Formation of 3D interconnectively macro/mesoporous TiO₂ sponges through gelation of lotus root starch toward CO₂ photoreduction into hydrocarbon fuels

Abstract

A particular TiO₂ sponge, consisting of macroporous framework with interconnected mesoporous channels, was fabricated through a co-gelation of lotus root starch (LRS) with TiO₂ precursor, followed by lyophilization and subsequent calcination. This strategy advantageously inherits both the traditional hard-templating technique for well-defined 3D predesigned macroporous architecture and soft-templating techniques for interpore connectivity. The resulting TiO₂ sponge exhibits about a 2.60 fold improvement in CO₂ photoconversion rate (CH₄: 5.13 ppm h⁻¹) compared to the referred TiO₂ (1.97 ppm h⁻¹) formed in the absence of the LRS. The generation rate of CH₄ over macro/mesoporous TiO₂ sponge could be further significantly enhanced to 11.95 ppm h⁻¹ by co-loading Pt (0.9 wt%) and Cu (1.7 wt%) as co-catalysts by improvement of the separations of the photogenerated electron-hole pairs. The higher photocatalytic activity of the macro/mesoporous TiO₂ sponge can be attributed to the following three reasons: (1) macroporous architecture favors gas diffusion of the reactants and the products; (2) macroporous architecture also promotes the multiple-reflection effect occurring inside the interior macrocavities, which enables trapping (or harvest) the incident light in the photocatalyst for a longer duration and bring forth more opportunities for light absorption; and (3) the mesoporous structure enhances gas capture/adsorption of the reactants and provides more reaction sites.

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Introduction

Photocatalytic reduction of CO₂ into hydrocarbon fuels, an artificial photosynthesis based on semiconductor photocatalysis, couples the reductive half-reaction of CO₂ fixation with a matched oxidative half-reaction such as water oxidation to achieve a carbon neutral cycle. In order to achieve high conversion efficiency, a series of appropriate photocatalysts have been designed through rational construction of novel nanostructures,^1–5 including our recent work with ultrathin nanoribbons (e.g. Zn₂GeO₄,⁶ Fe₂V₄O₁₃,⁷ Na₂V₆O₁₆·xH₂O (ref. 8)) and ultrathin nanosheets (e.g. Ti_0.91O₂-graphene,⁹ TiO₂-graphene,¹⁰ Bi₂WO₆,¹¹ and WO₃ (ref. 12)).

A plant leaf is considered a perfect system for decomposition of water to molecular oxygen, accompanied by reduction of CO₂ to carbohydrates and other carbon-rich products using sunlight as energy source.^13–16 Both the multi-scale pores and the interconnectivity between pores of the leaf scaffolds are important parameters for photosynthesis, which are facile for efficient mass flow. Thus, fabrication of innovative photocatalysts with multi-layer interconnected channels will enable one to exactly mimic the natural process of photosynthesis and make a breakthrough in the photoconversion efficiency of CO₂. Recently, we have demonstrated that mesoporous photocatalysts can significantly enhance the reaction efficiency owing to strong gas adsorption through the mesostructure and more reaction sites arising from high specific surface area.¹⁷ Ye and co-workers produced a 3D hierarchical network of interconnected pores of varying shapes, diameters, and orientations for improvement of CO₂ gas diffusion and light harvesting.^18,19

Lotus root starch (LRS) is granular, off-white or slightly grayish flour made from the roots of lotus water lilies (“Nelumbo nucifera”). When heated, aqueous dispersions of LRS become gel-like as a result of swelling and disruption of the starch granules. These changes are due to reassociation of amylase molecules in the aqueous system.²⁰ In this work, we design a particular TiO₂ sponge, consisting of the macroporous framework with interconnected mesopore channels through gelation of the LRS for photoconversion of CO₂ into methane.

The fabrication process is outlined in Scheme 1. Firstly, the heating of mixture of TiO₂ precursor, titanium(IV) (ammonium lactato) dihydroxide (TALD), with LRS granules formed a lump-free suspension. The suspension was subsequently cooled to room temperature to generate gelatin (Step 1: S1). The solid phase of the gelatin consists of the LRS–TALD hybrids with homogenous dispersity of TALD in the starch matrix through potential chemical interaction, e.g. hydrogen bond, between TALD and LRS. The LRS–TALD hybrid gel was fully dried by lyophilization to leave the well-defined macroporous network and create a sponge architecture with sufficient mechanical stability (S2). With calcination at 550 °C in air, the TALD was in situ decomposed into TiO₂ with an excellent replication of the macroporous LRS framework. The mesopore interconnectivity between TiO₂ macropores was also created simultaneously in the TiO₂ network scaffold through elimination of the embedded LRS (S3). This LRS gelation-based technique to macro/mesoporous TiO₂ sponges described herein advantageously inherits both the traditional hard-templating route for well-defined 3D predesigned macroporous architecture²¹ and soft-templating route for interpore connectivity.²² The macro/mesoporous TiO₂ sponge proves to have high efficiency of the photocatalytic activity toward reduction of CO₂ into renewable hydrocarbon fuels (methane: CH₄) in the presence of water vapor.

Scheme 1 Schematic of the procedure for preparing the particular TiO₂ sponges consisting of a macroporous framework with interconnected mesoporous channels.

Experimental

Preparation of 3D hierarchically macro/mesoporous TiO₂ sponge and referred TiO₂

The typical synthesis procedure of 3D hierarchically macro/mesoporous TiO₂ sponge was as follows: 1 g of LRS granules and 0.9 g of TALD were mixed with 8.1 g of distilled water in a beaker with stirring to form a lump-free suspension. The suspension was then heated on a water bath at 95 °C, and the stirring was continued until the starch gelatinized. The viscous starch paste was maintained at 95 °C for additional 10 min without further stirring. The starch paste was then removed from the water bath, naturally cooled to room temperature, and fully dried by lyophilization. The resulting LRS–TALD hybrid monolith was calcined at 550 °C for 5 h to get rid of the LRS and generate the macro/mesporous TiO₂ sponge. Moreover, TiO₂ samples formed in the absence of LRS gelatin (hereafter called “referred TiO₂”) were synthesized by direct calcinations of freeze-dried TALD and compared with TiO₂ sponges.

Characterization of 3D hierarchically macro/mesoporous TiO₂ sponge

Thermogravimetric analysis (TGA) was carried out (Pyris 1 DSC, PerkinElmer USA) at a heating rate of 20 °C min⁻¹ from 25 °C to 700 °C in air. The crystallographic phase of the as-prepared products was determined by powder X-ray diffraction (XRD) (Rigaku Ultima III, Japan) using Cu-Kα radiation (λ = 0.154178 nm) with scan rate of 10° min⁻¹ at 40 kV and 40 mA. The samples were analyzed with X-ray photoelectron spectroscopy (XPS) (K-Alpha, THERMO FISHER SCIENTIFIC). The XPS spectrum was calibrated with respect to the binding energy of the adventitious C1s peak at 284.8 eV. The morphology of the samples was observed by the field emission scanning electron microscopy (FE-SEM) (FEI NOVA NanoSEM230, USA) and transmission electron microscopy (TEM) (JEOL 3010, Japan). The specific surface area of the samples was measured by nitrogen sorption at 77 K on a surface area and porosity analyzer (Micromeritics TriStar, USA) and calculated by the BET method. The CO₂ absorption on the surface of the samples was evaluated by the above-mentioned adsorption apparatus under ambient pressure and 0 °C. Fourier transform infrared (FTIR) spectroscopy was conducted using a Nicolet NEXUS870 (USA) spectrometer.

Measurement of photocatalytic activity

In the photocatalytic reduction of CO₂, 0.1 g of sample was uniformly dispersed on the glass reactor with an area of 4.2 cm². A 300 W xenon arc lamp was used as the light source of photocatalytic reaction. The volume of the reaction system was about 230 mL. The reaction setup was vacuum-treated several times, and then the high purity CO₂ gas was added into the reaction setup until ambient pressure was reached. 0.4 mL of deionized water was injected into the reaction system as reducer. The as-prepared photocatalysts were allowed to equilibrate in the CO₂/H₂O atmosphere for several hours to ensure that the adsorption of gas molecules was complete. During the irradiation, about 1 mL of gas was continually taken from the reaction cell at given time intervals for subsequent CH₄ concentration analysis using a gas chromatograph (GC-2014, Shimadzu Corp, Japan).

Results and discussion

Fig. 1 and Fig. S1† showed the FE-SEM images of a series of the lyophilized pure LRS sponges obtained at various concentrations ranging from 0.5 wt% to 20 wt%. It was found that the self-assembling architecture of the starch sponge strongly depends on the initial starch concentration. The starch concentrations at 0.5 wt% and 1 wt% produce networks of interwoven nanofibers with diameters of tens and hundreds of nanometer, respectively (Fig. S1a and S1b†). With the concentration at 2 wt%, the sheet-like pieces begin to appear and coexist with interwoven fibers (Fig. S1c†). 5–20 wt% starch concentrations were found to be proper to construct 3D porous sponges with continuous walls (Fig. 1a–d, Fig. S1d and S1e†). Fig. 1 shows different magnification FE-SEM images of the LRS sponge typically formed at 10 wt%. The large-scale, well-defined, and macroporous network was clearly observed. The pore size ranges from 1 to 2 μm and the thickness of the pore wall is measured about 60–70 nm. The pore size and porosity of the sponge decrease with the increase of the starch concentration. Furthermore, increase in the concentration to 40 wt% leads to near disappearance of the porous structure.

Fig. 1 FE-SEM images of pure LRS sponges prepared from 10 wt% at different magnifications.

10 wt% LRS was typically selected for generation of TiO₂ sponge. The TALD was mixed with an aqueous dispersion of 10 wt% LRS granules, followed by heating, gelation, and subsequent lyophilization, which affords the formation of LRS–TALD hybrids. It was found that incorporation of TALD has no obvious influence on the gelatin morphology of the LRS, although TALD may partially hydrolyze during the gelation process (Fig. 2). The FTIR spectra show that native LRS granule exhibits several discernible absorbances at 1000–1100 cm⁻¹ and about 1700 cm⁻¹ (Fig. 3), which may be assigned to C–O and C=O stretching vibrations, respectively. The band assigned to C–O at around 1000 cm⁻¹ displays obvious blue shift for lyophilized LRS sponge and TALD-LRS hybrid sponge compared with native starch granule, which is attributed to the breaking of hydrogen bonding during gelatinization.²³ As for the band assigned to C=O at around 1700 cm⁻¹, the lyophilized LRS sponge shows no significant differences relative to the native starch granule. However, for the TALD-LRS hybrid sponge, there is an obvious red shift, indicating the interaction between TALD and starch species owing to the facile incorporation of the TALD into polysaccharides.²⁴ This demonstrates that the components of the LRS–TALD hybrid are intermingled with each other at a molecular level.

Fig. 2 FE-SEM images of as-prepared LRS–TALD hybrid sponge prepared from 10 wt% starch at different magnifications.

Fig. 3 FTIR spectra for (a) native LRS granules, (b) as-synthesized LRS sponge and (c) LRS–TALD hybrid sponge.

Calcination at 550 °C enables removal of the starch moieties, during which the TALD was in situ simultaneously decomposed into TiO₂. The thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) show that with the temperature rising, three weight loss regions were observed in the pure starch sponge (Fig. 4a). Based on the quantitative calculation of the weight loss in each region, the thermal decomposition processes were distinguished as follows. The weight loss region from 25 to 125 °C represents the evaporation of water previously adsorbed by the starch sponge. A notable weight loss region from 240 to 400 °C was attributed to the decomposition of the starch. A small peak appears at about 478 °C, which results from the further decomposition of the starch. This indicates that the LRS can be completely eliminated with calcination at 550 °C. Two new DTA peaks at 244 °C and 279 °C were observed in the TALD/starch sponge (Fig. 4b), which may be attributed to the step by step decomposition of TALD into TiO₂.

Fig. 4 The TGA (DTG) spectras of as-synthesized (a) LRS sponge and (b) LRS–TALD hybrid sponge.

Fig. 5 shows that the resulting TiO₂ sponge after calcinations easily retains the 3D hierarchically macroporous framework of the LRS. The TiO₂ sponge possesses smooth and continuous walls with thickness of ∼30 nm. The macropore size is about 1.5–2.5 μm, slightly bigger than that of the LRS template due to shrinkage of the framework after calcinations. The TEM image reveals that the TiO₂ wall was comprised of small nanoparticles with size ranging from 10 to 20 nm (Fig. 6). The wall network contains a considerable number of mesopores of generally 5–10 nm in pore size, which are absolutely penetrated through the wall and primarily originate from the removal of the LRS. The high-resolution TEM image further reveals well-defined lattice fringes of the TiO₂ nanoparticle and the presence of the mesopores. The X-ray diffraction (XRD) pattern of the TiO₂ sponge shows that all of the diffraction peaks can be indexed to anatase phase TiO₂ (JCPDS no. 21-1272), and no characteristic peaks of other impurities were observed, indicating that anatase was the only crystal phase present in the product (See ESI, Fig. S2†). The corresponding XPS spectrum demonstrates the presence of Ti 2p_3/2 and Ti 2p_1/2 at around 459.1 eV and 464.7 eV, respectively (Fig. S3†).

Fig. 5 FE-SEM images of as-prepared 3D interconnectively macro/mesoporous TiO₂ sponges templated by 10 wt% LRS at different magnifications.

Fig. 6 (a) TEM and (b) HRTEM images of as-prepared 3D interconnectively macro/mesoporous TiO₂ sponges templated by 10 wt% LRS.

Fig. 7 shows a typical N₂ gas adsorption–desorption isotherm of the resulting macro/mesoporous TiO₂ sponge. The isotherm displays the typical IV curve, which is ascribed to a predominantly mesoporous structure and is absent in the referred TiO₂ formed in the absence of the LRS. The presence of a pronounced hysteresis loop in the isotherm curve is indicative of a 3D intersection network of the TiO₂ sponge.²⁵ The measured mesopore size was centered on 2–10 nm, in good agreement with the HRTEM image. The TiO₂ sponge exhibits 31.4 m² g⁻¹ of surface area, ten times higher than the referred TiO₂ of 3.3 m² g⁻¹. The quantity of CO₂ adsorption of the TiO₂ sponge is 6.77 mg g⁻¹, which is also obviously higher than that of the referred TiO₂ of 1.67 mg g⁻¹. This indicates that the mesoporous architecture of the TiO₂ sponge can promote the photocatalytic conversion of CO₂ through the favored absorption of CO₂.

Fig. 7 N₂ adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset picture) of (a) as-prepared 3D interconnectively macro/mesoporous TiO₂ sponges and (b) referred TiO₂ formed in the absence of the LRS.

Photocatalytic conversion of CO₂ to renewable hydrocarbons using solar energy is one of the best solutions to both the global warming and the energy shortage problems. Generally, in the presence of water vapor, CO₂ could be photoreduced into CH₄ using the wide band gap semiconductor as a photocatalyst through water oxidation of 2H₂O → O₂ + 4H⁺ + 4e⁻_CB (E^o_redox = 0.82 V vs. NHE) and CO₂ reduction of CO₂ + 8e⁻ + 8H⁺ → CH₄ + 2H₂O (E^o_redox = −0.24 V vs. NHE). As a classic photocatalyst, the macro/mesoporous TiO₂ sponge was investigated for CO₂ photoreduction. Gas chromatographic analysis demonstrates that CH₄ was exclusively obtained as the reduction product without detectable CO, H₂, or C₂H₆ as secondary products. A control experiment with identical condition and in the absence of CO₂ shows no appearance of CH₄, proving that the carbon source was completely derived from the input CO₂. Fig. 8 shows that the gross yield in CH₄ increases with the photoreduction evolution time. The TiO₂ sponge (CH₄: 5.13 ppm h⁻¹) exhibits about a 2.60 fold improvement in conversion efficiency than the referred TiO₂ (1.97 ppm h⁻¹). The higher photocatalytic activity of the macro/mesoporous TiO₂ sponge can be attributed to the following three reasons: (1) macroporous architecture favors gas diffusion of the reactants and the products; (2) macroporous architecture also promotes the multiple-reflection effect, occurring inside the interior macrocavities, which enables trapping (or harvest) of the incident light in the photocatalyst for a longer duration and bring forth more opportunities for light absorption; (3) the mesoporous structure enhances gas capture/adsorption of the reactants and provides more reaction sites. It is noteworthy that the generation rate of CH₄ over macro/mesoporous TiO₂ sponge could be significantly enhanced to 11.95 ppm h⁻¹ by co-loading Pt (0.9 wt%) and Cu (1.7 wt%) as co-catalysts to improve the separations of the photogenerated electron–hole pairs.²⁶ It is expected that loading specific light-absorbing metal nanocatalysts into the present porous TiO₂ sponge may be further improved CO₂ conversion efficiency through photothermal reaction.²⁷

Fig. 8 CH₄ generation over (a) as-prepared 3D interconnectively macro/mesoporous TiO₂ sponges, (b) referred TiO₂ formed in the absence of the LRS and (c) 0.9 wt% Pt + 1.7 wt% Cu-coloaded 3D interconnectively macro/mesoporous TiO₂ sponges as a function of light irradiation time.

Conclusion

The 3D interconnectively macro/mesoporous TiO₂ sponges were synthesized using a facile, low cost, and environmental approach through the gelation process of LRS. This route to macro/mesoporous TiO₂ monoliths advantageously inherits both the traditional hard-templating technique for well-defined 3D predesigned macroporous architecture and soft-templating techniques for interpore connectivity. The hierarchically macro/mesoporous TiO₂ monolith exhibits a higher photocatalytic activity for the reduction of CO₂ into CH₄ in the presence of water than the referred TiO₂, benefiting from the favorable gas diffusion of the reactants and the products, more opportunities for light absorption, and more reaction sites.

Acknowledgements

This work was supported by the 973 Program (no. 2014CB239302, 2011CB933303, and 2013CB632404), the National Science Foundation of Jiangsu Province (no. BK2012015 and BK 20130053), the National Natural Science Foundation of China (no. 2147309, 51272101, 51202005), the College Postgraduate Research and Innovation Project of Jiangsu Province (no. CXZZ13_0033), the Provincial Science Key Foundation of Higher Education Institutions of Anhui (no. KJ2011A053), and the China Postdoctoral Science Foundation (no. 2012M521037).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06565d

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