Hierarchical TiO2/Ni(OH)2 composite fibers with enhanced photocatalytic CO2 reduction performance

Article abstract of DOI:10.1039/c7ta10073f

In the past few years, Ni(OH)₂ has been found to be an effective cocatalyst for photocatalytic hydrogen evolution. Herein, we report that it can also be used to enhance the photoreduction of CO₂ into chemical fuels. Vertically aligned Ni(OH)₂ nanosheets are deposited onto electrospinning TiO₂ nanofibers via simple wet-chemical precipitation to manufacture TiO₂/Ni(OH)₂ hybrid photocatalysts. The TiO₂ nanofibers can direct the ordered growth of Ni(OH)₂ nanosheets, which have a thickness of 20 nm and uniformly cover the surface of the TiO₂ substrate. The TiO₂/Ni(OH)₂ hierarchical composite displays remarkably improved photocatalytic CO₂ reduction activity compared to that displayed by pristine TiO₂ fibers. The bare TiO₂ can only produce methane and carbon monoxide (1.13 and 0.76 μmol h^-1 g^-1, respectively) upon CO₂ photoreduction. After loading 0.5 wt% Ni(OH)₂, the methane yield increases to 2.20 μmol h^-1 g^-1, meanwhile the CO yield is unchanged. Interestingly, alcohols (methanol and ethanol) also appear as products, in addition to CH₄ and CO, over the TiO₂/Ni(OH)₂ hybrid, and the maximum yield is reached with 15 wt% Ni(OH)₂ loading (0.58 and 0.37 μmol h^-1 g^-1 for methanol and ethanol, respectively). This can be ascribed to an enhanced charge separation efficiency and higher CO₂ capture capacity due to the presence of Ni(OH)₂. These results demonstrate that Ni(OH)₂ can not only improve the total CO₂ conversion efficiency, but can also alter the product selectivity upon photocatalysis. This work opens a new pathway for achieving high-efficiency photocatalytic CO₂ reduction with Ni(OH)₂ as a cocatalyst.

Full text of DOI:10.1039/c7ta10073f

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Hierarchical TiO2/Ni(OH)2 Composite Fiber with Enhanced
Photocatalytic CO2 Reduction Performance
Aiyun Meng,a Shuang Wu,a Bei Cheng,a Jiaguo Yu*ac and Jingsan Xu*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
In the past years, Ni(OH)2 has been found an effective cocatalyst for photocatalytic hydrogen evolution. Herein, we report
that it can also be used to enhance CO2 photoreduction into chemical fuels. Vertically aligned Ni(OH)2 nanosheets are
deposited onto electrospinning TiO2 nanofibers via simple wetꢀchemical precipitation to manufacture TiO2/Ni(OH)2 hybrid
photocatalysts. The TiO2 nanofibers can direct the ordered growth of Ni(OH)2 nanosheets, which have thickness of 20 nm
and uniformly cover the surface of TiO2 substrate. The TiO2/Ni(OH)2 hierarchical composite displays remarkably improved
photocatalytic CO2 reduction activity compared to pristine TiO2 fibers. The bare TiO2 can only produce methane and carbon
monoxide (1.13 and 0.76 ꢁmol hꢀ1 gꢀ1, respectively) upon CO2 photoreduction. After loading 0.5 wt% Ni(OH)2, the methane
yield increases to 2.20 ꢁmol·hꢀ1·gꢀ1, meanwhile with the CO yield unchanged. Interestingly, alcohols (methanol and ethanol)
also appear as products in addition to CH4 and CO over the TiO2/Ni(OH)2 hybrid, and the maximum yield is reached with 15
wt% Ni(OH)2 loaded (0.58 and 0.37 ꢁmol·hꢀ1·gꢀ1 for methanol and ethanol, respectively). This can be ascribed to the
enhanced charge separation efficiency and higher CO2 capture capacity owing to the presence of Ni(OH)2. These results
demonstrate that Ni(OH)2 can not only improve the total CO2 conversion efficiency, but also can alter the product selectivity
upon photocatalysis. This work opens a new pathway for achieving highꢀefficiency photocatalytic CO2 reduction with
Ni(OH)2 as a cocatalyst.
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attractive issue.
Introduction
In heterogeneous photocatalytic CO2 reduction, a hybrid
material is often used as the photocatalyst to collect light,
The harvest of solar energy and its subsequent conversion to
hydrocarbon fuels is a promising strategy to remedy the energy
shortage. Considering worsening greenhouse effect caused by
the increasing CO2 emission, photocatalytic CO2 reduction
using solar light is under extensive investigation to achieve
renewable energy generation as well as to reduce environment
pollution.1ꢀ12 For one thing, it is highly desirable to develop
new photocatalyts for improving the CO2 conversion efficiency.
For another, due to the generation of many possible
hydrocarbon compounds (methane, methanol, etc.), it is vital to
control the photoreduction process so that products can be
selectively obtained from CO2 reduction.13ꢀ16 In particular, the
photoconversion of CO2 to methanol is quite preferable due to
an increasing industrial demand for methanol such as the large
utilization of MTO (methanol to olefin) process.17,18 Therefore,
developing a highꢀefficiency photocatalyst for CO2 reduction
and ideally with high selectivity towards methanol is an
separate the photoinduced charge carriers and provide active
sites.19 Usually, a semiconductor material is responsible for the
first two functions while a cocatalyst serves as the active
sites.13,20ꢀ22 Therefore, the rational assembling of
a
semiconductor and a corresponding cocatalyst is a potential
way to reach high photocatalytic CO2 reduction activity. The
properly designed hybrid structures can harvest more solar light,
accelerate molecules diffusion and increase the active sites.23
As one type of efficient semiconductor photocatalyst, 1D TiO2
nanofibers have been widely used as a substrate to construct
heterostructured photocatalysts due to their extraordinary
physiochemical properties.24ꢀ33 For instance, the electrospun
TiO2 nanofibers are composed of closely stacked nanoparticles
with high porosity which is able to provide more active sites.34
Further, the intimate alignment of TiO2 nanocrystals in the
nanofibers is favorable to the electron transport.35
Recently, Ni(OH)2 has been frequently used as an effective
cocatalyst for enhancing photocatalytic H2 evolution via water
splitting due to more favorable electron transfer.36ꢀ39 However,
to the best of our knowledge, it has not been utilized in
photocatalytic CO2 reduction reaction. For CO2 reduction,
effective adsorption and activation of CO2 is a key step for
enhancing the efficiency.40ꢀ43 Since CO2 is an acidic molecule,
Ni(OH)2 can be a promising adsorber for CO2 molecules.
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Fig. 1 Schematic diagram for preparing hierarchical TiO2/Ni(OH)2
composite nanofibers.
Moreover, it was reported that transition metals can be efficient
cocatalysts in enhancing the activity and product selectivity for
CO2 photoreduction, which also makes Ni(OH)2 an alternative
due to its easy transformation from Ni(OH)2 to Ni cluster under
illumination.39,44,45 Thus, constructing bicomponent hierarchical
nanostructures that consists of TiO2 and Ni(OH)2 could be a
promising method to improve the conversion efficiency and
product selectivity for CO2 photoreduction.
In this context, 1D TiO2/Ni(OH)2 hybrid photocatalyst was
prepared via a simple electrospinning and solutionꢀprecipitation
method (Fig. 1). The Ni(OH)2 nanosheets were vertically
aligned and uniformly deposited onto the TiO2 fibers. The
TiO2/Ni(OH)2 heterostructure exhibited enhanced CO2
photoreduction activity and noteworthy selectivity over alcohol
production owing to the improved charge extraction and
directed charge transfer in the presence of Ni(OH)2 nanosheets.
The photocatalytic mechanism and the significant role of
Ni(OH)2 in the CO2 reduction were investigated and discussed.
Fig. 2 XRD patterns of samples T, N and TN15 (a). FESEM pictures
(b, c), EDX mapping (d), TEM (e) and HRTEM (f) images of sample
TN15.
at 24.6°, 34.9° and 59.7° can be ascribed to αꢀNi(OH)2 (JCPDS
No. 38ꢀ0715). As for sample T and TN15, the diffraction
patterns can be assigned to anatase TiO2 (JCPDS, No. 21ꢀ1272)
and rutile TiO2 (JCPDS, No. 21ꢀ1276), exhibiting the
coexistence of the two phases,35,46 with the rutile percentage of
ca. 7%. The diffraction peaks of αꢀNi(OH)2 was unobservable
in sample TN15 due to its weak crystallization. Besides, the
peak positions of TiO2 remained unchanged for sample TN15,
suggesting the deposition of Ni(OH)2 had no effect on the
structure of the TiO2 substrate. The morphology and
microstructure of the hybrids were studied by scanning electron
microscopy (SEM) and transmission electron microscopy
(TEM). SEM pictures (Fig. 2b, c) show the hierarchical
nanostructure of TN15 that is composed of 1D TiO2 nanofibers
and 2D Ni(OH)2 nanosheets with an overall diameter of 530 nm.
The Ni(OH)2 nanosheets had thicknesses around 20 nm and
were vertically aligned onto the surface of the TiO2 fibers. The
specific orientation can promote the lightꢀabsorption due to
multiple reflections and scattering of the incident light47 and
meanwhile maximize the exposed active sites of both TiO2 and
Ni(OH)2, which is highly beneficial to reactants adsorption
(CO2 and H2O) and the following photocatalysis reaction.29,48,49
This architecture may also promote the possible charge transfer
from TiO2 to Ni(OH)2 as well as from Ni(OH)2 to CO2.
Experimental
TiO2 nanofibers were prepared by an electrospinning method.
Typically, a homogeneous precursor consisting of polyvinyl
pyrrolidone (PVP), ethanol, glacial acetic acid and tetrabutyl
titanate was prepared. Electrospinning was then conducted
using the precursor solution, followed by calcination to remove
the organic compositions, and the asꢀobtained TiO2 nanofibers
were denoted as T. TiO2/Ni(OH)2 composite samples with
different weight ratios of Ni(OH)2 were synthesized by oil bath
at 90 oC for 6 h with TiO2 nanofiber as the substrate and
Ni(NO3)2·6H2O as Ni source, the obtained samples were
denoted as TNx (x = 0.5, 1, 1.5, 2, 15 wt%), where x means the
weight ratio of Ni(OH)2 with respect to TiO2. The Ni(OH)2
nanosheets were also prepared via a similar oil bath process but
without adding TiO2 nanofibers and were denoted as N. The
detailed fabrication procedure was given in Supporting
Information. To facilitate investigation, TN15 was chosen as
the typical sample due to its large weight ratio of Ni(OH)2 and
remarkable selective alcohol production.
Results and Discussion
Fig. 2a displays the Xꢀray diffraction (XRD) patterns of N, T
and TN15. For sample N, three weak diffraction peaks located
The energy dispersive Xꢀray (EDX) elemental mapping (Fig.
2d) identified the spatial distributions of Ti, O and Ni,
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suppressing the outward growth of Ni(OH)2 and facilitating inꢀ
plane sheets growth50. Thus, 2D Ni(OH)2 nanosheets grown in
a vertical mode, rather than covering on the surface of TiO2
fibers.
Further, light absorption of samples was measured by UVꢀvis
absorption (Fig. 3a). The absorption edge of sample TN15
remains almost unchanged compared with pure TiO2. Besides, a
weak absorption shoulder at ~450 nm and an absorption band in
the range of 600–800 nm were observed for sample TN15 and
N, resulting from the dꢀd transition of Ni.39
Fig.
3 (a) UVꢀvis spectra, (b) nitrogen adsorptionꢀdesorption
Fig. 3b presents the N2 adsorptionꢀdesorption isotherms and
corresponding poreꢀsize distribution curves (inset). All samples
exhibit isotherms of type IV, reflecting the existence of
mesopores (2–50 nm). The hysteresis loops of T at relative
pressures of 0.5~1 are of type H2, indicative of inkꢀbottleꢀlike
mesopores formed by TiO2 nanocrystals. The hysteresis loops
of TN15 and N samples vary from T: at relative pressures of
0.4~0.9, the shapes of hysteresis loops are of type H2,
indicating that inkꢀbottleꢀlike mesopores are formed by primary
Ni(OH)2 nanoparticles; at relative pressures of 0.9~1.0, The
hysteresis loops are of type H3, implying that slitꢀlike
mesopores are formed by the stacking of Ni(OH)2 nanosheets.
The derived data including specific surface area, pore volume
and pore size are summarized in Table S1.
isotherms of T, TN15 and N.
XPS measurements were performed to identify the surface
chemical status of elements of the photocatalysts. The survey
spectrum (Fig. 4a) of sample T shows the presence of Ti 2p
(459 eV) and O 1s (531 eV). For TN15, peaks attributed to Ni
2p (855 eV), O 1s (531 eV) and C 1s (285 eV) are found, while
Ti peaks became much less noticeable, due to the shielding
effect of the Ni(OH)2 nanosheets chemically grown on TiO2
surface. In fact, Ti element still exists with a lower intensity in
TN15 according to the Ti 2p highꢀresolution spectrum of TN15
(Fig. 4b). Fig. 4c depicts the high resolution O 1s spectra of T
and TN15. For pure TiO2, oxygen exists as lattice oxygen
(Ti−O−Ti, 529.6 eV) and surface hydroxyl oxygen (Ti−OH,
531.5 eV).51 As Ni(OH)2 nanosheets were deposited on TiO2
surface, oxygen exists as lattice oxygen (MꢀOꢀM, M = Ti or Ni,
529.6 eV) and hydroxyl oxygen (Ni−OH, 531.6 eV). The
intensity of the hydroxyl oxygen significantly increased.52 In
the Ni 2p region of TN15 (Fig. 4d, top), the binding energies at
Fig. 4 XPS survey spectra (a) and highꢀresolution XPS spectra of Ti
2p (b), O 1s (c) and Ni 2p (d) of T and TN15 samples.
suggesting the homogeneous distributions of Ni(OH)2 along
with the TiO2 fiber. The TEM images (Fig. 2e) further confirm
the hierarchical morphology of the in-situ grown TiO2/Ni(OH)2
hybrid. High resolution TEM (HRTEM, Fig. 2f) image clearly
demonstrates that the Ni(OH)2 nanosheets were well attached to
the TiO2 substrate, which remained stable after 30 min bath
sonication treatment. The low crystallinity of Ni(OH)2 can also
be observed, in accordance with the XRD results.
As a comparison, the morphology of individually prepared
Ni(OH)2 were also studied. The Ni(OH)2 sample synthesized in
the absence of the TiO2 support was in the form of aggregated,
nanosheetsꢀassembled spheres with ca. 1 ꢁm in diameter (Fig.
S1), completely distinct from the above observations. The
vertical growth of Ni(OH)2 can be explained by heterogeneous
nucleation and subsequent oriented crystal growth. The TiO2
fibers acted as backbones and provide sites for the preferential
growth of Ni(OH)2. At the beginning of the growth process,
NH3 and HCHO were generated from the decomposition of
hexamethylenetetramine and then Ni2+ ions reacted with the
OHꢀ that were released from NH3 hydrolysis, forming
amorphous Ni(OH)2 nanocrystal nucleus, which anchored to the
external surface of the TiO2 fibers. Then the NH3 molecules
adsorbed on the surface of Ni(OH)2 seeds by hydrogen bonds,
856.0 eV and 873.6 eV correspond to Ni 2p3/2 and Ni 2p1/2
,
respectively, indicating the generation of Ni(OH)2 phase.53
Moreover, the two shoulder peaks at about 861.8 eV and 880.2
Fig. 5 (a) FTIR spectra and (b) CO2 adsorption isotherms of T, TN15
and N samples.
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and S3). As displayed in Fig. 6a, CH4 and CO were detected as
products with a rate of 1.13 and 0.76 ꢁmol hꢀ1 gꢀ1 when T was
used as the photocatalyst. With 0.5 wt% Ni(OH)2 deposited
onto TiO2 nanofibers (TN0.5), the CH4 yield increased to 2.20
ꢁmol·hꢀ1·gꢀ1, which nearly doubled the reduction rate of T.
Besides, the CO yield almost remained unchanged (0.71
ꢁmol·hꢀ1·gꢀ1). Meanwhile, it is very interesting to note that
CH3OH and C2H5OH started to appear as coproducts with the
deposition of Ni(OH)2, with a generation rate of 0.11 and 0.15
ꢁmol·hꢀ1·gꢀ1, respectively. Further increase of the Ni(OH)2
amount (up to 15 wt%) resulted in the slow decrease of CH4
yield, but remained higher than bare TiO2. On the other hand, it
is noteworthy that the yield of CH3OH and C2H5OH tended to
improve with the increasing amount of Ni(OH)2 and the
maximum yield was reached when 15 wt% Ni(OH)2 was loaded
(0.58 ꢁmol·hꢀ1·gꢀ1 and 0.37 ꢁmol·hꢀ1·gꢀ1 for CH3OH and
C2H5OH, respectively). As a control, the photocatalytic activity
of pure Ni(OH)2 was also evaluated. CH3OH and C2H5OH were
detected as major products (0.15 and 0.44 ꢁmol·hꢀ1·gꢀ1,
respectively), while the total CO2 conversion efficiency was
much lower than that of the TiO2/Ni(OH)2 hybrids (Fig. 6a).
Table 1 shows the photocatalytic CO2 reduction performance of
various TiO2ꢀbased photocatalysts. It can be found that the
TiO2/Ni(OH)2 hybrid exhibits higher or comparable
photocatalytic CO2 conversion activities compared with most of
the recently reported TiO2ꢀbased photocatalyst. There are few
materials showing higher CO2 reduction efficiency,
nevertheless, these photocatalysts were modified with noble
metals such as Pt and Pd, which increased the cost and limited
the largeꢀscale usage. Therefore, the easilyꢀprepared, nonꢀnoble
metal cocatalyst of Ni(OH)2 can still be of great interest for
achieving highꢀefficiency CO2 reduction.
Moreover, the time courses of the photocatalytic chemical
fuel generation over sample T, N and TN15 were investigated
(Fig. 6bꢀd). The amounts of the evolved photocatalytic products
increased with the illumination time, indicating the continuous
photoꢀreduction of CO2. These results clearly demonstrate that
the overall photocatalytic CO2 conversion efficiency (to CH4,
CH3OH, C2H5OH and CO) can be greatly enhanced by
hybridizing TiO2 with Ni(OH)2. Moreover, the selectivity over
the products can be facilely tuned by adjusting the Ni(OH)2
amount. The related mechanism involving the vital role of the
vertically aligned Ni(OH)2 arrays is investigated and illustrated
below. Further, the wavelength dependent CO2 reduction
efficiency over sample TN15 was measured by using a
wavelength filter of 400 and 450 nm (Fig. S4), respectively. We
can see that reasonable amount of photoꢀreduction products
were still obtained, although the light with wavelength below
400 nm (the light that TiO2 is able to absorb) was filtered.
These result suggest that TiO2 played a major role for
photocatalytic CO2 reduction under fullꢀspectrum illumination
with Ni(OH)2 as a cocatalyst, whereas under longꢀwavelength
illumination Ni(OH)2 alone can also be a photocatalyst for CO2
reduction.
Fig. 6 Yields of CH4, CH3OH, C2H5OH and CO for photocatalytic
CO2 reduction over sample T, N and TNx (x=0.5, 1, 1.5, 2 and 15)
samples after 1 h irradiation (a). Time course of the products in the
photocatalytic conversion of CO2 over TN15 (b), T (c) and N (d).
eV can be ascribed to the satellite peaks of Ni 2p3/2 and Ni 2p1/2
Adsorption is the key step for the following photocatalytic
CO2 reduction processes. Fourier transform infrared
spectroscopy (FTIR) spectra were measured to quickly assess
the CO2 uptake capabilities. As shown in Fig. 5a, for all the
three samples, the broad peak at 3440 cmꢀ1 and the peak at 1623
cmꢀ1 are assigned to the O−H stretching vibration and H−O−H
bending vibration of water, respectively. For sample T, the
broad bands from 400 to 1000 cm−1 correspond to the special
vibration of TiO2.51 Moreover, the peak of sample N centered at
647 cmꢀ1 is indicative of Ni−O−H bending vibration,54 and the
sharp peak at 1382 cmꢀ1 is attributed to the CO2 stretching
vibration owing to the strong CO2 adsorption.52 As a result, the
TN15 shows an enhanced CO2 capture capacity compared to
pure TiO2 indicated by the appearance of the CO2 vibration
peak. This is highly desirable for improving CO2
photoreduction efficiency.
Quantitative CO2 uptakes of the samples were evaluated by
CO2 adsorption isotherms. As shown in Fig. 5b, all samples
exhibit an increase along with elevated CO2 pressure. Notably,
Ni(OH)2 nanosheets demonstrate a much higher CO2 adsorption
capacity due to its alkalinity and consequently TN15 has a
capacity almost twofold higher than that of TiO2 nanofibers,
agreeing well with the FTIR measurements. These results
demonstrate that Ni(OH)2 nanosheets can act as an effective
CO2 adsorbent and compensate the limited adsorption of TiO2
nanofibers.
.
Finally, the photocatalytic CO2 reduction activities of the asꢀ
prepared hybrids (50 mg) were evaluated using selfꢀdesigned
setups with a 350 W Xenon lamp as simulated sunlight (Fig. S2
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Table 1. Comparisons of photocatalytic CO2 reduction performance of various TiO2ꢀbased photocatalysts reported in 2014~2018.
Light intensity
(mW cmꢀ2)
Yield
Photocatalyst a
Reductant b
H2O vapor
Light c
Product
Year
Ref.
(ꢁmol hꢀ1 gꢀ1)
0.5% Ni(OH)2ꢀTiO2
350 W Xe lamp
40
CH4
CO
2.20
0.71
2018 This report
CH3OH
C2H5OH
CH4
0.11
0.15
1.35
0.156
0.241
0.090
1.94
4.8
TiO2 nanosheet
H2O vapor
H2O vapor
350 W Xe lamp
550 W Xe lamp
/ (not given)
100
2014
2014
55
56
hydrogenated TiO2
CH4
CO
mesoporous Co/TiO2
Pt/TiO2
H2O vapor
H2O vapor
H2O vapor
H2O vapor
300 W Xe lamp
300 W Xe lamp
350 W Xe lamp
300 W Xe lamp
/
CH4
2015
2015
2017
2017
57
58
59
60
CO
100
/
CH4
CO
0.1
Carbon/TiO2
hollow spheres
CH4
4.22
9.11
4.10
15.50
1.4
CH3OH
CH4
PdꢀTiO2
2.7
CO
1.5% AgꢀTiO2
1.2 nm CuPtꢀTiO2
Cu2O/TiO2
H2O vapor
H2O vapor
H2O vapor
300 W Xe Lamp
150 W Xe lamp
/
/
/
CH4
2016
2016
2017
61
62
63
CH4
11.35
2.11
1 kW Hg (Xe)
lamp
CO
PtꢀCu2O/TiO2
TiO2/gꢀC3N4
H2O vapor
H2O vapor
300 W Xe lamp
20.5
/
CH4
CO
1.42
0.05
2017
2017
64
65
300 W Xe lamp
CH4
CO
2.50
9.30
Notes: (a) TiO2 based photocatalysts; (b) solidꢀgas reaction systems, with H2O vapor as the reductant; (c) Xe lamps were used as
the light source.
Oxidation reactions by holes were also investigated. Fig. S5 against the photocatalytic CO2 reduction, indicated by no
shows the gas chromatogram results after 4 h of irradiation for changing of the crystal structure and morphology before and
photocatalytic CO2 reduction over sample T and TN15. It can after the photoreaction (Fig. S7).
be observed that O2 was detected in the photocatalytic process
In-situ diffuse reflectance infrared Fourier transform
for sample T (Fig. S5a), which was generated by water spectroscopy (DRIFTS) was carried out to monitor the CO2
oxidation (2 H2O→O2 + 4H+ + 4eꢀ). However, no O2 was adsorption and conversion over the photocatalysts. As shown in
observed for sample TN15 (Fig. S5b), and we consider that this Fig. 7a, without illumination (red line), only adsorbed H2O,
is because the holes were captured and quenched by the CO2 and carbonate species were detected on the TN15 surface.
hydroxyl groups from TiO2.66,67
The peaks at 1173 and 1272 cmꢀ1 are ascribed to adsorbed H2O
To determine the actual carbon source of the photocatalytic and CO2, respectively.68 The peaks at 1394 and 1715 cmꢀ1 are
products, isotope tracer experiment involving photocatalytic assigned to symmetric stretching vibration of monodentate
reduction of 13CO2 (12CO2 was also tested as a reference) was carbonate and bicarbonate species.69,70 Upon illumination for 60
performed and examined by GCꢀMS. As shown in Fig. S6, min (blue line), additional peaks associated to the reaction
peaks assigned to 13CH4 (m/z = 17), 13CH3OH (m/z = 33) and intermediates were observed. The peak at 1053 cmꢀ1 is
13C2H5OH (m/z = 47) were clearly observed when using 13CO2 attributed to stretching vibration of methoxy.71 The peaks at
as the carbon source. These results verified that the detected 1107, 1509, 1745 cmꢀ1 belong to the vibration of
photoꢀreduction products over the asꢀprepared materials formaldehyde.71,72 The peak at around 1458 cmꢀ1 was assigned
originated from the CO2 source gas rather than any to bending vibration of dioxymethylene.71 These results suggest
residual/contaminant carbon species. Moreover, it is worth that photocatalytic CO2 conversion on the surface of TN15
mentioning that the TiO2/Ni(OH)2 hybrid remained stable undergoes
a
multiꢀelectron reduction process (probably
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CO2→HCOOH→HCHO→CH3OH)8,59,73ꢀ76 involving reaction efficiency after loading of Ni(OH)2 nanosheets.
intermediates such as methoxy and formaldehyde. On the
Based on the above results, we propose a mechanism to better
contrary, no vibrations bands at 1458 cmꢀ1 and below 1100 cmꢀ1 understand the CO2 photocatalytic process. On one hand, TiO2
were observed for TiO2 under illumination (Fig. 7b, blue line), nanofibers alone (without Ni(OH)2) can harvest light and
suggesting the absence of the dioxymethylene and methoxy reduce CO2 into CH4 and CO with a rather low efficiency. On
intermediate, corresponding to the generation of CH3OH and the other, after the deposition of the Ni(OH)2 nanosheets, the
C2H5OH over the TN15. Although several peaks associated photogenerated electrons in TiO2 tend to migrate to Ni(OH)2,
with formaldehyde were also seen on the TiO2 surface (1506 resulting in partial reduction of Ni2+ ions into Ni0 atoms,
and 1745 cmꢀ1), the final product contained only CH4 and CO. forming Ni(OH)2/Ni clusters (step
①,
Fig. 8).39 The
These results demonstrate the conversion paths of CO2 and the Ni(OH)2/Ni clusters can be a cocatalyst and serve as electron
intermediates (e.g. formaldehyde) upon the photocatalyst sinks and accelerate the separation of photogenerated electrons
surfaces can be changed by the addition of Ni(OH)2, agreeing from TiO2 to the Ni(OH)2/Ni cluster and to the CO2 molecules.
with the aforementioned photocatalytic activity measurements.
The modified electron transfer is highly beneficial to the
The transient photocurrent response (Iꢀt) of the electrodes photocatalytic CO2 conversion, leading to much improved
deposited with sample T and TN15 was measured and recorded generation rate of the reduction products. Further increase of
(Fig. S8) under the irradiation of 365 nm LED light to evaluate the Ni(OH)2 loading amount suppressed the formation of CH4,
the separation and transfer efficiency of photogenerated charge while CH3OH and C2H5OH was preferably generated (step
②).
carriers. The onꢀoff cycles of photocurrent response were Herein, the formation of Ni0 clusters can be confirmed by XPS
reproducible over the chopped light illumination. The measurement of the TN15 sample before and after
photocurrent density of TN15 was over 7 times higher than that photocatalysis reaction. As shown in Fig. 4d (bottom), after
of pure TiO2, suggesting the promoted charge migration photocatalytic reaction, the Ni 2p3/2 binding energy of TN15
shifted to a lower binding energy of 855.3 eV, which can be
attributed to NiO. This can be explained by the fact that the Ni
clusters are unstable in air and they can be easily oxidized.
Therefore, it is reasonable that NiO (instead of Ni) was detected
after photocatalytic reaction.
Conclusions
In summary, a hierarchical TiO2/Ni(OH)2 hybrid photocatalyst
is prepared by depositing vertically aligned Ni(OH)2 nanosheets
onto the electrospun TiO2 fibers with a simple solution
precipitation method. We show that Ni(OH)2 can not only
Fig. 7 Inꢀsitu DRIFTS of sample (a) TN15 and (b) T. Black lines:
light off, without introducing CO2 gas. Red lines: light off, flowing
enhance photocatalytic H2 evolution as previously reported, but
also can remarkably improve the photocatalytic CO2 reduction
CO2 gas for 60 min. Blue lines: light on, flowing CO2 gas for 60 min.
as a cocatalyst on TiO2 nanofibers. It is found that the hybrids
possess stronger CO2 capture capacity compared with pure
TiO2 nanofibers. Further, a part of Ni(OH)2 can be reduced by
the photoꢀinduced electrons from TiO2 and then Ni(OH)2/Ni
clusters are formed, which can act as vital active sites and
decrease the recombination rate of photogenerated charge
carriers during photocatalytic process. Hence, the functions of
Ni(OH)2 are proposed to be increasing the density of CO2 on
the photocatalyst surface and favoring the charge transfer from
TiO2 nanofibers to Ni(OH)2 nanosheets and finally to CO2
molecules. Moreover, an enhanced alcohols generation
(CH3OH and C2H5OH) upon the hybrid photocatalyst can be
achieved, resulting from the Ni(OH)2 induced changing of the
CO2 conversion paths, which can be detected using the inꢀsitu
DRFITS. We believe this work provides a potential approach to
enhance both the efficiency and selectivity of photocatalytic
CO2 reduction by developing more transition metal based
compounds as cocatalysts.
Fig. 8 Schematic illustration for the separation and transfer of
photoinduced charge carriers over the TiO2/Ni(OH)2 nanofiber
system.
Acknowledgements
6 | J. Name., 2012, 00, 1-3
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ARTICLE
This work was supported by NSFC (51320105001, U1705251, 24 P. S. Archana, E. Naveen Kumar, C. Vijila, S. Ramakrishna,
21573170 and 21433007), the Natural Science Foundation of M. M. Yusoff and R. Jose, Dalton trans., 2013, 42, 1024ꢀ1032.
Hubei Province (2015CFA001), Innovative Research Funds of 25 T. A. Arun, A. A. Madhavan, D. K. Chacko, G. S. Anjusree, T.
SKLWUT (2017ꢀZDꢀ4) and the Fundamental Research Funds
for the Central Universities (2016ꢀYBꢀ005). J.X. is grateful to
G. Deepak, S. Thomas, S. V. Nair and A. S. Nair, Dalton
trans., 2014, 43, 4830ꢀ4837.
Australian Research Council for financial support 26 X. Wang, Q. Ni, D. Zeng, G. Liao and C. Xie, Appl. Surf. Sci.,
(DE160101488). The project is also supported by the State Key 2016, 389, 165ꢀ172.
Laboratory of Advanced Technology for Materials Synthesis 27 Y. Huang, J. Chen, W. Zou, L. Zhang, L. Hu, M. He, L. Gu, J.
and Processing (2018ꢀKFꢀ17).
Deng and X. Xing, Dalton trans., 2016, 45, 1160ꢀ1165.
28 S. W. Verbruggen, J. Photochem. Photobiol. C, 2015, 24, 64ꢀ
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29 A. B. Shahzad and R. Caue, J. Photochem. Photobiol. C, 2016,
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Liu, J. Mater. Chem. A, 2011, 21, 6922.
31 B. Liu, C. Feng, X. Zhang, L. Zhu, X. Wang, G. Zhang and D.
Xu, CrystEngComm, 2016, 18, 1215ꢀ1222.
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Graphical abstract
Hierarchical porous TiO2/Ni(OH)2 composite fiber photocatalyst with dramatically improved photocatalytic CO2
reduction activity and prominent alcohol production selectivity.