Synthesis and photocatalysis of mesoporous titania templated by natural rubber latex

Abstract

Although a few systems prepared by using biotemplates have been employed as mesoporous photocatalysts, the natural latex secreted by plants has not been used as a template to synthesize porous materials, let alone for the achievement of high photocatalytic activity under solar light. Herein, one of the most important biosynthesized polymers, natural rubber latex from the rubber tree, Hevea brasiliensis in Xishuangbanna was directly used as a structure-directing agent in synthesis of thermally stable mesostructured TiO₂. The as-prepared samples were characterized by XRD, FT-IR, XPS, N₂ adsorption/desorption, TEM and UV-vis DRS. The synthesized TiO₂ exhibited highly effective photocatalytic activity for the degradation of phenol and rhodamine B under solar light. Furthermore, addition of acetylacetone (ACA) during the synthesis significantly enhanced the photocatalytic activity. The reported strategies provide a new direction of using natural rubber latex as a template to prepare highly effective photocatalysts.

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1. Introduction

Since mesoporous TiO₂ was first synthesized by using the tetradecyl-phosphate surfactant as template, many efforts have been devoted to the synthesis of mesoporous TiO₂ because it usually shows higher surface areas and much more uniform and controllable pore size and pore morphologies compared with randomly organized forms of nanocrystalline TiO₂.¹ Up to now, the general synthetic strategies for mesoporous TiO₂ have been developed using a variety of templates, such as phosphates amine, ionic surfactants, block polymer, nonionic surfactants and nonsurfactant templates.² Recently, we have successfully used chrome azurol S³ and even commercial synthetic dyes⁴ as structure-directing agents in synthesis of mesostructured TiO₂. More recently, sodium salicylate was used as a template in the synthesis of mesoporous TiO₂ materials with well-defined crystal morphologies.⁵

On the other hand, with TiO₂ as photocatalyst, the light energy used must exceed the band gap of 3.2 eV, thus, UV light where λ < 387.5 nm is required. This greatly limits the use of sunlight as an energy source for the photoreactions. To develop more solar light efficient catalysts, it is very urgent to develop photocatalytic systems which are able to operate effectively under visible light irradiation. A number of systems such as doping with transition metal ions⁶ or nonmetallic ions (fluorine, nitrogen, carbon, sulfur, boron, phosphor, iodine),⁷ coupling with small band gap semiconductors, have been reported to serve as candidates for this application recently.

Biological templates have attracted considerable attention for the syntheses of porous inorganic materials in the recent several years, because they are generally inexpensive, abundant and environmentally benign.⁸ For example, mesoporous anatase TiO₂ prepared by using bacterial cellulose membranes as natural biotemplates exhibited good photocatalytic activity for photodegrading rhodamine B (RB) under UV light irradiation.⁹ Natural cellulosic substances were used for preparation of various effective photocatalysts.¹⁰ Green leaves have been used as templates to synthesize N-doped ZnO photocatalyst.¹¹ And we have synthesized mesoporous TiO₂ by using the plant skins as templates.¹² However, previous studies have not yet addressed the use of natural latex secreted by plants as templates to synthesize porous materials, let alone the achievement of highly photocatalytic activity under solar light.

Natural rubber latex is secreted by the rubber tree (Hevea brasiliensis) which is indigenous to the tropical rain forest and usually grows under full sunlight.¹³ Like other green plants, rubber tree can be considered as a plant factory for solar energy conversion and a carbon sink by virtue of the process of photosynthesis.¹⁴ Therefore, natural rubber latex may find potential application in the synthesis of highly efficient solar-light photocatalysts. Particularly, natural rubber latex as one of the most important biosynthesized polymers displays excellent chemical and physical properties.¹⁵ Introducing inorganic nano-fillers into natural rubber matrixes can exhibit a controllable combination of the benefits of polymers and those of inorganic phase.^15a,16 For example, a self-assembled natural rubber/silica nanocomposites was developed by latex-compounding techniques.¹⁵ Silver nanoparticles were formed in natural rubber matrix via photo reduction of film cast from natural rubber latex containing silver salt.^17a It is worthwhile to note that natural rubber latex was still left in the composite products and still the same as the original ones in these studies. What differentiates our work from others is that the natural rubber latex was directly used as template in our method. That means natural rubber latex was removed finally by calcination.

Polymer latex have also been employed as templates in synthesis of macroporous metal oxides, in particular, hollow spheres.¹⁸ For example, macroporous materials of TiO₂, silica and zirconia were synthesized by using the emulsion droplets as templates around which material is deposited through a sol–gel process.^18a Titania, zirconia and alumina samples with periodic three-dimensional arrays of macropores were synthesized from the corresponding metal alkoxides, using latex spheres as templates.^18b Porous TiO₂ hollow spheres by liquid phase deposition on polystyrene latex-stabilised pickering emulsions show photocatalytic activity at the UV induced degradation of dichloroacetic acid.^18c Submicrometer-sized TiO₂ hollow spheres with tunable shell thickness and smooth surfaces have been successfully synthesized by employing sulfonated polystyrene latex particles as a template in sol–gel method.^18d Cr/Ti hollow spheres prepared by using poly-(styrene-methyl acrylic acid) latex particles as a template exhibited considerable levels of the methylene blue decomposition (>35%) under visible light irradiation.^18e However, in these methods, latex spheres have to be synthesized first in an emulsifier-free emulsion process.

Natural rubber latex comprises 25–28 wt% water and particles with sizes ranging between 0.2 and 5 μm consists of cis-1,4-polyisoprene core naturally covered with a thin layer of adsorbed protein molecules,^17a any attempt to remove this adsorbed layer results in an immediate coagulation of the latex.¹⁷ Although chitosan, also a biopolymer, has been used as a template in mesoporous alumina macrospheres,¹⁹ it was dissolved in acid solution. While there are few reports on the preparation of mesoporous TiO₂ using basic conditions. Thus many synthetic challenges still need to be addressed in order to achieve good reproducibility in the synthesis of mesoporous TiO₂ by using natural rubber latex as template.

Herein, mesoporous TiO₂ templated by natural rubber latex with an addition of acetylacetone (ACA) was synthesized and denoted as MTiO₂/RL-ACA. Without any extra doping, mesoporous TiO₂ was also synthesized by natural rubber latex and denoted as MTiO₂/RL. For comparison, mesoporous TiO₂ templated by conventional dodecylamine (MTiO₂/DDA)²⁰ and the commercial Degussa P25 TiO₂ were evaluated by degrading of phenol and rhodamine B under solar light.

2. Experimental

2.1. Materials

Ammonia solution (25 wt% ammonia), were purchased from Tianjin Chemical Reagent Factory. Other chemical reagents such as tetrabutyl titanate (Ti(OBu)₄), acetylacetone (ACA), dodecylamine (DDA), phenol and rhodamine B were of analytical purity and were purchased from Tianjin Yong Da Chemical Reagent Development Centre. Ammoniated natural rubber latex with a total dry rubber content of 25% was supplied kindly by Mengpeng Farm, Xishuangbanna, Yunnan Province, China. All of them were used without further purification. Distilled water was used throughout this study.

2.2. Synthesis

MTiO₂/RL. In a typical preparation process of 1 mL natural rubber latex was dissolved in diluted ammonia solution containing 1 mL ammonia solution in 100 mL water, then 10 mL Ti(OBu)₄ was added drop-wise under continuous stirring. This solution was stirred and aged for 24 h, then transferred into a Teflon bottle and treated under autogenous pressure without stirring at 363 K for 7 days. The final solid product was filtrated, washed with water, dried at room temperature and calcined at 673 K in air for 6 h.

MTiO₂/RL-ACA was synthesized by following the same procedure with preparation of MTiO₂/RL by adding 10 mL ACA slowly before adding Ti(OBu)₄. MTiO₂/DDA was prepared as described in the literature.²⁰

2.3. Characterizations

X-ray powder diffraction (XRD) experiments were conducted on a D/max-3B spectrometer with Cu Kα radiation. Scans were made in the 2θ range 10–100° with a scan rate of 10° min⁻¹. FT-IR measurements were performed on a Thermo Nicolet 8700 instrument. Potassium bromide pellets containing 0.5% of the samples were used in FT-IR experiments and 32 scans were accumulated for each spectrum in transmission at a spectral resolution of 4 cm⁻¹. The spectrum of dry KBr was taken for background subtraction. X-ray photoelectron spectroscopy analysis (XPS) was performed on a PHI5500ESCA analyzer (Perkin-Elmer Physical Electronics). Pore size distributions, BET surface areas and pore volumes were measured by nitrogen adsorption/desorption using a NOVA 2000e gas sorption analyzer (Quantachrome Corp.). Prior to the analysis of nitrogen adsorption/desorption, the samples were degassed at 423 K for 1 h. Transmission electron micrographs (TEM) were recorded on a Hitachi H-800 microscope with 100 kV of acceleration to probe the mesoporosity of the samples. UV-vis diffuse reflectance spectra were measured at room temperature in air on a SHIMADZU UV-2401PC photometer over the range from 200 to 800 nm.

2.4. Photodegradation procedures

The photocatalytic experiments of phenol (or rhodamine B) were carried out in a glass beaker containing 50 mL of 10 ppm phenol (or rhodamine B) solution and 25 mg of photocatalyst. The suspensions were magnetically stirred in the dark for 24 h to attain adsorption/desorption equilibrium between phenol (or rhodamine B) and photocatalyst. After that, the reactions of the photocatalytic degradation of phenol (or rhodamine B) were carried out under solar light on sunny days between 11 a.m. and 3 p.m. where the solar intensity fluctuations were minimal. The sky was clear and the sunrays were very intense in this period in the city of Kunming (latitude: 102.7 °N; longitude: 25 °E; altitude: 1900 m), Yunnan Province of China. The intensity of outside solar light was measured using UV-B type double-channel UV irradiance at various time intervals of 5 h between 11 a.m. and 3 p.m. The solar irradiance in winter was 22 μW cm⁻² for 297 nm in UV range. All of the photocatalysis experiments were carried out on sunny days. Laboratory film was used to seal the beaker so that the volume of the solutions decreased little after the experiment. The solutions were stirred with a magnetic stirrer during the reaction process. The initial concentration of each solution was equal to 50 mg L⁻¹ phenol (or rhodamine B) with pH 6. Samples were analyzed after centrifugation (1500 rpm for 10 min). The degradation of phenol (or rhodamine B) was determined with the absorbance value at the wavelength of maximum absorbance for them by monitoring UV-vis spectrum over 200–800 nm. The decrease due to adsorption can be deducted after the adsorption equilibrium was achieved. Therefore, photodegradation yield is defined as:²¹
C_o is the initial concentration of dye, C_a is the concentration after photodegradation of dye and C_b is the decrease concentration because of the direct photolysis.

3. Results and discussion

3.1. Syntheses approach

Due to the high water content and immediate coagulation of natural rubber latex in acid or even neutral condition and ethanol solution, the surface chemistry of the rubber particles will be more difficult to control and the process to synthesize mesoporous TiO₂ samples by using natural rubber latex as template has to be different from those methods used in synthesis of mesoporous TiO₂.^1,2 For example, during hydrolysis of the titanium alkoxides, non-aqueous solvents such as ethanol² or highly acidic conditions² were often required to prevent the high reactivity of titania precursors. By contrast, alkaline ammonia solution was used in this work. In the preparing process we developed, dilute ammonia solution was used as solvent to control the coagulation of natural rubber latex. We demonstrate that dilute ammonia solution plays a pivotal role in the successful synthesis of mesostructured TiO₂ due to its relatively alkaline nature which is able to protect the rubber latex leading to a more well-defined mesostructure and robust inorganic wall. Since acetylacetone (ACA) was often used as hydrolysis reaction-decelerating reagents,²² acetylacetonate was also employed in a modified procedure before adding Ti(OBu)₄.

3.2. XRD measurements

X-ray diffraction patterns of MTiO₂/RL and MTiO₂/RL-ACA are shown in Fig. 1. The diffraction patterns of all prepared TiO₂ samples are consistent with the presence of anatase crystalline phase (JCPDS card no. 21-1272). However, a very small peak at 2θ = 30.8° oriented along (121) plane in MTiO₂/RL-ACA corresponds to the brookite phase of TiO₂.

Fig. 1 X-ray diffraction patterns of MTiO₂/RL and MTiO₂/RL-ACA.

3.3. FT-IR spectra

Fig. 2 presents FT-IR spectra of MTiO₂/RL-ACA, MTiO₂/RL and P25 TiO₂ between 400 and 4000 cm⁻¹. The peak at 400–1000 cm⁻¹ which is contribution from the vibrations of Ti–O.²² The peaks at 1638 cm⁻¹ and 3440 cm⁻¹, which can be attributed to the surface-adsorbed water and hydroxyl groups were observed.^23,24 Obviously, the bands assigned to cis-1,4-polyisoprene and other organic compounds in natural rubber latex was too weak to be observed inferred that natural rubber latex template was removed.

Fig. 2 FT-IR spectrums of MTiO₂/RL, MTiO₂/RL-ACA and P25 TiO₂.

3.4. XPS spectra

The XPS survey spectra of MTiO₂/RL and MTiO₂/RL-ACA are shown in Fig. 3. Both of the samples are composed of Ti, O and C elements, indicating self-doping of carbonaceous species into the resultant TiO₂. From the C 1s spectra depicted in Fig. 3c and (d), the major peak at 284.8 eV is related to C–C, C=C, and C–H bonds and the peaks at the binding energies around 286.3 and 287.9 eV were assigned to the C–O and C=O bonds, respectively. In comparison, the C 1s signals could not be observed in pure TiO₂ samples such as P25.²⁵

Fig. 3 XPS survey spectra of (a) MTiO₂/RL, (b) MTiO₂/RL-ACA and C 1s high-resolution spectra of (c) MTiO₂/RL, (d) MTiO₂/RL-ACA.

3.5. N₂ adsorption/desorption analysis

Using the preparing process we developed, both MTiO₂/RL and MTiO₂/RL-ACA were light-yellow in color. The textural properties of all the samples are summarized in Table 1.

Table 1 Summary of textural properties of synthesized TiO₂ samples

Material	BET surface area (m^2 g^−1)	Pore size (nm)	Pore volume (cm^3 g^−1)
MTiO2-RL	145.5	8.9	0.50
MTiO2/RL-ACA	146.4	4.5	0.20

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As a representative, the N₂ adsorption/desorption isotherms of MTiO₂/RL (a) and MTiO₂/RL-ACA (b) are shown in Fig. 4. Typical IV isotherms are observed indicating the mesoporous structure of them. This is in good agreement with those mesoporous TiO₂ templated by other traditional templates such as block copolymers or surfactants. The BJH pore size distribution of MTiO₂/RL shows two primary pore size distributions (Fig. 4a, inset) between 6.2 nm to 15.2 nm with a peak at 8.9 nm. And MTiO₂/RL-ACA exhibited a narrow pore size distribution curve between 2.8 nm and 6.4 nm with a peak at 4.5 nm. This may be attributed to that the addition of ACA reduces the rate of hydrolysis.

Fig. 4 N₂ adsorption/desorption isotherms and BJH pore-size distribution plots (inset) of (a) MTiO₂/RL and (b) MTiO₂/RL-ACA.

3.6. TEM

TEM images of MTiO₂/RL and MTiO₂/RL-ACA (Fig. 5) show that the particles are reasonably uniform in size of 10–20 nm and 5–10 nm, respectively. In contrast, mesoporous TiO₂ synthesized by dodecylamine (DDA) as a template (MTiO₂/DDA) particles are much bigger.²⁰ Moreover, the addition of ACA significantly reduced the aggregation because ACA acted as hydrolysis reaction-decelerating reagents.²² No regular mesopores could be observed probably because the mesoporous properties of the samples are attributed to slits and cracks between the TiO₂ nanocrystals. The HR-TEM images of the samples are shown in Fig. 5c and d. The lattice fringes of TiO₂ crystals in the focus are seen clearly.

Fig. 5 TEM images of (a) MTiO₂/RL, (b) MTiO₂/RL-ACA and HR-TEM images of (c) MTiO₂/RL, (d) MTiO₂/RL-ACA.

3.7. UV-vis spectroscopy

The diffuse reflectance UV-vis spectra of MTiO₂/RL and MTiO₂/RL-ACA are shown in Fig. 6. Obviously, using natural rubber latex as template can shift the absorption edge of MTiO₂/RL-ACA to the visible light range because the addition of ACA significantly enhanced the absorption in the visible region. For comparison, the diffuse reflectance UV-vis spectrum of P25 is also shown in Fig. 6. The absorption edges of MTiO₂/RL and MTiO₂/RL-ACA and P25 can be determined to be about 397.1 nm, 399.6 nm and 383.0 nm, corresponding to band gap of 2.58 eV, 2.56 eV and 2.67 eV, respectively. These observations justify why MTiO₂/RL-ACA can be potential candidates for performing photocatalytic activity under solar light.

Fig. 6 UV-vis spectrums of MTiO₂/RL, MTiO₂/RL-ACA and P25.

3.8. Photocatalytic performance

Fig. 7 illustrates the photocatalytic degradation of the phenol (a) and rhodamine B (b) with solar light in the presence of MTiO₂/RL and MTiO₂/RL-ACA. For comparison, a blank experiment was performed under illumination in the absence of catalysts. Compared with Degussa P25 TiO₂ and MTiO₂/DDA, the prepared mesoporous TiO₂ materials have higher degradation capacity for phenol and rhodamine B, particularly for the degradation of phenol. Moreover, MTiO₂/RL-ACA exhibited more significant photodegradation yield than MTiO₂/RL. For example, the photocatalytic degradation yield for rhodamine B of MTiO₂/RL-ACA with solar light irradiation is more than three times as much as the photodegradation yield of MTiO₂/RL. However, MTiO₂/DDA exhibited very low activity with solar light irradiation and P25 did not exhibit any meaningful activity for the degradation of rhodamine B under solar light.

Fig. 7 Photocatalytic degradation of (a) phenol and (b) rhodamine B under solar light.

3.9. Possible syntheses and photodegradation mechanisms

Ammonia solution plays a critical role in the formation of RL resin polymer nanospheres. As mentioned above, NH₄⁺ in the emulsion can not only accelerate the polymerization of RL, but also supply the positive charges that adhere to the outer surface of spheres to prevent the aggregation which cannot be provided by any other alkaline sources, such as NaOH. The formation of TiO₂ via such a simple procedure suggests that the natural rubber latex are the ideal building blocks for photocatalysts with solar-light photocatalytic activity even though natural rubber latex are expected to behave in a more complex manner than those conventional polymer latex templates.

Considering all of our experimental observations, we propose the following two-stage mechanism of the TiO₂ structure formation. At the first stage a “nucleus” of ordered phase appears when the upper surface of the thinning aqueous layer in the wetting film presses the latex particles toward the water–glass interface. Once the nucleus is formed, the second stage of crystal growth start a through directional motion of particles toward the ordered array. Fundamental understanding of the assembly mechanism and deep insights into the controlling parameters for forming different morphological porous superstructures deserve further investigations. It is reasonable to believe that these larger nanometersized precursor particles in the as-made organic–inorganic hybrid mesostructures play an important role in the enhancement of the crystallization of calcined frameworks.

The above results indicate that MTiO₂/RL-ACA and MTiO₂/RL exhibited better photoactivity under solar light than the commercial Degussa P25 TiO₂ and MTiO₂/DDA. The first explanation is that they shifted the absorption edge of TiO₂ to the visible-light range and have definite absorptions in the visible region. Thus, this in situ carbon transferred from natural rubber latex acted as a photosensitizer.²⁶ Moreover, the crystalline framework, high specific surface area of these new mesoporous TiO₂ are expected to afford better activity toward photocatalytic reactions. In particular, it is interesting to evoke some reasons why the MTiO₂/RL-ACA exhibited higher solar light photocatalytic activity than others. Firstly, the mixture of anatase with brookite and/or rutile might be beneficial.²⁶ Higher photocatalytic activities of anatase–rutile and anatase–brookite heterojunctions in contrast to pure anatase alone were reported previously.²⁷ This is mainly because the presence of interwoven crystallites reduced the recombination of photogenerated electrons and holes. Furthermore, photocatalytic activity might be enhanced by narrowing the pore distribution and reducing in particle size when ACA was added as hydrolysis reaction-decelerating reagents.¹⁹

4. Conclusions

In summary, mesoporous TiO₂ dioxide materials were synthesized using the natural rubber latex templating technique. It was found that MTiO₂/RL-ACA exhibited more effective photocatalytic activity in degrading of phenol and rhodamine B under solar light than MTiO₂/RL or MTiO₂/DDA, particular than the commercial Degussa P25 TiO_2. The addition of ACA significantly improved the formation of regular pore channels. The reported strategies combine sol–gel chemistry and self-assembly routes directly using natural rubber latex template which convert to carbonaceous materials after 400 °C calcination in the synthesis of thermally stable mesostructured TiO₂. This approach is simpler and potentially more active than conventional photosensitization. Furthermore, the natural rubber latex templating method proposed here represents a versatile approach of applying natural latex template in fabricating other mesoporous materials. This could open up new uses for mesoporous TiO₂ in applications such as treatment of polluted water, dye-sensitized solar cells, or other regions.

Acknowledgements

The authors thank the Natural Science Foundation of China (Project NSFC-YN U1033603, 21367024, 21403190) and Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province (IRTSTYN) for financial support.

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