ARTICLE pubs.acs.org/JPCC
First-Principles Investigation on Electronic Properties of Quantum Dot-Sensitized Solar Cells Based on Anatase TiO2 Nanotubes Cunku Dong,† Xin Li,*,†,‡ and Jingyao Qi*,‡ †
Department of Chemistry and ‡State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
bS Supporting Information ABSTRACT: The first-principles quantum mechanical investigation on CdS quantum dots (QDs) adsorbed on anatase TiO2 nanotubes (TiO2NTs) in quantum dot-sensitized solar cells (QDSSCs) was performed using the density functional theory (DFT) approach. The geometry and electronic coupling between a CdS QD and the TiO2NT have been examined for the first time, together with a detailed discussion of interfacial electron transfer and electron transport models along the TiO2NT. It has been found that adsorbate states are introduced in the band gap of the TiO2NT upon the adsorption of a CdS QD on the TiO2NT surface, and an electron transfers from the sulfur atoms of a CdS QD to the conduction band of the TiO2NT upon adsorption of visible light. The unique TiO2NT structure offers a onedimensional directional pathway for electron transport across the semiconductor substrate through titanium dx2 y2 and dz2 orbitals. Our work is of great benefit for understanding the charge separation process at the heterogeneous interface and the key factors to govern the electron transport efficiency in TiO2NTs.
1. INTRODUCTION Solar cells based on semiconductor nanocrystals with small band gaps have attracted great attention over the past decade as a promising alternative to traditional solid-state photovoltaic devices.1 5 Among these photovoltaic devices, quantum dot (QD)sensitized solar cells (QDSSCs) have a photovoltaic system of great interest, which consist of QDs with relatively small band gaps as light-harvesting sensitizers, and oxide semiconductors with large band gaps as a substrate (TiO2 and ZnO).6 9 In contrast with traditional organic and organometallic sensitizers, the QD exhibits unique size-dependent electronic and optical features, including a tunable band gap, high extinction coefficients, multiple exciton generation (MEG), higher stability, and an expanding light wavelength absorption range.10 12 In particular, multiple electron hole pair generation per photon of QDs by the impact ionization effect13,14 could increase the thermodynamic efficiency limit of these devices up to 44%,7 higher than the balance limit calculated by Schockley and Queisser (31%).15 In view of these unique features, QDSSCs have certain advantages over dye-sensitized solar cells (DSSCs) as the most promising third-generation photovoltaic devices. Nevertheless, to date, the energy conversion efficiency of QDSSCs (only about 4%)16 20 is still significantly lower than that of DSSCs (11.4%).21,22 Despite much effort devoted to selecting well-performed QDs and optimizing manufacturing conditions,23 28 photoelectronic conversion efficiency is not yet promoted remarkably. As a matter of fact, the performance of QDSSCs is greatly affected by electron collection in the electrode. This process r 2011 American Chemical Society
has proved much too difficult to control because of charge trapping on QD surfaces and charge recombination during transport through the oxide semiconductor substrate.29 In conventional TiO2 nanoparticle-based QDSSCs, electrons have to transport across a three-dimensional (3-D) nanoparticle boundary, limiting massive electron collection in the electrode. To address this problem, highly ordered one-dimensional (1-D) nanostructures (e.g., nanotubes, nanowires, and nanorods) have been exploited to enhance the electron transport efficiency. The anatase TiO2 nanotube (TiO2NT) has been reported to exhibit excellent photoninduced properties and higher charge collection efficiencies.30 33 The generally accepted explanation into TiO2NT-based QDSSCs with higher charge collection efficiencies is that the nanotube structure could offer a 1-D directional charge-carrier pathway toward electrodes, significantly reducing electron recombination during transport across the semiconductor substrate. Because of the difficulties to assign correct electronic responses at an atomistic resolution, the key factor in governing the electron transport efficiency in TiO2NTs is still unclear. Fortunately, first-principles simulations can fundamentally extend our ability to understand the electronic and optical properties of the photovoltaic conversion process at atomic-scale resolution. Thus far, the first-principles calculations of sensitized solar cells only focus on dye/TiO2 systems. De Angelis et al. reported a series of systematic density functional theory (DFT) computational Received: April 24, 2011 Revised: September 10, 2011 Published: September 12, 2011 20307
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The Journal of Physical Chemistry C investigations into dye sensitizer/TiO2 systems, which unveil the working mechanism of DSSCs and predict their electronic and optical properties.34 Meng and co-workers studied the electronic coupling and charge injection of natural dye/TiO2 nanowire species using time-dependent DFT.35 However, there are few theoretical reports on interfacial electron transfer and electron transport in QDSSCs, which plagues the further development in enhancing the conversion efficiency. Hence, a comprehensive insight into the electronic nature of these systems is very essential for an in-depth understanding of QDSSCs and for the generation of well-performed photovoltaic devices. In this work, we report a first-principles investigation on the adsorption of a QD cluster on the the anatase TiO2NT surface with the aim to study the electronic coupling between QDs and TiO2NTs in QDSSCs. The in situ deposition model is adopted in this study due to its simple synthetic process and homogeneous high surface coverage. CdS QDs (Cd2S2 clusters) are selected as the light sensitizers because of their relatively small band gap and the capacity to harvest photons in the visible and near-infrared regions. On the basis of the construction of modeled singlewalled TiO2NTs, we initially investigate the structural and electronic properties of the Cd2S2/TiO2NT complex. It is determined that adsorbate states are introduced in the band gap of the TiO2NT upon adsorption of a Cd2S2 cluster on the TiO2NT surface. The top of the valence band is dominated by the sulfur 2p orbital, while the bottom of the conduction band is of titanium 3d orbital character. A favored electron transfer from the sulfur 2p orbital to the titanium 3d orbital is expected to take place at the heterogeneous interface upon light adsorption. Significantly, photoexcited electrons could transport along the nanotube cylindrical wall through dx2 y2 and dz2 orbitals, illustrating that the TiO2NT structure offers a 1-D directional pathway for electron transport across the semiconductor substrate. Our work is expected to promote the further development of QDSSCs in enhancing electron transport efficiency and facilitating novel photovoltaic systems.
2. MODEL AND COMPUTATIONAL DETAILS The anatase (101) plane has been shown to demonstrate favorable adsorption of dye molecules and electron-accepting properties from adsorbed dye molecules and, therefore, is the plane from which we constructed our TiO2NT.36 The anatase single-walled TiO2NT (8, 0) was modeled by rolling an anatase (101) sheet along the [101] direction as described in the previous reports.37 39 However, the act of rolling a natively two-dimensional plane into a tubular shape (creation of a TiO2NT) in itself distorts the atomic coordinates compared with the anatase TiO2(101) surface, and therefore, our representative TiO2 geometry is described, most accurately, as a modified TiO2 anatase (101) plane. Interestingly, the implementation of TiO2 nanoparticles or nanotubes in DSSCs or QDSSCs is most commonly done with nonhighly faceted materials, which are also best described by distorted versions of pristine lattice planes. The TiO2NT (8, 0) model with a minimum structural unit (D8d) was adopted for the semiconductor substrate in QDSSCs. The CdS QD with the symmetry of D2h (Cd2S2) was chosen as the sensitizer in this study. Two direct adsorption models of Cd2S2 clusters were considered, perpendicular and parallel to the TiO2NT surface with the aim to investigate the influence on the structural configuration and electronic state exerted by adsorption models.
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All calculations were performed using the plane-wave pseudopotential method within the DFT framework. We used the generalized gradient approximation (GGA) parametrized with Becke’s three-parameter hybrid method with the Lee, Yang, and Parr exchange-correlation functional (B3LYP).40,41 The all-electron double numerical basis set with a polarized function (DNP) was chosen for DFT calculations. The real-space global cutoff radius was set to be 3.70 Å. For geometrical optimizations, the maximal forces exerted on all atoms and maximal displacements were converged to less than 0.054 eV and 0.005 Å, respectively. A tetragonal supercell with the size of 16 Å 25 Å cÅ was set, where c is 16.65 Å, equal to the minimum periodic unit in the TiO2NT (8, 0). The supercell includes 32 titanium and 64 oxygen atoms with the crystal form of (TiO2)32. The Brillouin zone was sampled by 1 1 2 special k-points using the Monkhorst Pack scheme for geometrical optimizations. To calculate the electronic properties of a clean TiO2NT and the Cd2S2/TiO2NT complex, the Brillouin zone was sampled by 4 4 2 special k-points.
3. RESULTS AND DISCUSSION 3.1. Geometric Structures of a Clean TiO2NT. The atomic arrangement of a clean TiO2NT is of great importance for the adsorption of QDs on its surface and 1-D electron transport along the nanotube. Thus, it is necessary to understand the atomic binding nature of TiO2NTs. Figure 1 shows the optimized geometrical configuration of a clean single-walled TiO2NT (8, 0) segment. It is found that the TiO2NT has coordinatively unsaturated atoms, for instance, 5-fold coordinated titanium (Ti5c) and 2-fold coordinated oxygen (O2c) atoms, as well as fully coordinated 3-fold oxygen (O3c). The clean TiO2NT is characterized by a triple-atom layer with the form of a (O Ti O) crystal structure. The inner and outer parts of the triple-atom layer consist of bridging O2c atoms, while the middle part is composed of O3c and Ti5c atoms. Furthermore, we also calculated Ti O bond distances in the TiO2NT to investigate atomic binding properties. The calculated Ti5c O3c bond length along the nanotube axis is calculated to be 1.98 2.08 Å, whereas Ti5c O2c bond distances along the circumference of the nanotube are shorter with respect to those of Ti5c O3c (1.85 Å). It is interesting to note that all Ti atoms in the TiO2NT are undercoordinated (Ti5c), greatly different from the anatase TiO2(101) surface where the fully coordinated titanium (Ti6c) atoms are exposed.39 As a matter of fact, the experimentally synthesized TiO2NT normally has a very large diameter that makes the nanotube surface approach a pristine (101) surface. The large diameter of the TiO2NT with a thick wall may maximize titanium coordination. For the TiO2NT with a relatively thick wall, the coordination of titanium atoms would be maximized with the increase of nanotube diameter as the titanium atoms inside the crystal wall are saturated. However, for thin-walled TiO2NTs, particularly ultrathin-walled TiO2NTs, the unsaturated (or uncoordinated) surface titanium atoms have a relative large proportion compared with the saturated (or coordinated) titanium atoms in the crystal wall. Yuan et al. reported the fabrication of TiO2NTs with an ultrathin tube wall (nearly 10 nm) that could load much more dye molecules and reduce the carrier recombination, leading to more efficient charge separation.25 The synthesized TiO2NT with an ultrathin-wall is beneficial for DSSCs and QDSSCs owing to the large specific surface area.23 28 The single-walled TiO2NT model adopted here is used to simulate ultrathin-walled TiO2NT materials. 20308
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Figure 1. Optimized geometry of a TiO2NT (8, 0) segment with the minimum repeated unit. The side view and top view of the TiO2NT are presented in the left and right panels, respectively. The oxygen atoms are displayed in red, while titanium atoms are presented in light gray. The triple-atom layer is embedded in the grass green circle. The scale bar is for both left and right panels.
Figure 2. Optimized geometries for a Cd2S2 cluster adsorbed on the surface of a TiO2NT (8, 0) in TCper (top panel) and TCpar (bottom panel) adsorption models . The marked atom labels of the TiO2NT (8, 0) in the TCpar model are identical to those in the TCper model.
3.2. Adsorption of a Cd2S2 Cluster on the TiO2NT Surface. It is expected that different adsorption models influence the electronic and optical activity of QDSSCs, similar to that of DSSCs.42,43 In this section, two initial structures of a Cd2S2 cluster on a
TiO2NT in perpendicular (TCper) and parallel (TCpar) adsorption models are considered to determine the most stable configurations. Some other initial structures with different orientations are optimized to these two particular stable structures 20309
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Table 1. Calculated Adsorption Energies (Eads), Bond Distances, and Dihedral Angle in a TiO2NT (8, 0) for the Adsorption of a Cd2S2 Cluster in TCper and TCpar Models a
a
adsorption model
Eads (eV)
Ti5c O3c1 (Å)
Ti5c S1 (Å)
TCper
1.20
2.09
2.60
TCpar
1.28
2.09
2.65
Cd1 O2c1 (Å)
Cd2 O2c2 (Å)
2.37
2.37
Cd2 O3c2 (Å)
S1 Cd1 Cd2 S2 ()
2.39
179.81 154.10
Atom labels in the table are marked in Figure 2.
Figure 3. Total density of states of a clean TiO2NT (8, 0) and the projected DOS on oxygen and titanium atoms. The black curve represents the total DOS of a clean TiO2NT, while the red and blue filled curves indicate the 3d-projected DOS of oxygen atoms and the 2p-projected DOS of titanium atoms, respectively. The Fermi level of an isolated TiO2NT is set at 0 eV, as denoted in the vertical black dotted line. VB and CB represent the mean valence band and conduction band, respectively.
Figure 4. HOMO and LUMO of a clean TiO2NT (8, 0) calculated at the Γ-point. The left panel displays the side view of electron density distributions, while the right panel is the top view of electron density distributions in the HOMO and LUMO.
(TCper and TCpar). It demonstrates that these two specific adsorption models are more preferable for the deposition of a
Cd2S2 cluster on a TiO2NT. Figure 2 shows the optimized geometries of the Cd2S2/TiO2NT complex with two adsorption models. 20310
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Figure 5. Total DOS of a clean TiO2NT (A), total and adsorbate-projected DOS of the Cd2S2/TiO2NT complex (B), and orbital contributions to the adsorbate-projected DOS (C). The black solid curves in (A) and (B): total DOS. The red filled and solid curves in (B) and (C): adsorbate-projected DOS. In (C), the 2p-projected DOS of sulfur atoms and the 4d-projected DOS of cadmium atoms are displayed in yellow and green filled curves, respectively. The dashed boxes at the top of the valence band represent parts of the DOS free of substrate states.
The calculated geometrical parameters of the Cd2S2/TiO2NT complex and adsorption energies for the corresponding models are reported in Table 1. In the TCper adsorption model, the sulfur atom (S1) of the absorbate is bound to the surface Ti5c with a Ti5c S1 bond distance of 2.60 Å, while the cadmium atom (Cd2) is bound to O3c2 with a Cd2 O3c2 bond length of 2.39 Å. The dihedral angle — S1 Cd1 Cd2 S2 is calculated to be 179.81, almost the same as that in the isolated Cd2S2 cluster. Hence, the formation of Ti5c S1 and Cd2 O3c2 bonds exerts a negligible influence on the geometric configuration of the Cd2S2 cluster in the TCper adsorption model. In the case of the TCpar adsorption model, the S1 atom bonds to the surface Ti5c with a bond length of 2.65 Å, whereas the Cd2S2 cluster forms two Cd O2c bonds with its neighboring O2c (Cd1 O2c1 and Cd2 O2c2) with a uniform bond distance of 2.37 Å. As shown in Figure 2, the Cd2S2 cluster is curving upward with a dihedral angle of 154.10 due to the formation of Cd O2c bonds. In both TCper and TCpar adsorption models, the underlying surface O3c atom bound to S1 (O3c1) is pushed down by about 0.5 Å from the surface due to the formation of a new Ti S bond. The calculated adsorption energies demonstrate that the TCpar configuration with an adsorption energy of 1.28 eV is more stable than the TCper configuration,
which is also confirmed by the formation of more chemical bonds. Nevertheless, the TCper configuration is only 0.08 eV lower in energy, indicating a possible coexistence of TCpar and TCper configurations in the Cd2S2/TiO2NT complex. These results clearly show that the Cd2S2 cluster can strongly couple with the TiO2NT, which is the requisite for excited electron injection into the substrate. Furthermore, TCper and TCpar configurations exhibit almost the same adsorption fashion where cadmium and sulfur atoms tend to bond to bridging O2c and Ti5c respectively. 3.3. Electronic Structures of a Pristine TiO2NT and Cd2S2/ TiO2NT Complexes. As the substrate for anchoring QDs in QDSSCs, it is of great importance to understand the electronic nature of a clean TiO2NT, which plays a key role in the electron transfer at the heterogeneous interface and electron transport across the substrate.10 Figure 3 reports the total density of states (DOS) of a clean TiO2NT and the projected DOS on oxygen and titanium atoms. We can notice the typical shape and intensity of the TiO2 crystal structure in the total DOS curves with distinct valence and conduction bands separated by a notable wide band gap. It is well known that titanium 3d orbitals in the TiO2 crystal bulk are split into a double high-energy level eg (dz2 and dx2 y2) and a triple lowerenergy level td (dxy, dyz, and dxz). From the total DOS, we note the similar energy level split for a clean TiO2NT near the Fermi level. 20311
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Figure 6. Γ-point orbital-isoamplitude surfaces of the HOMO and HOMO-1 for the Cd2S2/TiO2NT complex in the TCper model.
The valence band is dominated by the oxygen 2p orbital with a small contribution from the titanium 3d orbital (td). By contrast, the conduction band is dominated by the titanium 3d orbital (eg) with a small contribution from the oxygen 2p orbital. These features are quite similar to those of the anatase TiO2 surface slab or bulk.44 46 The TiO2NT (8, 0) has a direct band gap of 2.65 eV, in good agreement with the previous theoretical results.39 As the crystal orbitals at the top of the valence band and the bottom of the conduction band in the TiO2NT are of great importance for the electronic and optical activity in QDSSCs, we calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a clean TiO2NT at the Γ-point. In Figure 4, it is obviously seen that the electron density of the HOMO is localized on oxygen atoms with 2p orbital character, while the LUMO has the feature of titanium dx2 y2 orbitals. Moreover, the HOMO is mainly contributed from the oxygen 2p orbital in the middle and inner layers. These results are in line with the DOS analysis stated above. Figure 5 reports the total DOS and adsorbate-projected DOS for the adsorption of a Cd2S2 cluster on the TiO2NT surface in the TCper adsorption model. Interestingly, the TiO2NT retains the features of an oxide semiconductor after the deposition of a Cd2S2 cluster on its surface as the total DOS curve of the
substrate still has the typical intensity and shape of a clean TiO2NT. It is evident that the adsorbate states are introduced in the band gap of the clean TiO2NT from a comparison of the total DOS of the clean TiO2NT and the Cd2S2/TiO2NT complex. Moreover, the top part of the valence band of the Cd2S2/ TiO2NT complex clearly extends to the band gap of the clean TiO2NT, resulting in a decrease in the calculated band gap: from 2.65 eV for the clean TiO2NT to 0.89 eV for the Cd2S2/TiO2NT complex. The adsorbate-induced gap states have been also reported for dyes or metal deposition at the anatase TiO2 surface.46,47 These introduced states play a key role in optical excitation as the resulting narrow band gap could lead to the red shift of adsorption spectra of QDSSCs toward the visible region. Our calculated results are in line with the experimental observations where the adsorption spectrum is extended to the visible region after the deposition of CdS QDs on TiO2NTs.48 We examine the projected DOS of the adsorbate and substrate to understand the electronic coupling and orbital contributions. The adsorbate-projected DOS is the total DOS of the Cd2S2/ TiO2NT complex projected on the adsorbate Cd2S2 cluster. As observed in Figure 5B, the adsorbate-projected DOS is dispersed not only in the band gap of the clean TiO2NT but also at the top part of the valence band and the bottom part of the conduction 20312
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Figure 7. Γ-point orbital-isoamplitude surfaces of the LUMO and LUMO+1 for the Cd2S2/TiO2NT complex in the TCper model.
band. We can also see that the adsorbate-induced states couple with TiO2NT states in a wide energy range, indicating a strong electronic coupling between the adsorbate Cd2S2 cluster and the substrate TiO2NT. Upon adsorption of Cd2S2 on the clean TiO2NT surface, the Fermi level moved from the top of the valence band to the middle of the band gap and the band gap was partly filled by the Cd2S2 states, which are almost sulfur 2p-state contributions. Obviously, the top part of the valence band is dominated by the adsorbate states, and the substrate TiO2NT states lie in the bottom of the conduction band. This indicates a favored electron transfer from the sulfur 2p orbital of the Cd2S2 cluster to the conduction band of the TiO2NT, in agreement with the working principles of QDSSCs, in which, upon the adsorption of visible light, the electrons of the QD are excited to its conduction band and then injected into the conduction band of the semiconductor substrate. Figure 6 displays the HOMO and the next highest occupied molecular orbital (HOMO-1) of the Cd2S2 cluster adsorbed on the TiO2NT in the TCper adsorption model. By comparing the HOMO and HOMO-1 of the Cd2S2/TiO2NT complex with those of an isolated Cd2S2 cluster (see Figure S1, Supporting Information), it is interesting to note that these two unoccupied molecular orbitals are not influenced significantly by its adsorption on the TiO2NT surface. The electronic densities of the HOMO and HOMO-1 are almost localized around the Cd2S2 cluster with the character of the sulfur 2p orbital and the cadmium 4d orbital, in agreement with DOS analysis. This suggests that the electron in the ground state of the Cd2S2/TiO2NT complex is
localized on the Cd2S2 cluster that serves as the light sensitizer (i.e., electron donor) in the photoexcitation process. Figure 7 reports the LUMO and the next lowest occupied molecular orbital (LUMO+1) of the Cd2S2/TiO2NT complex. It is noted that the LUMO and LUMO+1 are dominated by TiO2NT states with the unique character of the conduction band of the TiO2NT. The calculated results demonstrate that the excited electron is injected into the conduction band of the TiO2NT upon photon adsorption, resulting in the charge separation at the Cd2S2/TiO2NT interface. Hence, our calculated results are in line with the classical chemical understanding of QDSSCs. The LUMO of the Cd2S2/TiO2NT complex has the character of the uniform dx2 y2 orbitals of titanium atoms in the cylindrical wall. The LUMO+1 is dominantly contributed from titanium dx2 y2 orbitals mixed with very limited dz2 orbitals. The similar picture holds in the case of other LUMOs in higher energy (see Figure S2, Supporting Information). We can see that the injected electron could transport across the cylindrical wall of the TiO2NT in the specific direction toward the electrode mainly through titanium dx2 y2 orbitals. The electron transport model in ultrathin TiO2NTs is very different from within the commonly used TiO2 nanoparticles. In the nanoparticles, the excited electron is localized on titanium atoms in the 3-D crystal network and, therefore, transports through titanium dx2 y2 and dz2 orbitals in random directions, increasing the probability of charge recombination. In the case of the TCpar adsorption model, the adsorbateintroduced states also emerge in the band gap of the TiO2NT. The shape and strength of the DOS of the Cd2S2/TiO2NT 20313
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Table 2. Calculated Atomic Charges (|e |) for Isolated and Adsorbed Cd2S2 Clusters atoms
isolated
adsorbed TCper
a
TCpar
S1a
0.606
0.538
0.560
S2
0.606
0.566
0.594
Cd1 Cd2
0.606 0.606
0.611 0.831
0.740 0.739
Net charge
0
0.344
0.325
Atom labels in the table are marked in Figure 2.
complex are similar to those in the TCper model (see Figure S3, Supporting Information). In addition, the adsorbate states also couple with the TiO2NT states in a wide energy range. By comparing with the DOS and adsorbate-projected DOS in the TCper model, the cadmium 4d orbital contributes less to the introduced states in the original band gap of the TiO2NT. The electron density distribution of the Cd2S2 cluster in HOMOs changes significantly with respect to the corresponding unoccupied molecular orbitals of the isolated cluster (see Figure S4, Supporting Information). It demonstrates that TiO2NT states interact with adsorbate states strongly, leading to the structural deformation and the change of orbital shape. We could also obtain the same conclusion in the electron transfer at the heterogeneous interface and transport along the TiO2NT as in the TCper model by analyzing the DOS and crystal orbitals. In addition, the electronic coupling of the adsorbate substrate complex can be obtained by analyzing the Mulliken charge population of both isolated and interacting systems. Table 2 lists the Mulliken atomic charges for the sulfur and cadmium atoms in isolated and adsorbed Cd2S2 clusters, respectively. The atomic charges in the Cd2S2 cluster significantly changes upon its adsorption on the TiO2NT. Compared with the isolated cluster, the atomic charges of the adsorbed Cd2S2 cluster change in both the TCpar model and the TCper model. It demonstrates a strong electronic coupling between the adsorbate and the substrate in both cases. The total charge of the Cd2S2 cluster significantly varies, from 0 |e | to 0.344 |e | in the TCper model and 0.325 |e | in the TCpar model, respectively, indicating an important electronic rearrangement between the adsorbate and the substrate.
4. CONCLUSION In this work, an investigation of a CdS cluster (Cd2 S 2 ) adsorbed on an anatase TiO2NT has been reported using the hybrid DFT approach in a periodic boundary condition. Two models (TCper and TCpar) are studied with an aim to explore the influence on the electronic nature of the Cd2S2/TiO2NT. We find that the Cd2S2 cluster bonds to Ti5c and bridging O2c atoms on the surface of the TiO2NT and a possible coexistence of TCpar and TCper configurations. Our analysis to the electronic structure of the Cd2S2/TiO2NT system is supported by the DOS and crystalline orbitals calculated at the Γ-point. Our work shows that adsorbate states are introduced in the original band gap of the TiO2NT upon adsorption of a Cd2S2 cluster on the TiO2NT, which could result in the red shift to the visible region for sunlight harvesting. The top of the valence band is dominated by the sulfur 2p orbital, while the bottom of the conduction band is of titanium 3d-orbital character. By investigating Γ-point crystalline
orbitals, an electron is found to transfer favorably from HOMOs (centered on the Cd2S2 cluster) to the conduction band of TiO2NT. Exactly, an electron transfers from sulfur 2p orbitals of the adsorbed Cd2S2 cluster to the titanium 3d orbital of the substrate, resulting in the charge separation at the heterogeneous interface. Moreover, the photoexcited electron could transport along the cylindrical ultrathin wall of the TiO2NT through titanium dx2 y2 and dz2 orbitals, and the TiO2NT structure offers a 1-D directional pathway for electron transport across the substrate. Ultrathin-walled TiO2NTs are beneficial not only for sensitizer adsorption but also for efficient electron transport. Experimentally synthesized ultrathin-walled TiO2NTs are expected to have great potential application in QDSSCs with better performance. Our study provides a computational approach to correctly depict the electronic coupling in the QDs/TiO2NT system and is expected to promote the further development of these photovoltaic devices in enhancing electron transport efficiency and to facilitate the design of new QDSSCs.
’ ASSOCIATED CONTENT
bS
Supporting Information. HOMO and HOMO-1 of an isolated Cd2S2 cluster, Γ-point orbital-isoamplitude surfaces of HOMOs and LUMOs for the Cd2S2/TiO2NT in the TCpar model, TDOS and adsorbate-projected DOS of the Cd2S2/ TiO2NT for the TCpar model, and HOMOs and LUMOs of the Cd2S2/TiO2NT in the TCpar adsorption model. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (X.L.),
[email protected] (J.Q.).
’ ACKNOWLEDGMENT This work was supported by the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2010TS06). The computations were supported by the Harbin Institute of Technology High Performance Computer Center. ’ REFERENCES (1) Nozik, A. J. Inorg. Chem. 2005, 44, 6893–6899. (2) Nozik, A. J. Chem. Phys. Lett. 2008, 457, 3–11. (3) Beard, M. C.; Ellingson, R. J. Laser Photonics Rev. 2008, 2, 377–399. (4) Luther, J. M.; Law, M.; Song, Q.; Perkins, C. L.; Beard, M. C.; Nozik, A. J. ACS Nano 2008, 2, 271–280. (5) Hillhouse, H. W.; Beard, M. C. Curr. Opin. Colloid Interface Sci. 2009, 14, 245–259. (6) Nozik, A. J. Phys. E 2002, 14, 115–120. (7) Klimov, V. I. J. Phys. Chem. B 2006, 110, 16827–16845. (8) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737–18753. (9) Hodes, G. J. Phys. Chem. C 2008, 112, 17778. (10) Mora-sero, I.; Gimenez, S.; Fabregat-santiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Acc. Chem. Res. 2009, 42, 1848–1857. (11) Yu, W. W.; Qu, L.; Guo, W.; Pen, X. Chem. Mater. 2003, 15, 2854–2860. (12) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Humphry-Baker, R.; Comte, P.; Aranyos, V.; Hagfeldt, A.; Nazeeruddin, M. K.; Gr€atzel, M. Adv. Mater. 2004, 16, 1806–1811. (13) Schaller, R. D.; Sykora, M.; Pietryga, J. M.; Klimov, V. I. Nano Lett. 2006, 6, 424–429. 20314
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