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S. Mostoni et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 534–545
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powders were firstly synthesized by a sol-gel method, evaluating
how structural, morphological, optical and surface properties can
be influenced by changing the zinc precursor (zinc acetate or
nitrate) and the acidity of the medium (nitric acid). Then, the
nanopowders were further modified adding a dopant to increase
their solar light adsorption (composed only by 5% of UV
component) and hence to improve the photocatalytic perform-
ances for in situ applications. Several articles [18,23–25] have
demonstrated the ability of bismuth dopant to reduce the band gap
of semiconductors, shifting the adsorption edge of the pure
semiconductor and thus modifying the separation rate of
photoinduced charge carriers. Before appreciating the possible
improvement under solar light due to the presence of bismuth, a
detailed study about the photocatalytic performances of bismuth-
doped nanopowders was done under UV light. Therefore, Bi-doped
samples were both prepared by traditional impregnation method
and by modification of the sol-gel route, introducing a bismuth
precursor (bismuth nitrate) directly in the synthetic path.
All the as-synthesized nanopowders were then tested as
photocatalysts, monitoring both the disappearance (by means of
Linear Sweep Voltammetry (LSV), as already reported in our
previous work [19]) and the mineralization degrees (by means of
Total Organic Carbon (TOC) technique). Finally, the by-products
identified by both HPLC/MS techniques on eluates (3 h and 6 h) and
FTIR analyses on used powders (after 6 h of photocatalysis process)
have allowed the proposal of a possible mechanism for o-toluidine
photoremoval, expanding the mechanism reported in the recent
literature [17,19].
continuously stirred for 24 h and subsequently centrifuged and
washed several times by the appropriate solvent (the same used
for the synthesis). Then, all the powders were dried in an oven at
90 ꢂC for a night and calcined at 300 ꢂC for 6 h under O2 stream (9
NL/h).
As concerns Bi-doped samples, a new synthetic route was
adopted to introduce Bi-dopant directly into ZnO lattice, instead of
the common impregnation method [18]. Thus, the suitable amount
of bismuth nitrate (Bi(NO3)3 5H2O) was utilized to have Bi/Zn
ꢀ
atomic ratio equal to 0.01 (as the most performing dopant
percentage [18]). To avoid the poor solubility of the bismuth salt in
both solvents (ethanol and water, used in the sol-gel method for
pure samples), acidic synthetic conditions (HNO3 65%wt, see
Table 1) were adopted. Therefore, the protocol previously reported
was modified in two points: i) the addition of nitric acid in the
initial solution of the zinc salt (so that the final concentration of
HNO3 is equal to 0.5 M) and ii) the addition of the bismuth acidic
solution before NaOH (0.5 M, Table 1). The as-prepared Bi-doped
ZnO compounds were labeled as Z_AcH_Bi (from zinc acetate) and
Z_NH_Bi (from zinc nitrate), respectively.
Moreover, due to the addition of nitric acid in both zinc salt
solutions, two further syntheses were carried out to obtain pure
zinc oxide, in order to understand if acidic conditions could modify
the properties of bare compounds. Thus, the ZnO nanopowders
were prepared using the same procedure described above with the
quantities reported in Table 1. These samples were respectively
named as Z_AcH and Z_NH.
2.2. Synthesis of Bi-doped ZnO via impregnation
2. Material and methods
In order to deeply study the role of the bismuth doping
procedures on the properties of the synthesized photocatalysts,
Z_Ac and Z_N calcined samples were also doped by impregnation
method. Indeed, according to what already reported in the
literature [26], Bi3+ cannot exist in aqueous solutions at pH higher
than 4 since it forms Bi(OH)3 compound, which is almost insoluble
and tends to precipitate. Thus, taking into account the occurrence
of basic conditions during the synthetic route described in the
previous paragraph, the impregnation method (in acidic con-
ditions) should be the more efficient one to dope ZnO nano-
particles with bismuth ions, due to their higher adsorption on zinc
oxide surface. Hence, the doping was realized by adding a 0.1 M
All chemicals were of reagent grade purity and were used
without further purification; doubly distilled water passed
through a Milli-Q apparatus was used to prepare solutions and
suspensions.
2.1. Synthesis of bare and Bi-doped ZnO nanoparticles via sol-gel route
In this work we adopted a sol-gel method, previously optimized
in our laboratory [19] for both bare and bismuth-doped ZnO
samples, based on the adoption of two different zinc precursors:
zinc acetate, Zn(CH3COO)2
ꢀ
2H2O (Z_Ac compounds) and zinc
nitrate, Zn(NO3)2 6H2O (Z_N compounds). A general synthetic
ꢀ
solution (1.23 mL) of Bi(NO3)3 5H2O (in HNO3 65%wt) to have a Bi/
ꢀ
procedure can be summarized as follows: the suitable amount of
zinc salt was dissolved under vigorous stirring (at 300 rpm) into
the appropriate solvent (i.e. ethanol or water for Z_Ac and Z_N,
respectively) and at different temperatures, accordingly to the
reagent used (Table 1). In the case of zinc acetate a cooling step in
water/ice bath (T ꢁ5 ꢂC) comes after, for 5 min. Then, a suitable
amount of 0.5 M NaOH (Table 1, 5th column, always in excess with
respect to Zn salt) was slowly added to the Zn2+ aqueous solution to
Zn molar ratio of about 0.01. Then, the powders were dried in an
oven at 90 ꢂC and calcined at 300 ꢂC for 6 h. Samples (labeled as
Z_Ac_Bi_impr and Z_N_Bi_impr, respectively) obtained after the
calcination step showed a very intense yellow color.
2.3. Sample characterizations
X-Ray Powder Diffraction (XRPD) analyses were performed on a
Philips PW 3710 Bragg-Brentano goniometer equipped with a
scintillation counter and 1ꢂ divergence slit, 0.2 mm receiving slit,
and 0.04ꢂ soller slit systems. We employed graphite-monochro-
reach
a pH around 9. The obtained whitish colloid was
mated Cu Ka radiation (Cu K 1 l= 1.54056 Å, K 2 l= 1.54433 Å) at
a a
Table 1
40 kV ꢃ 40 mA nominal X-rays power. Diffraction patterns were
collected between 20ꢂ and 80ꢂ with a step size of 0.1ꢂ and a total
counting time of about 1 h. Quanto fitting program and the
Scherrer equation were applied throughout to provide estimates of
the average domain sizes.
Synthetic conditions adopted for both bare (acidic and not) and Bi-doped ZnO
samples.
Sample
mol
Salt
T/ꢂC
Solvent
HNO3
NaOH
High-Resolution Transmission Electron Microscope/Scanning
Transmission Electron Microscope (HR-TEM) analyses were
performed on LIBRA 200 EFTEM (Zeiss) instrument operated at
200 kV accelerating voltage. The microscope is equipped with an
Energy-dispersive X-ray system (EDX – Oxford INCA Energy TEM
200) for elemental maps and analysis. The TEM grids were
Z_Ac
Z_N
Z_AcH
Z_NH
Z_AcH_Bi
Z_NH_Bi
0.015
0.050
0.015
0.050
0.015
0.050
3.4
5.6
3.4
5.6
3.4
5.6
–
3.4
3.4
11.4
14.0
11.4
14.0
80
25
80
25
80
25
–
0.10
0.10
0.14
0.12
Mostoni
