Photofunctional Materials Fabricated with Chalcopyrite-Type

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Photofunctional Materials Fabricated with Chalcopyrite-Type Semiconductor Nanoparticles Composed of AgInS2 and Its Solid Solutions Tsukasa Torimoto,*,† Tatsuya Kameyama,† and Susumu Kuwabata‡ †

Department of Crystalline Material Science, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8603, Japan ‡ Department of Applied Chemistry, Graduated School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan S Supporting Information *

ABSTRACT: Over the past decade, many researchers have made efforts to develop high-quality I−III−VI2 group chalcopyrite-type semiconductor nanoparticles of Cu-based or Ag-based materials, such as CuInS2, Cu(InGa)Se2, and AgInS2, due to their low toxicity, wide range of absorption from UV to visible or near-infrared regions, and tunable energy gap depending on the chemical composition. Ag-based chalcopyrite-type nanoparticles have attracted much attention because they have physicochemical properties that can be controlled in a manner similar to that for Cu-based nanoparticles but can be synthesized easily under milder reaction conditions than those of Cu-based ones. In this Perspective, we review recent works relating to the preparation of low-toxic nanoparticles of Ag-based chalcopyrite-type semiconductors and their solid solutions, and then we introduce their application as photofunctional materials. Tunability of optical properties has considerably contributed to recent advances in photofunctional applications of chalcopyrite-type semiconductor nanoparticles to sensors, photocatalysts, and solar cells.

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coefficient. For example, Cu-based semiconductors such as Cu(In,Ga)Se216,17 and Cu2ZnSnS418−20 have been utilized for the preparation of bulk thin-film solar cells. Kudo and coworkers reported that efficient photocatalysts for hydrogen evolution could be prepared with micrometer-sized particles of Ag-based semiconductors and their solid solution, such as AgInS 2 , (CuAg) x In 2 x Zn 2 ( 1 − 2 x ) S 2 , Ag 2 ZnSnS 4 , and (AgIn)xZn2(1−x)S2.21−25 Moreover, as one of the strategies to prepare semiconductor thin films at lower costs, colloidal nanoparticles of Cu-based chalcopyrite semiconductors have been developed, with nanoparticles being used as a precursor (an ink) to deposit uniform thin films followed by calcination. Recently, interest has also been shown in the preparation of nontoxic semiconductor nanoparticles, such as FeS 2 , Cu2ZnSnS4, Cu3SbS4, CuInS2, and AgInS2, with high quality to replace conventional Cd-based II−VI nanoparticles.26−32 Among various kinds of semiconductors investigated, chalcopyrite-type semiconductor nanoparticles have been a subject of intense research in recent years, because of their tunable optical properties by making solid solutions with different materials.30,33 Many researchers have exerted utmost efforts in developing high-quality chalcopyrite semiconductor nano-

ize-quantized semiconductor nanoparticles, so-called quantum dots, have a unique electronic energy structure that can be controlled by changing the size, and they have been intensively investigated for applications to photoluminescent materials,1−4 biomolecular markers,1,3−6 photocatalysts,7−9 light-emitting diodes,10,11 and photovoltaic devices.12−15 Many efforts have been made to develop high-quality semiconductor nanoparticles, mainly composed of binary elements, such as CdSe, CdTe, and PbS. Among the various kinds of synthetic procedures, thermal reactions of precursors in a hot organic solution have frequently been used to prepare highly photoluminescent nanoparticles with few defect sites. By using appropriate reaction conditions, particle size could be precisely controlled to obtain a desired optical property. However, most of the binary group semiconductor nanoparticles studied contained highly toxic elements, such as Cd, Pb, Se, and Te, limiting the range of their uses in practical applications. On the other hand, as for bulk semiconductor materials, multinary semiconductors have attracted attention to replace highly toxic materials such as CdTe and GaAs and/or to efficiently absorb solar light. Chalcopyrite-type I−III−VI2 group metal chalcogenide semiconductors and related materials are regarded as suitable alternatives because they exhibit a wide range of absorption from ultraviolet to visible or infrared wavelength regions, a tunable energy gap (Eg) depending on the chemical composition, good stability, and high absorption © XXXX American Chemical Society

Received: November 4, 2013 Accepted: December 20, 2013

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Nanoparticle Synthesis of I−III−VI2 Semiconductors and Their Solid Solutions. Many strategies have been developed for the syntheses of binary semiconductor nanoparticles, such as CdSe and CdTe, of high quality in solution phase. Particle size can be controlled by changing reaction conditions, such as the reaction temperature, kind of stabilizing agents, and concentration ratio of the precursors. For example, it has been reported that change in the Cd/Se ratio in preparation varied the size of resulting CdSe particles without considerable modification to the chemical composition of the particle cores themselves.37 I− III−VI2 nanoparticles, such as CuInS2,34,38−42 CuInSe2,38,43,44 CuGaSe2,43 AgInS2,36,39,45−50 and AgInSe2,51,52 are also obtained as colloidal solutions by using similar synthetic procedures. However, unlike the case of II−VI nanoparticle syntheses, change in the molar ratio of constituent elements in preparation frequently causes production of I−III−VI2 nanoparticles with nonstoichiometric composition. The difference in the reactivity of two or more metal precursors considerably influences the crystal growth of multinary semiconductors. So far, high-quality nanoparticles of I−III−VI2 semiconductors and related materials have been mainly prepared via thermolysis of precursors in hot organic solutions. To the best of our knowledge, the first successful preparation of luminescent CuInS2 nanoparticles was achieved by Castro et al. via a heating up method that seemed to be suitable for preparation on a large scale.34 A single-source precursor of (PPh3)2CuIn(SEt)4 was thermally decomposed in an organic solution at 200−250 °C. The size of CuInS2 particles increased from 2.1 to 4.0 nm with an increase in the reaction temperature from 200 to 250 °C. Thermolysis of a single-source precursor enables to release the same amount of Cu and In simultaneously in the reaction system, resulting in the predominant formation of CuInS2 but not binary compounds such as CuSx and InSx. The absorption spectra were blueshifted with a decrease in the particle size due to the quantum

particles of Cu-based or Ag-based multinary metal chalcogenide semiconductors. Castro et al. reported that CuInS2 nanoparticles prepared in a hot organic solution exhibited relatively intense photoluminescence (PL).34 Following this pioneer work, Maeda et al. focused on the preparation of solid solution nanoparticles between CuInS2 and ZnS to control their optical properties by changing the chemical composition, the peak wavelength of PL being blue-shifted with an increase in the content of ZnS in the solid solution particles.35 On the other hand, we have reported for the first time the preparation and PL properties of semiconductor nanoparticles of AgInS2 and their solid solution with ZnS, in which the ZnS-AgInS2 solid solution particles had tunable Eg depending on the chemical composition and then their PL color could be controlled from green to red.36 Ag-based chalcopyrite-type nanoparticles have attracted increasing attention, probably because they have physicochemical properties that can be controlled in a manner similar to that for Cu-based nanoparticles but can be synthesized easily under milder reaction conditions than those of Cu-based ones. In this Perspective, we review recent works relating to the preparation of low-toxic nanoparticles of Ag-based chalcopyrite-type semiconductors and their solid solutions and then introduce an application to photofunctional materials.

Ag-based chalcopyrite-type nanoparticles have physicochemical properties that can be controlled in a manner similar to that for Cu-based nanoparticles but can be synthesized easily under milder reaction conditions than those of Cu-based ones.

Table 1. Synthetic Conditions of Photoluminescent I−III−VI2-Based Nanoparticles

a

material

method

CuInS2 CuInS2 CuInS2 CuInS2 CuInSe2 ZnS-CuInS2 ZnSe-CuInSe2 CuInS2−CuInSe2 Cu(In,Ga)Se2 CuGaS2−CuInS2 Cu−In−Se nonstoichiometric CuInS2

heating up hot injection heating up heating up heating up heating up hot injection hot injection heating up heating up hot injection heating up

AgInS2 AgInSe2 AgInSe2 ZnS-AgInS2 ZnS-AgInS2 ZnS-AgInS2 ZnS-AgInS2 Zn-doped AgInS2 AgGaS2−AgInS2

hot injection heating up hot injection heating up hot injection hot injection hot injection heating up heating up

reaction temperature/ °C

precursor

ligands and solventsa

ref

200−250 60−210 182 160 240 160−280 300 240 240 230 280−360 210

single precursor: (PPh3)2CuIn(SEt)4 Cu(OAc), In stearate, sulfur Cu(acac)2, In(acac)3, sulfur CuI, In(Oac)3, diphenylphosphine sulfide CuCl, InCl3, selenium Zn(S2CNEt2)2, CuI, InI3 ZnCl2, CuCl, InCl3, selenium CuCl, InCl3, sulfur, selenium CuCl, InCl3, GaCl3, selenium CuI, In(OAc)3, Ga(acac)3, DDT CuC1, CuI, InCI3, InI3, (Me3Si)2Se CuI, In(OAc)3, DDT

hexanethiol, dioctylphthalate OA, OLA, TOP, DDT, ODE OLA, o-dichlorobenzene DDT OLA TOP, OLA, ODE OLA OA, OLA OLA DDT, ODE OLA, TOP DDT, OA, ODE

34 39 38 50 38 35 44 44 38 42 52 41

120 185 280 180 120−230 100 140−180 120−210 180−260

AgNO3, In stearate, sulfur single precursor: (PPh3)2AgIn(SeC{O}Ph)4 AgI, InI3, (Me3Si)2Se Single precursor: (AgIn)xZn2(1‑x)(S2CNEt2)4 AgNO3, In(acac)3, Zn stearate, sulfur Zn(OAc)2, AgNO3, In(Oac)3, thioacetamide Zn stearate, AgNO3, In(acac)3, sulfur Zn stearate, AgNO3, In(acac)3, DDT, sulfur single precursor: AgInyGa1‑y(S2CNEt2)4

DDT, OLA, ODE DDT, OLA OLA, TOP OLA DDT, ODE, TOP glutathione, water DDT, OA, OLA, ODE DDT, OA, TOP, ODE OLA

39 51 52 36 48 49 46 47 59

OA: oleic acid, OLA: oleylamine, TOP: trioctylphosphine, DDT: dodecanethiol, ODE: octadecene. 337

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que.54 This technique was previously developed for size control of II−VI semiconductor nanoparticles, such as CdS, CdSe, and CdTe: large particles are photoetched to smaller ones with monochromatic light irradiation until the irradiation light is not absorbed by the photoetched particles due to an increase in Eg along with a decrease in particle size (quantum size effect), so that the size of particles obtained after photoetching is simply determined by the wavelength of irradiation light.55−57 It is well-known that metal chalcogenide semiconductors are anodically photoetched by holes photogenerated in the particles in the presence of O2 as an electron acceptor. However, the photoetching reaction of AgInS2 hardly occurred in a basic aqueous solution without additional complexing agents, being different from the cases of binary semiconductor particles such as CdS and CdSe, probably because of the formation of Ag2O passivation layer on the particles that prevented further photoetching. The addition of NH3 as a complexing agent successfully promoted the size-selective photoetching of AgInS2 particles. Figure 1a and b show TEM images of AgInS2 particles before and after monochromatic light irradiation at the wavelength of 546 nm in the presence of O2 molecules. Figure 1c summarizes the changes in particle size by the irradiation of monochromatic light having various wavelengths as a function of absorption onset of the photoetched AgInS2 particles. The particles become smaller with a blue-shift of the absorption onset wavelength, the degree being dependent on the wavelength of irradiation light.

size effect. The CuInS2 particles exhibited relatively intense PL with a broad peak at around 700 nm, the PL quantum yield (PL QY) being 4.4%. Furthermore the decomposition of multiple precursors by a heating up procedure is also adoptable to prepare nearly monodisperse CuInS2 nanoparticles with a different size, in which the reactivity of Cu+ and In3+ monomers is tuned by selecting the kinds of stabilizers and/or the composition of mixture solvents.38,41 Hot injection techniques, first developed by Bawendi and coworkers to prepare cadmium chalcogenide nanoparticles of high quality with controlled size,53 are generally used for binary semiconductor nanoparticles with narrower size distribution. This strategy can be adopted for colloidal syntheses of I−III− VI2-based multinary semiconductor nanoparticles. Bawendi et al. prepared Cu−In−Se nanoparticles by injection of (Me3Si)2Se to a tri-n-octylphosphine/oleylamine mixture solution containing metal halides as precursors at 280−360 °C.52 The resulting particles of 2.0−3.5 nm in average diameter had an In-rich composition, the In/Cu ratio in the particles being remarkably varied both by the kind of counter halide ions in metal precursors used (I− or Cl−) and by the reaction temperature. The thus-obtained CuInSe2 particles exhibited intense PL with quantum yield up to 25%, the peak wavelength being tunable from red to near-infrared region. AgInSe2 nanoparticles of 3−6 nm in diameter could be prepared by a similar synthetic approach. Various strategies have been developed to precisely control particle size because the physicochemical properties of semiconductor nanoparticles are size-dependent due to the quantum size effect. A change in reaction conditions in preparation enabled the size of resulting I−III−VI2 semiconductor nanoparticles to be varied. Peng et al. reported that CuInS2 nanoparticles having various sizes from 2.0 to 20 nm were successfully prepared by the hot injection method.39 Elemental sulfur dissolved in oleylamine was quickly injected in the solution containing metal precursors at 180 °C in the presence of n-dodecanethiol as a ligand, resulting in the formation of chalcopyrite CuInS2 nanoparticles, and the sizes of CuInS2 nanoparticles could be controlled by the reaction temperature, concentration of the n-dodecanethiol, and growth time. The thiol compound acted as a reactivity-controlling agent for cationic species to produce the stoichiometric ratio of Cu/In. CuInS2 nanoparticles obtained in this process exhibited broad absorption spectra, but their absorption onset was blueshifted from 900 to 570 nm depending on their size, indicating that the energy structure of the particles was successfully controlled by their size. The conditions reported for the syntheses of I−III−VI2based semiconductor nanoparticles are summarized in Table 1. As mentioned above, Cu-based I−III−VI2 particles mostly required relatively high reaction temperatures (higher than ca. 180 °C) to be synthesized, even though several studies recently reported successful syntheses of CuInS2 nanoparticles at lower than 180 °C.35,39,50 In contrast, most of photoluminescent particles of Ag-based chalcopyrite-type semiconductors could be obtained at lower reaction temperatures of 100−180 °C. We have reported that thermal decomposition of (AgIn)(S2CN(C2H5)2)4 precursor powders in oleylamine at 180 °C produced highly photoluminescent AgInS2 nanoparticles that were spherical particles of 3−6 nm in diameter with tetragonal crystal structures.36 Furthermore, the size of as-prepared AgInS2 nanoparticles can be decreased by the size-selective photoetching techni-

The photoetching reaction of AgInS2 hardly occurred in a basic aqueous solution without additional complexing agents, being different from the cases of binary semiconductor particles such as CdS and CdSe. It is well-known that bulk I−III−VI2 group semiconductors can form a solid solution with II−VI group semiconductors or other I−III−VI2 group materials having different band gap energies. Modification of synthetic procedures for I−III−VI2 nanoparticles enables preparation of nanoparticles of such solid solution materials. This strategy is extremely useful to control the optical properties of I−III−VI2 nanoparticles. We have successfully prepared multinary semiconductor nanoparticles of solid solution between ZnS and AgInS2 (ZAIS) by the thermal decomposition of single precursors in hot organic solutions.36 The metal diethyldithiocarbamate complex of (AgIn)xZn2(1−x)(S2CN(C2H5)2)4 with different x values (0.4− 1.0) was used as a precursor and decomposed at 180 °C in an oleylamine solution. The content of ZnS in the resulting ZAIS nanoparticles increased with a decrease in the x value in the precursor, although the average size of ZAIS particles obtained was ca. 4−5 nm in diameter regardless of the chemical composition of x in the precursor. Figure 2 shows the absorption and PL spectra of thus-obtained ZAIS particles. The Eg of ZAIS particles was controlled from 1.77 to 2.58 eV by changing the fraction of ZnS in the particles. The ZAIS nanoparticles as prepared exhibited a broad PL peak with relatively high intensity, the wavelength of the PL peak being blue-shifted from 775 to 540 nm with a decrease in the x value from 1.0 to 0.4. No clear exciton peak was observed in the 338

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Figure 2. (a) Absorption spectra and (b) photoluminescence spectra of ZAIS nanoparticles. The value of x in (AgIn)xZn2(1−x)(S2CN(C2H5)2)4 used as a precursor is indicated in the figure. The excitation wavelength in PL measurement was 350 nm. The inset in panel a shows a photograph of UV-illuminated ZAIS nanoparticle solutions. Reprinted with permission from ref 36.

nanoparticles could be controlled by the composition of particles: an increase in the Ga content caused a blue-shift of both absorption and PL spectra, in which PL QY was more than 30% with a y value larger than 0.4. Here we summarize the changes in optical properties of I− III−VI2-semiconductor-based solid solutions with their chemical composition. Figure 3 shows a plot of wavelengths of the absorption onset (λonset) and the PL peak (λPL) reported in the literature as a function of chemical composition of the solid solution nanoparticles, where the value in the abscissa represents the fraction of a semiconductor having higher band gap energy in the solid solution. Both λonset and λPL are monotonously shifted to a shorter wavelength with an increase in the fraction of a semiconductor of higher Eg, indicating that the Eg of solid solution particles could be simply controlled in each case by changing the composition of particles and then the PL peak was also blue-shifted with an increase in Eg. It should be mentioned that the tunabilities of Eg and PL peak wavelength were more extensive for nanoparticles of solid solutions composed of ZnS as a component, mainly because ZnS has the largest Eg in the semiconductors listed in Figure 3. A large Stokes shift (Δλ > 50 nm) between λonset and λPL was observed for Ag- and Cu-based solid solution nanoparticles regardless of the chemical composition except for Cu-based ones having a smaller band gap (λonset > 600 nm). The most probable photoluminescent mechanism in I−III−VI2-based nanoparticles is donor−acceptor pair recombination, where photoexcited electrons and holes are trapped in deep intragap donor and acceptor levels followed by their radiative recombination, resulting in such a large Stokes shift. Synthesis of Nonstoichiometric I−III−VI2 Nanoparticles. The I−III−VI2 semiconductors are known to form intrinsic defects such as interstitial ions or vacancies. For example, several studies on bulk CuInS2 films have shown that Cu-rich or In-rich

Figure 1. TEM images of 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. (c) Relationship between average diameter and absorption onset of AgInS2 nanoparticles. The error bars represent standard deviations of the particles. The wavelength of monochromatic light for photoetching is indicated in the figures in the unit of nanometers. (Inset in panel c) Schematic illustration of the control of particle 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. Reprinted with permission from ref 54.

absorption spectra of ZAIS nanoparticles, being similar to other nanoparticles of I−III−VI2 semiconductors or their solid solution with II−VI semiconductors. Furthermore, even when the PL quantum yield was considerably high, most of the I− III−VI2 semiconductor particles and related materials exhibited broad PL peaks, being assigned to the radiative transitions in donor−acceptor levels and/or emission from trap sites but not to band edge emission. Solid solution nanoparticles between CdS and AgInS2 could also be prepared by the thermal decomposition of Cd(S2CN(C2H5 )2 )2 and AgIn(S2CN(C2H5)2)4 at 160 °C in the presence of oleylamine.58 Nanoparticles of I−III−VI2 group semiconductors can form solid solutions with other kinds of I−III−VI2 semiconductors. For Ag-based nanoparticles, the preparation of solid solution nanoparticles between AgInS2 and AgGaS2 has been reported by our group.59 The optical properties of AgInyGa1−yS2 339

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Figure 3. Plots of the absorption onset (λonset) (a) or PL peak (λPL) (b) as a function of the molar fraction of a semiconductor having higher Eg in Cu- or Ag-based solid solution nanoparticles. The solid solutions (AB) are composed of semiconductors of A and B, which are indicated in the inset of panel (b). Eg of component A is higher than that of component B. The onset wavelengths expected from Eg of bulk semiconductors are indicated in the figure as open triangles. The data of absorption onset or PL peak were obtained from ref 35, 36, 41, and 59.

nonstoichiometry of I−III−VI2 nanoparticles.61 Octylaminemodified AgInS2 nanoparticles having NAg/Nmetal = 0.37 exhibited a maximum quantum yield of ca. 70%. These particles probably contained the optimum amount of Ag vacancies acting as sites of donor−acceptor pair recombination with few surface defect sites for nonradiative recombination. A similar tendency was also observed for E g of CuInS 2 nanoparticles with different Cu/In ratios.41,60 Nonstoichiometric CuInS2 particles in the Cu-deficient region exhibited relatively intense PL, the optimum PL QY being at ca. 30%. Figure 4 shows wavelengths of the absorption onset (λonset) or the PL peak (λPL) reported in the literature as a function of

CuInS2 with deviation from stoichiometry resulted in the presence of defects, which consequently influenced the crystal structure and physicochemical properties. It has been clarified by DFT calculations for AgInS2, CuInS2, and solid solution between them that the valence band maximum was made up of hybrid orbitals of S 3p, Cu 3d, and Ag 4d, while the conduction band minimum was composed of In 5s5p hybridized with S 3p orbitals.21−23 Therefore, it is expected that controlling nonstoichiometry is also a good strategy for tuning optical properties of I−III−VI2 nanoparticles. Several studies have shown that the chemical compositions of these nanoparticles could be controlled by the molar ratio of precursors containing component elements in the preparation.41,60,61

Controlling nonstoichiometry is also a good strategy for tuning optical properties of I−III−VI2 nanoparticles. We have reported the influence of the nonstoichiometric composition of AgInS2 particles on their absorption and PL properties.61 Highly photoluminescent AgInS2 nanoparticles having various contents of Ag were successfully synthesized without significantly changing their average size, with the molar ratio of Ag ion to total metal ions in particles (NAg/Nmetal) varying from 0.07 to 0.5. The absorption spectra of AgInS2 particles were blue-shifted and their energy gap was enlarged from 1.7 to 2.1 eV with a decrease in the NAg/Nmetal ratio in particles, resulting in a blue shift of the PL peak wavelength from 830 to 650 nm. These changes can be reasonably understood from the change in the valence band level. Since the valence band maximum of AgInS2 was composed of hybrid orbitals of S 3p and Ag 4d as mentioned above, the decrease in the number of orbitals originating from Ag lowered the maximum level of the valence band, and then Eg of nonstoichiometric AgInS2 particles increased. The quantum yield of PL was also considerably dependent on the

Figure 4. Dependence of absorption onset (λonset) and PL peak (λPL) on the content of Ag or Cu in nonstoichiometric AgInS2 or CuInS2 nanoparticles. These data were obtained from the absorption onset or PL peak of the corresponding nanoparticles in refs 41 and 61.

the chemical compositions of nonstoichiometric AgInS2 or CuInS2 nanoparticles. Both λonset and λPL are monotonously shifted to a shorter wavelength with a decrease in the content of Ag or Cu in particles. These results indicated that tuning Eg of I−III−VI2 group nanoparticles by control of their chemical composition enabled the PL peak to be produced at a desired wavelength, being similar to the case of making solid solution particles of I−III−VI2-based materials. 340

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Enhancement of the Photoluminescence of I−III−VI2-based Nanoparticles. It is well-known that semiconductor nanoparticles have defect sites on their surface acting as nonradiative recombination sites, and elimination of such defect sites can thus enhance PL intensity. One of the useful strategies reported for II−VI semiconductors of CdSe and CdTe is surface coating with materials having a wider band gap energy, such as ZnS, as a shell layer. This strategy can also be applied to nanoparticles of I−III−VI2-based materials to increase their PL QY. For example, Peng et al. reported that CuInS2 nanoparticles, prepared by the hot injection method, exhibited intense PL by surface coating with a ZnS shell layer, in which PL QY increased from ca. 3% for original particles to ca. 30% for ZnScoated ones.39 The stability of nanoparticles in air could be improved by surface coating with a ZnS shell layer. Post-heat-treatment can be used to remove intrinsic defects inside I−III−VI2-based nanoparticles. We have reported that PL intensities of as-prepared nanoparticles of AgInS2 or its solid solution could be remarkably enhanced by heat treatment at an appropriate temperature. Figure 5a shows the absorption spectra of AgInS2 nanoparticles before and after heat treatment in oleylamine at various temperatures.45 The shape of the spectra was more structured with an increase in heating

temperature: the absorption edge became steep along with a blue shift of the absorption onset from 720 to 630 nm, probably due to a decrease in the amount of structural defects in the nanoparticles that produced, on the other hand, an absorption band below the original optical gap. The PL intensity of AgInS2 nanoparticles was also varied, the degree being greatly dependent on heating temperature (Figure 5b). After heat treatment at 180 °C, the PL intensity of particles was enhanced by almost 2-fold in comparison to that of as-prepared particles, but heat treatment at a temperature higher than 250 °C diminished PL intensity owing to the decrease in defect sites for radiative recombination of charge carriers, although the absorption edge became steep with a decrease in the amount of structural defects in the particles. Figure 5c shows changes in PL quantum yields of ZnSAgInS2 solid solution nanoparticles having various chemical compositions by postsynthesis treatment, where PL quantum yield is plotted as a function of PL peak wavelength. Regardless of the chemical composition of particles, the quantum yield of PL was increased by heat treatment at 180 °C. Moreover, surface coating of the resulting particles with a ZnS layer further enhanced PL quantum yields, indicating that surface defect sites remained on the particles after heat treatment and could be removed by deposition of a ZnS shell layer. Volcano-type dependence was observed for the relationship between PL quantum yield and chemical composition of particles. Optimum quantum yield of 66% or 80% was obtained for ZnS-AgInS2 solid solution nanoparticles prepared with x = 0.9 after heat treatment or ZnS coating, where PL peaks were observed at 650−700 nm, respectively. Sensors Based on Photoluminescence Quenching. As well as CdSe and CdTe nanoparticles, the intense photoluminescence of I−III−VI2 semiconductor nanoparticles and the tunability of PL color are attractive features for application to biomarkers, sensors, and LEDs. On the other hand, quenching of PL can easily occur depending on the environmental conditions, such as the concentration of additive redox species, pH of solvents, or environmental temperature, mainly because of their large surface-to-volume ratio of nanoparticles, unless the surfaces of semiconductor nanoparticles are completely covered by shell layers composed of insulating materials, such as SiO2 or polymers. This characteristic feature sometimes hinders the application of nanoparticles to biomarkers or LEDs, but is essential for the construction of sensing systems with high sensitivity. A glucose-sensing system could be prepared by utilizing the PL quenching behavior of ZnS−AgInS2 solid solution nanoparticles.62 The surface-modifying molecules on particles were changed from oleylamine to 3-mercaptopropionic acid (MPA) so as to be dissolved in aqueous solution. The resulting particles had negative surface charges to induce electrostatic interaction between ZAIS particles and ionic redox species in the solution. The addition of hydroxymethylferrocenium cation (Fc+CH2OH) to an aqueous solution containing luminescent ZAIS particles effectively quenched their PL, while its reduced form (FcCH2OH) did not cause any significant quenching. PL could be also quenched by 5-methylphenazium cation (MP+) but not by its reduced form (5-methyl-5,10-hydrophenazine, MPH). These results demonstrated that the redox states of these species can be detected by changes in the PL intensity of MPA-modified ZAIS particles, enabling the use of ZAIS particles as a luminescent reagent in chemosensors detecting redox-related reactions. As shown in Figure 6a, we fabricated a

Figure 5. (a) Absorption and (b) PL spectra of AgInS2 nanoparticles heat-treated at various temperatures. The excitation wavelength in PL was 365 nm. (c) Relationship between PL quantum yield and PL peak wavelength (λPL) of ZAIS nanoparticles prepared from precursors having different compositions. Samples are particles as-prepared (solid circles), heat-treated at 180 °C (open circles), or coated with a ZnS layer by heat treatment at 180 °C (solid squares). The number in the figure is the x value of (AgIn)xZn2(1−x)(S2CN(C2H5)2)4 used as a precursor. Reprinted with permission from ref 45. Copyright 2010 Royal Society of Chemistry. 341

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powers of the photogenerated electrons and holes, respectively, than those of bulk materials owing to the quantum size effect. Among the various kinds of semiconductor nanoparticles, many researchers have intensively investigated the photocatalytic activity of CdS nanoparticles, which were shown to have suitable Eg to effectively absorb solar light and sufficient reducing power of photogenerated electrons to induce hydrogen evolution.8,67,68 However, CdS is not good choice as a photocatalyst due to both its high toxicity and lack of stability against photocorrosion. Recently, photocatalytic activities of low-toxic semiconductor nanoparticles of I−III− VI2 and related materials have become an interesting research subject, because they are expected to be controlled by the chemical composition of materials or by the particle size. Furthermore, as aforementioned, AgInS2 nanoparticles have higher resistance to photocorrosion than that of II−VI nanoparticles, such as CdS and CdSe.54 As for bulk metal sulfide semiconductor photocatalysts, Kudo et al. prepared bulk particles of ZnS−AgInS2 or ZnS−CuInS2 solid solutions as visible light photocatalysts, which exhibited compositiondependent photocatalytic activities with relatively high hydrogen evolution rates under visible light irradiation.21−24 Teng et al. reported visible-light-driven photocatalytic activity of CuInS2 nanoparticles for H2 evolution in the presence of S2−/SO32− ions as a sacrificial agent.69 Although CuInS2 nanoparticles exhibited quite low photocatalytic activity without cocatalysts, a relatively large H2 evolution rate was observed with a quantum efficiency of ca. 4.7% by loading 0.5 wt.% Ru on the nanoparticles, which acted at an electron trap for subsequent reduction of H2O to H2. We have investigated the photocatalytic activity of ZnS− AgInS2 solid solution nanoparticles (particle size: ca. 4.4 nm) for H2 evolution.70 Under visible light irradiation in the presence of 2-propanol as a hole scavenger, the amount of H2 evolved was linearly increased with irradiation time, regardless of the chemical composition of the particles. However, the H2 evolution rate (R(H2)) was greatly dependent on the chemical composition of ZAIS particles. Figure 7a shows R(H2) of ZAIS nanoparticles having various chemical compositions as a function of the wavelength of absorption onset (λonset) of ZAIS particles, when powders of (AgIn)xZn2(1−x)(S2CN(C2H5)2)4 having various x values were used as the precursors for ZAIS preparation. Volcano-type dependence of R(H2) was observed, that is, with a blue shift in λonset from 700 to 540 nm (with a decrease in x of ZAIS particles from 1.0 to 0.7), R(H2) was enlarged, though a further blue shift of λonset from 540 nm decreased R(H2). The dependency of H2 evolution reaction on the kind of ZAIS particles was mainly due to the change in the band structure of ZAIS particles depending on their chemical composition. The photocatalytic activity was increased with a negative shift of the conduction band edge as the content of ZnS in ZAIS particles increased (that is, the x value decreased). However, the photocatalytic activity for H2 evolution was lessened as λonset became smaller than 540 nm (the value of x being smaller than 0.7) because the number of photons having energy larger than the energy gap of the ZAIS particles decreased in the irradiation light from the Xe lamp. The value of R(H2) obtained by ZAIS particles with x = 0.7 were almost comparable to that reported for a CdS nanoparticle photocatalyst loaded with Rh nanoparticles as a cocatalyst in the presence of 2-propanol. It should be noted that the photocatalytic activity for H2 evolution was relatively high even if ZAIS nanoparticles were used in the absence of

Figure 6. (a) Schematic illustration of the mechanism of a photoluminescent glucose sensor. (b) Time course of PL intensity at 655 nm (Ex = 450 nm) of a glucose-sensing solution containing MPA-ZAIS, MP+ (5 μmol/dm3), GDH (5 unit/cm3), and PBS (20 mmol/dm3, pH = 7.0). (c) Picture of photoluminescence in response to the addition of a glucose solution. Reprinted with permission from ref 62. Copyright 2009 Royal Society of Chemistry.

glucose-sensing system by using PL quenching behavior, in which a pyrroloquinoline quinone glucose dehydrogenase (PQQ-GDH) was employed as the enzyme and MP+ was used as the oxidizing agent.63 When glucose was added to the sensing system, glucose oxidation occurred, accompanied by the reduction of MP+ to MPH, resulting in recovery of the quenched PL emission of ZAIS particles (Figure 6b). The PL intensity was enlarged with an increase in the concentration of glucose added from 5 to 20 μmol/dm3, enabling the quantitative detection of glucose. This sensitivity is comparable to or higher than those of glucose sensors based on PL quenching of Cd-based binary nanoparticles.64 Figure 6c shows a picture of the initiation of PL by addition of drops of a glucose solution to the sensing system. In addition to this example, luminescent ZAIS nanoparticles have been used as a detection unit in various sensors, such as sensors for temperature measurements65 and metal ion detection.66 Photocatalysts. Semiconductor nanoparticles are promising photocatalysts for photodegradation of organic pollutants or production of renewable fuels by water splitting or organic synthesis because they have larger reducing and oxidizing 342

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and is plotted in Figure 7b as a function of λonset of ZAIS particles used. The value of fenhance monotonously increased with a red shift of λonset of ZAIS particles. Optimum fenhance was ca. 2.0 with the use of ZAIS particles prepared with x = 1.0. Considering that the LSPR peak appeared at ca. 550−560 nm for Au particles used, it was reasonable that ZAIS particles with λonset longer than ca. 570 nm were effectively photoexcited with the LSPR-induced electric field produced near Au particles. Photovoltaic Devices. Much interest has been shown in semiconductor nanoparticles for light-absorbing units in solar light energy conversion systems (quantum dot solar cells) because their light absorbing properties are adjustable to effectively utilize solar light energy. Furthermore, the discovery of efficient multiple exciton generation by single photons in semiconductor nanoparticles has initiated intensive research to fabricate highly efficient solar cells in which the efficiency of solar light energy conversion theoretically reaches ca. 44%,72 being much higher than that of the traditional ShockleyQueisser limit (32%) for conventional solar cells. So far, binary semiconductor nanoparticles of CdSe, CdTe, or PbS have been exclusively employed in the fabrication of quantum dot solar cells,12−15,73−77 but multinary nanoparticles of I−III−VI2 semiconductors have recently been considered to be potentially suitable as light absorbing units because they have direct band gaps with energies matching the solar spectrum as well as low toxicity.

Multinary nanoparticles of I−III− VI2 semiconductors have recently been considered to be potentially suitable as light absorbing units because they have direct band gaps with energies matching the solar spectrum as well as low toxicity.

Figure 7. (a) Photocatalytic H2 evolution rates with the use of ZAIS particles (open circles) and ZAIS particles immobilized on SiO2coated Au (solid circles) as a function of λonset of ZAIS particles used. (b) Relationship between fenhance and λonset of the ZAIS particles used. The experiments were performed by irradiation of light from a 300 W Xe lamp (λ > 350 nm). (c) Reaction mechanism of photocatalytic H2 evolution using ZAIS particles on SiO2-coated Au particles. The particle ratio of Au to ZAIS was 5.2 × 10−5. The values of x in the (AgIn)xZn2(1−x)(S2CN(C2H5)2)4 precursor used for the preparation of ZAIS particles are indicated in the figures. Reprinted with permission from ref 70.

Many reports on the fabrication of CuInS2 nanoparticlesensitized solar cells have been published.69,78−84 Porous metal oxide electrodes, such as TiO2 and ZnO, were sensitized with CuInS2 particles, and the resulting electrodes were used for fabrication of sandwich-type cells with an electrolyte solution containing appropriate redox species, such as sulfide/polysulfide, and a counter electrode. Sensitization with narrow band gap CuInS2 nanoparticles effectively occurred in a wide wavelength region below ca. 800 nm. In most cases, the solar light energy conversion efficiency reported for such solar cells was ca. 1% or less. Peng et al. prepared CuInS2 particles with sizes ranging from 2 to 8 nm, which were used as photosensitizers for TiO2 electrodes in a sensitized solar cell.83 The energy conversion efficiency of the resulting devices was dependent on the CuInS2 size, the optimum efficiency being ca. 0.72% when CuInS2 particles of 3.5 nm in diameter were used. In order to improve the conversion efficiency, Li and co-workers reported the sensitization of TiO2 electrodes with CuInS2 nanoparticles covered with a CdS buffer layer and a ZnS passivation layer, in which the energy conversion efficiency was increased to 4.2% from 0.32% for TiO2 electrodes with CuInS2 nanoparticles only.69 Kamat et al. found that surface coating of CuInS2 nanoparticles-porous TiO2 composite electrodes with a CdS layer gave higher conversion efficiency

additional cocatalysts, whereas for obtaining similar activity with use of CdS particles, noble metal nanoparticles of Rh were necessary to be loaded as a cocatalyst.71 Furthermore, the visible-light-driven photocatalytic activity of ZAIS nanoparticles can be enhanced by immobilization on Au particles (Figure 7c). It is well-known that photoexcitation of the localized surface plasmon resonance (LSPR) peak of Au particles produces an intensified electric field near Au particles, which enhances the photoexcitation probability of semiconductor nanoparticles located near Au particles. As shown in Figure 7a, immobilization of ZAIS particles on SiO2-coated Au particles increased R(H2), especially for the cases using ZAIS particles with x larger than 0.8, where the distance between ZAIS and Au particles was ca. 18 nm. The enhancement factor (fenhance) for R(H2) is calculated for each kind of ZAIS particles as the ratio of R(H2) of ZAIS immobilized on Au particles to that of ZAIS particles only 343

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of 3.91% than that of original CuInS2−TiO2 electrodes (1.14%).84 So far CdS layer as a passivation layer is the best choice to increase the performance of chalcopyrite-typenanoparticle-sensitized solar cells, but the alternatives to CdS are necessary to be developed for devices containing no highly toxic elements. Solid solution nanoparticles between ZnS and CuInS2 (ZCIS) could also be used as light absorbing materials in sensitized solar cells, and capping ZCIS nanoparticles with 1.3 ZnS monolayers increased the conversion efficiency to 0.71% from 0.28% obtained for particles without a ZnS layer due to the elimination of defect sites.78 We have reported that Ag-based semiconductor nanoparticles of AgInS2 or ZAIS acted as sensitizers in sensitized solar cells.85,86 These nanoparticles were immobilized on the ZnO rod surface by immersing ZnO rod electrodes in a solution containing nanoparticles without any additional crosslinking agents (Figure 8a). Sandwich-type solar cells were fabricated with ZAIS-loaded ZnO rod electrodes and an acetonitrile solution containing the redox couple of I−/I3−. Figure 8b shows current−voltage curves of resulting devices. The open circuit voltage was roughly constant at ca. 0.55 V regardless of the kind of ZAIS particles, while the short circuit current increased with increase in the x value. As shown in Figure 8c, the photoresponse of the obtained cells was tunable in a wide wavelength range of the visible light region by selecting the composition of ZAIS particles immobilized. The optimum energy conversion efficiency was ca. 0.72% in the case of using AgInS2 particles as a sensitizer, which exhibited photoresponse in a wider wavelength region than those of the other ZAIS particles studied. Different types of photovoltaic devices other than sensitized solar cells could be fabricated using Ag-based I−III−VI2 semiconductor nanoparticles. Peng and co-workers reported the fabrication of a heterojunction solar cell using AgInS2 nanoparticle ink as a precursor, in which an immobilized AgInS2 nanoparticle layer was heat-treated at 450 °C under Ar and sulfur atmospheres.87 The energy conversion efficiency was as low as ca. 0.50%. Furthermore, organic/inorganic hybrid bulk-heterojunction solar cells were fabricated by using Cudiffused AgInS2 nanoparticles and a hole conductive polymer (P3HT).88 The removal of Ag vacancies by the Cu doping improved the crystallinity of resulting particles, and the obtained device exhibited high power conversion efficiency of 1.1% as well as extended photoresponse to a longer wavelength at around 800 nm. These studies successfully demonstrated that Ag-based I− III−VI2 semiconductor nanoparticles were promising lightabsorbing materials in photovoltaic devices. Although the energy conversion efficiencies of devices fabricated with I−III− VI2-based nanoparticles are lower than those reported using conventional binary semiconductor nanoparticles, such as CdSe and PbS (ca.4−7%),73−77 these semiconductor particles have significant advantages for the fabrication of practical devices: the particles can be prepared without using highly toxic elements and their optical properties are tunable by changing the chemical composition. The energy conversion efficiency can probably be improved by appropriate selection of materials or redox species to transport photogenerated carriers from semiconductor nanoparticles to collecting electrodes. In this Perspective, we have briefly summarized several strategies for preparation of colloidal I−III−VI2-related nanoparticles, especially for preparation of Ag-based nanoparticles, with precisely controlled size and chemical composition to

Figure 8. (a) Schematic illustration of the mechanism of photocurrent generation in ZAIS-immobilized ZnO rod electrodes under visible light irradiation. (b) Current−voltage curves of sandwich-type cells prepared with ZAIS-immobilized ZnO rod electrodes under simulated solar irradiation (AM 1.5). (c) Action spectra of photocurrent obtained at the short circuit condition. The number in the figure is the x value of the (AgIn)xZn2(1−x)(S2CN(C2H5)2)4 precursor used in ZAIS preparation. Reprinted with permission from ref 86. Copyright 2012 Royal Society of Chemistry.

control their optical properties and electronic energy structure. These multinary semiconductor nanoparticles are attractive materials for practical applications due to the lack of highly toxic elements. For example, nanoparticles of I−III−VI2semicoductors and their solid solutions exhibited intense PL with high quantum yield, and their PL color could be changed in a wide range from UV−vis to NIR by controlling the chemical composition of particles, being advantageous in comparison to conventional luminophores such as organic dyes. A precise understanding of the PL mechanisms of these 344

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Young Scientists B (No. 25810133) and a Funding Program for Next Generation World-Leading Researchers (NEXT Program) from the Japan Society for the Promotion of Science.

nanoparticles is needed for further improvement of PL properties. One of the attractive applications utilizing electronic energy structure-tunable I−III−VI2-based semiconductor nanoparticles is fabrication of photoenergy conversion devices such as solar cells and photocatalysts due to their possibility to exceed the theoretical efficiency of conventional devices. As mentioned in the previous section, the DFT calculations for AgInS2 revealed that the valence band maximum was made up of hybrid orbitals originating from S and Ag atoms, and the conduction band minimum was those from In and S atoms. So it is expected that the potentials of valence band and conduction band will be individually controlled by varying both the size of particles and their nonstoichiometry (that is, the ratio of Ag to In), being different from the cases of conventional binary nanoparticles such as CdS and CdSe. The similar tunability is also expected for the solid solutions of ZnS−AgInS2 or CuInS2−AgInS2. Therefore elucidation and control of the size- and compositiondependent electronic energy structure of chalcopyrite-type nanoparticles will enable the exact alignment of energy levels of materials, which is useful for effectively transferring electrons and holes photogenerated in nanoparticles. Furthermore, for the improvement of solar light absorption, it is also necessary to adjust the energy gap of semiconductor particles by controlling the particle size or by making the solid solution with an appropriate composition. We believe these challenges will facilitate progress in the fabrication of efficient photoenergy conversion systems.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Tsukasa Torimoto has been a Professor of the Graduate School of Engineering at Nagoya University since 2005. He received his Ph.D. from Osaka University in 1994. His main research interests are the preparation of novel semiconductor and metal nanoparticles and their application to the energy conversion systems (http://www.apchem. nagoya-u.ac.jp/06-K-6/torimoto/index_eng.html). Tatsuya Kameyama is an assistant professor of the Graduate School of Engineering at Nagoya University. He received his Ph.D. from Nagoya University in 2011. He started his academic career at Nagoya University since 2012. His scientific interests include various aspects of (photo)electrochemistry of nanomaterials, with special focus on semiconductor nanocrystals and their hybrid assemblies. Susumu Kuwabata has been a Professor of the Graduate School of Engineering at Osaka University since 2002. He received his Dr. Eng. from Osaka University in 1991. His current research interests are centered on electrochemistry and functional nanomaterials, including design of the solid/liquid interface on the nanometer scale to enhance electron transfer and visualization of the electron-transfer reactions using ionic liquids. For further details, see http://www.chem.eng. osaka-u.ac.jp/∼elechem/. 345

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