Synthesis and characterization of Ni doped SnO2 microspheres with enhanced visible-light photocatalytic activity

Article abstract of DOI:10.1039/c5ra10268e

Nickel-doped tin dioxide (NDT) microspheres with various doping levels were prepared by hydrothermal synthesis and characterized by using X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, energy-dispersive spectroscopy analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and UV-vis absorption spectroscopy. The formation mechanism of NDT microspheres was discussed. All samples prepared with different Sn/Ni ratios shows higher photocatalytic activity than that of pure SnO₂ and pure NiO under visible light irradiation (λ > 420 nm). The photocatalytic activity is closely related to the composition of as-prepared photocatalysts. It is noted that the NDT microspheres with the doping content of 33.3 mol% exhibit the highest photoactivity. Moreover, this catalyst showed improved stability, and the activity did not weaken significantly after four recycles. The higher activity of NDT microspheres may be attributed to the combined effect of its demonstrated doping levels and appropriate band gap. And the doping of Ni²⁺ may facilitate the generation of electron and hole pairs and inhibit their recombination rate by acting as temporary trapping sites of photoinduced electrons.

Full text of DOI:10.1039/c5ra10268e

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ARTICLE
Synthesis and Characterization of Ni Doped SnO2
Microspheres with Enhanced Visible-light
Photocatalytic Activity
Cite this: DOI: 10.1039/x0xx00000x
Huan Chen, Liyong Ding, Wen Sun, Qingqing Jiang, Juncheng Hu*, Jinlin Li
Received 00th January 2012,
Accepted 00th January 2012
Nickel-doped tin dioxide (NDT) microspheres with various doping levels were prepared by
hydrothermal synthesis and characterized by using X-ray powder diffraction, scanning electron
microscopy, transmission electron microscopy, high-resolution transmission electron microscopy,
energy-dispersive spectroscopy analysis, Fourier Transform infrared spectroscopy, X-ray photoelectron
spectroscopy, and UV−vis absorption spectroscopy. The formation mechanism of NDT microspheres
was discussed. All samples prepared with different Sn/Ni ratios shows higher photocatalytic activity than
that of pure SnO2 and pure NiO under visible light irradiation (λ > 420 nm). The photocatalytic activity
closely relate to the composition of as-prepared photocatalysts. It is noted that the NDT microspheres
with the doping content of 33.3 mol% exhibits the highest photoactivity. Moreover, this catalyst showed
improved stability, and the activity did not weaken significantly after four recycles. The higher activity
of NDT microspheres may be attributed to the combined effect of its demonstrated doping levels,
appropriate band gap. And the doping of Ni2+ may facilitate the generation of electrons and holes pairs
and inhibit their recombination rate by act as a temporary trapping sites of photoinduced electrons.
DOI: 10.1039/x0xx00000x
www.rsc.org/
metals (e.g., Cr, Ni and Mn) or nonmetals (e.g., C, N and S) into
TiO2 to extend its absorption edge to visible region.12 In order to
Introduction
Environmental problems are harmful to human health. The organic
pollutants and toxic water pollutants are difficult to degrade by
natural means. Semiconductor photocatalysts have been widely used
to solve many environment-related issues recently because of low
toxicity, low cost, recyclability, and the ability to facilitate
multielectron transfer processes.1 Traditional semiconductor
materials, such as TiO2,2 Fe2O3 and CdS,3 suffer from low
responsive to visible light, high electron-hole recombination or
photocorrosion. Hence, researches in this field therefore pay more
attention on novel ones which are equipped with highly
photocatalytic activity. Because of sunlight’s distinct advantages of
being clean, accessible and inexhaustible4 for environmental
mediation.5 However, visible light accounts for about 50% of the
sunlight. To make full use of visible light, it is satisfying to exploit
novel visible light sensitive semiconductor photocatalysts.6 Because
of SnO2 with low cost, photochemical stability, high activity for
many photocatalytic reactions, it has raised very strong interest for
various technologically important applications in lithium batteries,7
gas sensing,8,9 as well as in photcatalysis.10,11 However, the instinct
band-gap of SnO2 (ca. 3.6 eV) limits the absorption ability of visible
light of the sunlight, which results in the relatively low efficiency.
Modification is one of the effective approaches to promote the
light absorption, which the preparation of visible light sensitive
photocatalysts could be based on. Such as by doping transition
improve visible light absorption ability of SnO2, efforts have been
done in the controlled synthesis of semiconductor oxides with doped
metals, such as Zn-doped SnO2 and Sn-doped Fe2O3.13,14 It has been
disclosed that several dopants (Mn, Al, Cr, Co, Mg, Cu, Fe) can
escort to enhance the properties of SnO2,15 which is able to improve
the photoactivity of photocatalysts, this is the origin of our research
impetus. Many papers have reported that nickel-doped tin dioxide
nannomaterials have outstanding gas sensing properties than that of
other materials.16 However, only few of them focused on the
photocatalytic properties of SnO2.
Herein, we successfully synthesized a series of nickel-doped tin
dioxide (NDT) microspheres for photo-degradation of rhodamine B
solution. The catalytic efficiency for the degradation of the dyes was
affected by different Ni dopant concentrations. At room temperature,
the as-prepared NDT microspheres have higher photocatalytic
activity compared to pure SnO2 and pure NiO. The causes of the
NDT microspheres for visible-light absorption were investigated and
discussed in terms of the changes of energy band structure. The
incorporation of Ni2+ into the SnO2 lattice can also provide them
with intriguing properties inherited from the synergetic effect and
the radius of Ni2+ is smaller than that of Sn4+. Furthermore, we
reported a simple one-pot method to prepare these catalysts.
Different from other method, this method has several advantages,
including low-temperature preparation, accurate control of the
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stoichiometry. For the preparation of NDT microspheres, the of 420 nm). At given intervals, 4 mL aliquots were collected,
hydrothermal method, where Ni2+ and Sn4+ were used as starting centrifuged, and then filtered to remove the catalyst particles for
reagents, had been demonstrated to be an effective approach. After analysis. The filtrates were finally analyzed using a UV-vis
calcinations, we can obtain NDT microspheres with high spectrophotometer (UV-2450).
crystallinity. The unique structures may show improved properties
and promising applications in many fields.
The formation of
samples was detected by a photoluminescence (PL) method using
coumarin as a probe molecule. Coumarincan readily react with OH
·OH on the surface of the UV illuminated
·
to produce the highly fluorescent product, 7-hydroxycoumarin
(7HC) (umbelliferone). The experimental procedure was similar to
the measurement of photocatalytic activity. In a typical process, 50
mg of the photocatalyst and coumarins (50 mL, 5.0 × 10−4 M) were
mixed under magnetic stirringfor 4 h under dark conditions. After
visible light irradiation for 10 min, the reactionsolution was filtered
to measure the increase in the PL intensity around 445 nm excited by
332 nm light.
Experimental
Synthesis of NDT Microspheres
In a typical synthesis of NDT microspheres, a solution having the
molar ratio of Sn/Ni = 2:1 was prepared by dissolving stoichiometric
amounts of SnCl4
·
5H2O and Ni(NO3)2
·
4H2O in 70 mL of absolute
4H2O totally
methanol. After the SnCl4
·
5H2O and Ni(NO3)2
·
dissolved, a certain amount of urea and benzyl alcohol was added to
the mixture in the ratio urea: BZ = 0.5:2 (molar ratio). After stirring
for 1 h, the solution was transferred to a teflon reactor (100 mL) and
autoclaved at 180 °C for 5 h. The resulting product was isolated by
centrifugation and washed three times with deionized water and
dried at 60 °C for 24 h. The product was calcined in air at 350 °C for
5 h. The preparations of other NDT microspheres were carried out
using the same procedure with different molar ratios of the metal
precursors.
To detect the active species during the photocatalytic reaction,
-
the presence of superoxide radicals (·O2 ), hydroxyl radicals (·OH)
and holes (h+) were investigated by adding benzoquinone (BQ),
isopropanol (IPA) and triethanol amine(TEOA) as scavengers,
respectively. The scavengers were introduced into the RhB solution
prior to the addition of the photocatalysts using a similar procedure
to that described in above.
Results and discussion
Characterization of the Catalyst
Fig. 1 shows the XRD patterns of as-synthesized samples.
Pattern (a) can be indexed to orthorhombic structure SnO2
(JCPDS 41-1445), characteristic peaks at 26.6◦, 33.9◦ and 51.8◦
can be assigned to the diffraction planes of (1 1 0), (1 0 1) and
(2 1 1).17 No other peaks are observed indicating that Sn(OH)4
formed firstly, and the further dehydration of Sn(OH)4
generated SnO2 nanoparticles. When Ni are introduced to the
synthesis, the formation of both SnO2 and Ni(OH)2 can be
observed for the as-synthesized microspheres before
calcination. After calcination at 350 °C for 5 h, with the
introduction of higher amounts of Ni, the characteristic peaks
become wider and shift toward lower 2θ values. These could be
attributed to the decrease of grain size and the insertion of
smaller radius atoms, like Ni2+ here. Moreover, the successive
peak shift indicates that the as-prepared product is not a
physical mixture, but the Ni species such are incorporated into
the lattice of SnO2 and form solid solutions probably. However,
the only observed diffraction peaks belong to SnO2, as showed
in Fig. 1. This further confirm the formation of a single phase
resulting in successful doping of Ni into SnO2. The slight shift
toward lower 2θ values are believed to be related to the
decrease of the length of the SnO2 crystal lattice, which is
attributed to a lower ionic radius of Ni2+ (0.69 Å) campared
with Sn4+ (0.83 Å). This further confirm that the Ni ions are
incorporated into the SnO2. This can be clearly observed from
the Fig. 1 inset.
The crystalline structure of the photocatalysis were characterized by
powder X–ray diffraction (XRD) employing a scanning rate of 0.05
°/s in a 2θ range from 10 to 80° , using a Bruker D8 advance
instrument equipped with monochromatized Cu-Kα radiation. The
morphologies and particle sizes of the samples were observed by an
SU-8000 field-emission scanning electron microscope (FESEM,
Hitachi, Japan) at an accelerating voltage of 3 kV. Energy dispersive
spectrum analysis (EDS) system was connected to the SEM.
Transmission electron microscopy (TEM) test was conducted by a
Tecnai G20 (FEI Co, the Netherlands) using an accelerating voltage
of 200 kV. X-ray photoelectron spectroscopy (XPS) was recorded on
a VG Multilab 2000 (VG Inc.) photoelectron spectrometer using
monochromatic Al Ka radiation undervacuum at 2 × 10−6 Pa. All the
binding energies were referenced to the C1s peak at 284.8 eV of the
surface adventitious carbon. The real composition of composites
were analyzed using a PerkinElmer NexION 300x Inductively
coupled plasma mass spectrometer (ICP-MS). The UV-vis DRS
were collected using a Shimadzu UV-2450 spectrophotometer from
200 to 800 nm using BaSO4 as background. The Fourier Transform
infrared spectroscopy (FTIR) were recorded on a Nicolet NEXUS
470 (Nicolet Instruments, Offenbach, Germany) FTIR ESR using
KBr as background.
Catalyst Activity Test
The photoactivity of the samples was tested by the degradation of
RhB under visible light irradiation. In a typical experiment, 50 mg of
the photocatalyst and 50 mL of aqueous solution of RhB (1.0 × 10−5
mol/L; 50 mL) were added into a flask, then the mixed solution was
oscillated in the dark overnight. After reaching adsorption
equilibrium, the photocatalytic reaction was initiated by irradiating
the system with a 350W xenon lamp (λ > 420 nm, use a cutoff filter
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constructed from fine particles. And the aggregations are relatively
uniform and tend to form regular spherical morphologies.
Fig. 1. XRD patterns of Ni-doped SnO2 (NDT) microspheres with
different ratios of Sn:Ni. The inset is the enlarged (101) peak of
Ni-doped SnO2.
Fig. 3. (a), (b) SEM images with different magnifications of NDT
microspheres (Sn/Ni = 2:1), (c) EDX spectra of NDT microspheres
(Sn/Ni = 2:1).
The microstructure of NDT microspheres (Sn/Ni=2:1) has been the
compositions of Sn0.67Ni0.33O1.67 are investigated by SEM, as showed
in Fig. 3. It is clearly that the microspheres’ external rough surfaces
are composed of tightly packed nanoparticles (Fig. 3a). From the
broken sphere in Fig. 3b, we can clearly find that the entire
microsphere is agglomerated compactly by small nanocrystals.
During the hydrothermal precipitation in the presence of
nanocrystals, which are uniformly assembled together into NDT
microspheres (Fig. 3b inset). EDS analysis of the sample indicated
the presence of Sn, Ni and O elements in the microspheres (Fig. 3c).
From the above analysis, it can be concluded that the final composite
was Ni doped SnO2 microspheres.
Fig. 4. (a), (b) TEM images of NDT microspheres (Sn/Ni = 2:1), (c)
HRTEM image, the inset is the FFT pattern.
Fig. 2. SEM images of the as-prepared NDT microspheres. (a), (b)
Sn/Ni of 5:1, (c) ,(d) Sn/Ni of 3:1, (e) Sn/Ni of 2:1 and (f) Sn/Ni of
1:1.
To shed light on the morphological feature of the samples, we resort
here to transmission electron microscopy (TEM), the results are
shown in Fig. 4. The small nanocrystals, as building blocks for the
microspheres (Fig. 4a), can be further clearly observed on the edge
of microsphere (Fig. 4b). It is the aggregation of these tiny
nanocrystals that constituting the NDT microspheres. It is
satisfactory and in agreement with the SEM images. The HRTEM
image (Fig. 4c) shows that the distances of the lattice fringes over a
large area are 0.335 and 0.264 nm, which are in agreement with the
lattice spacing of SnO2 (110) and (101) planes.
Then we studied the morphology of prepared samples. It can be seen
from Fig. 2 that these NDT samples are presented to be
microspheres, no other morphologies can be observed. However, the
sizes of corresponding NDT
microspheres are significantly
different. With the increase of Ni doping level, the average diameter
of NDT microspheres decreased gradually. For Sn/Ni of 5:1 and
Sn/Ni of 3:1, it is clearly that numerous microspheres with an
average diameter of about 5–6 µm and 3–4 µm respectively.
Moreover, it is obvious that the surface of the sphere was smooth.
For Sn/Ni of 2:1 ( 2µm) and Sn/Ni of 1:1 ( 1.5µm) , it clearly
displays the surfaces of spheres are rough and the spheres are
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prepared samples. Table 1. provides the metal compositions of the
samples measured by ICP-MS. The result shows that the final Sn/Ni
molar ratios of all the prepared samples are almost the same with the
initial ones.
Sample
During systhesis From ICP-MS
Sn:Ni=5:1
Sn:Ni=4:1
Sn:Ni=3:1
Sn:Ni=2:1
Sn:Ni=1:1
0.2
0.25
0.33
0.5
1
0.22
0.26
0.32
0.53
1.22
Table 1. The ratios of Ni/Sn from ICP-MS.
The UV-vis diffuse reflectance spectra of the calcined sample NDT
microspheres (Sn/Ni = 2:1) is showed in Fig. 6. As can be seen,
naked SnO2 has a good light absorption only in the ultraviolet region,
while the NDT microspheres shows better absorption performance
in the range of 400–600 nm. It indicates that doped Ni enhanced
significantly visible light absorption of SnO2 catalyst. The direct
band gap values of the SnO2 and NDT microspheres samples are
estimated from the (Ahυ)2 versus photon energy (hυ) plot as showed
in the insert of the Fig. 6. The band gap of the composites is
estimated to be 3.15 eV, which is lower than that of SnO2 (3.59 eV).
This indicates a band gap narrowing of the SnO2 due to the
introduction of Ni into the matrix of NDT microspheres. It indicates
that the samples have potential ability for photocatalytic
decomposition of organic contaminants under visible-light
irradiation, which is consistent with the apricot color of the sample.
Fig. 5. The corresponding high-resolution XPS spectra of (a) Sn 3d
(a1 for NDT microspheres (Sn/Ni = 2:1), a2 for SnO2 ), (b) O 1s (b1
for NDT microspheres (Sn/Ni = 2:1), b2 for SnO2 ), (c) Ni 2p for NDT
microspheres (Sn/Ni = 2:1).
To demonstrate the Ni dopant and its doping levels in the as-
prepared NDT microspheres, pure SnO2 and NDT microspheres
(Sn/Ni = 2:1) samples are investigated by XPS. The binding energies
(BE) values are measured for the Sn 3d, O 1s, and Ni 2p lines. Fig.
5a shows two peaks located at 486.7 eV and 495.2 eV, which can
attribute to Sn 3d5/2 and 3d3/2 states respectively.18,19 It indicates that
tin is in the Sn4+ statecoordinated by oxygen atoms. Fig. 5b provides
the information of binding energies and intensities of the surface
element of O 1s of SnO2 and NDT microspheres (Sn/Ni = 2:1). With
the introduction of Ni, the position of the peak shifts to a higher
energy compared with pure SnO2, Thus suggesting the coexistence
of different chemical environments. This could be ascribed to some
Sn4+ replaced by Ni2+ of the SnO2 lattice and the formation of
Sn−O−Ni bonds in the NDT samples. The difference in
electronegativity (χ) between Sn (χp = 1.96), O (χp = 3.44) and Ni
(χp = 1.91) results in a lower electron cloud density of O atoms, thus
the binding energy of O 1s (531.1 eV for NDT samples) increases
compared to pure SnO2 (530.9 eV). After peak deconvolution (Fig
5b), the O1s spectra reveal splitting into two bands. The major
component at 530.9 eV is typical for the SnO2 cassiterite network,
whereas the second minor band at higher BE (ca. 532.2 eV) is
mainly related to the presence of ≡Sn-OH species and/or to the O
atoms bonded to the Ni atoms which may arise from some Ni2+
replaced Sn4+ of the SnO2 lattice.20,21 In Fig. 5c, the binding energies
may also imply the formation of the NDT microspheres. The Ni 2p
XPS peak shows three components, at 854.9, 856.2 and 861.8 eV,
which can be referred as the main peak (856.2eV) and the two
satellite peaks (854.9 and 861.8 eV).22-24 The peak at 856.2 eV has
been associated with the presence of Ni3+ ions,25 Ni2+-OH species,26
Ni2+ vacancies,27 or as a result of a non-local screening mechanism,
while the satellite peak at 861.8 eV involves a charge transfer ligand-
metal.28,29 Thus, a different local environment of the Ni atoms in the
Ni-Sn-O catalyst can be inferred, further indicating the existence of
Ni in the sample.
Fig. 6. UV−vis DRS of (a) SnO2, (b) NDT microspheres (Sn/Ni = 2:1).
The inset is band gap evaluation from the plots of (Ahυ)2 versus
photon energy(hυ).
FT‐IR is used to investigate the functional groups of the as‐
synthesized samples. The further evidence for the existence of the
hydrophilic groups on the surface of samplewithout the calcination
come from FT-IR spectra in Fig. 7a, the peak at 3418 and 1628 cm−1
are the typical stretching vibration of hydroxyl groups. The intense
ICP analysis is explored to gain the compositions of the
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peaks at 1403 cm−1 are assigned to the typical asymmetrical and rates of RhB over Sn/Ni = 1:1 is slower than that over SnO2, but an
symmetrical stretching vibrations of C-O because of the COOH opposite positive results are obtained by comparison between other
groups. The double weaker peaks located at 1033 cm−1 is attributed doping contents and SnO2. Besides, no degradation of RhB occurred
to the C-O-C symmetrical stretching vibrations. The peak at 663 and in the absence ofcatalyst. According to Fig. 8 all the NDT
494 cm−1 are attributed to the Sn-O-H stretching caused by the microspheres sample have the highest photocatlyticactivity and
Sn(OH)2 impurities.30,31 The most remarkable differences between shows higher degradation compared to pure NiO. At 40 min the RhB
the three cures are the peaks at 3418 cm−1 and 1628 cm−1 in Fig. 8a, degradation percentage with Sn/Ni = 2:1 was about 100% within the
they disappeared after calcination, showed in Fig. 7b and 7c. The same time period. It is observed that the NDT microspheres (Sn/Ni =
peaks at <1000 cm−1 are all attributed to the Sn-O-Sn stretching.32 2:1) exhibited betterphotocatalytic performance than pure SnO2 and
Nevertheless, the relative intensity of the peaks decrease after doped other NDT microspheres.
nickel, implying that the coordination mode between Ni and SnO2
alsotransforms versus pure SnO2.
To quantitatively compare the photocatalytic activities of these
samples, the reaction rate constants (k) are calculated byadopting the
pseudofirst-order model as expressed by Eq. (1), which is generally
used for photocatalytic degradation processwhen the initial
concentration of pollutant is low,33
ln(C0/ Ct) = kt
(1)
where C0 and Ct are the concentrations of RhB aqueous solution
atirradiation times of 0 and t min, respectively. Table 1 shows the
values of k. As expected, the k values in Table 2 agree with the
above analysis results of Fig. 8.
Concentration
Amount of
k
Sample
of RhB
Photocatalysts(g)
(min−1)
(mol/L)
SnO2
0.05
0.05
0.05
0.05
0.05
0.05
1.0 × 10−5
1.0 × 10−5
1.0 × 10−5
1.0 × 10−5
1.0 × 10−5
1.0 × 10−5
0.015
0.023
0.016
0.022
0.071
0.010
Fig. 7. FTIR spectra of (a) Ni-doped SnO2 precursor, (b) the
corresponding Ni-doped SnO2 microspheres and (c) SnO2.
Sn:Ni=5:1
Sn:Ni=4:1
Sn:Ni=3:1
Sn:Ni=2:1
Sn:Ni=1:1
Table 2. Reaction rate constants (k) of samples prepared with
different Sn/Ni ratio.
In order to confirm the influence of adsorption, Fig. 9 shows the
change of RhB concentration in the presence of catalysts. With an
initial RhB concentration of 1.0 × 10−5 mol/L, the adsorption of RhB
over Ni-SnO2 catalysts only reach 15% after stirring for 3 h in dark.
Fig. 10 shows the photodegradation of RhB on different catalysts
under sunlight irradiation. The catalysts exhibite higher
photocatalytic activity than those under visible light, but they show
the same tendency, except for pure SnO2. It is observed that under
sunlight, the degradation efficiency of SnO2 was higher than that of
these Ni-doped SnO2 catalysts, whereas it is exactly the opposite
being observed under visible light. Compared with the light sources,
the most essential distinctness of them is the existence of UV light in
the sunlight. Therefore, we can contribute the relatively higher
photocatalytic efficiency of non-doped SnO2 to the UV light existing
in solar light. Thus, the synthesized catalysts should be pointed out
to be that, the Ni-doped SnO2 can be excited by both visible and UV
light and exhibited significant degradation efficiency under visible
light. It indicates that Ni doping improve the degradation efficiency
Fig. 8. Ct/C0 versus time curves of RhB photodegradation under
visible light ( λ > 420 nm) irradiation over catalyst free, pure SnO2,
pure NiO and Ni-doped SnO2 microspheres of different molar ratio
of Sn/Ni.
Fig. 8 shows the degradation of aqueous RhB (1.0×10−5 mol/L) in
the presence of 0.05 g of catalystunder visible light (λ > 420 nm)
irradiation. C0 and Ct is the concentrations of RhB aqueous solution
at the irradiation times of 0 (that is, after the dark adsorption
equilibrium) and t min, respectively. As can be seen from Fig. 8, the
photocatalytic activities of the as-prepared NDT microspheres
depended on their nickel contents. For example, the decolorization
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under visible light irradiation.
Fig. 11. The temporal evolution of the spectra during the
photodegradation of RhB over NDT microspheres (Sn/Ni = 2:1)
under visible light ( λ > 420 nm) irradiation.
Fig. 9. Concentrations of RhB against stirring time in the dark in the
presence of catalysts.
Fig. 10. Ct/C0 versus time curves of RhB photodegradation under
sunlight irradiation over catalyst-free, pure SnO2, pure NiO and Ni-
doped SnO2 microspheres with different molar ratios of Sn/Ni.
Fig. 12. Degradation percentage of RhB in the presence of NDT
microspheres (Sn/Ni = 2:1) under visible light ( λ > 420 nm)
irradiation.
Fig. 11 depicts the temporal evolution of the spectra during
thephotodegradation of RhB mediated by the NDT microspheres
(Sn/Ni = 2:1) under visible light irradiation ( λ > 420 nm). After 5
min irradiation, the maximum absorption band of the solution
gradually shiftedand the color of RhB aqueous solution change from
pink to light. Meanwhile, the absorption band gradually decreased,
which indicates that the ethyl groups of RhB were removedduring
irradiation.34 After reacting for 15–40 min, the adsorption peak of
the dye at around 496 nm decreased and the color of dye gradually
disappear with the increase of irradiationtime, whereas the
hypsochromic shifts of the absorption band are considerably
insignificant. It is presumed that the cleavage of thewhole
chromophore structure of RhB occurs over the photocatalysts. The
result demonstrates that the NDT microspheres could improve the
catalytic activity. As shown in Fig. 12, the catalyst can decompose
RhB dye efficiently without significant deactivation after fourth
times recycling.
In order to further insight into the photocatalytic mechanism, we
studied the active species involved in the photocatalytic process.
Characterised by strong oxidizing, hydroxyl radical (·OH) is deemed
to be one of the major active species in the photocatalysis process.35
By using coumarins as fluorescent probes, the concentration change
of ·OH generated on the surface of NDT potocatalysts can be
detected according to the following reaction process:
where coumarin is
a poorly fluorescent molecule and 7-
hydroxycoumarin is a fluorescent one.36 As shown in Fig. 13, it can
be seen that a gradual increase in the PL intensity at about 445 nm
with increasing irradiation time. However, no PL intensity increase
is observed in the absence of light irradiation or catalyst, which
confirms the formation of ·OH during the irradiation process.
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complexation between Sn4+ and OH– ions, and then the outer surface
is oxidized forming SnO2.38 In addition, Ni are introduced to the
2–
synthesis dissolved in the solution to form Ni(OH)4 ions.39 Under
hydrothermal conditions, because smaller seeds with larger curvature
possess higher surface energies, and thus increased tendency to
aggregate with nanocrystallites, and the process continues until the
microspheres are stabilized by lower their surface energy (Fig. 15,
Step 1). Finally, the metastable intermediatephase decomposed and
recrystallized to form microspheres morphologies.40 After the sample
is subsequently calcined at 350 °C, the release of the hydroxide of
the precursor and the oxidation of the organism (Fig. 15, Step 2). We
obtain the NDT microspheres structure.
Fig. 13. PL spectra changes observed during visible light illumination
of NDT microspheres (Sn/Ni = 2:1) microspheres in a 5×10−4
coumarin solution (excitation at 332 nm).
M
It is well know that ·O2 and h+ can also be generated besides
·
OH in
-
the photooxidation process.37 In order to find out which active
species dominate the degradation of RhB in the photocatalytic
Fig. 15. Illustration of the possible formation mechanism of Ni-
doped SnO2 microspheres.
reaction, IPA, BQ and TEOA were introduced as molecular
OH, ·O2 and h+, respectively. The produced
-
detectors to quench
·
active species would react with the scavengers preferentially and
hinder the photodegradation of RhB. As shown in Fig. 14, the RhB
degradation is markedly suppressed in the presence of BQ,
-
suggesting that ·O2 is primarily responsible for the degradation of
RhB. The decrease in photodegradation rate was observed with the
addition of IPA and TEOA, which revealed that ·
OH and h+
participated in the RhB degradation process. The active species
-
trapping experiments indicate that ·O2 is the most important reactive
specie for RhB degradation.
Fig. 16. Conduction and valence band edge potentials of the NDT
microspheres (Sn/Ni = 2:1) microspheres.
The calculated CB and VB edge potentials of the NDT microspheres
are illustrated in Fig. 16, the following equation is employed:
ECB = X-Ee -0.5Eg
(2)
(3)
Fig. 14. The photocatalytic activity of NDT microspheres (Sn/Ni =
2:1) microspheres with different quenchers.
EVB =ECB+Eg
Based on the experimental results, we propose a possible two-step
growth mechanism for the formation of NDT microspheres. The
where X is the electronegativity of the semiconductor which is the
geometric mean of the electronegativity of the constituent atoms,41
Ee is the energy of free electrons on the hydrogen scale, and the
value is 4.5 eV, and Eg is the band gap energy of the semiconductor.
starting stannum source used in our synthesis is the stannic chloride
2−
pentahydrate. First, Sn(OH)6
anions are formed by the
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It is known that SnO2 is an n-type direct wide band gap in SnO2 can also promote electron transfer. It is reported that doping
semiconductor (Eg=3.6 eV) while the band gap energy of NiO is 3.2 can create impurity level close to the valence band edges (dashed
eV. The conduction band of NiO (−2.7V vs. NHE)42 is more line shown in Scheme 1),47 the doping level of Ni expand the
negative than that of SnO2 (0.0 V vs. NHE) 43. The band gap of the absorption range of light and leading to more photo-generated holes
NDT microspheres can regulate the contents of Ni and Sn to and electrons. The reactive electrons (e-) from the conduction band
improve the activity. For the NDT microspheres (Sn/Ni=2) sample, reduce O2 to its radical anion, resulting in the oxidation of RhB at
the calculated results are ECB=-0.584 eV, EVB = 2.567eV based on last. NDT possess remarkable broader light absorption region than
above equation，the band gap detected (3.15 eV) is narrower than SnO2.
that of SnO2 (3.50 eV), and the CB energy level is more negative
than that of SnO2. Because the potential (-0.26 V vs NHE, pH = 0)
of Ni2+/Ni is lower than the CB level of the NDT microspheres, the
electrons in the CB can transfer to Ni clusters.44 These Ni clusters
can promote the separation and transfer of electrons, where O2 is
Conclusions
Nickel-doped tin dioxide microspheres with different amounts of
dopant are hydrothermally synthesized. The catalysts have superior
photocatalytic activity in the degradation of RhB under visible light
irradiation and shows much higher activity than pure SnO2 and NiO
at room temperature. The sample NDT microspheres with the 33.3%
of Ni doping exhibited the highest photoactivity. The high
photocatalytic activity can be attributed to the synergistic effect of its
demonstrated doping levels as well as the Ni doping. The doping
level of Ni of NDT are proposed to be responsible for its high
activity. The doped Ni plays a key role in narrowing the band gap
and reducing the recombination rate of photon-generated carriers,
thus promote the activity and stability of the catalyst. We expect that
the method and idea presented in this work will provide a new way
for the construction of highly efficient photocatalysts not only for
SnO2, but also for other semiconductor materials.
-
reduced to ·O2 .
Acknowledgements
This work was supported by Natural Science Foundation of
Hubei Province (2013CFA089).
Scheme 1. Diagram of the proposed photocatalytic mechanism of
Ni-doped SnO2 microspheres under visible-lightirradiation.
Notes and references
It is of interest to understand why the NDT microspheres series
exhibits the highest photocatalysis activity under identical visible-
light irradiation. As we know, the operative basis of photocatalysis is
the oxidation-reduction of photogenerated carriers generated by
optical excitation on catalysts. The electrons and holes will
recombined almost immediately if there is no interference factor.
Therefore, the high separation efficiency of photo-generated carriers
and wide visible light response range should be the significant
reasons that are responsible for this phenomenon.45 The schematic
diagram of the carriers generated and the mechanism of the
degradation process over NDT microsphere. The probable
mechanism of Ni doping effect is also proposed, which includes two
main ways, as shown in Scheme 1. The one is that Ni2+ with partially
filled d orbital creates an electron donor level in bandgap when they
partly entered the SnO2 lattice, the doped Ni2+ can act as atemporary
trapping site of photo-induced electrons and greatly suppress the
recombination of electron−hole pairs on the surface of the
photocatalyst. This is similar to the case for Ni doped TiO2.46 The
Key Laboratory of Catalysis and Materials Science of the State
Ethnic Affairs Commission & Ministry of Education, Hubei
Province, South-Central University for Nationalities, Wuhan
430074, PR China.
*Tel: +86 27 67841302; E-mail: jchu@mail.scuec.edu.cn
†
Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/b000000x/
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