Thin Amorphous TiO2 Shell on CdSe Nanocrystal Quantum Dots

Lin , Y. T.; Zeng , T. W.; Lai , W. Z.; Chen , C. W.; Lin , Y. Y.; Chang , Y. S.; Su , W. F. Efficient Photoinduced Charge ..... 1998, 80, 1928– 193...
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Thin Amorphous TiO2 Shell on CdSe Nanocrystal Quantum Dots Enhances Photocatalysis of Hydrogen Evolution from Water Sooho Lee,‡ Kangha Lee,‡ Whi Dong Kim, Seokwon Lee, Do Joong Shin, and Doh C. Lee* Department of Chemical and Biomolecular Engineering (BK21+ Program), KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-Gu, Daejeon 305-701, Korea S Supporting Information *

ABSTRACT: In this study, we designed and synthesized photocatalysts for hydrogen evolution from water by coating a thin layer of amorphous TiO2 (a-TiO2) on CdSe nanocrystals (NCs). The thin shell of a-TiO2 serves as a channel for charge carriers otherwise unutilized. Albeit a previous notion that a-TiO2 is a poor photocatalyst, the enhanced photocatalytic activity in the presence of a-TiO2 suggests that the material helps utilize the photogenerated charge carriers when it is in a form of thin shell on CdSe NCs. Type II band offset in CdSe/a-TiO2 appears to allow the electron in the conduction band of CdSe NCs to migrate over to that of a-TiO2, and the electron participates in the hydrogen production from water. Size of CdSe NCs influences the photocatalytic hydrogen evolution rate as the energy difference between the conduction bands of semiconductors becomes larger. Electron transfer from CdSe NCs to a-TiO2 layer is influenced by the level of the conductionband edge of CdSe NCs: the size dependence indicates that electron injection to TiO2 is facilitated with energy level offset between CdSe and TiO2, while smaller NCs have larger band gap and thus narrower spectral range of absorption. The interplay between charge-transfer rate and absorption cross-section should be considered in designing heterostructure NC-based photocatalysts for water splitting.

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INTRODUCTION Exciton dissociation is a key process in photocatalysis. Arguably, the most intuitive approach to facilitating separation of photogenerated electron−hole pairs involves creating semiconductor heterojunctions with a staggered type-II bandgap offset.1−3 To make use of both electrons and holes in a reaction, accessibility of reactant molecules to reduction or oxidation sites is also important. In that sense, semiconductor heterostructure with a type-II energy level offset and an “open” structure is a design prerequisite for improved photocatalysis. Recently, we synthesized PbSe/CdSe/CdS core/shell/shell nanoheterostructures with morphologies ranging from spherical to tetrapod to pyramid through the control of shell deposition and found that photocatalysis is indeed significantly influenced by the energy level offset and the morphology of the photocatalysts.4 Amorphous TiO2 (a-TiO2) can be an excellent material in an open hybrid composite to solve the challenge because they have a porous structure that enables a hole to participate in an oxidation reaction. However, a-TiO2 by itself shows relatively poor photocatalytic activity due to high density of atomic defects, which trap charge carriers blocking their path to surface. In fact, a-TiO2 exhibits several times slower photocatalysis than crystalline TiO2 does in many reactions.5,6 Nonetheless, the interest in utilizing a-TiO2 has remained unextinguished because of the ease with which it takes to synthesize the material: synthesizing a-TiO2 forgoes a calcination step necessary in crystallizing TiO2. One way to work around the defect trap sites in a-TiO2 is to use it in a form of thin shell. Because of relatively large adsorption stemming © XXXX American Chemical Society

from under-coordinated Ti atoms, a-TiO2 thin layer is a promising candidate that can complement existing photocatalysts.7 For example, Wang et al. studied PbSe/TiOx photocatalyst through rhodamine 6G reduction, in which a layer-by-layer method was employed to grow an amorphous TiOx layer on PbSe NCs.8 The authors concluded that a-TiOx enhanced photocatalytic activity of PbSe NCs for methylene blue reduction after forming type-II band offset with PbSe NCs. Now that a-TiO2 thin layers in contact with other semiconductor components have been reported to increase photocatalytic activity, a contemporary issue is to have control over the energy levels of the semiconductor pair. Quantum dots (QDs), semiconductor nanocrystals (NCs) whose size is in the quantum confinement regime, have size-tunable energy levels. The precision with which colloidal QDs can be synthesized and the theoretical understanding to estimate energy levels via the effective mass approximation have enabled controlled investigation of charge transfer through QD−TiO2 interfaces. The idea has been resonated by Robel et al., who demonstrated that charge-transfer rate from the conduction band of QDs to that of TiO2 increases with a decrease in the size of QDs via photoluminescence (PL) decay time analysis.9 Because this particular previous study used crystalline TiO2 and few studies acknowledged the improvement of photocatalytic efficiency using a-TiO2, it is of obvious importance to engineer energy levels and their offset between a-TiO2 and semiconductor NCs, Received: August 18, 2014 Revised: September 25, 2014

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1.5 light irradiation (ABET Technologies 10500), which was calibrated by one sun light intensity using light meter (TES 1337), was monitored via gas chromatography (GC, YL6500) by collecting the gas with a syringe from the headspace of the reactor by 250 μL as a function of illumination time. Characterization. UV−vis and IR spectra were recorded with a UV/vis spectrometer (Shimadzu UV3600), and PL emission spectra were obtained with a Dongwoo Optron SC100-LT77K spectrometer. To investigate PL decay dynamics, fluorescence lifetime spectrometer (Edinburgh Instruments FL920) was used. Transmission electron microscopy (TEM), elemental dispersive X-ray (EDX) mapping analysis, and highangle annular dark field (HAADF) scanning transmission electron microscopy (STEM) were employed to analyze the morphology of CdSe/a-TiO2 (Philips Tecnai F30 (300 kV) and Tiatan cubed G2 60−300 (200 kV)). Cyclic voltammograms were recorded using a potentiostat/galvanostat (BioLogic VSP). For the CV measurement, glassy carbon and platinum electrode were used as a working and counter electrode, respectively. The voltage was recorded versus Ag/ AgCl (in 3 M NaCl) reference electrode. The supporting electrode was 0.1 M TBAH in acetonitrile. Ultraviolet photoelectron spectroscopy (Sigma Probe) was used for band positions of CdSe/a-TiO2 composite. Surface area was examined using Brunauer, Emmett, and Teller (BET) pore size analyzer (Micromeritics ASAP 2020).

particularly in the context of developing efficient visible-active photocatalyst composites based on TiO2. We investigate the photocatalytic activity of CdSe NCs coated with an a-TiO2 thin layer for hydrogen evolution from water under the AM 1.5 solar spectrum.10−12 We coated CdSe NCs with a-TiO2 shell by reflux-assisted precipitation of TiO2 on CdSe NC surface. We compared the photocatalytic activity of CdSe/a-TiO2 with the mixtures of (i) CdSe NCs and P25 and (ii) CdSe NCs and a-TiO2, as the thin shell layer of a-TiO2 on CdSe NCs alleviates the issue of charge trapping. In addition, photoreduction of water using different size of CdSe NCs was examined to unveil the effect of charge-transfer rate on photocatalytic activity. Through the observation of photocatalytic efficiency, we gauge the potential of a-TiO2 as a photocatalyst.

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EXPERIMENTAL SECTION Chemicals. The following chemicals were purchased from Aldrich and used without additional treatment: acetylacetone (acac, >99%), acetonitrile (99.8%), cadmium oxide (CdO, 99.999%), N,N-dimethylforamide (DMF, >99.8%), mercaptoundecanoic acid (MUA, 95%), 1-octadecene (ODE, >99.9%), oleic acid (OA), selenium (Se, 99.999%), sodium sulfate (>98%), sodium sulfide, tetrabutyl ammonium hexafluorophosphate (TBAH, 98%), tetramethylammonium hydroxide pentahydrate (TMAH, >97%), titanium butoxide (TOB, 97%), trioctylamine (TOA), and trioctylphosphine (TOP, 97%). Synthesis of CdSe NCs. We synthesized CdSe NCs via a modified version of procedures described by Lim et al.13,14 0.203 g of CdO, 2 mL of OA, and 20 mL of TOA were mixed in a three-necked flask and degassed under vacuum at 80 °C for ∼1 h. At the same time, 0.2 g of Se was added in 1.2 mL of TOP and vigorously stirred in an Ar-filled glovebox. After degassing the mixture, we increased the temperature to 300 °C, followed by the rapid injection of TOP-Se solution. The reaction time was 90 s; then, the reaction was quenched. The final solution was washed with methanol and butanol two times. Ligand Exchange of CdSe NCs. Excess amount of MUA in methanol (100 mg/10 mL) was prepared for ligand exchange.15 The pH of the mixture was adjusted to 12 by adding TMAH. The solution was mixed with CdSe NCs dispersed in 1 mL of chloroform and was sonicated for a few seconds until the mixture turned optically clear. The CdSe NCs exchanged with MUA were collected by adding toluene via centrifugation. Passivation of CdSe NCs with a-TiO2. We referred to the procedures published in previous reports.16,17 93 μL of TOB and 28.5 μL of acac were first mixed in 5 mL of ethanol, and 1 mL of DMF was added in 4 mL of water in separate vials. After dispersing MUA-capped CdSe NCs in ethanol, we added the mixtures in the solution containing CdSe NCs in a threenecked flask. The mixture was heated and then refluxed at 84 °C until it looked turbid. The aggregated sample was collected through centrifugation and stored in vacuum. Photocatalytic Hydrogen Generation. Photocatalysts were dissolved in water at a concentration of 1 mg/mL. 0.35 M of sodium sulfate and 0.25 M of sodium sulfide were used as a sacrificial agent. We generally used 20 mg of photocatalysts in 20 mL of water containing hole scavengers. To ensure that the adsorption of reacting molecules on the surface reached an equilibrium, the mixture was purged with Ar gas and stirred in dark for 30 min. The amount of hydrogen evolved under AM

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RESULTS AND DISCUSSION Figure 1a shows the amount of hydrogen evolved from water as a function of irradiation time with four different photocatalyst samples: CdSe NCs; CdSe NCs mixed with a-TiO2 particles; CdSe NCs mixed with Degussa P25; and CdSe/a-TiO2 core/ shell structures. For H2 production from water, 20 mg of photocatalysts was mixed with 0.25 M of Na2S and 0.35 M of Na2SO3 as the sacrificial agents in 20 mL aqueous solution, and the mixture was irradiated under AM 1.5 illumination (1 mg/1 mL concentration of photocatalyst/water). Typically, a-TiO2 exhibits low photocatalytic activity despite large surface area and high adsorptivity, mainly because atomic defect sites trap electrons or holes. On the other hand, when it is in thin shell, aTiO2 is likely to trap fewer charges, and thus charge carriers become available to participate in the reduction and oxidation reactions, which results in higher H2 production. To clarify the effect of thickness on photocatalytic activity, we added more Ti precursor in the synthesis step. The concentration of CdSe NCs was determined by a simple exponential function,18 and hydrogen evolution rate was calculated based on unit mole number of CdSe NCs estimated from UV/vis spectra. When the amount of Ti precursor increases, photocatalytic hydrogen evolution rate decreases. (Figure S1 in the Supporting Information) Defining shell thickness is not trivial because aTiO2 randomly coated CdSe NCs. Notably, when the ratio of Ti to CdSe was 13 500, CdSe NCs were not distinguishable in TEM images, while Cd and Se were detected from EDX analysis. (Figure S2 in the Supporting Information) Not only did CdSe/a-TiO2 show some photocatalytic activity but it outperformed the case where CdSe NCs were mixed with crystalline TiO2. (Figure 1b). Staggered band offset in CdSe/aTiO2 induced efficient charge separation. A “thin” layer of aTiO2 indeed relieved charge trapping. These factors were very likely to help enhance photocatalytic hydrogen generation. The CdSe/a-TiO2 was synthesized via the following steps: (i) CdSe capped with MUA dispersed in ethanol was refluxed with B

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Figure 2. (a,b) TEM and HAADF STEM images of CdSe/a-TiO2. (c−e) Scanning EDX elemental mapping on image shown in panel b. (f) Overlayered mapping image of CdSe/a-TiO2: red, green, and blue dots represent Cd, Se, and Ti, respectively.

Figure 1. (a) H2 evolution as a function of time under AM 1.5 light irradiation using CdSe/a-TiO2 composites, CdSe NCs mixed with P25, CdSe NCs mixed with a-TiO2, and CdSe NCs in 20 mL of aqueous solution containing 0.25 M of Na2S and 0.35 M of Na2SO3. (b) Comparison of hydrogen evolution rate for the four samples. The number by each bar represents the hydrogen evolution rate using the respective photocatalyst in μmol/g·h.

Figure 3. UV/vis spectra of CdSe NCs and CdSe/a-TiO2. The slight increase in the UV region results from the a-TiO2.

conduction band of CdSe NCs to that of a-TiO2, while holes remain in the valence band of CdSe NCs. In the water splitting, the electrons in the conduction band of CdSe NCs are used to reduce the water to H2, whereas Na2S and Na2SO3 scavenge the holes, thereby preventing the surface oxidation of CdSe NCs (Figure 4d). Adsorption of HS− ions can also fix the Fermi level of CdSe NCs in sulfite/sulfide solution.22,23 Frame and Osterloh demonstrated using photocurrent measurement that sulfide ions can influence the potential in CdSe−MoS2 hybrid photoelectrode and subsequently H2 evolution.24 As a way to improve the photocatalytic activity with the use of QDs, core/shell structure has been used for type II band offset. However, charge carriers localized in the core component militate against efficient hydrogen evolution. In a previous study, we attributed tunneling of holes through the shell layer to the effective oxidation.4 Likewise, Chen et al. recently showed that the charges generated in active layer can move though thin TiO2 layer by tunneling.25 On a similar note, the porous a-TiO2 was introduced to CdSe NCs that allow a hole to react with Na2S and Na2SO3, which are electron donors. To obtain a more comprehensive set of evidence, we analyzed PL emission decay time. The inset in Figure 4b shows that the introduction of a-TiO2 to CdSe NCs creates a new nonradiative path. The relatively fast electron transfer from CdSe NCs to a-TiO2 is responsible for the shorter PL decay time and quenched PL, as was described in several similar

addition of Ti precursor under Ar; (ii) after 90 min the reaction was stopped and the CdSe NCs embedded in a-TiO2 were obtained. Figure 2a shows a TEM image of CdSe/a-TiO2 and elemental analysis results. A HAADF STEM image in Figure 2b shows CdSe/a-TiO2 in mapping region. The respective color of dots in Figure 2c−e represents Cd, Se, and Ti elements, measured from scanning elemental mapping for the region in Figure 2b. The overlayered mapping in Figure 2f verifies that CdSe NCs are coated with a-TiO2. UV/vis absorption spectroscopy demonstrated that a-TiO2 layer was formed onto CdSe NCs. (Figure 3) Because of the UV-active TiO2, optical density of CdSe NCs in UV region increased after CdSe/a-TiO2 composites were formed. That is, a slight increase in absorbance in the UV region (300−350 nm) results from aTiO2. Efficient charge separation relates to improved photocatalytic activity. Staggered band alignment leads to efficient charge separation. PL spectra and PL decay dynamics were measured to examine whether electron−hole pairs in the QDs are separated due to a-TiO2.20,21 As shown in Figure 4a, the MUAcapped CdSe NCs indicate the emission peak at 580 nm, and PL was quenched when they were passivated with a-TiO2. This observation demonstrates that electrons transfer from the C

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Figure 4. (a) PL emission spectra and (b) PL emission decay of CdSe NCs and CdSe/a-TiO2, excited at 488 and 470 nm, respectively. PL intensity was normalized at the point of the photoluminescence peak position. Full width at half-maximum (fwmh) of PL emission peak for CdSe NCs is 66 meV. (inset) Magnified view of PL decay dynamics at early time. (c) Cyclic voltammogram on 2.5 nm of CdSe NCs and CdSe/a-TiO2 in ethanol/ water mixture (2:3 in volume).19 The scan rate is 100 mV·s−1 and 0.1 M of TBAH as an electrolyte was used in acetonitrile. The point at which the slope dramatically changes after differentiation was set as the onset potential. (d) Schematic representation of photocatalytic hydrogen generation with CdSe/a-TiO2. Electrons generated in CdSe NCs are transferred to fully coated a-TiO2, whereas holes react with hole scavenger through the pore of a-TiO2.

systems.26−28 The radiative lifetimes are τCdSe 35 ns and τCdSe/a‑TiO2 46 ns. Electron energy level in a-TiO2 is also a crucial factor in influencing the photoreduction of water. The bandgap of aTiO2 is generally known to be larger than that of crystalline TiO2,29 whether anatase or rutile. However, there are tail states for conduction and valence band due to their overcoordinated Ti and positional disorder of O, respectively, in a-TiO2.29−31 As a result, their band edges where reduction and oxidation reactions occur have lacked accurate references. Cyclic voltammetry (CV) can be a good tool to reveal the redox potential.32 For example, Pron33 and Bard34 examined the electrochemical properties of colloidal semiconductor NCs. Table 1 summarizes that electrochemical gap from a cyclic voltammogram (shown in Figure 4c) measured on 2.5 nm of CdSe NCs and CdSe/a-TiO2 deposited on a glassy carbon electrode with a scan rate of 100 mV/s is smaller than optical

bandgap of CdSe NCs (Figure S3 and Table S1 in the Supporting Information). As Haram et al. explained in their study, peaks shift in CV as a result of decomposition of QDs and the redox potential measured electrochemically is smaller than optical band gap of the QDs.34 Trapping sites generated in the way of the decomposition of QDs accommodate a larger number of electrons and holes to participate in the reaction. Table 1 shows that the onset of reduction peak of CdSe NCs and CdSe/a-TiO2 are −0.88 V and −0.47 V, respectively. The result corroborates that type II band offset is formed by passivating CdSe NCs with TiO2 along with PL and ultraviolet photoelectron spectrum (UPS) (Figure S4 in the Supporting Information). In contrast, the onset of oxidation peaks is similar in the case of CdSe NCs and CdSe/a-TiO2. This can be interpreted that the oxidation occurs exclusively on CdSe NCs. Along these lines, Eithiraj and Geethalakshmi have reported that an a-TiO2 cluster has a very similar band edge level with an anatase phase, which means a-TiO2 shell in this study would form type-II band alignment with CdSe NCs.35 As previously explained, according to the type II band alignment, electrons generated in CdSe NCs are transferred to a-TiO2, but holes remain in CdSe NCs. Because the pore size of a-TiO2 determined with the Barrett−Joyner−Halenda (BJH) method is 2.6 nm whereas effective ionic radii of S2− and SO32− are 0.18 and 0.48 nm, respectively, S2− and SO32− ions used as a hole scavenger can travel in and out of the a-TiO2 layer and react with holes available on the surface of CdSe NCs25 (Figure S5 in the Supporting Information). Therefore, both photogenerated electron and hole can take part in the necessary reaction. For control experiment to investigate the effect of a-TiO2 shell on photocatalytic activity, CdSe NCs physically mixed

Table 1. Redox Potential Measured from Cyclic Voltammogram and Correlation between Optical and Electrochemical Bandgap Eox/Va Ered/Va Eg,el/eVb Eg,opt/eVc

CdSe NCs

CdSe/a-TiO2

1.46 −0.88 2.34 2.45

1.55 −0.47 2.02 2.45

a

Eox, Ered: onset potential of oxidation and reduction. bEg,el: electrochemical bandgap calculated from cyclic voltammetry. cEg,opt: optical bandgap determined with absorption spectroscopy. D

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Figure 5. (a) Photocatalytic activity of CdSe NCs and CdSe/a-TiO2 at varying QDs sizes under AM 1.5 light illumination. (b) Histogram summarizing hydrogen evolution rate with different CdSe NC sizes. (c) Absorbance-corrected relative hydrogen evolution rate of 2.3, 2.5, 3.0, and 3.5 nm of CdSe NCs as a function of energy difference between the conduction band levels of CdSe NCs and water reduction level. ((moles of evolved H2)/(overlap integral area multiplying photon flux of solar irradiation by absorbance of CdSe NCs * hour)). (inset) Photon flux of solar irradiance and absorption spectra of CdSe NCs (1 mg of CdSe NCs in 6 mL of water) (ref 37). (d) Energy level diagram based on reduction potentials of CdSe NCs measured using cyclic voltammetry.

with a-TiO2 and mixed with Degussa P25crystalline TiO2 were investigated (Figure 1a). In fact, when P25 or a-TiO2 alone is tested for the hydrogen production, they do not exhibit high activity for H2 production. a-TiO2 and P25 of H2 production rates are 0.09 and 0.19 μmol/g·h, respectively, whereas those of CdSe NCs mixed with a-TiO2 and with P25 are about 110 and 150 μmol/g·h, respectively (Figure S6 in the Supporting Information). It seems that by mixing CdSe NCs with a-TiO2 or P25 they might be in contact with each other, which results in type-II band offset and improved photocatalytic activity. The junction between CdSe NCs and a-TiO 2 influences the faster hydrogen production rate, and hydrogen production rate by charge separation is increased by 52%. Crystalline TiO2 as a photocatalyst generally has more advantages than a-TiO2 because they have less disorder and better electron-transfer property. However, the CdSe NCs mixed with P25 sample exhibited lower activity than CdSe NCs embedded in a-TiO2. This is attributed to the surface area and contact area between metal oxide and sensitizer. The form of shell onto a sensitizer provides much opportunity to transfer electron in that it can be from every direction, different from simple contact obtained by mixing with each other. To investigate the surface properties of a-TiO2, we measured nitrogen adsorption−desorption isotherms (Figure S5 in the Supporting Information). a-TiO2 in CdSe/a-TiO2 is classified as type IV isotherm and has mesoporous property according to the classification of IUPAC. The BET specific surface area of the composite materials is 96.4 m2/g. In general, the more reduction sites on the surface, the higher the photocatalytic activity because of their adsorptivity. Table S3 in the Supporting Information indicates that a-TiO2 has a larger

surface area than P25.36 Because reactions occur on the surface of the catalyst, it is important to transfer electron to the surface. If the oxide layer is thin enough, crystallinity of TiO2 in CdSe/ a-TiO2 did not seem to impact the photocatalytic reaction rate. That is, a thin a-TiO2 shell serves more as an active site than as charge traps. While Ti in crystalline TiO2 is six-fold and exists in the form of Ti4+, Ti atoms in a-TiO2 may stay undercoordinated and thus become very reactive with high H2O adsorbability.7 As a result, these defect-related sites at the surface can be considered to be considerably good photoactive sites in photocatalysis. Therefore, the activity of CdSe NCs covered with a-TiO2 is higher than that of only mixed sample with CdSe NCs and a-TiO2. To examine the effect of QD size on the hydrogen production rate with the effect of a-TiO2, we prepared and used QDs of varying size. Because of their size-tunable energy gaps, the electron-transfer rate between QD and a-TiO2 can also be controlled.9,26 To observe the influence of electron transfer rate on photocatalytic activity, we prepared 2.5, 3.0, and 3.5 nm of CdSe NCs that have small fwhm ranging from 40 to 70 meV (Figure S7 in the Supporting Information). The ratio of CdSe NCs to Ti precursor for a-TiO2 shell passivation was the same with respect to each size of QD. Photocatalytic test was also performed following the mentioned procedure above. Figure 5a refers to the photocatalytic activity with changing the size of QDs along with CdSe/a-TiO2. The smaller the size of QDs, the faster the photoreduction of water. The high conduction band level of small QDs results in higher electron transfer rate to the water reduction level. Robel et al. determined the charge injection rate into TiO2 with the assumption that electron transfer to TiO2 is the only additional E

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Table 2. Previous Reports on Photocatalytic Hydrogen Evolution photocatalyst

light source

incident light

activity (μmol/g·h)

CdSe NCs CdSe nanoribbons CdS-ZnO-CdO CdS QDs/g-C3N4 graphite/CdSe LaMnO3/CdS CdS-CNT ZnO-CdS

300 W Xe lamp 175 W Hg lamp 150 W Xe lamp 300 W Xe lamp AM 1.5G filter 150 W Xe lamp 200 W Xe lamp 300 W Xe lamp

λ > 320 nm λ > 400 nm

N-graphene/CdS CdSe/a-TiO2

300 W Xe lamp AM 1.5G filter

170 436 116 173 275 375 510 ZnO disk: 886 ZnO rod: 312 1050 436

λ λ λ λ

> > > >

400 420 420 420

nm nm nm nm

λ > 420 nm

reference 37 39 40 41 42 43 44 45 46 this study

particular study, thin amorphous oxide shell on relatively unstable surface of NCs makes sense particularly from the viewpoint of photocatalysis in aqueous media. Another major takeaway message from this study is that engineering band level offset in heterostructure photocatalyst composites is critically important in the development of efficient photocatlysts. Of particular interest in the community of photocatalysis is to understand how hot carriers or multiple excitons transfer at the heterojunctions between semiconductor components in composites. The ease with which the heterostructures with such precise energy level offset were prepared empowers the design strategy based on amorphous oxides, and thus enables further experimental studies to relate carrier dynamics to the photocatalytic consequences.

deactivation pathway for the excited-state interaction between CdSe and TiO2.9,26,38 As a result, it was revealed that the energy difference between two conduction bands influences the transfer rate. Zhao et al. investigated the size dependence of photocatalytic activity at the solid−liquid interface and revealed the effect of free energy on the reactivity of QDs.23 In this study, when the a-TiO2 was passivated onto CdSe NCs, similar trend was also shown. The effect of TiO2 shell showed dramatic increase in activity no matter what the size of QDs is. As light absorption of small QDs with large bandgap energy competes with charge injection rate, the amount of absorbed photon in the same mass of CdSe NCs is different. Relative hydrogen evolution rates considering different photon absorption calculated by multiplying photon flux of solar spectrum by respective absorbance of CdSe NCs were determined. Figure 5c shows that hydrogen evolution rate relatively increases as the energy difference between the conduction band level of CdSe NCs and water reduction level increases. That is, charge injection rate becomes higher with decreasing size of CdSe NCs. Figure 5d illustrates the correlation of different conduction-band level with the cause of higher hydrogen production rate.

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ASSOCIATED CONTENT

S Supporting Information *

Relative hydrogen evolution rate with changing amount of Ti precursor, N2 sorption analysis, photocatalytic activity of aTiO2 and P25, additional TEM image and UPS of CdSe/aTiO2, and UV−vis absorption spectra of 2.5, 3.0, and 3.5 nm of CdSe NCs. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS a-TiO2 in a form of thin shell, contrary to a prevailing notion that the amorphous oxide is a poor catalyst, enhances photocatalytic hydrogen production by forming staggered band offset with CdSe NCs. Rich crystalline disorder and defects in a-TiO2, generally a major culprit of the low photocatalytic activity of the material in bulk form, become less relevant when it was prepared as a thin shell. Because the synthesis of a-TiO2 is much more viable than that of crystalline TiO2, as the amorphous form can be prepared without a need to input energy for crystallization, a-TiO2 deserves to be investigated for further deployment. The facile synthesis of the sample with large surface area and porosity is an added benefit of exploiting a-TiO2 in heterostructure photocatalyst composites. As summarized in Table 2, the hydrogen evolution rate using CdSe/a-TiO2 in this study is on par with that of composites made of QDs and various crystalline metal oxides. This is a rather conservative understatement, considering that most of the other studies involved light sources with higher power than what was used in our study. The amorphous metal oxide shell can also enhance the stability of QDs for photocatalysis. Hu et al. recently demonstrated that amorphous TiO2 coating can stabilize Si, GaAs, and GaP in photoelectrochemical system.47 Although stability of QD-based photocatalysts is beyond the scope of this

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ‡

Sooho Lee and Kangha Lee contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) grants funded by the Korean government (Grants NRF2011-0030256 and NRF-2014R1A2A2A01006739), the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (no. NRF-2014M1A8A1049303), and the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20133030011330) This work was also funded by the Saudi Aramco-KAIST CO2 Management Center. F

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