The Electronic Structure and Photoinduced Electron Transfer Rate of

Subscriber access provided by UNIV OF WESTERN ONTARIO

Article

The Electronic Structure and Photoinduced Electron Transfer Rate of CdSe Quantum Dots on Single Crystal Rutile TiO: Dependence on the Crystal Orientation of the Substrate 2

Taro Toyoda, Witoon Yindeesuk, Keita Kamiyama, Kenji Katayama, Hisayoshi Kobayashi, Shuzi Hayase, and Qing Shen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09528 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 13, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

The Electronic Structure and Photoinduced Electron Transfer Rate of CdSe Quantum Dots on Single Crystal Rutile TiO2: Dependence on the Crystal Orientation of the Substrate Taro Toyoda,*,†,‡ Witoon Yindeesuk,†Keita Kamiyama,§ Kenji Katayama,# Hisayoshi Kobayashi,& Shuzi Hayase, //, ‡ and Qing Shen*,†,‡ †

Department of Engineering Science, The University of Electro-Communications 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan

§

Bunkoukeiki Co., Ltd, 4-8 Takakura, Hachioji, Tokyo 192-0033, Japan

#

Department of Applied Chemistry, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japan

&

Department of Chemistry and Materials Technology, Kyoto Institute of Technology,

Matsugasaki , Sakyo-ku, Kyoto 606-8585, Japan

//

Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology 2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan

‡

Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 45

ABSTRACT: Semiconductor quantum dots (QDs) have many desirable characteristics for use as sensitizers, such as enabling tuning of the bandgap based on the quantum confinement effect, a higher extinction coefficient, and facilitating charge injection as a result of the large dipole moment. Despite these potential advantages, no major advance in the efficiency of quantum-dotsensitized solar cells (QDSCs) has yet been reported. The poor efficiency can be attributed to electron transfer (ET) reactions that compete with the ideal energy generation cycle in QDSCs. Despite the great technological significance, the interfacial ET between QDs and inorganic species remains poorly understood. In this paper, we describe the electronic structure and the interactions between multiple sized CdSe QDs and single crystal rutile-TiO2 with (001), (110), and (111) orientations. Single crystal TiO2 is well characterized and is not only ideal for comparing the amount and the structure of the QDs, but is also useful for studying ET reactions. The rate of adsorption of CdSe QDs depends on the crystal orientation, although the average increase in diameter of the QDs is independent of the crystal orientation. The HOMO level is independent of the adsorption time. On the other hand, the value of the HOMO level depends on the crystal orientation of the R-TiO2 substrate. The ET rate constant increases as the change in free energy increases, and depends on the crystal orientation. This suggests that the mixing of the wavefunctions between the conduction band in the R-TiO2 and the LUMO level in the CdSe QDs depends on the crystal orientation.


2

Page 3 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION The increasing demand for renewable and low-cost energy has engendered some outstanding research in the field of solar cells. Nowadays, an intense effort aimed at developing thirdgeneration solar cells is being undertaken. Much attention has been devoted to dye-sensitized solar cells (DSCs) made from nanostructured TiO2 electrodes, firstly because of their high photovoltaic conversion efficiency, which exceeds 10%,1 and secondly, because the process to manufacture them is very simple. In DSCs, the application of organic dye molecules as a photosensitizer, nanostructured TiO2 as an electron transport layer, and an iodine redox couple for hole transport, has improved the light harvesting efficiency. The main undertaking for those developing next-generation solar cells is to improve the photovoltaic conversion efficiency, together with the long term stability. Another possible approach for third-generation solar cells is to replace organic dyes by inorganic substances with strong optical absorption characteristics and extended long term stability. When semiconductor materials are reduced to the nanoscale, thus creating quantum dots (QDs), new advantageous physical and chemical properties are realized as a consequence of the quantum confinement effect. Recently, as an alternative to organic dyes, QDs have been studied for their light harvesting capability and are an attractive alternative to molecular dyes as sensitizers (the application of quantum dots to sensitize solar cells: QDSCs).2-17 Based on the quantum confinement effect, QDs have desirable characteristics as sensitizers, since their bandgap (or HOMO-LUMO gap) can be tuned by controlling the size of the dots – the ability to design systems and devices with tailor-made electronic properties.18 This can be used to match the absorption spectrum to the spectral distribution of solar light. Moreover, compared to conventional metal-organic dyes, QDs possess higher extinction coefficients, the possibility of suppressing charge recombination, and the facilitation of charge


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 45

injection processes as a result of the large dipole moments created in the QDs during the electron-hole creation and separation processes,19,20 and, furthermore, an improved stability due to the inorganic nature of the QDs. These characteristics of QDs may be used to boost the lightto-energy conversion efficiency of QDSCs. Despite such potential advantages, no major advance in the photovoltaic conversion efficiency of QDSCs that equals or exceeds typical Ru-based DSCs (maximum value: ~ 12%) has yet been reported. The poor photovoltaic conversion efficiency can be attributed to electron transfer (ET) reactions that compete against the ideal energy generation cycle in QDSCs. Despite the great technological significance, interfacial ET remains poorly understood. This is mainly due to a lack of understanding of the surface chemistry of QD adsorption in nanocrystalline TiO2 films. Although the photovoltaic conversion efficiencies of QDSCs still lag behind those of DSCs, significant attention has been drawn to QDSCs as a result of their excellent characteristics, and they are regarded as promising thirdgeneration photovoltaic devices.12 Fundamental studies are urgently needed to shed light on the underlying physics and chemistry governing the poor initial performance of QDSCs when compared to DSCs. A fundamental understanding of the electronic structure of QDs adsorbed on nanocrystalline TiO2 substrates is lacking, and this deficiency needs to be addressed. Although the electronic interactions between QDs and organic molecules are well established,21,22 there have not been many studies on the interactions between QDs and inorganic species.10,23 Such interactions are fundamentally different from those in QD-organic molecular systems because inorganic materials possess continuum electronic states,10,24 as opposed to discrete states inherent to molecular acceptors. When QDs are implemented in QDSCs, ET reactions that occur at the interfaces between the TiO2 substrate and the QDs (nanohetero-structure) are intimately involved


4

Page 5 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with its function. The most important stages that determine the efficiency of QDSCs occur near the TiO2 surface. ET plays a key role in many research areas, and numerous efforts have been devoted to exploring the mechanisms for it. As a consequence, understanding the factors that drive ET in a system is critical to better understand and further exploit the unique properties of QDs. We know that nanostructured TiO2 plays a key role, since it offers a large surface area to adsorb a large amount of QDs for light harvesting. Nanoparticulate TiO2 collects the electrons from the QDs and transfers them to the electrodes.25 However, the structure and distribution of QDs on nanoparticulate TiO2 surfaces is difficult to determine since nanoparticulate TiO2 is a polycrystalline assembly and the surface is not flat. Nowadays anatase type TiO2 films are made of nanoparticles and exhibit random stacking of the {101} facets.26 Recently, high efficiency PbS QD heterojunction solar cells using anatase (001) TiO2 nanosheets have been reported, suggesting high reactivity of the (001) surface, with better photovoltaic performance (~ 4.7%) than TiO2 nanoparticles27 due to the higher ionic charge of (001).28 Also, a significant enhancement of the photoexcited ET from fluorophores to TiO2 nanocrystals via the reactive {001} facet with a factor of more than 10 in the quenching rate constant has been shown.29 Parkinson’s group has used dye and several QDs to sensitize single crystal surfaces to create a simplified model system for investigating the basic processes associated with ET into single crystal oxide electrodes.30-33 Well-characterized single crystal TiO2 is not only ideal for correlating the amount and structure of the QDs,34 but is also useful for studying the interactions between the QDs and TiO2, since the electronic structure of the surface of single crystal TiO2 has been well investigated.35 Nevertheless, a detailed analysis of the effect of surfaces with different crystal orientations on ET has not been done. Rutile type single crystal TiO2 (R-TiO2) was chosen as a model substrate. Figure 1 shows the basic-unit cell structure of R-TiO2 (tetragonal


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 45

structure) with lattice constants of a = 0.459 nm and c = 0.295 nm. Here, we describe the electronic structure of CdSe QDs on single crystal R-TiO2 and the interactions between multiple sized CdSe QDs and single crystal R-TiO2 with different orientations ((001), (110), and (111)) with the primary focus on the donor (CdSe QD) to acceptor (R-TiO2) ET rates. This paper is organized into three parts. In the first part, we describe the optical absorption characteristics measured using photoacoustic (PA) spectroscopy.36-38 Next, we determine the band gap or HOMO-LUMO gap (1st excitation energy) and the average diameter of the CdSe QDs on R-TiO2 with different crystal orientations. PA spectroscopy is a branch of photothermal (PT) spectroscopy in which heating the sample causes selective absorption of the optical energy. The PA technique detects the acoustic energy produced by heat generated through nonradiative processes in materials. The PA signal is less sensitive to light scattering effects than conventional spectroscopy signals. The sensitivity is higher for weak absorption than that of conventional techniques. Hence, the PA technique is useful not only for fundamental optical absorption edge characterization but also for sub bandgap characterization. In the second part, we describe the position of the valence band maximum (VBM) of single crystal R-TiO2 and the HOMO level of CdSe QDs on R-TiO2 determined using photoelectron yield (PY) spectroscopy.39,40 It is fundamentally important to determine the positions of the HOMO and LUMO levels of CdSe QDs on R-TiO2 with different crystal orientations. As PY spectroscopy does not need complex energy corrections in the data analysis, it is expected to have an advantage in the analysis of dielectric materials. By using the HOMO level of the CdSe QDs estimated from the PY measurements, the LUMO state of the CdSe QDs on R-TiO2 can be evaluated with the help of the PA measurements (HOMO-LUMO gap). Next, the change in free


6

Page 7 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


energy, ∆G, associated with the electron transfer from the CdSe QDs to the R-TiO2 substrate, is estimated. The value of ∆G correlates with the ET dynamics. In the third part, we describe the ET dynamics of CdSe QDs on R-TiO2 substrates with different crystal orientations using the improved-transient grating (TG) method.4,14,41-43 Basically, the TG method depends on the refractive index changes due to photoexcited carriers.41,44-46 In this method, a diffraction grating consisting of photoinduced charge carriers is utilized for monitoring the carrier dynamics. Although the TG measurement provides valuable information, it is not widely used because of the complicated apparatus required. The improved-TG method features: (1) very simple optical alignment to focus beams on the samples without lenses; (2) easy control of the phase difference between the probe and reference beams; (3) high stability of the phase due to the short optical path lengths of the probe and reference beams; (4) high sensitivity; and (5) evaluation of both electron and hole relaxation processes, under low pump light intensity.4,14,41-43 Comparison of the ET kinetics from the QDs to R-TiO2 with different crystal orientations as a function of ∆G leads to an understanding of the dependence of the charge injection dynamics and suggests possible ways to improve the photovoltaic conversion efficiency of QDSCs. EXPERIMENTAL SECTION Materials and Chemicals. The characteristics of single crystal R-TiO2 have already been reported.34 Single-crystal R-TiO2 wafers, 5 mm x 7 mm in area and 0.5 mm thick, with (001)-, (110)-, and (111)-cuts were obtained from Furuuchi Chemical Co., Ltd., Japan. The surface roughnesses of the (001), (110), and (111) faces were 0.322 nm, 0.356 nm, and 0.394 nm, respectively. Flat surfaces were obtained by washing them in acetone for 30 min, immersing


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 45

them in distilled water for 30 min, and treating them in ozone for 10 min. CdSe QDs were prepared and adsorbed on the surfaces of the wafers using a chemical bath deposition (CBD) technique2 according to a published procedure.34 In the CBD technique, a large contact area between the CdSe QDs and the TiO2 surface is usually obtained.9 An 80 mM sodium selenosulphate (Na2SeSO3) solution was prepared by dissolving elemental Se powder in a 200 mM Na2SO3 solution. Then, 80 mM CdSO4 and 120 mM of the trisodium salt of nitrilotriacetic acid [N(CH2COONa)3] were mixed with the Na2SeSO3 solution in a volume ratio of 1:1:1. The single crystal R-TiO2 was placed in a glass container filled with the final solution at 10ºC in the dark for various times (from 4 h to 28 h). The morphologies of the CdSe QDs on the R-TiO2 were observed by AFM (SPM-9700, Shimadzu Co., Japan). Figure 2 shows examples of AFM images of CdSe QDs adsorbed on (a) (001), (b) (110), and (c) (111) R-TiO2, respectively (the adsorption temperature and time were 10°C and 18 h, respectively). They show similar morphologies over an area of 1 µm2. Also, the thicknesses, ~18 nm, are similar to each other, indicating equal numbers of layers (~ 3) of CdSe. The crystal structure of CdSe QDs on R-TiO2 cannot be characterized by x-ray diffraction (XRD) measurements due to the small amount of CdSe QDs, so that we could not distinguish the structure, hexagonal or cubic. In the future, we are going to characterize the structure by reflection high energy electron diffraction (RHEED) measurements, since the HOMO and LUMO levels of CdSe QDs could strongly depend on the surface crystal structures. PA Spectroscopy Characterization. The optical absorption characteristics of CdSe QDs adsorbed on R-TiO2 with different crystal orientations were investigated using a single beam PA spectrometer. A typical gas-microphone PA technique was applied. A detailed explanation of the technique was reported in previous papers.17,47 The PA cell is made of an aluminum cylinder


8

Page 9 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with a small channel at the periphery into which a microphone is inserted. We used a 300 W xenon short arc lamp as the light source. Modulated monochromatic light (33 Hz) was focused onto the sample surface located inside the sealed PA cell. The PA signal was detected by first passing the output from the microphone through a preamplifier and then a lock-in amplifier synchronized with the modulation frequency. The data were averaged to improve the signal-tonoise ratio (S/N). The spectra were taken at room temperature in the wavelength range of 300 – 830 nm. The optical absorption length of the CdSe QDs for these wavelengths is longer than the thermal diffusion length and the thickness of the sample, indicating that the PA signal intensity is proportional to the optical absorption coefficient.47 The spectra were calibrated using the PA signals from a carbon black sheet that was proportional only to the incident light intensity.17,47 PY Spectroscopy Characterization. A detailed explanation of PY spectroscopy was given in a previous report.17 The PY spectra were collected using an ionized energy measurement system (BIP-KV201, Bunkoukeiki, Co., Ltd., Japan). For the PY measurements, a negative bias with respect to the grounded anode was applied to a base plate behind the sample. The number of photoelectrons was obtained by measuring the current needed to compensate for the photoexcited holes at the sample with an ammeter. In the PY measurements, the photoemission yield (Y) was measured as a function of photon energy (hν), and the value of the ionization potential (I) was determined from the onset of the PY spectrum. The PY spectrum around the photoelectric threshold I is expressed by the following equation Y = K (hν – I )n

(1)

where K is a constant and n is a parameter that mainly depends on the shape of the density of electronic states at the upper edge of the valence band and the transmission probability of


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 45

electrons across the surface.39 A quadratic function (n = 2) and a cubic function (n = 3) are used to fit the PY spectra of metals39 and organic materials,48 respectively. In this study, we employed the cubic function (n = 3) based on theoretical analyses.39,48 The Y1/3 plot as a function of hν obtained here shows a fairly linear relationship at the onset. The value of I was determined by extrapolating the linear part of Y1/3 to the baseline.17 An energy scan of the incident photons was performed while increasing the photon energy of the UV light (4 ~ 9.5 eV). The UV light was focused on the sample over an area of 1 × 3 mm2. All the measurements were performed in a vacuum chamber (~ 4 × 10-3 Pa) at room temperature. Improved-TG Characterization. A detailed explanation of the improved-TG method was given in a previous paper.14 The laser beam is separated into two parts for the pump and probe beams. The pump and probe laser pulse beams are set coaxially, before being trained on the transmission grating. For the pump beam, the spatial intensity profile has an interference pattern close to the far side of the transmission grating. When a sample is brought near the transmission grating surface, it is excited by the optical interference pattern. The probe beam is diffracted both by the transmission grating and by the grating induced on the sample. The two diffractions progress in the same direction and the time dependent diffraction intensity is detected. The laser source used in the improved-TG experiments was a regeneratively amplified titanium/sapphire laser (CPA-1000, Clark-MXR Inc., USA) with a fundamental wavelength of 775 nm, a repetition rate of 1 kHz, and a pulse width of 150 fs. The probe pulse (775 nm) was delayed by an optical delay line (0 ~ 400 ps). The pump pulse was generated using the travelling-wave optical parametric amplifier of a super fluorescence (TOPAS) system and was set at a wavelength of 520 nm, suitable for optical absorption in the CdSe QDs. The diameters of both the pump and probe lasers were 5 mm. It has previously been shown that a carrier depopulation mechanism,


10

Page 11 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


monitored by the TG technique, should be ascribed to one body processes (hole trapping and electron injection or trapping) under our experimental conditions of very low pump intensity (2 – 7 µJ/pulse). It was found that the dependence of the maximum signal intensity on the pump intensity was linear. In this case, we are able to separate the charge transfer/trapping from charge recombination and nonlinear processes, simplifying the data analysis. Also, the sample showed no apparent photodamage due to the pump intensity during the TG experiments. DFT Characterization. DFT calculations with periodic boundary conditions were carried out using a plane wave based program, Castep.49,50 The Perdew, Burke, and Ernzerhof (PBE) functional51,52 was used together with ultra-soft core potentials.53 The basis set cut-off energy was set to 300 eV. The electron configurations of the atoms are Ti: 3s23p64s23d2 and O: 2s22p4. Three slab models are shown in Figure 3. For the (001) and (110) slabs, the shape of unit cell is orthorhombic, whereas it is monoclinic for the (111). The lattice parameter c includes the vacuum region. The three slabs consist of seven, eleven and seventeen layers and include (TiO2)7, (TiO2)10, (TiO2)23 atoms, respectively. Geometrical optimization was carried out for all the atomic coordinates, and the lattice constants were fixed. RESULTS AND DISCUSSION PA characterization of CdSe QDs adsorbed on single crystal R-TiO2. In order to quantitatively estimate the amounts of CdSe QDs on different R-TiO2 surfaces, we investigate the absorbance spectrum measurements (absorbance = logI0/It, where I0 and It are the incident and transmitted light intensities). We set the sample just in front of the PA cell in which carbon black was situated, so that we could measure the transmission light intensity and evaluate the absorbance (transmission PA spectroscopy: T-PAS). By applying the T-PAS, the value of


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 45

absorbance of CdSe QDs on different R-TiO2 surfaces increases linearly with adsorption time. We assume that the adsorption time dependence of the absorbance corresponds to the rate of adsorption of CdSe QDs. The rates of adsorption of CdSe QDs thus determined from optical absorbance measurements were similar to our previous results.34 The rate of adsorption of CdSe QDs on the (111) surface (0.030/h) was higher than those on (110) (0.025/h) and (001) surfaces (0.023/h) within the accuracy of the experimental measurements.34 The shape of the PA spectrum was independent of the modulation frequency concerned, indicating that the PA spectrum reflects the optical absorption coefficient (the optical absorption length is longer than the thermal diffusion length). The obtained PA spectra of CdSe QDs on RTiO2 were similar to our recent results.34 Figure 4(a) shows example of the PA spectra for QDs on (001) R-TiO2 at a modulation frequency of 33 Hz with three different adsorption times (12, 18, and 24 h). The spectra are normalized at a photon energy of 3.0 eV. The HOMO-LUMO gap was estimated by the shoulder point indicated by the vertical arrow (↓).54 In the case of semiconductors, both for direct and indirect transitions, the band gap ( or HOMO-LUMO gap) as measured by the position of the shoulder (an inflection point) in the logarithmic PA spectrum agrees very well with the values reported in the literature.54 With increasing adsorption time, a redshift of the PA shoulder point (E1) can be observed which corresponds to the growth of the QDs. E1 is assumed to be the first excitation energy of the QDs. The value of E1 is important in characterizing both the LUMO state (details are in the PY characterization section) and the average diameter of the QDs. By applying the value of E1, the average diameter was estimated using the effective mass approximation (EMA).55,56 Figure 4(b) shows example of the adsorption time dependence of the average diameter of the QDs on (001) R-TiO2. Although the increases in diameter are similar to each other, slight differences can be observed in the initial growth stages.


12

Page 13 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In the case of QDs on (001) R-TiO2, the initial growth rate is somewhat lower than those on (110) and (111). In the Materials and Chemicals section, we pointed out that the thicknesses of the QDs on R-TiO2 with three different crystal orientations were similar to each other, being ~ 18 nm in the case of 18h adsorption time, indicating that the surface coverage was approximately two to three piles of CdSe QDs. Hence, the experimental condition for characterizing the ET rate of the QDs is well satisfied due to the small overlap of the QDs. In a wide variety of materials including ionic crystals, semiconductors, and organic materials, the PA signal intensity (P), which is proportional to the optical absorption coefficient near the fundamental absorption edge, is dependent on the photon energy (hν) according to Urbach rule57 P = P0 exp[R(hν – E0)]

(2)

where P0 and E0 are material-dependent constants, and R is a measure of the exponential optical absorption tail. This dependence refers to the low-energy side of the energy gap (or HOMOLUMO gap). In addition, the dimensionless parameter, σ, defined by σ = RkBT

(3)

where kB and T are Boltzmann’s constant and the absolute temperature, respectively, is a characteristic of the logarithmic slope (exponential tail) below the PA shoulder point (E1) and is called the steepness parameter. Study of the exponential tail gives basic information about the band structure, disorder, defects, impurities, and electron-phonon interactions.58-61 We assume that the value of the steepness parameter, σ, is a reflection of the disorder in the CdSe QDs. Thus, when the disorder increases, σ decreases. Figure 4(c) shows example of the adsorption time dependence of the steepness parameter, σ, of CdSe QDs on (001) R-TiO2. The value of σ


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 45

increases with increasing adsorption time, suggesting a decrease in the disorder of the QDs with adsorption time. The σ values after 28 h adsorption time are 0.40 (001), 0.31 (110), and 0.33 (111), respectively, indicating that the disorder in the QDs on the (001) substrate is lower than those on the (110) and (111) substrates. PY characterization of CdSe QDs adsorbed on single crystal R-TiO2. Figure 5 shows a schematic illustration of the density of states both in the R-TiO2 conduction band (D+(E)) and around the CdSe QD LUMO level (D-(E)). The valence band maximum (VBM) and the conduction band minimum (CBM) in the R-TiO2, and the HOMO/LUMO levels in the CdSe QDs are also shown. PY measurements were carried out to determine the position of the VBM in R-TiO2 for different crystal orientations. The positions obtained for the different orientations were similar, within the accuracy of the experimental measurements, to our previous results ((001): - 7.83 eV; (110): - 7.74 eV; (111): - 7.60 eV vs. vacuum level).34 These values are ~ 0.2 eV lower than the reported values for anatase TiO2.17,62,63 The position of the VBM for the (111) crystal orientation is higher than those for the (001) and (110) orientations. The position of the CBM in R-TiO2 is related to the enhancement in photoactivity.64 It is generally accepted that anatase TiO2 is the more active than R-TiO2 in photocatalysis studies. This enhancement in photoactivity is ascribable to the CBM of anatase TiO2 being higher than that of R-TiO2 by about 0.1 eV.64 These results suggest that the chemical reaction at the surface for the (111) orientation is faster than the other orientations due to the higher CBM. The HOMO energies of CdSe QDs adsorbed on R-TiO2 single crystal were determined. We measured the PY spectra of CdSe QDs in which the adsorption times were 12 h, 18 h, and 28 h. Figure 6(a) shows example of PY spectrum for CdSe QDs on (001) R-TiO2 with the adsorption time of 18 h. In this cases, the average sizes of the QDs was 5.8 nm. As the PY signal intensity


14

Page 15 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


had a good S/N ratio, the error bars are included within the points. The HOMO level was determined from the intersection of the baseline with the tangent to the spectra. Figure 6(b) shows example of the adsorption time dependence of the HOMO level for CdSe QDs on (001) R-TiO2. Although the HOMO level is independent of the adsorption time (or the average size of the QDs) within the accuracy of the measurements, the average value of the HOMO level depends on the crystal orientation, such that (001) < (110) < (111). Figure 6(c) shows the position of the VBM for R-TiO2 (─ line) and the average HOMO level of the QDs (─ line) for the different crystal orientations.

The HOMO level of CdSe QDs depends on the crystal

orientation, but the variation of the HOMO level with crystal orientation is different from the variation of the VBM in the R-TiO2. This indicates that the crystal binding and the combination of wavefunctions at the CdSe (Se4p)/TiO2 (O2p) interface depend on the crystal orientation. In order to determine the LUMO level, the first excitation energy (E1) deduced from the shoulder point in the PA spectrum is applied. Figure 7(a) shows example of the adsorption time dependence of the LUMO level for CdSe QDs on (001) R-TiO2. The LUMO level decreases with increasing adsorption time (or the average size of the QDs). The rate at which the LUMO level for the (111) crystal orientation decreases is less than those for the (001) and (110) orientations, also indicating differences in crystal binding and the mixing of wavefunctions at the CdSe (Cd5s+Se4p)/TiO2 (Ti3d) interface depending on crystal orientation. ∆G – the free energy change – is important for characterizing the electron transfer from the donor species (CdSe QDs) to the acceptor species (R-TiO2). In general, there are multiple factors that can contribute to the overall change in ∆G. First, the free energy for charging, ∆Gcharge, accounts for the energy difference associated with having non-neutral donors and acceptors following electron transfer. Second, the free energy for the Coulomb interaction, ∆Gcoulomb,


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 45

accounts for the energy needed to spatially separate the electron and hole. Thirdly, the change in electronic energy, ∆Gelectronic, accounts for the difference in energy between the initial and final electronic states.10 In the experiments, only ∆Gelectronic can be measured and the first and second terms can be neglected. ∆G is estimated to be the energy gap ∆E between the CBM of the TiO2 and the LUMO level of the CdSe QDs (Fig. 5).24 In order to determine the ∆G values for CdSe QDs on R-TiO2 with different crystal orientations, the bandgap of R-TiO2 (~ 3.0 eV) is used to determine the position of the CBM. Figure 7(b) shows example of the adsorption time dependence of –∆G for CdSe QDs on (001) R-TiO2. The value of –∆G decreases with increasing adsorption time (or the average size of the QDs). The values of –∆G for the (111) crystal orientation are lower than those for the (001) and (110) orientations, also indicating the dependence on crystal orientation of the crystal binding and the combination of wavefunctions at the CdSe QD/R-TiO2 interface already discussed in regard to the HOMO and LUMO levels. Improved-TG Characterization of CdSe QDs adsorbed on single crystal R-TiO2. Figure 8(a) shows example of the TG responses for CdSe QDs on (001) R-TiO2 surfaces in air (adsorption time: 18 h). Fast (~1 ps) and slow (within 100 ps) relaxation processes can be observed. The TG signal intensity S(t) is proportional to the change in refractive index (∆n(t)) due to the photoexcited carriers. ∆n(t) is determined to be a linear function of the concentration of free photogenerated carriers (electrons and holes) assuming Drude’s model.65,66 It should be noted that the relative contribution of each carrier to the TG signal is inversely proportional to the corresponding effective mass. Taking into account the effective masses of an electron and a hole in CdSe (0.13 m0 and 0.44 m0, respectively) and the effective mass of an electron in TiO2 (30 m0), the contribution of the TiO2 electrons to the TG signal can be neglected. As the effective mass of a hole in CdSe is more than three times that of an electron, the TG signal is dominated


16

Page 17 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by the response of the free electrons in CdSe QDs.65,66 Due to one body relaxation processes (injection or trapping), it seems appropriate that the TG signal intensity (S(t)) can be fitted with two exponential relaxation processes plus an offset (S0)

S(t) = A1exp (- ) + A2exp (-

) + S0

(4)

where A1, A2, and S0 are fitting parameters. A1, A2, and S0 correspond to fast, slow, and longer relaxation processes corresponding to recombination, respectively. τ1 and τ2 are the time constants of the fast and slow relaxation processes, respectively. The two exponential relaxation functions together with the offset term fit well with the TG experimental data using a leastsquares fit (─ lines in Fig. 8 (a)). Table 1 shows the least-squares best fit parameters of the TG responses of the CdSe QDs on R-TiO2 with different crystal orientations. The fast relaxation time constant τ1 is about 1ps and is independent of the R-TiO2 crystal orientation. A small increase in τ1 was observed with increasing adsorption time, and this tendency is similar for each crystal orientation. A fast relaxation process such as this was not observed in the TG response when we applied CdSe QDs to a nanoparticulate TiO2 substrate (polycrystalline).14,65,66 We measured the TG responses of single crystal R-TiO2 with different orientations without QDs adsorbed in order to investigate the effect of the single crystal. In this case, only the fast relaxation process with a relaxation time of about 1ps was observed. Thus the TG responses of the order of 1ps for single crystal R-TiO2 are similar to those obtained for CdSe QDs on R-TiO2. The fast relaxation process is mainly due to the optical Kerr effect, which is an effect in which the electric field is due to the light itself. This causes a variation of the refractive index that is proportional to the local irradiance of the light. The effect becomes significant with very intense beams such as those from lasers applied to single crystals (periodic space). The fast


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 45

relaxation processes of the order of several ps observed in nanoparticulate TiO2 substrates due to hole trapping65,66 were not observed in our TG measurements for CdSe QDs on R-TiO2. This is due to the strong overlap of the TG signal intensity with the optical Kerr effect at around 1ps. The slow relaxation time τ2 was between 30 ps and 80 ps. This is a reflection of the photoexcited electron relaxation process. τ2 increases with increasing adsorption time,10,65 and also depends on the crystal orientation. Increasing the adsorption time leads to both an increase in the diameter and in the number of QDs. As a general rule, the larger the size of the quantum dot, the smaller the ET rate constant. This can be rationalized by the fact that larger systems have a smaller percentage of their total charge density localized near the surface.67 This causes the value of –∆G to decrease as a result of the reduction in ∆Gcharge, so that the ET rate constant decreases. Assuming the only difference between CdSe QDs adsorbed on to TiO2 or SiO2 is the added ET pathway, we were able to calculate the apparent ET rate constants for CdSe QDs adsorbed on R-TiO2 using the following relationship68

ket =

( )

-

( )

(5)

where τ2(TiO2) and τ2(SiO2) are the relaxation times of the QDs adsorbed on R-TiO2 and SiO2, respectively. Using the relaxation time τ2 obtained here for QDs on R-TiO2 and those on SiO2 from the literature,43 we calculated the apparent ET rate constant ket from eq. (5). A full list of the calculated ket using eq. (5) is provided in Table 1. The calculated values of the ET rate constant ket here are somewhat smaller than those observed by Kamat et al,10 indicating the difference between adsorption on single crystal TiO2 and that on nanoparticulate TiO2. Figure 8(b) shows the dependence of the ET rate constant of CdSe QDs on the free energy change, –∆G. The ET


18

Page 19 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rate constant increases with increasing –∆G. The increase of the ET rate constant on the (111) surface is higher than those on the (001) and (110) surfaces, indicating differences in crystal binding and the combination of wavefunctions at the CdSe QDs/R-TiO2 interface. It was found that the symmetry of the excited states of the QDs to the orbitals of the substrate has an influence on the ET rate constant.67,69 The symmetry of the wavefunctions determines the charge density and the donor acceptor coupling. If the LUMO level for the CdSe QDs couples weakly with the Ti3d orbitals in R-TiO2, the ET rate constant is slow. On the other hand, if the coupling is strong, the ET rate constant shows higher electron injection.68 Hence, the coupling between the LUMO level in the CdSe QDs and the Ti3d orbitals in (111) R-TiO2 is stronger than the other crystal orientations ((001) and (110)). The ET rate constant ket can be written quantitatively24,70

ket =

ħ

() ()d

(6)

where () and D_(E) are the density of states in the conduction band of R-TiO2 and in the LUMO level of the CdSe QDs, respectively, already shown in Figure 5. u is the electron exchange integral defined as u = 〈 ()| !" |# ()〉

(7)

where (), # (), and !" are the wavefunction in the conduction band of R-TiO2 (Bloch function), that in the CdSe QDs (LUMO state), and the interaction Hamiltonian, respectively. There is a possibility that the electron exchange integral for the (111) orientation is higher than those for the other orientations ((001) and (110)), indicating stronger coupling between the LUMO level and the Ti3d orbitals for the (111) orientation due to higher s-d hybridization. In RTiO2, s-d hybridization occurs and the Ti3d electrons participate in chemical bonding. We


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 45

performed density functional theory (DFT) calculations for the partial density of states (PDOS) in R-TiO2 with different crystal orientations. Figure 9 shows the PDOS for the Ti3d and O2p bands of (a) (001), (b) (110), and (c) (111) R-TiO2. The calculation for R-TiO2 gave a bandgap (Eg) of 1.6 eV, which is an underestimate due to the shortcomings of the current DFT calculations. The DOS of the conduction band of R-TiO2 (D+(E) in eq. (6)) is dominated by Ti3d orbitals. PDOS of Ti3d in (001) and (110) show broad structures between 1.6 and 6 eV. However, the PDOS of Ti3d in (111) shows a somewhat localized structure between 1.6 and 3.6 eV, indicating the possibility that the PDOS in (111) is higher than the other crystal orientations ((001) and (110)). Consequently, there are two factors that enhance the ET rate constant of CdSe QDs on (111) R-TiO2. The first is the increase in the electron exchange integral and the second is the higher DOS of Ti3d orbitals due to higher s-d hybridization. Our studies have shown that the (111) surface orientation of R-TiO2 is suitable for the adsorption and the electron transfer of CdSe QDs. Hence, the application of assembly of R-TiO2 (111) nanosheets as photoanode is advantageous to contribute the better photovoltaic performance of CdSe QDSCs compared to the standard nanoparticulate TiO2 photoanode. The parameter S0 in eq. (4) corresponds to longer relaxation processes (recombination). The larger S0, the smaller the recombination rate. A full list of the calculated S0 using eq. (4) is provided in Table 1. S0 increases with increasing adsorption time, and also depends on the RTiO2 crystal orientation. The rate of increase of S0 on the (110) surface is less than on (001) and (111) surfaces, indicating that recombination rate on the (110) surface is possibly higher than the other crystal orientations. Figure 10 shows the dependence of the parameter S0 on the steepness parameter σ. S0 increases linearly with increasing σ. The rate of increase of S0 with respect to σ depends on the R-TiO2 crystal orientation. The smaller rate for the (110) surface with σ indicates


20

Page 21 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the possibility of higher recombination rate while the higher rate for the (111) surface with σ corresponds to lower recombination rate. Figure 10 shows the possibility that the recombination rate of the CdSe QDs on (001) R-TiO2 is higher than that on (111) although the disorder in the QDs on (001) is lower than that on (111). CONCLUSIONS In conclusion, we have shown that the adsorption, the electronic structure, and the photoinduced electron transfer rate constant of CdSe QDs depend on the crystal orientation of the R-TiO2 substrate. The rate of adsorption for CdSe QDs grown on the (111) surface is higher than those grown on (110) and (001) surfaces. Although the dependences of the average diameter of the QDs on adsorption time are similar to each other, slight differences can be observed in the initial growth stages. In the case of CdSe QDs on (001) R-TiO2, the initial growth rate is lower than those on (110) and (111) surfaces. Analysis of the exponential optical absorption tail (Urbach tail) suggests that the disorder decreases with increasing adsorption time. Although the HOMO level of CdSe QDs is independent of adsorption time (or the average size of the QDs), the value of the HOMO level depends on the R-TiO2 crystal orientation, and is such that (001) < (110) < (111). The variation of the HOMO level of CdSe QDs with crystal orientation is different from that of the position of the valence band maximum in R-TiO2. This suggests that the crystal binding and the mixing of wavefunctions at the CdSe (Se4p)/R-TiO2 (O2p) interface depend on the crystal orientation. The photoinduced electron transfer rate constant of CdSe QDs increases with increasing free energy. The increase in the photoinduced electron transfer rate constant of CdSe QDs on the (111) surface with respect to free energy is higher than those on (001) and (110) surfaces, indicating differences in crystal binding and the mixing of wavefunctions at the CdSe QD/R-TiO2 interface. There is a possibility that the electron exchange integral of CdSe


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 45

QDs for (111) R-TiO2 is higher than those for other crystal orientations ((001) and (110)), indicating stronger coupling between the LUMO level of the CdSe QDs and the Ti3d orbitals in (111) R-TiO2 due to higher s-d hybridization. Also, the recombination rate for CdSe QDs on the (111) surface is lower than those on (001) and (110) surfaces. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (T. T.); *E-mail: [email protected] (Q. S.) Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Part of this work was supported by Core Research for Evolutional Science and Technology (CREST), Japan Science Technology Agency (JST). Also, it was supported by JSPS Kakenhi Grant Number 26390016. We thank T. Amano and Y. Takeshita of Bunkoukeiki Co., Ltd. for cooperation with the PYS measurements. H. Ishii of Chiba University is acknowledged for his helpful discussions and comments on the photoelectron yield spectroscopy results.

REFERENCES (1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (version 39). Prog. Photovolt.: Res. Appl. 2012, 20, 12-20. (2) Shen, Q.; Arae, D.; Toyoda, T. Photosensitization of Nanostructured TiO2 with CdSe Quantum Dots: Effects of Microstructure and Electron Transport in TIO2 Substrates. J. Photochem. Photobiol. A: Chemistry 2004, 164, 75-80.


22

Page 23 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3) Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Rühle, S.; Cahen, D.; Hodes, G. Chemical Bath Deposited CdS/CdSe-Sensitized Porous TiO2 Solar Cells. J. Photochem. Photobiol. A: Chemistry 2006, 181, 306-313. (4) Diguna, L. J.; Shen, Q.; Sato, A.; Katayama, K.; Sawada, T.; Toyoda, T. Optical Absorption and Ultrafast Carrier Dynamics Characterization of CdSe Quantum Dots Deposited on Different Morphologies of Nanostructured TiO2 Films. Mater. Sci. Eng. C 2007, 27, 1514-1520. (5) Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T. High Efficiency of CdSe Quantum-DotSensitized TiO2 Inverse Opal Solar Cells. Appl. Phys. Lett. 2007, 91, 023116. (6) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe-TiO2 Architecture. J. Am. Chem. Soc. 2008, 130, 4007-4015. (7) Mora-Seró, I.; Giménez, S.; Fabregat-Santiago, F.; Gómez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1848-1857. (8) Mora-Seró, I.; Bisquert, J. Breakthroughs in the Development of Semiconductor-Sensitized Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3046-3052. (9) Tada, H.; Fujishima, M.; Kobayashi, H. Photodeposition of Metal Sulfide Quantum Dots on Titanium (Ⅳ) Dioxide and the Applications to Solar Energy Conversion. Chem. Soc. Rev. 2011, 40, 4232-4243.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 45

(10) Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V. Photoinduced Electron Transfer from Semiconductor Quantum Dots to Metal Oxide Nanoparticles. Proc. Nat. Acad. Sci. USA 2011, 108, 29-34. (11) Emin, S.; Singh, S. P.; Han., L.; Satoh, N.; Islam, A. Colloidal Quantum Dot Solar Cells. Sol. Energy 2011, 85, 1264-1282. (12) Toyoda, T.; Shen, Q. Quantum-Dot-Sensitized Solar Cells: Effect of Nanostructured TiO2 Morphologies on Photovoltaic Properties. J. Phys. Chem. Lett. 2012, 3, 1885-1893. (13) Yindeesuk, W.; Shen, Q.; Hayase, S.; Toyoda, T. Optical Absorption of CdSe Quantum Dots on Electrodes with Different Morphology. AIP Adv. 2013, 3, 102115. (14) Toyoda, T.; Onishi, Y.; Katayama, K.; Sawada, T.; Hayase, S.; Shen, Q. Photovoltaics and Photoexcited Carrier Dynamics of Double-Layered CdS/CdSe Quantum Dot-Sensitized Solar Cells. J. Mater. Sci. Eng. A 2013, 3, 601-608. (15) Becker, M. A.; Radich, J. G.; Bunker, B. A.; Kamat, P. V. How does a SILAR CdSe Film Grow? Tuning the Deposition Steps to Suppress Interfacial Charge Recombination in Solar Cells. J. Phys. Chem. Lett. 2014, 5, 1575-1582. (16) Fujishima, M.; Tanaka, K.; Sakami, N.; Wada, M.; Morii, K.; Hattori, T.; Sumida, Y.; Tada, H. Photocatalytic Current Doubling-Induced Generation of Uniform Selenium and Cadmium Selenide Quantum Dots on Titanium (Ⅳ) Oxide. J. Phys. Chem C 2014, 118, 8917-8924.


24

Page 25 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Toyoda, T.; Yindeesuk, W.; Okuno, T.; Akimoto, M.; Kamiyama, K.; Hayase, S.; Shen, Q. Electronic Structures of Two Types of TiO2 Electrodes: Inverse Opal and Nanoparticulate Cases. RSC Adv. 2015, 5, 49623-49632. (18) Yu, W. W.; Qu, L.; Guo, W.; Peng, X, Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854-2860. (19) Underwood, D. F.; Kippeny, T.; Rosental, S. J. Charge Carrier Dynamics in CdSe Nanocrystals: Implications for the Use of Quantum Dots in Novel Photovoltaics. Eur. Phys. J. D 2001, 16, 241-244. (20) Salant, A.; Shalom, M.; Tachan, Z.; Buhbut, S.; Zaban, A.; Banin, U. Quantum RodSensitized Solar Cell: Nanocrystal Shape Effect on the Photovoltaic Properties. Nano Lett. 2012, 12, 2095-2100. (21) Huang, J.; Huang, Z.-Q.; Yang, Y.; Zhu, H.-M.; Lian, T.-Q. Multiple Exciton Dissociation in CdSe Quantum Dots by Ultrafast Electron Transfer to Adsorbed Methylene Blue. J. Am Chem. Soc. 2010, 132, 4858-4864. (22) Huang, J.; Stockwell, D.; Huang, Z.-Q.; Mohler, D. L.; Lian, T.-Q. Photoinduced Ultrafast Electron Transfer from CdSe Quantum Dots to Re-Bipyridyl Complexes. J. Am Chem. Soc. 2008, 130, 5632-5633. (23) Tafen, D. N.; Long, R.; Prezhdo, O. V. Dimensionality of Nanoscale TiO2 Determines the Mechanism of Photoinduced Electron Injection from a CdSe Nanoparticle. Nano Lett. 2014, 14, 1790-1796.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 45

(24) Sakata, T.; Hashimoto, K.; Hiramoto, M. New Aspects of Electron Transfer on Semiconductor Surface: Dye-Sensitization System. J. Phys. Chem. 1990, 94, 3040-3045. (25) Tian, J.; Cao, G. Control of Nanostructures and Interfaces of Metal Oxide Semiconductors for Quantum-Dots-Sensitized Solar Cells. J. Phys. Chem. Lett. 2015, 6, 1859-1869. (26) Lazzeri, M.; Vittadini, A.; Selloni, A. Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces. Phys. Rev. B 2001, 63, 155409. (27) Etgar, L.; Zhang, W.; Gabriel, S.; Hickey, S. G.; Nazeeruddin, M. K; Eychmüller, A.; Liu, B.; Grätzel, M. High Efficiency Quantum Dot Heterojunction Solar Cell Using Anatase (001) Nanosheets. Adv. Mater. 2012, 24, 2202-2206. (28) Kavan, L.; Grätzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. Electrochemical and Photoelectrochemical Investigation of Single-Crystal Anatase. J. Am. Chem. Soc. 1996, 118, 6716-6723. (29) Maitani, M. M.; Tanaka, K.; Mochizuki, D.; Wada, Y. Enhancement of Photoexcited Charge Transfer by {001} Facet-Dominating TiO2 Nanoparticles. J. Phys. Chem. Lett. 2011, 2, 2655-2659. (30) Takeda, N.; Parkinson, B. A. Adsorption Morphology, Light Absorption, and Sensitization Yields for Squaraine Dyes on SnS2 Surfaces. J. Am. Chem. Soc. 2003, 125, 5559-5571. (31) Ushiroda, S.; Ruzycki, N.; Lu, Y.; Spitler, M. T.; Parkinson, B. A. Dye Sensitization of the Anatase (101) Crystal Surface by a Series of Dicarboxylated Thiacyanine Dyes. J. Am. Chem. Soc. 2005, 127, 5158-5168.


26

Page 27 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32) Sambur, J. B.; Parkinson, B. A. CdSe/ZnS Core/Shell Quantum Dot Sensitization of Low Index TiO2 Single Crystal Surfaces. J. Am Chem. Soc. 2010, 132, 2130-2131. (33) Sambur, J. B.; Averill, C. M.; Bradley, C.; Schuttlefield, J.; Lee, S. H.; Reynolds, J. R.; Schanze, K. S.; Parkinson, B. A. Interfacial Morphology and Photoelectrochemistry of Conjugated Polyelectrolytes Adsorbed on Single Crystal TiO2. Langmuir 2011, 27, 11906-11916. (34) Toyoda, T.; Yindeesuk, W.; Kamiyama, K.; Hayase, S.; Shen, Q. Effect of TiO2 Crystal Orientation on the Adsorption of CdSe Quantum Dots for Photosensitization Studied by the Photoacoustic and Photoelectron Yield Methods. J. Phys. Chem. C 2014, 118, 16680-16687. (35) Diebold, U. The Surface Science of Titanium Dioxide. Sur. Sci. Rep. 2003, 48, 53-229. (36) Inoue, Y.; Toyoda, T.; Morimoto, J. Evaluation of Co- and Pr-Doped Zinc Silicate Powders by Photoacoustic Spectroscopy. Jpn. J. Appl. Phys. 2006, 45, 4604-4608. (37) Toyoda, T.; Tsugawa, S.; Shen, Q. Photoacoustic Spectra of Au Quantum Dots Adsorbed on Nanostructured TiO2 Electrodes Together with the Photoelectrochemical Current Characteristics. J. Appl. Phys. 2009, 105, 034314. (38) Toyoda, T.; Oshikane, K.; Li, D.; Luo, Y.; Meng, Q.; Shen, Q. Photoacoustic and Photoelectrochemical Current Spectra of Combined CdS/CdSe Quantum Dots Adsorbed on Nanostructured TiO2 Electrodes, together with Photovoltaic Characteristics. J. Appl. Phys. 2010, 108, 114304. (39) Honda, M.; Kanai, K.; Komatsu, K.; Ouchi, Y.; Ishii, H.; Seki, K. Atmospheric Effect of Air, N2, O2, and Water Vapor on the Ionization Energy of Titanyl Phthalocyanine Thin Film Studied by Photoemission Yield Spectroscopy. J. Appl. Phys. 2007, 102, 103704.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 45

(40) Nakayama, Y.; Machida, S.; Minari, T.; Tsukagishi, K.; Noguchi, Y.; Ishii, H. Direct Observation of the Electronic States of Single Crystalline Rubrene under Ambient Condition by Photoelectron Yield Spectroscopy. Appl. Phys. Lett. 2008, 93, 173305. (41) Katayama, K.; Yamaguchi, M.; Sawada, T .Lens-Free Heterodyne Detection for Transient Grating Experiments. Appl. Phys. Lett. 2003, 82, 2775-2777. (42) Shen, Q.; Yanai, K.; Katayama, K.; Toyoda. T. Optical Absorption, Photosensitization, and Ultrafast Carrier Dynamic Investigations of CdSe Quantum Dots Grafted onto Nanostructured SnO2 Electrode and Fluorine-Doped Tin Oxide (FTO) Glass. Chem. Phys. Lett. 2007, 442, 89-96. (43) Shen, Q.; Katayama, K.; Sawada, T.; Toyoda, T.; Characterization of Electron Transfer from CdSe Quantum Dots to Nanostructured TiO2 Electrode Using a Near-Field Heterodyne Transient Grating Technique. Thin Solid Films 2008, 516, 5927-5930. (44) Aoyagi, Y.; Segawa, Y.; Namba, S. Determination of Diffusion Coefficients of an Exciton and Excitonic Molecule in CuCl by Picosecond Transient Grating Spectroscopy. Phys. Rev. B 1982, 25, 1453-1456. (45) Newell, V. J.; Rose, T. S.; Fayer, M. D. Surface Quenching of Optically Generated Carriers in Thin-Film Hydrogenated Amorphous Silicon: Picosecond Transient-Grating Experiments. Phys. Rev. B 1985, 32, 8035-8040. (46) Nakabayashi, S.; Komura, S.; Aoyagi, Y.; Kira, A. Transient Grating Method Applied to Electron-Transfer Dynamics at a Semiconductor/Liquid Interface. J. Phys. Chem. 1987, 91, 1696-1698.


28

Page 29 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(47) Rosencwaig, A.; Gersho, A. Theory of the Photoacoustic Effect with Solids. J. Appl. Phys. 1976, 47, 64-69. (48) Ballantyne, J. M. Effect of Phonon Energy Loss on Photoemissive Yield near Threshold. Phys. Rev. B 1972, 6, 1436-1455. (49) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Johnnopoulos, J. D. Iterative Minimization Techniques for ab initio Total-Energy Calculations: Molecular Dynamics and Conjugate Gradients. Rev. Mod. Phys. 1992, 64, 1045-1097. (50) Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M. C.; Akhmatskaya, E. V.; Nobes, R. H. Electronic Structure, Properties and Phase Stability of Inorganic Crystals: A Pseudopotential Plane-Wave Study. Int. J. Quantum Chem. 2000, 77, 895-910. (51) Perdew, P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (52) Perdew, P.; Burke, K.; Ernzerhof, M. Errata: Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1997, 78, 1396. (53) Vanderbilt, D. Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formation. Phys. Rev. B 1990, 41, 7892-7895. (54) Rosencwaig, A. Photoacoustic Spectroscopy – A New Tool for Investigation of Solids. Anal. Chem. 1975, 47, 592A-604A. (55) Ekimov, A. I.; Efros, AI. L.; Onushchenko, A. A. Quantum Size Effect in Semiconductor Microcrystals. Solid State Commun. 1985, 56, 921-924.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 45

(56) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E = sulfur, selenium, tellurium) Semiconductor Nanocrystals. J. Am. Chem. Soc. 1993, 115, 8706-8715. (57) Urbach, F. The Long-Wavelength Edge of Photographic Sensitivity and of the Electronic Absorption of Solids. Phys. Rev. 1953, 92, 1324. (58) Cody, G. D.; Tiedje, T.; Abeles, B.; Brooks, B.; Goldstein, Y. Disorder and the OpticalAbsorption Edge of Hydrogenated Amorphous Silicon, Phys. Rev. Lett. 1981, 47, 1480-1483. (59) Meeder, A.; Fuertes Marrόn, D.; Rumberg, A.; Lux-Steiner, M. Ch; Chu, V.; Conde, J. P. Direct Measurement of Urbach Tail and Gap State Absorption in CuGaSe2 Thin Films by Photothermal Deflection Spectroscopy and the Constant Photocurrent Method. J. Appl. Phys. 2002, 92, 3016-3020. (60) Drabold, D. A.; Li, Y.; Cai, B.; Zhang, M. Urbach Tails of Amorphous Silicon. Phys. Rev. B 2011, 83, 045201. (61) Jones, D. A.; Ung Lee, J. Observation of the Urbach Tail in the Effective Density of States in Carbon Nanotubes. Nano Lett. 2011, 11, 4176-4179. (62) Grätzel, M. Photochemical Cells. Nature 2001, 414, 338-344. (63) Chi, C-F.; Cho, H-W.; Teng, H.; Chuang, C-Y.; Chang, Y-M.; Hsu, Y-J.; Lee, Y-L. Energy Level Alignment, Electron Injection, and Charge Recombination Characteristics in CdS/CdSe Cosensitized TiO2 Photoelectrode. Appl. Phys. Lett. 2011, 98, 012101. (64) Bickley, R. I.; Gonzalez-Carreno, T.; Lees, J. S.; Palmisano, L.; Tilley, R. J. D. A Structural Investigation of Titanium Dioxide Photocatalysis. J. Solid State Chem. 1991, 92, 178-190.


30

Page 31 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(65) Guijarro, N.; Lana-Villarreal, T.; Shen, Q.; Toyoda, T.; Gόmez, R. Sensitization of Titanium Dioxide Photoanodes with Cadmium Selenide Quantum Dots Prepared by SILAR: Photoelectrochemical and Carrier Dynamics Studies. J. Phys. Chem. C 2010, 114, 21928-21937. (66) Guijarro, N.; Shen, Q.; Giménez, S.; Mora-Serό, I.; Bisquert, J.; Lana-Villarreal, T.; Toyoda, T.; Gόmez, R. Direct Correlation between Ultrafast Injection and Photoanode Performance in Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 22352-22360. (67) Akimov, A. V.; Neukirch, A. J.; Prezhdo, O. V. Theoretical Insights into Photoinduced Charge Transfer and Catalysis at Oxide Interfaces. Chem. Rev. 2013, 113, 4496-4565. (68) Pernik, D. R.; Tvrdy K.; Radich, J. G.; Kamat, P. V. Tracking the Adsorption and Electron Injection Rates of CdSe Quantum Dots on TiO2: Linked versus Direct Attachment. J. Phys. Chem. C 2011, 115, 13511-13519. (69) Rego, L. G. C.; Batista, V. S. Quantum Dynamics Simulations of Interfacial Electron Transfer in Sensitized TiO2 Semiconductors. J. Am Chem. Soc. 2003, 125, 7989-7997. (70) Hopfield, J. J. Electron Transfer between Biological Molecules by Thermally Activated Tunneling. Proc. Nat. Acad. Sci. USA 1974, 9, 3640-3644.


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 45

Figure 1. Basic unit-cell structure of rutile TiO2 with lattice constants of a = 0.459 nm and c = 0.295 nm.


32

Page 33 of 45


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 45

Figure 2. AFM images of CdSe quantum dots adsorbed on (a) (001), (b) (110), and (c) (111) single crystal rutile TiO2 (adsorption temperature: 10°C; adsorption time: 18 h).


34

Page 35 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Three slab models for (a) (001), (b) (110), and (c) (111) R-TiO2.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

Figure 4. (a) Photoacoustic spectra, (b) adsorption time dependence of the average diameter, and (c) adsorption time dependence of the steepness parameter of CdSe quantum dots on (001) single crystal rutile TiO2.


36

Page 37 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Schematic illustration of the density of states in both the R-TiO2 conduction band (D+(E)) and around the LUMO level in the CdSe quantum dots (D-(E)) together with the positions of the valence band maximum and the conduction band minimum in R-TiO2, and the HOMO/LUMO levels in the CdSe quantum dots.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

Figure 6. (a) Photoelectron yield spectrum (adsorption time: 18 h), (b) adsorption time dependence of the HOMO energy of CdSe quantum dots on single crystal R-TiO2, and (c) position of the valence band maximum for rutile TiO2 and the HOMO energy of CdSe quantum dots on the TiO2 substrate.


38

Page 39 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Adsorption time dependence of (a) the LUMO energy and (b) –∆G of CdSe QDs adsorbed on (001) single crystal R-TiO2.


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

Figure 8. (a) Transient grating response for CdSe quantum dots on (001) single crystal R-TiO2 (adsorption time: 18 h) and (b) free energy change (–∆G) dependence of the electron rate constant of CdSe quantum dots adsorbed on different crystal orientation of R-TiO2.


40

Page 41 of 45



41


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

Figure 9. Partial density of states for (a) (001), (b) (110), and (c) (111) R-TiO2.


42

Page 43 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Urbach parameter (σ) dependence of the longer relaxation process component (S0) for CdSe QDs on (001), (110), and (111) single crystal R-TiO2.


43


Page 44 of 45


44

Page 45 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Image


45

The Electronic Structure and Photoinduced Electron Transfer Rate of

Recommend Documents