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Changes in the structure and porosity of hollow spherical allophane under ... Calcination-induced changes in structure, morphology, and porosity of al...
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Synthesis, Self-Assembly, Transformation, and Functionalization of Nanoscale Artificial Allophane Spherules for Catalytic Applications Yao Zhou and Hua Chun Zeng* NUS Graduate School for Integrative Sciences and Engineering and Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore S Supporting Information *

ABSTRACT: Mesoporous materials with large surface area and chemical inertness are of great importance, and currently prevailing synthetic approaches involve usages of micelles as pore-directing agents to create such mesopores. In this work, allophanes, which are hollow aluminosilicate spherules of 3.5−5.5 nm in size, have been synthesized and assembled simultaneously for the first time in a controlled manner to generate mesoporous spherical allophane assemblages (MSAAs) with diameters of 445 ± 40 nm, specific surface area as high as 1032 m2/g, pore volume 1.104 mL/g at P/P0 = 0.975, and average mesopore size at 3.4 nm. Furthermore, the thus-prepared MSAA could be doped with transition metal ions to create a series of isomorphous derivatives; they could also be converted to aluminum-based hierarchical assemblages of layered double hydroxide easily. Different from the conventional channel-like mesopores, the new mesoporosity attained in MSAA is easily accessible because their mesopores are generated from the interparticle spaces of spherical building units of hollow spherules. Therefore, the mesoporous MSAA provides an excellent platform for construction of integrated nanocatalysts. Highly dispersed noble metal nanoclusters such as Pt, Au, and Pd could be deposited on the surface or in the interior mesopores of the MSAA. Excellent activity and stability of MSAA-based catalysts for Suzuki couplings and electrochemical sensing of H2O2 have been demonstrated using Pd/MSAA and Au/MSAA nanocomposites, respectively.

1. INTRODUCTION Mesoporous nanomaterials have found important applications in catalysis, separation, and drug delivery etc.1−4 In most of these applications, mesopores are created using soft or hard templates as pore-directing agents.2,3,5−7 In addition to the use of costly soft or hard templates, thus-obtained mesopores are often well-ordered and cylindrical in shape and are susceptible to pore plugging during the loading of the active components. On the other hand, creation of mesopores from stacking of repetitive nanoscale building blocks, that is, through selfassembly processes, represents an effective approach toward making hierarchical structures with well-defined architecture, relatively large geometric dimensions, and interconnected mesopores because such processes can be surfactant-free.8,9 Nevertheless, the self-assembly approach requires availability of uniform primary building blocks and strict controls over their self-assembly behaviors. As a consequence, there are only a few successful examples in this regard so far.8,9 In a closely related issue, aluminosilicates, as one of the most abundant clays on earth, have been fabricated into nanostructures with various structural merits and demonstrated a wide range of applications owing to their high surface areas and chemical inertness.10−14 In particular, allophane is a type of hydrous aluminosilicate originally derived from volcanic ash and weathered pumice.15,16 The morphology of allophane is characterized by their numerous hollow spherules with external diameters 3.5−5.5 nm and wall thickness of 0.7−1.0 nm.16−19 © 2017 American Chemical Society

The walls of the allophane spherules are perforated, with pore diameters around 0.3 nm, and on these wall perforations there are rich aluminol groups (i.e., Al−OH, Al−OH2) leading to their pH-dependent charge characteristics.20,21 Owing to their large specific surface areas, excellent stability, and active surface properties, allophanes have long been used as adsorbents for adsorption of various cations, anions, and organics.20−23 There are also numerous studies where allophanes are used for heterogeneous catalysis22,24−28 as well as for other technological applications.29 Conceptually, in view of their uniform shape, size, and inherent acidity, individual hollow allophane spherules could serve as primary building blocks for construction of mesoporous support in a self-assembly manner. Nonetheless, artificial allophanes are often synthesized using coprecipitation of alumina and silica from their respective inorganic sources or organic precursors.17,22,30 With limited control over their solidification kinetics, such synthetic allophanes often exist as large flocculates with irregular morphologies of micrometers in size. In this work, as depicted in Scheme 1, starting from the aluminum carboxylate polymeric spheres (Al−P1), we report simultaneous synthesis and assembly of the allophanes, which Received: May 11, 2017 Revised: June 16, 2017 Published: June 19, 2017 6076

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Chemistry of Materials Scheme 1. Process Flowchart of This Worka

down in laboratory air, the resulting precipitate was washed with ethanol and dried at 80 °C. 2.4. Synthesis of MSAA. Thirty milligrams of Al−Si−P2 and 120.0 mg urea were mixed well in 40.0 mL of deionized water, which was then transferred to a Teflon-lined stainless steel autoclave and kept at 150 °C for 15 h. After cooling down in laboratory air, the resultant precipitate was washed with water and dried at 80 °C to give the pristine MSAA. The pristine MSAA was kept in air at 550 °C for 4 h with a ramping rate of 2 °C/min to give the calcined MSAA. Different reaction conditions were also used in our control experiments. For instance, γ-Al2O3 or calcined Al−P1 was used to replace the original Al−P1, with detailed reaction conditions given in Supporting Information. 2.5. Doping MSAA with Transition Metal Ions. Six milligrams of the calcined MSAA, 28.0 mg of urea, 2.0−5.0 mL of NH4Cl (0.10 M), and 0.2 mL of trisodium citrate (0.10 M) were mixed in 10.0 mL of deionized water, into which 20−120 μL of NiSO4 (0.10 M) or 20− 80 μL of CoCl2 (0.10 M) was added; the mixture was transferred to a Teflon-lined stainless steel autoclave and was kept at 120 °C for 10 h. 2.6. Synthesis of LDH Assemblages from MSAA. Six milligrams of the calcined MSAA was dispersed in 10.0 mL of deionized water, into which 15.0−40.0 mg of urea and 20−200 μLof NiSO4 (0.10 M) or CoCl2 (0.10 M) were added and mixed. Subsequently, the mixture was transferred to a Teflon-lined stainless steel autoclave and was kept at 120 °C for 10 h. 2.7. Deposition of Noble Metal NCs on MSAA. For deposition of PtNCs, 1.0 mL of K2PtCl4 (2.0 mM) was mixed with 20.0 mL of aqueous solution of the calcined MSAA (0.25 g/L); after stirring for 2 min, the mixture was kept overnight at 80 °C. The Pt/MSAA nanocomposite was washed with water and dried at 80 °C. For deposition of AuNCs, alkaline solution of thiourea was first prepared by mixing thiourea (120 μL, 0.25 M) and NaOH (10 μL, 15 M) in 10.0 mL of deionized water. Then 1.2 mL of HAuCl4 (1 mM) was added into 20.0 mL of aqueous solution of the calcined MSAA (0.25 g/L), followed by addition of 0.5 mL of the above thiourea alkaline solution. After stirring for 2 min, the mixture was kept overnight at room temperature and then kept at 80 °C for 1 h. The Au/MSAA nanocomposite was washed with water and dried at 80 °C. For deposition of PdNCs, PdCl2 (50−100 μL, 10−50 mM) was mixed with 10 mg of the calcined MSAA. After being sonicated for 0.5 h, the mixture was then dried at 100 °C for 1 h and then calcined at 350 °C in air for 2 h. Subsequently, the nanocomposite was reduced in H2 at 200 °C for 2 h with a ramping rate of 2 °C/min to give the Pd/ MSAA nanocomposite. 2.8. Applications of M/MSAA Nanocomposite. Suzuki coupling reaction was selected as a model reaction to test the activity and stability of the Pd/MSAA nanocatalyst. Briefly, ethanol (10.0 mL), idobenzene (0.5 mM), phenylboronic acid (1 mM), K2CO3 (2 mM), n-dodecane as internal standard (0.5 mM), and 10 mg Pd/MSAA with 0.76% Pd loading were mixed and magnetically stirred in a glass reactor at 85 °C for 25 min. The mixture was then centrifuged at 6000 rpm for 8 min to separate the catalyst. The supernatant was analyzed by GC (Agilent-HP capillary column, Helium as carrier gas, initial temperature 70 °C, final temperature 250 °C, ramping rate 20 °C/min, FID temperature 280 °C) to calculate the yield. The catalyst, which was recovered by washing with copious water and ethanol, was recycled under the same reaction conditions for six times. H2O2 electrochemical sensor was also constructed using Au/MSAA nanocomposite. Briefly, 2.4 mg of the as-prepared Au/MSAA nanocomposite (Au% wt = 4.5%) was dispersed in 100 μL of 0.5 wt % Nafion aqueous solution; then 4 μL of such Au/MSAA solution was dropped on the polished GCE. After 1 h, another 4 μL of the Au/ MSAA was added, which was then dried overnight at ambient conditions. Electrochemical measurements were conducted using a computer-controlled potentiostat (Autolab, PGSTAT 302N) with a standard-three-electrode configuration. The counter and reference electrodes were Pt gauze and Ag/AgCl in saturated KCl, respectively. The Au/MSAA modified GCE as the working electrode was first subjected to cyclic voltammetry scan within −0.6−0.4 V (vs Ag/AgCl) for 30 cycles at 50 mV/s in 10.0 mL of PBS (pH = 7.0, purged with Ar

a (a) Al−P1, amorphous Al-carboxylic polymeric spheres; (b) Al−Si− P2, intermediate amorphous aluminosilicate solid spheres; (c) MSAA, mesoporous spherical allophane assemblages; (d) allophane spherule; (e) metal nanoclusters (NCs) deposited (g) in the internal mesopores and (f) on the external surface of the MSAA; and (g) MSAA doped with transition metal ions.

generate mesoporous spherical allophane assemblages (MSAAs) with well-defined shapes, narrow size distribution, high surface areas, and interconnected mesopores. The MSAA could be either isomorphously doped with different transition metal cations or be converted to hierarchical assemblages of another important class of clays, layered double hydroxides (LDHs). Because of their unique physicochemical surface properties, various noble metal nanoclusters (NCs) could be facilely loaded onto the surface or into the interspherule space of MSAA via surface modification-free and surfactant-free approaches, resulting in M/MSAA. To elucidate their catalytic activity and stability, the prepared catalysts were tested in liquid phase reactions and electrochemical sensing.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Aluminum tri-isopropoxide (≥98%), diethylene glycol (DEG), tetraethyl orthosilicate (TEOS), urea, K2PtCl4 (98%), HAuCl4·3H2O (>99.99%), NiSO4·6H2O, and trisodium citrate dihydrate were from Sigma-Aldrich; thiourea (>99%) and perfluorosulfonic acid−PTFE copolymer (5% w/w, Nafion) were from Alfa Aesar; NH4Cl was from BDH; CoCl2.6H2O was from Fluka; ethanol (analytic grade), acetone (analytic grade), H2O2 (30%), phosphate buffer solution (PBS, pH = 7.0), concentrated HNO3 (65%), and NaOH (>99%) were from Merck; and deionized water was collected through the Elga MicroMeg purified water system. 2.2. Synthesis of Al−P1 Spheres. The Al−P1 spheres were prepared according to our previous work with slight modification.31 Eighty milligrams of aluminum isopropoxide was added into 5.0 mL of DEG, followed by addition of 35.0 mL of acetone and 2.0 mL of concentrated HNO3 (65%); the mixture was stirred to get a clear solution, which was transferred to a Teflon-lined stainless steel autoclave and kept at 110 °C for 10 h and then at 140 °C for another 8 h. After cooling down in laboratory air, the resultant precipitate was washed with ethanol for four times and dried at 80 °C. 2.3. Synthesis of Al−Si−P2 Spheres. Twenty milligrams of the Al−P1 spheres together with 85.0 mg urea was mixed well in 20.0 mg of H2O and 12.0 mg of ethanol; toward the mixture, a 100 μL mixture of TEOS and ethanol (volume ratio 1:1) was added. After magnetically stirring for 5 min, the mixture was transferred to a Teflon-lined stainless steel autoclave and kept at 150 °C for 15 h. After cooling 6077

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Chemistry of Materials gas for 0.5 h; similarly thereafter). Cyclic voltammograms without or with 2.6 mM H2O2 in PBS were obtained. The chronoamperometric response of the working electrode to different concentrations of H2O2 was measured in 10.0 mL of PBS at constant potential of −0.3 V (vs Ag/AgCl). At every ∼2 min, an aliquot of freshly prepared H2O2 with a concentration of 5.0 mM was injected successively into the PBS. Note that at the first time 50 μL of H2O2 was added, which then was increased to 0.1 mL, 0.2 mL, 0.4, and 0.6 mL gradually. Baseline with steady-state-signal was obtained prior to injection of H2O2 and the solution was mildly stirred. 2.9. Characterization Methods. Morphologies of the prepared samples were observed by transmission electron microscopy (TEM, JEM-2010, 200 kV), high-resolution TEM (HRTEM, JEM-2100F, 200 kV), and field emission-scanning electron microscopy (FESEM, JSM6700F). Structural/bonding information was gathered by Fourier transformed infrared spectroscopy (FTIR; Bio-Rad FTS-3500ARX). Crystallographic information was obtained by powder X-ray diffraction (D8 Advanced, Bruker, Cu KR radiation at 1.5406 Å). X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical) analysis was conducted using a monochromatized Al Kα exciting radiation (hν = 1286.71 eV) with a constant analyzer-pass-energy of 40.0 eV. All binding energies were referenced to C 1s peak (BE set at 284.5 eV) arising from C−C bonds. Compositional determination based on the energy-dispersive X-ray (EDX) technique was an average result from six different areas of a sample. N2 adsorption−desorption experiment at 77.3 K was carried out using a surface area and pore size analyzer (Quantachrome Instruments NOVA 4200e). Prior to this study, samples were degassed overnight in flowing N2 at 150 °C. To determine its surface charge, calcined MSAA support was first dispersed in deionized water with different pH values, and then the charge measurement was performed in a zeta-potential analyzer (Zetasizer Nano-ZS, Malvern Instrument).

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of MSAA. Shown in Scheme 1 and Figure 1a, different from many metal silicates, which often start from SiO2 precursors,8,32 herein, aluminum carboxylate polymeric spheres (Al−P1) with a diameter of 320 ± 42 nm31 were used as the starting Al source and the template. The Al−P1 spheres are amorphous31 and potentially reactive (which deteriorates under a focused electron beam during our TEM observation). They are susceptible to further compositional and structural transformations. Depicted in Scheme 1b, after being treated with TEOS and urea at 150 °C for 15 h, Si was incorporated homogeneously into the Al−P1 sphere, forming aluminosilicate intermediate (Al−Si−P2, Figure 1b,c), as confirmed by element mapping and EDX analysis (Figure S1). The Al−Si−P2 spheres have an average diameter of 432 ± 40 nm (Figure 1b,c), which are evidently larger than the Al−P1 spheres, noting that their spherical morphology is still maintained. The surface of Al−Si−P2 looks smooth under TEM observation (Figure 1d). When the Al−Si−P2 was further hydrothermally treated with urea at 150 °C for 15 h, MSAAs with an average diameter close to that of Al−Si−P2 were formed (Scheme 1c). Shown in Figure 1e−j, different from Al− P1 and Al−Si−P2, the MSAA spheres look fluffy and have a rough surface due to structural transformation to the phase of allophane. At high resolutions, numerous small particulates with diameters around 5.0 nm could be clearly identified within each MSAA sphere (Figure 1j), and they show a higher image contrast at the boundary than at their central part, which reveals the nature of a hollow interior in each spherule (Figure 1k,l and Figure S2; also see latter sections). The observation of these tiny hollow spherules of aluminosilicate is a direct proof of formation of allophanes.17 The hollow spherules collapse under long exposure to an electron beam at high magnification.17 The

Figure 1. TEM and FESEM images of (a) the as-prepared amorphous Al−P1 spheres; (b−d) the intermediate Al−Si−P2 spheres; (e−j) pristine MSAA spheres; (k,l) assembled and freestanding hollow spherules of allophane observed at high resolutions; and (m) element mappings of Si, Al, and O in two MSAA spheres.

as-prepared MSAA samples are light brown in color due to the organic residues from Al−P1. After being calcined at 550 °C in air for 4 h, the MSAA become white in color, and their morphology and hierarchical assembly organization are well preserved after calcination at even higher temperature, for 6078

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Chemistry of Materials example, at 650 °C, demonstrating high thermal stability (Scheme 1c, Figure S3). Our EDX analysis confirms the chemical composition of the as-prepared MSAA (Figure S4), as the elemental mappings reveal homogeneous distribution of Al, Si, and O in the MSAA (Figure 1m). On the basis of the EDX data, the formula of the calcined MSAA can be denoted as Al2O3·(SiO2)1.36·(H2O)x (Figure S4). As shown in Figure 2a,

of 1.104 mL/g obtained at P/P0 = 0.975. In the pore size distribution profile, a pronounced peak is centered at 3.4 nm (Figure 2b). Because of the generation of such mesopores, importantly, the specific surface area of our MSAA sample is much larger than most of unorganized allophane agglomerations reported previously (which is often below 600 m2/g and very occasionally above 800 m2/g).20,27 Such a high specific surface area in our MSAA also verifies a thorough structural conversion of the aluminosilicate intermediate solid (i.e., Al− Si−P2) to hollow allophane spherules. As a type of amorphous aluminosilicate,17 XRD pattern of allophanes normally has a major band centered at 26−27° and two weak ones at around 40.0° and 66.5°. Consistently, the XRD pattern of our calcined MSAA also exhibits these three bands, with a major one at 24.8° and two others at 39.5° and 66.4°, respectively (Figure 2c-iii, the red curve). Compared with the pristine MSAA, which has a major band centered at 27.3° (Figure 2c-ii), the band shift to 24.8° in the calcined sample is known to be due to low Si/Al ratios or associated with the polymerization of silica component.33 The as-prepared MSAA spheres can be dispersed uniformly in water, forming a stable opalescent colloid. Surface properties which regulate the interaction between allophanes and external reactants are investigated with zeta-potential measurement. Because of the presence of aluminol groups, allophanes could either acquire or lose protons in response to pH changes.20,27 As can be seen, the point of zero charge of the calcined MSAA is found at pH = 4.5. Therefore, these MSAA spheres are positively or negatively charged below or above this pH value (Figure 2d). 3.2. Formation Mechanism of MSAA. To explore the formation mechanism of MSAA, the Al−Si−P2 intermediate, the pristine MSAA, and the calcined MSAA samples were characterized. Displayed in Figure 2e-i, FTIR spectrum of Al− Si−P2 (the red curve) reveals affluent presence of carboxylic groups. The bands at 1698 and 1295 cm−1 are due to stretching vibrations of CO and C−O of carboxylic groups, respectively (Figure 2e-i), both of which disappeared in the pristine and the calcined MSAA samples (Figure 2e-ii and iii). The intensive bands at 3128 and 1396 cm−1 are due to O−H stretching and bending vibrations of carboxylic groups, respectively. These two bands were weakened in the pristine MSAA (Figure 2e-ii), and they disappear almost completely in the calcined MSAA (Figure 2e-iii). The FTIR result reveals that a large portion of the carboxylic groups were removed during the conversion of Al−Si−P2 to allophanes. Presence of water was also observed in all the samples, as the band centered at 3451 cm−1 is due to O−H stretching of structural hydroxyl moiety and adsorbed water, and the bands at 1633 cm−1 are ascribed to O−H bending vibration of adsorbed water.27 Moreover, FTIR spectra of all the three samples (Figure 2e) demonstrate the presence of aluminosilicate. Specifically, the intensive band centered at 1050−1010 cm−1 is assigned to Si−O−Si stretching vibrations modified by tetrahedral Al in the framework; the position at 1050−1010 cm−1 indicates a high degree of substitution of Si with tetrahedral Al in the framework as Si−O−Si stretching vibrations in the absence of Al occurs at around 1100 cm−1.17 Consistently, the shoulder band at 880 cm−1 of the calcined MSAA (Figure 2e-iii) is attributed to stretching vibration of Si− O−(Al).30 The band at 567−571 cm−1 is ascribed to the vibration band of Al−O−Si stretching, which also verifies the corner linkage of [AlO4] and [SiO4] units in the aluminosilicate.34 In general, the FTIR spectra of our MSAA samples are in good agreement with those of natural or synthetic allophanes

Figure 2. (a) N2 adsorption−desorption isotherms; (b) volumetric pore size distribution (using the BJH method based on the desorption data) of the MSAA calcined at 550 °C in air for 4 h; (c) XRD pattern of (i) the Al−Si−P2, (ii) the pristine MSAA, and (iii) the calcined MSAA; (d) zeta potentials of the calcined MSAA versus pH values; and (e) FTIR spectra of (i) the Al−Si−P2, (ii) the pristine MSAA, and (iii) the calcined MSAA.

the adsorption of N2 to the MSAA increases almost linearly over the full measured range of P/P0. Therefore, this isotherm could not be classified into one of the six types of isotherms nomenclatured by IUPAC. However, an evident hysteresis loop is observed within the range of relative pressure (P/P0) from 0.4−0.6, proving the existence of mesopores in the MSAA (Figure 2a). The shape of the hysteresis loop belongs to the H2 type (IUPAC), which is associated with ink-bottle-like pores or voids between close-packed spherical particles. The deduced pore structure is also consistent with our TEM observation that the mesopores are originated from the assembled spherules in the MSAA. On the basis of the adsorption isotherm, the specific surface area (Brunauer−Emmett−Teller (BET) method) of the calcined sample is as high as 1032 m2/g, with the pore volume 6079

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Chemistry of Materials reported in literature, which show five major bands centered at 3451, 1633, 1044, 571, and 462 cm−1.24,33 The formation process was also investigated through XRD analysis. The XRD pattern of Al−Si−P2 has a major intensive band centered at about 25° (Figure 2c-i, the greenish-brown curve); some weak peaks of boehmite (AlOOH) phase were also present. In the absence of TEOS, AlOOH crystals were formed when Al−P1 was hydrothermally treated with only urea or mixture of urea and transition metal cations (Figure S5). This is because the polymeric precursor, Al−P1 (i.e., aluminum carboxylate), was attacked by OH− groups generated from urea hydrolysis and resulted in the boehmite phase eventually. When Al−P1 was treated with urea and TEOS (step 1 in Scheme 1), hydrolysis of TEOS in alkaline conditions gave rise to various anionic products such as [SiO4]4− (or more generally [Si(OH)4−nO n]n−, n = 0 and 1−4). At the same time, detachment of Al3+ (or depolymerization) from its organic ligand to form [Al(OH)4]− would take place in the Al−P1 matrix because of the basic medium. Condensation reaction between [Al(OH)4]− and [Si(OH)4−nO n]n− would then result in the Al−Si−P2. Interestingly, the intermediate spheres sampled at 2.5 and 5 h during the conversion of Al−P1 to Al−Si−P2 have an average size falling between the sizes of Al− P1 and Al−Si−P2 (Scheme 2b,c). The usage of Al−P1 is crucial for the formation of intermediate Al−Si−P2. As a comparison, when the amorphous Al−P1 was replaced by the commercial γ-Al2O3, no allophanes could be formed under the same condition (Figure S6). When the as-prepared Al−P1 was calcined (e.g., at 250 °C for 4 h) and then treated identically, only sparse allophanes spherules were formed on the surface of the resulting spheres (Figure S7). As a coordination polymer of aluminum carboxylate,31 the extensive presence of carboxylate groups makes the Al−P1 colloidal spheres amorphous and “soft”, which allows OH− and [SiO4]4− to penetrate deeply into the interior of the Al−P1 spheres and incorporate Si homogeneously. On the other hand, the resultant Al−Si−P2 (Figure S1) is also a key step toward the formation of MSAA. For comparison, SBA-15 and Al(NO3)3 were used as sources of silica and aluminum, respectively, which generates no allophanes under similar reaction conditions (Figure S8); only few allophane spherules could be found (Figure S9) when surfactants such as CTAB or Pluronic F-127 were introduced during the conversion of Al− P1 to Al−Si−P2, as in these processes TEOS partially condensed on the micelles (formed by these surfactants) as SiO2, consequently interfering the incorporation of Si into the Al−P1. The above comparative experiments illustrate the uniqueness and the critical role of our Al−P1 as both the source of Al and the template for simultaneous synthesis and selfassembly of allophanes. The morphological transformation from Al−Si−P2 to allophane spherules has been clearly demonstrated in Figure 1, though trace AlOOH impurity was still present in the pristine MSAA (Figure 2c-ii). Since the pristine MSAA spheres have almost the same size as the Al−Si−P2 intermediate, we speculate that the conversion of Al−Si−P2 to MSAA involves on-site structural transformation. TEM observation of the intermediate products during the conversion of Al−Si−P2 to the pristine MSAA confirms this speculation. At 2.5 h, the spheres well maintained their original size; nonetheless, they seem to become porous (Scheme 2d). At 5.0 h, most particles become quite perforated and porous (Scheme 2a-ii-1,e), and some allophane spherules could be spotted occasionally at this

Scheme 2. Process Flowchart and TEM Images of Intermediates during Conversiona

a (a) Process flowchart to convert (i) the Al−P1 to (ii) the Al−Si−P2 and then to (iii) the pristine MSAA (see Scheme 1): (i-1) Al−P1 sphere attacked by [SiO4]4− and OH− ions, (ii-1) a perforated Al−Si− P2 sphere with tiny gas bubbles within its pores; TEM images of intermediates sampled at (b) 2.5 h and (c) 5.0 h during the conversion of Al−P1 to Al−Si−P2; and TEM images of intermediates sampled at (d) 2.5 h and (e) 5.0 h during the conversion of Al−Si−P2 to the MSAA.

stage (Scheme 2e). During this process, the Al−Si−P2 spheres are etched by OH− released from urea hydrolysis, due to which the original aluminosilicate structure is partially depolymerized into aluminosilicate fabrics or even active monomers,11 forming the perforated intermediate spheres in Scheme 2e. More importantly, such perforated intermediates possess extensive interstitial spaces and solid−liquid interfaces which provide numerous sites for evolution of nanoscale gaseous bubbles (Scheme 2a-ii-1). The gas bubbles could serve as the template for the local rearrangement of the aluminosilicate fabrics. Our group reported the synthesis and assembly of Mn-silicate nanobubbles starting from SiO2 nanospheres and proved that the gas-forming species are necessary for the formation of such nanobubbles at hydrothermal conditions.8 In the present synthesis, a strong smell of ammonia was sensed due to hydrolysis of urea after the hydrothermal reaction. Moreover, the rich presence of caboxylate groups within Al−Si−P2 could also release CO2 gas bubbles. On this basis, we believe that the 6080

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Chemistry of Materials building up of the gas bubbles at the solid−liquid interface within the perforated Al−Si−P2 likely serves as the template for local rearrangement or recondensation of aluminosilicate fabrics (Scheme 2a-ii-1), leading to the formation of the small hollow spherules within each MSAA. To test this hypothesis, urea was replaced by NaOH alkaline solution to react with Al− Si−P2, the precursor particles were etched and became porous, but no hollow spherule was formed (Figure S10). The above proposed formation mechanism seems to be consistent with the fact that the natural allophanes are normally derived from volcanic ash or weathered pumice.15,16 3.3. Synthesis of Allophane-Based Derivatives by Reacting MSAA with Transition Metal Ions. Isomorphous doping enables modification of chemical composition and properties of solid matters, whereas it executes little impact on the original structure.32 Herein, doping the MSAA with other transition metal ions can be achieved (Scheme 1g) through hydrothermal treatment of the mixture of MSAA, urea, transition metal ions such as Co2+ or Ni2+, NH4Cl and sodium citrate. During the doping process, urea was hydrolyzed to release NH3 continuously. Transition metal ions such as Ni2+ and Co2+ precipitate easily under alkaline conditions. Therefore, NH4Cl and sodium citrate are introduced as NH4Cl could buffer the increasing pH value while the citrate ions would stabilize the transition metal ions via complexation. In Figure 3a−c for the case of Ni and in Figure 3d for the case of Co (Figure S11), all the doped MSAA preserve well their microscale spherical morphology and nanoscale allophane spherules (Figure S11). Because of the presence of metal dopants, the color of the products was changed accordingly. In Figure 3e, the pristinely white opalescent MSAA become light green for the Ni case and pink for the Co case. Therefore, it proves successful isomorphous doping of the original MSAA, through which the chemical composition of MSAA is partially changed. Consistently, the Al−O−Si vibrations in their FTIR spectra shift 23 and 30 cm−1 toward lower wavenumbers for the case of Ni and Co, respectively. This shift of FTIR band is attributed to the presence of divalent ions (Ni2+ and Co2+) in replacement of their high valency counterparts (Al3+ and Si4+). It is thus expected that the Al3+ and Si4+ would become less positive when Ni2+ and Co2+ are incorporated into the lattices (i.e., when forming M−O−Al and M−O−Si bond linkages), resulting in the elongation of Al−O bond and Si−O and thus longer Al−O−Si.32 Because the MSAA is highly porous and the dopants can diffuse deeply into its interior, allophane spherules in the MSAA could be modified evenly. Our EDX analysis (Figure S12) and element mappings (Figure 3g,h) of the doped MSAA also confirm the uniform distribution of dopant ions. Similarly, many MSAA derivatives with other transition metal dopants can also be produced through this method. It should be noted that when the weight ratio of the dopant to the original MSAA is too high, for example, at or over 12.0% for the case of Ni2+, the small spherules of allophane might become slightly deshaped (Figure S13). So far as we are concerned, this is the first report on successful isomorphous doping of transition metal ions into allophanes. When the calcined MSAA were hydrothermally treated with only urea and transition metal ions, for example, Ni2+ and Co2+, at 120 °C for 10 h, part of the allophanes was transformed to another mineral, layered double hydroxide (LDH), since the allophanes are able to release Al3+ ions. In Figure 4, when a relatively low dosage of transition metal ion was applied, thin nanosheets of Ni−Al-LDH (Figure 4a) or Co−Al-LDH (Figure

Figure 3. Doping of MSAA by transition metal ions: (a,b,c) TEM images of MSAA doped by Ni2+ and (d) by Co2+; (e) a color photograph of (left) the original MSAA colloid, (middle) the Nidoped MSAA, and (right) the Co-doped MSAA; (f) FTIR spectra of (i) the original MSAA colloid, (ii) the Ni doped MSAA, and (iii) the Co-doped MSAA; and elemental mappings of (g) Ni-doped MSAA and (h) Co-doped MSAA.

4b) were observed on the MSAA. When the dosage of metal was raised, the LDH nanosheets became increasingly dense whereas more allophane spherules were depleted (Figure 4c,d, where 19.6% of Ni2+ and 14.7% of Co2+ were applied, respectively). Their respective XRD patterns show strong presences of these synthetic anionic clays (Figure 4g-i and 4gii), though β-Al2O3 was found in the case of Ni. Formation of LDH relates closely with the presence of urea. Thermal hydrolysis of urea creates the necessary alkaline condition for releasing Al3+ ions from the allophane spherules, which then coprecipitate with the transition metal cations. Note that the formation of such 3D hierarchical LDH-based assemblages (or nanocomposites) in Figure 4b,c usually requires tedious multistep synthetic processes such as precoating of a target 6081

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with high surface areas are attractive for preparation of nanocatalysts.8,9 Nevertheless, in most cases surface modification of the targeted support, for example, via grafting of organic ligands, is unavoidable.36−39 Hierarchical siliceous nanoarchitectures created from the stacking of uniform nanoscale building blocks have been proven as excellent catalytic supports because of presence of easily accessible porosity. 40−42 Allophanes, because of their high surface areas and high chemical affinity toward various ionic species, are excellent natural sorption and support materials. Our synthetic MSAA, with their inherent intrinsic properties of allophanes and uniquely organized mesoporosity, provides an excellent platform for construction of nanocatalyst. As a demonstration, herein various noble metal nanoclusters (NCs) were loaded onto the calcined MSAA via surfactant-free and surface modification-free processes. Shown in Figure 5a,b, Pt/MSAA nanocomposite was obtained by simply incubating the MSAA colloid with K2PtCl4 at 80 °C overnight. The colloidal Pt/MSAA is light black in color. At high magnifications, numerous PtNCs as small as 2.0 nm can be observed on the MSAA (Figure 5b and Figure S14), and these PtNCs also form polycrystalline Pt nanocrystals with size around 5−10 nm (Figure 5a,b, Figure S14). Few unsupported PtNCs were found outside the MSAA. Our element mapping (Figure 5g), HRTEM observation (Figure S14d), and XPS analysis (Figure S14e) further confirm the deposited PtNCs. Note that no surfactant or reductant was used in this metal deposition. Thus, the allophanes not only serve as an adsorbent/support, but also assist an in situ solvolysis of Pt species by complexing PtCl42− with their surface aluminol groups.24 Previous research revealed that there is a strong attractive force between the PtNCs and the aluminol groups on allophanes, accompanied by the Pt−O covalent bonding,24 which is also supported by our XPS analysis (Figure S14e). Loading of AuNCs was achieved by incubating the mixture of HAuCl4, the MSAA and alkaline solution of thiourea at room temperature overnight and then at 80 °C for 1 h. In Figure 5c,d, dense monodispersed AuNCs of 3 nm were immobilized on the surface of MSAA (Figures S15 and S16). The deposited AuNCs are further affirmed by elemental analysis (EDX, Figure 5h), lattice parameter determination (HRTEM, Figure S15b), and chemical bonding investigation (XPS, Figure S15c). The above metal loading processes strongly depend on the electrostatic forces which regulate the adsorption of the metal ions on the MSAA. For instance, when the pH value is above the point of zero charge of the MSAA, the repulsive electrostatic forces would dominate the interaction between the negatively charged MSAA and the metal chloride complex anions such as AuCl4− and PtCl42−. As a consequence, nucleation of these noble metals would mainly occur in the solution rather than on the MSAA support, leading to extensive formation of AuNCs outside of the MSAA (Figure S17). The MSAA with large surface areas and rough surfaces also contribute to the formation of the highly dispersed metal NCs by providing an immense amount of active sites for heterogeneous nucleation. In addition, the strong interaction of MSAA with the supported metal NCs contributes to the product stabilization. For instance, most of deposited AuNCs could maintain their sizes even after calcined at 200 °C in H2 for 2 h (Figure S18). The hollow spherules of the MSAA were also kept well after the immobilization of metals. The above PtNCs or AuNCs are mainly deposited on the external surface

Figure 4. TEM images of: (a) Ni−Al-LDH and (b) Co−Al-LDH deposited on MSAA spheres, using 50 μL of NiSO4 and 25 μL of CoCl2, respectively; (c) the assembled Ni−Al-LDH and (d) Co−AlLDH formed with a higher dosage of transition metal ions; (e,f) MSAA/Ni−Al-LDH formed from addition of Ni2+ during the conversion of Al−Si−P2 to MSAA; and (g) XRD patterns of (g-i) the Ni−Al-LDH and (g-ii) the Co−Al-LDH. Numbers in (g) are the indexed diffractions of each LDH, and the peaks labeled with the blue lines in (g-i) belong to the β-Al2O3 impurity after transformation of allophanes.

core with first a layer of SiO2 and then a layer of AlOOH, or precoating of the core with a layer of carbon.35 Our current approach provides facile preparation of hierarchically organized MSAA/LDH nanocomposites. Additionally, MSAA/LDH nanocomposites could also be formed through a one-pot synthesis method, which treats the Al−Si−P2 together with urea and relevant transition metal precursors at 150 °C for 15 h. The thus formed products have LDH nanosheets perpendicularly intersected into the MSAA spheres (Figure 4e). Slightly different from the case in Figure 4a, the nanosheets of LDH are decorated with numerous tiny allophane spherules (Figure 4f). 3.4. Deposition of Noble Metal Nanoclusters on MSAA for Supported Catalysts. Mesoporous materials 6082

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the loading of Pd in the MSAA. Note that most mesopores generated with soft micelles are well-ordered and assume 1Dcylindrical shape which have only two openings.2,5,37 On the contrary, the resultant mesopores in our MSAA, which are originated from randomly stacking of individual allophane spherules, are actually disordered and interconnected with each other. This spells at least two advantages compared to those ordered cylindrical mesopores. As reported in Figure 5i, metal precursors could be driven easily into mesopores in the deep interior of MSAA, since the interconnected mesopores should have a smaller buildup of internal pressure from trapped air during the impregnation. Furthermore, our MSAA is expected to be more resistant to pore plugging caused by deposition of metal NCs or adsorption of bulky molecules. The above two structural merits enable the MSAA to serve as an attractive catalytic support. 3.5. Application of M-NCs/MSAA (M/MSAA) Nanocatalysts. To test its catalytic activity and chemical stability, the Pd/MSAA nanocomposite (Figure 5e,f,i) at a Pd-content of only 0.76% was used for Suzuki-coupling reaction between idobenzene and phenylboronic acid. With the mesopores at about 3.4 nm in this catalyst system, reactants and products could transport easily throughout the MSAA. In Figure 6a, 98% yield of biphenyl was achieved within 25 min with such a low Pd loading. The excellent catalytic activity of the Pd/MSAA can be attributed to the small size and high dispersity of the active PdNCs across the entire MSAA, as well as to the clean surface of allophane spherules since neither surface modification nor surfactant was involved during the preparation. Moreover, in

Figure 5. TEM images and their corresponding elemental mappings of (a,b,g) Pt, (c,d,h) Au NCs deposited on the surfaces of MSAA, and (e,f,i) PdNCs deposited in the interior of MSAA.

of the MSAA (Scheme 1f), as revealed by their EDX elemental analysis, which shows stronger intensities at the boundaries of mapping pattern. Using the conventional impregnationcalcination approach, the metal NCs could be deposited both on the surface and in the interior of the MSAA spheres (Scheme 1e). An example of this regard was shown in Figure 5e,f where well-dispersed PdNCs of 2 nm are homogeneously loaded in the interspherule spaces within the MSAA (Figure S19). Because of their small sizes, it is quite difficult to observe the PdNCs inside the MSAA. Nevertheless, we can still observe numerous imbedded PdNCs (which have a higher image contrast) in the interior at a higher magnification (Figure 5f). Convincingly, the EDX analysis in Figure 5i provides a homogeneous Pd elemental mapping across each MSAA, therefore confirming such a uniform distribution. Furthermore, HRTEM examination and XPS analysis (Figure S19) reaffirm

Figure 6. (a) Recycling test of Pd/MSAA nanocomposite for Suzuki coupling reaction between idobenzene and phenylboronic acid; (b,c) HRTEM images and (d) element mapping of the catalyst after being recycled for 6 times. Reaction conditions: 0.5 mM idobenzene, 1 mM phenyl boronic acid, 2 mM K2CO3, 0.5 mM n-dodecane as internal standard, 10 mL ethanol and 10 mg Pd/MSAA with 0.76% Pd loading, reaction time 25 min. 6083

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Chemistry of Materials Figure 6a, the catalyst was recycled for six times, and the yield was maintained excellently in all these cycles. The Pd/MSAA after the repeated use was further investigated, and its remarkable structural stability was confirmed. As shown in Figure 6b, the Pd/MSAA spheres remain well-dispersed, but no disassembly of allophane spherules is found, and their hollow feature, size and morphology are unaltered (Figure 6c and Figure S20). In Figure 6c, similar to the Pd/MSAA before the reaction (Figure 5e,f,i), no enlarged PdNCs could be seen (Figure S20), but the element mapping still reveals homogeneous existence of Pd within the MSAA (Figure 6d). The high chemical stability of the PdNCs could be explained from the following two aspects. First, immobilized within the confined interior mesopores of the MSAA, the PdNCs have less freedom and hence less chance to fuse with each other or falling off into the bulk solution. Second, as discussed earlier, the allophane spherules have a high chemical affinity toward various ionic species due to the rich presence of aluminol groups on their surfaces.20,21 Inorganic porous materials such as clays, SiO2 gels, zeolites have been proved as promising immobilization matrices because of their high mechanical, thermal and chemical stability, high surface area, good adsorption and penetrability.43−45 For example, in the field of chemical sensing, effective enzymatic or nonenzymatic sensors have been developed by electrode modification with such inorganic porous materials.43−46 In view of this progress, herein, we also expanded the application of allophane to the field of chemical sensing in addition to the coupling reaction results reported in Figure 6. In particular, we tested the Au/MSAA nanocomposite (Figure 5c,d,h) for electrochemical sensing of H2O2. The sensor was constructed by immobilization of the Au/MSAA nanocomposite on a polished GCE using Nafion aqueous solution. Typical cyclic voltammograms of the Au/MSAA modified GCE in the phosphate buffer solution (PBS, pH = 7.0) are shown in Figure 7a. The reduction current obtained in the presence of 2.6 mM H2O2 (Figure 7a-II) was greatly enhanced compared with that in the absence of H2O2 (Figure 7a-I), proving the occurrence of electrocatalytic reduction of H2O2. Chronoamperometric responses of the Au/MSAA-modified GCE to different concentrations of H2O2 are displayed in Figure 7b. With successive addition of H2O2 in each step, the reduction current increases accordingly, with steady signal obtained less than 8 s. Accordingly, thus obtained calibration line gives a linear range which falls within 25 μm to 2.2 mM with a good sensitivity of 30.6 μA·mM−1 ·cm−2 (R = 0.9992, and n = 27), and the detection limit was 1.1 μm at the signal-to-noise-ratio of 3 (Figure 7c). Such a result proves the potential of our mesoporous allophanes to serve as a matrix for construction of biosensors. Considering the excellent adsorption affinity of allophanes toward organic molecules47 such as various DNA,21,23 and enzymes,48,49 we believed various enzymatic biosensors could also be constructed using the MSAA as the immobilization matrix.

Figure 7. (a) Cyclic voltammograms of the as-prepared Au/MSAAmodified GCE in the absence and presence of 2.6 mM H2O2 in 10.0 mL PBS, scan rate 50 mV/s; (b) Chronometric response of the Au/ MSAA-modified GCE on successive addition of H2O2 into 10.0 mL PBS (pH = 7.0), the applied potential: − 0.3 V; and (c) calibration line of electrocatalytic current of H2O2 versus its concentration.

average mesopore size at around 3.4 nm. The formation mechanism of MSAA mainly involves the synthesis of the amorphous aluminosilicate intermediate spheres in the first step, followed by gas bubble-templated local structural rearrangement in the second step. Metal-doped derivatives are also made by isomorphously doping the MSAA with relevant transition metal ions; the MSAA could also be partially transformed to two types of hierarchically organized LDH/ MSAA nanocomposites. Because of their large specific surface area and the unique surface properties, the MSAA could be used as an effective catalytic support. Highly dispersed noble metal NCs such as Pt, Au, and Pd NCs can be immobilized easily on the external surface or introduced evenly across the interior part of the MSAA. With catalytically active components loaded on the external surface or inside the mesopores of the MSAA, the above prepared nanocomposites have shown great potential for a range of technological applications. For example, our Pd/MSAA nanocomposite has demonstrated its excellent catalytic activity and remarkable stability toward Suzuki coupling reactions. Furthermore, in this work, our Au/MSAA nanocomposite has been used in construction of electrochemical sensors for H2O2 detection.

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4. CONCLUSION In summary, using amorphous aluminum carboxylate polymeric spheres as the source of aluminum and the space-defined solid precursor, we have developed a two-step surfactant-free hydrothermal approach, which enables simultaneous synthesis and self-assembly of individual hollow allophones into disordered MSAAs with diameters of 445 ± 40 nm, specific surface area 1032 m2/g, pore volume 1.104 mL/g, and an

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01956. TEM images, HRTEM images, EDX spectrum, element mapping, XRD analysis of MSAA samples (PDF) 6084

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hua Chun Zeng: 0000-0002-0215-7760 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.Z. would like to thank NUS Graduate School (NGS) for Integrative Sciences and Engineering for providing her postgraduate scholarship. The authors gratefully acknowledge the financial support provided by the Ministry of Education, Singapore, National University of Singapore, and GSK Singapore. This project is also partially funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

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DOI: 10.1021/acs.chemmater.7b01956 Chem. Mater. 2017, 29, 6076−6086

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on June 30, 2017, with an error in Section 3.4. The corrected version was reposted on July 6, 2017.

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DOI: 10.1021/acs.chemmater.7b01956 Chem. Mater. 2017, 29, 6076−6086