Sunlight-driven photocatalytic degradation of Rhodamine B by BiOCl and TiO2 deposited on NiCr-LDH

ABSTRACT NiCr-LDH/BiOCl and NiCr-LDH/TiO2 nanocomposites were successfully synthesised by co-precipitation and hydrothermal methods. Their structure, morphology and optical properties were analysed by XRD, SEM, UV−vis, XPS and FTIR techniques. The Rhodamine B (RhB) dye was used as a model pollutant to evaluate the catalytic activity for the nanocomposites under sunlight irradiation for 60 min. The results showed a significant enhancement in the photocatalytic activity of NiCr-LDH/BiOCl (up to 96.57%), higher than that of NiCr-LDH/TiO2 (89.58%), and pure phase NiCr-LDH (26.09%). The nanocomposites exhibited a high catalytic performance than TiO2 and BiOCl pure phases. This funding was not related to the Bi or Ti content but to the dispersion of the pure phase on the surface of the NiCr-LDH. The enhanced photocatalytic activity of the NiCr-LDH/BiOCl naocomposite mainly attributed to the formation of heterojunction between LDH and BiOCl for efficient separation of photoinduced electrons and holes. Hydroxyl radicals and holes were the main active species. Both NiCr-LDH/BiOCl and NiCr-LDH/TiO2 photocatalysts exhibited high stability and reusability even after four cycles. Based on the experimental results, a possible photocatalytic mechanism on both nanocomposites was proposed.


Introduction
The water pollution by industrial effluents containing non-biodegradable organic dyes generated from dyeing process in paper, leather, textile, plastic, pharmaceutical, food and other industries has caused serious environmental problems.The impact of dyes in residual effluents, even at low concentration, can reduce photosynthetic process and increase toxicity [1].For this reason, the researchers are looking forward for inexpensive and suitable technologies to eliminate these reactive dyes.Among these technologies, we can cite the photocatalytic oxidation which is an advanced and highly effective technology for the degradation of a wide range of pollutants [2,3].Several research papers have been devoted to the use of solar energy directly as a clean and renewable free source of energy in this process [4].However, the catalytic activity of materials in the presence of solar light can be reduced by the electrons and holes recombination and the low absorption of sunlight [5].In this sense, the development of novel highly active visible light-sensitive photocatalysts becomes today as a primary goal for several researchers in order to put a limit to this pollution.To overcome this problem, many studies have been focused on semiconductor photocatalysts due to their potential application for solar energy conversion and degradation of pollutants [6].Among widely studied semiconductors in previous years BiOCl and TiO 2 are of great interest for scientific researchers due to their unique properties and potential applications in catalysis [7].Both materials have two different structures, for bismuth oxychloride (BiOCl) with space group P4/nmm has an open-layered structure where [Bi 2 O 2 ] 2+ slabs are interleaved by two slabs of Cl − , while for TiO 2 anatase with space group I 41 /amd, built on a network of corner-sharing or edge-sharing TiO 6 octahedra [8][9][10].
Previous works show that the pure phases of BiOCl and TiO 2 have exhibited a limited photocatalytic activity under solar irradiation, and therefore it has not been widely used for photocatalytic applications [11,12].In order to extend the wavelength response of these two semiconductors many researches were based on dye photosensitisation technique which gave a good result towards degradation and mineralisation of dyes under visible light [13,14].Recently, the self-sensitised degradation of orange II and Rhodamine B in presence of TiO 2 showed a good efficiency under visible light irradiation [15].Several methods have been employed to enhance the photocatalytic activity of materials.Among them, doping with metallic and non-metallic elements in order to modulate the optical absorption and to improve charge carrier's separation efficiency [16], combining materials to form heterojunctions to enhance excitation and hence the photocatalytic activity [17].In this context, the Fe 3 O 4 /BiOCl composite has been successfully synthesised to get higher efficiency on the decomposition of Rhodamine B than the pure materials [18], the TiO 2 nanoparticles doped with iodine exhibited highly efficient and stable degradation of Rhodamine B under direct sunlight [19].
In previous years, the layered double hydroxides (LDHs) materials have received much attention in catalysis due to their high anion exchange capacity, large specific surface area, non-toxicity, low cost and easy synthetic preparation method [20].The LDHs materials, well known as anionic clays, have the general formula of , where the bivalent and trivalent cations are respectively denoted by M 2+ and M 3+ , and A n-are the interlayer anions, and m the amount of water molecules present in the layered structure.This means that the layers are positively charged and recompensed by intercalation of hydrated anions [21].The LDH material can be considered as a semiconductor owing to their specific layered structure and chemical composition versatility.Although the LDHs have been widely used for pollutants adsorption and in catalysis may improve the catalytic reactions efficiency through the enhancement of the pollutant adsorption and hydroxyl radical (OH .) production.But, have a low photocatalytic activity due to the poor charge carrier mobility, rapid recombination of charge carriers, and lower electron hole transfer [22,23].For examples, the photocatalytic activity of NiFe-LDH under visible light of RhB can give a degradation rate of 50% [24] and 30% in another study [25].The photocatalytic performance of LDHs can be efficiently improved by various modifications such as doping with semiconductors or their immobilisation on selected substrates [26].The LDH has the capacity for high absorption in the spectral range owing to the presence of nano-sheets and metal to metal charge transfer (MMCT) [22].The isomorphous substitution effect of Cu 2+ by Co 2+ on photocatalytic properties in the CuCr-LDH structure showed the presence of Cu 2+ (Co 2+ )-O-Cr 3+ linkages which are responsible of degradation of pollutant over the LDH [27].Hence, in order to enhance the separation rate of electron-hole pairs and improving the interfacial kinetics of LDH-based photocatalysts, the best way is to form an heterostructure between LDH surface and semiconductors like nanoparticles of TiO 2 and BiOCl which can be an alternative way to produce an active low cost material, since the catalyst could be reused several times after filtration.
Several reports have been published on the LDH-based semiconductor materials for photocatalytic degradation of dyes, such as ZnCr-LDH surface decorated by TiO 2 and BiOCl used in the photodegradation of rhodamine B [28], AgCl-BiOCl-NiFe-LDH composites and its photocatalytic activity for the degradadtion of rhodamine B [29], methylene blue photodegradation by Cu 2 O/MgAl-LDH [30] and TiO 2 /Zn-Al LDH composite in conjunction with solar light for the degradation of rhodamine B [31].
Although much progress has been made, to the best of our knowledge, a rational design based on their semiconductor properties is rather limited, which restricts the exploration.The improvement of the intrinsic properties of LDHs relies on the identity of cations composing the layers [32].Among the prospective transition metals, the Cr 3+ cation has a special electronic configuration, which enhances charge transfer and electron capture [33].Particularly, the use of nickel within the LDHs structure gained substantial interest from the scientific view due to its electrochemical reactivity, especially towards the electrocatalytic oxidation of alcohols [34].Typically, nickel has a one-electron reversible redox couple, Ni(III)/ Ni(II), which is desirable for its electrocatalytic activity.Recently, Y. Wen et al. used NiCr-LDH, for the first time, as an efficient and stable bifunctional catalyst for the electrocatalytic oxidation and reduction of water [33].Y. Zhao et al. explore a family of visible-light responsive MCr-LDHs (M = Cu, Ni, Zn), synthesised by a scale-up method (SNAS), which displays remarkable photocatalytic activity for RSB, Congo Red, chlorinated phenol and salicylic acid sodium under visible-light irradiation [35].Y. Zhao et al. study the efficiency of M II M III -LDH (M II = Mg, Zn, Ni, Cu; M III = Al, Cr) nanosheets photocatalysts, for the photoreduction of N 2 to NH 3 in water at 25°C under visible-light irradiation [36].
In this work, we successfully synthesised two composites NiCr-LDH/BiOCl and NiCr-LDH/TiO 2 for photocatalytic degradation of Rhodamine B (RhB) under direct sunlight.The morphology, structure and optical properties of the synthesised composites were determined.Moreover, the photocatalytic activity of the composites has been studied and a mechanism of degradation of RhB was proposed.

Synthesis procedure
NiCr-LDH powder was prepared using the co-precipation method at room temperature.In typical synthesis, a mixed metal salts of Ni(NO 3 ) 2 • 6H 2 O (0.6 mol/L) and Cr(NO 3 ) 3 • 9H 2 O (0.3 mol/L) with Ni 2+ /Cr 3+ molar ratio of 2 were dissolved in 100 mL deionised water.At pH = 10, the resulting mixture was added dropwise, under vigorous stirring, to 100 ml of aqueous solutions of Na 2 CO 3 (0.2 mol) and NaOH (0.1 mol).The pH of the solution was adjusted by adding NaOH (3 M).Then, the resulting mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 80°C for 24 h for hydrothermally treated.The precipitated product was centrifuged, washed several times with deionised water and dried at 65°C for 24 h to obtain the NiCr-LDH [20].
The powders of NiCr-LDH/TiO 2 nanocomposites were obtained by the following procedure: 3 g of NiCr-LDH was suspended in 20 ml of ethanol, and an amount of terrabutyl titanate, dissolved in 10 ml of ethanol, was then added to NiCr-LDH and stirred for 24 h to obtain a homogeneous suspension with Ti 4+ /Cr 3+ ratio equal to 2. The obtained mixture was stirred in water bath at 80°C for 4 h, and transferred to an autoclave at a temperature of 180°C for 24 h.Finally, the solid reaction product was collected by filtration, rinsed with distilled water and ethanol several times and dried at 85°C in an oven for 24 h [28].
The NiCr-LDH/BiOCl nanocomposites with Bi +3 /Cr +3 ratio equal to 2, were obtained according to the following steps: firstly, 3 mmol of Bi (NO 3 ) 3 • 5H 2 O was dissolved in 30 ml of diethylene glycol and this solution was added dropwise to 50 ml of NiCr-LDH aqueous dispersion with magnetic stirring.Then, 3 mmol of NaCl dissolved in 10 ml of diethylene glycol was added at this mixture.After continuous stirring for 30 min, the mixture was hydrothermally treated at 180°C for 12 h.After the hydrothermal treatment, the solid product was separated from the solution by centrifugation and washed with distilled water and ethanol several times to remove the soluble salts.Finally, the solid was dried at 65°C for 24 h [28].
The pure phases of BiOCl and TiO 2 were synthesised by the following steps: For BiOCl, the same procedures used in the synthesis of NiCr-LDH/BiOCl nanocomposites was followed except adding the LDH material, and in a typical synthesis of TiO 2 : 3 mmol of tetrabutyltitanate was dissolved in 30 mL of ethanol, the mixed solution after 30 min of magnetic stirring was treated hydrothermally at 180°C for 24 h.The reaction product was collected by centrifugation, washed by deionised water several times and dried in a vacuum oven at 65°C [28].

Photocatalytic experiment
The catalytic potential of the nanomaterials was assessed through the degradation of RhB dye, as probe reaction, under sunlight irradiation and dark conditions.In typical process, 0.1 mg of catalyst was dispersed in RhB dye solution (50 mL, 5 mg/L) at pH = 7 and temperature 298 K. Prior to degradation experiments, the suspension was stirred in the dark for 30 min to ensure the establishment of adsorption/desorption equilibrium between the catalyst and RhB dye.Then the sample was exposed to sunlight irradiation and at regular time intervals, the sample was immediately centrifuged to remove catalyst particles, and RhB supernatant solution was analysed using HACH (DR) 5000 spectrophotometer.The percentage of dye degradation is calculated as follows [14]: where C 0 and C t are the concentrations (mg/L) of dye at initial and specified time (t).
The XRD patterns of synthesised NiCr-LDH/TiO 2 and NiCr-LDH/BiOCl nanocomposites (Figure 1) revealed the presence of all the characteristic diffraction peaks of the hydrotalcite phase such as (003), (006), (012), (015), (018), (110) and (113).The diffraction peaks of TiO 2 anatase were also observed in NiCr-LDH/TiO 2 [39].The structure of the NiCr-LDH was not affected by the incorporation of TiO 2 particles, but there is small change in lattice parameters with an increase in particle size (8.73 nm).For the second nanocomposite (Figure 1(b)), all the characteristic peaks of BiOCl were observed along with the LDH phase.Moreover, the intensity of the characteristic peaks of NiCr-LDH in the nanocomposite was reduced in comparison with those of LDH sample indicating the successful loading of BiOCl onto the LDH phase [25,29].
The FT-IR spectra of synthesised materials are shown in Figure 2. The broad band observed at 3441 cm −1 is attributed to the stretching vibration of the OH groups and the interlayer H 2 O molecules.The weak band at 1620 cm −1 is assigned to the bending vibration of interlayer H 2 O molecules in NiCr-LDH.The band at 1359 cm −1 , which appears strong is due to the ν 3 mode of the interlayer carbonate anions (CO 3 2-) [20].As shown in Figure 2(a), the FT-IR spectra of pure BiOCl shows a strong peak at about 528.33 cm −1 which is attributed to the ν(Bi-O) vibration of the chemical bond in BiOCl molecule [40].
For NiCr-LDH/BiOCl composite, the same intensity of ν(Bi-O) was observed indicating the presence of BiOCl in the composite.In the case of pure TiO 2 , the peak localised at 725 cm −1 was attributed to the Ti-O bond vibration [41].The absorption bands at 3344 cm −1 and 1636 cm −1 of NiCr-LDH /TiO 2 nanocomposite (Figure 2(b)) can be assigned to -OH of water molecules in the interlayer and physically adsorbed water.These two bands are also present in the FT-IR spectra of NiCr-LDH/BiOCl [38].The vibration bands at 733-512 cm −1 region are assigned to Ni-O, Cr-O and Ti-O chemical bond vibration [39].Moreover, the stretching vibration of carbonate anions CO 3 2− was located at 1354 cm −1 [21].UV−visible diffuse reflectance spectra of prepared photocatalysts are shown in Figure 3.For pure NiCr-LDH (Figure 3(a)), the spectrum is characterised by three absorption band regions located in the range 200-300, 300-500, and 500-700 nm.The first region could be assigned to the LMCT, involving transfer from O2p to Ni 3d t2g and from O 2p to Cr 3d t2g orbitals.The oxygen (O 2p) and Ni 2+ / Cr 3+ 3d t2g orbitals are usually defined as the valence and conduction bands, respectively.While the second region involving transitions from Ni 3d t2g to Ni 3d eg and from Cr 3+ 3d t2g to Cr 3+ 3d eg orbitals due to the transitions of the Ni 2+ and Cr 3+ ions respectively, in an octahedral field [27,40].The third region was due to the transition of Ni 2+ -O-Cr 3+ to Ni + -O-Cr 4+ generated from the induced MMCT (Metal to Metal Charge transfer).The oxo-bridged bimetallic LDH system can act as a photoinduced centre for the photocatalytic activity of LDH [27].Furthermore, the Ni 2+ ion which has a partial filled 3d orbital can have a significant role in increasing the photocatalytic efficiency [24,41].As shown in Figure 3, the NiCr-LDH/TiO 2 nanocomposite, has the same absorption bands compared to the NiCr-LDH, but a new strong absorption band has been observed at 300 nm.This new band corresponds to the electronic transmission O 2p → Ti3d t 2g .While for NiCr-LDH /BiOCl, a new less intense band appeared at 307 nm corresponding to the electronic transmission Cl 3p → Bi 6p [42].
The band gap energies (Eg) of a semiconductor material were calculated by fitting the absorption data with Kubelka−Munk absorbance and Tauc's plot via the following equation [43]: where α is the absorption coefficient, h is the Planck constant, ν is the energy of incident lights, A is an arbitrary constant, and Eg is the band gap energy of a semiconductor material.In Tauc's expression, the value of n signifies the kind of optical transition of a semiconductor (n = 1/2 for direct transition and n = 2 for indirect transition).The values of Eg for different materials can be determined by extrapolating the linear portion of the curve (Figure 3(b)).According to Figure 3(b), the Eg values of NiCr-LDH, NiCr-LDH/TiO 2 , NiCr-LDH/BiOCl, TiO 2 and BiOCl were calculated to be 3.38, 2.14, 2.5, 3.1, and 3.32 eV, respectively.
The values of the valence band energy (EVB) and the conduction band energy (ECB) of a semiconductor could be determined by the following equations [44,45,46]: Where χ is the absolute electronegativity of the atom semiconductor, which is the geometric mean of the electronegativity of each atom in the semiconductor.Herein, the electronegativity of an atom is the arithmetic mean of the atomic electron affinity and the first ionisation energy, and E e is the potential energy of free electrons in the standard hydrogen scale (~ 4.5 eV), E g is the bandgap energy.The photophysical parameters are shown in Table 1.
The valence state of elements and the chemical composition of synthesised phases are examined by X-ray photoelectron spectroscopy (XPS).The total survey of XPS spectrum of NiCr-LDH/BiOCl nanocomposite shown in Figure 4(a) confirms the existence of the elements Ni, Cr, C, Bi, Cl and O.
In Figure S2 (b) the XPS analysis of pure NiCr-LDH exhibited two peaks located at 856 and 879.13 eV attributed, respectively to Ni 2p 3/2 and Ni 2p 1/2, and two other satellite peaks situated at 862 and 874 eV.Furthermore, the presence of a satellite peak in the main line of Ni 2p indicates the presence of Ni cations in an octahedron coordination [NiO 6 ] [47,48].In Figure S2 (c), the two peaks corresponding to Cr 3+ ions located at 577.4 eV and 986.2 eV are assigned to Cr 2p 3/2 and Cr 2p 1/2 [49].For O 1s, the binding energy value of 531.53 eV can be attributed to the hydroxyl ions in the LDH material (Figures S2 (d)) [50].The C 1s XPS spectra (Figure S2 (e)) shows two peaks localised at 284.5 and 288.3.The first peak at 284.5 eV is assigned to the C-C coordination of the surface adventitious carbon.The second peak at 288.4 eV can be attributed to the inter-layer carbonate groups of the LDH [50].
In NiCr-LDH/TiO 2 nanocomposite, the XPS analysis exhibited two peaks corresponding to Ti 2p 3/2 and Ti 2p 1/2 localised, respectively at 458.3 eV and 464.6 eV.This result is compatible with Ti 4+ species in pure TiO 2 synthesised (Figure S3).For O 1s XPS spectra, the peaks at 529.4 and 532.4 eV indicate the presence of oxygen with different chemical states.The first peak was attributed to the oxygen of the lattice in the TiO 2 anatase structure, and the second peak was assigned to the oxygen species absorbed, and the proportion of the surface group -OH.(Figure S3) [51].In NiCr-LDH /BiOCl composite, two binding energy peaks localised at 164 eV and 158.9 eV are, respectively, assigned to Bi 4 f 5/2 and Bi 4 f 7/2.The Cl 2p XPS spectra shows two peaks at 197.4 eV and 199.1 eV, belonging respectively to Cl 2p3/2 and Cl 2p1/2 states.In O 1s spectrum, three peaks appeared at 529.3 eV, 530.6 eV and 531.4 eV, are attributed to Bi-O bonds in [Bi 2 O 2 ] 2+ slabs in BiOCl, Cl-O bonds and OH bonds of adsorbed water molecules, respectively (Figure 4).
Figure 5 shows the SEM images of BiOCl, TiO 2 , NiCr-LDH/TiO 2 and NiCr-LDH/BiOCl nanocomposites.As can be seen from Figure 5(a), the SEM image of BiOCl is composed of many sheets stacked together with smooth surface with 39 nm in thickness.The SEM image of TiO 2 nanoparticles (Figure 5 (Figure 5(d)) shows an irregular aggregation of the nano-sheets with BiOCl nano-flowers dispersed on the surface.

Photocatalytic activity of the synthesised materials
The photocatalytic studies of the prepared catalysts were tested against the degradation of rhodamine B under sunlight irradiation.The blank experiment shown in the Figure 6(a), indicated that about 24.6% of rhodamine B was eliminated by self-degradation.The adsorption test of the dye on the surface of the catalysts in the absence of light (dark conditions) was also checked.During the adsorption process, only a small amount of the dye was adsorbed by the nanocomposites.The adsorption efficiencies of NiCr-LDH, BiOCl, and NiCr-LDH/BiOCl were respectively, 14.88%, 28.67% and 20.45% and a negligible adsorption was observed for TiO 2 and NiCr-LDH/TiO 2 .During the sunlight irradiation for 60 min, the photodegradation rates for NiCr-LDH/BiOCl nanocomposite, BiOCl, TiO 2 and NiCr-LDH/TiO 2 nanocomposite, were respectively: 96.57%, 98.99% 96.45% and 89.58%.However, in the case of bare NiCr-LDH, only 26.09% of dye was degraded during the catalytic process.Hence, the catalytic activity of the nanocomposites was improved compared to non modified LDH phase.The reasons for this progress could be referred to the following parameters like morphology, surface area of BiOCl and TiO 2 nanoparticles dispersed on the LDH surface, the heterojunction and the interfacial charge transfer in the NiCr-LDH/BiOCl and NiCr-LDH/TiO 2 nanocomposites, and the effective reduction of recombination of photo-induced electrons and holes.However, several researches revealed that the photocatalytic activities of the nanocomposites were not dependent to their Bi and Ti contents.According to the literature, the photocatalytic activity of BiOCl-NiFe-LDH nanocomposite is higher than the pure BiOCl, this fact was not related to their Bi contents [25].The same study revealed that about 93.3% of degradation of rhodamine B under visible-light was obtained by BiOCl-NiFe-LDH in 120 min [25].Previous report with ZnAl-LDH/TiO 2 composite showed that only maximum 65% of rhodamine B dye could be degraded in 300 min [31].This shows that the prepared NiCr-LDH/BiOCl and NiCr-LDH/TiO 2 nanocomposites had better photocatalytic performance.To better understand the photocatalytic degradation reaction of Rhodamine B, the experimental data can be studied by applying the pseudo-first-order expression written as [45]: where C 0 represents the initial concentration of RhB, C t is RhB concentration at t instant and k is the apparent kinetic constant (min −1 ).The results were given in Figure 6 and Table 2. From Figure 6(b) and Table 2, it can be seen that the photocatalytic activity of NiCr-LDH/TiO 2 and NiCr-LDH/BiOCl nanocomposites towards the degradation of RhB were much higher than in the presence of NiCr-LDH pure phase.For the Figure 6(b), it was obvious that the decrease of the rate constant k from NiCr-LDH/BiOCl to NiCr-LDH/TiO 2 is observed, this result revealed that the surface of LDH modified by BiOCl exhibited improved sunlight photocatalytic activity.
Another aspect for this enhancement in the catalytic activity under sunlight are the morphology of BiOCl deposited on the LDH and the formation of electrons and holes on the surface of the photocatalyst which contributes a lot to a higher rhodamine B degradation [52,53].
In order to understand the effect of the catalyst dose on the photocatalytic degradation of rhodamine B dyes, several experiments were conducted by varying the mass of the catalyst at fixed dye concentration of 5 ppm. Figure 7 shows the percentage of dye as a function of the dose of the photocatalyst.As can be seen from Figure 7, the degradation efficiency increases with increasing the mass of the photocatalyst up to 0.1 g, while it decreases when this value is exceeded.This enhancement in the degradation efficiency can be explained by the fact when increasing the amount of catalyst up to 0,1 g, this leads to an increase in the number of active sites responsible for this improvement.Beyond the mass 0.1 g, there is a decrease in the efficiency of the photocatalytic degradation and this could be attributed to the saturation of the active sites due to the adsorption of all the RhB molecules on the surface of the catalyst.The increase in the amount of the photocatalyst particles also increases the darkness of the suspension, thus representing an inhibitory obstacle to the emission of the light field [54][55][56].Therefore, 0.1 g of the photocatalyst was selected as the optimal dose for the degradation of rhodamine B dye under the present experimental conditions.

Regeneration
The regeneration experiments have been realised to confirm the stability of the NiCr-LDH /TiO 2 and NiCr-LDH/BiOCl nanocomposites during the catalytic reactions.The experiences were carried out with the same ratio of catalyst and dye concentration.The recovered catalyst was washed several times with methanol and distilled water and dried in an oven at 60°C before reuse.
As seen in Figure 8, after four uses, the rate of photodecoloration was almost stable around 89.6% for NiCr-LDH/TiO 2 and 97.9% for NiCr-LDH/BiOCl after 60 min of reaction.This means that both catalysts have shown good catalytic performance without losing their effectiveness during four cycles.

Effect of different scavengers in the photocatalytic degradation
In these experiments, we have performed some reactions with scavengers in order to better understand the nature of the main active species that are responsible in catalytic event such as superoxide radicals ( − O 2

•
), hydroxide radicals (HO • ), holes (h + ) and photogenerated electrons (e − ). Figure 9(a) and (b) shows the role of different scavengers used to identify the active species in photodegradation by the nanocomposites under sunlight irradiation.After addition of Na 2 EDTA (as an h + hole scavenger), the rate of discoloration was decreased to 49% after 60 min of irradiation for NiCr-LDH/TiO 2 phase, and was attained 60.55% for NiCr-LDH/BiOCl phase.Moreover, about 60.8% and 49.52% degradation was observed in the presence of ethanol (as a HO • radical scavenger) for the NiCr-LDH /TiO 2 , and NiCr-LDH/BiOCl, respectively.And in the third reaction with ascorbic acid used as superoxide ( − O 2 • ) scavengers, about 77.

Photocatalytic degradation mechanism of RhB over the nanocomposites
According to the photocatalyst performance, a probable illustration for the enhanced photocatalytic mechanism under sunlight irradiation of the NiCr-LDH/BiOCl heterogeneous can be suggested and described in Scheme 1.The band gap energies of BiOCl and NiCr-LDH are deduced as 3.For NiCr-LDH/TiO 2 nanocomposite, the photocatalytic mechanism can be conducted by the same pathway as it is mentioned for NiCr-LDH/BiOCl.The only change in the mechanism is due to the VB potential of TiO 2 (2.69 eV vs. NHE), which is not sufficient to generate • OH radicals from the water molecules H 2 O/ • OH (+2.8 V/NHE) but is high enough to convert OH − to • OH radicals as the VB of the TiO 2 is more positive than the oxidation potential of OH − / • OH (+1.7 V/NHE).The CB potential of TiO 2 (-0. ) can be effectively able to degrade the RhB molecules.This funding was in good agreement with the results obtained for the effect of scavengers on the photocatalytic activity of both nanocomposites under sunlight irradiation.

Conclusions
Nano-heterostrcutured materials NiCr-LDH/BiOCl and NiCr-LDH/TiO 2 have been successfully elaborated via a co-precipitation and hydrothermal methods using a 2:2 molar ratio of Cr 2 + /Bi 3+ and Cr 2+ /Ti 4+ and their structure, morphology and optical properties were analysed by XRD, SEM, UV−vis, XPS, and FTIR techniques.The catalytic activity of the nanocomposites was evaluated by the degradation of Rhodamine B (RhB) dye under sunlight irradiation during 60 min.The results showed that the sequential catalytic performance was conducted as in the following order: R LDH-BIOCl > R LDH-TiO2 > R LDH .The high level of dispersion of TiO 2 and BiOCl pure phases on the surface of LDH was considered as the main factor which contributed to the high catalytic performance of the nanocomposites.The dispersed phases on the surface of LDH enhanced photocatalytic activity of the photocatalysts by the formation of heterojunction between LDH and BiOCl or TiO 2 which contributed efficiently in the separation of photoinduced electrons and holes.The major active species are the hydroxyl radicals and holes.The recycling experiments, show that the photocatalysts had high stability and reusability even after four cycles.The photocatalytic mechanism was mainly conducted by synergetic effect between hydroxyl radicals and holes.

Figure 6 .
Figure 6.(a) Photodegradation of RhB dye using different materials(b)the kinetic plots of the photocatalytic degradation.

Figure 7 .
Figure 7. Photodegradation of Rhodamine B dye as a function of irradiation time using different dose of materials.
4% of degradation achieved by NiCr-LDH/TiO 2 and 89.28% by NiCr-LDH/BiOCl during the same irradiation time.The photocatalytic degradation of rhodamine B on both nanocomposites was delayed by the presence of Na 2 -EDTA, ethanol and ascorbic acid.The results strongly indicated that h + , and HO • were the active main species involved in the photodegradation of rhodamine B. The superoxide − O 2• was less active in the photodegradation of rhodamine B.
7 and 3.32 eV, respectively.The transition of Ni 2+ -O-Cr 3+ to Ni + -O-Cr 4+ generated from the induced Metal to Metal Charge transfer (MMCT) in
4 eV) is more negative than the standard reduction potential of O 2 /O 2 •− (−0.28 V/NHE), thus the photoinduced electrons at the CB of TiO 2 can directly reduced O 2 into O 2 • − .These highly reactive species created over both materials (h +,• OH, O 2 •−

Table 1 .
E g , χ, E CB , and E VB for the nanomaterials.

Table 2 .
Kinetic parameters of photocatalytic degradation of rhodamine B over different materials.
NHE).In the contrast, the photoexcited holes in the VB of BiOCl can subsequently oxidise the H 2 O to • OH radicals, as the VB of the BiOCl is more positive than the oxidation potential of H 2 O/ • OH (+2.8 V/NHE).These highly reactive • OH species are able to oxidise or mineralise the RhB dye to CO 2 and H 2 O.Alternatively, to impulse additional reactive charge carriers and ROS radicals, the RhB degradation over NiCr-LDH /BiOCl micro-heterojunctions might occur via self-photosensitisation process.The energy band gap of RhB molecules (2.37 eV) is able to absorb sun visible light photons.Upon sunlight irradiation, the electrons of RhB are promoted to its excited state (LUMO level) to generate the excited electrons.Firstly, the excited electrons on the LUMO level, acting as an electron reservoir, can be transferred to the CB of BiOCl and then accumulate in the CB of NiCr-LDH owing to the more positive energy levels of the CB edges of BiOCl (0.023 eV) and NiCr-LDH (0.72 eV), compared with LUMO level (−1.42 eV) of RhB dye.The enriched excited electrons on the CB of NiCr-LDH are not able to capture the dissolved O 2 to O 2 •− due to the thermodynamic inconsistency of the reduction potential of O 2 /O 2 •− and CB of NiCr-LDH, as discussed in the direct photocatalysis process.On the Scheme 1.The degradation mechanism of RhB dye over BiOCl/NiCr-LDH.otherhand, the VB potential of BiOCl(3.28eV vs. NHE) is more positive than that of H 2 O/ • OH (+2.8 V/NHE) and OH − / • OH (+1.7 V/NHE), and thereby, the photoinduced holes can effectively transform H 2 O and hydroxyl anions to • OH radicals.Moreover, as the LUMO level (−1.42 eV) of RhB is more negative than the standard reduction potential of O 2 /O 2 •− (−0.28 V/NHE), the abundant excited electrons at the LUMO level can directly reduce the dissolved O 2 to O 2 •− , which further degrade or mineralise the RhB molecules.