11734
J. Phys. Chem. C 2007, 111, 11734-11741
PbSe Nanocrystal/TiOx Heterostructured Films: A Simple Route to Nanoscale Heterointerfaces and Photocatalysis Congjun Wang, Kwan-Wook Kwon, Michael L. Odlyzko, Bo Hyun Lee, and Moonsub Shim* Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ReceiVed: April 19, 2007; In Final Form: May 25, 2007
A simple approach to heterostructured thin film catalysts consisting of PbSe nanocrystals and layer-by-layer deposited TiOx is presented. Strong quantum confinement raises the conduction band of PbSe nanocrystals above that of TiOx, leading to a type II band offset. Photogenerated electrons in PbSe nanocrystals are transferred to catalytic TiOx which in turn initiate catalysis. Photocatalytic activity of the heterostructured films is found to be dependent on the size of the PbSe nanocrystals with the onset of catalysis coinciding at a photon energy of ∼2.5 times the band gap of the nanocrystals. Photocatalysis in the visible spectral region out to 650 nm is demonstrated.
1. Introduction TiO2 is one of the best-known catalysts.1 Technological advances in multiple areas from solar energy conversion to environmental remediation have been and continue to be made exploiting the exceptional properties of TiO2.2 However, as with most metal oxides, one limitation of TiO2 is the large band gap which renders clean solar energy driven processes inefficient.3 To activate TiO2 in the visible, several approaches have been examined. In photovoltaics, sensitization with visible light absorbers such as organic dye molecules is the prevalent solution.4 In photocatalysis, strategies include impurity doping and sensitization with dyes or pigments as well as developing composites with metal nanoparticles.5 While recent advances in doping TiO2 with impurities such as N and S to achieve new absorption features in the visible are promising,6 these approaches may have inherent limitations. For example, the new optical transitions in N-doped TiO2 involve localized N 2p states or other doping induced defect states rather than band gap narrowing for the anatase phase.7 For rutile TiO2, undesired widening of the band gap is predicted.7a Absorption crosssections in the visible may therefore be inevitably small. In addition, optical transitions involving localized states may leave the charge carriers inaccessible for subsequent catalysis. In these regards as well as in enhancing charge separation, sensitization with smaller band gap semiconductors may provide advantages. In particular, semiconductor nanocrystals (NCs) with large absorption cross-sections in the visible can provide efficient light absorption in addition to providing large interfacial areas for charge separation. In photovoltaics, many examples exist where semiconductor NCs are used in place of dye sensitizers especially to circumvent photobleaching problems.8 In addition to spatial proximity, type II band offset where the lowest electron state of the NC lies above the conduction band edge of TiO2 is required. Heterointerfaces with II-VI and IV-VI semiconductor NCs such as CdSe and PbS NCs have been shown to exhibit effective photoinduced charge separation.8 Since the very first requirement in both photovoltaics and photocatalysis is the efficient photoinduced charge separation, similar systems may be exploited for visible light induced catalysis. However, most studies * To whom correspondence
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along this line have not extended beyond CdS-based systems due to photostability problems of non-oxide semiconductors.5 Additives such as hole scavengers can prolong lifetimes.9 While photostability of sulfides and selenides, especially in aqueous systems, remains a challenge, at least as a model system, heterostructures incorporating quantum-confined semiconductor NCs are now particularly attractive since recent advances in synthesis allow NCs of various compositions with wellcontrolled size, size distribution, and shape which in turn allow tunable electronic structure. Combined with this size and shape dependent electronic structure, well-defined morphologies of heterodimers and other higher order nanocrystal heterostructures10 may provide exquisite control over the interfacial structure and directional charge separation between various catalytic materials. Hence semiconductor NC/TiO2 based heterostructures may serve as prototypes in rational design of tunable photocatalysts. Here, we demonstrate a facile route to heterostructured thin films of PbSe NCs and amorphous TiOx that exhibit photocatalytic activity in the visible spectral range beyond 600 nm. In addition to this unusual photoactivity, efficient charge separation in this system may be of use in carrier extraction for future photovoltaics that exploit PbSe or related NCs. 2. Experimental Section 2.1. Preparation and Characterization of PbSe NC/TiOx Photocatalyst. All chemicals except Degussa P25 TiO2, which was supplied by Degussa Corp., were purchased from Aldrich and Alfa Aesar and used without further purification. PbSe NCs were synthesized following established methods.11 Thin films of PbSe NCs were prepared by drop casting a hexane solution of NCs on glass slides. The glass slides with PbSe NC films were then immersed in a 10 mM 3-mercapto-1,2-propanediol solution in ethanol for ∼12 h. After rinsing with ethanol, amorphous TiOx was subsequently grown in a layer-by-layer fashion following ref 12. The PbSe NC films were first dipped in a 100 mM titanium(IV) isopropoxide (TIP) solution in 1:1 (v:v) toluene:ethanol for 10 min, rinsed with ethanol, and hydrolyzed in H2O for 1 min. We refer to these steps as one layer of TiOx deposition. For each additional layer of TiOx, the TIP/ethanol/H2O step was repeated. For a control experiment, PbSe NC/SnOx was also prepared. SnOx layers were deposited in a similar manner using tin isopropoxide in isopropyl alcohol
10.1021/jp073022h CCC: $37.00 © 2007 American Chemical Society Published on Web 07/13/2007
Photocatalysis with PbSe NC/TiOx Heterostructures
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SCHEME 1: Schematics of Photocatalysis Experiment Setup (Left) and the Structure of the Heterostructured Photocatalyst (Right)
as the Sn source. The UV/visible and near-infrared (IR) spectra were obtained with an Agilent 8453 diode array UV/vis spectrometer and a Thermo Nicolet Nexus Fourier transform IR spectrometer equipped with an InGaAs detector, respectively. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were carried out on a JEOL 2010 LaB6 TEM and a JEOL 2010F (S)TEM, respectively, both operating at 200 kV. X-ray photoelectron spectra (XPS) were recorded with Physical Electronics PHI 5400 X-ray photoelectron spectrometer. X-ray diffraction patterns were measured with Rigaku Geigerflex with a D-MAX system. Stanford Research Systems Inc. QCM 100 and QCM 200 quartz crystal microbalances were used to investigate the layer-by-layer deposition of TiOx and SnOx, respectively. 2.2. Photocatalysis Measurement. To evaluate the photocatalytic activity of heterostructured PbSe NC/TiOx films, photodegradation of rhodamine 6G (0.01 mM in water) was examined. A 200 W Spectra-Physics Xe arc lamp was used as the light source. Edmund Optics interference bandpass filters were utilized to select illumination wavelengths. The bandpass filters have full width at half-maximum (fwhm) of 80 nm except for the 400 nm filter, which has a fwhm of 50 nm. Two glass slides (∼5 × 25.4 mm2 each) with the catalyst deposited were placed against the two sidewalls of a 10 mm path length reduced volume fluorescence cuvette containing ∼0.5 mL of rhodamine 6G solution. The two slides were then illuminated with visible light perpendicular to the cuvette path length direction, and the absorption of rhodamine 6G was monitored periodically by taking UV/vis spectra along the path length direction of the cuvette (Scheme 1). The typical ratio of dye molecules to PbSe NCs is estimated to be about 100-1000 to 1 on the basis of the optical density of the NC films. Prior to illumination, the slides with the catalyst films were immersed in the rhodamine 6G solution in the dark up to ∼10 h to ensure that the dye adsorption on the catalyst surface has reached equilibrium. This was ensured by periodically collecting the absorption spectra of the solution until the dye absorption stabilized. To investigate the origin of the photocatalytic activity of PbSe NC/TiOx heterostructured films, various control experiments were performed. The “dye only” samples correspond to 0.01 mM rhodamine 6G solution in water that is exposed directly to light without any catalyst. The “TiOx only” sample corresponds to layer-by-layer deposited TiOx films without PbSe NCs as the catalyst equivalent. For these samples, the same three layers of TiOx deposition as the PbSe NC based heterostructured catalyst including 3-mercapto-1,2-propanediol was carried out on a 1 nm electron beam evaporated Au film with 1 nm Ti adhesion layer on a glass slide. The “Degussa” sample corresponds to commercial Degussa P25 TiO2 powder deposited directly on glass slides without PbSe NCs. To prepare the “bulk PbSe/TiOx” samples, macroscopic pieces of PbSe (millimeters in size) were ground to a powder, sonicated in ethanol, and drop-dried on a glass slide. The resulting film was processed
in the same manner as the PbSe NC based catalysts including the thiol adsorption. The “PbSe NCs only” samples correspond to PbSe NCs on glass slides with subsequent thiol adsorption treatment but without any TiOx. The “N2 saturated” samples correspond to PbSe NC/TiOx thin film catalysts that have been examined under N2 atmosphere with the dye solution being purged with N2 prior to and during the photocatalysis step. The “PbSe NC/SnOx” samples were prepared using layer-by-layer deposited SnOx on PbSe NC films on glass slide as described earlier. 3. Results and Discussion Charge separation across heterointerfaces of nanoscale semiconductors is of increasing interest for both photovoltaic and photocatalytic applications. To exploit large surface and interfacial areas afforded by nanometer length scale for photocatalysis, we examine thin films of semiconductor NCs and TiOx as a model system. A simple all solution deposition and large surface/interface areas of such films are particularly well-suited to screen various heterojunctions of nanoscale materials for catalytic activity. We first discuss the structure and composition of the thin film catalysts. We then discuss the catalytic activity of PbSe NC/TiOx thin films in the visible spectral region. To elucidate the possible mechanism of photocatalysis, the results obtained on various control experiments as well as the effect of PbSe NC size are discussed. 3.1. Structure and Composition of PbSe NC/TiOx Heterostructured Films. Figure 1 shows the layer-by-layer deposition of TiOx on a PbSe NC film followed by QCM measurements. On the basis of the measured frequency shifts, there is about a 5 times larger amount of TiOx deposited in each of the first two layers than the subsequent layers. This may be understood by considering the fact that the surface area of the PbSe NC film is larger initially compared to those of flat
Figure 1. Quartz crystal microbalance frequency shifts as a function of deposition of TiOx layers on PbSe films.
11736 J. Phys. Chem. C, Vol. 111, No. 31, 2007
Figure 2. XPS spectra of 4 nm PbSe NCs (A), three layers of TiOx only (B), and PbSe NCs with three layers of TiOx (C). All samples were prepared on heavily doped Si substrate.
surfaces. After deposition of two layers of TiOx, the surface area accessible for further deposition is reduced and becomes constant, resulting in consistent deposition of a smaller amount of TiOx after the second layer. The thickness of the TiOx film deposited in each cycle after layer 2 can be estimated from the relation -∆f ) Cftd, where -∆f is the measured frequency shift of ∼100 Hz, Cf ) 56.6 Hz µg-1 cm2 is the crystal sensitivity factor, t is the film thickness, and d is the density of TiOx (1.7 g/cm3 for bulk TiO2 based gel12b has been used here). The directly calculated thickness t ∼ 10 nm is ∼10 times larger than the previously reported value using similar technique.12b This may be attributed to the fact that the roughness and the porosity of the PbSe NC film significantly increase the surface area for TiOx deposition even after the first two layers compared
Wang et al. to a flat surface. Therefore, we estimate the actual TiOx thickness to be ∼1 nm for each layer deposition. To verify the composition of the heterostructured catalyst films, XPS spectra of PbSe NCs (Figure 2A), TiOx directly deposited on the substrate without NCs (Figure 2B), and the catalytically active heterostructured PbSe NC/TiOx film (Figure 2C) have been obtained. All distinguishable peaks are labeled with the corresponding elements. The results are consistent with TiOx species deposited on the PbSe NC films. EDS shown in Figure 3 further confirms the presence of TiOx. In addition, EDS collected at the spot labeled “B” in the right panel of Figure 3 indicates that TiOx is deposited even on parts of the substrate where there are no NCs present. A general requirement for exploiting quantum confinement effects in semiconductor NCs is that the processing conditions do not deteriorate the quality of PbSe NCs. For developing heterostructured photocatalysts, size, size distribution, high crystallinity, and optical absorption features must be maintained. TEM images shown in Figure 4 confirm that PbSe NCs maintain their structure, size, and size distribution after the layer-by-layer deposition steps with amorphous TiOx surrounding the NCs. Comparison of absorption spectra in solution and on glass slide after layer-by-layer deposition of TiOx shown in Figure 5 verifies that the strong optical absorption in the visible and the clear interband exciton transition of the PbSe NCs in the near IR are retained throughout the catalyst film fabrication steps. The high crystallinity of PbSe NCs even after TiOx deposition is evident in the XRD patterns (Figure 6). However, no diffraction pattern corresponding to either anatase or rutile TiO2 is observed in these heterostructured catalyst films. Therefore, we conclude that the as grown TiOx films are amorphous. Although crystalline TiO2 may be preferred for higher activity, amorphous TiOx is also significantly catalytically active.13 The strong confinement effect in high-quality PbSe NCs, on the other hand, is an essential component for forming type II band offset with TiOx for photoinduced charge separation. Therefore, no attempt has been made to convert amorphous TiOx to crystalline TiO2 by high-temperature annealing which can lead to sintering of NCs. 3.2. Photocatalysis with PbSe NC/TiOx Heterostructured Films. While the details of structure and composition discussed above verify that heterostructured thin films consisting of high-
Figure 3. EDS of PbSe NCs/TiOx. The image on the right indicates the locations of the sample where the corresponding spectra were taken.
Photocatalysis with PbSe NC/TiOx Heterostructures
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Figure 4. TEM images of PbSe nanocrystals before (A) and after (B) growth of three layers of TiOx.
Figure 5. UV/visible/near-IR spectrum of a film of 4 nm PbSe NCs with three layers of TiOx. The dotted curve is the absorption spectrum of the PbSe NCs in hexane solution. Spectra are offset for clarity.
Figure 6. XRD pattern of PbSe NCs with three layers of TiOx. Also shown are the expected patterns of PbSe and TiO2.
quality PbSe NC solids with a few layers of amorphous TiOx can be readily obtained, many additional requirements need to be met in order to enable visible light induced catalysis. The first such requirement is the efficient photoinduced charge separation. While bulk PbSe is expected to exhibit type I band offset with TiO2 and therefore no charge transfer is expected, the strong confinement effect leads to the 1Se state of PbSe NCs of diameter
