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One-pot synthesis of large-scaled Janus Ag–Ag2S nanoparticles and theirphotocatalytic properties†

Feiran Jiang,‡a Qiwei Tian,‡a Minghua Tang,a Zhigang Chen,a Jianmao Yangb and Junqing Hu*a

Received 27th May 2011, Accepted 7th October 2011

DOI: 10.1039/c1ce05632h

We report a facile, one-pot route based on thermal decomposition of

a single molecular precursor for preparing spherical, eggplant-sha-

ped Janus Ag–Ag2S nanoparticles; these Ag–Ag2S Janus-coupled

P25 TiO2 composites show a higher photocatalytic efficiency than

those of the Ag2S NPs coupled P25 TiO2 composites and pure

P25 TiO2.

In recent years, the synthesis of the Janus nanoparticles (JNPs)1 with

two or more different functional nanoparticles (NPs) contacting

intimately has attracted great research interests owing to their novel

properties that are not presented in individual component NPs.2–6

For individual component NPs, the electron is restricted to the

nanometre scale within a material; if two or more different compo-

nent NPs inter-grow, the small variation in electron transferring

across the interface between these two limited electron ‘nano-

reservoirs’ may lead to a drastic property change for each JNP.7

Therefore, the construction of JNPs offers an effectiveway to develop

new functional materials with novel physical and chemical properties

based not only on each NP dimension and shape but also on the

communication between the component NPs.7 So far, a considerable

effort has been devoted to synthesize JNPs that combine two ormore

different properties.7–10 In particular, as one of the most important

JNPs, the noble metal–semiconductor composite systems have been

investigated widely due to their well known remarkable optical,

electrical, magnetic and catalytic properties,11 such as Au–CdSe,2

Au–PbSe,12 Au–PbS,13 Au–ZnO,14 Au–Cu2S,15 Au–Ag2S,

16,17 FePt–

CdS,18 FePt–PbS/PbSe19 and Ag–Ag2S.20 Among these noble metal–

semiconductor JNPs, only one reference has been reported on the

synthesis of the Janus Ag–Ag2S nanoparticles with a complex

method, in which CuS hollow spheres converted from monodisperse

Cu2O solid spheres through a modified Kirkendall process were used

aState Key Laboratory for Modification of Chemical Fibers and PolymerMaterials, College of Materials Science and Engineering, DonghuaUniversity, Shanghai, 201620, China. E-mail: hu.junqing@dhu.edu.cn;Fax: +86-21-6779-2947; Tel: +86-21-6779-2947bResearch Center for Analysis and Measurement, Donghua University,Shanghai, 201620, China

† Electronic supplementary information (ESI) available: Detailedexperimental procedures; a photograph showing an eggplant; XRDpattern and TEM images of the Ag–Ag2S NPs obtained at othercondition; FTIR spectrum of the Ag–Ag2S JNPs. See DOI:10.1039/c1ce05632h

‡ These authors contributed equally to this work.

This journal is ª The Royal Society of Chemistry 2011

as a solid precursor of a sulfur source and the Ag–Ag2S heterodimers

were prepared through ion exchange and photo-assisted reduction.20

Owing to the excellent photoelectric and photocatalysis and good

chemical stability shown from Ag and Ag2S nanomaterials,21–29 it is

still necessary to explore a facile and rational synthetic strategy for the

large scale synthesis of the Janus Ag–Ag2S nanoparticles to explore

their novel properties and applications.

Herein, we report a simple controllable thermal decomposition

route for the one-pot synthesis of Janus Ag–Ag2S nanoparticles

without any templates. In a typical thermal decomposition process,

a pale yellow silver diethyldithiocarbamate (Ag–DEDTC) used as the

sourcematerial was injected into a hot oleylamine (OM)which served

as a reaction solution, and thermal decomposition was carried out at

180 �C for 30 min under a N2 atmosphere. Finally, a black solution

was formed and then the resulting powders were obtained by

centrifugation, washed with ethanol several times and dried in

a vacuum for further characterization.

The morphology of the as-synthesized Ag–Ag2S JNPs was

examined by transmission electronmicroscopy (TEM,Fig. 1). Fig. 1a

shows that the product is composed of monodisperse NPs possessing

Fig. 1 (a) TEM and (inset) high-magnification TEM images of the as-

obtained Ag–Ag2S JNPs, (b–d) TEM images of as-obtained Ag–Ag2S

JNPs at the same area on the copper grid with different tilting angles

(+21�, 0�, �21�) during the TEM imaging.

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a narrow size distribution (Fig. S1,† �15–25 nm), and most of the

NPs can be seen clearly with well-defined uniform Janus structures,

i.e., the different contrasts suggesting a different phase composition.

More and clearly detailed microstructure about the Janus Ag–Ag2S

NPs can be observed by a typical high-magnification TEM image

(the inset in Fig. 1a), where a small Ag particle, like a patch, contact

intimately on the Ag2S spheres (matrix) even without a distinguish-

able interface, and the axis length of the JNPs is �25 nm. To

demonstrate that all the NPs consist of two individual parts (Ag and

Ag2S), tilt-angle TEM examinations (Fig. 1b–d) were carried out. As

the tilted angle was changed from +21� to 0� and�21� by tilting thesample holder, the structural images of the Ag–Ag2S JNPs accord-

ingly and simultaneously changed since it was in fact a 2-D projection

on the screen of a 3-D real structure.When the tilted-angle was +21�,a sphere-shapedmorphology was found, Fig. 1b, because the Ag part

was hidden beneath the Ag2S part, and the projection of Ag2S and

Ag parts overlapped together when the axis of the Ag–Ag2S JNPs

was parallel to the electron beam.When the tilted-angle was changed

to�21�, the Janus structure was observed clearly since the axis of theAg–Ag2S JNPs was vertical or oblique to the electron beam, and the

projection of the two individual parts (Ag and Ag2S) could not

overlap completely, Fig. 1d. It is reasonable that all Ag–Ag2S

nanoparticles will display spherical, eggplant-shaped (Fig. S2,† ESI)

Janus structures if the sample plane is titled at a given angle.

The results of TEM imaging coupled with energy-dispersive X-ray

(EDX) spectroscopy line scan (Fig. 2a), elemental mapping (Fig. 2b)

and X-ray diffraction (XRD) pattern (Fig. 2c) give the further

detailed formation on the structure and local atomic composition of

the Janus Ag–Ag2S nanoparticles. The green arrow on the TEM

image shown in the inset of Fig. 2a indicates the scanning path of an

electron beam, and a clear presentation of the elemental distribution

is given by a plot of the EDX line scan signal versus the distance along

the axis of theAg–Ag2SNP.Overall, the EDX line scan profile shows

Fig. 2 (a) EDX line scan profile along the axis of a Janus Ag–Ag2S NP

indicated by a green arrow (inset), (b) STEM image and corresponding

EDX elemental mappings of Ag and S, (c) XRD pattern of the as-

prepared product and the standard Ag (JCPDS card, no. 65-8428, blue

bar) and Ag2S powders (JCPDS card, no. 65-2356, magenta bar).

7190 | CrystEngComm, 2011, 13, 7189–7193

that the signal peak of the S Ka1 locates at the middle regions of the

Ag–Ag2S NP, and the Ag La1 signal profile shows a similar trend

except for a considerable intensity increasing at the location of the Ag

patch. The clear distinct difference in the spatial profiles of Ag and S

is qualitatively consistent with well-defined structures. EDX

elemental mapping of a single nanoparticle (Fig. 2b) further confirms

that Ag and S are evenly distributed within Ag2S nanocrystals, while

the content of S sharply decreases and Ag has no noticeable decrease

at the location of Ag patch. The high-resolution TEM image

(Fig. S3†) revealed that the lattice fringes’ characteristic of the dark

patch on the surface of a nanocomposite was different from that of

thematrix. A resolved interplanar d-spacings of 0.238 nmon the dark

patch matches well the (1, 1, 1) lattice separation of a face-centered

cubic phase of Ag crystal, while d-spacings of �0.338 nm and

0.308 nm on the matrix corresponds to the (0, 1, 2) and (1, 1, 1) plane

separation of a monoclinic phase of Ag2S crystal, respectively. The

phase composition of the as-obtained NPs is further investigated by

XRDpattern. As shown in Fig. 2c, the XRDpattern of theAg–Ag2S

JNPs can be suitably indexed as a monoclinic structure of Ag2S with

lattice constants of a ¼ 4.23 �A, b ¼ 6.92 �A, and c ¼ 7.86 �A, which

agrees well with the reported values (a ¼ 4.23 �A, b ¼ 6.91 �A, c ¼7.87 �A) from JCPDS card no. 65-2356. Besides the diffraction peaks

due to Ag2S, the peaks (marked with A) in Fig. 2c located at 2q of

37.7�, 43.8� and 63.7� are assigned to diffraction from the (111), (200)

and (220) planes of face-centered cubic Ag crystals with lattice

constants of a ¼ 4.12 �A, which agree well with the reported values

(a¼ 4.12�A) from JCPDS card no. 65-8428. It indicates that theAg2S

and Ag coexist in the Ag–Ag2S JNPs. The above examinations

including EDX line scan, elemental mapping, high-resolution TEM

and XRD pattern indicate that the as-obtained NPs have the Janus

structure just described in Fig. 1.

In order to investigate the growth process, the products prepared at

different temperatures were collected for characterization. When the

pale yellow Ag–DEDTC dispersed in the OM at room-temperature

was injected into the hot OM at 90 �C, the color of the reaction

solution had no obvious change. TheXRDpattern (Fig. S4b†) of the

obtained product confirms the production of metallic Ag, coexisting

with the Ag–DEDTC precursor. The representative TEM images

presented in Fig. 3a–b show small and irregular Ag NPs with

a diameter of 1–4 nm. As the injection temperature was raised to

120 �C, the solution turned from pale yellow to black 5 min after the

injection. The XRD pattern (Fig. S4a†) reveals the formation of

Ag2S andAg in the absence of theAg–DEDTCprecursor, suggesting

the complete thermal decomposition of the Ag–DEDTC precursor.

The TEM images (Fig. 3c and 3d) show that well-defined sphere-

shaped core–shell Ag@Ag2S NPs with the diameter of�17 nm were

obtained. After further increasing the injected temperature to 150 �C,the color of the reaction solution was changed to black immediately.

Fig. 3e–f, the TEM images of the as-obtained NPs show clearly that

the structure of the NPs is still spherical core–shell Ag@Ag2S with

very small Ag ‘‘patches’’ on the surface of the NPs, and the diameter

was increased to �25 nm. In striking contrast, the TEM of the

product obtained at 180 �C presented in Fig. 1 shows a clear Janus

structure of Ag–Ag2S and the morphology evolves from the sphere

form obtained at 150 �C to eggplant-shaped at 180 �C. So, it isconcluded that themorphology, size, and structure of Ag–Ag2S JNPs

are mainly affected by the reaction temperature. To further validate

morphological evolution of Ag–Ag2S JNPs, a non-injection method

is also investigated. When the mixed solution of Ag–DEDTC and

This journal is ª The Royal Society of Chemistry 2011

Fig. 3 (a, c and e) TEM and (b, d and f) their high-magnification TEM

images of the Ag–Ag2S JNPs formed at different temperature: (a–b)

90 �C, (c–d) 120 �C and (e–f) 150 �C.

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OM was heated up to 180 �C at the rate of 10 �C min�1 and kept at

this temperature for 30 min, spherical particles with the diameter of

10–40 nm together with coupling and tripling of Ag–Ag2S NPs are

observed (Fig. S5a–b†). In the case in which the mixed solution was

firstly kept at 90 �C for 30 min, and then further heated up to 120 �Cfor another 30 min, the product exhibits a similar shape except with

a smaller diameter (Fig. S5c–d†).

From the above experimental results, a possible mechanism which

is similar to what has been known as the seed-mediated growth to

form core/shell NPs can be deduced, as described inFig. 4. Firstly, Ag

nuclei are formed at low temperature (Fig. 4b). OM is used as a mild

reducing agent as well as a surfactant; therefore the Ag–DEDTC

(Fig. 4a) is easily reduced to Ag when the Ag–DEDTC precursor is

dispersed in the OM. Secondly, the subsequent increase of the

temperature to 120–150 �C results in the decomposition of Ag–

Fig. 4 Schematic illustration of the formation of the Ag–Ag2S JNPs

from a single molecular precursor.

This journal is ª The Royal Society of Chemistry 2011

DEDTC in the presence of OM, and the generated Ag2S can be

deposited on the surface of the Ag nucleus to form the core/shell

Ag@Ag2S nanostructure (Fig. 4c). However, the obtained core/shell

Ag@Ag2S nanostructures are metastable because of the mismatch of

the lattice spacing. Lastly, after further increasing the temperature up

to 180 �C, Ag diffuses from the core of the Ag@Ag2S nanoparticle to

the surface of the Ag2S nanocrystal to form the Ag–Ag2S JNPs

(Fig. 4d). This diffusion process can be explained by a substitutional-

interstitial mechanism, which occurs in numerous systems including

the diffusion of elements in silicon and germanium,30 and theOstwald

ripeningmechanism, which is a phenomenonwhereby particles larger

than a critical size grow at the expense of smaller particles due to their

relative stabilization by the surface energy term.17 In the system of

core–shell Ag@Ag2S NPs, the mismatch of the lattice spacing causes

the system to be unstable. In order to get stability, the system will

be changed to a lower chemical potential and Gibbs free energy

when they obtain enough thermal energy at the higher temperature

(�180 �C). Taking the work function of Ag (�4.26 eV)20 and the

electron affinity of bulk Ag2S (�5.32 eV)31 into account, the align-

ment of energy levels inAg andAg2S is favorable for electron transfer

from Ag to Ag2S. After the electron transfer from Ag to Ag2S, Ag

ions coming from Ag nuclei substitute for Ag ions in the Ag2S lattice

and diffuse for some distance beforemoving to the next substitutional

site which is similar to Au–Ag2S heterodimers.17 Eventually, Ag ions

diffuse to the surface of Ag2S and reduce to Ag by capturing the

electrons, which undergo interstitial diffusion inAg2S lattices. For the

Ostwald ripening effect process, the diffusion process of Ag does not

lead to a homogeneous distribution of Ag on the particle surface but

forms a patch on the surface (Fig. 4d). The chemical potential and the

Gibbs free energy of the entire system can be lowered by this diffusion

process, so that the system becomes stable after forming theAg–Ag2S

JNPs.

As a result of the presence of the OM ligands, which have a weak

coordination ability, on their surfaces of the Ag–Ag2S JNPs

demonstrated by the Fourier transform infrared spectrum (Fig. S6†),

the Ag–Ag2S JNPs can be readily dispersed in chloroform (the inset

of Fig. 5) and easily transferred to water though the ligand exchange

method.32 The ultraviolet-visible-near-infrared (UV-Vis-NIR) spec-

trum (Fig. 5) indicates that the Ag–Ag2S JNPs absorb through the

Fig. 5 UV-Vis-NIR absorption spectra of chloroform dispersion of the

Ag–Ag2S JNPs, the insets showing the photo of chloroform dispersion of

the Ag–Ag2S JNPs.

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entire visible into the near-IR region which results in a black color of

the JNP material which is similar to what has been reported previ-

ously for the Ag2S quantum dots (QD).25 Such a wide absorbance

range of theAg–Ag2S JNPswill extend its applications inmore fields,

such as solar cells and photocatalysis.

Concerning the alignment of energy levels in Ag andAg2S which is

favorable for electron transfer from Ag to Ag2S and the wide

absorbance range, a prospective photocatalysis application of this

new material of Ag–Ag2S JNPs is found. Here, the composites of

Ag–Ag2S JNPs coupled with P25 TiO2 were used as an example to

explore their photocatalytic properties (see the Supporting Informa-

tion for details†) in the present study.We firstly coupled theAg–Ag2S

JNPs to the P25 TiO2 powders using the bifunctional linker molecule

(thioglycolic acid).32 And then 40 mg of Ag–Ag2S JNPs-coupled P25

TiO2 composites were added to 40.0 mL of methyl orange (MO)

aqueous solution (10mgL�1) in a 100mLbeaker. Finally, the formed

solution was illuminated with a 500 W Xe lamp. The MO concen-

tration (%) after various intervals of irradiation time could be esti-

mated using the following equation28:

MO concentration ð%Þ ¼ At

Ao

� 100%

where Ao is the initial MO absorbance at 464 nm and At is the

absorbance obtained after various intervals of irradiation time

(Fig. S7†). For a comparison, the photocatalytic activity of Ag2SNPs

coupled TiO2 composites was also recorded. As shown in Fig. 6, the

Ag–Ag2S JNPs-coupled P25 TiO2 composites decompose all theMO

molecules after 40 min of 500 W Xe lamp irradiation (red curve,

Fig. 6), while the photodecomposition rate of MO by Ag2S NPs-

coupled P25 TiO2 composites is 80% (blue curve, Fig. 6), whereas the

photodecomposition rate of pure P25 TiO2 powders is only 13%

according the previous results.28 The high photocatalytic activity of

the photocatalysts based on these Ag–Ag2S JNPs should be attrib-

uted to favorable electron transfer from TiO2 to Ag–Ag2S JNPs,

which is similar to the case of TiO2@Ag core/shell composites.29

In summary, we have demonstrated a facile, controllable thermal

decomposition method for the one-pot, large-scale synthesis of the

Ag–Ag2S JNPs from a single molecular precursor. In this synthetic

process, with the increase of the temperature, themorphology and the

structure of the synthesized NPs evolve from the spherical core/shell

Fig. 6 Plots of the decrease in MO concentration vs. irradiation time in

the presence Ag–Ag2S JNPs-coupled P25 TiO2 composites (a) and Ag2S

NPs-coupled P25 TiO2 composites (b) under 500 W Xe lamp irradiation.

7192 | CrystEngComm, 2011, 13, 7189–7193

Ag@Ag2S to eggplant-shaped Ag–Ag2S JNPs because of the diffu-

sion of Ag from the core to the surface due to the substitutional-

interstitial and theOstwald ripening process. This new strategywill be

useful for the synthesis of other semiconductor–metal hybrids and for

metal doping in semiconductor nanocrystals. Moreover, the Ag–

Ag2S JNPs-coupled P25 TiO2 composites show a higher photo-

catalytic efficiency than those of Ag2S NPs-coupled P25 TiO2

composites and pure P25 TiO2 powders, and it is expected to find new

applications in more fields such as solar cells and photocatalysis.

This work was supported by the National Natural Science

Foundation of China (Grant No. 21171035, 50902021 and

50872020), the Program for New Century Excellent Talents of the

University in China, the ‘‘Dawn’’ Program of the Shanghai Educa-

tion Commission (Grant No. 08SG32), the Science and Technology

Commission of Shanghai-based ‘‘Innovation Action Plan’’ Project

(Grant No. 10JC1400100), the ‘‘Chen Guang’’ project (Grant No.

09CG27) supported by the Shanghai Municipal Education

Commission and Shanghai EducationDevelopment Foundation, the

‘‘Dawn’’ Program of Shanghai Education Commission in China

(Grant No. 08SG32), the Program of Introducing Talents of Disci-

pline to Universities (No. 111-2-04) and Shanghai Leading Academic

Discipline Project (Grant No. B603).

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