Semiconducting properties of layered cadmium sulphide-based hybrid nanocomposites
- Zoraya López-Cabaña†1, 2,
- Clivia Marfa Sotomayor Torres†3, 4, 5 and
- Guillermo González†1, 2Email author
© López-Cabaña et al; licensee Springer. 2011
Received: 1 February 2011
Accepted: 6 September 2011
Published: 6 September 2011
A series of hybrid cadmium salt/cationic surfactant layered nanocomposites containing different concentrations of cadmium sulphide was prepared by exchanging chloride by sulphide ions in the layered precursor CdX x (OH) y (CnTA) z in a solid phase/gas reaction, resulting in a series of layered species exhibiting stoichiometries corresponding to CdS v X x (OH) y (CnTA) z , constituted by two-dimensional CdCl2/CdS ultra-thin sheets sandwiched between two self-assembled surfactant layers. The electronic structure of CdS in the nanocomposite is similar to that of bulk, but showing the expected features of two-dimensional confinement of the semiconductor. The nanocomposite band gap is found to depend in a non-linear manner on both the length of the hydrocarbon chain of the surfactant and the concentration of the sulphide in the inorganic sheet. The products show photocatalytic activity at least similar and usually better than that of "bulk" CdS in a factor of two.
During the last years, much effort has been invested on the development of strategies to assemble inorganic nanoparticles in well-defined arrays. In order to obtain technologically useful nanocrystal-based materials, their spatial orientation and arrangement need to be taken into account, in addition to the size and shape of the nanocrystal and their surface chemistry .
Numerous synthetic methods leading to semiconductor nanocrystalline II-VI materials, for example, CdS, ZnS, PbS and/or CdSe, have been reported where a number of templates have been used for forming and/or stabilising the nanoparticles, among them are mesoporous materials [2, 3], dendrimers , polymers [5, 6] or surfactants [7, 8]. Mesophases of lyotropic liquid crystals have been also used to produce CdS nanocrystals [9, 10]. In general, it is common to find organic-inorganic nanostructured composites which provide a rich source of new materials with promising technological applications . However, reports on layered arrangements of semiconductors like CdS are still scarce in literature. Among these, a method to obtain lamellae and dendrites of ZnS, starting from the layered precursor ZnS·(NH2CH2CH2NH2)0.5, has been reported , and cadmium chalcogenides in the solid state, i.e. with S, Se or Te, containing two ethylendiamine molecules per Cd atom, have also been prepared . In these products, the presence of amine avoids the structural collapse and helps to form the corresponding metal chalcogenide.
In this work we describe the synthesis of a series of layered single phases in which different amounts of CdS are confined in a CdCl2 matrix. The optical and photocatalytic properties of these nanocomposites are studied, as well as the dependence of the latter with the concentration of sulphur in the samples. It is found that these nanocomposites have better photocatalytic activity than "bulk" CdS.
Materials and chemicals
Cadmium chloride hydrated (Aldrich, Sigma-Aldrich Chemie GmbH, Steinheim, Germany), hexadecyltrimethylammonium bromide (CTAB, SigmaUltra 99%, Sigma-Aldrich, St. Louis, USA), 99%), octadecyltrimethylammonium bromide (OTAB, Aldrich, 97%, Sigma-Aldrich Chemie GmbH, Steinheim) (CTAB and OTAB abbreviated as CnTA with n = 16 or 18, respectively), iron(II) sulphide (Merck, Merck KGaA, Darmstadt, Germany) and hydrochloric acid (Merck) were used as received. Gaseous hydrogen sulphide was prepared in place by reaction of iron(II) sulphide with HCl 37% v/v and washed with deionised water.
Preparation of CdX x (OH) y (CnTA) z nanocomposites
Elemental analyses and stoichiometric formulae of precursor, Cd1X x (OH) y (CnTA) z , and nanocomposites, Cd1S v X x (OH) y (CnTA) z
Elemental analysis (%)
Cd1X3.5(OH)0.8(C19H42N)2.3 · 3H2O
CdCl2-CTAB + H2S
Cd1 X 3.2(OH)0.8(C21H46N)2·3H2O
CdCl2-OTAB + H2S
Preparation of CdS v X x (OH) y (CnTA)znanocomposites
The metallic sulphur-surfactant nanocomposites were prepared by bubbling gaseous H2S through an ethanol suspension of the precursor CdX x (OH) y (CnTA)z. The reaction was kept at room temperature under constant stirring for a period of 2, 4, 8, 16 or 24 h. The yellow solids obtained were separated by centrifugation, washed twice with ethanol, dried under vacuum and stored under argon. Analyses are reported in Table 1. CdS used as control sample along this work, named "bulk" CdS, was prepared from CdCl2 under the same conditions used for preparing the nanocomposites.
Fourier transformed infrared (FT-IR) spectra (4,000-500 cm-1) were recorded on a Bruker IFS 25 model infrared spectrophotometer (Bruker Optik GmbH, Ettlingen, Germany). Samples for FT-IR were prepared using pressed KBr disc technique. Raman vibrational spectra were performed on a Bruker Raman Fourier transform spectrometer RFS 100/S (Bruker Optik GmbH, Ettlingen, Germany). The samples grinded in agate mortar were put into sealed capillary glass tubes to be placed in the sample holder of the instrument. The beam of a Nd:YAG laser (λ = 1064 nm) was used as excitation source. X-ray powder diffraction analysis was performed using a Siemens D5000 diffractometer (Siemens company, Karlsruhe, Germany) with Cu Kα radiation (1.5418 Å, operation voltage 40 kV, current 30 mA). The morphology of the products was examined by scanning electron microscopy (SEM) using an S-5000 field-emission SEM (Hitachi Ltd., Japan),, operating at beam voltages between 1 and 10 kV. The chemical composition of the samples was determined by elemental chemical analysis (PerkinElmer 240C PerkinElmer Inc., California, USA) and atomic absorption spectrometry (Unicam 929, Agilent Technologies, USA).
Diffuse reflectance UV-visible (UV-vis) spectra were recorded using a Shimadzu UV-vis spectrophotometer, double beam model 2450 PC, equipped with an integrating sphere (Shimadzu Co., Tokyo, Japan. Barium sulphate was used in all cases as reference material. Spectra were recorded in the range of 200 to 800 nm at room temperature. Reflectance measurements were converted to absorption spectra using the Kubelka-Munk function .
The photoluminescence (PL) spectra were recorded at room temperature using a PerkinElmer spectrofluorometer, LS 55 model (PerkinElmer Inc., California, USA). This spectrometer is equipped with a 150-W Xenon lamp source, emission and excitation monochromator configurations and a photomultiplier tube (R-106). The spectral response was virtually flat in the examined spectral regions. Analysis of PL spectra was performed by deconvoluting the spectra by fitting experimental data points to a sum of n Gaussian functions using Origin 6.0 multi-peak fitting package; confidence criterion was adjusted R-square and reduced Chi-square values.
The photocatalytic activity of CdS-surfactant nanocomposites was tested using, as reaction model, the photodegradation of methylene blue. Experiments were performed typically using 50 ml of a 2 × 10-5 M aqueous solution of the dye and photocatalyst loadings in the range of 0.5 to 0.6 g/L. This solution was irradiated with a UV-visible light source emitting in the 270 to 310 nm range. The change in the concentration of methylene blue in the solution while irradiating was monitored by measuring the absorbance at regular intervals between 30 and 240 min using the UV-visible spectrometer mentioned in "Optical measurements".
Results and discussion
Structure of cadmium sulphide-surfactant nanocomposites
The nature of the CdCl2 crystal structure, in which bonding along the crystallographic c-axis is relatively weak, easily leads to mesostructured layered products in the presence of cationic surfactants . This feature was exploited in this work using the layered CdCl2/cationic surfactant as a precursor of the corresponding mixed CdCl2/CdS derivatives by specifically direct exchange of chloride by sulphide atoms in a solid phase/gas reaction. Given the high affinity of cadmium-ion for soft Lewis bases like sulphide, exchange reaction occurs spontaneously, and exothermically, under rather mild conditions. Thus, the layered structure of the precursor, as discussed below, remains practically unaltered. Varying the amount of added hydrogen sulphide, the relative concentration of sulphur in the sample may be regulated in the range of 6.62 to 12.51 atom% (see Table 1) without disrupting the pristine structure of the solid. In the following, we analyse the properties of products with the highest concentration of sulphur, i.e. in the range of 0.54 to 0.99 atoms of sulphur per cadmium ion.
Diffuse reflectance measurements
where R ∞ is the diffuse reflectance, and K and S are the absorption and diffusion coefficients, respectively, and R ∞ < 1 if K ≠ 0.
where α is the absorption coefficient, Eg is the band gap energy and m = 1 for allowed direct transitions. Since α(υ) is proportional to K/S, the band gap can be obtained from the plot (F(R ∞) × hv)2 against hv.
Band gap energies of "bulk" CdS and lamellar CdS/surfactant nanocomposites synthesised for this work
Sulphur content (%)
Eg ± 0.01 (eV)
CdS "bulk" (control sample)
2.34 (theoretical value, 2.42) 
Photoluminescence emission-excitation spectroscopy
The photocatalytic activity of CdS nanocomposites was tested using methylene blue as a model compound, which has been proved to be appropriate to study such processes . Complete decolouration accompanies photodegradation, thus permitting easy spectrometric determination of the degradation process and its irreversible character. The use of this dye is favourable since the regeneration of colour by oxygen or other oxidant is avoided. Our experiments were performed in aqueous media, and the progress of dye photodegradation was followed by observing the characteristic dye absorption peak intensity centred at 664 nm.
Results described in this paper show that by using adequate synthesis procedures, it is possible to obtain hybrid semiconducting cadmium sulphide nanocomposites in which CdS forms part of two-dimensional ultra-thin inorganic sheets sandwiched between two self-assembled surfactant layers. These nanostructures, containing predetermined amount of CdS, are found in bulk held together by van der Waals interactions, thus generating layered graphitic-like structures with inter-laminar distances which correlate well with the hydrocarbon chain length of the surfactant. The electronic structure of Cd, as deduced from absorption, excitation and emission spectra, is similar to that of bulk but shows "all" the features expected for a two-dimensional confinement of the semiconductor. The CdS band gap may be, to some extent, regulated by selecting both the length of the hydrocarbon chain of the surfactant and the concentration of the sulphide in the layers. The charge transfer ability of the nanocomposites, evaluated from the photocatalytic activity of these products, appears to be better than that of "bulk" cadmium sulphide. These results are encouraging in the search of methods to design and prepare tailor-made novel functional semiconducting materials.
band gap energy
scanning electron microscopy.
The authors acknowledge partially funding for this research by Universidad de Chile (VID, Convenio CSIC - University. of Chile 2009-2010), FONDECYT (contract 1090282), Basal Financing Program CONICYT, FB0807 (CEDENNA), and Millennium Science Nucleus, Basic and Applied Magnetism grant no. P10-061-F.
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