Electroless Synthesis of Cellulose-Metal Aerogel …...2 Functional aerogel metals can be prepared by a direct sol-gel approach1 or by adding metal salts to the precursor solution.7

1

Electroless Synthesis of Cellulose-Metal Aerogel Composites

M. Schestakow,1,a) F. Muench,2,a) C. Reimuth,2 L. Ratke,1 and W. Ensinger2

1Deutsches Zentrum für Luft- und Raumfahrt e.V., Institute of Materials Research, Linder Höhe, 51147, Köln, Germany

2Department of Materials and Geoscience, Technische Universität Darmstadt, Alarich-Weiss-Str. 2, 64287, Darmstadt,

Germany

An environmentally benign electroless plating procedure enables a dense coating of silver nanoparticles onto complex

cellulose aerogel structures. In the course of the nanoparticle deposition, the morphological characteristics of the aerogel are

preserved, such as the continuous self-supporting network structure. While achieving a high metal loading, the large specific

surface area as well as the low density is retained in the cellulose-metal aerogel composite. Due to the interesting features of

cellulose aerogel substrates (e.g. the accessibility of its open-porous network) and electroless plating (e.g. the possibility to

control the density, size and composition of the deposited metal nanoparticles), the outlined synthetic scheme provides a

facile and flexible route towards advanced materials in heterogeneous catalysis, plasmonics and sensing.

Porous noble metal nanostructures represent a remarkable material class composed of foams, sponges, aerogels, or

nanotube assemblies.1-5 Micro- and mesoporous metals produce highly active materials by means of their interfacial effects,

and for the presence of macropores that provide efficient pathways for mass transport.4 Metal aerogels or aerogel supported

metal particle assemblies that combine an exceptionally high surface area with multiple pore sizes with a monolithic, self-

supporting structure, have attracted special interest.1,3,6-8 Due to the wide functionality range and high degree of efficiency

that can be realized with nano-structured metals, this material class can meet emerging needs in energy storage,9 catalysis,10,11

sensor technology,12,13 plasmonics14 or drug delivery.4,15 For instance, Ag nanoparticle (NP) deposits on polysaccharidic

matrices address medical applications such as filtration, separation, drug delivery or plasmonics.7,15-18 Typical synthetic

strategies include the attachment of metal NPs to various template or substrate materials, sometimes followed by the template

removal.9,18-21 The results are highly active materials, but the procedures are often limited with regard to NP loading or

production volume leading to reduced performance or lack of scalability.

_____________________________

a) Authors to whom correspondence should be addressed. Electronic mail: [email protected], [email protected].

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MM

1Deutsches Zentrum für Luft- und Raumfahrt e.V., Institute of Materials Research, Linder Höhe, 51147,7 K

2Departm2 ent of Materials and Geoscience, Technische Universität Darmstadt, Alarich-WeWeWeWeWeWeWeWeWeWeWeWeWeWeWeeeeWeisisisisisissssisssisssssssssssssssssssss----StSS r. 2, 642

Germany

An environmentally benign electroless plating procedure enables a denssssssssseeeee eee ee eee eeee eee ccccoccocccccccccccccc attttttttiiiiiiiiiiiiiiinngngngngngngngnngnnngngngngggggggggggggggggg of ffffffff sisisisisiiiisisisisisisisisisisisisisisisiisisisillvllllllllllll er nanop

cellulose aerogel structures. In the course of the nanoparticle depositionnnnnnnnn,,,, ,, , ,, ,, thththhhhththththhththhhthhhhthhhheeeeeeeeeee eeee e ee momomoomoommmmomommmmmmmmommmmmmmmmmmmmmmm rprprprprprpppppprprpprprppprprprprprprprprprppphohohohohohohohohohohohohohhohhhohohhoohooohoohooologiiiiiiiiiiiiical characteri

preserved, such as the continuous self-ff supporting network structure. Whhhhhhhhhhhhhhhhhhhhhhililililililillilillililliliiiilii eeeeeeeeeeeeeeeeeee achieveveevvvvvvvvvvvvvvvvvevevevevevvvvininininininininiiniiiniinnng a high metal loa

surface area as well as the low density is retained in the cellulose------mememmemememememmmememememememmmememmmmmmm tatataaaatatatatatataataatataal aerorororooooooogegegegegeeeeegegegegeggegegegegegegegegeggelllllllllllllllllllllllllll cocococococoocococoococoocococccocccccc mposite. Due to the

cellulose aerogel substrates (e.g. the accessibility of its open-popopoooopopoooooopooopooopoppoooooop rororrrrrorrorrrroususususususssssssususususuususs nnnnnnnnnnnnnnnnnnnnneeetettetetttetetettetetetette woorrrkrkrkrkrkkkkkkrkrkrkkrkrkrkkkrkkkkrkrkkkkrr ) and electroless plating

control the density, size and composition of the depositedddddddddd mmmmmmmmmmmmmmmmmmmmmmmetallllllllll nnnnnnnnnnnnnnnnnnnnnnnnnnnnananaannananananananananananananananannananaanopopopoppopoppopoooooo arararararrararrrrararrar itititititititiiitittttittt cles), the outlined synthe

facile and flexible route towards advanced materials in heeeeetetetteteteeeeeteteteteeeeteeteeterororororororrorrroororororrorororrrorr gegeegeegegegegeeggggggggggggggggg neneneneneneneneneeeeeeneennnennnnnnnn ououououoououoouooououoooouoouououus cacacccacacacacacaaaacacacacaacacaccccaacatttttatttttttttttttttttttt lysis, plasmonics and sensing

Porous noble metal nanostructures repressssssssenenenenenenenenenenenenenenenenennenennenenenenenenennennennnnttttttttt tttttttttttttt aaaaaaaaaaaaaaaaaaa a aaaaaa rereeererererrrrrr mamamamamamamamamamamamamammamamamammammamaaamammarkrrrr able material class composed of foams,

nanotube assemblies.1-5 Micro- and mesopoooooooooororororororororororoororororororoororroroororororrr usususususususuusuusuusuuuuuuuuuuu mmmmmmmmmmmmmmmmmmmmmmmmmmeeetetetetettteteeeteetetetetete alalalalalalalalalaalalaaaalaalaalllllls produce highly active materials by means of t

and for the presence of macroporesssssssss ttttttttttttttthahahahahahahahahhahahahahhahaaahahaahahhah tttttttttttttttttttttt prprprprprprprprrprprprprrrprrprprprprprprpppp ovoooooooooooooooooooooooooo ide efefefefffffeffffffffffffffffffiffffffffffffffff cient pathways for mass transport.4 Metal aerogel

metal particle assemblies that cocococococococoococococoocococococccoc mmbmbmmbbmbmmmmm innnnnnnininniinininininininneeeeeeeeee eeeeeeeeeeeeeeeee ananaanananaaananananaananaananannannnnaannnnan eeeeeeeeexcxxcxcxcxcxcxccccxcxcxcxccxcxcxxccccxccccceptionally high surface area with multiple pore sizes w

supporting structure, have aaaaaaaaaaaaaaaaaaaaaattttttttttttttttttttttttttttttttttttttttttrrrrararaarr ctctctctctctctctcttttttctctcctctctctctctctccttedededededededededededededededededdededededddd speecicciciciciciciiciciiciciiiiiciciiciciicciaalaaaaaaaaaaaaaaaaaaaa interest.1,3,6-8 Due to the wide functionality range and hig

that can be realized witittttttttttttthhhhhh h hhhhhh hhhhhhh nanananannananannanananananannaanaaaaan nonnnnnnnnnnnnnnnnnnnnn -stststststtttttsttsttststststststttsttttrurururururururuururururuurururururururururrrr cccccccccccccccccccccccctut red metals, this material class can meet emerging needs in energy

sensor technology,y,y,y,y,y,y,y,y,y,yy,y,y,,y,y,,y,,y,1111111111111111111112222 1,111111111333333333333333333 plplplplplplplplplplplplplplplplpllplplplllllpllplplplplplpppp asasaassasasasasasasasasaasaasasasaasasasaasasasassmmmmmmommmmmmmmmmmmmm nics14 or drug delivery.4,15 For instance, Ag nanoparticle (NP) depos

matrices addresssssssss mmmmmmmmmmmmmmmmmmmmmmmededededededededededdededededededededeeedeedeeededede icicicicicicicicicccciciciciii al aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaapppppppppppppppppppppppppppppppppppppp lications such as filtration, separation, drug delivery or plasmonics.7,

strategies innncncnccccccccccccccncnnccnncllulululululllllllulullulude theeeeeeeeeeeeeee aaaaaaaaaaaaaaaattttttttttttttacacacacacacacccacacacccacacaccccaccchhhmh ent of metal NPs to various template or substrate materials, sometimes fo

removvvaaaaaavaaaavvaaaaavvvvavaalllllllll.l.lllllllllllllllllll 999999999 111111,11111188888888888888888888---------222221 ThThThThhhhhhhhThThhThhhhheeeeeeeeeeeeeeeee e rerereerererereererererererererereerereerer sssusssssssssssss lts are highly active materials, but the procedures are often limited with reg

prprrprprrprrprprprrrrrrprrrrodododoododododododododoodododoodoododdododdddoddddodo ucuucuuuucucuucucuuuuuuuccctiiononononnnnnnnnnnonnonononnn vvvvvvvvvvvvvvvvolololololooolololoolooolooolooooloolllllolumumumumumumumummumumumummmumumumummumumumme leading to reduced performance or lack of scalability.

___________________________ ______________________

2

Functional aerogel metals can be prepared by a direct sol-gel approach1 or by adding metal salts to the precursor solution.7

Supported nano-metal architectures are mostly realized by reduction of adsorbed metal ions17 or such methods as atomic

layer deposition,19,20 but none of these mention electroless plaiting (EP) of aerogels. EP is an easily scalable wet-chemical

process that does not require complex instrumentation and can specifically be designed to meet green chemistry

requirements.22 Due to its conformal seed-based deposition mechanism, it is well suited for the homogeneous metallization of

substrates enclosing complex pore networks.2 By adjusting the deposition time, the amount of plated metal can be controlled.

The method allows the deposition of numerous metals resulting in a variety of functionalities.2,13,20-23 In contrast to

conventional methods like in situ reduction or atomic layer deposition17,19 EP is capable of creating dense NP coatings and

thus enables considerably higher metal loading which is essential for electrical conductivity by percolation of conducting

paths as well as for highly active catalysts and sensors.2,11,13,24,25 Aerogels remarkably combine a high specific surface area,

with high porosity and accessibility of the entire pore system in a monolithic solid and therefore, qualify for outstanding

substrate materials. They are reported to effect responses in sensing, energy storage, and energy conversion orders of

magnitude faster than other porous solids.1 Porous noble metals architectures show outstanding performance in sensing

applications.12,19,26 Also, energy-related applications are conceivable.9 To meet sustainability requirements without sacrificing

performance, the use of biopolymers is promising. Cellulose is often favored for its abundancy and renewability as well as

biocompatibility and flexibility in functionalization.27 Cellulose nanocrystals or fibers efficiently support Pd or Ag NPs to

serve as catalysts in e.g. reduction of 4-nitrophenol or electrooxidation of methanol.11,17,18 Cellulose aerogels (CA) with such

characteristics as the nano-felt-like structure, adjustable in density, porosity and mechanical properties can be prepared from

easily recyclable aqueous salt hydrate melts.27-31 In this task CA substrates have two main obstacles, namely (i) fracturing of

the monolith in contact with typical, aqueous plating solutions due to strong capillary forces and (ii) sluggish diffusion of the

reactive bath components to the inner areas. In overcoming these difficulties we were able to apply an EP procedure to CA

substrate materials. Abrupt surface tension changes between different processing steps (seeding, washing, plating) was

avoided by solely working with a solvent mixture and thus avoiding fracturing. The CA, prepared as described in,29,30 was

first soaked with pure ethanol, which was continuously replaced with water until a 1:2 ratio (ethanol:water) was reached.

Intimate contact of the inner CA surface with the reagents was ensured by using a setup similar to column chromatography,

in which the substrate monolith is flushed with the reaction solutions. The environmentally friendly nature of CAs is

complemented by the EP reaction, which is performed at room temperature and uses unproblematic solvents, minimal

amounts of inorganic tin salts, and non-toxic tartrate for Ag reduction.

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requirements.22 Due to its conformal seed-based deposition mechanism, it is well suited for rr r r rrrr r ththththththtththtthththtththhthtthhhhhthhhthtthhhthhhhe homoge

substrates enclosing complex pore networks.2 By adjusting the deposition time, the ammouououououuuouuuuououuouououuuuuuuouunnnntnnnnnnnnnn oooooooooooofffffffffffffff ffff fff fffff plppplplplplplplplppplpplplpplplplplpllpllplpplppp ataaaaaaaaaa ed m

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conventional methods like in situ reduction or atomic layer deposition17,19 EPPPPPPPPPPP iiiiiiiiiiiiiiiiiiiiisssssssssssssss cacaccacccccccccccapapapapapaaaaaaaaapapaaaapaapapablblbblblbblblblblblblblblblblblblblblblblblblbbb ee eeee eeee eeeee ofofofofofoffffffofofoffffffoff ccccccccccccccccccccrrer ating d

thus enables considerably higher metal loading which is essential for electctctctririiriririiiiriiiriririririririririr cacacacacacacaacacacacacacacacacacaacacacacccaccc lllllllllllllllll l cocococccccococcoccocoocococoococooccccc ndndndndndndndndndndndndndndndnndddndndndndndndnddnducucucucucuccuccucucucuccucucucucucucucucuccuu titttttttttttttttttttt vity by perc

paths as well as for highly active catalysts and sensors.2,11,13,24,25 Aerogegegeeeeeeeeeegeeeegeeegeegeelsllslslslslslslsllllsllslssllssssss remararararrrrrrrrrrrrrrkakaakakkkakkakkkkakkkakakakkakkkakkakkkkkkakakk blblblblbllbllllllblblblllllyyyyyy yyy combine a high

with high porosity and accessibility of the entire pore system innnnnnnnnnn aaaaaaaaaaaaaaaaaaaaaaa mononononnonononnnonnonononnnnononnononnnololoololololololoololooololololllololololollllllllitittttiiitiiiitiiithihihihihihiihihihihihihihihihihh cccccccccccccc sssssossssssss lid and therefore, q

substrate materials. They are reported to effect responses in seseseseseseseeeeeseseesesesesseseeseesseseseensnsnsnnsnsnnsnnsnsnsnsnnssnsnsssssnsnssssnnsing,g,g,g,g,,,,,,g,,,,g,g,g,g eeeeeeeeeeeeeeeeenenenenenneneneneenenenneennnennneneeeeeeerrrgrrrrrr y storage, and energy

magnitude faster than other porous solids.1 Porous noblllblbllblbllllllbllblblblllleee eeeeeeeeeeeeee e ee eeeeeeee mememeeemeeeemeemeeeeetatatatatatatatatatatatatatatatatatatatatatatattt lslslsllsllslsllsllsllslssslslslsssss arcrccccccccccccccccchhhhhhhihihihhhhhihhhhhihhhiihhhhhhihihhhhh tectures show outstanding p

applications.12,19,26 Also, energy-related applications arererereeeeerereeeereeeee ccccccccccccccccccccccononononononononononnononoonoooooooo cececececececeeececececeeececececececcececeeivivivivivivivivivivivivivivvivivivivivivivivivivvvaababaabababababbbabbbabbbabababaaaba lelelelleleleleleleleleeleleleeeeeeeeeeeeee..9 To meet sustainability requireme

performance, the use of biopolymers is promisingg. CeCeCeCeCeCeCeCeCeCeCeCCCCeCeCeCeCeCCeCeCeCCCCeCeCCeCCCCCCCCC llllllllllllllllllllllllllllllllllllllllllllllllllululuululullullulullullulululululululululuululululu ososoooosooosooooooosososooooossssoso eeeeeee isisisissisisisisisisisisisisisisisisisisisisisiss often favored for its abundancy and r

biocompatibility and flexibility in functionalizzzzzzzzatatatatatatatatatatatatataatatatatatatattattatatatttattaa ioioioioioooioioioioioooiooioioioioiooiooioooiioiii n.n.n.n.nnn.n.nnnnnn.nn.nnnn 27 CCeCeCeCeCeCeCeCeCeeCeCeCeCeCeCeCeCeCeeCeCCeeeCeeeCCelllllllllllllll ulose nanocrystals or fibers efficiently sup

serve as catalysts in e.g. reduction of 4-nitrtrtrtrtrtrtrtrropopopopopopopopopopopopopppopopopopoopppoppopppppopopoopopphehehehhehehehehehehehhhehehehehehehhehehhehehehehehhheh nononononononononononononononononnononononnononon llllll l lll l llllllllll lll l ll ooooooooooooor rrrrrrrrrrr elllllllllllllectrooxidation of methanol.11,17,18 Cellulose ae

characteristics as the nano-felt-like sssssssssssssssssssstrtrtrtrtrtrtrtrtrtrtrtrtrtrrtrtrrrrucccccucucucucucucucucucucccucuuucuucuucuctutututututututututtutuututuuuuuututuutuututtt rererererererrrrrrrerrererrrrerrreree, adjustststststttttststttststststttststttstaaaba le in density, porosity and mechanical properties

easily recyclable aqueous salt hhhhhhhhhhhhydydydydydydyyydyddddydydydydddydyyddyddydydyyyyy rateteteteeeeeeteteteteteett mmmmmmmmmmmmmmmmmmmmmeleeeeeeeeeeeeeeeeeee ts.....222222222222777777777777-------31 In this task CA substrates have two main obstacles, n

the monolith in contact withththhhhthhthhhhhhhhhhhhhhh ttttttttttttttttttttyyyypypypypypypppyppyypypyyyyppypyyyyyy icicicicicicicicicicicicicicicicicicicicciiiicic lllalallalalalalalalaalaaalall,,,,,,,,, aqueueueueeeeeeeeeeueueeeeeeeeeooooooooouooooooooooooooo s plating solutions due to strong capillary forces and (ii) sl

reactive bath componenennnnennnnnnnnnnnnnenenntststsstssttstststststssss tttttttttttttto ooooooooooo thththhhhhhhhhthhhththhththeeeeeeee eeeeeeeeeeeee inininininininininiinininininnnninnnnnnnnnennn r aareas. In overcoming these difficulties we were able to apply a

substrate materialss.sss.ss.s.s.ssss..sss.ssss. AAAAAAAAAAAAAAAAAAAAAbrbrbrbrbrbrbrbrbrbrbrbrrbrbrbrbrbrbrbrbrbbbbbruupupupupuppupuupuppupupupupupuupupppupuuppupuuuuupupupupuppttttttt t ttttt tttttt surface tension changes between different processing steps (seeding, w

avoided by solelelellllllelelllellllly yyyyyyyyyy yy yyy yyyy y wwowowowowowowwwowowowwowoowowwowowowowowowowowoowooworkrkrkrkrrkrkrkrkrkrkrkkrkrkrkrkrkrkrkkrkkiniii g g wiwiwiwiwiwiwiwiwiwiwiwiwwiwiwwiwiwiwiwiwiwiwwwwwwwwww tthtttttttttt a solvent mixture and thus avoiding fracturing. The CA, prepared as

first soakeddddddddd wwwwwwwwwwwwwwwwwwith puuuuuurererererrerrrerrrerererrrererererrrerrrrrrerer eeeeeeeeeeeththhthththththththththhththhththhththhththhhanol, which was continuously replaced with water until a 1:2 ratio (ethano

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amounts oofoooo inorganic tin salts, and non-toxic tartrate for Ag reduction.

3

First, the self-supported biopolymeric backbone is seeded with Ag NPs by the reaction of superficially bound Sn(II) ions with

Ag(I) according to: Sn(II) + 2 Ag(I) Sn(IV) + 2 Ag↓. The CA disks were flushed with a sensitization solution consisting

of slightest SnCl2 amounts and trifluoroacetic acid, which results in Sn(II) adsorption. After thorough washing, the gel was

flushed with an activation solution (AgNO3 and NH3). This reaction is indicated by a change in color (white brown). After

a washing step, Ag deposition is performed by flushing the monolith with the plating solution which consists of a metal

source (AgNO3), a ligand (ethylenediamine), trifluoroacetic acid for pH adjustment and a reducing agent (potassium sodium

tartrate). During this time, the gel turns darker, indicating the growth of the NPs and an increase of the Ag loading.

FIG. 1. SEM analysis of pure CA (A) and CA-Ag composite (B), both with insets of higher magnification. TEM analysis of the CA-Ag composite in different magnification (C, D). The insets show the EDX analysis (C; the Cu signal stems from the TEM grid) and the particle size distribution (D, n=200).

After EP, the gel was washed with ethanol subjected to supercritical drying (110 bar, 50°C for 24 h). The dry CA with 3 wt.

% cellulose is white in color and deforms readily under compression, while the CA Ag-composite is less pliable and of black

color.

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a washing step, Ag deposition is performed by flushing the monolith with the plating sssssoloooololoololoolololoolloloollllooooolooloo utuuuuuuuuuuuuuuuuuuuuuuuuuuuu ion whi

source (AgNO3), a ligand (ethylenediamine), trifluoroacetic acid for pH adjustment annnnddd dd d dddddddddd d ddddd aaaaaaa aaaaaaaaaaaa reeeeeeeeeeedududududududuududududududududuududududdududduddduucicicicciciciccciccicicicciiicciciiingnnnnnnnnn ag

tartrate). During this time, the gel turns darker, indicating the growth of the NPs andddddddd aaaaaaaaaaaaaaaaaaannnnnnnnnn n n n nn nn nnnnnnnnn inininiininininiiininininininnninnnnnnnnini ccccrcrcrcrcrcrcrrrccrrccrrcrcrcrccc eeeeeaeeeeeeeeeeeee se of the

FIG. 1. SEM M ananaanananananannnnanananannananananannnannnananannaaaa alaalaaaaaaaaaaaaaaaa ysis oooooooooooff f ffffff ff f fff f f f fff pppuppppppppppppppppppppppppp re CA (A) and CA-Ag composite (B), both with insets of higher magnification. TEM ancomposssssssititittitititittititititititittitiittiitte e eee e eee e e e iiiniininininininininininininnnininnininninn ddddddddddddddddddddddddiffffffiffffifffififfifffifffffefefefefefefefefefefefeffefefefefffffffff rrerererrereeeerereeeereeeeerereerereeentntnnntntntnnnnntntnnntntnnnttttt mmmmmmmmmmmmmmmmmmmmmmmmaaaaagagagagagaaaagagaagaaagaaa nification (C, D). The insets show the EDX analysis (C; the Cu signal stems from the TEsize disssssssssstrttrtrtrtrtrrtrrtrttrrrtrrrtrrrtrrrttrrrrriiiibiiiii utiioi n (D(DDD(DDDD(DDDDDDDDDDDDD(( , n===n=n=n=n=n=n=n=n=n=n=222202020222202222222222222 0).

AfAfffAfffffAffffffAffffftetetetetetetetetetetetetetetetetetetetetetetetetettt r r rrrrrrrrrrrrrrr EPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPPPPEPP,, , ththththththtttththhhhhhthhhhthhthhhhhheeeeeeeeeeeeeeee gel was washed with ethanol subjected to supercritical drying (110 bar, 50°C for 24 h).

% celllllululululululululullllulululululululululululululososososoosoosoososoooosoosooosooosose is white in color and deforms readily under compression, while the CA Ag-composite is le

l

4

Taking advantage of the regular cylindrical shape of the aerogel monoliths, the densities are gravimetrically determined to be

57 kg m-³ for the CA and 93 kg m-³ for the CA-Ag composite. Furthermore, the porosity of 97 % for CA is slightly reduced

by Ag NP loading to yield 86 % in the composite. Both, the change in color and the increasing density indicate Ag NP

deposition onto the aerogel. SEM analysis reveals the CA to consist of a 3D network of aperiodically distributed cellulose

nano-fibrils (Fig. 1A). After EP, the structure remains fibrillar (Fig. 1B), whereas the pore size slightly decreases with Ag NP

loading. The CA-Ag composite, confirmed by EDX analysis, inset Fig. 1C, appears highly homogeneous in Ag NPs size and

distribution. However, from TEM image Fig. 1C the Ag NPs appear to be loosely mounted onto the fibrils, which may derive

from sample preparation performed by grinding in liquid nitrogen. The most frequently occurring particle size is the range of

15-25 nm making 73 % in total (inset Fig. 1D) and thus, implying uniform growth times and conditions for the Ag NPs.

Every fibril is covered with Ag NPs, indicating a very reliable and dense metal nucleation. We attribute this behavior to the

polar surface chemistry of the cellulose. The ubiquitous presence of hydroxy groups provides a high density of bonding sites

for the metal ions involved in the sensitization and activation steps, which in turn leads to an efficient template seeding.

FIG. 2. A) Specific surface area from BET results including BJH inset for the pore size distribution and B) XRD pattern of the CA-Ag composite showing characteristics of both components.

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nano-fibrils (Fig. 1A). After EP, the structure remains fibrillar (Fig. 1B), whereas the pore ssssssssssssssssssssiziziziziziziiziziiziziziizizziziizzzzzizzziziizzzizzzze slightly

loading. The CA-Ag composite, confirmed by EDX analysis, inset Fig. 1C, appears highghghghghghghghghghghghhghghghghghghghghghghhhhghhhhhhlylylylylylylylylyllylllylllylylyllyllyllyyy hhhhhhhhhhhhhhhomomomomomomomomomommmmmmmomomomomommmmmomommmmmoomogogogogogogoogoogoogogogooogggggggggeneo

distribution. However, from TEM image Fig. 1C the Ag NPs appear to be loosely momooooooooooooooununununununununununnunununuunununununnnunununuuuu tetettetetetetetetetetettetettetetettetetettt dddd ddddd d d dd d d d d ddd dddd ooooonooooooooooooo to the fib

from sample preparation performed by grinding in liquid nitrogen. The most ffffffrerererereeeeerererererereereererererequququququququququququququququququuququuququuuquqqqqq enenenneeeeeee tltltltltlllltltllltltltltltltltltltltllly yyyyyyyyy yy yyy y yyyyyyyyyy ococooococoooocoooooooococco cuuuuuuuuuuuuuuurrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr iniiiiii g parti

15-25 nm making 73 % in total (inset Fig. 1D) and thus, implying uniformrmrmrmmmmmrmmm ggggggggggggggggggggggggggggrrorororororooroorooorororroorororoorooroorororororowtwtwtwtwtwtwtwwtwtwtwtwtwwtwtwtwttwtwttwwwww hhhhhhhhhhhhhhhhhhhhhhhhh tititititititititititititititititititititttttt mmmmmmmmemmmmmemmmmmmmmmmmmm s and condi

Every fibril is covered with Ag NPs, indicating a very reliable and dennnsesseseseseseeeeeesesesesesssesesssss mmmmmetal nnnnnnnnnnnnnnnnnnnnnnnnuuuuuucuuuuuuuuuu eleeeleleeleleleleleeeation. We attribu

polar surface chemistry of the cellulose. The ubiquitous presence ofofofofofofofofofofofofoofofofofofofoffoffoofofooofof hydyddyddydydddydddydy rorororororororororororoororororrrororoooorrrr xyxxyxxyxyxxxyxxxxyxxxyxyxyxyxxyxxxxxxyxyxxyxyyyyy ggggggggggrorororororrorororororororororouuuuupuuuuuuuuu s provides a high d

FIG. 2222222222222....... AAA)AAAAA)A)A)A))A)A)A)A)A)AAAAA)A)AAAAAAAAA) Specific surface area from BET results including BJH inset for the pore size distribution and B) XRD pacomposititittttttittititttitittteeee e e eeeeee eeeeeeeeeeee shssssssssssssssssss owing characteristics of both components.

for the metal ions involved in the sensitization and activation stepepepeppppps,ssssssss,ss,s,s,s,s,s,s,sss,s,ss,ss,s wwwwwwwwwwwwwwwwwwwwwwwwwwhichchchchchhhhhhhhhhhhhchhhchhchhh iiiiiiiiiiiiinnnnnnnnnnnnnnnnn n tutttutttututututututututuuttttttt rn leads to an efficient te

5

Figure 2A shows physisorption isotherms of the CA and the CA-Ag composite confirming no change in type of physisorption

(IUPAC, type IV)32 and clearly supports the shift to smaller pore sizes from Fig 1. The specific surface area of 170 m² g-1

(CA) and 166 m² g-1 (CA-Ag) indicate marginal effects on the pore volume. XRD measurements (Fig. 2B) confirm the

material composition, showing reflexes of the CA (2θ < 35°) and Ag peaks (2θ > 35°).

To demonstrate the introduced metal functionality, the CA-Ag composite was employed as catalyst in the reduction of 4-

nitrophenol by NaBH4.33 4-nitrophenolate strongly absorbs light at 400 nm, while the product 4-aminophenolate does not

interfere in this spectral region. Accordingly, the reaction progress can be conveniently monitored by UV-Vis absorption

spectroscopy (Fig. 3A).

FIG. 3. A) UV-Vis spectra of the reaction solution before and after catalysis. B) Calculation of apparent rate constant, which corresponds to the negative slope of the fit.33 C) Catalyst activity over several reaction cycles (in % of the initial rate constant).

For the rate constant calculation, the absorbance at 400 nm was determined in intervals of 15 s. 3.36 mg of CA-Ag

catalyst was soaked with pure ethanol, which was continuously replaced with reaction solution (60 mM NaBH4 and 0.12 mM

4-nitrophenol in water). The catalytic experiments were performed by transferring the soaked CA-Ag to a quartz cuvette

containing 3 mL of fresh reaction solution (no stirring). Given a large excess of NaBH4, the reaction follows a pseudo-first

order rate law.33 This allows us to determine the catalysts’ apparent rate constant kapp by linearly fitting the natural logarithm

of the relative absorbance at 400 nm over time (Fig. 3B). With a kapp value of ≈ 1.2∙10-3 s-1, the catalytic activity can compete

with colloidal systems such as Ag NP doped carbon spheres (kapp = 1.69∙10-3 s-1),34 while providing the additional advantage

of simple catalyst recovery. As a result of the large surface area of the CA support, the nitrophenol conversion rate markedly

exceeds that of Ag NPs on macro-sized cellulose fibers (90 % conversion in approx. 30 min versus only slight conversion

FFFigigggggggggggggggggggggggggggggggggggggggurrururururrrurrrrrurrrrrrrrrrrruuurreeeeeeeeeeeee 2AAAA

(I((I(I(I(II(I(I(II(III(I(I(II(I(IIII(IIII(IIII((I(I((I((((((I(((I(I(I(III(IIIII(I(IIUPUUPUUUPUPUUUPPUUUUUUUUUUUUUUUUUPUUPPPPPACACACACACACACACACACCCACACACCACACCCACCCCCCCCCACCCCCCCCCACCCCCCCCACCACCACCCCCCCCCCCCACCACAAACACCCACCCCCCCCCCCCCCCCCC,,,,,

To demonstrate the introduced metal functionality, the CA-Ag composite was employeeeedd dd ddd d ddd d d d dddd d d d dd aaaaaaaasaaaaaaa catalyst

nitrophenol by NaBH4.33 4-nitrophenolate strongly absorbs light at 400 nm, while tttttttttttttttheheheheheheheeehehehehheehhehehehehehehhhe ppppppppppppppppppppppppprororrrorrorrorrrrrrrorrrrrrrrrrrodudududduduuuuuuctctctctctctctttctctctctctctctccccctcccccctc 44444444444444444444444-am

interfere in this spectral region. Accordingly, the reaction progress can be convnvnvvvvvvnvvvvvvvvvnvvvvvvnvvveneneneneneeneneeeneenennnennnnnieeeieeeieientntntntntntntntntntntnttntntntntnntntnnnnntntlllylylyllllllylylylylllllyllylylyllllylylylyllylylyly mmmmmmonitored

spectroscopy (Fig. 3A).

FIG. 3. A) UV-Vis spectra offffffff ttttttttttttttttthehhhehheheheheheheehehehhehehhehhhhh reaaaaaaaactctctctctctctctctctcctctctctctttioioooooiooioioiooioioioioiioioion nnnn nnnnnnnn n nnnn soooooooooolululululululululuululuululululululuutitittititiiititiitititiitititititttt on before and after catalysis. B) Calculation of apparent rate constathe negative slope of the fit.t..t......333333333333333333333333333 C)C)C)CC))))CC)C)))C)C)C)CC)C)C)C)C)C)C)C)C)C) CCCCCCCCCCCCCCCCCCCCCCCCCCataataaaaaaataaaaaaaaa alyssssssstttttttttttttttt t tttt acacaaaacaacacacaaaaactttititititititiitititittttt vity over several reaction cycles (in % of the initial rate constant).

For the rate connnnnnnnnnnnnnnnststststsstststtsttstststtststtstssss ananaanananananaaaanaaaannannt cacccacacaccaaaaacacacacaccaccccalcl ulatioioioioiioion, the absorbance at 400 nm was determined in intervals of 15

catalyst was soaaaaaaaaaakekekekkekekkkekkekekekekkekekkekekeeeeeeeekeddddd dddddddddddddd wiwiiwiiwiiiwiiwiwiiwiiiwiiiithththththththththththththttththththtthtthththhtthththhhhthth pururururrrrrrrrururrrurrururrurreeeeeeeeeeeeeeeeeeeeee eteeeeeeeeeeeeeeeeeeeeeeeeee hanol, which was continuously replaced with reaction solution (60 mM

4-nitrophenonononooooooonoonoooooooonoolllllll l lllllll iiininiininnnn waterererererrrrrrr)))))))))))))))))))))))))))..... .. .. ThThTThThTTThThThThThThThThThThTThThhThhThe catalytic experiments were performed by transferring the soaked CA-A

containininiininininininininiinininiininiiiiingngngngngngngngngngngggngnggggggggg 3333333333333333333333333333333333 mmmmmmmmmmmmmmmmmmmmmmLLLLLLLLLLLLLLLLLLLLLLLLLLLL ofofoofofoofooofofofofooofofoofofoffffof fffffffffffffffffffffrererererrererereeeererrerereererererererererereeer sh reaction solution (no stirring). Given a large excess of NaBH4, the reaction

order raaaaaaaaaaaaaaaateteteteteteeteeetetetetetetetetetetetett llllllllllllllllawaaaaaa .33333333333333333333333333 TTTThTTTTTTTTTTTTTTTTTTTTT is allows us to determine the catalysts’ apparent rate constant kapp by linearly fitting

of ttttttttttthehehehehehehehehehehhhehehhehehhhhheheheehheheeeeeeeeheh relelelelelelelelelelllleleeeeleeeee atatatatatatatatatatataatatataaatatttiviviiiviviviivivivivivivivivvvvivivivivivivivive eeeeee eeeeeeeeeeeeeeee e absorbance at 400 nm over time (Fig. 3B). With a kapp value of ≈ 1.2∙10-3 s-1, the catalyti

with colololllllllolllolollolollololololololololololololololololololoololololooooooloooidi al systems such as Ag NP doped carbon spheres (kapp = 1.69∙10-3 s-1),34 while providing th

6

after 1 day).17 In contrast to other monolith-supported NP systems,10 the CA-Ag catalyst also exhibits a good aging stability.

During 6 reaction cycles, the activity does not decline noticeably (Fig. 3C). The catalyst stability is likely related to the

dependable adhesion of the Ag NPs to the cellulose fibrils, which already endured the flow of various solutions in the course

of the metallization reaction.

In conclusion, we introduced a modified EP procedure as a facile and sustainable route to functionalize open-porous

fibrillar aerogel structures with a homogeneous layer of Ag NPs. The combination of CA supports and EP is strikingly fitting:

The polar surface chemistry of the cellulose promotes a dense nucleation and an efficient metal particle attachment, while the

conformal deposition is able to utilize the complex aerogel structure. As we have shown in a catalytic model reaction, the

hybrid biopolymer-metal material combines the functionality of the metal NPs with the benefits of the CA support. This

approach will be extended to other metals (Au, Pt) and other substrate aerogels to realize aerogel-metal composites of varying

morphology to be employed in plasmonic, sensing and catalytic applications.

ACKNOWLEDGMENTS

Thanks are due to Dr. René Tannert for fruitful discussion, Dr. Klemens Kelm and Philipp Watermeyer for TEM analysis,

Alexander Francke for XRD analysis, Dr. Umme Habiba Hossain for support with the Ag deposition, and Eva-Maria Felix

for help with the UV-Vis measurements.

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12535 (2014).

afafaffffffffffffffftetettetetttettttetttttttttttttttettttttttttttett r rrrrrrrr 11111111111111111111 daaaa

uDDuDDuDuDuDDDDuDuDuDuDDuDDDDDDuDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDuDDDDuDDDDDDDDDDuD ririrrriirrrrririiiiingngngnngggggggggggggggnggggggggnnngnnggngggggggggggggggggg 6666666666666

In conclusion, we introduced a modified EP procedure as a facile and sustainableeeeeeeeeeeeeeeeeeeee rrrrrrrrrrrrrrrrrrrrououououououououououououououuouuuuuouououooouoooo tetettttetttett to func

fibrillar aerogel structures with a homogeneous layer of Ag NPs. The combination off f f f f ff ffff f f ff fff CACCCACACCCCACACACACACACACACACACCCCCCCCCCCCCCCCC ssssssssupuuuupuupupupupupupupupupupupuuupupupuuuuupuu portrttttsssss ssssss ananananananannananananannanannannanannananaannananananananaand

The polar surface chemistry of the cellulose promotes a dense nucleation and an efefefefffffffffffefefefeffefeffffififififififfififififififififififfifififfificcicicicccicicicicicicciciciccciicicciccc eneeeeeeeeeeeee tttttttttt t memememememememememememememememmmemeemememememm tatatatatttatatttatatattal particleff

conformal deposition is able to utilize the complex aerogel structure. As wwweweweweweweweweeeeewewwwweewweweww hhhhhhhhhhhhhhhavvvveeeeeeee eeeeeeeeee shshshshshsshshshshshsshshshsshsshshshshhhowowwwwnnnnnnnnnn nnn nn nnnnn iniiiiiiiiiiii a catalyt

hybrid biopolymer-metal material combines the functionality of the mmmmmmmmmmmmmmmmmmmetetetetetettetttetteteteteeeteeeee lllllllall NNNNNNNNNNNNNNNNNNNNNNNNNNPs wwwwwwwwwwwwwwwwwwwwwititiiittiiititititttttititititititititiii hhhhhhhh hhhhhhhhhh hhhh the benefits of

approach will be extended to other metals (Au, Pt) and other substrararaaraaaaraaararaaaaaaaattetetettteteteteteteeeteteteettttette aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaerogogogggggggggggogogogggogoggelelllelellllelelelelelellllllleleleleele ssssssssssss s ssssss sss totototototototototottootottotottttottotooottotoo rrrrrrrrrrrrrreeeeeeaeeeeeeeeeeeeee lize aerogel-metal

morphology to be employed in plasmonic, sensing and catalyticc c cccc cc apaapapapapaaapapapapapapapapapapappppaappppppppplplpllpllllllpllllpllllpllp iiiiiciciiciciciciciciccciciiicicicii atatatatatatataatattatatataaaaaaaa iiiiiiiioiioioiiii nsss.

ACKNOWOWOWOWOWWWWWWWWWWOWOWOWWWWWWWWWWWWWWWLELELELELELEEEELELLLLELLLLLELLLLLLLLLLLLLLELEEDGDGDGDGDGDGDGDGDGDGGGGDGDGDGDGDDGGDGDGDGDGDGDGDGDGDGDGGMEMEMEMEMEMEMEMEMEMEEMEEMEMEMEEMEMEMEMMMEMMEMMMMMMMMM NNNNNNNNNTNNNNNNNNNNNNNNNN S

Thanks are due to Dr. René Tannert for fruitful dididididiiididiiidiiidiidiidiiiscscscscscscsscscsscsscscscscscscscsscsscscscccccccusuuuuuuuuuu siiiiononononnnnnnnnnnnonnnnnononnonononnn,,,,,,,,,,,, DrDDDDDDDDDDDDDDDDDDDDDDDDD . KlKlKlKKKKKlKKlK emens Kelm and Philipp Waterme

Alexander Francke for XRD analysis, Dr. Ummmmmmmmmmmmmmmmmmmmemememmmememmemememememmmmmmemmemememmmmmmee HHababababababbbbabababababababbababababbbabababbbbbabbbbbbibibibibbibibibbbibbiibibibibiibibibibbibibbibibibaaaaaaaaaaaaaaaaaaa HoHoHoHoHoHoHHoHooHHHoHoHoHHHoHoHoHoHoHoossssssssssssss ain for support with the Ag deposition

for help with the UV-Vis measurements.

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