J. Wang et al. / Catalysis Communications 46 (2014) 17–21
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3.2. UV–Vis absorption spectra and photoelectrochemical results
From Fig. 3a it can be noticed that the adsorption intensity of the
Cu2O/TNT samples is clearly stronger than bare TNTs through the entire
UV and visible light region, which demonstrate that the light absorption
performance has been improved greatly due to the deposition of the
Cu2O nanoparticles. It also can be noticed that with the electrodeposi-
tion charge increasing, the adsorption intensity of the Cu2O/TNT sam-
ples is increasing. However, the absorption intensity decreases when
the electrodeposition charge reached 1.5 C, which could be attributed
to the covered TiO2 nanotubes with too much Cu2O nanoparticles,
which lead to the enhancement of the light reflection and the reduction
of the light penetration into the TiO2 nanotubes.
From the photoelectrochemical results (see Fig. 3b), one can see that
there is no significant current in the dark, however, in the visible light
illumination, the photocurrents of all the Cu2O/TNT samples increase
significantly while the bare TNTs showed negligible photoresponse.
One can draw the conclusion that the photocurrent of Cu2O/TNTs is
greatly enhanced as compared with the TNTs, which may be ascribed
to the photo-generated carrier separation efficiency of the Cu2O/TNTs
by taking the advantage of the formation of heterostructure. Therefore,
it can be clearly inferred that the heterostructure construction is an ef-
fective way to improve the photoelectric performance.
3.3. Photocatalytic reduction of CO2
In order to evaluate the photocatalytic activity of converting CO2 into
hydrocarbon fuels by TNTs and Cu2O/TNTs, the conversion process was
investigated by using high power pulsed laser as the light source. It is
worth mentioning that laser excitation was a quite selective process
for the end product like CH3OH which is considered to be a future fuel
in the CO2 photocatalytic reduction. We selected the 1.0 C Cu2O/TNTs,
which exhibited the best photocatalytic activity in the CO2 photocatalyt-
ic reduction process. Comparative tests demonstrated that very little
product was found by using TNTs as photocatalysts, which is probably
due to the low conductive band edge potential of TiO2.
Fig. 3. UV–Vis DR spectra of the TNTs and Cu2O/TNTs with different electrodeposition
charges (a); Photocurrent density profiles of the TNTs and Cu2O/TNTs at a bias potential
of 0.0 V (vs. Ag/AgCl) under visible light (b).
3.3.1. Analysis and quantification of methanol product
Gas chromatography (GC) peak positions using standard methanol
and the calibration curve of methanol concentration vs GC peak area
are depicted in Fig. S-1. As shown in Fig. 4a, after every 2 h irradiation
for the Cu2O/TNT sample, the GC peaks of methanol from CO2 photore-
duction are obtained. All the GC peaks appear at exactly 2.46 min reten-
tion time and no other GC peaks were detected, suggesting that the
methanol is the only obtained product through the photocatalytic re-
duction of CO2. Fig. 4a also depicts that as the irradiation time increase
to 6 h, the GC methanol peak areas continuously grow to reach a max-
imum, which also indicates that the Cu2O/TNT samples exhibit better
photocatalytic activity than that of the TNTs. Fig. 4b depicts the concen-
tration and conversion efficiency trend of CO2 photoreduction into
methanol as a function of irradiation time. It demonstrates that the con-
centration of methanol increases with the irradiation time and reaches
to its maximum (55.15 μM/100 mL) at 6 h.
25.3°and 48.0° can be assigned to (101) and (200) planes of TiO2 having
anatase phase (JCPDS Card No. 84-1286) and the diffraction peaks at
40.2° and 53.0° are indexed to the (101) and (102) planes of Ti substrate
(JCPDS Card No. 44-1294), respectively. From the XRD curve of the
Cu2O/TNTs, it can be seen that the diffraction peaks at 27.4° could be
assigned to (110) planes of TiO2 of rutile phase (JCPDS Card No. 78-
2485), suggesting that Cu2O/TNT samples are consisted of anatase pre-
dominantly and a small amount of rutile. The diffraction peaks with 2θ
values of 36.4 and 42.3 can be indexed to (111) and (200) planes of
Cu2O (JCPDS Card No.05-0667) respectively. With the deposition charge
increases from 0.5 C to 1.5 C, the intensities of the Cu2O characteristic
peaks were also increased, which is resulted from the larger amount
of the nanocrystalline Cu2O deposited onto the nanotubes.
In Fig. 2a and b, SEM images illustrate that the TNTs are compact and
the average inner diameter is approximately 80 nm having the
length = 800 nm. From the top view (Fig 2c, e and g), the Cu2O nano-
particles can be clearly noticed which are formed on the surface of the
TNT arrays and the average side length of these Cu2O nanoparticles
with a polyhedral shape is about 80–100 nm. From the cross-sectional
image view (Fig 2d, f and h), it is evident that the entire TNTs including
the top, inner and outer walls of the TNTs are wrapped by the Cu2O
nanoparticles. By increasing the deposition charge from 0.5 to 1.0 C,
the nanotube walls become rougher and rougher and the amount of
the particles embedded into the structure seems also to be increasing.
When the electrodeposition charges reach to 1.5 C, a large amount of
Cu2O nanoparticles arranges so compactly that they almost cover all
the nanotube pores.
3.3.2. Conversion and photonic efficiency of CO2
The efficiency for CO2 conversion into methanol using Cu2O/TNTs
was also calculated. In a typical experiment, an amount of CO2 dis-
solved in 1 L distilled water at atmospheric pressure is 34 mmol, as
calculated by Henry's law. The CO2 pressure in our experiments
was 50 psi (3.4 atm), so the amount of CO2 dissolved in 100 mL
water would be 11.56 mmol. The CO2 conversion efficiency can be
estimated by the ratio of the methanol concentration to CO2 concen-
tration. As depicted in Fig. 4b, the maximum concentration of meth-
anol is 55.15 uM/100 mL and the maximum CO2 conversion efficiency
is about 0.48% after 6 h irradiation.
Wang, Junyi
