Enhancement of photocatalytic H2 production by metal complex electrostatic adsorption on TiO2 (B) nanosheets

Article abstract of DOI:10.1039/c8ta10421b

The assembly of cocatalysts on a semiconductor is a key way to promote the activity in solar hydrogen production. This process can be realized by anchoring metal complexes which can provide catalytically active sites. Despite the most recent advances, the performances and cycling stability of metal complex/semiconductor composite catalysts are still hindered by their poor contact or linkage. Herein, a series of metal complexes, such as [Co(NH₃)₅Cl]Cl₂, [Ni(NH₃)₆]Cl₂, and [Cu(NH₃)₄](OH)₂, are anchored on TiO₂ (B) nanosheets via a facile controllable electrostatic adsorption route. The resultant TiO₂ (B) composites exhibit significantly enhanced activity and stability in photocatalytic hydrogen production compared to the blank TiO₂ (B) counterpart. The [Co(NH₃)₅Cl]²⁺-TiO₂ (B) composite achieves a high activity of 3.9 mmol g^-1 after 8 h (with a turnover frequency (TOF) of 249 h^-1), which is about 57% of that of the benchmark Pt-loaded TiO₂ (B) catalyst. Further experimental and theoretical studies confirm that the strong contact between metal complexes with TiO₂ (B) can facilitate photoexcited electron transfer and separation, which is responsible for the improved activity. Desorption of hydrogen atoms from ammonium in metal complexes is kinetically favorable and they can accept photoexcited electrons to generate H₂. This study can open up a promising pathway to synthesize high-performance metal complex anchored semiconductors by electrostatic adsorption.

Full text of DOI:10.1039/c8ta10421b

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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JPolueransael odfoMnaotetraiadljsuCsht emmairsgtriynsA
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DOI: 10.1039/C8TA10421B
Journal of Materials Chemistry A
PAPER
Enhancement of photocatalytic H2 production by metal complex
electrostatic adsorption on TiO2 (B) nanosheets
Xiangchen Kong, a Zhonghui Gao,*b Yue Gong,a Huiming Huang,a Haifeng Wang,b Porun Liu,c Huajie
Received 00th January 20xx,
Accepted 00th January 20xx
Yin,c Zhenduo Cui,a Zhaoyang Li,a Yanqin Liang,a Shengli Zhu,*a Yunhui Huang *b and Xianjin Yang a
DOI: 10.1039/x0xx00000x
The assembly of cocatalysts on the semiconductor is a key way to promote the activity in solar hydrogen production. This
process can be realized by anchoring metal complexes which can provide catalytically active sites. Despite of the most recent
advances, the performances and cycle stability of metal complex/semiconductor composite catalysts are still hindered by
their poor contact or linkage. Herein, a series of metal complexes, such as [Co(NH3)5Cl]Cl2, [Ni(NH3)6]Cl2, and [Cu(NH3)4](OH)2,
are anchored on the TiO2 (B) nanosheets via a facile controllable electrostatic adsorption route. The resultant TiO2 (B)
composites exhibit significantly enhanced activity and stability in photocatalytic hydrogen production compared to the blank
TiO2 (B) counterpart. The [Co(NH3)5Cl]2+-TiO2 (B) composite achieves a high activity of 3.9 mmol/g (with turnover frequency
(TOF) of 249 h-1), which is about 57% of the benchmark Pt-loaded TiO2 (B) catalyst. Further experimental and theoretical
studies confirm that the strong contact between metal complexes with TiO2 (B) can facilitate photoexcited electrons transfer
and separation, which is responsible for the improved activity. Desorption of hydrogen atoms from ammonium in metal
complexes is kinetically favorable and they can accept photoexcited electrons to generate H2. This study can open up a
promising pathway to synthesize high-performance metal complexes anchored semiconductors by electrostatic adsorption.
www.rsc.org/
that single atoms well-dispersed on supports can provide more
active sites to remarkably boost photocatalytic activities with a
relatively low loading amount of metal components.22-25 In
Introduction
Photocatalytic hydrogen (H2) production from water is a
promising way for the sustainable solar energy conversion.1-6 To
addition, some metal complexes anchored on semiconductors
have been reported, such as Ni2+-dihydrolipoic acid,
metalloporphyrin, and bioinspired nickel phosphine type
complexes.26-29 However, these complexes have some
disadvantages, such as relatively complicated structure, low
decomposition voltage and poor cycling stability.30 Thus, to
create complex/semiconductor systems with improved
photocatalytic activity and long-term stability of for the H2
production is desirable but very challenging.
The efficient transfer of photoexcited electron from
semiconductor to metal complex plays an essential role in
achieving superior photocatalytic activity for hybrid
photocatalysts because the contact or linkage strength
between metal complex and semiconductor can determine the
order of photoexcited electron transfer from semiconductor to
metal complex.31-34 Chemical interaction and physical
adsorption are the main linkage modes for molecular complex
on the surface of semiconductor. It was reported that the close
contact of ruthenium complex on nitrogen-doped Ta2O5 could
facilitate the electron transfer in photocatalytic conversion of
CO2 to HCOOH.35 Despite of some promising findings, the
interaction between anchored metal complex and
semiconductor support is still unstable, which limits the charge
transfer to the associated interface and their potential wide
application. Hence, to strategically strengthen the linkage
realize
a highly efficient H2 evolution, an appropriate
photocatalytic system with low overpotential and low electron–
hole recombination rate is necessary.7-10 The system usually
composes of a semiconductor for solar light harvesting and a
cocatalyst as active center for receiving excited electrons. In
general, noble metals, such as Pt, Pd and Au, are suggested as
efficient cocatalysts, which could be anchored on the
semiconductor supports for photocatalytic H2 evolution.11-16 To
reduce the cost for large scale sustainable photocatalytic H2
production, developing noble-metal-free, earth-abundant
cocatalysts, such as Co, Ni, and Cu, is very crucial.17-20 However,
a novel material design for constructing these cocatalysts on
semiconductor is necessary due to their inferior catalytic
performance compared to noble metal counterparts. From the
microstructure perspective, the size of metal cocatalysts and
their dispersion on the support can influence their
photocatalytic properties significantly.21 One typical example is
a.School of Materials Science and Engineering, Tianjin University, Tianjin 300350,
China. Email: slzhu@tju.edu.cn
b.School of Materials Science and Engineering, Tongji University, Shanghai 201804,
China.
c. Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University,
Queensland 4222, Australia.
† Electronic Supplementary Information (ESI) available: Supplementary figures S1-
S12. See DOI: 10.1039/x0xx00000x
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between metal complex and semiconductor is critically electrostatic adsorption not only effectively links the metal
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DOI: 10.1039/C8TA10421B
important for the development of efficient and stable hybrid complexes with TiO2 (B), but also promotes the transfer of
photocatalysts.
photoexcited electrons. Density functional theory (DFT)
Electrostatic adsorption has been demonstrated as an efficient calculations demonstrate that the electrostatic adsorption of
and robust linkage approach for lots of molecular complexes on metal complex on the TiO2 (B) surface could promote
33, 36
different supports.7,
In this study, we demonstrate a photocatalytic H2 production on metal complex surface. This
general method for anchoring a series of transition metal simple method is expected to be capable of constructing stable
complexes on TiO2 (B) nanosheets by using electrostatic structure and well dispersing active transition metal complexes
adsorption through controlling the pH of the precursor solution. onto the surface of semiconductor, which can obtain an
The absorption of [Co(NH3)5Cl]Cl2 (Co(III)NC), [Ni(NH3)6]Cl2 effective transmission channel to accelerate the electrons
(Ni(II)NC), and [Cu(NH3)4](OH)2 (Cu(II)NO) complexes migration from semiconductor support to the transition metal
significantly enhances the photocatalytic H2 evolution of the complex for efficient hydrogen production.
Fig. 1 (a) Schematic illustration of synthesis procedure of Co(III)NC-TiO2 (B) nanosheets based on electrostatic adsorption; (b, c) TEM and HRTEM images of Co(III)NC-TiO2 (B)
nanosheets; (d) EDS mapping of Co, O and Ti elements of Co(III)NC-TiO2 (B); (e-f) AFM image of the Co(III)NC-TiO2 (B) nanosheet; (g) Zeta potential changes after electrostatic
adsorption of Co(III)NCs; (h) FT-IR spectra of the Co(III)NC, Ni(II)NC, and Cu(II)NO and after electrostatic adsorption on the surfaces of TiO2 (B) nanosheets.
pristine TiO2 (B) nanosheets in relatively alkaline condition. The
favor of the absorption of cationic Co, Ni, and Cu ammine
precursors.37 Zeta potential changes from electronegativity to
electroneutrality in this electrostatic adsorption process,
Results and discussion
The synthetic procedure of Co(III)NC-TiO2 (B) nanosheets hybrid
system based on electrostatic adsorption is schematically
depicted in Figure 1a. This facile synthetic procedure can be
used to adsorb a series of metal complexes on surface of TiO2
(B). The Co(III)NC-TiO2 (B) hybrid system is a representative
example. In this pathway, pH value is the most important
parameter and it could achieve the adsorption of series metal
complexes onto surface of TiO2 (B) nanosheets. As the pH is 12,
the OH groups of the TiO2 (B) become deprotonated. This would
implying the formation of metal complex layer with positive
charge on the surface of TiO2 (B) nanosheets. The two kinds of
electric charges would compensate each other (Figure 1g).
Figure 1b shows the typical TEM image of the as-synthesized
Co(III)NC-TiO2 (B) nanosheets. The HRTEM image of the as-
synthesized TiO2 nanosheets (Figure 1c) shows an interplanar
spacing of 3.59 Å, assignable to the feature (110) plane of TiO2
(B) crystal. It is evident that the Co(III)NC-TiO2 (B) nanosheets
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still possess 2D feature, and no metallic Co nanoparticles are Supporting Information). With the Co content up tVoie2w%ArtaicnledO4nl%ine,
formed on the surface of TiO2 (B). The TEM elemental mappings the photocatalytic properties of H2 DeOvI:o1l0u.t1i0o3n9/Co8TbAv1i0o4u2s1lBy
(Figure 1d) show that the Co, O, and Ti elements are evenly decrease, which due to the influence of saturation adsorption
distributed. In Figures 1e and 1f, the AFM image shows a well of complex on actual loading condition. Moreover, the loading
2D ultrathin feature of TiO2 (B) nanosheet with a thickness of amount of cocatalyst can influence its uniform dispersion,
about 1.5 nm. FT-IR spectra of the Co(III)NC, Ni(II)NC, and contact strength with TiO2 (B) and even the light absorption.
Cu(II)NO and their hybrids are shown in Figure 1h. The The 0.4%-Co(III)NC-TiO2 (B) exhibits excellent long-term stability
characteristic signals at 1610 and 1158cm-1 are ascribed to the according to the cycling photocatalytic test and long-term
bonded –NH3 groups inherited from Co(III)NC, Ni(II)NC, and irradiation test (as shown in Figure 3b). After 48 h irradiation
Cu(II)NO.38, 39 Same characteristic signals of –NH3 groups can be under full spectrum light, the 0.4%-Co(III)NC-TiO2 (B) still
founded in Co(III)NC-TiO2 (B), Ni(II)NC-TiO2 (B) and Cu(II)NO- maintains almost 100% activity with excellent H2 evolution rate,
TiO2 (B), which implies that the Co(III)NC, Ni(II)NC, and Cu(II)NO which is 10.1 times higher than that of pristine TiO2 (B) and even
should be anchored on the surface of TiO2 (B) through the reaches up to 57% of that of Pt-loaded TiO2 (B). The
electrostatic adsorption.40 After adsorption of metal (Co, Ni, Cu) photoluminescence (PL) properties of Co(III)NC-TiO2, Ni(II)NC-
complexes on TiO2 (B), the specific surface area of the TiO2 (B) TiO2 and Cu(II)NO-TiO2 (B) nanosheets are also investigated. The
was reduced by 20%. Their structures and properties (TEM, occurrence of fluorescent light is related to the recombination
XRD, BET, FTIR and XPS) are shown in Figures S1–S7 (Supporting of photoexcited electrons and holes. As shown in Figure 3d, the
Information).
emission wavelength peaks of both pristine TiO2 (B), Co(III)NC-
TiO2, Ni(II)NC-TiO2 and Cu(II)NO-TiO2 (B) locate at 400 nm with
an excitation wavelength of 320 nm. The PL emission intensity
of all samples is in an order of TiO2 (B)
＞Cu(II)NO-TiO2 (B)＞
Ni(II)NC-TiO2 (B) Co(III)NC-TiO2 (B). These results indicate that
＞
the charge carrier separation in Co(III)NC-TiO2, Ni(II)NC-TiO2 and
Cu(II)NO-TiO2 (B) would be more efficient than that in pristine
TiO2 (B). The enhanced charge carrier separation is attributed to
the prolonged excited-state lifetime of these metal ammine
complex-TiO2 (B). Moreover, the electrostatic linkage between
Co(III)NC, Ni(II)NC and Cu(II)NO complexes and TiO2 (B)
facilitates the photoexcited electrons transfer. The intensity of
the EPR signals can demonstrate the concentration of hydroxyl
radical (HO·) formed on the surface under illumination, as
shown in Figure 3e. The results suggest that the Co(III)NC-TiO2
Fig. 2(a) Fourier transform magnitudes of the Co K-edge EXAFS spectra of Co(III)NC-TiO2
(B) and the reference samples; (b) the optimal structure of Co(III)NC electrostatic
adsorbed on TiO2 (B) according to the EXAFS fitting results.
To characterize the bonding of Co(III)NC complex with TiO2 (B)
nanosheets, we performed extended EXAFS spectrometry.
Similar peaks of the Co-N at the region from 1 to 2 Å confirm the
presence of dispersed Co(III)NC complex absorbed on TiO2 (B).
However, the combined EXAFS and XPS results (Figure S4)
confirm that no Co-O bond is formed between Co(III)NC with
TiO2 (B) through the electrostatic adsorption. Figure 2b shows
the most possible structure of Co(III)NC-TiO2 (B) according to
the EXAFS fitting results.41, 42 The overall profile of the Co K-edge
XANES spectra of Co(III)NC and Co(III)NC-TiO2 (B) (Figure S8,
Supporting Information) indicates the presence of trivalent Co.
The photocatalytic H2 evolution performance was evaluated by
using these Co, Ni and Cu species modified TiO2 (B) nanosheets
with CH3OH sacrificial agents in a closed gas circulation system
under full spectrum light irradiation. Figure 3a shows the
photocatalytic H2 evolution activities of Co, Ni and Cu species
modified TiO2 (B) nanosheets. The Co(III)NC-TiO2 (B) nanosheets
exhibit the best photocatalytic H2 evolution performance of 3.9
mmol g-1 after 8 h.. Contrasting with other H2 evolution data of
TiO2 materials, it also has the superior performance in recent
five years (listed in Table S1). Through ICP-OES test, the real
loading with the Co, Ni and Cu species is about 0.4 at.% (Figure
S9 Supporting Information). The effect of loading amount of Co
species for H2 evolution performance is also investigated. The
results show that the hybrid system with 0.4 at.% of Co species
exhibits the highest H2 evolution performance (Figures S10,
(B) exhibits
a
relatively higher photoexcited carrier
concentration compared with pristine TiO2 (B). The Mott–
Schottky slopes (Figure 3f) of all samples show a positive slope,
indicating typical n-type semiconductors with electrons as
majority carriers. Co(III)NC-TiO2 (B) shows a smaller slope than
pristine TiO2 (B), suggesting a faster charge transfer and a higher
carrier density. Moreover, the carrier densities of the
corresponding samples are calculated according to the equation
(2). The obtained carrier densities of the pristine TiO2 (B),
Co(III)NC-TiO2 (B), Ni(II)NC-TiO2 (B), and Cu(II)NO-TiO2 (B) are
1.46×1018, 0.44×1018, 0.724×1018 and 1.04×1018 cm-3,
respectively, indicating that Co(III)NC-TiO2 (B) has a better
conductivity than pristine TiO2 (B). Also this should facilitate the
charge separation and transfer. The AQE results of pristine TiO2
(B) and complex (Co, Ni and Cu)-TiO2 (B) are shown in Figure S11
(Supporting Information). The Co(III)NC-TiO2 (B) has the best
photocatalytic activity among other samples with an AQE of 4.2%
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(400nm). According to the above experimental analysis, it decomposition rate under various reaction time is shown in
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should be demonstrated that the superior photocatalytic Figures S12 (Supporting Information).DOItI: 1i0s.10o3b9/vCio8TuAs104th21aBt
activity of Co(III)NC-TiO2 (B) is resulted from the electrostatic decomposition of Co(III)NC-TiO2 (B) during the photocatalytic
adsorption of Co(III)NC on the surface of TiO2 (B) nanosheets. reaction is very little. Such a robust contact between Co(III)NC
Fig. 3 (a) photocatalytic H2 production, (b) photocatalytic long-term cycling tests, (c) turnover number of the H2 production, (d) photoluminescence (PL) spectra, (e) electron
paramagnetic resonance (EPR) spectra and (f) Mott–Schottky plots over pristine TiO2 (B), Co(III)NC-TiO2 (B), Ni(II)NC-TiO2 (B) and Cu(II)NO-TiO2 (B) nanosheets under irradiation of
full spectrum light.
with TiO2 (B) can synergistically accelerate the transfer of
photoexcited electrons.
The optical properties of as-synthesized TiO2 (B) and Co(III)NC-
TiO2 (B) samples are detected by diffuse reflectance ultraviolet–
visible (UV) spectroscopy (see Figure 4a). The band-edge
absorption of pristine TiO2 (B) is around 366 nm. The Co(III)NC-
TiO2 (B) shows obvious visible light absorption (the band-edge
absorption is about 700 nm) due to the electrostatic adsorption
of Co(III)NC. The plots of transformed Kubelka–Munk function
versus the energy of light (Figure 4b) give band-gap energies of
2.88 and 3.16 eV for Co(III)NC-TiO2 (B) and TiO2 (B), respectively.
The optical properties of Ni(II)NC-TiO2 (B) and Cu(II)NO-TiO2 (B)
are shown in the Figure S13 (Supporting Information). In
addition, the valence band positions of the corresponding
samples are determined by linear extrapolation of the leading
edges of valence band (VB) XPS spectra to the base lines. The
valence band values of 2.37 and 2.97 eV are marked for
Co(III)NC-TiO2 (B) and TiO2 (B), respectively. Based on the results
from band gaps, VB XPS spectra and Mott–Schottky plots, the
detailed band diagrams of the Co(III)NC-TiO2 (B) is depicted in
Fig.4 (a, b) Diffuse reflectance ultraviolet-visible (UV) spectra of pristine TiO2 (B),
Co(III)NC-TiO2 (B) and Co(III)NC samples ;(c) XPS valence band spectrum of pristine TiO2
(B) and Co(III)NC-TiO2 (B); (d) Schematic illustration for the energy band diagrams of
Figure 4d. The valence band maximum (VBM) of the TiO2 (B) was
detected by the XPS measurement and the HOMO, LUMO
energy levels of the Co(III)CN were confirmed by using the DFT
calculation. According to DFT results, the band gap of Co(III)CN
is 0.53 eV, which shows a high electron conductivity. The
Co(III)NC-TiO2 (B).
Moreover, the Co concentration in the solution after
photocatalytic reaction has been determined by using ICP. The
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photogenerated electrons transfer from the conduction band to in neutral and alkaline solution (pH=7 and 12), which is higher
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DOI: 10.1039/C8TA10421B
the LUMO energy level of the Co(III)CN easily, although there is than that (0.2 mmol h ) in acid solution (pH=3). This further
a tiny energy barrier of 0.2 eV between conduction band of TiO2 illustrates that the robust linkage between TiO2 (B) nanosheets
(B) with LUMO energy level. According to the previous studies and metal complex is affected via the electrostatic adsorption,
43, 44, reactions with barriers less than 0.8 eV can proceed which depends on the pH value of testing solution. In the
readily. In our system, the tiny energy barrier of 0.2 eV between photocatalytic hydrogen production, the free energy of atomic
Co(III)CN with TiO2 (B), the Co(III)CN is the appropriate hydrogen adsorption was calculated to analyze the outstanding
candidate as cocatalyst to improve the photocatalytic activity. H2 evolution property of Co(III)NC-TiO2(B) nanosheets. We find
The above-mentioned PL spectra (Figure 3d) also give the that when the dissociative hydrogen atoms are bound to the
evidence that the band bending changes the interfacial electric hexacoordination Co atoms from Co(III)NC, the adsorption
field. It is speculated that the Co(III)NC is kinetically favorable in energy is very high, which is quite hard to adsorb hydrogen
the H2 evolution process, as shown in Figure 5. We deduce that atoms in solution. Moreover, the adsorption energy of
in this hybrid system, TiO2 (B) works for light absorption, the hydrogen atoms on Cl in Co(III)NC reaches up to 2.08 eV
LUMO of Co(III)NC for photoexcite electrons transfer, and calculated by DFT, indicating that absorption of hydrogen atoms
Co(III)NC for proton reduction. The improved electron–hole on Co(III)NC is thermodynamic unfavorable. However, DFT
separation in these metal ammine complex-TiO2 (B) compared calculations also show that the free energy is 0.08 eV, which is
with pristine TiO2 (B) is further demonstrated by transient quite close to that of MoS2 (Figure 5a). The well-dispersed
photocurrent (Figure S14, Supporting Information). It is Co(III)NC species electrostatic adsorption on TiO2 (B)
noteworthy that Co(III)NC electrostatic adsorption on TiO2 (B) nanosheets are kinetically favorable in the H2 evolution through
can not only largely facilitate the photoexcited electron transfer a dehydrogenation of the ammonium intermediate pathway.
but also prolong the active excited-state lifetime.
Figure 5a shows a very low energy barrier of about 0.08 eV. This
metastable state indicates the weak adsorption of H in the
ammonium intermediate. The individual Co(III)NC molecule is in
Figure 5b (STAGE-I) and the TiO2 (B) support is not displayed.
The photoexcite electrons transfer from the conduction band of
TiO2 (B) to the ammonium intermediate (-NH3) through the Co
and then the hydrogen on ammonium is reduced to H2. (STAGE-
II). From thermodynamic modeling in Figure 5b,
dehydrogenation of the ammonium intermediate is proposed
to be the turnover-limiting step (TOLS) in photocatalytic H2
evolution (as shown in the yellow and blue arrow). Meanwhile,
the protons from H2O/CH3OH solution can chemisorb on
ammonium intermediate to achieve a cycle and back to the
original stage (as shown in red arrow).
Conclusions
We developed a facile approach to anchor well-dispersed
Co(III)NC onto TiO2 (B) surface through electrostatic adsorption.
To extend this strategy as a general approach, Ni(II)NC and
Cu(II)NO were also used as cocatalysts to be anchored on TiO2
(B) surface. The resulting Co(III)NC-TiO2(B) exhibits a highly
stable photocatalytic H2 evolution rate of 0.5 mmol h-1 and TOF
of 249 h-1, exceeding those of pristine TiO2 (B) (0.05 mmol h-1
and TOF of 25 h-1). The PL, EPR and Mott–Schottky
investigations disclose that the superior photocatalytic H2
evolution of these hybrids is originated from the strong
electrostatic adsorption between metal complex with TiO2,
which can lead to the efficient charge transfer and the
separation of the photoexcited electrons and holes. The
combined theoretical and experimental studies demonstrate
that H2 evolution via a dehydrogenation of the ammonium
intermediate in the anchored Co(III)NC species is kinetically
favorable (with a free energy of 0.08 eV). This hybrid system
consisting of earth-abundant elements shows a potential
application on the sustainable photocatalytic hydrogen
production.
Fig.5(a) The calculated free energy diagram for hydrogen production; (b) Proposed
possible mechanism of hydrogen formation scheme on the anchored Co(III)NC (the
TiO2 (B) structure is not displayed in here).
To further confirm the stability of this hybrid system originated
from the electrostatic adsorption, we studied the effect of
solution pH on the H2 evolution performance by adjusting the
H2O/CH3OH solution with HCl. Since the surface charge of TiO2
is sensitive to the pH of solution, the Co(III)NC would be
desorbed on TiO2 (B) in acid condition (pH<6). Figure S15
(Supporting Information) shows that the H2 evolution activity
decreases with pH value, implying the desorption of Co(III)NC
anchored on the TiO2 (B) surface. The Co(III)NC-TiO2(B)
nanosheets show a super high H2 evolution rate of 0.5mmol h-1
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TiO2 (B) nanosheets were analyzed by inductively coupled
DOI: 10.1039/C8TA10421B
plasma optical emission spectrometry (ICP-OES, SPS3520UV-
Experimental
2.1 Synthesis of TiO2 (B) material.
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DD, SII Nano Technology Inc., Japan). X-ray absorption fine
structure (EXAFS) signals of the Co K-edge were recorded at the
14 W1 beam line of Beijing Synchrotron Radiation Facility (BSRF,
China) in the fluorescence mode for Co(III)NC-TiO2 (B) sample,
and in the transmission mode for metallic cobalt foil and cobalt
oxide. The electron beam energy was 3.5 GeV with a mean
stored current of 300 mA. The energy of the X-ray was tuned by
using a fixed-exit double-crystal Si (111) monochromator.
Metallic cobalt and cobalt oxide were used for energy
calibration. The intensities of the incident and fluorescence X-
ray were monitored by using a standard N2-flled ion chamber
and an Ar-filled Lytle-type detector, respectively. Fourier
transformed-infrared spectra (FT-IR) were collected on a
thermo-scientific iS10 FT-IR spectrometer. Surface chemical
compositions were measured by X-ray photoelectron
spectroscopy (XPS) by using Al Kα radiation (PHI5000 Versa
Probe). All the binding energies were calibrated using C 1s peak
(BE = 284.8 eV) as standard. Ultraviolet-visible (UV-vis) diffuse
reflectance absorption spectra (DRS) were recorded by using
BaSO4 as the reflectance standard reference with the region of
Ultrathin TiO2 (B) nanosheets were prepared based on our
previous work.45 0.5 mL TiCl4 (Alfa Aesar, 99.6%) was dropped
in 15 mL ethylene glycol (EG, Alfa Aesar, 99%) at room
temperature. After that, 0.5 mL deionized water was added into
the mixture and they were transferred into a 20 mL Teflon-lined
stainless-steel autoclave. The sealed autoclave was heated at
150 °C for 4 h. A white product was collected by centrifugation
and washed by water and ethanol. The product was finally dried
in vacuum at 60 °C for 24 h.
2.2 Preparation of the metallic complexes.
[Co(NH3)5Cl]Cl2: 8 g CoCl2 (Alfa Aesar, 99.6%) and 4 g NH4Cl (Alfa
Aesar, 99%) were added in 25 mL NH4OH solution (25-30%). 13
mL hydrogen peroxide (H2O2) solution was added in the
solution. A brick-red precipitate was collected by centrifugation
and dried in vacuum at 60 °C for 24 h.
[Ni(NH3)6]Cl2: 10 g NiCl4 (Alfa Aesar, 99.6%) and 10 mL NH4OH
(25%-30%) were mixed in 10 mL ice-cold deionized water. A
violet product was collected by centrifugation and dried in
vacuum at 60 °C for 24 h.
[Cu(NH3)4](OH)2: 10 g CuSO4 (Alfa Aesar, 99.6%) was added in
the solution consisting of 10 mL deionized water and 6 mL
NH4OH (25-30%). A sapphire product was collected through a
lyophilization process for 24 h.
2.3 Electrostatic adsorption of metallic complexes on the TiO2 (B)
nanosheets.
100 mg TiO2 (B) powder was dispersed into 100 ml water at
room temperature. The pH of solutions was adjusted to 11.5-12
at room temperature by using NH4OH solution. The nominal
concentrations of metal complexes in the solutions are 0.7, 3.5
and 7% (atomic ratio of metallic complex to TiO2 (B)),
respectively. After being stirred for 2 h, the products were
collected and putted in vacuum for drying.
200–800 nm by
a UV-vis spectrophotometer (UV-2700,
Shimadzu). Photoluminescence (PL) spectra excited at 400 nm
were recorded on a Spex Fluorolog spectrofluorometer. The
electron paramagnetic resonance (EPR) spectra were measured
by JEOL JES-FA200 EPR spectrometer operating at about 9.0 GHz
and 77 K. For EPR, the samples were dispersed in purified
methanol with 5, 5-dimethyl-lpyrroline-N-oxide (DMPO) as
trapper. And the magnetic field was calibrated with Mn (II) in
MgO. Determination of the elements concentration was
performed using inductively coupled plasma mass spectrometry
(ICP-MS, Thermo Scientific, 7000 Series).
2.6 Evaluation of photocatalytic activity for H2.
The photocatalytic activities of 25 mg Co(III)NC-TiO2 (B),
Ni(II)NC-TiO2 (B), Cu(II)NO-TiO2 (B) and Pt-TiO2 (B) samples were
carried out, respectively, in a closed gas circulation system with
an external-irradiation type of a glass reactor (Pefectlight-6A,
China). The photocatalytic test solution contained 100 mL H2O
and 30 mL CH3OH as sacrifice agent. The light source was a 300
W Xenon arc lamp with an AM 1.5 filter (Oriel, USA) and 150
mW cm-3 as simulated sunlight source. The as-formed gas was
analyzed by an online gas chromatograph (GC-8A; Shimadzu)
2.4 Photodeposition of Pt nanoparticles on TiO2 (B) nanosheets. 35
μL PtCl2 (5 mM) solution was added into dispersion consisting
of 20 mg TiO2 (B) and 50 mL H2O. Pt nanoparticles were loaded
onto TiO2 (B) successfully by using a photocatalytic reduction
method with a Xenon arc lamp (150 mW cm-3).
2.5 Characterizations.
The morphology and structure of the samples were
characterized by transmission electron microscopy (TEM) and
high-resolution TEM (HR-TEM) (JEOL-2100 electron microscope
with an accelerating voltage of 200 kV). Energy-dispersive X-ray
spectrometry (EDS) mapping measurement was used to detect
the surface element distribution. The crystallographic
structures of the Co(III)NC-TiO2 (B), Ni(II)NC-TiO2 (B) and
Cu(II)NO-TiO2 (B) were measured by X-ray diffraction (XRD,
Bruker D8 Advance) with Cu Kα radiation. The thickness of TiO2
(B) nanosheets was measured by atomic force microscopy
(AFM, SPA-300HV). The surface potentials of the samples
dispersed in mixed water/ethanol (1/1 volume ratio) solution
were determined by Delsa Nano Zeta Potential (Beckman
Coulter, USA). The actual amounts of Co, Ni and Cu loaded on
equipped with
a thermal conductivity detector (TCD).
Photoelectrochemical measurements of the samples were
performed on an electrochemical workstation (Gamry Interface
1000) with standard three-electrode system: the Co(III)NC-TiO2
(B), Ni(II)NC-TiO2 (B) and Cu(II)NO-TiO2 (B) samples as work
electrodes respectively, Ag/AgCl as reference electrode, and
platinum foil as opposite electrode. The electrolyte was 0.1 M
Na2SO4 aqueous solution. For preparation of the
photoelectrodes, 4 mg corresponding sample was dissolved in
1 mL alcohol and 0.01 mL naphthol solution. The mixtures were
6 | J. Mater.Chem.A
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dip-coated onto a 1 × 1 cm2 FTO glass electrode to form a film
and then dried at room temperature.
2.7 Computational formula of photoconversion efficiency.
The mass activity or specific activity of the catalysts can be
expressed by turnover frequency (TOF), which is usually defined
by the number of reacted molecules to that of an active site and
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (51172159) and Recruitment Program of
Global Experts “1000 Talents Plan” of China (WQ20121200052).
View Article Online
DOI: 10.1039/C8TA10421B
calculated as following Eq. (1):
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10.
11.
12.
13.
14.
15.
16.
17.
풆− + 푯+ →
ퟏퟐ 푯ퟐ
(3)
18.
19.
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Eq. (4).
∆푮 = ∆푬 + ∆풁푷푬 −푻
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푯ퟐ
ퟐ
phase at standard conditions.
Conflicts of interest
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 2018
J. Mater.Chem.A | 7
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