II. STRATEGIES FOR SYNTHESIS OF TiO₂ NANOSHELLS

Since the reporting of several pioneering works in the early 2000s, there have been many further reports on the formation of a TiO₂ layer on the surface of nanomaterials to produce core–shell nanostructures.Reference Pastoriza-Santos, Koktysh, Mamedov, Giersig, Kotov and Liz-Marzán^20–Reference Sakai, Kanda, Shibata, Ohkubo and Abe²² Most of these efforts have been devoted to depositing the nanoscale TiO₂ layer, formed by limited hydrolysis–condensation kinetics using small amounts of sol-gel precursors, on the modified surface of solid (metal, metal oxide, polymer, and/or carbon) cores.Reference Zheng, Meng and Tang^18, Reference Lee, Othman, Eom, Lee, Kim and Kim^19, Reference Mayya, Gittins, Dibaj and Caruso^23–Reference Lu, Yin and Xia²⁵ Liz-Marzan et al. achieved a coating of a thin TiO₂ layer on simultaneously reduced silver (Ag) nanoparticles.Reference Pastoriza-Santos, Koktysh, Mamedov, Giersig, Kotov and Liz-Marzán²⁰ Xia et al. synthesized hollow TiO₂ nanoshells by hydrolyzing a TiO₂ precursor and then depositing the product onto the surface of close-packed polystyrene (PS) spheres, followed by rapid evaporation of solvent and chemical etching of the sacrificial PS cores.Reference Zhong, Yin, Gates and Xia²⁶

TiO₂ shells can also be produced without the use of special templates.Reference Yang and Zeng^27, Reference Hu, Ge, Sun, Zhang and Yin²⁸ Similar to the SiO₂ case which we reported recently, the core portion of TiO₂ spheres can be preferentially removed to form hollow capsules through a “surface-protected etching” process.Reference Hu, Ge, Sun, Zhang and Yin^28, Reference Zhang, Zhang, Ge and Yin²⁹ The Ostwald ripening mechanism has also been considered as a method for transforming solid TiO₂ particles into hollow ones without using specific templates.Reference Yang and Zeng^27, Reference Li and Zeng^30, Reference Li and Zeng³¹

A. Template-free synthesis

We have developed a template-free method for preparing TiO₂ hollow shells by heating sol-gel-derived TiO₂ microspheres with poly(acrylic acid) (PAA) in diethylene glycol (DEG).Reference Hu, Ge, Sun, Zhang and Yin²⁸ Initially, sol-gel-derived TiO₂ microspheres are solid particles containing micropores [Fig. 1(a)]. DEG is used as both a solvent and an etching agent because amorphous TiO₂ particles prepared through a sol-gel process can be dissolved in pure DEG at high temperatures. Interestingly, heating TiO₂ microspheres in the presence of PAA slows down the dissolution and eventually produces hollow shells [Fig. 1(b)]. Upon heating to high temperatures, PAA molecules can bind to the surface of TiO₂ spheres and act as protecting agents during the subsequent etching. With its multiple carboxylate groups on a polymer chain, PAA acts as a crosslinker to connect adjacent TiO₂ nanoparticles into a stable shell and protects them against rapid dissolution by DEG. Dissolution of TiO₂ preferentially occurs inside the microsphere after DEG penetrates through the protected outer layer and reacts with unprotected core material. Thus, preferential removal of core material through the formation of soluble titanate species leaves behind hollow shells. The shells become thinner after continuous heating at high temperature and are eventually completely dissolved by the solvent. As the PAA chains are stretchable, the separation between adjacent surface nanoparticles increases after the removal of the cores. This explains why the diameter of the shells is significantly larger in comparison to the primary microspheres [Fig 1(b), inset]. Also, because the PAA chains are flexible, the shells are unable to support themselves mechanically when they are dried on a solid substrate.

FIG. 1.

Dark-field optical microscopy images of (a) the primary TiO₂ microspheres and (b) the corresponding hollow shells. The insets are TEM images of the primary particles and hollow shells. Adapted with permission from Ref. Reference Hu, Ge, Sun, Zhang and Yin28© 2007 American Chemical Society.

Ostwald ripening is a classical phenomenon in solid dispersions or liquid sols in which small crystals or sol particles dissolve and regrow on larger ones. For some time now, the Ostwald ripening mechanism has been widely used for explaining the formation of hollow nanostructures. Zeng et al. first proposed the mechanism in the formation of hollow TiO₂ shells through the hydrothermal treatment of titanium tetrafluoride (TiF₄) solution.Reference Yang and Zeng²⁷ The TiF₄ precursor can first be hydrolyzed and condensed to produce solid TiO₂ microspheres. Core portions of the TiO₂ spheres can be continuously dissolved and then redeposited on the outer region to yield hollow TiO₂ shells. Li and Zeng have also extended the hollowing process via Ostwald ripening to the synthesis of hollow tin (Sn)-doped TiO₂ shell structures by using a similar hydrothermal treatment.Reference Li and Zeng³¹ The ripening strategy has been extended to the preparation of other oxides including silicon dioxide (SiO₂),Reference Zhang, Ge, Hu, Zhang, Aloni and Yin³² tin dioxide (SnO₂),Reference Lou, Wang, Yuan, Lee and Archer³³ cuprous oxide (Cu₂O),Reference Chang, Teo and Zeng^34, Reference Teo, Chang and Zeng³⁵ cobalt (II,III) oxide (Co₃O₄),Reference Liu and Zeng³⁶ and many othersReference Zhang, Wang, Goebl and Yin³⁷ through a similar formation mechanism.

B. Template-assisted method

Compared with the template-free synthesis, the template-assisted method represents a more universal process for synthesizing nanostructured shells.Reference Joo, Zhang, Lee, Dahl, Zaera and Yin^38, Reference Joo, Zhang, Dahl, Lee, Goebl, Zaera and Yin³⁹ In fact, template-assisted synthesis has been the most commonly used method during the past decades for the preparation of shell structures. The principle of this strategy is straightforward and involves several sequential steps: (i) preparing the original templates; (ii) depositing the shell materials on the surface of the templates; (iii) selective etching to remove the templates. Various types of colloidal particles have been reported as having been experimented as core templates including gold (Au) nanoparticles,Reference Mayya, Gittins and Caruso²¹ silver (Ag) nanoparticles,Reference Pastoriza-Santos, Koktysh, Mamedov, Giersig, Kotov and Liz-Marzán²⁰ PS,Reference Wang, Chen and Tang^16, Reference Imhof¹⁷ glucose-derived carbon,Reference Zheng, Meng and Tang^18, Reference Ao, Xu, Fu and Yuan^40, Reference Ao, Xu, Fu and Yuan⁴¹ and silica.Reference Ye, Zhang, Hu, Ge, Lu, He, Chen and Yin^6, Reference Lee, Othman, Eom, Lee, Kim and Kim^19, Reference Joo, Zhang, Lee, Dahl, Zaera and Yin^38, Reference Joo, Zhang, Dahl, Lee, Goebl, Zaera and Yin³⁹ Among them, uniform silica spheres prepared by the Stöber method have been the most popular template due to the ease of coating them with TiO₂ materials as well as their convenient removal by base etching.Reference Ye, Zhang, Hu, Ge, Lu, He, Chen and Yin^6, Reference Lee, Othman, Eom, Lee, Kim and Kim^19, Reference Lee, Joo, Yin and Zaera⁴² Deposition of TiO₂ layers on the surface of silica spheres can be easily achieved through a sol-gel process which uses titanium tetrabutoxide (TBOT) as the TiO₂ precursor and hydroxypropyl cellulose (HPC) as surfactant.Reference Ye, Zhang, Hu, Ge, Lu, He, Chen and Yin^6, Reference Lee, Othman, Eom, Lee, Kim and Kim^19, Reference Lee, Joo, Yin and Zaera⁴² The thickness of the TiO₂ shells can be conveniently increased by carrying out multiple coating steps. Hollow TiO₂ shells can then be obtained using an aqueous solution of NaOH to remove the silica cores [Fig. 2(a)]. In a typical synthesis, a single run of the TiO₂ coating procedure produces ∼25 nm TiO₂ shells [Fig. 2(b)], while repeating the coating steps three and five times can increase the shell thickness to ∼50 and ∼70 nm, respectively [Figs. 2(c) and 2(d)].Reference Joo, Zhang, Lee, Dahl, Zaera and Yin³⁸

FIG. 2.

(a) Schematic illustration showing the typical template-assisted synthesis for hollow TiO₂ shell nanostructure and corresponding TEM images of TiO₂ hollow spheres with different thicknesses prepared using multiple sol-gel coating processes: (b) once (c) three times, and (d) five times.

III. CRYSTALLINITY CONTROL

Typical sol-gel processes, which prepare TiO₂ shell nanostructures by direct hydrolysis and condensation of titanium alkoxides, cannot yield crystalline structures. However, good crystallinity is an important feature required for many practical applications including sensors, energy storage, and photocatalysis.Reference Wang, Chen and Tang^16, Reference Yang, Niu, Lu, Hu and Han^43, Reference Kamata, Lu and Xia⁴⁴ There is no doubt that anatase phase TiO₂ is considered to be the most active phase due to its higher reduction potential and lower recombination rate of electron-hole pairs in photocatalytic applications even though the rutile phase has the advantage of better absorption in the visible regime due to a lower band-gap energy. To achieve high activity in photocatalysis, it is important for the material to have a high surface area with well-developed porosity, which provides a large number of redox reaction sites on the catalyst surface as well as ensures easy diffusion of reactant molecules. It is therefore believed that well-crystallized anatase TiO₂ with large surface area and porosity is the preferred form for photocatalysis.

Calcination is considered to be a simple method to enhance the crystallinity of TiO₂ shells. Nevertheless, direct calcination of either hollow TiO₂ or SiO₂@TiO₂ core–shell structures cannot produce a stable shell morphology with the targeted anatase crystalline phase because the thin TiO₂ layer can be damaged during extensive structural reorganization at high calcination temperatures. Recently we have developed a “protected calcination method” to achieve crystalline TiO₂ shells with porous characteristics [Fig. 3(a)].Reference Joo, Zhang, Lee, Dahl, Zaera and Yin³⁸ The “protected calcination” process not only improves the crystallinity of the samples but also brings additional benefits such as high surface area and well-developed mesoscale porosity. Compared with direct calcination, the major modification we introduced into this process is to protect the TiO₂ shells with an additional outer silica layer. Calcination of SiO₂@TiO₂@SiO₂ composites followed by chemical etching produced well-defined hollow TiO₂ shells containing mesopores which could be easily observed by transmission electron microscopy (TEM) imaging [Figs. 3(b)–3(d)]. Our results suggest that the outer silica layer not only suppresses the crystal growth of TiO₂ but also inhibits phase transformation. Similar to colloidal silica, the sol-gel-derived TiO₂ layer contains micropores with diameters typically below 2 nm. Silicate species may be able to penetrate into these pores and condense in the TiO₂ framework during the outer silica coating. The penetrated silica may then act as a barrier, preventing the extensive structural reorganization and growth of amorphous TiO₂ into large crystal grains during calcination. This behavior may result in smaller anatase grains than those produced at similar calcination temperatures without the protection of silica.

FIG. 3.

(a) Schematic illustration of silica-protected calcination procedure and corresponding TEM images of the product at each step: (b) SiO₂@TiO₂ (c) SiO₂@TiO₂@SiO₂ after calcination, and (d) final hollow TiO₂ after the removal of the SiO₂.

The presence of a silica layer significantly limits the growth of anatase grains to below ∼5 nm even at a calcination temperature as high as 900 °C. Although the crystalline hollow TiO₂ structures have considerably large surface areas (above 300 m²/g), the small grain size is not optimal for reactions such as the photocatalytic decomposition of organic molecules. For increasing the crystallinity of TiO₂ shells in a controllable manner, we have further developed a “partial etching and recalcination” process.Reference Joo, Zhang, Dahl, Lee, Goebl, Zaera and Yin³⁹ When the TiO₂ shell is coated with an additional layer of silica, the silica that penetrates into or near the TiO₂ layer has a more disrupted Si–O–Si network than silica in other regions, making it more vulnerable to etching in base solution. Indeed, preferential etching can be achieved, as evidenced by the appearance of a small gap between the TiO₂ layer and both the outer and inner SiO₂ layer in the TEM image [Fig. 4(a)]. The degree of preferential etching can be easily controlled by both the duration of etching and concentration of etchants. The preferential removal of surrounding silica in or near the TiO₂ layer allows further growth of anatase crystal domains during the recalcination process. Figures 4(b)–4(d) are TEM images showing the different sizes of TiO₂ crystal grains produced by recalcining a partially etched SiO₂@TiO₂@SiO₂ sample at different temperatures followed by the complete removal of SiO₂ using NaOH. Compared with the original TiO₂ shell samples produced simply by using the silica-protected calcination process [∼4 nm grain size, Fig. 3(d)], the crystal grain size increased to 7 and 10 nm upon recalcination for 4 h at 700 and 800 °C, respectively [Figs. 4(b) and 4(c)]. Prolonged heating at 800 °C for a longer time period (16 h) gives crystal grains with an average size above 14 nm [Fig. 4(d)]. As discussed in Sec. IV, the TiO₂ nanostructured shells with controllable crystallinity are particularly interesting for photocatalytic applications due to their large active sites.

FIG. 4.

TEM images of (a) partially etched SiO₂@TiO₂@SiO₂, and (b–d) final TiO₂ shell nanostructure after recalcining SiO₂@TiO₂@SiO₂(partially etched) at different conditions and NaOH etching: (b) 700 °C for 4 h, (c) 800 °C for 4 h, and (d) 800 °C for 16 h.

IV. PHOTOCATALYTIC APPLICATION AND ACTIVITY ENHANCEMENT

TiO₂ is the most studied and most promising photocatalytic material, which can absorb UV light to promote redox reactions. Its high photocatalytic activity can be improved by tuning several characteristics including surface area, surface properties, crystalline phase, and crystallinity.Reference Joo, Zhang, Dahl, Lee, Goebl, Zaera and Yin³⁹ Nanostructured TiO₂ shells are good candidates for photocatalysis due to their relatively large surface area, excellent water dispersity, and controllable anatase crystallinity. The hollow TiO₂ shells prepared through the “partial etching and recalcination” process show a well-developed anatase phase with the grain size tunable by adjusting the recalcination temperature and time [Figs. 5(a) and 5(b)]. When used for photocatalytic degradation of dye molecules such as Rhodamine B (RhB), the TiO₂ nanoshells significantly enhanced the decomposition rate while the control sample without catalyst showed the lowest apparent rate constant (k) values [Figs. 5(b) and 5(c)]. As expected, TiO₂ shells prepared without partial etching and recalcination [Sample A in Fig. 5(a)] showed the lowest k values among TiO₂ catalysts tested, while k continuously increased as recalcination temperature increased. Eventually, TiO₂ shells recalcined at 800 °C for 16 h (Sample E) showed the largest k value, indicating the highest catalytic efficiency. The improved catalytic activity should be attributed to the enhanced crystallinity of the anatase grains, resulting in better charge separation during photocatalysis. Although it is generally accepted in catalysis field that the activity of a catalyst benefits from a higher specific area, our studies however have clearly shown that photocatalytic activity can be promoted more by enhanced crystallinity due to better charge separation, even though the value of surface area decreases correspondingly.

FIG. 5.

(a) X-ray diffraction patterns, (b) calculated crystallite grain size and apparent rate constant value (k) during photocatalytic RhB decomposition, and (c) photocatalytic reaction results of TiO₂ samples: TiO₂ shells (A) without recalcination process, (B) recalcined at 600 °C for 4 h, (C) recalcined at 700 °C for 4 h, (D) recalcined at 800 °C for 4 h, (E) recalcined at 800 °C for 16 h. Adapted with permission from Ref. Reference Joo, Zhang, Dahl, Lee, Goebl, Zaera and Yin39© 2011 Royal Society of Chemistry.

The synthesis of highly active visible-light-responsive photocatalysts has attracted intensive interest because they hold the promise of more efficient harvesting of solar energy. Modification of TiO₂ through nonmetal anion doping, metal cation doping or plasmonic metal decoration lowers its band-gap energy and allows for absorption of photon energy in the visible regime, thus utilizing a greater portion of the solar spectrum. Recently, we demonstrated the nitrogen doping of porous TiO₂ nanocrystal clusters by NH₃ treatment.Reference Zhang, Joo, Lu, Dahl, Oliveira, Ye and Yin⁴⁵ After gas phase treatment at high temperature, the white powder became yellowish, indicating the successful N doping which yielded particles capable of visible light absorption. It has been known that metal nanoparticles such as Au or Ag can enhance the activity of photocatalysts in visible light due to their plasmonic properties. By combining nonmetal doping with the plasmonic effect, we have designed SiO₂/Au/N-doped TiO₂ shell nanostructures for improving photocatalytic activity.Reference Zhang, Lima, Lee, Zaera, Chi and Yin⁴⁶ The catalyst has a sandwich structure that comprises a SiO₂ core, a layer of decorated gold nanoparticles (AuNPs), and a doped TiO₂ nanocrystalline shell. The TiO₂ shell was doped with both N and C using an unconventional method that involved introducing 3-aminopropyl-triethoxysilane (APTES), which originally acted as a ligand for immobilization of AuNPs on the surface of the SiO₂ support, but upon subsequent decomposition at high temperature served as a source of both the N and C dopants. Eventually, the sandwich-structured catalysts showed excellent catalytic activity for oxidation of organic compounds under UV, visible and direct sunlight. The enhanced photocatalytic efficiency can be attributed to the interfacial nonmetal doping which improves visible light absorption, and to the plasmonic metal decoration, which enhances light harvesting and charge separation.