Anisotropic Crystal Growth, Optical Absorption, and Ground-State

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Anisotropic Crystal Growth, Optical Absorption, and Ground State Energy Level of CdSe Quantum Dots Adsorbed on the (001) and (102) Surfaces of Anatase-TiO: Quantum Dot-Sensitization System 2

Taro Toyoda, Qing Shen, Motoki Hironaka, Keita Kamiyama, Hisayoshi Kobayashi, Yasushi Hirose, and Shuzi Hayase J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07378 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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The Journal of Physical Chemistry

Anisotropic Crystal Growth, Optical Absorption, and Ground State Energy Level of CdSe Quantum Dots Adsorbed on the (001) and (102) Surfaces of Anatase-TiO2: Quantum Dot-Sensitization System Taro Toyoda,*,†Qing Shen,*,† Motoki Hironaka,† Keita Kamiyama, §Hisayoshi Kobayashi,┴ Yasushi Hirose,# and Shuzi Hayase // †Department

of Engineering Science, The University of Electro-Communications,

1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan §Bunkoukeiki

┴Department

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

of Chemistry and Materials Technology, Kyoto Institute of Technology,

Matsugasaki, Sakyoku, Kyoto 606-8585, Japan

#Department

of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033,

Japan //Graduate

School of Life Science and Systems Engineering, Kyushu Institute of Technology,

2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan

ACS Paragon Plus Environment

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ABSTRACT: In this study we focus on the effect of the crystal orientation of TiO2 on the optical absorption and ground state energy levels in systems comprising CdSe quantum dots (QDs) adsorbed on the (001) and (102) surfaces of anatase-TiO2 (A-TiO2) (quantum dotsensitization system). The optical absorption characteristics indicate that the crystal growth shows the orientation dependence of A-TiO2 surface. Firstly, the adsorption rate of CdSe QDs on A-TiO2(001) with adsorption time is higher than that on A-TiO2(102) in agreement with our DFT calculations ((001) » (101) > (102)). Secondly, the rate at which the diameter of the CdSe QDs increases with adsorption time on A-TiO2(001) is lower than that on A-TiO2(102), indicating the existence of higher negative crystal growth (dissolve) on the former. The ground state energy levels of CdSe QDs on A-TiO2(102) are lower than those on A-TiO2(001), indicating the difference between the permittivities of different surfaces of A-TiO2. The energy levels of the excited states of CdSe QDs on A-TiO2(102) are lower than the conduction band minimum of ATiO2(102), suggesting that photo-sensitization by the excited states of CdSe QDs on ATiO2(102) is not possible.


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INTRODUCTION Owing to the size-dependent quantum confinement effect, semiconductor quantum dots (QDs) have a tunable energy gap and high extinction coefficients, indicating the possibility of using them as sensitizers for solar cells. Moreover, the electron injection process benefits from large dipole moments facilitating the separation processes of the electron-hole pairs.1,2 These characteristics of QDs supply them with the possibility of boosting the photovoltaic conversion efficiency of quantum dot-sensitized solar cells (QDSCs). A number of researchers have proposed QDSCs over two decades.3-9 Although QDs have such promising property, the photovoltaic conversion efficiencies are still lower than those of dye-sensitized solar cells (DSCs). Metal oxide nanoparticulate electrodes play a key role in applications to QDSCs, because the large surface area with a large amount of QDs adsorbed on them affords effective light harvesting.10 Conventional metal oxide nanoparticulate electrodes (TiO2, ZnO etc.) have heterogeneous distributions of defects, an abundance of grain boundaries, unknown adsorption sites, and exposed surfaces with polycrystalline assemblies.11 The size, shape, and charge of the QDs is also heterogeneous.11 These complexities make it difficult to understand in detail the fundamental properties of QDs adsorbed on metal oxide nanoparticulate electrodes. However, it is important and necessary to understand the fundamental properties of these nanoheterostructures, since studies of these can help researchers not only to assemble multilayer-QD solar cells but also to fabricate new types (e.g. perovskite one) tackling one of the most valuable issues in photovoltaic research. There are also reports that the morphology of the oxide electrode has an impact on electron transfer due to changes in the dielectric constant and the different densities of the band-edge-states.8,12 It should be noted that structural properties of electrodes and different sensitization methods affect the photovoltaic properties13,14 and multiple exciton


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collection.15 First of all, it is necessary to characterize the dependencies of the optical absorption and the ground state energy level of the QDs on the size of the QDs on well characterized single crystal metal oxide surfaces, where the electronic structure has been thoroughly investigated.11,16 These characterizations are important not only for fundamental studies but also for improving the photovoltaic properties of QDSCs. Nevertheless, a detailed study of the influence of electrodes with different crystal orientations (in other words, different circumstance for the QDs) has seldom been studied except in a few reports.17-25 The present study focuses on electrode surfaces with different crystal orientations in systems comprising CdSe QDs adsorbed on epitaxial thin films of (001)- and (102)-oriented single crystal anatase-TiO2 (A-TiO2) and the effect the orientation has on optical absorption and the ground state energy level. We chose CdSe QDs as the sensitizer because these are the most extensively investigated. A-TiO2 is more highly active than rutile-TiO2 (R-TiO2), so is generally used as the oxide nanoparticulate electrode for QDSCs, DSCs, and photocatalytic materials. However, the reasons for the differences in photocatalytic activity between anatase and rutile are still being debated.26,27 It is necessary to obtain information on optical absorption to study the electronic states of QDs on A-TiO2. We applied photoacoustic (PA) spectroscopy to characterize the optical absorption, not only in the bandgap but also in the sub bandgap region.28-30 PA spectroscopy detects a signal directly proportional to the produced thermal energy (photothermal phenomena) as a result of nonradiative processes after optical absorption. Although the absolute value of the optical absorption cannot be measured using the PA method, it is more sensitive than conventional absorbance spectroscopy. The effects of scattering on the optical absorption measurements can be minimized by employing PA spectroscopy rather than conventional transmission spectroscopy. In addition, photoelectron yield (PY) spectroscopy was applied to


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determine the absolute ground state energy level of the QDs. While UV-vis optical absorption spectra determine the energies of the bands relative to each other and give information only at the band edge, they do not give the absolute energy level. PY spectroscopy is usually used to determine the ground state energy level of bulk semiconductors and metals. However, it has not been applied to examine the absolute ground state energy level of QDs as a function of size. With PY measurements we expect to get information on the ground state energy levels of QDs.31 EXPERIMENTAL SECTION Materials and Chemicals. It is not easy to get good quality single crystal A-TiO2, so (001) and (102)-oriented A-TiO2 epitaxial films were fabricated by pulsed laser deposition on the (100) and (110) planes of single crystalline LaAlO3 (LAO) substrates, respectively. Details of the growth condition have been reported elsewhere.32,33 Briefly, a KrF excimer laser (Coherent, COMPex pro 50, λ = 248 nm, 2.0 mJ cm-2 pulse-1, 2 Hz) was used for ablating a TiO2 target. The growth temperature and oxygen partial pressure were set to 600°C and 1.0 × 10-3 Torr, respectively. The thickness of the films was around 50 nm. Epitaxial growth of (001)- and (102)oriented A-TiO2 without any impurity phase was confirmed by x-ray diffraction (XRD) measurements with a 4-axis x-ray diffractometer. The surface morphology of the films was evaluated by an atomic force microscope (SPM-9700, Shimadzu Corporation, Japan) to observe the effect of roughness on optical absorption and ground state energy of CdSe QDs. CdSe QDs were adsorbed on the surfaces of the thin epitaxial films using a chemical bath deposition (CBD) technique.34 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


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Na2SeSO3 solution in a volume ratio of 1:1:1. The single crystal A-TiO2 was placed in a glass container filled with the final solution at 10ºC in the dark for various times (18 ~ 72 h). PA Spectroscopy Characterization. The optical absorption was investigated using a single beam PA spectrometer with a gas-microphone technique.35 Monochromatic light at 33 Hz was focused onto the sample surface in the sealed PA cell with a sensitive acoustic microphone built into one wall (sample cell volume, ~ 0.67 cm3). Changes in the sample temperature due to the photothermal effect cause pressure changes in the PA cell that are converted into an electrical signal by the microphone. The spectra were taken at room temperature in the wavelength range of 350 – 800 nm. The PA signal intensity is proportional to the optical absorption coefficient due to the relationship between the optical absorption length and the thermal diffusion length (by adjustment of experimental parameters).35 The spectra were calibrated using the PA signals from a carbon black sheet. The PA signals from a carbon black sheet was proportional to incident light intensity because absorption length is shorter than thermal diffusion length.35 This normalization eliminates the spectral variation of the illumination source. As PA spectroscopy involves indirect measurements, it does not estimate the absolute value of the optical absorption. So we applied a UV-vis spectrometer (V-670 spectrometer, JASCO, Japan) to investigate the absorbance (Abs) measurements to quantitatively estimate the optical absorption characteristics at a photon energy of 3.0 eV. In addition, Abs is related to the adsorption thickness of CdSe QDs discussed later. PY Spectroscopy Characterization. The PY spectra were collected using an ionization energy measurement system (BIP-KV201, Bunkoukeiki Co., Ltd., Japan).21 In the PY measurements, the photoemission yield (Y) was measured as a function of photon energy (hν, 4 ~9.5 eV), and the value of the ionization energy (I) was determined from the onset of the PY spectrum. I is defined as the minimum energy for removing an electron from the system. The


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number of photoelectrons was obtained using an amperometer to measure the current needed to compensate for the photoexcited holes generated in the sample. A negative voltage of -50 V was applied to the base plate to prevent carrier recombination of photoelectrons emitted from the sample surface with photogenerated holes in the sample. All the measurements were performed in a vacuum chamber (~ 4 × 10-3 Pa) at room temperature. Computational Method. DFT calculations with periodic boundary conditions were carried out using a plane wave band program, Castep.36,37 The Perdew, Burke, and Ernzerhof (PBE) functional38,39 was used together with ultra-soft core potentials.40 The basis set cut-off energy was set to 300 and 340 eV for geometry optimization and the post energy calculation. The electron configurations of atoms were O: 2s22p4, Ti: 3s23p64s23d2, Se: 4s24p4, and Cd: 4d105s2. RESULTS AND DISCUSSION Morphology Characterization of CdSe QDs on Single Crystal A-TiO2. The (001)- and (102)oriented anatase TiO2 epitaxial thin films (A-TiO2(001) and A-TiO2(102)) have thicknesses of ~ 50 nm. The AFM images of each of these show grain-like structures and the root mean square roughness (RMS) are 0.55 nm for A-TiO2(001) and 1.87 nm for A-TiO2(102). The larger RMS of A-TiO2(102) is probably due to the larger lattice mismatch and the double-domain microstructure.32 From the top view of AFM images, the sizes of the crystalline grain are ~ 78 nm (A-TiO2(001)) and ~ 56 nm (A-TiO2(102)), respectively. The resistivity of both A-TiO2 is around 1 MΩ (ρ ~ 1 Ωcm), indicating the appropriate stoichiometry. The resistivity is so high that Hall measurement is impossible to evaluate carrier concentration. We anticipate the carrier concentrations of both A-TiO2 are less than 1017 cm-3 by the analogy that the carrier concentration is around 1018 cm-3 at ρ ~ 10-2 Ωcm. Figure 1(a) and 1(b) show AFM images of CdSe QDs


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adsorbed on A-TiO2(001) and A-TiO2(102), respectively (adsorption time, 18 h), which show grain-like structures. From the top view of AFM images of CdSe QDs on A-TiO2, the sizes of the aggregation are ~ 31 nm (A-TiO2(001)) and ~ 34 nm (A-TiO2(102)), respectively, indicating no correlation with A-TiO2 morphology. Abs Characterization of CdSe QDs on Single Crystal A-TiO2. Firstly, Abs measurements were carried out to estimate the absolute value of the optical absorption. Abs is defined as Abs = log10(I0/It), where I0 and It are the incident and transmitted light intensity. As a result, Abs is the product of the optical absorption coefficient (α) and the thickness (L) of the material (Abs = αLlog10 e). In reference 41, optical absorption coefficients greater than the energy gap are sizeindependent, whereas, in the energy gap region, the optical absorption coefficient is sizedependent.41 The adsorption time dependence of the Abs of CdSe QDs at a photon energy of 3.0 eV, higher than the energy gap to avoid the size-dependent change in α, is shown in Figures 1(c) for QDs on A-TiO2(001) and 1(d) for QDs on A-TiO2(102). Here, we can assume that Abs is proportional to the thickness (due to adsorption) of CdSe QDs on A-TiO2. Figure 1(e) shows the Abs of CdSe QDs for an adsorption time of 24 h on A-TiO2(001) and A-TiO2(102). Assuming that 24 h adsorption corresponds to the seed initial growth, the seed initial growth is independent of the orientation of the substrate. This trend is different from that of PbS QDs on different surface orientations of R-TiO2.24 There is a difference between the Abs gradients (adsorption rate for adsorption time) for CdSe QDs on A-TiO2(001) and on A-TiO2(102) in Figures 1(c) and 1(d), indicating the different adsorption rates. Figure 1(f) shows the adsorption rates at which the Abs of CdSe QDs on A-TiO2(001) and A-TiO2(102). The adsorption rate of CdSe QDs on A-TiO2(001) is higher than that on A-TiO2(102), indicating differences in adsorption. Therefore, the adsorption of CdSe QDs on A-TiO2 is surface orientation dependent, corresponding to different surface energies.


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Hence, DFT calculations of the adsorption energy with periodic boundary conditions were carried out using a plane wave band program. The atomic compositions of the slabs were (TiO2)100(CdSe)13 and (TiO2)110(CdSe)13 for the (001) and (102) orientations, respectively, together with (TiO2)64(CdSe)13 for the (101) orientation for comparison (see Figure S1, Supporting Information). The adsorption energies were estimated by the energy difference, i.e., ΔE = E(TiO2/CdSe) – E(TiO2) – E(CdSe), and found to be -10.9 and +0.99 eV for A-TiO2(001) and ATiO2(102), respectively. We conclude that the adsorption energy is negative large value on (001) corresponds to higher adsorption energy. On the other hand, that on (102) shows positive value corresponding unstable situation in DFT calculations. The adsorption was very strong on (001), whereas unstable on (102), qualitatively similar to the results in Figure 1(f). For comparison, the adsorption energy for A-TiO2(101) (typical facet) was additionally estimated to be -0.70 eV. The order of adsorption energy is (001) » (101) > (102) according to our DFT calculations, which is similar to the experimental results (Figure 1(f)). Also, the projected densities of states (PDOS) are roughly almost the same for the three slab models, and was obtained for the (TiO2)64(CdSe)13 slab (see Figure S2, Supporting Information). The four band edges appear and can be seen as the valence band maximum (VBM) (O2p) and the conduction band minimum (CBM) (Ti3d) of ATiO2, and the ground state (Se4p) and excited state (Cd5s5p) energy levels of CdSe. PA Spectroscopy Characterization of CdSe QDs on Single Crystal A-TiO2. The optical absorption characterization of the A-TiO2(001) and A-TiO2(102) substrates was done using PA spectroscopy (see Figure S3, Supporting Information). The logarithmic PA intensity varies linearly as a function of photon energy in two regions (2.8 – 3.2 eV and 3.2 - 3.7 eV) and has two peak positions at around 3.2 eV and 3.5 eV, respectively, in both A-TiO2(001) and ATiO2(102). Therefore, similar optical absorption processes can be seen both in A-TiO2(001) and


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A-TiO2(102). The peak position around 3.2 eV corresponds to the indirect band transition (Γ3 → Χ1b) and the peak around 3.5 eV corresponds to the direct band transition (Χ1a → Χ1b).21,42 The positions of Γ3, Χ1a, and X1b are shown in the short energy diagram of A-TiO2 (see Figure S4, Supporting Information).21,42 Therefore, the electronic structures in A-TiO2(001) and ATiO2(102) are not influenced by the morphology difference shown in Figure 1(a) and 1(b). Figures 2(a)-2(c) and 2(f)-2(h) show PA spectra of CdSe QDs on A-TiO2(001) and ATiO2(102), respectively, with different adsorption times ((a),(f): 24 h; (b),(g): 48 h; (c),(h): 72 h). The spectra are normalized to a photon energy of 3.0 eV. The energy gap between the ground and excited states was estimated from the knee point (E1) indicated by the vertical arrow (↑). In general, the energy gap measured by the knee position in the logarithmic PA spectrum agreed well with the values reported in the literature.43 To confirm the reliability of PA spectroscopy, we measured the PA spectrum of commercial bulk CdSe powders with size of a few μm. The PA spectrum showed that the knee point appeared at around 1.7 eV, which is in good agreement with the band-gap energy reported elsewhere. In spite of the aggregation of CdSe QDs on ATiO2(001) and A-TiO2(102), the electronic structure of CdSe QDs is independent of the aggregation since the position of E1 is higher than bulk bandgap (~ 1.7 eV) due to quantum confinement effect, and the trend is similar to the reported results.22,23 Here, E1 corresponds to the first excitation energy of CdSe QDs between excited state energy level and ground state energy level. A lower energy shift of E1 (or longer wavelength shift: red shift) appears when the adsorption time increases. This is related to the reduction in the quantum confinement effect with the growth of the CdSe QDs. The average diameter was calculated from the value of E1 using the effective mass approximation.44 Figures 2(d) and 2(i) show the adsorption time dependence of the average diameters of CdSe QDs on A-TiO2(001) and A-TiO2(102), respectively. There is a


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possibility of a size distribution in synthesized CdSe QDs. If there is any size distribution, the position of E1 depends on the modulation frequency.35 However, our PA measurements suggest that there is a few size distribution because the position of E1 is independent of the modulation frequency. When the adsorption time increases, the diameter increases and then shows a tendency toward saturation in both cases. In general, the volume of the crystal increases with adsorption time in the case of solution growth. Hence, crystal diameter increases with the cubic root of adsorption time (t1/3). However, the experimental results shown in Figure 2(d) and 2(i) show the deviation from the normal solution growth. We propose an assumption that there is another growth mechanism (negative or positive) including seed initial growth and it is proportional to the adsorption time (t). The time dependence of CdSe QD diameter is assumed to be composed of two terms, linear and cubic root dependences. The CdSe QD diameter can be written from the assumption as R = C1t + C2t1/3,

(1)

where C1 and C2 are the fitting parameters. C2 corresponds to the parameter for normal solution growth and C1 is an adjustable parameter for taking account of saturation effect.45 The fitting values of C1 of CdSe QDs on A-TiO2(001) and (102) are -0.215 and 0.006, respectively. C1 of CdSe QDs on A-TiO2(001) shows negative value, indicating the suppression of the CdSe QDs crystal growth. On the other hand, that of CdSe QDs on A-TiO2(102) shows around zero, indicating the lower suppression in the adsorption time up to 72 h. The Gibbs-Thomson effect indicates the existence of both size dependent growth and critical size. According to the GibbsThomson effect, nanocrystals smaller than critical size have negative growth rates (dissolve), while larger nanocrystals grow at rates depending on size.45,46 When the size is the same as the critical size, the growth is zero. The fitting values of C2 of CdSe QDs on A-TiO2(001) and (102)


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are 1.98 and 1.87, respectively, which are similar to each other. The difference of crystal growth of CdSe QDs on A-TiO2(001) and (102) is due to the appearance of suppression (negative growth). The kinetics of crystal growth is affected strongly by the variation in the surface energy, provided that the crystallites are sufficiently small so that the Gibbs-Thomson effect is significant.45,46 The results suggest the difference of surface energy of CdSe QDs on different ATiO2 surface orientations. The optical absorption below the fundamental absorption region increases exponentially in semiconductors (Urbach tail).47 Studying the Urbach tail gives basic information on the disorder, defects, impurities, and electron-phonon interactions.23,48-50 The PA signal intensity (P) in the Urbach tail region is expressed by the following equation:49

P = P0 exp

[

𝜎(ℎ𝜈 ― ℎ𝜈0) 𝑘𝐵𝑇

],

(2)

where h is Planck’s constant (hν, incident photon energy), kB is the Boltzmann’s constant, T is absolute temperature, and P0 and ν0 are fitting parameters. σ is a characteristic of the logarithmic slope (Urbach tail) and is called the steepness parameter. Figures 2(e) and 2(j) show the dependence of the value of σ on the adsorption time for CdSe QDs on A-TiO2(001) and ATiO2(102), respectively. We assume firstly that the value of σ is related to the structural disorder in the CdSe QDs. A lower σ corresponds to a broader Urbach tail and hence to higher structural disorder.51 σ increases with increasing adsorption time in both cases, indicating a decrease in structural disorder. The rate at which the steepness parameter increases for A-TiO2(102) is higher (~ twice) than that for (001). This indicates that there is less structural disorder in CdSe QDs on A-TiO2 (102) than those on A-TiO2(001) with increasing adsorption time (increasing average


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diameter). Therefore, the structural disorder of CdSe QDs is not influenced by the aggregations and the morphologies of A-TiO2 surfaces (grain-like structures). Another possibility for the adsorption time dependence of σ is the effect of electro-phonon interactions.49 The experimental results suggest that the electron-phonon interaction for CdSe QDs on A-TiO2(102) decreases with increasing adsorption time (increasing average diameter). In the future, we need to know the temperature dependence of σ in order to clarify the nature of the structural disorder and/or electron-phonon interactions of CdSe QDs with different adsorption times on different substrate orientations.49 Figure 3 shows a schematic illustration of the crystal growth of CdSe QDs on ATiO2(001) and A-TiO2(102) at an adsorption time of 72 h. Figure 3 is inference and we did not consider layer detachment and crack. In Figure 3, we emphasize different piled thickness and size of CdSe QDs on different substrate (A-TiO2(001) and A-TiO2(102)). PY Spectroscopy Characterization of CdSe QDs on Single Crystal A-TiO2. We applied PY spectroscopy to determine the adsorption-dependent (size-dependent) ground state energy levels of CdSe QDs on A-TiO2(001) and A-TiO2(102). An understanding of the absolute energy levels is important to obtain a complete picture of the electronic structure. Colored vertical lines in Figure 4 correspond to the index to overview the slope of PY spectra for lower energy region (between red and yellow lines) and higher energy region (between green and purple lines). Figures 4(a) and 4(e) show PY spectra (Yield1/3 vs. photon energy) of A-TiO2(001) and ATiO2(102) substrates, respectively. We employed Yield1/3 based on a theoretical analysis.52,53 The value of the valence band maximum (VBM) for A-TiO2 was determined by extrapolating the linear part of Yield1/3 to the base line. We applied straight lines above a photon energy of 8.3 eV (linear region). Figures 4(b)-4(d) and 4(f)-4(h) show PY spectra of CdSe QDs on A-TiO2(001) and A-TiO2(102), respectively with different adsorption times ((b),(f): 24 h; (c),(g): 48 h; (d),(h):


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72 h). The value of the ground state energy level of the CdSe QDs was determined by extrapolating the linear part of Yield1/3 to the base line. We applied straight lines above photon energies of 6.0 eV for A-TiO2(001) and 7.5 eV for A-TiO2(102) (linear region). The PY spectra of CdSe QDs on A-TiO2(001) with adsorption times of 48 h and 72 h (Figures 4(c) and 4(d)) have meandering curves in the higher photon energy region possibly due to higher insulating characteristics in these specimens. Figures 5(a) and 5(b) show the alignment of the ground state (blue lines) and excited state (red lines) energy levels of CdSe QDs with different adsorption times together with the VBMs and conduction band minimums (CBMs) of the A-TiO2(001) and A-TiO2(102) substrates (error in energy level is ±0.02 eV). The first excited energy levels of the CdSe QDs were deduced from the PA characterization of E1. The ground state energy levels and excited state energy levels of CdSe QDs on A-TiO2(102) are lower than those on A-TiO2(001). Also, the excited energy levels of CdSe QDs on A-TiO2(102) are lower than the CBM of ATiO2(102), suggesting that sensitization by photoelectrons transfer from the excited levels of CdSe QDs to the CBM of A-TiO2(102) is not possible. Our preliminary transient grating (TG) characterization based on ultrafast carrier dynamics22,23 also suggests the impossibility of sensitization for A-TiO2(102) surface. TG characterization shows that the relative component of CdSe QDs on A-TiO2(102) is lower (~ 0.44) than that of A-TiO2(001) (~ 0.74), suggesting a few electron transfer. From the results of Figure 5(a), there is a possibility of higher photovoltaic conversion efficiency QD heterojunction solar cell when A-TiO2(001) nanosheets are applied as elecrtrode.23 The more negative value of the ground state energy level of CdSe QDs on ATiO2(102) than on A-TiO2(001) is possibly due to the difference between the permittivities of the A-TiO2(001) and A-TiO2(102) substrates and the Coulomb interactions between the CdSe QDs and A-TiO2. The adsorption time dependence of the ground and excited state energy levels of


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CdSe QDs on A-TiO2(001) and A-TiO2(102) are shown in Figures 5(c) and 5(d). Figures 5(c) and 5(d) show that the ground state energy levels of the CdSe QDs shift upward to vacuum level (positive polarization) with increasing adsorption time. The trend of upward shit to vacuum level on A-TiO2(001) is similar to that on A-TiO2(102). On the other hand, the excited state energy levels are shown to be independent of adsorption time within the experimental accuracy. There is a possibility of correlation between the positive polarization and the structural disorder of CdSe QDs. Also, these results are not explained by the simplest effective mass approximation (EMA) model which predicts the upward shift of the ground state energy level is approximately three times smaller than the downward shift of the excited state energy level.23 In this case, the EMA model is inaccurate and overestimates the quantum confinement effect. The effective density of states (N(E)) is obtained by derivative of the corresponding PY spectrum.54 The absolute values and the photon energy dependence of N(E) of the A-TiO2(001) and A-TiO2(102) substrates are similar to each other (see Figure S5(c) and S5(d), Supporting Information) in spite of the different surface morphology. Blue lines in Figures S5(c) and S5(d) show photon energy dependence (above 8.0 eV) of N(E). Also, we carried out a preliminary evaluation of N(E) of CdSe QDs with an adsorption time of 24 h on A-TiO2(001) and ATiO2(102). Blue lines in Figures S6(c) and S6(d) show photon energy dependence (above 8.0 eV) of N(E). The distribution of N(E) below the ground state energy level on A-TiO2(102) is somewhat localized (steeper slope of blue line) compared to that on A-TiO2(001), suggesting a difference between the CdSe QDs and A-TiO2 with different surface orientations (see Figures S6(c) and S6(d), Supporting Information). CONCLUSIONS


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The adsorption rate of CdSe QDs on A-TiO2(001) is higher than that on A-TiO2(102), indicating differences in adsorption and/or crystal growth. Therefore, the adsorption of CdSe QDs on ATiO2 is related to the orientation of the A-TiO2, corresponding to different surface energies due to the different electronic structures. DFT calculations of the adsorption energy with periodic boundary conditions were carried out. DFT calculations indicates that the adsorption energy was very strong on (001), whereas unstable on (102) ((001) » (101) > (102)), qualitatively similar to the experimental results of CdSe QDs adsorption on A-TiO2. On the other hand, in cases of CdSe QDs on A-TiO2(001) and A-TiO2(102), the average diameter increases with increasing adsorption time. The rate at which the diameter increases for A-TiO2(102) is higher than that for A-TiO2(001), although the adsorption rate for A-TiO2(102) is lower than that for A-TiO2(001). This suggests the existence of higher negative crystal growth (dissolve) on A-TiO2(001) according to the Gibbs-Thomson effect. The rate at which the steepness parameter (logarithmic slope of the optical absorption below the fundamental absorption edge) increases for ATiO2(102) is higher (~ twice) than that for (001). This indicates that there is less structural disorder in CdSe QDs on A-TiO2 (102) than those on A-TiO2(001) with increasing adsorption time. The ground state energy levels and excited state energy levels of CdSe QDs on ATiO2(102) are lower than those on A-TiO2(001). The ground state energy levels of the CdSe QDs shift upward (positive polarization) with increasing adsorption time and are similar to each other. There is a possibility of correlation between the positive polarization and the structural disorder of CdSe QDs. On the other hand, the excited state energy levels are shown to be independent of adsorption time within the experimental accuracy. ASSOCIATED CONTENT Supporting Information


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The three unit cells used for the calculations (Figure S1) and the projected density of states for Ti, O, Cd, and Se atoms from the top to the bottom of the band gap region (Figure S2). Photoacoustic spectra of A-TiO2(001)/LAO(100) and A-TiO2(102)/LAO(110) (Figure S3) and a short energy diagram of A-TiO2 (Figure S4). Photoelectron yield spectra and the effective density of states of A-TiO2(001) and A-TiO2(102) substrates (Figure S5) and those of CdSe QDs on A-TiO2(001) and A-TiO2(102) (Figure S6). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (T. T.); Tel.: +81-42-443-5464 *E-mail: [email protected] (Q. S.); Tel. +81-42-443-5471 ORCID Taro Toyoda: 0000-0002-2067-3689; Qing Shen: 0000-0001-8359-3275 Yasushi Hirose: 0000-0002-0792-4631 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Part of this work was supported by Core Research for Evolutional Science and Technology (CREST) program, Japan Science Technology Agency (JST). The work was also supported by JSPS Kakenhi Grant Numbers 26390016 and 17K04988. N. Nakazawa (Univ. ElectroCommun.) is acknowledged for helping with the absorbance measurements and useful comments. The authors want to thank D. Katsuzawa and T. Wakasugi (Univ. Tokyo) for helping with the preparation of A-TiO2 epitaxial films. We thank T. Amano and Y. Takeshita (Bunkoukeiki Co., Ltd.) for cooperation with the photoelectron yield spectroscopy measurements.


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PbS QDs/A-TiO QDs/A-TiO22 (001) (001) (a)(a)CdSe

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(b) CdSe QDs/A-TiO2 (102)

Figure 1. (a, b) AFM images of CdSe QDs adsorbed on (a) A-TiO2(001) and (b) A-TiO2(102) (adsorption time, 18 h). (c, d) Adsorption time dependence of absorbance at a photon energy of 3.0 eV for CdSe QDs adsorbed on (c) A-TiO2(001) and (d) A-TiO2(102). (e) Absorbance of CdSe QDs with an adsorption time of 24 h on A-TiO2(001) and (102). (f) Absorbance rate of CdSe QDs adsorbed on A-TiO2(001) and (102).


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Figure 2. (a–c) Photoacoustic spectra of CdSe QDs on A-TiO2(001) with different adsorption times from 24 – 72 h. (f–h), and those of CdSe QDs on A-TiO2(102) with different adsorption times from 24 – 72 h. Adsorption time dependence of average diameter of CdSe QDs on (d) A-TiO2(001) and (j) A-TiO2(102), and those of steepness parameter of CdSe QDs on (e) A-TiO2 (001) and (j) A-TiO2(102). ACS Paragon Plus Environment

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Figure 3. Schematic illustration of the crystal growth of CdSe QDs on A-TiO2(001) and (102) (adsorption time, 72 h).


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Figure 4. Photoelectron yield spectra of (a) A-TiO2(001) and (b) A-TiO2(102). (b-d) Photoelectron yield spectra for CdSe QDs on A-TiO2(001) with different adsorption times from 24 – 72 h. (f-h) Photoelectron yield spectra for CdSe QDs on A-TiO2(102) with different adsorption times from 24 – 72 h.


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Figure 5. (a-b) Alignment of energy levels of CdSe QDs on (a) A-TiO2(001) and (b) ATiO2(102) with different adsorption times from 24 – 72 h together with the positions of the valence band maxima of A-TiO2(001) and (102). (c-d) Adsorption time dependence of the ground state energy levels (blue lines) and the excited state energy levels (red lines) of CdSe QDs on (c) A-TiO2(001) and (d)(102).

TOC Graphic


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Anisotropic Crystal Growth, Optical Absorption, and Ground-State

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