Efficient photocatalytic hydrogen production over Ni@C/TiO2 nanocomposite under visible light irradiation

Article abstract of DOI:10.1016/j.cplett.2011.01.007

A novel visible-light-driven carbon-coated Ni (Ni@C)/TiO₂ nanocomposite photocatalyst with enhanced photoactivity for hydrogen production was synthesized and characterized for the first time. The resultant Ni@C/TiO₂ nanocomposites ar

Full text of DOI:10.1016/j.cplett.2011.01.007

Chemical Physics Letters 503 (2011) 262–265
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Efficient photocatalytic hydrogen production over Ni@C/TiO2 nanocomposite
under visible light irradiation
⇑
⇑
Peng Zeng, Xungao Zhang , Xiaohu Zhang, Bo Chai, Tianyou Peng
College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel visible-light-driven carbon-coated Ni (Ni@C)/TiO2 nanocomposite photocatalyst with enhanced
photoactivity for hydrogen production was synthesized and characterized for the first time. The resultant
Ni@C/TiO2 nanocomposites are composed of nanorods with an average diameter of ca. 10 nm and length
in the range of 40–100 nm, and exhibit remarkable photostability in an aqueous suspension by using tri-
Received 27 November 2010
In final form 3 January 2011
Available online 6 January 2011
ethanolamine (TEOA) as a sacrificial reagent. Moreover, a hydrogen generation rate of up to 300 l
mol hÀ1
over 5 wt% Ni@C/TiO2 without Pt-loading is achieved under visible light (k P 420 nm) irradiation.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
photogenerated carriers and photosensitization just like the above-
mentioned MWCNTs/TiO2, and the carbon-coated Ni nanocrystal-
As a chemical stable, environmentally friendly, abundant and
cost effective photocatalyst, titania (TiO2) can only absorb UV light
with low quantum efficiency due to its wide energy-gap (ca.
3.2 eV), which limits its photocatalytic H2 production application.
Therefore, the development of visible-light-driven photocatalysts
has received much attention due to its potential applications of a
hydrogen energy conversion system from solar energy [1,2]. More-
over, various noble metal species such as Pt and RuO2 have been
used in most cases as cocatalysts to promote H2 production by
reducing the over-potential loss [3,4], but many meaningful
endeavours have focused on developing photocatalysts with non-
noble metals (e.g. Ni) as cocatalyst [5,6].
lites can act as cocatalyst [5,6], whereas less work has been done
on the photocatalytic H2 production, which is thought to be one
of the best approaches for obtaining clean and sustainable energy
[1–4]. To the best of our knowledge, for the first time in this Letter
we employed a facile hydrothermal process to fabricate a novel
nanocomposite containing TiO2 and Ni@C in situ through a surfac-
tant-mediated templating route [7,12]. The obtained Ni@C/TiO2
nanocomposite was used as photocatalyst for H2 production in tri-
ethanolamine (TEOA) solution under visible-light irradiation, and
the effect of Ni@C on the efficiency of photocatalytic H2 evolution
over the nanocomposites was studied in detail.
Recently, both steady visible-light-driven photoactivity and en-
hanced H2 production efficiency over Pt-loaded multiwalled car-
bon nanotubes (MWCNTs)/TiO2 nanocomposites have been
achieved in our group [7]. MWCNTs in the nanocomposite can
make TiO2 respondent also to visible light region, but it is still nec-
essary to load noble metal Pt as H2 evolution promoter. Our previ-
ous researches have shown especially chemical and thermal
stability when ferromagnetic metal (such as Fe, Co and Ni) was
coated with carbon [8], these carbon-coated ferromagnetic metal
(denoted as metal@C) materials have been used as high-density
magnetic recording media, magnetically separable catalysts and
biomedical applications for years [9–11]. It is reasonable to expect
that Ni@C/TiO2 nanocomposite photocatalyst would enhance the
visible light response and photocatalytic activity for H2 production
due to the synergistic effect of suppression of the recombination of
2. Experimental details
2.1. Material preparation
Ni@C was prepared by an AC arc discharge method under He
gas atmosphere according to our previous publication (ESI, Figure
S1) [8], and pretreated by 2 M HCl solution in order to remove
the uncoated Ni. The Ni@C contains 67.9 wt% cubic Ni nanocrystal-
lites, which is encapsulated by graphite-like carbon, with particle
sizes in the range of 10–50 nm [8]. Ni@C/TiO2 containing various
Ni@C contents were hydrothermally prepared through a cetyltri-
methylammonium bromide (CTAB)/Ti(SO4)2 system, similar to
our previous processes [7,12] except that the initial pH value was
adjusted to 8.0 by ammonia.
2.2. Material characterization
⇑
Corresponding authors. Fax: +86 27 6875 2237.
X-ray powder diffraction (XRD) patterns of Ni@C/TiO2 were ob-
E-mail addresses:
xgzhang66@whu.edu.cn (X. Zhang),
typeng@whu.edu.cn
(T. Peng).
tained on a Bruker D8 advance X-ray diffractometer with a Cu-Ka
0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2011.01.007

P. Zeng et al. / Chemical Physics Letters 503 (2011) 262–265
263
radiation at 40 kV and 100 mA. Field emission scanning electron
2.3. Photocatalytic activity evaluation
microscopy (FESEM) images were obtained on a JSM-7400F elec-
tron microscope. Transmission electron microscopy (TEM) images
were obtained on a LaB6 JEM-2010(HT)-FEF electron microscope.
X-ray photoelectron spectra (XPS) were recorded on a Kratos
Photocatalytic reactions were carried out in an outer irradia-
tion-type photoreactor (Pyrex) connected a closed gas-circulation
system. A 300 W Xe-lamp (PLS-SXE300, Beijing Trusttech Co., Chi-
na) equipped with a cutoff filter (Kenko L-42, k P 420 nm) was ap-
plied as the light source. The photocatalytic reaction was
performed in 100 mL water suspension containing 15 vol% trietha-
nolamine (TEOA) and photocatalyst (150 mg). Then the suspension
was thoroughly degassed to remove air, and irradiated from the
top of the reactor system. The photocatalytic H2 evolution rate
was determined by using SP-6800A gas chromatograph (TCD
detector, 5 Å molecular sieve columns and Ar carrier).
ASIS-HS X-ray photoelectron spectroscope equipped with
a
standard and monochromatic source (Al-K ) operated at 150 W
a
(15 kV, 10 mA). The UV–Vis diffuse reflectance absorption spectra
(DRS) were recorded by a Cary 5000 UV–Vis-NIR spectrophoto-
meter equipped with an integrating sphere. The carbon and nickel
element contents were detected by using Bruker S4 Pioneer X-ray
fluorescence (XRF) spectrometer in a standardless mode.
3. Results and discussion
3.1. XRD result analyses
Figure 1 shows the XRD patterns of Ni@C/TiO2 nanocomposites
containing different Ni@C contents. The main diffraction peaks of
various products can be ascribed to anatase TiO2, while the peaks
at 2h = 26.5° for the graphite-like carbon can also be detected once
the Ni@C content is more than 5 wt%. When the Ni@C content is
larger than 10 wt%, metal Ni species can be seen from the XRD pat-
tern, and those nanocomposites with Ni@C content lower than
10 wt% do not show the relative Ni diffraction peaks, implying that
part Ni might be oxidized and/or lost during the preparation pro-
cess. This assumption can be further validated by the XRF analysis
results (ESI, Table S1), in which Ni content in various nanocompos-
ites is slightly lower than its addition amount.
3.2. Morphology and component analyses
Figure 2a and b shows the FESEM images of the pristine TiO2
and 5 wt% Ni@C/TiO2. It is noteworthy that the pristine TiO2 de-
rived from the present experiment condition shows nanorod-like
Figure 1. XRD patterns of Ni@C/TiO2 nanocomposites containing different Ni@C
contents.
Figure 2. FESEM and TEM images of various products. a: Pristine TiO2; b and c: 5 wt% Ni@C/TiO2; d: EDX spectrum of 5 wt% Ni@C/TiO2.

264
P. Zeng et al. / Chemical Physics Letters 503 (2011) 262–265
morphologies with average diameter of ca. 10 nm and length in the
range of 40–120 nm (Figure 2a). After addition of 5 wt% Ni@C, the
nanocomposite shows shorter nanorod-like structures co-existing
with large amount of nanoparticles (Figure 2b), indicating addition
of Ni@C can retard the formation of nanorod-like TiO2 during the
hydrothermal process. TEM images can offer further insight into
the morphology and microstructure of Ni@C/TiO2. As can be seen
from Figure 2c, the obtained nanocomposite shows nanorod-like
structures coexisting with nanoparticles as observed from Figure
2b. The inserted selected area electron diffraction (SEAD) pattern
taken from Figure 2c reveals the multicrystalline feature of TiO2.
The HRTEM image inserted in Figure 2c shows that most Ni@C
microstructures are maintained during the present conditions,
and the Fast Fourier Transform (FFT) pattern for the Ni@C indicates
the sample containing metal Ni species. The energy dispersive X-
ray (EDX) spectrum in Figure 2d also indicates there are obviously
C and Ni elements in the nanocomposite.
XPS can supply some proofs of above suggestions (ESI, Figure
S2). There is no obvious XPS peak due to Ni species observed from
5 wt% Ni@C/TiO2, implying that most Ni species are enveloped by
the graphite-like carbon and maintained its original structure.
Even for 20 wt% Ni@C/TiO2, XPS pattern of Ni species is quite indis-
tinct as shown in Figure 3. The XPS peaks of 853.9 eV and 855.4 eV
may be assigned to NiO and Ni2O3, respectively. Possibly, NiOx as
cocatalysts can promote the H2 production as reported in previous
literatures [13–15]. Based on above observations, it is reasonable
to conjecture that most Ni@C maintained its original structures,
whereas small part Ni@C changed into metal oxide clusters highly
dispersed on TiO2 during the preparation procedure.
3.3. Absorption spectra analyses
Figure 4 shows the UV–Vis diffuse reflectance absorption spec-
tra (DRS) of Ni@C/TiO2 nanocomposites containing different Ni@C
contents. Upon enhancing the Ni@C content, the absorption inten-
sity of the nanocomposite gradually increases, plausibly due to the
color changed to darker. It can be observed that introducing Ni@C
to TiO2 could enhance the visible light absorption ability of the
nanocomposite with extended absorption tail into the whole range
of visible light region. This absorption enhancement at wave-
lengths greater than 400 nm would have an influence on the fun-
damental process of formation of electron/hole pairs and their
separation in the photocatalytic process under light illumination
[7].
3.4. Photocatalytic activity and stability
Figure 3. XPS spectrum for Ni 2p of 20 wt% Ni@C/TiO2 nanocomposite.
The photoactivities for H2 evolution from an aqueous suspen-
sion containing TEOA and Ni@C/TiO2 with different Ni@C contents
are achieved under visible-light irradiation. Effects of Ni@C content
and photocatalyst addition amount are also investigated (ESI, Fig-
ures S3 and S4). The maximum photocatalytic H2 evolution effi-
ciency is obtained from 5 wt% Ni@C/TiO2 suspension (150 mg). It
is possible that this nanocomposite achieved the optimum light
absorption capability and photogenerated carriers’ separation effi-
ciency [7]. Further experiments show that the pristine Ni@C and
the physical mixture of 5 wt% Ni@C and TiO2 also exhibits detect-
able photoactivity for H2 production; whereas the hydrothermally
prepared nanocomposites containing TiO2 and single soot (amor-
phous carbon, prepared by an AC arc discharge under the same
conditions of Ni@C without addition of Ni power) or Ni powders
(instead of Ni@C) show no detectable photocatalytic H2 production
as shown in Table 1. Even the mixture of soot and Ni powders with
the same content as Ni@C is introduced into the hydrothermal
reaction system, the as-made product still shows no obvious
photoactivity. However, 5 wt% Ni@C/TiO2 shows the highest H2
evolution efficiency among all above products. Therefore, it can
be conjectured that the graphite-like carbon could act as photosen-
sitizer, similar to the MWCNTs absorbing the visible light, which
Figure 4. UV–Vis diffuse reflectance absorption spectra (DRS) of Ni@C/TiO2
nanocomposites containing different Ni@C contents.
Table 1
Photocatalytic H2 evolution activities of various products under visible-light irradiation.
a
b
c
d
Photocatalysts
H2 evolution/
Soot/TiO2
0
Ni/TiO2
0
Ni@C
21.5
(Soot + Ni)/TiO2
0
5 wt%Ni@C/TiO2
300
5 wt%Ni@C + TiO2
34.4
l
mol hÀ1
Photocatalytic conditions: 100 mL aqueous suspension containing 15 vol% TEOA and 150 mg catalyst, k P 420 nm light irradiation 5 h.
a
Soot was prepared by an AC arc discharge process under the same conditions as Ni@C without addition of Ni power.
Ni/TiO2 was prepared by using Ni powder instead of Ni@C.
Soot and Ni powder with the same content as Ni@C were introduced into the hydrothermal system.
Physical mixture of 5 wt% Ni@C and TiO2, and TiO2 was prepared under the same hydrothermal conditions without addition of Ni@C.
b
c
d

P. Zeng et al. / Chemical Physics Letters 503 (2011) 262–265
265
4. Conclusion
A novel Ni@C/TiO2 nanocomposite was successfully synthesized
via a hydrothermal treatment. The experimental results demon-
strate addition of Ni@C benefits the enhancement of visible light
adsorption and photogenerated carrier separation. Steady visible-
light-driven photoactivity and enhanced H2 production efficiency
were both achieved from the present nanocomposite. These results
suggested interesting possibilities for the preparation of more effi-
cient panchromatic photocatalysts with better durability and more
broader light absorption range (e.g. including UV light) in compar-
ison with the organic dye-sensitized semiconductors, which usu-
ally cannot survive from the UV light illumination. Thus, as a
novel and stable artificial visible-light-driven photocatalyst for
H2 production, Ni@C/TiO2 seems to suggest a way in making pan-
chromatic respondent and low-cost (without noble metal-loading)
photocatalysts. Future investigation on its systematic characteriza-
tion and photocatalytic mechanism is under progress.
Figure 5. Time course of the photocatalytic H2 production from 100 mL aqueous
suspension containing TEOA (15 vol%) and 5 wt% Ni@C/TiO2 (150 mg) under visible
light (k P 420 nm) irradiation.
Acknowledgments
This work was supported by the NSFC (20973128), Program for
New Century Excellent Talents in University (NCET-07-0637), and
Independence Innovation Program (2081003) of Wuhan Univer-
sity, China.
can inject the photogenerated electrons into TiO2 conduction band
as mentioned in our previous paper [7]. Meanwhile, the synergetic
effect of the intrinsic properties of components in the present
nanocomposite is also beneficial for the electron transfer in the
conduction band to reduce the water molecules for H2 production
[7,16,17].
Appendix A. Supplementary data
Figure 5 shows the time course of photocatalytic H2 production
over 5 wt% Ni@C/TiO2. As can be seen, a steady visible-light-driven
photoactivity and enhanced H2 production efficiency over Ni@C/
Electronic Supplementary Information (ESI): details of Prepara-
tion procedures and photocatalytic reaction condition optimiza-
tion for the Ni@C/TiO2 nanocomposite. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.cplett.2011.01.007.
TiO2 are both achieved, and more than 4500
tained during 15 h photoreaction with H2 production efficiency
of 300
mol hÀ1, whereas the pristine Ni@C only produced about
21.5
mol hÀ1. Moreover, the photoactivities for H2 production
lmol of H2 was ob-
l
l
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was used instead of TEOA, although the hydrogen evolution rate
was lower.
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