Article
pubs.acs.org/JPCC

Preferential Photooxidation of CO in Hydrogen across the Crystalline
Face Boundary over Spheroidal ZnO Promoted by Cu Ions
Yusuke Yoshida,† Takaomi Itoi,‡ and Yasuo Izumi*,††Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Department of Mechanical Engineering, Graduate School of Engineering, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522,
Japan

‡

ABSTRACT: The design and synthesis of nanoscale zinc oxides (ZnOs) and their applications to photocatalysis are widely
explored. However, the photocatalytic controls needed to design appropriate crystalline faces and promoter sites for eachcatalytic reaction step using ZnO have been rarely reported. This study optimized photocatalytic hydrogen puriﬁcation using thepreferential oxidation (PROX) of CO impurities among nanoscale disk-like, spheroidal, and rod-like ZnO promoted by adsorbed
Cu ions. Four key factors were examined: (1) the diﬀusion length to the surface of separated electrons and holes induced bylight; (2) the crystalline face where formate was formed from CO and its stability; (3) the crystalline face where Cu ionsadsorbed for trapping electrons and reducing O2; and (4) the frequency factor of “charge recombination as cations and anions”
derived from photogenerated holes and electrons, respectively, at the boundary of crystalline faces. The optimal photocatalyst forhydrogen puriﬁcation (CO PROX) was determined to be Cu-spheroidal ZnO. An eﬃcient photocatalytic cycle was obtained byincreasing the frequency factor between unstable unidentate formate on the (0001̅) face and Cu ions adsorbed on neighboring
unsaturated {101̅1} faces.

1. INTRODUCTION
Various nanoparticles, nanotubes, and nanorods of zinc oxides
have been extensively synthesized1−5 for potential applicationsin optical and electric materials,4,5 photovoltaics,6,7 andphotocatalysts.8 The electric potentials at the bottom of the
conduction band (CB) and at the top of the valence band (VB)of ZnO are similar to those of TiO2; therefore, the band gap
values are also similar (in the range of 3.0−3.2 eV).7,9 In
contrast to utilizing the strong photooxidation power of TiO2,
the possibility of selective photooxidation utilizing the strong
photooxidation power of ZnO is not well recognized.
Recently, spheroidal ZnO with an average major axis of 47
nm and an average minor axis of 22 nm was preferably grown inthe direction of the [0 0 0 1] axis of wurtzite. The spheroidal
ZnO with 0.5 wt % of adsorbed Cu2+ ions preferentiallyphotooxidized 9700 ppm of CO to 0.35 ppm in 6.3 kPa of H2
in 5 h.10 The achieved concentration of CO satisﬁed theregulation for the use of H2 in fuel cells. The photocatalytic roleof Cu ions was suggested to be as a photogenerated electron
trap that provides electrons to the O2 and/or O2-derived
intermediate species. Unlike in the proposed preferential© 2015 American Chemical Society

oxidation (PROX) mechanism in which CO adsorbs on
metal nanoparticles (e.g., Au on TiO2) and reacts with O2 at
the boundary between metal nanoparticles and the support,11,12
formate species were produced via eq 1:10,12
CO + HO(surface) → HCO2 (surface)(1)

To complete the photocatalytic cycle, protons derived fromone-electron-oxidized formate species eqs 2 and 3) must
transfer and combine with the O2-derived species to restorehydroxy groups (eqs 4 and 5:
HCO2 (surface) + h+ → H+CO2 (surface)(2)

H+CO2 (surface) → CO2 + H+(surface)(3)

O2 → O2‡(surface)(4)

Received: July 27, 2015
Revised: August 31, 2015
Published: August 31, 2015
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O2‡(surface) + 2H+(surface) + 2e− → → → 2HO(surface)sulfate heptahydrate (30 mL; purity >99.5%, Kanto Chemical)
was added to 1.33 M NaOH aqueous solution (90 mL). The
suspension was maintained at 333 K for 2 h. The white
precipitate was ﬁltered using JGWP04700, washed with
deionized water, and dried at 333 K for 12 h. The obtained
white powder was denoted as rod-like ZnO.
Cu ions were adsorbed on disk-like, spheroidal, or rod-likeZnO by mixing 0.150 g of ZnO with 0.40 mM Cu nitrateaqueous solution (30 mL; purity >99.9%, Wako Pure
Chemical) for 30 min, ﬁltrating using JGWP04700, washingwith deionized water (total 100 mL), and drying at 333 K for 8
h. The obtained powder samples were denoted as Cu-disk-like
ZnO, Cu-spheroidal ZnO, and Cu-rod-like ZnO; thecorresponding Cu uptake values were 100, 100, and 64%(0.50, 0.50, and 0.32 wt %), respectively, based on the X-ray
absorbance (the edge jump values in XANES). The Cu content
was typically 0.50 wt % for Cu-disk-like ZnO and Cu-spheroidal
ZnO, but samples containing 0.10 wt % were also prepared bymixing 0.150 g of spheroidal ZnO with 0.080 mM Cu nitrateaqueous solution (30 mL).
The photoreduction of Ag+ ions using the Cu-spheroidal
ZnO sample was monitored by photodeposition from theaqueous Ag nitrate solution. Ag nitrate (1.0 mg, > 99.8%, Wako
Pure Chemical) was dissolved in 30 mL of deionized water, and
0.1 mL of ethanol and 50 mg of Cu-spheroidal ZnO wereadded to the solution. The suspension was irradiated fromabove by UV−visible light from a 300-W xenon arc lamp
(model OPM2-500XQ, Ushio) for 10 min. The distance
between the exit of the lamp house and the surface of thesuspension was 40 cm. The suspension was then ﬁltrated using
a JGWP04700 ﬁlter, washed with 100 mL of deionized water,and dried at 290 K for 72 h. A grayish blue powder was
obtained for TEM observation.
2.2. Photocatalyst Characterization. Nitrogen adsorption isotherm measurements were performed at 77 K in thepressure range of 1.0−90 kPa in a vacuum system connected to
diﬀusion and rotary pumps (10−6 Pa) and equipped with acapacitance manometer (Models CCMT-1000A and GM-2001,
ULVAC). The Brunauer−Emmett−Teller (BET) surface area
(SBET) was calculated using eight-point measurements between
10 and 46 kPa (P/P0 = 0.10−0.45) in the adsorption isotherm.
The samples (40−200 mg) were evacuated, and the temperature was elevated at a rate of 4 K min−1 to 393 K andmaintained at 393 K for 2 h before the measurements.
Scanning electron microscopy (SEM) images were observed
using a model JSM-6510A (JEOL) at the Center for Analytical
Instrumentation, Chiba University. A tungsten ﬁlament wasused in the electron gun and the electron accelerating voltage
was 15 kV. TEM images were obtained using a model H-7650(Hitachi). The samples were mounted on Cu mesh (150 meshper inch) coated with a copolymer ﬁlm of poly(vinyl alcohol)
and formaldehyde (Formvar, Monsanto) and coated withcarbon. A tungsten ﬁlament was used in the electron gun, andthe electron accelerating voltage was 100 kV. The magniﬁcation
ranged between 60000× and 600000×. High-angle annular darkﬁeld (HAADF) scanning TEM (STEM) and HR-TEM images
were also observed using a model JEM-2100F (JEOL).
Chemical compositions and elemental distributions wereanalyzed with EDS using a Si(Li) detector equipped in the
TEM. X-ray diﬀraction (XRD) patterns were observed using a
D8 ADVANCE diﬀractometer (Bruker) and a Mini-Flex
diﬀractometer (Rigaku).

(5)

In this study, disk-like, spheroidal, and rod-like ZnO
nanoparticles were systematically synthesized to evaluate thegeometric eﬀects on the photo-PROX reaction of CO. Under
light irradiation, charge separation occurs, and the charges
diﬀuse to the surface. The diﬀusion lengths toward the polar (00 0 1) and (0 0 0 1̅) faces are shorter for the disk-like andspheroidal ZnO, while the diﬀusion lengths toward thenonpolar {101̅0} faces are shorter for spheroidal and rod-likeZnO (see table of contents graphic). To evaluate the diﬀusioneﬃciency, the electron diﬀusion toward adsorbed Cu2+ sites wasmonitored by X-ray absorption near-edge structure (XANES).
The production of unstable intermediate formate species via eq
1 was followed by Fourier transform infrared (FTIR)
spectroscopy.
Furthermore, the population of adsorbed Cu ions on eachspheroidal ZnO face was investigated using high-resolution(HR) transmission electron microscopy (TEM) equipped withenergy dispersive spectroscopy (EDS) to compare theelemental mappings of Cu Kα and Zn Kα. Furthermore, theinteraction of O2 with under-coordinated ZnO surfaces was
simulated by the reductive precipitation of Ag+ ions via
irradiation with ultraviolet (UV)−visible light over such
surfaces and monitored by TEM.
The major ﬁnding of this study was that spheroidal ZnO was
the best photo-PROX catalyst due to (1) the shorter
photogenerated hole diﬀusion toward the unstable formate
species that were dominant on the (0 0 0 1̅) face; (2) the
shorter photogenerated electron diﬀusion toward adsorbed
Cu2+ and/or O2-derived species on the under-coordinated facesof ZnO (e.g., {101̅1}); and (3) the minimal distance between
formate species and under-coordinated faces of ZnO.

2. EXPERIMENTAL SECTION
2.1. Photocatalyst Preparation. Disk-like ZnO wassynthesized via the procedure reported in ref 13. Brieﬂy, 2.0mmol of zinc nitrate hexahydrate (>99.9%, Wako Pure
Chemical), 10 mmol of urea (>99%, Wako Pure Chemical),
10 mmol of cetyltrimethylammonium bromide (>98%, Wako
Pure Chemical), and 5.0 mL of 1-butanol (>99%, Wako Pure
Chemical) were dissolved in 50 mL of deionized water (<0.06
μS cm−1). This solution was stirred for 2 h at 290 K and thentransferred into a Teﬂon-lined autoclave (TVS-N2−100,
Taiatsu Techno; inner volume 100 mL) and kept at 393 K
for 15 h. The solution was then cooled to 290 K, and theobtained white powder was ﬁltrated using a 0.2-μm-pore
polytetraﬂuoroethylene membrane ﬁlter (Omnipore
JGWP04700, Millipore) and washed several times with
deionized water (total 500 mL) and ethanol (total 300 mL).
The powder was calcined at 573 K for 5 h. The obtained
powder was denoted as disk-like ZnO.
The synthetic procedures for the spheroidal and rod-likeZnO were previously reported in ref 10. Brieﬂy, zinc acetate
dihydrate (11.6 g; purity >99%, Wako Pure Chemical) wassuspended in ethanol (110 mL). A 0.50 M sodium hydroxide
ethanol solution (50 mL; purity >97%, Kanto Chemical) was
added to the Zn2+ solution and agitated using ultrasound (430W, 38 kHz). The mixed suspension was reﬂuxed at 353 K for
72 h. The obtained white precipitate was ﬁltered using
JGWP04700, washed with deionized water, and dried at 333
K for 12 h. The obtained white powder was denoted asspheroidal ZnO. Separately, a 0.20 M aqueous solution of Zn
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Table 1. Structural Data for ZnO and ZnO with Adsorbed Cu

a

Cu

SBET

entry

samplewt %(m2 g−1)tb

tmajortminor(101̅0):(0002):(101̅1)

A
B
C
D
E
F
G

disk-like ZnO
spheroidal ZnO
rod-like ZnO
FZO-50a
Cu-disk-like ZnO
Cu-spheroidal ZnO
Cu-rod-like ZnO

0
0
0
0
0.50
0.50
0.32

75
35
20
50
47
39
32

24
39
48
21

1700
47
440
20

20
22
80
20

100:81:156
100:121:189
100:115:181
100:93:175

34

35

24

100:96:192

average particle size (nm)

XRD peak ratio

ZnO > 97.4%, Ishihara Sangyo Co. bBased on Scherrer equation t = 0.9λ/(peak width × cos θBragg) for (101̅1) peak.

Thus, the photoreduction rates of CuII sites in the three ZnO
samples were evaluated. A control experiment was performed
separately for a sample under argon at atmospheric pressure.
The distance between the center of the sample and the exit
window of the lamp house (Model UI-502Q) was 42 mm. The
scan steps were ∼9, ∼ 0.3, and ∼2.5 eV in the pre-edge, edge,
and postedge regions, respectively. The data accumulation timewas 1.0 s for each data point. The Cu K-edge absorption energy
value was calibrated to 8980.3 eV for the spectrum of Cu
metal.14,15 The energy position of the monochromator was
reproduced within ±0.1 eV.
The XAFS data were analyzed using an XDAP package.16
The pre-edge background was approximated by the modiﬁed
Victoreen function (C2/E2) + (C1/E) + C0. The background of
postedge oscillation was approximated by a smoothing spline
function calculated using eq 7 for the number of data points:

UV−visible and FTIR spectra were measured using a model
V-650 (JASCO) spectrophotometer equipped with an integrating sphere (ISV-469, JASCO) for diﬀuse reﬂectance measurements in the wavelength region between 200 and 800 nm and amodel FT/IR-4200 spectrophotometer equipped with amercury−cadmium−tellurium-M detector (JASCO) in thewavenumber region between 4000 and 650 cm−1, respectively.
For the FTIR measurements, the energy resolution was set to 2
cm−1. A photocatalyst sample disk (ϕ = 20 mm, 35−150 mg)
was positioned perpendicular to the IR beam in a quartz reactor
equipped with transparent NaCl windows on both sides andconnected to a vacuum system (10−6 Pa). First, the sample disk
was evacuated for 2 h at 290 K in the reactor, and then 4.6 kPa
of CO was introduced.
Cu K-edge X-ray absorption ﬁne structure (XAFS) spectra
were measured at 290 K on beamlines NW10A and 9C in thetransmission mode at the Photon Factory Advanced Ring
(PFAR) and Photon Factory (PF) at the High Energy
Accelerator Research Organization (Tsukuba). Si(3 1 1) andSi(1 1 1) double-crystal monochromators were inserted in the
X-ray beam path. The storage ring energy and ring current at
PFAR were 6.5 GeV and 50 mA, respectively; at PF, therespective values were 2.5 GeV and 450 mA. The X-ray
intensity was maintained at 65% of the maximum ﬂux using a
Piezo translator set on the crystal. The slit opening size in frontof the I0 ionization chamber was 1 mm (vertical) × 2 mm(horizontal). The I0 and Itransmit ionization chambers (Oken)
were purged with N2 and a gas mixture of N2 (85%) + Ar
(15%), respectively.
The sample disk was set in a Pyrex glass cell equipped with a
25-μm-thick polyethylene naphthalate window (Q51-25,
Teijin). The sample was vertically positioned at the center
between the exit window of the I0 ionization chamber and theentrance window of the Itransmit ionization chamber and tilted tomake the X-ray incident angle to the sample surface equal to45°. The sample was illuminated with an Xe arc lamp (500 W;
model UI-502Q, Ushio) placed perpendicular to the X-ray
beam path in the same horizontal plane with initial CO, O2, andH2 pressures of 46 Pa, 92 Pa, and 4.5 kPa, respectively. During
the irradiation, a shoulder peak intensity at 8983.1 eV gradually
grew. The CuI site ratio was evaluated from a linear
approximation of peak intensity at 8983.1 eV for CuII andCuI sites based on eq 6:

data points∑
i=1(μxi − BGi)2
exp( −0.075ki 2)≤ smoothing factor(7)

where k is the wavenumber of the photoelectrons.
Multiple-shell curve-ﬁt analyses were performed for the
Fourier-ﬁltered k3-weighted extended XAFS (EXAFS) data inboth k- and R-space using empirical amplitude and phase-shiftparameters extracted from the EXAFS data for CuO. Theinteratomic distances (R) and the associated coordination
numbers (N) for the Cu−O and Cu(−O−)Cu pairs were set toan R value of 0.1956 nm with an N value of 4 and to an R valueof 0.3028 nm with an N value of 10, respectively, on the basis of
their crystal structures.17 The σ2 value was referenced to themodel parameter (Δσ2). The many-body reduction factor S02
was assumed to be equal for both the sample and the reference.
2.3. Photo-PROX Reaction Tests. The photo-PROX
reaction of CO in predominant hydrogen was performed in aclosed circulating system equipped with a quartz reactor
comprising 23.8 cm2 of the ﬂat bottom section.18−22 The totalinternal volume of the system was 143 mL. Photocatalyst
powder (50 mg) was homogeneously spread at the bottom of
the reactor and evacuated (10−6 Pa) for 2 h prior to thephotocatalytic test, and reaction gas (H2 370 μmol; CO 3.7
μmol; O2 4.5 μmol) was introduced. The photocatalyst wasirradiated by UV−visible light from a 500-W xenon arc lamp
(UI-502Q). The distance between the exit of the lamp houseand the photocatalyst was 24 cm, and the light intensity at thecenter of the photocatalyst was 42 mW cm−2. The products andreactants were analyzed using columns of 13X-S molecularsieve and polyethylene glycol (PEG)-6000 supported on Flusin
P (GL Sciences) set in the online gas chromatograph equippedwith a thermal conductivity detector (Shimadzu, model GC-

Cu I − site ratio
peak intensity@8983.1eV − 0.048(for Cu II 100%)≈0.21(for Cu I 100%) − 0.048(for Cu II 100%)
(6)
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8A). The photocatalytic activity (in units of mol) was evaluatedbased on eqs 8 and 9:
CO conversion(%) = 100 × (nCO,0h − nCO,3h)/nCO,0h(8)

Selectivity(%) = 100 × nCO2 /(nCO2 + n H2O)

(9)

Furthermore, ﬁrst-order reactions were assumed (eq 10), andthe rate constants were evaluated by the ﬁt to eq 11:−

dPCO, tdt

= k(PCO, t − PCO,final)(10)

The diﬀerential eq 10 is solved by integration
PCO, t − PCO,final
PCO,initial − PCO,final= e−kt(11)

3. RESULTS
3.1. SEM and XRD Characterization. The obtained
spheroidal and rod-like ZnO crystallites were anisotropic withmean sizes of 47 nm × 22 nm and 440 nm × 80 nm,
respectively (Table 1B,C).10 In contrast, based on the SEM
image (Figure 1A), the synthesized disk-like ZnO consisted of
nanoplates with a mean thickness of 20 nm and a mean
diameter of 1.7 μm (Table 1A).
Three XRD peaks appeared in the 2θB range (θB: Bragg
angle) of 29°−39° for disk-like, spheroidal, and rod-like ZnO;
these peaks were assigned to the 101̅0, 0002, and 101̅1
reﬂections of wurtzite ZnO (Figure 2a−c), respectively. Amongthe three peaks for disk-like ZnO grown in the direction
perpendicular to the c-axis, the 0002 peak intensity was weaker
than the intensities of the 101̅0 and 101̅1 peaks (Figure 2a,Table 1A). The 101̅1 reﬂection was the most intense among
the three peaks, especially for spheroidal, rod-like, and FZO-50
ZnO (Table 1B−D). The 101̅1 reﬂection was even more
intense for Cu-spheroidal ZnO than that for spheroidal ZnO(Table 1B, F).
3.2. Monitoring of Distribution of Adsorbed Cu Ionsand Photoreduction Sites via Ag+ Precipitation. The
distribution of Cu ions adsorbed on spheroidal ZnO wasobserved by HAADF-STEM and elemental mapping using EDS(Figure 3A). In the HAADF-STEM image of Cu-spheroidal
ZnO, spheroidal particles with sizes of 16−38 nm in the
direction of major axis and 7.6−21 nm in the direction of minor
axis were observed. The Zn Kα, Cu Kα, and O Kα mappings
distributed in the upper left region, similar to in the HAADFSTEM image of Cu-spheroidal ZnO (Figure 3A).
The ED spectra of a ∼5 nm selected area for Cu-spheroidal
ZnO (red circle: edge; blue circle: center; and green circle:edge on the other side for spheroids) were also measured(Figure 3B). The Cu Kα and Zn Kα intensities were comparedamong the selected areas on the spheroidal ZnO. It should be
noted that the Cu Kα peak appeared at 8.0 keV in the blanktest in the absence of sample due to the Cu mesh of the sample
mount (Figure 3B, top right). Therefore, the Cu Kα peakintensity for the blank test was subtracted from the Cu Kα peakintensity for the Cu-spheroidal ZnO sample. After thisadjustment, the peak ratio of (Cu Kα)/(Zn Kα) at the edgeof Cu-spheroidal ZnO was 0.44 ± 0.15 (Figure 3B, red andgreen circles), whereas the ratio at the center of Cu-spheroidal
ZnO was 0.32 ± 0.06 (Figure 3B, blue circle). Taking error into

Figure 1. (A) SEM image of disk-like ZnO. (B) TEM images ofminimal uptake of Ag+ ion on Cu-spheroidal ZnO [(a)−(d)] by UV−
visible irradiation for 10 min. (C) Histogram of the population ofphotodeposited Ag particles on each zone of spheroidal ZnO from theedge to center.

Figure 2. XRD patterns of disk-like ZnO (a), spheroidal ZnO (b), rodlike ZnO (c), and FZO-50 (d).

account, the peak ratio of (Cu Kα)/(Zn Kα) is greater at theedge than at the center.
Minimal uptake of Ag+ ions on Cu-spheroidal ZnO under theirradiation of UV−visible light was monitored by TEM (Figure
1B). In average for the ZnO spheroids, the zone from the edge
to 12.7(±4.3)% was {101̅1} planes and that from 12.7(±4.3)%
to the center (50%) was {101̅0} planes. In the image, the21588

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Figure 3. (A) HAADF-STEM image and the elemental mappings for Cu Kα, Zn Kα, and O Kα in the same view as the HAADF-STEM image for
Cu-spheroidal ZnO. (B) HR-TEM images and EDS spectra for a ∼ 5 nm selected area (red, blue, and green circles) of Cu-spheroidal ZnO.

deposited Ag0 nanoparticles populated mostly on {101̅1} slopes
and also at the boundary between slopes of the {101̅1} and{101̅0} planes and the population gradually decreased to thecenter of ZnO spheroids (see the histogram in Figure 1C).
These results suggest that the preferable reduction sites of Ag+
were on the unsaturated interface sites between the {1011̅ } and{101̅0} planes and/or relatively unsaturated {101̅1} planecompared to {101̅0} planes.
3.3. FTIR Spectra of the Three ZnOs in CO. CO gas (4.6kPa) was introduced into the ZnO (Figure 4) to clearlymonitor the adsorbed species. 4.6 kPa corresponded to by 100
times greater than the CO pressure (46 Pa) for the photoPROX reaction tests in this study. When 46 Pa of CO wasintroduced to ZnO, it was diﬃcult to quantitatively determine
the surface species due to the background modulation resulting
from the amount change of Zn carbonate at the surface.
After 1 h of CO introduction, peaks appeared at 1629, 1604,
1397, and 1342 cm−1 in the spectrum of disk-like ZnO (Figure
4A1), at 1641 and 1605 cm−1 in the spectrum of spheroidal
ZnO (Figure 4C1), and at 1638, 1606, and 1393 cm−1 in the
spectrum of rod-like ZnO (Figure 4E1) in the wavenumber
range of 1700−1300 cm−1. Based on the spectra, these peaks
were assigned to two types of surface formate species including
Cu-ZnOs. Subsequently, the CO gas was evacuated (Figure
4A2−F2). The intercorrelated peaks appearing at 1649−1620
cm−1 and 1442−1393 cm−1 were assigned to the antisymmetric[νas(OCO)] and symmetric stretching vibration [νs(OCO)] of
formate “Species A”. The other intercorrelated peaks appearing
at 1606−1592 cm−1 and 1361−1342 cm−1 were assigned to theantisymmetric [νas(OCO)] and symmetric stretching vibration[νs(OCO)] of formate “Species B”. The background was notalways suﬃciently ﬂat due to the eﬀects of residual carbonate
species adsorbed on basic ZnO (not shown).

Species A and B were formed on ZnO via the adsorption of
CO gas. No signiﬁcant diﬀerences were observed between theintensities of Species A and/or B for each sample per sample
amount (100 mg) at 0.023−0.030 for νas(OCO) and 0.024−
0.045 for νs(OCO). After the evacuation of CO gas, Species A
and B decreased/disappeared in the spectra of all the ZnO
samples (Figure 4A2, C2, E2). For disk-like and rod-like ZnO,
only Species A remained, and its intensity decreased to 5%−
40% and to 48%−56%, respectively, of the intensity under CO.
Conversely, 63% of Species A and 61% of Species B remainedin the case of spheroidal ZnO.
3.4. FTIR Spectra for Three Cu-ZnOs in CO. Surface
formate Species A and B were also conﬁrmed in Cu-ZnO
samples in 4.6 kPa of CO, and the νas(OCO) peak intensities
per sample amount compared to the corresponding peak for
ZnO under CO increased by 1.5−2.8 times because of theeﬀects of the Cu ions (Table 2).
In Cu-disk-like ZnO, the peaks at 1642 and 1400 cm−1 wereassigned as Species A; their intensities decreased by 20%−79%
of those under CO after the removal of CO gas (Figure 4B1,
2). Species B was not clearly observed/resolved.
In Cu-spheroidal ZnO under CO, Species A appeared at
1643 and 1442 cm−1, and Species B appeared at 1602 and 1361
cm−1 (Figure 4D1). In addition, the peak due to CuI−CO
appeared at 2090 cm−1. After the evacuation of CO gas, 56%−
74% of the peaks due to Species B remained, in clear contrast
to Species A, which totally disappeared (Figure 4D2). The
spectrum in Figure 4D2 taken under dark did not change
signiﬁcantly if the Cu-spheroidal ZnO sample disk was
irradiated by UV−visible light. Because the signal/backgroundratio became worse if the samples were irradiated by UV−
visible light, FTIR spectra taken only under dark were shown inthis study. As the trend for Species A was not observed forspheroidal ZnO, the added Cu ions destabilized Species A.
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and 4.5 kPa of H2. Prior to the irradiation, a tiny 1s-3d
quadrupole-transition pre-edge peak appeared at 8978.3−
8978.6 eV for all the samples (Figure 5B-a, a′), indicatingthat the Cu valence state was II for all the incipient samples.
The 1s-3d pre-edge peak intensity and a shoulder peak at8983.1 eV for as-prepared Cu-spheroidal ZnO negligiblychanged if the sample was evacuated at 290 K for 2 h (Figure
5A2a, a′; B2a, a′), demonstrating that evacuation eﬀects on the
Cu state were negligible. During 55 min of irradiation, the preedge peak intensity decreased and a shoulder peak at 8983.1 eV
gradually grew for all the Cu-ZnOs. The CuI site ratio wasevaluated based on eq 6 (see the Experimental Section).
Assuming ﬁrst-order reaction kinetics for the reduction ([CuII]= [CuII]initial e−kCu reductiont), the rate constant kCu reduction wasdetermined to be 0.0082, 0.0076, and 0.0032 min−1 for Cudisk-like ZnO, Cu-spheroidal ZnO, and Cu-rod-like ZnO,
respectively (Table 3E, G, H).
As a control test, the reduction of CuII sites was also
monitored for Cu-spheroidal ZnO under Ar gas and UV−
visible irradiation. The reduction of CuII sites was not clear, andthe k value was determined to be below the detection limit(<0.0005 min−1), which was signiﬁcantly smaller than that
under the mixed reaction gas of CO, O2, and H2 (0.0076 min−1;
Table 3G). Thus, the presence of CO and/or H2 as a reducing
reagent was the requisite for charge separation in Cu-ZnO
samples and the reduction of CuII to CuI.
3.6. Determination of Site Structure by EXAFS
Spectra. The Cu K-edge EXAFS spectra were measured for
the Cu-spheroidal ZnO photocatalyst freshly evacuated at 290
K for 2 h (Figure 6A) and for the sample under CO (46 Pa),O2 (92 Pa), and H2 (4.5 kPa) for 55 min irradiated with an Xe
arc lamp (Figure 6B). Based on the curve-ﬁt analyses, the ﬁrstshell coordination distances of the Cu−O interatomic pair were0.1922 nm with an N value of 3.7 for the fresh sample indarkness (Table 4 and Figures 6A3, 4) and 0.195 nm with an N
value of 3.2 for the sample tested photocatalytically for 55 min(Table 4 and Figures 6B3, 4).
The second-shell coordination of the Cu(−O−)Zn interatomic pair was not clearly observed in the Fourier transform forthe fresh sample (Figure 6A2). In contrast, a weak peak at 0.27
nm (phase-shift uncorrected; Figure 6B2) was nicely ﬁt at 0.300
nm with an N value of 2.4 as Cu(−O−)Zn interatomic pair for
the Cu-spheroidal ZnO subjected to the photo-PROX test with
CO for 55 min (Table 4 and Figure 6B3, 4). No peaks
associated with the metallic Cu−Cu interatomic pair at ∼0.256
nm appeared in the spectra of either sample (Figure 6A2, B2).
For Cu-disk-like ZnO, the interatomic distance of the Cu−O
pair was 0.193 nm with an N value of 4.3 for the fresh sample.
The data negligibly changed to 0.194 nm with an N value of 3.8
for the sample after photocatalytic test for 55 min. The
Cu(−O−)Zn coordination was signiﬁcant only for sample after
photocatalytic test (at 0.3176 nm with the N value of 3.2),
similar to the data for Cu-spheoridal ZnO (Table 4).
For Cu-rod-like ZnO, the R(Cu−O) value was slightly
longer: 0.1952 nm with an N value of 3.2 for the fresh sample.
The data negligibly changed to 0.1957 nm with an N value of
3.6 for the sample after photocatalytic test for 55 min. The
Cu(−O−)Zn coordination was not signiﬁcant both before andafter photocatalytic test (Table 4). In summary, the Cu−O
interatomic pair at 0.1922−0.1957 nm with the N values of
3.2−4.3 for Cu-ZnOs in this study suggested major CuO-likecoordination [N(Cu−O) = 4; Table 4] rather than Cu2O-likecoordination [N(Cu−O) = 2].

Figure 4. FTIR spectra under dark of disk-like ZnO (A), Cu-disk-likeZnO (B), spheroidal ZnO (C), Cu-spheroidal ZnO (D), rod-like ZnO(E), and Cu-rod-like ZnO (F). The conditions are under 4.6 kPa of
CO for 1 h (1) and the sample from 1 after being evacuated for 10 min(2).

In the case of Cu-rod-like ZnO under CO, Species A
appeared at 1644 and 1409 cm−1, while the shoulder peaks ofSpecies B appeared at 1598 and 1359 cm−1 (Figure 4F1). As for
the Cu-spheroidal ZnO, a CuI−CO peak was observed at 2088
cm−1. In addition, 24%−39% of the peaks due to Species A
remained after the evacuation of CO gas, whereas Species B
disappeared (Figure 4F2).
The stabilities of Species A and B under vacuum were
diﬀerent in each sample. Species A was unstable on spheroidal
ZnO, especially Cu ions-adsorbed ZnO. In contrast, Species B
was stable on spheroidal ZnO, likely on the (1011̅ ) face,because of the diﬀerent exposed crystal faces. Species A wasrelatively stable on Cu-disk-like ZnO and Cu-rod-like ZnO;
however, its peak intensity was weak.
3.5. Monitoring of Cu Valence State by XANES. The
photoreduction rates of Cu sites were compared among Cudisk-like ZnO, Cu-spheroidal ZnO, and Cu-rod-like ZnO
irradiated by UV−visible light under 46 Pa of CO, 92 Pa of O2,
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Table 2. Wavenumber and Intensity of FTIR Peaks for Disk-Like, Spheroidal, and Rod-Like ZnOs and Cu-Supported ZnOswith CO and Reference Samples under Various Conditions
peaks (cm−1)(peak height)νas(OCO)
entry

gas (pressure)+δ(CH)

νs(OCO)
ν(CH)+δ(CH)

ν(CO)(A) disk-like ZnOa
1
CO (4.6 kPa)

2

1629,
(0.025)(1604)

1 was evacuated

1 was evacuated2090(0.052)2930(0.005)2857(0.004)2089(0.028)

1 was evacuated2
acetaldehyde(I) Cu/SiO2
1
HCO2H(J) Cu/ZnO/SiO2
1
HCO2H
2

acetaldehyde

Species A

Species A
Species A

Species A
Species B
Species A
Species B

1643,(0.046)1602(0.035)1592(0.026)1442,(0.039)1361(0.039)1343(0.022)

Species A

1638,(0.025)1606(0.024)1620(0.012)1393(0.045)1395(0.025)Species A

1644,(0.062)
1598(0.008)1649(0.015)1409,(0.085)1359(0.007)1400(0.033)Species A

1580

1365

Species B

1625,
1587
1602

1374
1372

Species B
Species B

27
28

Species B
Species B

Species A
Species B

this work
2088(0.020)

1 was evacuated(G) ZnO
1
HCO2H(H) ZnO/SiO2
1
HCO2H

1400
(0.066)1399(0.052)

Species B

this work(F) Cu-rod-like ZnO (0.5 wt % Cu)c1
CO (4.6 kPa)2

1642
(0.069)1643(0.014)

Species A

this work(E) rod-like ZnOc
1
CO (4.6 kPa)2

refthis work

1 was evacuated

1 was evacuated1620(0.010)1397,(0.040)1342(0.024)1395(0.002)1641,(0.030)1605(0.023)1631(0.019)1603(0.014)(D) Cu-spheroidal ZnO (0.5 wt %Cu)b1
CO (4.6 kPa)2

assignment

this work(C) spheroidal ZnOb
1
CO (4.6 kPa)2

νs(OCO)

this work(B) Cu-disk-like ZnO (0.5 wt % Cu)a1
CO (4.6 kPa)
2

νas(OCO)

2091(0.004)
2960

Species B
Species A

27

2939

2859

1558

1359

Species B on Cu

2939

2897,
2858
2900,
2857

1626,
1582
1595,
1626,

1360
1385,

Species B
Species B on Zn

2969,
2936

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Table 2. continued
peaks (cm−1)(peak height)νas(OCO)
entry

gas (pressure)

ν(CH)+δ(CH)(K) Cu/ZnO
1
CO2 + H2 (1:9) (101 kPa)a,b,cνs(OCO)

2970

2880

ν(CO)

+δ(CH)

νas(OCO)νs(OCO)

assignment

1557

1358

Species B on Cu

1580

1365

Species B

2740

ref

24

Peak height is converted value if 100 mg of photocatalyst sample is used (35−40 mg,a 120−150 mg,b and 60−120 mgc used in measurements).

Figure 5. (A) Normalized Cu K-edge XANES for Cu-disk-like (1), Cu-spheroidal (2), and Cu-rod-like ZnO (3) as prepared (a), evacuated at 290 K
for 2 h before irradiation (a′), at 5 min (b), 21 min (c), 38 min (d), and 55 min (e) from the start of UV−visible light irradiation from a 500-W Xe
arc lamp with the initial pressures of CO (46 Pa), O2 (92 Pa), and H2 (4.5 kPa), at 1 min after irradiation (f), and completely reduced CuI-spheroidal
ZnO (g). (B) Expanded view of the pre-edge region in panel A. (C) The intensity change in the shoulder peak at 8983.1 eV during irradiation by
UV−visible light for Cu-disk-like (•), Cu-spheroidal (⧫), and Cu-rod-like ZnOs (■).

Table 3. Results of Photo-PROX Reactions for 55 mina and the Reduction Rates of Cu Ion Sites for Irradiation under CO, O2,
and H2
1st order

CO

CO

kPROX

removal rateb

conv.c

Cu

a

entry

photocatalysts

wt %

A
B
C
D
E
F
G
H

disk-like ZnO
spheroidal ZnO
rod-like ZnO
FZO-50Cu-disk-like ZnO
Cu-spheroidal ZnO

0
0
0
0
0.50
0.10
0.50
0.32

Cu-rod-like ZnO

(10

−3

−1

min )

6.0
4.3
0.1
4.4
17
31
25
2.1

(μmol h−1gcat−1)

28
25
2.4
29
59
97
82
16

Selec.d

Quantum

1st order

eﬃciency

kCu reduction

(%)

(%)

(%)

59
57
2.3
57
91
97
97
30

98
97
51
96
99
96
99
100

0.017
0.015
0.001
0.018
0.036
0.059
0.050
0.010

(10

−3

ref.

−1

min )
10
10
10

8.2
10
7.6
3.2

H2 6.4 kPa, CO 64 Pa, and O2 76 Pa. bIn 0.5 h of reaction. cConversion. dSelectivity.

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rate and conversion decreased in the following order (Table
3A−D):
disk‐like ZnO > FZO − 50
> spheroidal ZnO(12)≫ rod‐like ZnO

For disk-like ZnO, a shorter crystalline growth length in thec-direction (Table 1, tminor) corresponded to a better photoPROX performance, suggesting the importance of photogenerated charge diﬀusion in the c-direction of wurtzite-type
ZnO. In the time course using ZnOs (Figure 7A−C), theamount of decrease in CO nearly corresponded to amount of
CO2 formation. However, the decreases in the amounts of CO
and O2 were nearly equivalent, with the exception of in rod-likeZnO (even more excess of O2 decrease) despite the 2:1
reaction stoichiometry (2CO + O2 → 2CO2), suggesting that
O2 was adsorbed on the ZnO surface.
When Cu was loaded over various ZnOs, the CO removal
rates and conversions were enhanced by Cu and decreased in
the following order (Table 3E−H):
Cu‐spheroidal ZnO (0.10 wt % Cu)> Cu‐spheroidal ZnO (0.50 wt % Cu)> Cu‐disk‐like ZnO
Figure 6. Cu K-edge EXAFS spectra for Cu-spheroidal ZnO (0.5 wt %Cu): (A) a fresh sample evacuated at 290 K for 2 h; and (B) a sample
in CO (46 Pa) + O2 (92 Pa) + H2 (4.5 kPa) irradiated with UV−
visible light for 55 min. (1) The k3-weighted EXAFS χ-function, (2) its
associated Fourier transform, and (3, 4) the best-ﬁt results in (3) kspace and (4) R-space. The thin red lines represent the experimental
values, and the thick blue lines represent the calculated values in panels(3) and (4). The solid lines represent the magnitude, and the dottedlines (- - -) represent the imaginary part in panels (2) and (4).

(13)

≫ Cu‐rod‐like ZnO

The selectivity toward CO2 was not complete using ZnOs:
51−98% (Table 3A−D) and in fact minor water was formed
using spheroidal and rod-like ZnOs (Figures 7B, C). In
contrast, the CO photo-PROX selectivity was improved to behigher than 96% for all the Cu-ZnO photocatalysts (Table 3E−
H). The stoichiometry of the reaction (2CO + O2 → 2CO2)
was preserved in the time course of Cu-ZnOs (Figures 7D−F),
with the exception of the latter part of reaction test for Cuspheroidal ZnO because CO was almost consumed after 1.5 h(Figure 7E).10

3.7. Photo-PROX Tests Using the ZnOs and Cu-ZnOs.
In photo-PROX reaction tests among ZnOs, the CO removal

Table 4. Best-Fit Results of Cu K-Edge EXAFS Analyses for Cu-ZnOsa
Cu(−O−)M(M = Zn, Cu)

Cu−O
Δσ 2

sampleN(nm)(10−5 nm2)

Cu-disk-like ZnO, fresh

4.3(±1.2)
3.8(±1.0)
3.7(±0.4)
3.2(±0.5)
3.2(±0.3)
3.6(±0.4)
4
2

0.193(±0.001)
0.194(±0.001)
0.1922(±0.0004)
0.195(±0.001)
0.1952(±0.0004)
0.1957(±0.0005)
0.19560.18488

5.4(±4.9)
4.0(±4.4)−3.8(±0.6)−1.2(±1.3)−0.4(±0.9)
0.7(±1.1)

usedb
Cu-spheroidal ZnO, fresh
usedb
Cu-rod-like ZnO, fresh
usedb
CuO
Cu2O
a

R

N

3.2
(±0.6)

2.4(±0.3)

10
12

R

Δσ 2

(nm)

(10−5 nm2)

0.3176(±0.0004)

0.300(±0.001)

goodness of ﬁt

ref

1000

this work

150

this work

1600

this work

200

this work

1000

this work

400

this work

6.1(±3.2)−4.5(±2.1)

0.30280.30191

17
17

The values in parahthesis are ﬁt errors. bAfter photocatalysis in CO (46 Pa) + O2 (92 Pa) + H2 (4.5 kPa) for 55 min.
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2K). Formate species are widely accepted to be the reaction
intermediate in the synthesis of methanol from CO (and/or
CO2) and hydrogen over Cu-ZnO catalysts via eq 15:25
2O(surface) + H 2 + CO
→ HCO2 (surface) + OH(surface)The νas(OCO) and νs(OCO) peaks were within/very near
the wavenumber range for formate Species B observed under
4.6 kPa of CO: νas(OCO) at 1606−1592 cm−1 and νs(OCO) at
1361−1342 cm−1 (Table 2). Species B was also reported as
νas(OCO) between 1587 and 1558 cm−1 and νs(OCO)
between 1374 and 1359 cm−1 for ZnO,26 ZnO/SiO2,27 Cu/
SiO2,28 and Cu/ZnO/SiO227 in gaseous formic acid (Table
2G−J), respectively. In the literature,25−28 Species B isconsidered to be bridging formate (Scheme 1A2). The minor
diﬀerence in νas(OCO) wavenumber in this study compared tothose in refs 26−28 can be attributed to the diﬀerent surface;
the surface hydroxy group to react with CO in this study versus
the surface consisted of Zn-terminating sites and O-terminatingsite to react dissociatively with HC(O)O−H.26−28
A pair of Species B peaks was observed at 1602 and 1361
cm−1 for Cu-spheroidal ZnO in 4.6 kPa of CO (Figure 4D1 andTable 2D1). The two peaks remained at 1592 and 1343 cm−1
in vacuum (Figure 4D2 and Table 2D2). In contrast, anotherpair of peaks corresponding to Species A was observed at 1643
and 1442 cm−1 in 4.6 kPa of CO and totally disappeared in
vacuum.
The νas(OCO) and νs(OCO) peaks for formate Species A
under 4.6 kPa of CO ranged from 1649 to 1620 cm−1 and1442−1393 cm−1, respectively (Table 2). Species A wasunstable or totally disappeared under vacuum for Cu-spheroidal
ZnO. Compared to the relatively stable Species B, Species A
can be considered as unidentate formate (Scheme 1A2). The
preferable formation of unidentate formate on the ZnO (0 0 01)̅ face has been reported.23,24
Under CO gas, the formate Species A was formed by thereaction of CO molecules with surface hydroxy groups (i.e., theZnO (0001̅) face on disk-like and rod-like ZnO), whereas
formic acid dissociatively adsorbs as Species A onto Znterminating surface sites (i.e., the ZnO (0001) face)29 or onto
Cu ions over the ZnO surface. Thus, Species A was formed via
the dissociative adsorption of formic acid on surface Zn and O
ions to form Zn−O−C(O)H and hydroxy species, whereas
CO inserted into surface hydroxy groups under CO. Thischemical diﬀerence led to the diﬀerent populations of Species A
derived from CO and formic acid.
The νas(OCO) and νs(OCO) peak intensities associated with
Species A and B for spheroidal ZnO (0.014−0.030) were 54%−66% (adjusted for sample weight) of that of corresponding
peaks for Cu-spheroidal ZnO when the samples were under 4.6
kPa of CO (Figure 4D1, C1) or when the gaseous CO wasevacuated (Figure 4D2, C2). Therefore, half as much formate
species should be formed on or assisted by the Cu-ion sites for
Cu-spheroidal ZnO.
In the comparison of total νas(OCO) and νs(OCO) peakintensities associated with Species A and B in 4.6 kPa of CO or
after the evacuation, no clear trend was found among disk-like,spheroidal, and rod-like ZnOs (total peak intensity: 0.053−
0.094 in CO and 0.012−0.037 after evacuation; Table 2A, C, E)
and among Cu-disk-like ZnO, Cu-spheroidal ZnO, and rod-likeZnO (total peak intensity: 0.135−0.162 in CO and 0.048−
0.066 after evacuation; Table 2B, D, F). The total peakFigure 7. Time course of the PROX reaction of CO in H2 using disklike ZnO (A), spheroidal ZnO (B),10 rod-like ZnO (C),10 Cu-disk-likeZnO (D), Cu-spheroidal ZnO (E),10 and Cu-rod-like ZnO (F)illuminated with UV−visible light. Reaction gases: CO 3.7 μmol (63Pa), H2 370 μmol (6.3 kPa), and O2 4.5 μmol (76 Pa).

4. DISCUSSION
4.1. Photogenerated Electron Diﬀusion. Based on thegrowth of the shoulder peak at 8983.1 eV in the in situ XANES
spectra (Figure 5), the photoreduction rates of CuII sites in thethree ZnO samples followed the order (Table 3):
Cu‐disk‐like ZnO ≈ Cu‐spheroidal ZnO> Cu‐rod‐like ZnO

(15)

(14)

The thin ZnO disks (mean thickness = 20 nm) of the Cudisk-like ZnO and nanocrystallites of the Cu-spheroidal ZnO(mean major axis 35 nm × minor axis 24 nm; Table 1F) wereapparently advantageous for the diﬀusion of photogenerated
electrons from bulk to surface Cu sites. The diﬀerence in charge
diﬀusion path length is the major reason for the poor
photocatalytic activity obtained using Cu-rod-like ZnO.
The diﬀusion path of photogenerated electrons from bulk tosurface was visualized using TEM as the reductive photodeposition of Ag nanoparticles on Cu-spheroidal ZnO sample
starting from Ag+ ions (Figure 1B). As compared to the zones
of edge, {101̅1} faces, and {101̅0} faces in ZnO spheroids(Figure 1C), the Ag nanoparticles populated mostly near theboundary between slopes of the {1011̅ } and {1010̅ } faces andon {101̅1} faces, suggesting that these relatively unsaturated
zones on ZnO spheroids on each end are the key photoreduction sites for Ag+ and possibly for O2. The population ofdeposited Ag nanoparticles gradually decreased to the center of
ZnO spheroids probably because a part of photoreduced Ag
nanoparticles on unsaturated zones noted above diﬀused to{101̅0} planes and stabilized (Figure 1B, C).
4.2. Assignments of Formate Species. In the FTIR
spectrum for Cu-supported ZnO in CO2 + H2,23,24 theabsorption peaks of formate species at 1580 and 1365 cm−1
were assigned to νas(OCO) and νs(OCO), respectively (Table
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oxidation reaction because Cu ions serve as electron traps anddonor, as demonstrated by XANES, Ag+ photodeposition, and
HR-TEM combined with elemental mapping. Furthermore, no
reduced products from CO, e.g., formaldehyde, methanol, or
methane, were detected in this study. Thus, the relatively
unstable Species A formed on the Zn sites by the CO insertion
into surface hydroxy groups under CO is the potential
intermediate to photooxidation to CO2. The well-satisﬁed,
polar (0001̅) face should be favorable to form Species A on Zn(Scheme 1A1).
4.3. Coordination and Distribution of Adsorbed Cu
Sites. In the Fourier transform of the Cu K-edge EXAFS
spectrum of the fresh Cu-spheroidal ZnO evacuated for 2 h(Figure 6A2), only the Cu−O interatomic pair was observed,
and the N value was 3.7 based on the curve-ﬁt analysis. The
XANES spectra (pre-edge peak at 8978.3−8978.6 eV and ashoulder peak at 8983.1 eV) for as-prepared Cu-spheroidal
ZnO negligibly changed if the sample was evacuated for 2 h at
290 K (Figure 5A2a, a′; B2a, a′). Thus, the Cu sites were
exclusively in the valence II under these conditions.10
The N values for Cu−O for the fresh Cu-ZnOs were 4.3−3.2
similar to 4 for CuO (Table 3). Slightly lower N value for fresh
Cu-rod-like ZnO (3.2) should be due to desorption ofadsorbed water, O2, or CO2 by evacuation at 290 K. The
N(Cu−O) value of 3 was reported for Cu-ZnO10,30,31 andCuO−ZnO/Al2O3.32
After the sample was irradiated with UV−visible light for 55
min (Figure 6B2), the interatomic distance of Cu−O changed
negligibly from 0.1922 to 0.195 nm. The N value for Cu−O
slightly decreased to 3.2, and the Cu(−O−)Zn pair wasobserved at 0.300 nm based on the curve-ﬁt analysis. No
metallic Cu−Cu bond(s) was observed throughout this study(Table 4). As evaluated by XANES (Figure 5), 31% of CuII sites
in fresh Cu-spheroidal ZnO was reduced to CuI after
photocatalytic test for 55 min. Thus, theoretical N value for
Cu−O pair in Cu-spheroidal ZnO after photocatalytic test
would be 4 × 0.69 + 2 × 0.31 = 3.38,17 almost in agreementwith the experimental N(Cu−O) value of 3.2 (Table 4). The
photoreduction to CuI in CO + O2 + H2 (0.0076 min−1; Table
3G) negligibly proceeded under Ar (<0.0005 min −1 ),demonstrating the reduction proceeded, e.g.

Scheme 1. Proposed Surface Reaction Steps for the PhotoPROX Reaction of CO in Hydrogen Gas via Formate
Species A and the Route to Accumulate Inactive Formate
Species B for Cu-ZnOs

2Cu II − O + H 2 + 2e− → 2Cu I − OH(16)

It should be noted that the amount of CuI sites wasmonitored (Figure 5) in the dynamic equilibrium of CO photoPROX irradiated by UV−visible light (Scheme 1A). The
reduction rates monitored in Figure 5C would be for CuI sitesthat did not quickly reoxidize for the O2 reduction (Scheme
1A4, 7).10
The HR-TEM results and elemental mapping using Cu Kα(Figure 3B) indicated that CuII ions were well dispersed on
spheroidal ZnO. As a result, more Cu was populated on theedge region of the spheroid (i.e., (0001̅) and {101̅1}) than in
the central region of the spheroid. In relation to the Cu
distribution, photodeposition sites for Ag+ were observed
predominantly on {101̅1} faces and the interface to {101̅0}
faces. Thus, Cu ions preferably adsorb on the unsaturated{101̅1} faces of spheroidal ZnO during catalyst preparation andserve as electron trap sites for Ag+ ions. Furthermore, thecharges generated by light irradiation in nanospheroidal ZnO
easily reached the {101̅1} face with a relatively shorter diﬀusion
length. The mean size of ZnO spheroids was 47 nm × 39 nm

intensities did not correlate to the CO photo-PROX activity
order (Table 3, eqs 12 and 13). Conversely, species B appeared
only for spheroidal ZnO and Cu-spheroidal ZnO after
evacuation of CO (Table 2C2, D2) among all samples
measured for Figure 4. Thus, the preference to Species B anddestabilization of Species A by added Cu (Table 2C2, D2)should be closely related to the best CO photo-PROX
performance using Cu-spheroidal ZnO.
It should be noted that the formates (and CO) adsorbed on
Cu ions (Figure 4) cannot be the intermediate in the CO
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The Journal of Physical Chemistry C(ZnO) or 35 nm × 24 nm (Cu-ZnO), by far shorter comparedto the reported electron diﬀusion length of 50−140 μm.33
Conversely, Cu sites also interacted with CO. CuI sites
formed by the desorption of the surface O atoms of spheroidal
ZnO adsorbed CO, as evidenced by an FTIR peak at 2090−
2089 cm−1 (Figure 4D). The O vacancy on ZnO was reported
in vacuum at 433 K34 and in helium at 513−533 K.35 Owing to
the presence of reducible CuII sites over ZnO surface, formed O
vacancy in vacuum at 290 K should result in the CuI−CO peakin this study. The O vacancy formation on Cu-ZnO in vacuum
was also reported, but at 873 K.36 As discussed above based on
Figure 5A2a, a′; B2a, a′, predominant Cu sites in Cu-spheroidal
ZnO were valence II before the photocatalytic tests.
Furthermore, no reduced products from CO were detected inthis study. Conversely, we did not detect any Cu0 sites in
XANES (Figure 5) and EXAFS (Figure 6) after CO photoPROX tests that are plausible if CuI−CO species was theintermediate to CO2. Furthermore, no CuI−CO peak was
observed for decently active Cu-disk-like ZnO (Table 3E)
under CO + O2 + H2 and UV−visible light, like Figure 4A2, in
contrast that weak CuI−CO peak appeared for quite poor Curod-like ZnO under the conditions, like Figure 4F2. Thus, the
CuI−CO species were inert not to participate in catalysis andthe CuI sites need to be free for the coordination of O2(Scheme 1A3) to work as reduction sites.
The FTIR peak intensity [νas(OCO), νs(OCO)] due toformate Species A and B increased by 1.5−1.9 times after the
addition of 0.5 wt % Cu to spheroidal ZnO (Table 2). We
suspect that the formate Species A (unidentate) was mainly
associated with Cu (Figure 4D1) and unstable in vacuum(Figure 4D2). The preferable Cu distribution on edge regions(i.e., (0001̅) and {101̅1)) rather than in the center (Figure 3B)
along with the preferable population of unstable Species A on
the well-satisﬁed, polar (0001̅) face are the key factors inconverting CO to CO2 during the photo-PROX reaction(Scheme 1A1, 2).
4.4. Proposed CO PROX Reaction Mechanism. The
following photo-PROX reaction steps are proposed: formate
Species A is formed37 (Scheme 1A1 → 2; 4 → 5; eq 1) andoxidized by photogenerated holes (Scheme 1A2 → 3; 5 → 6;eq 2); surface Cu2+ sites, which populate preferably on
unsaturated edge sites of spheroidal ZnO, e.g., {101̅1} faces,
are reduced by photogenerated electrons (2 → 3; 5 → 6);10
and O2 is reduced by electrons via Cu-ion trapping sites
coupled with protons of oxidized formate species H+CO2 acrossthe crystalline face boundary (3 → 4 → 5 → 6 → 7) toregenerate two surface hydroxy groups (Scheme 1A7 → 1; eq
5). The separation of photooxidation and photoreduction sites
was reported depending on the crystal face (e.g., {0 0 1} and {10 1}) of TiO2.38−41 In the case of Cu-spheroidal ZnO, we
suspect that the Species A on the (0001̅) face and Cu sites onthe {101̅1} face should be very close, enabling proton transfer(Scheme 1A3 and 6) and hydroxy restoration (7 → 1). In acontext that a proton transfer controlled the catalysis, this
proposed mechanism including proton transfer from formate
species is closely related to CO PROX mechanism on Au/TiO2
catalyst, whereas proton transfer from carboxy intermediate on
Au was proposed in ref 12. Although following eq 17 is fourelectron process

2CO + O2 → 2CO2hydroxy groups eqs 1−5. Thus, the quantum eﬃciency of thisreaction was calculated to 0.059% for Cu-spheroidal ZnO(Table 3F). It should be noted ∼81% of incident UV−visible
light was transmitted through the photocatalyst under theconditions of CO PROX tests in this study,10 and the eﬀective
quantum eﬃciency would be 0.31% for Cu-spheroidal ZnO.
Species B should be accumulated (Scheme 1A2) and eventually
become inert.
The CO removal rate and rate constant kPROX for Cuspheroidal ZnO (97 μmol h−1 gcat−1 and 0.031 min−1) could
not reach the values using Au-TiO2 (1200 μmol h−1 gcat−1 and
0.141 min−1) by 12−4.5 times under similar reaction conditions(11 mg, 290 K, initial pressure H2 6.3 kPa, CO 63 Pa, O2 150
Pa, no light),1 but the kPROX for Cu-spheroidal ZnO was on thesame order as the one for CuO/CeO242 (0.085 min−1) undersimilar reaction conditions (50 mg, 323 K, initial pressure H2
6.3 kPa, CO 63 Pa, O2 76 Pa, no light).43 The Cu-spheroidal
ZnO photocatalysts sustainably substitute inexpensive Cu andZn for Au11,12 or Ce42 under the condition that enough area isavailable for the irradiation of sunlight.
The photogenerated holes would not reach Species A(Scheme 1B) in rod-like ZnO (mean length = 440 nm andmean diameter = 80 nm; Table 1C) based on the hole diﬀusion
length of 240−440 nm.44,45 The length in the [0 0 0 1]
direction of the rods should limit the CO PROX activity forrod-like ZnO and Cu-rod-like ZnO (eqs 12 and 13).
Conversely, formate Species A were clearly observed for Cudisk-like ZnO (Figure 4B), likely from the reaction of CO withthe predominant (0001)̅ face. In fact, such Species A weresuggested to be good intermediates to CO2 in the CO photoPROX reaction for ZnOs (eq 12). Cu (0.5 wt %) was alsostably adsorbed on disk-like ZnO (Table 1E); however, Cudisk-like ZnO was inferior to Cu-spheroidal ZnO in terms of
CO photo-PROX activity (eq 13). One possible explanation isthat the eﬀective separation of photooxidation and photoreduction sites was diﬃcult for Cu-disk-like ZnO compared tothe photooxidation of Species A on the (0001)̅ face and thephotoreduction of O2 on Cu ions on the {101̅1} faces for Cuspheroidal ZnO (Scheme 1A).

5. CONCLUSIONS
The anisotropic eﬀects of ZnO on the CO photo-PROX
reaction using Cu-ZnO along with the distribution and the roleof adsorbed CuII ions were investigated in this study. The orderof CO photo-PROX activity was as follows: disk-like ZnO >spheroidal ZnO ≫ rod-like ZnO. Adsorbed Cu ions improved
the photocatalytic performance in all ZnOs, and the new orderof activity became Cu-spheroidal ZnO (0.10 wt % of Cu) > Cuspheroidal ZnO (0.50 wt % of Cu > Cu-disk-like ZnO ≫ Curod-like ZnO. For comparison, the photogenerated electrontransfer rates to surface CuII sites for Cu-ZnO were monitored
using XANES and followed the order: Cu-disk-like ZnO ≈ Cuspheroidal ZnO > Cu-rod-like ZnO.
The distributions of Cu2+ sites and reduction sites for Ag+
ions over spheroidal ZnO crystallites were investigated using
elemental mapping and TEM. Cu2+ ions populated more at theedge regions rather than at the center regions of spheroids.
EXAFS demonstrated that the Cu ions were atomicallydispersed on ZnO. The Ag particles formed under irradiation
with UV−visible light in the presence of ethanol deposited
predominantly over the slope {101̅1} faces and the interface to{101̅0} faces of spheroidal ZnO crystallites. Thus, {101̅1} faces
were suggested to be the key photoreduction sites for O2.

(17)

reaction mechanism proposed in Scheme 1 proceeds by two
photons to form two pairs of electrons and holes utilizing
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Conversely, two types of formate species were detected for
ZnO and Cu-ZnO samples under CO gas using FTIR via thereaction of CO and surface hydroxy groups. Species A, whichappeared at 1649−1620 cm−1 and 1442−1393 cm−1, wasrelatively unstable and assigned to unidentate formate. Species
B, which appeared at 1606−1592 cm−1 and 1361−1342 cm−1,
was assigned to bridging formate. The well-satisﬁed, polar(0001̅) face should be favorable to form unidentate formate onZn.
A plausible reaction mechanism for the CO PROX reaction
using the most eﬃcient Cu-spheroidal ZnO photocatalyst was
proposed as follows. Species A was formed on the (0001̅) faceof spheroidal ZnO via the reaction between CO and surfacehydroxy groups and oxidized by photogenerated holes. The
surface Cu2+ sites, which formed preferably on the unsaturated{101̅1} edge faces of spheroidal ZnO, trapped and donatedphotogenerated electrons to O2. Charge recombination
between cations and anions then occurred via proton transferfrom the oxidized formate species H+CO2 to [O2−]‡ species
near the boundary of the (0001̅) and {101̅1} faces. One more
proton transfer coupled to CO oxidation to [HO2−]‡ species
would restore two hydroxy groups to complete a catalytic cycle(Scheme 1A).

■

AUTHOR INFORMATION

Corresponding Author*E-mail: yizumi@faculty.chiba-u.jp. Fax: +81-43-290-2783.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
The authors are grateful for the ﬁnancial support from theGrant-in-Aid for Scientiﬁc Research C (26410204 and22550117) and the Research Fellowship for Young Scientists(25·489; Y.Y.) from the Japan Society for the Promotion of
Science and approval of the Photon Factory Proposal Review
Committee for X-ray measurements (2013G159 and2011G083).

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