ATR-FTIR and XPS Evaluation of Alkyl Immobilization by Hydrosilylation on n-Si(111) for Photoelectrochemical Cell Electrode

doi:10.4236/jsemat.2013.33A002

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Journal of Surface Engineered Materials and Advanced Technology, 2013, 3, 7-10

http://dx.doi.org/10.4236/jsemat.2013.33A002 Published Online September 2013 (http://www.scirp.org/journal/jsemat)

ATR-FTIR and XPS Evaluation of Alkyl Immobilization

by Hydrosilylation on n-Si(111) for Photoelectrochemical

Cell Electrode

Toshihito Ohtake

Department of Mechanical Systems Engineering, Faculty of Engineering, Aich University of Technology, Gamagori, Japan.

Email: [email protected]

Received July 16th, 2013; revised August 20th, 2013; accepted August 30th, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

An electrode development was needed with high efficiency and low costs as next generation solar cells. We attempted

the electrode fabrication for a photoelectrochemical cell by immobilizing functional groups on a n-Si(111) surface. The

immobilization was applied by hydrosilylation terminated with ester (hydrophobic) or carbonyl (hydrophilic) group.

Results confirmed the immobilization on the Si surface by measuring ATR-FTIR and XPS.

Keywords: ATR-FTIR; XPS; Si Surface; Hydrosilylation; Immobilization; Photoelectrochemical Cell

1. Introduction

We are faced with problems about energy and global

environment, and solar energy has been noticed to solve

them. Solar power generation is studied actively, and

high efficient solar cells are produced recently. However,

there are high costs to be complicated manufacturing

processes or the use of expensive materials for transpar-

ent conductive oxide including indium etc. Hence, solar

cell developments of high efficiently and low costs are

required to spread them largely [1,2].

It is known that photoelectrochemical cell (PEC) by

using n-Si electrode onto supporting with Pt shows high

open circuit photovoltage over 0.6 V comparing with

conventional p-n junction Si solar cell [3]. PEC is inter-

ested with very low costs, but the stability of Si electrode

in electrolytic solution is not so good to form SiO2 layer

on the surface by oxidation of Si.

Researches have been reported about alkyl termination

on Si surface [4-9]. Especially bio-tips for detecting

DNA or protein etc. are applied by immobilizing func-

tional groups [10]. On the other hand, it is known that the

immobilization particularly makes Si surface stable for

oxidation [5,11,12]. Furthermore, controls of electrode

properties are expected for the termination by the func-

tional groups on Si surface.

In this study, termination with ester or carboxy group

was carried out by a hydrosilylation on hydrogen-termi-

nated Si surface. The termination was estimated by ATR-

FTIR and XPS measurement, simultaneously with evalua-

tion of Si surface oxidation.

2. Methodology

A n-Si(111) was performed by RCA washing to termi-

nate with hydrogen after washed with aceton to remove

organic matters. The Si was etched to termination of hy-

drogen by 5% HF solution for 5 min and 40% NH4F for

15 min. The ester termination directly was made use of

the hydrosilylation on the hydrogen-terminated Si in

Scheme 1. The Si was immersed in 0.1 M methyl

propiolate/hexan solution for 24 h at room temperature

under Ar atmosphere in schlenk flask, after washed with

diethyl ether. Furthermore, the immersed time was

Scheme 1. Termination process by hydrosilylation.

ATR-FTIR and XPS Evaluation of Alkyl Immobilization by Hydrosilylation on N-Si(111) for

Photoelectrochemical Cell Electrode

changed from 2 h to 15 h in methyl propiolate/hexan

solution. Carbonyl group termination on the hydro-

gen-terminated Si was performed by the hydrosilylation

with an unsaturated carboxylic acid. Similarly, the Si was

immersed in 0.1 M propiolic acid/hexan solution for 24 h.

The Si surface was measured by MIR-FTIR and XPS,

and cumulative number was 256 and 64 respectively.

3. Results and Discussion

3.1. MIR-FTIR Measurement

The ester termination on Si surface was estimated about

C=C, C=O and Si-H stretching vibration. Figure 1 shows

MIR-FTIR spectra of the n-Si(111) surface terminated

with methyl propiolate by 24 h immersing time in line (a)

and terminated with hydrogen by RCA washing in line

(b). This result would show the ester termination by

applying methyl propiolate because peaks of C=O at

1728 and C=C at 1588 stretching vibration based on the

ester was appeared, and Si-H stretching vibration at 2083

cm−1 was disappeared simultaneously.

Furthermore, Figure 2 shows the spectra of the termi-

nation by a various methyl propiolate immersing time

from 1 to 24 h. We obtained that the intensity of C=O

and C=C peaks was increasing and that of Si-H peak was

decreasing with immersing time. These results would

conclude that the ester termination by the hydrosilylation

progressed instead of the hydrogen termination on Si

surface.

The carbonyl group termination on Si surface was

similarly evaluated in Figure 3 line (a). The spectra

showed peaks of C=O at 1709 cm−1 and C=C at 1586

stretching vibration based on the carbonyl group, hence

the Si surface would be immobilized by the carbonyl

group. Moreover, spectra of the sample applied in 0.01

M NaOH solution obtained a peak of C=O based on

-COO- at about 1550 cm−1 instead of decrease in the

Figure 1. MIR-FTIR spectra of (a); methyl propiolate ter-

mination, (b): hydrogen termination on n-Si(111) surface.

2200 2100 20001900 1800 17001600 1500

C=C

Si-H

C=O

Wavenumbe r/c m-1

Absorbance

0.01

Figure 2. Dependence of immersing time in methyl propi-

olate on MIR-FTIR spectra. The time is 1, 3, 6, 9, 12, 15, 18,

21, 24 h from the front row.

Figure 3. MIR-FTIR spectra on Si surface (a): carboxy

group termination, (b): after immersing in 0.01 M NaOH

and (c): after immersing in 2.0 M HCl subsequently.

peak of C=O based on -COOH at 1709 cm−1 showed in

Figure 3 line (b). Subsequently, spectra of the sample

applied in 2.0 M HCl aq showed the peak of C=O based

on -COOH at 1709 cm−1 instead of disappear at the peak

based on -COO- at about 1550 cm−1 in Figure 3 line (c).

Hence, the carbonyl group terminated on the Si surface

will cause acid-base reaction with the NaOH or HCl so-

lution. Then, a peak of C≡C stretching vibration at 2119

cm−1 disappeared by the acid-base reaction. However the

peak is not clear yet, dimers formed between the immo-

bilized carbonyl group and an unreacted propiolic acid

might exist on the Si surface in Scheme 2. Consequently,

the peak based on C≡C at 2119 cm−1 might be appeared

by the termination, and disappeared by acid-base reac-

tions.

3.2. XPS Measurement

Figure 4 shows XPS spectra of C1s about the ester ter-

mination (a) and the carbonyl group termination (b) on

the Si surface, which indicated three peaks of C-H, C-O

ATR-FTIR and XPS Evaluation of Alkyl Immobilization by Hydrosilylation on N-Si(111) for

Photoelectrochemical Cell Electrode

Scheme 2. Possibility of dimers formed between the immo-

bilized carboxy group and an unreacted propiolic acid.

(a)

(b)

Figure 4. XPS spectra of C1s (a); the ester, (b): the carbonyl

group termination on n-Si(111) surface.

and O=C-O [13]. These results would confirm the termi-

nation by the ester or carbonyl group followed by the

MIR-FTIR measurements in the same way. The oxida-

tion on the Si surface was estimated by the spectra of

Si2p in Figure 5. A spectrum for (a); the hydrogen ter-

minated on the Si surface showed no oxidation at about

103 eV except a main peak at about 100 eV [14]. Al-

though a spectrum for (b); the ester terminated on the Si

surface indicated a peak barely at about 103 eV, the sur-

face would not be effected by the oxidation. A spectrum

for (c); the carbonyl group terminated on the Si surface

was obtained slightly at about 103 eV, which will be

caused by immersing in the propiolic acid/hexan solu-

(a)

(b)

(c)

Figure 5. XPS spectra of Si2p (a); the hydrogen, (b); the

ester, (c); the carbonyl group termination on n-Si(111) sur-

face.

tion. The peak for the carbonyl group termination was

larger than that of the ester termination. This result might

show a difference of hydrophobic or hydrophilic func-

tional groups. These terminated Si would be applied as

the functional electrodes.

4. Conclusion

The hydrosilylation on the hydrogen-terminated Si re-

acted as the ester termination in the propiolate/hexan

ATR-FTIR and XPS Evaluation of Alkyl Immobilization by Hydrosilylation on N-Si(111) for

Photoelectrochemical Cell Electrode

solution, and was confirmed by the MIR-FTIR meas-

urements. Similarly, the carbonyl group termination was

also performed and estimated in the propiolic acid/hexan

solution, and the Si surface was immobilized by the hy-

drophobic or hydrophilic functional groups. Although the

Si surface was hardly oxidized by XPS measurement, it

will be necessary to form more pure Si surface.

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