JP6993787B2 - Evaluation method of photocatalytic activity - Google Patents

Description

The present invention relates to a method for evaluating photocatalytic activity.

Artificial photosynthesis is a reaction technology that obtains hydrogen from water by sunlight in a form that imitates photosynthesis of plants, or obtains organic substances from carbon dioxide as a raw material, and is expected as a dream technology that produces sustainable useful resources. .. In particular, the technology for producing hydrogen from water by photocatalytic reaction contributes most to the cost, and it is said that it is commercially feasible if the solar energy conversion efficiency exceeds 10%.

However, the current solar energy conversion efficiency has not reached the target, and further breakthrough is required to achieve the target. In order to improve the energy conversion efficiency, the absorption wavelength region of the photocatalyst should be expanded to the near infrared region, and the ratio of the number of electrons used in the reaction to the number of absorbed photons, that is, the quantum yield should be improved. Is necessary, and in particular, it is most important to improve the quantum yield of the latter. However, it is said that the decrease in quantum yield is caused by the recombination and deactivation of electrons and holes (hereinafter collectively referred to as "carriers") generated by light irradiation in the photocatalyst. However, the clear factors for improving the quantum yield are unclear.

Therefore, in recent years, attempts have been made to clarify by carrier dynamics analysis of photocatalysts what factors contribute to the improvement of quantum yield, that is, the improvement of photocatalytic activity. For example, in Non-Patent Documents 1 and 2, the life of the generated carrier is measured by the transient absorption spectrum of the photocatalyst, and the carrier life and the photocatalytic activity are discussed. Further, in Non-Patent Document 3, a transmission type transient absorption spectrum is measured for an anatase type titanium oxide nanocrystal film, and a spectrum derived from a carrier is separated and discussed.

M.Salmi, N.Tkachenko, V.Vehmanen, R.Lamminmaki, S.Karvinen, H.Lemmetyinen, (2004) "The effect of calcination on photocatalytic activity of TiO2 particles: femtosecond study," J. Photochem. Photobiol., A. 163, 395-401. A.Yamakata, J.J.M.Vequizo, H.Matsunaga, (2015) ”Distinctive Behavior of photogenerated electrons and holes in anatase rutile TiO2 powders,” J. Phys. Chem., C.119, 24538-24545 T.Yoshihara, R.Katoh, A.Furube, Y.Tamaki, M.Murai, K.Hara, S.Murata, H.Arakawa, M.Tachiya, (2004) ”Identification of reactive species in photoexecited nanocrystalline TiO2 films by wide-wavelength-range (400-2500nm) transient absorption spectroscopy, ”J. Phys. Chem., B.108, 3817-3823

The photocatalytic reaction is caused by charge separation between excited electrons in a high energy state and holes in the photocatalyst, and the excited electrons cause a reduction reaction and the holes cause an oxidation reaction. However, in the conventional analysis method, there is a problem that the specific behavior of excited electrons and holes is not clear for the generated carriers, and therefore the correlation between the photocatalytic activity and the carrier lifetime is unclear.

An object of the present invention is to provide a method for evaluating photocatalytic activity that can clarify the behavior of excited electrons or holes generated in a photocatalyst.

In the present invention, a plurality of procedures 1 for obtaining time-series data of the diffused reflection spectrum of a photocatalyst measured by pump-probe spectroscopy and a plurality of time-series data of the diffused reflection spectrum obtained in the above procedure 1 are applied to a predetermined model. It is a method for evaluating photocatalytic activity, which comprises a procedure 2 for separating into spectral components and a procedure 3 for extracting pump-probe electrons or holes generated in the photocatalyst from a plurality of spectral components separated in the above-mentioned procedure 2.

According to the present invention, the time-series data of the diffused reflection spectrum of the photocatalyst is separated into a plurality of spectral components, and those derived from the excited electrons or holes generated in the photocatalyst are extracted from them, and the spectral components are analyzed. By doing so, the behavior of excited electrons or holes can be clarified.

It is a figure which shows the catalytic action of a photocatalyst. It is a figure which shows the structure of the diffuse reflection spectrum measuring apparatus. It is a figure which shows the irradiation timing of a pump light and a probe light. It is a figure which shows the transient absorption spectrum. It is a figure which shows the time series data of the diffuse reflection spectrum of titanium oxide. It is a figure which shows the time-dependent change of the 1st spectrum component of titanium oxide. It is a figure which shows the time-dependent change of the 2nd spectrum component of titanium oxide. It is a figure which shows the time-dependent change of the 3rd spectrum component of titanium oxide. It is a figure which shows the 2nd spectrum component of a sample 1. FIG. It is a figure which shows the 2nd spectrum component of a sample 2. It is a figure which shows the 2nd spectrum component of a sample 3. It is a figure which shows the 2nd spectrum component of a sample 4. It is a graph which shows the relationship between the time constant of the 2nd spectrum component, and the oxygen generation rate.

Hereinafter, embodiments will be described in detail.

As shown in FIG. 1, when the photocatalyst C is irradiated with light of a predetermined wavelength, the electron e ⁻ in the ground state absorbs the light and becomes an excited state having a high energy level. Charges are separated into the excited electron e ^- and the hole h ⁺ after it is removed. Then, the photocatalytic reaction is promoted by the carriers consisting of these excited electrons e- ^and holes h ⁺ . For example, when the photocatalyst C is in contact with the aqueous phase W, the excited electrons e ^- are utilized in the reduction reaction. Hydrogen H ₂ is generated from hydrogen ion H ⁺ , and hole h ⁺ is used for the oxidation reaction to generate hydrogen ion H ⁺ and oxygen O ₂ from water H ₂ O. On the other hand, the excited electrons e- ^and hole h ⁺ of the carriers generated in the photocatalyst C move within the same energy level, respectively, or are inactivated by recombination, and are directly involved in the photochemical reaction. It also shows behaviors that are not involved and that inhibit photochemical reactions. In order to effectively promote the photocatalytic reaction by the photocatalyst C ^, it is desirable to use the ^{photocatalyst} C having high photocatalytic activity. It is necessary to evaluate the photocatalytic activity of the converted photocatalyst C.

The method for evaluating the photocatalytic activity according to the embodiment that meets the above object includes the following procedures 1 to 3.

(Procedure 1)
In the method for evaluating the photocatalytic activity according to the embodiment, as step 1, time-series data of the diffuse reflection spectrum of the photocatalyst measured by pump-probe spectroscopy, that is, a transient absorption spectrum is obtained.

FIG. 2 shows an example of a diffuse reflection spectrum measuring device 10 used for measuring a diffuse reflection spectrum of a photocatalyst by pump-probe spectroscopy.

The diffuse reflection spectrum measuring device 10 includes a sample table 11, a pump light source 12, a probe light source 13, a lens member 14, a colored glass filter 15, an off-axis parabolic mirror 16, a spectroscope 17, a detector 18, and a data output unit. It is equipped with 19. The spectroscope 17 and the detector 18 are connected by an optical waveguide _LO , and the detector 18 and the data output unit 19 are connected by an electric wire _LE . Further, the pump light source 12 and the probe light source 13 are connected to a light emission control unit (not shown).

The photocatalyst of the sample S to be measured is set in the sample table 11.

The pump light source 12 is composed of a pulse laser that emits a pulsed pump light. Examples of such a pulse laser include a solid-state laser, a semiconductor laser, a fiber laser, and the like. The pump light source 12 is provided in an oblique direction of the sample table, and irradiates the sample S on the sample table 11 with pump light from the oblique direction. The pump light source 12 may be provided in the facing direction of the sample table 11 and may be configured to irradiate the sample S on the sample table 11 with the pump light from the facing direction.

The probe light source 13 is composed of, for example, a white light source, a halogen lamp, or the like that emits pulsed probe light in a predetermined wavelength range. The probe light source 13 is provided in the oblique direction of the sample table 11, and irradiates the sample S on the sample table 11 with the probe light from the oblique direction. The probe light source 13 may be provided in the facing direction of the sample table and may be configured to irradiate the sample S on the sample table 11 from the facing direction. However, from the viewpoint of increasing the amount of diffuse reflected light collected, the probe light source 13 is preferably provided so as to irradiate the sample S on the sample table 11 from an oblique direction.

The lens member 14 is provided so as to face the sample table 11, and collects the diffusely reflected light of the probe light irradiated to the sample S on the sample table 11.

The number of openings of the lens member 14 is preferably 0.10 or more, more preferably 0.20 or more, still more preferably 0, from the viewpoint of increasing the amount of collected diffused reflected light regardless of the wavelength and performing measurement at a high SN ratio. It is .30 or more, and is preferably 0.70 or less, more preferably 0.60 or less, still more preferably 0.50 or less, from the viewpoint of facilitating the incident of the pump light and the probe light without causing steric obstruction. The numerical aperture of the lens member 14 is preferably 0.10 or more and 0.70 or less, more preferably 0.20 or more and 0.60 or less, and further preferably 0.30 or more and 0.50 or less.

Further, in the lens member 14, the difference (absolute value) between the focal length for light having a wavelength of 650 nm and the focal length for light having a wavelength of 800 nm is preferably 50 μm or less from the viewpoint of increasing the amount of condensed reflected light collected regardless of the wavelength. , More preferably 20 μm or less, more preferably 5 μm or less, still more preferably 1 μm or less.

From the viewpoint of measuring at a high SN ratio, the lens member 14 is preferably composed of an objective lens for a microscope provided so that the objective side is the sample S side.

When the lens member 14 is composed of an objective lens for a microscope, the working distance of the lens member 14 composed of an objective lens for a microscope is a viewpoint that facilitates incident of pump light and probe light without causing steric obstruction. Therefore, it is preferably 10.0 mm or more, more preferably 15.0 mm or more, still more preferably 20.0 mm or more, and preferably 50 mm or less from the viewpoint of increasing the amount of condensed reflected light collected regardless of the wavelength. It is preferably 40.0 mm or less, more preferably 30.0 mm or less. The working distance of the lens member 14 is preferably 10.0 mm or more and 40.0 mm or less, more preferably 15.0 mm or more and 40.0 mm or less, and further preferably 20.0 mm or more and 30.0 mm or less. The distance from the tip of the lens member 14 to the sample S on the sample table 11 is preferably the same as this working distance.

Further, the magnification of the lens member 14 composed of the objective lens for a microscope is preferably 5 times or more, more preferably 10 times or more, and more preferably 10 times or more, from the viewpoint of increasing the amount of condensed reflected light collected regardless of the wavelength. From the same viewpoint, it is preferably 50 times or less, more preferably 30 times or less. The magnification of the lens member 14 is preferably 5 times or more and 50 times or less, and more preferably 10 times or more and 30 times or less.

The colored glass filter 15 is provided at a distance from the lens member 14 on the side opposite to the sample S side, and absorbs and cuts an unnecessary wavelength component of the diffuse reflected light from the lens member 14.

The off-axis parabolic mirror 16 is provided at a distance from the colored glass filter 15 on the side opposite to the sample S side, and reflects the diffusely reflected light from the colored glass filter 15 to release the side off-axis. Focuses on the external focal point of the parabolic mirror 16.

The spectroscope 17 has a spectroscope main body 17a including a dispersion element such as a prism or a diffraction grating, an optical fiber cable 17b extending from the spectroscope main body 17a, and a head member 17c provided at the tip thereof, and extends from the spectroscope main body 17a. The light receiving surface of the head member 17c at the tip of the optical fiber cable 17b is positioned at the focal point of the off-axis spectroscopic mirror 16. The spectroscope 17 receives diffusely reflected light from the off-axis parabolic mirror 16 by the head member 17c, takes it into the spectroscope main body 17a via the optical fiber cable 17b, and disperses the diffusely reflected light by the dispersion element.

The detector 18 includes a photoelectric conversion element, and converts the diffuse reflected light dispersed from the spectroscope 17 into electrical signal information by the photoelectric conversion element. It should be noted that this electric signal information corresponds to the light intensity distribution information in the wavelength range of the diffusely reflected light and therefore the probe light.

The data output unit 19 is composed of a computer, performs predetermined data processing on the electric signal information from the detector 18, and outputs a diffuse reflection spectrum.

A method for measuring time-series data of the diffuse reflection spectrum by pump-probe spectroscopy using the diffuse reflection spectrum measuring device 10 having the above configuration will be described.

First, for example, a thick film-like or powder-like photocatalyst sample S is set on the sample table 11. The sample S is preferably provided under vacuum conditions (for example, an atmosphere of ^10-3 Torr or less), and may be placed in a quartz cell or the like.

Then, as in the first wave shown in FIG. 3, a pulsed pump light is emitted from the pump light source 12, and after irradiating the sample S on the sample table 11 with the pulsed light, the probe light source 13 emits a pulsed light. After _a delay time of t1 after emitting the probe light of the sample S and irradiating the photocatalyst of the sample S with the pump light, the sample S on the sample table 11 is irradiated with the probe light.

At this time, in the sample S on the sample table 11, the electrons contained in the photocatalyst are excited when irradiated with the pump light, and the excited electrons are used for the reaction or deactivated with the passage of time to the basal state. However, the state after the delay time _t1 after irradiating the photocatalyst of the sample S with the pump light is detected by the diffusely reflected light of the probe light irradiated and diffusely reflected by the sample S. .. After passing through the lens member 14, the diffusely reflected light is input to the spectroscope 17 through the colored glass filter 15 and the off-axis parabolic mirror 16 in order, and is dispersed. The diffuse reflected light that has been separated is converted into electrical signal information by the detector 18. On the other hand, at any stage, the reference electrical signal information of the diffuse reflected light dispersed when the sample S is irradiated with only the probe light without irradiating the pump light is obtained. In the data output unit 19, in the wavelength range of the probe light, the transient is changed from the light intensity R ₀ of the diffuse reflected light based on the reference electric signal information and the light intensity R of the diffuse reflected light based on the electric signal information when the pump light is irradiated. The absorption intensity value is calculated as (R ₀ −R) / R ₀ , and as a result, the diffuse reflection spectrum with the delay time t ₁ is output as shown in FIG.

Similarly, as in the second and third waves shown in FIG. 3, the delay time from the irradiation of the sample S with the pump light to the irradiation of the probe light is quantified to t ₂ or t ₃ with respect to the sample S. And irradiate the probe light. At this time, as shown in FIG. 4, the data output unit 19 outputs the diffuse reflection spectrum at the delay times t ₂ and t ₃ .

The measurement of the time-series data of this diffused reflection spectrum is performed after irradiating the photocatalyst of sample S with pump light from the viewpoint of effectively comparing the relationship between the photocatalyst activity and specific carrier species such as excited electrons and holes. The range of nanoseconds or more and 1 millisecond or less is preferably time-divided, and the range of 1 nanosecond or more and 100 microseconds or less is more preferably time-divided. The interval between time division measurements is, for example, 500 picoseconds or more and 10 nanoseconds or less. This interval may be constant or non-constant. From the viewpoint of reducing the amount of measurement data to be stored and achieving both measurement accuracy, it is preferable to set this interval so that it becomes longer as the delay time t becomes longer.

By the transient absorption spectrum measurement as described above, when the photocatalyst titanium oxide is specifically used as the sample S, time-series data of the diffuse reflection spectrum as shown in FIG. 5 can be obtained. In FIG. 5, the diffuse reflection spectra at delay times t of 2 ns, 10 ns, 50 ns, 100 ns, and 200 ns are shown by thick lines.

(Procedure 2)
In the method for evaluating the photocatalytic activity according to the embodiment, as step 2, the time-series data of the diffuse reflection spectrum obtained in step 1 is applied to a predetermined model and separated into a plurality of spectral components.

The time-series data of the diffuse reflection spectrum obtained by the transient absorption spectrum measurement contains information in which the behaviors of the generation and deactivation of excited electrons and holes inside and on the surface of the photocatalyst are all combined. Therefore, in this procedure 2, the time-series data of the diffuse reflection spectrum including the complexed information is separated into a plurality of spectral components.

The time series data of the diffuse reflection spectrum can be applied to, for example, a model generalized by the following equation (1).

ｎ_ｃｏｍｐ：成分数
ｃ_ｌ,λ（ｔ，θ）：ｌ成分の装置に関わるパラメータθ及び時間ｔに依存する関数
ε_ｌ（λ）：ｌ成分のスペクトル

n _comp : Number of components cl _{, λ} (t, θ): Functions related to the device of l components θ and time t-dependent function ε _l (λ): Spectrum of l components

Specifically, when this model is a model in which the number of components is n _comp = 3 and each of them is expressed as the sum of a plurality of spectral components that decrease exponentially independently of each other, the following equation (2) is used. A differential equation as shown can be established, and by solving this, the following equation (3) composed of the first to third terms can be obtained.

τ_ｌ：時定数

τ _l : Time constant

ｃ_ｌ ^ＤＡＳ（θ）：ｌ成分の装置に関わるパラメータθに依存する関数

cl ^DAS (θ): A function that depends on the parameter θ related to the device of the _l component.

Then, when the time-series data of the diffuse reflection spectrum when the photocatalyst titanium oxide shown in FIG. 5 is used as a sample is fitted to the model of this equation (3) by the least squares method or the like, the three items 1 to 3 are obtained. Can be separated into spectral components of. Further, assuming that the first to third terms of the formula (3) are sequentially the first to third spectral components, they can be shown separately in FIGS. 6A to 6C, respectively. Further, the time constant τ (time required for the initial transient absorption intensity value to decrease to 1 / e times the initial transient absorption intensity value), which is an index of the rate of decrease of each spectral component, can be obtained. In this case, the time constant τ ₁ = 3 ns of the first spectrum component, the time constant τ ₂ = 59 ns of the second spectrum component, and the time constant τ ₃ = 34 μs of the third spectrum component.

(Procedure 3)
In the method for evaluating the photocatalytic activity according to the embodiment, as the procedure 3, those derived from the excited electrons or holes generated in the photocatalyst are extracted from the plurality of spectral components separated in the procedure 2.

In this procedure 3, for each of the plurality of spectral components separated in step 2, the relationship between the excited electrons generated in the photocatalyst and each of the holes is verified, their attribution is performed, and those derived from the excited electrons or holes are extracted. And associate.

This extraction can be performed on a photocatalyst in the presence of an electron scavenger or hole scavenger based on a contrast with a plurality of spectral components obtained according to steps 1 and 2. That is, the spectral component whose time constant τ becomes smaller in the presence of the electron scavenger is predicted to have shortened the lifetime of excited electrons due to the presence of the electron scavenger. can. Further, the spectral component whose time constant τ becomes smaller in the presence of the hole scavenger is predicted to have shortened the life of the hole due to the presence of the hole scavenger, and thus can be attributed to the one derived from the hole. Further, the spectral component in which the time constant τ does not change regardless of the presence of the electron scavenger or the hole scavenger can be attributed to that derived from the carrier inside the photocatalyst. Here, examples of the electron scavenger include oxygen, methylviologen, and the like. Examples of the hole scavenger include methanol, thiocyanate and the like.

Specifically, the time constant τ ₂ is obtained for the second spectral component shown in FIG. 6B, which is obtained by separating the time series data of the diffused reflection spectrum when the photocatalyst titanium oxide shown in FIG. 5 is used as a sample into three spectral components. Is 59 ns, but it has been confirmed that the time constant τ ₂ is as short as 6 ns in the presence of methanol, which is a highly reactive chemical species with holes on the surface of the photocatalyst, that is, in the presence of a hole trapping agent. Therefore, this second spectral component can be attributed to and extracted from the hole.

This extraction can also be performed on a photocatalyst carrying an cocatalyst capable of storing excited electrons or holes, based on a contrast with a plurality of spectral components obtained according to steps 1 and 2. That is, since it is expected that the life of the holes in the photocatalyst is extended, the component whose time constant τ is increased by supporting the co-catalyst capable of storing excited electrons can be attributed to those derived from the holes. can. In addition, components whose time constant τ is increased by supporting a co-catalyst capable of storing holes are expected to have a longer lifetime of excited electrons in the photocatalyst, so they should be attributed to those derived from excited electrons. Can be done. Furthermore, the spectral component whose time constant τ does not change with either the excited electrons or the co-catalyst capable of storing holes can be attributed to those derived from the carriers inside the photocatalyst. Here, examples of the co-catalyst capable of storing excited electrons include platinum, gold, and silver. Examples of the co-catalyst capable of storing holes include cobalt oxide, iridium oxide and the like.

According to the method for evaluating the photocatalytic activity according to the above embodiment, the time-series data of the diffused reflection spectrum of the photocatalyst is separated into a plurality of spectral components, and the excited electrons or holes generated in the photocatalyst are derived from them. Since the substance is extracted, the behavior of excited electrons or holes can be clarified by analyzing the spectral component thereof.

Then, in this procedure 3, the time constant τ of the spectral component derived from the excited electrons or holes generated in the photocatalyst can be evaluated as a lifetime index of the excited electrons or holes. As a result, it is possible to clarify the correlation between the photocatalytic activity and the lifetime of excited electrons and the lifetime of holes, which are specific carrier species. For example, it can be evaluated that the larger the time constant τ is, the higher the energy state can be obtained without deactivation, and therefore the higher the photocatalytic activity is.

Further, in the photocatalyst, when the pump light is irradiated, the electrons trapped in the trap level between the valence band and the conduction band and the electrons existing in the conduction band level are excited to a higher energy level. It also shows the behavior. Therefore, their behavior is reflected in the spectral components derived from excited electrons. Therefore, for example, when comparing photocatalysts having the same band structure, if there is a spectral component derived from excited electrons that absorbs a large amount on the short wavelength side, it means that the electrons are trapped in a deep trap level. It can be evaluated that the reducing ability is low. In the case of a hall, it can be evaluated in the same way.

Furthermore, since the time constants τ of excited electrons and holes on the surface of the photocatalyst are on the order of nanoseconds and milliseconds, the inventors have divided the time range of 1 nanosecond or more and 1 millisecond or less into the photocatalyst itself. It was found that by measuring the transient absorption spectrum, the time scale directly involved in photocatalytic activity can be analyzed, and the relationship between specific carrier species such as excited electrons and holes and photocatalytic activity can be effectively compared.

In the above embodiment, the model to which the time-series data of the diffused reflection spectrum is applied is a model represented as the sum of a plurality of spectral components, each of which decreases exponentially independently of each other. The present invention is not particularly limited to this, and for example, as shown by the differential equation of the following equation (4), the first spectral component generates the second spectral component, and then the second spectral component is sequentially generated. May be configured as a model in which a third spectrum component is generated and the third spectrum component decreases exponentially, and as shown by the differential equation of the following equation (5), the first spectrum It may be configured as a model in which the first spectral component also generates the third spectral component in parallel with the component generating the second spectral component, and the third spectral component decreases exponentially.

Ｋ_１，Ｋ_２：定数

K ₁ , K ₂ : Constant

Ｋ_１２，Ｋ_１３，Ｋ_２３：定数

K ₁₂ , K ₁₃ , K ₂₃ : Constant

The following configurations are further disclosed with respect to the above-described embodiment.

<1> Procedure 1 for obtaining time-series data of the diffuse reflection spectrum of the photocatalyst measured by pump-probe spectroscopy, and
Step 2 in which the time-series data of the diffuse reflection spectrum obtained in step 1 is applied to a predetermined model and separated into a plurality of spectral components, and
Procedure 3 for extracting excited electrons or holes generated in the photocatalyst from the plurality of spectral components separated in step 2 and
A method for evaluating photocatalytic activity.

<2> The method for evaluating photocatalytic activity according to <1>, wherein the photocatalyst sample is provided under vacuum conditions in the measurement by the pump-probe spectroscopy.

<3> The time-series data of the diffused reflection spectrum is preferably in the range of 1 nanosecond or more and 1 millisecond or less, more preferably in the range of 1 nanosecond or more and 100 microseconds or less after the photocatalyst is irradiated with pump light. The method for evaluating photocatalytic activity according to <1> or <2>, which includes data measured in a time-divided manner.

<4> The model is a model generalized by the following formula (1). The method for evaluating photocatalytic activity according to any one of <1> to <5>.

ｎ_ｃｏｍｐ：成分数
ｃ_ｌ,λ（ｔ，θ）：ｌ成分の装置に関わるパラメータθ及び時間ｔに依存する関数
ε_ｌ（λ）：ｌ成分のスペクトル

n _comp : Number of components cl _{, λ} (t, θ): Functions related to the device of l components θ and time t-dependent function ε _l (λ): Spectrum of l components

<5> The model is a model in which the time-series data of the diffuse reflection spectrum is represented as the sum of the plurality of spectral components in which each of them decreases exponentially independently of each other <1> to <4. > The method for evaluating photocatalytic activity according to any one of.

<6> The method for evaluating photocatalytic activity according to <5>, wherein in the procedure 3, the time constant of the spectral component derived from the excited electrons or holes generated in the photocatalyst is used as the lifetime index of the excited electrons or holes.

<7> The extraction in the procedure 3 is performed based on the comparison of the photocatalyst in the presence of an electron scavenger or a hole scavenger with a plurality of spectral components obtained according to the procedures 1 and 2 <1> to <6. > The method for evaluating photocatalytic activity according to any one of.

<8> The method for evaluating photocatalytic activity according to <7>, wherein the electron scavenger contains oxygen or methylviologen.

<9> The method for evaluating photocatalytic activity according to <7>, wherein the hole scavenger contains methanol or thiocyanate.

(Diffuse reflection spectrum measuring device)
A sub-nanosecond transient absorption spectrometer (manufactured by EOS Ultrafast Systems) is equipped with a titanium sapphire laser (manufactured by Hurricane-X Spectra Physics) and an optical parametric amplifier (manufactured by OPA800CF Spectra Physics) as a pump light source, and a lens member. Objective lens for microscopes (M Plan Apo NIR 20 x Mitsutoyo Co., Ltd., numerical aperture 0.40, working distance 20.0 mm, difference between focal length for light with wavelength of 650 nm and focal length for light with wavelength of 800 nm 0. A diffused reflection spectrum measuring device having the same configuration as shown in FIG. 1 provided with the objective side facing the sample side (70 μm) was prepared.

(sample)
Four types of powdered rutile-type titanium oxide samples from different manufacturers were used as photocatalyst samples 1 to 4, respectively.

(Test method)
<Analysis test of photocatalyst>
The following analytical tests were performed on each of the photocatalyst samples 1 to 4.

About 200 mg of a photocatalyst sample was placed in a quartz cell, the inside was replaced with argon gas, and then the inside of the cell was depressurized to ^10-3 Torr or less and sealed. This was set on the sample table of the diffuse reflection spectrum measuring device.

Then, according to the procedure 1 of the above embodiment, the pump light (wavelength 355 nm, light intensity per pulse 0.15 μJ, pulse width about 200 fs, beam diameter about 20 μm) and probe light (wavelength 390 to 900 nm, pulse) are applied to the sample. The diffuse reflection spectrum by pump-probe spectroscopy was measured in time series by irradiating with a width of about 500 ps and a beam diameter of about 10 μm). The delay time was variable in the range of 1 to 100 μs, the number of plots was 200, and the integration time was 60 minutes.

Subsequently, according to the procedure 2 of the above embodiment, the time series data of the diffuse reflection spectrum obtained in the procedure 1 was applied to the model of the equation (3) and separated into three first to third spectrum components.

Then, as described in the procedure 3 of the above embodiment, since the second spectral component is attributed to the one derived from the hole on the surface of the photocatalyst, attention was paid to each of the second spectral components of the samples 1 to 4.

<Photocatalyst performance test>
The following performance tests were performed on each of Samples 1 to 4.

To a glass reaction vessel, 180 ml of water, a photocatalyst sample of 1.5% by mass based on the water, and 56 μmol of sodium iodate were added, and then the pH was adjusted to 11 with sodium hydroxide.

Then, under an argon gas atmosphere, the sample is irradiated with light having an irradiation light intensity of 250 mW / cm ² from an ultra-high pressure mercury lamp (SX-UID502HUV manufactured by Ushio Denki) via an ultraviolet light transmission visible absorption filter (UG). The generation rate of oxygen gas was quantified by a gas chromatograph (GC-8A manufactured by Shimadzu Corporation) of a TCD detector.

(Test results)
7A-D show the second spectral components of Samples 1-4. Table 1 shows the time constant τ ₂ and the oxygen gas generation rate of each of the second spectral components of Samples 1 to 4.

According to FIGS. 7A to 7D, it can be seen that the spectra of Samples 1, 3 and 4 are similar to each other, whereas only Sample 2 absorbs a large amount at a wavelength of around 420 nm. It is considered that this is because the holes on the surface of the photocatalyst are deeply trapped, and it is predicted that the oxidizing ability is low. Comparing the oxygen generation rates by the water splitting reaction, it can be seen that the oxygen production rate of Sample 2 is extremely lower than that of Samples 1, 3, and 4.

FIG. 8 shows the relationship between the time constant τ ₂ of the second spectral component in the samples 1, 3 and 4 and the oxygen gas generation rate.

According to FIG. 8, in the samples 1, 3 and 4 having similar spectral shapes, it can be seen that the longer the time constant τ ₂ , that is, the longer the life of the holes on the surface of the photocatalyst, the higher the oxygen generation rate. This is because the reaction between the holes on the surface of the photocatalyst and water is the rate-determining process of this reaction. Therefore, extending the life of the holes increases the oxygen generation rate, that is, increases the photocatalytic activity. It is considered to suggest that it contributes directly to. In Sample 2, the time constant τ ₂ is 995 ns, and the life of the holes on the surface of the photocatalyst appears to be very long numerically. However, as described above, the photocatalyst has lost its oxidizing ability. It is considered that the activity is low.

The present invention is useful in the technical field of methods for evaluating photocatalytic activity.

10 Diffuse reflection spectrum measuring device 11 Sample stand 12 Pump light source 13 Probe light source 14 Lens member 15 Color glass filter 16 Off-axis parabolic mirror 17 Spectrometer 17a Spectrometer body 17b Optical fiber cable 17c Head member 18 Detector 19 Data output unit LE _Electric wire L _O Optical waveguide S Sample

Claims (4)

Step 1 to obtain time-series data of the diffuse reflection spectrum of the photocatalyst measured by pump-probe spectroscopy, and
Step 2 in which the time-series data of the diffuse reflection spectrum obtained in step 1 is applied to a predetermined model and separated into a plurality of spectral components, and
Procedure 3 for extracting excited electrons or holes generated in the photocatalyst from the plurality of spectral components separated in step 2 and
It is a method for evaluating photocatalytic activity having
The time-series data of the diffuse reflection spectrum includes data measured by time-dividing the range of 1 nanosecond or more and 1 millisecond or less after irradiating the photocatalyst with pump light.
The model is a model in which the time-series data of the diffuse reflection spectrum is expressed as the sum of the plurality of spectral components, each of which decreases exponentially independently of each other, and is generalized by the following equation (1). can be,

n _comp : Number of components
cl _{, λ} (t, θ): A function that depends on the parameters θ related to the device of the l component and the time t.
ε _l (λ): spectrum of l component
The extraction in the procedure 3 is performed based on the comparison of the photocatalyst in the presence of an electron scavenger or a hole scavenger with a plurality of spectral components obtained according to the procedures 1 and 2.
In step 3, a method for evaluating photocatalytic activity using the time constant of a spectral component derived from excited electrons or holes generated in the photocatalyst as a lifetime index of the excited electrons or holes . The method for evaluating photocatalytic activity according to claim 1 , wherein the electron scavenger contains oxygen or methylviologen. The method for evaluating photocatalytic activity according to claim 1 , wherein the hole scavenger contains methanol or thiocyanate. The method for evaluating photocatalytic activity according to any one of claims 1 to 3 , wherein the photocatalyst sample is provided under vacuum conditions in the measurement by the pump-probe spectroscopy.

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