according to the absorbance monitored at 625 nm (the peak
absorption of indophenol blue). This effect is attributed to the
III
II
regeneration of Fe (clay) from Fe (clay) [eqn. (3)]
FeII(clay) ? FeIII(clay) + e2
(3)
after the photooxidation of DEPD by FeIII(clay) [eqn. (4)]
FeIII(clay) + DEPD ? FeII(clay) + DEPD+
(4)
The concentration ratio of DEPD to a-naphthanol is also an
important factor for the performance of the clay film. Experi-
ments showed that as [DEPD]/[a-naphthanol] approaches 3+2,
a better contrast between the illuminated regions and the
background could be obtained.
This filmed electrode appeared to be reusable since the image
could be erased after dipping in dilute (1 mM) sulfuric acid [Fig.
1(b)] and once the electrode was reexposed a clear image could
be restored [Fig. 1 (c)]. Long-term tests [Fig. 1 (d)], in addition,
Fig. 2 J–V curves for DEPD (1 mM, pH 4): (i) with the electrode prepared
with the bare clay particles, in the dark; (ii) with the clay|AcH electrode, in
+
the dark; (iii) with the electrode as in (a), under illumination; (iv) with the
+
21
+
clay|AcH electrode as in (b), under illumination. Scan rate: 50 mV s
.
showed that although the clay|AcH electrode gradually loses
+
its sensitivity (tentatively ascribed to leaching of AcH ), it
could be reused more than twenty times. In summary, such very
cheap iron-containing clay minerals after proper tailoring, may
be useful materials for image-recording.
We thank the National Science Council, Republic of China
and Chinese Petroleum Company for financial support (grant
number: NSC 88-2113-M-003-016; NSC 88-CPC-M-
0
03-002).
Notes and references
1
2
G. L. Closs and J. R. Miller, Science, 1988, 240, 440.
M. R. Wasielewski, D. G. Johnson, W. A. Svec, K. M. Kersey, D. C.
Cragg and D. W. Minsek, in Photochemical Energy Conversion, ed.,
J. R. Norris, Jr. and D. Mieisel, Elsevier, New York, 1989, pp.
Fig. 3 Time-resolved laser spectroscopy (pH 4, acetic acid–acetate buffer,
+
+
I ≈ 0.1 M, lem = 500 nm) for free AcH (a) and clay|AcH (b).
1
135–1147.
3
4
D. Gust and T. A. Moore, Science, 1989, 244, 35.
M. R. Wasielewski, M. P. Niemczyk, W. A. Svec and E. B. Pewitt,
J. Am. Chem. Soc., 1985, 107, 5562.
AcH+*(clay) + Fe2+(clay) ? AcH.(clay) + Fe3+(clay)
The lack of an electron transfer product from AcH * in the clay
(2)
+
.
5 E. Danielson, C. M. Elliot, J. W. Merkert and T. J. Meyer, J. Am. Chem.
Soc., 1987, 109, 2519.
6
7
is believed to be a consequence of the fast reaction of AcH with
3+
III
Fe species in the Fe sites in the clay. To prevent the back
reaction of eqn. (2) from occurring, an anodic regeneration for
M. A. Fox, Adv. Photochem., 1986, 13, 237.
Chemical Sensors, ed., T. E. Edmonds, Chapman & Hall, New York,
+
13
.
AcH (at E° ≈ 20.4 V vs. SCE, pH 4) from AcH appears to
1
988.
U. Schaller, E. Bakker, U. E. Spichiger and E. Pretsch, Anal. Chem.,
994, 66, 391.
S.-C. Shyu and C. M. Wang, J. Electrochem. Soc., 1997, 144, 3419.
be necessary.
8
9
The photoquality in terms of the contrast between the blue
regions and the background is a function of the electrode
potential, pH and the concentration ratio of the reactants. For
instance, a high pH environment greatly favored the formation
of indophenol blue. However, for a solution pH of > 6, the
formation of indophenol blue became too rapid to be controlled.
As a consequence, a lower pH environment (pH 4), although
less favorable for the formation of indophenol blue on the clay
electrode, was, in turn, less interfering. The electrode potential
is another key factor determining the image quality. Control
experiments showed that the formation of indophenol blue
tended to a limiting rate as the electrode potential approached
1
10 S.-C. Shyu and C. M. Wang, J. Electroanal. Chem., 1997, 440, 27.
11 S.-C. Shyu and C. M. Wang, J. Electrochem. Soc., 1998, 145, 154.
12 C. F. Cheng and C. M. Wang, J. Electroanal. Chem., 1999, 466, 82.
1
3 Y. W. Teng, I.-J. Chang and C. M. Wang, J. Phys. Chem. B, 1997, 101,
0386.
4 C. S. Ouyang and C. M. Wang, J. Electrochem. Soc., 1998, 145,
654.
1
1
2
1
1
5 R. D. Theys and G. Sosnovsky, Chem. Rev., 1997, 97, 83.
6 Braving the Elements, ed., H. B. Gray, J. D. Simon and W. C. Trogler,
University Science Books, Sausalito, 1995, pp. 300–314.
III/II
(clay)
10
0
.4 V vs. SCE (i.e. the formal potential of Fe
)
Communication a908503c
28
Chem. Commun., 2000, 27–28