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Feb 12, 2018 - Through this method, various materials, such as 3DOM/m Pt, Pd, Ni2P, carbon and N/P codoped carbon (NPC), have been successfully synthe...
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General Synthesis of 3D Ordered Macro-/Mesoporous Materials by Templating Mesoporous Silica Confined in Opals Tingting Sun, Nannan Shan, Lianbin Xu, Jiexin Wang, Jianfeng Chen, Anvar A. Zakhidov, and Ray H. Baughman Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04829 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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Chemistry of Materials

General Synthesis of 3D Ordered Macro-/Mesoporous Materials by Templating Mesoporous Silica Confined in Opals Tingting Sun,† Nannan Shan,†,‡ Lianbin Xu,*,† Jiexin Wang,† Jianfeng Chen,† Anvar A. Zakhidov,§,║,# and Ray H. Baughman§ †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China ‡

Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506, United States

§

The Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, Texas 75083, United States

║

ITMO University, St. Petersburg 197101, Russia

#

National University of Science and Technology “MISIS”, Moscow 119049, Russia

ABSTRACT: Three-dimensionally (3D) ordered macro-/mesoporous (3DOM/m) materials, which combine the advantages of high surface area of the mesopores, efficient mass transport within the macropores, and strong structural stability of the 3D interconnected porous frameworks, have shown remarkable performances in a wide range of applications. Herein, we demonstrate a novel dual-hard-templating technique as a general synthetic strategy for constructing the 3DOM/m materials by nanocasting mesoporous silica confined within the regular voids of a silica colloidal crystal (opal). Through this method, various materials, such as 3DOM/m Pt, Pd, Ni2P, carbon and N/P co-doped carbon (NPC), have been successfully synthesized. The as-prepared 3DOM/m Ni2P and NPC are demonstrated to exhibit outstanding electrocatalytic performance for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), respectively.

INTRODUCTION Ordered mesoporous materials have received widespread attention for applications in diverse fields because of their distinctive structures and properties, including high specific surface area, controllable pore structure and uniform pore distribution, as well as good thermal and mechanical stabilities.1,2 Templating strategies based on hard templates (e.g., mesoporous silica and carbon) or soft templates (e.g., nonionic surfactants) are commonly applied for the synthesis of porous materials having ordered mesostructures.3-6 Compared with the softtemplating method, the hard templating one (also called the nanocasting method) is a quite fascinating route that is widely deployable for synthesizing ordered mesoporous materials containing various components, such as metals,7 carbon,8 metal oxides,9 metal nitrides,10 and metal sulfides.11 Constructing hierarchical porous materials by incorporating macropores in mesoporous materials has been demonstrated to be an attractive strategy for enhancing the performance of mesoporous materials for practical applications, since the macropores can improve mass transport and reduce diffusion limitations present in purely mesoporous materials, while the mesopores enable high surface areas.12-15 Various synthetic approaches have been developed to fabricate the hierarchically structured

macro-/mesoporous materials. Typically, the dualtemplating synthesis route, which applies cooperative colloidal crystal (opal) templating (hard-templating) and surfactant templating (soft-templating) techniques, is employed for preparing hierarchical materials possessing three-dimensionally (3D) ordered interconnected macroand mesoporous structure.16,17 Owing to their ordered and tailored pore architectures at both macro- and mesoscales, 3D ordered macro-/mesoporous (3DOM/m) materials (e.g., TiO2, carbon, and metals) have shown remarkable performance boosts in numerous applications, such as catalysis, energy storage and conversion, sensing, adsorption, and separation.18-21 However, the application of the above hard-soft dual-templating approach is limited by the lack of suitable soft-templating synthesis techniques for many materials, as well as the difficulty in con trolling the hydrolysis and condensation reactions for most nonsiliceous precursors to create ordered mesostructures.22,23 Herein, we report a novel dual-hard-templating approach to prepare the 3DOM/m materials, which combines the colloidal crystal templating and nanocasting techniques by using silica opal as the macroporous mold and mesoporous silica filled in the opal as the mesostructural template. This method provides a general strategy for fabricating the 3DOM/m materials, removing the necessity of employing soft templates, which may not be

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applicable in the synthesis of many materials having mesoporous structures. Figure 1 briefly describes the synthetic procedure for the 3DOM/m materials. A mesoporous silica precursor containing triblock copolymer Pluronic F127 and tetraethoxysilane (TEOS) was first infiltrated into the void spaces of the silica opal, followed by aging and calcination processes to form the mesoporous silica networks inside the opal. The resulting mesoporous silica/opal composite was then filled with metals (e.g., Pt and Pd), alloys (e.g., Ni2P), and carbon-based materials (e.g., carbon and N/P co-doped carbon (NPC)) by corresponding chemical methods. Subsequent removal of the silica templates (silica opal and mesoporous silica) produced the freestanding 3DOM/m materials (for more details, see the Supporting Information). This study reports for the first time the synthesis of the 3DOM/m Ni2P and NPC through the dual-hard-templating technique, which are difficult to prepare by other methods, such as the hard-soft dualtemplating method. Also, the use of the 3DOM/m Ni2P and NPC as highly effective electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), respectively, has been demonstrated.

Figure 1. Schematic representation of the preparation of three-dimensionally ordered macro-/mesoporous materials.

RESULTS AND DISCUSSION Figure 2a and S1a show the typical scanning electron microscopy (SEM) images of the synthesized mesoporous silica/opal composite. The interstices of the ordered close-packed silica spheres of opal are filled with mesoporous silica. The highly ordered mesoporous structure can be observed in the high-magnification SEM images of the composite (Figure 2b and S1b). The SEM images of the 3DOM/m Pt (Figure 2c,d), Pd (Figure S2a), Ni2P (Figure S2b,c), C (Figure S2d), and NPC (Figure S2e,f) shows that all of these as-prepared 3DOM/m materials have wellordered macroporous structures with interconnecting windows between adjacent spherical voids. Transmission electron microscopy (TEM) and high resolution TEM (HR-TEM) were performed to gain further insight into the structural information of the obtained 3DOM/m materials. The TEM image of the mesoporous silica/opal composite (Figure 2e) reveals that the interstitial spaces between the silica opal spheres are filled with highly ordered mesoporous silica having pore size of 7–10 nm. Figure 2f-i, and Figure S3 are TEM images of the 3DOM/m Pt, Pd, Ni2P, NPC, and C, respectively. Uniform macropores and well-ordered mesopores in the

macropore walls are present in the TEM images, indicating the hierarchically ordered macro-/mesostructures of the materials. The mesopore diameters of the 3DOM/m materials are in the range of 3–5 nm and the average sizes of the Pt, Pd, Ni2P and NPC nanoparticles are approximately 9 nm. The Ni2P particle size of the 3DOM/m Ni2P is smaller than that of the Ni2P nanoparticles (NPs) (~16 nm, Figure S5) used for comparative studies of the electrocatalytic properties for the HER. The selected area electron diffraction (SAED) patterns of the 3DOM/m Pt, Pd, and Ni2P (insets in Figure 2f-h respectively) reveal the polycrystalline face-centered cubic (fcc) structures of the materials with homogeneous rings corresponding to the diffraction over many small metal particles in the macropore walls. The HR-TEM images of the 3DOM/m Pt, Pd, and Ni2P (Figure 2j-l, respectively) show the lattice fringes in the mesopore walls, and it is observed that the fringe distances agree with the corresponding d-spacings of adjacent crystal planes for Pt, Pd, and Ni2P.24-26 The elemental mapping of the 3DOM/m Ni2P (Figure 2m) displays homogeneous distribution of Ni and P across the selected area. The uniform distribution of C, N and P in the 3DOM/m NPC is also confirmed by the mapping images shown in Figure 2n. The powder X-ray diffraction (XRD) patterns of the 3DOM/m Pt, Pd, and Ni2P are presented in Figure 3a-c, respectively, which exhibit the diffraction peaks that can be readily indexed to the fcc phases of Pt, Pd, and Ni2P (JCPDS cards, No. 65-2868, 65-2867, and 03-0953, respectively). The average particle size for the 3DOM/m Pt, Pd, and Ni2P estimated using the Scherrer equation is ~9 nm, which agrees well with the HR-TEM observations. The XRD patterns of both the 3DOM/m NPC (Figure 3e) and 3DOM/m C (Figure S6b) have two broadened peaks centered at about 2θ of 24.2° and 43.3°, characteristic of the carbon materials having a low degree of graphitization.27 The presence of peaks at around 0.51° in the low-angle Xray diffraction patterns for the 3DOM/m Pt, Ni2P, NPC, and mesoporous silica/opal (Figure S7) indicates the existence of ordered mesostructures in the samples. The nitrogen adsorption−desorption isotherms of the 3DOM/m Ni2P, NPC, and C exhibit type IV isotherm characteristics, indicating the mesoporous structure of the samples (Figure S6).28 The Brunauer–Emmett–Teller (BET) surface area and total pore volume of the prepared 3DOM/m materials are summarized in the Table S1. The pore size distribution (PSD) curves for the 3DOM/m Pt, Ni2P, NPC, and C (insets of Figure S8a-d, respectively), obtained from the desorption branches of the isotherms by using the Barrett-Joyner-Halenda (BJH) model, show narrow distributions of pores centered around 3.8 nm, which is consistent with the TEM results. The XPS spectra of the 3DOM/m Ni2P and NPC are presented in Figure 4. In the Ni 2p spectrum of 3DOM/m Ni2P (Figure 4a), the peaks at approximately 852.6 eV and

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Chemistry of Materials

Figure 2. SEM images of (a,b) mesoporous silica/opal composite, and (c,d) 3DOM/m Pt. TEM images of (e) mesoporous silica /opal composite, (f) 3DOM/m Pt, (g) 3DOM/m Pd, (h) 3DOM/m Ni2P, and (i) 3DOM/m NPC. The insets in (f-h) are SAED patterns. HR-TEM images of (j) 3DOM/m Pt, (k) 3DOM/m Pd, and (l) 3DOM/m Ni2P. (m) High-angle annular dark-field scanning TEM (HAADF-STEM) image of 3DOM/m Ni2P and the corresponding element maps for Ni and P. (n) HAADF-STEM image of 3DOM/m NPC and the corresponding element maps for C, N, and P.

855.8 eV for Ni 2p3/2 are assigned to Niδ+ in Ni2P and Ni2+ in surface oxidized Ni species, respectively.29 In the P spectrum of 3DOM/m Ni2P (Figure 4b), the peak at around 129.5 eV corresponds to Pδ- in Ni2P, whereas the other peak centered at 133.2 eV is attributed to the P5+ in surface phosphorus oxides.30 The presence of Ni2+ and P5+ species suggests the partial surface oxidation of Ni2P in air. The N 1s XPS spectrum of 3DOM/m NPC (Figure 4c) can be fitted to pyridinic N (N1) at 398.3 eV, pyrrolic N (N2) at 399.5 eV, graphitic N (N3) at 400.1 eV, and oxidized N (N0) at 402.7 eV, respectively.31 The P 2p XPS spectrum of the 3DOM/m NPC (Figure 4d) are divided into two peaks centering at 132.1 eV and 133.2 eV, which are assigned to the P-C and P-O bond, respectively.32 The XPS derived surface composition percentages (at.%) of the 3DOM/m Ni2P and NPC are summarized in Table S2. The electrochemically active surface areas (ECSAs) of the 3DOM/m Pt and Pd were estimated by using cyclic voltammetry in Ar-saturated 0.5 M H2SO4 solution. As shown in Figure S11, the 3DOM/m Pt and Pd exhibit typical hydrogen adsorption/desorption and oxidation/reduc-

Figure 3. XRD patterns of (a) 3DOM/m Pt, (b) 3DOM/m Pd, and (c) 3DOM/m Ni2P. (d) Standard pattern of Ni2P. (e) XRD pattern of 3DOM/m NPC. XRD patterns were obtained using Cu-Kα radiation with a wavelength of 1.5418 Å.

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tion behavior, respectively. The ECSAs of the 3DOM/m Pt and Pd are calculated to be 19.2 m2 g−1 and 37.4 m2 g−1, respectively. The electrocatalytic HER performance of the 3DOM/m Ni2P was examined in 0.5 M Ar-saturated H2SO4 solution. The catalytic HER activity of the Ni2P nanoparticles (NPs), bare GC, and commercial 20 wt.% Pt/C were also evaluated as control experiments. Figure 5a shows the polarization curves of the different catalysts. A current density of 10 mA cm−2 can be achieved at an overpotential of merely 92 mV for the 3DOM/m Ni2P, much smaller than that of the Ni2P NPs (225 mV). The Tafel plots were used to probe the catalytic kinetics for the HER (Figure S12). The Pt/C catalyst gives a Tafel slope of ~31 mV dec−1, which is in line with the reported values.33 The Tafel slope of the 3DOM/- m Ni2P is determined to be 41 mV dec−1, indicative of a super HER activity with VolmerHeyrovsky mechanism.34 The Electrochemical impedance spectroscopy (EIS) measurements were conducted to further examine the electrode kinetics under HER operating conditions (Figure S13). The width of the semicircle in the Nyquist plots reveals that the 3DOM/m Ni2P has a much lower chargetransfer resistance (Rct) than that of the Ni2P NPs, suggesting the higher charge transport efficiency of the 3DOM/m Ni2P. Based on the BET surface areas (Figure S8b and S9), the 3DOM/m Ni2P gives a specific activity of 0.066 mA cm−2 at the overpotential of 100 mV, about 6 times that of the Ni2P NPs (0.011 mA cm−2). The catalytic HER activity of the 3DOM/m Ni2P is also better than that of the current most Ni2P-based and other nanostructured non-precious-metal HER electrocatalysts (Table S3), revealing that the 3DOM/m Ni2P is an excellent electrocatalyst for HER. Furthermore, the 3DOM/m Ni2P catalyst exhibits negligible overpotential increase after 10 hours of continuous operation under a static current density of 10 mA cm−2, in contrast to a much sharper activity loss of the Ni2P NPs catalyst (Figure 5b), indicating the high durability of the 3DOM/m Ni2P in catalysing HER under acidic medium. The enhanced HER catalytic performance of the

Figure 4. High-resolution XPS spectra of Ni 2p (a) and P 2p (b) for 3DOM/m Ni2P. High-resolution XPS spectra of N 1s (c) and P 2p (d) for 3DOM/m NPC.

Figure 5. (a) HER polarization curves of bare glassy carbon (GC), commercial Pt/C, 3DOM/m Ni2P, and Ni2P NPs in Arsaturated 0.5 M H2SO4 solution (2000 rpm, scan rate: 2 mV −1 s ). (b) Time-dependent catalytic overpotential curves for 3DOM/m Ni2P and Ni2P NPs at a static current density of -10 −2 mA cm for 10 hours. (c) ORR polarization curves of 3DOM/m NPC, 3DOM NPC, 3DOM/m C, and commercial Pt/C in O2-saturated 0.1 M KOH solution (1600 rpm, scan −1 rate: 5 mV s ). (d) The effects of 5000 continuous cycles (be−1 tween 0.2 and 1.0 V at 100 mV s ) on the ORR polarization curves of 3DOM/m NPC and Pt/C in O2-saturated 0.1 M KOH solution.

3DOM/m Ni2P may benefit from its structural advantages of the unique 3D ordered interconnected macropores and mesopores, such as large surface area, improved molecular/ionic transport and active site accessibility, and high electrical conductivity and structural stability (Figure S14a,b).35,36 The oxygen reduction reaction (ORR) electrocatalytic properties for the 3DOM/m NPC were evaluated using a rotating disk electrode technique in O2-purged 0.1 M KOH. The 3D ordered macroporous (3DOM) NPC, 3DOM/m C, and commercial Pt/C catalysts were also tested for comparison. Figure 5c reveals the polarization curves of the studied samples. The 3DOM/m NPC possesses the highest ORR activity in terms of the half-wave potential (E1/2) and diffusion-limited current density, demonstrating the significance of co-doping with N-P and 3D ordered interconnected macro/mesoporous in catalyzing ORR. The E1/2 of 3DOM/m NPC (0.86 V) is more positive than or comparable to that of the other reported nonprecious-metal ORR catalysts (Table S4), suggesting superior ORR electrocatalytic activity of the 3DOM/m NPC. Accelerated durability measurements were conducted by taking potential cycles between 0.2 and 1.0 V vs. RHE at a scan rate of 100 mV s−1 (Figure 5d). After 5000 continuous cycles, the half-wave potential on 3DOM/m NPC only negatively shifts by ~4 mV, much less than that on Pt/C (~25 mV), implying that the 3DOM/m NPC has outstanding catalytic durability. The high durability of the 3DOM/m NPC could be attributed to the scarcely noticeable mechanical deformation and swelling of the 3D interconnected macro- and mesoporous materials, in which

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Chemistry of Materials the morphology and porous structure were still well kept even after the durability testing (Figure S14c,d).

CONCLUSIONS In summary, we have developed an effective dual-hardtemplating strategy for the construction of threedimensionally ordered macro-/mesoporous (3DOM/m) materials by using the mesoporous silica/silica opal composite as sacrificing template. Various 3DOM/m materials, such as Pt, Pd, Ni2P, carbon, and N/P co-doped carbon (NPC), have been successfully fabricated. Because of the large surface area, efficient mass/charge transport, and high structural stability of the unique 3D interconnected network architecture with ordered macropores and mesopores, the 3DOM/m Ni2P and NPC exhibit remarkable electrocatalytic HER and ORR performance, respectively. It is believed that the present synthetic approach, possibly in combination with other techniques like electrodeposition and melt infiltration, will allow access to many other 3DOM/m materials having technologically important properties.

EXPERIMENTAL SECTION Chemicals. Amphiphilic triblock copolymer Pluronic F127 (PEO127-PPO70-PEO127) and Nafion solution (5 wt.%) were purchased from Sigma-Aldrich Co. Chloroplatinic acid hexahydrate (H2PtCl6•6H2O, 37 wt.%) was purchased from Tianjin Guangfu Fine Chemical Research Institute. Palladium chloride (PdCl2, 60%), hydrazine hydrate (N2H4•H2O, 98%), nickel (II) chloride hexahydrate (NiCl2•6H2O, 98%), sodium hypophosphite (NaH2PO2, 99%), phytic acid (C6H18O24P6) were purchased from Shanghai Aladdin Biochemmical Technology Co., Ltd. Ammonia solution (NH3•H2O, 28%) and urea (CO(NH2)2, 99%) were purchased from Beijing Tongguang Fine Chemical Co. Tetraethoxysilane (TEOS, 98%), sucrose (C12H22O11), potassium hydroxide (KOH, 98%), hydrofluoric acid (HF, 40%), nitric acid (HNO3, 68%), hydrochloric acid (HCl, 37%), sulfuric acid (H2SO4, 98%), and ethanol (99.7%) were obtained from Beijing Chemical Works. All chemicals were used as received without further purification. Preparation of silica colloidal crystal (opal). Silica opal was prepared by published methods.37 Monodisperse silica spheres with a diameter of ca. 290 nm was initially prepared from hydrolysis of TEOS. The spheres were then formed into close-packed lattices through a sedimentation process over several months. This precipitate was then sintered at 120 oC for 2 days and then 750 oC for 4 h, producing a robust opalescent piece that could be readily cut into smaller sections (