Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

Ag–In–Ga–S nanocrystals embedded in a Ga₂O₃ matrix produced a narrow band-edge photoluminescence, the intensity being significantly enlarged by Na⁺ doping, and they effectively served as an emitting layer in green quantum dot-light-emitting diodes.

DOI： 10.1039/d3na00755c

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

Narrowing the emission peak width and adjusting the peak position play a key role in the chromaticity and color accuracy of display devices with the use of quantum dot light-emitting diodes (QD-LEDs). In this study, we developed multinary Cu–In–Ga–S (CIGS) QDs showing a narrow photoluminescence (PL) peak by controlling the Cu fraction, i.e., Cu/(In+Ga), and the ratio of In to Ga composing the QDs. The energy gap of CIGS QDs was enlarged from 1.74 to 2.77 eV with a decrease in the In/(In+Ga) ratio from 1.0 to 0. The PL intensity was remarkably dependent on the Cu fraction, and the PL peak width was dependent on the In/(In+Ga) ratio. The sharpest PL peak at 668 nm with a full width at half maximum (fwhm) of 0.23 eV was obtained for CIGS QDs prepared with ratios of Cu/(In+Ga) = 0.3 and In/(In+Ga) = 0.7, being much narrower than those previously reported with CIGS QDs, fwhm of >0.4 eV. The PL quantum yield of CIGS QDs, 8.3%, was increased to 27% and 46% without a PL peak broadening by surface coating with GaSx and Ga–Zn–S shells, respectively. Considering a large Stokes shift of >0.5 eV and the predominant PL decay component of ∼200–400 ns, the narrow PL peak was assignable to the emission from intragap states. QD-LEDs fabricated with CIGS QDs surface-coated with GaSx shells showed a red color with a narrow emission peak at 688 nm with a fwhm of 0.24 eV.

DOI： 10.1063/5.0144271

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Nano-objects, such as quantum dots (QDs), are essential units for the construction of functional materials and devices in current technologies. The establishment of a versatile scheme to sort desired components from a crude product is crucial for bringing out the full potential of the original materials. However, it is still challenging to separate QDs with the same composition on the basis of size and to sort QDs with the same size but different optical properties. Here, we demonstrate such sorting for the first time by combining plasmonic optical trapping with thin-layer chromatography (TLC), which is a widely used tool. LED photoexcitation of the localized surface plasmon resonance of Au nanoparticles immobilized on a TLC plate affected the distance QDs traveled depending on the wavelength and intensity of irradiated light, which led to clear separation according to the size and/or optical properties of the QDs. Since optical property-based separation cannot be achieved by conventional chromatography, in which the interactions between stationary phases of chromatographs and QDs are simply based on differences in the size or surface functionality of the QDs, the present strategy will be a key solution for the establishment of a versatile scheme for sorting nano-objects.

File： 20220722_NPG Asia Materialsにハイライト-01.pdf

DOI： 10.1038/s41427-022-00414-3

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Other Link： https://www.nature.com/articles/s41427-022-00414-3

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

AuRh alloy nanoparticles, prepared by simultaneous sputter deposition of Au and Rh metals on an ionic liquid, exhibited a surface composition-dependent electrocatalytic activity.

DOI： 10.1039/d2cp01461k

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PubMed

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Nanoparticles composed of molybdenum oxide, MoOx, were successfully prepared by room-temperature ionic liquid (RTIL)/metal sputtering followed by heat treatment. Hydroxyl groups in RTIL molecules retarded the coalescence between MoOx NPs during heat treatment at 473 K in air, while the oxidation state of Mo species in MoOx nanoparticles (NPs) could be modified by changing the heat treatment time. An LSPR peak was observed at 840 nm in the near-IR region for MoOx NPs of 55 nm or larger in size that were annealed in a hydroxyl-functionalized RTIL. Photoexcitation of the LSPR peak of MoOx NPs induced electron transfer from NPs to ITO electrodes.

DOI： 10.1039/d0ra05165a

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Language：English   Publishing type：Research paper (scientific journal)  

We prepared a solid-state Z-scheme photocatalyst in which zinc rhodium oxide (ZnRh2O4) and bismuth vanadium oxide (Bi4V2O11) that served as hydrogen (H2) and oxygen (O2) evolution photocatalysts, respectively, were connected with gold (Au) nanoparticles. The Au nanoparticles were prepared by sputtering in an ionic liquid, N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide, to generate Au/ZnRh2O4/Au/Bi4V2O11 with various amounts of Au in the 12 mol. %-29 mol. % range (vs 1.0 mol ZnRh2O4 + 0.2 mol Bi4V2O11). Au/ZnRh2O4/Au/Bi4V2O11 photocatalyzed overall pure-water splitting under irradiation with red light at a wavelength of 700 nm, and the dependence of the amounts of Au on the apparent quantum efficiency tended to increase in the measurement range.

DOI： 10.1063/5.0010100

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PubMed

Authorship：Lead author   Publishing type：Research paper (scientific journal)  

Plasmon-induced charge separation (PICS) at the interface between a plasmonic nanoparticle and a semiconductor becomes less efficient as the plasmon resonance wavelength increases, because the energy of a photon may not be sufficiently higher than the interfacial Schottky barrier height. In this study, we developed PICS photocathodes by coating Au nanoparticles of different sizes on an ITO electrode with a thin TiO layer, and applied negative potentials to those photocathodes so as to suppress back electron transfer and improve the PICS photocurrent responses. The photocurrent enhancement factor was increased as the particle size was decreased, and enhancement of about two orders of magnitude was observed for small Au nanoparticles when bias voltage of 0.5 V was applied. In some cases the photocurrent enhancement was accompanied by a slight redshift of the photocurrent peak, which was caused by a lowered barrier. This technique would be useful for tuning the photocurrents when it is applied to devices such as electrochemical LSPR sensors and photodetectors. 2

DOI： 10.1039/c9pp00098d

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PubMed

Authorship：Lead author   Publishing type：Research paper (scientific journal)  

In order to improve the performance of localized surface plasmon resonance (LSPR) sensors, lattice plasmons of plasmonic nanoparticle (NP) arrays are often used. The lattice plasmon sensors based on coupling between LSPR of each NP and diffraction of the NP array generally have a higher ability to detect local refractive index changes than conventional LSPR sensors based on random NP ensembles. However, the surface selectivity of the lattice plasmon sensors is lower than that of the LSPR sensors because the local electric field of the former extends far from the particle array surface. In this work, we develop a novel plasmonic-diffractive hybrid sensor that can simultaneously determine both local and bulk refractive indices from two output signals, namely, extinction dip and peak, the wavelengths of which are differently dependent on those indices. The hybrid sensor can evaluate the refractive index changes in the immediate vicinity of the sensor surface by canceling the difference in the bulk refractive index between the sample and standard solutions.

DOI： 10.1021/acsanm.8b01829

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Plasmon-induced charge separation (PICS), in which an energetic electron is injected from a plasmonic nanoparticle (NP) to a semiconductor on contact, is often inhibited by a protecting agent adsorbed on the NP. We addressed this issue for an Ag nanocube-TiO system by coating it with a thin Au layer or by inserting the Au layer between the nanocubes (NCs) and TiO . Both of the electrodes exhibit much higher photocurrents due to PICS than the electrodes without the Au film or the Ag NCs. These photocurrent enhancements can be explained in terms of PICS with accelerated electron transfer, in which electron injection from the Ag NCs or Ag@Au core-shell NCs to TiO is promoted by the Au film, or PICS enhanced by a nanoantenna effect, in which the electron injection from the Au film to TiO is enhanced by optical near field generated by the Ag NC. 2 2 2 2

DOI： 10.1117/1.JPE.6.042505

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Publishing type：Research paper (scientific journal)  

Potentiometric and conductometric sensors based on localized surface plasmon resonance were developed. The sensors can be applied to coloured and turbid samples because light need not pass through the sample solution.

DOI： 10.1039/c5cc01020a

Scopus

PubMed

Authorship：Lead author  

DOI： 10.1149/MA2024-02674718mtgabs

DOI： 10.1149/MA2024-02593969mtgabs

DOI： 10.1149/MA2024-02684800mtgabs

DOI： 10.1149/MA2024-02594001mtgabs

DOI： 10.1149/MA2024-02674719mtgabs

DOI： 10.1149/MA2024-02573827mtgabs

DOI： 10.1149/MA2024-02593973mtgabs

DOI： 10.1149/MA2024-02513558mtgabs

DOI： 10.1149/MA2024-02513561mtgabs

Language：English   Publishing type：Research paper, summary (international conference)   Publisher：The Electrochemical Society  

Semiconductor nanocrystals, often referred to as quantum dots (QDs), exhibit unique physicochemical properties due to the quantum size effect when their size is less than 10 nm. In our previous works, we successfully synthesized Zn-Ag-In-S (ZAIS) solid solution QDs with a tunable energy gap (E_g) and then reported the photocatalytic activity for H₂ evolution depending on their morphology and particle composition [1,2]. However, ZAIS QDs have limited light absorption with wavelengths less than ca. 600 nm, restricting their utilization of near-infrared light in solar light. In this study, we report the preparation of near-IR-responsive QD photocatalysts with use of Zn-Ag-In-Se (ZAISe) solid solution semiconductors. Precise controls over their morphology and particle composition enabled to vary their photocatalytic H₂ evolution activity.

ZAISe QDs with different compositions and particle shapes (sphere or rod) were synthesized by thermal decomposition of corresponding metal acetates and selenourea in a mixture solution of oleylamine and dodecane thiol at 250 ^oC. The chemical composition of QDs was controlled by varying the Zn fraction in metal precursors, ensuring nearly stoichiometric composition.

The obtained QDs with a rod shape had lengths varying from ca. 10 to 100 nm, correlating with an increase in the Zn fraction. The onset wavelengths of absorption spectra of the ZAISe QDs were red-shifted from 500 nm to 900 nm with decreasing Zn fraction, indicating that the E_g of ZAISe QDs was controlled between visible and near-infrared wavelength regions.

The QDs modified with dodecane thiol showed poor dispersibility in water and almost no photocatalytic activity for H₂ evolution. In contrast, the surface modification of QDs with 3-mercaptopropionic acid enabled to uniformly disperse in an aqueous solution. The photocatalytic activity of ZAISe QDs was investigated for H₂ evolution as a model reaction. The irradiation to ZAISe QDs in an aqueous solution containing Na₂S and Na₂SO₃ as hole scavengers was carried out using Xe lamp light (λ> 350 nm). The H₂ evolution rate exhibited a remarkable dependence on both the composition and particle shape of ZAISe QDs. The photocatalytic activity showed a tendency to increase with the enlargement of the E_g of QDs, probably due to the negative shift of conduction band minimum potential. The optimum H₂ evolution activity was obtained with rod-shaped QDs having the E_g of 1.65 eV, being twice as large as that of spherical QDs with a similar E_g. This behavior can be attributed to the change of the crystal facets exposed on the QD surface.

References Kameyama, T. Takahashi, T. Machida, Y. Kamiya, T. Yamamoto, S. Kuwabata, and T. Torimoto, J. Phys. Chem. C, 2015, 119, 24740-24749.

Torimoto, Y. Kamiya, T. Kameyama, H. Nishi, T. Uematsu, S. Kuwabata, and T. Shibayama, ACS Appl. Mater. Interfaces 2016 , 8, 27151-27161.

DOI： 10.1149/MA2024-01351939mtgabs

Language：English   Publishing type：Research paper, summary (international conference)   Publisher：The Electrochemical Society  

Introduction

Multinary quantum dots (QDs) have been promising materials as an efficient light absorber in solar light energy conversion systems. These QDs including Cu(In,Ga)(S,Se)₂ and Cu₂ZnSn(S,Se)₄ QDs, consisting of less-toxic elements, are known for their high absorption coefficient and excellent processability. However, the cation disorder in Cu₂ZnSn(S,Se)₄ has been observed to cause a bandgap fluctuation and an emergence of additional antisite defects¹. An effective approach to mitigating such disorder involves the substitution of Cu or Zn with larger cations. Here we focus on Ag₂MnSnS₄ (AMTS) due to similar optical properties with Cu₂ZnSn(S,Se)₄ QDs. The energy gap of bulk AMTS was reported to ca. 2.0 eV², making it suitable for efficient solar light energy conversion systems. However, creating AMTS structures can be challenging, often requiring high temperatures or pressures. Thus, a simpler synthesis route for preparing QDs will offer a new perspective in addressing this issue, and the size reduction will enable to tune their optical bandgap. In this study, we report for the first time the solution-phase preparation of AMTS QDs. The resulting QDs exhibited composition-dependent optical properties depending on the Ag/metal ratio.

Experimental

The synthesis of AMTS QDs was conducted by a facile colloidal heating-up method. Precursors of corresponding metal complexes and sulfur powder were dispersed in a mixture solvent of 1-dodecanethiol (DDT) and oleylamine (OLAm), followed by the heat treatment at more than 423 K. Thus-obtained solution was subjected to centrifugation to remove large precipitates, and target QDs were isolated from the supernatant by adding methanol and ethanol as a non-solvent, followed by centrifugation.

Results and Discussion

AMTS QDs with diameters less than 6 nm were obtained. The absorption spectra of AMTS QDs varied with the Ag/metal ratio. The onset wavelength was blue-shifted from ca. 670 nm to ca. 500 nm with a decrease in the Ag fraction. In the case of Ag/metal= 0.50, that is, the stoichiometric Ag₂MnSnS₄, the bandgap was determined to 2.0 eV, being similar to the bulk bandgap of monoclinic Ag₂MnSnS₄ ². The optical bandgap was monotonously increased from 2.0 eV to 2.4 eV as the Ag/metal ratio decreased from 0.50 to 0.05.

The electronic energy structure of AMTS QDs was determined from the ionization energy of the QDs, which was evaluated from the onset energy of the photoelectron yield spectra in air. The conduction band minimum level shifted from –3.5 eV to –3.1 eV with a decrease in the Ag fraction, while the valence band maximum was constant at ca. –5.5 eV. The crystal structure and particle morphology were almost unchanged even when the particle composition was varied, indicating that the change in bandgap resulted from the composition. The stability of QDs was improved by surface coating with a ZnS shell. Subsequent ligand exchange with a hydrophilic molecule enabled the uniform dispersion of QDs in an aqueous solution without changing their optical properties and chemical composition. These findings open up possibilities for utilizing ATMS QDs in various applications such as photocatalysis and bioimaging in aqueous media.

Reference

¹ J. Kumar, and S. Ingole, J. Alloys Compd., 865, 158113 (2021).

² Frank N. Keutsch, Dan Topa, Rie Takagi Fredrickson, E. Makovicky, and W.H. Paar, Mineral. Mag. 83(2), 233–238 (2019).

DOI： 10.1149/MA2024-01131080mtgabs

Authorship：Lead author   Language：English   Publishing type：Research paper, summary (international conference)   Publisher：The Electrochemical Society  

Introduction

When semiconductor nanoparticles are reduced to a size of several nanometers or less, the physico-chemical properties start to change by the quantum confinement. The energy gap of particles is enlarged and their absorption spectrum is blue-shifted with a decrease in the particle size. These nanoparticles have been intensively investigated for developing efficient light energy conversion systems, such as photocatalysts [1] and photovoltaics [2]. Especially, high-efficiency quantum dot solar cells fabricated with PbS and PbSe nanoparticles, which showed the responsivity in near-IR region, have attracted much attention [3]. However, the contents of highly toxic element, Pb and Se, limit their use in a wide range of applications [4].

Recently, a multinary AgBiS₂ semiconductor can absorb visible and near-IR lights and seems to be a promising material for efficient light energy conversion [5], but there are few reports to prepare size-quantized nanoparticles and control their particle size. In this study, we report the preparation of AgBiS₂ nanoparticles using a liquid-phase chemical synthesis method. Their size was varied depending on the preparation conditions. Furthermore, we investigated their photoelectrochemical properties as functions of particle size and composition.

Experimental

Precursors of bismuth acetate (Bi(OAc)₃), silver acetate (Ag(OAc)), and sulfur powder, were dispersed in a mixture solvent of 1-dodecanethiol (DDT) and oleylamine (OLAm), followed by the heat treatment at 373, 393, 423 or 473 K for 30 min. Thus-obtained AgBiS₂ nanoparticles were isolated by the addition of methanol as a non-solvent.

Results and Discussion

The particle size and composition of AgBiS₂ nanoparticles were controlled by changing the heating temperature. With an increase of the heating temperature from 373 K to 473 K, the particle size was enlarged from 3.2 nm to 8.1 nm, accompanied by the increase of the Ag fraction in nanoparticles to the stoichiometric value. The onset wavelength in the absorption spectra of AgBiS₂ nanoparticles was shifted from 1000 nm to 1200 nm with an increase in the particle size, resulting in the decrease of bandgap from 1.45 eV to 1.05 eV. Since the estimated bandgap values were larger than that of the bulk AgBiS₂ (0.80 eV) [6], it was found that the obtained AgBiS₂ nanoparticles showed a quantum size effect. The XRD analysis revealed that the obtained nanoparticles exhibited the cubic AgBiS₂ crystal structure, regardless of the reaction temperature.

The Ag/Bi ratio in the preparation also changed the particle size of obtained AgBiS₂ nanoparticles as well as their chemical composition. By reacting precursors with Ag/(Ag+Bi) = 0.33 at 423 K, almost stoichiometric AgBiS₂ nanoparticles were formed, that is, Ag: Bi: S = 1: 1: 2. In contrast, when Ag/(Ag+Bi) ratio in preparation was decreased from 0.5 to 0.25, the Ag fraction in nanoparticles decreased from the stoichiometric value of Ag/Bi=1, that is, non-stoichiometric AgBiS₂ nanoparticles were formed. The particle size tended to decrease with a decrease in the Ag/(Ag+Bi) ratio in preparation.

We carried out the photoelectrochemical measurement of AgBiS₂ nanoparticles immobilized on ITO electrodes. Anodic or cathodic photocurrents were detected, depending on the Ag fraction in nanoparticles. The energy levels of AgBiS₂ nanoparticles were determined by measuring the photoelectron yield spectroscopy in air. The onset potentials of photocurrent generation were significantly different from the levels of conduction band minimum or valence band maximum, suggesting that the thus-obtained AgBiS₂ nanoparticles contained the large amount of defect sites which acted as a carrier recombination site.

Acknowledgments

This work is supported by the New Energy and Industrial Technology Development Organization (NEDO).

References

(1) S. Chen, D. Huang, P. Xu, W. Xue, L. Lei, M. Cheng, R. Wang, X. Liu, and R. Deng., J. Mater. Chem. A, 8, 2286 (2020).

(2) G. H. Carey, A. L. Abdelhady, Z. Ning, S. M. Thon, O. M. Bakr, and E. H. Sargent, Chem. Rev., 115, 12732 (2015).

(3) H. Lee, H. J. Song, M. Shim, and C. Lee, Energy Environ. Sci., 13, 404 (2020).

(4) F. M. Winnik and D. Maysinger, Acc. Chem. Res., 46, 672 (2013).

(5) M. Bernechea, N. Miller, G. Xercavins, D. So, A. Stavrinadis, and G. Konstantatos, Nat. Photonics, 10, 521 (2016).

(6) S. N. Guin and K. Biswas, Chem. Mater., 25, 3225 (2013).

DOI： 10.1149/ma2022-0113934mtgabs

Authorship：Lead author   Language：English   Publishing type：Research paper, summary (international conference)   Publisher：The Electrochemical Society  

Introduction

Incident lights interact with noble metal nanoparticles (NPs) to form collective oscillations of free electrons, creating a strong electromagnetic field on their surface. This phenomenon is called localized surface plasmon resonance (LSPR). Recently metal oxide NPs with appreciable concentration of free electrons have been also reported to exhibit LSPR peaks, the peak wavelengths of which were controlled from the visible to near-IR regions.[1] When a plasmonic NP was combined with a semiconductor, the excitation of LSPR could cause the injections of electrons and/or holes from plasmonic NPs to the semiconductor, that is, the plasmon-induced charge separation (PICS).[2] Since the PICS was reported using plasmonic metal oxide NPs,[3] the intense research has been devoted to studying plasmonic metal and metal oxide NPs for the application to photovoltaics, photocatalyts, and biosensors.

Recently we reported the preparation of noble metal NPs by metal sputtering deposition onto room-temperature ionic liquids (RTIL) under a reduced pressure (RTIL/metal sputtering), where NPs of several nanometers in size were stably and uniformly dispersed without additional stabilizing agents.[4] This technique enabled the clean preparation of plasmonic metal NPs, such as Au[4], Ag[5], and AgAu alloy[6]. In this study, we apply the RTIL/metal sputtering to the preparation of molybdenum oxide (MoO_x) NPs. Thus-obtained NPs exhibited the LSPR peak, the excitation of which induced the PICS.

Experimental

The sputter deposition of molybdenum were carried out on the surface of 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate (HyEMI-BF₄) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF₄) used as RTILs for 1 h with a discharge current of 30 mA under an argon pressure of 3.0 Pa. The deposited NPs were partially oxidized by the heat treatment at 473 K for 30 min in air, resulting in the formation of MoO_x NPs. The photoelectrochemical properties were measured in a 0.5 M Na₂SO₄ aqueous solution with the ITO electrode immobilized with MoO_x NPs as working electrodes, a Pt counter electrode and a Ag/AgCl reference electrode. The photocurrents were detected with monochromatic light irradiation under the potential application at +0.5 V vs Ag/AgCl.

Results and Discussion

TEM measurements revealed that Mo NPs sputter-deposited in HyEMI-BF₄ had an average diameter of 6.8 nm and then the heat treatment at 473 K increased the particle size to ca. 65 nm. XPS spectra of thus-obtained particles indicated that the thus-obtained NPs were composed of MoO_x containing Mo(V) and Mo(VI) species. The MoO_x NPs prepared in HyEMI-BF₄ showed a LSPR peak at around 840 nm, the peak wavelength of which was close to that of LSPR reported for chemically synthesized MoO_3-x NPs.[1] On the other hand, MoO_x NPs prepared in EMI-BF₄ showed no LSPR peak in the visible and near-IR regions. Anodic photocurrents were observed by the irradiation to MoO_x NP-immobilized ITO electrodes with wavelength shorter than ca. 1000 nm. Regardless of the kinds of MoO_x NPs, the action spectra of photocurrents showed the increase of IPCE at the wavelength below 600 nm due to the photoexcitation of interband transition of MoO₃, being in agreement with their extinction spectra. Furthermore, the MoO_x NPs prepared in HyEMI-BF₄ showed a peak at 840 nm, the peak wavelength of which agreed with that of their LSPR peak. In contrast, MoO_x NPs formed in EMI-BF₄ exhibited no photocurrent by the irradiation of near-IR lights.

Conclusively, we successfully prepared the MoO_x NPs with RTIL/metal sputtering technique followed by the heating treatment. The NPs formed in HyEMI-BF₄ showed the LSPR peak in near-IR region, the photoexcitation of which induced the PICS from the MoO_x NPs to the ITO electrodes.

References

(1) A. Agrawal, S. H. Cho, O. Zandi, S. Ghosh, R. W. Johns, and D. J. Milliron, Chem. Rev., 118, 3121 (2018).

(2) Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005).

(3) S. H. Lee, H. Nishi, and T. Tatsuma, Nanoscale, 10, 2841 (2018).

(4) T. Torimoto, K. Okazaki, T. Kiyama, K. Hirahara, N. Tanaka, and S. Kuwabata, Appl. Phys. Lett., 89, 243117 (2006).

(5) T. Suzuki, K. Okazaki, T. Kiyama, S. Kuwabata, and T. Torimoto, Electrochemistry, 77, 636 (2009).

(6) K. Okazaki, T. Kiyama, K. Hirahara, N. Tanaka, S. Kuwabata, and T. Torimoto, Chem. Commun., 6, 691 (2008).

DOI： 10.1149/ma2020-02592962mtgabs

Other Link： https://iopscience.iop.org/article/10.1149/MA2020-02592962mtgabs/pdf

Language：English   Publishing type：Research paper, summary (international conference)   Publisher：The Electrochemical Society  

Since the Fujishima effect was reported in 1972 [1], researches on photocatalytic water splitting using sunlight have been progressing as a method for producing hydrogen (H₂), which attracts attention as an alternative clean energy source for fossil fuels. For the effective use of sunlight, we have investigated an all-solid-state Z-scheme photocatalyst that can be sensitive to long-wavelength, low-energy light and can decompose pure water (without chemicals as a sacrificial agent or pH adjuster). In our previous study, we fabricated the Z-scheme photocatalyst, consisting of zinc rhodium oxide (ZnRh₂O₄) and bismuth vanadium oxide (Bi₄V₂O₁₁) as a H₂- and oxygen (O₂)-evolution photocatalyst, respectively, and gold (Au) as a solid redox mediator (ZnRh₂O₄/Au/Bi₄V₂O₁₁). The photocatalyst has successfully achieved the overall pure water splitting irradiated with red light (wavelength 700 nm) via Au, which mediated the transfer of photoexcited electrons from the conduction band (CB) of Bi₄V₂O₁₁ to the valence band (VB) of ZnRh₂O₄. Then, photogenerated holes in the VB of Bi₄V₂O₁₁ and photoexcited electrons in the CB of ZnRh₂O₄ effectively oxidized and reduced water, respectively, to achieve overall water splitting [2].

One of the co-authors (T.T.) reported a simple technique using a sputtering method to synthesize Au nanoparticles (Au_sp), much smaller than commercially available Au [3]. In the synthesis procedures described below, some part of Au_sp, supported only on the surface of ZnRh₂O₄, functions as a co-catalyst for H₂ evolution. Thus, it was expected that Au_sp could be homogeneously distributed as the redox mediator and co-catalyst, resulting in the improvement of the water splitting reaction. In this study, various concentrations of Au_sp in ZnRh₂O₄/Au_sp/Bi₄V₂O₁₁ were prepared, and the dependence of Au_sp concentration on water-splitting activity was evaluated.

Au_sp were evenly deposited by sputtering onto an ionic liquid (PP13-TFSI; N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide). Before performing the process, treatments of heating and stirring were performed in a vacuum atmosphere so as to extract air and water from the ionic liquid. ZnRh₂O₄ and Bi₄V₂O₁₁ powders were prepared by a solid-state reaction method. ZnO and Rh₂O₃ powders, and Bi₂O₃ and V₂O₅ powders as starting materials were uniformly mixed in a stoichiometric ratio to prepare ZnRh₂O₄ and Bi₄V₂O₁₁, respectively. The mixture was pelletized and calcined at 1150 °C for ZnRh₂O₄ for 24 h and 850 °C for Bi₄V₂O₁₁ for 8 h, then pulverized to obtain ZnRh₂O₄ and Bi₄V₂O₁₁ powders. ZnRh₂O₄ were added to the prepared Au_sp containing in PP13-TFSI and the mixture was heated and stirred in a nitrogen atmosphere in a test tube for 280 °C for 1 h (the expected molar percentage of Au to ZnRh₂O₄ was 2, 10, 12, and 17 mol%). Next, PP13-TFSI was removed by centrifugation and drying overnight to produce ZnRh₂O₄+Au_sp. ZnRh₂O₄/Au_sp was obtained by calcining the ZnRh₂O₄+Au_sp at 850 ° C for 2 h to strengthen the connection. ZnRh₂O₄/Au_sp/Bi₄V₂O₁₁ was produced by mixing Bi₄V₂O₁₁with ZnRh₂O₄/Au_sp at a ratio of 20 mol% with respect to ZnRh₂O₄ and following the same procedure as for producing Bi₄V₂O₁₁.

As-prepared Au_sp size distribution was determined by measuring the size of individual particles in the TEM image, and the average particle size was 2.3 nm. The average particle size of Au_sp containing in ZnRh₂O₄+Au_sp, ZnRh₂O₄/Au_sp and ZnRh₂O₄/Au_sp/Bi₄V₂O₁₁ after heat treatment was 9.0 nm, 27 nm, and 42 nm. Thus, it was confirmed that Au_sp could be used in the Z-scheme photocatalyst with the particle size much smaller than that of the commercial Au (300-500 nm) [2]. The amount of Au containing in ZnRh₂O₄/Au_sp/Bi₄V₂O₁₁ was estimated from XPS. Since Au_sp is mainly bonded to the surface of ZnRh₂O₄, we estimated the Au_sp amount based on the Bi 4f peak of Bi₄V₂O₁₁, which is unlikely to be affected by the change of Au_sp amount when measured by XPS. Then, the estimated Au_sp amounts were 12, 18, 20, and 29 mol% (vs. 1.0 mol of ZnRh₂O₄ + 0.2 mol of Bi₄V₂O₁₁). All photocatalysts were confirmed that H₂and O₂ were generated from pure water at a ratio of approximately 2 : 1 by irradiation of red light with a wavelength of 700 nm. The water splitting activity tended to be proportional to the amount of Au_sp estimated from XPS. These will be discussed in detail at the conference.

[1] A. Fujishima and K. Honda, Nature, 238, 37 (1972)

[2] K. Kamijyo, T. Takashima, M. Yoda, J. Osaki, and H. Irie, Chem. Commun., 54, 7999 (2018)

[3] T. Torimoto, K. Okazaki, T. Kiyama, K. Hirahara, N. Tanaka, and S. Kuwabata, Appl. Phys. Lett., 89, 243117 (2006)

DOI： 10.1149/ma2020-02613118mtgabs

Other Link： https://iopscience.iop.org/article/10.1149/MA2020-02613118mtgabs/pdf

Authorship：Lead author   Language：English   Publishing type：Research paper, summary (international conference)   Publisher：The Electrochemical Society  

Introduction

Light incident on metal thin films or nanoparticles (NPs) may induce a collective oscillation of conduction electrons that can lead to generation of strong electromagnetic fields in the vicinity of the metal surfaces. This phenomenon is commonly referred to as surface plasmon resonance (SPR) for metal films and localized surface plasmon resonance (LSPR) for metal NPs. These plasmon resonances have been applied to affinity-based sensing due to their sharp response to refractive index changes in the vicinity of the metal surface. Although LSPR sensors can be smaller and less expensive than SPR sensors, they are not suitable for colored or turbid sample solutions in general because light must pass through the sample to an optical detector. In this study, LSPR sensors that directly output potential changes were developed (Figure 1).¹ The sensors are based on plasmon-induced charge separation (PICS),²in which excited conduction electrons in metal NPs transfer to the semiconductor in direct contact with the NPs. Since light need not pass through the sample solution, the sensor can be applied to colored and turbid sample liquids.

Experimental

TiO₂ thin films (~70 nm thick) were prepared on ITO-coated glass plates by a spray pyrolysis method from 2-propanol containing titanium diisopropoxide bis(acetylacetonate) followed by annealing at 500 °C for 30 min. Citrate-protected Au NPs (40 nm) were adsorbed onto the TiO₂ surface by immersing the substrate in a Au NP dispersion at pH 2.7 for 6 h. Au@TiO₂ core-shell NPs were also used instead of Au NPs. The Au NPs were covered with 5–7 nm-thick TiO₂shells.

The open-circuit potential (OCP) measurements were conducted with the Au NP/TiO₂/ITO working electrode in water (refractive index n = 1.33) and a Ag|AgCl reference electrode. The working electrode was irradiated with light of different wavelengths, and a negative potential shift upon the light irradiation was measured. Refractive index of the solution was changed by adding glycerol (n= 1.47) to water and peak shifts in the photopotential action spectra were detected.

Results and Discussion

a) Sensors with Au NPs. The Au NP/TiO₂/ITO electrode was irradiated with light and a negative shift of the OCP was observed. The photoresponse is ascribed to injection of electrons from resonant Au NPs to the conduction band of TiO₂ due to PICS. In Figure 2, typical potential shifts –ΔE at t = 60 s (light irradiation was started at t = 0 s) are plotted against the energy of irradiated photons. The photopotential action spectrum is in good agreement with the LSPR absorption spectrum in shape. Furthermore, the peak in the action spectrum was redshifted in response to an increase in the refractive index of the sample solution. This peak wavelength shift was plotted against the refractive index of solutions, and a linear relationship was obtained. The refractive index sensitivity was calculated to be 33 ± 4 nm RIU^-1 (refractive index unit). The sensor was applied to coffee as a colored and turbid sample. The Au/TiO₂film in coffee did not show any extinction peak but a clear photopotential peak.

b) Sensors with Au@TiO₂ NPs. Since the refractive index sensitivity was not sufficiently high, we replaced the Au NPs with Au@TiO₂ NPs. As a result, the sensitivity was increased to 72 nm RIU^-1. In the case of the bare Au NPs, electron oscillation is somewhat localized at the bottom of each NP, at which the NP is in contact with TiO₂, so that certain volume of the sensing region is occupied by the TiO₂ substrate. On the other hand, the electron oscillation is delocalized over the whole NP surface for the Au@TiO₂ NPs because the Au core is fully covered with the TiO₂ shell, and therefore the masking by the TiO₂substrate is less significant.

Conclusion and Future Plan

The LSPR sensor which directly output electrical signals from TiO₂loaded with Au NPs was developed. The photopotential peaks were obtained on the basis of PICS. The response peak redshifted linearly as the refractive index of the sample solution increased. Since light need not pass through a sample solution, the sensor can be applied to a colored and turbid sample liquid. Work is currently underway to improve the sensor performance by using anisotropic NPs.

References

(1) T. Tatsuma, Y. Katagi, S. Watanabe, K. Akiyoshi, T. Kawawaki, H. Nishi, and E. Kazuma, Chem. Commun., 51, 6100 (2015).

(2) Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005).

Figure Caption. (a) Mechanism of the potentiometric LSPR sensor. (b) Typical photopotential action spectra of a Au/TiO₂ film.

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Figure 1

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DOI： 10.1149/ma2016-02/53/3919

Other Link： https://iopscience.iop.org/article/10.1149/MA2016-02/53/3919/pdf

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