Magnetic-Field-Regulated TiO2 {100} Facets: A Strategy for C-C Coupling in CO2 Photocatalytic Conversion

Article abstract of DOI:10.1016/j.chempr.2020.06.033

Solar-driven CO₂ conversion is an attractive option for producing usable fuels and chemicals. However, traditionally synthesized TiO₂ materials suffer from the low activity of CO₂ conversion into multi-carbon products. Here, for the first time, strong magnetic fields were introduced into TiO₂ synthesis, and new TiO₂{100} facets containing more active low-coordinate Ti atoms were developed. This is beneficial to the coupling of adsorbed CO?, which enables highly efficient CO₂ conversion into C₂H₅OH with a yield rate of 6.16 μmol g ^?1 h^?1.The greenhouse effect has attracted global attention. According to the Intergovernmental Panel on Climate Change, the annual growth of the greenhouse effect is 1.6percent, and it is predicted that CO₂ emissions will increase by 40percent to 110percent by 2030. Simultaneously, a global energy crisis is approaching. The pressure of environmental stewardship and the shortage of energy make the development of renewable fuels an inevitable trend. An interesting strategy is photochemical CO₂ conversion, which uses solar energy to convert CO₂ into high value-added usable fuels under mild conditions. However, CO₂ conversion into multi-carbon products remains a long-term challenge. Introducing strong magnetic field photocatalyst synthesis creates favorable conditions for the coupling of adsorbed CO? intermediates in the CO₂ photocatalytic conversion process. This strategy exhibits significant potential in the CO₂ photocatalytic conversion as well as the rational design of novel materials.Solar-driven CO₂ conversion is an attractive option for producing usable fuels and chemicals. However, traditionally synthesized TiO₂ materials suffer from the low activity of CO₂ conversion into multi-carbon products. In this study, for the first time, strong magnetic fields were introduced into the synthesis of TiO₂. By regulating the splitting ratio of high-angle and low-angle quantum orbitals, we developed a new type of TiO₂{100} facets containing more active low-coordinate Ti atoms. In-situ Fourier transform infrared spectroscopy (FTIR) and DFT calculations both revealed that the interfacial charge redistribution and lattice structure of such TiO₂{100} are beneficial to the coupling of adsorbed CO?. This enables highly efficient conversion of CO₂ into C₂H₅OH with a yield rate of 6.16 μmol g^?1 h^?1, which is 22-fold higher than that of pristine TiO₂. This strategy provides a new platform for desirable photocatalyst synthesis and furthers our understanding of the relationship between atomic orbital control and CO₂ conversion.

Full text of DOI:10.1016/j.chempr.2020.06.033

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Article
Magnetic-Field-Regulated TiO2 {100} Facets:
A Strategy for C-C Coupling in CO2
Photocatalytic Conversion
Meng-Pei Jiang, Ke-Ke Huang,
Jing-Hai Liu, ..., Zhi-Bin Geng,
Xiang-Yan Hou, Shou-Hua Feng
shfeng@jlu.edu.cn
HIGHLIGHTS
Strong magnetic fields were
introduced into TiO2 synthesis for
the first time
New TiO2 {100} facets, containing
more active low-coordinate Ti
atoms, were developed
The TiO2 {100} facets were
beneficial to the coupling of
adsorbed CO* intermediates
Solar-driven CO2 conversion is an attractive option for producing usable fuels and
chemicals. However, traditionally synthesized TiO2 materials suffer from the low
activity of CO2 conversion into multi-carbon products. Here, for the first time,
strong magnetic fields were introduced into TiO2 synthesis, and new TiO2{100}
facets containing more active low-coordinate Ti atoms were developed. This is
beneficial to the coupling of adsorbed CO*, which enables highly efficient CO2
conversion into C2H5OH with a yield rate of 6.16 mmol g À1 hÀ1
.
Jiang et al., Chem 6, 1–12
September 10, 2020 ª 2020 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2020.06.033

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Article
Magnetic-Field-Regulated TiO2 {100}
Facets: A Strategy for C-C Coupling
in CO2 Photocatalytic Conversion
Meng-Pei Jiang,1 Ke-Ke Huang,1 Jing-Hai Liu,2 Dan Wang,3 Ying Wang,4 Xia Wang,2 Zhi-Da Li,5
Xi-Yang Wang,1 Zhi-Bin Geng,1 Xiang-Yan Hou,1 and Shou-Hua Feng1,6,
*
SUMMARY
The Bigger Picture
The greenhouse effect has
attracted global attention.
Solar-driven CO2 conversion is an attractive option for producing us-
able fuels and chemicals. However, traditionally synthesized TiO2 ma-
terials suffer from the low activity of CO2 conversion into multi-carbon
products. In this study, for the first time, strong magnetic fields were
introduced into the synthesis of TiO2. By regulating the splitting ratio
of high-angle and low-angle quantum orbitals, we developed a new
type of TiO2{100} facets containing more active low-coordinate Ti
atoms. In-situ Fourier transform infrared spectroscopy (FTIR) and DFT
calculations both revealed that the interfacial charge redistribution
and lattice structure of such TiO2{100} are beneficial to the coupling
of adsorbed CO*. This enables highly efficient conversion of CO2 into
C2H5OH with a yield rate of 6.16 mmol gꢀ1 hꢀ1, which is 22-fold higher
than that of pristine TiO2. This strategy provides a new platform for
desirable photocatalyst synthesis and furthers our understanding of
the relationship between atomic orbital control and CO2 conversion.
According to the
Intergovernmental Panel on
Climate Change, the annual
growth of the greenhouse effect is
1.6%, and it is predicted that CO2
emissions will increase by 40% to
110% by 2030. Simultaneously, a
global energy crisis is
approaching. The pressure of
environmental stewardship and
the shortage of energy make the
development of renewable fuels
an inevitable trend. An interesting
strategy is photochemical CO2
conversion, which uses solar
energy to convert CO2 into high
value-added usable fuels under
mild conditions. However, CO2
conversion into multi-carbon
products remains a long-term
challenge. Introducing strong
magnetic field photocatalyst
synthesis creates favorable
conditions for the coupling of
adsorbed CO* intermediates in
the CO2 photocatalytic
INTRODUCTION
Solar-driven CO2 photocatalysis, which mimics natural photosynthesis, involves the
conversion of CO2 into reusable CO or hydrocarbons (such as CH4 and C2H5OH), of-
fering a potentially ‘‘green’’ approach to mitigating global energy challenges and
ecological deterioration.1 Anatase TiO2, a high-profile photocatalyst, has been
investigated extensively in the photocatalytic conversion of CO2 owing to its
outstanding surface electronic catalytic performance.2,3 The TiO2 {101} facets, which
have a low energy of 0.44 J mꢀ12, are almost 100% exposed, but they contain only
50% highly active Ti5c (pentacoordinate Ti) (Figure S1). This significantly restricts the
catalytic performance and is a bottleneck that must be addressed.4
Therefore, Yang and co-workers explored increasing the exposure of {001} facets,
which have a higher energy of 0.90 J mꢀ12, by changing the crystal growth orienta-
tion to obtain more highly active Ti5c sites.5 The TiO2 {001} and {101} surface junction
were reported as capable of converting CO2 into CH4 production with a high yield
conversion process. This strategy
exhibits significant potential in the
CO2 photocatalytic conversion as
well as the rational design of novel
materials.
rate of 1.35 mmol gꢀ1 hꢀ1 6,7
Moreover, some researchers also found that co-
.
exposed {100} and {001} facets can result in the special CO2 reduction product
CH3OH at 1.5 mmol gꢀ1 hꢀ1.8 All of above information demonstrates the substantial
dependence of the product distribution on the TiO2 facets and the crucial role of the
coordinate environment of active Ti atoms.
The Zeeman effect is an effective means of controlling quantum orbital directional split-
ting.9 The mechanism for the above induction can be attributed to the electrons rotating
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around the nucleus, resulting in a different magnetic angular momentum under an
external magnetic field. The addition of angular momentum makes the splitting of
high-angle quantum orbits more favorable than that of low-angle quantum orbits.10
The synthesis of TiO2 with a special structure by controlling the directional splitting of
two low-angular quantum orbits and four high-angular quantum orbits is highly
desirable.
In this study, for the first time, the Zeeman-effect-assisted anodic oxidation
method was used in the synthesis of TiO2. By controlling the directional splitting
of orbits in the TiO2, we obtained TiO2{100} facets containing more low-coordinate
Ti atoms that are beneficial to the coupling of adsorbed CO* intermediates. Conse-
quently, the yield rate of conversion of the CO2 to C2H5OH was enhanced into
6.16 mmol gꢀ1 hꢀ1, which is 22-fold higher than that of pristine TiO2.
RESULTS AND DISCUSSION
Formation of TiO2{100} under Strong Magnetic Field
The TiO2 nanotubes (NTs) used herein were prepared by magnetic-field-assisted anodic
oxidation, and their morphological features differed significantly from those of traditional
anodized TiO2 NTs, as confirmed by field emission scanning electron microscopy (FE-
SEM)and transmission electron microscopy (TEM) (Figures 1A–1D). Figure1A clearly illus-
trates that the diameter of a single TiO2 NT is approximately 50 nm and the wall thickness
is 5ꢁ6 nm, but after exposure to the magnetic field, the TiO2 NT wall changed from
smooth to porous and was composed of nanocrystals with a diameter of 5ꢁ6 nm (Fig-
ure 1B). Additional SEM images are shown in Figure S2. The TEM images in Figures 1C
and 1D show that unlike the original tetragonal biconical anatase phase, the TiO2 nano-
crystals prepared by the Zeeman effect exhibit a tetragonal double-sided prism structure
with exposed {001} facets or a cubic structure with exposed {100} facets. Additional high-
resolutionTEM (HRTEM) and correspondingselected-area diffraction (SEAD) patterns are
shown in Figure S3 and this information, in conjunction with X-ray diffraction (XRD) data,
can be used to calculate the facet exposure ratio.11 The specific lattice distortion of the
TiO2 induced by the strong magnetic field can be directly identified by advanced aberra-
tion-corrected scanning transmission electron microscopy (AC-STEM) and the corre-
sponding high-angle annular dark-field scanning transmission electron microscopy
(HAADF-STEM) images (Figures 1E and 1F). As shown in the HAADF-STEM image
1State Key Laboratory of Inorganic Synthesis and
Preparative Chemistry, International Joint
enlarged from the yellow cyan frame (Figure 1G), a perfectly one-to-one correspondence
exists between the light spots and Ti atoms at the {100} facets (Figure 1G). Furthermore,
thelightspotsfromthepurplecyanframeshowthesameperfectcorrespondencewiththe
Ti in the TiO2 {101} facets (Figure 1H), providing evidence that the strong magnetic field
could facilitate the transformation of TiO2 {100} facets from the inner TiO2 {101} facets.
Research Laboratory of Nano Micro Architecture
Chemistry, NMAC, College of Chemistry, Jilin
University, Changchun, P.R. China
2Inner Mongolia Key Laboratory of Carbon
Nanomaterials, Nano Innovation Institute (NII),
College of Chemistry and Chemical Engineering,
Inner Mongolia University for Nationalities, Tong
Liao, P.R. China
The Zeeman effect at different magnetic field strengths caused the p orbitals with
different angular quantum numbers in the TiO2 to undergo different degrees of split-
ting. The core-level X-ray photoelectron spectroscopy (XPS) spectrum of the Ti 2p
shows a high-angle quantum orbit, 2p3/2, at 458.5 eV and a low-angle quantum
orbit, 2p1/2, at 464.1 eV (Figure 2A). The corresponding XPS survey spectra are
shown in Figure S4. The XPS Ti 2p contour map in Figure 2B shows that the areas
of the Ti 2p3/2 and Ti 2p1/2 orbitals increase to different degrees as the magnetic field
strength increases. The linear fitting shown in Figure 2C suggests that the 2p1/2 and
2p3/2 orbital ratio decreases as the magnetic field increases. For example, when the
magnetic field strength is increased to 1,000 Gs, the ratio of 2p1/2 to 2p3/2 decreases
from 0.58 to 0.44, implying that the p3/2 orbitals experience more splitting than does
p1/2 because p3/2 has more angular quantum numbers than p1/2. The XRD patterns of
3State Key Laboratory of Biochemical
Engineering, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing, P.R. China
4Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Changchun, P.R.
China
5Provincial Key Laboratory of Oil & Gas Chemical
Technology, College of Chemistry & Chemical
Engineering, Northeast Petroleum University,
Daqing, P.R. China
6Lead Contact
*Correspondence: shfeng@jlu.edu.cn
https://doi.org/10.1016/j.chempr.2020.06.033
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Figure 1. SEM and STEM Images of TiO2 Prepared under Magnetic Field
(A) SEM images of original TiO2 NTs prepared without adding a magnetic field.
(B–D) SEM (B) and TEM ([C] and [D]) images of TiO2 prepared under a magnetic field strength of
1,000 Gs.
(E and F) AC-STEM (E) and HAADF-STEM (F) images of TiO2 prepared under a magnetic field
strength of 1,000 Gs.
(G and H) Images enlarged from the area marked with yellow frame (G) and purple frame (H),
respectively.
the prepared TiO2 can be indexed to anatase TiO2 (JCPDS no. 21-1272) (Figure S5).
As the magnetic intensity is increased during the preparation process, the exposure
ratio of the {100} crystal facets significantly increases (Figure S6). When the magnetic
field strength is 1,000 Gs, the exposure ratio of the {101} crystal facet decreases from
100% to 24.87%, whereas the exposure ratio of the new {100} crystal facet increases
from 0% to 48.01% (Figures S7–S9; Tables S1 and S2).
Formation Mechanism and Interfacial Charge Redistribution of Photocatalysts
The change in the surface energy can be used to reveal the formation of the {100} fac-
ets under the strong magnetic field. As shown in Figure 2D and Table S3, when the
magnetic moment is less than 8 mB, the crystal facet energy of the TiO2 {100} is larger
than that of the {101}, which is consistent with the existing literature. However, when
the magnetic moment exceeds 8 mB, this trend is reversed, and the crystal facet en-
ergy of the {100} gradually becomes lower than that of the {101}, implying that the
{100} facets are more stable under a strong magnetic field.12 To further understand
the interfacial charge redistribution, the ultraviolet photoelectron spectrometer
(UPS) work function (F), DOS plots, and UV-vis spectrum are used to provide the
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Figure 2. Investigation of Formation Mechanism and Charge Redistribution of As-Made TiO2
Samples
(A) Core-level XPS spectra of Ti 2p.
(B) Contour map of Ti 2p XPS.
(C) The ratio of Ti 2p1/2 to p3/2 in TiO2 prepared under different magnetic field strengths.
(D) Curves of energy difference of {100} and {101} at various magnetic moments.
(E) Secondary electron cutoff (Ecutoff).
(F) TDOS and partial DOS of TiO2 {101} and {001} facets.
location of the Fermi level (Ef), conduction band (CB), and valence band (VB), respec-
tively (Figures 2E, 2F, and S10–S14). The corresponding schematic illustration of the
electronic band structures of the TiO2 prepared under 0 and 1,000 Gs are shown in
Figure S15, which shows the newly formed surface heterojunction lead the directional
transfer of photo-generated electrons and holes to facilitate photocatalytic CO2
reduction, and this is also confirmed by the increased lifetime because the slower
recombination of photo-generated electrons and holes facilitated the efficient sep-
aration of photo-generated electrons and holes (Figure S16; Table S4).13
Active Low-Coordinate Ti Atoms
The hybrid orbital theory indicates the formation of low-coordinate Ti atoms under the
strong magnetic field. When the high-angle quantum orbital of the Ti splits more than
the low-angle quantum orbital, the Ti-O bonds in the horizontal direction have more
connection possibilities, which causes lattice distortion (Figure S17).14,15 The Raman
spectra further explains the formation process of low-coordinate Ti. As shown in Figures
4
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Figure 3. Structural Characterization of As-Made TiO2 Samples
(A–F) Raman spectra ([A] and [B]), schematic of Raman active modes (C), XPS spectra of Ti 2p (D),
XPS spectra of O1s (E), and ESR spectra (F) in which the signal at g = 1.996 corresponds to the low-
coordinate Ti atoms.
(G) Best fit for Ti k-edge extended X-ray absorption fine structure (EXAFS) date for TiO2 prepared
under different magnetic field strengths: Fourier transforms (FTs) for experimental and modeled
EXAFS data.
3A–3C, the vibrations of the nondegenerate B1g and A1g, which are located at Raman
shifts of 395 and 529 cmꢀ1, markedly increase with the increasing magnetic field strength
duringpreparation. Thisimpliesthatmoreconnectionpossibilities inthe horizontaldirec-
tion enhance the chaos in the vertical vibration.16 The XPS of the TiO2 prepared under
different magnetic fields demonstrates that as the magnetic field increases, the Ti-O
bonding energy exhibits an obvious downshift relative to that of pristine TiO2, indicating
the lengthening of the Ti-O bonds as well as the formation of a lower-coordinated envi-
ronment of Ti atoms (Figure 3D).17 This lengthening of the Ti-O bonds in the magnetic
fieldcontributestobalancing the chaos,asillustratedinFigures 3A and3B, and alsoplays
the role of reducing the system energy. Moreover, the corresponding O1s XPS illustrates
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that the lengthening of the Ti-O bonds is inevitably accompanied by the formation of
new-bonding O with high energy (Figure 3E). The electron spin resonance (ESR) further
directly verify the magnetic-field-introduced-low-coordinated environment because
the signalat g = 1.996 increases continuouslywith the enhancementofthe magneticfield
strength (Figure 3F).2 To determine the specific coordinated environment of the mag-
netic-field-introduced-low-coordinated Ti further. The X-ray absorption far-edge struc-
ture (XAFS) was used to detect the k-edge of the elemental Ti (Figure S18). The best-
fitting results of Ti k-edge (Figure 3G; Table S5) suggest that, as the magnetic field
˚
strength increases, the Ti-O distance increases from 1.97 G 0.01 to 2.27 G 0.08 A and
the coordination numbers (CN) decreased from 6 to 3 (Figure 3G; Table S5).18,19 The cor-
responding schematic of the specific coordination-environment changes with the mag-
netic field also is shown in Figure S17, which directly unveils the relationship between
the low-coordinate TiO2 {100} facets and strong magnetic field. The corresponding
X-ray absorption near-edge (XANES) of element O is also performed, which demon-
strates a new transition R, as the O2p orbital hybridized with Ti3p is formed with the intro-
duction of the magnetic field (Figure S19).20
Visible-Light-Driven CO2 Reduction and Photoelectric Performance Test
Tofurther exploretherelationshipbetween theseTiO2{100} facetsand the corresponding
CO2 photocatalytic performance, a series of samples were used for the photocatalytic
conversion of CO2. Gas chromatography (GC) and gas chromatography/mass spectrom-
etry (GC/MS) were used to analyze the photocatalytic products.21 The relevant GC and
GC/MS spectra and calibration curves are shown in Figures S20 and S21, and the yield
of each CO2 conversion product is displayed in Table S6. The relationship between the
product yield and magnetic field strength (Figure 4A) shows that the ratio of CO2 photo-
catalytic products is strongly dependent on the magnetic field strength during the prep-
aration process. In contrast to the C1 products (CH4, CO, COOH, and CH3OH) from the
photocatalytic conversion of CO2 mentioned in most other research, a large amount of
C2H5OH was obtained. Furthermore, as the magnetic field strength increases, that is,
the (100) crystal facet exposure ratio increases, the C2H5OH yield gradually increases,
and CH4 formation is suppressed. Amazingly, when the field strength was increased
from 0 to 1,000 Gs, the amount of C2H5OH produced increased from 0.28 to 6.16 mmol
gꢀ1 ꢀ1, and the reaction selectivity increased from 6.63% to 66.71% (Figure 4B). The
h
CH4 production rate steadily declined from 3.0 to 1.6 mmol gꢀ1 hꢀ1, while its selectivity
decreased from 71.06% to 17.33%. To further validate the origin of C2H5OH from CO2
reduction, 13CO2 isotopic-labelingexperimentswere also carried out, andthe GCspectra
of C2H5OH are shown in Figure 4C. The GC-MS spectra in Figure 4D confirm that the
ethanol product originate from the C-C coupling of CO2 rather than impurities, because
the introduced 13CO2 and 12CO2 released from the NaHCO3 together formed 13C2H5OH
(m/z = 48), 12CH313CH2OH+13CH313CH2O (m/z = 47), 12CH213CH2O (m/z = 46), and
13CH313CH2 (m/z = 30.6).22 Moreover, the product distribution under different irradiation
wavelengths and the cycle performance are also given in Figures S22 and S23.
To further clarify the photoelectrochemical (PEC) properties of the samples pre-
pared under different magnetic fields, cyclic voltammetry (CV) was performed at
different scan rates (Figure S24). When the magnetic field strength was 0 Gs, only
two reduction peaks were visible in the CV curves, that is, the peak of the reduction
of CO2 to CO at ꢁ ꢀ0.12 V (versus RHE) and the peak of the reduction of H2O to H2 at
ꢁ ꢀ0.45 V (versus RHE) (Figure S25). As the magnetic field strength increases, a new
CO2 reduction peak, namely CO2/C2H5OH, appeared at ꢁ 0.08 V (versus RHE), and
the reduction peak increases as the magnetic field strength increases (Figure S24).23
The CV results match the previous trend in the product distribution (Figure 4A),
i.e., as the field strength increases, the exposure ratio of the {100} facet increases,
6
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Figure 4. Visible-Light-Driven CO2 Reduction and Photoelectric Performance Test
(A and B) Rate of photocatalytic products (A) and selectivity (B) on TiO2 prepared at different
magnetic strengths.
(C and D) Gas chromatograms (C) and GC-MS spectra (D) of C2H5OH when the reactant solution
was 12CO2 and 13CO2 saturated 0.1M NaHCO3, respectively.
(E) Randles-Sevcik plots of C2H5OH and CO in photoelectric performance testing.
resulting in a better photocatalytic performance for the conversion of CO2 into
C2H5OH.24,25 To quantify the relationship between the active area of the photoelec-
trode and magnetic field, the Randles-Sevcik equation (Equation 1) was used to pro-
cess the CV data obtained at different scan rates (Figure S24).26
ip = 2.693105n3/2AD1/2Cv1/2
(Equation 1)
where A is the active area of the electrode, D is the diffusion constant, C is the peak
current density (ip), and v is the scan rate.
We obtained the relationships between the peak current density (iP) of C2H5OH and CO
and the square of the scan rate (v1/2) (Figures 4E and 4F). Since the slope obtained by
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Figure 5. DFT Calculation and In Situ Infrared Results for the Mechanism of C-C Coupling
(A) Total In-situ Fourier transform infrared spectroscopy (FTIR) on pristine TiO2 and strong magnetic field prepared TiO2.
(B) In-situ FTIR of CO* at 2,080–2,200 cmꢀ1 on pristine TiO2.
(C) In-situ FTIR of CO* at 2,080–2,200 cmꢀ1 on strong magnetic field prepared TiO2.
(D) Free energy diagrams for CO2 reduction and the corresponding energetically favorable structures (blue, Ti; red, O; dark blue, C; and white, H).
linear fitting is proportional to the real active area, assuming that the actual generated
area of C2H5OH was A when the field strength was 833 Gs, the area is excepted to be
2.23 A when the magnetic field strength was increased to 1,000 Gs. However, the actual
active areaforCOproductiondecreasedfromA to0.48A,revealingthattheformation of
CO and C2H5OH exhibit a competitive relationship. The current transformation effi-
ciency (ƞ) values obtained from the calculated linear sweep voltammetry (LSV) data using
Equation 2 (Figures S24 and S26) are shown in Figure S23: 27
JðCO2Þ ꢀ JðN2Þ
h =
(Equation 2)
JðCO2Þ
where J(CO2) is the current density of CO2 conversion in a 0.1 mol NaHCO3 solution
saturated with CO2, and J(N2) is the current density in an N2-saturated 0.05 mol
NaSO4 solution.
The current transformation efficiency of C2H5OH under PEC conditions increased
with the magnetic field strength. When the magnetic field strength was 833 Gs,
the current transformation efficiency of C2H5OH under PEC conditions is 32.2%,
and an increase of only 167 Gs in the magnetic field strength increased the efficiency
from 32.3% to 53.1% (Figure S27). The Faraday efficiency under the corresponding
test conditions is also given in Figure S28 and Table S7.
Mechanism of Photocatalytic CO2 Reduction and C-C Coupling
Combined with Figure 4E, the in-situ infrared results further demonstrate the C-C
coupling in the CO2 photocatalysis on the TiO2 prepared under a strong magnetic field,
owing to the continuously weakened signal assigned to CO*(2,120 cmꢀ1 28
enhanced signal assigned to COCO* (1,720cmꢀ1) and29 *C2H5OH (1,041 and
)
and
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1,410 cmꢀ1) (Figure 5A).30,31 The C-C coupling originates from more adsorption modes
of CO* intermediates on the TiO2 prepared under a strong magnetic field in comparison
to the pristine TiO2 (Figures 5B and 5C), which promotes the connection of CO* inter-
mediates rather than rapid desorption to CO product. Based on the previous analysis,
we established a structural model (Figures S29 and S30) and performed density func-
tional theory (DFT) calculations. Figure 5D illustrates the free energy diagram of CO2
on the prepared Ti {100} facets, and the specific path is shown below:
CO2/*COOH/CO*/*COCO/*COCOH/*COCHOH/*COCH2OH/
CH2CH2OH/CH3CH2OH,
The connection of CO* is the rate-determining step of the reaction, which is consis-
tent with the previous analysis in Figure 4E. The dicarbon product C2H5OH can form
more easily on the low-coordinate {100} facet because the formation of the CO* in-
termediate only needs to overcome an energy barrier of 2 eV but needs to overcome
2.8 eV on the original {101} facet. Subsequently, the unstable CO* species could
further combine to form *COCO on these special TiO2 {100} facets. Moreover, as
described previously, the formed heterojunction allows the TiO2 to utilize light en-
ergy more effectively to transfer additional photo-generated electrons for the reac-
tion, whereby the selectivity and yield of the photocatalytic conversion of CO2 into
C2H5OH are greatly enhanced.
Conclusion
In summary, introducing strong magnetic fields into the preparation of TiO2 results
in the directional splitting of the electron orbitals in Ti atoms, which eventually leads
to the generation of more low-coordinate TiO2 {100}. The experimental results and
density functional theory (DFT) calculations both reveal that this newly created TiO2
structure and the electronic reconstruction enhanced the coupling of the CO* inter-
mediates leading to the yield rate of CO2 into C2H5OH improving 22-fold compared
to the pristine TiO2. This study has pioneered controllable facet engineering under a
magnetic field and provides an effective method for the highly selective and active
photocatalytic conversion of CO2 into C2H5OH.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2020.06.033.
EXPERIMENTAL PROCEDURES
Resource Availability
Lead Contact
Further information and requests for resources and reagents should be directed to
and will be fulfilled by the Lead Contact, Prof. Shouhua Feng (shfeng @ jlu.edu.cn).
Materials Availability
This study did not generate new unique reagents.
Data and Code Availability
This study did not generate datasets or codes.
General Procedures
Synthesis of the TiO2 Electrode
Anodization process: polished Ti pieces (2 3 2 cm2) were ultrasonically washed succes-
sively with acetone, ethanol, and deionized water for 15 min. A cleaned Ti piece (2 3
2 cm2) was used as the positive electrode, and a Pt piece (2 3 2 cm2) was used as the
negative electrode. The positive and negative electrodes were parallel and spaced
2 cm apart. The reaction solution contained 0.2 g of NH4F, 60 ml of ethylene glycol,
Chem 6, 1–12, September 10, 2020
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Please cite this article in press as: Jiang et al., Magnetic-Field-Regulated TiO2 {100} Facets: A Strategy for C-C Coupling in CO2 Photocatalytic
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and 1.2 ml of deionized water (DI) for anodization of the Ti at room temperature. A
voltage of 30 V was applied to the Ti piece for a 30 min. During this process, a strong
electromagnet (China, XDA-160/50) was placed 4 cm from the Ti plate to provide a
magnetic field for the anodization process. A surface magnetometer (American,
BELL4100) was attached to the center of the Ti plate to determine the actual magnetic
field strength on the plate during the process. The field strengths at the Ti electrode
were 0 Gs, 83 Gs, 167 Gs, 250 Gs, 333 Gs, 418 Gs, 500 Gs, 583 Gs, 665 Gs, 750 Gs,
833 Gs, 918 Gs, and 1,000 Gs. The electrode sheets after anodization were dried under
nitrogen for 2 h and used for subsequent experiments. A schematic of the magnetic
field-assisted material preparation is shown in Figure S32.
Calcination process: the samples were placed in a muffle furnace with a temperature-
program function, and the muffle furnace was heated from room temperature to
450ꢂC at a heating rate of 12.5 ꢂC/min under an air atmosphere. Once the temperature
reached 450ꢂC, it was maintained for 2 h. When the muffle furnace was cooled to room
temperature, the electrode sheets were removed for use.
Photocatalytic Conversion of CO2 and Photoelectric Performance Test
A custom-made airtight, round-bottomed quartz container was used as the reaction de-
vice. When performing the photoelectric test, the prepared TiO2 was placed in the
quartz reactor 1 cm from the bottom. The Pt piece was used as an electrode, and the
reference electrode was Hg/HgO, Moreover, an electrochemical workstation (ZEN-
NIUM, Zahne, Germany) was used to provide the bias-voltage for the three-electrode
system, a UV-enhanced Xenon light (PLs-SXE300UV, Perfect light, China) was used as
the reaction light source, and CO2 saturated 0.1 M NaHCO3 was used as the reaction
solution. The schematic diagram of this process was shown in Figure S33. The photoca-
talytic reaction conditions were basically the same as those in the photoelectric perfor-
mance test, except that no external voltage was required from the electrochemical work-
station and the samples had to be placed.
Characterization and Product Detection Methods
Characterization methods: SEM (JSM-6700F, Jeol, Japan) and TEM (TecnaiG2 S-
Twin F20, FEI, Netherlands) were used to examine the morphology, while HRTEM,
SEAD, and XRD (D/MAX2550, Rigaku, Japan) were used to analyze the crystal struc-
ture. Structural evolution at the atomic scale was carried out by JEM ARM200F
(JEOL, Tokyo, Japan), equipped with double CEOS (Heidelberg, Germany) probe
aberration correctors. The high-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF STEM) was performed at 200 kV. The acquired raw data
were processed with Gatan Micrograph Suite software (GMS 3). XPS (ESCALAB250,
American) and Raman spectroscopy (INVIA, Renishaw, Britain) were used for the
analysis of the orbitals. HR-UPS (R3000, PREVAC sp.z.0.0, Poland) was used to deter-
mine the location of the Fermi level (EF). Steady/Transient State Fluorescen-
ce(FLS920, Edinburgh Instruments, UK) was used to study the fluorescence lifetime.
XAS (beamline BL14W1 of Shanghai Synchrotron Radiation Facility in China) and ESR
(JEOR JES-FA200) were used to study the electronic states and bond structures. GC-
MS (TraceDSQ, Finnigan, USA), GC (6890N, Agilent, USA), and HPCL (1290-micro
TOFQ2, Bruker, Germany) were used for the product analysis.
Detection method: the GC-MS instrument equipped with a flame ionization detector
and a PEG-20 m GC column was used for the detection of hydrocarbon products.
The inlet temperature was adjusted to 150ꢂC, the column temperature was set to
80ꢂC, nitrogen was used as the carrier gas, and the gas flow rate was set to 0.2 L
minꢀ1. The H2 product was detected by GC equipped with a TCD detector. The
HCOOH product was detected by HPCL equipped with a UV detector (wavelength:
210 nm, mobile phase: mixture of 92% KH2PO4, and 7% acetonitrile).
10
Chem 6, 1–12, September 10, 2020

Please cite this article in press as: Jiang et al., Magnetic-Field-Regulated TiO2 {100} Facets: A Strategy for C-C Coupling in CO2 Photocatalytic
Conversion, Chem (2020), https://doi.org/10.1016/j.chempr.2020.06.033
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Article
In-situ FTIR Experiments
Fourier transform infrared spectroscopy (FTIR) experiments were conducted on a
BRUKER V70 Fourier Transform Infrared Spectrometer, which was equipped with a
HARRICK In-situ infrared high-temperature reaction cell and a mercury-cadmium-
telluride MCT detector. The sample was pretreated at 473 K for 0.5 h under Ar to re-
move moisture and impurities. Thereafter, the sample was cooled to 303 K to obtain
a background spectrum. Subsequently, a CO2-H2O gas mixture was introduced for
the adsorption of CO* for 1 h. After being purged by a pure Ar stream for 0.5 h, the
sample was maintained sequentially, with recording done every 5 min.
Calculation Detail
The DOS plots of the TiO2 {100} and {101} facets were obtained by first-principle DFT
calculations, for which the CASTEP package based on the plance-wave-pseudo po-
tential approach was used. The subsequent mechanism of the CO2 photocatalytic
conversion was also calculated by DFT using the VASP GGA+PBE with a cutoff en-
˚
ergy of 440eV and a (43431) k-point grid on a 20 A vacuum.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China
(21427802, 21671076, 21621001, and 21831003). The authors thank Beamline
BL14W1 (Shanghai Synchrotron Radiation Facility) for providing beam time. The au-
thors also thank the NSRL beamline BL11U and BL12B-a (National Synchrotron Ra-
diation Laboratory) for providing beam time.
AUTHOR CONTRIBUTIONS
S.-H.F. conceived and designed the research. M.-P.J. carried out most of the exper-
iments and analyzed the data. J.-H.L., D.W., Y.W., and X.W. performed the DFT cal-
culations and analyzed the related data. Z.-D.L. helped conduct the photoelectric
performance test. X.-Y.W. and Z.-B.G. carried out the XAS experiments. All authors
were involved in the discussion of the results and commented on the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: October 31, 2019
Revised: December 10, 2019
Accepted: June 24, 2020
Published: July 21, 2020
REFERENCES
1. Yuan, L., Fei, Z., and Cheng, Z. (2018). Electric
power generation via asymmetric moisturizing
of graphene oxide for flexible, printable and
portable electronics. Energy Environ. Sci. 11,
1730–1735.
4. Ong, W.J., Tan, L.L., Chai, S.P., Yong, S.T., and
Mohamed, A.R. (2014). Highly reactive {001}
facets of TiO2-based composites: synthesis,
formation mechanism and characterization.
Nanoscale 6, 1946–2008.
renewable hydrocarbon fuels: state-of-the-art
accomplishment, challenges, and prospects.
Adv. Mater. 26, 4607–4626.
8. Xu, Q.L., Yu, J.G., Zhang, J., Zhang, J.F., and
Liu, G. (2015). Cubic anatase TiO2 nanocrystals
with enhanced photocatalytic CO2 reduction
activity. Chem. Commun. (Camb.) 51, 7950–
7953.
5. Yang, H.G., Sun, C.H., Qiao, S.Z., Zou, J., Liu,
G., Smith, S.C., Cheng, H.M., and Lu, G.Q.
(2008). Anatase TiO2 single crystals with a large
percentage of reactive facets. Nature 453,
638–641.
2. Guo, Y., Chen, S.W., Yu, Y.G., Tian, H., Zhao, Y.,
Ren, J.C., Huang, C., Bian, H., Huang, M., An,
L., et al. (2019). Hydrogen-location-sensitive
modulation of the redox reactivity for oxygen-
deficient TiO2. J. Am. Chem. Soc. 141, 8407–
8411.
9. Chen, Y.L., Wang, Z.W., Chen, S., Ren, H., Li, B.,
Yan, W., Zhang, G., Jiang, J., and Zou, C.
(2018). Electric-field control of Li-doping
induced phase transition in VO2 film with
crystal facet-dependence. Nano Energy 51,
300–307.
6. Yu, J., Low, J., Xiao, W., Zhou, P., and Jaroniec,
M. (2014). Enhanced photocatalytic CO2-
reduction activity of anatase TiO2 by Co-
exposed {001} and {101} facets. J. Am. Chem.
Soc. 136, 8839–8842.
3. Wang, S.L., Luo, X., Zhou, X., Zhu, Y., Chi,
X., Chen, W., Wu, K., Liu, Z., Quek, S.Y.,
and Xu, G.Q. (2017). Fabrication and
properties of a free-standing two-
dimensional Titania. J. Am. Chem. Soc. 139,
15414–15419.
10. Kohler, F., Blaum, K., Block, M., Chenmarev, S.,
¨
7. Tu, W., Zhou, Y., and Zou, Z. (2014).
Photocatalytic conversion of CO2 into
Eliseev, S., Glazov, D.A., Goncharov, M., Hou,
J., Kracke, A., Nesterenko, D.A., et al. (2016).
Chem 6, 1–12, September 10, 2020
11

Please cite this article in press as: Jiang et al., Magnetic-Field-Regulated TiO2 {100} Facets: A Strategy for C-C Coupling in CO2 Photocatalytic
Conversion, Chem (2020), https://doi.org/10.1016/j.chempr.2020.06.033
ll
Article
Isotope dependence of the Zeeman effect in
defect structure of oxide nanowires on SrTiO3. 25. Latempa, T.J., Rani, S., Bao, N., and Grimes,
lithium-like calcium. Nat. Commun. 7, 10246.
Phys. Rev. Lett. 107, 086102.
C.A. (2012). Generation of fuel from CO2
saturated liquids using a p-Si nanowire k n-TiO2
nanotube array photoelectrochemical cell.
Nanoscale 4, 2245–2250.
11. Xiang, Q., Yu, J., and Jaroniec, M. (2011).
Tunable photocatalytic selectivity of TiO2 films
consisted of flower-like microspheres with
exposed {001} facets. Chem. Commun.
(Camb.) 47, 4532–4534.
18. Ceotto, M., Presti, L.L., and Cappelletti, G.
(2012). About the nitrogen location in
nanocrystalline N-doped TiO2: combined DFT
and EXAFS approach. J. Phys. Chem. C 116,
1764–1771.
26. Jiang, M., Wu, H., Li, Z., Ji, D., Li, W., Liu, Y.,
Yuan, D., Wang, B., and Zhang, Z. (2018). Highly
selective photoelectrochemical conversion of
carbon dioxide to formic acid. ACS Sustainable
Chem. Eng. 6, 82–87.
12. Guo, L., Yang, Z., Marcus, K., Li, Z., Luo, B.,
Zhou, L., Wang, X., Du, Y., and Yang, Y. (2018).
MoS2/TiO2 heterostructures as nonmetal
plasmonic photocatalysts for highly efficient
hydrogen evolution. Energy Environ. Sci. 11,
106–114.
19. Chen, L.X., Rajh, T., and Wang, Z.Y. (1997).
XAFS studies of surface structures of TiO2
nanoparticles and photocatalytic reduction of
metal ions. J. Phys. Chem. B 101, 10688–10697.
27. Shen, Q., Chen, Z., Huang, X., Liu, M., and
Zhao, G. (2015). High-yield and selective
photo-electrocatalytic reduction of CO2 to
formate by metallic copper decorated Co3O4
nanotube arrays. Environ. Sci. Technol. 49,
5828–5835.
20. Kruger, P., Sluban, M., Umek, P., Guttmann, P.,
¨
and Bittencourt, C. (2017). Chemical bond
modification upon phase transformation of
TiO2 nanoribbons revealed by nanoscale X-ray
linear dichroism. J. Phys. Chem. C 121, 17038–
17042.
13. Dong, F., Zhao, Z.W., Xiong, T., Ni, Z., Zhang,
W., Sun, Y., and Ho, W.K. (2013). In situ
Construction of g-C3N4/g-C3N4 metal-free
heterojunction for enhanced visible-light
photocatalysis. ACS Appl. Mater. Interfaces 5,
11392–11401.
28. Chang, X.X., and Wang, T. (2018). Tuning
Cu/Cu2O interfaces for the reduction of
carbon dioxide to methanol in aqueous
solutions. Angew. Chem. Int. Ed. Engl. 57,
15455–15459.
21. Calvinho, K.U.D., Laursen, A.B., Yap, K.M.K.,
Goetjen, T.A., Hwang, S., Murali, N., Mejia-
Sosa, B., Lubarski, A., Teeluck, K.M., Hall, E.S.,
et al. (2018). Selective CO2 reduction to C3 and
14. Geisinger, K.L., Gibbs, G.V., and Navrotsky, A.
(1985). A molecular orbital study of bond
length and angle variations in framework
structures. Phys. Chem. Minerals 11, 266–283.
C4 oxyhydrocarbons on nickel phosphides at
29. Wang, L.X., He, S.X., Wang, L., Lei, Y., Meng, X.,
and Xiao, F. (2019). Cobalt-nickel catalysts for
selective hydrogenation of carbon dioxide into
ethanol. ACS Catal. 9, 11335–11340.
overpotentials as low as 10 mV. Energy Environ.
Sci. 11, 2550–2559.
15. Woicik, J.C., Nelson, E.J., Kronik, L., Jain, M.,
Chelikowsky, J.R., Heskett, D., Berman, L.E.,
and Herman, G.S. (2002). Hybridization and
bond-orbital components in site-specific X-ray
photoelectron spectra of rutile TiO2. Phys. Rev.
Lett. 89, 077401.
22. Guo, Q., Liang, F., and Wu, L.Z. (2019). Efficient
and selective CO2 reduction integrated with
organic synthesis by solar energy. J. Energy
Chem. 5, 2605–2616.
30. Kattel, S., Yan, B., Yang, Y., Chen, J.G., and Liu,
P. (2016). Optimizing binding energies of key
intermediates for CO2 hydrogenation to
methanol over oxide-supported copper. J. Am.
Chem. Soc. 138, 12440–12450.
16. Liu, B.T., Huang, Y.J., Wen, Y., Du, L., Zeng, W.,
Shi, Y., Zhang, F., Zhu, G., Xu, X., and Wang, Y.
(2012). Highly dispersive {001} facets-exposed
nanocrystalline TiO2 on high quality graphene
as a high performance photocatalyst. J. Mater.
Chem. 22, 7484–7491.
23. Qiao, J., Liu, Y., Hong, F., and Zhang, J. (2014).
A review of catalysts for the electroreduction of
carbon dioxide to produce low-carbon fuels.
Chem. Soc. Rev. 43, 631–675.
31. Das, T., and Deo, G. (2011). Synthesis,
characterization and in situ DRIFTS during the
CO2 hydrogenation reaction over supported
cobalt catalysts. J. Mol. Catal. A Chem. 350,
75–82.
24. Bard, A.J., Parsons, R., and Jordan, J. (1985).
Standard Potentials in Agueous Solutions (CRC
Press).
17. Marshall, M.S.J., Becerra-Toledo, A.E., Marks,
L.D., and Castell, M.R. (2011). Surface and
12
Chem 6, 1–12, September 10, 2020