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Summary

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A synthesis method for cellulose nanofiber biotemplated palladium composite aerogels is presented.The resulting composite aerogel materials offer potential for catalysis, sensing, and hydrogen gas storage applications.

Abstract

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Here, a method to synthesize cellulose nanofiber biotemplated palladium composite aerogels is presented. Noble metal aerogel synthesis methods often result in fragile aerogels with poor shape control. The use of carboxymethylated cellulose nanofibers (CNFs) to form a covalently bonded hydrogel allows for the reduction of metal ions such as palladium on the CNFs with control over both nanostructure and macroscopic aerogel monolith shape after supercritical drying. Crosslinking the carboxymethylated cellulose nanofibers is achieved using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in the presence of ethylenediamine. The CNF hydrogels maintain their shape throughout synthesis steps including covalent crosslinking, equilibration with precursor ions, metal reduction with high concentration reducing agent, rinsing in water, ethanol solvent exchange, and CO₂ supercritical drying. Varying the precursor palladium ion concentration allows for control over the metal content in the final aerogel composite through a direct ion chemical reduction rather than relying on the relatively slow coalescence of pre-formed nanoparticles used in other sol-gel techniques. With diffusion as the basis to introduce and remove chemical species into and out of the hydrogel, this method is suitable for smaller bulk geometries and thin films. Characterization of the cellulose nanofiber-palladium composite aerogels with scanning electron microscopy, X-ray diffractometry, thermal gravimetric analysis, nitrogen gas adsorption, electrochemical impedance spectroscopy, and cyclic voltammetry indicates a high surface area, metallized palladium porous structure.

Introduction

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Aerogels, first reported by Kistler, offer porous structures orders of magnitude less dense than their bulk material counterparts^1,2,3. Noble metal aerogels have attracted scientific interest for their potential in power and energy, catalytic, and sensor applications. Noble metal aerogels have recently been synthesized via two basic strategies. One strategy is to induce the coalescence of pre-formed nanoparticles^4,5,6,7. Sol-gel coalescence of nanoparticles can be driven by linker molecules, changes in solution ionic strength, or simple nanoparticle surface free energy minimization^7,8,9. The other strategy is to form aerogels in a single reduction step from metal precursor solutions^{9,10,11,12,13}. This approach has also been used to form bimetallic and alloy noble metal aerogels. The first strategy is generally slow and may require up to many weeks for nanoparticle coalescence¹⁴. The direct reduction approach, while generally more rapid, suffers from poor shape control over the macroscopic aerogel monolith.

One possible synthesis approach to address challenges with control of noble metal aerogel macroscopic shape and nanostructure is to employ biotemplating¹⁵. Biotemplating uses biological molecules ranging from collagen, gelatin, DNA, viruses, to cellulose to provide a shape-directing template for the synthesis of nanostructures, where the resulting metal-based nanostructures assume the geometry of the biological template molecule^16,17. Cellulose nanofibers are appealing as a biotemplate given the high natural abundance of cellulosic materials, their high aspect ratio linear geometry, and ability to chemically functionalize their glucose monomers^{18,19,20,21,22,23}. Cellulose nanofibers (CNF) have been used to synthesize three dimensional TiO₂ nanowires for photoanodes²⁴, silver nanowires for transparent paper electronics²⁵, and palladium aerogel composites for catalysis²⁶. Further, TEMPO-oxidized cellulose nanofibers have been used both as a biotemplate and reducing agent in the preparation of palladium decorated CNF aerogels²⁷.

Here, a method to synthesize cellulose nanofiber biotemplated palladium composite aerogels is presented²⁶. Fragile aerogels with poor shape control occurs for a range noble metal aerogel synthesis methods. Carboxymethylated cellulose nanofibers (CNFs) used to form a covalent hydrogel allow for the reduction of metal ions such as palladium on the CNFs providing control over both nanostructure and macroscopic aerogel monolith shape after supercritical drying. Carboxymethylated cellulose nanofiber crosslinking is achieved using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in the presence of ethylenediamine as a linker molecule between CNFs. The CNF hydrogels maintain their shape throughout the synthesis steps including covalent crosslinking, equilibration with precursor ions, metal reduction with high concentration reducing agent, rinsing in water, ethanol solvent exchange, and CO₂ supercritical drying. Precursor ion concentration variation allows for control over the final aerogel metal content through a direct ion reduction rather than relying on the relatively slow coalescence of pre-formed nanoparticles used in sol-gel methods. With diffusion as the basis to introduce and remove chemical species into and out of the hydrogel, this method is suitable for smaller bulk geometries and thin films. Characterization of the cellulose nanofiber-palladium composite aerogels with scanning electron microscopy, X-ray diffractometry, thermal gravimetric analysis, nitrogen gas adsorption, electrochemical impedance spectroscopy, and cyclic voltammetry indicates a high surface area, metalized palladium porous structure.

Protocol

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​CAUTION: Consult all relevant safety data sheets (SDS) before use. Use appropriate safety practices when performing chemical reactions, to include the use of a fume hood and personal protective equipment (PPE). Rapid hydrogen gas evolution can cause high pressure in reaction tubes causing caps to pop and solutions to spray out. Ensure that reaction tubes remain open and pointed away from the experimenter as specified in the protocol.

1. Cellulose nanofiber hydrogel preparation

1. Preparation of cellulose nanofiber solution: Prepare 3% (w/w) cellulose nanofiber solution by mixing 1.5 g of carboxymethyl cellulose nanofibers with 50 mL of deionized water. Shake the solution and vortex for 1 min. Sonicate the solution in a bath sonicator at ambient temperature for 24 h to ensure complete mixing.
2. Preparation of cross-linking solution: First add 0.959 g of EDC and 0.195 g of 2-(N-morpholino)ethanesulfonic acid (MES) buffer to 2.833 mL of deionized water. Vortex. Add 0.167 mL of ethylenediamine. Vortex for 15 s. Adjust the final volume to 10 mL and pH to 4.5 by adding 1.0 M HCl and deionized water.
   NOTE: Final crosslinking solution concentrations are 0.5 M EDC, 0.25 M ethylenediamine, and 0.1 M MES buffer.
3. Centrifugation of cellulose nanofiber solution: Pipette 0.25 mL of the 3 % (w/w) cellulose nanofiber solution into each of 6 microfuge tubes (1.7 mL or 2.0 mL). Centrifuge the microfuge tubes for 20 min at 21,000 x g. Remove excess water above the compacted CNFs with a pipette avoiding contact with the top surface.
   NOTE: After centrifuging, the cellulose nanofiber solutions present a distinct interface between the concentrated CNF's and the clear supernatant. Based on removal of the excess water, the final CNF concentration will be approximately 3.8 %.
4. Cross-link the cellulose nanofiber hydrogels. Pipette 1.0 mL of the EDC and diamine crosslinking solution above the compacted cellulose nanofibers in each of the microfuge tubes. Wait at least 24 h for the crosslinking solution to diffuse through the gels and crosslink the CNFs.
5. Gel rinsing: Remove the crosslinking solution supernatant in the microfuge tubes with a pipette. With the microfuge tube caps open, immerse the microfuge tubes containing the crosslinked CNF gels in 1 L of deionized water for at least 24 h to remove excess crosslinking solution from within the CNF hydrogels.
6. Fourier-transform infrared (FTIR) spectroscopy: Place approximately 0.5 mL of 3% (w/w) CNF solution in deionized water on the sample stage and scan percent transmittance for 650 – 4000 cm^-1. Use the same scan conditions and repeat for a CNF crosslinked hydrogel from Step 1.5.

2. Preparation of cellulose nanofiber – palladium composite hydrogels

1. Prepare Pd(NH₃)₄Cl₂ solution. Prepare 10 mL of 1.0 M Pd(NH₃)Cl₂ solution. Vortex the solution for 15 s. Dilute 1.0 M Pd(NH₃)Cl₂ solution to 1 mL volumes at 1, 10, 50, 100, 500, and 1000 mM.
   NOTE: 1.0 M NaPdCl₄ solution and respective dilutions may be used and results in similar final aerogel structures.
2. Equilibrate cellulose nanofiber hydrogels in palladium solutions. Pipette 1 mL of the 1, 10, 50, 100, 500, and 1000 mM Pd(NH₃)Cl₂ solutions on the top of the cellulose nanofiber hydrogels in the microfuge tubes. Wait at least 24 h for the palladium solution to equilibrate within the hydrogels.
3. Prepare NaBH₄ reducing agent solution. Prepare 60 mL of 2 M NaBH₄ solution. Aliquot 10 mL of NaBH₄ solution into each of six 15 mL conical tubes.
   NOTE: The 2 M NaBH₄ solution is a highly concentrated reducing agent solution and should be handled within a chemical fume hood. Spontaneous decomposition and hydrogen gas evolution will be observed. Ensure that the tubes are pointed away from the experimenter and that proper PPE is worn.
4. First reduction of palladium salts on cellulose nanofiber hydrogels: Invert the microfuge tubes with the palladium equilibrated CNF hydrogels and gently tap to remove the hydrogels. In a chemical fume hood, with flat tweezers, place each of the palladium equilibrated CNF hydrogels into each of the 15 mL conical tubes with 10 mL of NaBH₄ solution. Allow the reduction to proceed for 24 h.
   NOTE: Upon placing the palladium equilibrated CNF gels into the 2 M NaBH₄ solution, violent hydrogen gas evolution will occur. Ensure that reaction tubes remain open and pointed away from the experimenter.
5. Prepare second NaBH₄ reducing agent solution. Prepare 60 mL of 0.5 M NaBH₄ solution. Aliquot 10 mL of NaBH₄ solution into each of six 15 mL conical tubes.
6. Second reduction of palladium salts on cellulose nanofiber hydrogels: In a fume hood, using a pair of flat tweezers transfer each of hydrogels from the 2 M NaBH₄ solutions into the 0.5 M NaBH₄ solutions. Allow the reduction to proceed for 24 h.
   NOTE: The initially reduced CNF gels in the 2 M NaBH₄ solution will be mechanically stable during the transfer step. However, light pressure should be used with the flat tweezers during solution transfer steps to avoid gel compaction.
7. Rinse the cellulose nanofiber-palladium composite gels. Using flat tweezers, transfer each of the reduced palladium-CNF gels into 50 mL deionized water in conical tubes. Exchange deionized water after 12 h and allow the gels to rinse for at least an additional 12 h.
8. Perform ethanol solvent exchange in cellulose nanofiber-palladium gels. Use flat tweezers to transfer the rinsed CNF-palladium gels successively into 50 mL of 25%, 50%, 75%, and 100% ethanol solutions with at least 6 h in each solution.

3. Aerogel preparation

1. After solvent exchange with ethanol, dry the CNF-palladium gels using CO₂ in a supercritical dryer with a set point of 35 °C and 1200 psi. After supercritical drying is complete, allow the chamber to equilibrate for at least 12 h prior to opening and removal of the aerogels.
   NOTE: Occasionally, the 500 mM and 1000 mM samples have been observed to combust when removed from the supercritical dryer which is attributed to the presence of palladium hydride. The 12 h supercritical chamber equilibration is intended to allow for outgassing of hydrogen.

4. Composite aerogel material characterization

1. Scanning electron microscopy (SEM): Cut the CNF-palladium aerogel with a razor blade to obtain a thin film approximately 1 – 2 mm thick. Affix the thin film sample with carbon tape on a SEM sample stub. Initially use an accelerating voltage of 15 kV and beam current of 2.7 – 5.4 pA to perform imaging.
2. X-ray diffractometry (XRD): Place the CNF-palladium aerogel in a sample holder and align the top of the aerogel with the top of holder. Alternatively, place a thin film sample section, as in Step 4.1, on a glass slide. Perform XRD scans for diffraction angles 2Θ from 5 ° to 90 ° at 45 kV and 40 mA with Cu K_α radiation (1.54060 Å), a 2 θ step size of 0.0130 °, and 20 s per step.
3. Thermal gravimetric analysis (TGA): Place the aerogel sample in the instrument crucible. Perform analysis by flowing nitrogen gas at 60 mL/min and heating at 10 °/min from ambient temperature to 700 °C.
4. Nitrogen gas adsorption-desorption: Degas the samples for 24 h at room temperature. Use nitrogen at -196 °C as the test gas with equilibration times for adsorption and desorption of 60 s and 120 s, respectively.
   NOTE: Elevated degas temperatures are not recommended to avoid decomposition of the cellulose nanofibers.
5. Electrochemical characterization.
   1. Immerse the aerogel samples in 0.5 M H₂SO₄ electrolyte for 24 h.
   2. Use a 3-electrode cell with a Ag/AgCl (3 M NaCl) reference electrode, a 0.5 mm diameter Pt wire auxiliary/counter electrode, and a lacquer coated 0.5 mm diameter platinum working electrode. Place the lacquer coated wire with a 1 mm exposed tip in contact with the top surface of the aerogel at the bottom of the electrochemical vial¹².
   3. Perform electrochemical impedance spectroscopy (EIS) from 1 MHz to 1 mHz with a 10 mV sine wave.
   4. Perform cyclic voltammetry (CV) using a voltage range of −0.2 to 1.2 V (vs. Ag/AgCl) with scan rates of 10, 25, 50, 75, and 100 mV/s.

Representative Results

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The scheme to covalently crosslink cellulose nanofibers with EDC in the presence of ethylenediamine is depicted in Figure 1. EDC crosslinking results in an amide bond between a carboxyl and primary amine functional group. Given that the carboxymethyl cellulose nanofibers possess only carboxyl groups for crosslinking, the presence of a diamine linker molecule such as ethylenediamine is essential to covalently link two adjacent CNFs via two amide bonds. To conf…

Discussion

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The noble metal cellulose nanofiber biotemplated aerogel synthesis method presented here results in stable aerogel composites with tunable metal composition. The covalent crosslinking of the compacted cellulose nanofibers after centrifugation results in hydrogels that are mechanically durable during the subsequent synthesis steps of palladium ion equilibration, electrochemical reduction, rinsing, solvent exchange, and supercritical drying. The hydrogel stability is vital during the electrochemical reduction step given th…

Materials

0.5 mm platinum wire electrode	BASi	MW-4130	Used for auxillery electrode and separately for lacquer coating and use as a working electrode
1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC)	Sigma-Aldrich	1892-57-5
2-(N-morpholino)ethanesulfonic acid (MES)	Sigma-Aldrich	117961-21-4
Ag/AgCl (3M NaCl) Reference Electrode	BASi	MF-2052
Carboxymethyl cellulose, TEMPO Cellulose Nanofibrils, Dry Powder	University of Maine Process Development Center	No 8
Ethanol, 200 proof	PHARMCO-AAPER	241000200
Ethylenediamine	Sigma-Aldrich	107-15-3
Fourier-Transform Infrared (FTIR) Spectrometer, Frontier	Perkin Elmer	L1280044
Hydrochloric Acid	CORCO	7647-01-0
Na2PdCl4	Sigma-Aldrich	13820-40-1
NaBH4	Sigma-Aldrich	16940-66-2
Pd(NH3)4Cl2	Sigma-Aldrich	13933-31-8
Potentiostat	Biologic-USA	VMP-3	Electrochemical analysis-EIS, CV
Scanning Electron Mciroscope (SEM) Helios 600 Nanolab	ThermoFisher Scientific
Supercritical Dryer	Leica	EM CPD300	Aerogel supercritical drying with CO2
Surface and Pore Analyzer	Quantachrome	NOVA 4000e	Nitrogen gas adsorption
Thermal Gravimetric Analysis	TA instruments	TGA Q500
Ultrasonic Cleaner	MTI	EQ-VGT-1860QTD
XRD	PanAlytical	Empyrean	X-ray diffractometry