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Capped CuInS2 quantum dots for H2 evolution from water under visible light illumination
Journal of Alloys and Compounds 550 (2013) 326–330
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Capped CuInS2 quantum dots for H2 evolution from water under visible light illumination Tzung-Luen Li a, Cheng-Da Cai a, Te-Fu Yeh a, Hsisheng Teng a,b,⇑ a b
Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 1 June 2012 Received in revised form 20 September 2012 Accepted 28 October 2012 Available online 6 November 2012 Keywords: Photosynthesis Photocatalyst H2 production Water splitting CuInS2 Quantum dots
a b s t r a c t This study demonstrates H2 evolution from water decomposition catalyzed by capped CuInS2 quantum dots (QDs) that are highly dispersed in a polysulfide aqueous solution. The CuInS2 QDs, which are obtained from solvothermal synthesis, have a size of 4.3 nm and a band gap of 1.97 eV. For photosynthetic H2 evolution in the aqueous solution, the QDs are capped with a multidentate ligand (3-mercaptopropionic acid), which has a thiol end for attaching the QDs and a hydrophilic carboxylic end for dispersion in water. The capped QDs exhibit low activity in catalyzing H2 evolution under visible illumination. After photodepositing 0.5 wt.% Ru, the capped QDs are active in producing H2 with illumination. This demonstrates that the photogenerated electrons travel through the capping reagent to generate deposited Ru, which subsequently serves as an electron trap for H2 evolution. A heterostructure formed by attaching the capped QDs on TiO2 nanoparticles, followed by coating CdS with photodeposition, exhibits a high quantum efficiency of 41% for H2 evolution from the polysulfide solution. These results demonstrate the potential for photosynthesis and phototherapy in biologic in vivo or microfluidic systems based on this capped QD material. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor quantum dots (QDs) have attracted considerable attention in photo-conversion applications because of their unique optical and electrical properties, which are distinct from bulk materials [1–5]. Nanometer-sized scale allows electrons to be confined to a small box, and quantization effect transforms energy band of bulk materials into discrete energy states [6–9]. This provides possibilities of generating multiple excitons from single-photon absorption. Because of their small size, QDs have been used as sensing materials in biologic in vivo systems, microreactors and nanoelectro-mechanical systems [10–15]. For photosynthetic or photovoltaic applications [16–18], numerous previous studies used QDs as a sensitizer in photoelectrochemical cells to expand the spectral absorption range of the photoelectrode [19–21]. This sensitizer role confined the application of QDs in systems associated with biologic in vivo or microfluidic environments. In addition, the implementation of QDs in aqueous media was hampered by the use of hydrophobic coordinating solvents for QD synthesis [22,23]. Cap-exchanging the hydrophobic surface of QDs with multidentate hydrophilic ligands have facilitated the use of QDs as imaging ⇑ Corresponding author at: Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan. Tel.: +886 6 2385371, fax: +886 6 2344496. E-mail address: [email protected] (H. Teng). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.157
or sensing agents in biologic systems or microreactors containing aqueous media [10–14,23]. The use of such water-soluble QDs as a photosynthetic or phototherapy agent can substantially expand the utility of QDs as the leading role for these biologic or microreactor systems. Investigation on the photosynthetic performance of capped QDs suspended in an aqueous solution is crucial to evaluate the feasibility of directly using QDs in a fluidic system without attaching them to any stationary substrate. Among semiconductor QDs, II–VI-type chalcogenides (such as CdSe, CdS, and PbS) and ternary chalcopyrites CuME2 (M = Ga, In; E = S, Se, Te) are often chosen for photo-conversion applications owing to their high quantum yield and suitable band gap matching the solar spectrum [24,25]. CuInS2 is one of the crucial ternary chalcopyrites because of its high absorption coefficient and excellent solar energy conversion efficiency [26]. Synthesis of CuInS2 QDs through thermolysis of various single or multisource precursors is commonly performed [27,28]. We have reported facile solvothermal synthesis of nearly monodisperse colloidal CuInS2 QDs with diameters smaller than 5 nm [29]. The main feature of this synthesis is to use excessive sulfur relative to copper and indium, and conduct the reaction at a relatively low temperature. The synthesized QDs were cap-exchanged with 3-mercaptopropionic acid (MPA), which has a thiol end for attaching the QDs and a carboxylic end to make the capped QDs hydrophilic. Previous studies have reported that the MPA linker facilitates electron
T.-L. Li et al. / Journal of Alloys and Compounds 550 (2013) 326–330
transport from QDs attached the thiol end to the other end through the multidentate linkage [5,21,29]. In photosynthesis (such as H2 generation from aqueous solutions) using suspended semiconductor media, the electron–hole pairs induced from light absorption proceed with diffusion and separation for chemical reactions at the particle surface [30–33]. The conduction band level of CuInS2 QDs is sufficiently high for H2 evolution from water reduction [5,29]. However, the chemical reactions involving the photogenerated charges at the interface of capped CuInS2 QDs have not been explored. This study suspended capped CuInS2 QDs in an aqueous polysulfide (S2/SO32) solution and investigated the feasibility of using the capped QDs as the electron donor for water reduction under visible-light illumination [34,35]. 2. Experimental section 2.1. Preparation of colloidal CuInS2 QDs Optimized synthesis recipe is briefly described as follows. CuCl (0.01 mmol) and InCl3 (0.01 mmol) were dissolved in oleylamine (0.198 mL) at room temperature. The resulting mixture was heated to 120 °C and maintained for 1 h with vigorous magnetic stirring to obtain a clear mixture. A total of 30 mL of hexane was added to a Teflon-lined stainless steel autoclave with a capacity of 200 mL, and the mixture was subsequently added under magnetic stirring. A sulfur solution (1 mmol dissolved in 1 mL oleylamine) was injected into the reaction mixture quickly. Solvothermal synthesis of CuInS2 QDs was conducted in the sealed autoclave at 150 °C for 1 h. After the autoclave was cooled to room temperature, the product was separated through addition of a mixture of ethanol/methanol (1:2, v/v) followed by centrifuging. The supernatant was decanted, and the precipitate was redispersed in hexane for characteristic analysis without any size-selective precipitation. For the subsequent evaluation of photosynthetic performance of the capped CuInS2 QDs in aqueous solution, the hydrophobic OA ligands on the QD surface were exchanged with MPA. In the exchange with MPA, dried OA-capped CuInS2 QDs were dispersed in a methanol solution of MPA (60 mM) and tetramethylammonium hydroxide (70 mM) [36], and the mixture was subsequently sonicated for 30 min to obtain a clear dispersion containing MPA-capped CuInS2 QDs. The MPA-capped QDs were precipitated with an addition of ethyl acetate/hexane (12/50, v/v) solution and redispersed in water.
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and sulfuric acid (0.1 M) at room temperature for 12 h. The MPA-coated TiO2 powder was subsequently added to a methanol solution containing MPA-capped CuInS2 QDs for 24 h to entrap the QDs. The CuInS2 QDs-coated TiO2 nanoparticles were mixed with a ethanol solution containing 0.01 M Cd(NO3)2 and 0.0025 M sulfur, and subsequently irradiated with the 450 W high-pressure mercury lamp for 12 h to photo-deposit CdS [40]. We subjected the resulting TiO2/CuInS2-QDs/CdS nanoparticles to photocatalytic reaction in the same manner as that for the capped CuInS2 QDs without using Ru cocatalyst.
3. Results and discussion Fig. 1(a) shows the TEM image of the synthesized CuInS2 QDs, which have a particle size of 4.3 ± 0.3 nm. Energy-dispersive X-ray spectroscopy analysis (Fig. 2) revealed that the atomic composition ratio of Cu, In, and S was 1.2:1.0:2.3. This ratio indicates the presence of intrinsic defects in the QD specimens. However, considering the large surface-to-volume ratio of QDs, the atomic composition ratio observed in the present study is consistent with CuInS2 stoichiometry. Fig. 3(a) shows the evolution of absorption spectrum of the colloidal CuInS2 QDs. The spectrum exhibits a broad shoulder with a long tail to lower energy. Fig. 3(b) shows the first derivative of the absorption spectrum. The local maximum can be qualitatively regarded as the band-edge absorption position,
2.2. Characterization Transmission electron microscopy (TEM) and high-resolution TEM were performed on Hitachi H-7500 (Japan) operating at 80 kV and Jeol 2100F (Japan) at 200 kV, respectively. The samples for TEM and high-resolution TEM were prepared by placing a drop of a dilute sample on a carbon-film-coated grid (the samples were diluted in hexane and ethanol, respectively). Elemental composition was determined with energy-dispersive X-ray spectroscopy (EDS) attached to the Jeol JSM6700F scanning electron microscope (Japan). The resulting colloidal solutions were placed in 1-cm quartz cuvette for the optical analysis. The UV–vis absorption spectra were recorded using a Hitachi U-4100 (Japan) spectrophotometer. 2.3. Photosynthetic reaction Photocatalytic reactions were conducted at approximately 25 °C in a gas-closed inner irradiation system. The light source was a 450 W high-pressure mercury lamp. A jacket between the mercury lamp and the reaction chamber was filled with flowing, thermostatted aqueous NaNO2 solution (1 M) as a filter to block UV light [37]. Only visible light (k > 400 nm) reached the reaction chamber for photocatalytic reactions. In the reaction using the capped CuInS2 QDs as the photocatalyst, the QD powders were magnetically stirred in the reaction chamber containing 500 mL of Na2S (0.35 M) and Na2SO3 (0.25 M) aqueous solution for H2 evolution. Ru cocatalyst was loaded by a photodeposition method in situ, using (NH4)2RuCl6 as reagent [38]. The amount of Ru cocatalyst was approximately 0.5 wt.%. The amounts of evolved H2 were determined by using gas chromatography (Hewlett– Packard 7890, USA; molecular sieve 5A column, thermal conductivity detector, argon carrier gas). The light spectrum irradiated on the photocatalytic reaction system was obtained by using a photodetector (Oriel, model 71964, USA) coupled with a Cornerstone 130 monochromator (Oriel) with a bandwidth of 10 nm. This study calculated the photon flux of each wavelength interval and thus the total photon flux by incorporating the irradiation spectrum and the light power irradiated on the reacting system [39]. To enhance the light conversion efficiency, this study attached the CuInS2 QDs to TiO2 nanoparticles, followed by CdS deposition to passivate the CuInS2 QDs. We first attached the MPA linker molecules to a TiO2 powder (P25, Degussa, Japan; average size 25 nm) by mixing P25 with an acetonitrile solution of MPA (1 M)
Fig. 1. (a) TEM image of the as-synthesized CuInS2 QDs grown at 150 °C. (b) Highresolution TEM image of the Ru-deposited CuInS2 QDs, showing crystal lattice of both CuInS2 and Ru.
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Fig. 2. Energy-dispersive X-ray spectroscopy analysis of the as-synthesized CuInS2 QDs.
solution. The conduction band edge to the vacuum for bulk CuInS2 is approximately 3.7 eV [42,43]. Because the effective mass of the hole (mh = 1.3 m0, where m0 is the electron mass) is considerably greater than that of electrons (me = 0.16 m0) in CuInS2, the upward shift in the conduction band edge is predominantly responsible for the enlargement of the band gap in the quantum-confined CuInS2 QDs [44,45]. Therefore, based on the band gap of 1.97 eV, the conduction band edge to the vacuum is approximately 3.3 eV for the synthesized CuInS2 QDs (4.3 nm). The energy difference between a semiconductor conduction band edge and the energy level for water reduction (4.5 eV to vacuum, Fig. 4) is the driving force for the injection of photogenerated electrons from the semiconductor into the aqueous solution [39,44,46]. Based on the energy levels shown in Fig. 4, the CuInS2 QDs-water interface has an electroninjection driving force of approximately 1.2 eV, which is sufficiently large to justify the feasibility of using excited CuInS2 QDs as an electron donor for H2O reduction. However, the density of the interacting sites on the QD surface governs the rate of H2 evolution. This is discussed in this paper. The MPA-capped CuInS2 QDs were used as a photocatalyst in the proposed H2 evolution system with the presence of a sulfide/ sulfite (S2/SO32) sacrificial reagent under visible light
Fig. 3. (a) Optical absorption spectrum of the colloidal CuInS2 QDs grown at 150 °C. (b) The first derivative of the absorption spectrum from panel (a), showing a local maximum at 630 nm.
which was located at 630 nm (or 1.97 eV). The distinct shift of the absorption wavelength from that of bulk CuInS2 (826 nm) indicates that the size of the particles synthesized in this study was in the quantum confinement region (smaller than 8.1 nm for CuInS2) [41]. Fig. 4 shows a schematic energy diagram to elucidate the probable charge transfer mechanisms involved in the irradiated CuInS2 QDs for a photosynthetic reaction, H2 evolution from an aqueous
Fig. 4. A schematic energy diagram elucidating the probable charge transfer mechanisms involved in the irradiated CuInS2 QDs for a photosynthetic reaction of H2 evolution from a polysulfide aqueous solution. The CuInS2 QDs can absorb visible light and the photogenerated electrons and holes subsequently separate and migrate to surface reaction sites. The conduction band (CB) is sufficiently high for H2 evolution by electron transportation through capping molecules and Ru. Holes in the valence band (VB) oxidize the sacrificial reducing reagent (Re.) to an oxidized form (Ox.).
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(k > 400 nm) illumination. The possible mechanisms for the reactions occurring in the aqueous solution can be summarized as follows [47–49]:
6H2 O þ 6e ! 6OH þ 3H2 þ
2 2 þ þ H2 O þ 6h ! SO2 3SO2 3 þ 2S 4 þ 2S2 O3 þ 2H
ðR1Þ ðR2Þ
Fig. 5 shows the H2 evolution over the CuInS2 QDs under visible light irradiation. The amount of H2 evolution was low, indicating the low rate of the R1 reaction. After photodepositing 0.5 wt.% Ru in situ on the QDs, H2 evolution from the illuminated Ru/CuInS2QDs was strong and stable, with a H2 rate of 418 lmol h1. The increase in H2 evolution was attributed to the linkage formation between the bifunctional capping reagent, MPA, and the Ru cocatalyst surface. The MPA molecule has the carboxyl and thiol ends to facilitate the binding of CuInS2 QDs (with the thiol group) to other particles. The Ru4+ ions can interact with the carboxyl end, which serves as an acceptor in the linkage. Under illumination, the photogenerated electrons can be transferred through the linkage to grow Ru particles, which later become an electron trap for reducing H2O to H2. Fig. 1(b) shows the presence of Ru nanoparticles in the CuInS2 QD specimen. The figure indicates that the Ru seeds grown on various QDs coalesce into a larger Ru nanoparticle that has a size comparable to that of the CuInS2 QDs. We assumed that numerous Ru seeds were present on the QD surface but invisible with TEM inspection. Both the Ru seeds and nanoparticles serve as the electron trap for H2 evolution. This study calculated the photon fluxes of the irradiation with wavelength below 630 nm (i.e., with energy above 1.97 eV). The photon flux was 17.64 mmol h1. The apparent quantum efficiency for H2 evolution over the CuInS2 QDs from the polysulfide solution (Fig. 5) was calculated as 4.74%. The quantum efficiency value is not high, mainly because of incomplete light absorption, which was attributed to the low concentration of the QDs and the lack of light scattering among the QDs. The turnover number, defined as Eq. (1) [48], for the Ru/CuInS2-QDs was 2.25 at 9 h of reaction.
Fig. 6. A conceptual schematic elucidating the probable charge transfer mechanisms involved in the irradiated TiO2/CuInS2-QDs/CdS heterostructure for a photosynthetic reaction of H2 evolution from a polysulfide aqueous solution. The CuInS2-QDs/CdS site can absorb visible light and the photogenerated electrons are injected into the TiO2 nanoparticle and subsequently travel through the TiO2 conduction band for reducing H2O to H2. The CuInS2-QDs/CdS site can be regenerated by reacting with the sacrificial reducing reagent (Re.), which is oxidized to an oxidized form (Ox.).
The turnover number provides the molar quantity of electrons transferred per mole of CuInS2. The turnover number was larger than unity to demonstrate a stable photocatalytic reaction for this
H2 evolution process, rather than photocorrosion. This study demonstrates that when modified with a hydrophilic ligand and metallic cocatalyst, QDs can effectively donate photogenerated charges for interaction with the surrounding species in an aqueous environment. To our knowledge, this is the first example to demonstrate H2 evolution from water over illuminated QDs in a dispersion system. Our previous study demonstrated that photoelectrode consisting of a nanocrystalline TiO2 film co-sensitized with CuInS2 QDs and CdS layers effectively converted light energy in polysulfide aqueous solutions [5]. In the photoelectrochemical cell, a voltage bias facilitated electron transport in the TiO2 film, and promoted electron injection from photoexcited CuInS2 QDs into TiO2. The present study attached CuInS2 QDs to TiO2 nanoparticles with the linker MPA, and passivated the attached CuInS2 QDs by photodepositing a CdS layer on the QDs [40]. Fig. 6 shows the conceptual schematic of the TiO2/CuInS2-QDs/CdS heterostructure for photocatalytic H2 evolution from water with the presence of the polysulfide sacrificial reagent. Under visible-light irradiation, the electrons generated from the active CuInS2-QDs/CdS sites travel through the TiO2 conduction band and then react with H2O to generate H2 on the TiO2 surface. A suspension system could not apply a voltage bias to assist charge transport. The sacrificial reagent regenerates the CuInS2-QDs/CdS sites by scavenging the photogenerated holes.
Fig. 5. Photocatalytic H2 evolution from an Na2S (0.35 M)/Na2SO3 (0.25 M) aqueous solution (500 mL) under visible light (k > 400 nm) irradiation over 800 mg of MPAcapped CuInS2 QD samples with 0.5 wt.% Ru deposition (a) and without Ru deposition (b).
Fig. 7. Photocatalytic H2 evolution from a Na2S (0.35 M)/Na2SO3 (0.25 M) aqueous solution (500 mL) under visible light (k > 400 nm) irradiation over 900 mg of TiO2/ CuInS2-QDs/CdS nanoparticles.
Number of reacted electrons Amount of CuInS2 photocatalyst Molar quantity of evolved H2 2 ¼ Molar quantity of used CuInS2
Turnover number ¼
ð1Þ
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Fig. 7 shows the H2 evolution over the TiO2/CuInS2-QDs/CdS nanoparticles under visible light irradiation. The evolution was strong with a stable H2 rate of 3610 lmol h1, which corresponds to an apparent quantum efficiency of 41%. The passivation of QDs by CdS along with the strong attachment of the MPA carboxylic end on TiO2 resulted in effective electron injection from the QDs to TiO2 for H2 generation (see Fig. 6). In addition, the deposited CdS may have contribued to the conversion of the irradaited visible light [5,21]. This high quantum efficiency demonstrates that the CuInS2 QDs are a promising medium for converting light to chemical energy in a suspension system when they form heterojunctions with other semiconductors and are appropriately passivated. To sum up, the use of QDs as active media in photosynthetic systems lays the foundation for a microfluidic photo-monitored synthesis strategy that allows for the delicate synthesis or reaction of materials in nanoelectro-mechanical systems with non-intrusive energy and low-disturbance energy supply. 4. Conclusions This study demonstrated that capped CuInS2 QDs can serve as a medium for photosynthetic reactions. The bifunctional capping reagent MPA has a thiol end to bind CuInS2 QDs and the other carboxyl end to make the QDs water-soluble or to bind other electron-accepting particles. When suspending the capped QDs in an aqueous solution containing polysulfide and Ru4+ ions with visible light irradiation, the photogenerated electrons in the QDs traveled through the linker MPA to grow Ru particles, which subsequently became an electron trap for reducing H2O to H2. The growth of Ru particles on the capped QDs dramatically promoted the H2 evolution rate, which corresponded to an apparent quantum efficiency of 4.7%. The present study attached the QDs to TiO2 nanoparticles, followed by photodepositing CdS for passivation. This TiO2/CuInS2QDs/CdS heterostructure resulted in photosynthetic H2 evolution at an apparent quantum efficiency of 41%. The results of this study revealed the viability of using suspended QDs as the photosynthesis or photosensing media in a microfluidic system with non-intrusive and low-disturbance energy supply.
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Acknowledgments
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This research is supported by the National Science Council of Taiwan (98-2221-E-006-110-MY3, 101-3113-E-007-006, 1013113-E-006-010, and 101-3113-E-006-011) and the Bureau of Energy, Ministry of Economic Affairs, Taiwan (101-D0204-2).
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Capped CuInS2 quantum dots for H2 evolution from water under visible light illumination Tzung-Luen Li a, Cheng-Da Cai a, Te-Fu Yeh a, Hsisheng Teng a,b,⇑ a b
Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 1 June 2012 Received in revised form 20 September 2012 Accepted 28 October 2012 Available online 6 November 2012 Keywords: Photosynthesis Photocatalyst H2 production Water splitting CuInS2 Quantum dots
a b s t r a c t This study demonstrates H2 evolution from water decomposition catalyzed by capped CuInS2 quantum dots (QDs) that are highly dispersed in a polysulfide aqueous solution. The CuInS2 QDs, which are obtained from solvothermal synthesis, have a size of 4.3 nm and a band gap of 1.97 eV. For photosynthetic H2 evolution in the aqueous solution, the QDs are capped with a multidentate ligand (3-mercaptopropionic acid), which has a thiol end for attaching the QDs and a hydrophilic carboxylic end for dispersion in water. The capped QDs exhibit low activity in catalyzing H2 evolution under visible illumination. After photodepositing 0.5 wt.% Ru, the capped QDs are active in producing H2 with illumination. This demonstrates that the photogenerated electrons travel through the capping reagent to generate deposited Ru, which subsequently serves as an electron trap for H2 evolution. A heterostructure formed by attaching the capped QDs on TiO2 nanoparticles, followed by coating CdS with photodeposition, exhibits a high quantum efficiency of 41% for H2 evolution from the polysulfide solution. These results demonstrate the potential for photosynthesis and phototherapy in biologic in vivo or microfluidic systems based on this capped QD material. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor quantum dots (QDs) have attracted considerable attention in photo-conversion applications because of their unique optical and electrical properties, which are distinct from bulk materials [1–5]. Nanometer-sized scale allows electrons to be confined to a small box, and quantization effect transforms energy band of bulk materials into discrete energy states [6–9]. This provides possibilities of generating multiple excitons from single-photon absorption. Because of their small size, QDs have been used as sensing materials in biologic in vivo systems, microreactors and nanoelectro-mechanical systems [10–15]. For photosynthetic or photovoltaic applications [16–18], numerous previous studies used QDs as a sensitizer in photoelectrochemical cells to expand the spectral absorption range of the photoelectrode [19–21]. This sensitizer role confined the application of QDs in systems associated with biologic in vivo or microfluidic environments. In addition, the implementation of QDs in aqueous media was hampered by the use of hydrophobic coordinating solvents for QD synthesis [22,23]. Cap-exchanging the hydrophobic surface of QDs with multidentate hydrophilic ligands have facilitated the use of QDs as imaging ⇑ Corresponding author at: Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan. Tel.: +886 6 2385371, fax: +886 6 2344496. E-mail address: [email protected] (H. Teng). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.157
or sensing agents in biologic systems or microreactors containing aqueous media [10–14,23]. The use of such water-soluble QDs as a photosynthetic or phototherapy agent can substantially expand the utility of QDs as the leading role for these biologic or microreactor systems. Investigation on the photosynthetic performance of capped QDs suspended in an aqueous solution is crucial to evaluate the feasibility of directly using QDs in a fluidic system without attaching them to any stationary substrate. Among semiconductor QDs, II–VI-type chalcogenides (such as CdSe, CdS, and PbS) and ternary chalcopyrites CuME2 (M = Ga, In; E = S, Se, Te) are often chosen for photo-conversion applications owing to their high quantum yield and suitable band gap matching the solar spectrum [24,25]. CuInS2 is one of the crucial ternary chalcopyrites because of its high absorption coefficient and excellent solar energy conversion efficiency [26]. Synthesis of CuInS2 QDs through thermolysis of various single or multisource precursors is commonly performed [27,28]. We have reported facile solvothermal synthesis of nearly monodisperse colloidal CuInS2 QDs with diameters smaller than 5 nm [29]. The main feature of this synthesis is to use excessive sulfur relative to copper and indium, and conduct the reaction at a relatively low temperature. The synthesized QDs were cap-exchanged with 3-mercaptopropionic acid (MPA), which has a thiol end for attaching the QDs and a carboxylic end to make the capped QDs hydrophilic. Previous studies have reported that the MPA linker facilitates electron
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transport from QDs attached the thiol end to the other end through the multidentate linkage [5,21,29]. In photosynthesis (such as H2 generation from aqueous solutions) using suspended semiconductor media, the electron–hole pairs induced from light absorption proceed with diffusion and separation for chemical reactions at the particle surface [30–33]. The conduction band level of CuInS2 QDs is sufficiently high for H2 evolution from water reduction [5,29]. However, the chemical reactions involving the photogenerated charges at the interface of capped CuInS2 QDs have not been explored. This study suspended capped CuInS2 QDs in an aqueous polysulfide (S2/SO32) solution and investigated the feasibility of using the capped QDs as the electron donor for water reduction under visible-light illumination [34,35]. 2. Experimental section 2.1. Preparation of colloidal CuInS2 QDs Optimized synthesis recipe is briefly described as follows. CuCl (0.01 mmol) and InCl3 (0.01 mmol) were dissolved in oleylamine (0.198 mL) at room temperature. The resulting mixture was heated to 120 °C and maintained for 1 h with vigorous magnetic stirring to obtain a clear mixture. A total of 30 mL of hexane was added to a Teflon-lined stainless steel autoclave with a capacity of 200 mL, and the mixture was subsequently added under magnetic stirring. A sulfur solution (1 mmol dissolved in 1 mL oleylamine) was injected into the reaction mixture quickly. Solvothermal synthesis of CuInS2 QDs was conducted in the sealed autoclave at 150 °C for 1 h. After the autoclave was cooled to room temperature, the product was separated through addition of a mixture of ethanol/methanol (1:2, v/v) followed by centrifuging. The supernatant was decanted, and the precipitate was redispersed in hexane for characteristic analysis without any size-selective precipitation. For the subsequent evaluation of photosynthetic performance of the capped CuInS2 QDs in aqueous solution, the hydrophobic OA ligands on the QD surface were exchanged with MPA. In the exchange with MPA, dried OA-capped CuInS2 QDs were dispersed in a methanol solution of MPA (60 mM) and tetramethylammonium hydroxide (70 mM) [36], and the mixture was subsequently sonicated for 30 min to obtain a clear dispersion containing MPA-capped CuInS2 QDs. The MPA-capped QDs were precipitated with an addition of ethyl acetate/hexane (12/50, v/v) solution and redispersed in water.
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and sulfuric acid (0.1 M) at room temperature for 12 h. The MPA-coated TiO2 powder was subsequently added to a methanol solution containing MPA-capped CuInS2 QDs for 24 h to entrap the QDs. The CuInS2 QDs-coated TiO2 nanoparticles were mixed with a ethanol solution containing 0.01 M Cd(NO3)2 and 0.0025 M sulfur, and subsequently irradiated with the 450 W high-pressure mercury lamp for 12 h to photo-deposit CdS [40]. We subjected the resulting TiO2/CuInS2-QDs/CdS nanoparticles to photocatalytic reaction in the same manner as that for the capped CuInS2 QDs without using Ru cocatalyst.
3. Results and discussion Fig. 1(a) shows the TEM image of the synthesized CuInS2 QDs, which have a particle size of 4.3 ± 0.3 nm. Energy-dispersive X-ray spectroscopy analysis (Fig. 2) revealed that the atomic composition ratio of Cu, In, and S was 1.2:1.0:2.3. This ratio indicates the presence of intrinsic defects in the QD specimens. However, considering the large surface-to-volume ratio of QDs, the atomic composition ratio observed in the present study is consistent with CuInS2 stoichiometry. Fig. 3(a) shows the evolution of absorption spectrum of the colloidal CuInS2 QDs. The spectrum exhibits a broad shoulder with a long tail to lower energy. Fig. 3(b) shows the first derivative of the absorption spectrum. The local maximum can be qualitatively regarded as the band-edge absorption position,
2.2. Characterization Transmission electron microscopy (TEM) and high-resolution TEM were performed on Hitachi H-7500 (Japan) operating at 80 kV and Jeol 2100F (Japan) at 200 kV, respectively. The samples for TEM and high-resolution TEM were prepared by placing a drop of a dilute sample on a carbon-film-coated grid (the samples were diluted in hexane and ethanol, respectively). Elemental composition was determined with energy-dispersive X-ray spectroscopy (EDS) attached to the Jeol JSM6700F scanning electron microscope (Japan). The resulting colloidal solutions were placed in 1-cm quartz cuvette for the optical analysis. The UV–vis absorption spectra were recorded using a Hitachi U-4100 (Japan) spectrophotometer. 2.3. Photosynthetic reaction Photocatalytic reactions were conducted at approximately 25 °C in a gas-closed inner irradiation system. The light source was a 450 W high-pressure mercury lamp. A jacket between the mercury lamp and the reaction chamber was filled with flowing, thermostatted aqueous NaNO2 solution (1 M) as a filter to block UV light [37]. Only visible light (k > 400 nm) reached the reaction chamber for photocatalytic reactions. In the reaction using the capped CuInS2 QDs as the photocatalyst, the QD powders were magnetically stirred in the reaction chamber containing 500 mL of Na2S (0.35 M) and Na2SO3 (0.25 M) aqueous solution for H2 evolution. Ru cocatalyst was loaded by a photodeposition method in situ, using (NH4)2RuCl6 as reagent [38]. The amount of Ru cocatalyst was approximately 0.5 wt.%. The amounts of evolved H2 were determined by using gas chromatography (Hewlett– Packard 7890, USA; molecular sieve 5A column, thermal conductivity detector, argon carrier gas). The light spectrum irradiated on the photocatalytic reaction system was obtained by using a photodetector (Oriel, model 71964, USA) coupled with a Cornerstone 130 monochromator (Oriel) with a bandwidth of 10 nm. This study calculated the photon flux of each wavelength interval and thus the total photon flux by incorporating the irradiation spectrum and the light power irradiated on the reacting system [39]. To enhance the light conversion efficiency, this study attached the CuInS2 QDs to TiO2 nanoparticles, followed by CdS deposition to passivate the CuInS2 QDs. We first attached the MPA linker molecules to a TiO2 powder (P25, Degussa, Japan; average size 25 nm) by mixing P25 with an acetonitrile solution of MPA (1 M)
Fig. 1. (a) TEM image of the as-synthesized CuInS2 QDs grown at 150 °C. (b) Highresolution TEM image of the Ru-deposited CuInS2 QDs, showing crystal lattice of both CuInS2 and Ru.
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Fig. 2. Energy-dispersive X-ray spectroscopy analysis of the as-synthesized CuInS2 QDs.
solution. The conduction band edge to the vacuum for bulk CuInS2 is approximately 3.7 eV [42,43]. Because the effective mass of the hole (mh = 1.3 m0, where m0 is the electron mass) is considerably greater than that of electrons (me = 0.16 m0) in CuInS2, the upward shift in the conduction band edge is predominantly responsible for the enlargement of the band gap in the quantum-confined CuInS2 QDs [44,45]. Therefore, based on the band gap of 1.97 eV, the conduction band edge to the vacuum is approximately 3.3 eV for the synthesized CuInS2 QDs (4.3 nm). The energy difference between a semiconductor conduction band edge and the energy level for water reduction (4.5 eV to vacuum, Fig. 4) is the driving force for the injection of photogenerated electrons from the semiconductor into the aqueous solution [39,44,46]. Based on the energy levels shown in Fig. 4, the CuInS2 QDs-water interface has an electroninjection driving force of approximately 1.2 eV, which is sufficiently large to justify the feasibility of using excited CuInS2 QDs as an electron donor for H2O reduction. However, the density of the interacting sites on the QD surface governs the rate of H2 evolution. This is discussed in this paper. The MPA-capped CuInS2 QDs were used as a photocatalyst in the proposed H2 evolution system with the presence of a sulfide/ sulfite (S2/SO32) sacrificial reagent under visible light
Fig. 3. (a) Optical absorption spectrum of the colloidal CuInS2 QDs grown at 150 °C. (b) The first derivative of the absorption spectrum from panel (a), showing a local maximum at 630 nm.
which was located at 630 nm (or 1.97 eV). The distinct shift of the absorption wavelength from that of bulk CuInS2 (826 nm) indicates that the size of the particles synthesized in this study was in the quantum confinement region (smaller than 8.1 nm for CuInS2) [41]. Fig. 4 shows a schematic energy diagram to elucidate the probable charge transfer mechanisms involved in the irradiated CuInS2 QDs for a photosynthetic reaction, H2 evolution from an aqueous
Fig. 4. A schematic energy diagram elucidating the probable charge transfer mechanisms involved in the irradiated CuInS2 QDs for a photosynthetic reaction of H2 evolution from a polysulfide aqueous solution. The CuInS2 QDs can absorb visible light and the photogenerated electrons and holes subsequently separate and migrate to surface reaction sites. The conduction band (CB) is sufficiently high for H2 evolution by electron transportation through capping molecules and Ru. Holes in the valence band (VB) oxidize the sacrificial reducing reagent (Re.) to an oxidized form (Ox.).
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(k > 400 nm) illumination. The possible mechanisms for the reactions occurring in the aqueous solution can be summarized as follows [47–49]:
6H2 O þ 6e ! 6OH þ 3H2 þ
2 2 þ þ H2 O þ 6h ! SO2 3SO2 3 þ 2S 4 þ 2S2 O3 þ 2H
ðR1Þ ðR2Þ
Fig. 5 shows the H2 evolution over the CuInS2 QDs under visible light irradiation. The amount of H2 evolution was low, indicating the low rate of the R1 reaction. After photodepositing 0.5 wt.% Ru in situ on the QDs, H2 evolution from the illuminated Ru/CuInS2QDs was strong and stable, with a H2 rate of 418 lmol h1. The increase in H2 evolution was attributed to the linkage formation between the bifunctional capping reagent, MPA, and the Ru cocatalyst surface. The MPA molecule has the carboxyl and thiol ends to facilitate the binding of CuInS2 QDs (with the thiol group) to other particles. The Ru4+ ions can interact with the carboxyl end, which serves as an acceptor in the linkage. Under illumination, the photogenerated electrons can be transferred through the linkage to grow Ru particles, which later become an electron trap for reducing H2O to H2. Fig. 1(b) shows the presence of Ru nanoparticles in the CuInS2 QD specimen. The figure indicates that the Ru seeds grown on various QDs coalesce into a larger Ru nanoparticle that has a size comparable to that of the CuInS2 QDs. We assumed that numerous Ru seeds were present on the QD surface but invisible with TEM inspection. Both the Ru seeds and nanoparticles serve as the electron trap for H2 evolution. This study calculated the photon fluxes of the irradiation with wavelength below 630 nm (i.e., with energy above 1.97 eV). The photon flux was 17.64 mmol h1. The apparent quantum efficiency for H2 evolution over the CuInS2 QDs from the polysulfide solution (Fig. 5) was calculated as 4.74%. The quantum efficiency value is not high, mainly because of incomplete light absorption, which was attributed to the low concentration of the QDs and the lack of light scattering among the QDs. The turnover number, defined as Eq. (1) [48], for the Ru/CuInS2-QDs was 2.25 at 9 h of reaction.
Fig. 6. A conceptual schematic elucidating the probable charge transfer mechanisms involved in the irradiated TiO2/CuInS2-QDs/CdS heterostructure for a photosynthetic reaction of H2 evolution from a polysulfide aqueous solution. The CuInS2-QDs/CdS site can absorb visible light and the photogenerated electrons are injected into the TiO2 nanoparticle and subsequently travel through the TiO2 conduction band for reducing H2O to H2. The CuInS2-QDs/CdS site can be regenerated by reacting with the sacrificial reducing reagent (Re.), which is oxidized to an oxidized form (Ox.).
The turnover number provides the molar quantity of electrons transferred per mole of CuInS2. The turnover number was larger than unity to demonstrate a stable photocatalytic reaction for this
H2 evolution process, rather than photocorrosion. This study demonstrates that when modified with a hydrophilic ligand and metallic cocatalyst, QDs can effectively donate photogenerated charges for interaction with the surrounding species in an aqueous environment. To our knowledge, this is the first example to demonstrate H2 evolution from water over illuminated QDs in a dispersion system. Our previous study demonstrated that photoelectrode consisting of a nanocrystalline TiO2 film co-sensitized with CuInS2 QDs and CdS layers effectively converted light energy in polysulfide aqueous solutions [5]. In the photoelectrochemical cell, a voltage bias facilitated electron transport in the TiO2 film, and promoted electron injection from photoexcited CuInS2 QDs into TiO2. The present study attached CuInS2 QDs to TiO2 nanoparticles with the linker MPA, and passivated the attached CuInS2 QDs by photodepositing a CdS layer on the QDs [40]. Fig. 6 shows the conceptual schematic of the TiO2/CuInS2-QDs/CdS heterostructure for photocatalytic H2 evolution from water with the presence of the polysulfide sacrificial reagent. Under visible-light irradiation, the electrons generated from the active CuInS2-QDs/CdS sites travel through the TiO2 conduction band and then react with H2O to generate H2 on the TiO2 surface. A suspension system could not apply a voltage bias to assist charge transport. The sacrificial reagent regenerates the CuInS2-QDs/CdS sites by scavenging the photogenerated holes.
Fig. 5. Photocatalytic H2 evolution from an Na2S (0.35 M)/Na2SO3 (0.25 M) aqueous solution (500 mL) under visible light (k > 400 nm) irradiation over 800 mg of MPAcapped CuInS2 QD samples with 0.5 wt.% Ru deposition (a) and without Ru deposition (b).
Fig. 7. Photocatalytic H2 evolution from a Na2S (0.35 M)/Na2SO3 (0.25 M) aqueous solution (500 mL) under visible light (k > 400 nm) irradiation over 900 mg of TiO2/ CuInS2-QDs/CdS nanoparticles.
Number of reacted electrons Amount of CuInS2 photocatalyst Molar quantity of evolved H2 2 ¼ Molar quantity of used CuInS2
Turnover number ¼
ð1Þ
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Fig. 7 shows the H2 evolution over the TiO2/CuInS2-QDs/CdS nanoparticles under visible light irradiation. The evolution was strong with a stable H2 rate of 3610 lmol h1, which corresponds to an apparent quantum efficiency of 41%. The passivation of QDs by CdS along with the strong attachment of the MPA carboxylic end on TiO2 resulted in effective electron injection from the QDs to TiO2 for H2 generation (see Fig. 6). In addition, the deposited CdS may have contribued to the conversion of the irradaited visible light [5,21]. This high quantum efficiency demonstrates that the CuInS2 QDs are a promising medium for converting light to chemical energy in a suspension system when they form heterojunctions with other semiconductors and are appropriately passivated. To sum up, the use of QDs as active media in photosynthetic systems lays the foundation for a microfluidic photo-monitored synthesis strategy that allows for the delicate synthesis or reaction of materials in nanoelectro-mechanical systems with non-intrusive energy and low-disturbance energy supply. 4. Conclusions This study demonstrated that capped CuInS2 QDs can serve as a medium for photosynthetic reactions. The bifunctional capping reagent MPA has a thiol end to bind CuInS2 QDs and the other carboxyl end to make the QDs water-soluble or to bind other electron-accepting particles. When suspending the capped QDs in an aqueous solution containing polysulfide and Ru4+ ions with visible light irradiation, the photogenerated electrons in the QDs traveled through the linker MPA to grow Ru particles, which subsequently became an electron trap for reducing H2O to H2. The growth of Ru particles on the capped QDs dramatically promoted the H2 evolution rate, which corresponded to an apparent quantum efficiency of 4.7%. The present study attached the QDs to TiO2 nanoparticles, followed by photodepositing CdS for passivation. This TiO2/CuInS2QDs/CdS heterostructure resulted in photosynthetic H2 evolution at an apparent quantum efficiency of 41%. The results of this study revealed the viability of using suspended QDs as the photosynthesis or photosensing media in a microfluidic system with non-intrusive and low-disturbance energy supply.
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Acknowledgments
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This research is supported by the National Science Council of Taiwan (98-2221-E-006-110-MY3, 101-3113-E-007-006, 1013113-E-006-010, and 101-3113-E-006-011) and the Bureau of Energy, Ministry of Economic Affairs, Taiwan (101-D0204-2).
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