実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report

提出日 Date of Report

課題番号 Project No.

装置責任者 Name of responsible person

2012A0023

Norifumi Yamada

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Neutron reflectometry of the adsorbed hydrogen and the oxide film

BL 16

on Pt polycrystalline electrode

実施日 Date of Experiment

実験責任者名 Name of principal investigator

11/7/2012～11/10/2012

Nagahiro Hoshi
所属 Affiliation
Graduate School of Engineering, Chiba University
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.
Pt film (100 nm) deposited on Si(111) block (36 mmφ、2 mm thick) doped with P.

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

<Purpose>
Pt electrodes give redox peaks in the voltammograms before the onset potential of hydrogen evolution.

Thesepeaks have been ascribed to the adsorption and desorption of hydrogen atoms. Adsorbed hydrogen atoms affectthe catalytic activity remarkably; elucidation of the nature of adsorbed hydrogen atoms is important for theunderstanding of the origin of electrocatalysis.

However, the existence of the adsorbed hydrogen has not beenverified on Pt electrodes in electrochemical environments. In this research subject, we have tried to detect theadsorbed hydrogen on Pt electrode using neutron reflectometry.
<Experimental Methods>
Si(111) is chemically polished to an atomically flat surface. Pt film (100 nm thick) is deposited on Si(111)
using ion beam coater.
Electrochemical cell for neutron reflectometry was made of quartz. The design of the cell is shown in Fig. 1.
Counter and reference electrodes are Pt wire. Electrolytic solution is 0.1 M DClO4 in D2O.
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2. 実験方法及び結果（つづき） Experimental method and results (continued)
In the neutron reflection measurements, the scattering lengthof the electrolytic solution is controlled to be the same as that of
the Pt film by changing the ratio of H2O/D2O at hydrogen freepotentials.
<Results and Discussion>
Fig. 2 shows voltammogram of Pt/Si(111) electrode in the cellfor neutron reflectometry.
solution.

O2 gas in the air is dissolved in theThe base line of the voltammogram is tilted because

the current due to the O2 reduction reaction is overlapped withthe voltammogram.
A reduction peak due to the adsorption of hydrogen (H+ + Pt+ e- → H-Pt) is found below -0.25 V (vs Pt). An oxidation
peak below -0.5 V (vs Pt) originates from the desorption of+-hydrogen (H-Pt → H + Pt + e ). Small oxidation peak above

Fig. 1

Scheme of electrochemical cell forneutron reflectometry.

0 V(vs Pt) can be assigned to the formation of the oxide film（PtOH）. Therefore, we measured a background data without adsorbed hydrogen at 0 V (vs Pt).
Fig. 3 shows neutron reflectometry profiles at various potentials.

The profile at 0 V (vs Pt), where nohydrogen is adsorbed, gives no fringe in 0.1 M DClO4 in 100% D2O. At -0.70 and -0.80V (vs Pt) where thecoverage of adsorbed hydrogen is high, the profiles are almost the same as that at 0 V (vs Pt). The profile doesnot change at 0.60 V(vs Pt) at which the oxide film PtOH will be formed.
We cannot detect the neutron reflectometry profile of adsorbed hydrogen in these experiments.

Theroughness of the Pt film prepared using ion beam coater is ±1 nm. The surface is too roughened to observe avery thin Pt-H layer.

We must prepare atomically flat Pt film on Si block to measure neutron reflection ofadsorbed hydrogen atoms.

Fig. 2 Voltammogram of Pt/Si(111) in 0.1 M
Fig. 3 Neutron reflectometry profile of Pt/Si(111) atDClO4 in 100% D2O.

various potentials (0.1 M DClO4 in D2O).
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