Metal oxide mesocrystals with tailored structures and properties for energy conversion and storage applications

Mesocrystals (MCs) are superstructures with a crystallographically ordered alignment of nanoparticles. Owing to their organized structures, MCs posses some unique characteristics such as a high surface area, pore accessibility, and good electronic conductivity and thermal stability; thus, MCs could be beneﬁcial for many areas of research and application. This review begins with a description of the common synthesis strategies for, and characterization and fundamental properties of metal oxide MCs. Newly developed analytical methods (that is, photoconductive atomic force microscopy and single-molecule, single-particle ﬂuorescence microscopy) for unraveling the charge transport and photocatalytic properties of individual MCs are then introduced. Further, recent developments in the applications of various metal oxide MCs, especially in the ﬁelds of energy conversion and storage, are also reviewed. Finally, several perspectives in terms of future research on MCs are highlighted.

Among metal oxides, TiO 2 is used in many applications that deal with environmental and energy problems (for example, photodegradation of pollutants, 67 water splitting for H 2 evolution, 68 dyesensitized solar cells 68 and lithium-ion batteries 69 ) owing to its chemical stability as well as its low cost and non-toxicity. When TiO 2 is illuminated with ultraviolet (UV) light, electron-hole pairs are created simultaneously. Only a small part of the photogenerated charge carriers spatially separate via diffusion and facilitate surface reactions to generate reactive species (for example, O 2 À and OH) for subsequent oxidation/reduction processes on TiO 2 surfaces. 70 Thus, highly active photocatalysts require high charge separation efficiency in addition to a high specific surface area for the adsorption of reagents. Controlled crystal growth also determines the exposed facets of crystals as well as their shape and size, which have different surface physicochemical properties. 71,72 As Yang et al. 73 successfully synthesized anatase TiO 2 single crystals with highly exposed reactive {001} facets, enormous efforts have been devoted to the synthesis of anatase TiO 2 crystals with different crystal planes exposed for better performance. 74 Furthermore, mesoporous TiO 2 single crystals with well-defined facets have been prepared and utilized in dye-sensitized solar cells and photocatalysis for H 2 evolution and degradation of dyes. 75,76 It has been suggested that the dense packing of TiO 2 nanocrystals (NCs) enhances the photocatalytic activity and performance of dyesensitized solar cells owing to the efficient interparticle electron transfer between NCs. [77][78][79][80][81][82][83] For example, the Choi group reported that mesoporous TiO 2 consisting of compactly packed nanoparticles showed higher photocatalytic activity for H 2 evolution than that shown by colloidal and commercial TiO 2 samples in both UV and visible light (dye-sensitized) systems. 78 In addition, transient photocurrent measurements and random walk numerical simulations have shown that the ordering of a nanocrystalline structure significantly influences charge transport and recombination, which are closely related to the performance of photovoltaic cells. 82,83 Zaban and co-workers 82 found that the highest degree of ordering in porous TiO 2 nanocrystalline films correlates with the highest values of effective diffusion coefficient. Indeed, MCs consisting of NCs highly oriented in the same direction are an ideal system to study this issue.
In this review, we focus on the synthesis and structures of metal oxide MCs, mostly TiO 2 MCs, and their applications for energy conversion and storage. We selected examples, which showed the most marked enhancement of physical properties in applications. For instance, owing to their high specific surface area and mesoporous structures, TiO 2 MCs could show higher photocatalytic activity for H 2 evolution and organics degradation than that shown by bulk single crystals. The higher photocatalytic activity of TiO 2 MCs is strongly related to the facile interparticle electron transport between NCs in the MC superstructure. Finally, we summarize our contribution together with some future research directions in various aspects.

SYNTHESIS OF MCS
In this section, we briefly overview the synthesis of metal oxide MCs by several different approaches. As the details of the synthesis procedures for and formation mechanisms of MCs are beyond the scope of this article, we request interested readers to refer to the comprehensive reviews cited in Introduction section. 6

Metal oxide mesocrystals T Tachikawa and T Majima
The method most commonly used to synthesize MC materials is based on hydrothermal/solvothermal treatments in which a solution is maintained at a specific temperature and aged for a specific period of time. The resulting precipitates are then collected and washed to remove impurities. In some cases, the precipitates require further annealing. For example, the Qi group synthesized spindle-shaped anatase TiO 2 MCs by the solvothermal treatment of a solution of tetrabutyl titanate in acetic acid and subsequent calcinations. As illustrated in Figure 2a, the reaction between tetrabutyl titanate and acetic acid first forms unstable titanium acetate complexes by ligand exchange/substitution, concomitant with the release of butanol. 16 The concomitant Ti-O-Ti condensation forms transient amorphous fiberlike precursors. Flower-like precursors are then produced at the expense of the transient precursors and release soluble titaniumcontaining species for the nucleation and growth of anatase NCs. These NCs undergo oriented aggregation along the [001] direction, consequently resulting in the formation of spindle-shaped anatase MCs (Figures 2b and c). The Brunauer-Emmett-Teller surface area and pore volume of the synthesized TiO 2 MCs were 114 m 2 g À1 and 0.14 m 3 g À1 , respectively. Further, using the solvothermal method, Li and co-workers 17 synthesized single crystal-like anatase TiO 2 MCs with a high surface area of 180 m 2 g À1 and a uniform pore diameter of 3.4 nm. These TiO 2 MCs had either massive or granular structures in which most of the high-energy {001}surfaces were hidden in the bulk.
The O'Brien group were the first to prepare anatase TiO 2 MCs via the topotactic transformation of NH 4 TiOF 3 crystals. 13 The topotactic transformation is a solid-state transformation in which the final product is structurally and orientationally related to the starting material. Figure 3 illustrates the topotactic transformation of NH 4 TiOF 3 to anatase TiO 2 MC. As the structures of NH 4 TiOF 3 and anatase TiO 2 are very similar, the positions of Ti atoms in the {001} planes are well matched with each other. Using a similar synthesis strategy, Yu et al. 19 synthesized layered anatase TiO 2 nanosheets with exposed {001} facets by a simple hydrothermal method, followed by calcination.
In 2012, Bian et al. 21 significantly improved the synthesis method for anatase TiO 2 MCs with dominant {001} facets. Their improved method is illustrated in Figure 4a; plate-like TiO 2 MCs are obtained by annealing a thin layer of an aqueous solution containing TiF 4 , NH 4 F and NH 4 NO 3 (without surfactants) on a silicon wafer. In this one-step annealing process, the material undergoes changes in two stages. First, the precursors of Ti 4 þ , F À , NH 4 þ and H 2 O begin a series of combination reactions during evaporation of water at low temperatures to form NH 4 TiOF 3 . With further increase in annealing temperature, NH 4 TiOF 3 is topotactically transformed into anatase TiO 2 . When large amounts of N, H and F atoms are removed, the volume of nanoparticles decreases and gaps or pores are formed between nanoparticles, resulting in the formation of porous TiO 2 Figure 3 Schematic illustration of the topotactic transformation of NH 4 TiOF 3 to anatase TiO 2 mesocrystal (MC). Reprinted with permission from reference Zhou et al. 13 Copyright 2008 American Chemical Society.

Metal oxide mesocrystals T Tachikawa and T Majima
MCs that consist of anatase single-crystalline nanoparticles with the {001} surfaces as dominant facets.
They characterized the structures of the synthesized TiO 2 MCs by scanning electron microscopy (SEM) and transmission electron microscopy. The TiO 2 MCs showed a plate-like structure several micrometers in size and 50-300 nm in thickness ( Figure 4b). The MCs were composed of an ordered alignment of anatase TiO 2 NCs with an average size of approximately 40 nm. A porous structure with pore diameters of several nanometers was confirmed by highresolution scanning electron microscopy and transmission electron microscopy images (Figures 4c and d). It is worth mentioning here that a selected-area electron diffraction pattern of the crystal (inset) corresponded to single-crystal anatase along the [001] zone axis, thus suggesting the formation of anatase TiO 2 with the {001} facet exposed. An oriented arrangement of NCs decreases the grain boundaries in a sample. The pure phase of anatase was thus retained after calcination at 800 1C. The enhanced phase stability was explained by the assumption that the transformation to the rutile phase was inhibited by the elimination of interfacial nucleation sites. 16 To understand the formation mechanism of anatase TiO 2 MCs, they compared the scanning electron microscopy images, X-ray diffraction patterns, N 2 adsorption-desorption isotherms and pore size distribution of the samples obtained at different synthesis temperatures (Figures 4e and f). NH 4 TiOF 3 growth started at 250 1C with a mixture NH 4 TiOF 3 and NH 4 NO 3 during the evaporation of water (Figure 4e). Pure NH 4 TiOF 3 with a plate-like structure was observed at 300 1C, but it did not have any pore structure on the surface and its specific surface area was very low (B0.4 m 2 g À1 ) ( Figure 4f). After annealing at 400 1C, the obtained product was pure anatase TiO 2 (Figure 4e) and had a porous structure with a specific surface area of 74 m 2 g À1 and a pore size of approximately 5 nm. Furthermore, no apparent change in morphology was observed, indicating the topotactic transformation from NH 4 TiOF 3 to TiO 2 by the removal of N, H and F atoms from the crystal lattice. This removal of atoms resulted in the creation of space between TiO 2 NCs, and thus, a porous structure was formed in the crystal. Upon further increasing the calcination temperature, the morphology and crystalline phase of TiO 2 remained unchanged, while the mutual fusion of NCs led to an increase in the particle size, a decrease in the specific surface area and an increase in the pore size ( Figure 4f).
Another important progress is the formation of MCs via electrochemical deposition. 46,49 For example, Fang et al. 46 reported an external electric field-driven particle-mediated bottom-up approach for synthesis of Ag 2 O MCs with different morphologies. Figure 5 shows the scanning electron microscopy images of the as-prepared Ag 2 O MCs under different applied potentials and growth times. The morphologies of the products obtained under different conditions are remarkably different (rhombic hexahedron, cube and dodecahedron), and the rough surface of MCs reveals a nonclassical crystallization process (insets of Figure 5).
An external magnetic field has also been applied to induce the selfassembly of g-Fe 2 O 3 nanocubes into micrometer-sized MC structures. 41 The thickness of the MCs is controllable by varying the concentration of the nanoparticle dispersion as well as the duration of the applied magnetic field.

OPTICAL PROPERTIES OF MCS
MCs are expected to exhibit some interesting and unique optical properties owing to their structural features. Wu et al. 26 synthesized core-shell-structured ZnO MC microspheres by a hydrothermal method in the presence of a water-soluble polymer. The whole surface of these MCs was composed of densely packed ZnO nanoplatelets aligned perpendicularly to the microspheres. Interestingly, these ZnO MCs exhibited strong mechanical resonance and radiative emission at B0.36 THz under 514.5-nm continuous-wave laser irradiation. This property originated from the laser-induced coherent vibration of the ZnO nanoplatelets.
Hu et al. 84 demonstrated that AgIn(WO 4 ) 2 MCs exhibit a white emission in the visible region when excited by 460-nm light. Their photoluminescence (PL) strongly correlated with the surface nanostructures of outgrowths; a larger amount of outgrowths led to stronger emission intensities. The MCs with high surface-to-volume ratios had more surface defects, which were responsible for the radiative recombination of charge carriers. In addition, Li et al. 24 found that the PL properties of hollow-type ZnO MCs are largely governed by the number and nature of defects in the ZnO lattice.
Bian et al. 85 investigated the recombination of photogenerated electrons and holes in plate-like TiO 2 MCs that yields distinct PL in the visible region. Figure 6a shows a typical emission image acquired for two partially overlapping TiO 2 MCs during 380-nm photoexcitation in ambient air. Spectral measurements revealed a broad emission band at the center positions of the crystal at around 450-600 nm (Figure 6b), which originated from surface states. 86,87 The average PL lifetimes of TiO 2 MCs and NCs were around 5.9 and 2.0 ns, respectively ( Figure 6c). Shorter (longer) PL lifetime of the samples indicates relatively faster (slower) charge recombination. Thus, it was concluded that the MC superstructure significantly improved charge separation efficiency. They also found that the average lifetime measured at the center of the MCs (5.5 ns) was slightly shorter than that measured at their edge (6.9 ns). The location-dependent nature of PL implied that charges trapped near the edge were subjected to a different recombination probability as compared with those trapped at the center.

ELECTRON TRANSPORT IN MC
Experiments on a single assembly of NCs revealed the superior physicochemical properties of ordered superstructures that are unattainable using conventional disordered systems. To explore the intricate relationship between structure and function, Bian et al. 21 investigated the photoconductivity of individual TiO 2 MCs by means of conductive atomic force microscopy, with the instrument equipped with a UV light source ( Figure 7a). As shown in Figures 7b, a significant photocurrent response was observed in case of UV irradiation on a single plate-like TiO 2 MC on an indium tin oxide electrode, while there was no measurable current response without UV irradiation. From the thickness dependence of the photocurrent, the photoconductivity was calculated to be 2 Â 10 À2 O À1 m À1 in air, which was several orders greater than that of aggregated TiO 2 NCs with a similar size (B200 nm) under the same conditions (Figure 7b). This result suggested that electron transport ability in TiO 2 MCs was largely improved possibly because of the intimate contact between NCs and/or between NCs and electrodes.
Single-molecule, single-particle fluorescence micro(spectro)scopy has emerged as a powerful tool for obtaining information about the structure and dynamics of individual objects. 88,89 Bian et al. 85 applied this technique to determine the location of reactive sites on plate-like TiO 2 MCs by using a redox-responsive fluorogenic probe. By accepting electrons from photoexcited TiO 2 , nonfluorescent 8-(3,4dinitrophenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-sindacene (DN-BODIPY; fluorescence quantum yield F fl o10 À4 in methanol) was reduced to form highly fluorescent 4-hydroxyamino-3-nitrophenyl-BODIPY (HN-BODIPY; F fl ¼ 0.50 in methanol)  Figures 8b and c, most of the fluorescence spots, that is, catalytically active sites, were found to be located near the edges of the MCs. This interesting finding was consistent with the observation that Au and Pt nanoparticles were preferentially photodeposited at the edges of TiO 2 MCs with {101} facets (Figures 8e  and f). In addition, the average rate of the formation of fluorescent products on the basal surfaces decreased from 5.6 to 2.3 molecules mm À2 s À1 upon Au loading on TiO 2 MCs, whereas the reaction rate on the lateral surfaces increased from 11 to 19 molecules mm À2 s À1 (Figure 8d). These differences strongly supported the mechanism in which photogenerated electrons on the basal surfaces of TiO 2 MCs can rapidly migrate to Au nanoparticles loaded on the lateral surfaces through the NC network. This anisotropic electron transport greatly improved the photocatalytic performance to be better than that of conventional disordered systems (Figures 8g and h). In fact, the metal-nanoparticle-loaded TiO 2 MC composites exhibited higher photocatalytic activity than the NC-based composites. Further, the loading amounts of noble metals required to achieve 50% of the photocatalytic degradation of 4-chlorophenol were compared under the same experimental conditions. The optimum loading amounts of metals on TiO 2 MCs (for example, 0.04 and 0.2 wt% for Au and Pt, respectively) were about 10 times lower than those on TiO 2 NCs (for example, 0.4 and 4.0 wt% for Au and Pt, respectively). This is desirable for the practical use of TiO 2 MCs as photocatalysts because of the high price of noble metals.

LITHIUM STORAGE PROPERTIES OF MCS
MCs can be used in Li-ion batteries as either anode or cathode materials. The MC structure provides not only facile electronic conduction but also fast Li ion transport between the MC electrode and electrolyte. For example, Ye et al. 16 prepared anatase TiO 2 MCs (see Figure 2) and used them as an anode material for a high-power Li-ion battery. As shown in Figure 9, the MC electrode exhibited better rate capability than a TiO 2 nanoparticle electrode at high current rates. The uniformly porous structure of MCs facilitated their contact with the electrolyte and hence was also advantageous for fast Li-ion transport. The TiO 2 MCs delivered specific discharge capacities of 164.9 and 151.7 mA h g À1 at 1 and 2 1C, respectively; these values were larger than those reported for TiO 2 hollow spheres and were comparable with those reported for mesoporous spheres.
Duan et al. 40 prepared high-stability a-Fe 2 O 3 MCs by a facile solvothermal method without polymer additives, and evaluated the discharge cycling performance of an electrode fabricated using a-Fe 2 O 3 MCs with different morphologies. Rhombic a-Fe 2 O 3 MCs showed the best cycling stability owing to closed and intracrystalline porosity.

PHOTOCATALYTIC PERFORMANCE OF MCS
The design and preparation of mesoporous TiO 2 photocatalysts have attracted increasing interest in recent years owing to their exceptional properties and widespread potential applications. Tartaj 18 synthesized anatase TiO 2 MCs that exhibited good photocatalytic activity for organics degradation under UV light irradiation owing to their high crystallinity. Owing to their unique structural features, TiO 2 MCs  maintain a large specific surface area along with their good crystallinity.
Yu et al. prepared rutile TiO 2 MCs by a microwave-assisted hydrothermal method. 15 The TiO 2 MCs are composed of rutile nanowires with an average aspect ratio of approximately 270. These MCs showed good photocatalytic activity and stability for NO removal under UV or simulated solar light irradiation. Au modification of these TiO 2 MCs led to a 50% improvement in their photocatalytic performance. The researchers ascribed the high photocatalytic ability to the following factors: (i) a high aspect ratio of the rutile TiO 2 nanowires that allowed fast charge transport; (ii) a large effective surface area of the TiO 2 MCs that resulted in easier diffusive transport of photogenerated holes to the target molecules; and (iii) efficient penetration of light and diffusion of NO molecules into the photocatalyst because of open pores.
The above-mentioned plate-like TiO 2 MCs exhibited decent performance for the photocatalytic oxidation of 4-chlorophenol and rhodamine B as well as for the photocatalytic reduction of Cr 6 þ in water. 21 The photocatalytic activities of these TiO 2 MCs exhibited a 100-300% enhancement relative to that of anatase TiO 2 NCs with dominant {001} facets and a similar specific surface area; the observed marked enhancement in activity could not be attributed to the specific surface area of the NCs. The photocatalytic performance of the TiO 2 MCs was nearly similar to that of benchmark P25 TiO 2 (Figures 10a-c). Time-resolved diffuse reflectance spectroscopy was used to directly measure the lifetime of a charge-separated state; this measure is a criterion for evaluating the efficiency of photocatalytic reactions. 70 As shown in Figure 10d, the TiO 2 MCs exhibited a broad transient absorption band in the visible to near-infrared range upon 355-nm laser excitation, which represented the overlapping of trapped holes (mainly 440-600 nm) and trapped electrons (mainly 660-900 nm). 70 4-(Methylthio)phenyl methanol (MTPM) was then selected as the probe molecule to estimate the lifetime of the charge-separated state. 70 As can be clearly seen in Figure 10e, the 550-nm absorption band of the MTPM radical cation (MTPM þ ) was indicative of one-electron oxidation of adsorbed MTPM by photogenerated holes. The half-lives of MTPM þ were determined to be B2 ms for the TiO 2 MC systems, which was much longer than that of TiO 2 NC (B0.5 ms). Hence, TiO 2 MCs could exhibit greatly increased photocatalytic activity owing to their superstructure that enhances charge separation.
Another interesting example is that of anatase TiO 2 MCs with different proportion of {001} and {101} facets synthesized by Chen et al. 22 The proportion of {101} facets was tuned by adjusting the solvothermal periods. As shown in Figure 11, the TiO 2 MCs with a high proportion of {101} facets possessed higher photooxidation/ reduction activity than those with a lower proportion. This result was explained in terms of the synergistic effect of Ti 3 þ and the proportion of {101} facets. In addition, the normalized photocatalytic activity of  Metal oxide mesocrystals T Tachikawa and T Majima TiO 2 MCs was higher than that of TiO 2 nanopolycrystals when the proportion of {101} facets was equal; this indicated that the structural integrity of the crystals had a key role in determining the photocatalytic activity. Liu and co-workers 90 synthesized hollow-type anatase TiO 2 MCs with dominant {101} facets via a new route by using PO 4 3 À /F À as morphology-controlling agents. The hollow MCs were more active in H 2 evolution from water splitting and CH 4 generation from photoreduction of CO 2 , but were less active in O 2 evolution from water splitting than hollow single crystals with a similar surface area. Interestingly, this reaction preference could be attributed to the fact that the hollow MCs had higher conduction and valence band edges than the hollow single crystals.

MC-BASED COMPOSITE MATERIALS
The direct growth method using topotactic transformation is suitable for constructing MC assemblies or layers on a variety of support materials. In 2008, Liu and Zeng 91 developed a mild one-pot solution approach to prepare anatase TiO 2 MCs on multiwalled carbon nanotubes (CNTs) with controllable surface coverage, surface area, crystal orientation and TiO 2 /CNTs ratio. CNTs were mixed with a TiF 4 aqueous solution and held over an ultrasonic water bath for 30 min. The subsequent aging at 60 1C for 20 h produced CNTs covered with closely arranged TiO 2 crystallites with sizes in the range 2-4 nm (Figures 12a and b). The as-prepared TiO 2 /CNTs nanocomposites showed better performance for photocatalytic degradation of an organic dye than P25 and CNTs because of (i) the high specific surface area that provided abundant adsorption sites for reactants; (ii) the porous structure that allowed efficient transport of reactants and products; (iii) the oriented arrangement of TiO 2 NCs that minimized light reflections and allowed light transmission to deeper parts of the catalyst; and (iv) surface defect sites and conductive CNTs support  Metal oxide mesocrystals T Tachikawa and T Majima that may have served as electron reservoirs to suppress the recombination of electron-hole pairs. Yang et al. 92 prepared graphene-TiO 2 MC composites by a facile template-free process based on a combination of sol-gel and solvothermal methods (Figures 12c and  d). The resulting products exhibited a uniform distribution of nanoporous anatase TiO 2 MCs on the graphene sheets. Composites prepared in the presence of different amounts of graphene oxide exhibited higher photocatalytic activity for photocatalytic degradation of rhodamine B than pure TiO 2 MCs and P25.

SUMMARY AND PERSPECTIVES
The focus of current researches in the field of nanoscience and nanotechnology is shifting from the synthesis of individual NCs to the preparation and characterization of their MC superstructures and the realization of their applications. Although self-assembly of NCs by utilizing nanoscale attractive forces provides a simple approach for the fabrication of MCs, the synthesis procedures are sometimes very complicated, and hence, their large-scale applications are limited. A highly reproducible, facile synthesis of MCs with a controlled shape and size is strongly desirable for fundamental research and practical applications. One of the alternative strategies for fabricating such MCs is top-down fabrication through topotactic transformation.
MCs are also an ideal platform for constructing multifunctional materials that incorporate a variety of functional materials. For instance, the development of composite MCs consisting of two or more different types of metal oxide NCs (for example, composites of p-and n-type semiconductors) opens up exciting new opportunities for designing and constructing much more efficient photocatalysts and photovoltaic devices. Such binary nanoparticle superstructures have been well developed for several metals and chalcogenides but not yet for semiconductor metal oxides. [93][94][95] Experiments on a single-particle assembly have revealed that ordered superstructures produce a high yield of photogenerated charges and have high photoconductivity, which are difficult to achieve using traditional disordered systems consisting of crystalline nanoparticles owing to the inevitable occurrence of charge recombination at the internal interface. The Majima group used singlemolecule, single-particle fluorescence microscopy to show that photogenerated electrons could reach reactive sites over a micrometer distance and are preferentially trapped at the edge of plate-like TiO 2 MCs, in which {101} facets are predominantly exposed. 21,85 This anisotropic electron transport significantly retarded the charge recombination with holes, thereby resulting in enhanced photocatalytic activity. The excellent charge/molecular transport properties of MCs thus hold great promise for energy conversion and storage applications. Further development of the synthesis methods for MCs and understanding of their fundamental properties will lead to the production of innovative materials with potential applications in energy conversion and storage, catalysis and sensing.