PAPER www.rsc.org/materials | Journal of Materials Chemistry

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Photochemical growth of cadmium-rich CdS nanotubes at the air–waterinterface and their use in photocatalysis†

Yuying Huang,ab Fengqiang Sun,*ab Hongjuan Wang,ab Yong He,ab Laisheng Li,a Zhenxun Huang,ab

Qingsong Wuab and Jimmy C. Yuc

Received 20th April 2009, Accepted 8th July 2009

First published as an Advance Article on the web 4th August 2009

DOI: 10.1039/b907871a

Cadmium-rich CdS nanotubes were directly obtained at the air–water interface by a new

photochemical route. Under ultraviolet light irradiation, branch-like lamellas were first formed on the

surface of the precursor solution, and then they bent into nanotubes because of the composition

difference between the two sides of lamellas during photochemical reactions and sulfur–air reactions.

A typical nanotube has one spherical seal and one open end. Most of the cadmium was contained in the

tubes and a little was doped in the tube-walls during their formation. Such nanotubes showed higher

photocatalytic activity than the corresponding pure CdS nanotubes in the photodegradation of

methylene blue because of the existence metal cadmium. This route is green, template/surfactant-free,

reproducible and can be extended to prepare other binary compound semiconductor nanostructures

containing elements that can react either with air or another gas.

Introduction

Cadmium sulfide (CdS), an important semiconductor with

a band gap of 2.52 eV at room temperature, has attracted

considerable interest in photocatalysis,1–3 light-emitting diodes,4

solar cells,5 and some other optoelectronic applications.6,7 In

order to be more suitable for applications or acquire the

enhanced properties, many CdS nanostructures with different

morphologies have been synthesized and applied.8–10 CdS

nanotubes, as an important nanostructure for many areas of

technology, had been fabricated by template/surfactant-assisted

methods11–18 such as solution precipitation, chemical vapor

deposition, microwave etc. Obviously, all these methods must

deal with the fabrication and removal of templates or surfac-

tants, which limit the efficient research and large-area applica-

tions of nanotubes, for example, no reports on the photocatalysis

of CdS nanotubes has been found. Moreover, all the as-prepared

nanotubes with variations in composition and the related prop-

erties were not discussed. In general, doping with certain metals

is an efficient way to further enhance the properties of some

semiconductor nanostructures, for example the Mn-doped

CdS19–21 nanocrystals and nanowires used in photoluminescence.

Therefore, it is necessary and challengeable for the high-output,

low-cost and simple synthesis of CdS based nanotubes with

certain compositions.

aSchool of Chemistry and Environment, South China Normal University,Guangzhou 510006, P. R. China. E-mail: fqsun@scnu.edu.cn;fengqiangsun@yahoo.com.cnbKey Lab of Electrochemical Technology on Energy Storage and PowerGeneration in GuangDong Universities, Guangzhou 510006, P. R. ChinacDepartment of Chemistry and the Centre of Novel Functional Molecules,The Chinese University of Hongkong, Shatin, New Territories,Hongkong, P. R. China

† Electronic supplementary information (ESI) available: Electronmicroscopy images, XRD spectrum and BET measurements. See DOI:10.1039/b907871a

This journal is ª The Royal Society of Chemistry 2009

Here, we introduce a new photochemical route to synthesize

cadmium-rich (Cd-rich) CdS nanotubes. Over the past few years,

ultraviolet (UV) light irradiation techniques have emerged as

a green22 and effective way of controlling the morphology of noble

metals and some semiconductors at room temperature, such as

gold,23,24 silver,25 CdS,26,27 CdSe,28 HgS,29 etc. However, all these

methods are limited to solutions and must be assisted by the

templates or surfactants. Direct fabrication of regular nano-

structures with photochemical techniques is still difficult up to

now. In the present research, we draw on a different approach to

directly synthesize Cd-rich CdS nanotubes with the assistance of

air–water interfaces. Though in several works30,31 air–water

interfaces have ever been used to prepare template/surfactant-

induced heavy metal (gold and silver) nanostructures, their

characteristics were not fully considered and they served only as

a medium for transferring free-standing films to solid substrates.

In fact, on one hand, the air–water interface itself provides a site

for crystal nuclei formation; on the other hand, which may be

important for semiconductors, the composition of materials

formed at the interface might be affected by air, which may further

induce the formation of certain structures.32,33 For CdS reported

here, a kind of lamella rich in cadmium and deficiencies at the

interface was formed through photochemical and sulfur–air

reactions, and the lamella could then be bent into nanotubes under

the slight temperature variation caused by UV irradiation.34,35

The cadmium could be doped in the tube-walls and filled in the

interior of tubes at the same time. A new and attractive application

of the CdS nanotubes is in photocatalysis, and they have shown

higher activity in the photodegradation of methylene blue (MB).

Experimental section

Preparation of Cd-rich nanotubes

In a typical synthesis, as shown in Scheme 1, a precursor solution

composed of 0.1 M CdSO4 and 0.6 M Na2S2O3 is put into a petri

J. Mater. Chem., 2009, 19, 6901–6906 | 6901

Scheme 1 Strategy of photochemical preparation of CdS nanotubes at

the air–water interface. (a) UV light irradiated the precursor solution;

(b) picking up the as-formed film; (c) Cd-rich CdS nanotubes; (d) pure

CdS nanotubes.

Fig. 1 (a) A photo of the film formed on the surface of a precursor

solution; (b) SEM image of a piece of random film sample from (a);

(c) SEM image of the final product after the film was washed and broken.

The inset is an amplified SEM image; (d) the corresponding XRD spectra

of the final products.

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dish. The dish is then placed under a tube-type UV lamp (Philips,

254 nm, 8 W, 0.734 mWcm�2) (Scheme 1a) in a box. When the

lamp is turned on, a photochemical reaction occurs mainly at the

surface. After 24 h, a gray film composed of Cd-rich CdS

nanotubes had formed over the whole surface. This is lifted off

(Scheme 1b) and washed with deionized water, and the film then

breaks and disperses in the water. Finally, the products are

collected to further characterize and applied after drying

(Scheme 1c). When these nanotubes were put into 0.1 M nitric

acid, pure CdS nanotubes (yellow sample) formed after several

minutes.

Photocatalysis

The photodegradation activity of the as-prepared Cd-rich and

pure CdS nanotubes was tested using a methylene blue solution

at 3 � 10�5 M, and a 0.05 g sample was put into a column-like

container containing 200 ml methylene blue. The mixture inside

the container was maintained in suspension by magnetic stirring.

A quartz cold trap refrigerator with a continuous flow of

water was set into the suspension. The light source was put inside

the quartz cold trap. We used a 500 W mercury lamp for the

UV light to test the photocatalytic activity in the UV range, and

a 500 W xenon lamp for the visible range light strength. The

degradation process was controlled by monitoring the absor-

bance (at l ¼ 665 nm) characteristic of the target which was

proportional to its concentration in the solution.

Characterization

After washing drastically more than 5 times in a centrifugal

machine and then in an ultrasonic bath, the morphology and size

of the samples were characterized by SEM (JSM-6330F). For

further insight into the microstructure of the samples, TEM and

HRTEM (JEM-2010HR) observations were performed. The

phase identification of the samples was further investigated using

X-ray powder diffraction with Cu Ka radiation (l ¼ 1.54178).

The chemical composition of the nanostructures was analyzed

using EDS.

Results and discussion

Fig. 1a shows the photo of the as-prepared gray film on the

surface of the solution. It covers all the observed area. Fig. 1b

shows the representative scanning electron microscopy (SEM)

images of a sample randomly selected from the film. Obviously,

the film is composed of one-dimensional (1D) nanostructures

6902 | J. Mater. Chem., 2009, 19, 6901–6906

and the longest length exceeds 20 mm. After the film was broken

(Scheme 1c), these 1D structures can not be destroyed, as shown

in Fig. 1c. By carefully observing, some openings can be seen

(inset of Fig. 1c), so we could confirm the 1D nanostructure

might be a kind of nanotube. A single nanotube is clearly

uniform throughout its entire length and outer diameters vary

from 80 to 180 nm. Most tubes have a spherical top. An X-ray

diffraction (XRD) pattern (Fig. 1d) of the sample shows that the

tubes are composed of CdS and metal cadmium.

Transmission electron microscopy (TEM) and high-resolution

transmission electron microscopy (HRTEM) images of the

products are shown in Fig. 2. The tube structure with its

80–180 nm outer diameter and 50–100 nm inner diameter is

clearly displayed in Fig. 2a. One end of the tube is sealed with

a spherical top just like a test tube and the whole tube is unevenly

filled with materials different from that of the wall. A typical tube

is shown in the inset in Fig. 2a. Three obvious regions are found:

the top is an empty pure tube (region 1), while the middle and

lower regions (regions 2 and 3) are filled with certain materials.

Energy dispersive spectroscopy (EDS) analyses of the different

regions are shown in Fig. 2b. The Cd:S molar ratio of the cor-

responding region has been found to be ca. 1.5:1, 5.6:1 and 7.7:1,

with a standard deviation of ca. 1%. Clearly, the whole tube

structure is rich in cadmium. Combining this information with

the XRD characterization (Fig. 1d), we can speculate that the

tube wall is composed of CdS and a little metal cadmium,

whereas the tube might be filled with pure cadmium (region 3)

and a mixture of CdS and cadmium (region 2). Fig. 2c shows the

other end of the tube. The spherical seal is integrated with the

tube wall without a clear boundary, which suggests that both

may be composed of the same or similar materials. A HRTEM

image of a region of the spherical seal (region 4) is shown

in Fig. 2d. The lattice fringe allows identification of the

This journal is ª The Royal Society of Chemistry 2009

Fig. 2 Images of Cd-rich CdS nanotubes: (a) TEM image; (b) EDS spectra of different regions of a single nanotube shown in the inset of (a) where Eds1,

Eds2 and Eds3 are EDS spectra of regions 1, 2 and 3 respectively; (c) TEM image of one end of a single tube; (d) HRTEM image of region 4 in (c);

(e) HRTEM image of region 5 in (c); (f) a magnified HRTEM image of circled region in (e).

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crystallographic spacing of CdS or metal cadmium. According to

the Fourier transform of the full image (inset of Fig. 2d), nearly

all fringe distances calculated are d ¼ 0.34 nm, matching the

(111) crystallographic plane of CdS. The adjacent fringes with

the same orientation constitute nanoclusters, and different CdS

nanoclusters are closely connected to each other. This reveals

that the spherical seal is almost entirely composed of poly-

crystalline CdS. The tube wall has a similar composition (Fig. 2e

corresponding to region 5, Fig. 2c), but a cadmium nanocluster

with a fringe distance of 0.28 nm, corresponding to the Cd (002)

plane, is found in the observed area (Fig. 2f).

If such tubes are washed with dilute nitric acid, the metal

cadmium is removed and pure CdS tubes can be obtained. The

sample shown in Fig. 3 comes from that shown in Fig. 1 but has

been washed in 0.1 M HNO3. The overall one-dimensional

structure and morphological homogeneity of the precursors are

preserved in these products (Fig. 3a). The XRD measurement

(Fig. 3b) shows that the product is composed only of CdS.

Further characterization with TEM (inset of Fig. 3a), clearly

Fig. 3 Characterization of pure CdS nanotubes: (a) SEM image and

TEM image of a single nanotube (inset); (b) XRD spectrum.

This journal is ª The Royal Society of Chemistry 2009

indicates a pure CdS tube. The tube structure is well preserved

and there is no filler, which serves to confirm that the original

metal cadmium took up most of the inside of the tube, as shown

in Fig. 2a.

In order to discover the growth mechanism, we observed

samples that were irradiated for different numbers of hours, as

shown in Fig. 4a–d. After irradiation for one hour, there were

mainly dendritic lamellas, instead of tubes, formed at the air–

water interface (Fig. 4a). Surprisingly, however, two hours later,

curved lamellas combined with tubes appeared (Fig. 4b).

Fig. 4 SEM images of films floating on surfaces of precursor solution

after irradiation for different time periods: (a) 1 h; (b) 2 h; (c) 5 h; (d) 6 h.

J. Mater. Chem., 2009, 19, 6901–6906 | 6903

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When irradiated for five hours, many curved tubes were obtained

(Fig. 4c).36 Six hours later, straight tubes with small spherical

seals like the final product shown in Fig. 1a had formed (Fig. 4d).

Notably, some tubes could be found attached to a stem rather

than free standing in the observed area. In certain areas, we

occasionally found a tube that had not formed completely, as

shown in the inset of Fig. 4d, where a groove structure, like

a branch, was combined with a lamella.

Based on these phenomena and the character of the air–water

interface, we propose a growth mechanism of the Cd-rich CdS

nanotube in a sketch shown in Scheme 2. In general, crystal

nuclei form preferentially on a specific interface, such as the

air–water interface, where a suitable concentration of precursors

is present. When UV irradiates the precursor, as shown in

Scheme 2A, a series of photochemical reactions would occur.37,38

The S2O32� ions are considered to adsorb photons and be

dissociated under irradiation.

S2O32� + hn 0 S + SO3

2� (1)

The S2O32� ions also supply solvated electrons.

2S2O32� + hn 0 S4O6

2� + 2e� (2)

SO32� + S2O3

2� + hv 0 S3O62� + 2e� (3)

And then,

Cd 2+ + S + 2e� ¼ CdS (4)

However, at the air–water interface, the sulfur from the photo-

degradation of S2O32� possesses high activity and may be

Scheme 2 Schematic illustration of the formation of Cd-rich CdS

nanotubes: (A) UV light irradiated on the surface of the precursor

solution; (B) dendrite lamellas at the air–water interface; (C) typical

dendrite lamella from (B); (D) an amplified single branch lamella;

(E) branch lamella begins to bend; (F) branch lamella becomes a tube;

(G) production of dissociated sulfur near free end of tube; (H) typical

nanotube; (I) obtained nanotubes.

6904 | J. Mater. Chem., 2009, 19, 6901–6906

oxidized by oxygen during CdS formation. More photo-gener-

ated electrons would combine with Cd2+ directly. Two additional

reactions (5) and (6) must occur at the surface of the solution.

This leads to a deficiency of sulfur and a richness of cadmium in

the products, as shown in Fig. 1 and 2.

S + O2 0 SO2 (5)

Cd 2+ + 2e� ¼ Cd (6)

Thus, at the air–water interface, Cd and CdS nuclei form

randomly on the homogeneous surface, where they grow by

adsorbing new ions to form a kind of lamella in the local area.

Because the strength of the UV light is inevitably unevenly

distributed in the micro/nano range, the growth rate in different

directions naturally differs, which leads to the formation of

dendritic lamella, as shown in Fig. 4a and Scheme 2B and 2C.

Then each branch preferably grows to form a kind of belt-like

lamella (Scheme 2D). One end grows freely and tends to float on

the surface. The other is fastened to the original sheet slightly

below the surface. The lamella is composed of cadmium,

vacancies and CdS, but the cadmium and vacancies are always

inclined to form on the upper side, following reactions (5) and

(6). A composition gradient forms from the bottom to the top of

the lamella. Obviously, the side towards the air has more

vacancies from the absence of sulfur, and there is a stress

difference between the two sides of the lamella. As a result, the

branched lamella is gradually bent from the free end to the

fastened end under the slight temperature variation of about 4 �C

(see ESI†) induced by the photochemical reactions. Fig. 4b and

Scheme 2E clearly show some bent but unclosed lamella struc-

tures. The atom arrangement on the convex side of the bent

lamella becomes loose and allows more atoms to be ‘‘inserted’’,

which continuously pushes the lamella bending. At the same

time, atoms continue to be deposited on the edges of the lamella.

Finally, the edges close and the lamella becomes a tube, as shown

in Fig. 4c and Scheme 2F. Subsequently, atoms are deposited on

the outside wall of the tube to gradually thicken it. The inner wall

is rich in cadmium, so cadmium is still preferentially deposited on

it from the precursor solution in the tube, so that the tube is

mainly filled with metal cadmium. More dissociated sulfur is

produced in the tube for the same reason, and floats to the

opening of the tube, accompanying the filling of cadmium

(Scheme 2G), which leads to a large quantity of sulfur congre-

gating near the opening of the tube. The abundant sulfur can

quickly combine with Cd2+ to form CdS on the tip, and hence

a spherical seal forms (Scheme 2H and Fig. 4d). Because the

bending begins at the free end of the lamella branch, cadmium in

the tube mainly fills the spherically sealed end. Once the tube

comes into being, the sulfur formed cannot escape from the tube,

so it combines with Cd2+ to form CdS in the tube, as shown in

region 2, Fig. 1. When the nanotube film is lifted off and

subsequently washed, nanotubes can be broken from the trunk.

Tubes with one sealed end and one open end are hence formed

(Scheme 2I).

Further experiments prove that the formation of Cd-rich CdS

nanotubes is influenced by proximity to the air–water interface,

ambience, UV light power, concentrations and molar ratios of

different precursors. These factors are all related to the release of

This journal is ª The Royal Society of Chemistry 2009

Fig. 5 Photodegradation activity and UV-vis diffuse reflectance spectra of Cd-rich and pure nanotubes; 0.05 g samples were used for the degradation of

200 ml 3 � 10�5 M methylene blue with different light sources: (a) with 500 W mercury lamp; (b) with 500 W xenon lamp; (c) with UV-vis diffuse

reflectance spectra.

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sulfur and the production of electrons, and they all further the

formation of cadmium. Within the solution, there is an excess of

sulfur and no film is formed, so that only CdS and sulfur particles

with irregular shapes can form before the whole surface is

covered with a nanotube film. When a solution with the same

concentration as shown in Fig. 1 is set in a N2 surrounded

container, the sulfur formed was not lost but reacted with

cadmium, so that a yellow film instead of a gray film formed on

the surface, which shows that little or no dissociated cadmium

was produced. As a result, no composition gradient formed, and

hence no nanotubes. This further proves the ‘‘lamella bending’’

growth mechanism we present. As long as the UV power is higher

than 0.050 mW/cm2, nanotubes can always form at the air–water

interface; otherwise, very few electrons are produced so that only

a CdS film, or none at all, forms. For a similar reason, when the

precursor concentration is below 0.0125 M, nothing is produced

on the surface. Provided the concentration of one of the two

precursors is kept at a relatively higher value, for example, 0.1 M,

nanotubes can be always obtained, despite variations in

concentration of the other, but the output and the cadmium

content (as seen by the color) changes with the ratio between the

two precursors.

Interestingly, both the Cd-rich and the pure CdS nanotubes

possess a high photocatalytic activity, as measured by the

decomposition of methylene blue (MB). Under UV light, Cd-rich

CdS nanotubes exhibited a degradation efficiency of up to 98% in

only 24 min, and the same weight of pure CdS nanotubes reached

up to 95% (Fig. 5a). Under visible light, the degradation effi-

ciency falls a lot and the corresponding data were 60% and 46%

respectively in 120 min (Fig. 5b). Obviously, the Cd-rich CdS

nanotubes have a higher photocatalytic activity than the pure

nanotubes in UV, and especially in the visible range. The BET

(Brunauer–Emmett–Teller) measurement (not shown here)

showed that the surface area of the non-doped pure CdS nano-

tubes was 43.6974 m2 g�1 while that of Cd-rich CdS nanotubes

was 9.4105 m2 g�1. Therefore, the significantly higher surface area

of the CdS nanotube was not the only factor responsible for the

higher photocatalytic activity, and the existence of metal

cadmium could be critical. These results are consistent with the

UV-vis diffuse reflectance spectroscopy as shown in Fig. 5c. The

absorbance of the Cd-rich CdS nanotubes was higher than that

of the non-doped pure CdS nanotubes in the visible range.

Clearly, doping with metal cadmium increases the absorbance

efficiency for visible light. On the other hand, cadmium, like

some other doping metals,21,39 might hinder the recombination of

This journal is ª The Royal Society of Chemistry 2009

photo-generated electrons and holes. As a result, Cd-rich CdS

nanotubes have higher photocatalytic activity, although they

have a reduced surface area.

Conclusion

In summary, we have developed a novel photochemical route to

directly prepare Cd-rich CdS nanotubes with one sealed end and

one open end selectively at the air–water interface. Under UV

light irradiation, large areas of tubes could form on surfaces of

precursors by a ‘‘lamella bending’’ mechanism which involves

photochemical and sulfur–air reactions. CdS constitutes the tube

structure and metal cadmium fills the interior or dopes the wall of

the tube. These structured materials can be used as efficient

photocatalysts for the degradation of MB. This method may

offer a new route for the direct growth of binary compound

semiconductor nanostructures containing an element that can

react with certain gases such as CuS, SnO2, ZnS, etc. These

nanostructured materials could be effectively used in some

optoelectron-related fields, for example, photocatalysis, gas-

sensors and light-emission devices.

Acknowledgements

This work was co-supported by the Program for New Century

Excellent Talents in University (NCET-07-0317) and the

National Natural Science Foundation of China (Grant No.

20773042 and No. 50502032).

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