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Ordered mesoporous metal oxides: synthesis and applications Yu Ren,ab Zhen Ma*c and Peter G. Bruce*b

Downloaded by Indian Institute of Technology Chennai on 04 June 2012 Published on 31 May 2012 on http://pubs.rsc.org | doi:10.1039/C2CS35086F

Received 20th March 2012 DOI: 10.1039/c2cs35086f Great progress has been made in the preparation and application of ordered mesoporous metal oxides during the past decade. However, the applications of these novel and interesting materials have not been reviewed comprehensively in the literature. In the current review we first describe different methods for the preparation of ordered mesoporous metal oxides; we then review their applications in energy conversion and storage, catalysis, sensing, adsorption and separation. The correlations between the textural properties of ordered mesoporous metal oxides and their specific performance are highlighted in different examples, including the rate of Li intercalation, sensing, and the magnetic properties. These results demonstrate that the mesoporosity has a direct impact on the properties and potential applications of such materials. Although the scope of the current review is limited to ordered mesoporous metal oxides, we believe that the information may be useful for those working in a number of fields.

1. Introduction Since the discovery of ordered mesoporous silicas (e.g., MCM-41 and SBA-15) in the 1990s,1,2 mesoporous materials have attracted much interest owing to their wide range of applications.3–8 a

National Institute of Clean-and-low-carbon Energy (NICE), Beijing, 102209, China. E-mail: [email protected]; Fax: +86 10 57339649-9664; Tel: +86 10 57339664 b School of Chemistry and EaStChem, University of St Andrews, St Andrews, Fife KY16 9ST, UK. E-mail: [email protected]; Fax: +44 (0)1334 463808; Tel: +44 (0)1334 463825 c Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, P.R. China. E-mail: [email protected]; Tel: +86 21 65642997

Yu Ren received MS (2003) from Fudan University in the field of porous silicates, adsorption, and catalysis (Prof. Heyong He). After 3 years in the coating industry, he went to the UK and worked on Li-ion batteries and nanoionics. He obtained a PhD degree (2010) under the supervision of Prof. Peter Bruce and continued as a Postdoc thereafter. Since October 2011, Dr Ren has joined the National Institute of Clean-and-low-carbon Energy Yu Ren (NICE), Shenhua Group, China. Currently Dr Ren has broad research interests in energy storage (Li-ion batteries, supercapacitors), materials chemistry, heterogeneous catalysis, and surface chemistry. This journal is

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Although numerous methods have been developed to prepare silica mesostructures,9 the preparation of non-silica mesoporous materials is more challenging. Whilst the hydrolysis and condensation of silica precursors (e.g., tetraethyl orthosilicate in water) can be well controlled, and the resulting silicas are thermally stable during calcination, the hydrolysis and condensation of non-silica precursors (e.g., metal alkoxides) are generally difficult to control.5 The beginning of the 21st century saw considerable progress in the preparation of ordered non-silica mesoporous materials.10–15 For instance, Yang et al. used a soft templating method to obtain a series of semicrystalline mesoporous metal oxides.16 Ryoo and co-workers developed a hard templating method for

Zhen Ma

Zhen Ma received BS and MS degrees from Fudan University in 1998 and 2001, respectively. He obtained a PhD degree from University of California-Riverside in 2006 and became a Postdoc at Oak Ridge National Laboratory thereafter. He has been an Associate Professor at Fudan University since 2009. Prof. Ma has broad research interests in heterogeneous catalysis, materials chemistry, and surface chemistry.

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the preparation of mesoporous carbons.17–20 Zhu et al. employed a hard templating method to prepare ordered mesoporous Cr2O3.21 Grosso et al. used a soft templating method to synthesize ordered crystalline mesoporous SrTiO3, MgTa2O6, and CoxTi1xO2x.22 Among non-silica mesoporous materials, mesoporous transition metal oxides are particularly important because they possess d-shell electrons confined to nanosized walls, redox active internal surfaces, and connected pore networks. With these attributes they exhibit many interesting properties in energy conversion and storage, catalysis, sensing, adsorption, separation, and magnetic devices.23–29 Here we summarize the synthesis and applications of ordered mesoporous metal oxides.

2. Preparation of ordered mesoporous metal oxides 2.1 Preparation of mesoporous metal oxides using soft templates The preparation of ordered mesostructured metal oxides (WO3, Sb2O5, Fe2O3, etc.) using soft templates (cationic surfactants such as alkyltrimethylammonium surfactants; anionic surfactants such as C16H33SO3H) was first reported in 1994.23 Two general synthetic pathways have been explored: direct co-condensation with surfactants of opposite charges and indirect co-condensation of similarly charged species mediated by the intercalation of counter ions (Na+ and K+) at the surfactant–inorganic interface. However, the mesostructures collapsed when the surfactant was removed by calcination. In 1995, Antonelli and Ying reported an ordered mesoporous TiO2 from which the template could be removed by calcination.30 In their synthesis, tetradecyl-phosphate was used as a template and acetylacetone was added to decrease the hydrolysis rate of titanium acetylacetonate tris-isopropoxide. This so-called ‘‘ligand-assisted’’ method was subsequently adopted to prepare ordered mesoporous Nb2O5.25 The dodecylamine–Nb precursor monomer formed micelle in an ethanol solution. The long dodecyl tail is hydrophobic and tends to face the center of the micelle. The Nb–N head is hydrophilic,

Peter Bruce received his BS (1979) and PhD (1982) from Aberdeen University. Then he carried out postdoctoral research at University of Oxford (1982–1985) with Prof. John Goodenough. Currently Peter Bruce is Wardlaw Professor of Chemistry at the University of St Andrews where he leads a research group dedicated to materials chemistry, especially ionically conducting solids (both extended arrays and Peter G. Bruce polymers). He has been elected to the Royal Society and the Royal Society of Edinburgh, has held research fellowships from the Royal Society, the Royal Society of Edinburgh, and a number of awards, medals and prizes, both national and international. Chem. Soc. Rev.

facing the ethanol solution. Mesoporous Nb2O5 was obtained after the condensation of Nb species and removal of the amine template. This ‘‘ligand-assisted’’ method was also used to prepare mesoporous Ta2O5, VOx, and phosphated ZrO2 with ordered and amorphous walls.31–34 Another widely used soft templating method is Evaporation Induced Self-Assembly (EISA),35 especially useful in the formation of ordered semicrystalline mesoporous metal oxides. In 1998, Yang et al. prepared ordered mesoporous metal oxides (e.g., TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, WO3, HfO2, SnO2, SiAlO3.5, SiTiO4, ZrTiO4, Al2TiO5, ZrW2O8) with semicrystalline walls using commercial poly(alkylene oxide) block copolymers as templates.16,36 Sanchez, Antonietti, and their co-workers used a semi-commercial ‘KLE’ template to synthesize ordered crystalline mesoporous metal oxides.13,37 They used in situ small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), and Fourier transform infrared (FTIR) spectroscopy to investigate the mechanism of the EISA process (Fig. 1),22 and divided this process into three steps: (a) preparation of stable solutions containing the KLE3739 copolymer (PBH79-b-PEO89, PBH = hydrogenated poly(butadiene) template) and the inorganic precursors at the appropriate stoichiometry; (b) evaporation induced selfassembly associated with dip-coating. Evaporation induced the progressive concentration of inorganic precursors into a homogeneous, flexible, and poorly condensed network surrounding the surfactant mesophase (M1xM2yOz(OR)w where R = CnH2n+1, n = 0–4); (c) a treatment step involving pre-consolidation, template removal, and network crystallization. In this way, complex metal oxide nano-crystalline films (e.g., SrTiO3, MgTa2O6) were obtained.22 2.2 Preparation of mesoporous metal oxides using hard templates The use of hard templates to synthesize mesoporous materials has brought new possibilities for the preparation of novel mesostructured materials.5,10,38 The hard templating method is also called nanocasting,10,11 exotemplating,39 or the repeated templating method.15 Porous Al2O3 membranes prepared by anodic oxidation (AAO) were used initially as a mold to prepare carbons, metals, or other nanostructures by electrodeposition,40 Chemical Vapor Deposition (CVD),41–43 or Atomic Layer Deposition (ALD).44 The pore sizes of the resulting materials are typically between 15 and 150 nm. Based on this methodology, 3D mesoporous carbon (CMK-1, Fig. 2b)17 and mesoporous Pt networks (Fig. 2c)45 were prepared using mesoporous silica MCM-48 as the template. Fig. 2a shows a schematic diagram,15 in which SBA-15 is used as an example to represent hard templates. First the silica mesopores are infiltrated with a precursor solution; the precursor is converted to a solid by reduction or decomposition inside the pores. Then the mesoporous silica template is removed using an aqueous NaOH or HF solution, and after washing, a material replicating the mesostructure of the hard template is obtained. The presence of disordered micropores between the 1D mesopores of SBA-15 ensures that the replica carbon or metal oxide nanowire arrays are connected by bridges so as to form mesopores (such as those in CMK-3).46 This journal is

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Fig. 1 Scheme of three critical steps in the meso-crystallization process (M1xM2yOn/2).22 (a) Preparation of stable initial solutions. (b) Evaporation induced self-assembly associated to dip-coating. (c) Thermal treatment step including (1) heating at low temperature induces the departure of residual volatile species and the pre-stiffening of the matrix. (2) Heating at moderate temperature allows the departure of the template and the creation of the porosity around a dense amorphous mixed oxide matrix. (3) Heating above the temperature of crystallization triggers the nucleation and progressive growth of crystalline nanoparticles through diffused sintering. Reproduced with permission of Macmillan from ref. 22.

Otherwise only carbon nanowires could be obtained as for MCM-41 templated carbon materials.18 Mesoporous silicas with different pore architectures (e.g., MCM-41, MCM-48, SBA-15, SBA-16, KIT-6, FDU-12) or mesoporous carbons (CMK-1, CMK-3) have been used as hard templates. The first preparation of ordered highly crystalline

Fig. 2 (a) A schematic diagram of hard templating synthesis using SBA-15;15 TEM images of (b) mesoporous carbon CMK-117 and (c) Pt network prepared using MCM-48 silica as the hard template.45 Reproduced with permission of the American Chemical Society from (a) ref. 15; (b) ref. 17; and (c) ref. 45.

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porous transition metal oxides via a hard templating method was reported in 2003.21 Amine functionalized SBA-15 was used as a hard template and K2Cr2O7 was introduced into the mesopores quantitatively. Mesoporous single crystalline Cr2O3, consisting of nanowire arrays, connected by short bridges, was obtained (Fig. 3).21 Later, mesoporous WO347 and a-Fe2O348 were prepared in a similar way. In a hard templating process, it is not easy to fill the mesoporous silica template completely, because there are complex interactions between the silica and filtrated metal ion precursor: hydrogen bonding, Coulombic interactions, coordinating interaction, and van der Waals interaction.10 Therefore, different methods have been developed to improve the impregnation and minimize the loading outside pores. Metal ion precursors can interact with the fresh silica template

Fig. 3 (a) Mesostructure model and (b) HRTEM image for mesoporous Cr2O3 prepared using SBA-15 silica template.21 The arrows indicate the small ‘‘bridges’’ between the wires. Reproduced with permission of the Royal Society of Chemistry from ref. 21.

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through weak Coulombic interactions with Si–OH. In order to introduce the maximum quantity of metal ion precursors for good replication, one way is to retain as many silanols as possible.49,50 The functionalization (post synthesis grafting or one-pot preparation) of the mesoporous silica templates by certain organic groups (for instance, –NH2, –CHQCH2) may lead to a strong interaction between the organic group and metal ion precursor, thus improving the loading of metal ion precursor.21,51–53 Ordered mesoporous metal oxides can also be prepared without surface functionalization using the two-solvent method, solvent evaporation method, solid–liquid method, impregnation–precipitation–calcination method, and combustion method. In a ‘‘two-solvent’’ method, a suspension of mesoporous silica in dry hexane is mixed with a concentrated aqueous solution of metal nitrate. Generally, the solution volume is equal to the silica pore volume to maximize the impregnation quantity and prevent the growth of metal oxides out of the pores.54,55 The precursor ions will be ‘‘pushed into’’ pores as much as possible by the hexane.55,56 The solvent evaporation method involves fully mixing the mesoporous silica with a selected metal nitrate in ethanol.57 The nitrate precursor is expected to migrate into the pores by capillary condensation during the slow evaporation of ethanol. In a solid–liquid method, a metal nitrate is ground with a mesoporous silica template, and is expected to move into the pores of silica after melting when the mixed solid is heated to a temperature above the melting point of the precursor.58 The method is limited to precursors (e.g., Ce(NO3)36H2O, Cr(NO3)39H2O, Co(NO3)26H2O, Ni(NO3)26H2O) with melting points lower than their decomposition temperatures. The impregnation–precipitation–calcination method employs low cost metal chlorides as the starting materials. First, mesoporous silica is impregnated with a metal chloride and dried, treated with NH3 (gas or solution) to convert the chloride into hydroxide, and then calcined to form the

corresponding metal oxide. The whole process is generally repeated several times and a mesoporous metal oxide is obtained after removing the silica template. That method was first developed to prepare mesoporous CeO2,59 and was later applied to the preparation of mesoporous Co3O4 and NiO.60 Special methods are occasionally needed to prepare a certain metal oxide. For instance, it is difficult to prepare ordered crystalline mesoporous CuO directly using the hard templating method. Lai et al. used CMK-3 as a hard template to prepare mesoporous CuO and removed the carbon template by calcination in air at 500 1C for 48 h.61 Our group attempted to repeat the above process, only to get bulk CuO or disordered CuO. Probably when mesoporous carbon is used as a hard template, there will be problems due to poor solution infiltration and loss of ordered mesostructure after calcination.62 Therefore, a combustion method was developed (Fig. 4).63 First, KIT-6 was impregnated with an aqueous Cu(NO3)2 solution. After drying, the copper precursor–silica composite was exposed to NH3 vapor from an aqueous ammonia solution at room temperature for 1 h and again dried. This impregnation–ammoniation– drying process was repeated twice more. Then the resulting composite was calcined to obtain the CuO/silica nanostructure and the silica template was removed by 0.1 mol L1 of hot NaOH solution (60–80 1C). The concentration of the leaching agent is important: a 2 mol L1 NaOH or 10% HF solution dissolved the CuO nanostructure and silica template completely in a short time.63 Having summarized several preparation methods, the question now arises as to how to choose from the above methods. The solvent evaporation and ‘‘two solvent’’ methods offer a simple approach that does not rely on strict reaction conditions. On the other hand, the surface functionalization (post synthesis grafting) entails processing under an inert gas21 and the impregnation–precipitation–calcination method has problems associated with its long preparation time and possibility of product contamination by residual chlorine.

Fig. 4 Preparation of ordered mesoporous CuO by a combustion method from KIT-6.63 Attention: This reaction involves rigid combustion, therefore care should be taken when dealing with calcination. A large and open crucible needs to be used.

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2.3

Reinforced crystallization method

One problem for amorphous mesoporous metal oxides is the stability of the mesostructures during crystallization, thus a silica/carbon enforcement method was developed (Fig. 5).64 In procedure (a), the mesopores are first filled with a carbon percursor (e.g. sucrose), followed by calcination in an inert atmosphere for crystallization, and the formed carbon can stabilize the mesostructure. Then the filled carbon is removed by calcination in air. In procedure (b), a silica layer is coated to strengthen the structure, and after crystallization by calcination, removed by treatment with an aqueous NaOH or HF solution. Highly crystalline ordered mesoporous Nb2O5, Ta2O5, (Ta, Nb)2O5, Al2TiO5, TiZr2O6, and TiNb2O6 were obtained using both procedures.64 Recently, Lee et al. prepared ordered crystalline TiO2 and Nb2O5 in a ‘‘one-pot’’ synthesis using block copolymers with an sp2-hybridized carbon-containing hydrophobic block as the structure-directing agents.65 The polymer converted to a sturdy, amorphous carbon material upon appropriate heating. The carbon formed in situ is sufficient to keep the pores of the oxides intact while crystallizing at temperatures as high as 1000 1C. In this way, highly crystalline ordered mesoporous TiO2 and Nb2O5 were obtained, with surface areas and average pore diameter of 90 m2 g1 (23 nm) and 54 m2 g1 (35 nm), respectively. 2.4

Post synthesis solid–solid conversion

A post-synthesis solid–solid conversion method was developed to prepare transition metal oxides with mixed valences or valences not easily accessed directly from solution. For instance, mesoporous Fe3O4 was prepared by reducing mesoporous a-Fe2O3 in 5% H2 (balanced with 95% Ar) at 350 1C for 1 h.66 Ordered mesoporous g-Fe2O3 was obtained after further mild oxidation under air at 150 1C for 2 h. Mesoporous Mn3O4 was prepared by reducing mesoporous Mn2O3 in 5% H2 at 280 1C for 3 h.67 Schu¨th and co-workers prepared ordered mesoporous CoO by the thermal treatment of mesoporous Co3O4 with hot glycerol as a reducing agent.68 Recently, Bruce and co-workers showed that ordered mesoporous CoO, Mn3O4 and Cu–Cu2O could be obtained by reducing mesoporous Co3O4, b-MnO2, and CuO, respectively, in 5% H2.69

Another method for the preparation of low-valence ordered mesoporous metal oxides is to reduce a metal oxide precursor with mesoporous silica as a hard template.70,71 Mesoporous MoO2 was prepared by reducing H3PMo12O40/KIT-6 nanocomposites in 10% H2 (90% N2) at 500 1C and then removing the KIT-6 using aqueous HF.70 Ordered mesoporous WO3x was prepared by reducing the WO3–KIT-6 nanocomposite in 4% H2 (96% N2) at 600 1C for 4 h and then removing the KIT-6 by aqueous HF.71 Lithiated mesoporous transition metal oxides can be prepared via a post-synthesis reaction with LiOH. For example, mesoporous Mn3O4 and Co3O4 were converted to ordered mesoporous Li–Mn–O spinel and low temperature LiCoO2 (spinel structure).57,72 Ordered mesoporous LixNi2/3Co2/3Mn2/3O4 spinel was prepared using mesoporous NiCoMnO4 as the starting material.63 These lithiated mesoporous metal oxides kept the original spinel structure. In contrast, the solid–solid conversion of rutile MnO2 with LiOH led to LiMn2O4 with a collapsed mesostructure.73 The solid–solid conversion may involve a significant change of structure. For example, the reduction of Mn2O3 (corundum structure) to Mn3O4 (spinel structure) involves a significant loss of oxygen and change of the O2 stacking from hexagonal to cubic close packing.67 Such a transformation requires shearing of the close packed planes from AB (hcp) to ABC (ccp) stacking. The ability to do so while preserving the mesostructure demonstrates that the thin walls (B7.5 nm) can accommodate the strain of such a structural transformation without severe fracture. The loss of oxygen on transformation leads to shrinkage of the walls (i.e., larger pores) while preserving the basic pore shape, further testifying to the flexibility of the mesostructure in its accommodation of structural changes. 2.5 Nanoparticle self-assembly The preparation of mesoporous metal oxides via a soft templating method involves different reactions (hydrolysis of metal– organic salts, condensation of the metal ions, self-assembly of the template) sensitive to reaction conditions. If the selfassembly processing is based on monodisperse nanoparticles, the process will be less sensitive to the variable reaction conditions.74 Such a method has four steps: (1) formation of

Fig. 5 Schematic illustration of the strategy for reinforced crystallization of mesoporous metal oxides: (a) back filling of carbon, (b) coating of silica species, (c) crystallization, and (d) removal of the reinforcement (carbon or the silica).64 Reproduced with permission of the American Chemical Society from ref. 64.

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monodisperse metal oxide nanoparticles; (2) mixing of the template and nanoparticles; (3) assembly of nanoparticles into ordered mesoporous metal oxides; (4) removal of the template by calcination, etc. Corma and co-workers prepared ordered mesoporous CeO2 with a BET surface area of 160 m2 g1 and pore volume of 0.35 cm3 g1 by self-assembly of 5 nm CeO2 nanoparticles using Pluronic P123 as a template.75 They further prepared mesostructured materials using CeO2, ZrO2, and CeO2–Al(OH)3 nanoparticles functionalized by di-functional amino acid species as building blocks.76 Alternatively, CeO2 and SnO2 nanoparticles without surface functionalization were self-assembled into ordered crystalline mesoporous metal oxides.77,78 2.6

Comparison of the soft and hard templating methods

The advantages of the soft templating method are that the templates can be of low cost, the synthesis is relatively easy and can be carried out under mild conditions, and a variety of mesoporous structures are possible depending on the template and the composition of the solution. The main disadvantages are that their syntheses are based on complicated sol–gel processes, and the hydrolysis and polymerization of transitional metal ions are difficult to control. Their products usually have amorphous or semi-crystalline walls and poor thermal stability,79 and the synthesis is generally sensitive to the relative humidity when using commercial PEO based copolymers.80 Highly crystalline metal oxides could be obtained using designed polymer templates such as KLE with much higher thermal stability.22,81–85 A hard templating procedure offers some advantages. The mesostructures of the target materials can be controlled by choosing hard templates with desired structures. In addition, mesoporous metal oxides with highly crystalline walls may be obtained because the mesoporous silicas used as hard templates are stable to suffering high temperature to allow many metal oxides to crystallize. The post-synthesis solid–solid conversion enables the preparation of low-valence metal oxides and lithiated mesoporous transition metal oxides, greatly enriching the range of mesoporous materials and their applications.57,66,67,72,86 However, the hard templating method also has disadvantages. First, the targeting mesoporous metal oxides must be stable to NaOH or HF solutions used to remove the silica template, although mesoporous carbon, which can be removed by calcination, can be used as a hard template instead. For example, the synthesis of mesoporous ZnO, MgO, and Al2O3 has not been possible using mesoporous silica, while such mesoporous metal oxides were obtained using mesoporous carbon CMK-3 as a template.62,87–90 Second, a solution step is still required to introduce the transition metal ion precursor and this limits the range of materials to those stable in solutions. Also, materials (e.g., lithiated mesoporous metal oxides, mesoporous ZnO and Al2O3) for which the precursors react with the mesoporous silica cannot be synthesized. When using mesoporous carbon as a hard template, a main disadvantage is the poor wetting of the pore walls by the aqueous precursor solution. Chem. Soc. Rev.

2.7 Controlling the pore size and wall thickness of mesoporous metal oxides via the hard templating method The pore sizes and wall thicknesses of mesoporous metal oxides can be tuned via several methods including controlling the pore size and wall thickness of the silica hard template, and changing the crystallization temperatures of mesoporous metal oxides. The pore size and wall thickness of the silica are generally varied by altering the hydrothermal conditions used during the synthesis,1,2,91,92 by using a series of surfactants with different chain lengths2,93 or changing the calcination temperature.19,94,95 For instance, Rumplecker et al. prepared a series of mesoporous Co3O4 with wall thicknesses from 4 to 10 nm, pore size from 3 nm to 10 nm using different KIT-6 templates obtained under different hydrothermal treatment temperatures.96 A similar method was used to prepare a series of ordered mesoporous b-MnO2 with pore sizes between 3 nm and 11 nm, wall thicknesses between 4.7 and 10.1 nm.97 This method is extended to the synthesis of mesoporous Co3O4 with different pore sizes using SBA-15, KIT-6, and AMS-10 as hard templates.98 Mesoporous silica KIT-6 templates calcined between 500 and 1000 1C were used to obtain Co3O4, with pore size and wall thickness varied systematically in the range of 3.7–11.9 nm and 2.2–8.2 nm, respectively.95 Yue et al. used KIT-6 as a hard template to prepare mesoporous rutiles with different pore sizes (4.9–7.2 nm) by variation of the rutile crystallization temperatures between 100 and 600 1C.99 It is possible to prepare mesoporous b-MnO2 and NiO with a bimodal pore size distribution and to vary the ratio of the two types of pores (Fig. 6) by controlling the textural properties of KIT-6 template.97,100 KIT-6 contains two sets of mesopores connected by microporous channels,91,92,101 and the prevalence of these channels varies with the hydrothermal synthesis conditions. The presence of a large number of microporous channels in KIT-6 ensures complete filling of both sets of mesopores, resulting in mesoporous metal oxides with a pore diameter of B3.4 nm on casting, whereas fewer microporous channels in KIT-6 result in mesoporous metal oxides with a pore diameter of 11 nm (Fig. 6).55,96,100 In this way mesoporous b-MnO2 and NiO with different proportions of large (ca. 11 nm) to small (ca. 3.4 nm) diameter pores were prepared.97,100 A more detailed explanation of how different pore sizes may be generated is given in the cited ref. 96, 100 and 102. Using a recent developed tricontinuous mesoporous silica as the hard template,103 it may be possible to prepare trimodal mesoporous metal oxides.

3. Applications of ordered mesoporous metal oxides Commercial bulk metal oxides have been widely used in energy conversion and storage, catalysis, sensing, adsorption and separation, etc.104 However, the surface area and particle size may have a large impact on such applications. With higher surface areas and ordered pore structures, ordered mesoporous metal oxides may demonstrate better performance. Here we highlight the applications of ordered mesoporous metal oxides in energy conversion and storage, catalysis, This journal is

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Fig. 6 Mechanism by which two pore sizes in mesoporous metal oxides can occur using KIT-6 as a hard template.100 Here the synthesis of mesoporous NiO is used as an example. Reproduced with permission of the American Chemical Society from ref. 100.

sensing, adsorption and separation. The comparison with bulk materials will be made where possible. 3.1

Energy conversion and storage

3.1.1 Solar cells. Solar cells may be divided into two categories: photovoltaics in which incident electromagnetic radiation is transformed directly into an electric current and Dye Sensitized Solar Cells (DSSCs) which utilize a light adsorbing dye with electron transfer to a cathode through an electrochemical reaction thus sending out a current (Fig. 7). Mesoporous metal oxides can play a role in the latter case.105–107

Fig. 7 Schematic illustration for the operation of the dye-sensitized solar cells (DSSC).105 The photoanode, made of a mesoporous dyesensitized semiconductor, receives electrons from the photo-excited dye which is thereby oxidized, and which in turn oxidizes the mediator, a redox species dissolved in the electrolyte. The mediator is regenerated by reduction at the cathode by the electrons circulated through the external circuit. Reproduced with permission of Macmillan from ref. 105.

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In DSSCs, mesoporous TiO2 films are used to accommodate the dye.108 The anode is typically composed of a transparent conductive glass anode (e.g. indium-tin oxide glass), attached to a film of mesoporous TiO2 employing randomly oriented nanoparticles between which the light-harvesting dyes are loaded. The Pt counter electrode is separated from the anode by a liquid electrolyte (Fig. 7).105 That acts to shuttle electrons from the counter electrode to regenerate the oxidised dye species following charge injection, and complete the circuit. In 1998, an organic hole transporting molecule, 2,2 0 ,7,7 0 tetrakis(N,N-di-p-methoxyphenylamine)-9,9 0 -spirobifluorene (spiro-MeOTAD), was developed to replace the liquid electrolyte and the resulting solid-state DSSCs demonstrate a high quantum yield of 33%.109 Randomly oriented TiO2 nanoparticles have detrimental boundaries between nanoparticles, which could limit the rate of electron transport out of the active layer. Thus direct utilization of highly crystalline mesoporous metal oxides (e.g., CeO2, TiO2, SnO2) in DSSCs has been reported recently.75,110–114 Corma et al. found that ordered mesoporous CeO2 prepared using CeO2 nanoparticles (5 nm) as the building block showed a photovoltaic response, whereas conventional CeO2 did not.75 The better photovoltaic behaviour of nanocrystalline mesoporous CeO2 is probably due to its high surface/ grain-boundary area that enhances the electronic transport properties of nanocrystalline CeO2. Zukalova´ et al. demonstrated that Pluronic P123 templated disordered mesoporous TiO2 film enhanced solar conversion efficiency by about 50% compared to that of traditional films of the same thickness made from randomly oriented anatase nanocrystals when used in conventional liquid DSSCs.110 Crossland et al. used a diblock copolymer poly(4-fluorostyreneblock-D,L-lactide) (PFS-b-PLA) to prepare an ordered bicontinuous gyroid anatase network by electrochemical deposition within the pores of a thin film template.111 This thin DSSC (B400 nm thick) exhibited up to 1.7% power conversion efficiency. Chem. Soc. Rev.


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The impact of the TiO2 mesostructures was also investigated in a liquid DSSC, and the power efficiency was found to be the highest for gyroid structure while lowest for a quasi-1D nanowire system.112 Recently, Snaith and co-workers constructed a solid-state DSSC based on disordered113 and ordered mesoporous TiO2114 prepared using a diblock copolymer poly(isoprene-b-ethylene oxide) (PI-b-PEO) as a soft template. First, they investigated the impact of the morphology and crystallinity of the TiO2 film in solid state DSSC. The increased crystallinity of the mesoporous anatase film improved the performance of DSSC. However, the direct utilization of the copolymer templated TiO2 film has limitations associated with film thickness (several hundred nanometres) due to stress-induced cracking. Therefore, monolithic mesoporous crystalline TiO2 was ground down and processed into a paste to coat the substrates, forming crack-free, albeit rough, films up to many micrometres in thickness.114 Both of the DSSC outperformed the nanoparticle-based solid state DSSC, and they used organic dye and organic solid hole transporter avoiding the problem of electrolyte leakage. A detailed discussion of progress in mesoporous TiO2 based DSSCs is summarized in the cited ref. 115. SnO2 has higher electron mobility and more positive conduction band edge position than TiO2, both of which could allow a SnO2 photo anode DSSC to demonstrate superior performance than the one based on the TiO2 photoanode. Ordered mesoporous SnO2 with a 3D bicontinuous cubic mesostructure, high surface area, and crystalline framework was synthesized using KIT-6 as a hard template and employed in a photo anode for DSSC.116 Coating an ultrathin TiO2 or Al2O3 layer by ALD on the mesoporous SnO2 photoanode greatly improved the open-circuit voltage, short-circuit current, and fill factor, leading to more than a 3-fold improvement in the energy conversion efficiency. The superior photovoltaic performance of the surface modified mesoporous SnO2 photoanode was attributed mainly to inhibited electron recombination caused by passivation of reactive surface states and increased dye loading. DSSCs fabricated with a conventional nanoparticle SnO2/TiO2 photoanode exhibited a similar trend but with about 30% lower energy conversion efficiency due to low dye loading and poor light scattering.116 3.1.2 Electrode materials for lithium ion batteries. It is advantageous to use ordered mesoporous metal oxides as electrode materials for rechargeable Li-ion batteries.8 First, the grains of mesoporous metal oxide are micron-sized particles hence the fabrication of mesoporous metal oxides into composite electrodes is similar to that of dense micron sized particles, resulting in a similar packing density. Second, the micron sized particles maintain good contact between particles ensuring efficient electron transport. Third, the thin (nanometre sized) walls ensure short diffusion distances for Li+ on intercalation, leading to fast electrode reactions. Fourth, the electrolyte can flood the pores, leading to a high electrolyte– electrode contact area and therefore fast electrode reactions. Finally, the ordered mesopores have the same dimensions (monodispersed) as the walls, thus the transport of Li+ is identical everywhere within the pore and within the wall. Such an ordered structure ensures that a mesopore/wall size may be Chem. Soc. Rev.

selected so as to be large enough to permit efficient Li+ transport but not too large to waste volume. In this way, an electrode on optimum balance between high rate performance and high volumetric capacity can be obtained. A number of ordered mesoporous metal oxide electrode materials have been reported. Cathodes include mesoporous low temperature LiCoO2 spinel,57 b-MnO2,97,117,118 LixNi2/3Co2/3Mn2/3O4 spinel,63 and Li1.12Mn1.88O4 spinel;72 anodes include mesoporous Co3O4,119,120 Fe2O3,121 MoO2,70 NiO,122 SnO2,123–125 and TiO2.99,126,127 In a lithium ion battery, the only source of Li is the cathode. However, the syntheses of Li containing ternary oxides possess a challenge. If hard templating is used, then a Li source is introduced and it will react with the template. To avoid this problem, post template synthesis was employed. For instance, mesoporous low temperature LiCoO2 spinel was prepared at 400 1C by the lithiation of mesoporous Co3O4 with LiOH for 1 h.57 Crucially the ordered mesoporous structure was retained while converting Co3O4 to low temperature LiCoO2. The resulting mesoporous LiCoO2 exhibited better properties as a cathode compared with low temperature LiCoO2 nanoparticulates. Ordered mesoporous Li1.12Mn1.88O4 spinel was synthesized and used as a cathode.72 Spinel with this particular composition is of interest, because the oxidation state of Mn is higher than that in stoichiometric LiMn2O4 and the associated lower Mn3+ content reduces the dissolution of manganese in the electrolyte.128–130 The mesoporous Li1.12Mn1.88O4 retained the ability to cycle, storing 50% more Li than the equivalent Li1.12Mn1.88O4 bulk phase at a charge–discharge rate of 3000 mA g1 (30 C, fully charge or discharge in 2 minutes, Fig. 8a).72 The volumetric energy density (energy stored pore unit volume) of mesoporous Li1.12Mn1.88O4 at high rate was 10% higher than that of the bulk despite the low weight density of the mesoporous form. Remarkably, despite the high surface area (90 m2 g1), the capacity retention at low rate (30 mA g1) and 50 1C was comparable to the bulk material and much better than Li1.12Mn1.88O4 nanoparticles (Fig. 8b).72 Evidently the combination of nanometre and micrometre structures of the mesopores offers the advantages of both length scales. Ordered mesoporous b-MnO2 was prepared using KIT-6 as a hard template, and used as cathode for Li-ion batteries.117,118 Bulk b-MnO2 is electrochemically inert. However, mesoporous b-MnO2 (rutile structure) with a highly ordered pore structure (3.4 nm pore diameter) and highly crystalline walls (B8 nm) was capable of reversibly accommodating lithium, up to a composition of Li0.92MnO2 (equivalent to a charge storage of 284 mA h g1).118 Recently, the influences of mesoporous pore diameter and wall thickness on lithium intercalation rate were investigated, using mesoporous b-MnO2 as an example (more details in Section 4.1).97 On intercalating lithium into mesoporous b-MnO2, the material transformed from the tetragonal rutile structure to a new phase, mesoporous LixMnO2-b with orthorhombic symmetry, with a volume increase of ca. 30%. Sayle et al. generated full atomistic models for b-MnO2 nanomaterials, and predicted that the b-MnO2 host should be symmetrically porous and heavily twinned to maximize its This journal is

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Fig. 8 (a) Rate capability for mesoporous Li1.12Mn1.88O4 (K), bulk Li1.05Mn1.95O4 (’), and bulk Li1.12Mn1.88O4 (m); capacity retention expressed as percentage capacity at 30 mA g1 (0.30 C); (b) cycling data for mesoporous Li1.12Mn1.88O4 (*), bulk Li1.12Mn1.88O4 (K), nanoparticulate Li1.12Mn1.88O4 (m), bulk Li1.05Mn1.95O4 (.) and nanoparticulate Li1.05Mn1.95O4 (’), at 50 1C at 30 mA g1 (0.30 C) between 3 and 4.3 V.72 Reproduced with permission of Wiley from ref. 72.

electrochemical properties,131 in good agreement with the experimental observations.97 In addition, it was predicted that there exists a ‘‘critical (wall) thickness’’ for MnO2 nanomaterials (ca. 100 nm) above which the strain associated with Li insertion is accommodated via a plastic deformation of the host lattice leading to capacity fading upon cycling, which could explain the severe capacity fading in bulk b-MnO2. On the contrary, mesoporous b-MnO2 has a symmetrical pore structure (Ia3d) and a wall thickness of 8 nm, enabling the elastic deformation during intercalation/deintercalation.131,132 Some ordered mesoporous transition metal oxides have been used as anode materials operating via a conversion/ displacement reaction, in which metal oxides are fully reduced to form metal clusters imbedded in a Li2O matrix (Li + MxOy 2 xM + yLi2O).133 Examples include Co3O4,119,120 Fe2O3,121 MoO2,70 and NiO.122 They generally show higher capacities than graphite but suffer from problems associated with low rate capability and a large voltage gap between charge and discharge.133–137 Jiao et al. examined the factors influencing the rate of the a-Fe2O3 anode using mesoporous, nanoparticle, and bulk forms of a-Fe2O3, and demonstrated that the electron transport to and within the particles was the rate-limiting step.121 This journal is

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A composite electrode based on mesoporous a-Fe2O3 can store 700 mA h g1 at a rate of 3000 mA g1. Sn-based anodes have received extensive interests as they proceed by a Li/Sn alloy and de-alloy mechanism. If SnO2 is used as an anode material, the total procedure is 4Li + SnO2 Sn + 2Li2O, 4.4Li + Sn - Li4.4Sn.138–140 Fan et al. used a SnOx/CMK-3 nanocomposite as an anode for a lithium battery and demonstrated poor cycling stability—only remaining 200 mA h g1 after 100 cycles.123 Pure mesoporous and nanowire SnO2 anode materials were also prepared.124 The charge capacities of these two anodes were similar at 800 mA h g1, but mesoporous SnO2 showed much improved cycle life performance and rate capabilities because of its higher surface area (160 vs. 80 m2 g1). The capacity retention of mesoporous SnO2 was 98%, compared with 31% for the SnO2 nanowires at a 10 C rate (= 4000 mA g1). With certain amount of silica residue (3.9–6 wt%) in the mesoporous SnO2, dramatic reduction in capacity fading after prolonged cycling was realized.125 Titanium oxides or titanates are also favored as anodes because of their superior safety and ability to sustain high charge–discharge rates compared with graphite. Li can be intercalated into bulk anatase (TiO2), but only to Li0.5TiO2 and with poor cycleability. In contrary, mesoporous anatase reaches a composition of approaching LiTiO2 with good cycleability.126 The ability to store Li in mesoporous anatase was compared with 6 nm anatase nanoparticles, and the former was found to be superior, exhibiting significantly higher volumetric capacities than the nanoparticulate materials, up to twice the capacity at the highest rates, despite the lower intrinsic density of mesoporous anatase. 3.1.3 Supercapacitors. There are generally two types of electrochemical supercapacitors, i.e., double layer and redox supercapacitors (pseudo supercapacitors). In the former case, the charge is stored in the electrical double layer of each electrode while in the latter there is a degree of intercalation that close to battery. The advantage of the redox supercapacitors is that they have high energy density, although still much less than a battery. Supercapacitors offer much faster charging than a battery and therefore should be used for high power, low energy applications. Vettraino et al. studied the electrochemistry of ordered amorphous mesoporous TiO2, Nb2O5, and Ta2O5, and found that only mesoporous TiO2 showed reversible redox behavior.141 In another study, crystalline V2O5 with hierarchical mesopores was synthesized by using a CTAB–BMIC cotemplate (CTAB = cetyltrimethylammonium bromide, BMIC = 1-butyl-3-methylimidazolium chloride) and it demonstrated a large capacitance of 225 F g1.142 Mesoporous metal films (e.g. Pt, Ni) can be prepared by electrodeposition templated by a liquid crystal.143 Ordered mesoporous Ni/Ni(OH)2 films were synthesized by surface autocorrosion of the above electrodeposited mesoporous Ni, delivering a capacitance of 257 mC cm2.144 Similar ordered mesoporous cobalt hydroxide films were electrodeposited and delivered a capacitance of 2646 F g1.145 Ordered mesoporous NiO films could be prepared by heating the above electrodeposited mesoporous Ni films in air at various temperatures.146 The specific capacitance of the nickel oxide films showed a maximum value of 590 F g1 after annealing at 250 1C for 1.5 h. Chem. Soc. Rev.


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Manganese oxides have also been used widely as supercapacitor materials. Three types of porous MnO2 (ordered mesoporous b-MnO2, disordered mesoporous g-MnO2, and disordered porous a-MnO2) were synthesized using hard template, self-assembly, and co-precipitation methods, respectively.147 The capacitance profiles are dependent on their micropore channels (1 1, 1 2, and 2 2 for ordered mesoporous b-MnO2, disordered mesoporous g-MnO2, and disordered porous a-MnO2, respectively). Ordered mesoporous b-MnO2 showed the best rate capability due to the ordered mesostructure. Similarly, ordered mesoporous Co3O4 nanoarrays prepared with SBA-15 as a hard template provided a capacitance of about 250 F g1, approximately four times larger than that of Co3O4 prepared by direct calcination of Co(NO3)26H2O (58 F g1).148 Mesoporous Co3O4 templated from KIT-6 demonstrated a higher capacitance of 370 F g1 probably due to its larger surface area (185 vs. 83 m2 g1 for the above-mentioned Co3O4 nanoarray).120 Brezesinski et al. used an EISA method to prepare amorphous or crystalline WO3, anatase, a-MoO3, CeO2, etc., and used them as electrodes in supercapacitors.149–153 The charge storage of ordered crystalline mesoporous WO3 was highly reversible, over 95%, and the coloration efficiency was ca. 40 mF cm2.149 They also synthesized mesoporous disordered anatase TiO2 thin films using either sol–gel reagents or preformed nanocrystals as building blocks.150 When mesoscale porosity was created in a TiO2 material with dense walls (rather than porous walls derived from the aggregation of nanocrystals), the insertion capacities were comparable to those of templated nanocrystal films, but the capacitance was much lower.150 Recently, Brezesinski and co-workers found that mesoporous CeO2 films exhibited reasonable levels of pseudocapacitive charge storage and much higher capacities than samples prepared without any polymer template.152 Mesoporous Nb2O5, Ta2O5, solid solutions of Nb and Ta oxides, and a-MoO3 have also been investigated.151,153 The mesoporous a-MoO3 supercapacitor exhibited two charge storage mechanisms, the double-layer and redox pseudocapacitance charge storage.153 Electrochemical measurements were used to calculate capacitance of 605, 330, and 215 C g1 for mesoporous crystalline, mesoporous amorphous, and non-porous crystalline films, respectively (Fig. 9a).153 The capacitive charge-storage properties of ordered mesoporous a-MoO3

films were superior to those of either mesoporous amorphous a-MoO3 or non-porous crystalline a-MoO3.153 The total amount of charge storage (gravimetrically normalized) as a function of charging time for crystalline a-MoO3 was much greater than the corresponding mesoporous amorphous films (Fig. 9b).153 The capacitive contribution for the mesoporous crystalline films was three times larger than that of the mesoporous amorphous material (Fig. 9c).153 Both crystalline and amorphous mesoporous a-MoO3 showed redox pseudocapacitance, whereas the iso-oriented layered crystalline domains enabled lithium ions to be inserted into the van der Waals gaps of the mesoporous a-MoO3. Such intercalation pseudocapacitance occurred on the same timescale as surface redox pseudocapacitance, so the charge-storage capacity was increased without compromising the charge–discharge kinetics.153 3.2 Catalysis Despite the high internal surface areas of mesoporous transition metal oxides and the redox activity of their surfaces, the investigation of ordered mesoporous metal oxides as catalysts or catalyst supports has been limited.154,155 Studies to date have investigated in detail as catalysts in redox reactions, acid catalysis, and photocatalysis. 3.2.1 Redox reactions. CO oxidation is a general probe reaction for the evaluation of metal oxide catalysts due to its simplicity. Mesoporous CeO2 with ordered 3D pore structure replicated from KIT-6 showed high activity for CO oxidation as its T50 (temperature of 50% CO conversion) value was 250 1C, lower than that on the CeO2 prepared by the decomposition of cerium nitrate (T50 = 333 1C).156 CuO loaded mesoporous CeO2 showed even higher activity as the T50 value of the optimized catalyst (20% CuO loading) was only 116 1C. In another work, highly ordered mesoporous Ce1xZrxO2 samples with various Ce/Zr ratios were synthesized using an EISA method.157 Pt nanoparticle loaded mesoporous Ce0.5Zr0.5O2 showed even higher activities in CO oxidation (T50 = 100 1C). Recently, ordered crystalline mesoporous CeO2, Co3O4, Cr2O3, CuO, Fe2O3, b-MnO2, Mn2O3, Mn3O4, NiO, and NiCoMnO4 were investigated as catalysts for CO oxidation.158 Among them, mesoporous Co3O4, b-MnO2, and NiO showed appreciable CO conversions below 0 1C (Fig. 10). Schu¨th and co-workers synthesized three ordered mesoporous Co3O4 with three KIT-6 hard

Fig. 9 Electrochemical characterization of sol–gel derived ordered mesoporous MoO3 films.153 (a) Cyclic voltammograms for different MoO3 systems at a sweep rate of 1 mV s1. A refers to the mesoporous amorphous MoO3 film; C refers to the mesoporous crystalline a-MoO3 films; N is a non-porous crystalline a-MoO3 film. (b) Kinetic behavior of the various mesoporous films. (c) The capacitive contribution to the total charge stored is plotted as a function of charging time for the same films as in b. Reproduced with permission of Macmillan from ref. 153.

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between the Fe2O3 species and the CeO2 support, as well as by the formation of methoxy group intermediates on the CeO2 surface. Ordered mesoporous metal oxides with amorphous walls, prepared by a ligand assisted method, also exhibited activity as a redox active catalyst, for instance, N2 fixation at room temperature.163–168 Commercially, N2 activation in the production of ammonia employs iron oxide based Haber catalysts under high temperatures (400–500 1C) and high pressures (around 200 atm). It is surprising to discover that N2 can be activated at room temperature by reduced mesoporous Nb2O5 obtained by reducing mesoporous Nb2O5 with bis(toluene)Nb.163 The center for N2 activation is Nb2+ confirmed by XPS. H2O readily reacted with the adsorbed nitride (formed after N2 adsorption) to form ammonia. Reduced mesoporous TiO2,164 mesoporous Ta2O5–TiOx oxide composite,168 Fe3+ doped Ta2O5–TiOx composite,166 Ru and Rh doped mesoporous Ta2O5165,167 were also used as catalysts for N2 activation. The other reactions have also been employed for the investigation of the catalytic activity of mesoporous metal oxides, most of which are the total oxidation of organic molecules, e.g. alkane,169,170 alkene,171 formaldehyde,172 acetone,172,173 etc.

Fig. 10 Comparison of CO conversions on different mesoporous metal oxides of morphologies pretreated at 400 1C.158 Reproduced with permission of Springer from ref. 158.

templates that differ in their pore size and surface area (by changing the hydrothermal temperatures of the KIT-6).159 The CO oxidation activity clearly depended on surface areas and pore diameters – the mesoporous Co3O4 with largest BET surface and pore size had the highest activity (specific rate of the catalyst at 20 1C was 0.8 mmol g1 h1 mistakenly calculated as 0.1 mmol g1 h1 in our publication158). Sun et al. also found that mesoporous Co3O4 with a larger BET surface area and larger pores exhibited better CO oxidation performance than the ones with smaller BET surface area and pores.160 Methanol decomposition/oxidation is another popular probe reaction for mesoporous metal oxides. Roggenbuck et al. synthesized mesoporous CeO2 using CMK-3 carbon as a hard template.161 The product exhibited uniform pores with a diameter of ca. 5 nm in a two-dimensional hexagonal periodic arrangement, as well as interparticle porosity, broadly distributed around ca. 35 nm; the specific surface area was 148 m2 g1. The activity of mesoporous CeO2 when used as a catalyst for methanol decomposition was substantially higher than that of a commercial CeO2 with a low surface area of 6 m2 g1. Mesoporous CeO2 or MgO supported iron oxide nanoparticles for methanol decomposition was investigated later.162 The most pronounced catalytic activity was found for Fe/mesoporous CeO2, probably determined by the existence of intensive electron exchange at the interface This journal is

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3.2.2 Mesoporous solid acid catalysts. Ordered mesoporous metal oxides have been used as supports for loading heteropolyacids.174,175 A catalyst consisting of ordered mesoporous Co3O4 and tungstophosphoric acid (H3PW12O40, HPW) was prepared by a hard templating method and used in the direct decomposition of N2O.174 The HPW content could be tuned between 0 and 36%, with the 6%HPW/ mesoporous Co3O4 catalyst reaching the highest activity (N2O conversion of 58%, TOF of B6.5 103 s1) at 400 1C. A series of well-ordered mesoporous alumina–HPW composite frameworks have been prepared.175 The Keggin clusters were incorporated into the mesoporous alumina walls. The chemically bonding of the HPW to the alumina matrix is responsible for the outstanding stability of these materials against water-leaching. The mesoporous surfaces exhibited exceptional acidity that arises from the unique alumina–HPW composite structure. The surface acidity of the composites was enhanced with the increased loading of HPW, as reflected in the higher catalytic activity towards isopropanol dehydration.175 H2SO4 or H3PO4 treated mesoporous Nb2O5 and Ta2O5 were investigated for benzylation reactions.176,177 The acid site density of sulfated mesoporous Nb2O5 was 31.78 mmol g1, higher than that of bulk sulfated zirconia. Ordered amorphous mesoporous Ta2O5 with different pore sizes was prepared by variation of the carbon chain length of the primary alkyl amine templates (C6, C12, and C18). Sulfuric acid treated Ta2O5 samples were used as alkylation catalysts. The 1-dodecylamine templated sample exhibited the highest selectivity for the alkylation of benzene with dodecene, ascribed to shape selectivity.177 In another study, 1-dodecylamine templated mesoporous Ta2O5 treated by sulfuric acid exhibited not only the best catalytic activity but also the highest selectivity in the 1-hexene isomerization to trans-2-hexene.178 After reaction at 70 1C for 4 h, the product ratio of trans-2-hexene/cis-2-hexane was ca. 4 for 1-dodecylamine templated H2SO4/mesoporous Ta2O5, much higher than those of H2SO4/mesoporous Ta2O5 templated by n-hexylamine and n-octadecylamine (less than 1). Chem. Soc. Rev.


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3.2.3 Photocatalysis. In photocatalysis, the catalyst creates electron–hole pairs under light radiation, which in aqueous media generates free radicals (hydroxyl radicals: OH) able to undergo secondary reactions. Photoelectrolysis of water (water splitting) for the production of H2 and O2 by semiconductor metal oxides (e.g., TiO2, Ta2O5, WO3, BiVO4) has been intensively studied for over forty years.179 Kondo and Domen et al. prepared ordered mesoporous Ta2O5 with amorphous and crystalline walls as a water decomposition catalyst under UV irradiation.64,180–186 The water splitting activity of mesoporous Ta2O5 with the amorphous walls was quite low; H2 being generated at a rate of 50 mmol h1 (no oxygen evolution). The activity increased to 150 mmol h1 for H2 and 73 mmol h1 for O2 after loading with NiO (4 wt%).181 Later, ordered mesoporous Ta2O5 with highly crystalline walls was synthesized by strengthening the amorphous framework with a silica or carbon layer (see Section 3.3.1),64,185,186 and the photocatalytic water splitting was improved (30 mmol h1 for H2 and 10 mmol h1 for O2).186 The best photocatalytic activity was achieved on NiO(3.0 wt%)/mesoporous crystalline Ta2O5, with 3360 mmol h1 of H2 and 1630 mmol h1 of O2 (evolution in the first hour).64,186 Crystalline mesoporous anatase was also used as a water splitting catalyst with addition of 0.5% Pt cocatalyst, and the H2 evolution was 5.5 times quicker than bulk anatase under optimal conditions.187 Bare mesoporous metal oxides can also be used as photocatalysts with organic molecules. Mesoporous TiO2 and Nb2O5 with amorphous walls were prepared by a ligandassisted templating method and tested as photocatalysts for oxidative dehydrogenation of 2-propanol to acetone.188 However, the quantum yields of these catalysts (0.0026 and 0.0041, respectively) were lower than that of Degussa P25 (0.45), due to their low crystallinity. A mesoporous anatase carbon composite was used in the degradation of Rhodamine B.189 Photocatalytic activity of anatase with a cubic ordered mesoporous structure and formed as a thin film by an EISA method was tested through UV-induced photo-degradation of methylene blue and lauric acid.190 Crystalline mesoporous anatase was prepared using KIT-6 and SBA-15 as hard templates and exhibited superior photocatalytic activity.191 Using the oxidation of toluene to benzaldehyde in liquid phase as an example, the photocatalytic activity of commercial TiO2 (P-25), polycrystalline TiO2, and single crystalline TiO2 with disordered, 2D ordered, and 3D ordered mesoporous channels were compared (Fig. 11a).191 The single crystalline mesoporous TiO2 material had comparable surface areas (160–180 m2 g1) and pore diameters (3.4–4.5 nm), whereas the polycrystalline sample showed a higher surface area of 200 m2 g1 and a slightly smaller pore diameter of 2.6 nm. The single-crystal like TiO2 samples exhibited a two- to threefold increases of activity relative to that of polycrystalline TiO2 or P-25. To further probe the effect of the crystallinity and porous structure, a larger-size substrate, cinnamyl alcohol, was used. The conversion of cinnamyl alcohol into cinnamaldehyde systematically increased from photocatalyst 1 to 5 (Fig. 11b).191 Conversion of only 20–30% was achieved for commercial P-25 and polycrystalline TiO2, but almost 100% conversion was achieved for 3D single crystalline mesoporous TiO2 (catalyst 5), thus Chem. Soc. Rev.

Fig. 11 (a) Photocatalytic conversions of the oxidations of toluene to benzaldehyde and (b) cinnamyl alcohol to cinnamaldehyde in liquid phase in the presence of (1) commercial P-25 (50 m2 g1), (2) polycrystalline TiO2 (201 m2 g1), and single-crystal-like TiO2 with (3) disordered (180 m2 g1), (4) ordered 2D (179 m2 g1), and (5) ordered 3D (165 m2 g1) mesoporous structure.191 Reproduced with permission of Wiley from ref. 191.

confirming that the 3D mesostructure endows the catalyst with outstanding performance. When loaded with other component, e.g. Au or semiconductor nanoparticles, the photocatalytic activity of mesoporous metal oxide catalysts can be improved. When gold nanoparticles were loaded onto mesoporous TiO2, the catalyst demonstrated better photocatalytic activity than bare mesoporous TiO2.192 The conversion efficiency of phenol oxidation increased continuously from 22% to 95% when the Au content was increased from 0 to 0.5%. CdS quantum dots loaded onto ordered mesoporous TiO2 also showed excellent photocatalytic efficiency for both oxidation of NO in air and degradation of organic compounds in aqueous solution under visible light irradiation.193 3.3 Gas sensing Semiconductor metal oxide-based gas sensors (Fig. 12)194 are mechanically robust, relatively inexpensive and they offer

Fig. 12 Photographs (top) and schematic drawing (bottom) of an example sensor substrate (UST Umweltsensortechnik GmbH, Germany).205 The sensor material is deposited on the interdigitated Pt electrode structure, for example, by drop-coating. Reproduced with permission of Wiley from ref. 205.

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excellent sensing capabilities. When a chemical reaction takes place on semiconductor surfaces, there will be electron transfer between the gas molecules and semiconductor surfaces, resulting in the variation of electric conductivity/resistivity. In this way the gas concentration can be determined. Because gas sensing includes adsorption and catalytic reactions on surfaces, one of the main parameters determining the sensitivity of a semiconductor sensor is its surface area. A series of ordered SnO2 mesopores were prepared using n-cetylpyridinium chloride or polyethylene glycol as soft templates.195–199 Phosphoric acid treatment or addition of mesitylene in the precursor solution improved the thermal stability.197 The H2 sensitivity of the mesoporous SnO2-based sensor was superior to that of a conventional SnO2 sensor.195 The H2 sensitivity of mesoporous SnO2 was largely dependent on its specific surface area, the sensitivity being the highest for the largest specific surface area.196 With the same surface area, larger pores were more sensitive than smaller pores. However, the electrical resistance of mesoporous SnO2 was close to the limit for practical measurements. Coating of mesoporous SnO2 on conventional SnO2 powder improved the gas-sensing properties, while maintaining the sensor resistance in air.197 Rossinyol and co-workers prepared mesoporous WO3 using both SBA-15 and KIT-6 as hard templates.200 The WO3 replica of KIT-6 displayed a higher response rate and shorter response time for NO2 sensing than the WO3 replica of SBA-15 (Fig. 13),201 may be due to the higher surface area and/or 3D pore structure of the KIT-6 templated materials. The introduction of Cr on both of WO3 materials increased the sensitivity and response time. Recently, CaO/mesoporous In2O3 was used as a CO2 sensor.202 The response of CO2 concentrations between 300 and 5000 ppm increased greatly with the introduction of alkaline CaO due to the formation of bicarbonate that induced a large change in the material resistivity. Tiemann and co-workers developed several ordered mesoporous metal oxides using a soft template or hard templating method.203–207 The ordered mesoporous SnO2 templated by CTAB was more sensitive to CO and exhibited much higher stability against the humidity of the test gas than commercial SnO2 based sensors.203 They also prepared ordered mesoporous ZnO and Co3O4 by a hard templating method for CO and/or NO2 sensing.204,205 Mesoporous ZnO templated from a mesoporous carbon (CMK-3) hard template showed a higher sensitivity to both gases as compared with the bulk sensor.204 Mesoporous Co3O4, synthesized by using mesoporous SBA-15 silica as the template, demonstrated higher sensitivity to CO at lower operation temperatures than a nonporous Co3O4 sample.205 Recently, the same authors also prepared ordered crystalline mesoporous In2O3 as a methane gas sensor.206,207 The sensitivity was correlated not only with the surface-to-volume ratio, but also with the nanoscopic morphology (for more details see Section 4.2). 3.4

Adsorption and separation

Ordered mesoporous materials can be used as stationary phases in high performance liquid chromatography208–211 or as adsorbents for heavy metals, anions, organic pollutants, and gases (H2 and CO2).212 For instance, ferrihydrite prepared This journal is

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Fig. 13 (a) Dynamic response and (b) response time of pure SBA-15 and KIT-6 WO3 replicas to different concentrations of NO2 in synthetic air at 230 1C.201 Reproduced with permission of Wiley from ref. 201.

by using silica gel as a hard template is an excellent sorbent for the removal of heavy metals from waste streams.213,214 The pore size and pore uniformity of mesoporous materials may make them suitable for the separation of macromolecules. Given the properties of mesoporous solids as adsorbent and for separation, it is surprising that they have not been reported often. NO shows great promise for the development of new therapies for several human diseases. One particularly promising method for delivering NO locally uses triggered release of gas stored in porous materials.215 We investigated the NO adsorption and release behavior of various mesoporous metal oxides.63 Mesoporous a-Fe2O3, b-MnO2, Mn2O3, Mn3O4, and NiO exhibited little or no adsorption of NO. Mesoporous Cr2O3 and Co3O4 demonstrated modest adsorption capacities (0.12 and 0.58 mmol g1, respectively). On the other hand, the adsorption capacities of mesoporous Cu–Cu2O and CuO were much higher (0.97 and 1.25 mmol g1, respectively). The NO adsorption/desorption isotherms of both CuO and Cu–Cu2O showed strong hysteresis on the desorption branch, indicative of strong interaction between NO and the metal sites (Fig. 14a). Almost all the NO adsorption was retained even when the external pressure of NO was decreased. CO adsorption on mesoporous CuO was also investigated between 30 and 120 1C (Fig. 14b). Type II isotherms are Chem. Soc. Rev.

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4. Relationship between textural properties of the mesopores and their performance The textural properties of mesoporous metal oxides have an impact on their performance in various applications,207 so control of the nanosize/structure in mesoporous metal oxides is important. Below we use several examples to demonstrate the importance of mesostructure controlling for their applications.


4.1 Influence of size on the rate of mesoporous electrodes for lithium batteries

Fig. 14 (a) NO adsorption isotherms on mesoporous Cu–Cu2O and CuO at 25 1C;63 (b) CO adsorption over mesoporous CuO at different temperatures.

observed. The adsorption capacity increased almost linearly when the temperature was increased from 30 to 80 1C, peaking at 80 1C (ca. 0.75 mmol g1), and decreased quickly above 80 1C. This work demonstrated that copper containing porous materials are active for CO adsorption and have potential medical applications.

The influence of the pore size and wall thickness of mesoporous intercalation electrodes on their rate capability was investigated using b-MnO2 as an example.97 First, a series of mesoporous b-MnO2 materials, with pore sizes ranging from 3.4 to 28 nm, and wall thicknesses from 4.7 to 30 nm were prepared. Although Li cannot be intercalated into bulk (mm) b-MnO2, it can be intercalated into the wall of the mesoporous b-MnO2 (rutile structure). The first Li intercalation is associated with a two-phase reaction (tetragonal to orthorhombic phase) while sequent cycling of Li intercalation occurred within a single phase (orthorhombic).97 By synthesizing mesoporous b-MnO2 materials with a range of wall thicknesses from 5.0 to 8.5 nm while maintaining pore size at 3.4 0.1 nm, it was shown that the thinner walls permitted higher rate capability of the materials investigated for 2-phase intercalation (Fig. 15a).97 Similarly, the rate capability increased with increasing pore diameter (from 3.4 to 28 nm) for 2-phase intercalation, consistent with the expectation that larger pores permit more facile Li+ transport. For the single phase intercalation little variation of rate capability with variation of pore size and wall thickness was observed.97 It was observed that the first discharge capacities (amount of Li intercalation) increased with increasing wall thickness (from 4.7 to 8.5 nm), then decreased somewhat until remaining constant (pore width > 10 nm, Fig. 15b). A similar behaviour was disclosed for mesoporous b-MnO2 with a 1D pore structure prepared from SBA-15, for which capacities increased from 225 mA h g1 (wall thickness 7.5 nm) to 265 mA h g1

Fig. 15 (a) The first discharge capacity for different mesoporous b-MnO2 electrodes as a function of rate. Note the capacity is expressed as a percentage of the first discharge capacity at the lowest rate (15 mA g1).97 (b) Variation of the first discharge capacity as a function of mesoporous b-MnO2 wall thickness. Reproduced with permission of the American Chemical Society from ref. 97.

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(wall thickness 10.1 nm) at a low rate of intercalation (15 mA g1). The explanation lies in the relative proportion of ‘‘bulk’’ and ‘‘near surface’’ regions within the walls of the mesoporous solids and that the latter is a poor host for Li. The thinnest pore wall b-MnO2 has the largest proportion of distorted ‘‘near-surface’’ structure compared with ‘‘bulk’’ structure so it has the lowest first discharge capacity. When the wall thickness is over 10 nm, a further increase in wall thickness does not increase the capacity, attributed to the fact that as the walls become thicker the proportion of ‘‘bulk’’ structure dominates and hence the capacity saturates. 4.2 Sensing activity of mesoporous metal oxides— consideration of mesoporosity parameters (surface area, pore diameter, and wall thickness) 2D and 3D mesoporous WO3 were obtained by using 2D SBA-15 (P6mm) silica or 3D KIT-6 silica (Ia3d) as hard templates.48,79,119,200 When mesoporous WO3 was used as sensing materials for NO2,201,216 3D WO3 displayed a higher response rate and a lower response time to NO2 than the SBA-15 replica. The higher surface area of the KIT-6 replica and its 3D mesostructure may account for such a behavior.201 Recently, Tiemann et al. prepared 2D and 3D ordered crystalline mesoporous In2O3 as methane gas sensors.206,207 The sensitivity correlated with the surface-to-volume ratio and the nanoscopic morphology. Fig. 16a demonstrates a linear correlation between the CH4 sensitivity and the surface area of mesoporous In2O3. The influences of pore diameters and pore wall thickness on CH4 sensitivity (surface normalized) were investigated (Fig. 16(b and c)). Since both the pore width and wall thickness changed concurrently, it is difficult to determine which factor is decisive. The results from theoretical studies

suggested that the wall thickness rather than the pore diameter was the determining factor. The sensitivity dependence on mesostructure parameters of the mesoporous In2O3 materials opens up a new avenue for creating sensors with tailored properties.207 4.3 Correlation between nanoscale structure and magnetic properties There are a few reports concerning the magnetic properties of ordered mesoporous metal oxides,51,67,79,100,120,217–223 but the methods (including the variation of the phase composition and calcination temperatures) used for tuning the magnetic properties have been limited.79,220,221 Ordered mesoporous a-Fe2O3 samples with single crystalline walls (calcined at 600 1C for 6 h) or polycrystalline walls (calcined at 500 1C for 3 h) were prepared.79 The magnetic behavior of mesoporous a-Fe2O3 with single crystalline walls is unique. Although the wall thickness was less than 8 nm, it had longrange magnetic ordering similar to bulk a-Fe2O3. However, the confined dimensions were sufficient to suppress the Morin transition (a magnetic phase transition in bulk (mm) a-Fe2O3 hematite where the antiferromagnetic ordering re-organizes from being aligned perpendicular to the c-axis to be aligned parallel to the c-axis below the Morin transition temperature) present. The magnetic behavior of mesoporous a-Fe2O3 with polycrystalline walls (ca. 6 nm) was consistent with that of a-Fe2O3 nanoparticles of less than 8 nm, that is, no long-range magnetic order, absence of a Morin transition, and the presence of superparamagnetic behavior. A series of mesoporous NiO–NiCo2O4–Co3O4 composites were synthesized by using SBA-15 silica as a hard template.220 Their magnetic properties, e.g., saturation magnetization and coercivity, can be easily tuned given the ferrimagnetic (NiCo2O4) and antiferromagnetic (NiO and Co3O4) character of the constituents. Quickel et al. prepared a series of mesoporous CoFe2O4 thin film using KLE as a soft template.221 The domain sizes of the crystallites were tunable from 6 to 15 nm by variation of annealing temperature, a control which comes at little cost to the ordering of the mesostructure. Increases in crystalline domain size could directly be correlated with increases in room temperature coercivity. The room temperature coercive widths increased as the domains grow from 700 Oe at 500 1C to 1200 Oe at 700 1C.

5. Summary and perspective

Fig. 16 Correlation between the sensor signals and structural parameters of the In2O3 samples: (a) sensitivity vs. specific BET surface area; (b) surface-normalized sensitivity vs. average pore width; (c) surface-normalized sensitivity vs. average pore wall thickness.207 Reproduced with permission of Wiley from ref. 207.

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The introduction of mesoporous silicas in the 1990s opened a new chapter in materials. Although the synthesis of mesoporous metal oxides is generally more demanding than that of mesoporous silicas, they can exhibit unique physicochemical properties and therefore a wide range of applications. In recent years, a variety of mesoporous metal oxides have been synthesized by soft and hard templating methods,10,15 and their properties and applications investigated. The present review focuses on ordered mesoporous metal oxides; disordered mesoporous metal oxides and ordered non-oxide mesostructures, including CdS,224 WS2,225 MoS2,225 CoN,226 CrN,226 InN,227 GaN,228 and carbon,17–20,46,92,229–231 are not covered. The hard templating method has been used extensively and successfully Chem. Soc. Rev.


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to synthesize ordered metal oxide mesopores but it is not a direct one-step method, the preparation is expensive, and difficult to scale up. This encourages investigation of alternative synthesis methods in future work. Also, the thermal stability of most pure mesoporous metal oxides (except mesoporous Nb2O5 and Ta2O5) is generally poor such that these metal oxides cannot be used as long-life catalysts at high temperatures (e.g. >500 1C). These shortcomings limit their potential applications. This is probably why there are relatively few reports evaluating long lifetime catalytic stability of mesoporous metal oxide catalysts.172,232,233 An alternative is to prepare mesoporous silica supported metal oxides directly.234 Of course, there are many applications near room temperature (e.g., NO release, Li ion battery) for which thermal stability is not a problem. The review highlights that the pore size, pore structure, crystallinity, and pore volume can all influence the properties in applications, ranging from the Li-ion battery, through catalysts, sensing, and adsorption to magnetic devices. A logical consequence of the significance of texture is the importance of preparing mesopores containing extra-large pores or having pores in multi scales. For example, Sonnauer et al. reported a mesoporous MOF exhibiting giant cages with diameters of up to 4.6 nm, although the windows connecting these cages are much smaller with dimensions of about 2.0 nm.235 Other Covalent Organic Frameworks (COF) with mesopore dimensions have also been described.236 There is intense interest in zeolites or silicas with micro–meso dual porosity. Recently, there has been a report of ordered mesoporous zeolites with ordered microporous walls.237 The challenge is to prepare ordered crystalline metal oxides with micro–meso dual/multi porosity, which will have interesting magnetic, catalytic, electronic, and adsorption properties in a single material. The next decades of the 21st century will definitely see extensive new development in these directions.

Acknowledgements Y. Ren is indebted to Shenhua Group for financial support. Z. Ma is grateful for the financial support by National Natural Science Foundation of China (Grant No. 21007011 and 21177028) and Doctoral Fund of Ministry of Education in China (Grant No. 20100071120012). P. G. Bruce is grateful to the EPSRC including SUPERGEN and the programme grant ‘‘Nanoionics’’ for financial support. We also thank Prof. Ricardo Morales and Dr Edward Crossland for useful comment.

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