Nanocomposites of layered clays and cadmium sulfide: similarities and differences in formation,...

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Microporous and Mesoporous Materials 108 (2008) 168–182

Nanocomposites of layered clays and cadmium sulfide: Similaritiesand differences in formation, structure and properties

Zhaohui Han a,*, Huaiyong Zhu b, Kyle R. Ratinac c, Simon P. Ringer c,Jeffrey Shi d, Jiangwen Liu b

a Nanochemistry Research Institute, Department of Applied Chemistry, Curtin University of Technology, WA 6102, Australiab School of Physical and Chemical Sciences, Queensland University of Technology, Qld. 4001, Australia

c Australian Key Centre for Microscopy and Microanalysis, The University of Sydney, NSW 2006, Australiad Department of Chemical Engineering, The University of Sydney, NSW 2006, Australia

Received 11 October 2006; received in revised form 2 April 2007; accepted 2 April 2007Available online 21 April 2007

Abstract

Four series of nanocomposites of layered clays and cadmium sulfide have been prepared from laponite, saponite, hectorite and mont-morillonite. These nanocomposites have been characterized in terms of the sizes and morphologies of the sulfide species, their texturalfeatures, and light absorption properties. From this detailed study, the similarities and differences in structure and properties of the fourtypes of composite have been determined. The composites consist of CdS pillars and CdS nanoparticles, which tend to increase in size asthe amount of complex precursor increases. The sizes of the sulfide prepared from the same amount of complex differ across the fourseries of composites due to the effect of the different clays on precursor exchange, and on the subsequent nucleation and growth ofCdS particles. The composites show the characteristic light absorbance of CdS, and absorbance increases with the sulfide content.The composites are generally mesoporous. Each series of composites maintains a similar structure to its parent clay, but the specific sur-face areas, total pore volumes and average pore sizes of the composites vary with the amount of complex precursor and type of clays usedfor the synthesis.Crown Copyright � 2007 Published by Elsevier Inc. All rights reserved.

Keywords: Nanocomposite; Cadmium sulfide; Clay; Laponite; Saponite; Hectorite; Montmorillonite

1. Introduction

Layered clays are versatile minerals with applicationsthat include sorbents and absorbents, rheological controlof coatings and cosmetics, and synthesis of nanomaterials.The widespread use of layered clays arises from theirunique properties. These derive from their layered crystalstructures, which result in plate-like morphologies(‘‘layers’’ or ‘‘platelets’’), and their varied chemistries andstructural charges. A key class of these clays is the smectitesor 2:1 layered clays, in which each layer comprises two

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* Corresponding author. Tel.: +61 8 9266 1280; fax: +61 8 9266 2300.E-mail address: [email protected] (Z. Han).

tetrahedral silica sheets bonded to a central sheet of octahe-drally-coordinated cations (predominantly Mg2+ or Al3+).Each layer is negatively charged due to isomorphousreplacement of the octahedral and/or tetrahedral cationsby those of a lower charge, and electroneutrality is main-tained by positively charged cations that lie within theinterlayer spaces (‘‘galleries’’) between the clay layers [1].The variety of smectites is due to the spectrum of differenttypes, amounts and locations of isomorphous substitutionsthat can occur within the layer structure. For instance, sap-onite and hectorite are trioctahedral clays in which alloctahedral sites are occupied, predominantly by Mg2+ ions.However, the structural charge in saponite arises from sub-stitution of Al3+ ions for the tetrahedrally-coordinated Si4+

ions, whereas the negative charge in hectorite occurs in the

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Z. Han et al. / Microporous and Mesoporous Materials 108 (2008) 168–182 169

octahedral sheet when Li+ ions substitute for Mg2+ ions[2]. The dioctahedral smectites nominally contain twothirds of the octahedral sites filled by Al3+ ions, with fur-ther substitutions generating specific types of clays. Thus,montmorillonite is derived from substitution of Mg2+

and Fe2+ ions for the octahedral Al3+ ions. In additionto such naturally occurring clays, synthetic versions arealso available; for example, laponite is a synthetic hectorite,but it has far smaller platelet dimensions, leading to muchlarger accessible surface areas [3] and much higher viscosi-ties in aqueous suspension [4].

Given their nanoscale dimensions, high surface areasand unique chemistries, such layered clays have foundapplications as catalysts and catalyst supports by them-selves or in composite structures [5–8]. Frequently, the syn-thesis of such composite catalysts takes advantage of thecation exchange capacities (CECs) of the clays; in aqueoussuspensions, the interlayer cations can be exchanged forhydrated polycations [9,10], or positively-charged sol parti-cles [11,12]. Subsequent heating, typically at temperaturesexceeding 400 �C, converts the exchanged species intometal oxide clusters or so-called pillars, which permanentlyprop open the silicate layers and thereby increase the acces-sible surface area of the catalysts. A large variety of poly-cations derived from elements such as Al, Cr, Ti, Zr, Ga,Si, Fe and Cu have been used as pillaring precursors toproduce pillared clays [10,13–15]. More recently, layeredclays have been combined with other species in pursuit ofvarious functional materials; these include compositesmade with mesoporous silica [16], noble metal nanoparti-cles [17], metal complexes [18,19], and polymers [20–22].Further modification of pillared clays can also generateunique properties. For instance, vanadium pentoxide orp-toluenesulfonic acid has been incorporated into pillaredclays to produce porous materials with oxidant or stronglyacidic characters for use as catalysts in organic chemistry[23]; alternatively, incorporation of platinum into alu-mina-pillared clays creates catalysts for benzene hydroge-nation [24].

By combining layered clays with metal sulfides, sulfide-pillared clays can be produced; however, these havereceived limited attention to date. Nonetheless, the avail-able work shows that these composites are promising mate-rials that could have important industrial applications. Forinstance, an iron-sulfide-pillared clay was found to success-fully remove nickel and vanadium from a heavy crude oilduring demetallization [25,26]. Similarly, a chromium-sulfide-pillared clay has proven to be a suitable catalystfor thiophene hydrodesulfurization [27]. In recent work,we reported a straightforward wet-chemical route for thepreparation of nanocomposites of (i) montmorilloniteand CdS [28] and (ii) other layered clays and various metalsulfide species including cobalt sulfide, nickel sulfide, zincsulfide and lead sulfide [29]. The semiconducting propertiesof metal sulfides means that these clay-sulfide compositeshold promise as visible-light photocatalysts for producinghydrogen from water, particularly as the photocatalytic

performance of CdS in layered-composites is better thanthe performance of CdS alone [19,20]. Consequently, thereis need to further investigate the properties of nanocom-posites made from layered clays and metal sulfide semicon-ductors; for this study, we have chosen CdS as theexemplar sulfide for synthesis of the composites.

It is known that the nature of the clay and the synthesisconditions have a significant effect on the properties of thefinal composites [30]. Our previous work mainly discussedthe preparation of an extensive range of composites com-posed of different metal sulfides and different clays. In thiswork, we focused on CdS-clay nanocomposites preparedfrom synthetic laponite, and from natural saponite, hector-ite and montmorillonite, to systematically investigate anddiscuss similarities and differences in formation, structuresand properties the nanocomposites derived from differentclays. Nanocomposites of montmorillonite and CdS, whichhave been synthesized previously [28], were prepared againin the work by a slightly different method, to allow directcomparison of four series of nanocomposites from four dif-ferent layered clays.

2. Experimental

2.1. Materials

Synthetic laponite (Laponite RD) was supplied by FernzSpecialty Chemicals, Australia; saponite (Lake E) waskindly provided by Wateroo Minerals, Australia; hectorite(SHCa-1) was from the Source Clay Minerals Repository,University of Missouri, Columbia, USA; and montmoril-lonite was supplied by ECC International Ltd. Key proper-ties of the clays and their aqueous suspensions aresummarized in Table 1. Cadmium chloride, CdCl2, andthiourea, NH2CSNH2, were of analytical purity and sup-plied by Aldrich.

2.2. Preparation of samples

Controlled amounts of CdCl2 and NH2CSNH2, at amolar ratio of 1:2, were dissolved into 60 mL of deionizedwater to form a cadmium-thiourea complex, Cd[NH2-CSNH2]2Cl2; this precursor yields CdS under hydrothermalconditions. In this study, the amounts of the complex usedfor the synthesis of composites were 0.1, 0.2, 0.3, 0.4, 0.5,and 1.0 mmol/g clay, while the amount of the clay wasmaintained constant at 2.0 g. The clay powder was slowlydispersed into the solution of Cd[NH2CSNH2]2Cl2 com-plex with vigorous stirring, which was continued for30 min to allow ion exchange to proceed between the com-plex cations and the exchangeable cations in the clay. Theresulting aqueous suspension then was transferred into an80-mL, Teflon-lined autoclave, more water was added untilthe volume of the suspension was approximately 68 mL,and then the autoclave was sealed and heated to 170 �Cfor 6 h. After this hydrothermal process, the solids in thereacted mixtures were separated by centrifugation, washed

Table 1Some important characteristics of the clays used for preparation of the composites

Clay Structural formula BET surfacearea (m2/g)a

CEC As-received(meq/100 g)b

Viscosity ofsuspension(mPa s)c

Initial pH ofsuspension

Laponite RD Na0.67K0.01(Si7.95Al0.05)–(Mg5.48Li0.38Ti0.01)O20(OH)4 336.7 55 690 9Saponite

(Lake E)Nax(Si8�xAlx)(Mg6)O20(OH)4 (generic) 173.4 110 5 7

Hectorite(SHCa-1)

Na0.66(Si7.96Al0.04)(Al0.04Mg5.3Li0.66)O20(OH)4 70.7 44 3 8

Montmorillonite(MC BP)

Na0:63K0:05Ca0:09ðSi7:72Al0:28ÞðAl3:01Fe3þ0:49Mg0:46ÞO20ðOHÞ4 53.4 76 5 7

a Determined from nitrogen gas adsorption isotherms for P/P0 data between 0.05 and 0.2 with the BET equation. Measurements were made on aQuantachrome Autosorb-1.

b Based on data from suppliers.c Two grams of each clay were dispersed into 70 mL deionized water during stirring. The suspensions were equilibrated for two days, and redispersed by

sonication before measurement with a Brookfield Model DV-III Programmable Rheometer at 25 �C. Each value listed is the average of 10 individualmeasurements at a spindle (type SC4-18) speed of 250 rpm.

170 Z. Han et al. / Microporous and Mesoporous Materials 108 (2008) 168–182

with deionized water, and desiccated by vacuum evapora-tion at ambient temperature to produce ‘‘as-desiccated’’samples. A portion of each as-desiccated sample was fur-ther heated at 170 �C in air for 1 h to eliminate the waterfrom the interlayer regions of the samples, producing a par-allel series of ‘‘air-dried’’ samples. For comparison pur-poses, a suspension of each layered clay without anyCd[NH2CSNH2]2Cl2 complex was autoclaved under thesame conditions, collected and treated by the same proce-dures to generate four reference clays. The XRD data dis-cussed in the text, unless otherwise specified, are from the‘‘air-dried’’ samples.

2.3. Characterization

The viscosity of each reference clay suspension was mea-sured with a Brookfield Model DV-III Rheometer as theaverage of 10 measurements at a spindle (type SC4-18)speed of 250 rpm; the mean particle size was measured witha Malvern Mastersizer from two thousand scans. X-ray dif-fraction (XRD) patterns of the samples were recorded on aShimadzu XRD-6000 diffractometer equipped with agraphite-monochromated, rotating copper anode (Ka radi-ation, k = 1.5418 A) under a fixed power source (40 kV,40 mA). The samples were scanned at a rate of 1�/min overa range of 2.5–32.5�, covering the main diffraction peaks ofthe clay and CdS nanocrystals. Transmission electronmicroscope (TEM) images, electron diffraction patterns,and energy dispersive X-ray spectra (EDXS) were recordedon a Philips CM12 electron microscope, operating at120 kV. High resolution TEM (HRTEM) images weretaken on a JEOL JEM-3000F field-emission electronmicroscope under an accelerating voltage of 300 kV. TheTEM samples were prepared by sonication of the powdersin ethanol and then evaporation of a drop of the suspen-sion on a holey carbon film supported by a copper grid.In order to determine the particle size distributions of theCdS particles in the composites, 35–45 images were takenfor selected samples, representing approximately 150 parti-

cles in each case. The maximum horizontal distance of eachparticle (effectively a projected diameter) was measured onthe digital images with the software analySIS Pro 3.2 (SoftImaging System GmbH, Munster, Germany). To detectlow contents of Cd and S within the galleries, EDXS werealso recorded on a VG HB601 scanning transmission elec-tron microscope (STEM) operating at 100 kV. Diffusereflectance UV–Vis spectra of the samples were recordedon a Varian Cary 5E UV–Vis–NIR spectrophotometer.The data were collected in the range of 200–800 nm withbarium sulfate as a reference. Nitrogen adsorption–desorp-tion isotherms were obtained at �196 �C on a Quanta-chrome Autosorb-1 surface area and pore size analyzerusing a static adsorption procedure. The samples weredegassed at 150 �C in vacuum below 10�3 Torr overnightprior to measurement. The specific surface area was calcu-lated by the BET equation from data in the P/P0 rangebetween 0.05 and 0.2.

3. Results

Four series of nanocomposites were prepared and aredenoted as CdS/laponite, CdS/saponite, CdS/hectoriteand CdS/montmorillonite. These names are prefaced witha number and hyphen, e.g., 0.5-CdS/laponite, to indicatethe initial amount of the precursor complex, 0.5 mmol/gclay, used in the synthesis of the composite sample. Thecomposites maintained the layered structure of the parentclays, and consisted of small CdS pillars within the clay gal-leries and of CdS nanoparticles external to the clay tac-toids, as demonstrated by XRD and TEM data.

3.1. XRD traces of the samples

Fig. 1 illustrates XRD patterns of the four series ofnanocomposites and their corresponding parent-clay refer-ences. Except for pure laponite and the CdS/laponite com-posites, which do not display a significant basal peak dueto the small size of the laponite platelets, all the XRD

2θ (deg) 2θ (deg) 2θ (deg) 2θ (deg)

Fig. 1. XRD patterns of the various clay reference materials and the corresponding samples synthesized from the clays and the complex precursor at:(a) 0.1 mmol/g clay, (b) 0.2 mmol/g clay, (c) 0.3 mmol/g clay, (d) 0.4 mmol/g clay, (e) 0.5 mmol/g clay and (f) 1.0 mmol/g clay. The ‘‘*’’ denotes hexagonalCdS.


patterns show a measurable basal (001) peak. The presenceof these peaks confirms that the composites retained thelayered structure of the parent clays. These compositeshave larger basal spacings, d001, than those of the corre-sponding reference clays, as reflected in the small shifts ofthe diffraction peaks to lower angles shown in Fig. 1. Thisexpansion of the basal spacing is evidence for the existenceof small CdS pillars in the products [28,31]. The magnitudeof the peak shift, Dd001, depends on the amount of theCd[NH2CSNH2]2Cl2 complex used for the synthesis. Table2 summarizes the Dd001 values for all the samples, whichwere obtained by subtracting the d001 value of the respec-tive parent-clay reference from the d001 value of each sam-ple. Despite the scatter in the data, there is a general trendto increasing Dd001 values with increasing amounts of com-plex in each series of composites. Thus, we conclude thatthe size of CdS pillars increases with the amount of thecomplex used for the synthesis of the composites, irrespec-tive of clay type.

Though the overall shifts in d001 shown in Table 2 areextremely small, it is important to compare these shiftswith those expected theoretically for full exchange of theinterlayer cations with the bivalent complex. On the basisof (i) the CECs listed in Table 1; (ii) a theoretical totalsurface area for smectites of approximately 750 m2/g [32];(iii) the density for hexagonal CdS of 4.83 g/cm3, [33] and(iv) the assumption that the CdS forms a uniform layerof the hexagonal phase throughout the tactoid galleries,complete exchange of each type of clay should generatelayer thicknesses of 0.2–0.4 A. Of course, the measured

Table 2Changes of d001 values of the samples against parent clay references, Dd001,as a function of the amount of the complex precursor used duringsynthesis

Amount of precursor(mmol/g clay)

Dd001 (A)

Saponite Hectorite Montmorillonite

0 0 0 00.1 0.2 0.1 0.40.2 0.2 0.4 0.60.3 0.3 0.4 1.00.4 0.4 1.0 1.20.5 0.5 0.0 1.31.0 1.2 0.9 1.1

interlayer thicknesses are larger than these expected valuesbecause, the CdS does not form a uniform layer within thegalleries, especially given that the calculated thicknesses areless than the unit cell dimensions for either hexagonal orcubic CdS. In reality, the CdS forms pillars, thereby reduc-ing the interfacial energies of the composites. The generalbroadening of the basal peaks in the nanocomposites(Fig. 1) is consistent with CdS pillars distributed through-out the galleries; a decrease in the registry of the plateletswould be expected owing to flexing of the platelets aroundthe irregularly spaced CdS pillars.

The XRD patterns of Fig. 1 also reveal that the compos-ites contain crystalline CdS particles as well. Compared withthe clay references, characteristic diffraction peaks of hexag-onal CdS are present in all four series of composite samples.As the amount of the Cd[NH2CSNH2]2Cl2 complex used forthe synthesis increases, the diffraction peaks become shar-per, suggesting progressively larger CdS particles developin each series of composites. The hexagonal CdS polycrys-tals in these composites are comparable to those producedin montmorillonite during synthesis with H2S gas, whereonly the hexagonal CdS phase was formed [34].

3.2. Electron microscopy and microanalysis of the samples

The existence of CdS pillars in the galleries of the com-posites is supported by STEM analysis. For simplicity inthe elemental analysis, the 0.5-CdS/laponite sample wasselected as a representative composite because laponite isa synthetic clay with a well-defined composition. Fig. 2 pre-sents a STEM image of a general region in the 0.5-CdS/laponite sample that was free from CdS particles and thecorresponding energy spectrum. Besides the dominantpeaks of Mg and Si, which are due to the laponite, the spec-trum clearly shows very small peaks of Cd and S; thisstrongly suggests the presence of CdS species in the sample.Given the absence of CdS nanoparticles in this region, theCd and S can be ascribed to the CdS pillars between thelaponite layers. The weak signals of Cd and S in the EDXSare consistent with the low content of CdS pillars in thecomposite. Interestingly, the high sensitivity of the STEMalso reveals residual Cl in the composite, which was theanion associated with the bivalent Cd½NH2CSNH2�2þ2 com-plex used in the synthesis.

Fig. 2. STEM image of a region of the 0.5-CdS/laponite sample that is free from CdS particles (left), and EDXS of a selected area in the region (right).


TEM imaging allows direct observation of the size andshape of CdS nanoparticles in the nanocomposites. Over-all, the morphologies of the four series of composites arequite similar, though the particle sizes vary with clay type(see below). To avoid repetition, we only present imagesof representative samples here. Fig. 3 displays TEM imagesof a series of CdS/laponite samples prepared from differentamounts of the Cd[NH2CSNH2]2Cl2 complex precursor.The CdS nanoparticles, which show a darker contrast thanthe laponite due to mass-thickness and diffraction effects,are apparent in the samples. EDXS and electron diffraction

Fig. 3. TEM images of typical CdS/laponite nanocomposites: (a) 0.2-CdS/lapo(e) 1.0-CdS/laponite.

(not shown) also confirm that the dark nanoparticles arepolycrystalline CdS. The sizes of the sulfide nanoparticlesgenerally increase with the amount of complex precursorused for the synthesis. The CdS particles in the compositessynthesized from smaller amounts of the complex precur-sor are generally short dendrites, such as those shown inFig. 3a and b; higher amounts of the complex precursortend to produce larger and more aggregated particles, suchas those shown in Fig. 3e. In general, the CdS particles arewell dispersed in the composites, as revealed by the series ofTEM images.

nite, (b) 0.3-CdS/laponite, (c) 0.4-CdS/laponite, (d) 0.5-CdS/laponite and


3.3. Nitrogen adsorption/desorption isotherms of the samples

The hysteresis loops of the composites display similarshapes to their respective parent-clay reference, suggestingthat the composites are also mesoporous and retain similarmorphologies to the reference clays. Fig. 4 plots nitrogenadsorption/desorption isotherms of examples from the fourseries of samples. The specific surface area, pore volumeand mean pore size of the samples can be calculated fromthe nitrogen isotherms, as summarized in Table 3. Thesedata show that the porosity and texture of the four seriesof composites are highly variable. For example, the surfaceareas range from as much as 426 m2/g down to only 35 m2/g; pore volumes range from 0.297 mL/g to as little as0.066 mL/g; and mean pore sizes range from 20.1 nm downto 2.6 nm.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

60

80

100

120

140

160

180

200

CdS/Laponite CdS/Laponite Laponite clay

Vol

ume

(mL/

g)

Relative Pressure, p/po

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.030

40

50

60

70

80

90

100

110

120

CdS/Saponite CdS/Saponite Saponite clay

Vol

ume

(mL/

g)


Fig. 4. N2 adsorption isotherms of the parent clay references (h) and the cor0.5 mmol/g clay (”) and 1 mmol/g clay (j).

Table 3Textural properties of the four types of clays and their corresponding nanocoprecursor

Sample Total pore volume (mL/g) Avera

Laponite 0.221 2.60.1-CdS/laponite 0.241 2.80.2-CdS/laponite 0.281 2.70.3-CdS/laponite 0.277 2.60.4-CdS/laponite 0.273 2.70.5-CdS/laponite 0.265 2.71.0-CdS/laponite 0.297 2.9

Saponite 0.140 3.20.1-CdS/saponite 0.153 3.20.2-CdS/saponite 0.159 3.40.3-CdS/saponite 0.158 3.40.4-CdS/saponite 0.152 3.50.5-CdS/saponite 0.159 3.81.0-CdS/saponite 0.163 3.7

Hectorite 0.068 3.90.1-CdS/hectorite 0.073 4.40.2-CdS/hectorite 0.079 4.60.3-CdS/hectorite 0.077 4.60.4-CdS/hectorite 0.076 4.60.5-CdS/hectorite 0.067 5.01.0-CdS/hectorite 0.066 5.0

Montmorillonite 0.163 12.20.1-CdS/montmorillonite 0.155 13.20.2-CdS/montmorillonite 0.176 15.80.3-CdS/montmorillonite 0.184 15.50.4-CdS/montmorillonite 0.201 20.10.5-CdS/montmorillonite 0.152 15.91.0-CdS/montmorillonite 0.144 16.5

3.4. UV–Vis absorbance of the samples

In contrast to the low absorbance of the clay references,all the composites show the expected strong absorption atapproximately 480 nm that is characteristic of CdS,although the absolute absorbance and the absorptiononsets of the composites vary with CdS content and withthe type of clay. As a representative series, Fig. 5a presentsthe absorbance of the CdS/laponite samples as a functionof the amount of the complex precursor. The absorbanceincreases following the increase in the amount of the com-plex used for the synthesis, which parallels the nominalCdS content in the composites. This trend holds for all fourfamilies of composites and simply results from the increasein photon absorption with increasing CdS content. Thoughthe overall shapes of the spectra of are quite similar for the

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

10

15

20

25

30

35

40

45

CdS/Hectorite CdS/Hectorite Hectorite clay

Vol

ume

(mL/

g)


0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

10

20

30

40

50

60

70

80

90

100

110

CdS/Montmorillonite CdS/Montmorillonite Montmorillonite clay

Vol

ume

(mL/

g)


responding typical composites synthesized from the complex precursor at

mposites with CdS synthesized by using various amounts of the complex

ge pore size (nm) d = ±0.1 nm BET specific surface area (m2/g)

336.7339.5418.9425.9403.1390.4416.9

173.4189.4187.5184.4172.4168.1173.6

70.765.768.667.865.753.553.1

53.447.044.547.540.038.334.9

400 500 600 700 800

1.0-CdS/Laponite 0.5-CdS/Laponite 0.4-CdS/Laponite 0.3-CdS/Laponite 0.2-CdS/Laponite 0.1-CdS/Laponite Laponite clay

Abs

orba

nce

Wavelength (nm)

300 400 500 600 700 800

0.5-CdS/Laponite 0.5-CdS/Hectorite 0.5-CdS/Saponite 0.5-CdS/Montmorillonite

Abs

orba

nce

Wavelength (nm)

a b

Fig. 5. Diffuse reflectance UV–Vis absorption spectra of (a) CdS/laponite samples synthesized from varied amounts of complex and (b) the fournanocomposites synthesized from the complex at 0.5 mmol/g clay.

Fig. 6. The shift in basal spacing, Dd001, as a function of amount ofprecursor complex.


four series of composites, they differ in onset location andpeak absorbance (Fig. 5b), suggesting that the absorbanceis influenced by clay type.

4. Discussion

4.1. Formation of the nanocomposites

The basic synthesis process may be summarized as fol-lows. Because all the layered clays contain exchangeablecations, some of the Cd½NH2CSNH2�2þ2 cations access theinterlayer spaces of the clay tactoids by means of ionexchange. Any residual complex establishes an equilibriumstate between adsorption on the external surfaces of theclay particles and remaining in solution. Consequently,decomposition of the complex precursor to CdS yields bothsulfide species confined within the interlayer space of theclays as well as larger sulfide particles external to the claytactoids. The development of the nanocomposites is dis-cussed below in terms of the ion exchange process at roomtemperature and the succeeding hydrothermal process at170 �C.

4.1.1. Ion exchange process

The first stage of the synthesis process is cationexchange. The use of Cd½NH2CSNH2�2þ2 complex allowsexploitation of the cation exchange capacities (CECs) ofthe clays to intercalate some of the precursor species. Inthis system, the high charge and small size of the organiccomplex means that the driving force for adsorption inthe system is predominantly electrostatic. Thus, the theo-retical limit of the exchange process, the exchange capacity,is half of the CEC of the clay, because two charge sites canaccommodate one bivalent complex cation at most. Aseach complex molecule contains only one Cd atom andone S atom, the galleries receive a low loading of CdS, lead-ing to small basal shifts as observed above.

Practically, for any of the composite samples made inthis work, the degree of exchange relative to the exchangecapacity depends on a variety of thermodynamic and

kinetic factors. Thermodynamically, the fraction of precur-sor cations relative to the number of Na+ ions in the clay iscrucial in controlling the final exchange equilibrium: thehigher the ionic fraction of precursor, the higher the degreeof exchange [35]. Thus, although the galleries can accom-modate all the precursor cations at any initial concentra-tion below half of the CEC, complete exchange of allavailable cations will not occur due to the nature of theequilibrium between the Cd½NH2CSNH2�2þ2 ions and Na+

ions, leaving a residual concentration of complex in solu-tion. This is consistent with the formation of nanoparticlesat the lowest precursor concentrations used in all systems,as demonstrated by the presence of the diffraction peaks ofCdS particles shown in Fig. 1.

For each clay series, the Dd001 values should increasewith increasing initial amounts of precursor until finallyreaching approximately constant values once saturationof the CEC occurs. These transition points should providethe best estimate for the concentration of precursor corre-sponding to saturation of the available exchange sites. It isevident from the data plotted in Fig. 6 that, despite thescatter, each series tends to form a linearly ascendingregion followed, in the case of hectorite and montmorillon-


ite, by a plateau region of constant Dd001. It seems reason-able to conclude that the onset of the plateaus correspondto saturation of the exchange capacities of the clays.Crucially, the theoretical saturation points, which are halfthe CECs of the clays, lie below the transition points inthe curves for all three cases. This is consistent with incom-plete exchange of precursor at low concentrations as wasdiscussed above. Thus, although only approximate, thesetransition points provide the best estimate for theconcentration of precursor that corresponds to saturationof the exchange sites: approximately 0.4 mmol/g for hect-orite and montmorillonite, and somewhere between 0.5and 1.0 mmol/g for saponite. It is assumed that the lapo-nite, with its similar composition and CEC to hectorite,also achieved approximate ‘‘saturation’’ of exchange sitesat a nominal precursor concentration of 0.4 mmol/g ofclay.

4.1.2. Hydrothermal process

The next stage of the process is the hydrothermal treat-ment, during which the autoclaves are allowed to react at170 �C. For a pure aqueous solution of thiourea, this tem-perature would cause rapid decomposition of thiourea toammonium and thiocyanate ions [36]. In the presence ofCd2+ ions, however, the reaction is more complicated.Much of the work related to this decomposition processhas been done in alkaline solutions in which Cd2+ is com-plexed with NH3, as these systems are most amenable toformation of quality CdS films. Ignoring the spectatorNH3 species, the expected overall reaction under these con-ditions is: [37,38]

Cd2þ+ CS(NH2)2 + 2OH!CdS + NCNH2 + 2H2O ð1Þ

The cyanamide (NCNH2) then can further decompose tourea, ammonia and carbon dioxide [38]. Because the sam-ples used in this study are initially free from ammoniaand are neutral to slightly alkaline – pH values of 7–9 asshown in Table 1 – perhaps a better overall decompositionreaction is:

Cd[CS(NH2)2]2Cl2 + 2H2O!CdS + 2NH4Cl + CO2 ð2Þ

This reaction may be expressed by the following two sub-reactions, which represent the decomposition of thioureato H2S and then its reaction with Cd2+ ions to form CdS:

CS(NH2)2 + 2H2O! 2NH3+CO2 + H2S ð3Þ

Cd2þ+ H2S!CdS + 2Hþ ð4Þ

Realistically, the presence of the clays further complicatesthe above reactions. For example, CdS nanoparticles andthin films are known to form on substrates by heteroge-neous nucleation after the adsorption of thiourea and Cd(or its complexes) [37,38]. Given the large surface areasassociated with the clays and the exchange, as well as pos-sible adsorption, of the Cd½CSðNH2Þ2�

2þ2 onto the external

surfaces of the clay particles, a similar process is likely to bethe dominant nucleation mechanism in the nanocomposite

systems. Furthermore, differences in the native pH valuesof the clays could influence decomposition rates. For in-stance, the more alkaline environment of the laponitesuspensions (pH 9) would tend to drive reactions (3) and(4) towards the formation of CdS.

4.2. CdS pillars and CdS particles in the composites

As explained in Section 3, there is a general trend toincreasing d001 values with amount of precursor complex,illustrative of growing CdS pillars. The average size ofthe CdS nanoparticles also clearly increases with theamount of precursor complex. It appears that the shapeof the CdS particles in the composites is determined mainlyby the amount of precursor, rather than by the propertiesof the clay hosts, because the CdS particles produced fromsimilar starting materials without the clays [39] have similarmorphologies to those observed in this work. However,despite the similar trends within each series, the data clearlyshow variations across the four groups of samples due tothe influence of the clay raw materials on the finalproducts.

4.2.1. The CdS pillars

The theoretical maximum of Cd½NH2CSNH2�2þ2 cationexchange within the clays varies significantly with the dif-ferent CECs of the four clays. As listed in Table 1, saponitehas the largest CEC (110 meq/100 g of clay), due to its highlayer charge density, while the CECs of the other threeclays are substantially smaller (44–76 meq/100 g of clay).Consequently, the saponite clay with its large CEC wouldbe expected to admit more intragallery complex and, as aconsequence, possibly generate larger CdS pillars thanthe other clays. Thus, at or above the saturation ofexchange sites of all four clays, it is reasonable to expectthat the Dd001 value of the saponite composite might belarger than those of the hectorite and montmorillonitecomposites. On the basis of the data in Table 2 for1.0 mmol of complex, this expectation appears to be true;Dd001 increases from 0.9 A for the 1.0-CdS/hectoritenanocomposite to 1.1 A for the 1.0-CdS/montmorillonitenanocomposite to 1.2 A for the 1.0-CdS/saponite nano-composite. This difference is more obvious when the finald001 values of the 1.0-CdS/Clay samples are normalizedto the thickness of the silicate layer (9.5 A) rather thanthe d001 values of the reference clays. These numbers showthat the nominal pillar size increases from 1.4 A for themontmorillonite nanocomposite to 1.7 A for the hectoritenanocomposite to 3.6 A for the saponite nanocomposite.Clearly this interpretation should be treated with cautionin recognition of the relatively small changes in basal spac-ings involved and because the ion exchange process isaffected by factors apart from the CEC, such as plateletsize, degree of swelling and charge location. Nonetheless,these data still support the intuitive concept that clays withlarger CECs tend to generate larger CdS pillars throughthis wet-chemical process.


4.2.2. The CdS particles

Across the four series of composites, the sizes of the CdSnanoparticles vary from tens of nanometers to hundredsnanometers. For an individual composite, the particle sizedistribution is much narrower; the 0.5-CdS/laponite samplein Fig. 7a has particle diameters from 20 to 90 nm. For thecomposites prepared from a specific type of clay, the CdSparticles become larger when the amount of the complexprecursor is increased for the synthesis; this is due to the factthat a higher feed of the complex precursor yields more CdSnuclei upon decomposition and offers more material forcrystal growth, thereby yielding larger particles. Fig. 7bsummarizes the mean particle size as a function of precursorconcentration. For comparison, Fig. 7c plots the crystallitesizes of CdS, as estimated from the full-width-at-half-max-imum of the 0 01 peak of the CdS (Fig. 1). It is apparent thatthe mean particle size is systematically larger, typically 3–6times, than the corresponding crystallite size. Visually, thesevalues appear reasonably consistent with the multiplebranches of individual CdS dendrites shown in Fig. 3, whichusually have 3–4 branches.

The sizes of the CdS nanoparticles (or crystallites) varysignificantly across the four composites synthesized at afixed concentration of Cd[NH2CSNH2]2Cl2 complex. Hect-orite tends to produce the largest particles, laponite thesmallest and saponite and montmorillonite generally pro-duce intermediate particle sizes. These differences can beexplained by the effects of the different clay types on thenucleation and growth of the sulfide nanoparticles.

18 24 30 36 42 48 54 60 66 72 78 84 900

5

10

15

20

25

30

Per

cent

age

in th

e S

tats

itc P

artic

les

(%)

Sizes of CdS Particles in CdS/Laponite (nm)

0.2 0.3 0.4 0.520

40

60

80

100

120

140

Ave

rage

CdS

Par

ticle

Siz

es (

nm)

Amount of the Com

a b

Fig. 7. (a) Size distribution of CdS nanoparticles in the 0.5-CdS/laponite sa(c) calculated sizes of CdS crystallites in the nanocomposites.

Table 4Calculations of thiourea decomposition and CdS nucleation behavior for the fclay

Sample Equilibriumconcentration of Cd2+

and H2S, Ce(M)

Residualconcentration ofCS(NH2)2, C(M)

Rate ofCS(NH2)2

decomposit

1.0-Laponite 3.7 · 10�14 2.2 · 10�2 6.5 · 10�6

1.0-Saponite 3.7 · 10�12 1.3 · 10�2 3.9 · 10�6

1.0-Hectorite 3.7 · 10�13 2.3 · 10�2 6.8 · 10�6

1.0-Montmorillonite 3.7 · 10�12 1.8· 10�2 5.4 · 10�6

During the hydrothermal process, the rate of heteroge-neous nucleation on the clay surfaces, which ultimatelydetermines CdS particle number and size, depends predom-inantly on the supersaturation of the solution and thenumber of sites available for nucleation [40]. The supersat-uration of a solution is quantified in the supersaturationratio, S, which is the ratio of the actual amount of reactingspecies in solution, C, to the equilibrium solubility of thereacting species, Ce. For CdS, the dissolution reactionobtained by reversing Eq. (4) can be described by an acidsolubility product, Kspa, of 1.38 · 10�9 (from Kspa for thedissolution of CdS to HS� of 1.26 · 10�16 and K1 for thedissociation of H2S to HS� of 9.1 · 10�8) [33,41]. Combin-ing Kspa with the initial pH values of the clay suspensions inTable 1 allows the calculation of the equilibrium concen-trations of Cd2+ and H2S as listed in Column 2, Table 4.

The other information required to estimate the supersat-uration ratio is the actual concentration of reacting species,e.g., H2S and Cd2+, in solution. In the present system, thisis determined by the rate of decomposition of the thiourea,which is affected by temperature and thiourea concentra-tion, and probably by the Cd2+ ions and the clay particles.As a first approximation, however, we can ignore the lastfactors and use available data on the decomposition ofthiourea solutions to allow for the effects of temperatureand concentration. Shaw and Walker showed that thedecomposition rate is first order with respect to thioureaconcentration [36]. Fitting the Arrhenius model to theirdata gives:

0.6 0.7 0.8 0.9 1.0

hectorite montmorillonite saponite laponite

plex (mmol/g clay)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.08

12

16

20

24

28

32 montmorillonite hectorite saponite laponite

Cal

cula

ted

CdS

Cry

stal

lite

Siz

es (

nm)

Amount of the Complex (mmol/g clay)

c

mple, (b) average sizes of CdS particles as determined from TEM and

our samples made with an initial level of precursor complex of 1.0 mmol/g

ion (M/s)

Supersaturationratio, S

Ratio of ln�3 S

to that of saponiteRatio of A � S � e�B=ln2S

to that of saponite

1.7 · 108 0.4 4.1 · 108

1.1 · 106 1.0 1.01.8 · 107 0.6 8.7 · 104

1.5 · 106 0.9 1.6


k ¼ 1� 1014e�ð17963=T Þ with R2 ¼ 1 ð5Þ

in which T is in units of Kelvin. For the purposes of thiswork, it is more straightforward to derive a variant of thisequation in which k is proportional to T in units of �C. Fol-lowing a logarithmic transform, the final relationship is:

lnðkÞ ¼ 0:1227T � 27:887 with R2 ¼ 0:9991 ð6Þ

After the room-temperature autoclaves were placed intothe oven, they were allowed to naturally attain thermalequilibrium at 170 �C; this took place during the first hourof the hydrothermal treatment. On this basis, and assuminga linear increase in temperature to 170 �C as a first approx-imation for mathematical simplicity, the equation for heat-ing rate, T = 0.0403t + 25, can be combined with Eq. (6) torelate k and reaction time, t. Iteratively solving this newequation for the critical time to complete decompositionof the thiourea, tc, gives 56 min. This corresponds to anautoclave temperature of 161 �C and a mean rate constantfor decomposition of km = 2.96 · 10�4 s�1. Thus, thioureadecomposition would be complete just prior to the auto-claves reaching peak temperature. In practice, the likelycatalytic effects of Cd2+ ions and/or the clay, along withthe non-linearity of the heat transfer, means the actualdecomposition time probably would have been shorterthan this. Nevertheless, the estimated decomposition timeprovides an easy and conservative basis for the followingorder-of-magnitude calculation.

The foregoing mean rate constant allows estimation ofthe amount of reacting species in solution for any initialthiourea concentration, as is necessary to understand thenucleation behavior in each of the four series of samples.To illustrate the point, consider the four samples madefrom a starting precursor level of 1.0 mmol/g of clay. Fromthe discussion of the cation exchange process above, thislevel corresponds to approximate saturation of exchangesites across all clays, giving estimated residual precursorconcentrations as listed in Column 3, Table 4. Multiplica-tion of these concentrations by km generates the mean rateof thiourea decomposition, listed in Column 4, Table 4.Assuming that, on average, nucleation removes the reac-tants at an equivalent rate to that generated by decomposi-tion, the rate of decomposition represents an effectiveequilibrium supersaturation. On this basis, the supersatura-tion ratio, S, is calculated as the ratio of the data in Col-umns 3 and 4, Table 4, with the results shown in Column 5.

To determine the available surface area for heteroge-neous nucleation in each system, it is necessary to recall thatthe ion exchange process involved bivalent precursor cat-ions, which electrostatically bind the platelets togetherand severely limit the aqueous swelling of tactoids [42,43].Consequently, the clay particles in the samples certainlywere tactoids rather than individual platelets at the end ofthe exchange period. Moreover, the chosen approach of dis-persing the dry clay powders directly into the solution of thecomplex implies that the tactoids in solution resemble thoseof the original dry powders. The combination of particle

wetting and dispersion with simultaneous cation exchangemakes it probable that the original tactoids would not havedelaminated extensively in solution. Therefore, the accessi-ble surface area for the nucleation process in each series ofsamples should be comparable with, though not the sameas, the BET surface areas measured from gas adsorptionon dry reference clays (Table 1). As a first approximation,therefore, the surface area for heterogeneous nucleation ineach sample is taken to be the surface area of the corre-sponding clay measured from gas adsorption, assumingthat the nucleation site density per unit surface area isequivalent in each clay system.

The first-order dependence of thiourea decompositionrate on concentration means that the nucleation rate, J,is proportional to the total number of nuclei formed ineach sample over the entire decomposition period. To fur-ther simplify the calculations, we will assume that the vari-ations in contact angle of the CdS nuclei across thedifferent types of clay platelets are negligible. Under theseassumptions and the foregoing arguments, it can be shownthat: [40]

vn / ln�3S ð7Þ

and

J / A � S � e�B=ln2S ð8Þ

where vn is the critical nuclei volume, A is the surface areafor heterogeneous nucleation in the system, and B is a ther-modynamic constant with an approximate value of 5800 forheterogeneous nucleation. From the data given in Tables 1and 4, therefore, it is possible to estimate the relative nucleivolumes and rates of nucleation for samples containing dif-ferent types of clay. The last two columns of Table 4 sum-marize the relative nuclei volumes (Eq. (7)) and therelative rates of nucleation (Eq. (8)) for each clay systemin comparison with saponite, which has the lowest esti-mated supersaturation ratio. Several points are evidentfrom these data. The nuclei size increases in the order lapo-nite to hectorite to montmorillonite to saponite. However,the maximum variation in relative nuclei diameter is only0.9–1.0 (0.4–1.0 on a volume basis); apparently, nuclei vol-ume is relatively insensitive to the changes in supersatura-tion ratio given in Table 4. By contrast, J is stronglyaffected by supersaturation and clay external surface area,so that the relative nucleation rate varies over eight ordersof magnitude across the four clay types. In this case, the rateof nucleation (and, therefore, the total number of nuclei)decreases in the order laponite to hectorite to montmoril-lonite to saponite; practically speaking, however, montmo-rillonite and saponite have the same nucleation rates.Despite the various assumptions and approximations madein these calculations, the trends observed are probably cor-rect. Therefore, we can conclude that laponite tends to pro-duce the most nuclei, which have the smallest size; hectoritetends to produce significantly fewer nuclei, which have aslightly larger size; and montmorillonite and saponite are

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

20

40

60

80

100

120

140

160

180

CdS/Laponite CdS/Saponite CdS/Hectorite CdS/Montmorillonite

Vol

ume

(mL/

g)


Fig. 8. N2 adsorption isotherms of the composites synthesized from fourdifferent clays at a complex precursor concentration of 0.4 mmol/g clay.


comparable, producing the fewest nuclei, with slightly lar-ger dimensions again.

From cessation of thiourea decomposition in the hydro-thermal reaction, the nuclei have approximately 5 h togrow into the final nanoparticles. This growth occurs bytwo basic mechanisms: Ostwald ripening and orientedattachment [44]. Ostwald ripening is the growth of largerparticles by dissolution of smaller particles, due to theirhigher solubility. Oriented attachment occurs when adja-cent nanoparticles rotate in solution to align correspondingcrystallographic directions and then join along the com-mon face by forming chemical bonds [45]. This mechanismhas been shown to occur for small nanoparticles of TiO2

[45], FeOOH [46], and ZnS [44]. The rate of orientatedattachment depends on the probability of particle contactin appropriate orientations and, therefore, increases withthe number of nucleated nanoparticles. On the basis ofthe earlier calculations, laponite, which nucleates the great-est number of particles that also have the smallest size,should display the highest rate of orientated attachmentand Ostwald ripening, leading to the largest final particles.In fact, this is not the case, as evident from Fig. 7b, becauseof the very high viscosities of the laponite gels (Table 1).Mechanistically, orientated attachment and Ostwald ripen-ing require diffusion of nanoparticles and ions, respectively.The laponite gels hinder mass diffusion, because diffusivityis approximately inversely proportional to viscosity. There-fore, the nucleated particles can only grow slowly by orien-tated attachment and Ostwald ripening and, consequently,the laponite systems produce the smallest particles. For theremaining three clay types, the similarly low suspension vis-cosities mean that viscosity is not a determining factor inparticle growth. Therefore, we expect orientated attach-ment and Ostwald ripening to occur most rapidly in thehectorite samples, which, after laponite, have the next larg-est amount of nucleated particles of smallest size; this isconfirmed by the data in Fig. 7b. Finally, as montmorillon-ite and saponite have significantly smaller numbers ofnuclei, also of the largest sizes, we expect reduced ratesof orientated attachment and Oswald ripening, leading tosmaller final particles than for hectorite. Evidently the highviscosities in the laponite samples dominate the particlenumber and size effects because the montmorillonite andsaponite samples ultimately produce larger nanoparticlesthan the laponite systems.

On the basis of the number of available nucleated parti-cles and the suspension viscosities, the rate of orientatedattachment should increase in the order laponite to mont-morillonite and saponite to hectorite. Consequently, themean number of crystallites per particle should increase inthis order too. The mean number of crystallites per particlecan be estimated from the data in Fig. 7b divided by thedata in Fig. 7c. Despite variations within series, the overallmean number of crystallites per particle for each of the fourseries is 3.5 for laponite, 4.7 for montmorillonite and sapo-nite, and 6.4 for hectorite; this trend is entirely consistentwith the expected order for increasing oriented attachment.

4.3. Textures of the nanocomposites

For each series of composites, some of the textural fea-tures exhibit noteworthy trends. As shown in Table 3, thesurface areas of the CdS/laponite nanocomposites tend toincrease with the amount of complex. This could be dueto the CdS nanoparticles disrupting the formation of tightlaponite agglomerates, thereby creating more voids in thecomposites; this is consistent with the increased pore vol-umes of the composites. In contrast, the surface areas ofthe CdS/hectorite nanocomposites decrease as the amountof the complex increases. Simultaneously, there is a largeincrease in pore size, but relatively little change in the totalpore volume. Consequently, the decrease in composite sur-face area is caused by the loss of pore surface area associ-ated with having fewer, larger pores in the compositescaused by the growth of the large CdS nanoparticles inthe clay. The CdS/montmorillonite composites display sim-ilar trends to the CdS/hectorite system. Finally, the porevolumes of the CdS/saponite composites are all larger thanthat of the saponite reference, while the average pore size,and occasionally the specific surface area, also tends toincrease for the composites. Thus, the increased pore vol-ume of CdS/saponite composites appears to be caused bythe creation of more and larger pores during formationof the composites.

The composites prepared with different clays display dis-tinct isotherm shapes – as shown in the representative iso-therms in Fig. 8 – revealing profound differences in porestructures. The CdS/laponite nanocomposite has the larg-est adsorption capacity, reflecting its large porosity. Theadsorption of this sample is high at low relative pressuresand this generally indicates a large specific surface areaand/or a large amount of micropores (<2 nm) [47].Between P/P0 of 0.3 and 0.8, the adsorption increases sub-stantially, but only shows slight further increases at relativepressures higher than 0.8. This means that the solid pos-sesses numerous mesopores of several nanometers in size,but has few pores larger than 10 nm. According to the clas-sification of de Boer, the isotherms of CdS/laponite com-posites display a ‘‘Type E’’ hysteresis loop, which is


associated with irregularly-shaped mesopores [47]. The iso-therm of the CdS/saponite nanocomposite shows highadsorption at low relative pressures, and then increasesslightly between the relative pressures of 0.2 and 0.8. Itexhibits a ‘‘Type B’’ hysteresis loop, which is characteristicof slit-shaped mesopores. Because the hysteresis is not largein terms of the adsorption capacity, the contribution of theslit-shaped mesopores to specific surface area is not impor-tant. The strong adsorption at low relative pressures is dueto the small micropores in the solid, which contribute mostof the specific surface area. According to the d001 valuefrom the XRD pattern of the 0.4-CdS/saponite sample,the interlayer space is approximately 2.9 A, which is smal-ler than the kinetic diameter of nitrogen molecules(3.64 A). However, the d001 peak is broad, so that the inter-layer spacing has a distribution of sizes. This is confirmedby the HRTEM of the 0.5-CdS/saponite nanocompositesample shown in Fig. 9. The lamella stacking of the sapo-nite layers is evident and confirms the basal spacing of lay-ers is not uniform, varying from 1.1 nm to 1.3 nm. It isprobable that a considerable fraction of the tactoids con-tains larger interlayer spacings, which allow nitrogen mol-ecules to penetrate into the pillared galleries; these are themicropores detectable by nitrogen adsorption and thatcause the strong adsorption at low relative pressures.

To further understand the different textures, the micro-pore volume can be derived from the t-plot method [48].

Fig. 9. HRTEM image of a section of the 0.5-CdS/saponite sample showing1.3 nm.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40

20

40

60

80

100

120

140

160

180

Ads

orpt

ion,

cm3 (

STP

)/g

t, nm

CdS/Laponite CdS/Saponite

Fig. 10. t-Plots of the nanocomposites prepared at a co

In Fig. 10, a tangent line (dashed line) is drawn for eacht-plot; the intercept of this on the adsorption axis indicatesthe micropore volume. The micropores in the CdS/saponitenanocomposite account for a considerable fraction of itsoverall pore volume. The specific surface area from themicropores derived from the slope of the tangent line is54 m2/g; this is nearly one third of the total surface area.The large amount of micropores in the CdS/saponite nano-composite must come from intercalation of CdS pillars thatare large enough to prop open the clay layers and allownitrogen molecules to access significant fractions of inter-layer space. The XRD patterns of the CdS/saponite nano-composites (Fig. 1) confirm that the saponite clay layersremain intact during the synthesis, which supports the con-clusion that the micropores are due to CdS intercalation. Incontrast, micropores account for only a small part of thesurface areas in the CdS/hectorite and CdS/montmorillon-ite nanocomposites as shown by the pore size distributionsof the samples in Fig. 11 (calculated according to Ref. [42]).Even in un-pillared layered clays, the imperfect stacking ofthe clay layers creates small volumes of slit-shaped microp-ores approximately 1 nm across, i.e., about the thickness ofa clay layer. Therefore, the limited micropores in the CdS/hectorite and CdS/montmorillonite nanocomposites canlikely be accounted for by such stacking defects alone.Finally, the pores in the CdS/laponite nanocomposite arethe voids between the small clay platelets and the CdS

a layered structure in which the basal spacing varies between 1.1 nm and

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40

10

20

30

40

50

Ads

orpt

ion,

cm3 (

ST

P)/

g

t, nm

CdS/Hectorite CdS/Montmorillonite

mplex precursor concentration of 0.4 mmol/g clay.

2 4 6 8 10 12 140.00

0.01

0.02

0.03

0.04

0.05

dV/d

Dh,

cm3 g

-1 n

m-1

Hydraulic diameter, nm

CdS/Laponite CdS/Saponite CdS/Hectorite CdS/Montmorillonite

Fig. 11. The pore size distributions of the samples derived from thenitrogen adsorption isotherms in Fig. 8.


nanoparticles; some fall into the micropore range, but mostof them are mesopores of several nanometers in size.

These varied structures indicate that the textural fea-tures of the nanocomposites can be tailored by manipulat-ing the type of clay and the quantity of complex precursorfor the synthesis. This is an important finding because thesize and distribution of the pores are influential factorsin, for example, the catalytic performance of such nano-composite materials [49].

Table 5Key characteristics of the UV–Vis absorbance spectra of the nanocomposites

Sample Total absorbance Abs

Laponite 00.1-CdS/laponite 0.95 5430.2-CdS/laponite 1.05 5480.3-CdS/laponite 1.30 5470.4-CdS/laponite 1.47 5510.5-CdS/laponite 1.50 5571.0-CdS/laponite 1.57 559

Saponite 00.1-CdS/saponite 0.35 5320.2-CdS/saponite 0.48 5360.3-CdS/saponite 0.49 5380.4-CdS/saponite 0.65 5430.5-CdS/saponite 0.73 5431.0-CdS/saponite 0.78 544

Hectorite 00.1-CdS/hectorite 0.46 5410.2-CdS/hectorite 0.75 5490.3-CdS/hectorite 0.89 5590.4-CdS/hectorite 0.94 5610.5-CdS/hectorite 1.06 5591.0-CdS/hectorite 0.98 561

Montmorillonite 00.1-CdS/montmorillonite 0.28 5330.2-CdS/montmorillonite 0.36 5350.3-CdS/montmorillonite 0.47 5370.4-CdS/montmorillonite 0.57 5360.5-CdS/montmorillonite 0.59 5361.0-CdS/montmorillonite 0.82 541

a Determined as the intersection of the two lines extrapolated from the lineab Estimated from the minimum in the first derivative of the absorbance spec

4.4. Light absorption

Despite the fact that all of the composites show theabsorption onset of CdS nanoparticles, the four series exhi-bit large differences in the maximum absorbance at anyfixed precursor concentration (e.g., Fig. 5b). Assumingthe composites represent physical mixtures, i.e., Beer’slaw is obeyed, we can removed the influence of the clayson the overall absorption of photons by simply subtractingthe absorbance of the corresponding reference clay. Theo-retically, the resulting spectra allow direct comparison ofthe absorbance by the CdS nanoparticles independent ofthe clays; key parameters for these adjusted spectra aregiven in Table 5. It is evident from these data that theabsorbance generally increases with CdS content withinany of the four series.

It is expected that larger amounts and smaller sizes ofnanoparticles should yield samples with larger absorbancevalues. Referring to the data listed in Table 5, it is evidentthat this is partially true in that the CdS/laponite samples,which have the smallest average particle sizes of all sys-tems, have the greatest absorbance. Also, as expected,the CdS/saponite and CdS/montmorillonite series, whichhave similar mean particle sizes, have similar absorbancevalues. However, the CdS/hectorite series doesn’t follow

as a function of complex concentration

orption onset wavelength (nm)a Exciton wavelength (nm)b

522518509513509508

503504507508508508

511510515514515512

506507505504503505

r portions on either side of the absorption onset.trum [50].


the expected trend because it has intermediate absorbancevalues despite having the largest CdS particles. There aretwo obvious explanations for this. One is that the effect ofsize and amount of nanoparticles is moderated by theaccessibility of the particles to the radiation, whichdepends on the texture and openness of the nanocompos-ites. In other words, Beer’s law might not apply due todifferent levels of obscuration of the nanoparticles bythe clay as the morphologies of the nanocompositesvaried with precursor concentration. Another possibilityis that, despite having the largest mean particle size ofall clay types, the hectorite could have a significant pro-portion of small particles that contribute to increasedabsorbance when compared with the saponite and mont-morillonite samples. A statistical examination of the mea-sured particle size distributions confirms that this is truefor the samples made from intermediate levels of precur-sor. For the 0.3-CdS/Clay samples, for instance, thesmallest 10% of the CdS particles lie below 21 nm forthe hectorite, below 37 nm for the saponite and below43 nm for the montmorillonite; the same order occurs atthe first quartile (25%).

5. Conclusion

A systematic series of clay-CdS nanocomposites hasbeen prepared from four types of clay and differentamounts of complex precursor. The synthesis processinvolves room-temperature ion exchange followed by ahydrothermal treatment. All composites maintained thelayered structure of the parent clays, and consisted ofsmall CdS pillars within the clay galleries and CdS nano-particles external to the clay tactoids. The sizes of bothforms of CdS tend to increase as the amount of complexprecursor for the synthesis increases; thus, the size andmorphology of the CdS can be controlled to some extentby varying the amount of the complex precursor. Thesizes of CdS nanoparticles in the composites also dependon the clay hosts; for the composites prepared from a con-stant amount of the complex precursor, the sulfide parti-cles in CdS/laponite are smallest, those in the CdS/saponite and CdS/montmorillonite composites are inter-mediate, and those in CdS/hectorite composites are larg-est. We have interpreted these trends in terms of theeffects of the different clay types on the nucleation andgrowth of the sulfide nanoparticles. The nanocompositestend to retain the basic mesoporous nature as the hostclays, but vary in specific surface area, pore volume andpore size, depending upon the clay and the amount ofcomplex precursor used for the synthesis. Overall, thecomposites show similar light absorption properties, witheach displaying the characteristic absorption of CdS nano-particles, though absorption is influenced by both the clayand the amount of complex used. The varied microstruc-tures of the four series of nanocomposites suggest that themorphologies of the materials could be tailored for spe-cific applications.

Acknowledgements

This work was supported by a discovery Project(DP0210065) from the Australian Research Council(ARC) and by the Curtin Research Fellow Scheme. Dr.Zhu is indebted to the ARC for a QEII Fellowship.

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