Enhanced selectivity of methane production for photocatalytic reduction by the piezoelectric effect

Article abstract of DOI:10.1039/c7cc05112c

Under simultaneous full arc light and ultrasonic irradiation, photo-generated electrons are brought together by piezoelectric potential, and thus dense electrons induce the reduction of carbon dioxide on the surface of piezoelectric semiconductors, result

Full text of DOI:10.1039/c7cc05112c

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Enhanced Selectivity of Methane Production for Photocatalytic
Reduction by Piezoelectric Effect
Received 00th January 20xx,
Accepted 00th January 20xx
Kaiqiang Wang, Zhibin Fang, Xueyan Huang, Wenhui Feng, Yaozhu Wang, Bo Wang, and Ping Liu*
DOI: 10.1039/x0xx00000x
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Under simultaneous irradiation of full arc light and ultrasonic, the centers that results in an electric field.11 The origin of the non-
photo-generated electrons are brought together by piezoelectric zero electric field is a break in the inversion symmetry which
potential, thus dense electrons involve the reduction of carbon lead to electric dipoles within the material, the strength of
dioxide on the surface of piezoelectric semiconductor, resulting in which changes as the material is strained.11 As already
the improved selectivity of methane production to a greater reported, the piezopotential can effectively influence charge
degree.
transport resulting in significantly enhanced optoelectronic
performances. This is known as the piezophototronic effect
presence in piezoelectric-semiconductor materials.12-19 It is
generally accepted that objects will migrate and gather
together with a specific direction when confronted with a
force, and so do the photo-generated electrons and holes. If
Reduction of CO2 has attracted much attention under the
current background of the depletion of fossil resources and
the increase of emissions of CO2. The photocatalytic
conversion of CO2 using solar energy is the most promising
route for the transformation of CO2 to specific fuels and
chemicals.1 However, one of the severe problems in limiting
practical applications of CO2 photoreduction technology is the
low selective yields of particular products. To solve this issue,
much effort has been made, such as loading co-catalysts,2-7
doping ions or metals,8 and optimizing synthesized methods of
catalysts.9, 10 Among various treatment solutions, loading co-
catalysts is the most attractive technology for its green and
facile feature. It has been demonstrated that the
photocatalytic selectivity of particular products can be
influenced by loading co-catalysts.2-7 For example, methane
was selectively produced by loading metallic copper acted as
electron traps resulting in an increased probability of multiple
electrons reactions.2 The method of loading co-catalysts has
shown improved effect greatly, while the high cost and
stringent operation on loading co-catalysts may restrain the
practical applications at industrial level. The demand for new
low-cost and effortless technology in enhancing the selective
yields of particular chemicals is rapidly increasing.
the photo-generated carriers are introduced into
a
piezoelectric field, their movements may be influenced by the
piezopotential formed in piezoelectric field. Similar to the
above-mentioned impact of metallic copper, the local density
of photo-generated electrons may be increased by the
oriented driving effect of piezopotential, which would be
beneficial for the multi-electron reactions.
Herein, for the first time, we establish a novel fundamental
piezo-photocatalytic mechanism that can selectively convert
carbon dioxide to methane under both photonic and
mechanical energy through coupling the piezoelectric and
photocatalytic properties of ZnO nanorods. With the
assistance of piezopotential, the selectivity of methane
production is improved. Such an economic and green method
of energy treatment shows greatly potential applications in
harvesting mechanical energy in the natural environment, such
as noise, vibration, wind, tide, sonic wave and atmospheric
pressure.18, 20-23
The hydrothermal or solvothermal methods are low-cost
and convenient, yet robust methods for the synthesis and
morphological transformation of many nanostructures.24 In the
current study, ZnO nanorods were prepared by a solvothermal
method.25 The XRD pattern of pure ZnO is given in (Fig. S1a,
ESI†). All the diffraction peaks can be well-indexed to
hexagonal phase of ZnO (space group: P63mc) crystal. As
shown in the original SEM image (Fig. 1a), straight and smooth
rods with ca. 20-30 nm in diameter and 0.5–1 μm in length are
synthesized successfully. The HRTEM image in Fig. 1b obviously
Piezoelectrics are a class of functional materials which have
great potential in many fields. Under applied deformation,
there is a separation between the positive and negative charge
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on
Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002,
P. R. China. E-mail: liuping@fzu.edu.cn;
Tel & Fax number: +86-591-83779239.
†
Electronic Supplementary Informaꢀon (ESI) available: Sample preparation,
evolution tests of gas-phase products, detailed characterization, and Fig. S1
See DOI: 10.1039/x0xx00000x
–S8.
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photoreduction process, eight electrons are required in the
formation of each CH4 molecule,26-30 and only two electrons
are needed for each CO molecule evolution,1, 28-31 which means
the transformation of CH4 need more electrons than that of
CO. In our experiment, CO is detected as the major gas-phase
product while CH4 as the minor product. Therefore, the
photocatalytic reduction of CO2 to CO may be a dynamic
favored process in the present system.32 In order to
demonstrate the reduction of CO2 is induced by photocatalytic
progress, we carry out several blank experiments under L and
L+S irradiation. CO and CH4 are negligibly detected in the
absence of the following three substances individually, which
are catalyst, full arc light irradiation and CO2. These related
results confirm that CO and CH4 are formed by the
photocatalytic reduction of CO2 using ZnO NRs.
confirm that ZnO sample grow along the direction of [0 0 0 1]
plane forming a rod-like structure with a typical lattice of 0.26
nm. The optical absorbance of ZnO NRs is measured by UV-
visible absorption spectra (Fig. S2, ESI†). ZnO NRs have a wide-
band gap (3.15 eV at room temperature) and almost do not
absorb visible light. Hence, ultraviolet light is essential for the
optical irradiation.
Fig. 1 Morphology of the solvothermally synthesized ZnO NRs
with (a) SEM image and (b) HRTEM image.
To demonstrate the effect of ultrasonic irradiation on the
photocatalytic performance of ZnO NRs, the reductive reaction
of CO2 is evaluated respectively under ultrasonic irradiation
(40 kHz, 50 W), full arc
（UV-Vis ）light irradiation, and
Fig. 2 Reduction of CO2 by 20 mg ZnO NRs with (a) productive
rates of CO and CH4 and (b) selective percentage of CH4 under
different conditions.
simultaneous irradiation of full arc light and ultrasonic. In our
approach, the piezopotential in ZnO NRs is periodically altered
by applying ultrasonic wave, the schematic diagram of reaction
device is illustrated in (Fig. S3, ESI†). In our experimental
conditions, CO and CH4 are dectected as the gas-phase
products of reducing CO2. As shown in Fig. 2a, the productive
rates of CO and CH4 are 0.492 μl∙h-1 and 0.0391 μl∙h-1
respectively under full arc light irradiation (L). While under the
condition of simultaneous irradiation of full arc light and
ultrasonic (L+S), the corresponding values turn to 0.714 μl∙h-1
and 0.112 μl∙h-1 respectively. Under the condition of L+S, the
yields of CO and CH4 are increased, yet the production of CH4
rises to a greater degree. Compared with L condition, the
selectivity of methane production is increased from 7.36% to
13.56% under the condition of L+S as demonstrated in Fig. 2b,
When illuminated by full arc light with sufficient photonic
energy and appropriate wavelength, photo-generated
electrons (e-) and holes (h+) are created on the surface of ZnO
NRs. The electrons in the valence band are excited to the
conduction band, leaving an equal number of holes in the
valence band. As a periodic force is applied on ZnO, the
nanorods are forced to bend and form a piezoelectric field
across its width with negative piezopotential on the
compressive strain region (blue) and positive piezopotential on
the tensile strain region (red) as shown in the top left of Fig.
3.33 The piezoelectric field provides a driving force for
attracting electrons to migrate to the positive surface and the
holes flow away to the negative surface,33, 34 thus causes the
internal movement of the photo-generated carriers with a
specific orientation. From the infrared spectra and Raman
spectroscopy of ZnO NRs (Fig. S4 and Fig. S5, ESI†), no
characteristic absorption peaks of the organic residues are
detected, which demonstrates that the surface of ZnO are
which almost increases doubled. However, as
a
piezoelectrochemical catalyst, only using ultrasonic irradiation
(S), the productive rates of CO and CH4 are limited, which
below the detection limit of the chromatography.
The photocatalytic reduction of carbon dioxide is usually
thought to be a multiple electrons reaction. During the
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clean. Holes directly oxidize H2O molecules and the electrons would be assembled together so that the production of CH4
react with CO2, generating O2, CO and CH4. This piezo-driven increases.
carrier migration reduces the recombination rate of photo-
generated electrons and holes, and more carriers migrate to
the surface of ZnO NRs, hence, the yields of CO and CH4 are
increased. Increasing the density of electrons can indeed
improve the selectivity of CH4 production which has been
testified by experiments (Fig. S6 and Fig. S7, ESI†). The
migration of the photo-generated carriers to the surface along
the opposite direction by piezoelectric potential increases the
local density of electrons, which is beneficial for the
production of CH4 requiring eight electrons to be transformed.
Fig. 4 SEM image of ZnO nanoparticles.
Fig. 3 Schematic diagram illustrating the distribution of photo-
generated carriers in ZnO NRs under L and L+S condition.
For sole ZnO material, the decay lifetime of carriers is in
nanosecond scale,35, 36 while the switching frequency of the
bending direction for the generated piezoelectric field caused
by the applied periodic ultrasound is 25 μs (40 kHz). The
generation and movement speed of photo-generated carriers
is obviously faster than that of direction alternation of the
piezoelectric field. For one carrier reaction, the migrated and
reactive progress can be regarded in a static electric field with
a constant intensity and direction. When the force direction productive rates of CO and CH4 and (b) selective percentage of
Fig. 5 Reduction of CO2 by 20 mg ZnO particles with (a)
CH4 under different conditions of non-rod shaped ZnO.
change to the other side, the essence of this chemical reaction
has not changed, just the properties of surface charge become
opposite.
Compared with ZnO NRs, wurtzite ZnO nanoparticles (Fig.
S1b, ESI†) with non-rod shapes (Fig. 4) show lower selectivity
for methane production. As shown in Fig. 5a, the productive
rates of CO and CH4 are 0.232 μl∙h-1 and 0.0162 μl∙h-1
respectively under L condition. While under the condition of
L+S, the corresponding values increase to 0.322 μl∙h-1 and
0.0316 μl∙h-1 respectively. Compared with L condition, the
selectivity of methane production is increased from 6.53% to
8.94% under the condition of L+S (Fig. 5b). The ZnO NRs
synthesized by solvothermal method not only produce more
gas-phase products but also have a higher selectivity of
methane production under the condition of L+S. The large
amounts of ZnO nanoparticles have no uniform c-axis
direction, which is similar to the polycrystalline structure.
Fibers with longer lengths exhibit a greater bending curvature
than that of shorter fiber lengths when under the same
For photocatalytic reaction, the absorption of reactants and
desorption of products are conducted on the local area of
catalyst surface. While under the full arc light irradiation
without the assistance of piezopotential, no driving force make
the photo-generated electrons migrate to the surface of ZnO
NRs directionally, causing
a relatively lower density of
electrons on the local surface area compared with L+S
condition (as shown in the bottom right of Figure 3). As a
periodic force applied on ZnO NRs, oriented migration of
electrons caused by piezoelectric potential is essential to
increase the local electron density that improves the selectivity
of methane. With the assistance of piezopotential, the sparse
electrons difficult to reach the reductive degree of yielding CH4
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applied force in a vibration event.37 Due to this property, ZnO
NRs built up a higher piezopotential than particles with equal
mechanical vibration. It is for this reason that non-rod shaped
ZnO nanoparticles exhibit relatively limited selectivity for
reducing CH4. In addition, the photocatalytic performance and
the selectivity of CH4 production have not been improved by
anatase TiO2 (none piezoelectric properties) under L+S
condition (Fig. S8, ESI†). These results further confirm that the
enhancement of photocatalytic efficiency of ZnO arises from
the piezo-assistance.
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