Investigation of photoelectrocatalytic activity of Cu2O nanoparticles for p-nitrophenol using rotating ring-disk electrode and application for electrocatalytic determination

Article abstract of DOI:10.1016/j.electacta.2010.09.051

A cuprous oxide (Cu₂O) nanoparticles modified Pt rotating ring-disk electrode (RRDE) was successfully fabricated, and the electrocatalytic determination of p-nitrophenol (PNP) using this electrode was developed. Cu₂O nanoparticles were obtained by reducing the copper-citrate complex with hydrazine hydrate (N₂H₄·H ₂O) in a template-free process. The hydrodynamic differential pulse voltammetry (HDPV) technique was applied for in situ monitor the photoelectrochemical behavior of PNP under visible light using nano-Cu ₂O modified Pt RRDE as working electrode. PNP undergoes photoelectrocatalytic degradation on nano-Cu₂O modified disk to give electroactive p-hydroxylamino phenol species which is compulsive transported and can only be detected at ring electrode at around 0.05 V with oxidation signal. The effects of illumination time, applied bias potential, rotation rates and pH of the reaction medium have been discussed. Under optimized conditions for electrocatalytic determination, the anodic current is linear with PNP concentration in the range of 1.0 × 10^-5 to 1.0 × 10 ^-3 M, with a detection limit of 1.0 × 10^-7 M and good precision (RSD = 2.8%, n = 10). The detection limit could be improved to 1.0 × 10^-8 M by given illumination time. The proposed nano-Cu₂O modified RRDE can be potentially applied for electrochemical detection of p-nitrophenol. And it also indicated that modified RRDE technique is a promising way for photoelecrocatalytic degradation and mechanism analysis of organic pollutants.

Full text of DOI:10.1016/j.electacta.2010.09.051

Electrochimica Acta 56 (2010) 953–958
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Investigation of photoelectrocatalytic activity of Cu2O nanoparticles for
p-nitrophenol using rotating ring-disk electrode and application for
electrocatalytic determination
Yong-e Gu, Yuzhen Zhang, Fengyuan Zhang, Jinping Wei, Chunming Wang∗, Yongling Du, Weichun Ye
Department of Chemistry, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2010
Received in revised form
14 September 2010
Accepted 16 September 2010
Available online 24 September 2010
A cuprous oxide (Cu2O) nanoparticles modified Pt rotating ring-disk electrode (RRDE) was successfully
fabricated, and the electrocatalytic determination of p-nitrophenol (PNP) using this electrode was devel-
oped. Cu2O nanoparticles were obtained by reducing the copper–citrate complex with hydrazine hydrate
(N2H4·H2O) in a template-free process. The hydrodynamic differential pulse voltammetry (HDPV) tech-
nique was applied for in situ monitor the photoelectrochemical behavior of PNP under visible light using
nano-Cu2O modified Pt RRDE as working electrode. PNP undergoes photoelectrocatalytic degradation
on nano-Cu2O modified disk to give electroactive p-hydroxylamino phenol species which is compulsive
transported and can only be detected at ring electrode at around 0.05 V with oxidation signal. The effects
of illumination time, applied bias potential, rotation rates and pH of the reaction medium have been dis-
cussed. Under optimized conditions for electrocatalytic determination, the anodic current is linear with
PNP concentration in the range of 1.0 × 10−5 to 1.0 × 10−3 M, with a detection limit of 1.0 × 10−7 M and
good precision (RSD = 2.8%, n = 10). The detection limit could be improved to 1.0 × 10−8 M by given illu-
mination time. The proposed nano-Cu2O modified RRDE can be potentially applied for electrochemical
detection of p-nitrophenol. And it also indicated that modified RRDE technique is a promising way for
photoelecrocatalytic degradation and mechanism analysis of organic pollutants.
Keywords:
Cu2O
Photoelectrocatalysis
Rotating ring-disk electrode
Determination
p-Nitrophenol
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
nanoporous gold [12] and zircomia (ZrO2) nanoparticles [13]. How-
ever, no publication has reported on using bare ring nano-Cu2O
p-Nitrophenol is one of the priority pollutants included in
the US environmental protection Agency (EPA) due to their toxic
effects on humans, animals and plants [1]. Generally, there have
been some methods for degradation of p-nitrophenol includ-
ing biodegradation [2], photocatalytic destruction [3], microwave
assisted oxidation [4], electrocatalytic oxidation [5] and photoelec-
trocatalytic degradation [6]. At present, various methods have been
developed for detection of p-nitrophenol and intermediates of PNP
degradation, such as liquid chromatography and GC–MS methods
[7] are commonly used. For detection of p-nitrophenol, electroana-
lytical techniques based on various chemically modified electrodes
[8–10] have been proposed for direct PNP determination, with the
analytical signal being derived from the 4-electron reduction of the
nitro group. Several applications and electroanalytical methodolo-
gies employing microelectrode as working electrode for indirect
quantification via the hydroxylamine absorbed product of PNP
were recently reported, such as multiwall carbon nanotube [11],
modified disk Pt RRDE for determination of PNP via monitoring the
intermediates of photoelectrocatalytic degradation.
As a p-type semiconductor with a band gap of 2.17 eV [14], Cu2O
is a promising material with potential applications in solar con-
version [15], catalysis [16], and sensing [17]. Besides, Cu2O has a
great potential application in photocatalytic degradation of organic
pollutants under visible light [18–20]. Some electrocatalytic behav-
iors of Cu2O have also been investigated [21,22]. Our previous
work [23] has proved that Cu2O exhibited potential photoelec-
trocatalytic property. The rotating ring-disk electrode technique
has become one of the important tools for detection of reaction
intermediates and study of electrode reaction mechanism since
Frumkin and Nekrasov proposed in 1957. This is a good “Occurred
– collection” technology to use ring electrode to monitor the inter-
mediates formed on disk electrode. Our group has investigated the
photoelectrocatalytic degradation behavior of PNP with nano-TiO2
modified film at rotating ring-disk electrode under UV light [24].
In this study here, we employed a nano-Cu2O modified Pt
RRDE for photoelectrocatalytic degradation of p-nitrophenol, and
found the electrode also could be applied for electrocatalytic
determination of p-nitrophenol via monitoring the intermedi-
∗
Corresponding author. Tel.: +86 931 8911895; fax: +86 931 8912582.
E-mail address: wangcm@lzu.edu.cn (C. Wang).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.09.051

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Y.-e. Gu et al. / Electrochimica Acta 56 (2010) 953–958
ates. The Cu2O nanoparticles were obtained by reducing the
copper–citrate complex with N2H4·H2O under alkaline conditions.
The morphology of the Cu2O was observed using scanning electron
microscope (SEM). The structure and the chemical composition
of the Cu2O nanoparticles were characterized by X-ray diffrac-
tion (XRD) and X-ray photoelectron spectroscopy (XPS). The results
showed that p-nitrophenol was photoelectrocatalytic degraded to
p-hydroxylamino phenol on nano-Cu2O modified disk; the inter-
mediate was simultaneously collected and detected at bare Pt-ring
electrode due to its further oxidation signal at about 0.05 V. And
we found this signal could be applied for electrocatalytic deter-
mination of p-nitrophenol. The present work firstly revealed that
nano-Cu2O has photoelectrocatalytic ability for degradation and in
situ determination of PNP. And it also indicated that modified RRDE
technique could be considered as an important way to investigate
the mechanism of the photoelectrocatalytic degradation of organic
pollutants.
Scheme 1. The schematic of RRDE and the electrode reaction mechanism of p-
nitrophenol occurred on it during photoelectrocatalysis.
2. Experimental
they exhibit rough surface composed of smaller nanoparticles. No
impurities were attached to the surface, attributed to the method
fabricated in a template-free process and without any surfactant.
The XRD pattern is displayed in Fig. 2a. All diffraction peaks in
the pattern can be perfectly indexed as the cubic phase Cu2O (JCPDS
File No. 05-0667). No other diffraction peaks arising from possible
impurities such as Cu and CuO are detected, indicating that the
products are pure Cu2O. The peaks with 2ꢀ values of 29.68◦, 36.54◦,
42.46◦, 61.62◦, 73.86◦ and 77.12◦ correspond to the crystal planes
of 110, 111, 200, 220, 311 and 222 of crystalline Cu2O, respectively.
XPS was used to further investigate the chemical composition of
the Cu2O nanoparticles. The core level Cu2p photoemission spec-
trum is presented in Fig. 2b. It can be clearly observed that two
peaks assigned to Cu2p1/2 and Cu2p3/2 appear at 952.62 and
932.77 eV, respectively, which is corrected with reference to C1s
(284.6 eV). In combination with the main peak appears at 332.66 eV
in the Auger spectrum, it can be confirmed that Cu2O nanoparticles
are highly pure. The result is in good agreement with data observed
for Cu2O [25].
p-Nitrophenol (analytical standard) and nafion-117(5 wt.%) was
purchased from Aldrich reagent company. All of the other com-
pounds were AR grade and used without further purification.
Milli-Q water was used throughout.
The Cu2O nanoparticles were synthesized by reducing
copper–citrate complex with N2H4·H2O under alkaline condi-
tion at room temperature, with template-free and non-surfactant.
In
a
typical procedure, 200 mL of 0.05 M CuSO4·5H2O and
0.1 M C6H5Na3O7·2H2O (Na3Cit), the pH was set to 12.00, adjusted
by additions of 4 M NaOH solution. Then 2.0 mL N2H4·H2O was
added dropwise into the above solution with magnetic stirring for
1 h. The orange yellow power thus obtained. They were collected
and washed with Milli-Q water several times and then dried in a
vacuum oven at 50 ◦C for 12 h.
The photoelectrochemical measurements were carried out at
room temperature in a quartz glass reactor using 50 mL of 0.1 M
Na2SO4 (pH 5.8) as the electrolyte; a rotating ring-disk electrode
system (Pine Instrument Co.) and a CHI 7101c electrochemical
workstation (Shanghai Chenhua Instrumental Co., China) in a four-
electrode system. A daylight lamp (24 W, Shanghai, China) was
located about 50 mm far off the bottom of quartz glass reactor as
visible light source. Solution was purged with N2 for 25 min before
the experiments and a weak stream of N2 was maintained during
the experiments. A fan cooler system was also employed.
A Pt-ring (i.d. and o.d., 7 and 8.6 mm, respectively) electrode
served as working electrode. About 15 ␮L of nafion-Cu2O mixing
solution in ethanol, which was composed of 0.5 wt.% Nafion and
15 mg mL−1 Cu2O by ultrasonic mixing for 1 h, was dropped care-
fully onto the clean Pt-disk electrode (6 mm diameter) and dried in
air. The schematic of working electrodes is shown in Scheme 1. The
counter electrode was a Pt wire, and an Ag/AgCl electrode (satu-
rated KCl) served as a reference electrode. HDPV experiments were
performed with rotation of the working electrode at 1600 rpm and
at a scan rate of 20 mV s−1. The DPV parameters were 50 mV pulse
amplitude and 50 ms pulse width.
3.2. Photoelectrochemical behavior of PNP on nano-Cu2O
modified Pt RRDE
Fig. 3 shows the DPV on bare ring electrode with nano-Cu2O
modified disk and non-modified disk in the absence and presence
of PNP, using 0.1 M Na2SO4 as the electrolyte. For the non-modified
Pt RRDE, the DPV in the absence and presence of 1 × 10−4 M PNP
were recorded as curves a and b. The peak located at about −0.1 V
was related to O2, the inset shows electrochemical behavior of Pt
ring electrode in blank solution with different time of purged N2.
While for the nano-Cu2O modified Pt RRDE, curve c represents the
DPV performance of Pt ring electrode without PNP, which indi-
cates that the peak appeared at about +0.3 V is attributed to the
electrochemical reaction of Cu2O and comes from the formation of
CuO layer. When PNP was added (curve d), an obvious anodic peak
was observed at around 0.05 V, which has been well-defined as
an oxidation signal of generated hydroxylamine to nitroso species
[26].
3. Results and discussion
3.3. Effect of the illumination time
3.1. Characterization of the Cu2O nanoparticles
Fig. 4 shows the corresponding DPV photoelectrochemical
responses in 0.1 M Na2SO4 solution containing 1 × 10−4 M PNP
with different illumination time. The Pt disk electrode modified
with nano-Cu2O retained the bias potential of 0.2 V herein. At the
very start without illumination, the anodic peak at 0.004 V was
observed remarkably. The peak current is much larger than that
Fig. 1a shows the typical top-view SEM image of the as-
obtained Cu2O nanaparticles by reducing copper–citrate complex
with N2H4·H2O. As can be seen, Cu2O nanaparticles were uniformly
dispersed, with diameter of 80–120 nm. Fig. 1b shows the high-
magnification of the Cu2O nanaparticles, we could observe that

Y.-e. Gu et al. / Electrochimica Acta 56 (2010) 953–958
955
Fig. 1. SEM images of Cu2O nanoparticles obtained by reducing copper–citrate complex with N2H4·H2O: (a) at low magnification and (b) at high magnification.
obtained in Fig. 3d (without applied potential), which indicated
that the applied bias potential enhanced the catalytic performance
of Cu2O nanoparticles. The obvious increase of the peak currents
with the more illumination time during 20 min (curves a–e) can
be attributed to the cumulative effects of the intermediate prod-
uct. Whereafter, the anodic peak currents decreased gradually with
more illumination time during the following 40 min (e–i). The
results imply that the further degradation of p-hydroxylamino phe-
nol occurs during the process. Apparently, the p-hydroxylamino
phenol was electrochemically oxidized to p-nitrosophenol.
3.4. Effect of bias potential
Fig. 5 shows the dependence of the anodic peak current at the
Pt ring electrode on continuous illumination time in 0.1 M Na2SO4
solution containing 1 × 10−4 M PNP. Different bias potentials rang-
ing from −0.1 to 0.5 V were applied on the nano-Cu2O modified
Pt disk electrode. As shown in Fig. 5, at −0.1, 0, or 0.1 V, the peak
current reached the max in 30 min and then decreased gradually.
While at 0.2, 0.3, 0.4 or 0.5 V, the peak current reached the max in
20 min, which indicated that the increased positive potential has
more forceful effect on the nano-Cu2O, especially at +0.3 V. The
photocatalytic performance at 0 V of the Cu2O for the degradation
of PNP is relatively slow and ineffective compared with the photo-
electrocatalytic process. The response current at +0.3 V increased
sharply but its change was inconspicuous when decreased, so the
photoelectrocatalytic degradation at +0.2 V is more effective. This
is because when the bias potential E > 0.2 V, Cu2O may be partially
Fig. 3. DPV obtained at bare ring with non-modified (a, b) and nano-Cu2O modified
disk (c, d) Pt RRDE; solutions: (a, c) 0.1 M Na2SO4, (b, d) 0.1 M Na2SO4 + 1 × 10−4
M
PNP. Solutions were purged N2 for 25 min. Inset: non-modified Pt RRDE ring elec-
trode in 0.1 M Na2SO4 with different time of purged with N2: 0 min, 10 min and
25 min.
oxidized to CuO which grows according to a high field mechanism;
Strehblow et al. reported that Cu2O composition was found with
a Cu (II) oxide overlayer at E > 0.2 V [27]. Thus +0.2 V is the opti-
mal bias potential to applied on Cu2O modified Pt disk. The result
Fig. 2. (a) XRD pattern and (b) XPS spectrum of the as prepared Cu2O nanoparticles.

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Y.-e. Gu et al. / Electrochimica Acta 56 (2010) 953–958
Fig. 4. The DPV responses of 0.1 M Na2SO4 solution containing 1 × 10−4 M PNP at bare ring nano-Cu2O modified disk Pt RRDE with different visible light illumination time
(a → i) 0, 2, 5, 10, 20, 30, 40, 50 and 60 min. Applied potential: 0.2 V; rotation rate: 1600 rpm.
exhibited at −0.1 V is not as good as that at 0 V, which reflected that
the negative bias potential applied on the nano-Cu2O modified Pt
disk electrode has counteracted effects on direct photolysis. When
negative bias potential applied, although it can play a promote role
on the electron movement, but it also increase the recombination
probability of the electron–hole pairs and lessen the effective holes.
coefficient and the PNP concentration, respectively, and ꢁ is the
kinetic viscosity of the electrolyte solution. As shown in Fig. 6b, the
anodic peak current is linear with ω1/2, defined k as the slope, thus,
k = 0.62nFAC0D2/3ꢁ−1/6
(3)
0
3.5. Influence of rotation rates and solution pH
The linear regression equation was Ipa = −0.6127 + 0.3963ω1/2
,
Fig. 6a shows the DPV responses of 1 × 10−4 M PNP present for
various rotation rates recorded at bare ring electrode when the disk
electrode was modified with nano-Cu2O and had an applied poten-
tial of 0.2 V. This was operated without illumination as illumination
time has effect on peak current. The data was analyzed using the
with a correlation coefficient of 0.9997, indicating that the elec-
trode progress is controlled by diffusion [28]. The electrons
transfer number was calculated to be 1.8 (approximately 2),
demonstrating that p-hydroxylamino phenol was transferred to p-
nitrosophenol by two-electron-transfer process. The parameters
used in this calculation were the following: diffusion coeffi-
cient D = 4.32 × 10−5 cm2 s−1, concentration of PNP C0 = 1 × 10−4 M,
kinetic viscosity ꢁ = 0.01 cm2 s−1, rotating ring area A = 0.196 cm2.
The electro-reduction of PNP is a proton-participating reaction,
so the effect of pH on Ep and Ip was examined in an electrolyte
composed by 0.1 M Na2SO4 + 1 × 10−4 M PNP. The disk electrode
modified with nano-Cu2O had an applied potential of 0.2 V, oper-
ating under visible light illumination. The successive DPV responses
at different pH value were displayed in Fig. 7a, the magnification
of line groups 3, 4 and 5 was displayed in Fig. 7b. The pH was
adjusted using 0.1 M H2SO4 or NaOH. The results showed that the
potentials shifted negatively with the increase of solution pH, indi-
cating that protons take part in the electrode reaction process. The
anodic peak potential is proportional with the solution pH. The lin-
ear regression equation was Epa = 0.320–0.054 pH, the value of slope
demonstrated that the electrode process is equal proton–electron
transfer involved in the p-hydroxylamino phenol oxidation. The
peak current decreased sharply with the pH increasing. At low pH
(line groups 1 and 2), the currents were high but the electrode
become unstable, the peak currents were decreased much with
scan times. This may be ascribed to that electroreduction of PNP
and redox of its intermediates proceed easily at low pH values than
those of alkaline and neutral conditions [12,29], which results in the
high current. On the other hand, Cu2O was disproportionate easily
at acid condition. When pH was 5.8, we found the peak currents
were increased with scan times, which attributed to the influence
of the visible light. A new peak II appeared at about 0.1 V, which
also existed at low rotation rate in Fig. 6a. We preliminarily specu-
late that this peak is related with adsorption. At pH > 6 (line groups
4 and 5), the peak currents were too low, and the interference of
peak II was serious. Therefore, pH 5.8 was chosen as the optimal
value.
Koutecky–Levich equation [28], plotted as peak current versus ω1/2
,
and the result is shown in Fig. 6b.
1
Ilim
1
ILev
1
IK
=
+
(1)
where
ILev = 0.62nFAC0D2/3
ꢁ
−1/6ω1/2
(2)
0
The parameter I is the measured limiting current, ω is the rota-
tion rate, n, D0 and C0 are the electron number, the diffusion
Fig. 5. The dependence of the oxidation peak currents at Pt ring electrode on con-
tinuous illumination time under different disk bias potential (a) −0.1 V, (b) 0 V, (c)
+0.1 V, (d) +0.2 V, (e) +0.3 V, (f) +0.4 V and (g) +0.5 V.

Y.-e. Gu et al. / Electrochimica Acta 56 (2010) 953–958
957
Fig. 6. (a) DPV responses of 1 × 10−4 M PNP at bare ring Cu2O modified disk Pt RRDE with various rotation rates. (b) Plots of peak current versus ω1/2. Curve numbers represent
rpm rotation rates of: (a) 500, (b) 700, (c) 900, (d) 1200, (e) 1400, (f) 1600 and (g) 1800 rpm. Applied potential: 0.2 V; without visible light illumination.
Fig. 7. (a) Successive DPV responses of 1 × 10−4 M PNP at bare ring nano-Cu2O modified disk Pt RRDE at different pH value (1) 4.15, (2) 5.05, (3) 5.8, (4) 6.48 and (5) 7.25.
Inset shows variation of potential versus pH. Bias potential: +0.2 V, under visible light; rotation rate: 1600 rpm; (b) the magnification of (3) 5.8, (4) 6.48 and (5) 7.25.
3.6. Electrode reaction mechanism of p-nitrophenol on Pt RRDE
mechanism is shown in Scheme 1. The reduction of the nitro group
to hydroxylamine (reaction (1)) had been established with the
successive 2e− steps [30]. The calculated value of electrode process
of p-hydroxylamino phenol oxidation is 2e− and 2H+. The prob-
able electrochemical oxidation reaction of p-hydroxylamino
phenol on Pt ring electrode was described as reaction
(2).
On nano-Cu2O modified disk, PNP undergoes photoelectro-
catalytic degradation to give a p-hydroxylamino phenol species,
the intermediate product is compulsive transported and detected
simultaneously at bare Pt ring electrode at about 0.05 V as
oxidation signal. The schematic of RRDE and possible reaction
Fig. 8. (a) DPV responses of increasing PNP concentration, from a → j, 2, 8, 16, 30, 40, 50, 60, 70, 80, 90, 100 × 10−5 M, respectively. (b) The dependence of Ipa on PNP
concentration.

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3.7. Electrocatalytic determination of PNP
the intermediates and study the photoelecrocatalytic degradation
mechanism of organic pollutants.
Fig. 8 displays the DPV responses of the Pt ring electrode in
increasing concentrations of PNP solution, with nano-Cu2O modi-
fied disk retained the bias potential of 0.2 V. At low concentrations
two peaks are evident, one at 0.05 V and the other at 0.15 V. At
higher concentrations, the peak at 0.05 V dominates. One possible
explanation of this phenomenon is that the peak at 0.15 V is com-
parable to the postwave adsorption peak first studied by Brdicka
[31] and later by Wopschall and Shain [32]. They observed a diffu-
sive peak from the bulk solution and a postwave adsorption peak
from the species adsorbed to the surface of the electrode. The cur-
rent phenomenon we observed is much analogous, so the peak at
0.15 V is identified as the oxidation of adsorbed p-hydroxylamino
phenol species on the Pt ring electrode surface. The peak at 0.05 V
is controlled by diffusion. As stabilized by the adsorption energy
[33], the oxidation of the adsorbed species requires a high energy
and occurs at a more positive potential.
The dependence of Ipa on PNP concentration was given in Fig. 8b.
The results showed that Ipa was proportional to concentration of
PNP in the range of 1.0 × 10−5 to 1.0 × 10−3 M. The linear regres-
sion equation being Ipa (␮A) = 64.1127 + 1.3964C, with a correlation
coefficient of 0.9985. The detection limit was 1.0 × 10−7 M, it could
be improved to 1.0 × 10−8 M if given illumination time. The relative
standard deviation of 10 successive scans was 2.8% for 5.0 × 10−5 M
PNP, indicating excellent reproducibility of the nano-Cu2O modi-
fied RRDE. The result showed that bare ring nano-Cu2O modified
disk Pt RRDE technique could be successfully used to monitor p-
nitrophenol by the oxidation signal of p-hydroxylamino phenol
generated from photoelectrocatalytic degradation.
Acknowledgement
This work was supported by the National natural Science Foun-
dation of China (Grant No. 20775030).
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4. Conclusions
In summary, the elecrocatalytic determination of PNP was suc-
cessfully carried out at bare ring nano-Cu2O modified disk Pt RRDE,
using the oxidation signal of p-hydroxylamino phenol produced by
photoelectrocatalytic degradation of PNP. This intermediate could
be oxidized at low potential thus avoiding matrix interference.
The applied bias potential and visible light enhanced the anodic
peak currents compared to that without light and bias potential.
The present work shows that Cu2O nanoparticles are potentially
applied for monitor the hazardous pollutant p-nitrophenol ascribed
to its photoelectrocatalytic property. Our findings suggest that
Cu2O become a new photoelectrocatalytic material for degra-
dation of organic pollutants in environmental remediation, and
the modified RRDE technique is a promising way for monitor
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