B. Krishnakumar, T. Imae / Applied Catalysis A: General 486 (2014) 170–175
171
several methods have been developed [3,5,23–25]. For this reason,
reactive silane coupling agents have often been used as they can
form stable chemical bonds between both inorganic (metal oxide)
and organic materials. To date, literatures related to surface func-
tionalization of metal oxides have largely been focused on silica
particles [26,27], and some molecular treatments of ZnO particles
have been investigated [28,29].
Therefore, in the present investigation, for the first time, the sur-
face of ZnO prepared by simple physical grinding was chemically
modified by PAMAM dendrimer using glycidoxypropyltrimethoxy
silane (GPTMS) as a binding agent. The chemical modification of
particle (ZnO) surface, which can be classified as surface graft-
ing, is the most promising method because of the strong covalent
bond between particle surface and PAMAM dendrimer. The surface
modification reduces its surface energy and hence the agglomera-
tion, resulting in the dispersibility of ZnO particles. To the best of
our knowledge, this is the first report on the synthesis of chem-
ically modified dendrimer-ZnO nanocomposite and its use in the
degradation of naphthol blue black (NBB) dye under UV light illu-
mination.
10 min. The formed zinc hydroxide was washed several times with
water and ethanol, filtered and dried at room temperature for one
◦
day. The crystals were calcined for 3 h at 200 C in the muffle furnace
◦
−1
heated at the rate of 20 C min
.
About 100 mg of ZnO nano rods were dispersed in 10 mL of
ethanol under ultrasonication for 10 min, followed by addition of
100 mg of GPTMS to this solution (Scheme 1). After ultrasonication
for 3 h, the unreacted GPTMS was removed by centrifugation. The
precipitate of GPTMS-modified ZnO was washed three times with
ethanol to complete removal of unreacted GPTMS.
About 100 mg of surface modified ZnO nano rods were dis-
persed in 20 mL of ethanol and 10 L of G4-PAMAM dendrimer
◦
was added to the dispersion. The mixture was stirred for 6 h at 50
C (Scheme 1). The chemically modified dendrimer-ZnO nanocom-
posite was collected by centrifugation and dried in air for one day.
2.4. Photodegradation experiments
Photocatalytic activities of the as-obtained photocatalysts were
evaluated by the degradation of NBB azo dye under a HOYA-SCHOTT
Ex 250 photoreactor (UV light source <400 nm). In each experi-
ment, the reaction suspension containing photocatalyst (10 mg) in
a 50 mL NBB solution (30 ppm) was magnetically stirred in the dark
for 30 min to ensure the adsorption equilibrium between photocat-
alyst powders and NBB. Then the solution was exposed to the UV
light irradiation, and aliquots (4 mL) were sampled at given time
intervals and centrifuged to remove photocatalyst powders. The
filtrates were analyzed by recording the absorbance variations for
an absorption band (320 nm) of NBB.
2
. Experimental
2.1. Materials
Zinc acetate dihydrate (99%) and sodium hydroxide were
obtained from Acros Chemicals. A 10 wt% methanol solution of
fourth generation (G4) amine-terminated PAMAM dendrimer, gly-
cidoxypropyltrimethoxy silane (GPTMS), and azo dye NBB (the
structure of the dye and its absorption maxima are given in Figure
S1, see Supplementary data) were obtained from Aldrich Chemicals
and used as received. The ultra-pure (Milli Q) water was used to
3. Results and discussion
prepare experimental solutions. An aqueous H SO4 or NaOH solu-
2
tion was used for adjusting the pH of the solution before irradiation.
3.1. Characterization of catalyst
2
.2. Analytical methods
The prepared ZnO nano rods and dendrimer-ZnO nanocompos-
ite were characterized by different characterization techniques.
XRD patterns of ZnO nano rod and dendrimer-ZnO nanocomposite
Fourier transform-infrared (FT-IR) absorption spectra were
◦
recorded on a Thermo Nicolet Nexus 6700 instrument. The crystal
phase analysis was done by a wide-angle X-ray diffraction (XRD)
are shown in Fig. 1. The diffraction peaks of ZnO (Fig. 1a) at 31.66 ,
◦
◦
◦
34.35 , 36.13 and 56.47 correspond to (1 0 0), (0 0 2), (1 0 1) and
(1 1 0) planes of wurtzite ZnO (JCPDS 89-0511). The diffraction pat-
tern of dendrimer-ZnO nanocomposite has no difference from that
of ZnO nano rods, as shown in Fig. 1b. This reveals that dendrimer-
ZnO nanocomposite has a wurtzite structure as well as ZnO nano
rods without any disturbance by dendrimer. IR studies were per-
formed to investigate the presence of PAMAM dendrimer in ZnO.
The IR spectra of ZnO nano rods and dendrimer-ZnO nanocom-
posite are given in Fig. 2a and b, respectively. Absorption bands
(
Bruker D2 Phaser, USA) at 10 mA current and 30 kV voltage with
a monochromatic CuK␣ radiation (ꢀ = 1.5405 A˚ ) with a 2ꢁ step of
◦ ◦
.05 per 1 s, and a scan range of 2ꢁ = 20–80 . On an observa-
0
tion with a JEOL JSM-6500F cold field emission scanning electron
microscope (FE-SEM), the samples were mounted on a gold plat-
form placed in chamber. The transmission electron microscopic
(
TEM) observation was carried out on a Hitachi H-7000, Japan,
with an acceleration voltage of 100 kV. The specimens were pre-
pared by depositing a drop of the suspension of sample powder,
which was ultrasonically dispersed in acetone for 10 min, on a
carbon-coated copper grid, followed by drying at room tempera-
ture. The ultraviolet-visible absorption spectra were recorded using
a Jasco V-670 series UV spectrometer. For the measurement of dif-
fuse reflectance spectra (DRS), the powder sample was dispersed
in acetone and the quartz plate was dipped in the concentrated
dispersion and dried in an air.
−
1
at 3384, 1548 and 1398 cm can be attributed to the characteris-
tic bands of surface hydroxyls of ZnO in both cases [1]. There are
−
1
three new bands at 3502, 3328 and 1616 cm in dendrimer-ZnO
nanocomposite (Fig. 2b), which can be attributed to the charac-
teristic bands of free and hydrogen bonded amino ( NH2) and
amide group (CONH) of G4-PAMAM dendrimer [30]. In addition,
−
1
dendrimer-ZnO has a strong absorption band at 1116 cm
due
to antisymmetric stretching vibration of O Si O framework from
GPTMS coupling agent [31].
2
.3. Synthesis of dendrimer-ZnO nanocomposite
The morphology of ZnO nano rods and PAMAM-ZnO was
observed by FE-SEM and TEM and the images are given in Fig. 3.
Both ZnO and PAMAM-ZnO had rod like structure, but the size
of the rods decreased after the chemical modification by PAMAM
dendrimer. It may be due to sonication effect during the sur-
face modification. Under sonication, the elongated rods may be
broken to decrease in length. The length of the nano rods was
from 100 to 300 nm and the width of the rods was approxi-
mately 20–40 nm, while the size of dendrimer-ZnO composite
ZnO was prepared by simple physical grinding of zinc acetate
with sodium hydroxide using pestle and mortar (Scheme S1, see
Supplementary data) under solvent free condition. About 5 mmol
(
2
0.9174 g) of zinc acetate dihydrate was taken in mortar, and
0 mmol (0.8 g) of sodium hydroxide pellets were added. Then the
mixture was ground in a mortar with pestle at room temperature.
The grinding was continued for 1 h with 5 min interval for each