ORIGINAL PAPER

Cellulose/silver nanoparticles composite microspheres:eco-friendly synthesis and catalytic application

Junjie Wu • Ning Zhao • Xiaoli Zhang •

Jian Xu

Received: 7 April 2012 / Accepted: 29 May 2012 / Published online: 7 June 2012

� Springer Science+Business Media B.V. 2012

Abstract Cellulose/silver nanoparticles (Ag NPs)

composites were prepared and their catalytic perfor-

mance was evaluated. Porous cellulose microspheres,

fabricated from NaOH/thiourea aqueous solution by a

sol–gel transition processing, were served as supports

for Ag NPs synthesis by an eco-friendly hydrothermal

method. The regenerated cellulose microspheres were

designed as reducing reagent for hydrothermal reduc-

tion and also micro-reactors for controlling growth of

Ag NPs. The structure and properties of obtained

composite microspheres were characterized by Opti-

cal microscopy, UV–visible spectroscopy, WXRD,

SEM, TEM and TG. The results indicated that Ag NPs

were integrated successfully and dispersed uniformly

in the cellulose matrix. Their size (8.3–18.6 nm), size

distribution (3.4–7.7 nm), and content (1.1–4.9 wt%)

were tunable by tailoring of the initial concentration of

AgNO3. Moreover, the shape, integrity and thermal

stability were firmly preserved for the obtained

composite microspheres. The catalytic performance

of the as-prepared cellulose/Ag composite micro-

spheres was examined through a model reaction of

4-nitrophenol reduction in the presence of NaBH4.

The composites microspheres exhibited good catalytic

activity, which is much high than that of hydrogel/Ag

NPs composites and comparable with polymer core–

shell particles loading Ag NPs.

Keywords Cellulose � Ag nanoparticles �Hydrothermal method � Catalysis

Introduction

Silver nanoparticles (Ag NPs) have attracted much

attention owing to their unique properties and intrigu-

ing applications in the field of catalysis (Corain et al.

2008), surface enhanced Raman scattering (SERS)

(Cao et al. 2002; Nie and Emery 1997), antimicrobial

(Kumar et al. 2008), and printable electronics

(Grouchko et al. 2011; Polavarapu et al. 2011), etc.

For example, Ag NPs exhibits more negative reduc-

tion potential and higher specific surface area than

bulk Ag, making it an effective catalyst in various

electron transfer chemical reactions (Corain et al.

2008). However, it seems impossible to use colloidal

Ag NPs as a catalyst directly due to huge challenges in

coagulation and aggregation (Ajayan et al. 2004). To

solve these problems, a classic method is immobilization

J. Wu � N. Zhao (&) � X. Zhang � J. Xu (&)

Beijing National Laboratory for Molecular Sciences,

Laboratory of Polymer Physics and Chemistry,

Institute of Chemistry, Chinese Academy of Sciences,

Beijing 100190, People’s Republic of China

e-mail: zhaoning@iccas.ac.cn

J. Xu

e-mail: jxu@iccas.ac.cn

J. Wu

Graduate University of the Chinese Academy of Science,

Beijing 100049, People’s Republic of China

123

Cellulose (2012) 19:1239–1249

DOI 10.1007/s10570-012-9731-3

or encapsulation of Ag NPs on/into supporting matri-

ces. Hybrid catalysts integrated Ag NPs with inorganic

and organic supports have been successfully devel-

oped (Lu et al. 2007b; Sharma et al. 2009; Tang et al.

2010; Zhang et al. 2009). For instance, Lu et al.

(2006a, b, c, 2007a) prepared a serious core–shell

polymer particles, which the core consist of poly(sty-

rene) (PS) and the shell consist of hydrophilic polymer

networks or brushes, to load Ag NPs in the shell. These

nanocomposite particles displayed great catalytic

activity for the reduction of 4-nitrophenol by sodium

borohydride (NaBH4).

Cellulose, the most abundant and most exploited

biomass in nature, has attracted more and more

interests both in scientific and industrial applications

(Chang et al. 2009; Ke et al. 2009; Zeng et al. 2010).

Utilization of cellulose is certainly benefited for

environmental protection and sustainable develop-

ment owing to it is biocompatible, biodegradable and

renewable resource. The recently developed some

low-toxicity and effective cellulose solvents [e.g.,

ionic liquid (Swatloski et al. 2002; Zhang et al. 2005)

and alkali aqueous solution system (Cai et al. 2008;

Lue et al. 2007; Qi et al. 2011; Yan and Gao 2008)]

provided much more ways to design and fabricate

functional cellulose materials. Cellulose also is a

fascinating supporting matrix for Ag NPs. The pres-

ence of a large number of hydroxyl and ether groups

on cellulose chain can effectively stabilize Ag NPs via

strong interactions. In addition, cellulose is easily to

form porous structure which is able to control the size

and shape of Ag NPs in the case of in situ synthesis. In

view of the superiority of cellulose as a carrier, plenty

of Ag NPs hybrid materials with different styles of

cellulose scaffold including fibers (Song et al. 2012;

Sureshkumar et al. 2010), films (Maneerung et al.

2008; Marques et al. 2008; Yang et al. 2012),

hydrogels (Cai et al. 2009), and nanocrystal (Liu

et al. 2011; Shin et al. 2008) have been prepared. Ag

NPs were synthesized by chemical reduction using of

additional reducing reagents (Maneerung et al. 2008;

Shin et al. 2008) or ‘‘greener’’ approach through

hydrothermal method by using cellulose itself as

reductant (Cai et al. 2009). The reported cellulose/Ag

composites were applied as antimicrobial materials

(Sureshkumar et al. 2010; Yang et al. 2012), SERS

substrates (Marques et al. 2008), and electrochemical

sensor for DNA hybridization (Liu et al. 2011).

However, to the best of knowledge, the catalytic

performance of cellulose/Ag composites has not been

reported.

In this article, we attempt to fabricate cellulose/Ag

NPs composite and determine its catalytic property.

To obtain highly active catalysts, the matrices of

porous cellulose microspheres with high specific

surface area were prepared by a sol–gel transition

(SGT) method from NaOH/thiourea aqueous solution.

In view of the importance of environmental protection,

the eco-friendly hydrothermal method was employed

to generate Ag NPs without additional reducing

reagents. The structure and property of obtained

hybrid microspheres were characterized by optical

microscopy, UV–visible spectroscopy, X-ray diffrac-

tion, electron microscopy, and thermogravimetry (TG)

analysis. The catalytic activity of cellulose/Ag NPs

composites was examined by model reaction of

reduction of 4-nitrophenol in the presence of NaBH4.

Experimental section

Materials

Cellulose (cotton linter pulp) with a viscosity-average

molecular weight (Mg) of 1.01 9 105 g/mol was

provided by Hubei Chemical Fiber Group Ltd.

(Xiangfan, China). Silver nitrate (AgNO3), 4-nitro-

phenol and sodium borohydride (NaBH4) and other

chemical reagents of analytical grade were purchased

from commercial sources in China and used without

further purification. Deionized water was used for all

experiments.

Preparation of regenerated cellulose hydrogel

films and microspheres

Eight gram cellulose was dispersed in 200 g of

4.5 wt% NaOH/9.5 wt% thiourea aqueous solution

that pre-cooled to -8 �C and then stirred vigorously

for 5 min to obtain a transparent cellulose solution.

The solution was spread on a glass plate with a

thickness of 0.5 mm and coagulated by a diluted HCl

aqueous solution (10 wt%), followed by washing

thoroughly with water to get transparent cellulose

hydrogel film. The regenerated cellulose microspheres

were fabricated by a previously reported STG method

(Luo et al. 2009). 300 mL of paraffin oil and 15 mL of

Span 80 were mixed in a reactor and stirred at

1240 Cellulose (2012) 19:1239–1249

123

1,000 rpm for 1 h to get a well-mixed suspension.

50 mL of cellulose solution prepared was dropped into

the suspension with a speed of 1 mL/min. The

suspension was kept stirring for 10 h at the same

stirring speed to make a transformation of cellulose

solution to gel state. Subsequently, 10 wt% HCl was

added into the suspension until pH = 7 to form

regenerated cellulose microspheres. After removing

the supernatant liquid paraffin, cellulose microspheres

deposited in the bottom were collected. The cellulose

microspheres were washed with water and acetone

several times to completely remove the residual

paraffin oil, Span 80 and HCl. The regenerated

cellulose microspheres were coded as CMS.

In situ generation of Ag NPs in cellulose matrices

by hydrothermal treatment

The Ag NPs embedded in cellulose films or micro-

spheres were synthesized by in situ hydrothermal

reduction of AgNO3 as follows. 1 g hydrated cellulose

films or microspheres (water content 90 %) were

immersed in 10 mL of aqueous AgNO3 solution in a

Teflon-sealed reaction vessel and heated at 80 �C for

24 h. After cooled to room temperature, the resultant

cellulose/Ag composite films and microspheres were

washed with water thoroughly. The obtained cellu-

lose/Ag composite microspheres were denoted as

CMS/Ag-x, where x = 1, 5, 15, and 25 corresponds to

the initial AgNO3 concentration of 10, 50, 150, and

250 mM, respectively. The prepared cellulose and

cellulose/Ag composites were stored in water at 4 �C

or freeze-dried under vacuum at -60 �C for structure

characterization.

Characterizations

Optical photomicrographs of CMS and CMS/Ag

microspheres were observed using an optical micros-

copy with a single reflex camera (BX51, Olympus,

Germany). The histograms, mean sizes, and standard

deviations of 200 microspheres were analyzed by the

software of ImageJ. UV–visible characterization was

performed on a TU-1901 UV–visible spectrophotom-

eter (Purkinje General Instrument Co., Ltd., Beijing,

China). Scanning electron microscopy (SEM) images

were acquired from a scanning electron microscopy

(JSM 6700F JOEL, Japan) on an accelerating voltage

of 5 kV. The freeze-dried microspheres were coated

with Pt for SEM observation. Transmission electron

microscopy (TEM) images were accumulated by a

transmission electron microscopy (JEM 2011F JOEL,

Japan) operated at 200 kV. The sample for the TEM

examination was prepared as follows: the freeze-dried

microspheres were imbedded in methyl methacrylate

monomers containing 0.1 % (w/w) AIBN as initiator.

After cured by two steps, lasting 12 h each, at

progressively higher temperatures of 60 and 80 �C,

the embedded specimen was ultrathin-sectioned by a

LEICA Ultracut-E using a glass knife at room

temperature. The ultrathin sections approximately

100 nm thick (golden color) were mounted on copper

grids with carbon support for TEM observation. The

histograms, mean sizes, and standard deviations were

obtained by analyzing 100 Ag NPs in highly magnified

TEM images using ImageJ. Wide-angle X-ray dif-

fraction (XRD) analysis was performed on X-ray

diffractometer (Riguku D/max-II, Japan) using a

CuKa target at 40 V and 200 mA. The mean size of

crystalline (D) was determined by Scherrer’s formula:

D ¼ KkB cos h

ð1Þ

here, K is a dimensionless shape factor of 0.9,

k = 0.15 nm is the wavelength of Ni-filtered CuKaradiation, B is the full width at half-maximum in

radians of each diffraction peak, and h is the Bragg

angle. Thermogravimetric (TG) analysis was carried

out by thermogravimetric analyzer (Pyris 1, Pekin-

Elmer Co., USA). The sample was put in a platinum

pan and heated from 25 to 600 �C (20 �C/min) under

nitrogen and air atmosphere.

Catalytic reduction of 4-nitrophenol

in the presence of NaBH4

The microspheres used for catalytic reduction were

obtained via exchanging the water in hydrated micro-

spheres to acetone and then drying under vacuum at

ambient temperature for 24 h. The catalytic 4-nitro-

phenol reduction was carried out in a quartz cuvette

and monitored using UV–visible spectroscopy.

0.75 mL of fresh NaBH4 aqueous solution (0.4 M)

was mixed with 1.5 mL of CMS or CMS/Ag-x aqueous

dispersion with different concentrations. Subse-

quently, 0.75 mL of 4-nitrophenol aqueous solution

(4 9 10-4 M) was added. As a result, the concentra-

tion of 4-nitrophenol and NaBH4 in the reaction

Cellulose (2012) 19:1239–1249 1241

123

solution was 1 9 10-4 and 0.1 M, respectively. The

reaction was monitored at decided time intervals.

Results and discussion

Formation of Ag nanoparticles in cellulose

matrices

The reducing capacity of regenerated cellulose from

NaOH/thiourea aqueous solution is assessed using

cellulose hydrogel film firstly. The transparent cellu-

lose hydrogel films turned to yellow after hydrother-

mal treatment with 50 mM AgNO3 aqueous solution

for 24 h. The appearance of plasmon resonance peak

at 406 nm in UV–visible spectra of hydrothermal

treated film proves the formation of Ag NPs in

cellulose matrices (Fig. 1a). This result indicates that

cellulose is able to reduce Ag ions to Ag under

hydrothermal environment. The hydrothermal treated

cellulose microspheres (CMS–Ag-x) show light yel-

low to yellowish-brown colors (insert in Fig. 1a),

demonstrating the incorporation of Ag NPs in cellu-

lose microspheres. The darkening in color of CMS/

Ag-x may attribute to the increase of content and

particle size of incorporated Ag NPs (Cai et al. 2009).

It is should been point out that Ag NPs neither

appeared in solution nor deposited on the wall of

reaction vessel. These phenomena indicate the reduc-

tion of Ag ions to Ag NPs is induced by cellulose and

cellulose is a good stabilizer for generated Ag NPs.

Figure 1b, c show the optical photomicrographs of

CMS and CMS/Ag-5 in water. Both microspheres

present good spherical shape and no obvious differ-

ence in size (from 22.1 ± 8.8 to 21.4 ± 10.0 lm after

Ag NPs incorporation). The results suggest the low

hydrothermal temperature of 80 �C did not destroy the

cellulose microspheres during synthesis of Ag NPs.

X-ray diffraction results further certify the forma-

tion of Ag NP in cellulose microspheres by hydro-

thermal treatment. Regenerated cellulose exhibits a

typical cellulose II crystal with three characteristic

diffraction peaks at 12.2, 20.1, and 21.8� (Fig. 2a)

(Liang et al. 2008; Wu et al. 2010). CMS/Ag-1

displays a peak at 2h = 38.1� (Fig. 2b) that assigned

to the (111) lattice plane of Ag. Other characteristic

peaks of Ag crystallite at 44.3, 64.5, and 77.4� appear

on the XRD patterns of CMS/Ag-5, CMS/Ag-15 and

CMS/Ag-25, indexed as (200), (220), and (311) planes

of Ag, respectively (Saha et al. 2010). The average

size of Ag NPs calculated by Scherrer’s formula

(Eq. 1) is 7.8, 11.7, 16.5, and 19.3 nm for CMS/Ag-1,

5, 15 and 25, respectively. The size of Ag NPs

increases with the increase of initial AgNO3 concen-

tration, indicating that the size of Ag NPs in cellulose

microspheres can be controlled by tailoring the

concentration of AgNO3 solution. However, the

intensities of cellulose crystal peaks of CMS/Ag-

x decrease with the increase of AgNO3 concentration,

implying the cellulose crystal was partly broken by the

Fig. 1 UV–visible spectra (a) of initial (a) and Ag NPs

incorporated (b) cellulose hydrogel film. Insert in (a) is the

photographs of CMS (c), CMS/Ag-1 (d), CMS/Ag-5 (e), CMS/

Ag-15 (f), and CMS/Ag-25 (g) in water. Images b, c are the

optical photomicrographs of CMS and CMS/Ag-5 dispersed in

water, respectively. The scan bar is 200 lm. Inserts in images

b and c are the particle size histograms of CMS and CMS/Ag-5,

respectively

1242 Cellulose (2012) 19:1239–1249

123

introduction of Ag NPs. We propose that some Ag ions

may diffuse into the crystalline region of cellulose.

The Ag NPs generated from these Ag ions will impair

the ordered packing of cellulose chains. The decrease

of cellulose crystallinity in composites of cellulose

with other nanoparticles has also been observed in

previous reports (Chang et al. 2009; Zeng et al. 2010).

Microstructure of cellulose/Ag composite

microspheres

The surface morphology of composite microspheres

and shape and size of Ag NPs inside cellulose matrix

were observed through SEM and TEM. Figure 3

shows the SEM images of CMS and CMS/Ag-x. All

samples exhibit intact spherical structure (inserts in

Fig. 3) suggests that cellulose microspheres were not

been destroyed during the incorporation of Ag NPs.

CMS shows rough surface with many nanopores with

size of 20–30 nm (Fig. 3a, b). The morphology is in

consisting with our previous reported cellulose hydro-

gel membrane fabricated from NaOH/thiourea aque-

ous solution (Liang et al. 2007). Cellulose chain is

considered to be a flexible coil but not worn-like

semistiff conformation in NaOH/thiourea system (Lue

et al. 2007). Therefore, cellulose is facile to aggregate

and tends to form dense structure during the STG.

Figure 3c–f reveals the morphology evolution of the

surface of CMS/Ag-x with the change of AgNO3

concentration. CMS/Ag-1 and CMS/Ag-5 show sim-

ilar morphology with small sized nano pores as CMS

(Fig. 3c, d). In contrast, nanoparticles append on the

surface of CMS/Ag-15 (Fig. 3e) and CMS/Ag-25

(Fig. 3e, f). The diameters of some appeared particles

are larger than 30 nm.

Figure 4 shows the TEM images and particle size

histograms of Ag NPs in composites microspheres.

Spherical Ag NPs are dispersed well in cellulose

matrix without obvious aggregation (Fig. 4a–d), sug-

gesting the Ag NPs is well segregated by cellulose

matrix. With the increase of initial AgNO3 concen-

tration from 10 to 250 mM, the particle size increases

gradually (from 8.3 ± 3.4 to 18.6 ± 7.7 nm)

(Fig. 4e–h). The values agree well with the results

obtained from XRD measurement and verify the fine

turning of the size of Ag NPs. The average particle size

for all samples is less than 20 nm. It is speculate that

the nano pores of cellulose matrix are main reaction

sites during Ag NPs synthesis. Therefore, the growth

of most Ag NPs embedded inside the cellulose matrix

was restricted in the 20–30 nm nanopores resulting in

small average particle size of obtained Ag NPs.

However, at high initial AgNO3 concentration (e. g.

150 and 250 mM), small amount of large particles

(size larger than 30 nm) can be observed (Fig. 4g, h).

These large particles maybe generate from Ag ions

adsorbed on the surface of cellulose microspheres.

Due to the lack of spatial restriction, these Ag NPs can

continuously grow at high AgNO3 concentration to

generate relatively large particles.

Thermal stability analyses

The thermal stability of catalyst is an important

criterion for application in practice. In view of this,

the thermal decomposition behaviors of composite

microspheres were investigated by TG in nitrogen

and air atmosphere, respectively. The TG and DTG

curves of CMS and CMS/Ag-x are shown in Fig. 5.

For all samples, the small weight losses below

150 �C are caused by the evaporation of absorbed

water in microspheres. Under nitrogen, the decom-

position behavior of CMS/Ag-x is almost the same

as CMS and most weight loss occurs at 330–380 �C

(Fig. 5a, b). The increase of remaining char of

CMS/Ag-x at 600 �C with x (inserts in Fig. 5a, c)

validates the increase of Ag content in composite

microspheres.

It is interesting that the decomposition of cellulose

in air distinct apparently from that under nitrogen. On

Fig. 2 XRD patterns of CMS (a), CMS/Ag-1 (b), CMS/Ag-5

(c), CMS/Ag-15 (d), and CMS/Ag-25 (e)

Cellulose (2012) 19:1239–1249 1243

123

the one hand, cellulose was totally decomposed in air

(the residue of CMS at 600 �C is 4.25 and 0.03 wt% in

nitrogen and air, respectively, seen Fig. 5a, c). Ignor-

ing the oxidation of Ag NPs, the weight content of Ag

in CMS/Ag-x can be estimated from TG curve in air

test (Fig. 5c), and the value is 1.1, 2.5, 3.8, and

4.9 wt% for CMS/Ag-1, CMS/Ag-5, CMS/Ag-15, and

CMS/Ag-25, respectively. On the other hand, against

the one step decomposition in nitrogen, the decompo-

sition of cellulose is split into two stages in air. The

first stage at 340–370 �C is carbonization of cellulose

and the second stage at 400–600 �C is ascribed to

burning of the char (Fig. 5c). Moreover, the range of

carbonizing temperature of CMS/Ag-x narrows down

and the burning temperature shifts to lower temper-

ature, as shown in Fig. 5d, with the increase of the Ag

content. The accelerated decomposition of CMS/Ag-

x can be explained by the catalytic effect of Ag NPs.

Another contributive factor for this acceleration

possibly is the damage of cellulose crystal by incor-

poration of Ag NPs (as discussed in XRD analysis).

Nevertheless, all samples have adequate thermal

stability because the decomposition did not occur

below 300 �C.

Fig. 3 SEM images of CMS (a, b), CMS/Ag-1 (c), CMS/Ag-5 (d), CMS/Ag-15 (e), and CMS/Ag-25 (f). Inserts illustrate the integral

morphology of CMS and CMS/Ag-x

1244 Cellulose (2012) 19:1239–1249

123

Fig. 4 TEM images and particle size histograms of Ag NPs in CMS/Ag-1 (a, e), CMS/Ag-5 (b, f), CMS/Ag-15 (c, g), and CMS/Ag-25

(d, h)

Fig. 5 TG and DTG curves

of CMS and CMS/Ag-

x under nitrogen (a, b) and

air (c, d): CMS (a); CMS/

Ag-1 (b); CMS/Ag-5 (c);

CMS/Ag-15 (d); and CMS/

Ag-25 (e)

Cellulose (2012) 19:1239–1249 1245

123

Catalytic properties of CMS/Ag composite

microspheres

Catalytic performance of the as-prepared cellulose/Ag

composite microspheres was examined through the

model reaction of 4-nitrophenol reduced by NaBH4.

This reaction is a thermodynamically favorable (E0 for

4-nitrophenol/4-aminophenol and BO2-/BH4

- is -

0.76 and -1.33 V, respectively) but kinetically

restricted process in the absence of catalyst. Before

reduction, 4-nitrophenol is translated into 4-nitrophe-

nol anion by addition of alkaline NaBH4. Then, the

resulted 4-nitrophenol anion is reduced by NaBH4 in

the presence of catalyst as the following equation

(Pradhan et al. 2002):

ð2Þ

The advantage of this model reaction is the reactant

4-nitrophenol anion (kmax = 400 nm) and the product

4-aminophenol (kmax = 300 nm) can be easily mon-

itored by spectrophotometry. If the concentration of

NaBH4 is largely exceed that of 4-nitrophenol, the

reaction should be first order with regard to the

4-nitrophenol concentration. Moreover, the apparent

kinetic rate constant kapp will be strictly proportional

to the total surface S of all metal nanoparticles due to

the catalytic reduction proceeds on the surface of

metal NPs. The apparent rate constant kapp and k1 can

be defined by equation (Lu et al. 2006c, 2007a):

� dCt

dt¼ KCt ¼ K1SCt ð3Þ

here Ct is concentration of 4-nitrophenol at time t, k1 is

the rate constant normalized to S, which is the total

surface area of Ag nanoparticles of unit volume in

reactions. For the calculation of S the bulk density of

silver (q = 10.5 9 103 kg/m3) has been used. TG

results have been used to estimate the silver content in

composite microspheres and TEM results have been

used to determine the diameter of Ag NPs.

In most cases, the decrease of absorption at 400 nm

was used to figure out rate constant. However, the

matrix here is porous cellulose microspheres, a known

bio-adsorbent, which may adsorb reactant of 4-nitro-

phenol anion. Therefore, a critical step to study the

influence of cellulose microspheres without Ag NPs

on the reaction must be taken in advance. Figure 6a

shows the time dependent UV–visible spectra of the

reaction carried out in the presence of CMS. The light

yellow 4-nitrophenol aqueous solution shows absorp-

tion maximum at 317 nm, while the solution turned to

yellow green and the peak red shifts to 400 nm after

addition of NaBH4 immediately, implying the forma-

tion of 4-nitrophenol anion (Pradhan et al. 2002). The

absorption intensity at 400 nm decreases and then

remains unaltered nearly after 24 h staying. However,

it is clear that the peak at 300 nm ascribed to

4-aminophenol does not appear even after 48 h,

suggesting 4-nitrophenol cannot be reduced in the

absence of Ag NPs and the decrease of absorption is

derived from the adsorption of 4-nitrophenol ion by

cellulose. Figure 6b reveals a typical evolution of the

UV–visible spectra of reacting solution with time

using excess NaBH4 (CNaBH4=C4-nitrophenol = 1,000)

and 100 mg/L catalyst of CMS/Ag-5. The successive

decrease of absorption at 400 nm accompanying with

the increase at 300 nm indicates the catalytic reduc-

tion of 4-nitrophenol to 4-aminophenol. The isosbestic

points at 252, 280, 318 nm demonstrate that

Fig. 6 UV–visible spectra

of 4-NP aqueous with CMS

and CMS/Ag-5 in the

presence of 0.1 M NaBH4.

Conditions:

C4-NP = 1.0 9 10-4 M,

CCMS or CMS/Ag-

5 = 100 mg/L

1246 Cellulose (2012) 19:1239–1249

123

4-aminophenol is the sole product of the reaction

(Saha et al. 2010). The fast reduction of 4-nitrophenol

in the presence of CMS/Ag-5 (the absorption at

400 nm decreased more than 96 % within 11 min)

illustrates the influence of adsorption by cellulose

matrix on reducing reaction can be neglected. Here,

the curve of t = 0 min in Fig. 6b is missed because the

reaction with catalyst is too quick to measure the initial

state accurately. However, we tested the absorption

spectra of the mixed solution without microspheres

and found there is no difference from the spectra of

Fig. 6a at t = 0 min. Therefore, the absorption value

at 400 nm in Fig. 6a at t = 0 min has been served as

A0 for the following calculations.

Figure 7 shows the plots of ln(Ct/C0) versus t with

varied concentration of CMS/Ag-5. All curves reveal

the first-order kinetics (the correlation coefficients of

the fitted lines are more than 0.98). It is interesting to

note that no induction time relates to the activation of

the catalyst by NaBH4 was found in all cases. Here, the

catalyst was activated by mixing CMS/Ag-5 with

NaBH4 aqueous solution before the addition of

4-nitrophenol. By fitting the slope of these curves,

the corresponding apparent rate constants kapp can be

obtained.

Figure 8 shows the apparent rate constant kapp as a

function of the concentration of CMS/Ag-x and the

total surface S of Ag NPs in CMS/Ag-x, respectively.

Linear dependence between kapp and CCMS/Ag-x is

observed in all cases of CMS/Ag-x and the slope

increases with x (Fig. 8a). However, reducing kapp to

the total surface S of Ag NPs in catalyst, a master curve

could fit to all of data (Fig. 8b). This linear relation-

ship corroborates the assumption given by Eq. 3. The

result accords with the above mentioned mechanism

that catalytic reduction occurs on the surface of Ag

NPs. The k1 of cellulose/Ag composite microspheres

was obtained from the slope of fitted line in Fig. 8b.

The catalytic activity of Ag NPs embedded in

cellulose microspheres (4.42 9 10-2 s-1 m-2 L) is

much higher than that supported by bulk polymer

hydrogels (7.31 or 7.80 9 10-5 s-1 m-2 L) (Lu et al.

2007b), and comparable with that immobilized in PS-

NIPA microgel core–shell particles (5.02 9

10-2 s-1 m-2 L) (Lu et al. 2006b) and PS-PEGMA/

PAA brush core–shell particles (7.27 or 7.81 9

10-2 s-1 m-2L) (Lu et al. 2006c, 2007a). The large

specific surface area of porous cellulose microspheres

is favorable for the diffusion of reactant which can

reach nanoparticles rapidly and the high catalytic

reduction can be expected.

Conclusion

Cellulose/Ag NPs composite microspheres have been

fabricated successfully by combination of sol–gel

Fig. 7 The relationship between ln(Ct/C0) and reaction time

with different concentration of CMS/Ag-5: 10 (a), 50 (b), 100

(c), 150 (d), and 200 mg/L (e). Conditions: C4-NP = 1.0 9 10-4

M, CNaBH4 = 0.1 M. Herein, Ct/C0 were directly obtained from

the relative intensity of the corresponding absorbance of At/A0 at

400 nm

Fig. 8 Plot of apparent rate

constant kapp versus the

concentration of cellulose/

Ag composite microspheres

(a) and the total surface area

S of Ag NPs normalized to

the unit volume (b) in

reaction: CMS/Ag-1

(squares), CMS/Ag-5

(circles), CMS/Ag-15

(triangles) and CMS/Ag-25

(diamonds)

Cellulose (2012) 19:1239–1249 1247

123

method and hydrothermal treatment. Porous cellulose

microspheres were served as reducing reagent and

micro-reactors during in situ generation of Ag NPs. Ag

nanocrystal dispersed well in cellulose matrix and

their size, distribution, and content were tunable by

tailoring of the initial AgNO3 concentration. The

incorporation of Ag NPs did not destroy the shape and

integrity of cellulose microspheres due to the mild

hydrothermal temperature. The obtained cellulose/Ag

composite microspheres maintain adequate thermal

stability and did not decompose below 300 �C. The Ag

NPs in cellulose microspheres show good catalytic

activity due to the large specific surface area of porous

cellulose microspheres. In view of the sustainable,

biodegradable and readily available of cellulose as

well as the facile and eco-friendly preparation, the

hybrid material proposed here is a promising candi-

date for catalytic applications in practice. Our research

is also potentially expanded to prepare other new

functional cellulose nanocomposites.

Acknowledgments This work was supported by The National

Natural Science Foundation of China (Grant No. 50821062,

21121001), Ministry of Science and Technology (2009

CB623401) and Open-end Fund of Engineering Research

Center of Biomass Modified Materials, Sichuan Province (No.

2210zxfk22).

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