ARTICLE pubs.acs.org/Langmuir
Anisotropy Property of CdS and PbS Quantum Dots Encapsulated in Silica Film with Uniformly Oriented Mesochannel Feng Shan, Xuemin Lu,* Qian Zhang, Bin Su, and Qinghua Lu* School of Chemistry and Chemical Engineering, the State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: In this paper, hybrid film of mesoporous silica film with oriented mesochannels and semiconductor quantum dot has been prepared. Encapsulation of CdS and PbS within the oriented mesochannels leads to a regular arrangement at the macro scale. The hybrid film thus obtained showed remarkable anisotropic photoelectronic properties due to the confinement effect of the oriented mesochannels. Furthermore, due to the independence of the orientations of the mesochannels on the substrate, bilayer films containing both CdS and PbS could be prepared. This design has allowed an extension of the range of light absorption by the thin film as well as an amplification of the response to external photoelectronic effects. Such a hybrid film may prove useful in the design of anisotropic electrodes and electronic nanodevices.
1. INTRODUCTION Semiconductor quantum dots (QDs) have been intensively studied due to their potential applications including lasers, sensors, and solar cells. Recently, it was proposed that such material processing anisotropic property such as optical, magnetic, or electronic is very valuable and will open up a new dimension for applications.15 A few methods have been reported to control the anisotropic property based on control of the QD shape.613 On the other hand, compared with the complex control of QD shape, regular arrangement of QDs is also an effective method to create the anisotropic property. Chen et al. reported the regular arrangement of CdSe QDs on micropattern grating and found obvious optical anisotropy of the obtained film.5 In the past decades, mesoporous silica film (MSF) with onedimensional (1D) oriented mesochannels at macro scale received increasing attention due to its potential application in preparing hybrid film which shows anisotropic response to outside stimuli. In such a mesoporous film, the mesochannels were controlled to be oriented in one direction at macro scale. The oriented mesochannels often lead to the manifestation of axis-dependent properties of the encapsulated functional molecules. Various species, such as dye molecules,14 semiconductor polymers,1518 and metal nanowires,19,20 have been encapsulated in oriented mesochannels. If the QDs can be encapsulated into the oriented mesochannels, it would be possible to control the anisotropic property of these materials, and this could open up a new possibility for the creation of polarization dependent optoelectronic devices based on semiconductor quantum dots. In this paper, we prepared hybrid film of semiconductor materials and MSF with oriented mesochannels. CdS and PbS were in situ encapsulated in the oriented mesochannels using an ion-exchange process. We have investigated the photoelectronic properties of such hybrid thin films to confirm the effect of anisotropic orientation of the mesochannels on the electrochemical r 2011 American Chemical Society
properties of CdS and PbS. Results have proven that QDs displayed obviously anisotropic photoelectronic properties when the film was submitted to different biases. Furthermore, a complex architecture based on introducing various QDs with different absorption wavelengths into the oriented mesochannels, occupying different layers of bilayer films, has also been fabricated. Experimental results have proven that the encapsulation of different QDs in various layers of MSF can effectively amplify the response of the hybrid thin film to external photoelectric stimuli, which should prove highly beneficial for designing optic and electronic nanodevices.
2. EXPERIMENTAL SECTION Surfactant P123 (polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer (EO20-PO70-EO20)) was dissolved in ethanol (5 mL) and the solution was stirred for 20 min at room temperature. Tetraethoxysilane (TEOS) was added to ethanol (5 mL) under acidic conditions, and this solution was also stirred for 20 min at room temperature. The two solutions were then mixed together. The final molar ratio of the reactants was TEOS/P123/H2O/HCl/EtOH = 1:0.01:6.5:0.01:22. The precursor solution was stirred for 3 h, and then oriented mesostructured silica films were prepared by hot air flow over the glass or FTO substrates. The film samples were first aged at 20 °C and 50% relative humidity for 24 h and then at 130 °C for a further 3 h (heating rate: 1 °C/min). A CdS QD-loaded oriented silica film was prepared through an ionexchange process.6,21 The as-prepared MSF was refluxed in a 0.1 M solution of cadmium acetate in methanol at 80 °C for 24 h. The resultant film was extensively washed with hot methanol and distilled water to remove any remaining Cd2+ from the surface and then dried at 120 °C Received: August 29, 2011 Revised: October 25, 2011 Published: November 27, 2011 812
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for 12 h to remove the adsorbed water. The sample was then treated with H2S to produce CdS inside the MSF. A PbS QD-loaded MSF was prepared by the same method using 0.1 m lead acetate. Bilayer hybrid oriented MSFs were prepared in a layer-by-layer manner. A few drops of sol precursor were placed on a CdS QD-loaded MSF and a hot air flow was applied parallel to the first layer mesochannel direction to form the second oriented MSF layer. The obtained bilayer film was aged at 20 °C and 50% relative humidity for 24 h and then at 130 °C for a further 3 h to consolidate the silica walls. The bilayer film, consisting of a lower layer of CdS-loaded MSF and an upper layer of asprepared MSF with identical mesochannel orientation, was refluxed in a 0.1 M solution of lead acetate in methanol and then reacted with H2S to encapsulate PbS in the upper layer MSF. The assembly thus obtained is hereinafter referred to as MSF/CdS/PbS. A bilayer MSF/PbS/CdS sample was prepared in the same manner, which consisted of a lower layer of PbS-loaded MSF and an upper layer of CdS-loaded MSF. X-ray diffraction (XRD) patterns were recorded at a scanning rate of 1°/min using a Rigaku D/max-2200/PC diffractometer with Cu-Kα radiation operating at 40 kV and 20 mA. In-plane XRD measurements were performed using Cr-Kα radiation (D8 Discover GADDS, General Area Detector Diffraction System, Bruker) using diffraction radiation. The degree of orientation (D.O. (%)) of the mesochannels was calculated with the following equation:22 D:O: ¼
∑
360° Wi 100% 360°
Figure 1. UV/vis absorption spectrum of MSF/CdS. The inset shows photographic images of MSF, MSF/Cd2+, and MSF/CdS.
ð1Þ
where Wi (degrees) is the full-width at half-maximum (fwhm) of the diffraction peak in the in-plane XRD profile, summed over i, the number of peaks. Transmission electron micrographs (TEM) and energydispersive X-ray spectra (EDS) were obtained on a JEOL TEM-2100 operating at an accelerating voltage of 200 keV. For TEM studies, the cross-sectional specimens were artificially thinned first, and then sectioned on an ultramicrotome by ion etching. UV/visible absorption spectra were measured on a Perkin-Elmer Lambda 750S spectrophotometer. Photoluminescence spectra were obtained using a Perkin-Elmer LS 50B fluorescence spectrophotometer. The excitation wavelength was 450 nm and vertically polarized by a quartz linear polarizer. The sample was held with mesochannel orientation parallel or perpendicular to the excitation polarization by rotating the sample. Photoelectrochemical studies were carried out in a double-electrode cell with a Pt-coated FTO glass as counter electrode using a computercontrolled electrochemical analyzer (CHI660C Instruments). Pt-coated counter electrodes were prepared by thermal decomposition.23 A drop of 10 mm H2PtCl6 in 2-propanol was spread on the FTO glass by spincoating and then the substrate was heated at 400 °C for 15 min in air. The CdS QD-loaded working electrodes were sealed within sandwich cells with a 30 μm spacer by using Pt-coated FTO glass as the counter electrode. An aqueous polysulfide solution containing 0.5 M Na2S, 0.125 M S, and 0.2 M KCl was used as an electrolyte for photoelectrochemical measurements. The photovoltaic performance was measured under simulated AM 1.5 solar illumination with an intensity of 100 mW/cm2. The incident light intensity was adjusted with a silicon reference solar cell produced by NREL. The scan rate was 0.05 V/s.
Figure 2. Low-angle XRD profiles of (A) oriented MSF, (B) MSF/Cd2+, (C) MSF/CdS, and (D) MSF/CdS/PbS. The inset shows the (200) diffraction bands.
parallel to the substrate for 10 s to prepare a mesostructured film. In the experiment, the speed and temperature of the air flow were controlled at 19.5 m/s and 70 °C, respectively. Cadmium ions were introduced within the air-flow-treated MSF by refluxing in methanolic cadmium acetate solution. The surfactant P123 was concurrently extracted and substituted by cadmium acetate. Finally, treatment of the film samples with H2S in a vacuum chamber ensured the synthesis of stable CdS on the channel surface and pore walls of the oriented mesostructured films. The color of the CdS-loaded MSF (MSF/CdS) was indicative of the occurrence of a size-quantization effect, since it was brighter than that of the bulk material (Figure 1). The MSF/CdS hybrid film exhibited the onset of absorption at about 480 nm, which is obviously blue-shifted relative to that of bulk CdS particles; the quantum size effect is due to the CdS particle size being restricted by the pore channels of MSF.6,21 Based on the relationship between band gap energy and particle size for CdS, the size of CdS QDs in the pores was calculated to be 4 nm.25 Figure 2 shows the low-angle X-ray diffraction (XRD) profiles for as-prepared MSF, Cd2+-loaded MSF (MSF/Cd2+), MSF/ CdS, and MSF/CdS/PbS. The XRD patterns for the MSF and MSF/Cd2+ display a strong (100) diffraction and a weak (200) diffraction, which are characteristics of the two-dimensional hexagonal (p6mm) structure. This shows that the primary SBA-15 mesoporous silica thin film was maintained with high quality during the functionalization of the channel surface with Cd2+.
3. RESULTS AND DISCUSSION 3.1. MSF Encapsulated with Different QDs. A mesostructured silica thin film with oriented mesochannels was prepared according to a previously reported method.24 Briefly, a surfactant/silica composite film was prepared on a FTO glass substrate by employing a jet of hot, strong air flow. As the standard procedure, a droplet of sol silica precursor solution was pipetted onto a FTO glass, and then a hot, strong air flow was applied 813
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Figure 3. Wide-angle XRD patterns of (A) oriented MSF, (B) MSF/ Cd2+, (C) MSF/CdS, and (D) MSF/CdS/PbS.
Figure 4. Φ-Scanning in-plane XRD patterns of MSF, MSF/Cd2+, MSF/CdS, and MSF/CdS/PbS.
The decrease in peak intensities of MSF/Cd2+ and MSF/CdS confirmed the pore filling of the host material, because pore filling can reduce the scattering contrast between the pores and the walls of a mesoporous material.10 Additionally, the peaks of MSF/Cd2+ and MSF/CdS were slightly shifted to higher angle, which may be attributed to partial contraction of the framework during functionalization of the MSF. A similar phenomenon was also observed for the MSF/CdS/PbS sample. Figure 3 shows the wide-angle XRD profiles of the MSF, MSF/ Cd2+, MSF/CdS, and MSF/CdS/PbS samples. In Figure 3A and B, the very broad XRD peak comes from the diffraction of the amorphous wall material of SBA-15. The broad diffraction peaks in Figure 3C at 26.4°, 43.8°, and 51.72° (corresponding to the (111), (220), and (311) lines, respectively) are due to cubic crystalline CdS [JCPDS No. 652887], which proved that CdS had been efficiently introduced into the mesochannels of the mesoporous silica film. We further investigated the bilayer MSF with CdS and PbS encapsulated in different layers. Here, the sequence of the two layers was glass/MSF-CdS/MSF-PbS. The XRD profile of this film is shown in Figure 3D, which can be indexed to cubic crystalline CdS and galena phase PbS [JCPDS No. 050592], while the (111) and (220) reflections of PbS are overlapped with the (111) and (220) reflections of CdS. 3.2. Orientation of Mesochannels after QDs Encapsulation. The orientation of mesochannels in the film before and after the introduction of CdS was first estimated by Φ-scanning in-plane XRD. Two diffraction peaks were observed at Φ values of 90° and 270°, as shown in Figure 4. The D.O. of the mesochannels of the MSF was calculated to be 91.1%, indicating that uniaxially oriented mesochannels were well formed.26 After the incorporation of Cd2+ into the mesochannels of the MSF, two diffraction peaks also appeared in the same positions. However, the peaks became broader, and the calculated D.O. of MSF/Cd2+ was 73.9%. After treatment with H2S, the orientation of the mesochannels was almost unchanged and the D.O. of MSF/CdS was calculated to be 69.6%. For the bilayer MSF with CdS and PbS, after the incorporation of Pb2+ ions, the D.O. was almost the same as that for the MSF/CdS single layer. After reaction with H2S, the calculated D.O. of the bilayer MSF/CdS/PbS was 68.0%. The formation of oriented mesochannels is believed to be caused by the shear force generated by the high-speed air flow.24 Under this shear force, micelles formed from surfactant P123 are aligned along the air-flow direction and an MSF with oriented mesochannels is obtained on the support substrate.
In other words, the orientation of mesochannels is mainly controlled by the air flow, rather than the nature of the substrate. For the single and bilayer hybrid MSFs, the decrease in the degree of orientation was mainly due to the introduction of QDs. Therefore, it is practical to prepare multilayer oriented MSFs with different encapsulated semiconductor materials using this method. Figure 5a and b shows cross-sectional TEM images of oriented MSF and MSF/CdS. By comparing these images, it can be seen that the regularity of the MSF host was maintained during the formation of CdS. The cross-sectional TEM images also demonstrate that uniaxially aligned CdS nanorods were confined in the MSF channels along the mesochannel orientation in a highly dispersed state. The average diameter of the CdS nanorod was about 4 nm, which is consistent with the result from UV/vis analysis. The EDS spectrum in Figure 5c features the characteristic peaks of the elements Cd and S. Based on the EDS data, the concentration ratio of Cd to Si was calculated as 0.11:1. The stoichiometric ratio for Cd and S was thus 1:0.95, slightly larger than 1, which may be due to a surplus of unreacted Cd2+ ions during the H2S treatment. 3.3. Anisotropic Fluorescence of CdS Encapsulated into Oriented MSF. In a previous report, Tolbert et al. demonstrated that a hybrid film showed an anisotropic response after the confinement of semiconductor macromolecules in the oriented mesochannels.16 Kuroda et al. reported similar results when dye molecules or metal nanowires were introduced into the oriented mesochannels.14,19 Here, in order to investigate the effect of the oriented mesochannels on the encapsulated QDs we measured the fluorescence of the CdS in the MSF. We used vertically polarized light of wavelength 450 nm to excite the MSF/CdS, as shown in Figure 6. When the polarization of the exciting light was parallel to the direction of mesochannels, a strong fluorescence emission was seen at a wavelength of 631 nm; however, when the sample was rotated by 90° with the direction of the mesochannels perpendicular to the polarization of the exciting light, only weak fluorescence emission was seen. The ratio between the parallel and perpendicular directions was 3.33. As shown in Figure 5b, CdS nanorods were uniaxially oriented along the mesochannels alignment direction and the anisotropic fluorescence results were in agreement with earlier polarization measurement on ordered nanorod arrays.2731 We ascribe the observed anisotropic fluorescence originated from the anisotropy of the unidirectionally aligned CdS nanorods in which the transition dipole orientation is parallel to the nanorod long axis. When the excitation polarization is parallel to the mesochannel direction, the fluorescence is 814
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Figure 5. Cross-sectional TEM images of (a) pure oriented MSF and (b) oriented MSF/CdS. Insets: schematic illustrations of oriented MSF and MSF/ CdS. (c) EDS spectrum of MSF/CdS.
Figure 6. Fluorescence spectra of CdS encapsulated in oriented mesoporous silica films; the orientation direction of the CdS-loaded oriented MSF was parallel (green line) or perpendicular (red line) to the polarization direction of the incident light. The excitation wavelength was 450 nm.
expected to be maximal, since the transition dipole is parallel to the vertically polarized incident light, while minimal fluorescence is obtained with the excitation polarization is perpendicular to the transition dipole.15 This anisotropic fluorescence of CdS in MSF proved that the orientation of the mesochannels exerted a spatial influence on the growth of the CdS. 3.4. Photoelectrochemical Properties of QDs Encapsulated in MSF. In order to investigate the effect of mesochannel orientation on the properties of the MSF/CdS electrodes, linearsweep voltammetry measurements were made on sandwichstructured cells in the dark and under light illumination (Figure 7a,b). The electric potential was applied parallel and perpendicular to the mesochannel orientation, respectively. As shown in Figure 7c, the voltammetric responses were different, depending not only on the mesochannel orientation but also on the light irradiation. The current density at a potential of 1.0 V in the dark was 0.154 mA/cm2 when the potential was applied parallel to the mesochannel orientation. In contrast, the current density was only 0.032 mA/cm2 when the potential was applied perpendicular to the mesochannel orientation, which demonstrates the anisotropic property of the CdS-MSF electrodes. Under light irradiation, the current intensity was increased due to the photovoltaic effect of the CdS. The current densities of the illustrated cell were 0.342 and 0.098 mA/cm2, respectively, when the electrical potential of 1.0 V was applied parallel and perpendicular to the mesochannels. The dichroic ratio of the current density was estimated to be ∼3.5, and this result clearly shows the preferential anisotropy of the MSF/CdS electrode.
Figure 7. Schematic illustration of the photoelectrochemical measurement of MSF/CdS under light illumination with the electrical potential applied parallel to the mesochannel orientation (a) and perpendicular to the mesochannel orientation (b); (c) linear-sweep voltammetric curves of MSF/CdS with mesochannels parallel and perpendicular to the applied bias; (d) illustration of charge-transfer process and band energies (not to scale) of SiO2 in comparison with CdS.
In such a MSF/CdS hybrid film, the CdS nanorods were confined in the mesochannels and isolated from each other. As the silica walls were chemically and electrochemically inert under the experimental conditions, the observed current of the FTO electrodes covered by MSF with a 2D hexagonal mesostructure was due to sieving effects and permselective properties under the applied electric potential, although the value was very low and depended on the porous structure of the film.32 Here, the incorporation of CdS into MSF can enhance the current density even under dark conditions, which is due to the semiconducting property of CdS. Semiconductor nanocrystals are capable of injecting excited electrons into a wider gap semiconductor.33,34 The injected electrons are easily and effectively transferred between the CdS nanorods along the mesochannel direction under an applied bias (Figure 7d). This property of MSF hybrid materials is anticipated to be very useful if electrodes can be deposited at the ends of the mesochannels. The response of the oriented MSF/CdS electrode to ON OFF cycles of visible illumination was further investigated under 1.0 V bias, with the electrical potential applied parallel to the 815
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Figure 8. Photoresponse of an oriented MSF/CdS electrode with mesochannels parallel to the 1.0 V applied bias. Figure 10. Left: Linear-sweep voltammetric curves of (A) MSF/CdS/ PbS, (B) MSF/PbS/CdS, (D) MSF/PbS, and (E) MSF/CdS with mesochannels parallel to the applied bias, and (C) MSF/CdS/PbS with mesochannels perpendicular to the applied bias under light illumination. Right: Schematic illustration of the process of light irradiating different layered films.
the linear-sweep voltammetric curves of single MSF/CdS and MSF/PbS electrodes. Both of the bilayer films exhibited higher current density than the MSF/CdS and MSF/PbS single electrodes. In addition to the broader light absorption range and higher absorbance, the increased film thickness is also an important factor. Interestingly, it was also found that the MSF/CdS/PbS electrode produced a higher current density compared with the MSF/PbS/CdS electrode. When white light enters the cell, the CdS QDs absorb the portion of the light with shorter wavelengths. Light with longer wavelengths, which is transmitted through the initial MSF/CdS layer, may then be absorbed by the subsequent MSF/PbS layer. However, in the reversed configuration, only a small portion of the light with shorter wavelengths can be absorbed by the MSF/CdS layer, which induces fewer injected electrons from CdS layer and hence lower current density. From this result, we can suppose that this method has potential for use over the full spectrum, and it may be used to improve the performance of quantum dot solar cells.
Figure 9. UV/vis spectra of single-layer MSF/CdS, MSF/PbS, and bilayer MSF/CdS/PbS.
mesochannel orientation. Under light illumination, the photocurrent responded immediately and increased vertically. When the light was turned off, the photocurrent decreased vertically after a small decay (Figure 8). The observed current decay may be due to a diffusion-related problem.33 As mentioned above, the mesochannel orientation in MSFs is controlled only by the air-flow direction, so it is possible to prepare multilayered MSFs encapsulating different functional molecules. We further investigated the effect of bilayer oriented mesostructured films containing both CdS and PbS on the photoelectrochemical properties of the MSF. PbS shows a broad absorption from 400 to 800 nm, so that coencapsulation of PbS and CdS led to a widening of the light-absorption properties of the MSF. As shown in Figure 9, a cosensitization effect of CdS and PbS was clearly evident from the extension of the absorption range and the increased absorbance. When an MSF/PbS layer was deposited on an MSF/CdS layer, the resulting bilayer film showed an absorption edge close to that of MSF/PbS, but its absorbance was higher than those of single-layer MSF/CdS and MSF/PbS films (Figure 9). Linear-sweep voltammetry measurements under light illumination were also made on bilayer MSF hybrid films with mesochannels parallel to the applied bias. To this end, we prepared bilayer MSFs with different configurations, as shown in Figure 10. In one, the CdS layer is adjacent to the FTO substrate (MSF/ CdS/PbS); in the other, the PbS layer is adjacent to the FTO substrate (MSF/PbS/CdS). For comparison, we also measured
4. CONCLUSIONS In summary, MSF/semiconductor hybrid film has been prepared by incorporating CdS and PbS into the oriented mesochannels of MSFs. The degree of orientation of the MSF was slightly decreased after the introduction of CdS and PbS. The resulting hybrid thin film showed pronounced anisotropic behavior in its electrochemical response as well as in its photo response under light irradiation. We have also successfully selectively positioned two types of QDs, CdS and PbS, in the multilayer oriented mesoporous silica film in order to absorb a broad spectrum of the incident light. Such a strategy leads to the enhancement of the hybrid film to eternal stimuli. This kind of film should be useful for designing anisotropic electrodes and electronic nanodevices. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected]. 816
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’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Science Fund for Distinguished Young Scholars (50925310), the National Science Foundation of China (50902094), 973 project (2009CB93043), and the Shanghai Leading Academic Discipline Project (No. B202).
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