ORI GIN AL

Surface functionalization of bamboowith nanostructured ZnO

Yan Yu • Zehui Jiang • Ge Wang • Genlin Tian •

Hankun Wang • Ye Song

Received: 12 April 2010 / Published online: 18 October 2011

� Springer-Verlag 2011

Abstract Imparting excellent preservative performances to bamboo is the key to

expand the applications of this extraordinary non-wood forest resource. This study

reports on the formation of ZnO-nanostructured network films on the surface of

bamboo via a simple two-step process. This process consists of the generation of

ZnO seeds on the bamboo surface followed by a solution treatment to promote the

crystal growth. The morphology and chemical composition of the ZnO films were

studied by field-emission scanning electron microscopy combined with energy-

dispersive X-ray analysis and X-ray diffraction. Accelerated weathering was used to

evaluate the photostability of the treated wood. The antifungal and antibacterial

performances were also examined. The results indicate that the approach can

simultaneously furnish bamboo with excellent photostability and antifungal and

antibacterial performances. The growth mechanism of ZnO-nanostructured net-

work films on the uneven and chemically complicated surface of bamboo was also

discussed.

Introduction

It is increasingly accepted that nanotechnology will have a significant impact on the

forest product industry (Moon et al. 2006). UV protection, pest and fungi control are

thought to be the most promising areas to be improved by nanotechnology (Roughley

2005). Several studies have focused on the application of nanotechnology to wood

Y. Yu (&) � Z. Jiang � G. Wang � G. Tian � H. Wang

Department of Biomaterials, International Center for Bamboo and Rattan, No. 8,

Futong Dong Da Jie Street, Wangjing Area, Chaoyang District, Beijing 100102, China

e-mail: yuyan1975830@googlemail.com

Y. Song

School of Materials, Central South Forestry Science and Technology University,

Changsha 410004, China

123

Wood Sci Technol (2012) 46:781–790

DOI 10.1007/s00226-011-0446-7

protection and achieved some successes (Liu et al. 2003; Giorgi et al. 2005; Cai et al.

2007; Kumar et al. 2008). ZnO is a wide band gap (3.37 eV) semiconductor with a

large exciton binding energy (60 meV). Nanostructured ZnO has great potential for

many practical applications, such as dye sensitized solar cells, piezoelectric

transducers, UV-light emitters, chemical and gas sensors, and transparent conductive

coating (Ozgur et al. 2005). It also exhibits intensive ultraviolet absorption and can

potentially be utilized as UV-shielding materials and antibacterial agents (Kim and

Osterloh 2005). Wang et al. (2004) demonstrated that oriented hexagonal ZnO

nanorods could be grown onto cotton fabrics using low-temperature aqueous

solutions. Lu et al. (2006) further proposed a novel approach to fabricate ZnO/

polystyrene nanohybrid coatings on cotton fabrics and provided the treated fabrics

with ultrahigh UV protection properties and superior washing fastness. Paper could

also be provided with excellent antibacterial properties when coated with ZnO

nanoparticles (Ghule et al. 2006).

Bamboo is one of the most important non-wood forest resources in the world,

growing faster than almost all the trees on earth. Bamboo can reach a maximum

height of 15–30 m within 2–4 months and full-stand maturity within 3 to 5 years.

There is intensive interest in utilizing bamboo as an alternative raw material to

wood due to its rapid growth rate, high strength and surface hardness, and superior

flexibility (Liese 1987). However, bamboo is much more susceptible to the attack by

fungi and insects due to its higher sugar and starch content compared with wood,

which results in degraded performance, shortened service life, and reduced value

(Liese and Kumar 1998). In addition, bamboo is very sensitive to the ultraviolet

irradiation during outdoor service. Furthermore, conferring antibacterial perfor-

mance on bamboo or bamboo-based products will significantly increase their added

value by extending their applications to some high-risk environments, such as

medical and related health care.

This work reports for the first time on the formation of ZnO nanonetwork films on

the surface of bamboo via a modified low-temperature aqueous solution route. The

final purpose was to explore whether bamboo can be simultaneously functionalized

with photostability and antifungal and antibacterial performances by being coated

with nanostructured ZnO.

Materials and methods

Formation of ZnO Crystal seeds on bamboo

Bamboo blocks with dimensions of 20 mm (L) 9 20 mm (T) 9 5.8 mm (R) were

rinsed ultrasonically in deionized water for 20 min and oven-dried at 60�C for

3–6 h. All chemicals were used as received without further purification. ZnO films

were grown on bamboo using a modified simple two-step process consisting of seed

coating in ZnO nanosol and crystal growth in a zinc salt aqueous solution (Greene

et al. 2003). The ZnO nanosol was prepared with reference to the method proposed

by Pacholski et al. (2002). A NaOH solution in methanol (MeOH) (0.03 M) was

added slowly to a solution of zinc acetate dihydrate (0.01 M) in MeOH at about

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60�C and stirred for 3 h to form a transparent homogeneous solution. Bamboo

samples were then immersed in the sol at room temperature for 1 h followed by heat

treatment in air at 100�C for 3 h. The above procedure was repeated 3 times to form

ZnO nanocrystal seeds on bamboo, which will act as crystal nuclei to facilitate the

growth of ZnO in the growth solution.

Growth of ZnO nanonetwork films on bamboo

The seed-coated samples were then immersed in an aqueous growth solution

containing equal mol of zinc nitrate six hydrate (Zn(NO3)2�6H2O) and methenamine

((CH2)6N4). The solution temperature was kept at 90�C for 6 h. The mol

concentration of the zinc salt solution was set at 0.020 M. Finally, the samples

were rinsed with deionized water and dried at 60�C for 3 h.

The appearance and texture of the treated bamboo stays unchanged after the

treatment. No attempts were made to measure the thickness of ZnO films since the

rough and uneven surface of bamboo makes it almost impossible to obtain reliable

thickness data. However, the weight gain after treatment was measured to be

approximately 1.43%. This value was calculated by taking into account the weight

loss of bamboo extractives measured by extracting bamboo samples in aqueous

solution under similar pH value, temperature, and time.

Characterization

Structure and chemical compositions

A field-emission scanning electron microscope (XL30-FEG-SEM, FEI) combined

with energy-dispersive X-ray analysis (EDXA) was used to examine the morphol-

ogies and elemental compositions of the films. A Philips X’pert diffractometer was

used to evaluate the crystallization behavior of the formed films.

Evaluation of photostability

An accelerated weathering test chamber (Atlas, Germany) was used to accelerate

the photo discoloration of bamboo samples. The radiation intensity was set at

42 W/m2 and the chamber temperature at 40�C. The samples were fixed in stainless

steel holders and rotated around the fixed xenon light source in non-turning mode at

65% relative humidity for periods ranging from 0 to 120 h. The changes in the

surface color of the bamboo with irradiation time were determined using a color

meter (BYK-6834, Germany). CIELAB L�, a�, b�, and E� parameters were

measured at five locations of each specimen and average value was calculated. Five

specimens were measured for every treatment condition. In the CIELAB system, L�

axis represents the lightness, and a� and b� are the chromaticity coordinates. L�, a�

and b� values are used to calculate the overall color changes DE� using the

following equation:

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DE� ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðDa�Þ2 þ ðDb�Þ2 þ ðDL�Þ2q

ð1Þ

where Da�, Db�, and DL� are the difference in initial and final values of a�, b�, and

L�, respectively. A lower DE� value corresponds to lower color changes and

indicates better photostability.

Evaluation of antifungal performance

A very convenient and quick method for the evaluation of antifungal performance of

bamboo was adopted here based on the fact that molds grow much quicker on bamboo

than on wood in a high moisture environment. This makes fungal inoculation to

bamboo unnecessary, while it is usually required for wood. Both the control and

treated samples were water-saturated and then placed in a sealed vessel with a relative

humidity of 95 ± 2% and a temperature of 23–25�C. The vessel was placed in a

common laboratory with normal illumination in the daytime and a dark break at night

for 35 days. Pictures and qualitative description were taken every day at the initial

stage and could be extended to intervals of 3–5 days at the final stage.

Evaluation of antibacterial performance

Antibacterial performance was evaluated according to the Japanese industrial standard

‘‘Antimicrobial products—test for antimicrobial activity and efficacy’’ (JIS Z 2801:

2000). Escherichia coli (ATCC 25922) was inoculated on both the ZnO-treated samples

and the control ones. The inoculated samples were placed in a 90-mm culture dish at a

relative humidity of more than 90% and at 35 ± 1�C for 24 h. The cell viability of

bacteria was determined based on the number of colonies developed on the nutrient

broth agar plates.

Results and discussion

The ZnO nanonetwork films on bamboo

Figure 1a shows the microstructure of a longitudinal section of untreated bamboo.

Vascular bundles that have the function of mechanical reinforcement are embedded

in the matrix of ground parenchyma. Figure 1b shows that nanonetwork films are

formed on the surface of bamboo using the present reaction conditions. The film

was actually composed of randomly oriented irregular sheets with a wall thickness

normally less than 50 nm. In order to further ascertain whether the fabricated

nanostructured films were ZnO, the chemical elements and crystalline structure of

the films were determined by energy-dispersive X-ray analysis (EDXA) and X-ray

diffraction (XRD). Figure 2 indicates that zinc, oxygen, carbon, and platinum can

be detected with EDXA from the treated samples. Platinum is sure to come from the

conductive layer on the surface of the samples for SEM observation. Carbon signals

and some of the oxygen signals are believed to originate from the bamboo substrate

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underneath. Though EDXA is capable of determining which elements exist in the

films, it cannot tell whether there are Zn(OH)2 or other crystalline impurities mixed

with ZnO. The X-ray patterns in Fig. 3 show the typical diffraction peaks of

wurtzite structure of ZnO. The cellulose characteristic peak (22.4�) from bamboo

could also be clearly observed. No characteristic peaks were observed for the other

possible impurities such as Zn(OH)2. The X-ray diffraction patterns identified that

the films formed on the surface of bamboo were pure ZnO.

The growth of ZnO nanonetwork films is thought to be based on the heterogeneous

nucleation and subsequent crystal growth on bamboo, respectively, involved in the

two-step process of seed coating in ZnO nanosol and crystal growth in the zinc nitrate

aqueous solution. Similar approach has been successfully used for the fabrication of

oriented ZnO nanorod arrays or nanowires on various inorganic and synthetic polymer

substrates (Vayssieres 2003; Choy et al. 2003). Normally, ZnO nanorods are the

predominant morphology grown on smooth inorganic substrates if a similar approach

is used. However, the growth of ZnO films on bamboo seems to be more complicated.

Fig. 1 SEM image showing the morphology of longitudinal section of untreated bamboo (a) and thenanostructured networks (b) formed on the treated samples

Fig. 2 Surface elementalcompositions of bamboo coatedwith nanonetwork films

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Though it is hard to accurately describe the full growth process of ZnO presently, it can

at least be inferred with confidence that the growth process should be to some extent

similar to that involved in the growth of ZnO nanorods. The growth mechanism of ZnO

nanorods has been described based on oriented attachment of preformed quasi-

spherical ZnO nanoparticles and the mechanism of layer-to-layer growth (Pacholski

et al. 2002; Li et al. 2005). During the process, the following chemical reactions are

involved:

ðCH2Þ6N4 þ 6H2O$ 6HCHOþ 4NH3 ð2Þ

NH3 þ H2O$ NHþ4 þ OH� ð3Þ

2OH� þ Zn2þ ! ZnOðsÞ þ H2O ð4ÞFor the present reaction system, a film of ZnO nanoparticles will first form on the

surface of bamboo after it was treated in ZnO nanosol, serving as nucleation sites

for the subsequent growth of ZnO crystals shown in Fig. 4. The preformed ZnO

nanoparticles grow and transform into irregular nanosheets in the growth solution.

These nanosheets are randomly oriented due to the unevenness of the bamboo

surface, leading to the network structures observed most frequently. Occasionally,

ZnO nanoparticles directly grow into nanorods/nanowires and form similar network

structures. The assumption that nanoparticles could directly grow into nanorods/

nanowires is strongly supported by the SEM image in Fig. 5.

20 25 30 35 40 45 50 55 60 65

2Theta (°)

Inte

nsity

(co

unts

)100

002101

102 110103

112

Cellulose microcrystal

ZnO

Fig. 3 X-ray diffraction patterns of bamboo coated with nanonetwork films

1 μm

Fig. 4 Schematic representations of the growth process of ZnO nanonetwork films on bamboo

786 Wood Sci Technol (2012) 46:781–790

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Photostability

Figure 6 shows the effect of photo irradiation on the color changes in control samples

and the samples coated with ZnO nanonetwork films. A positive DL� indicates a

tendency of an object color toward white, while negative values determine the color

shift toward black. For Da�, positive values indicate a tendency toward red, while

negative values determine the color shift toward green. Positive values of Db� indicate

a tendency toward yellow, while negative values determine the color shift toward

black. The color changes in the samples coated with ZnO nanonetwork films are

significantly less than those of the controls after 120-h irradiation, which indicates that

ZnO films effectively improve the photostability of bamboo. The total color difference

250 nm

Fig. 5 Schematic representations of the growth process of ZnO nanorods/nanowires on bamboo

0 20 40 60 80 100 1200

3

6

9

12

15

18

ΔE*

Irradiation time (h)

Nanostructure networks Control

0 20 40 60 80 100 120

0

2

4

6

Δa*

Irradiation time (h)

Nanostructure networks Control

-2

0

2

4

6

8

10

12

14

Δb*

Irradiation time (h)

Nanostructure networks Control

0 20 40 60 80 100 120

0 20 40 60 80 100 120-15

-12

-9

-6

-3

0

3

Δ L*

Irradiation time (h)

Nanostructure networks Control

Fig. 6 Variation in CIELAB parameters DL�, Da�, Db�, and DE� at different irradiation times forbamboo coated with ZnO nanonetwork films and the control one

Wood Sci Technol (2012) 46:781–790 787

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DE� observed was less than 1/9 of the control samples. The results of this study

demonstrate that ZnO-nanostructured films have strong capability in ultraviolet

shielding, which can be explained by their high separation efficiency of electron and

hole pairs due to nanosized effects.

Antifungal performance

Mold fungi are universally detected in water-damaged wood constructions (Andersson

et al. 1997). This problem becomes much more serious during the utilization of bamboo

resources. For page consideration, only the photographs taken on 0, 4th, 11th, and 35th

day are presented here. For the control samples, mold appeared within 24 h and quickly

covered the whole surface in less than 11 days (Fig. 7a). However, there was almost no

visible mold growth on the surface of the samples coated with nanostructured networks

(Fig. 7b). The above results indicate that the ZnO nanonetwork films possess excellent

antifungal capability.

Antibacterial performance

Table 1 shows the antibacterial activity for E. coli. of bamboo coated with ZnO

nanonetwork films. Viable cell counts on the bamboo coated with ZnO nanonetwork

films were in less than 25 after 24 h, while the cell numbers on the control samples

increased from 5.5 9 105 to 9.0 9 106 during the same period. The improved

antibacterial performance of the treated samples might be partially attributed to the

inherently antibacterial activity of ZnO. Release of H2O2 has been proposed as the

main mechanism responsible for the antibacterial activity of ZnO bulk materials

Fig. 7 Photographs showing mold growth on control bamboo (a) and the bamboo coated with ZnOnanonetwork films (b). The numbers in the upper corners of the photographs indicate the duration ofexposure in days

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(Yamamoto 2001). However, Brayner et al. (2006) recently reported that ZnO

nanoparticles could enhance antibacterial activity through damaging cell membrane,

leading to the cellular internalization of these nanoparticles. The damage of cell

membrane of a gram-positive bacterium Bacillus atrophaeus by ZnO nanorods

array was recently observed (Tam et al. 2008). However, the mechanism responsible

for antibacterial activity of nanonetwork ZnO is still not fully understood.

Conclusion

An aqueous solution route to grow ZnO nanonetwork films onto bamboo substrates at

low temperature was presented. The formed ZnO nanonetwork films were composed

of randomly oriented irregular nanosheets and occasionally of nanowires/nanorods. It

was further demonstrated that the present approach can simultaneously furnish

bamboo with excellent photostability and antifungal and antibacterial activities.

Acknowledgments We would like to thank the National Natural and Science Foundation of China

(30871971) and the 11th Five Years Key Technology R&D Program of China (2006BAD19B05) for the

financial support. We greatly appreciate the help of Dr. J. Jakes and Jane O’Dell for the revision of the

manuscript.

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0 h 24 h

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Nanostructured networks – \20 [5.7 [99

Values are the average of three replication experiments

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