MSN, Bi/TiMSN_600 and Bi/Ti-MSN_800 samples. The relative error is 0.1.

MSN Ti-MSN

Bi/Ti-MSN_600

Bi/Ti-MSN_800

Si

33%

27%

30%

32.3%

O

67%

67%

65%

64%

Ti

-

6%

2%

1.5%

Bi

-

-

3%

2.2%

XPS spectra of MSN (Figure S5) displays Si2p and O1s bands centred at 103.3 eV and 532.7 eV of BE, respectively, typical values for silica matrix.41 The quantitative analysis agrees with the presence of silicon dioxide, being the atomic fraction ratio O/Si about 2. In the Ti-MSN sample, besides the Si2p and O1S bands, a Ti2p doublet is clearly visible (Figure S6), whose Ti2p3/2 peak is centered at 458.6 eV of BE. This value is compatible with Ti4+ presence.41 The Si2p band is the same as for MSN sample. O1s peak is the overlap of two different components: the first one, less intense, centered at 530.2 eV, is in agreement with presence of TiO2;41 the second one, more intense and centered at 532.7 eV, is related to oxygen atoms forming SiO2. The calculated stoichiometry of titanium oxide is in agreement with the presence of Ti dioxide, within experimental error. Converting the atomic fraction to weight percentage, it results that TiO2 represents about 23% of the sample instead of the loaded 10%. Since XPS analysis can provide information only on the first 5-10 nm from the sample surface, it means that the surface of the sample is richer in TiO2 than the core.


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In addition to Si, O, and Ti XPS related bands, Bi/Ti-MSN_600 sample shows also bands due to Bi, in particular Bi4d and Bi4f (Figure 4b and Figure 4c). The 4d5/2 and 4f7/2 doublet components fall at 441.8 eV and 159.0 eV of BE, respectively: these values agree with the presence of Bi3+.41 Even if the Bi4f band is usually used for the quantitative analysis, in our case we are forced to use Bi4d5/2 band since the former strongly interferes with Si2s signal. Ti2p3/2 doublet component (Figure 4c) was centered around 458.0 eV of BE, thus suggesting the presence of Ti4+.41 O1s band can be seen as the overlap of two components (Figure 4a): the first one, due to oxygen atoms forming TiO2, Bi2O3, and/or mixed compounds such as bismuth silicate or titanate, corresponds to about 13% of the total intensity; the second band is related to oxygen forming SiO2, having 87% of the total O1s intensity. The hypothesis of the presence of TiO2, Bi2O3, and/or more complex oxides is in agreement also with the related atomic fractions, as it can be deduced by the data reported in Table 2. If compared to Ti-MSN sample, here the amount of titanium at the surface of the sample decreases from 6% to 2%. This result is not depending on the fitting parameters: by fixing larger values of the Ti2p band intensity, we could not obtain an acceptable fit of the experimental spectrum. This suggested that titanium compounds are deeper inside the silica matrix, probably because of the closing of the surface mesopores as a consequence of bismuth introduction. Moreover, the amount of Bi is 3%, slightly higher than Ti: this is compatible with the possibility that a part of oxidized bismuth forms bismuth titanate, while the remaining part is forming bismuth silicate.


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Figure 4. XPS O1s (a), Si2s+Bi4f (b) and Bi4d+Ti2p (c) bands revealed by the Bi/Ti-MSN_600 sample.

The bands detected for Bi/Ti-MSN_800 (Figure S7) show very similar BE position of those found for Bi/Ti-MSN_600. As reported in Table 2, the amount of both Ti and Bi decreases, passing from 2.0% to 1.5% and from 3.0% to 2.2%, respectively. Also in this case, the difference is not an artifice of the fitting procedure, as evidenced also by a direct comparison of the spectra recorded for the two different samples. This suggested that bismuth titanate could be more and more covered by silica, located deeper and deeper below the surface of the silica particle. In summary, the XPS experimental findings are in agreement with the hypothesis previously reported and depicted in the schematic illustration in Scheme 1: as the temperature of the thermal treatment increases, bismuth titanates nanocrystals grow inside the silica network and simultaneously the Bi3+ ions acts as a low-melting point agent for silica, leading to a self-sealing process.


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Scheme 1. Schematic process of the growth of BixTiyOz into MSN: (i) MSN is impregnated with TTIP and calcined at 550°C to form TiO2 nanoparticles (TiO2-MSN); (ii) subsequently, the adding of Bi(NO3)3·5H2O and the thermal treatment at 600°C lead to the formation of mainly Bi2Ti2O7 inside the silica channels, furthermore in the inner part the mesoporous structure is still kept, while at the surface a self-sealing process occurs (Bi/Ti-MSN_600); (iii) finally, with the arising calcination temperature to 800°C, also a small quantity of Bi4Ti3O12 phase grows, a complete collapse of the inner mesoporous structure occurs and the self-sealing process in the particles surface is completed (Bi/Ti-MSN_800).

Optical reflectivity measurements were performed on the powder samples to obtain absorption information and to evaluate the corresponding bandgap. In Figure 5a, the spectra of Ti-MSN and Bi/Ti-MSN_800 samples are compared. It is notable that the formation of BixTiyOz spread the absorption to the UVA region - only partially accessible by the Ti-MSN sample - with a significant theoretical impact on the UV-filter properties. Moreover, taking into account the fact that anatase phase and both Bi2Ti2O7 and Bi4Ti3O12 phases are indirect bandgap semiconductors,30 from the Tauc plot45 of the Kubelka-Munk absorption (K-M), plotting (K-M · hν)1/2 versus hν, we estimated the bandgap energy of the systems. Following this approach, absorption edges at about 3 eV are observed for Bi/Ti-MSN_y samples: compared to Ti-MSN sample, that have an absorption edge at about 3.65 eV, the Bi/TiMSN_y samples show a significant blue shift. The bandgap energies obtained for Ti-MSN and Bi/Ti-MSN_y samples are higher than the values reported in literature for both anatase (≈3.2


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eV)46 and bismuth titanates (≈2.9 eV for Bi2Ti2O7 and ≈3.1 eV for Bi4Ti3O12 respectively).30 This can be due to the quantum confinement of the nanocrystals well dispersed into the silica matrix. Even if the bandgap energy of Bi/Ti-MSN_y samples is higher than the bulk one, the red shift of the UV absorption edge to longer wavelength with respect to the value of the Ti-MSN is remarkable.

Figure 5. DR-UV spectra of Ti-MSN and Bi/Ti-MSN_800 (a) and Tauc plot of the KubelkaMunk function vs hν energy of the samples (b).

In order to verify whether the introduction of the bismuth ion on Ti-MSN has also influenced the photocatalytic activity of the system, the degradation of MO was tested, comparing the Ti-MSN and the Bi/Ti-MSN_y samples. As shown in Figure 6, after 30 min of equilibration in the dark, no alterations of the dye are recorded. The Ti-MSN sample displays a high photocatalytic activity toward MO, which is completely degraded after 90 minutes of UV irradiation. Although,


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BixTiyOz and Bi2SiO5 systems are well known photocatalysts,36,47-50 with the introduction of the bismuth ions and the consequent formation of Bi-composites (Bi/Ti-MSN_y samples), a strong reduction on the photodegradation of the dye occurs. In particular, the trend observed for the MO degradation shows that, as the calcination temperature increases, the photocatalytic efficiency tails off. After 240 minutes under UV light, there is a degradation of about 30% for Bi/TiMSN_500, 29% for Bi/Ti-MSN_600, 27% for Bi/Ti-MSN_700 and 11% for Bi/Ti-MSN_800. The results from photocatalytic degradation, theoretically inconsistent with the photocatalytic ability of the BixTiyOz and Bi2SiO5, are in agreement with the scenario in which: i.

at low annealing temperatures the pores of the MSN are almost open and the MO is able to reach the photocatalysts;

ii.

increasing the calcination temperature, bismuth titanates grow and at the same time a self-sealing process starts to occur;

iii.

at the highest calcination temperatures the silica network collapses, the photocatalysts are embedded into the silica matrix and the self-sealing prevent the contact between MO and the active phases.

The photocatalytic tests demonstrate that this new system can be considered safe thank to the self-sealing process able to isolate the photocatalytic nanocrystals from the environmental molecules, thus causing the photocatalytic inactivity.


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Figure 6. Performances of adsorption and MO photodegradation on Ti-MSN and Bi/Ti-MSN_y series under UV irradiation. The error bars represent standard deviation from three replicas for each measurement.

CONCLUSIONS In summary, a novel approach for the development of bismuth-based safe self-sealing nanomaterials as inorganic sunscreen UV filters is reported. The growth mechanism of BixTiyOz nanocrystals has been analyzed in detail as a function of temperature, displaying a progressive and preferential stabilization of the Bi2Ti2O7 crystalline structure. The concomitance loss of the silica mesoporous structure and the key role of bismuth ions in the self-sealing process were analyzed by means of N2 adsorption-desorption, TEM, and XPS analysis. The double role of bismuth ions in the spreading of the UV shield properties and the suppression of the photocatalytic properties make the proposed nanosystem a suitable candidate for the development of the next generation of nanomaterials for the sun shields. Moreover, the ability of bismuth to act as local low-melting ion for silica network, leading to a self-sealing


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process, can be exploited in the design of a wide range UV filter nanomaterials with the aim of cutting their photocatalytic ability.

ASSOCIATED CONTENT Supporting Information. Additional XRD spectra, Rietveld refinement XRPD patterns, N2 adsorption-desorption isotherms, BJH pore size distribution, TEM images and XPS spectra are shown. The quantitative determination by SF-ICP-MS and XRPD, the comparison of textural properties and the XPS binding energy are also reported.

AUTHOR INFORMATION Corresponding Author * Alvise Benedetti: E-mail: [email protected]; Fax: +39 041 234 6747; Tel: +39 041 234 8544. Patrizia Canton: E-mail: [email protected]; Fax: +39 041 234 6747; Tel: +39 041 234 6790.

ACKNOWLEDGMENT The authors acknowledge Mr Tiziano Finotto for XRPD measurements, Mrs Martina Marchiori for nitrogen physisorption measurements, Prof. Antonella Glisenti for the XPS measurements and Dr. Beatrice Bocca and Dr. Flavia Ruggieri for the SF-ICP-MS analysis. The authors thank Dr. Loretta Storaro for having made available the instruments for the photocatalytic experiments. The authors would like to thank also Prof. Enrico Sabbioni.


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For Table of Contents Only


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Formation and Controlled Growth of Bismuth Titanate Phases into

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