INDIUM-ZINC-OXIDE NANOCRYSTALLITES: PREPARATION, … · 2018-04-04 · 3 Indium-zinc-oxide nanocrystallites Article no. 608 photogenerated electrons react with dissolved oxygen molecules

INDIUM-ZINC-OXIDE NANOCRYSTALLITES: PREPARATION,

PROPERTIES AND VISIBLE-LIGHT-GENERATED

PHOTOCATALYTIC EFFICIENCY IN DEGRADATION

OF PSYCHOACTIVE DRUGS FROM WATER SYSTEMS

T. IVETIĆ1, N. FINČUR2, B. MILJEVIĆ3, LJ. ĐAČANIN FAR1, S. LUKIĆ-PETROVIĆ1,

B. ABRAMOVIĆ2

1University of Novi Sad, Faculty of Sciences, Department of Physics,

Trg Dositeja Obradovića 4, 21000 Novi Sad, Serbia

E-mails: [email protected]; [email protected]; [email protected] 2University of Novi Sad, Faculty of Science, Department of Chemistry,

Biochemistry and Environmental Protection,

Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia

E-mails: [email protected]; [email protected] 3University of Novi Sad, Faculty of Technology, Department of Materials Engineering,

Bulevar Cara Lazara 1, 21000 Novi Sad, Serbia

E-mail: [email protected]

Received September 27, 2017

Abstract. In this work, we studied the structure, band gap, particle size, and

morphology of the indium-zinc-oxide (IZO) nanocrystalline powders (NCPs) prepared

via solid-state method. Materials characterization was performed by using a variety of

techniques (XRD, Raman, SEM-EDS, and UV-Vis). The estimated values of the

optical band gaps of the obtained IZO NCPs indicated a possibility of their

photocatalytic activity under solar irradiation. According to this, the efficiency of the

obtained NCPs as photocatalysts in the degradation of alprazolam and amitriptyline,

the active components one of the most prescribed psychoactive drugs nowadays,

under simulated solar irradiation, was explored.

Key words: Oxide material, semiconductor, photocatalysis, psychoactive drugs.

1. INTRODUCTION

Heterogeneous photocatalysis (HPC) belongs to the well-established

wastewater treatment technology known as advanced oxidation processes (AOPs).

Besides, AOPs include photolysis, ozonization, Fenton’s oxidation, electro-

chemical oxidation, ultrasound irradiation and wet air oxidation [1, 2]. HPC

method degrades or otherwise transforms the organic pollutants in water to usually

less toxic chemicals or a completely harmless carbon dioxide and water, and

inorganic ions depending on the structure of molecules.

Romanian Journal of Physics 63, 608 (2018)

Article no. 608 T. Ivetić et al. 2

HPC requires the presence of a metal oxide semiconductor in contaminated

water and its illumination with either UV or visible light source. The effectiveness

of HPC depends on the physicochemical properties (mostly electronic structure,

size, surface area, morphology, and composition) of the used metal oxide material

made in the form of powder or thin film. Therefore, the preparation procedure

employed for the fabrication of these semiconductor materials is crucial for

obtaining the best photocatalytic performance. The latest industrial trends for HPC

are encouraging the preparation of ecologically affordable photocatalysts via

inexpensive routes such as it is mechanochemical processing, with desired

characteristics to induce the oxidation of the organic pollutants under visible light

irradiation [3, 4].

In the pursuit of the most effective and suitable aforementioned

semiconductor material that supports the photodegradation of the organic

pollutants, zinc oxide (ZnO) was found to be one of the most promising candidates

to replace the commonly used material for this purpose, titanium dioxide (TiO2)

known as "the gold standard" photocatalyst [5]. Major drawbacks in ZnO

photocatalytic performance are related to its UV light requirements for the band

gap excitation, fast recombination of the charge carriers that diminishes the

effectiveness of the degradation processes, and photo-corrosion vulnerability [5].

These barriers could be overcome by tailoring the ZnO electronic and surface-bulk

structure and related photogenerated charge carrier transfer pathways. In this

regard, the substitutional ZnO doping with various metal cations like Mn, Ga, Ag,

Cd, Mg, and/or ZnO coupling with other metal oxide semiconductors with different

band gaps to form heterojunctions like ZnO/TiO2, ZnO/SnO2, ZnO/CuO,

ZnO/Fe2O3, have been studied so far [5, 6]. Furthermore, it was already reported

that modification of ZnO with In2O3 greatly improves its stability, reduces its size

and enables better control of the charge carrier’s recombination for enhancement of

ZnO photocatalytic efficiency [6, 7]. Progress in making a better ZnO

photocatalytic performance with In2O3 could be achieved firstly by changing the

coordination environment of the Zn2+

ions in the ZnO lattice via defect engineering

and/or indium doping. The group-III impurities like In act as shallow donors when

substituting Zn site in ZnO [8]. Donor impurities introduce new electronic energy

levels within the band gap states and modify the electronic band structure by

shifting the band gap towards the visible region [3−5, 9] (illustrated in Fig. 1a).

In3+

-doping, then makes possible for ZnO to absorb more visible light photons,

excites the electrons (e−) from valence band (VB) to the conduction band (CB) and

consequently leaves holes (h+) in VB. Then, holes react with surrounding water

molecules (H2O) and produce strong oxidizing agents, hydroxyl radicals (•OH),

species mostly responsible for the transformation of the organic substances via

HPC into CO2 and H2O and other less harmful compounds. Simultaneously,

3 Indium-zinc-oxide nanocrystallites Article no. 608

photogenerated electrons react with dissolved oxygen molecules and create

superoxide radical anions (

2O ) that in the further chain reaction mechanism

produce even more •OH for degradation of the organic pollutants in water systems.

Fig. 1 – Photogenerated charge carrier pathways in a) In-ZnO and b) ZnO/In2O3 NCPs when present

in the aqueous solution of the organic pollutants (VB−valence band; CB−conduction band;

NHE−normal hydrogen electrode).

Another way for enhancing ZnO photocatalytic activity by In2O3

modification is ZnO and In2O3 coupling. This idea originates from the

consideration of ZnO and In2O3 VB upper edge and the CB lower edge redox

potentials, and the charge carrier’s movement ways across their interfaces when

these two oxide semiconductors are coupled together [6, 7]. As presented in

Fig. 1b, the CB bottom of ZnO (around –0.2 eV) lays bellow the CB bottom of

In2O3 (–0.6 eV) and VB top of ZnO (3.0 eV) lies below the corresponding band

edge of In2O3 (2.2 eV) with respect to normal hydrogen electrode scale (NHE).

This surface contact type ZnO/In2O3 coupling, on one hand, enables more efficient

visible light absorption through In2O3, as its band gap is 2.8 eV [6], and on the

other creates the conditions for achieving more efficient charge carrier separation

mechanism. Upon visible light illumination, electrons excited from VB to CB of

In2O3 are transferred via interfaces into the CB of ZnO [6], while formed holes stay

in VB. In this way, the electrons-hole pairs (e−−h

+) formed upon visible light


excitation are effectively separated which gives more time for their efficient impact

on the surrounding aqueous solution through the oxidation/reduction reactions and

improves the degradation rate of the organic pollutants under visible light

irradiation.

Alprazolam and amitriptyline are representatives of a large group of

psychoactive drugs that include narcoleptics, ataractics/tranquilizers, hypnotics,

sedatives, and antidepressants. Alprazolam is a new generation benzodiazepine that

possesses anxiolytic, sedative and hypnotic properties [10−13], while amitriptyline

is one of the most used tricyclic antidepressants for treating disorders like

depression, anxiety, and a variety of chronic pain syndromes [14, 15].

Pharmaceuticals used for treating mental disorders and diseases i.e. their active

components (ACs) are continuously introduced into the environment during their

manufacturing, disposal or after use by human and animal excretions, and are

increasingly becoming an ecological problem [16−19]. Namely, ACs are found in

drinking and environmental waters in many countries, and it was established that

the main water pollution with these ACs originates from the wastewater treatment

plants [16]. Apparently, ACs are not completely removed by conventional water

purification procedures and hence, supplementary methods like HPC are more than

needed. Furthermore, HPC using semiconductor nanoparticles has already shown

most effectiveness in removing pharmaceutical contaminants from wastewaters [1].

This work explores the possibility of using the simple, sustainable and eco-

friendly solid-state mechanochemical processing for the preparation of indium-

zinc-oxide nanocrystallites (NCs), and their application as photocatalyst in the

HPC of selected active components of psychoactive drugs under simulated solar

irradiation (SSI). The obtained NCs structure, morphology, and optical properties

were examined in detail using a variety of techniques for materials

characterization. The solid-state mechanochemical method for NCs preparation

used in this study is a good alternative to a solution-based method for

manufacturing the functionally tailored nanomaterials [10, 11, 14, 20, 21].

2. EXPERIMENTAL PROCEDURE

2.1. SAMPLE PREPARATION

Two different mechanochemical procedures were employed for obtaining

indium-zinc-oxide NCs. In the first procedure starting precursors, ZnO (Sigma-

Aldrich; purity 99.9% powder and particle size 1 m) and In2O3 (Alfa Aesar;

purity 99.99% and metal basis 325 mesh powder), were stoichiometrically mixed

to achieve about 5% (w/w) of indium doping in an agate mortar for 10 min, pressed

under 50 kg/cm2 load, annealed at two different temperatures (700 and 950 °C for


1 h) in air atmosphere, and ground again for 10 min (marked as In-ZnO-700 and

In-ZnO-950 samples, respectively). The second preparation procedure was

designed for preparing the contact-type ZnO/In2O3 NCs mixture and it was based

on the recent applied mechanochemical route for obtaining ZnO/SnO2 NCs

photocatalyst mixture [14]. The starting precursors, in the molar ratio of

ZnO:In2O3=2:1 (37 wt.% of ZnO and 63 wt.% of In2O3), were ground for 10 min

in an agate mortar, annealed at 700 °C for 2 h, and additionally ground for 10 min

(marked as ZnO/In2O3 sample).

2.2. MATERIALS CHARACTERIZATION

X-ray diffraction (XRD) was carried out using Philips PW 1050 instrument,

with Cu Kα1,2 radiation, and a step scan mode of 0.02°/s in angular range

2θ = 1090° which enabled good profile fitting using PDXL and HighScore Plus

(PANanalytical) software. Scanning electron microscope, SEM (JEOL JSM-

6460LV) equipped with an energy-dispersive spectrometer (EDS) was used to

investigate the morphology, microstructure and elemental concentration of the

obtained samples. The Raman spectra of the In-ZnO samples were measured using

the Centice MMS Raman spectrometer equipped with charge-coupled device

(CCD) as a detector. A diode laser operating at 785 nm (1.58 eV) was used as the

excitation source. The diffuse reflectance spectra (DRS) were obtained using the

Ocean Optics QE65000 High-sensitivity Fiber Optic Spectrometer, and in

accordance with it, the Kubelka-Munk function was estimated using Spectra Suite

Ocean Optics operating software. All measurements were carried out at room

temperature.

2.3. MEASUREMENTS OF PHOTOCATALYTIC ACTIVITY

The photocatalytic activities of In-ZnO and ZnO/In2O3 samples were

evaluated by degradation of the aqueous alprazolam solution (8-chloro-1-methyl-6-

phenyl-4H-[1,2,4]triazole[4,3,-α]-[1,4]-benzodiazepine, CAS No. 28981-97-7,

C17H13ClN4, Mr = 308.765, 98%, Sigma-Aldrich). Besides, the photocatalytic

activity of ZnO/In2O3 sample was evaluated by degradation of the aqueous

amitriptyline solution (3-(10,11-dihydro-5H-dibenzo[a,d][7]annulen-5-ylidene)-

N,N-dimethylpropan-1-amine hydrochloride, C20H24ClN, Mr = 313.9, CAS No.

549-18-8, 98%, Sigma-Aldrich). In both cases, the initial concentration of

investigated pharmaceutically active components (alprazolam/amitriptyline) was

0.03 mmol/L, while the catalyst loading was 1.0 mg/mL.


A cell made of Pyrex glass (total volume of ca. 40 mL) with a plain window

for the light beam focus, magnetic stirring bar, and the water-circulating jacket was

used in the photocatalytic degradation experiments. Uniform dispersion with the

photocatalyst NCs and adsorption equilibrium was made by sonication (50 Hz) of

the suspension, prior to the illumination, in dark for 30 min. From the beginning,

the suspension was thermostated at 250.5 °C in O2 stream (3.0 mL/min). The

obtained suspension was then illuminated by simulated solar irradiation using a 50

W halogen lamp (Philips). During irradiation, the stirring and streaming with O2

constant rate flow were continued. Kinetic studies of alprazolam/amitriptyline

photodegradation were monitored by ultra fast liquid chromatography with UV/Vis

diode array detector (UFLC−DAD) set at 222 nm (wavelength of alprazolam

maximum absorption) and at 206 nm (wavelength of amitriptyline maximum

absorption). The aliquots of 0.5 mL of each reaction mixtures were taken at the

beginning of the experiment and then at certain time intervals up to 60 min, filtered

through a Millipore (Millex-GV, 0.22 m) membrane filter to remove NCs, and

samples were injected and analyzed with UFLC-Shimadzu. Procedures for

UFLC−DAD analysis were described previously in the case of alprazolam [10],

and in the case of amitriptyline [14].

3. RESULTS AND DISCUSSION

3.1. STRUCTURE AND MORPHOLOGY

Figure 2 shows XRD patterns of the obtained In-ZnO samples. The main

diffraction peaks were indexed as hexagonal wurtzite ZnO (JCPDS #36-1451)

phase. Few diffraction peaks (marked with an asterisk in Fig. 2) were found to

belong to the cubic In2O3 (JCPDS #44-1087) phase.

The Williamson-Hall (W-H) analysis [22] was employed to the following

ZnO peaks of the XRD data: (100), (002), (101), (102), (110) and (103), in order to

estimate the crystallite size [23, 24] of the ZnO particles in In-ZnO samples

(Fig. 3).

The obtained values from W-H analysis for ZnO crystallite sizes showed a

decrease with the increase of the annealing temperature during preparation

procedure when In2O3 modification of ZnO was at a doping level, which was

confirmed by Rietveld refinement as well (Table 1).

Two factors are often considered to have an influence on ZnO doping,

electronegativity and ionic radius of the doping metal ions, which is for In3+

slightly higher (1.78, and 0.081 nm, respectively) than those of Zn2+

(1.65, and

0.074 nm) [25]. When doped ions have larger radii they tend to occupy the grain

boundaries of expansion lattices, form a diffusion barrier and suppress the growth


of the ZnO crystal. The difference in oxidation states of In3+

and Zn2+

ions further

enhances the lattice distortion. There is a possibility of In2O3 decomposition when

heated above 850 °C, and a creation of In2+

ion that is chemically even more

similar to the Zn2+

ion [26]. According to this, as the temperature rises there is a

chance for more indium ions (both In2+

and In3+

) to enter the ZnO lattice and

replace Zn2+

ions. ZnO lattice parameters change can be used to study the doping

effects (Table 1) (ZnO reference values from JCPDS #36-1451 are

a = b = 3.24982; c = 5.20661). For example, the c-axis lattice constant usually

decreases in the case of doped ions with smaller ionic radii (In-ZnO-950) as they

tend to occupy the grain boundaries of the compression lattices, and usually

increases for doped ions with larger ionic radii (In-ZnO-700) as they tend to

occupy grain boundaries of the expansion lattices [25].

Table 1

The results of the structural analysis for In-ZnO samples

Structural properties Rietveld refinement W-H analysis

In-ZnO-700 In-ZnO-950 In-ZnO-700 In-ZnO-950

Crystallite size, nm 66.5(6) 41.5(3) 96.29±16.05 66.34±18.09

Microstrain (%) 0.102(7) 0.128(13) 0.117±0.15 1.02±0.37

Unit cell parameter a = b = 3.2517(3)

c = 5.2090(5)

a = b = 3.2527(4)

c = 5.2046(7)

Weight % (ZnO) 96.9 97.2

Weight % (In2O3) 3.1 2.8

Fig. 2 – XRD patterns of a) In-ZnO-700 and b) In-ZnO-950 samples.


Fig. 3 – The W-H analysis (left) of a) In-ZnO-700, and b) In-ZnO-950 samples. Fit to the data,

the crystalline size (D) is extracted from the y-intercept of the fit as D = Kλ/(y-intercept); K = 0.9

and λ = 0.1540598 nm. SEM images (right) of a) In-ZnO-700 and b) In-ZnO-950 samples.

a) b)

Fig. 4 – The EDS analysis of a) In-ZnO-700, and b) In-ZnO-950 samples.


Table 2

EDS elemental composition for In-ZnO samples

Sample Element Weight %

In-ZnO-700

O 16.41

Zn 77.89

In 5.70

In-ZnO-950

O 20.37

Zn 76.04

In 3.59

The micrographs (Fig. 3, right) of the obtained In-ZnO samples show a

nonuniform distribution of particle size and shape typical for mechanochemical

powder processing. The particles annealed at 700 °C are bigger in size than

particles annealed at 950 °C, which follows the trend observed with the crystallite

sizes in XRD, probably because of stronger interaction between In2O3 and ZnO at

higher temperature (950 °C) that creates internal strain (Table 1) and significantly

reduces the ZnO particle size [6].

EDS measurements confirm the incorporation of In into the zinc oxide lattice

since the characteristic peaks corresponding to In peaks are identified as well

(Fig. 4). Table 2 shows the weight percentage of the elements present in In-ZnO

samples.

Deeper insight and confirmation of the observed effects, when In was

introduced to ZnO lattice, was achieved by Raman spectroscopy. As Raman

spectra of both In-ZnO NCs samples (Fig. 5) coincide with typical Raman

spectrum of ZnO [10, 14, 27], in terms of the shape and number of peaks, further

analysis (Table 3) was performed only for the peak position shifting and

broadening of the first-order ZnO Raman modes that indicate the change in lattice

constants (E1 (TO), A1 (TO)), band structure (E2high

), and those strongly affected by

defects (A1 (LO), E1 (LO)).

The shift of A1(TO) and E1(TO) band positions and theirs broadening

confirmed the change in lattice parameters of ZnO as they reflect the strength of

the polar lattice bonds [10]. The E2high

is a band characteristic of ZnO wurtzite

phase and its broadening indicates the band structure change. The two symmetry

types of LO modes were hard to resolve probably because powders have tilted

orientation [10] and interact and create one single mode i.e. quasi-LO mode.

According to the found LO mode positions, we assumed it has mostly E1 symmetry

and it was assigned to E1(LO) mode. Its position (in cm−1

) remained almost

unaffected by the change in the annealing temperature.


However, a decrease in FWHM of E1(LO) indicates the decrease in intrinsic

defects including oxygen vacancies and interstitials [10] with the increase of the

annealing temperature. Raman analysis goes in favor with the XRD result where

the lattice parameters change was found and assumed to be caused by Zn2+

ion

substitution with In3+

/In2+

in ZnO.

Fig. 5 – Raman spectra of a) In-ZnO-700, and b) In-ZnO-950 NCs

(black dots-experimental points, straight red line-fitted),

further analyzed Raman modes are noted.

Table 3

Frequency (in cm−1) of selected peaks from the simultaneous fitting of Raman spectra

(numbers in brackets denote the FWHM) and proposed mode symmetry assignment

based on the reported data for ZnO [14, 27]

Mode symmetry

assignment In-ZnO-700 In-ZnO-950

A1(TO) 372.5 (18.4) 382.1 (27.9)

E1(TO) 418.6 (39.6) 426.2 (49.1)

E2high 430.7 (10.8) 441.2 (11.1)

E1(LO) 578.5 (49.5) 578.6 (42.8)

XRD pattern of ZnO/In2O3 sample (Fig. 6, left) confirms, as expected,

a much more defined mixture of ZnO and In2O3 phases.


Fig. 6 – XRD pattern (left) and SEM images with different magnifications (right)

of the ZnO/In2O3 NCs mixture.

The results of the structure and quantitative phase analysis via Rietveld

refinement are summarized in Table 4 (lattice parameters, atomic position, and

phase weight fractions). Table 4 also contains results of the W-H analysis for the

crystallite size and microstrain of both ZnO and In2O3 phases in the catalyst

mixture and confirms their nano-crystallinity. SEM images (Fig. 6, right) of

ZnO/In2O3 NCs mixture show typical powder-like agglomeration and non-evenly

distribution of particle size and shape.

Table 4

The results of the structural analysis of the ZnO/In2O3 NCs mixture

Phase ZnO In2O3

Unit cell parameters a = b = 3.2509(1)

c = 5.2086(3) a = b = c = 10.1226(3)

Weight % 45.6% 54.4%

Crystallite size (nm) 93±9 nm 75±7 nm

Microstrain (%) 0.101±0.09 0.074±0.013

3.2. OPTICAL BAND GAP MEASUREMENTS

Diffuse reflectance spectra of In-ZnO NCs (Fig. 7) show redshift as the

temperature of annealing is increased due to intensified introduction of In ions into

ZnO lattice. DRS of all obtained NCs (exemplary spectra are shown in Fig. 7) were


used to estimated the optical band gap energies (Eg) by plotting (F·h)2 vs. photon

energy (inset in Fig. 7), where F is the Kubelka-Munk function defined as

(1R)2/2R and R is measured diffuse reflectance. Optical band gap energies

obtained by extrapolation of F(R) = 0 were 3.2(3) eV and 3.0(7) eV for In-ZnO-

700 and In-ZnO-950, respectively (which is in accordance with the broadening of

the E2high

Raman mode). The decrease in the optical band gap, therefore, originates

from the formation of the shallow levels inside the band gap due to impurity In

atoms intensified entering at higher temperatures.

Likewise, optical band gap energy for the ZnO/In2O3 NCs mixture was

estimated to be 3.1(6) eV. Values of the optical absorption thresholds (380 nm, 404

nm, and 392 nm) then calculated from λg = 1240/Eg, indicated a possibility of

photocatalytic activity of the obtained indium-zinc-oxide NCs under solar

irradiation.

Fig. 7 – Reflectance spectra of In-ZnO NCs with inserted plots of [F(R)·h]2 vs. photon energy.

3.3. PHOTOCATALYTIC EFFICIENCY

The influence of variations in the obtained indium-zinc-oxide NCs structure

and morphology (achieved by changing the annealing temperature during

preparation and concentration of the starting precursors) on its photocatalytic

properties were investigated by measuring the efficiency of alprazolam and

amitriptyline photodegradation under simulated solar irradiation (Figs. 8 and 9). As

can be seen in Fig. 8, the In-ZnO-950 NCs catalyst showed no photocatalytic

activity in degradation of alprazolam since the rate of direct photolysis was almost

the same as in its presence, while the In-ZnO-700 NCs catalyst showed better

photocatalytic activity under SSI. It proved to be even slightly better compared to


the most frequently used oxide semiconductor for photodegradation nowadays,

TiO2 Degussa P25 [30], (Fig. 8). Namely, after 60 min of irradiation in presence of

In-ZnO-700, 41.2% of alprazolam was removed, while in presence of TiO2

Degussa P25, 35.3% of alprazolam was degraded. The reasons for decreased In-

ZnO NCs photocatalytic efficiency when prepared at higher annealing temperature

than 700 °C, seems to be a result of several factors. Raman analysis confirms that

the number of intrinsic defects, such as oxygen vacancies, decreases with

temperature increase, as like its contribution to the band tailing. Therefore, there

must be an optimum number of intrinsic defects such as oxygen vacancies that

could be obtained by preparation procedure, for a good photocatalytic performance.

In the case of In-ZnO-950, this number is obviously reduced probably mainly due

to higher annealing temperature employed. It is a known fact that the electron-hole

pair separation that contributes to higher photocatalytic performance, is dependent

upon defects, like oxygen vacancies that act as charge traps and prevent their

recombination [28, 29]. Additionally, possibly formed In2+

in In-ZnO-950 is

relatively unstable compared to In3+

and tends to transfer the trapped charge

carriers to the adsorbed O2 in order to regenerate In3+

and thus acts as

recombination center which also decreases the photocatalytic activity [29].

Evidently, the desired improvements in physical properties, achieved by changing

the mechanochemical condition of In-ZnO NCs preparation (e.g. the shift of the

optical band gap of In-ZnO-950 towards the visible region (Fig. 7), and reduction

in particle size) are overshadowed by effects unfavorable for photocatalytic

performance, such as loss of intrinsic defects (oxygen vacancies), as well as the

formation of In2+

ions that act as recombination centers.

Fig. 8 – The efficiency of direct photolysis and photocatalytic degradation of alprazolam

(co = 0.03 mmol/L) in the presence of In-ZnO NCs, ZnO/In2O3, and TiO2 Degussa P25 (1.0 mg/mL),

using SSI. The inset represents the structural formula of alprazolam.


The significantly better result was obtained when ZnO/In2O3 NCs mixture

was used as a photocatalyst in photocatalytic degradation of alprazolam. Namely,

by using ZnO/In2O3, 53.7% of alprazolam was removed after 60 min of irradiation.

Bearing in mind that ZnO/In2O3 showed the highest efficiency in degradation of

alprazolam and in order to investigate the influence of substrate type on the process

of photocatalytic degradation, efficiency of mentioned photocatalyst was

investigated in degradation of amitriptyline under simulated solar irradiation.

As can be seen in Fig. 9, after 60 min of irradiation, 72.7% of amitriptyline

was removed, while 43.6% of amitriptyline removal was achieved in the presence

of TiO2 Degussa P25 under the same experimental conditions. Obtained results

indicate the significance of investigated influence of substate type, since ZnO/In2O3

showed better efficiency in degradation of amitriptyline. Compared to the

efficiency of TiO2 Degussa P25, results indicate almost 30% increase in the

efficiency of ZnO/In2O3 NCs in case of amitriptyline, and almost 20% in case of

alprazolam, which seems significant. TiO2 Degussa P25 itself is a mixture of two

allotropes of titanium dioxide (TiO2), anatase and rutile phases. Degussa P25

contains anatase and rutile in 3:1 ratio with average crystallite sizes of about

20 nm, according to the producerʼs specification, with specific surface area of

53.2 m2/g and total pore volume 0.134 cm

3/g [31]. Possibly, the coupling of ZnO

with In2O3 allows both the conditions needed for better visible light absorption

through In2O3 band gap excitation and effective charge carriers separation

mechanism that improve photocatalytic performance of the investigated

psychoactive drugs.

Fig. 9 – The efficiency of direct photolysis and photocatalytic degradation of amitriptyline

(co = 0.03 mmol/L) in the presence of ZnO/In2O3 NCs mixture and TiO2 Degussa P25 (1.0 mg/mL),

using SSI. The inset represents the structural formula of amitriptyline.


4. CONCLUSIONS

To summarize, this paper reports the structure and optical properties of

mechanochemically prepared indium-zinc-oxide NCs. XRD, Raman, SEM-EDS,

and UV-Vis spectroscopy were employed to characterize the obtained NCs. The

potential application of the obtained NCs in photocatalytic processes was examined

by determining the photocatalytic degradation kinetics of alprazolam and

amitriptyline in the water solutions, for the first time to the best of our knowledge

by using the obtained indium-zinc-oxide NCs and under simulated solar irradiation.

The results show, in the case of In-ZnO samples, that the annealing temperature

should be not higher than 700 °C. Even though the particle sizes are significantly

reduced at the higher temperature and optical band gap set for better visible light

absorption, it badly affects the photocatalytic activity as unstable In2+

that

accelerates the recombination rate and diminishes the photocatalytic efficiency is

formed. The higher photocatalytic efficiency was obtained in the case of forming

the contact-type structure between ZnO and In2O3 (ZnO/In2O3 NCs mixture),

which seems to be a result of a synergetic effect of both visible light activation

through In2O3 absorption and charge separation mechanism.

Acknowledgments. The authors are grateful to the APV Provincial Secretariat for Higher

Education and Scientific Research for partly financing this work, and acknowledge the support of the

Ministry of Education, Science and Technological Development of the Republic of Serbia (Project

numbers: ON 172042, ON 171022 and III 45020).

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