The photocatalytic activity of semiconductor nanoparticles such as quantum dots (QDs) has been intensively studied over the past decade. The main advantage of QDs is their large surface area, which increases their photocatalytic efficiency. However, the quantum size effect of QDs smaller than the Bohr exciton radius also significantly influences their photocatalytic activity. Photocatalytic reactions on semiconductor QDs involve excitation of electrons from the valence to conduction band, by absorbing photons with energy equal to or higher than the transition energy between the corresponding electron and hole states. Generated electrons and holes at corresponding quantum levels can recombine, or partake in oxidation and reduction reactions of photocatalytic processes on the QDs surface.
ZnxCd1-xS is a direct band gap semiconductor with a gap energy between 2.4 (CdS) and 3.7 (ZnS) eV at 300 K [1, 2, 3]. Similarly to ZnS and CdS, ZnxCd1-xS exhibits photocatalytic properties (see below). The advantage of ZnxCd1-xS photocatalysts is its larger excitation radius of between 2.5 nm (ZnS) and 3 nm (CdS), compared with photocatalysts such as TiO2 or ZnO with exciton Bohr radii nearer 2 nm. This allows quantum size effects to be exploited for QDs smaller than 3 nm, so that their electron or hole energies can be adjusted for photocatalytic reactions. Another advantage is the decreased probability of radiative recombination of carriers in QDs smaller than 3 nm and compositions close to CdS [4], which can increase their photocatalytic efficiency.
Syntheses and applications of ZnxCd1-xS nanostructures have been thoroughly described [5, 6, 7, 8, 9, 10]. ZnxCd1−xS nanoparticles show promise in the visible light photocatalytic production of hydrogen [11, 12]. The syntheses and applications of ZnxCd1−xS nanoparticles were comprehensively reviewed by Fang et al. [13]. We previously investigated the potential of ZnS and CdS QDs in the photocatalytic reduction of CO2 [14, 15]. A ZnS nanocomposite containing the clay mineral montmorillonite (MMT) was also tested for the photodecomposition of phenol [16]. Combinations of ZnS and CdS in core/shell CdS/ZnS nanostructures were studied for the photocatalytic decomposition of methylene blue [17].
The current study investigates how the photocatalytic activity of ZnxCd1-xS QDs depends on their composition x. MB is used for this purpose, and can be decomposed by hydroxyl radicals formed by the reaction of electrons and holes, according to the scheme published in Ref. [18]. The photocatalytic activity of ZnxCd1-xS QDs depends on their composition. The experimental results are explained by calculating the QD sizes, and thus the energy levels of electrons and holes involved. To the best of our knowledge, this approach combining reaction kinetics results and quantum calculations has not previously been used to study ternary semiconductor QDs.
All solvents and chemicals were of analytical reagent grade. Zinc acetate, cadmium acetate, and sodium sulphide were from Lachema, Czech Republic, and cetyltrimethylammonium bromide (CTBA) was from Sigma-Aldrich, USA. All water used to prepare solutions was deionized by reverse osmosis (Aqua Osmotic, Czech Republic). MMT was purchased from the Source Clays Repository of the Clay Mineral Society (West Lafayette, USA).
ZnxCd1-xS QDs were prepared by precipitating aqueous solutions of zinc and cadmium acetates with sodium sulfide, in the presence of CTAB. We previously found that CTAB created positively charged bilayers around the QDs, thus stabilizing the resulting colloid dispersions [19]. The CTAB concentration was held at ~4 mmol/L, to ensure the formation of micelles. The Zn(Cd):S:CTAB molar ratio was 1:1.5:2 [15].
The QDs were deposited on MMT (QDs/MMT) prior to characterization. Depositions of ZnS, CdS, and ZnS/CdS (core/shell) QDs covered by positively charged CTAB layers were attracted to the negatively charged MMT surface, as described previously [14, 15, 16, 17, 19]. The resulting QDs/MMT solid samples were easily handled, stored and analysed by powder X-ray diffraction (XRD) and transmission and scanning electron microscopies (TEM and SEM).
Ultraviolet-visible (UV-Vis) absorption spectra of ZnS, CdS, and ZnxCd1-xS dispersions were measured in 1-cm-pathlength quartz cuvettes using a UV-VIS spectrometer Lambda 25 (Perkin Elmer, USA), at 200‒800 nm. Photoluminescence spectra of CdS and ZnxCd1-xS nanoparticles were measured by a spectrometer FLS920 (Edinburgh Instrument Ltd., UK) equipped with a 450-W Xenon lamp (Xe900). The excitation wavelength was 365 nm, and the excitation and emission slit widths were 3 nm. A 395-nm absorption filter was used.
Powder XRD patterns were recorded using Co Kα radiation (λ = 0.1789 nm) with a Bruker D8 Advance diffractometer (Bruker AXS), equipped with a fast position sensitive detector VÅNTEC 1. Measurements were carried out in reflection mode, on powder samples pressed in a rotational holder. Phase compositions were evaluated using the database PDF 2 Release 2004 (International Centre for Diffraction Data).
High resolution TEM images of ZnxCd1-xS QDs as QDs/MMT samples were collected on a JEM 220FS microscope (Jeol, Japan), operated at 200 kV. QDs/MMT particles were dispersed in ethanol, and droplets of the resulting dispersion deposited on a TEM grid with a carbon holey support film, using an ultrasonic sprayer.
SEM was performed with aXL 30 Philips SEM microscope (the Netherlands) equipped with a Robinson backscattered electron detector, to examine the ZnxCd1-xS morphologies and Zn and S compositions. Powder samples were first coated with gold and palladium in an ionization chamber. Elemental analysis was performed using energy dispersive X-ray spectroscopy (EDX).
The photocatalytic activity of colloidal dispersions of ZnxCd1-xS QDs was evaluated by their degradation of MB under UV irradiation, with a maximum emission wavelength at 365 nm (Hg lamp HSC-1L Pen-Ray, UVP, Germany). Experiments were performed in a stirred batch reactor open to the air. In a typical experiment, a colloidal dispersion of ZnxCd1-xS QDs was added to an aqueous solution of MB, to give a total volume of 100 mL. The initial concentrations of MB and ZnxCd1-xS were 8×10‒3 and 2 mmol/L, respectively. Before photocatalysis, the MB solution in contact with the QDs was stirred for 10 min in the dark, to reach adsorption equilibrium and homogenize the dispersion [17]. The UV lamp was then switched on. The reactor temperature was kept at 18 °C.
QD energy levels were calculated based on the spherical effective mass approximation. In the case of infinite potential barriers, the energy of a particle on the n-th quantum level can be expressed as:
A more precise calculation considers finite barriers, with electrons and holes partly penetrating into the barrier region. In the current case, the ZnxCd1-xS QD represents the potential well region, and water surrounding the QD represents the barrier. The particle in a quantum potential well can be described by the Schrödinger equation:
where ħ =1.05 × 10‒34 J·s is the reduced Planck constant, Ψ is the particle wave function of the quantum system, Ñ is the differential operator, r is the coordinate, m is the particles mass, V is the potential energy, and E is the energy of the state Ψ. In the current case, the particle is the charge carrier (electron or hole), the quantum well is the ZnxCd1-xS QD of radius R, and the barrier is the surrounding water.
The particle wave function (Ψ) and particle momentum (1/m(R)δΨ(R)/δR) must be continuous on the well/barrier interface. Thus, the following equations are valid for the same types of carrier wave functions (1st and 3rd quantum states):
or for an odd carrier wave function (2nd quantum state in the current case):
where mW= mW m0 is the mass of an electron or hole in a quantum well (in the ZnxCd1-xS QD of given composition in the current case), mW is the particle effective mass, mB = mBm0 is the particle mass in water, since mB in water can be supposed to be 1, En is the energy of the particle on the n-th quantum level, and ΔEc,v represents the conduction or valence band discontinuity.
The conduction band discontinuity for water and CdS was taken as 3.0 eV for the conduction band and 2.5 eV for the valence band, according to Ref. [20]. The schematic diagram of the energy levels in a ZnxCd1-xS QD is shown in Fig. 1. Material parameters for the calculations, such as the conduction and valence band energies, carrier effective masses and band gap energies of CdS and ZnS were taken from Zgaren et al. [21]. ZnxCd1-xS parameters were approximated by linear interpolation.
The equations mentioned above were used to calculate the ZnxCd1-xS QD radius from a known ground state transition energy (Et1), corresponding to the absorption edge energy. The equations were also used to calculate the excited state transition energies between corresponding electron and hole states, for the specific ZnxCd1-xS QD composition and radius. Since Etn represents the transition energy between the n-th electron and n-th hole quantum level, then:
where Eg (bulk) is the gap energy of the ZnxCd1-xS bulk semiconductor of composition x. Eqs. (3) and (4) were solved numerically using the Maple software package.
Another approach was used to calculate the QD radii, based on the Brus equation [25]. The ground state transition energy of a QD representing a quantum well with infinite potential barriers for electrons and holes was calculated as:
where Et and Eg are the ground state transition energy of the QD and gap energy of the bulk semiconductor, respectively, h is Planck’s constant, R is the radius of the nanoparticle, me and mh are the effective masses of the electron and hole, respectively, e is the charge of an electron, εr is the dielectric constant (5.7 for CdS and 5.4 for ZnS), and ε0 is the permittivity of a vacuum.
To calculate the ZnxCd1-xS QD radii, we assumed that the effective masses of electron and holes, dielectric constant and gap energy were linearly dependent on the composition x. The calculated bulk gap energies used in Eqs. (5) and (6), and the QD radii for quantum wells with finite and infinite energy barriers, are summarized in Table 1.
UV-Vis absorption spectra of colloidal QD dispersions were recorded immediately after precipitation. Typical UV-Vis absorption spectra of CdS, ZnS and ZnxCd1-xS dispersions are shown in Fig. 2. The spectra of the QDs were used to evaluate the QD ground state transition energy (absorption edge) using the Tauc equation [26]:
where e is the molar extinction coefficient obtained from the Beer-Lambert law, hn is the incident photon energy, C is a constant, Et is the QD ground state transition energy, and p depends on the type of transition. For direct semiconductors like ZnS and CdS, then p = 1/2. The usual method for determining Et involves plotting (ehv)1/p against hv. The obtained Et values were used to estimate the radii of nanoparticles, according to Eqs. (3) and (4). A red shift in the transition energy Et1 (absorption edge) with decreasing x was observed in the UV-Vis absorption spectra.
The photoluminescence spectra in Fig. 3 show that increasing the Zn content increased the emission intensity. Small peaks at ~418 nm were likely caused by Raman scattering of water molecules. The high intensity emission at ~580 nm was probably caused by surface defects and traps, with slow radiative recombination. The decay time of these wide maxima was typically several µs. Under low excitation intensity, slow transitions dominated the spectra, and band edge luminescence was negligible. Under high excitation intensity (laser source), these energy levels were saturated, and much faster excitonic transitions become dominant. This phenomenon has been observed in other materials with deep defect luminescence [24]. Similar spectra under low intensity excitation from a Xe lamp, and a comparison of different decay times for different excitation intensities, have been reported previously [25].
Complexes with deep acceptor surface states were probably responsible for the broad luminescence maxima. The position and intensity of the broad maximum slightly differed between samples, because the concentration and energy of deep defects depended on the QD composition and specific surface. The lowest intensity broad peak was observed for the CdS QDs, probably because of their lowest surface area. Different kinds of deep defects were responsible for the photoluminescence of the ZnS QDs. Only electrons contributed to the photocatalytic decomposition of MB, so deep acceptor levels could increase the photocatalytic activity of the QDs, by decreasing the radiative recombination rate.
The QDs were unable to be deposited on non-diffractive silicon or glass. Thus, ZnxCd1-xS QDs were deposited on MMT and studied by XRD (Fig. 4). The intensities of diffraction peaks characteristic of the QDs were low, and were largely obscured by the stronger diffraction peaks of MMT and quartz [26]. No ZnS and CdS diffractions were observed, indicating that there was no phase separation of ZnS and CdS within ZnxCd1-xS. The XRD patterns showed that the QDs were of hexagonal phase, with the strongest (101) peak for zinc cadmium sulphide observed at 2θ = 34.3° (PDF 00-04901302).
The SEM image (Fig. 5(a)) shows Zn0.4Cd0.6S QDs deposited on MMT. The EDX spectrum (Fig. 5(b)) indicated the presence of Zn, Cd, and S in the ZnxCd1-xS QDs. The C and Br peaks were due to the presence of the CTAB stabilizer. Other elements were components of montmorillonite. EDX indicated that the true composition of Zn0.2Cd0.8S was Zn0.21Cd0.79S, and that of Zn0.4Cd0.6S was Zn0.38Cd0.62S.
The QDs/MMT samples were examined by TEM. Fig. 6 shows a TEM image of the Zn0.3Cd0.7S QDs, and their corresponding size distribution. The average sizes of the Zn0.1Cd0.9S, Zn0.3Cd0.7S, Zn0.4Cd0.6S, and Zn0.7Cd0.3S particles estimated from the TEM images were 5.0, 5.4, 5.2, and 4.2 nm, respectively. The corresponding radii were approximately 2.1‒2.5 nm, which were consistent with the calculated radii shown in Table 1. QDs sizes estimated from the TEM images may have been influenced by the thickness of the adsorbed CTAB layer [19].
The photocatalytic efficiencies of the CdS, ZnS, and ZnxCd1-xS QDs were investigated by their photocatalytic decomposition of MB. The rate of this heterogeneous reaction (<r>) can be described by the Langmuir-Hinshelwood equation [27, 28, 29]:
where kobs is the observed kinetic rate constant, and cMB is the MB concentration. kobs was calculated by fitting the measured data, using the integrated rate equation based on first order kinetics (lnc0/c = kobst, where c0 and c are the concentrations of MB at time t = t and t = 0, respectively). The same photocatalytic experiments were performed using the CdS and ZnS QDs for comparison, and also in the absence of QDs to observe any MB photocatalysis, which was found to be negligible.
Aqueous dispersions of ZnxCd1-xS QDs were prepared with x ranging from 0 (CdS) to 1 (ZnS) in 0.1 increments. The kinetic rate constants for the decomposition of MB are shown in Fig. 7. Increasing Zn content in ZnxCd1-xS QDs was expected to increase the photocatalytic activity, owing to the higher electron energy, as discussed below. However, three distinct regions were observed, each with different dependence of photocatalytic activity on QD composition.
In region A (x ≤ 0.3), the photocatalytic efficiency decreased with increasing Zn content. The expected increase in photocatalytic efficiency was observed in region B, and efficiency reached a maximum at x = 0.6. Efficiency then decreased continually with increasing Zn content in region C.
The QD sizes were necessary to explain this result. QD radii were derived from the measured transition energies determined using Eq. (5), and are summarized in Table 1 and Fig. 8. This energy was equal to the transition energy between the ground (1st) electron and hole levels (Et1) (Fig. 1). QD radii were calculated using Eqs. (3) and (4), and also by approximation with infinite energy barriers using Eq. (6). The radii calculated using these two methods are compared in Fig. 8. Values were similar for larger QDs, but deviated for smaller QDs. The penetration of electrons into the energy barriers (water) was more significant for smaller QDs. Approximation with a finite barrier and penetration of carriers into the energy barrier region was considered to yield more precise QD radii values, so were used as input parameters for further calculations. The calculated ZnxCd1-xS QD radii decreased with increasing Zn content.
We calculated the transition energies between the electron and hole energy levels for the first three corresponding electron and hole quantum states, for the series of ZnxCd1-xS QDs. These results are shown in Fig. 9. When the quantum levels were close to the barrier energy, they were much less influenced by the QD composition. Larger QDs possessed quantum levels located deeper within the potential well. Thus, their transition energies were more influenced by the gap energy, which corresponded to the ZnxCd1-xS composition. The composition of smaller QDs (R = 1.3‒2.7 nm) only slightly influenced their properties.
We now consider the dependence of photocatalytic activity on ZnxCd1-xS QD composition in the different regions in Fig. 7. The excitation energy was compared with the transition energies between electron and hole quantum levels in the QDs, as shown in Fig. 9. The three sets of curves show the calculated transition energies of the 1st, 2nd, and 3rd quantum states for ZnxCd1-xS QDs with x = 0, 0.2, 0.5, and 1. Squares represent experimental data for the lowest energy transition between the first electron and hole levels. These transition energies (Et1) were derived from the UV-Vis absorption spectra. Circles represent the calculated excited states transition (Et2) (between the 2nd electron quantum state and 2nd hole quantum state). Black lines connecting these points show approximate transition energies of the QDs, which could theoretically be prepared using the current method. These black curves show that the QD size decreased with increasing x (crossovers of straight and structured lines of calculated transition energies).
The transition energies of the QDs were compared with the excitation energy, to explain the photocatalytic efficiencies of the QDs. Two crossovers of black lines with excitation energy defined the three regions with different numbers of quantum states, which could be excited. For QD radii > 2.5 nm in region I, photons with excitation energy of 3.4 eV were sufficient to generate electron-hole pairs in two quantum states. In region II, QDs had radii of 1.7‒2.5 nm. These QDs could absorb the photon energy, but it was only sufficient for generating electron-hole pairs in the ground quantum states. For QDs with smaller radii in region III, the photon energy was insufficient to generate electron-holes pair in any quantum state. These three regions are closely related to regions A‒C exhibiting different photocatalytic behaviour in Fig. 7.
Comparing regions in Figs. 7 and 9 must be done while considering that the QD size distribution was not sharp, and the QD radius calculated from Et1 reflected the maximum of the QD size distribution (Fig. 6). In region A of Fig. 7, the QD Zn content was < 0.3, and the QD radius was > 2.5 nm. Thus, the electron-hole pairs could be generated in two quantum states, corresponding to region I in Fig. 9. With increasing Zn content, the QD radius decreased to the QD size limit, where only one quantum dot level could be activated by the excitation. In this region, the number of QDs where two quantum levels could be excited decreased with increasing Zn content. The second quantum level contained three times more electron states than the first. The number of excited electrons decreased steeply, and could not be compensated by the increased QD density and increased surface area, as a result of the decreasing QD size. The fewer available electrons excited by the incident irradiation also suppressed the photocatalytic activity. This was consistent with the dependence of photocatalytic activity on QD composition in region A of Fig. 7.
When almost all QDs crossed this limit, only the ground quantum level in the QDs could be excited (region B in Fig. 7, and region II in Fig. 9). In region B, further increasing the Zn content increased the photocatalytic efficiency, mainly due to the increased QD surface area. This was inversely proportional to the decreasing QD radius and increased QD density in the dispersion. The increased penetration of electrons into aqueous solution, and the increased probability of QD excitation with increased Et1,also contributed.
The maximum photocatalytic activity was achieved for the QDs with x = 0.6. Here, Et1 for the 1st quantum level was equal to with excitation energy, which increased the excitation probability of this quantum state. The number of QDs able to be excited sharply decreased at higher Zn content. No electron-hole pairs were generated in the majority of these QDs, and their photocatalytic activity under the current excitation decreased (region C in Fig. 7, corresponding to region III in Fig. 8).
Aqueous dispersions of ZnxCd1-xS QDs (x = 0‒1) were prepared by precipitating zinc and cadmium acetate with sodium sulphide in the presence of CTAB. The transition energies between the first electron and hole quantum levels indicated that the QD size was influenced by the ZnxCd1-xS composition. QD size decreased with increasing Zn content. The photocatalytic activity of the ZnxCd1-xS QDs did not simply increase with increasing Zn content. The maximal photocatalytic decomposition of MB was observed for x = 0.6, when the QD transition energy was equal to photon energy (3.4 eV). Three different ZnxCd1-xS composition regions exhibited different dependences of photocatalytic activity on Zn content. The number of quantum levels which could be excited influenced the photocatalytic activity of the ZnxCd1-xS QDs. Engineering QDs with suitable composition and size is important for maximizing their photocatalytic activity. These findings aid our understanding of processes involved in photocatalysis. They provide a way to optimize QD properties, and achieve high photocatalytic activity with respect to the incident irradiation.
The authors thank Dr. K. Mamulová Kutláková and Mrs. M. Heliová (both from Nanotechnology Centre, VŠB-Technical University of Ostrava) for measuring the XRD patterns and the SEM analysis, respectively.
2015, Vol. 36 Issue (3): 328-335
PDF (938 KB)
2015, Vol. 36 Issue (3): 328-335
PDF (938 KB)