MnO2 nanoarrays: an efficient catalyst electrode for nitrite electroreduction toward sensing and NH3 synthesis applications

Article abstract of DOI:10.1039/c8cc05837g

It is highly attractive to develop efficient electrocatalysts for the sensitive and selective detection of nitrite. In this communication, we report that an MnO₂ nanoarray on titanium mesh (MnO₂ NA/TM) is an efficient catalyst electrode for the electroreduction of nitrite. Electrochemical measurements demonstrate that the constructed MnO₂ NA/TM sensor offers a superior sensing performance, having a short response time of 3 s, a wide detection range of 1.0 μM to 5.0 mM, a low detection limit of 1.5 nM (S/N = 3), and a response sensitivity of 10?301 μA mM^?1 cm^?2, with satisfactory selectivity, specificity, and reproducibility. This electrochemical system is also capable of catalyzing the electrochemical reduction of nitrite to NH₃ with a transformation efficiency of 6%.

Full text of DOI:10.1039/c8cc05837g

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MnO2 nanoarray: an efficient catalyst electrode for nitrite
electroreduction toward sensing and NH3 synthesis applications
Rui Wang,¶a Zao Wang,¶a,b Xiaojiao Xiang,a Rong Zhang,b Xifeng Shic and Xuping Suna*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
It is highly attractive to develop efficient electrocatalysts for
Manganese oxides are very attractive alternative to noble
sensitive and selective detection of nitrite. In this communication, metal catalysts due to their merits, including molecular
we report that MnO2 nanoarray on titanium mesh (MnO2 NA/TM) adsorption, ion-changing, suitably high density and purity,
is an efficient catalyst electrode for electroreduction of nitrite. electrochemical and magnetic properties.12 Previous reports
Electrochemical measurements demonstrate that the constructed have illustrated that MnO2 nanostructures are potential
MnO2 NA/TM sensor offers superior sensing performances, having catalysts for constructing nitrite sensor, which based on
a short response time of 3 s, a wide detection range of 1.0 μM to electrochemical oxidation of nitrite.13-18 Contrast with anodic
5.0 mM, a low detection limit of 1.5 nM (S/N = 3), a response oxidation, cathodic catalytic reduction can not only avoid
sensitivity of 10301 μA mM–1 cm–2, with satisfactory selectivity, secondary reactions,19 but could produce NH3 as fertilizer. To
specificity, and reproducibility. This electrochemical system is also the best of our knowledge, however, their application toward
capable of catalyzing the electrochemical reduction of nitrite to electrochemical nitrite detection based on cathodic catalytic
NH3 with a transformation efficiency of 6%.
reduction has scarcely reported yet.
In this communication, we report the first use of MnO2
nanoarray supported on titanium mesh (MnO2 NA/TM) as an
efficient catalyst electrode for nitrite reduction under neutral
condition. As an electrochemical sensor for nitrite, such MnO2
NA/TM exhibits superior sensing performance with a rapid
response time of 3 s, a wide range of 1.0 μM to 5.0 mM, a low
detection limit of 1.5 nM (S/N = 3), a response sensitivity of
10301 μA mM–1 cm–2, with satisfactory selective specificity and
reproducibility. Interestingly, this electrochemical system also
offers a nitrite-to-NH3 transformation efficiency of 6%.
As an important necessity for preservatives and fertilizing
reagents for food, nitrites are widely presented in
environment.1 The rapid increasing pollution of ground water
resources for human consumption by nitrites due to the
anthropogenic activities from agriculture (by using nitrogen
based fertilizer) and waste water from industry is receiving
worldwide attention.2 Noticeable, nitrites are an essential
precursor for the formation of carcino genic N-nitrosamine.3
Therefore, it is of great importance to accurately monitor
nitrite for public health, environmental and food industries.
Several analytical techniques have been developed for nitrite
determination including fluorescence,4 chemiluminescence,5,6
capillary electrophoresis,7 and chromatography.8 However,
expensive equipments, tedious detection procedures and
often time consuming have limited the further applications of
these analytical methods. Compared to these methods, the
electrochemical method can provide compact, relatively
inexpensive, reliable, sensitive and real-time analysis.9-11
Fig. 1a shows the X-ray diffraction (XRD) patterns for MnO2
NA/TM, suggesting the presence of MnO2 (JCPDS. 44–0141)
and metallic titanium. The scanning electron microscopy (SEM)
analysis indicates that the entire surface of TM (Fig. S1) is
completely covered by MnO2 nanowires array (Fig. 1b). The
transmission electron microscopy (TEM) image (Fig. 1c) also
confirms the formation of MnO2 nanowire. From the high
resolution TEM (HRTEM) image presents in Fig. 1d, we can
clearly resolve the lattice with an interplanar spacing of 0.236
nm indexed to the (211) plane of MnO2. Energy-dispersive X-
ray (EDX) elemental mapping images (Fig. 1e) demonstrate the
homogenous distribution of Mn and O elements throughout
the entire nanoarray. Fig. 1f shows the X-ray photoelectron
spectroscopy (XPS) survey spectrum for MnO2. The Mn 2p
spectrum (Fig. 1g) exhibits binding energies (BEs) at 641.6 and
653.4 eV eV for Mn 2p3/2 and Mn 2p1/2, respectively, consistent
with the characteristic peaks of MnO2.20 The BEs located at
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531.9 and 530.1 eV are assigned to the O 1s region (Fig. 1h). further illustrates that the cathodic peak current density
These results demonstrate the successful growth of MnO2 increases linearly as the rise of NaNO2 concentration, as shown
nanowires array on TM.
in Fig. 2c. Although previous work reported that platinum wire
as counter electrode could cause anodic dissolution and
subsequent Pt deposition at the working electrode,21 using
graphite rod as the counter electrode in our present study
gives consistent experimental result (Fig. S2). It indicates that
platinum wire has no effect on sensing performance.
Fig. 3a and 3b present typical current-time plots of MnO2
NA/TM electrode in 0.1 M Na2SO4 with consecutive step
changes of NaNO2 concentrations at an applied potential of –
1.75 V. The electrode shows a fast amperometric response
toward NaNO2 and can achieve steady state current density
within 3 s. The calibration curve for our NaNO2 sensor is shown
in Fig. 3c, which gives a linear dependence on NaNO2
concentration in the range of 1.0 μM to 5.0 mM (R2 = 0.9938).
The detection limit and sensitivity were calculated as 1.5 nM
(S/N = 3) and 10301 μA mM–1 cm–2, respectively. These values
compare favorably to the behaviors of most reported
electrochemical NaNO2 sensors (Table S1). Anti-interference is
another important factor for analysis applications.22 The anti-
interference properties were assessed by successive additions
of 1.0 mM NaNO2 and 3.0 mM other interferences in 0.1 M
Na2SO4. As shown in Fig. 3d, these compounds have no
interference on NaNO2 determination.
Fig. 1. (a) XRD pattern for MnO2 NA/TM. (b) SEM images for MnO2 NA/TM. (c)
TEM and (d) HRTEM images for MnO2. (e) SEM and corresponding EDX elemental
mapping images of Mn, O, and Ti for MnO2/TM. (f) XPS survey spectrum for
MnO2. XPS spectra of MnO2 in the (g) Mn 2p and (h) O 1s regions.
Fig. 2. (a) LSV curves of blank TM (curves 1 and 2) and MnO2 NA/TM (curves 3
and 4) in 0.1 M Na2SO4 (pH: 7.0) with the absence (curves 1 and 3) and presence
(curves 2 and 4) of 4.0 mM NaNO2 (scan rate: 30 mV s–1). (b) LSV curves for
MnO2/TM in 0.1 M Na2SO4 with the presence of varied NaNO2 concentrations: 0,
2, 4, 6, 8, 10, and 12 mM (from inner to outer) at a scan rate of 30 mV s–1. (c)
Corresponding calibration curve.
Fig. 3. (a, b) Amperometric responses and (c) the corresponding calibration curve
of MnO2 NA/TM electrode to successive additions of NaNO2 at −1.75 V in 0.1 M
Na2SO4. (d) Amperometric responses of MnO2 NA/TM to the successive addition
of 1.0 mM NaNO2, 3.0 mM Na2SO4, 3.0 mM Na2CO3, 3.0 mM NaAc, 3.0 mM
MgSO4, 3.0 mM Na2SO4, and 1.0 mM NaNO2 at −1.75 V in 0.1 M Na2SO4.
The electrocatalytic properties of MnO2 NA/TM electrode
for NaNO2 reduction were tested with platinum wire as
counter electrode and Ag/AgCl as reference electrode. Fig. 2a
shows linear sweep cycle voltammetry (LSV) curves of MnO2
NA/TM and blank TM in 0.1 M Na2SO4 with the absence (curve
1, 3) and presence (curve 2, 4) of 4.0 mM NaNO2 at a scan rate
of 30 mV s–1 within the potential rang of 0.0 to –2.0 V.
Obviously, blank TM only shows very small increase in cathodic
current after introducing 4.0 mM NaNO2. In sharp contrast,
MnO2 NA/TM offers greatly enhanced cathodic current in the
presence of 4.0 mM NaNO2. These results confirm that MnO2
is efficient for NaNO2 electro-reduction. Fig. 2b shows the LSV
curves of MnO2 NA/TM toward different concentrations of
NaNO2 in 0.1 M Na2SO4. Clearly, the current densities of
reduction peak increase with increased NaNO2 concentration
in a range of 0 to 12 mM. The corresponding calibration curve
Furthermore, the reproductivity of this nitrite senor was
assessed by detecting 10 mM sample with five prepared
electrodes independently, and the relative standard deviation
is 1.5%, showing the satisfactory reproducibility. Considering
potential practical application, we also conducted stability test.
As shown in Fig. S3, the currents percents remains the same
after 30 days, indicating a good stability. In addition,
chronoamperometry experiments were conducted to evaluate
a continuous stability of MnO2 NA/TM in 0.1 M NaSO4
containing 5.0 mM NaNO2 at –1.75 V. As shown in Fig. S4,
there is no obviously decrease within at least 13 h, indicating a
satisfactory stability.
2 | J. Name., 2012, 00, 1-3
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To assess its practicability, MnO2 NA/TM was applied to This work was supported by the National Natural Science
detect NaNO2 in river water and tap water. As observed, the Foundation of China (No. 21575137). We also appreciate Hui
reduction peak current densities increased as the successive Wang from the Analytical & Testing Centre of Sichuan
addition of NaNO2 into the river water and tap water (Fig. S5a University for her helping with SEM characterization.
and S5c). And the derived calibration curve indicated that the
peak current densities increase linearly as the increased
Conflicts of interest
NaNO2 concentration in a range of 0.0 to 5.0 mM (Fig. S5b and
S5d). These observation strongly demonstrate that the MnO2
There are no conflicts to declare.
NA/TM electrode is a good candidate for practical samples
analysis.
NH3 is an essential building block for manufacturing
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performances for nitrite detection, but it is also effective to
convert nitrite to NH3 with a transformation efficiency of 6%.
This work would open up an exciting new avenue to explore
the use of transition metal oxides as an attractive catalyst
materials for electroreduction of nitrite toward sensing and
NH3 synthesis applications.
Acknowledgements
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