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Enhanced Photocatalytic Activity of Zn-Ag-In-S Semiconductor Nanocrystals with a Dumbbell-Shaped Heterostructure Tatsuya Kameyama, Seiya Koyama, Takahisa Yamamoto, Susumu Kuwabata, and Tsukasa Torimoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00255 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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The Journal of Physical Chemistry

Enhanced Photocatalytic Activity of Zn-Ag-In-S Semiconductor Nanocrystals with a DumbbellShaped Heterostructure Tatsuya Kameyama,1 Seiya Koyama,1 Takahisa Yamamoto,1 Susumu Kuwabata2, and Tsukasa Torimoto*1 1

Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan.

2

Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan.

Abstract

Nanocrystals with a heterojunction of type-II band alignment have attracted much interest for the construction of solar energy conversion systems because photogenerated electrons and holes can be effectively separated at the heterojunction. Here, we report the preparation of heterostructured multinary nanocrystals composed of ZnS-AgInS2 solid solution (ZAIS), which have tunability of the electronic energy structure depending on their chemical composition. Ellipsoidal ZAIS nanocrystal domains with various compositions were epitaxially grown on both termini of preformed ZAIS nanorods as seeds. The resulting nanocrystals had a dumbbell shape comprised

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of two ellipsoidal nanocrystals (ca. 4~6 nm in width × 7~11 nm in length) connected by a nanorod (ca. 4 nm in width × 16~23 nm in length). Since the Zn fraction in the chemical composition was larger in the rod part than in the two tip parts in the nano-dumbbell, heterojunctions of type-II or quasi-type II were formed at the interface between the rod and tip parts, as confirmed by photoluminescence lifetime measurements. Analysis of the energy structure of ZAIS nanodumbbells indicated that photogenerated electrons and holes were effectively and spatially separated at the heterojunction, resulting in localization of the photogenerated electrons in ellipsoidal ZAIS tips in the nano-dumbbell but delocalization of holes over the whole nanocrystal. The photocatalytic H2 evolution rate of ZAIS nano-dumbbells was remarkably improved in comparison to those of single-component counterparts, ZAIS nanorods or free ellipsoidal ZAIS nanocrystals with similar sizes, by tuning the chemical composition of the tip parts to optimize the energy structure of the type-II heterojunction for H2 evolution.

Introduction

Visible-light-driven photocatalysts have been intensively investigated in order to develop efficient systems for conversion of solar energy to chemical energy.1-7 Considerable efforts have been made by many researchers to improve the photocatalytic activities of such photocatalysts. One successful strategy has been to utilize size-quantized semiconductor nanocrystals as photocatalysts, because such nanocrystals have exhibited tunable size-dependent physicochemical properties, unlike to corresponding bulk particles, due to the quantum size effects.8-16 Control of the size and morphology of these nanocrystals is important to obtain optimized photocatalytic activity. Deposition of metals17-22 or semiconductors23-33 on a semiconductor nanocrystal, by which


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various types of heterojunction are produced, is another strategy for enhancing their photochemical properties. At the interfaces between different semiconductor nanocrystals, band alignment is tunable depending on the relative positions of conduction and valence band edges between the original and deposited semiconductors.12,26,27 Among the various heterojunctions, a staggered band alignment, called a type-II heterojunction, observed at the interfaces such as CdSe/CdS and CdTe/CdS

interfaces,

is

useful

for

effective

separation

of

photogenerated

charge

carriers.24,27,28,31,32,34-39 Amirav and co-workers reported that the photocatalytic activity of CdSe@CdS dot-in-rod nanocrystals was superior to that of pure CdS nanorods because holes photogenerated in the CdS rod part were spatially confined in the CdSe part completely incorporated in the CdS rod crystal.37,38 Energy levels at the heterojunction were shown by Zamkv et al. to influence the photocatalytic activity of heterostructured nanocrystals: asymmetric dumbbell-shaped nanorods of ZnSe/CdS/Pt exhibited higher H2 evolution activity than that of ZnTe/CdS/Pt nanorods because of the larger oxidizing power of localized holes in ZnSe tips than in ZnTe tips.36 On the other hand, much interest has been shown in nanocrystals of I-III-VI-based metal chalcogenide semiconductors, such as CuInS2, AgInS2, and their solid solutions with ZnS, as alternatives to conventional binary nanoparticles, such as CdSe, CdTe, and PbS, because they are composed of elements with less toxicity and exhibit physicochemical properties that can be controlled by their chemical composition as well as by their size.10,11,15,40-51 Since the absorption properties of multinary metal chalcogenide nanocrystals vary in a wide wavelength range from visible light to near-infrared light, these nanocrystals are useful materials as components to fabricate nanoscale heterojunctions.52-57 An appropriate combination of I-III-VI2 nanocrystals and binary semiconductor nanocrystals can produce a type-II heterojunction, accomplishing spatial


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and efficient charge separation of photogenerated electrons and holes. For example, Teranishi and co-workers reported that holes photogenerated in the CuInS2/CdS heterotetrapod nanocrystals were localized in CuInS2 core but that photoexcited electrons were delocalized in whole particle.55 It was reported by Pradhan et al. that the photocatalytic activities for H2 evolution of AgGaSe2 and AgInSe2 nanorods were improved by deposition of CdSe on the ends of the nanorods due to the formation of a type-II heterostructure in the resulting hybrid nanocrystals.56 Heterostructured nanocrystals of dot-in-rod-structured CuInSe2/CuInS2, composed of two ternary I-III-VI semiconductors, were successfully prepared by Donega et al. via sequential cation exchange, in which photogenerated charge carriers were funneled in the CuInSe2 core to emit near-IR photoluminescence.54 We have recently reported single-crystalline nanocrystals composed of ZnS-AgInS2 solid solution ((AgIn)xZn2(1-x)S2, ZAIS), the optical and photocatalytic properties of which were tunable by changing their particle size, morphology, and chemical composition.58-60 Their photocatalytic activity for H2 evolution was dependent on the chemical composition and increased with nanocrystal morphology in the order of rice-seed shape < sphere < rod.60 The energy levels of conduction band and valence band edges of ZAIS nanocrystals were also continuously varied by particle size and chemical composition, accompanied by a slight modification in the lattice constant due to a change in the chemical composition.59 This finding is of importance for ZAIS nanocrystals to design the heterojunction: band alignment at the heterojunction between two ZAIS nanocrystals is potentially controlled, without significant lattice strain, by changing the chemical composition of one of the nanocrystals. However, such an attempt has not been demonstrated yet and there has been no report on photocatalytic activity of heterostructured ZAIS nanocrystals. Here, we report the selective growth of ZAIS nanocrystal domains with different compositions on both


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termini of ZAIS nanorods, the heterostructure being similar to a dumbbell shape. The carrier dynamics is considerably modulated by the heterojunction formed in ZAIS nanocrystals. The photocatalytic activity for H2 evolution of dumbbell-shaped ZAIS nanocrystals was improved by precise control of the chemical composition in comparison to the photocatalytic activities of starting ZAIS nanorods and ellipsoidal ZAIS nanocrystals as counterparts.

Experimental Methods Materials Indium(III) acetate was purchased from Aldrich. Oleylamine (OLA) was obtained from Tokyo Chemical Industry. Zinc(II) acetate, 1-dodecanethiol (DDT), 1,3-dibutylthiourea (DBTU), and elemental sulfur were supplied by Wako Chemicals. Other chemicals were purchased from Kishida Reagents Chemicals. All reagents were used as received. Aqueous solutions were prepared with purified water just before use by a Millipore Milli-Q system. Preparation of ZAIS nanorods with the experimentally determined composition of x= 0.24. Rod-shaped ZAIS nanocrystals (ZAIS nanorods) were prepared by a two-step heating-up process according to our previous procedure.60 All reactions were carried out under an N2 atmosphere. Briefly, powders of Zn(CH3COO)2, Ag(CH3COO), and In(CH3COO)3 with a mole ratio of Zn:Ag:In = 1.0 : 0.5 : 0.5 (total metal ions: 0.20 mmol) were put into a test tube with a 2.5cm3 portion of OLA and a 0.50-cm3 portion of DDT as a surface modification agent, followed by heat treatment at 150 °C with vigorous stirring to yield a homogeneous transparent solution. After a 0.30-cm3 DDT solution containing elemental sulfur (0.050 mmol) and DBTU (0.15 mmol) had been swiftly added to the solution, the resulting mixture solution was heat-treated for 30 min at


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150 °C with stirring (first heating step). Finally, the reaction temperature was quickly raised to 250 °C at a rate of ca. 1.2 °C s−1 and the solution was further stirred for 3 min (second heating step). The solution was cooled to room temperature, and then the thus-obtained suspension was subjected to centrifugation at 4000 rpm for 5 min. The precipitates were washed with methanol several times and then redissolved in chloroform (3.0 cm3). On the basis of elemental analysis, the value of x in the formula of (AgIn)xZn2(1−x)S2 was determined to be x= 0.24 for thus-obtained ZAIS nanorods, denoted here as ZAIS(0.24) nanorods in the present study. Preparation of ZAIS nano-dumbbells and nano-ellipsoids with the ratio of precursor metal ions, xp, of 1.0. Dumbbell-shaped ZAIS nanocrystals were prepared with the use of ZAIS(0.24) nanorods via a seeded growth process. Mixture powders of Zn(CH3COO)2, Ag(CH3COO), and In(CH3COO)3 with a mole ratio of Zn:Ag:In = 2(1- xp) : xp : xp were used as metal precursors. ZAIS(0.24) nanorods (3.8 × 10-9 mol(particles)) as seed crystals were put into a test tube with metal precursor powders (xp= 1.0, total metal ions of 0.10 mmol) and thiourea (0.10 mmol). After adding a mixture solution of OLA (2.7 cm3) and DDT (0.30 cm3), the resulting suspension was heat-treated for 8.0 min at 170 °C with vigorous stirring under an N2 atmosphere. After cooling the solution to room temperature, the thus-obtained suspension was subjected to centrifugation at 4000 rpm for 5 min. The precipitates were washed with methanol several times and then redissolved in chloroform (3.0 cm3). The obtained chloroform solution contained a mixture of single-component ellipsoidal ZAIS nanocrystals (ZAIS nano-ellipsoids) and dumbbell-shaped ZAIS nanocrystals (ZAIS nanodumbbells), the latter nanostructure being composed of two ellipsoidal ZAIS nanocrystals


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connected by a ZAIS nanorod. Thus, the size-selective precipitation technique was used for separation of particles with individual shapes. A portion of methanol as a non-solvent, typically ca. 0.05~0.1 cm3, was mixed with the obtained chloroform solution, followed by centrifugation to isolate the precipitates containing enriched ZAIS nano-dumbbells. Smaller ZAIS nano-ellipsoids as a by-product were isolated from the supernatant by vacuum evaporation of the solvent. Separated portions of ZAIS nanocrystals were again dissolved individually in chloroform (3.0 cm3). With repetition of these procedures several times, purified ZAIS nanocrystals with only one shape, dumbbell or ellipsoid, were obtained. Preparation of ZAIS nano-dumbbells and nano-ellipsoids with different xp values of 0.40 ~ 0.60. The above-mentioned preparation procedure was slightly modified to prepare ZAIS nanodumbbells having ZAIS tips of different chemical compositions. Mixture powders of Zn(CH3COO)2, Ag(CH3COO), and In(CH3COO)3 with a mole ratio of Zn:Ag:In = 2(1- xp) : xp : xp were used as metal precursors. ZAIS(0.24) nanorods (3.8 × 10-9 mol(particles)), metal precursor (total metal ions: 0.10 mmol), and thiourea (0.10 mmol) were added to a test tube containing OLA (2.9 cm3) and DDT (0.10 cm3). The resulting suspension was heat-treated with vigorous stirring under an N2 atmosphere for 30 min at 150 °C and then for 10 min at 250 °C. The thus-obtained ZAIS nano-dumbbells were isolated from nano-ellipsoids as a by-product in the same way as that described above. Characterization of ZAIS nanocrystals. A transmission electron microscope (TEM, Hitachi, H-7650) was used to evaluate the size distribution of nanocrystals at an operation voltage of 100 kV. TEM samples were prepared by


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dropping a chloroform solution containing nanocrystals onto a copper TEM grid covered with an amorphous carbon overlayer (Okenshoji Co., Ltd., ELS-C10 STEM Cu100P grid), followed by drying under vacuum. Images of high-resolution bright-field TEM and high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) were obtained by a Cscorrected HR-STEM (JEOL Co. Ltd., ARM-200F) with an acceleration voltage at 200 kV. Energydispersive X-ray spectroscopy (EDS) analysis was simultaneously carried out during the HAADFSTEM measurements. X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (Rigaku, SmartLab-3K) using Cu Kα radiation. Samples for XRD measurement were prepared by loading nanocrystals on a low-background Si sample holder. The chemical composition of ZAIS nanocrystals was determined by X-ray fluorescence spectroscopy (Rigaku, EDXL-300) or energy dispersive X-ray spectroscopy using a Hitachi SU-1500 scanning electron microscope equipped with an EDS analyzer (Horiba, Emax Energy EX-250). The ionization energy of ZAIS nanocrystals was measured by photoelectron yield spectroscopy in air with the use of a photoelectron spectrometer (Riken Keiki, AC-2). For the ionization energy measurement, nanocrystal solutions were spread on a fluorine-doped tin oxide conducting glass substrate, followed by drying with a nitrogen flow. ZAIS nanocrystals were uniformly dispersed in chloroform for measurements of absorption and photoluminescence (PL) spectra. A spectrophotometer (Agilent Technology 8453A) was used to obtain UV-visible absorption spectra. Photoluminescence (PL) spectra were measured with a photonic multichannel analyzer (HAMAMATSU, PMA-12) at an excitation wavelength of 365 nm. The absolute PL quantum yield (QY) was acquired with an absolute PL QY measurement system (HAMAMATSU C9920-03) with excitation wavelength of 365 nm. PL decay curves at


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room temperature were recorded by a time-correlated single-photon counting apparatus (HAMAMATSU, Quantaurus-Tau) equipped with photodiodes. Photocatalytic H2 evolution with ZAIS nanocrystals. Excess amounts of OLA and DDT adsorbed on the particle surface were removed by dissolving ZAIS nanocrystals in toluene followed by refluxing for 1 h. A portion of methanol was added to the ZAIS nanocrystal toluene solution at room temperature, and then the precipitates were washed several times with methanol and once with 2-propanol. Thus-obtained wet ZAIS nanocrystals precipitates, containing 4.5 μmol as the total amount of Ag, In and Zn, were suspended in a 5.0-cm3 portion of water/2-propanol (1:1) mixture solution containing 50 mmol dm−3 Na2S, in which 2-propanol and sulfide ion worked as hole scavengers. The suspension was deaerated with Ar bubbling for 30 min and then irradiated by a 300 W Xe lamp (λ > 350 nm, light intensity of 500 mW cm−2) under an Ar atmosphere with vigorous magnetic stirring at room temperature. The amount of H2 evolved was measured by an Agilent micro-GC 3000A gas chromatograph equipped with a molecular sieve 5A column.

Results and discussion Characterization of ZAIS nano-dumbbells and nano-ellipsoids with xp of 1.0 in the preparation. Figure 1a show a representative TEM image of starting ZAIS nanorods with the composition of x= 0.24. These nanorods with a width of 4.0 ± 0.9 nm and length of 22 ± 8.5 nm were used as seed crystals for the deposition of ZAIS nanocrystals with different compositions via a heterogeneous nucleation process. Heat treatment of ZAIS(0.24) nanorods in an OLA-DDT


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mixture solution containing the precursors at 170 °C for 8.0 min resulted in the formation of a mixture of ZAIS nanocrystals with different shapes, as shown in Fig. 1b. Many small nanocrystals with ellipsoid or elongated polygon shapes were observed, the sizes of which were 1100 nm (not shown), but deposition of such spherical particles as tips on the termini of starting nanorods was scarcely observed (Fig. S2a). On the other hand, heat treatment for 15 min at 170 °C caused excessive crystal growth on the ends of individual rods, resulting in the formation of non-uniform dumbbell-shaped particles, accompanied by coalescence of ZAIS nanocrystals into larger teardrop-shaped nanocrystals (ca. 15 nm in width × ca. 30 nm in length) (Fig. S2b). Thus, we concluded that well-controlled ZAIS nano-dumbbells could be prepared by appropriate selection of reaction conditions, including reaction temperature and reaction time. Figure 2 shows XRD patterns of starting ZAIS(0.24) nanorods, ZAIS nano-ellipsoids, and ZAIS nano-dumbbells. As reported in our previous paper,60 ZAIS nanorods had a hexagonal wurtzite crystal structure, with each diffraction peak being located between those corresponding to hexagonal ZnS and AgInS2 crystals. The ZAIS nano-ellipsoids also showed a hexagonal crystal structure similar to that of ZAIS nanorods, but individual diffraction peaks were shifted to a lower diffraction angle. These observations indicated that the nano-ellipsoids were composed of a solid solution between ZnS and AgInS2, the Zn fraction in their composition being smaller than that of starting ZAIS(0.24) nanorods. On the other hand, ZAIS nano-dumbbells exhibited sets of diffraction peaks assignable to both the starting ZAIS(0.24) nanorods and ZAIS nano-ellipsoids. Since the dumbbell-shaped nanocrystals were not a simple mixture of these nanocrystals, the results suggested that the starting ZAIS(0.24) nanorods were incorporated in the dumbbell structure as a rod part without significant changes in the crystal structure and chemical composition,


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while ellipsoidal ZAIS tips were grown on the ends of the nanorods and their crystal structure and chemical composition were the same as those of free ZAIS nano-ellipsoids simultaneously formed

(11-22)

(10-13)

(11-20)

(10-12)

(10-11)

(0002)

(10-10)

as a by-product. This was confirmed by following high-resolution TEM measurements.

Hexagonal ZnS

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b) (c) Hexagonal AgInS2

20

30

40

50

2 / degree (Cu K)

60

Figure 2. XRD patterns of starting ZAIS(0.24) nanorods (a) and purified ZAIS nanocrystals of nano-dumbbells (b) and nano-ellipsoids (c) obtained by a size-selective precipitation technique. Heat treatment used for the preparation of ellipsoidal and dumbbell-shaped nanocrystals was carried out at 170 °C for 8 min with xp= 1.0. XRD patterns of bulk materials are also shown for hexagonal ZnS (PDF# 00-010-043) and hexagonal AgInS2 (PDF# 01-089-502).


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The chemical compositions of starting ZAIS(0.24) nanorods and purified ZAIS nanocrystals of nano-ellipsoids and nano-dumbbells prepared with xp= 1.0 are shown in Table 1. Regardless of particle morphology, the obtained nanocrystals had nearly stoichiometric ratios of Zn : Ag : In as expected from the solid solution between ZnS and AgInS2. ZAIS nano-ellipsoids had a Zn : Ag : In ratio of 8 : 48 : 44, giving the experimentally obtained x value of 0.92 in the composition formula, (AgIn)xZn2(1-x)S2, which was slightly smaller than xp in the preparation, 1.0. Here, ZAIS nanoellipsoids with an experimentally determined x value are represented as ZAIS(x) nano-ellipsoids. It should be noted that the nano-ellipsoids contained a small amount of Zn even in the case of the preparation of xp= 1.0, that is, the addition of only Ag(CH3COO) and In(CH3COO)3 as metal precursors for crystal growth. This indicated that partial dissolution of starting ZAIS nanorods occurred to liberate Zn2+ ions in the solution during the heat treatment. The ZAIS nano-dumbbells had an intermediate composition between the starting nanorods and the nano-ellipsoids in the preparation of xp= 1.0, as expected from their XRD pattern.


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Table 1. Experimentally obtained chemical compositions, dimensions, and energy gaps of a nanorod, nano-ellipsoids, and nano-dumbbells composed of ZAIS solid solution.

structure

(a)

xp

composition of particles

dimensions x in chemical (b) formula

Zn : Ag : In

rod or rod part

ellipsoid or tip part

(width / nm ) ×(length / nm)

(width / nm ) ×(length / nm)

Eg / eV

rod

----

76 : 12 : 12

0.24

(4.0±0.90) ×(22±8.5)

----

2.92

ellipsoid

1.0

8 : 48 : 44

0.92

----

(5.2±0.61) ×(8.9±1.3)

1.96

dumbbell

1.0

34 : 36 : 30 0.92/0.24/0.92(c)

(4.1±0.90) ×(23±7.0)

(5.6±0.85) ×(11±2.1)

1.93

ellipsoid

0.60

38 : 28 : 34

----

(3.8±0.65) ×(7.2±1.5)

2.17

dumbbell

0.60

63 : 17 : 20 0.62/0.24/0.62(c)

(3.9±0.53) ×(16±4.0)

(4.9±1.0) ×(7.0±2.2)

2.18

ellipsoid

0.50

54 : 20 : 26

----

(4.2±0.56) ×(8.0±1.5)

2.23

dumbbell

0.50

67 : 15 : 18 0.46/0.24/0.46(c)

(3.8±0.62) ×(17±5.1)

(4.7±0.86) ×(9.7±3.5)

2.24

ellipsoid

0.40

61 : 17 : 22

----

(3.4±0.50 ) ×(6.5±1.2)

2.50

dumbbell

0.40

71 : 12 : 18 0.39/0.24/0.39(c)

(3.7±0.50) ×(19±6.3)

(4.3±0.70) ×(7.9±2.7)

2.49

0.62

0.46

0.39

(a) The metal precursor powders were added with molar ratio of Zn(CH3COO)2 : Ag(CH3COO) : In(CH3COO)3 = 2(1- xp) : xp : xp : in the preparation of dumbbellstructured nanocrystals. (b) The value was calculated from experimentally obtained chemical composition. (c) The spatial change of the composition, denoted as x(tip)/x(rod)/x(tip), was estimated for a dumbbell-shaped nanocrystal.

The interface between rod part and ellipsoidal tip part in a dumbbell-shaped particle was investigated by high-resolution TEM measurements. Figure 3a shows a bright field image for a ZAIS nano-dumbbell obtained by high-resolution TEM measurement. The starting ZAIS nanorods


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were reported in our previous paper to have a long axis along the direction and a short axis parallel to .60 Continuous lattice fringes were observed throughout the particle without showing grain boundary inside the particles, even at the interfaces between the rod part and the ellipsoidal tip parts deposited on the termini of the nanorod. This indicated that each nanodumbbell was also composed of a single crystal of ZnS-AgInS2 solid solution, in which the ellipsoidal tips were epitaxially grown on both termini of a nanorod. The interplanar spacing of the lattice fringes observed in the ellipsoidal tip domain was 0.351 nm, which was assigned to that of {10-10} planes, being slightly larger than that in the rod domain, 0.336 nm, as indicated in the inset of Fig. 3a. Furthermore, the HAADF-STEM measurements enabled visualization of the spatial change of chemical composition between the rod part and ellipsoidal tip parts of a ZAIS nano-dumbbell. As shown in Fig. 3b, the ellipsoidal tip in a dumbbell nanocrystal had a brighter image contrast than that of the rod part, indicating a larger content of heavy elements, Ag and In. The lattice was coherently grown through the interface between the rod and tip parts, though the outermost layer of the ellipsoidal tip part did not show lattice fringes, probably due to the presence of lattice distortion or the deposition of an amorphous layer. The lattice fringes shown in Fig. 3b were assigned to {0002} planes. From cross-sectional intensity profiles of the HAADF-STEM image (Fig. S3), the average values of interplanar spacings in the rod and ellipsoidal tip parts in the dumbbell nanocrystal were determined to be 0.316 nm and 0.330 nm, respectively, which were roughly consistent with the lattice distances of {0002} planes in ZAIS(0.24) nanorods and ZAIS(0.92) nano-ellipsoids of 0.314 nm and 0.334 nm, respectively, determined from XRD peaks in Fig. 2. Although the lattice mismatch between {0002} planes of ZAIS(0.24) and ZAIS(0.92) crystals was expected to be as large as 6.4% from the XRD analysis, no misfit dislocation was observed at the heterojunction due to the relaxation of lattice strain in a nano-sized crystal.


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Nanoscale EDS analysis carried out during HAADF-STEM measurement revealed that the ratios of Zn : Ag : In in the rod and ellipsoidal tip parts of dumbbell nanocrystal were 73 : 15 : 12 and 9 : 49 : 42, respectively, being in good agreement with those of starting ZAIS(0.24) nanorods and free ZAIS(0.92) nano-ellipsoids (Table 1), respectively. Therefore, we concluded that the ZAIS nanodumbbells were produced by epitaxial crystal growth of ellipsoidal ZAIS tips on both termini of starting ZAIS(0.24) nanorods, the chemical composition being spatially and steeply modified between the rod and tip parts. Furthermore, the finding that the chemical composition of the ZAIS tips on nanorods was very similar to that of the free ZAIS nano-ellipsoids formed as a by-product is of significant importance to estimate the spatial modulation of the chemical composition in a ZAIS nano-dumbbell. Here, we denote the spatial composition change in a dumbbell-shaped nanocrystal as ZAIS(x(tip)/x(rod)/x(tip)), for example, ZAIS(0.92/0.24/0.92), by assuming that the chemical composition of each part is the same as that experimentally obtained for the corresponding single-component counterpart, nanorod or nano-ellipsoid.


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0.336 nm

0.351 nm

(a) 10 nm

d= 0.316 nm

30 nm

(b) 5 nm

d= 0.330 nm

Figure 3. (a) A bright field HR-TEM image with atomic resolution for a ZAIS nano-dumbbell. (Inset) High magnification image of a tip on one end of the nanorod. (b) High-resolution HAADFSTEM image of the interface between the rod part and the ellipsoidal tip part in a ZAIS nanodumbbell. (Inset) A wide area image of the same nano-dumbbell crystal as that shown in panel b. Sample nanocrystals were prepared by heat treatment at 170 °C for 8 min with xp= 1.0.


18

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Ag In

ZAIS(0.24) nanorod

Ag+, In3+, Zn2+, S2-

Zn S

In

Heating

Zn

Ag S Zn In Ag

ZAIS(x/0.24/x) nano-dumbbell

S

ZAIS(x) nano-ellipsoid

Epitaxial growth

Figure 4. Schematic illustration of the formation mechanism of ZAIS nano-dumbbells and nanoellipsoids.

We propose the formation mechanism of ZAIS nano-dumbbells with a spatially tuned chemical composition as shown in Fig. 4. Each terminus surface of the nanorod has a larger amount of defect sites due to its high curvature than those in the facets on its side, acting as a starting point for further growth of the ZAIS crystal. Heat treatment can decompose precursors to form active metal ions and sulfide ion species in the solution, accompanied by partial dissolution of the starting ZAIS nanorods. Some active species are epitaxially deposited on the both ends of a ZAIS nanorod to form ellipsoidal ZAIS tips, the chemical composition of which is dependent on the molar ratio of corresponding metal acetates as a precursor used in the crystal growth and is different from that of ZAIS(0.24) nanorods as a host, while the remaining active species can form an ellipsoidal


19


Page 20 of 44

nanocrystal through homogeneous nucleation, the composition of which is consistent with that of ZAIS tips on the nanorod.

Control of the optical properties of ZAIS nano-dumbbells by compositional modulation The tunability of the electronic energy structure of ZAIS nanocrystals depending on the chemical composition59,60 is one of the most advantageous properties for application to light energy conversion systems. We prepared a variety of ZAIS nano-dumbbells to investigate the influence of compositional modulation in the tip parts on their photochemical properties, with the xp value of the metal ion ratio in the precursor being systematically varied in the preparation of nanodumbbells. Regardless of the xp value used, anisotropic nanocrystals with similar dumbbell shapes were formed, accompanied by simultaneous formation of ZAIS nano-ellipsoids as a by-product. Figure 5 shows TEM images of ZAIS nano-dumbbells and nano-ellipsoids prepared with different xp values, which were purified by the size-selective precipitation. The dimensions of ZAIS nanodumbbells and nano-ellipsoids as well as their chemical composition are also shown in Table 1. ZAIS nano-ellipsoids prepared with xp of 0.40~0.60 (Figs. 5b, d and f) had similar dimensions with an average width of 3.4~4.2 nm and average length of 6.5~8.0 nm, being slightly smaller than those of nano-ellipsoids prepared with xp= 1.0 due to the difference of reaction conditions. On the other hand, ZAIS nano-dumbbells, shown in Figs. 5a, c and e, had a structure composed of a rod part and two ellipsoidal tips of similar sizes. Although some of particles had irregularly-grown tip parts, the average dimensions of tip parts of dumbbell nanocrystals prepared with xp of 0.40~0.60 were slightly larger than those of corresponding nano-ellipsoids, and then their rod parts had the


20

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similar average lengths of ca. 16~19 nm, which were a little shorter than that of the rod part, 23 nm, of ZAIS nano-dumbbell prepared with xp=1.0.

Nano-dumbbell

(a)

Nano-ellipsoid

(b)

xp= 0.60

50 nm

50 nm

(c)

(d)

xp= 0.50

50 nm

(e)

50 nm

(f)

xp= 0.40

50 nm

50 nm

Figure 5. Wide-area TEM images of ZAIS nano-dumbbells prepared with different xp values and corresponding ZAIS nano-ellipsoids simultaneously formed as a by-product. The xp value used in the preparation is shown beside panels. ZAIS nanocrystals of each shape were separated from asprepared mixture nanocrystals by a size-selective precipitation technique.


21


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XRD measurements (Fig. S4) revealed that ZAIS nano-ellipsoids had a hexagonal wurtzite crystal structure, and individual diffraction peaks were shifted to a higher diffraction angle with a decrease in the xp value used in the preparation due to an increase in the Zn fraction. The individual XRD patterns of ZAIS nano-dumbbells could be expressed by the sum of those of ZAIS(0.24) nanorods and ZAIS nano-ellipsoids prepared with the same xp as that of nano-dumbbells, being similar to the XRD pattern of ZAIS(0.92/0.24/0.92) nano-dumbbells prepared with xp= 1.0 in Fig. 2. The chemical compositions of ZAIS nano-dumbbells and nano-ellipsoids prepared with xp= 0.40~0.60 are also shown in Table 1. Each kind of nanocrystal had an almost stoichiometric ratio of Zn : Ag : In. By analogy to the above-mentioned ZAIS(0.92/0.24/0.92) nano-dumbbells prepared with xp=1.0, it is reasonable to speculate that the spatial compositional changes of a nanodumbbell crystal prepared with xp= 0.40, 0.50, or 0.60 were comparable to those of the particles consisting of two ZAIS nano-ellipsoids prepared with the corresponding xp values connected by a starting ZAIS(0.24) nanorod. Thus we use the same abbreviation of ZAIS(x(tip)/x(rod)/x(tip)) with experimentally determined x values for nano-dumbbells prepared with xp=0.40~0.60 in the preparation in the following discussion.


22

(iv)

(iii)

(0) 300

400

(ii) (i)

500

600

Absorbance (normalized)

(a)

(c)

(iii)

300

700

400

400

500

600

700

800

Wavelength / nm

500

600

700

Wavelength / nm

(b)

900

1000

PL Intensity (normalized)

(0) (iv) (iii) (ii) (i)

(ii) (i)

(iv)

Wavelength / nm

PL Intensity (normalized)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Absorbance (normalized)

Page 23 of 44

(iv) (iii) (ii)

400

500

600

(d)

(i)

700

800

900

1000

Wavelength / nm

Figure 6. Absorption (a,c) and photoluminescence (PL) spectra (b,d) of ZAIS(x/0.24/x) nanodumbbells (a,b) and ZAIS(x) nano-ellipsoids (c,d). The x values of samples are 0.92 (i), 0.62 (ii), 0.46 (iii), and 0.39 (iv). For comparison, the absorption spectrum and PL spectrum of the starting ZAIS(0.24) nanorods (curves (0)) are also shown in the panels a and b, respectively. The absorption spectra were normalized at 300 nm. The PL spectra were obtained by excitation at 365 nm.

Figures 6a and c show absorption spectra of ZAIS(x/0.24/x) nano-dumbbells and ZAIS(x) nano-ellipsoids, respectively. In both cases of nano-dumbbells and nano-ellipsoids, the absorption spectra were blue-shifted with a decrease in the experimentally determined x value, that is, with an increase in the Zn fraction. Their absorption onsets were located at the longer wavelength side of that of starting ZAIS(0.24) nanorods, 440 nm. However, when a comparison was made for each


23


Page 24 of 44

x value, the absorption onsets were the same for ZAIS(x/0.24/x) nano-dumbbells and ZAIS(x) nano-ellipsoids, indicating that Eg of the ellipsoidal ZAIS tips grown on the nanorod agreed well with that of corresponding free ZAIS(x) nano-ellipsoids formed in the solution as a by-product, regardless of the x value. Eg was determined by the onset energy of a Tauc plot (Fig. S5), as shown in Table 1. By changing the chemical composition from x= 0.92 to 0.39, the obtained Eg values were increased from 1.93 to 2.49 eV for ZAIS(x/0.24/x) nano-dumbbells and from 1.96 to 2.50 eV for ZAIS(x) nano-ellipsoids. It should be noted that ZAIS(x/0.24/x) nano-dumbbells had an Eg value smaller than that of starting ZAIS(0.24) nanorods, but Eg of nano-dumbbells was very similar to that of ZAIS(x) nano-ellipsoids when a comparison was made for the same x value. This was in good agreement with the assumption of the similarity in compositions of the ellipsoidal tips in nano-dumbbells and the free nano-ellipsoids. Figures 6b and d show PL spectra of nanodumbbells and nano-ellipsoids with various compositions. Each kind of nanocrystal exhibited a broad PL peak attributed to emission with donor-acceptor pair (DAP) recombination and/or emission via defect sites. The peak wavelength was blue-shifted with an increase in Eg of nanocrystals. Being similar to the behavior in the absorption spectra, the individual PL peak wavelengths of ZAIS(x/0.24/x) nano-dumbbells agreed well with those of corresponding ZAIS(x) nano-ellipsoids but were greatly red-shifted from that of the stating ZAIS(0.24) nanorods. Since Eg of the rod part was larger than that of ZAIS tips in nano-dumbbells due to the larger Zn fraction, these results suggested that the photogenerated electrons and/or holes in the rod parts of nanodumbbells were transferred to ZAIS tips, being subjected to radiative recombination.


24

70 60

/ cps 0.5

30 20

Yield

0.5

/ cps

5.37 10

50 40 30

5.33

20

(c)

30

0.5

0.5

0.5

40

40

(b) / cps

(a)

0.5

50

Yield

20

5.55 10

10

0 4

4.5

5

5.5

6

6.5

0 4

4.5

Energy / eV

5

5.5

6

6.5

0 4

4.5

5

5.5

6

6.5

Energy / eV

Energy / eV

(d) Composition x= 0.92 0.24 0.92

e-

-2.45

x= 0.39 0.24

e-3.37 -3.05

0.39

-2.45

ECB 2H＋/H2

-3.68 Energy / eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Yield

Page 25 of 44

(pH 12.8)

-5.37

-5.37

-5.33

h+

ZAIS(0.92/0.24/0.92)

-5.55

h+

EVB

ZAIS(0.39/0.24/0.39)

Figure 7. Photoelectron yield spectra of PYSA measurement. Samples used were ZAIS(0.24) nanorods (a), ZAIS(0.92) nano-ellipsoids (b), and ZAIS(0.39) nano-ellipsoids (c). (d) Schematic illustrations of the electronic energy structures of nano-dumbbells of ZAIS(0.92/0.24/0.92) and ZAIS(0.39/0.24/0.39) estimated from the energy structures of corresponding single-component counterparts.

Evaluation of the electronic energy structure of dumbbell-shaped ZAIS nanocrystals is important for the application to solar light energy conversion systems, because the nanostructured heterojunctions between semiconductors with different energy gaps can form the conduction band and valence band offsets. However, the presence of two components with different energy gaps in


25


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ZAIS nano-dumbbells made it difficult to directly determine their ionic energy. Thus, we estimated the electronic energy structure of ZAIS nano-dumbbells based on the ionic energy and energy gap of starting ZAIS nanorods and free ZAIS nano-ellipsoids, which are assumed to comprise dumbbell-shaped nanocrystals. Among the various methods reported, photoelectron yield spectroscopy in air (PYSA) is an effective method for determining the electronic energy structures of semiconductor nanocrystals.61,62 Figures 7a-c show PYSA spectra of ZAIS nanorods and nanoellipsoids. From the onset energy of the photoelectron emission, the ionization energies can be determined to be 5.37, 5.33 and 5.55eV for ZAIS(0.24) nanorods, ZAIS(0.92) nano-ellipsoids and ZAIS(0.39) nano-ellipsoids, respectively. The opposite sign of ionization energy should correspond to the energy level of the valence band edge (EVB) of a semiconductor relative to the vacuum level, and then that of the conduction band edge (ECB) is approximately estimated by subtracting Eg from EVB. The thus-determined energy levels of EVB and ECB were -5.37 and -2.45 eV for starting ZAIS(0.24) nanorods, -5.33 and -3.37 eV for ZAIS(0.92) nano-ellipsoids, and 5.55 and -3.05 eV for ZAIS(0.39) nano-ellipsoids, respectively. Since the ZAIS tips grown on the nanorod termini were assumed to have both chemical composition and energy gap similar to those of corresponding free nano-ellipsoids, the electronic energy structures of ZAIS nano-dumbbells are deduced by the combination of ZAIS(0.24) nanorods and nano-ellipsoids of ZAIS(0.92) or ZAIS(0.39) as constituent units. As shown in Fig. 7d, the nano-dumbbells of ZAIS(0.39/0.24/0.39) have a staggered band alignment assignable to a type-II structure, while the EVB levels at the heterojunction in ZAIS(0.92/0.24/0.92) nano-dumbbells were very similar each other, their band alignment being assigned to a quasi-type II. In both nano-dumbbells, large energy differences of ECB were observed at the heterojunction, indicating the localization of photogenerated electrons in the ZAIS tips grown on the rod termini. In contrast, it was suggested that photogenerated holes


26

Page 27 of 44

were delocalized over the whole dumbbell nanocrystal, because the offsets in EVB levels at the heterojunctions for both kinds of nano-dumbbells were very small in comparison to those of ECB. Thus, charge separation of photogenerated electrons and holes is expected to effectively occur in dumbbell-shaped ZAIS nanocrystals, resulting in longer lifetimes of photogenerated carriers, which can potentially increase the possibility of photocatalytic reactions with redox species at the particle surface.

3

10

PL intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ZAIS(0.24) nanorod ZAIS(0.39) nano-ellipsoid ZAIS(0.39/0.24/0.39) nano-dumbbell

2

10

0

5

10

Time / s

15

Figure 8. PL intensity decay profiles of starting ZAIS(0.24) nanorods, ZAIS(0.39) nano-ellipsoids, and ZAIS(0.39/0.24/0.39) nano-dumbbells, measured at corresponding PL peak wavelengths.


27


Page 28 of 44

Carrier dynamics in the dumbbell-shaped ZAIS nanocrystals was remarkably different from those of single-component counterparts. Figure 8 shows representative PL intensity decay profiles, measured at corresponding peak wavelengths. The PL intensity of ZAIS(0.39/0.24/0.39) nanodumbbells decayed more slowly than those of ZAIS(0.39) nano-ellipsoids and ZAIS(0.24) nanorods. Since each curve could be fitted well with a triple exponential function with the fitting parameters listed in Table S1, we calculated average PL lifetime, τave, by the following equation:

 ave

A  A

2

n n

n

n n

n

,

where τn represents the decay lifetime of the PL emission and An represents the amplitude corresponding to the lifetime. The results as well as PL QYs are shown in Table 2. As expected, τave of each kind of nano-dumbbell was larger than the values of corresponding nano-ellipsoids and starting nanorods. Furthermore, the PL QYs of nano-dumbbells were smaller than those of nano-ellipsoids. These results agree with previously reported results for hybrid semiconductor nanocrystals with a type-II heterostructure, such as CdTe/CdSe, CdTe/CdS and ZnSe/CdS, in which photogenerated charge carriers were spatially separated in the nanocrystals, resulting in the increase of PL lifetimes due to the smaller overlap between electron and hole wavefunctions.24,26,31,34,35 The rate constants of radiative and non-radiative recombination processes, krad and knr, respectively, were estimated from τave and PL QYs. The starting ZAIS nanorods had larger values of both krad and knr than those of nano-dumbbells. When a comparison was made for a constant x value for both the ZAIS tips in dumbbell nanocrystals and the nanoellipsoids, both krad and knr decreased in the order of nano-ellipsoid > nano-dumbbell. These results could be understood well by the effective charge separation at the heterojunctions originating from the band alignments of quasi-type-II or type-II: In the dumbbell nanostructure, the photogenerated


28

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electrons were localized in the ellipsoidal tips grown on the termini of nanorods and the holes were delocalized over the whole particles, resulting in a reduced possibility of recombination due to a decreased overlap of the electron and hole wavefunctions. It should be noted that the decrease in the x value of ZAIS(x) nano-ellipsoids enlarged both krad and knr, being consistent with the tendency of spherical ZAIS nanocrystals of ca. 5.4 nm in size reported in our previous paper.63 This suggested that the numbers of recombination centers for both radiative and non-radiative transitions became large with an increase in the Zn fraction in ZAIS nanocrystals.

Table 2. Average lifetimes of PL decay, PL quantum yields, and rate constants of radiative and non-radiative recombination for various kinds of ZAIS nanocrystals.

Structure

x in chemical formula

τave (µs)

PL QY (%)

krad (105 s-1)(b)

2.1±0.25

0.56±0.067

knr (105 s-1)(c)

rod

0.24

0.37

ellipsoid

0.92

2.2

36±1.0

1.6±0.05

2.9±0.05

dumbbell

0.92/0.24/0.92(a)

3.2

15±0.4

0.46±0.011

2.7±0.01

ellipsoid

0.62

1.7

38±0.5

2.3±0.03

3.7±0.03

dumbbell

0.62/0.24/0.62(a)

2.9

12±0.3

0.43±0.010

3.1±0.01

ellipsoid

0.39

0.71

25±1.2

3.5±0.17

dumbbell

0.39/0.24/0.39(a)

4.6

7.5±0.97

0.17±0.021

26±0.1

11±0.2 2.0±0.02

(a) The spatial change of the composition, denoted as x(tip)/x(rod)/x(tip), was estimated for a dumbbell-shaped nanocrystal. (b) krad = PLQY/τave (c) knr = (1/τave) - krad


29


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Photocatalytic activity of ZAIS nano-dumbbells with heterojunctions It was reported in our previous papers that the photocatalytic activity of ZAIS nanocrystals was tunable depending on their composition, particle size, and particle morphology.59,60 However, only single-component ZAIS nanocrystals have so far been used as photocatalysts. An understanding of the influence of a heterojunction on the photocatalytic activity gives an important insight for designing the nanostructure of multi-component ZAIS nanocrystals. In this study, we investigated the photocatalytic activity of dumbbell-shaped ZAIS nanocrystals for H2 evolution and clarified the influence of their energy structure. Figures 9a and b shows time courses of the amount of H2 evolution by irradiation to various kinds of ZAIS nanocrystals. Since the amount of H2 linearly increased with elapse of irradiation time in each case, the ZAIS nanocrystals worked as stable photocatalysts under the experimental conditions used in the present study. Regardless of the composition, ZAIS(x/0.24/x) nano-dumbbells exhibited higher photocatalytic activity than corresponding ZAIS(x) nano-ellipsoids or particle mixtures of ZAIS(0.24) nanorods and ZAIS(x) nano-ellipsoids with a particle number ratio of 1 : 2 when the comparison was made for a constant x value. These results indicated that the ZAIS tips in nano-dumbbells had sufficient potential of ECB to reduce H+ on the surface and then the reaction probability of photogenerated electrons was enlarged by the increase of carrier lifetimes due to the effective charge separation at the heterojunctions of type-II or quasi-type II. The amount of evolved H2 for the case of ZAIS(0.39/0.24/0.39) nano-dumbbells reached ca. 290 µmol after 10 h irradiation, which was considerably larger than the molar amount of photocatalyst used, 4.5µmol. Furthermore, after the ZAIS nano-dumbbell suspension had been purged with pure Ar gas at the irradiation of 10 h, H2 evolution was repeatedly induced at a slightly lower rate with the Xe lamp irradiation, regardless of the chemical composition of nano-dumbbells (Fig. S6). TEM measurement of the photocatalyst


30

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particles collected after the photoreaction revealed that most nano-dumbbells had the original dumbbell shape (Fig. S7). Furthermore, the nano-dumbbells of ZAIS(0.92/0.24/0.92) and ZAIS(0.39/0.24/0.39) after the reaction had molar ratios of Zn : Ag : In= 36 : 36 : 27 and 71 : 12 : 17, respectively, being almost the same as those of as-prepared particles (Table 1). These results indicated that H2 evolution proceeded photocatalytically by the photoexcitation of ZAIS nanodumbbells.


31


300

(a) x= 0.92

Amount of H / mol

250

2

200 150 100

(iii)

50

(i) (iv) (ii)

0

0

2

4

6

8

10

Irradiation time / h

300

(i)

(b) x= 0.39 Amount of H / mol

250

(ii)

2

200

(iv)

150 100

(iii)

50 0

0

2

4

6

8

10


x= 0.92 0.39 0.92 0.39 0.92 0.39 30

(c)

R(H2) / mol h-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 44

20 10 0

(iii)

(i)

(ii)

(iv)

Figure 9. (a,b) Time courses of the amount of H2 evolution by irradiation to ZAIS nanocrystals. The x values of the samples are 0.92 and 0.39 in panels (a) and (b), respectively. (c) Dependence of R(H2) on the kind of ZAIS nanocrystals used. Samples were (i) ZAIS(x/0.24/x) nano-dumbbells, (ii) ZAIS(x) nano-ellipsoids, (iii) ZAIS(0.24) nanorods, and (iv) mixtures of ZAIS(0.24) nanorods and ZAIS(x) nano-ellipsoids (1 : 2).


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The H2 evolution rate (R(H2)) was determined from the slope of linear portion in the time course curves (Fig. 9c). The ZAIS(0.39/0.24/0.39) nano-dumbbells exhibited R(H2) of 32 µmol h1

, being ca. 3.7 times higher than that of the starting ZAIS(0.24) nanorods, 8.5 µmol h-1, while

ZAIS(0.92/0.24/0.92) nano-dumbbells gave a lower value of R(H2), 3.2 µmol h-1, than ZAIS(0.24) nanorods. The ZAIS nano-ellipsoids also exhibited significant composition-dependent photocatalytic activity: the R(H2) value increased from 0.54 µmol h-1 at x= 0.92 to 23 µmol h-1 at x= 0.39, owing to the shift of ECB from -3.37 eV to -3.05 eV with an increase in the Zn fraction as aforementioned. Considering this composition-dependent photocatalytic activity of nanoellipsoids, the change in the activity of ZAIS nano-dumbbells could be explained by the difference in ECB levels of the tips in nano-dumbbells. Based on the energy structures of nano-dumbbells shown in Fig. 7d, photogenerated electrons, localized in ellipsoidal ZAIS tips grown on the rod termini, reacted with H+ to produce H2 or otherwise were consumed by the recombination with holes. The energy level of the reduction of H+ to H2 under the present experimental condition (pH 12.8) is estimated to -3.68 eV. Since ECB levels of ZAIS tip parts in ZAIS(0.39/0.24/0.39) and ZAIS(0.92/0.24/0.92) nano-dumbbells were -3.05 and -3.37 eV, respectively, the driving force to reduce H+ is much larger for ZAIS(0.39/0.24/0.39) than for ZAIS(0.92/0.24/0.92) nano-dumbbells, resulting in the increase of photocatalytic activity in the order of ZAIS(0.92/0.24/0.92) < ZAIS(0.39/0.24/0.39) nano-dumbbells in a manner similar to that for ZAIS nano-ellipsoids.

Conclusion


33


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We have successfully developed a procedure for synthesis of dumbbell-shaped ZAIS nanocrystals via a seeded growth process, in which ellipsoidal ZAIS tips were epitaxially grown on both termini of ZAIS nanorods used as seeds. The chemical composition of ZAIS tips in nanodumbbells was controllable, without significant changes in the dimension and composition of the rod part, by changing the metal ion ratio in the precursor used for the seeded growth process. The molar ratio of Zn : Ag : In in ZAIS nano-dumbbells was spatially modified: the fraction of Zn was larger in the rod part than in the tips on the nanorod, resulting in modulation of electronic energy structure at the interfaces between the rod and tip parts to form heterojunctions of type-II or quasitype-II. The optical properties and carrier dynamics of nano-dumbbells were remarkably influenced by the chemical composition of the tip parts. The photogenerated electrons and holes were effectively and spatially separated at the heterojunction, resulting in the localization of photogenerated electrons in ellipsoidal ZAIS tips in the nano-dumbbells. The photocatalytic H2 evolution rate of ZAIS nano-dumbbells was also controllable, and precise tuning of the chemical composition of the ZAIS tips on nanorods considerably improved their photocatalytic activity. The most important advantage of multinary Zn-Ag-In-S nanocrystals for the fabrication of heterojunctions is spatial tunability of the energy structure without significant lattice strains just by changing the chemical composition. This is not expected for heterojunctions between conventional binary semiconductors such as CdSe and CdTe. The findings presented in this paper will be important for the development of novel heterostructured nanocrystals, especially, those composed of multinary semiconductors, for utilization in solar light energy conversion systems.

ASSOCIATED CONTENT


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The Supporting Information is available free of charge on the ACS Publications website at DOI: number: TEM images of ZAIS nano-dumbbells prepared with different reaction conditions, XRD patterns, Tauc plots, fitting parameters of PL decay curves, and time curses of the amount of H2 evolution.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by JSPS KAKENHI Grand Numbers JP16H06507 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation”, JP17H05254, JP15H03876 and JP16H06052. PYSA measurement, which was performed at Riken Keiki Co., Ltd., was kindly supported by Dr. Yoshiyuki Nakajima and Mr. Yoshiharu Iwai (Riken Keiki Co., Ltd.).


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Enhanced Photocatalytic Activity of Zn-Ag-In-S ... - ACS Publications

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