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High Coverage Formation of CdS Quantum Dots on TiO by the Photocatalytic Growth of Preformed Seeds 2

Musashi Fujishima, Yasunari Nakabayashi, Kouichi Takayama, Hisayoshi Kobayashi, and Hiroaki Tada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04091 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

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The Journal of Physical Chemistry

High Coverage Formation of CdS Quantum Dots on TiO2 by the Photocatalytic Growth of Preformed Seeds Musashi Fujishima,a Yasunari Nakabayashi,a Kouichi Takayama,a Hisayoshi Kobayashi,b Hiroaki Tada a * Department of Applied Chemistry, School of Science and Engineering, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan b Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan a

Supporting Information Placeholder ABSTRACT: Photoelectrochemical experiments and density functional theory calculations indicated that visible-light irradiation of the CdS quantum dots (QDs)-TiO2 direct coupling system (CdS/TiO2) causes the electron injection from the valence band (VB) of CdS into the conduction band (CB) of TiO2 (path 2) in addition to the inter-CB electron transfer from CdS to TiO2 (path 1). Path 2 can be induced by the sub-bandgap excitation of CdS QDs to extend the spectral response of the CdS/TiO2 system. For path 2 as well as path 1 to effectively work, CdS QDs should be directly deposited on the TiO2 surface with high coverage. According to the guideline, a photocatalytic growth of the preformed seed (PCGS) technique has been developed. Transmission electron microscopy observation and X-ray photoelectron spectroscopy measurements of the CdS/TiO2 prepared by the PCGS technique indicated that the TiO2 surface is highly covered by CdS QDs. The technique was applied to mesoporous TiO2 nanocrystalline films (mp-TiO2) to yield CdS/mp-TiO2. CdS QD-sensitized photoelectrochemical (QD-SPEC) cells with a structure of CdS/mp-TiO2 (photoanode) | aqueous sulfide solution | Ag/AgCl (reference electrode) | Pt (cathode) were fabricated. The rate of hydrogen (H2) generation in the QD-SPEC cell under illumination of simulated sunlight (AM 1.5 one sun, λ > 430 nm) increases with an increase in the TiO2surface coverage by CdS QDs.

INTRODUCTION Nanohybrids comprising quantum dots (QDs) and TiO2 have attracted much interest as photocataltyic materials because the charge separation can be enhanced by the interfacial charge transfer between them. A representative of them is the CdS QD-loaded TiO2 (CdS/TiO2) system, which has mainly been studied in particulate systems.1-4 Also, a great deal of attention has been focused on the QD-loaded TiO2 nanostructures such as mesoporous TiO2 nanocrystalline film (mp-TiO2) for the application as the photoanode of photoelectrochemical (PEC) cells for solar energy conversion.5,6 The deep understanding of the mechanism on the visible-light-induced electron injection from CdS QDs to TiO2 underpins the improvement in the performances of the photocatalysts and photoanodes. As shown in Scheme 1, we have shown in the CdSe/TiO2 system that the sub-bandgap excitation-induced electron injection from the VB of CdSe to the CB of TiO2 (path 2) occurs in addition to the inter-CB electron transfer (path 1).7 However, the detailed mechanism in the CdS/TiO2 system remains unknown. If path 2 exists also in the CdS/TiO2 system, the high-coverage formation of CdS QDs on TiO2 can be expected to enhance the performance in the same manner as the CdSe/TiO2 system.8 On the other hand, the developments of the technology for synthesizing nanostructured scaffolds of QDs9 and the PEC cell architecture10-12 are in rapid progress. From a viewpoint of chemistry, the study on the formation of QDs on the TiO2 sur-

Scheme 1. Visible-light-induced electron injection from metal chalcogenide quantum dots to TiO2.

face is of great importance, but it is rather limited.13,14 The successive ionic layer adsorption and reaction (SILAR) method has usually been used for the preparation of QD/mp-TiO2. However, it is inappropriate for the application to the particulate system since the adsorption-washing-centrifugationreaction-washing-centrifugation cycle must be repeated many times. The photodeposition (PD) technique has recently been developed for conveniently preparing QD/TiO2 and QD/ZnO in the particulate and film forms.15-19 The application of the PD technique is currently being widen for preparing nanoscale composites of metal sulfide QDs with conducting polymers20

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and metal-organic frameworks.21 Also, metal selenide QDs have successfully been photodeposited on TiO222,23 by developing the preceding works.24,25 In the CdS/TiO2 photocatalysts, both the surfaces of CdS and TiO2 should be exposed to the surrounding media for the oxidation and reduction to occur.26 In the application as the photoanode of PEC cells, the high TiO2-surface coverage by QDs is desirable to reduce the loss due to the back electron transfer from TiO2 to the oxidant in the electrolyte solution.27 A problem in the present PD technique is that the TiO2-surface coverage with QDs is fairly low. Here we show that the photocatalytic growth of the CdS seeds preformed by the SILAR method (PCGS) remarkably increases the TiO2-surface coverage by CdS QDs. Further, the coverage effect on the performance of CdS/TiO2 as the photoanode in the CdS QD-SPEC cell for H2 generation from water was studied.

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JEM-3010 at an applied voltage of 300 kV. Energy dispersive X-ray (EDX) element mapping was performed for samples with respect to Ti, O, Cd, and S. X-Ray diffraction (XRD) patterns were obtained with a Rigaku SmartLab X-Ray diffractometer. X-Ray photoelectron spectroscopic (XPS) measurements were performed using a Kratos Axis Nova X-ray photoelectron spectrometer with a monochromated Al Ka X-ray source (hν = 1486.6 eV) operated at 15 kV and 10 mA. The take-off angle was 90°, and multiplex spectra were obtained for Cd3d and Ti2p photopeaks. UV/Vis diffuse reflectance spectra were recorded on a Hitachi U-4000 spectrophotometer. Photoelectrochemical measurements. Three-electrode PEC cells consisting of CdS/mp-TiO2 photoanode | 0.1 M Na2S + 5.0 × 10-2 M Na2SO3 + 0.1 M Na2SO4 (aqueous electrolyte solution) | Ag/AgCl (reference electrode) | Pt (cathode) were fabricated. Illumination of one sun (λ > 430 nm, AM 1.5, 100 mW cm-2) was carried out using 500W-Xe short arc lamp (UXL-500D-O, Ushio Inc.) as a light source. J-E curves with or without illumination was followed for the PEC cells connected with a potentio/galvanostat (HZ-5000, Hokuto Denko). Action spectra of IPCE were measured under the illumination of monochromatic light on the CdS/mp-TiO2 working electrode. A xenon lamp with a monochromator (fwhm = 10 nm, HM-5, JASCO) was used as a light source. The short-circuit current at the rest potential was measured as a function of excitation wavelength (λ/nm), and the IPCE was calculated using the following equation:

EXPERIMENTAL SECTION Preparation of CdS/TiO2 through the SILAR and PD methods. Pastes containing anatase TiO2 particles with a mean size of 20 nm (PST-18NR, Nikki Syokubai Kasei) was coated on indium tin oxide (ITO)-coated glass substrates ( 320 nm) was carried out for 3 h with a high-pressure mercury lamp (H400P, Toshiba) at 298 K; the light intensity integrated from 310 to 400 nm (I310-400 nm) was 3.8 mW cm-2. After irradiation, the films were pulled out from the solution and the particles were recovered by centrifugation. The resulting films and particles were washed with ethanol several times to be dried under vacuum. This deposition procedure is named as SILAR → PD (N) in the text. For the purpose of comparison, CdS/TiO2 were also prepared through the reverse deposition procedure, i.e., PD → SILAR(N), and through the sole deposition methods, i.e., SILAR(N) and PD. Characterization of CdS/TiO2. The amount of Cd in the deposits was determined by inductively coupled plasma (ICP) spectroscopy (ICPS-7500, Shimadzu). Transmission electron microscopic (TEM) observation was carried out with a JEOL

IPCE[%] =

J ph ( E ) N A hc IFλ

×100

(1)

where Jph(E) is the photocurrent at an electrode potential of E, NA is Avogadro constant, I (W cm-2) is light intensity, F is Faraday constant, h is Planck constant, and c is speed of light. The amount of hydrogen evolved at the Pt cathode at E = -0.6 V vs. Ag/AgCl was quantified by volumetric analysis as follows: a glass buret (volume: 25 mL) with its open end connected to a glass funnel was used to collect hydrogen evolved from cathode. The buret was placed in the electrolyte solution with its open end directing to the cathode and its tip end to the air. Before measurements, air evacuation and filling with the electrolyte solution were carried out for the buret. Hydrogen bubbles evolved from the cathode were collected in the buret throughout the experiment. Also, hydrogen bubbles adhering on the cathode surface were harvested carefully. Sum of the volume of hydrogen evolved was measured every 10 min by reading the position of the solution surface on the scale engraved on the buret. DFT Calculations. DFT calculations with the periodic boundary conditions were carried out by using a plane wavebased program, Castep.28 The Perdew–Burke–Ernzerhof (PBE) functional29 was used together with ultrasoft-core potentials.30 The basis set cutoff energies were set to 300 eV. The electron configurations of the atoms were Cd4d105s2, S3s23p4, O2s22p4, and Ti 3s23p63d24s2. Geometry optimization was carried out with respect to all atomic coordinates and the lattice constants were fixed. Since GGA functionals underestimate bandgap energies for some semiconductors,31 the GGA+U method32 was employed for CdS and CdS/TiO2 systems. The Hubbard U potential was applied only to the S3p AOs. The unit cell of CdS was optimized using the normal PBE functional (no U value), and then series of energy calculations were carried out with different U values from 3.5 to 6.0


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eV. The U value of 5.0 eV was adopted, as it gave a bandgap of 2.472 eV, which was the closest to the experimental values [2.42 eV (rt) or 2.58 eV (0 K)]. The TiO2 part was modeled by anatase (101) surface, and the slab model was represented by a unit cell consisting of (TiO2)64 (See Figure 2). The lattice con-

result was also observed in the CdS/TiO2 nanotube array sys(A)

(B)

stants were a= 15.104 Å, b=20.420 Å, α =88.98 ° , and

β=γ=90°. As to the direction to surface normal, c was set to 30 Å including the vacuum region. A few cluster models of CdS with different sizes were investigated, however, the results with (CdS)13 were discussed in this work. The geometry was optimization with the PBE functional, and optical absorption, that is, excitation spectra were calculated with the PBE+U functional for the optimized structure and gradually separated structures between (CdS)13 and (TiO2)64. The d value, that is, the length of a Cd–O bond was 2.482 Å for the optimized structure. The geometry optimization with the PBE+U functional led to a longer d value, and to less consistency with the experimental results. Among the parameters in simulation of spectra, absorption (rather than reflection) and polarized light modes were adopted with the smearing width of 0.1 eV. The scissor operator was not used.

Figure 2. (A) Side and (B) top views of unit cell structure of (CdS)13/(TiO2)64. Ti, O, Cd, and S atoms are shown as gray, red, pale brown, and yellow spheres, respectively. H atoms are omitted in the cell.

tem, where the Eon (= 2.2 eV) is smaller than the Eg for the bulk-state CdS.35 To clarify the origin, we performed DFT calculations for a model coupling system of CdS QDs and anatase TiO2, in which a (CdS)13 cluster is supported on a (TiO2)64 slab. Figure 2 shows the optimized structure of the (CdS)13 cluster on the anatase (101) surface. The nearest distance between the cluster and the TiO2 surface is 2.48 Å. The projected density of states (PDOS) for (CdS)13/(TiO2)64 shows that the coupling with the CdS cluster decreases the bandgap of TiO2 (Figure S2). Also, the VB maximum is composed of S3p atomic orbitals, which

RESULTS AND DISCUSSION 1. Electron injection mechanism in the directly coupled CdS/TiO2 system Firstly, the mp-TiO2 film was coated on ITO electrode. CdS QDs were photodeposited on mp-TiO2 by adding mercaptoacetic acide (MAA) with varying concentration.8,33 The sample prepared at a MAA concentration of C is designated as CdS(C)/mp-TiO2. The particle size of CdS decreases with an increase in C: the mean radius (r) of CdS particles is 6.0 nm at C = 0, r = 4.9 nm at C = 0.01 mM, and r = 4.0 nm at C = 0.1 mM (Figure S1). The basic PEC property of the CdS/mp-TiO2 photoanode was evaluated for a three-electrode QD-SPEC cells with a structure of photoanode | 0.1 mol dm-3 Na2S + 5.0 × 10-2 mol dm-3 Na2SO3 + 0.1 mol dm-3 Na2SO4 (aqueous electrolyte solution) | Ag/AgCl (reference electrode) | Pt (cathode). Figure 1 compares the action spectra of IPCE for the QDSPEC cells using the CdS/mp-TiO2 photoanodes and their absorption spectra. The bandgap (Eg) for CdS(C = 0)/mp-TiO2 sample is ~2.4 eV, which is close to the value of the bulk-state

(A)

(B)

(C)

(D)

(E)

(F)

Figure 3. Electron density contour maps of orbitals at lower and upper parts of the O2p band, HOMO, and LUMO for (CdS)13/(TiO2)64. (A) HOMO-25: E = -0.625 eV, (B) HOMO-15: E = -0.519 eV, (C) HOMO5: E = -0.372 eV, (D) HOMO-1: E = -0.142 eV, (E) HOMO: E = 0.0 eV, (F) LUMO: E = +2.475 eV.

Figure 1. IPCE action spectra (solid circle) and UV-vis absorption spectra (solid line) of CdS(C)/TiO2.

CdS.34 The increase in C significantly increases the Eg due to the quantum size effect. Interestingly, the photon energy at the photocurrent onset (Eon) is 2.3 ± 0.1 eV, and the energy gap between the Eg and Eon increases with increasing C. A similar

is higher than the top of the O2p band by ~0.3 eV. The CB minimum is composed of the Ti3d atomic orbitals in the same manner as pure TiO2. Figure 3 shows the electron density contour maps of orbitals at the upper part of the VB band, and the



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HOMO and LUMO for (CdS)13/(TiO2)64. The upper parts of the VB band (HOMO-25–HOMO-1) consist of O2p and/or S3p orbitals. The HOMO is mainly localized on the S atoms which are at the peripheral of cluster and at the interface to TiO2. Importantly, the chemical bond through the overlap of the S3p and Ti3d orbitals is present between the CdS cluster and the TiO2 surface. On the other hand, the LUMO is localized on Ti atoms typical to the CB minimum of TiO2. Further the Eon value in the IPCE action spectra (2.3 ± 0.1 eV) is comparable with the HOMO-LUMO gap energy for the (CdS)13/(TiO2)64 cluster (2.475 eV). Therefore, the absorption near the tail edge of CdS/TiO2 can be assigned to the interfacial electronic transition from CdS QD levels to TiO2.

intensifies at d = 2.48 Å for the optimized structure. Evidently, the absorption tail is induced by neither the CdS cluster nor TiO2 but the interfacial bonds between them.

2. High coverage formation of CdS QDs on TiO2

Figure 6. Cd3d-XPS (A) and Ti2p-XPS (B) spectra for CdS/mp-TiO2 prepared by various methods.

The CdS seeds were formed on the TiO2 surface by the SILAR method, and then, grown by the subsequent PD (SILAR(N) → PD). For comparison, samples were prepared by reversing the order of the steps (PD → SILAR (N)). The SILAR cycle number (N) was varied, while the irradiation time was fixed at 3 h in the PD process. The amount of CdS loaded on mp-TiO2 (X) was determined by inductively coupled plasma spectroscopy (Figure S4). The X in the SILAR sample increases according to the equation of X (µg cm-2) ≈ 3.9N, and the value for the PD sample was ~87 µg cm-2. The X value for the PD → SILAR (N) sample was comparable to that for the SILAR (N) → PD sample at the same N, being expressed by X (µg cm-2) ≈ 5.7N + 87. Figure 5 shows transmission electron micrographs (TEM) for the PD, SILAR (N = 10), PD → SILAR (N = 10), and SILAR → PD (N = 10) samples, and the CdS particle size distributions were compared in Figure S5. The deposits formed on TiO2 surface were confirmed to be hexagonal CdS by energy dispersive X-ray (EDX) analyses and X-ray diffraction (XRD) measurements (Figures S6 and S7). The comparison of images Figure 5 (A) and (B) indicates that the density of observable CdS particles in the SILAR (N = 10) sample is higher than that in the PD sample. The particle size distribution indicates that fairly uniform CdS particles with a mean

Figure 4. DFT-calculated excitation spectra for (CdS)13/(TiO2)64 with varying the distance between CdS cluster and TiO2 surface (d): Red, d = 2.48 Å; orange, d ~ 3 Å; green, d ~ 4 Å; blue, d ~ 5 Å; violet, d ~ 6 Å. (B)

(A) (C)

(B)

(A)

The kinetics or the efficiency of the interfacial chargetransfer is sensitive to the distance of the CdS cluster from the TiO2 surface (d).15 Figure 4 shows the DFT-calculated excitation spectra for (CdS)13/(TiO2)64 clusters with the d value varied. At d ≥ 4 Å, the absorption edge is located at ~2.7 eV, which is in agreement with the Eg for the CdS cluster (Figure

(A)

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(B)

(D)

Figure 5. TEM images for the CdS/TiO2 samples prepared by PD (A), SILAR (N = 10) (B), PD → SILAR (N = 10) (C), and SILAR → PD (N = 10) (D).

Figure 7. UV-visible absorption spectra for the PD and SILAR (N = 1, 5, 10) samples (A) and for PD → SILAR (N = 1, 5, 10) and SILAR (N = 1, 5, 10) → PD samples (B).

S3). Surprisingly, at d ≈ 3 Å, weak absorption appears in the visible range from 2.4 to 3 eV, and further, the absorption

particle diameter of 8.9 nm are formed on TiO2 in the PD


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sample, while particles larger than 10 nm are present in the SILAR sample. In image Figure 5 (C), the CdS particle density increases somewhat as compared to the PD sample. From image (D), it is seen that the CdS particles preformed by the SILAR method act as seeds to grow during the subsequent PD process. The morphology of the samples was further observed by high resolution (HR)-TEM (Figure S8). It is noteworthy that the SILAR → PD process yields flattened CdS particles with a contact angle against TiO2 surface significantly lower than those for the particles prepared by the other three methods. To gain the information about the TiO2-surface coverage by CdS QDs, X-ray photoelectron specroscopy (XPS) spectra were measured. Figure 6 shows Cd3d-XPS (A) and Ti2p-XPS (B) spectra for the CdS/mp-TiO2 samples prepared by various methods. In spectra (A), two signals are observed at binding energies (EB) of 405.1 and 411.8 eV corresponding to the emissions from the Cd3d5/2 and Cd3d3/2 orbitals of CdS, repsectively. The signal intensity is on the order of SILAR (N = 10) < PD < SILAR (N = 10) → PD ≈ PD → SILAR (N = 10). In spectra (B), there are emissions at EB = 458.6 eV and 464.5 eV from the Ti2p3/2 and Ti2p1/2 orbitals of TiO2, respectively. The order of the Ti-signal intensity is opposite to that of the Cd-signal intensity, i.e., SILAR (N = 10) → PD ≈ PD → SILAR (N = 10) < PD < SILAR (N = 10) 430 nm, AM 1.5, one sun). Figure 8A shows the J-E curves for the SILAR and PD photoanode cells. The photocurrent in the SILAR photoanode cell increases with increasing N, but is significantly smaller than that in the PD photoande cell. Interestingly, as shown in Figure 8B, the photocurrent for the SILAR (N) → PD photoanode cell is significantly greater than that for the PD → SILAR (N) photoanode at N = 5 and 10. The quantity of H2 evolved was also measured under the same conditions. Figure 8C shows time courses for H2 generation in the QD-SPEC cells with the photoanodes prepared by the SILAR and PD methods at E = -0.6 V vs. Ag/AgCl. In every cell, the amount of H2 increases in proportion to irradiation time, and the rates for H2 generation paralleled the photocurrents. Also, to check the stability of the photoanode during the PEC experiment, Raman and UVvisible absorption spectra were measured (Figure S9). As a result, we could not find any notable change in the spectra for the photoanode before and after the PEC experiment. These results indicate that S2- ions work as a good hole scavenger to effectively suppress the photocorrosion of CdS QDs.36 The SILAR photoanode cell yields H2 generation with the rates of

Figure 8. (A, B) Current (J)-potential (E) curves in the QD-SPEC cells using CdS/mp-TiO2/ITO as the photoanode under illumination of one sun (λ > 430 nm, AM 1.5, 100 mW cm-2). (C, D) Time courses for H2 generation in the QD-SPEC cells using the CdS/mp-TiO2/ITO photoanodes under the same conditions.

(CdS/mp-TiO2/ITO) is important for the application as the photoanode of the QD-SPEC cells. Figure 7A shows UV-



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0.043 mL h-1 cm-2 at N = 5 and 0.080 mL h-1 cm-2 at N = 10, which are significantly smaller than the rate of 0.19 mL h-1 cm2 for the PD photoanode cell. This can mainly be attributed to the larger visible-light harvesting for the PD photoanode than the SILAR photoanodes. Figure 8D compares the rate of H2 generation in the QD-SPEC cells using the photoanodes prepared by the PD → SILAR (N) and SILAR (N) → PD methods. At N = 1, the rates are comparable. At N = 5 and 10, the rate for the SILAR → PD photoanode is much greater than that for the PD → SILAR photoanode in spite that the visiblelight harvesting of the former is smaller than that of the latter. The relation between the H2 generation rate in the CdS QDSPEC cell and the ICd3d5/2/ITi2p3/2 for the photoanode was examined. As shown in Figure 9, a clear positive correlation is observed between them. Very recently, the importance of the TiO2-surface coverage with QDs has also been shown by the mercaptoacetic acid-surface modified PD technique in the CdSe QD-SPEC cell for H2 generation from water.8 The increase in the coverage enhances the electron transfer from CdS to TiO2 and the hole transfer from CdS to S2- ions in the electrolyte solution because of the increase in the CdS-TiO2 and electrolyte solution-CdS interfacial areas. Also, the loss due to the back electron transfer from TiO2 to the oxidant in the electrolyte solution can be reduced since CdS has a higher conduction band edge in energy than TiO2 acts as an energy barrier.27 Consequently, the H2 generation would be enhanced with increasing coverage.

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Figure 9. Plots of H2 generation rate versus ICd3d5/2/ITi2p3/2. The ratio of ICd3d5/2/ITi2p3/2 was calculated from the data shown in Figure 6.

ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research (C) No. 15K05654 and MEXT-Supported Program for the Strategic Research Foundation at Private Universities.

REFERENCES 1 Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. All-SolidState Z-Scheme in CdS-Au-TiO2 Three-Component Nanojunction System. Nat. Mater. 2006, 5, 782-786. 2 Park, H.; Choi, W.; Hoffmann, M. R. Effects of the Preparation Method of the Ternary CdS/TiO2/Pt Hybrid Photocatalysts on Visible Light-Induced Hydrogen Production. J. Mater. Chem. 2008, 18, 23792385. 3 Kannaiyan, D.; Kim, E.; Won, N.; Kim, K. W.; Jang, Y. H.; Cha, M.A.; Ryu, D. Y.; Kim, S.; Kim, D. H. On the Synergistic Coupling Properties of Composite CdS/TiO2 Nanoparticle Arrays Confined in Nanopatterned Hybrid Thin Films. J. Mater. Chem. 2010, 20, 677-682. 4 Park, H.; Kim, Y. K.; Choi, W. Reversing CdS Preparation Order and Its Effects on Photocatalytic Hydrogen Production of CdS/Pt-TiO2 Hybrids Under Visible Light. J. Phys. Chem. C 2011, 115, 6141-6148. 5 Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737-18753. 6 Rühle, S.; Shalom, M.; Zaban, A. Quantum-Dot-Sensitized Solar Cells. ChemPhysChem 2010, 11, 2290-2304. 7 Yoshii, M.; Kobayashi, H.; Tada, H. Sub-Bandgap Excitation-Induced Electron Injection from CdSe Quantum Dots to TiO2 in the Directly Coupled System. ChemPhysChem 2015, 16, 1846-1851. 8 Yoshii, M.; Murata, Y.; Nakabayashi, Y.; Ikeda, T.; Fujishima, M.; Tada, H. Coverage Control of CdSe Quantum Dots in the Photodeposition on TiO2 for the Photoelectrochemical Solar Hydrogen Generation. J. Colloid Intereface Sci. 2016, 474, 34-40. 9 Grimes, C. A.; Varghese, O. K.; Ranjan, S. Light, Water, Hydrogen-The Solar Generation of Hydrogen by Water Photoelectrolysis, Springer, New York, 2008. 10 Paracchino, A.; Laporte, V.; Sivula, K.; Grätzel, M.; Thimsen, E. Highly Active Oxide Photocathode for Photoelectrochemical Water Reduction. Nat. Mater. 2011, 10, 456-461. 11 Raja, R.; Sudhagar, P.; devadoss, A.; Terashima, C.; Shrestha, L. K.; Nakata, K.; Jayavel, R.; Ariga, K.; Fujishima, A. Pt-Free Solar Driven Photoelectrochemical Hydrogen Fuel Generation Using 1T MoS2 CoCatalyst Assembled CdS QDs/TiO2 Photoelectrode. Chem. Commun. 2015, 51, 522-525. 12 Peerakiatkhajohn, P.; Butburee, T.; Yun, J.-H.; Chen, H.; Richards, R. M.; Wang, L. A Hybrid Photoelectrode with Plasmonic Au@TiO2 NanoParticles for Enhanced Photoelectrochemical Water Splitting. J. Mater. Chem. A 2015, 3, 20127-20133.

CONCLUSIONS This study has reported a photocatalytic growth of the preformed seeds (PCGS) technique for forming CdS QDs on the TiO2 surface with high coverage, and shown that the rate of H2 generation in the CdS QD-SPEC cell increases with an increase in the coverage. We anticipate that the present design and the technique for preparing the directly coupled CdS/TiO2 system are useful for the applications to not only the photoanode of QD-SPEC cells but also photocatalysts for the solar-to-chemical conversion.

ASSOCIATED CONTENT AUTHOR INFORMATION Supporting Information Plots of mean particle radius of CdS versus MAA concentration; PDOS for (CdS)13/(TiO2)64, (CdS)13, and (TiO2)64; plots of the amount of CdS loaded on mp-TiO2 versus SILAR cycle number; particle size distribution of CdS QDs and TiO2 (A100); (HR-)TEM images, EDX element mapping, X-ray diffraction patterns, Raman spectra, and UV-vis absorption spectra for CdS/mp-TiO2; This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author TEL: +81-6-6721-2332, FAX: +81-6-6727-2024 E-mail: [email protected]


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The Journal of Physical Chemistry Controlled PbS Quantum Dots on TiO2. ChemPhysChem 2010, 11, 2349-2352. 34 Katsikas, K.; Eychmuller, A.; Giersig, M.; Weller, H. Discrete Excitonic Transitions in Quantum-Sized CdS Particles. Chem. Phys. Lett. 1990, 172, 201-204. 35 Baker, D. R.; Kamat, P. V. Photosensitization of TiO2 Nanostructures with CdS Quantum Dots: Particulate versus Tubular Support Architectures. Adv. Funct. Mater. 2009, 19, 805-811. 36 Tachibana, Y.; Akiyama, H. Y.; Ohtsuka, Y.; Torimoto, T.; Kuwabata, S. CdS Quantum Dots Sensitized TiO2 Sandwich Type Photoelectrochemical Solar Cells. Chem. Lett. 2007, 36, 88-89.

13 Lee, H.; Wang, M.; Chen, P.; Gamelin, D. R.; Zakeeruddin, S. M.; Grätzel, M.; Nazeeruddin, M. K. Efficient CdSe Quantum DotSensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process. Nano Lett. 2009, 9, 4221-4227. 14 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. 15 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. 16 Ho, W.; Yu, J. C.; Lin, J.; Yu, J.; Li, P. Preparation and Photocatalytic Behavior of MoS2 and WS2 Nanocluster Sensitized TiO2. Langmuir 2004, 20, 5865-5869. 17 Zhukovskiy, M. A.; Stroyuk, A. L.; Shvalagin, V. V.; Smirnova, N. P.; Lytvyn, O. S.; Eremenko, A. M. Photocatalytic Growth of CdS, PbS, and CuxS Nanoparticles on the Nanocrystalline TiO2 Films. J. Photochem. Photobiol. A, Chem. 2009, 203, 137-144. 18 Zhu, H.; Yang, B.; Xu, J.; Fu, Z.; Wen, M.; Guo, T.; Fu, S.; Zuo, J.; Zhang, S. Construction of Z-scheme Type CdS–Au–TiO2 Hollow Nanorod Arrays with Enhanced Photocatalytic Activity. Appl. Catal. B 2009, 90, 463-469. 19 Kozytskiy, A. V.; Stroyuk, O. L.; Skoryk, M. A.; Dzhagan, V. M.; Kuchmiy, S. Y.; Zahn, D. R. T. Photochemical Formation and Photoelectrochemical Properties of TiO2/Sb2S3 Heterostructures. J. Photochem. Photobiol. A: Chem. 2015, 303-304, 8-16. 20 Varga, A.; Endrődi, B.; Hornok, V.; Visy, C.; Janáky, C. Controlled Photocatalytic Deposition of CdS Nanoparticles on Poly(3hexylthiophene) Nanofibers: a Versatile Approach to Obtain Organic/Inorganic Hybrid Semiconductor Assemblies. J. Phys. Chem. C 2015, 119, 28020-28027. 21 Wang, H.; Yuan, X.; Wu, Y.; Chen, X.; Leng, L.; Zeng, G. Photodeposition of Metal Sulfides on Titanium Metal–Organic Frameworks for Excellent Visiblelight-Driven Photocatalytic Cr(VI) Reduction. RSC Adv. 2015, 5, 32531-32535. 22 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. 23 Tanaka, K.; Jin-nouchi, Y.; Fujishima, M.; Tada, H. Lead Selenide– Titanium Dioxide Heteronanojunction Formation by Photocatalytic Current Doubling-Induced Two-Step Photodeposition Technique. J. Colloid Interface Sci. 2015, 457, 248-253. 24 Somasundaram, S.; Chenthamarakshan, C. R.; Tacconi, N. R.; Ming, Y.; Rajeshwar, K. Photoassisted Deposition of Chalcogenide Semiconductors on the Titanium Dioxide Surface: Mechanistic and Other Aspects. Chem. Mater. 2004, 16, 3846-3852. 25 Tan, T. T. Y.; Zaw, M.; Beydoun, D.; Amal, R. The Formation of Nano-Sized Selenium–Titanium Dioxide Composite Semiconductors by Photocatalysis. J. Nanoparticle Res. 2002, 4, 541-552. 26 Hattori, A.; Tokihisa, Y.; Tada, H.; Ito, S. Acceleration of Oxidations and Retardation of Reductions in Photocatalysis of a TiO2/SnO2 Bilayer ‐Type Catalyst. J. Electrochem. Soc. 2000, 147, 2279-2283. 27 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. 28 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. 29 Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 30 Laasonen, K.; Car, R.; Lee, C.; Vanderbilt, D. Implementation of Ultrasoft Pseudopotentials in ab initio Molecular Dynamics. Phys. Rev. B 1991, 43, 6796-6799. 31 Ouendadji, S; Ghemid, S; Meradji, H; Hassan, F. E. H. Theoretical Sudy of Structural, Eectronic, and Thermal Properties of CdS, CdSe and CdTe Compounds. Comput. Mater. Sci. 2011, 50, 1460-1466. 32 Mosey, N. J.; Carter, E. A. Ab initio Evaluation of Coulomb and Exchange Parameters for DFT+U Calculations. Phys. Rev. B 2007, 76, 155123-1 - 155123-13. 33 Jin-nouchi, Y.; Akita, T.; Tada, H. Ultrafast Photodeposition of Size-



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Potentio/ galvanostat

mp-TiO2

Iphoto

ITO CB

Visible-light

H2

CdS QD (SILAR→PD)

Path1 Seed growth

ee-

Cd2+

CdS seed S2-

PD

Path2 h

S22S2-

SILAR

Pt

CdS QD

CdS seed (SILAR)

+

VB


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High Coverage Formation of CdS Quantum Dots ... - ACS Publications

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