On/Off switching mechanism of photoemission from QDs using photochromic molecules

TEM images of CsPbBr3 QDs

ON/OFF switching of photoemission using quantum dots coupled with photochromic molecules

Colloidal quantum dots (QDs) are commercially available as bioimaging materials because of their higher stability and higher PL quantum efficiency than organic dyes, and their application to displays has also started. Among them, perovskite-type metal halide (CsPbX3, X = Cl, Br, I) quantum dots are attracting much attention because they have the 100% fluorescence quantum yield and extremely sharp emission half-width, and multicolor emission is possible by controlling the anion composition. We found that photoemission from the QDs can be switched on/off by combining them with photochromic (PC) molecules. Changes in color of PC molecules by photostimulation efficiently modulates the photoemission from the QDs. Currently, to improve the efficiency of this process, we are synthesizing various organic molecules that can bind to the quantum dots, and studying the applications of the hybrid to on/off control of multicolor/white emission, bioimaging, display materials, etc.

Chem. Commun. 55, 8060-8063 (2019).

ACS Mater. Lett. 2, 727-735 (2020).

ACS Applied Nano Materials, in press (2020).

Synthesis of monodispersed SnO2 nanocrystals and WO3 nanolamellae for gas sensing

Gas sensing with nanostructured materials

We are making nanostructured materials such as quantum dots, nanotubes, nanorods, and nanolamellae for detecting volatile organic compounds (VOCs) and pollutant gases in the atmospheric air.Controlling the size and morphology of nanomaterials leads to improved sensitivity and selectivity. 

Chem. Mater. 22, 2662-2667 (2010).
Appl. Phys. Express 6, 047201 (2013).

J. Phys. Chem. C, 117, 17574-17582 (2013).

ACS Appl. Mater. Inter. 6, 5319-5326 (2014).

Langmuir 30, 2571-2579 (2014).

J. Phys. Chem. C 118, 2554–2563 (2014).

ACS Appl. Mater. Interfaces 7, 5863-5869 (2015).

Anal. Chem. 87, 8407–8415 (2015).

Crystal Growth & Design 16, 4203 (2016).

ACS Appl. Mater. Interfaces 8, 35485 (2016).

Adv. Funct. Mater. 28, 1704620 (2018).

ACS Sensors 5, 1624-1633 (2020).

Device structures of GO-based gas sensor and electrozyer

Development of electrochemical devices using graphene oxide

Nanocarbon materials such as fullerenes, carbon nanotubes, and graphene have been in the spotlight as multifunctional materials. Among these nanocarbon materials, we have focused on graphene oxide (GO), one of graphene derivatives. It has been discovered by a group of Kumamoto University that GO exhibits a good proton conductivity. We also found that GO has a mixed electron-proton conductivity when it is partially reduced by light or heat. Utilizing this property, we have developed an electrochemical hydrogen sensor based on a free-standing stacked GO nanosheet membrane. The GO membrane sandwiched with Pt/C electrode layers worked as a selective hydrogen sensor operative at room temperature according to the mixed potential theory. We also constructed an electrolysis device by attaching platinum and iridium electrocatalysts to the GO membrane. The device efficiently electrolyzed humid air to produce hydrogen and oxygen at room temperature with a 100% Faradaic efficiency.

Chem. Mater. 26, 5598-5604 (2014).

J. Mater. Chem. A 3, 20892-20895 (2015).

ACS Omega 2, 2994-3001 (2017).

ACS Sustainable Chem. Eng. 6, 11753-11758 (2018).

ACS Appl. Nano Mater. 3, 4292–4304 (2020).

Esterification of fattyacids using a GO catalyst under microwave irradiation

Conversion of biomass into more valuable products with graphene oxide

Taking advantage of GO's excellent proton conductivity, we investigated the use of GO as a solid acid catalyst for biomass conversion. Under microwave irradiation, GO showed a good activity for esterification of fatty acids (biodiesel production) and conversion of cellulose into glucose. Furthermore, we found that the conversion proceeds more efficiently by the combination of subcritical water treatment and microwave irradiation. In addition, this process can be applied to the hydrolysis of fucoidan, which is a polysaccharide component of wakame seaweed, and glycosides.

Green Chem. 19, 3831-3843 (2017).       

Energy Fuels 32, 3599-3607 (2018).　

Catal. Sci. Technol. 8, 5434-5444 (2018).

RSC advances 9, 30325-30334 (2019).

Analysis of a standard gas containing propofol using a CO2 sensor combined with combustion catalysts

Oxygen separation membrane based on mixed conducting BaFeO3

Applications of ceramic ionic conductors for electrochemical devices

Na3Zr2SiPO12 is a sodium ion conducting material, which is known as NASICON (Na ion Super Ionic Conductor）developed by Prof. Goodenough at the University of Texas.We have developed a high performance NASICON-based CO2 sensor that can detect 1 to 10000 ppm CO2 in air without interference from other gases like water and hydrocarbons.Our current interest is to analyze the concentrations of standard organic gases by combining combustion catalysts with the CO2 sensor.The catalytic combustion of organic gases at the catalyst layer produces CO2, which is then detected by the CO2 sensor and the sensor signal is transduced as the concentration of organic molecules in the gas, as shown in the scheme. Recently, we have reported that this system can also be used to evaluate the performance of CO2 capture materials.

We are also working on research and development of oxygen separation membranes using mixed-conducting ceramic oxygen conductors. We have reported a separation membrane that can extract only oxygen from air at 700-900C by using a La-doped BaFeO3 membrane device with catalyst layers.

Anal. Chem. 82, 3315-3319 (2010).

Anal. Methods 3, 1887-1892 (2011).  

Sensors and Actuators B: Chem. 128105 (2020).

Adv. Mater. 22, 2367-2370 (2010).

J. Solid State Chem. 183, 2426-2431 (2010).

ACS Appl. Mater. Interfaces 2, 2849–2853 (2010).

Ceramics Int. 41,7830–7835 (2015).

Photorecovery process using a heteropoyacid-surfactant hybrid as a photocatalyst

Organic-inorganic hybrid for photocalysis at liquid-liquid interfaces

We have demonstrated that heteropolyacids combined with surfactants work as a photocatalyst at an interface between an organic solution and water containing noble metal ions.
This reaction system preferentially produces sheet-like noble metal particles (nanosheets), which is indicative of the occurrence of the reaction at the interface.
 

The develop method is applicable to photo-recovery of noble metals from waste solutions.This is a potentially green and cheap method in that catalysts can be recycled, few amounts of chemicals are used, and the reaction is driven by light.
We also found that the addition of thiol compounds significantly improved the recovery rate because of strong interaction between metals and thiols.

Langmuir 24, 7648-7650 (2008).
J. Phys. Chem. C 113, 19986-19993 (2009).
Langmuir 29, 2128-2135 (2013).

Chem. Eur. J. 21, 7462-7469 (2015).

J. Phys. Chem. C 121, 13515-13523 (2017).

Synthesis of Cu2ZnSnS4 nanocrystals and their application for printed solar cells

Printed solar cells based on sulfide nanocrystals

Cu2ZnSnS4 (CZTS) has been attracting much attention because of its good light absorption properties.Another advantage is that CZS is not composed of toxic or rare elements, which makes CZTS a promising absorber layer in compound semiconductor solar cells.We are currently synthesizing CZTS nanocrystals with good quality, which can be applied to thin film fabrication.Making thin films from nanocrystals using a simple coating process is expected to drastically decrease the fabrication cost of compound semiconductor solar cells.Major challenges are development of feasible, reproducible routes to get high quality heterojunction films.

J. Phys. Chem. C 118, 804-810 (2014).

Inorg. Chem 54, 7840-7845 (2015).

ChemistrySelect 1, 86-93 (2016).

Extraction of natural products using SCCO2 and subcritical water

Tunable physical properties of supercritical fluids

Separation and reaction processes in supercritical fluid

One of our many research interests deal with the chemical engineering aspects of sub and supercritical fluid technologies and their applications to effective biomass utilization, bioresources recovery, chemical recycling and development of energy-saving processes. Specifically, supercritical fluids are applied to extraction of valuable compounds from natural products, fractionation of essential oils, biomass decomposition, bio-oil production, plastic recycling and bitumen upgrading to name a few.

Supercritical Fluid Technology for Energy and 　　Environmental Applications, 235-247 (2014)

Sep. Pur. Tech., 125, 319-325 (2014)

J. Agric. Food Chem. 61 (24), 5792-5797 (2013)

J. Supercrit. Fluids, 66, 215-220 (2012)

Microwave-solvothermal synthesis of biofuel

Mechanism of solvent-free microwave extraction of essential oil

Microwave-mediated separation and reaction processes

Microwave irradiation, under ambient  or high pressure conditions, is applied to extraction of functional natural products and to synthesis of biofuels such as biodiesel, bioETBE and GTBE. 

Green Energy and Technology, 117-130 (2013)

Ind. Eng. Chem. Res., 52, 7940-7946 (2013)

Chemical Engineering and Science, 1(1), 122-16 (2013)

Biofuel Production-Recent Developments and Prospects, 415-436(2011)