Article

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Synthesis of Titanium Dioxide/Cadmium Sulfide Nanosphere

Particles for Photocatalyst Applications
Thi Thuy Duong Vu,†,‡ Frej Mighri,*,†,‡ Abdellah Ajji,‡,∥ and Trong-On Do†,§
†
Department of Chemical Engineering, Laval University, Quebec, Quebec G1V 0A6, Canada
‡
Center for Applied Research on Polymers and Composites (CREPEC); §Centre in Green Chemistry and Catalysis (CGCC);
∥
Department of Chemical Engineering, École Polytechnique of Montreal, C.P. 6079, Montreal, Quebec H3C 3A7, Canada

ABSTRACT: Semiconductor nanocomposites, which are composed of titanium dioxide (TiO2) nanorods, cadmium sulphide
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(CdS) nanoparticles (NPs), and Ni clusters, were synthesized. The following steps were adopted: (i) surfactant-capped TiO2
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nanorods with controlled length were synthesized in an autoclave using oleic acid and amino hexanoic acid as surfactants. By
using a ligand-exchange procedure, in which nitrosonium tetrafluoroborate (NOBF4) was used to replace the original surfactants,
hydrophilic NOBF4−TiO2 nanorods were obtained; (ii) the resulting nanorods were deposited with CdS NPs and (iii) then
deposited selectively with Ni clusters (as cocatalyst) on the nanocomposite surface. Under visible-light illumination of the
nanocomposite, the generated electrons from the conduction band of CdS are transferred to TiO2 via TiO2/CdS interface, then
to metallic Ni cluster. As a result, the electron−hole separation was highly enhanced, leading to a Ni−TiO2/CdS nanocomposite
with high photocatalytic performance for the production of hydrogen (H2).

1. INTRODUCTION effective.12 With proper band structures, the TiO2/CdS

As one of the most abundant elements with a high energy nanocomposite exhibits good properties in photocatalysis,
leading to an improved photoproduction of H2 under visible
efficiency, hydrogen (H2) generated via solar water splitting has
light.13−17
currently attracted attention. Hydrogen energy yield is reported
Herein, we describe new non-noble metal−nanocomposites
up to 122 kJ/g, which is largely higher than that of other fuels,
(NCs) as highly efficient and stable visible-light driven
such as gasoline (40 kJ/g).1 So, H2 is presently considered as
photocatalysts. These NCs are composed of TiO2 nanorods,
one on the future ideal fuel candidates for the energy
CdS NPs, and Ni clusters. An important advantage of TiO2
generation. Moreover, solar water splitting is environmentally nanorod-based nanocomposites is that CdS NPs are evenly
friendly and has great potential for low-cost and clean hydrogen dispersed on nanorod surface with strong bonding, and
production. In addition, H2 can be easily distributed over large cocatalyst Ni clusters are selectively deposited on the surface
distances through pipelines or via tankers. It can also be stored of these nanorods. This configuration can improve the
in gaseous, liquid, or metal hydride forms, thus providing a efficiency of electron transfer from the sensitized CdS NPs to
huge market potential. TiO2 and then to Ni clusters. As anticipated, Ni−TiO2/CdS
In a photocatalytic H2 production reaction from water, the nanocomposites developed in the present work exhibit
chemical reaction is induced by photoirradiation in the enhanced H2 production from water under visible light using
presence of a photocatalyst. With a relative narrow band gap ethanol as a sacrificial reagent.
of 2.4 eV, CdS is one of the sulfide-based semiconductors,
which have promising applications in photocatalysis.2−6
However, CdS alone shows very low H2 generation rates due
2. EXPERIMENTAL SECTION
to the rapid recombination of photogenerated electrons and 2.1. Materials. All chemicals were used as received without
holes, which causes a lack of H2 evolution sites. Good further purification or distillation. Titanium(IV) butoxide (TB,
performances were mostly achieved in the presence of noble 97%), oleic acid (OA, 90%), 6-aminohexanoic acid (6AHA),
metal cocatalysts, such as platinum (Pt), palladium (Pd), and cadmium acetate dehydrate, thioamide, and nitrosonium
nickel (Ni). Among various strategies to improve the tetrafluoroborate solution (NOBF4) were purchased from
photocatalytic activity of CdS, the most efficient method is to Aldrich. Absolute ethanol, N,N-dimethylformamide (DMF),
promote the charge separation of photogenerated electrons and dichloromethane, hexane, and toluene, were, respectively,
holes by coupling CdS with other semiconductors with purchased from Brampton Canada, Fisher Scientific Canada,
adequate flat potentials, such as TiO2,7,8 zinc oxide (ZnO),9 and Anachemia Canada. All of them were of analytical grade.
or graphene.10,11 In such systems, electrons from the 2.2. Synthesis of Length-Controlled TiO2 Nanorods
conduction band of CdS can be transferred to other Using Oleic Acid and 6-Aminohexanoic Acid as
semiconductors or graphene, leading to improved electron−
hole separation, which could enhance the generation rate of H2. Received: November 2, 2013
TiO2 has been widely used as a photocatalyst due to its high Revised: January 28, 2014
photostability and oxidation efficiency, and its abundance and Accepted: February 19, 2014
noncorrosives. It is also environmental friendly and cost- Published: February 19, 2014

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Scheme 1. Sketch for the Preparation of TiO2/CdS Nanocomposites

Surfactants. Capped-TiO2 nanorods were synthesized at low were obtained on a JEOL JEM 1230 operated at 120 kV.
temperatures using the solvothermal method. Oleic acid (OA), Samples were prepared as follows: a drop of a dilute toluene
and 6-aminohexanoic acid (6AHA) were used as surfactants dispersion of nanocrystals were deposited onto a 200 mesh
with various molar ratios. A mixture of 1 mmol TB, 6AHA, OA, carbon-coated copper grid then evaporated immediately at
and absolute ethanol (EtOH) with desired precursor molar ambient temperature. Elemental dispersive spectrum (SEM-
ratios was mixed well and stirred for 30 min under room EDX) analysis was obtained from a JEOL 6360 instrument
temperature before being transferred into a Teflon-lined working at 3 kV. Powder X-ray diffraction (XRD) patterns of
stainless steel autoclave. The autoclave also contained about the samples were obtained on a Bruker SMART APEXII X-ray
5−10 mL of EtOH in order to keep equilibrium in the mixture diffractometer equipped with a Cu Kα radiation source (λ =
and to avoid any change in EtOH concentration during the 1.5418 Å) in the 2θ range of 5−20° at a scan rate of 1.0°/min.
crystallization process. The synthesis process was set at 140 °C All samples were dried at 65 °C overnight to eliminate guest
for 18 h. After that, the autoclave was cooled down slowly to solvent molecules on the surface of particles before the XRD
room temperature, and samples were collected and washed scan. Fourier transform infrared absorption spectra (FTIR)
several times using ethanol and toluene. were measured with a FTS 45 infrared spectrophotometer in
2.3. Development of TiO2 Nanorods by Ligand the spectral range of 4000−400 cm−1. The thermal analyses of
Exchange Reaction. Typically, 5 mL of dichloromethane the as-made TiO2 nanorods, CdS NPs, and hybrid TiO2/CdS
solution of NOBF4 (0.01M) was added to hexane solvent NCs were carried out at a heating rate of 10 °C/min up to 900
containing capped-TiO2 nanorods at room temperature. The °C under an oxygen flow using a Perkin-Elmer TGA
mixture was then gently shaken until the precipitation of the thermogravimetric analyzer. The UV−visible spectra of the
TiO2 nanorods. These nanorods quickly become insoluble and nanostructures were recorded for the powder sample on a Cary
are collected through centrifugation. Then, they were 300 Bio UV−visible spectrophotometer, and pure magnesium
redispersed in DMF hydrophilic solvent. To purify the TiO2 oxide (MgO) was used as a blank. ζ-Potential measurements
nanorods, DMF solutions were washed through the addition of were performed with a Zetasizer Nano ZS in water at 25 °C.
a mixture of toluene and ethanol 95% until precipitation occurs Nitrogen adsorption/desorption isotherms of the samples were
then followed by centrifugation. This process was repeated few obtained using with a Quantachrome Autosorb-1 system, after
times. Finally, the collected TiO2 nanorods were dried degassing at 100 °C and 10−5 mmHg for at least 5 h. The
overnight in oven at 65 °C to remove residual solvent specific surface areas (SBET) of the samples were calculated
molecules. from adsorption isotherm data using the standard Brunauer−
2.4. Synthesis of Colloidal Hybrid TiO2/CdS Nano- Emmett−Teller (BET) method. XPS characterization was
composite. A mixture of 4.5 mmol of NOBF4-capped-TiO2 carried out in order to analyze the chemical composition of
nanorods dispersed in 10 mL of DMF, and 9 mmol cadmium composite, as well as the electronic state of Ni in the sample.
acetate dihydrate was stirred under room temperature for 2 h. XPS measurement was done in an evacuated ion-pumped
Subsequently, 9 mmol thioamide was added to the mixture and chamber at 1 × 10−9 Torr of Kratos Axis-Ultra instrument. The
let under stirring for an additional 3 h in order to ensure a X-ray source is a monochromatic Al source (Al Kα, hv = 1486.6
complete reaction. The precipitated TiO2/CdS nanocrystals eV) operated at 300 W. The binding energy of samples was
were washed few times using toluene and ethanol 95%, and measured by fixing an internal reference C1s peak at 285.0 eV.
then collected by centrifugation. For the separate constituents after background subtraction, all
2.5. Synthesis of Ni−TiO2/CdS by a Photodeposition the peaks were deconvoluted by means of standard CasaXPS
Method. Typically, Ni(NO3)2 was added to the solution software.
containing TiO2/CdS. Because the surface of TiO2 is negative, 2.7. Photocatalysis Characterization (Photocatalytic
positive charge Ni2+ is selectively absorbed on the TiO2 surface, H2 Evolution). Before photocatalytic characterization, the
leading to the formation of TiO2/CdS−Ni2+. This solution is surfactants adsorbed on samples were eliminated. These
then illuminated with visible light for 1.5 h. As the potential of samples were dried overnight at 65 °C and used as such for
Ni2+/Ni is lower than the conduction band level of TiO2, the photocatalytic measurement. Visible-light-induced H2 evolution
electrons from the latter can effectively reduce Ni2+ species was carried out in 80 mL septum-sealed glass vials. A mixture of
adsorbed on their surface, forming a metallic Ni cluster.18 20 mg of sample and 3% Ni2+ were dispersed well in 27 mL of
2.6. Characterization. Transmission Electron Microscopy aqueous solution containing ethanol (25 wt%). The vial was
(TEM) Images of TiO2 nanorods, and hybrid TiO2/CdS NCs deoxygenated using nitrogen and then placed in front of 300 W
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Xe lamp with a 420 nm cutoff filter (FSQ-GG420) for catalytic

reaction. Gaseous products were then identified by collecting
0.5 mL of the gas in the headspace of the vials. This gas was
then analyzed by gas chromatography (GC) using a thermal
conductivity detector (TCD) for the quantification of H2 with
Ni as the carrier.

3. RESULTS AND DISCUSSIONS

Scheme 1 shows the procedure adopted for the synthesis of
surfactant capped-TiO2 nanorods by the hydrolysis of a titania
precursor followed by a solvothermal reaction in autoclave.
First, an ethanol solution of titanium(IV) butoxide (TB) was
modified by hydrolysis with OA and 6AHA as surfactants. The
hydrolysis process helped to yield three-dimensional polymeric
titania skeletons, which acted as the seeds for titania growth. To
obtain the desired TiO2 uniform sizes of particles, the
subsequent solvothermal process was carefully controlled with
presetting the reaction time (18 h) and temperature (140 °C).
It was observed that TiO2 nanorods were always achieved with
the use of OA and 6AHA as surfactants.

3.1. TEM, FTIR, AND BET CHARACTERIZATION

Figure 1 shows TEM image of the obtained TiO2 nanorods
before sonication. As seen in the TEM image, these nanorods

Figure 2. TEM images of synthesized TiO2 nanorods after sonication:

(a) 3 × 40 nm nanorods for TB:OA:6AHA molar concentration of
1:7:3 and (b) 3 × 10 nm nanorods for TB:OA:6AHA molar
concentration of 1:7:10.

and 6AHA surfactants have selective bindings to the different

Figure 1. TEM image of the synthesized TiO2 nanorods before faces of TiO2. Joo et al.20 reported that OA binds strongly to
sonication. the TiO2 {001} faces, while 6AHA binding is more favored on
{101} faces. When the concentration of 6AHA is high
were attached together in a parallel configuration to form big (OA:6AHA molar ratio = 7:10), the strong adhesion of
aggregation. This is different from the results obtained by Dinh 6AHA to the low surface energy {101} face, compared to the
et al.19 who showed well-dispersed TiO2 nanorods by using OA adhesion of OA to {001} face, leads to a less progressive TiO2
and oleylamine as surfactants. The aggregation obtained in our growth along {001} direction to form TiO2 nanorods with
approach may be due to the replacement of oleylamine by the short length. By decreasing the molar concentration of 6AHA,
6AHA surfactant. the adhesion of 6AHA to the low surface energy {101}
Figure 2 also shows TEM images of TiO2 nanorod samples decreases while the adhesion of OA to {001} is kept the same.
obtained with different molar TB:OA:6AHA ratios after a few The growth along {001} is then preserved, leading to longer
minutes of sonication. As seen in Figure 2, by varying the molar TiO2 nanorod shape.21
ratio between TB, OA, and 6AHA, different sizes of TiO2 Because OA and 6AHA were used as capping agents, the
nanorods were observed. For a TB:OA:6AHA molar ratio of hydrophobic surfactant capped- TiO2 nanorods were soluble in
1:7:3, TiO2 nanorods of 3 × 40 nm were achieved (Figure 2a). nonpolar hydrophobic solvents, such as toluene and hexane.
When the concentration of 6AHA was increased from 3 to 10 However, after being treated with dichloromethane solution of
(e.g., from 1:7:3 to 1:7:10), while the TB and OA NOBF4, TiO2 nanorods precipitated immediately in hexane
concentrations kept the same, the shape of TiO2 nanorods solvent after gentle shaking indicating that NOBF4 has replaced
did not change; however, the length of the nanorod was the original hydrophobic surfactant capped to the nanorod
decreased from 40 to 10 nm (Figure 2b). Hence, it could be surface. This also indicates a dramatic change in surface
assumed that the length of TiO2 nanorods is controlled by the properties of these NPs, from hydrophobic to hydrophilic. As
molar ratio OA:6AHA. Also, it should be mentioned that OA seen in Figure 3, it was observed that NOBF4 capped-TiO2
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Figure 3. (a) Surfactant-capped TiO2 nanorods dissolved in toluene;

(b) TiO2 nanorods after NOBF4 treatment dissolved in DMF.

nanorods were easily redissolved in DMF solvent as well as in

water. This is considered as an advantage during the deposit
process of CdS on the surface of TiO2 nanorods since both
cadmium acetate and thioamide are well dissolved in DMF. A
higher dispersion of the initial precursors in the media (TiO2,
Cd2+, S2−) increases the chance to achieve uniform TiO2
nanorods with a higher dispersion of CdS on their surface.
To analyze the surface properties of TiO2 nanorods, FTIR
Figure 4. FTIR of (a) capped-TiO2 nanorod synthesized using OA
characterization was done for the samples before and after
and 6AHA as surfactants; (b) TiO2/CdS nanoparticles.
surfactant treatment. The corresponding results are shown in
Figure 4. FTIR spectra of the capped TiO2 nanorods before
surface treatment with NOBF4 and those of OA and 6AHA H stretching vibration at 2800−3000 cm−1 was observed after
surfactants are shown in Figure 4a. The small peaks at 3004 CdS deposition, as compared to that of the sample before
cm−1 were observed in the both FTIR spectra of OA and deposition. This could be due to NOBF4 treatment process
6AHA, corresponding to the stretching of =CH bond. The where CdS deposition was able to remove some residues of OA
sharp vibrations bands at 2916 and 2857 cm−1 are attributed to and 6AHA molecules attached to TiO2 nanorods surface (see
the asymmetric and symmetric CH bonds in methylene Figure 4). As will be presented later (TGA characterization),
groups (CH2),22 respectively. The peaks at 1714 and 1282 this could explain the difference of weight loss between TiO2
cm−1 in the spectrum of OA are assigned to CO and CO nanorods and TiO2/CdS nanocomposite. Furthermore, in
stretching and those appearing at 1463 and 936 cm−1 are due to comparison with the FTIR spectrum of TiO2 nanorods before
in-plane and out-of-plane OH. Compared to the commercial NOBF4 treatment, there is a small peak at around 1000 cm−1,
P25TiO2, our synthesized TiO2 nanorods are identified by which is assigned to BF4− anions. Furthermore, no peak is
the additional peaks at 3004, 2922, 2853, and 1465 cm−1 due to observed around 2100−2200 cm−1, which is normally ascribed
the presence of capping ligand on the surface. In addition, the to NO+. This is an indication that surfactant exchange was
peak appearing at 1608 cm−1 indicates the existence of between the organic ligands and inorganic BF4−, not with NO+.
carboxylic acid salt on the surface of surfactant capped-TiO2 The big peak at around 3050 cm−1 on the FTIR spectrum of
nanorods, which is the result of the reaction between the OA TiO2/CdS, which is similar to the peak observed for
surfactant and TiO 2 during the solvothermal process. commercial TiO2 nanorods, is attributed to the water absorbed
Furthermore, a weak peak at 1041 cm−1 in the sample of on the surface of TiO2/CdS nanocomposite.
surfactant capped-TiO2 nanorods, which corresponds to that of Figure 5 shows TEM image and Brunauer−Emmett−Teller
CN bonds in the amine groups, proves the existence of (BET) adsorption/desorption isotherm curves for the sample
amine on their surface (resulting from the 6AHA surfactant). of TiO2/CdS nanocomposites. As seen in Figure 5a, TiO2/CdS
Figure 4b shows the FTIR spectrum of the TiO2/CdS NCs nanoparticles were aggregated to form hollow nanospheres with
after surface treatment. No essential peak characteristic of −C− a uniform diameter of around 150 nm. When water sonication
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Figure 6. XRD characterization of (a) TiO2 nanorod and (b) TiO2/

CdS nanocomposite.

51.9° (311 planes).13 Hence, with those peaks shown in the

XRD pattern of CdS NPs, we can conclude that CdS NPs are in
cubic phase.
XRD patterns of the TiO2/CdS nanocomposites confirm the
Figure 5. (a) TEM image of TiO2/CdS nanocomposite and (b) BET presence of CdS and TiO2. However, when mixed with high
characterization of TiO2, CdS, and TiO2/CdS nanocomposite with the concentration of CdS NPs, the intensity of the diffraction peaks
inset is their corresponding pore size distribution. at 48° was very low, which could be due to the attachment of
CdS on the surface of TiO2 nanorods. In the XRD spectrum of
was performed, hollow nanospheres were separated from each our TiO2/CdS nanocomposite, three broad and symmetric
other. However, single nanospheres were not separated into peaks were observed at 2θ = 26.5° (111 planes), 43.9° (220
single NPs by sonication at low frequency ultrasound. Because planes), and 51.9° (331 planes), corresponding to the cubic
the TiO2/CdS hollow nanospheres are composed of a large phase of CdS. The absence of planes referring to hexagonal
number of nanoparticles, a high surface area can be expected, as structured CdS indicates the presence of only cubic CdS
shown in Figure 5b. The BET specific surface area is 146 m2/g, nanoparticles in the sample. Furthermore, the broadening of
which is much higher than that of TiO2 nanorod (27.5 m2/g) the peaks is due to the CdS nanosize in the TiO2/CdS
and of CdS cubic (34.7 m2/g). The surface area results are in nanocomposite.
agreement with the observation from the isotherm figure, which 3.3. XPS and SEM-EDX Characterization. The XPS
shows that the isotherms of TiO2/CdS shift up compared to survey spectrum (Figure 7a) shows the existence of Ti, O, Cd,
those of TiO2 and CdS. S, Ni, and C elements in the sample. Also, the high-resolution
The pore size distribution curves (see inset, Figure 5b) XPS spectrum of Ni 2p3/2 peak at 856.4 eV confirms the
calculated from the desorption branch of the nitrogen presence of Ni in the sample (Figure 7b), mainly from
isotherms by the BJH method show a wide range of pore NiO.26,27 The formation of NiO could be due to the
diameters (from 5 to 237 nm) with a peak at a pore diameter of photoinduced electrons in the conduction band of TiO2
about 166 nm. Meanwhile, a distinct hysteresis loop can be transferred to Ni2+ clusters causing the reduction of a part of
observed between adsorption and desorption branches, in the Ni2+ clusters to NiO atoms due to their instability in the air.27
range of 0.8 to 1 nm, which is an indication of mesostructured In addition, the Ti2p and O1s peaks are respectively found at
the TiO2/CdS nanospheres.23,24 458.6 and 530.95 eV, which are compatible with the assignment
3.2. XRD Characterization. Figure 6 shows XRD patterns to TiO2. Cd3d (405.1 eV) and S2p (161.95 eV) peaks are
of TiO2 nanorods, CdS NPs, and TiO2/CdS nanocomposites. reported to be compatible with CdS. The observation of C1s
XRD patterns of TiO2 nanorods exhibit strong diffraction peaks element is due to the surfactant capped on the surface of the
at 25° and 48°, indicating a TiO2 anatase phase. All peaks were sample, and also from the adventitious hydrocarbon in the XPS
in good agreement with the standard spectrum for TiO2 instrument itself. The XPS peak at 686.91 eV is ascribed to F−
(JCPDS nos 88-1175 and 84-1286). Meanwhile, it is known ions coming from NOBF4 during surfactant treatment process.
that CdS NPs possess the hexagonal phase with (002) as the The presence of Ni in the sample was also confirmed from
preferential crystalline plane with two main peaks at 28.3° (101 the SEM-EDX elemental analytical spectrum (Figure 8). This
planes) and 48.1° (103 planes),25 while the cubic phase has spectrum shows that the intensity of Ni peak is small compared
three main peaks at 26.5° (111 planes), 43.9° (220 planes), and to the other elements. This is due to the small amount of Ni
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surface treatment and TiO2/CdS nanocomposites were

investigated by UV/vis absorption and photoluminescence
(PL) characterization techniques. The UV−visible absorption
spectrum (Figure 9a) has been performed to measure the
photoresponse of TiO2 nanorods after their loading with CdS.
The absorption edge for anatase TiO2 nanorod is approx-
imately 380 nm (3.12 eV), which has no significant absorption
in visible-light region. However, the spectrum of CdS exhibits a
broad absorption band around 530 nm (2.32 eV), indicating
the effective photoabsorption property in the visible region.
Basically, the spectrum of TiO2/CdS nanocomposite is a
combination of TiO2 and CdS spectra. The absorption edges of
the TiO2/CdS nanocomposite are at approximately 547 nm
(2.23 eV), which is around 15 nm red-shift than that of CdS.
This probably results from the coupling between CdS and
TiO2.
Figure 9b shows the PL emission spectra for CdS and TiO2/
CdS nanocomposites at room temperature under light
excitation at a wavelength of 380 nm. According to the PL of
both CdS and TiO2/CdS sample, PL peak of TiO2/CdS
exhibited much weaker intensity than of that of CdS. The
decrease in PL intensity indicates a better PL quenching, which
also indicates a decrease in light emission of the material or a
coupling between CdS and TiO2 with a better charge transfer
between these two nanoparticles. As discussion above, the
efficient charge transfer from CdS to TiO2 conduction band
could effectively separate the photoinduced electrons from
holes in the CdS semiconductor. Thus, the decrease in PL
intensity also could be ascribed to the lower recombination
probability of photoinduced electrons and holes in the TiO2/
Figure 7. (a) XPS characterization of Ni-TiO2/CdS nanocomposite CdS nanocomposite.28
and (b) high-resolution XPS of Ni. 3.5. Thermal Gravimetric (TGA) and ζ-Potential
Characterization. Thermal gravimetric characterization of
cluster deposited on the TiO2/CdS composite, which is only 3 synthesized capped TiO2 nanorods, CdS NPs, and TiO2/CdS
wt%. nanocomposites are summarized in TGA curves of Figure 10,
3.4. UV/Vis and Photoluminescence (PL) Character- which were obtained at a heating rate of 10 °C/min under O2
izations. The optical properties of TiO2 nanorods before atmosphere. All the three curves show an initial weight loss

Figure 8. SEM-EDX characterization of Ni−TiO2/CdS nanocomposite.

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evaporation of residual solvent. A non-negligible gain in mass

was also observed between 400 and 750 °C, which is an
indication of the formation of cadmium sulfate (CdSO4)
through the following reaction (eq 1).29 The decomposition of
CdSO4 starts at 750 °C leading to a further decrease in mass.
CdS(g) + 2O2 → CdSO4 (1)
The TGA behavior of the TiO2/CdS nanocomposite is
basically a combination of TiO2 and CdS behaviors. The weight
loss below 200 °C could be attributed to the water absorbed on
the surface of particles, while weight loss from 200 to 400 °C
could be due to the loss of the rest of surfactant on the surface
of TiO2. The mass increase observed at the same temperature
level corresponding to the increase in CdS mass, is due, as
mentioned above, to the formation of the intermediate product,
CdSO4.
The ζ-potential curves of TiO2 nanorods before and after
NOBF4 treatment, CdS NPs, and TiO2/CdS nanocomposites
are shown in Figure 11. According to these curves, the charge

Figure 9. (a) UV−vis spectra of TiO2, CdS, and TiO2/CdS. (b)

Photoluminescence (PL) emission spectra under excitation at a
wavelength of 380 nm for CdS and TiO2/CdS nanocomposite. Figure 11. ζ-Potential distributions in aqueous solution at pH ∼ 5 of
TiO2 nanorods before and after treatment with NOBF4 surfactant,
CdS NPs, and TiO2/CdS nanocomposite.

surface potential of TiO2 nanorods before surfactant exchange

was zero at pH = 5. However, when treated with NOBF4, the
surface of TiO2 nanorods was negatively charged, which is in
agreement with the results reported by Dong et al.30 Because
the surface of TiO2/CdS is negatively charged, Ni clusters were
selectively deposited (by using photodeposition technique) as
cocatalysts on the surface of TiO2/CdS composite. In this case,
Ni2+ is selectively adsorbed on the surface of TiO2 nanorods,
not on the surface of CdS (because the ζ-potential of CdS is
zero), due to the electrokinetic potential preferable in colloidal
systems. Under visible-light illumination, the generated
electrons from the conduction band of CdS are transferred to
the conduction band of TiO2. Because the conduction band
Figure 10. TGA characterization of (black) TiO2 nanorods, (blue) level of Ni2+/Ni is lower than that of TiO2, the electrons from
CdS NPs, and (red) TiO2/CdS nanocomposites. the conduction band of TiO2 are able to reduce Ni2+ to form
metallic Ni clusters on the surface of TiO2 nanorods (Scheme
starting at around 50 °C, which could be attributed to the water 1).
absorbed on the surface of the nanoparticles. For TiO2 3.6. Photocatalytic Activity. The photocatalytic activity of
nanorods, the most significant weight loss obviously occurred TiO2, CdS, and TiO2/CdS nanocomposite with Ni cocatalyst
between 200 and 480 °C and corresponds to OA surfactants. for H2 generation were carried out under visible-light
For higher temperatures (>480 °C), the very small weight loss irradiation (λ > 420 nm) using ethanol as a sacrificial reagent.
could be attributed to the decomposition of residual product As seen from Figure 12a, TiO2 nanorods are not able to
traces that forms a sheath over the TiO2 nanorods. For CdS generate H2 because TiO2 nanorods do not absorb visible light
NPs, the TGA spectrum shows that the main mass decrease and consequently could not generate electron−hole to support
occurred below 400 °C, which could be mainly due to the the H2 evolution. Besides, CdS alone shows very low H2
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times higher than the production for the Ni−CdS system. The
rate of Ni−TiO2/CdS photocatalytic activity is also reported to
be faster compared to that of Ni−CdS, which could be due to a
better charge transfer between CdS and TiO2, as shown and
discussed above (Figure 9). The photocatalytic performance of
TiO2/CdS without a Ni cocatalyst using ethanol as a sacrificial
reagent was also carried out; however, the H2 production
evolution maybe was too low, and so we would not be able to
detect the signal of activity. In other words, without using Ni as
a cocatalyst, the composite TiO2/CdS is not active for
photocatalytic H2 production using visible light.
To investigate the stability of Ni−TiO2/CdS samples, a series
of tests composed of four cycles with intermittent deoxygena-
tion were carried out without catalyst regeneration. Between
each cycle, the reaction system was bubbled with N2 to remove
H2. As shown in Figure 12b, the results show good stability for
the photocatalyst up to 15 h of irradiation without noticeable
catalytic deactivity; however, after 15 h of reaction, the activity
is decreased by about 50%. Even though the photocatalyst was
decreased after 15 h of irradiation, this achievement is still
considered as a good improvement for the photocatalytic
activity of metal sulfides, which are often unstable for
conventional CdS photocatalysts, due to the reduction of
metal cations in metal sulfides by generated electrons, and the
oxidation of S2− by generated holes.31−33
In Ni−TiO2/CdS nanocomposite, with the support of TiO2
nanorods, the photo-oxidation is avoided due to the electrons
transfer from the conduction band of CdS to that of TiO2 and
then to the metallic cocatalyst (Ni), therefore it would prevent
Cd2+ from reduction. In addition, under visible-light illumina-
Figure 12. (a) Comparison of the activity of H2 evolution using tion, only CdS with a small bandgap energy of 2.4 eV can
different photocatalysts; (b) H2 production from TiO2/CdS-Ni generate holes in the valence band (VB). However, because the
photocatalyst monitored over 18 h. Each 4.5 h, the react ion system VB of CdS (+1.5 V vs SHE) is smaller than the VB of TiO2
is bubbled with N2 to remove the H2 inside.
(+3.4 V vs SHE),34,35 these holes in the VB of CdS cannot be
transferred to the VB of TiO2. Thus, Ni clusters, which are only
generation rates, only 0.77 μmol·h−1·g−1 after 4.5 h of reaction. located on the surface of TiO2, are cannot be oxidized by holes
The low rate could be due to the rapid recombination of in the VB of CdS NPs. Therefore, with those mentioned special
photogenerated electrons and holes, which resulted in the lack features above, it is not surprising to see that Ni−TiO2/CdS
of H2 evolution sites.31,32 The coupling of CdS with TiO2 nanocomposite exhibits not only high activity but also good
nanorods shows a big improvement in H2 production; around stability in the photocatalyst production of H2 up to 15 h of
33.63 μmol·h−1·g−1 of H2 was evolved, which is around 44 irradiation.

Figure 13. Mechanism illustration of the activity of Ni−TiO2/CdS under visible light for the production of H2; inset is the potential redox energy
corresponding to CdS, TiO2, and H+/H2.

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Article

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