Document Type : Original Research Paper


1 Department of Applied Sciences and Humanities Indira Gandhi Delhi Technical University Delhi-110006 Ph: 08826629331

2 applied sciences and humanities department, indira gandhi delhi technical university for wome, delhi, india.110006


Mg(OH)2/CdS heteronanostructures have been successfully synthesized by a novel precipitation method and the synthesis involves three steps. The first step involves the synthesis of Mg (OH)2nanoparticles using homogeneous precipitation method. Then, surface-modifying agent citric acid was used to functionalise Mg (OH)2. Finally, the cadmium sulfide (CdS) shell was deposited on the surface modified Mg (OH)2by co-precipitation method. The Mg(OH)2/CdS heteronanostructures were characterized using X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and photoluminescence spectroscopy. DRS results showed blue shift of CdS band gap absorption with respect to bulk CdS. XPS results showed evidence for the binding energies of Mg(OH)2, Cd and S. The Mg (OH)2/CdS heteronanostructures was explored as catalyst for sunlight induced photocatalytic degradation of β- naphthol pollutant. The batch of 0.2 mg/ mL of Mg (OH)2/CdS heteronanostructures maintained at pH 8.5 showed maximum photodegradation efficiency (75 ± 2.1 %). Higher photocatalytic degradation efficiency for Mg(OH)2/CdS heteronanostructures could be due to incorporation of CdS and increased reactive oxygen species (ROS) generation. The reusability of the Mg (OH)2/CdS heteronanostructures was also tested, and they show stability for up to three cycles without loss of efficiency.


Water contamination is one of the major problems of the whole world and it is due to industrial and radioactive wastes, inadequate sewage treatment, marine dumping issues, etc. [1,2]. Mg(OH)2 nanomaterials being nontoxic, noncorrosive, eco-friendly, and highly stable have been used earlier for water purification [3, 4]. There have been various methods reported for the synthesis of Mg(OH)2 nanoparticles and their nanocomposites. Mg(OH)2 nanopowders having platelet-like morphology were synthesized from the serpentinite mineral via solvothermal reaction [5]. However, Mg (OH)2 use is restricted to the UV region due to its wide bandgap. Some studies were carried out to narrow down the band gap of semiconductors by coupling them with metals, to use them in the visible region. In this respect, cobalt was doped into Mg (OH)2 nanoparticles (NPs) using the co-precipitation method for narrowing the bandgap of Mg (OH)2[6]. In this scenario, Mg(OH)2/2 dimensional (2D) heterostructures such as Mg(OH)2/MoS2, Mg(OH)2/WS2, Mg(OH)2/AlN were found to be useful due to their optical and electronic properties [7-9]. Blue P/Mg(OH)2 heterostructures were used as an efficient visible photocatalyst for water splitting [10]. CuInS2/Mg(OH)2 nanosheets were synthesized via hydrothermal approach and used for enhanced visible-light photocatalytic degradation of Tetracycline antibiotic [11]. Zero valent iron supported on Mg(OH)2 was synthesized by the chemical precipitation method for the enhanced removal of Pb(II) from an aqueous solution [12]. Carbon cloth-supported Mg(OH) 2 were synthesized via electrodeposition and were used for rare earth metal Europium (Ⅲ) recovery from an aqueous solution [13]. High-gravity technology was used for the fabrication of Mg(OH)2/graphene oxide composite and was used for the removal of dyes [14]. TiO2 / Mg(OH)2 hetero nanostructures were synthesized by simple thermal hydrolysis method and used for the catalytic degradation of chemical warfare agents DMMP, 2-CEES, and 2-CEPS [15]. A strong light absorptive Mg(OH)2 micro flowers supported by Ruthenium nanoparticles were synthesized for the photothermal carbon dioxide hydrogenation [16]. In order to convert the UV-based photocatalyst like Mg (OH)2 to visible photocatalyst and to obtain extraordinary optical and electronic properties, it is necessary to couple it with the semiconductors having a narrow bandgap. Furthermore, the separation of photoexcited electron-hole pairs could be achieved using Mg(OH)2 based heterostructure. In this regard, CdS is a tremendous preference since it has a bandgap of about 2.42 eV, and also combining it with large bandgap materials, could induce excellent optical and electrical properties [17]. The characteristics such as morphology, electronic and optical properties of the nanomaterials need to be considered for attaining good efficiency in applications like photocatalysis, water splitting, etc. under natural sunlight [18-20]. Besides, Mg (OH)2/CdS heterostructures were capable of transferring their charge from one semiconductor to another leading to the prevention of exciton recombination, simultaneously tuning the bandgap of Mg (OH)2 to the visible region. Commonly, during the synthesis of nanoparticles, citric acid, a tridentate carboxylic acid, was used as a stabilizing ligand. The excellent characteristics possessed by the citrate ions are good surface binding capacity, prevention of agglomeration, and bringing water solubility to the synthesized nanoparticles. β-naphthol, one of the naphthalene derivatives, which is widely used in industrial chemicals and used extensively in dyestuffs manufacturing, pharmaceutical production, and biogeochemical processes is found to be hazardous. This compound’s maximum allowable limit from the effluent of the factories is 3 ppm [21]. The techniques based on the usage of biological methods, membranes, and evaporation methods, which have been carried out to eliminate β-naphthol in water have some limitations due to secondary environmental pollution [22]. In this regard, Praseodymium oxide nanostructures were prepared via a thermal treatment approach and their photocatalytic activity was investigated by the degradation of 2-naphthol under ultraviolet light irradiation [23]. Nanocrystalline cadmium molybdate was prepared by ultrasonic method their photocatalytic degradation against 2-naphthol under visible light irradiation was carried out [24]. Both the rutile and anatase forms of TiO2 nanoparticles stabilized on activated carbon using microwave energy were synthesized and used in the removal of β-naphthol from wastewater, under A 20 W power UV-C light [21]. N-TiO2/SiO2 and nitrogen-doped N-TiO2/SiO2 were prepared by sol-gel method and their photocatalytic activities against β-naphthol were evaluated with a 25 W lamp (Natural light PT 2191-ExoTerra) as the simulated solar light source [25]. 
In this paper, we present a simple, economical method for the synthesis of Mg (OH)2/CdS hetero nanostructures and were characterized and further explored as sunlight-driven photocatalysts for the degradation of β-naphthol in an aqueous solution.  
●    Synthesis of Mg(OH)2/CdS hetero nanost-ructures using a simple, economical, chemical precipitation method
●    Characterization of synthesized material using X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), and photoluminescence spectroscopy, etc.
●    Sunlight-induced photocatalytic degradation of β- naphthol pollutant.
●    Determination of hydroxyl radicals using the terephthalic acid assay. 
●    Determination of the role of Histidine as reactive oxygen species (ROS) scavenger. 
β-naphthol, Sodium sulfide were purchased from SRL Chem Ltd., India. Magnesium Nitrate (Fischer), citric acid (99 %, Himedia, LR), cadmium chloride (99 %, Lobachemiepvt. Ltd., AR), sodium hydroxide (98 %, Rankem, AR), Mercaptoacetic acid (MAA) (Fischer), Terephthalic acid (CDH chemicals), acetone (99 %, Qualigens) was used as received. 

Synthesis of Mg (OH)2/CdS hetero nanostructures
The Mg (OH)2/CdS hetero nanostructures were synthesized using the following three steps 

Synthesis of Mg (OH)2nanoparticles
A standard solution of 0.1M concentration of magnesium nitrate was prepared by dissolving 6.41gm of magnesium nitrate in 25mL of water and was stirred at 80 0C for 1 h to obtain a clear solution. Then the uniform solution of 0.1 M NaOH was added drop by drop to the above solution. Further, the resultant mixture was stirred for an extra 2h, to attain completion of the reaction. Then, the resultant solution was collected and centrifuged for 15 min at 5000 rpm and washed with sterilized double-distilled water. Finally, the obtained white precipitate was dried in a hot air oven at 70 0C overnight.

Surface modification of Mg (OH)2NPs
The surface modification of Mg (OH)2 nanoparticles was carried out using citric acid as the surface-modifying agent. Initially, Mg (OH)2 of concentration 1 mM was prepared by dissolving 0.8 gm of Mg (OH)2in 50 mL of distilled water and then 0.05gm of citric acid was added. The obtained mixture was stirred at room temperature for 4 h. The contents formed were filtered, washed, and dried in the oven at 70 oC.

Synthesis ofMg (OH)2/CdSheteronanostructures using precipitation method 
To 50 mM CdCl2 aqueous solution, 87.2 mL of mercaptoacetic acid (MAA) was added dropwise. The above solution was stirred for 30 min. The pH of the resulting solution was raised to 8.0 using 0.1 N NaOH. Further, Mg (OH)2 was added to the above solution and then the aqueous suspension of sodium sulfide was added and stirred for 3 h. Further, acetone was added, to precipitate the synthesized hetero nanostructures. Finally, the nanocomposite was separated by centrifuging at 4000 rpm for 5 min. The supernatant was discarded and the yellow-colored precipitate so obtained was washed with deionized water and centrifuged. The obtained residue was stored at room temperature till further use. This batch of the sample will be henceforth referred to as Mg (OH)2/CdS hetero nanostructures. 

Characterization of Mg (OH)2/CdSheteronanostructures
The X-ray diffraction measurements of the Mg (OH)2/CdS hetero nanostructures were performed with (M. Smartlab, Rigaku) operated at 40 kV using graphite monochromatized Cu Kα radiation source with a wavelength of 1.54 Å in a wide-angle region from 10° to 90° on a 2θ scale. The morphology of Mg (OH)2/CdS hetero nanostructures were characterized by using field emission scanning electron microscopy coupled with energy dispersive X-ray analyzer VY.05 (SIGMA). The absorption spectroscopy of the as-synthesized nanoparticle dispersion was measured by UV–vis spectrophotometer (Cary 100 UV Vis) in the range of 200–800 nm shown in Table 1. The bandgap of the synthesized hetero nanostructures was determined from diffuse reflectance spectra (Shimadzu, UV-2450) with BaSO4 as the reference scattering material. The emission spectra are recorded using a fluorescence spectrophotometer (Shimadzu, RF-5301 PC) at an excitation wavelength (λex) of 315 nm. The excitation and emission slit width of 5 nm was maintained for all the measurements. The chemical constituents in the Mg (OH)2/CdS hetero nanostructures were determined from X-ray photoelectron spectroscopy (XPS) using a multi-technique surface analysis system of Omicron Nanotechnology, Germany. The XPS analyses were carried out using a monochromatic Al Kα X-ray source of 1486.7 eV, with an operating voltage of 15 kV and emission current of 15 mA. The samples for XPS analysis of Mg(OH)2/CdS hetero nanostructures were made as pellets using a hydraulic press. The transmission electron microscopy (TEM) images were recorded using FEI Technai-G2 microscope operated at 200 kV and its corresponding nature was determined by SAED attachment. The samples for TEM analysis were prepared by placing a drop of diluted Mg(OH)2/CdS hetero nanostructures dispersed on a carbon-coated 150 mesh copper grid and dried at room temperature.  
Photocatalytic degradation study 
The degradation of β-naphthol– a model pollutant was studied using Mg(OH)2/CdS hetero nanostructures under natural sunlight. In our study, β-naphthol pollutant of 7 x 10-5 M was prepared, and the batches of the test solution were treated with Mg (OH)2/CdS hetero nanostructures (MC) of increasing concentrations of 0.1mg/mL (MC1), 0.2 mg/mL(MC2), 0.3 mg/mL of Mg (OH)2/CdS hetero nanostructures (MC3) and compared with bare Mg (OH)2 NPs and control sample (without nanoparticles). The pollutant treated with different batches of NPs was first ultrasonicated for 20 min in dark to ensure minimal ROS formation and followed by stirring for 40 min for proper adsorption and desorption equilibrium. All the batches of test solutions were continuously stirred in the presence of sunlight between 12:00 noon and 2:00 pm at IGDTUW campus, Delhi in March 2021 when the intensity of the sunlight fluctuation is low. The intensity of sunlight at Delhi (the latitude 28° 40’ 2.96” N and longitude 77° 13’ 40.96” E respectively) in March is 1366 Watt/m2.
The time-dependent degradation studies of the pollutant were carried out by measuring the concentration of a pollutant from the test sample at an interval of 30 min. The obtained aliquot was centrifuged at 15000 rpm at 25 oC for 5 min in Beckman CoulterTMAllegraTM X-22 R to separate the dispersed nanoparticles and its absorption maxima were measured at λabs=223 nm. The decomposition of the pollutant was compared with a positive control containing the same concentration of pollutants without NPs. The effect of degradation of pollutant by Mg (OH)2/CdS was also compared with a negative control containing the same concentration of pollutant treated with Mg (OH)2 NPs. An experiment was performed in the absence of sunlight, to study the effect of light. All analyses were performed in triplicate and the results are presented as mean and standard deviation of three analyses. 

Measurement of OH radicals (ROS indicator)
The batches of 0.2 mg/mL each of Mg (OH)2 NPs and Mg (OH)2/CdS hetero nanostructures were sonicated for 10 min in 3 mM solution of terephthalic acid prepared in 0.01 M NaOH solution. The obtained solution was continuously stirred for 30 min to form adducts between terephthalic acid and OH* radicals. The dispersed solution was centrifuged at 7000 rpm for 5 min and the emission spectrum of the supernatant was recorded using Shimadzu fluorescence spectrophotometer (RF-5301 PC) at an excitation wavelength (λexc) of 315 nm. A control experiment was carried out by measuring the intensity of the emission spectrum of the reaction mixture before UV illumination. The total number of OH radicals generated during the reaction was a measure of the fluorescence intensity of adducts formed between the OH radical and terephthalic acid [26]. All analyses were performed in triplicate and the results are presented as mean and standard deviation of three analyses. 

XRD patterns of pure Mg (OH)2 and Mg (OH)2/CdS hetero nanostructures were shown in Fig. 1. Pure Mg (OH)2nanostructures (JCPDS file No: 00-044-1482) [7] showed reflections at 18.77°, 32.90°, 38.04°, 50.90°, 58.68°, 62.09°, 68.31°, 72.17°and 81.33° are ascribed to (001), (100), (101), (102), (110), (111), (103), (201) and (202) planes respectively. The intense peak at 2θ = 38.04°confirmed the preferred orientation of Mg (OH)2 facet along the (101) plane. Pure CdS showed reflections at 26.44°, 43.80°, and 52.00° corresponding to (111), (220), and (311) planes of cubic CdS (JCPDS file no: 01-080-0019) [27] (Fig.S1.) In the XRD patterns of Mg (OH)2/CdS hetero nanostructures, both Mg (OH)2 and CdS reflections are observed. Fig. 1b. shows that the XRD peaks due to CdS in the hetero nanostructures are more intense and less broad. The crystallite size of Mg (OH)2 and CdS in the Mg (OH)2/CdS samples was calculated using Debye–Scherrer equation using the XRD peaks at 2θ = 38.04° for Mg (OH)2 (101) and at 2θ = 26.44° for CdS (111). The average crystallite size of pure Mg (OH)2 is higher (14 nm) compared to that in the Mg (OH)2/CdS hetero nanostructures (3 nm approx.).
The FE-SEM images of Mg (OH)2/CdS samples are shown in Fig. 2. Pure Mg (OH)2 showed platelet-like morphology, after surface modification with citric acid (Fig.S2). In the presence of citric acid, the crystallinity and precipitation dispersibility of Mg (OH)2 was improved drastically [28]. In the final step of the synthesis, using the precipitation method, nanoneedles were obtained. Our results were in good agreement with the following data [29]. Herein the crystal growth was observed, in which the length of Mg (OH)2nanoneedles was about 1000 nm and the diameter was found to be 20 ± 5 nm. It was also observed that some of the agglomerated CdS nanoparticles got deposited irregularly on the surface of Mg (OH)2. The elemental composition of Mg (OH)2/CdS hetero nanostructures was studied using EDX analysis (Fig. 2d). All the samples showed the presence of elements such as Mg, O, Cd, S, etc. The TEM and SAED images of Mg (OH)2/CdS samples were shown in Fig.3. The SAED patterns of Mg (OH)2/CdS hetero nanostructures confirmed the amorphous nature of the material and this could be due to the higher deposition of CdS NPs on the surface of Mg (OH)2 /CdS. Moreover, from the XRD pattern of CdS, as shown in Fig. S1. it’s clear that the obtained CdS nanomaterial was amorphous. Further, the determination of chemical state and chemical composition of Mg (OH)2/CdS sample was studied by XPS (Fig. 4), where Mg, O, Cd, S binding energy peaks and their oxidation states were revealed. Fig. 4a shows the detailed scan of Mg (OH)2 /CdS heteronanostructures, wherein the characteristic peaks of Mg (KLL), Cd 3p,3d, O 1s, and S2p were observed. Mg showed a binding energy peak at 51 eV corresponding to 2p(Fig.4c), which confirmed the presence of Mg+2. Further, the binding energy value of 1304eV was observed for Mg 1s(Fig.4d) and the value 50.5 eV was related to Mg-O/OH species in the spectra. For O 1s the Binding energy value 532.6 eV was observed(Fig.4b), which proved the existence of hydroxide form [30]. Similarly, the binding energy peaks of Cd 3d3d5/2 and Cd 3d3d3/2 were recorded at 407eV and 412 eV and for S2p it was recorded at 161.5 eV and 162.7 eV, which showed the presence of sulfur in S2- form (Fig.4e,f). Our results corroborated well with the following data [31].
The DRS and absorption spectra of Mg (OH) 2 /CdS hetero nanostructures were shown in Fig. 5. The bandgap values of pure Mg (OH)2 /CdS hetero nanostructures were calculated using the Kubelka-Munk plot. Pure Mg (OH)2 and CdS have direct bulk band gap values of 5.7 and2.42 eV respectively [7,32]. In the present study, pure Mg (OH)2 / CdS hetero nanostructures possess a bandgap of 2.45 eV. The Mg (OH)2 in the Mg (OH)2/CdS hetero nanostructures showed about 3.1 eV blue shift with respect to bulk Mg (OH)2 (Eg = 5.5 eV). In Mg (OH)2/CdS hetero nanostructures, the blue shift of the bandgap absorption of CdS is ascribed to the quantum size effect [33]. The absorption maxima showed two peaks in Mg(OH)2/CdS samples which were found to be 238 nm and 408 respectively (Fig.5a). The higher catalytic activity could be achieved through efficient charge separation. 

Photocatalytic activity: effect of sunlight and concentration of the photocatalyst
The degradation kinetics of β-naphthol by the batches of Mg (OH)2/CdS hetero nanostructures were studied for 2.5 h and were compared with the photodegradation of the control batch, where inβ-naphthol was not treated with nanoparticles (data shown in Fig. 6a).  The pollutant was treated with photocatalyst Mg (OH)2/CdS hetero nanostructures (MC)of increasing concentrations of 0.1 mg/mL (MC1), 0.2 mg/mL (MC2), 0.3 mg/mL of Mg (OH)2/CdS hetero nanostructures (MC3) and compared with bare Mg (OH)2 NPs and control sample (without nanoparticles). The kinetics of β-naphthol degradation was also studied (Fig. 6b). The higher photocatalytic activity was shown by Mg (OH)2/CdS hetero nanostructures compared to pure Mg (OH)2 platelets (Fig.6b). Photodegradation of β-naphthol followed first-order kinetics and is evident from the linear relationship between ln (Co/C) and irradiation time in Mg (OH)2/CdS hetero nanostructures [34,35]. The noticeable first-order rate constant (k) for the degradation of β-naphthol was calculated as 0.005, 0.009, 0.009 min-1 for MC1, MC2, MC3 respectively showing no further increase in efficiency for MC3. Among all three samples, the highest degradation efficiency and rate constant were shown by MC2 and MC3 samples making MC2 the optimum concentration.   
The photodegradation efficiencies of Mg (OH)2/CdS hetero nanostructures of 0.2 mg/mL were determined to be 75 ± 3.4 %, which followed first-order kinetics (Fig. 6b) and were much higher than that of the Mg (OH)2 of the same concentration (52 ± 1.2 %). It may be remarked here that the batch of 0.3 mg/mL of Mg (OH)2/CdS hetero nanostructures did not show a considerable increase in the degradation efficiency (75 ± 2.8 %). Absorption spectra of degradation of β-naphthol in natural light was shown in Fig. S3. However, after particular catalyst content, a reduction in the penetration of the light has been observed in the solution due to the creation of opacity; Consequently, a decrease in the number of radicals leading to a slight decrease in the degradation efficiency was noticed. In the absence of sunlight, the degradation of β-naphthol by Mg (OH)2/CdS (MC2), through adsorption was found to be 27.8 ± 1.5% after 1h. 
Mechanism of β-naphthol degradation
When sunlight illuminated on Mg (OH)2/CdS hetero nanostructures, electrons (e–) and holes (h+) were generated. The produced excitons either recombine or create superoxide radicals (O2• −) due to electron-mediated reduction of surface oxygen. Subsequently, hydroxyl radicals (OH*) were generated, which might act as primary oxidants for catalyzing the degradation reaction of dyes and organic compounds [36,37]. Therefore e– h+ recombination needs to be suppressed to attain efficient photocatalytic degradation. When Mg (OH)2/CdS samples are exposed to sunlight, the transfer of electrons from the conduction band of CdS to that of Mg (OH)2 has occurred facilitating the prevention of electron-hole pair recombination. The conduction band of Mg (OH)2 possesses more electrons and no free holes were available in the valence band of Mg (OH)2for the recombination. The electrons could be utilized for the reduction of oxygen bound to the surface of Mg (OH)2 and resulted in the formation of highly reactive superoxide anions followed by generation of reactive oxygen species (ROS), e.g., hydroxyl radical (OH•). Similarly, the holes can oxidize adsorbed water on the CdS to form highly reactive hydroxyl radical (OH•), and eventually participate in photocatalytic degradation of β-naphthol. 
The possible mechanism for the photodegradation of β-naphthol was given in Scheme 1. In the present case, the real VB and CB of Mg (OH)2/CdS hetero nanostructures and bare Mg (OH)2 and CdS have been estimated theoretically 

EVB = X – 4.5 + 0.5 Eg   

ECB = EVB – Eg  

Here, EVB, ECB, and Eg correspond to the valence band, conduction band, and bandgap of the individual semiconductor. X represents the geometrical summation of individual elements of the corresponding semiconductor. The VB and CB of Mg (OH)2/CdS hetero nanostructures and bare Mg (OH) 2 and CdS are presented in Table S1. The observed VB and CB band positions indicate that Mg (OH)2/CdS possess a lower conduction band and lower valence band than bare Mg (OH)2, which facilitates fast electron transport to the surface and enhances the photocatalytic activity towards photodegradation of β-naphthol. 

ROS induced photocatalytic effect by Mg (OH)2/CdS hetero nanostructures
Nanoparticles usually generate reactive oxygen species (ROS) in water, when they interact with light [38]. In the present study, hydroxyl radicals were generated upon the interaction of 0.2 mg/mL Mg (OH)2/CdS hetero nanostructures dispersed in aqueous media. The formation of OH* was detected by the terephthalic acid assay and was corroborated well with the data corresponding to hydroxyl radical-mediated dye degradation using Se doped ZnO NPs [39]. The intensity of the fluorescent emission peak measured at λnm= 425 nm corresponds to the formation of an adduct between terephthalic acid and the OH* generated in the aqueous media. The order of the emission peak intensities was found to be Mg (OH)2/CdS>Mg (OH)2 as shown in Fig.7, which also follows the same tendency for the order of OH* formed in the reaction mixture. Further, the amount of OH* generated was found to be more, when the reaction was carried out in the presence of sunlight compared to dark conditions. This indicated that the higher photodegradation efficiency of Mg (OH)2/CdS was due to higher OH* generation in aqueous media. 
In order to ascertain the role of ROS generation toward photodegradation of β-naphthol, an experiment was designed to scavenge the generated ROS in the β-naphthol solution by histidine, which is a well-known scavenger of hydroxyl radicals and singlet oxygen [39]. However, in this study, it is important to ascertain that histidine itself does not exhibit any photocatalytic activity. In this regard, a control experiment is kept which resulted in negligible degradation of pollutant treated with 1.0 mg of histidine per 30 mL of the solution without photocatalyst (Mg (OH)2/CdS heteronanostructures), illuminated by natural sunlight. The extent of degradation was similar to the blank sample comprising pollutant illuminated under natural sunlight, without histidine and photocatalyst (Fig. 8). Degradation efficiencies of the test sample comprising 0.2 mg/mL of photocatalyst (Mg (OH)2/CdS heteronanostructures) in β-naphthol solution, illuminated with natural sunlight, decreased with an increase in the histidine concentration (0.4 mg/30 mL to 1.0 mg/30 mL in β-naphthol solution). Notably, the batch of β-naphthol solution treated with 1.0 mg/30 mL histidine showed 80% inhibition of photodegradation as compared to the batch without histidine addition. These results, together with the terephthalic acid assay confirmed ROS-induced mechanism for photodegradation of β-naphthol. The time-dependent kinetic study of the test solution was carried out using the same UV-visible spectrophotometer. All of the analyses were performed in triplicate and the obtained results are represented as a mean and standard deviation of three analyses. 

Reusability and Stability of the Mg (OH)2/CdSheteronanostructures 
After the β-naphthol degradation, the catalyst was separated from the reaction flask through centrifugation, washed several times with water, and reused for the next cycle. The reusability of the catalyst towards the β-naphthol degradation was monitored by UV/Vis spectroscopy, and the results of up to three cycles are shown in Fig. 9.  For the catalytic degradation of β-naphthol, the catalytic activities decreased by 3 and 4 %, respectively, after three cycles; therefore, the Mg (OH)2/CdSheteronanostructures have high stability, reusability, and applicability for practical applications. 
A novel and facile nanoscale photocatalyst Mg (OH)2/CdS was developed by precipitation method, which exhibited enhanced photodegradation of a model pollutant– β-naphthol. The FE-SEM images of Mg (OH)2/ CdS samples exhibited a cauliflower-shaped structure in some portions, whereas Mg (OH)2 showed bristle-shaped nanoneedles. The length of Mg (OH)2nanoneedles is about 1000 nm and the diameter is 20 ± 5 nm. The photocatalyst concentration of 0.2 mg/mL, at pH 8.5 was optimum for photocatalytic degradation of β-naphthol.  The degradation efficiency of β-naphthol depended on photocatalyst concentration, sunlight illumination. The photodegradation phenomenon was attributed to the generation of hydroxyl radicals, favored by a blue shift in the bandgap absorption of CdS. The role of ROS generation towards photocatalytic activity was proved by showing an increase in the generation of OH* by terephthalic acid assay. The higher catalytic activity of MC2 is attributed to the synergistic interaction between Mg (OH)2 and CdS and efficient charge separation. The present synthetic method is inexpensive and the synthesized Mg (OH)2/CdS hetero nanostructures could not only be used as a well visible photocatalyst but also as an efficient photothermal material.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

The authors are thankful to Indira Gandhi Delhi Technical University for Women for providing the facilities. The authors also give thanks to the Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia, New Delhi, and for providing the instrumental facilities. 
Supplementary data to this article can be found online at

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