Study of solids and surfaces by photoacoustic spectroscopy

173

JoumalofMolecuhrStructure,79(1982)173-176 E&evier9cientificPublishing Company,Amsterdam-_PrintedinTheNetherlands

STUDY OF SOLIDS AND SURFACES

BY PHOTOACOUSTIC

SPECTROSCOPY

C.N.R. RAO, P. GANGULY AND K. .JAGANNATHA.N

Solid

State & Structural

Bangalore-

Chemistry

Unit,

Indian Institute

of Science,

(India)

ABSTRACT Photoacoustic a variety

spectroscopy

of solids.

dyes, amorphous several by

by

oxide materials.

study

this

The systems

chalcogenides

of dye

Acidic

studied

on

to study the electronic

include powders

of intensely

and oxide gels besides polycrystalline

Surface

adsorption

active oxides.

has been employed

sensitivity

oxide

surfaces

of the and

technique

determination

and basic sites on catalyst

spectra

of

coloured

samples

of

has been examined of surface

areas

surfaces have also been

of

estimated

technique.

INTRODUCTION Photoacoustic optical

spectroscopy

spectra

methods.

The

of solids technique

surface phenomena. of solids

and

laboratory RESULTS

AND

which

is surface

employing

an

sensitive

and

indigenous

also

the spectrum

characteristic the spectrum

of CrpOg,

samples

Cr3+ show characteristic

is

by conventional

likely

to become

to study spectroscopic

useful

some of the results

spectrometer

LaCr03

bands of

to study

of our

fabricated

of oxides. coloured

to obtain.

systems can be avoided

various

the transition

of PbCrO4 which is an intensely

highly absorbing

and

although with poor resolution.

spectra of Fe3+ in a variety

spectra of such dyes are difficult

preparing

to examine

technique

in

studies

this

DISCUSSION

containing

with

difficult

useful

1).

PAS of polycrystalline

gives

are

as a very

In this article, we describe

surfaces

(ref.

has emerged

(PAS)

other

oxide

metal ion;

systems

chromia

We could similarly

gel

obtain

We have been able to record dye;

transmission

optical

The problem of signal saturation by mixing with inert absorbers

G:

by

thin films.

We have recorded

the spectra of a series of arsenic

absorption

eaesshift

absorption

edges are associated

to longer wavelengths

0022-2860/82/000~000/$02.75@1982

chalcogenide

in the amorphous

state

with intensity maxima as expected, Elsevier ScientificPubLishing Company

glasses.

The

(Fig. 1).

The

but the maxima

174

are not prominent

in some cases.

It must be noted that

obtain spectra of such chalcogenides

Fig. 1.

Normalized

PA spectra of amorphous

We have recorded absorbed

an observation and Gersho

difficult

to

spectroscopy.

As2S3 B and such dyes

The spectra show differences

the surface sensitivity

is that monolayers

is

blue, rhodamine

and other oxide surfaces.

oxide surface suggesting

it

or reflectance

and crystalline

the spectra of methylene

on aldna

interesting

by transmission

of the technique.

of dyes show unexpected

which is not readily understood

with the

What is more

phase-dependence

(Fig. Z),

in terms of the theory of Rosencwaig

(ref. 2).

Intensities

of PAS bands of crystal violet absorbed

the concentration feature to monitor

of the dye in solution.

We have been able to exploit

surface areas of oxides.

of the intensities

when there is complete

at maximum

coverage

this

We show in Fig. 3 how the normalized

intensity of the 580 nm band varies with the initial dye reaching a maximum

on oxide surfaces vary with

solution concentration

coverage of the surface.

for any alumina

of the

The ratio

samples is found to correspond

to the ratio of their surface areas. We have investigated

the use of PAS in determining

catalysts by n-butylamine catalyst employing n-butylamine,

titration.

bromocresol

green

Typical

surface acidities

spectra obtained with a H2S04/Si02

are shown in Fig. 4.

two bands at 550 nm and 420 nm are observed.

corresponds

to the acid form for the first titration range

indicator.

In the second titration

region,

of oxide

Before

the addition of

The 550 nm band (lower pKhg+) of the

the band at 600 nm corresponds

to the

175

A,nm

Fig. 2 (a) Normalized PA spectrum of phenolphthalein absorbed on 2% Na20 - Al203 surface from an ether solution. (b) Phase <&I) dependence of the signal.

r 0.06

fs 0.06 & z 0.04 m 8 0.02

n1. Q.01 OS g of Crystal Violet

i per HI0mlsdn

Fig. 3. PA intensity versus concentration was adsorbed on an alumina surface.

of solution

from which crystal violet

176

A,nm

Fig. 4. PA spectra of bromocreson green adsorbed on HgS04/Si02 amount of n-butylamine (in millimole/g of solid).

base

form

it has We

while

been

have

the band

possible

also

of Ni/A1203,

employed

at

420

run corresponds

to determine PAS

acidity

to study

dimethylglyoxjme

was

metal

to the

distribution states

as a function

acid

form.

of

several

on oxide

used

to identify

Ni

National

Science

Academy

By this oxide in

surfaces;

on the

of

procedure

catalysts. the case

surface.

ACKNOWLEDGEMENT The

authors

thank

the

Indlan

for

support

of this

research. REFERENCES 1 2

P. Ganguly and C.N.R. Rae, Proc. Indian Acad. Sci (Chem. Sci), A. Rosencwaig and A. Gersho, J. APQ~. Phys., 47(1976) 64.

the

QO(lQ81)

153.