Photocatalytic Current Doubling-Induced Generation of Uniform

Apr 10, 2014 - Large aggregates of Se particles (∼100 nm) are generated on TiO2 from aqueous and 2-methyl-2-propanol solutions of H2SeO3 by UV-light...
1 downloads 11 Views 4MB Size
Download PDF
Article pubs.acs.org/JPCC

Photocatalytic Current Doubling-Induced Generation of Uniform Selenium and Cadmium Selenide Quantum Dots on Titanium(IV) Oxide Musashi Fujishima,† Kentaro Tanaka,† Naoki Sakami,† Masataka Wada,† Katsuyuki Morii,‡ Takanori Hattori,‡ Yasutaka Sumida,‡ and Hiroaki Tada†,* †

Department of Applied Chemistry, School of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan ‡ Advanced Materials Research Center, Nippon Shokubai Company, Ltd., 5-8, Nishi Otabi-cho, Suita, Osaka 564-8512, Japan S Supporting Information *

ABSTRACT: We have developed a current doubling-induced two step photodeposition (CD2PD) technique for forming selenium quantum dots (QDs) and metal selenide QDs on TiO2, and proposed a reaction mechanism. Large aggregates of Se particles (∼100 nm) are generated on TiO2 from aqueous and 2-methyl-2-propanol solutions of H2SeO3 by UV-light irradiation. In contrast, highly dispersed selenium QDs are formed on TiO2 from the H2SeO3 ethanol and methanol solutions (Se/TiO2). The mean particle size increases with an increase in irradiation time (tp1) to reach 8.7 nm at tp1 = 2 h. The rates of Se photodeposition in the latter solvents are much faster than those in the latter solvents. These striking differences can be attributed to the current doubling effect of ethanol and methanol by photoelectrochemical measurements. Subsequent UV-light irradiation of Se(tp1 = 20 min)/TiO2 in ethanol and methanol solutions containing Cd2+ ions converts the Se QDs into homogeneous CdSe QDs (∼2 nm). The application of this in situ CD-2PD technique to the mesoporous TiO2 nanocrystalline film enables the uniform incorporation of CdSe QDs into the film (CdSe/mp-TiO2). QD-sensitized solar cells employing the CdSe/mp-TiO2 photoanodes afford much higher power conversion efficiencies than that using a photoanode prepared in the aqueous solution.

1. INTRODUCTION Recent serious environmental and energy issues have given impetus to the researches on solar catalysts1 and solar cells2 using TiO2-based nanoarchitectures. Se3 and Se-modified TiO2 nanoparticles4,5 have been found to exhibit UV- and visiblelight activities for the decomposition of organic pollutants in water. Also, sensitization of mesoporous TiO2 nanocrystalline film (mp-TiO2) by metal sulfides and selenides has attracted much interest because of the possible applications to quantum dot-sensitized solar cells (QD-SSCs)6−9 and photoelectrochemical cells (QD-SPECs) for hydrogen production.10,11 The key to increasing the conversion efficiencies is to develop the technique enabling deposition of the QDs smaller than the pore diameter of mp-TiO2 (∼20 nm) uniformly throughout mpTiO2.12,13 To this end, several in situ methods including successive ionic layer adsorption and reaction (SILAR),14−17 chemical bath deposition (CBD),18 and chemical vapor deposition19 have been developed so far. In these processes, the chemical reaction between the precursors of metal chalcogenides is initiated from the approaches to the mesopores of mp-TiO2. This is apt to incur wide size distribution of the QDs toward the direction of depth and the pore-blocking. We have recently developed the one-step photodeposition technique for preparing metal sulfide QDloaded TiO213 with superior photoelectrochemical properties.20 In addition to the simplicity and environmental benignity,21 this © 2014 American Chemical Society

technique has a great feature that metal sulfide QDs can be deposited uniformly on the inner surfaces of mp-TiO2 without pore-blocking.22 QD-SSC22 and QD-SPECs10,11 employing the metal sulfide QD-loaded mp-TiO2 as the photoanode afford high conversion efficiencies. On the other hand, metal selenides may be more effective than metal sulfides as a photosensitizer, since the former possesses smaller band gap. However, the onestep photodeposition technique is inapplicable to the preparation of metal selenide QDs on TiO2 because of the difficulty in producing selenide ions, served as precursor ions in this technique, from selenium salts containing Se(IV) or Se(VI) ions due to complex pH-dependent multistep electron transfer reaction in addition to the instability. Interestingly, the research groups of Rajeshwar23 and Amal24 have reported a method for photodepositing selenium particles on TiO2, further transforming them into metal selenide particles in aqueous solutions. Although their processes are very useful for water purification, the samples are inappropriate for the photocatalysts and photoanodes because of large Se particle size distribution with particles larger than100 nm.24 Here we present a CD-2PD technique for depositing metal selenide QDs on TiO2 in a high dispersion state. At the first Received: November 1, 2013 Revised: April 9, 2014 Published: April 10, 2014 8917

dx.doi.org/10.1021/jp410794j | J. Phys. Chem. C 2014, 118, 8917−8924

The Journal of Physical Chemistry C

Article

solution was determined by inductively coupled plasma spectroscopy (ICPS-7500, Shimadzu). 2.3. Photoelectrochemical Measurement. Photochronopotentiometry (PCP) profiles were measured on a photoelectrochemical (PEC) cell connected with a potentio/ galvanostat (HZ-5000, Hokuto Denko). The PEC was designed using mp-TiO2/FTO photoelectrode, a Pt counter electrode, and a Ag/AgCl reference electrode. PCP measurements were carried out under deaerated conditions in ethanol solution or aqueous solution containing a 0.1 M NaClO4 supporting electrolyte and 1.36 mM H2SeO3 after argon bubbling for 0.5 h in the dark. Illumination at one sun (AM 1.5, 100 mW cm−2) was started by using a solar simulator (PEC-L10, Peccell technologies, Inc.) after a constant potential had been reached in the dark. Electrochemical response with irradiation was followed for the PEC connected with a potentio/galvanostat. 2.4. Fabrication and Performance Evaluation of CdSe QD-Sensitized Solar Cells. Photocurrent−voltage (J−E) curves were measured under illumination by a solar simulator (PEC-L10, Peccell technologies, Inc.) at one sun (AM 1.5, 100 mW cm−2) for the sandwich-type photoelectrochemical solar cells (photoanode|2 M Na2S + 3 M S (solvent = water)| cathode). CdSe QD/mp-TiO2 films were used as the photoanodes. Prior to use, ZnS thin films were coated on the photoanodes by the procedure previously reported.22 The active area of the cell was 0.16 cm2. CuS-deposited mesoporous Sb-doped SnO2 nanocrystalline films (CuS/mp-SnO2) were used as the cathode. Sb-doped SnO2 particles with a specific surface area of 7.2 m2 g−1 and a mean size of 105 nm (SN-100P, Ishihara Sangyo) were mixed with poly(ethylene glycol) (PEG20,000, 0.5 g), Triton X-100 (0.25 mL), and a few drops of acetylacetone. The obtained paste was coated on FTO electrodes (12 Ω/□) by a squeegee method, and the sample was heated in air at 773 K for 1 h to form mp-SnO2/FTO. CuS/mp-SnO2 films were prepared by the following procedure. The mp-SnO2/FTO film was immersed in an aqueous solution (20 mL) of CuSO4 (0.1 M) at room temperature for 1 min, and then the film was washed with water and dried in air. Subsequently, the electrode was immersed in an aqueous solution (20 mL) of Na2S (0.1 M) at room temperature for 1 min, and then the film was washed with water and dried in air. Such an immersion cycle was repeated 10 times to obtain CuS/ mp-SnO2 films. The potentio/galvanostat (HZ-5000, Hokuto Denko) was used to record the J−E characteristics.

step, Se QDs ( 1 h with poisonous H2Se hardly liberated (this fact guarantees the safety of this process). The optical property of Se/TiO2 is important in connection with the photocatalytic activity.3−5 Figure 6A shows UV−visible

Figure 7. Photochronopotentiometry profiles of mp-TiO2/ITO in (a) aqueous solution and (b) ethanol solution containing 0.1 mol dm−3 NaClO4.

reference electrode potential below. The potential at the photostationary state reaches −0.89 ± 0.01 V in the ethanol solution, while it is −0.72 ± 0.01 V in the aqueous solution. Table 1 summarizes the dark potential (Edark), the potential at Table 1. Edark and Epss, and ΔE (=Epss − Edark) Values for mpTiO2/FTO Electrodes in Various Solvents solvents water methanol ethanol 2-methyl-2-propanola

Edark/V vs Ag/AgCl

Epss/V vs Ag/AgCl

± ± ± ±

−0.72 ± 0.01 −0.88 ± 0.02 −0.89 ± 0.01 −0.76

0.03 0.19 0.19 0.01

0.04 0.02 0.05 0.02

ΔE/V −0.75 −1.06 −1.03 −0.78

± ± ± ±

0.05 0.02 0.01 0.02

a

10 vol % acetonitrile solution (2-methyl-2-propanol:acetonitrile = 1:9 v/v) was used to dissolve NaClO4 supporting electrolyte.

the photostationary state (Epss), and the shift with irradiation (ΔE = Epss − Edark) in various solvents. Alcohols with αhydrogens show the current doubling effect, but those without α-hydrogens do not.26 The ΔE values for the current doubling solvents (methanol and ethanol) reach ca. −1.0 V, whereas the values for noncurrent doubling solvents (water and 2-methyl-2propanol) are −0.76 ± 0.02 V. The Epss is on the order of methanol ≈ ethanol 1 h, the

Figure 6. (A) UV−visible spectra of the Se/TiO2 samples prepared by changing irradiation time (tp1, from left to right tp1= 0, 0.5, 1, 2, 5 h). F(R∞) denotes the Kubelka−Munk function. (B) Band gap (Eg) of the Se/TiO2 samples as a function of tp1.

absorption spectra of the Se/TiO2 samples prepared by changing tp1 in the ethanol solution. Visible-light absorption appears with the Se deposition at λ < 700 nm. As a result of the increase in tp1, the absorption intensity increases with the absorption edge red-shifted. Figure 6B shows plots of band gap (Eg) vs tp1. At tp1 < 2 h, the Eg decreases with increasing tp1 to approach the value for bulk Se (1.95 eV).25 In this manner, the size quantization appears at tp1 < 2 h, and these TiO2 nanoparticles with highly dispersed Se QDs can be expected as a photocatalyst.3−5 To clarify the origin for the striking downsizing of Se particles in the ethanol (and methanol) solution systems, the potential of the mp-TiO2 coated on fluorine-doped tin oxide (mp-TiO2/FTO) electrodes was measured in 0.1 mol dm−3 NaClO4 solutions.13 Figure 7 shows photochronopotentiometry (PCP) profiles of the mp-TiO2/FTO electrodes in aqueous and ethanol solutions. UV-light irradiation of mpTiO2/FTO gives rise to a drastic negative shift in the electrode potential (E vs Ag/AgCl) due to the oxidation of the solvent by the valence band (VB) holes in TiO2. The electrode potential and redox potentials are shown with respect to the Ag/AgCl 8920

dx.doi.org/10.1021/jp410794j | J. Phys. Chem. C 2014, 118, 8917−8924

The Journal of Physical Chemistry C

Article

Figure 9A shows the Se 3d-X-ray photoelectron (XP) spectra of the CdSe/TiO2 samples prepared by changing tp2 from the

dissolution-redeposition is pronounced with a decrease in the SeO32− concentration. Once Se QDs are formed, they are also excited by the UV-light irradiation (eq 5). Smaller Se QDs (SeS) are reduced to Se2− ions (E0(Se/Se2− = −0.87 V)27 by the excited electrons in the CB(SeS) with highly negative E (−1.85 V for bulk Se) (eq 6).25 The growth of the larger Se QDs (SeL) stems from the reoxidation of the Se2− ions by the holes in the VB(SeL) (E = +0.10 V for bulk Se) (eq 7). The self-dissolution of smaller Se QDs could be enhanced because of the higher lying CB-edge and the smaller lattice energy. We assumed the recombination of h+VB(SeS) and e−CB(SeL) occurs via TiO2 (eqs 8 and 9). Clearly, the key factor for forming Se QDs on TiO2 is the ΔE or Epss value.

hν TiO2 → e−CB(TiO2 ) + h+ VB(TiO2 )

(1)

C2H5OH + h+ VB(TiO2 ) → CH3·CHOH + H+

(2)

CH3·CHOH → CH3CHO + e−CB(TiO2 ) + H+

(3)

SeO32 − + 4e−CB(TiO2 ) + 6H+ → Se + 3H 2O

(4)

hν Se → e−CB(Se) + h+ VB(Se)

(5)

SeS + 2e−CB(SeS) → Se 2 −

(6)

Se 2 − + 2h+ VB(SeL) → SeL

(7)

e−CB(SeL) → e−CB(TiO2 )

(8)

e−CB(TiO2 ) + h+ VB(SeS) → heat

(9)

Figure 9. (A) Se 3d-XP spectra and (B) UV−visible absorption spectra of CdSe/TiO2 prepared from Cd(ClO4)2 ethanol solutions. F(R∞) denotes the Kubelka−Munk function.

At the second stage, Se QDs are converted to QDs of metal selenides such as CdSe. Se QD(tp1 = 20 min)/TiO2 with d < 1 nm was used as a precursor. When UV-light irradiation was irradiated to Se QD(tp1 = 20 min)/TiO2 in ethanol solution containing metal ions, the atomic ratio of Cd to Se in the deposits approached approximately unity at irradiation time (tp2) > 1 h. In Figure 1c, a broad diffraction peak indexed as the (220) plane of cubic CdSe (PDF No. 01−077−7287) appears, while the Se(021) peak disappears (Figure 1b). As shown by Figure 8A, CdSe QDs (∼2 nm) are formed with the high dispersion state of Se QDs on the TiO2 surface maintained. Also, Figure 8B shows high resolution-TEM exhibiting clear lattice fringes with the nearest distance of 0.34 nm, which is in agreement with the value for the (111) plane of cubic CdSe.

Cd(ClO4)2 ethanol solution. In the spectrum for Se/TiO2, Se 3d3/2, and 3d5/2 signals of Se QDs are located at the binding energies of 55.6 and 54.8 eV, respectively. At tp2 > 0.5 h, the signals shift to 54.3 and 53.5 eV, which accord with the literature values for CdSe.28 Figure 9B shows UV−visible absorption spectra for the CdSe/TiO2 samples prepared from the Cd(ClO4)2 ethanol solution. The visible-light absorption remarkably intensifies as a result of the transformation of Se (indirect band gap semiconductor)29 into CdSe (direct band gap semiconductor).30 The absorption edge blueshifts at tp2 < 2 h as compared with the value of ∼710 nm for bulk CdSe31 because of the electron confinement. The present in situ CD-2PD technique can favorably be applied to mp-TiO2. Scanning electron microscpic observation confirmed that a uniform mp-TiO2 film with thickness of ∼2.5 μm is formed on FTO electrodes. Figure 10 shows electron probe microanalysis (EPMA) of CdSe/mp-TiO2 samples prepared from ethanol solution. Se and Cd are homogeneously distributed toward the depth of the mp-TiO2 films. Evidently, homogeneous CdSe QDs are formed by the second step photoirradiation of Se/TiO2 in the ethanol solution containing Cd2+ ions. This may be strange because the light intensity in mp-TiO2 decreases in the direction of depth with the absorption. The dye-sensitized SCs and QD-SSCs are major carrier devices, and the diffusion length of the electrons in mp-TiO2

Figure 8. TEM (A) and high resolution-TEM (B) of CdSe/TiO2 prepared from Cd(ClO4)2 ethanol solution. 8921

dx.doi.org/10.1021/jp410794j | J. Phys. Chem. C 2014, 118, 8917−8924

The Journal of Physical Chemistry C

Article

cm−2) The open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and the power conversion efficiency (η) for the cells are summarized in Table 2. The η values for the methanol and ethanol systems are much greater than that for the water system. The superior performance of the former can be explained in terms of the current doubling effect of the solvent used for the synthesis of CdSe/mp-TiO2. First is the efficient visible-light absorption because of the large amount deposition of CdSe on TiO2, which is mainly responsible for the larger Jsc value. Second, the small CdSe QDs would enable the permeation of the electrolyte solution into the mesopores of mp-TiO2 without pore-blocking. This is a prerequisite for the loaded CdSe QDs to effectively act as a photosensitizer. Third, the size quantization can increase the driving force of the electron injection from CdSe to TiO2,13 which can also contribute to the increase in Jsc. As expected from the diode equation applicable to the photoelectrochemical cells,33 Voc also increases with the increase in Jsc. Forth, high coverage of TiO2 surface with CdSe QDs minimizes the back electron transfer from TiO2 to Sx2− ions. This factor would reduce the loss or increase in FF although the gain is slight. Consequently, the higher η values in the methanol and ethanol systems are ascribable to the formation of CdSe QDs with a large loading amount and uniform distribution throughout the mp-TiO2 film. The method for preparing the CdSe/mp-TiO2 photoanode can be classified into the in situ ones (CBD, SILAR and photodeposition) and the ex situ one referred to as selfassembled method (SAM). CdSe QDs are directly deposited on TiO2 in the former methods, whereas they are indirectly bonded to the TiO2 surface through linker molecules in the latter method. To evaluate the quality of the present CdSe QD/ mp-TiO2 photoanode, the power conversion was compared with those for the cells using the photoanodes prepared by the other methods and the polysulfide electrolyte. The η for the cell using the photoanode prepared by the CD-2PD from the ethanol solution (1.8 ± 0.2%) is comparable with the values for the cells employing the CBD (η ∼ 1.8%)34 and SILAR (η ∼ 1.9%)35 photoanodes, while it is much higher than that for the cell using a SAM photoanode (0.43 < η < 0.83%).36 The interfacial electron transfer between CdSe QDs and TiO2 is very sensitive to the surface-to-surface distance13 and the interfacial bonding state.37 We previously showed by density functional theory calculations for the model coupling system of CdS QDs and TiO2 that Cd−O and Cd−Ti bonds are formed at the interface.13 Similar chemical bonds could also be formed at the CdSe-TiO2 interface of CdSe/mp-TiO2 prepared by the in situ methods including CD-2PD. At any rate, the direct contact of CdSe QDs and TiO2 without linker molecules is favorable for the photoinduced interfacial electron transfer from CdSe to TiO2.38 This kinetic factor would partly contribute to the higher conversion efficiency of the in situ CdSe QD-grown samples than the ex-situ attached sample. In recent years, highly ordered anodic TiO2 nanotube arrays (NTAs), in comparison to nanoparticulate systems, have been revealed in DSSCs to have superior photoelectric properties and light-harvesting efficiency.39−41 The application of the present PD technique to TiO2 NTA-based QD-SSCs can be expected to further improve the conversion efficiency. Further, the photodeposition technique has a unique feature that patterned metal sulfide and selenide QDs can be formed on the surface of TiO2 films.

Figure 10. Cd (left) and Se (right) depth profiles measured by EPMA for the cross-section of CdSe/mp-TiO2/FTO prepared using ethanol solutions.

reaches ∼25 μm (at the present light intensity of ∼6.5 × 1015 cm−2 s−1),32 which is larger than the thickness of mp-TiO2 by approximately one-order of magnitude. Similar situation would hold for this photodeposition processes, where the holes in the VB(TiO2) are effectively scavenged by ethanol (eq 2). Also, the injection of another electron from the resulting radical into the CB(TiO2) (eq 3), the hole concentration in TiO2 would be very small at the photostationary state. In addition, the lower surface tension of ethanol (21.97 mN m−2) than water (71.99 mN m−2) should enhance the permeation of the reaction solutions into the mesopores. Consequently, homogeneous generation of CB-electrons or Se2− ions throughout the mpTiO2 is achieved at the photostationary state (eq 6). In the presence of excess Cd2+ ions, the resulting Se2− ions instantly react with them to form uniform CdSe QDs in the inner surfaces of mp-TiO2 (eq 10). This process is categorized into the “ionic route” in contrast to the “atomic route” in the onestep photodeposition of metal sulfide QDs on TiO2.13 Se 2 − + Cd2 + → CdSe

(10)

CdSe/mp-TiO2 samples were prepared from the aqueous, methanol and ethanol solutions of Cd(ClO4)2. By using the CdSe/mp-TiO2 as a photoanode, CdSe QD-SSCs with a structure of photoanode|2 M Na2S + 3 M S (solvent = water)| CuS/SnO2 film were fabricated. The polysulfide electrolyte solution (Sx2−/S2−) can be suitably used because of the good stability of the chalcogenide QDs in it. As a counter electrode, a copper sulfide-deposited thin film was used because of its high electrocatalytic activity for S2−/Sx2− redox reaction.9 Figure 11 compares the photocurrent−voltage (Jph−E) curves for the cells using CdSe/mp-TiO2 photoanodes prepared from different solvents under illumination of one sun (AM 1.5, 100 mW

Figure 11. Jph−V curves for the solar cells using CdSe/mp-TiO2 photoanode prepared from different solvents. 8922

dx.doi.org/10.1021/jp410794j | J. Phys. Chem. C 2014, 118, 8917−8924

The Journal of Physical Chemistry C

Article

Table 2. Cell Performances Obtained from the Jph−V Measurements solvents

Voc/V

Jsc/mA cm−2

FF

η/%

ethanol methanol water mp-TiO2

−0.41 ± 0.01 −0.34 ± 0.01 −0.31 −0.26

11.5 ± 1.18 9.08 ± 0.10 6.77 ± 1.35 3.73

0.37 ± 0.01 0.40 0.36 ± 0.01 0.34

1.76 ± 0.21 1.25 ± 0.01 0.76 ± 0.17 0.33

(8) Mora-Sero, I.; Gimenez, S.; Fabregat-Snatiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert. Recombination in Quantum Dot Sensitized Solar Cells. J. Acc. Chem. Res. 2009, 42, 1848−1857. (9) 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. (10) Jin-nouchi, Y.; Hattori, T.; Sumida, Y.; Fujishima, M.; Tada, H. PbS Quantum Dot-Sensitized Photoelectrochemical Cell for Hydrogen Production from Water under Illumination of Simulated Sunlight. ChemPhysChem 2010, 11, 3592−3595. (11) Nagasuna, K.; Akita, T.; Fujishima, M.; Tada, H. Photodeposition of Ag2S Quantum Dots and Application to Photoelectrochemical Cells for Hydrogen Production under Simulated Sunlight. Langmuir 2011, 27, 7294−7300. (12) Könenkamp, R. Nanostructured and Photoelectrochemical Systems for Solar Photon Conversion; Archer, M. D., Nozik, A. J. Eds.; Imperial College Press: London, 2008. (13) 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. (14) Vogel, R.; Hoyer, P.; Weller, H. Quantum-Sized PbS, CdS, Ag2S, Sb2S3, and Bi2S3 Particles as Sensitizers for Various Nanoporous WideBandgap Semiconductors. J. Phys. Chem. 1994, 98, 3183−3188. (15) 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. (16) Guijarro, N.; Lana-Villarreal, T.; Shen, Q.; Toyoda, T.; Gomez, 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. (17) Guijarro, N.; Lana-Villarreal, T.; Lutz, T.; Haque, S. A.; Gomez, R. Sensitization of TiO2 with PbSe Quantum Dots by SILAR: How Mercaptophenol Improves Charge Separation. J. Phys. Chem. Lett. 2012, 3, 3367−3372. (18) Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Ruhle, S.; Cahen, D.; Hodes, G. Chemical Bath Deposited CdS/CdSe-Sensitized Porous TiO2 Solar Cells. J. Photochem. Photobiol., A 2006, 181, 306−313. (19) Zhang, Q.; Su, J.; Zhang, X.; Li, J.; Zhang, A.; Gao, Y. Chemical Vapor Deposition of a PbSe/CdS/Nitrogen-Doped TiO2 Nanorod Array Photoelectrode and Its Band-Edge Level Structure. New J. Chem. 2012, 36, 2302−2307. (20) Kozytskiy, A. V.; Stroyuk, A. L.; Kuchmy, S. Ya.; Streltsov, E. A.; Skorik, N. A.; Mosalyuk, V. O. Effect of the Method of Preparation of ZnO/CdS and TiO2/CdS Film Nanoheterostructures on their Photoelectrochemical Properties. Theor. Exp. Chem. 2013, 49, 165− 171. (21) Ma, B.; Wang, L.; Dong, H.; Gao, H.; Geng, Y.; Zhu, Y.; Qiu, Y. Photocatalysis of PbS Quantum Dots in a Quantum Dot-Sensitized Solar Cell: Photovoltaic Performance and Characteristics. Phys. Chem. Chem. Phys. 2011, 13, 2656−2658. (22) Jin-nouchi, Y.; Naya, S.-i.; Tada, H. Quantum-Dot-Sensitized Solar Cell Using a Photoanode Prepared by in Situ Photodeposition of CdS on Nanocrystalline TiO2 Films. J. Phys. Chem. C 2010, 114, 16837−16842. (23) Chenthamarakshan, C. R.; Ming, Y.; Rajeshwar, K. Underpotential Photocatalytic Deposition: A New Preparative Route to Composite Semiconductors. Chem. Mater. 2000, 12, 3538−3549.

4. CONCLUSIONS This study has shown that UV irradiation of TiO2 in the H2SeO3 ethanol (or methanol) solution yields uniform Se QDs, which can further be converted to CdSe QDs by the subsequent irradiation in ethanol (or methanol) solution containing Cd2+ ions. We proposed a photoinduced deposition-dissolution-redeposition mechanism for the first step deposition of Se QDs on TiO2, and an ionic process for the second step conversion of Se QDs to CdSe QDs. This study would open up a new in situ route for preparing Se QD- and metal selenide QD-loaded mp-TiO2 widely applicable for the photocatalysts, QD-SSCs, and QD-SPECs for hydrogen production.

■

ASSOCIATED CONTENT

* Supporting Information S

Gaseous H2Se trapping experiments by aqueous solutions. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*(H.T.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

H.T. acknowledges Ishihara Sangyo Co. for the gift of anatase TiO2 particles (A-100) and Sb-doped SnO2 particles (SN100P). This work was partially supported by a Grant-in-Aid for Scientific Research (C) No. 24550239, and Nippon Sheet Glass Foundation for Materials Science and Engineering, and by Sumitomo Foundation.

(1) Kubacka, A.; Fernández-García, M.; Colón, G. Advanced Nanoarchitectures for Solar Photocatalytic Applications. Chem. Rev. 2012, 112, 1555−1614. (2) Hodes, G. Comparison of Dye- and Semiconductor-Sensitized Porous Nanocrystalline Liquid Junction Solar Cells. J. Phys. Chem. C 2008, 112, 17778−17787. (3) Chiou, Y.-D.; Hsu, Y.-J. Room-Temperature Synthesis of SingleCrystalline Se Nanorods with Remarkable Photocatalytic Properties. Appl. Catal., B 2011, 105, 211−219. (4) Stengl, V.; Bakardjieva, S.; Bludska, J. Se and Te-Modified Titania for Photocatalytic Applications. J. Mater. Sci. 2011, 46, 3523−3536. (5) Rockafellow, E. M.; Haywood, J. M.; Witte, T.; Houk, R. S.; Jenks, W. S. Selenium-Modified TiO2 and Its Impact on Photocatalysis. Langmuir 2010, 26, 19052−19059. (6) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737−18753. (7) Rühle, S.; Shalom, M.; Zaban, A. Quantum-Dot-Sensitized Solar Cells. ChemPhysChem 2010, 11, 2290−2304. 8923

dx.doi.org/10.1021/jp410794j | J. Phys. Chem. C 2014, 118, 8917−8924

The Journal of Physical Chemistry C

Article

(24) 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. (25) Streltsov, E. A.; Poznyak, S. K.; Osipovich, N. P. Photoinduced and Dark Underpotential Deposition of Lead on Selenium. J. Electroanal. Chem. 2002, 518, 103−114. (26) Miyake, M.; Yoneyama, H.; Tamura, H. Two Step Oxidation Reactons of Alcohols on an Illuminated Rutile Electrode. Chem. Lett. 1976, 635−640. (27) Denki Kagaku Binran; Electrochemical Society of Japan: Maruzen, Tokyo, 2000. (28) Galian, R. E.; de la Guardia, M.; Perez-Prieto, J. Size Reduction of CdSe/ZnS Core-Shell Quantum Dots Photosensitized by Benzophenone: Where Does the Cd(0) Go? Langmuir 2011, 27, 1942−1945. (29) Fischer, R. Absorption and Electroabsorption of Trigonal Selenium Near the Fundamental Absorption Edge [and Luminescence]. Phys. Rev. B 1972, 5, 3087−3094. (30) Zhao, L.; Hu, L.; Fang, X. Growth and Device Application of CdSe Nanostructures. Adv. Funct. Mater. 2012, 22, 1551−1566. (31) Guyot-Sinnest, P.; Lhuillier, E.; Liu, H. J. Chem. Phys. 2012, 137, 1547041−6. (32) Peter, L. M.; Wijayantha, K. G. U. Electron Transport and Back Reaction in Dye Sensitised Nanocrystalline Photovoltaic Cells. Electrochim. Acta 2000, 45, 4543−4551. (33) Memming, R. Photoelectrochemical Solar Energy Conversion. Top. Curr. Chem. 1988, 143, 79−112. (34) Shen, Q.; Yamada, A.; Tamura, S.; Toyoda, T. CdSe Quantum Dot-Sensitized Solar Cell Employing TiO2 Nanotube WorkingElectrode and Cu2S Counter-Electrode. Appl. Phys. Lett. 2010, 97, 123107 1−3. (35) Liu, C.; Mu, L.; Jia, J.; Zhou, X.; Lin, Y. Boosting the Cell Efficiency of CdSe Quantum Dot Sensitized Solar Cell via a Modified ZnS Post-Treatment. Electrochim. Acta 2013, 111, 179−184. (36) Coughlin, K. M.; Nevins, J. S.; Watson, D. F. Aqueous-Phase Linker-Assisted Attachment of Cysteinate(2−)-Capped CdSe Quantum Dots to TiO2 for Quantum Dot-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 8649−8654. (37) Tada, H.; Jin, Q.; Iwaszuk, A.; Nolan, M. Molecular Scale Transition Metal Oxide Nanocluster-Surface Modified Titanium Dioxide as Solar Activated Environmental Catalysts. J. Phys. Chem. C 2014, DOI: 10.1021/j412312m. (38) Hines, D. A.; Kamat, P. V. Quantum Dot Surface Chemistry: Ligand Effects and Electron Transfer Reactions. J. Phys. Chem. C 2013, 117, 14418−14426. (39) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Use of Highly-Ordered TiO2 Nanotube Arrays in Dye-Sensitized Solar Cells. Nano Lett. 2006, 6, 215−218. (40) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Enhanced Charge-Collection Efficiencies and Light Scattering in Dye-Sensitized Solar Cells Using Oriented TiO2 Nanotubes Arrays. Nano Lett. 2007, 7, 69−74. (41) 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.

8924

dx.doi.org/10.1021/jp410794j | J. Phys. Chem. C 2014, 118, 8917−8924