Design of ordered Ag/AgBr nanostructures coupled activated carbon with enhanced charge carriers separation efficiency for photodegradation of tetracycline under visible light

Article abstract of DOI:10.1016/j.molliq.2019.112032

Oriented design of ordered Ag/AgBr photocatalyst with high separation of photoinduced charge carriers is crucial for high catalytic performance. In this work, ordered Ag/AgBr nanostructures were synthesized by a facile thermal polyol method in the presence of char-microwave assisted activated carbon (AABR-ACK). The influence of four synthesis key variables comprising of the mass of surfactant, the temperature, the mass of catalyst support and time, and their simultaneous interactions on the photodegradation of tetracycline (TC) were investigated using response surface methodology (RSM). The time and temperature had significant influence on the TC photocatalytic removal under visible light, as observed from the RSM model. The analysis of XRD, FTIR, SEM, optical and electrochemical properties for AABR-ACK composites evidenced the formation of ordered nanostructures with enhanced charge carriers separation efficiency. It was found that 92% of TC was decomposed after 180 min by optimized AABR-ACK 11. Furthermore, the inorganic phosphate anion retards the degradation performance of AABR-ACK 11 on TC removal. The AABR-ACK 11 composite also exhibits higher activity and reusability for degradation of TC without any major reduction in the performance. This work provides new insight into engineering ordered Ag/AgBr nanostructures through optimization of synthesis variables, with potential application for degradation of antibiotic pollutants.

Full text of DOI:10.1016/j.molliq.2019.112032

Journal Pre-proof
Design of ordered Ag/AgBr nanostructures coupled activated carbon with enhanced
charge carriers separation efficiency for photodegradation of tetracycline under visible
light
S.O. Sanni, E.L. Viljoen, A.E. Ofomaja
PII:
S0167-7322(19)34009-7
DOI:
https://doi.org/10.1016/j.molliq.2019.112032
MOLLIQ 112032
Reference:
To appear in: Journal of Molecular Liquids
Received Date: 18 July 2019
Revised Date: 17 October 2019
Accepted Date: 28 October 2019
Please cite this article as: S.O. Sanni, E.L. Viljoen, A.E. Ofomaja, Design of ordered Ag/AgBr
nanostructures coupled activated carbon with enhanced charge carriers separation efficiency for
photodegradation of tetracycline under visible light, Journal of Molecular Liquids (2019), doi: https://
doi.org/10.1016/j.molliq.2019.112032.
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Ordered Ag/AgBr nanospheres coupled activated carbon with high charge carriers separation
for enhanced photocatalytic performance.

Design of Ordered Ag/AgBr Nanostructures Coupled Activated Carbon with Enhanced
Charge Carriers Separation Efficiency for Photodegradation of Tetracycline under Visible
Light
Sanni S.O., Viljoen, E.L, Ofomaja A.E*
Biosorption and Wastewater Treatment Research Laboratory, Department of Chemistry, Faculty
of Applied and Computer Sciences, Vaal University of Technology, Private. Bag X021,
Vanderbijlpark-1900, South Africa.
*Corresponding author’s address: E-mail aus_ofomaja@yahoo.com; augustineo@vut.ac.za
Phone: +27 745408161
ABSTRACT
Oriented design of ordered Ag/AgBr photocatalyst with high separation of photoinduced charge
carriers is crucial for high catalytic performance. In this work, ordered Ag/AgBr nanostructures
were synthesized by a facile thermal polyol method in the presence of char-microwave assisted
activated carbon (AABR-ACK). The influence of four synthesis key variables comprising of the
mass of surfactant, the temperature, the mass of catalyst support and time, and their simultaneous
interactions on the photodegradation of tetracycline (TC) were investigated using response
surface methodology (RSM). The time and temperature had significant influence on the TC
photocatalytic removal under visible light, as observed from the RSM model. The analysis of
XRD, FTIR, SEM, optical and electrochemical properties for AABR-ACK composites
evidenced the formation of ordered nanostructures with enhanced charge carriers separation
efficiency. It was found that 92% of TC was decomposed after 180 min by optimized AABR-
ACK 11. Furthermore, the inorganic phosphate anion retards the degradation performance of
AABR-ACK 11 on TC removal. The AABR-ACK 11 composite also exhibits higher activity and
reusability for degradation of TC without any major reduction in the performance. This work
1

provides new insight into engineering ordered Ag/AgBr nanostructures through optimization of
synthesis variables, with potential application for degradation of antibiotic pollutants.
Keywords: Ordered Ag/AgBr nanostructures, Thermal polyol, Response surface methodology,
Tetracycline, Visible light
2

1. INTRODUCTION
Pharmaceutical antibiotics are widely used in the environment to protect human health and
promotes livestock growth, whilst partial metabolism by the human or animal body results in a
large fraction of these antibiotics being discharged into the environment by excretion from urine
and faeces, thus creating water pollution [1, 2]. Tetracycline (TC), a commonly used antibiotic
with bio-refractory properties, poses a greater threat to the environment [3, 4]. As such, effective
removal of TC is crucial to protect the environment and human health. The development of
visible light driven (VLD) photocatalyst materials with strong catalytic activity in the full solar
spectrum is gaining interest for their application in photocatalytic degradation of TC antibiotics
[5, 6]. These VLD photocatalysts are viewed as attractive alternatives to the conventional
photocatalyst materials which operate only in the ultraviolet region [7]. The unique surface
plasmon resonance (SPR) effect of noble-metal nanoparticles (e.g. Ag, Au, Pt) has shown
potential attributes as a VLD photocatalyst material for environmental remediation [8-10]. A
typical example is Ag anchored on the surface of AgX which combines to form a narrow
bandgap photocatalyst resulting in a highly efficient visible light response plasmonic
photocatalysts (Ag/AgX, X= Cl, Br and I) [11, 12]. Ag/AgBr catalyst are known to be more
efficient than Ag/AgCl due to the lower electron affinity of Bro atom and the small band gap as
compared with Clo atoms [13, 14].
Nevertheless, key challenges of Ag/AgBr for the practical application includes high
recombination of photo-induced electron-hole pairs and irregular nanostructures, resulting in low
photocatalytic activities [15]. The coupling of the Ag/AgBr catalyst with carbonaceous support
such as graphene oxide, carbon nanotube, and activated carbon is considered an effective
approach to overcoming these problems [16-19]. Activated carbon [20], among these
carbonaceous materials, has attracted focus due to its abundance, non-toxicity, low cost, rich
3

oxygenated functional groups and electron conductivity [21]. The 3D interconnected carbon
structure with high porosity, and the high surface area of AC from renewable source [22, 23]
makes it an appropriate candidate as a catalyst support. Previous work has shown that the
biomass AC support form ordered nanostructure composites with a photocatalyst, which also
facilitates the separation of charge carriers and prolongs the lifetime of electron-hole pairs [21,
24-26] with high catalytic activities. However, the conventional photo-reduction process for
dispersing plasmonic Ag/AgX catalyst on carbonaceous materials results in irregular
morphologies, agglomeration, and aggregation of actives sites that affect degradation activities
[27-29].
The synthesis of stable and efficient Ag/AgBr plasmonic photocatalysts with ordered
nanostructures have been achieved through a host of synthetic routes [30-34]. Recently, a
thermal polyol route has shown exceptional design of isotropically or anisotropically
nanostructures of Ag/AgBr [34, 35], that are often difficult to obtain through the traditional
photo-reduction process [13]. The polyol solvent used in this synthetic approach can also
influence the nanostructures control, whilst behaving as a reducing agent to achieve metallic Ag
particles and still being coherent with other heating approaches [32, 36, 37]. Of emphasis is that
the preparation main variables such as synthesis time [36, 38], reaction temperature [34, 38],
structural directing agent [34, 36], and catalyst support can also engineer ordered nanostructures
with fast interfacial charge separation and enhance the photocatalytic activities of Ag/AgX
catalyst. Thus, the optimization of synthesis parameters is desirable to obtain high photocatalytic
activities for the Ag/AgBr catalyst materials on organic pollutant removal.
The conventional optimization method was previously employed for preparation factors
influencing Ag/AgBr catalytic performance, demand time and cost especially with large amounts
4

of variables [34]. Response surface methodology (RSM) is an appropriate statistical technique
for an experimental design with a large number of variables, evaluation of simultaneous
interaction of preparation conditions and optimization of the results to yield the desirable
response [39]. However, there have been no reports in the literature on the application of RSM
that correlates the relationship between synthesis variables and photocatalytic performance using
regulated Ag/AgBr coupled biomass activated carbon (AABR-ACK) composites on TC
antibiotic removal.
In this study, AABR-ACK composites were prepared via a thermal polyol route, with the
purpose of designing ordered nanostructures and improving their charge carrier’s separation
efficiency. RSM experimental design was first applied to investigate and optimize the relevant
preparation factors of AABR-ACK composites associated with a removal efficiency of TC in an
aqueous solution under visible light irradiation. The integral properties of the prepared
composites comprising of the crystal phase, morphology structure, optical and electrochemical
characteristics were investigated in detail, based on observations from the RSM findings. The
most appropriate photocatalyst (AABR-ACK) exhibited enhanced photocatalytic activity, which
was attributed to the ordered nanospheres structure with enhanced separation of photo-induced
charge carriers. The ordered AABR-ACK photocatalyst will be a promising choice for antibiotic
removal in environmental fields.
2. Experimental section
2.1 Materials
AgNO3 was obtained from the Merck Chemical Company. Hexadecyltrimethylammonium
bromide (HTAB, 99%), polyvinylpyrrolidone (PVP, M.W. 58000; K29-32), and ethylene glycol
were purchased from Acros. Ethanol, potassium hydroxide (KOH, 99%), hydrochloric acid (HCl,
5

99%), sodium hydroxide isopropyl alcohol [40], Ethylenediaminetetraacetic acid disodium salt
(EDTA 2Na),
benzoquinone (BQ) and tetracycline hydrochloride (99%) were purchased from Sigma Aldrich.
All the reagents were used as received and water used in all these experiments was purified with
a Millipore system.
2.2.1 Activated Carbon Synthesis
Activated carbon from impregnated pine cone biochar with potassium hydroxide (ACK) was
prepared from pine cone biomass via a two-step heating approach [41]. The pine cone was
placed inside aluminum foil, carbonized at 600 °C for 2 hr in a tubular furnace (Carbolite Gero)
under N2 flow (50 mL/min) at a heating rate of 10 °C/min was to obtain the biochar (BCR). The
BCR obtained was cooled, washed repeatedly with distilled water and dried overnight at 80∘C.
The dried BCR was sieved to average particles less than 180 ꢀm, while 5 g of sieved BCR was
mixed with 25 mL solution of 2M KOH. This mixture was stirred for 24 hr, and dried in an oven
o
at 80 C for 12 hr to remove any remaining moisture present in the material. The dried sample
was placed in Duran bottle under nitrogen atmosphere flow (50 mL/min), then pyrolyzed in the
microwave reactor (model LG MH8042GM, maximum power of 1000 W at a frequency of 2450
MHz) at a constant power of 400 W for 16 minutes. The sample produced was cooled to room
temperature, washed with 0.1 M HCl (99%, Sigma Aldrich) and hot distilled water until the pH
was between 6-7. The washed activated carbon (ACK) was then dried at 105 °C overnight and
then stored in an airtight container.
2.2.2 Preparation of AABR-ACK
The synthesis of Ag/AgBr on ACK (AABR-ACK) was carried out through a modified thermal
polyol route [34]. Typically, 18 mL of ethylene glycol was poured into a round-bottom flask
6

which was heated at 60 ̊C for 30 min. Different weights of PVP (0.12, 0.26 and 0.4 g) and 480
mg of HTAB were added to the solution. Different masses of ACK (0.01, 0.03 and 0.06 g) were
then added to the mixture. After proper mixing to completely dissolve the PVP and HTAB, 3 mL
ethylene glycol solution containing silver nitrate was slowly added drop-wise to the above
mixture. The solution was maintained at 60 °C for an additional 30 min and the mixture was then
heated at different temperatures (100, 140 and 180 °C) in line with an experimental design using
RSM. After attainment of the desired temperature, different reaction times of 10, 17.5 and 25
min was used for further generation of metallic Ag particles. The reaction vessel was cooled
down to room temperature in the air. The resulting catalysts were collected by centrifugation
(REMI bench top centrifuge-R-8D) and washed thoroughly with ethanol and dried in an oven for
12 hr. In addition, other comparative samples (AABR and AgBr catalyst) were obtained under
similar preparation routes without ACK and light illumination.
2.3 Characterizations
The morphology and composition of the samples were recorded by scanning electron
microscopy (Zeiss Leo 1430 VP) and energy dispersive X-ray spectrometer (EDS, INCA). The
crystal structures and phase data for the AABR-ACK photocatalysts were determined by
Shimadzu X-ray 700 (XRD) with Cu Kαradiation in the 2θ range of 20–80 at 40 Kv and 40 MA.
X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Thermo
Scientific K-Alpha spectrometer with an Al Kα X-ray source. The functional group of the
samples was investigated by a Fourier transform infrared spectrometer (Perkin Elmer spectrum
400) within the range of 600–4000 cm−1. The UV–visible spectrum of the samples was recorded
using Ocean Optics high-resolution spectrometer (Maya 2000) equipped with an integrating
sphere accessory, using BaSO4 as a reference within the wavelength range of 350 to 800 nm. The
7

photoluminescence [1] emission spectra of samples were detected with a Fluorescence
spectrophotometer (FP-8600 Spectrofluorometer, Jasco) using a Xenon lamp as the excitation
source at room temperature, and the excitation wavelength was 365 nm. Electrochemical
impedance spectroscopy (EIS) was carried out in a three-way electrode system using a Biologic
SP 240 potentiostat workstation. The working, reference and counter electrode were glassy
carbon electrode, Ag/AgCl (in saturated KCl) and platinum wire respectively. The glassy carbon
electrode (GCE) was thoroughly cleaned with 0.3 and 1-micron alumina powder to eliminate
impurities, while the catalyst material (5 mg) was dispersed in a 0.5 ml (Dimethylformamide
solution) solution using an ultrasonication machine to obtain a homogenous solution, and 20 µL
of it was drop cast on the clean GCE. Linear sweep voltammetry was swept linearly from 0.5 to
2.0 V vs SCE at a scan rate of 50 mV s−1 using 0.2 M sodium sulphate. The EIS experiments
were carried out between frequency ranges of 100 KHz to 10 mHz with a perturbation amplitude
of 5 mV in 5 mM ferrocyanide containing 0.1 M KCl solution.
2.4 Photocatalytic Activity Evaluation
The photocatalytic activity of the prepared samples was evaluated by the degradation of the
Tetracycline (TC) antibiotic in the aqueous solution under the irradiation of 36 W white LED
light strips. The initial TC concentration was 15 mg/L and the photocatalyst loading was 0.3g/L
in 150 mL of TC solution. Prior to irradiation, the mixed solution was ultrasonicated then mildly
stirred in the fabricated reactor for 60 min in the dark to reach the adsorption/desorption
equilibrium of TC on the photocatalyst surface. The adsorbed solution was further subjected to
degradation by switching on the LED light under stirring for 3 hr. The sample solution (3 mL)
was withdrawn at given time intervals (30 min) and centrifuged to remove suspended particles.
8

The concentration of TC was determined by measuring the absorbance of the solution in a UV-
visible spectrophotometer at 376 nm. To better understand the advantages of the optimized
AABR-ACK from the RSM design, reference Ag/AgBr (AABR), AgBr and titanium dioxide on
dealuminated Clinoptilolite (TiO2/HCP) photocatalysts from previous work [40] were used to
compare activity on the TC. Different radical scavengers were added to the TC solution, to
determine which radical species were mostly responsible for the enhanced activity of active
photocatalyst. Benzoquinone (BQ), isopropanol and disodium ethylenediaminetetraacetic acid
(EDTA-Na2) at one mmole were dispersed in TC solution added photocatalyst to scavenge
−
superoxide radicals (•O2 ), hydroxyl radicals (•OH) and holes (h+), respectively, followed by the
photocatalytic activity test. Total organic carbon (TOC) was analyzed by Teledyne Tekmar
analyzer.
2.5 Response surface methodology
The RSM design known as central composite design (CCD) was used in this study, to estimate
the influence and interaction of the synthetic variables on the response (degradation efficiency of
TC). The synthetic variables selected as main parameters include a mass of PVP, reaction
temperature, the mass of carbon and time. The variables are defined as a districted numeric
parameter and studied in three levels as indicated in Table 1, while the variables chosen were
based on literature published in the past [34, 38]
The experiment comprises of 30 runs as presented by design-expert software (version 7) in
Table 2. The response was utilized to develop an empirical model which correlated the response
to the synthetic process variables using a second-degree polynomial equation as given by Eq. 1
4
4
4
4
Y = b + b x + b x2 +
b x x
ij i j
(1)
∑ i ∑
∑∑
0
i
ii
i
i=1
i=1
i=1 j=i1
9

where Y is the predicted response for degradation percentage of TC, bo is constant coefficient, bi
the linear coefficient, bij is factor coefficient, bii is quadratic coefficient, while
and are the
xi
j
x i
variables which determine the response Y.
3.0 Results and Discussions
3.1 Model fitting on the degradation of TC
Based on the sequential model sum of squares, the quadratic model was selected as the highest
order polynomial where the additional term is significant and the model is not aliased. The
regression analysis of the CCD for the quadratic model with the significant terms on degradation
percentage (DP) for TC are shown in Eq. 2 below:
87.58 + 2.78B − 3.14D − 4.38BD −32.06B2 − 7.57D2
DP
(2)
=
The linear terms of B, D, the interaction terms of BD and the quadratic terms of B2 and D2
were the significant variables for the degradation efficiency for TC, hence insignificant effects
were ignored as shown in Eq. 2 and Table S1. Further details on model authentication from
analysis of variance (ANOVA) is highlighted in supplementary information SI. The highest
coefficient values of time and temperature from the regression analysis in Eq. 3 and their high F
values from ANOVA (Table S1), shows their major contributions toward TC degradation. The
time determines the size growth of AABR dispersed on the ACK [42], which also ensures strong
contact between the components and further reducing migration distance of charge carriers.
Meanwhile, the formation of unique morphologies with an intimate contact structure between
AABR and ACK will be governed by the reaction temperature [34, 36], which simultaneously
enhances separation of electron-hole pairs and promotes radicals formation. Overall, both time
10

and temperature are advantageous towards enhancing the degradation of TC from the RSM
design.
3.1.1 Variables interaction influence on TC degradation efficiency
Three-dimensional (3D) response surface was used to investigate the influence of the synthesis
variables and their interactions on the degradation efficiency of TC removal. The significant
interactions of synthesis variables from Eq. 3 on the response are presented in Fig. 1b.
Fig. 1b shows the influence of temperature and time on degradation efficiency of TC at a
fixed mass of carbon and PVP (around 0.03 g and 0.26 g). Increasing both variables
simultaneously from 100 - 180 °C and 10 – 25 min, the degradation percentage increases from
49.44 to 92.08% and 81.98 to 92.08% respectively, whilst further increase for both variables
decreased to 57.26 and 73.73%. The highest degradation efficiency is achieved when the
temperature is maintained at 140 °C and the time extended for 17.50 min. From the 3D surface
plots findings, the time and temperature play a significant role in the photocatalytic performance
of AABR-ACK composites on TC removal. The importance of time and temperature as
preparation variables were not regarded fully from past literature until now. Therefore, in this
study, the focus was placed on the characterization of prepared AABR-ACK samples with time
and temperature variation to understand how these preparation variables influence the catalytic
activity on TC removal (Table 2)i.
3.2 Characterization
3.2.1 Phase, morphology and surface composition analysis
The XRD patterns of the AABR-ACK composites at different times (AABR-ACK 7, AABR-
ACK 11 and AABR-ACK 14) and temperatures (AABR-ACK 21, AABR-ACK 11 and AABR-
ACK 26) at the constant mass of PVP and carbon (0.26 and 0.03 g) are shown in Fig. 2a-b. The
11

XRD pattern of the prepared composites is comprised of pure face center cubic phase of AgBr
with space group: Fm-3m (225) [43]. The peaks at 2θ values of 26.6, 31.1, 44.26, 54.8, 64.8, and
73.4° are in indexed with planes of (111), (200), (220), (222), (400) and (420) cubic structure of
AgBr (JPCDS 079-0148) respectively. The diffraction peaks of metallic cubic Ag at 38.1, 44.2,
64.4 and 77.8o are ascribed to (111), (200), (220) and (311) planes (JCPDS 071-3762, space
group: Fm-3m (225)). In this study, the diffraction peak of Ag at 38.1o is distinct, highlighting
the abundance of metallic Ag in the composites as compared to other reports [15, 44]. Metallic
Ag peaks at 111, 200, 220 and 311 widen with an increase in time and temperature, which is
similar to other reports [45]. A further increase in time to 25 min (AABR-ACK-14), results in a
negative shift of XRD peaks for AgBr and Ag by 0.2o, which is ascribed to the close interaction
between AgBr and metallic Ag [46]. The diffraction peaks of AgBr and metallic Ag were
enhanced with AABR-ACK 11 compared to other composites, highlighting the formation of
ordered nanostructures along with reduced particle diameter, which is further evidenced by the
morphology discussion in Fig 3a-f.
The FTIR spectra for AABR-ACK composites at different times (10, 17.5 and 25 min) and
temperatures (100, 140 and 180 ºC) are shown in Fig. 2c-d. For ACK, five absorption bands are
located at 3340, 1733, 1648, 1341 and 1135 cm−1 that are ascribed to -OH stretching, -C=O from
carbonyl stretching, -COO from carboxyl stretching, the –C-OH and –C-O-C stretching
vibrations [47]. The –C=O carbonyl stretching, -C-OH and –C-O-C stretching band for ACK
shifts to lower wavenumber in the AABR-ACK composites, which confirms the interaction
between the AABR particles and ACK. The ACK, as the capping molecules with abundant
oxygenated functional groups such as -COO, -C-OH and –C-O-C partake in AABR defined
nanostructures. This leads to a decrease in agglomeration, decrease in size distribution and
12

decrease the rate of AABR particle growth. The bands at 2918 and 2839 cm−1 that are ascribed to
the stretching vibration of C-H bonds in -CH3 and -CH2 groups from the PVP molecules [48] are
pronounced for AABR-ACK 7, 11, and 14 composites. The band at 1450 cm−1 corresponds with
anti-symmetric deformation vibration of the -CH3 group of the PVP molecules [49] are also
present for AABR-ACK composites at different times (Fig. 2c). The presence of PVP molecules
causes a steric effect [50] between the Ag+ and Br-, as a result, rapid growth and agglomeration
of AgBr crystal particles are prohibited with the formation of ordered nanostructures (as
presented in SEM analysis in Fig.3a-c). However, these mentioned bands (-CH3 and -CH2
groups for the PVP molecules) are weakened for AABR-ACK 21 and AABR-ACK 26
composites (Fig. 2d), due to irregular morphology, agglomeration (evidenced from SEM image
in Fig. 3d and f) and also PVP polymerizes at a higher temperature [51].
The SEM morphology of the as-prepared AABR-ACK composites prepared at different times
(AABR-ACK 7, AABR-ACK 11 and AABR-ACK 14) and temperatures (AABR-ACK 21,
AABR-ACK 11 and AABR-ACK 26) are shown in Fig. 3a-f. The SEM images of prepared
AABR-ACK composites at different times (10-25 min) are all well-distributed nanospheres
morphology (Fig. 3a-c) with a particle diameter range between 162-201 nm. The morphology of
the AABR-ACK 7 are large nanospheres with a particle diameter of 201 nm. The AABR-ACK
composites nanostructures remains unchanged with the variation of time (from 17.5 to 25 min)
as observed with the previous report [42], but the particle diameter for AABR-ACK 11 and 14
are 162 and 198 nm. The AABR-ACK 26 (100 °C) is comprised of aggregated morphologies of
cubes, spheres and triangles (Fig. 3d) with a particle diameter of 262 nm. When the temperature
was increased to 140 °C (AABR-ACK 11), the formation of ordered AABR nanospheres
anchored on the ACK surface (Fig. 3e) with a particle diameter of 162 nm. Furthermore, with an
13

increase in temperature to 180 °C (AABR-ACK 21), the spheres agglomerate together into the
formation of octahedral morphology (Fig. 3f) with a particle diameter of 586 nm. The uniform
nanospheres morphology for AABR-ACK 11 with reduced particle diameter will shorten the
migration of charge carriers and enhanced the degradation activities [42] as presented in Table 2.
The AABR-ACK unique ordered shape and reduced particle diameter is better in comparison to
Ag/AgBr dispersed on commercial activated carbon [19, 52], reduced graphene oxide [53] and
carbon nanotube [18] from previous work. The pure AABR particles (Fig. S1a) are an irregular
octahedral shape, aggregated with a particle diameter of 1.3 ꢀm. Herein, the ACK (Fig. S1b)
interconnected carbon network structure with abundant oxygenated functional groups will aid in
the favorable immobilization of ordered AABR nanostructure for AABR-ACK composites.
The formation process of AABR-ACK composite is presented in Scheme 1. When the AgNO3
solution is added dropwise to the suspension mixture of HTAB and PVP in the presence of ACK
at an initial temperature of 60 °C, AgBr nanoparticles are generated via an ion exchange/
precipitation reaction. Moreover, the partial reduction of Ag+ from formed AgBr to metallic Ag
occurs upon ramping up the temperature to the desired target under ambient light condition to
form the Ag/AgBr. The increased reaction temperature, PVP and ACK significantly strain the
fast diffusion rate of generated AgBr, thus resulting in the formation of ordered Ag/AgBr
nanoparticles on the ACK surface.
The TEM images of the ACK and AABR-ACK 11 composite are shown in Fig. 4a and b. Fig.
4a present the porous ACK material, while Fig. 4b shows the TEM image for AABR-ACK 11 is
spheres shaped nanoparticles, in which the AABR particles with a diameter of around 3-5 nm
uniformly distributed on the ACK surface. A close identification of the AABR-ACK 11
nanostructure as observed by HRTEM (Fig. 4c)a shows the lattice fringes with an interplanar
14

spacing of 0.238 and 0.285 nm, which are attributed to Ag (111) and AgBr (200) planes. Herein,
intimate contact between AABR and ACK was formed for AABR-ACK 11, which favors
accelerated electron migration from the former to the latter, which results in photo-prompted
electron-hole pairs separation and enhanced photocatalytic activity.
The surface chemical composition and valence states of AABR-ACK 11 composite were
analyzed by X-ray photoelectron spectroscopy (XPS) in Fig. 5. The survey scan spectrum (Fig.
5a) indicates the presence of Ag, Br, and C in the sample. However, the high-resolution XPS
spectra of Ag 3d (Fig. 5b) has two symmetrical peaks at binding energies of 366.87 eV and
372.80 eV for Ag 3d5/2 and Ag 3d3/2 respectively [54]. These two peaks are further divided into
two different peaks. The peaks at 366.87and 372.80 eV are attributed to Ag+, while the peaks at
366.78 and 372.80 eV could be ascribed to metallic Ago [18, 55, 56]. The Br 3d spectra (Fig. 5c)
at 67.27 eV and 67.99 eV would be attributed to Br 3d5/2 and Br 3d3/2 respectively. [57]. Fig. 5d
shows the high-resolution XPS spectra of C 1s. The main peak at 284.6 eV is attributed to the
C=C/C-C bond with graphitic carbons (sp2 orbital), which originate from the amorphous carbon
phase [58]. The peaks at 286.1, 287.6 and 289 eV are ascribed to C-C (sp3 orbital), C-O, and
C=O respectively. The corresponding XPS survey scan of optimized AABR-ACK 11 with other
composites from Table 2 are also shown in Fig. S2. Based on the observation of XRD, SEM and
XPS analysis, these findings show the hybridization of AABR with ACK, and the existence of
metal Ag⸰ , Ag+, and Br- in the composite.
3.2.2 Optical and Electrical Properties of AABR-ACK Composites
Fig. 6a-b illustrates the UV–Vis DRS spectra of as-prepared AABR-ACK composites at
different times (AABR-ACK 7, AABR-ACK 11 and AABR-ACK 14) and temperatures (AABR-
ACK 21, AABR-ACK 11 and AABR-ACK 26). The AABR-ACK composites exhibited strong
15

absorbance edge in the visible region within the range of 400 to 800 nm. This is ascribed to the
intense surface plasmon resonance (SPR) of the metallic Ag particles, caused by a thermal
reduction of AgBr by EG [59, 60]. The SPR of Ag particles is influenced by the ordered
nanostructure and particle diameter as evidenced by SEM analysis (Fig. 3a-f). The AABR-ACK
X (X =7, 14, 21, 26) composites absorption edges in the visible region were reduced as
compared to AABR-ACK 11 (Fig. 6a-b), which endow the latter to efficiently utilize the visible
light for enhanced photodegradation of TC [61]. The strong hybridization between AABR and
ACK interconnected carbon network acting as light transmittance channels [62], also boosted the
visible light harvesting ability of the AABR within the AABR-ACK 11 (Fig. S3a). The band gap
energy (Eg) of AABR-ACK and AABR were calculated using the Kubelka-Munk [63] equation
below:
n
α
h
ν
= A(h
ν
− Eg)2
(3)
where
α,
հ
,
ʋ
, A and Eg are the optical absorption coefficient, Plank’s constant, light frequency,
a constant and band gap energy. Also n is equal to 1 and 4 for direct and indirect bandgap
semiconductor, hence the AgBr catalyst is an indirect bandgap semiconductor [64] and the plot
of (αһν)a2 versus photon energy (һν) is shown in Fig. S3b. The corresponding band gap for
AABR-ACK 11 and AABR are 2.34 eV and 2.4 eV respectively.
The VB and CB positions for AABR-ACK 11 are determined through the following empirical
formula:
EVB = X − EO + 0.5Eg
(4)
16

ECB = EVB − Eg
(5)
where EVB and ECB is the edge potential of the valence band (VB) and conduction band (CB)
respectively, and X represents the absolute electronegativity of the semiconductor. The X value
for AgBr is calculated at 5.8 eV. EO is the energy of free electrons on the hydrogen scale (about
4.5 V) [63] and Eg is the band gap energy of the semiconductor. The EVB and ECB values of
AABR are calculated to be 2.47 and 0.13 V vs. NHE, respectively.
The recombination of photo-induced electrons and holes pairs [65] affects the photocatalytic
activity of a photocatalyst. The intensity of AABR-ACK composites according to different times
can be ranked as (AABR-ACK 11 < AABR-ACK 14 < AABR2-ACK 7; Fig. 7a) and at different
temperatures (AABR-ACK 11 < AABR2-ACK 26 < AABR2-ACK 21; Fig. 7b). AABR-ACK
11 composite exhibits a lower intensity, which indicates that the recombination of photo-induced
electron-hole pairs was effectively inhibited [66] and correlates with its enhanced degradation of
the TC in Table 2. The ordered nanospheres for AABR-ACK 11 composites retard the
recombination of electron-hole pairs, which further boost the photocatalytic performance. The
ACK support also effectively facilitates separation of photo-induced charge carriers from
AABR-spheres judging from the PL spectra of AABR and AABR-ACK 11 composite (Fig. S4).
To further study the influence of facile tuning AABR-ACK nanostructures on the separation
of photo-prompted electron-hole pairs and electron conductivity, photo-electrochemical analysis
was also carried out. The photo-electrochemical response as well as the curve under dark are
presented in Fig. 7c-d. The photocurrent density-voltage curve was measured in the potential
range of 0.5–2.0 V vs SCE. The AABR-ACK 11 composite showed the highest photocurrent
density of 1.07 mA/cm2, which is higher than that of AABR-ACK X relatives (X = 14, 7, 26 and
17

21) and AABR under visible light irradiation, suggesting its higher electron-hole pairs separation
efficiency. The high photocurrent density for AABR-ACK 11 further verifies that the ordered
nanostructures of AABR and interconnected network structure of ACK significantly boost the
photo-electrochemical performance for high degradation activities.
The EIS Nyquist plots of AABR-ACK composites are shown in Fig. 8a-b. The rank of
smallest semicircle arc radius according to the fitted charge resistance values (Table S2 for the
composites at different times are: AABR-ACK 11 < AABR-ACK 14 < AABR-ACK 7, while for
temperatures: AABR-ACK 11 < AABR-ACK 26 < AABR-ACK 21. The smallest semicircle arc
radius implies that effective electron conductivity and enhanced separation efficiency of photo-
prompted electron-hole pairs [67] occurred with the AABR-ACK 11 nanospheres composite. In
addition, the AABR-ACK 11 composite displays smaller arc radius as compared to pure AABR
in Fig.S5. This means that an efficient charge carriers separation was actualized via the intimate
contact between AABR and ACK support [68] for AABR-ACK 11 nanospheres. ACK
conductive support in the AABR-ACK 11 composite will also effectively enhance the separation
of photoinduced charge carriers and improves the photocatalytic performance [69].
The corresponding Bode plots in Fig. 8c shows much lower |Z| trend for AABR-ACK 11 than
that of AABR-ACK relatives and AABR. The lower |Z| trend implies faster electron-hole pairs
separation, which confirms that the ordered nanostructure for AABR and the conductive ACK
promotes the charge carriers separation properties of AABR. The hybridization of AABR with
ACK enhanced the transportation rate of photo-excited electrons through an interfacial
interaction between AABR nanospheres and the conductive ACK support [43], prompting
reactive species formation for higher photocatalytic activities for AABR-ACK 11 composite.
18

Overall, the TC high degradation efficiency using AABR-ACK 11 is advanced by ordered
nanostructures morphology with strong synergistic interactions between components of the
composite. The ordered AABR nanospheres further enhanced visible light harvesting and fast
electron transfer in the presence of conductive ACK, along with a suppressed recombination rate
of charge carriers. The analysis of optical, photo-electrochemical, morphology results and RSM
approach evidence suggests that AABR-ACK 11 composite is the active photocatalyst for the
degradation of TC in this study.
3.3 Photocatalytic Activities
The visible light photocatalytic activities of AABR-ACK 11 composite and other as-prepared
samples were further investigated on the degradation of TC (15 mg/L, 150 ml), as presented in
Fig. 9a. The degradation of a TC without photocatalyst under visible light condition cannot be
effectively oxidized as observed in Fig. 9a. The photocatalytic activity of AABR, AgBr and
reference TiO2/HCP exhibited a lower degradation rate of less than 60% in 180 min. However,
the AABR-ACK 11 showed a higher photocatalytic rate of 92.08% within 180 min than AABR
and other samples under the same conditions; hence, close contact of ordered AABR with ACK
enhanced the oxidation degradation of the TC. To quantitatively evaluate the photodegradation
kinetics of TC over the prepared samples, the experiment values were fitted to both pseudo-first-
order and pseudo-second-order kinetic feature [6, 70]. The pseudo-second-order model in Fig.
9b fitted better to describe the photocatalytic process of TC removal as compared the pseudo-
first-order model (Fig. S6a). The degradation rate constant for the as-prepared samples are
ranked as: AABR-ACK 11 > AABR > AgBr > TiO2/HCP. AABR-ACK 11 composite has the
highest rate constant (0.0044 L mg-1 min-1), which is 9.4 times fold more than AABR (0.0005 L
mg-1 min-1). The absorption spectra of TC over AABR-ACK 11 composite is shown in Fig. 9c,
19

the main band of TC at 376 nm reduces with an increase in irradiation time up to 180 min during
the photocatalytic process. The reduction of the TC main band at 376 nm suggests that the
phenolic-diketone group are easily broken down [71, 72]. This indicates that the AABR-ACK 11
is a promising photocatalyst for the treatment of antibiotic wastewater. The enhanced
photocatalytic performance for AABR-ACK 11 is attributed to the design of ordered
nanospheres structure, which promotes interfacial charge separation and prolonging the lifetime
of photo-carriers [73, 74].
In real water situations, inorganic ions may also be present and therefore their potential
influence on the activity of the photocatalyst was investigated. In this work, the influence of
inorganic anions on the photocatalytic performance of AABR-ACK 11 composite on the removal
of TC was also carried out using several sodium salts of sulfate, sulphite, bicarbonate, chloride,
and phosphate at a concentration of 20 mM as presented in Fig.9d. All the inorganic anions used
in this work, have an inhibitory effect on the photocatalyst with the degradation of TC, as the
degradation rate reduced from 92% to 49%. This inhibitory effect is attributed to the scavenging
of free reactive oxidative species (h+ and
˙OH) on the photocatalyst surface, which further
transforms into inactive oxidative species and lowers the degradation % [75, 76]. The negative
effects of these inorganic anions were found to be in the order of phosphate > chloride >
bicarbonate > sulphite > sulphate. Among the inorganic anions, the highest inhibitory effect was
with phosphate and chloride ions, which is ascribed to their high potential [77, 78] to occupy the
active sites of AABR-ACK 11 composite as compared to other anions.
The mineralization of TC in the photodegradation process was further assessed by the change
of the total organic carbon (TOC) concentration as shown in Fig. S6b. The initial TOC value of
TC solution is 40.31mg/L. After 180 min adsorption in the dark / irradiation, the value decreases
20

to 36.67, 5.29 and 32.63 mg/L for AABR-ACK 11 without light, AABR-ACK 11 n and AABR
under visible light illumination respectively, while calculated TOC removal efficiency is 86.88%
and 19.06% for AABR-ACK 11 and AABR. It shows that the degraded TC pollutant is broken
down into CO2 and H2O, highlighting the complete mineralization attributes of the AABR-ACK
11 composite. The stability and reusability of a photocatalyst are critical for a cost-efficient
commercial process. Therefore, the reusability of AABR-ACK 11 composite was evaluated for
the degradation of TC in five consecutive cycles. After each cycle, the photocatalysts were
recovered by centrifugation, washing, and drying. Fig. 10a.shows that the degradation % of
AABR-ACK 11 exhibited no significant reduction in the catalysts ability to degrade TC, 8.9%
loss was observed after 5 cycles, which originated from small catalyst mass losses during the
washing and drying process. Therefore, the XRD, FTIR and UV-vis DRS spectra (Fig. 10b-d)
for the reused AABR-ACK 11 (five cycle times) shows that the phase, structural and optical
properties remain unchanged after the photocatalytic process. The ordered AABR-ACK 11
nanospheres exhibit higher activity and reusability for the degradation of TC under LED visible
light illumination, which is of great value for the practical application for the antibiotic
degradation in wastewater. The prepared material AABR-ACK 11 achieved comparable
photocatalytic activity for TC removal when compared with other nanocomposite photocatalysts
in Table S3.
3.4 Mechanism of Enhanced Photocatalytic Activity
In order to understand the photocatalytic mechanism, active species quenching experiments
were performed. The quenchers used in this work comprised of EDTA-Na2 for h+, IPA for
˙OH
−
and BQ for ˙O2 [79, 80] respectively. As shown in Fig. 11, both BQ and EDTA- Na2 quenchers
greatly inhibited the degradation of TC, and weakly reduced the activities on TC removal after
21

−
the addition of IPA. It’s concluded that the ˙O2 and h+ are the main active species in the
photocatalytic degradation of TC using AABR-ACK 11, while the ˙OH plays a secondary role.
−
Invariably, O2 , h+, and ˙OH reactive species all participate in TC degradation (as a mixture).
Based on the above quencher experiment, a tentative reaction mechanism for the degradation
of TC under visible light irradiation using AABR-ACK 11 nanospheres structure is proposed, as
presented in Fig. 12. The TC molecules adsorb on the Ag and AgBr particles with close
proximity to the ACK through π–π, electrostatic or hydrogen-bonding interactions [41, 81].
Under visible light irradiation, both Ag and AgBr particles in AABR-ACK 11 composite are
photoexcited to produce electron-hole pairs [10, 82]. The energy of excited electrons generated
by the Ag SPR effect reached to 1.0–4.0 eV [83], which means that the generated electrons have
enough energy to jump onto the CB of AgBr. Injected electrons in the CB of AgBr cannot trap
O2 on its surface, since the CB potential of AgBr (0.13 eV) is more positive than the potential of
−
O2/ O2 (-0.33 eV vs NHE) [84] and ACK presence as an acceptor of photo-induced electrons
promotes the migration of electrons away from the conduction band (CB) of AgBr. The
recombination with photo-excited holes is prevented while allowing the electrons to react with
O2 molecules on the ACK surface to form superoxide radical species and other reactive oxygen
species that attacks the adsorbed TC directly [11, 85, 86]. This electron transfer process
significantly prolongs the lifetime of photogenerated charge carriers for intimate contact with the
TC pollutant, which results in an elevated photocatalytic activity of AABR-ACK 11 under
visible light illumination. The photo-induced holes accumulated at the Ag VB [87] directly
participate in the oxidation of the TC. In addition, the photo-induced holes in VB AgBr are more
positive to oxidize H2O to form ˙OH radicals (OH−/ OH, 2.4ꢁV vs NHE) [88]. This reactive
hydroxyl species (˙OH) produced by H2O with photo-induced holes also aid in the degradation of
22

TC [89]. The positively charged holes on the AgBr surface also oxidize Br- to Bro, as Bro is a
reactive species that can oxidize TC directly due to its high oxidation potential, which is
consistent with other reports [90, 91]. Overall, the regulated nanospheres structure for AABR-
ACK 11 composite, separation and transfer of photo-induced electron-hole pairs enhanced the
photocatalytic efficiency and stability.
4. Conclusions
In conclusion, AABR-ACK nanocomposites with close interfacial contact between the AABR
particles and carbon were synthesized through a thermal polyol route. The synthesis variables
were optimized using a RSM experimental design to obtain the highest degradation of
tetracycline (TC) antibiotic under visible light irradiation. The time and temperature play a major
role in improving the catalyst properties to enhance the degradation of TC using the AABR-ACK
composites. Characterization techniques such as TEM, SEM, UV-VIS spectroscopy, PL
spectroscopy, electrochemical studies showed that the AABR-ACK 11 composite had an ordered
nanospheres morphology with reduced particle diameter, enhanced visible light harvesting along
with a high separation of photo-induced electron-hole pairs, which explains its high catalytic
activity on TC removal in comparison to the other composites. The coupling of AABR into the
ACK interconnected carbon network, significantly aids in the uniform distribution of AABR
particles, which furthers promotes interfacial charge separation and prolongs the lifetime of
photo-carriers. The AABR-ACK 11 catalyst had a significantly higher visible light
photocatalytic activity for TC removal than those of AABR, AgBr and reference TiO2/HCP
photocatalysts. The phosphate inorganic anion decreases the performance of AABR-ACK 11
more than other inorganic ions such as sulfate, sulphite, bicarbonate, and chloride. The
23

−
quenching experiments with scavengers indicated that h+ and ˙O2 were the main radicals
responsible for the degradation process. In summary, the ordered nanospheres structure of
AABR-ACK 11 with enhanced charge carriers separation efficiency is the major factor that
enhanced the activity for TC removal by oxidation degradation under visible light irradiation.
Acknowledgments
The National Research Foundation of South Africa with the grant number (99330 and 111330), the Sasol
University Collaboration Programme and the Vaal University of Technology financially supported this
work. The authors are very grateful to Prof Kaushik Mallick (University of Johannesburg) for
electrochemistry guidance.
24

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