Synthesis of nitrogen-doped ZnO by sol-gel method: Characterization and its application on visible photocatalytic degradation of 2,4-D and picloram herbicides

Article abstract of DOI:10.1039/c4pp00273c

In this work, nitrogen-doped ZnO material was synthesized by the sol-gel method using zinc acetate as the precursor and urea as the nitrogen source (15, 20, 25 and 30% wt.). For comparative purposes, bare ZnO was also prepared. The influence of N doping on structural, morphological, optical and photocatalytic properties was investigated. The synthesized catalysts were characterized by XRD, SEM-EDS, diffuse reflectance UV-Vis spectroscopy, BET and XPS analysis. The photocatalytic activity of N-doped ZnO catalysts was evaluated during the degradation of a mixture of herbicides (2,4-D and picloram) under visible radiation ≥400 nm. The photo-absorption wavelength range of the N-doped ZnO samples was shifted to longer wavelength compared to those of the unmodified ZnO. Among different amounts of dopant agent, the 30% N-doped ZnO material showed higher visible-light activity compared with pure ZnO. Several degradation by-products were identified by using HPLC and ESI-MS/MS. The enhancement of visible photocatalytic activity of the N-doped ZnO semiconductor could be mainly due to their capability in reducing the electron-hole pair recombination. This journal is

Full text of DOI:10.1039/c4pp00273c

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Synthesis of nitrogen-doped ZnO by sol–gel
method: characterization and its application on
visible photocatalytic degradation of 2,4-D and
picloram herbicides†
Cite this: DOI: 10.1039/c4pp00273c
J. J. Macías-Sánchez,a L. Hinojosa-Reyes,a A. Caballero-Quintero,b W. de la Cruz,c
E. Ruiz-Ruiz,a A. Hernández-Ramíreza and J. L. Guzmán-Mar*a
In this work, nitrogen-doped ZnO material was synthesized by the sol–gel method using zinc acetate as
the precursor and urea as the nitrogen source (15, 20, 25 and 30% wt.). For comparative purposes, bare
ZnO was also prepared. The influence of N doping on structural, morphological, optical and photo-
catalytic properties was investigated. The synthesized catalysts were characterized by XRD, SEM-EDS,
diffuse reflectance UV-Vis spectroscopy, BET and XPS analysis. The photocatalytic activity of N-doped
ZnO catalysts was evaluated during the degradation of a mixture of herbicides (2,4-D and picloram) under
visible radiation ≥400 nm. The photo-absorption wavelength range of the N-doped ZnO samples was
shifted to longer wavelength compared to those of the unmodified ZnO. Among different amounts of
dopant agent, the 30% N-doped ZnO material showed higher visible-light activity compared with pure
ZnO. Several degradation by-products were identified by using HPLC and ESI-MS/MS. The enhancement
of visible photocatalytic activity of the N-doped ZnO semiconductor could be mainly due to their capa-
bility in reducing the electron–hole pair recombination.
Received 14th July 2014,
Accepted 9th October 2014
DOI: 10.1039/c4pp00273c
www.rsc.org/pps
Additionally, some problems still remain to be solved in the
application of ZnO semiconductors, such as the fast recombi-
Introduction
Zinc oxide (ZnO) is a widely used photocatalyst owing to its nation of photogenerated electron–hole pairs.3 For that reason
high activity and low cost, and has attracted much attention in is important to evaluate new modified ZnO catalysts with
the field of environmental research, where it is commonly improved photocatalytic activity and enhanced activities under
used for the degradation of various recalcitrant organic pollu- both UV and visible light irradiation.2–5
tants such as herbicides in aqueous solution.1
Recent investigations to activate ZnO have been focused on
However, the photocatalytic activity of ZnO is limited to doping ZnO with transition metals such as manganese, ruthe-
UV irradiation because it is a semiconductor with a wide band- nium and vanadium using sol–gel or co-precipitation
gap of about 3.2 eV and can only absorb UV light with λ ≤ methods.4,6,7 However, the thermal instability and the increase
387 nm.2 Therefore visible light responsive catalysts are highly of carrier recombination centers greatly limit the performance
desired due to the low cost of energy involved and the possi- of transition metal dopants.8 One way to avoid this limitation
bility of using solar energy to activate the photocatalyst. is to dope non-metals such as nitrogen, sulphur, and carbon
into the substitutional sites in the crystal structure of the
photocatalyst.9–12
Therefore, the anionic non-metal impurities are presented
as better options for incorporation as dopants into the substi-
aUniversidad Autónoma de Nuevo León, UANL, Facultad de Ciencias Químicas, Cd.
Universitaria, San Nicolás de los Garza, Nuevo León C.P. 66451, Mexico.
E-mail: jorge.guzmanmr@uanl.edu.mx
tutional sites in the crystal structure of a photocatalyst because
of their closer position to the oxygen in the periodic table.9
The incorporation of this type of dopant in the crystal struc-
ture of the semiconductor promotes narrowing of the band
gap, causing its activation with lower energy radiation.13 Nitro-
gen-doped ZnO catalyst (N-ZnO) has been synthesized by
thermal decomposition of zinc nitrate at 350 °C. This material
showed improvement over undoped ZnO in its optical
bProcuraduría General de Justicia del Estado de Nuevo León, Laboratorio de Química
Forense, Criminalística y Servicios Periciales, Av. Gonzalitos 452, Col. Residencial
Galerías, Monterrey, NL C.P. 66451, Mexico
cUniversidad Nacional Autónoma de México, UNAM, Centro de Nanociencias y
Nanotecnología, Km 107 Carretera Tijuana-Ensenada, C.P. 22800 Baja California,
Mexico
†This paper is published as part of the themed issue of contributions from the
8th European Meeting on Solar Chemistry and Photocatalysis: Environmental
Applications held in Thessaloniki, Greece, June 2014.
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absorption and photocatalytic activity on the reduction of solutions. All other reagents were reagent grade and used as
Cr(VI) and the oxidation of methyl orange under visible light received. All solutions were prepared with ultrapure water
radiation.14 Thin films of N-ZnO were also synthesized via (18 MΩ cm−1) from a Millipore Milli-Q system.
spray pyrolysis technique in aqueous medium using zinc
Synthesis of ZnO and N-ZnO
acetate and ammonium acetate as precursors. The rate of
degradation of toluene over N-doped ZnO was higher as com- N-ZnO material was prepared by the sol–gel method using zinc
pared to that of pure ZnO under solar illumination.12
acetate as the precursor and urea as the nitrogen source; the
In this work, the synthesis of N-ZnO photocatalyst by the [N]/[Zn] ratios of the doped materials calculated on the basis
sol–gel method was proposed since this method allows the of the weight percent used in the starting solution were 15, 20,
incorporation of the dopant into the crystal lattice of the 25 and 30%. The technique is briefly described as follows:
material15 using urea as nitrogen precursor. Among the nitro- 13.5 g of zinc acetate and urea was dissolved in 250 mL of
gen sources, urea is an inexpensive reagent that has demon- water. An aqueous solution (50% v/v) of NH4OH was added
strated to lead to controlled nucleation and growth of under continuous magnetic stirring to reach a pH of 8.5. The
nanocrystals, allowing the formation of well-ordered meso- reaction mixture was kept at room temperature until the gela-
porous structures.16 To the best of our knowledge, the success- tion was started. The obtained gel was aged for 24 h at room
ful incorporation of nitrogen in TiO2 using the sol–gel process temperature to complete the polymerization, filtered, washed
has already been demonstrated,17–19 but there are no reports with 0.1 M NH4NO3, and subsequently dried by heating slowly
in synthesizing N-ZnO photocatalyst by the sol–gel method. In to 90 °C until the solvent was completely evaporated. The
the sol–gel process, the doping levels as well as the size of resulting powder was calcined at 400 °C for 5 h in air with
nanoparticles can be easily controlled depending on the reac- heating rate of 10 °C min−1. The pure ZnO catalyst was pre-
tion conditions such as the solvents used, pH, temperature, pared by the same procedure without addition of urea.
and hydrolysis rate.15 Moreover, the utilization of N-ZnO
Catalyst characterization
photocatalyst in degrading 2,4-dichlorophenoxyacetic acid
(2,4-D) and 4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid The structural properties of the catalysts were studied by X-ray
(picloram) mixture in aqueous solution under visible radiation diffraction (XRD) (Siemens D5000) using Cu Kα radiation (λ =
has not been reported so far. 2,4-D is extensively used world- 1.5418 Å). BET surface areas were calculated from N2 adsorp-
wide due to its low cost for selective weed control in gardens tion–desorption isotherms obtained on an Autosorb-1 instru-
and farmlands. It has led to its frequent detection in ground- ment (Quantachrome Co., Boynton Beach, FL, USA). The band
water.3,20 Picloram is a potent herbicide that effectively con- gap energy (Eg) values of the studied catalysts were calculated
trols the growth of broadleaf weed and woody plants in from the UV-Vis diffuse reflectance spectra using a Thermo
pastures and rangelands. Picloram is poorly bound to soil and Scientific Evolution 300 UV-Vis spectrophotometer (Thermo
its water solubility (440 mg L−1) makes it a potentially mobile Fisher Scientific, Massachusetts, USA) equipped with an inte-
compound. These properties, combined with being relatively grating sphere TFS-Praying Mantis. Spectralon sample was
persistence in soil, mean that the use of picloram can cause used as the reference. The morphology of the catalysts was
groundwater contamination.13,21 Commercial formulations examined using JEOL JSM-6701F scanning electron microscopy
usually available in Mexico contain these herbicides in mixture (SEM) and energy dispersive spectroscopy (EDS) on powder
e.g. Tordon 101, Tordon 472, etc. Therefore, the photocatalytic samples directly sprinkled over aluminium foil under high
degradation of a 2,4-D and picloram mixture was undertaken vacuum and accelerating voltage of 12 kV. X-ray photoelectron
under visible radiation using N-ZnO catalyst. An attempt was spectroscopy (XPS) analyses were performed on a multi-tech-
also made to identify the intermediate products formed nique system (SPECS) equipped with a dual Mg/Al X-ray source
during the photocatalytic process through HPLC and ESI-MS/ and hemispherical PHOIBOS 150 WAL analyzer operating in
MS analysis.
the fixed analyzer transmission (FAT) mode. The spectra were
obtained at 1486.74 eV using Al Kα as the X-ray source. The
anode was operated at a voltage of 14.5 kV and source power
level was set to 350 W. The binding energies were referenced to
the carbon (C1s) peak at 285.0 eV to compensate for charging
effects and the peak positions were compared to standard
Experimental section
Chemicals
Zinc acetate (99.6% purity) was purchased from J. T. Baker. values for identification of different elements and their oxi-
Ammonium hydroxide (50% v/v) reagent grade was from Reac- dation states.
tivos Monterrey. Urea (99% purity), acetonitrile (HPLC grade),
sodium hydroxide (97% purity) and phosphoric acid (85%, v/v)
Evaluation of the photocatalytic activity
were from Sigma-Aldrich. 2,4-D (98% purity) and picloram The photocatalytic experiments were performed in a Pyrex
(99.6%) analytical grade herbicides were obtained from Spec- reaction vessel (200 mL capacity) containing an aqueous solu-
trum and Fluka, respectively. Stock solutions of the herbicides tion of a mixture of 20 mg L−1 2,4-D and 5 mg L−1 picloram.
(100 mg L−1) were prepared in volumetric glassware. The The solution pH was adjusted to 7 by the addition of 0.1 M
required working standards were prepared daily from the stock NH4OH. As radiation source, a visible 25 W tungsten–halogen
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(Philips Electronics, 760 W m−2) lamp was used, which emits
polychromatic radiation in the spectral range from 400 to
700 nm (with the maximum in the wavelength range
540–620 nm).
The on-line monitoring of the concentration of each herbi-
cide was performed using an automated multisyringe chroma-
tographic system (MSC).22 The MSC equipment consisted of a
multisyringe module (BU-4 S, Crison; Alella, Barcelona, Spain)
with four 5 mL syringes (S1–S4) (Hamilton, Bonaduz, Switzer-
land) and an 8-port multi-position selection valve (MPV,
Crison Instruments, Alella, Spain), each of which was con-
nected at the head to a three-way solenoid valve (N-Research,
Caldway, NJ, USA).
The detection system consisted of a DT-1000CE deuterium
light source (Analytical Instrument System, Flemington, NJ,
USA) and a USB2000 CCD spectrophotometer (Ocean Optics,
Dunedin, FL, USA) connected to a flow-through quartz cell
(178.711-QS; Hellma, NY, USA). Measurements were recorded
at 250 nm. Autoanalysis v5.0 software (Sciware, Palma de Mal-
lorca, Spain) was used for instrument control, data acquisition,
and data processing. Quantitative analysis was based on peak
height. Chromatographic separation of 2,4-D and picloram
was performed on an Onyx™ C18 monolithic column (25 ×
4.6 mm, Phenomenex) with a 10 × 4.6 mm monolithic guard
column. The manifold was built from 0.8 and 1.5 mm i.d.
Teflon tubing. The mobile phase used for chromatographic
Fig. 1 XRD patterns of ZnO and N-doped ZnO catalysts prepared by
sol–gel method.
Table 1 Specific surface area, crystallite size and Eg values for N-
doped ZnO and pure ZnO
BET area
Crystallite
size (nm)
Sample
ZnO
(m2 g−1
)
Eg value (eV)
18.50
13.77
18.72
25.80
21.95
36.20
31.13
28.54
27.63
28.84
3.06
2.97
2.85
2.90
2.87
separation was 5 mM H3PO4–acetonitrile (70 : 30, v/v) with a 15% N-ZnO
flow rate of 1 mL min−1. The photocatalytic reactions were con-
20% N-ZnO
25% N-ZnO
30% N-ZnO
ducted in triplicate.
The mineralization percentage was assessed by determining
the total organic carbon (TOC) content in the aqueous solution
with a TOC-V CSH Shimadzu Analyzer.
good crystallinity. The introduction of nitrogen atoms did not
change the ZnO crystal structure. Crystallite sizes of the N-con-
taining ZnO were calculated from the X-ray peak broadening of
the (101) diffraction peak using the Scherrer formula.23 The
crystallite sizes of N-containing ZnO are shown in Table 1
along with other physical characteristics. It can be seen from
Table 1 that the crystallite size of N-ZnO materials decreased
with the increase of nitrogen present in the material, up to
25%. This result was consistent with those reported by Qin
et al.,11 who indicated that during preparation of N-doped ZnO
catalysts, the addition of urea, which behaves as a surfactant,
could inhibit the growth of ZnO.
BET surface areas are also given in Table 1. The specific
surface area varied from 18.50 to 25.80 m2 g−1, and its
maximum value was observed on 25% N-doped ZnO. This
value is higher than the value of 18.95 m2 g−1 for pure ZnO,
indicating that the introduction of nitrogen facilitated the for-
mation of particles with porous surfaces. Optical characteriz-
ation was performed by measuring the UV-Vis diffuse
reflectance spectrum to determine the Eg value of each
material using the Kubelka–Munk function F(R) (1).24
Generated carboxylic acids were detected by ion-exclusion
chromatography using a Bio-Rad Aminex HPX-87H column
(300 × 7.8 mm) with UV detection (210 nm) using 14 mM
H3PO4 mobile phase at 0.4 mL min−1
.
The by-products from the photocatalytic degradation were
identified by an ESI-MS/MS mass spectrometer AB Sciex API
4000 operating in the negative ion mode. The reaction aliquots
were directly infused into the ion source at a flow rate of 20 μL
min−1 using a micro syringe (Hamilton Company, Reno, NV,
USA) and the related mass spectra obtained as an average of
1463 scans, each one requiring 0.2 s. Typical ESI conditions
were as follows: heated capillary temperature, 200 °C; sheath
gas (N2) at a flow rate of 20 mL min−1; spray voltage, −5.5 kV;
nebulizing gas, and curtain gas flows were at instrument set-
tings of 30, and 10 mL min−1, respectively.
Results and discussion
Characterization
The diffraction patterns of ZnO and N-doped ZnO (15, 20, 25
and 30%) semiconductors are shown in Fig. 1.
FðRÞ ¼ ð1 ꢀ RÞ=2R
ð1Þ
It can be seen that N-containing ZnO powder exhibits domi- where R is the magnitude of reflectance and F(R) is pro-
nant diffraction peaks of wurtzite phase (JCPDS: 36-1451) with portional to the extinction coefficient (α). The Kubelka–Munk
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Fig. 2 SEM images at 50 000× magnification of (a) ZnO, (b) 15%
N-ZnO, (c) 20% N-ZnO, (d) 25% N-ZnO and (e) 30% N-ZnO.
function can be obtained by multiplying the F(R) function by
hν using the corresponding coefficient (n) associated with an
electronic transition as follows: (F(R) × hν)n; for ZnO, the value
for the direct allowed sample transition is n = 1/2.24,25
As can be seen in Table 1, the Eg values of the catalysts were
noted to shift to lower energies with increasing the nitrogen
content in the catalyst. The increased amount of visible light
energy absorbed can be related to two phenomena: introduc-
tion of surface structural defects and lattice alteration during
the modification procedure. In fact, the Eg value decreased by
0.19 eV for the 30% N-doped sample compared to its value for
the ZnO catalyst.
The catalysts were also analysed by SEM-EDS. Although the
elemental analysis detected the presence of Zn, C and O (data
no shown), changes in the morphology of the catalysts were
noticed when the concentration of nitrogen increased from 15
to 30%. The N-ZnO photocatalysts consisted of agglomerates
of primary particles with uniform rod-like shape of approxi-
mately 80–100 nm (Fig. 2).
Fig. 3 XPS spectra of the (a) N1s, (b) Zn2p and (c) O1s core levels of the
30% N-ZnO catalyst. For (b) and (c) the ZnO spectrum is also shown as a
reference.
3/2
The presence of nitrogen in the crystalline lattice of N-ZnO
and its concentration was further confirmed by XPS. From the presence of N–H compounds in the N-ZnO catalyst. From the
peak intensity and using the photo-ionization constants for each
asymmetric broad peak from 398 to 402 eV with maximum at element (Zn, O and N), the N percentage present in the sample
high resolution scanning XPS spectra of N1s, a characteristic
11,14
398.5 eV related to N1s
was observed in the N-ZnO was calculated. The N percentage for 30% N-doped ZnO was
0.9%. This value is similar to the obtained by Chen et al.14
material, which suggests that more than one chemical state of
N is present;26 although the peak in the XPS survey profile was through high resolution scanning XPS analysis, which reported
weak, the peak position and its broadening were clear in the an amount of 1.0 at% for a low intensity spectra. XPS is a surface
chemical analysis technique that detects the elemental compo-
N
1s scan profile (Fig. 3a). Chen et al. reported a binding energy
of 398.8 eV for the nitrogen substitution on the oxygen sub- sition of the surface of materials (usually the top 1–10 nm).
lattice (NO).14 The peak of 399.6 eV is a typical value of the N1s
binding energy in amines;26 thus is reasonable to expect the
High resolution XPS spectra of the Zn2p and O1s core lines
3/2
are shown in Fig. 3b and c, respectively. The core line of Zn2p
3/2
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for pure ZnO was located at 1021.8 eV (Fig. 3b). The asymmetric
Zn2p peak for nitrogen doped ZnO was located at 1022 eV and
3/2
showed superposition of Zn–O and Zn–N bonds, revealing incor-
poration of N in the ZnO catalyst.27 In Fig. 3c the high resolu-
tion scanning XPS spectrum of O1s after deconvolution shows
that oxygen exists at least in three forms with the following
binding energies: 530.5, 532.0, 533.5 eV. The peak at 530.5 eV is
mainly assigned to the oxygen atoms coordinated with Zn
atoms. The peak at 532 eV corresponds to oxygen adsorbed on
sample surface, probably as (–OH). The peak at 533.5 eV is likely
to be due to water molecules on the surface14,28
Photocatalytic evaluation
The photocatalytic activities of the N-ZnO materials were com-
pared to that of ZnO sol–gel under visible irradiation for the
degradation of a mixture of 20 mg L−1 2,4-D and 5 mg L−1
picloram at pH 7 in aqueous solution using 1 g L−1 of catalyst.
The initial concentration of the substrates was taken on basis
of the concentration ratio of these herbicides in commercial
formulations, where the compounds are available as mixture.
An experimental design full factorial 32 was preliminary
applied to evaluate the effect of the catalyst amount added
(0.5, 1.0 and 1.5 g L−1) and solution pH (5, 7 and 9). Although
the pH and the catalyst amount were not dominant factors
affecting the degradation rate of the pesticide mixture, it was
found that the highest degradation of both compounds was
observed at pH between 7.0 and 8.5 using 1 g L−1 of catalyst
amount. The photocatalytic degradation of 2,4-D and picloram
over the N-doped ZnO materials is shown in Fig. 4. As can be
seen in the figure, the photocatalytic activity of 30% N-doped
ZnO powder was slightly higher than that of the pure ZnO. The
30% N-doped ZnO reached 100% degradation for 2,4-D and
picloram in 210 and 240 min, respectively. The results showed
that the efficiency of the photocatalytic degradation is influ-
enced by the molecular structure of the pollutant. The herbi-
cide 2,4-D was more susceptible to hydroxyl radical attack.
Conversely, the picloram oxidation reaction was slower due to
greater stability of the picloram molecule conferred by the co-
valently bound chlorine atoms.
Fig. 4 Photocatalytic degradation of (a) 2,4-D (20 mg L−1) and (b)
picloram (5 mg L−1) mixture at pH 7 and 1 g L−1 catalyst mass under
visible radiation with N-doped ZnO catalysts with different N loadings.
ZnO. Consequently, higher mineralization was obtained in
the presence of 30% N-doped ZnO after 270 min of reaction:
57% mineralization of the herbicide mixture vs. 0.2% TOC
abatement with the ZnO material (Table 2). The high degra-
dation and mineralization efficiency under visible light radi-
ation (λ > 400 nm) of the N-doped ZnO can be explained by the
presence of nitrogen species. Additionally, the N-ZnO material
showed smaller crystallite size and larger surface area com-
pared with pure ZnO, which could improve the photoactivity.
The suppression of electron–hole recombination and gen-
eration of more hydroxyl radicals in the N-doped samples
could play an important role in the enhanced rate of degra-
dation under visible radiation.12
The photocatalytic degradation rate of pesticide compounds
can be explained by a Langmuir–Hinshelwood (LH) pseudo-
first-order kinetics model1,3,21,25 with the following eqn (2):
lnðC0=CÞ ¼ kobs
t
ð2Þ
Table 2 Rate constant (kobs) and mineralization percentage values for
the photocatalytic degradation of 2,4-D and picloram mixture under
visible light irradiation using N-doped catalysts
where C0 is the equilibrium concentration of herbicides
(mg L−1) after 1 h in dark adsorption, C is the concentration of
the herbicides remaining in the solution at irradiation time
t (min), and kobs is the observed rate constant (min−1). The
summary of the pseudo-first-order kinetic values of the pre-
pared ZnO catalysts under visible-light irradiation is shown in
Table 2. The rate of degradation increases gradually with the
doped level of N. The maximum kobs value for 2,4-D and
picloram were found during the photocatalytic degradation
using 30% N-ZnO. As is clearly evident from this data the
N-ZnO was more active under visible-light compared with pure
2,4-D
Picloram
TOC
abatement
(%)
kobs × 10−3
kobs × 10−3
Catalyst
(min−1
)
R
(min−1
)
R
ZnO
15% N-ZnO
20% N-ZnO 12.4
25% N-ZnO 10.4
30% N-ZnO 15.0
10.7
9.8
0.9726 7.5
0.9953 7.3
0.9761 8.1
0.9831 5.8
0.9820 9.1
0.9735 0.2
0.9535 29
0.9544 27
0.9588 36
0.9798 57
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XPS measurements demonstrated the presence of nitrogen in
the N-doped ZnO material. The photocatalytic performance of
modified N-ZnO nanoparticles under visible light irradiation
was demonstrated by the oxidation of picloram and 2,4-D.
Among different amounts of dopant agent, the 30% N-ZnO
photocatalyst indicated the highest degradation efficiency due
to its lower band gap energy and higher surface area. The
•
identified by-products were transformed by OH radicals into
carboxylic acids such as oxalic, oxamic, acetic and formic acid.
Conclusively, ZnO nanoparticles doped with 30% nitrogen can
be an efficient photocatalyst for the removal of organic herbi-
cides under visible light irradiation due to its capability for
reducing electron–hole pair recombination.
Acknowledgements
The authors thank the financial support from PAICYT-UANL
CN847-11 and CONACYT projects (181057 and 177990). J. J.
Macías-Sánchez acknowledges the granted scholarship by
CONACYT-Mexico.
Fig. 5 Proposed fragmentation pathways of (a) 2,4-D and (b) picloram
in the presence of 30% N-doped ZnO catalyst under visible radiation.
Notes and references
The formation of organic intermediates during the photo-
catalytic treatment of 2,4-D and picloram using 30% N-doped
ZnO material was monitored by ESI-MS/MS. During this
determination each herbicide was treated individually. Based
on the information provided by the MS (the m/z values of the
ionic species) and MS/MS data (the fragmentation profiles of
the mass-selected ions), the proposed chemical structures of
reaction products are indicated in Fig. 5. The identified inter-
mediates during the degradation of 2,4-D were 2,4-dichloro-
phenol (1), which is subsequently degraded to form 2-chloro-
1,4-benzoquinone (2) and chlorohydroquinone (3). Likewise,
these compounds were also suggested during the degradation
by oxalate-mediated photooxidation29 and advanced electro-
chemical oxidation processes.30 On the other hand, ESI-MS/
MS analysis of the irradiated aqueous solution of picloram
showed the formation of two main products: 4-amino-2,3,5-tri-
chloropyridine (4) and 4-aminopyridine-2-carboxylic acid (5).
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13,21
degradation by TiO2
and in the electro-Fenton process.31
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