Tunable Photoelectrochemical Properties of Chalcopyrite AgInS2

Article pubs.acs.org/JPCC

Tunable Photoelectrochemical Properties of Chalcopyrite AgInS2 Nanoparticles Size-Controlled with a Photoetching Technique Tsukasa Torimoto,*,† Masaki Tada,† Meilin Dai,† Tatsuya Kameyama,† Shushi Suzuki,†,‡ and Susumu Kuwabata‡,§ †

Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan Japan Science & Technology Agency, CREST, Kawaguchi, Saitama 3320012, Japan § Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan ‡

ABSTRACT: Size-selective photoetching enabled precise size control of chalcopyrite AgInS2 nanoparticles in a basic aqueous solution containing ammonia by selecting the wavelength of monochromatic irradiation light in the range between 650 and 520 nm. The energy gap of the photoetched particles was enlarged from 1.8 to 2.4 eV with a decrease in particle size from 5.1 to 2.7 nm due to the quantum size effect, while the crystal structure and chemical composition of the particles were unchanged after the photoetching processes. AgInS2 nanoparticle-immobilized electrodes exhibited an anodic photocurrent similar to that of n-type semiconductors, the onset potential of which was negatively shifted with a decrease in particle size. Potentials of the conduction band edge (ECB) and the valence band edge (EVB) of AgInS2 nanoparticles were estimated from their photoelectrochemical measurements, and they exhibited remarkable size dependence: with a decrease in particle size from 5.1 to 2.7 nm, ECB was shifted negatively from −0.6 to −1.0 V vs Ag/AgCl, accompanied by a positive shift of EVB from 1.3 to 1.7 V vs Ag/AgCl.

1. INTRODUCTION

trochemical properties, from which ECB and EVB of CdS were determined as a function of particle size. Recently, nanoparticles of ternary I−III−VI semiconductors, such as CuInS2 and AgInS2, and related materials have attracted much attention for applications in solar light energy conversion systems, such as solar cells and photocatalysts, because of their low toxicity and large absorption coefficient in the wavelength from visible to near-infrared regions.18−22 These nanoparticles have been synthesized in solution phase and have been reported to exhibit size-dependent photoluminescence properties due to quantum size effects. One of the most promising applications of I−III−VI nanoparticles is as a light-absorbing material for quantum dot solar cells, and it is therefore necessary to determine the electronic energy structure of the nanoparticles for designing and fabricating efficient devices. However, the details have not been investigated. One of the reasons is the difficulty in controlling particles size without any changes in the chemical composition or crystal structure of semiconductor nanoparticles consisting of more than three elements.18,23,24 It is well-known that I−III−VI ternary semiconductors, such as CuInS2 and AgInS2, form nonstoichiometric compounds,23,25−27 the physicochemical properties of which are greatly dependent on the chemical

Semiconductor nanoparticles have been key materials in the development of next-generation electronic and optoelectronic devices.1−5 Intensive studies have been performed for the syntheses of binary semiconductor nanoparticles of high quality, such as CdS, CdSe, CdTe, PbS, and InAs, and determination of their size-dependent physicochemical properties.6−12 For photovoltaic applications, it is important for the design of efficient devices to clarify changes in potentials of the conduction band edge (ECB) and the valence band edge (EVB) with decrease in the size of semiconductor nanoparticles.13 Various techniques to clarify such changes, including photoelectrochemical measurement,14,15 cyclic voltammetry,16 photoemission spectroscopy13 and pulse radiolysis,17 have been developed. For example, Nozik and co-workers17 determined the electronic energy structure of CdTe nanoparticles with pulse radiolysis in which the potentials of ECB and EVB exhibited negative and positive shifts, respectively, with a decrease in particle size due to the quantum size effect. Jasieniak and coworkers13 clarified the size-dependent potential shifts for CdSe, CdTe, PbS, and PbSe using photoelectron spectroscopy in air. Furthermore, photoelectrochemical measurement of a semiconductor nanoparticle-modified electrode is a convenient method and enables estimation of the flat band potential from the onset potential of photocurrents. We previously reported15 that CdS nanoparticles exhibited size-dependent photoelec© 2012 American Chemical Society

Received: July 24, 2012 Revised: September 10, 2012 Published: September 14, 2012 21895

dx.doi.org/10.1021/jp307305q | J. Phys. Chem. C 2012, 116, 21895−21902

The Journal of Physical Chemistry C

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cm3 portion of 1.0 mol dm−3 MES aqueous solution and then stirred for 12 h. The obtained suspension was subjected to centrifugation, followed by dissolution of precipitates in water. 2.3. Size-Selective Photoetching of AgInS2 Nanoparticles. MES-modified AgInS2 nanoparticles were dissolved in an aqueous solution containing 10 mmol dm−3 HMP to give an absorbance of 0.2 at 550 nm, followed by pH adjustment with NaOH or NH3 aqueous solution. A 3.0 cm3 portion of the thus-obtained aqueous solution was put into a quartz cuvette (1.0 cm × 1.0 cm × 4.0 cm) and irradiated with monochromatic light with various wavelengths under gentle bubbling of oxygen gas, which acted as an electron acceptor for electrons photogenerated in AgInS2 particles. The light sources used in this study were a 500 W high-pressure Hg lamp (USHIO, Optical Modulex USH-500SC) and a 300 W Xe lamp (EAGLE, PE300BF). The former was used for irradiation of monochromatic light at 546 nm, one of the Hg emission lines, that was extracted from an Hg lamp through an interference filter. On the other hand, monochromatic lights at 650, 600, 570, and 520 nm were obtained from the Xe lamp through interference filters. The light intensity was ∼10 mW cm−2 in the wavelength region of photoetching. 2.4. Characterization of Particles. Absorption spectra were obtained by measuring the particle solutions in 10 mm path-length quartz cuvettes with an Agilent Technology 8453A UV−visible spectrophotometer. PL spectra were acquired with a photonic multichannel analyzer (Hamamatsu, PMA-12) by monochromatic light irradiation (λ = 430 nm) at room temperature. The morphology of the obtained particles and their size distribution were determined using a Hitachi H7650 transmission electron microscope (TEM) with an acceleration voltage at 100 kV. Samples for TEM observation were prepared by dropping the particle solution onto a carbon-coated copper grid, followed by complete drying under decreased pressure. The chemical composition of AgInS2 particles was determined by X-ray fluorescence spectroscopy (Rigaku, EDXL-300). Crystal structures of nanoparticles were analyzed using a powder X-ray diffraction (XRD) instrument (Rigaku, 2100HL) equipped with an X-ray tube (Cu Kα radiation: λ = 1.540 59 Å, 40 kV, 30 mA). The particle solutions were dropped on a lowbackground silicon sample holder and dried for XRD measurement. 2.5. Photoelectrochemical Measurements. After sizeselective photoetching, a portion of 1.0 mol dm−3 MES aqueous solution was mixed with an equal volume of the solution containing photoetched AgInS2 nanoparticles, followed by stirring overnight. The MES-modified AgInS2 nanoparticles in the solution were purified with ultrafiltration using a Millipore PM-10 filter, and then the resulting aqueous solution containing AgInS2 particles was spread on an F-doped SnO2 (FTO) electrode, followed by drying under vacuum. Photoelectrochemical properties of AgInS2 particle-immobilized FTO electrodes were measured in a acetonitrile solution containing 0.10 mol dm−3 LiClO4 and 0.10 mol dm−3 triethanol amine as a hole scavenger. The potential was determined against an Ag/AgCl (sat. KCl) reference electrode, and a Pt wire was used as a counter electrode. Photocurrents were measured under a nitrogen atmosphere using the lock-in technique: the photocurrent was detected with a potentiostat (Hokuto Denko, HA-151) and amplified with a lock-in amplifier (NF circuit, LI5640) by extracting the signal that was synchronized with irradiation. Irradiation was carried out by chopping at 7 Hz a light (λ > 350 nm) that was obtained by

compositions, and also exhibit polymorphism.28−32 Therefore, changes in the reaction conditions for chemical syntheses of I− III−VI nanoparticles having various sizes frequently caused modification of the chemical composition or crystal structure of resulting particles.24,33,34 We have developed a size-selective photoetching technique to precisely control the size of quantized particles of II−VI semiconductors, such as CdS,35,36 CdSe,37 and CdTe,38 in which large particles were photoetched to smaller ones with monochromatic light irradiation until the irradiating light could not be absorbed by the photoetched particles due to an increase in the energy gap along with a decrease in particle size. For example, the size of CdS nanoparticles can be adjusted from 3.7 to 1.7 nm with relatively narrow size distributions by selecting the wavelength of monochromatic light used for the photoetching in the range from 514 to 365 nm.35,36 This technique enables the particle size of metal chalcogenide semiconductors to be changed but is principally expected to retain the chemical composition or crystal structure of original particles during the photoetching reactions, and the resulting particles therefore seem to be suitable for investigation of their size-dependent properties. We report for the first time precise size control of chalcopyrite semiconductor nanoparticles of AgInS2 by using the size-selective photoetching technique. The particles had the same chemical composition and crystal structure as those before photoetching. Photoelectrochemical measurements of the resulting particles enabled determination of the potentials of ECB and EVB as a function of particle size.

2. EXPERIMENTAL SECTION 2.1. Materials. Chemicals of silver nitrate, indium(III) nitrate trihydrate, and sodium N,N-diethyldithiocarbamate trihydrate were purchased from Wako Pure Chemical Industries. Oleylamine (OLA) and sodium 2-mercaptoethane sulfonate (MES) were obtained from Tokyo Chemical Industry. Sodium hexametaphosphate (HMP) was supplied by Nacalai Tesque. Other chemicals were purchased from Kishida Chemical. All chemicals were used without further purification. Aqueous solutions were prepared with water purified by a Millipore Milli-Q system. 2.2. Synthesis of AgInS2 Nanoparticles. Nanoparticles of an AgInS2 semiconductor were synthesized by thermal decomposition of a single-source precursor in oleylamine solutions with a slight modification of our previously reported method.31 A 50 mg portion of the precursor powder of AgIn[S2CN(C2H5)2]4 was suspended in a 3.0 cm3 portion of OLA, followed by heat treatment at 180 °C for 3 min with vigorous stirring in a N2 atmosphere. After the suspension had been cooled down to room temperature, the precipitates were removed by centrifugation. The thus-obtained supernatant was diluted by adding the same volume of methanol, and then precipitated AgInS2 particles were isolated by centrifugation, followed by washing with methanol several times. The thusobtained particles were surface-modified with the primary amine used for the preparation and then dissolved in chloroform (1.0 cm3). The surface modifier OLA was replaced by MES to dissolve AgInS2 particles in an aqueous solution according to our previously reported procedure.39 A 0.2 cm3 portion of AgInS2 particle chloroform solution was mixed with 2-propanol (2.0 cm3). The thus-obtained solution was added dropwise to a 2.0 21896



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shorter wavelength region, and then finally the absorption onset was shifted to 540 nm, which almost agreed with the wavelength of the irradiation light. Further irradiation did not induce an appreciable shift of the absorption spectra. These facts indicated that the size-selective photoetching was successfully performed; large AgInS2 particles were photoetched to smaller particles until the irradiated photons could not be absorbed by AgInS2 particles due to enlargement of the energy gap along with a decrease in particle size, that is, the quantum size effect. This interpretation was confirmed by TEM observation. Figure 2 shows TEM images of particles obtained

passing light from a 300 W Xe lamp (EAGLE, PE300BF) through a UV cutoff filter. The irradiation intensity at the electrode surface was 150 mW cm−2. Action spectra of the photocurrent were obtained with monochromatic light irradiation using a Xe lamp equipped with a monochromator (JASCO, CT-10T). The incident photon-to-current efficiency (IPCE) was calculated by dividing the number of electrons detected in the photocurrents by that of incident photons.

3. RESULTS AND DISCUSSION 3.1. Size-Selective Photoetching of AgInS2 Nanoparticles. We reported previously that HMP-stabilized CdS particles were photoetched with monochromatic light and that the sizes of the resulting nanoparticles were easily controlled by choosing the wavelength of irradiation light.35 Therefore, in the present study, at first irradiation to AgInS2 nanoparticles was carried out in a similar solution, a NaOH aqueous solution (pH 9.5) containing 10 mmol dm−3 HMP. As shown in Figure 1a,

Figure 2. TEM images of MES-modified AgInS2 nanoparticles before (a) and after (b) size-selective photoetching at 546 nm. The size distribution of particles is shown on the right side of the corresponding TEM image.

before and after irradiation at 546 nm in the NH3 solution (pH 9.5). Original AgInS2 particles were spherical with an average diameter (dav) of 5.1 nm and standard deviation (σ) of 1.2 nm, while particles having a smaller average size and narrower size distribution (dav = 3.0 nm, σ = 0.6 nm) were produced after size-selective photoetching at 546 nm irradiation. It should be noted that a slight increase in absorbance was observed in wavelength longer than ca. 600 nm in Figure 1b. This originated from light scattering probably caused by the little formation of aggregated particles or the partial deposition of metal oxide or metal hydroxide made from metal ions released by the photocorrosion of AgInS2 particles. Figure 1c shows the time course of the shift of λonset of AgInS2 nanoparticles with monochromatic light irradiation at 546 nm in various kinds of aqueous solutions. Size-selective photoetching of AgInS2 nanoparticles was successfully carried out in aqueous solutions containing NH3, in which λonset was blue-shifted to ca. 540 nm, the photoetching rate being enhanced with higher pH of NH3 aqueous solutions, that is, higher concentration of NH3 in solutions. On the other hand, in aqueous solutions, the pH of which was adjusted by NaOH, monochromatic light irradiation at 546 nm did not cause a shift of λonset to 540 nm: at pH 9.5, λonset was not changed at all in a NaOH solution, while a partial shift of λonset up to ca. 600 nm was observed in the solution at pH 6.3.

Figure 1. (a, b) Changes in absorption spectra of MES-modified AgInS2 nanoparticles in 10 mmol HMP aqueous solutions (pH 9.5) with monochromatic light irradiation at 546 nm. The pH values of the solutions were adjusted with NaOH (a) or NH3 (b). Irradiation time in the unit of hours is indicated in the figures. The arrows show the wavelength of irradiation light (546 nm). (c) Time course of the shift of the absorption onset of AgInS2 nanoparticles along with sizeselective photoetching at 546 nm in solutions with pH at 9.5 (i) and 6.3 (ii) adjusted by NaOH and in solutions with pH at 9.5 (iii) and 11.8 (iv) adjusted by NH3.

there was little change in the absorption spectra in the whole wavelength region even for irradiation of monochromatic light at 546 nm for 6 h. On the other hand, irradiation of 546 nm light to AgInS2 nanoparticles in NH3 aqueous solution (pH 9.5) caused a blue shift of the absorption onset (λonset) from ca. 680 nm, accompanied by a decrease in the absorption at a 21897



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It is well-known that metal chalcogenide semiconductor particles are anodically corroded by irradiation in the presence of O2 as an electron scavenger.35 Though photo-oxidation products of AgInS2 particles were not analyzed in the present experiment, it was suggested from the previous investigation of metal sulfide semiconductors that the photogenerated holes in metal sulfide particles oxidized themselves to produce SO42− as an oxidation product as well as the formation of the corresponding metal ions which were released into aqueous solutions, while the photogenerated electrons were consumed to reduce the dissolved O2. Thus, the net reaction for the photocorrosion of AgInS2 particles is reasonably assumed to be eq 1.

Figure 3. Changes in PL spectra of MES-modified AgInS2 nanoparticles in 10 mmol HMP aqueous solutions with monochromatic light irradiation at 546 nm. The pH of the solution was adjusted with NH3 to 9.5. Irradiation time in the unit of hours is indicated in the figures. PL was measured under a N2 atmosphere by excitation with monochromatic light at 430 nm.

hv

AgInS2 + 4O2 → Ag + + In 3 + + 2SO4 2 −

(1)

Since Ag+ ions form insoluble Ag2O in a basic aqueous solution,40 the results shown in Figure 1 can be reasonably explained in terms of the complexation of released Ag+ ions with NH3 in solutions. In the absence of NH3 in solutions, the Ag+ ions released along with the photocorrosion of AgInS2 produced precipitates of corresponding insoluble metal oxide of Ag2O at pH 9.5 on the particle surface, resulting in the formation of passivation films on the surface that prevented further photoetching of AgInS2. Lowering pH to 6.3 from 9.5 decreased the amount of Ag2O deposition, but the deposits still prevented efficient photoetching of AgInS2. On the other hand, a complexing agent of NH3 can stabilize Ag+ ions in solutions to form Ag(NH3)2+, so that increase in the NH3 concentration in solution resulted in diminution in the amount of insoluble species of Ag2O deposited on AgInS2 particles during the photoetching process. This mechanism can explain the larger photoetching rate of AgInS2 particles in solutions with a higher concentration of NH3. It should be noted that the photoetched particles of AgInS2 exhibited broad absorption spectra as did the original particles, though the size distribution became remarkably narrowed. These results were different from the those obtained for photoetching of II−VI semiconductor particles,35−38,41 such as CdS, CdSe, and CdTe, in which the photoetched particles exhibited clear absorption peaks in the structured absorption spectra because of the nearly monodispersed size distribution. Considering the narrowed size distribution, the broad features observed in the absorption spectra were attributed in part to factors other than the size distribution of particles, such as defect sites inside the particles. It was reported in our previous paper19 that the broad absorption spectra of AgInS2 particles prepared at 180 °C became more structured with an increase in heating temperature higher than 250 °C, probably due to the removal of structural defect sites that exhibited an absorption band below the original optical gap. Unfortunately, the AgInS2 nanoparticles obtained after sizeselective photoetching did not show any photoluminescence (PL) at room temperature. Figure 3 shows the changes in PL spectra of AgInS2 particles with irradiation of 546 nm monochromatic light. A broad emission peak, assigned to the radiative transition in donor−acceptor levels, was observed for original AgInS2 nanoparticles. With irradiation at 546 nm, the PL intensity remarkably decreased with a slight blue shift of the emission peak from ca. 800 to ca. 770 nm, and finally no PL peak was detected with irradiation for more than 6 h. This behavior was opposite to that in the cases of size-selective photoetching of CdSe37 and CdTe,38 in which band edge

emission was observed and its intensity was remarkably enhanced by the photoetching of particles along with a blue shift of the PL peak. The reason why PL intensity was diminished by photoetching of AgInS2 remains unclear, but a possible explanation would be the formation of nonradiative recombination sites of charge carriers on the surface of AgInS2 particles. As aforementioned, the partial deposition of metal oxide or metal hydroxide, which was made from metal ions released by the photocorrosion of AgInS2 particles, caused the little aggregation of particles to induce a slight increase in absorbance in wavelength longer than ca. 600 nm (Figure 1b). Since the stoichiometry on the surface of CdSe nanoparticles has been reported to greatly affect the PL properties of nanoparticles,41 it was suggested that the small amount of deposition of metal oxide or metal hydroxide induced the nonstoichiometry on the surface of AgInS2 particles and then acted as nonradiative recombination sites, resulting in the decrease in the PL intensity. Figure 4a shows absorption spectra of AgInS2 nanoparticles photoetched by monochromatic light irradiation at various wavelengths. Irradiation at a shorter wavelength caused a larger blue shift of absorption spectra. In each case, λonset of photoetched nanoparticles almost agreed with the wavelength of monochromatic light used. Figure 4b shows the relationship between the size of photoetched particles and their λonset. The average size of AgInS2 particles could be adjusted from 5.1 to 2.7 nm by decreasing the wavelength of irradiation light. This indicated that size-selective photoetching of AgInS2 particles was successfully carried out by using monochromatic light in the wavelength range of 520−650 nm, and then large particles were photoetched to smaller ones for which the size was simply determined by the wavelength of irradiation light. The chemical composition of photoetched particles was almost unchanged after size-selective photoetching. The molar ratio of Ag+ ions to total metal ions (NAg/Nmetal) in the original particles was 0.43, while the photoetched particles had almost the same chemical composition with NAg/Nmetal of ca. 0.46, regardless of the wavelength of irradiation light used. These results suggested that original AgInS2 nanoparticles had Ag vacancies as defect sites in the particles and then the sizeselective photoetching did not cause selective dissolution of Ag+ or In3+, resulting in a constant NAg/Nmetal ratio before and after photoetching. Figure 5 shows XRD patterns of MES-modified AgInS2 particles obtained. Regardless of the irradiation wavelength, the particles exhibited three broad diffraction peaks at 2θ = 26.7, 44.8, and 52.3°, which were assignable to diffractions of (112), (204), and (312) planes, respectively, of a 21898



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in particle size due to photoetching of AgInS2 did not cause any change in the chemical composition or crystal structure. Figure 6 shows the increment in the energy gap, ΔEg, of AgInS2 nanoparticles from the bulk value (1.8 eV)43 as a

Figure 6. Relationship between energy gap increase (ΔEg) and particle size of AgInS2 particles (solid circles). The solid line represents theoretically calculated ΔEg by using electron and hole effective masses reported for an analogue chalcopyrite semiconductor of CuInS2.

function of particle size, where ΔEg was obtained by subtracting the bulk energy gap (1.8 eV) from the energy gap (Eg) determined from λonset of AgInS2 (Figure 4). It is wellknown that the quantum size effect becomes remarkable when the diameter of semiconductor particles is smaller than twice the exciton Bohr radius (aB). Though effective masses of bulk AgInS2 have not been reported, we can roughly estimate aB of AgInS2 to be 3.6 nm, by using the dielectric constant of εr = 9.644 for AgInS2 and the effective masses of electron and hole, which were reported for a I−III−VI chalcopyrite analogue of CuInS2, me* = 0.16mo and mh* = 1.30mo,45 respectively (mo representing the free electron mass). As shown in Figure 4, the original particles with dav of 5.1 nm, the size of which was roughly comparable to calculated 2aB (7.2 nm), gave an energy gap similar to that of the bulk material, while a decrease in dav with photoetching drastically increased ΔEg. These results indicated that enlargement of the energy gap with a decrease in size was due to the exciton confinement in small AgInS2 particles. Figure 6 also shows the theoretically predicted ΔEg previously reported in the literature,45 which was calculated by effective mass approximation with the finite-depth-well model using electron and hole effective masses of a I−III−VI chalcopyrite analogue of CuInS2. Calculation with the infinitepotential-well model, well-known as the Brus equation,46 gave considerable overestimation of ΔEg values (not shown). The experimentally observed change in ΔEg for AgInS2 with decreasing particle size was roughly in agreement with the profile obtained by the theoretical calculation. This suggested that the energy gap of AgInS2 nanoparticles could be estimated by the relationship between particle size and calculated ΔEg even if precise theoretical calculation for AgInS2 could not be performed owing to the lack of reported values for effective masses of electron and holes. 3.2. Size-Dependent Photoelectrochemical Properties of AgInS2 Nanoparticles. As mentioned previously, sizeselective photoetching enabled the preparation of sizequantized AgInS2 nanoparticles with the desired size in the range of 2.7−5.1 nm. As for II−VI nanoparticles, such as CdS and CdTe nanoparticles, it has been reported that decrease in the size induced either a negative shift of the potential of ECB or a positive shift of EVB.17,35 Thus, it is important for photovoltaic

Figure 4. (a) Absorption spectra of MES-modified AgInS2 nanoparticles obtained by size-selective photoetching using monochromatic light with various wavelengths. (b) Relationship between average diameter and absorption onset of AgInS2 nanoparticles. The error bars represent standard deviations of the particles. Dotted lines in panel a indicate the baselines used for the determination of absorption onsets. The wavelength of monochromatic light for photoetching is indicated in the figures in the unit of nanometers. (Inset in panel b) Schematic illustration of the control of particles size with size-selective photoetching by changing the wavelength of irradiation light from λ1 to λ2 (λ1 > λ2). The increase in energy of monochromatic light from hc/λ1 to hc/λ2 decreases the size of photoetched particles.

Figure 5. XRD patterns of MES-modified AgInS2 particles before and after photoetching with monochromatic light of various wavelengths. The wavelength of monochromatic light for photoetching is indicated in the figures in the unit of nanometers.

tetragonal AgInS2 crystal (chalcopyrite type42), being similar to XRD peaks of the original particles. The peak width became slightly broader for particles photoetched with monochromatic light at a shorter wavelength due to the formation of particles of smaller size. Consequently, it was concluded that the decrease 21899



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application of AgInS2 nanoparticles to understand the sizedependent changes in their electric energy structure. Figure 7 shows photocurrent−potential curves of FTO electrodes modified with various kinds of AgInS2 nanoparticles.

Figure 8. Action spectra of the photocurrent obtained for AgInS2 particles immobilized on FTO electrodes. Particles used were original particles (a) and those photoetched at 600 (b) and 546 nm (c). The spectra were measured at 0.5 V vs Ag/AgCl. The arrows show the onset wavelength of photocurrent generation.

(EFB) of the semiconductor. Therefore, for electrodes modified with n-type semiconductor nanoparticles, it was reasonably assumed that the potential of EFB was equal to the onset potential of anodic photocurrents.15 Since EFB could be comparable to ECB as reported in many instances of n-type semiconductors, ECB was estimated from the onset potential of anodic photocurrents shown in Figure 7. Then EVB is obtained by using the following equation (eq 2)15,48

Figure 7. Photocurrent−potential curves for AgInS2 particles immobilized on FTO electrodes. Particles used were original particles (a) and those photoetched at 600 (b) and 546 nm (c). The arrows show onset potential of the photocurrent.

In all kinds of particles, an anodic photocurrent was observed, the intensity being increased with positive potential application. These results indicated that AgInS2 nanoparticles exhibited a photoresponse similar to that of n-type semiconductors electrodes. However, the onset potential of the anodic photocurrent varied depending on the size of AgInS2 particles used: with a decrease in the size of AgInS2 particles from 5.1 nm (original particles) to 3.0 nm (those photoetched with 546 nm), the onset potential was negatively shifted from −0.60 to −1.0 V vs Ag/AgCl. Action spectra of the anodic photocurrent are shown in Figure 8. Each onset wavelength was in good agreement with the absorption onset of the corresponding kind of AgInS2 particles (Figure 4a), indicating that the immobilized nanoparticles acted as photosensitizers without coalescence to larger particles and exhibited a size-dependent visible-light photoresponse. It has been reported that the bulk film of chalcopyrite AgInS2 exhibited the potential of EFB at −0.54 V vs Ag/AgCl.47 Since original AgInS2 nanoparticles (dav of 5.1 nm) had EFB at ca. −0.6 V vs Ag/AgCl (Figure 7), it is thought that AgInS2 particles with a diameter larger than 5.1 nm possessed an electric energy structure similar to the bulk properties, being in good agreement with the size-dependent energy gap of AgInS2 (Figure 6). In bulk n-type semiconductor electrodes, photogenerated electrons diffuse inside the semiconductor and then can be injected into a contacting metal electrode, unless the electrode potential is more negative than that of the flat band potential

Eg = E VB − ECB + ECoulomb

(2)

Here Eg was the energy gap of photoetched AgInS2 particles determined from λonset (Figure 4), ECoulomb is the Coulomb interaction energy between an electron and a hole which is given by −1.8e2/(2πεrεod)48 (εo: the permittivity of free space), and d and e represent the particle diameter and electronic charge, respectively. Figure 9 shows the potentials of ECB and EVB as a function of average diameter of AgInS2 nanoparticles. With a decrease in particle size from 5.1 to 2.7 nm, Eg of AgInS2 nanoparticles was enlarged from 1.8 to 2.4 eV, and then ECB was shifted negatively from −0.6 to −1.0 V vs Ag/AgCl, accompanied by a positive shift of EVB from 1.3 to 1.7 V vs Ag/ AgCl.

4. CONCLUSION We successfully controlled the size of spherical AgInS2 nanoparticles in the range of 5.1−2.7 nm by selecting the wavelength of monochromatic light used for the size-selective photoetching. The crystal structure and chemical composition of the particles were unchanged during the photoetching process. These features cannot be achieved by conventional solution-phase syntheses of recently developed chalcopyrite semiconductor nanoparticles, such as AgInS2, CuInS2, and their 21900



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Figure 9. Potentials of ECB (solid circles) and EVB (open circles) as a function of average diameter of AgInS2 nanoparticles. Solid lines are drawn as eye guides.

solid solution with ZnS or other chalcopyrite semiconductors, in which solution-phase synthetic methods enabled control of the particle size of chalcopyrite nanoparticles but frequently did not prevent modification of their chemical composition and crystal structure. Thus, the size-selective photoetching technique could provide an optimum size series of chalcopyrite nanoparticles for investigation of their size-dependent physicochemical properties. Furthermore, by measuring the photoelectrochemical properties of nanoparticles immobilized on electrodes, it was found for the first time that size-quantized AgInS2 nanoparticles, exhibiting photoelectrochemical activities similar to those of ntype semiconductors, also had size-dependent potentials of the conduction and valence band edges, in which negative and positive shifts occurred with a decrease in particle size. Chalcopyrite nanoparticles are attractive for light-absorbing materials for light-energy conversion systems, and our findings will therefore be important for designing and constructing quantum-dot solar cells and photocatalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Funding Program for Next Generation World-Leading Researchers from the Japan Society for the Promotion of Science. M.D. expresses her appreciation to The Program for Leading Graduate Schools of Nagoya University.

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REFERENCES

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Tunable Photoelectrochemical Properties of Chalcopyrite AgInS2

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