1.1 Introduction

The prevalence of antibiotics in the surface and municipal water poses huge risks to our environment and human health at large. Trace level and long accumulation of these antibiotics in the environment have adverse effects towards both aquatic and terrestrial organisms [1]. Tetracycline is a commonly used antibiotic that has been detected in the ecosystem [2]. The complete removal of this antibiotic using conventional techniques still have their pitfalls, hence the need for effective treatment method that can be applied on a large scale is required [2]. Semiconductor photocatalysis under advanced oxidation processes (AOPs) has shown promising attributes for tetracycline removal [3]. However, the development of efficient and sustainable visible light driven photocatalysts that can fully explore the potential of the solar spectrum is considered as one of the promising strategies toward solving the future energy shortages. Huge attention is channeled towards the design of Ag-based photocatalysts (Ag/AgX, X = Cl, Br, I), which respond intensively to the visible light due to surface plasmon resonance (SPR) of Ag nanoparticles (NPs) anchored on the large band gap of the photocatalysts [4, 5].

Ag/AgBr is an important photocatalyst with exceptional attributes as compared to Ag/AgX (X = Cl, I) [6, 7], with less emphasis on the synthetic approach for well-defined Ag/AgBr nanostructures. The synthetic approach plays a huge role in the development of this highly efficient photocatalyst. However, the rapid reaction in the formation of AgBr nanospecies from conventional post-treatment route (photoinduced reduction) is still challenging for effective distribution and growth of the metallic silver nanoparticles (Ag NPs) [5]. The well tailor surface plasmon resonance properties (SPR) of Ag NPs are dependent on controlled microstructure (shape, size, and composition) of Ag/AgX nanocomposite. The controlled synthesis of Ag/AgBr nanospecies with different sizes and defined morphologies have been achieved through a host of approaches [7,8,9]. However, the SPR attributes of produced Ag NPs anchored on AgBr NPs from these preceding works [7,8,9,10] have limited significance on separation of charge carriers and as such high recombination rate of electron and holes is still evident. Therefore, there is need to disperse Ag/AgBr NPs on the clean surface material, which can participate in the controlled microstructure of Ag/AgBr NPs. This material should also boost the separation efficiency of charge carrier and behaves as a transport medium, and boosts their overall stability in the catalytic process.

The application of carbonaceous materials with abundant oxygenated functional groups as a template, promoter, and support in the dispersion of Ag/AgBr nanoparticles [6, 11,12,13], has shown exceptional attributes to overcome the challenges of previous works [7,8,9]. In view of the outstanding properties of these carbonaceous materials, this work focuses on the utilization of activated carbon as template and support in the controlled microstructure of Ag/AgBr nanoparticles. The Ag/AgBr-AC nanocomposites were prepared through two different approaches (thermal polyol and template-assisted method) for photocatalytic degradation of tetracycline antibiotics under visible light irradiation. Ag/AgBr-AC nanocomposite from thermal polyol route displays higher photocatalytic activity on tetracycline than the nanocomposite from template assisted route and Ag/AgBr alone under visible light irradiation. The synergy of a well-controlled microstructure of Ag/AgBr NPs and the AC support acting as an acceptor of the photogenerated electrons from the Ag/AgBr, effectively enhanced the catalytic properties of the nanocomposite synthesized from thermal polyol route. A plausible reaction mechanism was proposed based on the scavenging experiment.

1.2 Experimental Section

1.2.1 Materials

AgNO3 (99%) was purchased from Merck whereas hexadecyltrimethylammonium bromide (HTAB, 99%), polyvinyl-pyrolidine PVP (K29-32), ethylene glycol (EG, 99%) were purchased from Acros. Ethanol (98%), potassium hydroxide (KOH, 99%), hydrochloric acid, sodium hydroxide and tetracycline hydrochloride (99%) were bought from Sigma Aldrich. All the reagents were used as received and water used in all these experiments was purified with a Millipore system.

1.2.2 Activated Carbon Synthesis

The chemical activation of pine cone biomass (PCB, 5 g) was conducted by impregnating pine cone in 25 mL solution of 2 M KOH at a ratio of 2.24:1 (g KOH/g PCB). The prepared solution was impregnated for 24 h and the solution was later dried in the oven at 80 °C for 12 h to eliminate remaining moisture present in the material. A sample of KOH impregnated pine cone placed in a Duran bottle under nitrogen atmosphere flow was pyrolyzed in the microwave reactor (model LG MH8042GM), at a constant power of 400 W for 16 min. The pine cone activated carbon produced was cooled to room temperature, washed with 0.1 M HCl and hot distilled water to remove any impurities until the pH was between 6 and 7. Then the washed activated carbon obtained was dried at 105 °C overnight and then stored in an airtight container.

1.2.3 Preparation of Ag/AgBr-AC Through Thermal Polyol Route

The synthesis of Ag/AgBr-AC was carried out through a modified thermal polyol route [13]. 18 mL of ethylene glycol was poured into a round-bottom flask which was heated at 60 °C for 30 min. PVP (0.12 g) and 480 mg of HTAB were added to the solution, then AC (0.06 g) was added to the stirred mixture. After proper mixing to completely dissolve the PVP and HTAB, 3 mL ethylene glycol solution containing silver nitrate was slowly added dropwise to the above mixture. The solution was maintained at 60 °C for an additional 30 min and the mixture was then heated to a temperature of 140 °C. After attainment of the desired temperature, the reaction was further allowed to proceed for 14 min. The reaction vessel was taken out and cooled down to room temperature in the air. The resulting solids were collected by centrifugation (REMI bench top centrifuge-R-8D), washed thoroughly with ethanol and dried in an oven for 12 h. The synthesized nanocomposite was coded as plasmon K1 and Ag–AgBr was synthesized through the same route described above without the addition of AC.

1.2.4 Preparation of Ag/AgBr-AC Through Template Assisted Route

The Ag/AgBr coupled AC nanocomposite was synthesized through a modified template-assisted method with respect to the literature [14]. Typically, 12 mL ethylene glycol solution (EG) was heated in a round-bottom flask for 30 min at a temperature of 65 °C. HTAB (0.26 g) and AC (0.09 g) were sequentially added to the EG stirred solution. After homogenization, 0.2 g of silver nitrate mixed with 1 M ammonium hydroxide solution (2.50 mL) was added dropwise to the stirred solution and further allowed to stir for 6 h under ambient light for formation of Ag0 NPs on nucleated AgBr. The precipitated solution was allowed to cool down, separated by centrifugation (6000 rpm, 10 min), further washed with ethanol and deionized water repeatedly, then dried in an vacuum oven at 60 °C overnight. The synthesized nanocomposite was stored in an airtight container in the dark and denoted as plasmon K2.

1.2.5 Characterization of the Synthesized Materials

The morphology and composition properties of the nanocomposite materials were determined using scanning electron microscopy (SEM, Zeiss Leo 1430 VP) and energy dispersive X-ray spectrometer (EDS, INCA). Shimadzu X-ray 700 (XRD) with Cu Kα radiation was used to determine the crystal structure and phase data for the photocatalysts in the 2θ range of 20–80° at 40 kV and 40 mA. The crystallite size of the nanocomposites was estimated from the Scherrer equation by using the most intense reflection peak [15]. The functional groups on the prepared samples were investigated by Fourier transform infrared spectrometer (Perkin Elmer spectrum 400) within the range of 600–4000 cm−1. The UV–visible spectra of the nanocomposites was recorded using Ocean Optics high-resolution spectrometer (Maya 2000) equipped with an integrating sphere accessory, using BaSO4 as a reference. 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 EIS experiments were carried out between frequency ranges of 100 kHz to 40 MHz with a perturbation amplitude of 5 mV in 5 mM ferrocyanide containing 0.1 M KCl solution.

1.2.6 Photocatalytic Activity Evaluation

The photocatalytic activity of the prepared samples was evaluated on the degradation of tetracycline (TC) in aqueous solution under the irradiation of 36 W white visible LED light strips. The initial TC concentration was 15 mg/L with a catalyst loading of 0.3 g/L in 150 mL of TC solution. Prior to irradiation, the mixed solution was ultra- sonicated, then stirred in the reactor for 1 h 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 h, while 3 mL of the sample solution was withdrawn at given time intervals (30 min) and centrifuged to remove suspended particles. The concentration of TC left was determined by measuring the absorbance of the solution in a UV–visible spectrophotometer at 356 nm. The percentage degradation of tetracycline was calculated using the Eq. 1.1 below:

$$\% R = 1 - \frac{{C_{t} }}{{C_{o} }} \times 100\%$$
(1.1)

where Co is denoted as the initial concentration of TC solution that reached adsorption-desorption equilibrium and Ct denoted as the concentration at reaction time t.

1.3 Characterization

1.3.1 Morphology, Size and Composition Analysis

The SEM micrographs of plasmon K1 and K2 are shown in Fig. 1.1a, b, where the formation of nanospheres morphology is well-defined in Fig. 1.1a for plasmon K1 with an average diameter of 162 nm. However, the morphologies of plasmon K2 are irregular spheres with an average particle size of 180 nm (Fig. 1.1b). The reduced particle size and uniform morphologies for plasmon K1 as compared to plasmon K2 are ascribed to moderate synthesis temperature and PVP influence, as these two parameters hamper irregular growth of Ag/AgBr nanoparticles that yield formation of uniform nanostructures. The moderate synthesis temperature from previous work has resulted in the formation of isotropically shaped Ag/AgBr nanoparticles [13, 16], whilst PVP acts as a capping agent [8] in particle size reduction and formation of well-defined shaped nanoparticles. Plasmon K1 with small particle size and uniform nanospheres morphology will have speedier charge transfer, facilitating adsorption of the pollutant on the surface of catalyst and creation of active sites in the catalytic system for enhanced degradation of tetracycline pollutant in this study.

Fig. 1.1
figure 1

SEM images and EDX spectrum of plasmon K1 (a, c) and plasmon K2 (b, d)

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The elemental components of the plasmon nanocomposites (K1 and K2) were investigated by energy dispersive X-ray spectroscopy (EDX) analysis. As depicted in Fig. 1.1c, d, Ag, Br and presence of C element from activated carbon are identified in the Ag/AgBr-AC nanocomposites. The elemental analysis also reveals no other elements in the prepared Ag/AgBr-AC photocatalyst, which confirms the high purity of the prepared products, while the atomic ratio between Ag and Br was more than 1:1 for the nanocomposite materials based on the semi-quantitative analysis (Table 1.1). This further confirms the formation of metallic Ag NPs anchored on AgBr in the nanocomposite. However, the plasmon K2 possesses more content of Ag NPs ratio and higher percentage reduction of Ag NPs as compared to plasmon K1. A few amounts of metallic Ag are formed from AgBr NPs under ambient light with plasmon K1, while more Ag NPs from reduction of rich AgBr NPs are formed with plasmon K2. This is ascribed to ammonium hydroxide ionizing the carboxylic functional groups on AC surface leading to the formation of more carboxylate anions. The formed carboxylate anions act as additional capping sites for AgBr NPs dispersion and growth through electrostatic interactions between the carboxylate anions and silver cations [14].

Table 1.1 Elemental composition of Ag/AgBr-AC nanocomposites synthesized from both methods
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1.3.2 Crystal Phase and Structures by X-Ray Diffraction

The crystal structures and the existence of metallic Ag in the nanocomposites were established with XRD investigation. As shown in Fig. 1.2, the XRD pattern of the Ag/AgBr-AC nanocomposites indicated distinct diffraction peaks that are ascribed to face centered cubic phase of AgBr (JPCDS 079-0148) and metallic Ag NPs (JCPDS 071-3762) [18]. These peaks at the 2θ include 26.6° (111), 31.1° (200), 44.3° (220), 54.8° (222), 64.8° (400), 73.4° (420) for AgBr and 38.0° (111), 44.2° (200), 64.4° (220) and 77.8° (311) all belong to Ag NPs [19]. The XRD results show that the formed nanocomposites are composed of abundant Ag NPs based on the distinct peaks of Ag and AgBr nanoparticles. The intensity ratio of I(200)/I(220) of AgBr peaks for plasmon K1 and K2 are about 1.06 and 0.68, which are higher than that of Ag/AgBr NPs at 0.42. This observation confirms that synthesized Ag/AgBr materials growth directions are <100> direction with high exposed facet of {111} AgBr NPs [20]. This {111} facets of AgBr from previous work describe the facets as possessing higher surface energy than facets of {100} and {110} with enhanced catalytic activities [21]. As a result, the plasmon K1 with a high ratio of the {111} facet AgBr NPs will exhibit higher catalytic activity as compared to plasmon K2 and Ag/AgBr with lower ratios.

Fig. 1.2
figure 2

XRD patterns of the synthesized Ag/AgBr-AC nanocomposites and Ag/AgBr

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1.3.3 Fourier-Transform Infrared (FTIR) Analysis

The FTIR analysis confirms the synergistic interaction between AC and Ag/AgBr NPs. The AC [22] displays a broad peak at 3333 cm−1 which corresponds to O–H stretching, while the carbonyl stretching peak is at 1706 cm−1 [23] and the peak at 1567 cm−1 is ascribed to the carboxylic (–COO) stretching vibration (Fig. 1.3). The characteristic peaks of C–OH and C–O–C are found at 1367 and 1227 cm−1 respectively. The carbonyl-stretching band shifted to high wavenumber in the Ag/AgBr-AC nanocomposites (Fig. 1.3), which confirms the formation of the nanocomposite as observed with other reports [24, 25] using carbonaceous material in hybridization of plasmonic materials.

Fig. 1.3
figure 3

FTIR spectrum of AC, plasmon K1 and K2

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1.3.4 Optical Properties of Ag/AgBr-AC

The as-prepared nanocomposites (plasmon K1 and K2) display strong absorptions both in the UV and visible regions as shown in Fig. 1.4. This further confirms the presence of metallic Ag NPs in both nanocomposites, which generates surface plasmon resonance (SPR) absorption in the visible region [26]. As the tailored SPR properties of Ag NPs anchored on AgBr are influenced by controlled microstructure (size, morphology, and composition) [7], plasmon K1 nanocomposite exhibits more intense absorption in the visible region as compared with plasmon K2, which can be ascribed to oriented SPR properties of Ag NPs. The as-prepared plasmon K1 on the basis of controlled microstructures will possess higher photocatalytic activity than plasmon K2 in the whole solar spectrum region.

Fig. 1.4
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UV–Vis diffusive reflection spectra of plasmon K1 and plasmon K2

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1.3.5 Electrochemical Properties

The electrochemical impedance spectra (EIS) Nyquist plots and equivalent circuit utilized in fitting the curve are shown in Fig. 1.5, where Rs denotes the series resistance of electrolyte, Rd is the diffusion resistance, Rct is the charge transfer resistance, while CPE, Cf, and Ws are the constant phase element, chemical capacitance and Warburg resistance between the photoanode and electrolyte respectively. The plasmon K1 displays a smaller arc radius as compared to plasmon K2 and Ag/AgBr in the EIS Nyquist plots (Fig. 1.5), which signify a reduced charge transfer resistance with enhanced interfacial separation of charge carriers. The charge transfer resistance (Rct) calculated from the equivalent circuit model values for plasmon K1, K2 and Ag/AgBr are given in Table 1.2. Plasmon K1 has least Rct values with the highest efficiency for electron-hole separation for enhancing the generation of reactive active species for tetracycline (TC) degradation. This is ascribed to the well-controlled microstructure of Ag/AgBr nanoparticles with exposed active sites and also the synergetic coupling of Ag and AgBr with activated carbon. The carbon support behaves as a transport medium in the separation of charge carriers from the Ag/AgBr nanoparticles.

Fig. 1.5
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EIS Nyquist plots of plasmon K1, plasmon K2 and Ag/AgBr with an equivalent circuit diagram

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Table 1.2 The charge resistance values of different Ag/AgBr samples in accordance with a series circuit
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1.3.6 Photocatalytic Degradation of Tetracycline

The photocatalytic activities of the as-prepared samples were assessed by degradation of tetracycline (TC) under visible light irradiation. As shown in Fig. 1.6a, after 180 min of degradation of TC, only 24.66% removal was achieved in the absence of the photocatalyst, indicating that TC molecule is highly stable under visible light irradiation. However, with other photocatalysts prepared in this work, the decrease in concentration of TC as a function of time is evident in Fig. 1.6a. The Ag/AgBr showed visible light activity on the degradation of TC (59.82%), while the dispersion of Ag/AgBr on AC using above described synthetic route improves the degradation rate significantly. Plasmon K1 induces more degradation rate (92.08%) than plasmon K2 (81.12%) due to the tailored SPR properties of Ag NPs and also restrained recombination of photogenerated charge carriers by the activated carbon in transferring the photogenerated charge (e−) away which enhances the charge separation efficiency. Overall, plasmon K1 photocatalytic performance was higher than that of plasmon K2, Ag/AgBr and photolysis in the degradation of TC.

Fig. 1.6
figure 6

a Photocatalytic activity of as-prepared nanocomposites on TC degradation under visible light and b decrease in maximum absorption peak of TC at different irradiation times using plasmon K1 as a photocatalyst

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Figure 1.6b shows the UV–Vis absorption plot of TC degradation in the presence of plasmon K1 under visible light irradiation. The characteristic maximum absorption peak of TC (376 nm) decreases rapidly as the degradation time increases. The catalytic degradation rates of TC over the as-prepared photocatalyst and photolysis all obey the pseudo-first-order kinetics [27] (Fig. 1.7a). The equation model is ln(C/Co) = −kt, where Co is the initial concentration of TC, C is the concentration at the irradiation time t, and the slope (k) is the apparent first-order rate constant (min−1). The k values for dispersion of Ag/AgBr on AC nanocomposites are higher than Ag/AgBr and photolysis (Fig. 1.7b). The k value for plasmon K1 is 2, 3 and 9 times faster than plasmon K2, Ag/AgBr and photolysis respectively in this work.

Fig. 1.7
figure 7

a Pseudo-first-order reaction kinetics and b corresponding apparent rate constants of as-prepared nanocomposites for TC degradation

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1.3.7 Photocatalytic Mechanism

For further confirmation of active reactive species responsible for enhanced photodegradation of TC over plasmon K1 and plasmon K2 under visible light irradiation, a host of different scavenger solutions (1 mmol) such as benzoquinone (BQ), isopropanol (IPA) and ethylenediaminetetraacetic acid disodium (EDTA-Na2) were added to TC solution to quench \(^{ \cdot } {{\text{O}}_{2}}^{ - }\), ·OH, and h+ in the degradation process. The degradation rate of TC decreases significantly to 9–14% and 32–40% in the presence of BQ and EDTA-Na2 respectively (Fig. 1.8a), which indicates \(^{ \cdot } {{\text{O}}_{2}}^{ - }\) and h+ greater influence in the removal of TC. However, the degradation rate of TC was not affected by the addition of IPA, indicating that ·OH is not the major reactive radical and combination of all the scavengers highlights their participation in TC degradation. Overall, the \(^{ \cdot } {{\text{O}}_{2}}^{ - }\) is majorly responsible for enhanced TC degradation, as its significant role is further explored in this study.

Fig. 1.8
figure 8

a Photocatalytic degradation of TC solution over plasmon K1 and K2 with different scavengers and b quantification of \(^{ \cdot } {{\text{O}}_{2}}^{ - }\) reactive species using NBT

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To confirm the rate of formation of \(^{ \cdot } {{\text{O}}_{2}}^{ - }\) in this study, degradation of nitroblue tetrazolium (NBT) to diformazan [28] under visible light irradiation was carried out. The generation rate of \(^{ \cdot } {{\text{O}}_{2}}^{ - }\) is measured by the decrease in absorbance peak of NBT (259 nm) with degradation time. A higher amount of reactive \(^{ \cdot } {{\text{O}}_{2}}^{ - }\) is produced with plasmon K1 (Fig. 1.9b) with faster decline in absorbance peak, which is in tune with its enhanced degradation rate on TC as compared to plasmon K2, Ag/AgBr and photolysis.

Fig. 1.9
figure 9

The degradation rate of TC solution by plasmon K1 photocatalyst a five recycling runs and b plausible photocatalytic reaction mechanism

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Furthermore, the stability of plasmon K1 was evaluated. The nanocomposite still keeps its original catalytic performance after 5 cycle runs on TC degradation as shown in Fig. 1.9a under visible light irradiation. Our findings reveal that the plasmon K1 will have an extraordinary photocatalytic performance for a host of organic compounds elimination with good stability under visible light irradiation.

Figure 1.9b illustrates the plausible reaction mechanism of plasmon K1 on the degradation of TC under visible light irradiation. The TC solution was adsorbed on the surface of the AC through electrostatic adsorption [29]. The AC and the smaller particle size of the overall nanocomposite increase the active sites between TC and photocatalyst, with the generation of reactive species for enhanced degradation of TC. Under visible light irradiation, both Ag and AgBr can generate photogenerated electron-hole pairs, while the controlled SPR attributes of Ag NPs result in efficient interfacial separation of photogenerated charge carriers [5]. There is first the transport of separated electrons from photoexcited Ag NPs into the conduction band (CB) of AgBr through the Schottky barrier. Then the AC further transfers the electron away from the CB of AgBr, which reacts with adsorbed oxygen to generate the reactive superoxide species, which is a strong oxidant species that degrades the adsorbed TC effectively [30, 31]. The holes left behind on the surface of Ag and in the valence band (VB) of AgBr [32] react with H2O to form ·OH; this formed reactive species will also degrade TC effectively under visible light irradiation. The positively charged holes also oxidize Br− to Br0, a reactive species that can also oxidize TC and is reduced back to Br ions again [7].

From the analysis and discussion described above, the tailored SPR properties of Ag NPs from the controlled microstructure of Ag/AgBr-AC nanocomposite, speedier interfacial separation of photogenerated charge carriers and large generation of reactive superoxide species in plasmon K1 from thermal polyol route all resulted in higher photocatalytic activity on TC removal in this study.

1.4 Conclusion

In summary, the microstructure of Ag/AgBr-AC nanocomposites (plasmon K1 and K2) were effectively controlled in this study through two different synthetic approaches (thermal polyol and template-assisted method), then subsequently utilized in the degradation of tetracycline under visible light irradiation. Both plasmon K1 and K2 have spherical shape with the distribution of Ag/AgBr nanoparticles with an average particle size between 160 and 190 nm on the surface of activated carbon as support. However, based on uniform nanospheres morphology, smaller particle size, a more exposed facet of AgBr and restrained recombination of charge carriers in the generation of reactive species, plasmon K1 displays exceptional enhanced photocatalytic degradation of tetracycline as compared to plasmon K2 and Ag/AgBr under visible light. The activated carbon presence in the nanocomposite significantly boosted the transport and separation efficiency of charge carrier, and the overall stability in the catalytic process after several repetitive cycles.