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ZnO Nanoparticle Fortified Highly Permeable Carbon/Silica Monoliths as a Flow-Through Media Srujan Singh, Kunal Mondal, and Ashutosh Sharma Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01361 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017
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ZnO Nanoparticle Fortified Highly Permeable Carbon/Silica Monoliths as a Flow-Through Media
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Srujan Singh, Kunal Mondal‡ and Ashutosh Sharma*
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Department of Chemical Engineering
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Indian Institute of Technology Kanpur, Kanpur 208016, India
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Abstract
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We demonstrate a facile one-pot synthesis of porous “flow-through” ZnO nanoparticle
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impregnated carbon/silica monoliths with high mechanical strength and interconnected end-
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to-end pores decorated with functional and catalytic nanoparticles. The materials and
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conditions for the synthesis were tailored to achieve the desired properties of high mechanical
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strength, good flow-through permeability, and crack-free morphology. Monoliths were
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prepared from a resorcinol formaldehyde rout but with the addition of tetraethyl-orthosilicate
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and a metal oxide precursor, ZnCl2. The monoliths were ambient dried and carbonized under
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optimized conditions to suppress cracks. Compressive tests of both the resin and carbonized
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monoliths were performed to examine the effect of the metal oxide precursor on the
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mechanical properties. The permeability of the monoliths was determined to verify their
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utility as a flow-through material. The monoliths exhibited a high compressive modulus of
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~30 MPa compared with conventional carbon aerogels and a permeability of ~10-12 m2.
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Various characterization techniques were used to analyze the surface morphology, pore
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texture and chemical composition of the monoliths. Finally, Ag nanoparticles were
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incorporated in the monoliths to demonstrate an example of a “flow-through” catalysis
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application where controlled catalytic conversion of para-nitrophenol into para-aminophenol
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could be achieved in a continuous flow reactor mode.
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Keywords: Carbon Monoliths, High Mechanical Strength, Permeability, Sol-gel
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*Corresponding author email:
[email protected] 1 ACS Paragon Plus Environment
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Introduction
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Porous materials have long been explored and have held a prominent position in material
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research and industries because of their indispensability in the fields of catalysis,1,
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adsorption,3 electrochemistry,4 gas storage,5, 6 and chromatography7. With the introduction of
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resorcinol formaldehyde (RF)-derived low-density carbon aerogels8 obtained via the sol–gel
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route, porous carbonaceous materials have gained popularity among porous silica, zeolites,
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metal–organic frameworks because of the ease of synthesis and better control over their pore
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texture.9 Attributes such as high thermal and chemical stability, light weight resulting from
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large pore volumes, and high surface area make carbon all the more appropriate.
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Hard templating,10 also known as
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synthesizing porous carbon, in which an organic precursor is impregnated into a porous pre-
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synthesized inorganic template and carbonized before the removal of the template yields a
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negative/inverted replica similar to that obtained in negative casting; hence, the name. This
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method produces porous carbon with consistent mesoporosity.11 The interconnected pore
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network achieved using this method is responsible for the remarkable properties, i.e., light
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weight, low thermal conductivity, and transport properties; however, at the same time, it
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compromises the mechanical strength of these aerogels, leading to their fragility and
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explaining why they are mostly obtained in powder form.12
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Previous works have reported improvement in the mechanical properties of carbon aerogels
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by incorporation of carbon nanotubes (CNTs)13, inorganic oxides14, and carbon nanofibers15.
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However, much less effort has been directed to synthesis of monoliths that exhibit good
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mechanical strength and good permeability, thus making them suitable for the applications
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such as catalytic beds and flow through electrodes that allow accessing internal area. The
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advantage of working with monoliths instead of powdered samples is that they can be formed
“nano-casting,” is a fairly common technique for
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into any desired shape, eliminating the need for binders and artificial support tape to achieve
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the desired configuration.
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In the present work, monoliths were synthesized that consist of a bicontinuous network of
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carbon and silica using RF and tetraethyl-orthosilicate (TEOS) as their respective precursors.
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ZnO was incorporated into this network to enhance the mechanical properties of the
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monoliths. A detailed analysis of the effect of the metal oxide concentration on the
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mechanical properties of the monoliths was performed. The carbon network essentially forms
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the backbone of the monoliths, providing structural integrity, and the addition of a metal
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oxide precursor provides enhanced mechanical strength. The silica network is responsible for
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creating the required permeable architecture for a flow-through assembly, acts as a medium
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to facilitate homogenous distribution of the different chemical species within the monolith
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bulk, and adds to the structural integrity. Ag nanoparticles were incorporated into the
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monoliths and a “flow-through” assembly was set up to study the catalytic conversion of
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para-nitrophenol (PNP) into para-aminophenol (AP). Clearly, other functional/catalytic
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particles can also be incorporated in the monoliths for other continuous flow reactor
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applications.
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Experimental
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Materials
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Resorcinol (99%), Tetraethyl-orthosilicate (TEOS; 98%), 3-Aminopropyltriethoxysilane
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(APTES; 99%) were purchased from Sigma Aldrich. Acetone (99.5%) was purchased from
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Fisher Scientific. Zinc Chloride (97%) was purchased from Thomas Baker Chemicals Pvt.
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Ltd. Para-nitrophenol (98%) and Formaldehyde (37-41% w/v) were purchased from Loba
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Chemie. Sodium Borohydride (≥95%) was purchased from Merck. Silver nitrate (99.8%)
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was purchased from Qualigens Fine Chemicals. Milli-Q (ρ =18.2 MΩ cm) deionized (DI) 3 ACS Paragon Plus Environment
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water was used throughout for preparing any water based solution. All reagents were of
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analytical grade.
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Synthesis of monoliths
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The simple technique of sol-gel synthesis was utilized for preparation of the monoliths. 734
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mg of resorcinol (R) was added to 4 ml of acetone (Ac). Zinc oxide precursor (Z) i.e. ZnCl2
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was added accordingly to maintain a Z/R (w/w) value of 0.10, 0.25, 0.50, 0.75 and 1.00. Let
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this solution prepared so far be referred to as solution “A”. From this step onwards three
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different types of monoliths were obtained depending on the reagents added. Adding 1.5 ml
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of formaldehyde (F) to A and keeping it in an oven at 60 0C for 12 h produced the first type
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of monoliths as shown in fig. S1(b-f) referred to as RF-Zn(Z/R). However, addition of silica
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precursors to A i.e. 2.5 ml of TEOS and 300 μl of APTES followed by addition of 1.5 ml of
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formaldehyde (F) and spending 12 h in oven at 60 0C produced another type of monolith as
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shown in fig. S1(h-l) referred to as RF-SiO2-Zn(Z/R). For the synthesis of the final type of
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monoliths, i.e. RF-SiO2-Zn-Ag, the (Z/R) value was chosen to be 0.25 and instead of
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formaldehyde, a 1M solution of silver nitrate in formaldehyde was prepared and 1.5 ml of
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this solution was used. All the reagents were added and mixed thoroughly in 15 ml centrifuge
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tubes and allowed for gelation to take place. From these synthesized monoliths, the ones with
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Z/R value of 0.25 were chosen for carbonization, the reason for doing this and the
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carbonization process have been discussed in the later sections.
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Post Synthesis Steps: Drying and Carbonization of Monoliths
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Drying and carbonization of the synthesized RF monoliths was performed to obtain the
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corresponding carbonized monoliths which were subsequently used for different
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characterizations. The RF monoliths were simply allowed to dry within their molds (in which
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they were synthesized) for approximately 5 days. The top of the mold was covered with a 4 ACS Paragon Plus Environment
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perforated paraffin film to allow for slow drying. Henceforth, the dried RF monoliths
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(Z/R=0.25) were carbonized at 700 0C in an inert atmosphere of Ar (flowrate: 150 cc/min)
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with a holding time of 2h. The heating rate was 3.5 0C/min.
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Monolith Sample preparation for different characterizations
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Samples of RF-Zn, RF-SiO2-Zn and their respective carbonized forms were prepared in the
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form of cylindrical units by cutting out an arbitrary cross section (fig. 1a) from the entire
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monolith, for compressive tests and for measuring their permeability. The bulk density of the
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monoliths were calculated as ρ = m/v, where ρ, m and v represent bulk density, mass and
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volume (v = πR2H where R and H are radius and height of the monoliths) respectively. For
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measuring the permeability, the monoliths were sandwiched between two T-shaped holders
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and the entire assembly was sealed using epoxy resin, which has been referred to as the “flow
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through” assembly as depicted in fig. 1b.
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Figure 1. (a) Step by step fabrication of the “flow through” assembly. (b) Components of the flow through assembly.
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Preparation of para-nitrophenol (PNP) and NaBH4 for kinetics study
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An aliquot of 40 μl of 0.01 M PNP (pale yellow colour) was taken from a stock solution and
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added to 40 ml of a freshly prepared solution of 0.1M NaBH4 (colourless). The appearance of
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an intense yellow colour manifests the conversion of nitrophenol molecules into
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nitrophenolate anions. As shown in fig. S2 this solution was pumped using a peristaltic pump
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and made to flow through the catalytic “flow through” assembly (made up of the C-SiO2-
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ZnO-Ag monolith) at different flowrates and collected in culture tubes. 2 ml of this collected
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solution was required for determining the absorbance and thereby the conversion. For all
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absorbance measurements a Varian Cary 50 Bio UV-Vis Spectrophotometer was used.
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General Characterizations
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The compressive tests of the monoliths were done using an INSTRON 1195 testing machine
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at room temperature and at a rate of 0.5mm/min. Field Emission Scanning Microscopy
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(FESEM, Quanta 200, Zeiss Germany) was used for analysing the surface morphology of the
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different monolith samples prepared. Powder X-ray diffraction (XRD) measurements were
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performed using X’Pert Pro, PAN-analytical, Netherlands X-ray system with Cu Kα
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radiation. Transmission electron microscopy (TEM) was carried out using a Tecnai G2, USA
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microscope. BET (Brunauer–Emmett–Teller) surface area and pore size distribution were
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determined using Autosorb iQ (Quantachrome Instruments, USA). Typically, an
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approximately 40 mg chunk of the monolithic sample was used for BET surface area analysis
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and N2 (77K) was used as the gaseous adsorbate for surface probing. X-ray photoelectron
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spectroscopy (XPS) was carried out using PHI 5000 Versa Probe II (Ikon Analytical
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Equipments Pvt. Ltd. New Delhi). Perkin Elmer Spectrum Two FTIR spectrometer was used
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for the purpose of Fourier Transform Infrared spectroscopy and its software was used for
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analysing the peaks.FTIR spectroscopy of the RF monoliths (as well as their carbonized 6 ACS Paragon Plus Environment
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forms) of all possible combinations was performed using KBr as the background for
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measurements.
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Results and Discussion
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Drying of the monoliths in the moulds allowed minimal exposure to the atmosphere ensuring
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a slow rate of drying. The monoliths were given sufficient time to dry in order to prevent any
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crack formation due to development of internal stresses on account of solvent molecules
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leaving the bulk of the monoliths. Over time as the monoliths dried, their volume gradually
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shrunk and the entire structure could be removed from the molds as crack-free monoliths (fig.
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S1).
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Differences in the physical properties of the two systems of monoliths were apparent. The
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RF–Zn monoliths closely resembled the parent RF resin – honey colored, hard-glassy
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textured and translucent in nature. However, the RF–SiO2–Zn appeared more like clay with a
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rough texture. The RF–Zn monoliths were much denser compared with their counterparts, as
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indicated in Table 1. Notably, at a Z/R value of 0.25, the shrinkage was maximum (fig. 2a);
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the bulk density also reached a maximum in the RF–Zn monolith series, which is discussed in
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the next section.
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Figure 2. (a) Volume shrinkage observed in RF-Zn monoliths at different Z/R values of (i) 0 (ii) 0.10 (iii) 0.25 (iv) 0.50 (v) 0.75 (vi) 1.00. (b) Cracks formed in monolith after carbonization of RF-Zn to form C-ZnO (c) Crack-free monolith obtained after carbonization of RF-SiO2-Zn to form C-SiO2-ZnO.
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The carbonization of dried monoliths leads to evolution of gases such as carbon monoxide,
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hydrogen, and water vapor from within the bulk of the monolith and can cause it to crack.
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Even then, the C–ZnO monoliths cracked upon carbonization (fig. 2b); however, the C–SiO2–
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ZnO monoliths were obtained as crack-free single monolithic units at least for Z/R < 0.5 (fig.
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2c). This finding can be attributed to the fact that carbon has a higher thermal expansion
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coefficient than fused silica and ZnO (wurtzite) crystals.16-18 The phenolic network shrinks as
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it becomes carbonized. However, at the same time, growth of ZnO crystallites occurs. Both
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these processes are conflicting, and the unmatched expansion and contraction of the two
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different phases create internal stresses leading to cracks. The scanning electron microscopy
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(SEM) image in fig. 5a indicates that the ZnO species was squeezed out of the bulk to form
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crystallites on the carbon surface rather than having a uniform distribution within the bulk.
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However, for the C–SiO2–ZnO monoliths, the ZnO species appears to be well distributed
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within the bulk. The silica network retains and supports the structure as the phenolic network
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shrinks, allowing the local shrinkage of the carbonized species into the pores created by the
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framework of the silica network because of the higher thermal expansion coefficient of
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carbon. Retention of the structural integrity by the silica network thus prevents crack
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formation. However, at higher Z/R values, larger crystallites of ZnO are formed, and a similar
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phenomenon occurs as in the case of C–ZnO. Thus, we selected a Z/R value of 0.25 for all
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our syntheses and related studies.
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The compressive modulus of the RF monoliths increased significantly upon increasing the
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Z/R ratio. As observed in Table 1, the compressive modulus increased for the RF–Zn
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monoliths, reaching a maximum for a Z/R value of 0.5 before starting to decline. A somewhat
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similar trend is also observed for the bulk density of the monoliths. The compressive modulus
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increased as the bulk density increased as expected. However, the trend did not exhibit a
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power law density dependence as for the low-density aerogels reported by Pekala et al.19
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Table 1. Comparison of bulk densities and compressive moduli of the monoliths.
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RF-Zn Monoliths
RF-SiO2-Zn Monoliths
RF
Compressive Modulus (MPa) 95.7
Bulk Density (g/cc) 1.1
RF-SiO2
Compressive Modulus (MPa) 1.1
Bulk Density (g/cc) 0.26
RF-Zn (0.10)
185.3
1.55
RF-SiO2-Zn (0.10)
4.5
0.32
RF-Zn (0.25)
351.8
1.68
RF-SiO2-Zn (0.25)
284.2
0.53
RF-Zn (0.50)
361.9
1.65
RF-SiO2-Zn (0.50)
581.5
0.70
RF-Zn (0.75)
300.2
1.59
RF-SiO2-Zn (0.75)
417.2
0.82
RF-Zn (1.00)
234.4
1.46
RF-SiO2-Zn (1.00)
295.3
1.25
Specimen
Specimen
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Figure 3. Stress v/s Strain plots for (a) RF-Zn monoliths and (b) RF-SiO2-Zn monoliths with varying concentration of ZnO precursor (c) C only and C-ZnO obtained by carbonization of RF and RF-Zn(0.25) respectively (d) C-SiO2, C-SiO2-ZnO and C-SiO2-ZnO-Ag obtained by carbonization of RF-SiO2, RF-SiO2-Zn(0.25) and RF-SiO2-Zn-Ag respectively.
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The abrupt increase in the modulus and density can be attributed to the additional role of the
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divalent Zn2+ ions, which form coordination bonds with the oxygen-containing functional
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groups -OH of the RF chain as shown in fig. 4a. This bonding leads to more compact packing
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within the monolith bulk as the RF chains are arranged and held tightly in space by the
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linkages with the metal ions, which is responsible for the enhanced mechanical properties
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reported in some previous works.20, 21 The metal ions also prevent the need for the addition
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of a catalyst, as they assist in the gelation process because of their acidity.22 A similar trend
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was observed for the RF–SiO2–Zn monoliths. However, the maximum compressive modulus
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attained was 581.5 MPa which is substantially higher than that of RF–Zn at 361.9 MPa. This
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enhancement can be attributed to the probable hydrogen bonds present between the silanol
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groups in the silica network (fig. 4b) and the hydroxyl groups of the RF chain, providing
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additional strength to the monoliths. However, as the concentration of the metal ions
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continues to increase within the bulk, there might not be any sites remaining for coordination
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to occur. This phenomenon leads to the agglomeration of these ions, which is detrimental to 10 ACS Paragon Plus Environment
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the structure as these agglomerates can act as stress concentrators and hinder a uniform stress
2
distribution resulting in inferior mechanical properties.23
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Figure 4. Two major networks that form the backbone of the monoliths. (a) RF or phenolic network with Zn2+ metal ions linking polymer chain via coordination bonds (b) Silica network formed by hydrolysis of TEOS.
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The RF monoliths exhibited a smooth and broad plastic region (fig. 3a), indicating that the
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RF chains could slide over one another once the applied load overcame the coordination
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bonds. In contrast, the RF–SiO2–Zn monoliths exhibited a narrow and slightly distorted
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plastic region (fig. 3b). This finding most likely occurred because the presence of two
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different networks did not allow for much movement of the polymeric chains. The RF–SiO2–
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Zn monoliths contain many more voids than the RF-Zn monoliths, as evident from their low
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bulk densities. The distortion in the plastic region indicates that the monolith was repeatedly
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cracking yet recovering. The energy was being absorbed as the structure was collapsing into
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the pores, eventually leading to complete failure after all the pores were filled.
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For the carbonized monoliths, the C–ZnO monoliths naturally exhibited poor mechanical
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properties compared with C-only because of the crack formation (fig. 3c), as discussed
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earlier. However, for the C–SiO2 monoliths, we see remarkable improvement, with
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compressive moduli of C–SiO2–ZnO (𝜌𝜌𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 = 0.58 g/cc) and C–SiO2–ZnO–Ag (𝜌𝜌𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 = 0.63 g/cc) of 32.3 and 36.1 MPa, respectively (fig. 5d) compared with the value of 0.37 MPa
for C–SiO2 (𝜌𝜌𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 = 0.16 g/cc). Their bulk densities were much lower than those of carbon xerogels, suggesting the presence of voids and pores within the bulk.
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Fig. S3a presents all the spectra for the RF monoliths, and fig. S3b presents those of their
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carbonized forms. The broad peak observed in the range 3200–3600 cm-1 in both spectra
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corresponds to the OH stretching. The peaks at 460 cm-1 and 1050 cm-1 in the RF and
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carbonized monoliths (with TEOS and (3-aminopropyl)triethoxysilane (APTES) added) are
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attributed to O–Si–O stretching and Si–O–Si stretching, respectively, confirming the presence
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of a silica moiety within the bulk of the monolith formed from the hydrolysis of the
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precursors. In addition, the peak at 1561 cm-1 is due to the stretching of NH bonds present in
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the silica network caused by APTES.24_ENREF_2,25 The peaks at 2860 and 2922 cm-1 can be
13
related to CH2 stretching and bending vibrations, respectively. For the RF monoliths, the peak
14
(shoulder) at 1222 cm-1 directly adjacent to Si-O–Si stretching corresponds to methylene
15
ether linkages within the RF polymer chain, indicating that appropriate condensation has
16
occurred within the bulk.26
17
The surface morphology of the carbonized monoliths was analyzed. In fig. 5a, the ZnO
18
species appears to erupt out from within the bulk of the C–ZnO monolith and forms
19
crystallites on the carbon surface, as displayed in fig. 5b. The size of these crystallites varied
20
greatly within the micrometer range, and especially at high Z/R values, the crystallites
21
agglomerated to form much larger structures. This behavior leads to the formation of two
22
distinct entities, i.e., the carbon network and ZnO crystals, rather than a homogenous
23
distribution, which also leads to crack formation (circled) in the monoliths, as observed in fig.
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5c.
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Figure 5. SEM micrographs of (a) C-ZnO showing ZnO species emerging out of the monolith bulk (b) C-ZnO with ZnO crystallites formed on the carbon surface (c) C-ZnO at Z/R ratio much higher than 0.25 showing crack (circled) formed (d) Silica framework obtained by calcining RF-SiO2-ZnO (e) and (f) C-SiO2-ZnO surface with visible flow through channels, (g) and (h) C-SiO2-ZnO-Ag surface showing dispersion of silver nanoparticles with visible ZnO structure (arrow marked) (i) Magnified image of ‘h’ showing silver nanoparticles immobilized to the monolith matrix.
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The carbon entity in the C–ZnO monoliths appears rather rigid and compact, devoid of any
11
major channels or pores; however, the C–SiO2–ZnO monoliths have a much different
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morphology. The incorporation of the silica moiety into the monolith bulk opens up void
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spaces because of the local shrinkage of the carbon entity into the silica framework, as
14
discussed earlier (figs. 5e, 5f). These voids help in forming transport pathways throughout the
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bulk, interconnecting the pores similar to a branched network. In addition, there are no
16
distinct ZnO crystallites separating out from the network as for C–ZnO, which suggests that
17
the silica moiety helps in avoiding agglomeration of the ZnO species and provides
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homogeneity to the bulk. This phenomenon is evident from the SiO2 network (fig. 5d) 13 ACS Paragon Plus Environment
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obtained by calcination of the C–SiO2–ZnO, which contains many macropores (in which the
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carbon entity resided before calcination because of local shrinkage) and lacks any visible
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ZnO crystals. This suggests a uniform distribution and minimal crystal growth of ZnO, as
4
confirmed by X-ray diffraction (XRD) analysis (fig. 7). Figs. 5g, 5h, and 5i show the surface
5
morphology of the C–SiO2–ZnO–Ag monolith. Ag nanoparticles are distributed throughout
6
the monolith bulk both on the surface (fig. 5i) and beneath it (fig. 5g). Small crystallites of
7
ZnO can also be observed among the Ag nanoparticles (marked by the arrow in fig. 5h),
8
emphasizing the stabilizing effect of the silica moiety and its role in imparting a homogenous
9
distribution.
10 11 12 13
Figure 6. TEM micrographs of C-SiO2-ZnO-Ag (a) showing uniform distribution of silver nanoparticles (circled) throughout the monolith bulk (b) showing small ZnO crystallite emphasizing stabilizing effect of silica.
14 15
TEM analysis of the C–SiO2–ZnO–Ag monoliths was performed to obtain insight into the
16
arrangement of entities throughout the bulk. Fig. 6 clearly shows Ag nanoparticles (circled)
17
well dispersed throughout the monoliths, avoiding their agglomeration which is essential for
18
good catalytic activity. In addition, one can see the small columnar structure of ZnO
19
(indicated by the arrow, fig. 6b) smaller than 1 μm, which once again highlights the
20
stabilizing effect of silica in suppressing the crystal growth of ZnO.
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The XRD patterns of the carbonized monoliths are compared in fig. 7. The broad peaks
2
observed at 2θ values of approximately 26° and 44° in all the monoliths are typical of carbon
3
derived from RF polymers and can be attributed to (002) and (101) planes, respectively.27 For
4
the C–ZnO monoliths, the distinct peaks at 31.6°, 34.3°, and 36.1° and faint peaks at 56.5°,
5
62.7°, and 67.9° are the characteristic peaks of ZnO and correspond to (100), (002), (101),
6
(110), and (103) planes, respectively.28
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Figure 7. XRD pattern of all carbonized monoliths
9
The distinct peaks indicate the presence of crystalline ZnO structures, which is also clear
10
from the SEM micrographs. For C–SiO2, there are no peaks, suggesting an amorphous phase
11
throughout. The C–SiO2–ZnO exhibited a much less intense peak at 36.2°, corresponding to
12
the (101) plane of ZnO, which accentuates the stabilizing effect of the silica moiety in the
13
monolith bulk. The presence of silica in the immediate surroundings of ZnO limits its
14
crystalline growth, thus resulting in the reduced intensity of the peak.29 This phenomenon
15
also explains why no distinct ZnO crystallites were visible in the SEM micrographs of C–
16
SiO2–ZnO monoliths as for C–ZnO. Finally, for C–SiO2–ZnO–Ag, the very intense peaks
17
observed at 38.1°, 44.3°, 64.4°, and 77.4° can be related to the (111), (200), (220), and (311)
18
planes of Ag, respectively.30,31
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1
X-ray photoelectron spectroscopy (XPS) measurements of the monoliths were performed to
2
determine the chemical compositions of the monoliths. It was assumed that the surface
3
composition would approximately represent the composition of the bulk as the monoliths
4
were uniform and homogenous in composition. The wide-scan analysis of the C–SiO2–ZnO–
5
Ag monolith revealed core-level photoelectrons with distinct peaks corresponding to C, O, Si,
6
Zn, and Ag (fig. S4a). The binding energy values were matched with the NIST XPS database
7
for recognition of the respective elemental states. Fig. S4(b–f) present the detailed scans for
8
C-1s, O-1s, Si-2p, Zn-2p, and Ag-3d regions, respectively. The binding energy (B.E.) scale
9
was calibrated using C-1s (285 eV) as a reference. The asymmetrical C-1s peak was
10
deconvoluted into two peaks with B.E. values of 284.8 and 286 eV, indicating the presence of
11
C–C and C–O bonds, respectively, as observed in fig. S4b. The O-1s peak (fig. S4c) was
12
deconvoluted into two peaks with B.E. values of 533.1 and 530.9 eV, corresponding to SiO2
13
and ZnO, respectively. The Si-2p peak (fig. S4d) at 103.6 eV confirms the presence of a silica
14
entity in the form of SiO2 32 . The Zn 2p level peaks of 2p1/2 and 2p3/2 observed at 1045.6 and
15
1022.6 eV in fig. S4e are due to ZnO
16
peaks of 3d3/2 and 3d5/2 at 374.6 and 368.6 eV, respectively. This result indicates that the Ag
17
is present in the metallic Ag form, which is essentially the catalyst for the catalytic reduction
18
of PNP.34
19
The N2 adsorption isotherms of C-SiO2, C-SiO2-ZnO and C–SiO2–ZnO–Ag monolith samples
20
are presented in fig. S5 (a,b,c) respectively. All the isotherms displayed a hysteresis loop
21
which is typical of a Type IV isotherm and is a signature of mesoporous materials.35, 36 It is
22
also interesting to observe that the desorption branch of the isotherms do not match the
23
adsorption branch, suggesting the presence of ultramicropores that do not empty even at the
24
low-pressure range. Thus, a residual volume is observed in the desorption branch (pore
25
diameter < 0.8 nm).37 The pore size distribution was determined using the Barrett–Joyner–
33
. In addition, in fig. S4f, we can observe the Ag-3d
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Halenda method (inset) and reveals two major systems of pores — one in the ~40 Å domain,
2
which contributes to mesoporosity and is essential for the transport of species and the other in
3
the < 20 Å domain, which suggests the presence of micropores and ultramicropores and is
4
consistent with the findings we inferred from the isotherm. The microporosity observed is a
5
characteristic contribution of the carbon network.38 However, mesoporosity is mostly caused
6
by the silica network.
7
The Brunauer–Emmett–Teller surface area of the C-SiO2, C-SiO2-ZnO and C-SiO2-ZnO-Ag
8
samples were found to be 258, 152 and 238 m2/g respectively. The marked decrease in
9
surface area may be attributed to presence of entities like ZnO microstructures and Ag
10
nanoparticles within the monolithic matrix which might be compromising the pore density. It
11
can also be observed that C-SiO2 contains mesopores predominantly with a pore width of ~60
12
Å while in case of C-SiO2-ZnO and C-SiO2-ZnO-Ag it was ~40 Å. Additionally, the
13
hysteresis loop in case of C-SiO2 resembles Type H1 which suggests presence of
14
agglomerates of nearly uniform spheres while in the case of C-SiO2-ZnO and C-SiO2-ZnO-
15
Ag, it resembles Type H4 indicating the presence of narrow slit like pores.36
16
The permeability of the prepared “flow-through” assembly of monoliths was calculated using
17
Darcy’s law:39
18
Q =
19
where Q is the flow rate of fluid through the monolith, K is the medium’s permeability
20
(Darcy’s number), ΔP is pressure drop across the monolith, L is the length of the monolith
21
across which pressure drop is measured, A is the cross-sectional area of flow, and μ is the
22
viscosity of the fluid flowing through the monolith. A simple setup was created, and water
23
was allowed to flow through the “flow-through” assembly, as illustrated in fig. 8b. The flow
𝐾𝐾∆𝑃𝑃𝑃𝑃 𝜇𝜇𝜇𝜇
,
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1
was solely due to the action of gravity. The height of the water column above the monolith
2
surface was converted into its equivalent pressure units, and the flow rate was measured at
3
varying pressures.
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4 5 6
Figure 8. (a) Plot of flowrate v/s pressure for the carbonized monoliths – C-SiO2 and C-SiO2ZnO-Ag to measure and compare their permeability (b) Set up for determining permeability
7 8
From the plot, one can observe that the flow rate exhibited a linear relationship with the
9
pressure difference, which is consistent with Darcy’s law. The flow rate at very low pressure
10
difference (
