A facile chemical synthesis of nanoflake NiS2 layers and their photocatalytic activity

A single-phase and crystalline NiS2 nanoflake layer was produced by a facile and novel approach consisting of a two-step growth process. First, a Ni(OH)2 layer was synthesized by a chemical bath deposition approach using a nickel precursor and ammonia as the starting solution. In a second step, the obtained Ni(OH)2 layer was transformed into a NiS2 layer by a sulfurization process at 450 °C for 1 h. The XRD analysis showed a single-phase NiS2 layer with no additional peaks related to any secondary phases. Raman and X-ray photoelectron spectroscopy further confirmed the formation of a single-phase NiS2 layer. SEM revealed that the NiS2 layer consisted of overlapping nanoflakes. The optical bandgap of the NiS2 layer was evaluated with the Kubelka–Munk function from the diffuse reflectance spectrum (DRS) and was estimated to be around 1.19 eV, making NiS2 suitable for the photodegradation of organic pollutants under solar light. The NiS2 nanoflake layer showed photocatalytic activity for the degradation of phenol under solar irradiation at natural pH 6. The NiS2 nanoflake layer exhibited good solar light photocatalytic activity in the photodegradation of phenol as a model organic pollutant.


Introduction
In the last decades, nanostructured transition metal suldes (NTMSs) have received considerable attention in different elds because of their unique optical, magnetic and catalytic properties. 1-3 The properties of these materials are strongly dependent on the dimension, size, and morphologies of fabricated materials, 4-6 making these materials very promising for numerous advanced applications such as adsorbents for dye removal, 7 supercapacitors, 8 rechargeable lithium-ion batteries, 9 hydrodesulfurization catalysts, 10 hydrogen evolution reaction, 4, 11,12 and catalysts in the degradation of organic dyes. 13 Metal sulde materials such as zinc sulde, 14 manganese sulde, 15 silver sulde, 16 iron sulde, 17 molybdenum sulde, 18 nickel sulde, 12 and copper suldes, have been reported and studied extensively. 19 Among the metal suldes, nickel suldes are more favorable in terms of earth-abundant resources, forming numerous phases such as NiS, NiS 2 , Ni 3 S 2 , Ni 3 S 4 , Ni 7 S 6 , and Ni 9 S 8 , which are suitable as alternative materials for different applications. [20][21][22][23] Nickel disulde (NiS 2 ) crystallizes in a pyrite-like structure (FeS 2 ), with a cubic phase Pa 3 symmetry. 10,24,25 Nanostructured pyrite NiS 2 with a cubic structure has interesting optical, electronic, and magnetic properties. 3, 10,26 NiS 2 nanostructures with controlled morphology such as nanoparticles, nanowires, nanosheets and hollow microspheres, [27][28][29] have been considered as promising semiconducting materials for catalytic applications due to their low-cost, nontoxicity and chemical stability. 30,31 However, the catalytic performance of NiS 2 in the degradation of organic pollutants such as endocrine disrupting compounds (EDCs) is still less competitive compared to other catalytic materials based on phosphides and noble metals. 4,13 In this regard, numerous techniques have been used to develop and fabricate nickel sulde nanostructures with good physical and chemical properties including hydrothermal methods, 23,32,33 solvothermal, 34 decomposition of single-source precursors, 35 microwave-assisted synthesis, 36,37 solventless route in air, 38 sonochemical, 39 and ultrasonic spray pyrolysis. 10 Most of these methods are suitable for preparing nickel suldes in powder form with other different phases sometimes accompanying. 22,29,38 The different possible phases of nickel sulde make the synthesis of single-phase nickel disulde very complicated. 20,29,34 Therefore, the demand for an alternative approach to prepare a single phase of nickel disulde layers with a high specic area and uniform morphology is still a major challenge, and will open doors to various opportunities for advanced applications.
Endocrine disrupting compounds (EDCs) such as phenol and its derivatives are a category of dangerous persistent organic pollutants, which are usually present in low concentrations in water environments. Phenol molecules are considered very harmful to human health, marine creatures and living organisms due to their carcinogenic, mutagenic, stability, and bioaccumulation nature, even in low concentrations. Phenolic compounds are discharged to ecology through effluent from many industries for instance, paint production, processing of petroleum, tanning, and pharmaceuticals. [40][41][42] The conventional treatment is not very effective for the removal of these hazardous pollutants. Thus, the development of novel techniques is essential to address this issue. Morphological control is one of the effective approaches for promoting the photodegradation of phenol using NiS 2 with nanoake morphology. One of the big problems in the photocatalyst process is separation and recovery restriction of the photocatalyst from effluent aer the treatment process. Therefore, herein, this problem is overcome through immobilizing prepared nickel sulde on a glass substrate as layers.
In this study, single-phase NiS 2 nanoake layers were successfully processed via a facile two-step fabrication process. First, Ni(OH) 2 nanoake layers were grown on glass substrates by the chemical bath deposition method, followed by the phase transformation of Ni(OH) 2 into NiS 2 via a sulfurization process. Structural, morphological and optical properties as well as the catalytic activity of the obtained NiS 2 layer were studied. The unique nanoake-like morphology of NiS 2 serves as an efficient photocatalyst for the degradation of destructive organic pollutants (phenol as the model organic compound).

NiS 2 layers deposition
First, the chemical bath deposition (CBD) approach was used for the synthesis of the nanoake-structured nickel hydroxide layer. The glass substrates were ultrasonically cleaned using acetone and ethyl alcohol for 20 min followed by distilled water, then dried with nitrogen gas, prior to loading into the reaction bath. 0.1 M aqueous solution of nickel chloride (NiCl 2 $6H 2 O -Sigma Aldrich) and ammonia solution were used as the source of Ni 2+ and complexing agent for layer deposition, respectively. In a typical experimental procedure, the ammonia solution was added drop wise into the nickel chloride solution under continuous magnetic stirring to produce a clear and homogeneous aqueous solution as the starting solution. The precleaned glass substrates were vertically immersed in the solution bath at optimum deposition temperature, (T d ¼ 50 C) and pH 11 during the synthesis process. Aer 2 h, the blue-colored solution changed to a greenish white color with the formation of Ni(OH) 2 layer on the surface of the substrate by the adsorption and nucleation of the nickel cations on the substrate. The as-deposited Ni(OH) 2 layers were transferred into a tube furnace with excessive sulfur powder and subsequently sulfurized at 450 C for 1 h in nitrogen atmosphere to obtain a nickel sulde layer. 43,44 For the ease of understanding, the facile CBD and synthesis process for the nanoake structured NiS 2 layer is schematically described in Fig. 1.
The formation mechanism of the nanoake-structured nickel disulde layer is divided into two processes: rst, the formation of a nickel hydroxide phase via the chemical bath deposition route, as indicated by the following equations: The detailed mechanism of the formation of nickel hydroxide by chemical bath deposition (CBD) can be found elsewhere. 45,46 Second, the transformation of as deposited Ni(OH) 2 layers to nickel disulde 2 due to reaction of Ni(OH) 2 with sulfur atoms at 450 C according to the following reactions:

Characterization of prepared immobilized NiS 2
In this study, the structural investigation and phase identication of the as-prepared NiS 2 layer was analyzed by X-ray powder diffraction (XRD) with a Panalytical X'Pert diffractometer using Cu Ka1 radiation at 45 kV and 40 mA. Scanning electron microscopy (SEM) (QUANTA FEG250) was used for the surface morphology imaging of the obtained layers. X-ray photoelectron spectroscopy (XPS) was collected on K-Alpha (Themo Fisher Scientic, USA) with monochromatic X-ray Al K-alpha radiation at pressure 10 À9 mbar to determine the elemental composition and electronic states of the NiS 2 layer. Raman analysis was performed on a confocal Raman microscope model WITec Alpha 300 RA under the laser excitation of 532 nm. Diffuse reectance spectra were carried out using a UV/Vis/NIR spectrophotometer (Jasco V770) in the wavelength range 250-1000 nm.
Evaluation of the photocatalytic performance of the asprepared immobilized NiS 2 layer The photocatalytic performance of the as-prepared NiS 2 layer was established by photodegradation of phenol as a model organic pollutant. For this purpose, the NiS 2 slide is primarily xed by a silicon adhesive on a 2 cm-height edge inside a 150 mL beaker. Aer that, 90 mL of 10 mg L À1 phenol solution was placed in dark and stirred by a magnetic stirrer for 30 min to achieve adsorption desorption equilibrium. The beaker was irradiated vertically in a solar system (UVA CUBE 400, Dr Hönle AG UV Technology, Germany) equipped with a halogen lamp (model: SOL 500), which is simulated to the natural sunlight (1000 W m À2 ). At denite time intervals, a 1 mL sample was withdrawn from the beaker and 1 mL double distilled water was inserted instead to keep the distance between the light source and meniscus of solution constant all over the experiment duration. Phenol concentration in the withdrawn samples was determined by a high-performance liquid chromatograph (HPLC, Agilent 1260, USA) equipped with an analytical column Zorbax reverse-phase C18 and a diode-array detector at 280 nm wavelength. Each point was measured in triplet and the average was recorded. The column temperature was kept at 25 C during the analysis. Gradient elution was obtained using water (mobile phase A) and acetonitrile (mobile phase B). 75% A mobile phase was eluted for 1 min, and then decreased to 60% A for 2 min. The ow rate of the mobile phase was kept at 0.5 mL min À1 . The generation of redox reactive species by NiS 2 aer solar irradiation excitation was inspected by 1 mmol ammonium oxalate (AO) as the hole (h + ) scavenger agent, 1 mmol para-benzoquinon (p-BQ) as the superoxide radical (O 2 c À ) scavenger and 1 mmol isopropyl alcohol (IPA) as the hydroxyl radical (cOH) scavenger.

Results and discussion
Structural and elemental composition properties The XRD and Raman data for the as-prepared NiS 2 layer are presented in Fig. 2. The XRD pattern of the NiS 2 layer (Fig. 2(a)) show sharp and dominant characteristic peaks of the NiS 2 cubic structure (JCPDS card no. 00-011-0099), 45 with no additional peaks related to any other crystalline nickel compounds such as nickel oxide, nickel hydroxide and other phases of nickel suldes, indicating the complete transformation of the Ni(OH) 2 phase to NiS 2 phase. [47][48][49] The XRD analysis well matched with reported studies in literature. 28,30,31,50 The average crystallite size (D) of the NiS 2 layer was calculated using the Scherrer-Debye formula (eqn (6)) for the (200) reection plane.
where K is the Debye constant, l is the X-ray wavelength, b is the line broadening at full width at half maximum of the diffraction peak, and q is the Bragg's angle. 51 The calculated crystallite size of the NiS 2 layers was approximately 26 nm. The surface Raman spectrum measured at room temperature of the NiS 2 layer (Fig. 2(b)) shows the dominant characteristic peaks of the NiS 2 phase. 20 The peaks at 279 and 476 cm À1 are assigned to E g and A g photons, respectively. The observed peaks shied towards a lower frequency compared with the NiS 2 single crystal. The obtained spectrum shows no noticeable characteristic peaks related to possible secondary phases and is consistent with previous reports. 11 ,28 In order to study the elemental compositions and electronic states of the nanoake NiS 2 layer, X-ray photoelectron spectroscopy (XPS) measurements were performed in this study. Fig. 3(a) presents the high-resolution XPS spectrum of Ni 2p for the nanostructured NiS 2 layer, which has two main peaks appearing at 854.12 and 871.63 eV, tting to the binding energy of Ni 2p 3/2 and Ni 2p 1/2 , respectively. In addition, both Ni 2p 3/2 and Ni 2p 1/2 have shake-up satellite peaks located at 860.1 eV and 875.59 eV, respectively. Peak tting analysis to separate overlapping peaks was made for the Ni 2p 3/2 component, which indicates that it can be de-convoluted into a pair of peaks located at 854.12 and 856.05 eV, corresponding to Ni 2+ and Ni 3+ in NiS 2 , respectively. The existence of Ni 3+ results from the surface oxidation of NiS 2 , which is in agreement with literature. The collected XPS results of the deconvolution of Ni 2p are in agreement with the reported binding energy values for Ni 2+ and Ni 3+ . 4, 36 In addition, the spectral deconvolution of the S 2p spectrum (Fig. 3(b)) consists of two strong peaks at 162.91 (S 2p 3/2 ) and 164.38 eV (S 2p 1/2 ), implying the presence of unsaturated S atoms on the Ni-S and S-S bonds in NiS 2 . These results t well with NiS 2 single crystal XPS data. 22,29,52 Morphological properties The morphology of the NiS 2 layer was investigated by SEM. Fig. 4(a-d) shows the SEM images of the surface with different magnication, and cross section of the NiS 2 layer, synthesized on the glass substrate. The top view images of the as-prepared NiS 2 layer show that the surface of the NiS 2 sample reveals a rough nanoake-like structure, with homogeneous and uniform distribution as well as a pinhole free layer. Moreover, the magnied view images show that the cross-linked nano-akes are compact and uniform, resulting in a network architecture on the substrates. Also, the rough nanoake edges observed clearly in Fig. 4(c) can be associated with the sulfurization process of the as-deposited Ni(OH) 2 layer as a result of gas release and dehydration during annealing, leading to the formation of NiS 2 with a high surface area structure. 44 The high surface area and rough morphology can signicantly inuence the photocatalytic performance of materials. 20,28 A cross-sectional image (Fig. 4(d)) exhibits that the NiS 2 layer has a uniform thickness in the range of approximately 950 nm.

Optical properties
The energy bandgap of NiS 2 was derived from the diffuse reectance of the obtained layer using the Kubelka-Munk (KM) function 53,54 , as indicated by the following equation: where F(R) is the (KM) function, R is the diffused reectance, a is the absorption coefficient, and S is the scattering coefficient. The optical band gap energy (E g ) of the NiS 2 layer can be calculated by the Tauc's equation: 55 where (hn) is the incident photon energy, a is the absorption coefficient, A is a constant, (E g ) is the optical band gap energy. Based on the KM function and Tauc's equation, the optical bandgap energy of the NiS 2 layer can be estimated using the following equation: The plots of (F(R)hn) 2 vs. hn for indirect allowed transition are shown in Fig. 5. It was found that the estimated value of E g for the NiS 2 layer was 1.19 eV, which is in agreement with reported values. 33,56 The low E g value would allow the utilization of this material in photocatalytic applications under solar radiation. 29,57 Photocatalytic activity measurements The photocatalytic activity performance of the NiS 2 sample (5 cm 2 ) was examined using phenol as the model organic pollutant, two NiS 2 samples and at natural pH of 10 mg L À1 phenol. The variation of phenol relative concentration (C/C 0 ) is offered in Fig. 6 with the matching values of the 1 st order apparent rate constants. Phenol presented insignicant  photolysis under solar light. On the other hand, the rate of phenol photodegradation under solar light in the presence of the as-prepared NiS 2 layer was improved. This is due to the presence of the as-prepared NiS 2 slide, which absorbs solar light and photogenerates e À /h + pairs utilized in photodegradation.
The main active species used in pollutant photodegradation are e À , h + , cOH and O 2 c À . The active species produced by NiS 2 are identied in Fig. 7. The active species identication was done by adding 1 mmol of each scavenger agent (AO, p-BQ and IPA) with 10 mg L À1 phenol and NiS 2 layer compared to the experiment done without any scavenger. As shown in Fig. 7, the primary active species is O 2 c À and h + is a secondary species, which are used as redox species in phenol photodegradation. Therefore, the proposed mechanism of the photocatalytic reactions is indicated by the following equations: NiS 2 (e À ) + O 2 / NiS 2 + O 2 c À O 2 c À + h + + phenol / photodegradation product On the other hand, the NiS 2 reusability process is a very important issue, making the treatment process more economical. Fig. 8 shows a ve cycle reusability test for NiS 2 phenol photodegradation. The removal efficiency was slightly decreased aer the rst cycle. Thereaer, there was no change in the phenol removal efficiency aer each cycle.

Conclusion
A NiS 2 layer with a nanoake-like structure was successfully synthesized by a facile two-step growth process. The Ni(OH) 2 layer was deposited on a glass substrate by chemical bath deposition, followed by a sulfurization process to obtain a single phase NiS 2 layer. The XRD and Raman analysis conrmed the formation of single-phase NiS 2 . SEM revealed that the NiS 2 layer consisted of overlapping nanoakes. XPS measurements revealed that the observed peaks from Ni 2p and S 2p spectra were attributed to NiS 2 . The NiS 2 displayed a narrow optical bandgap of 1.19 eV. The NiS 2 nanoake layer showed photocatalytic activity for the degradation of phenol under the irradiation of solar light at natural pH 6. The NiS 2 nanoake layer exhibited good solar light photocatalytic degradation of phenol with good stability and reusability. The as-prepared NiS 2 layer can absorb solar irradiation and generate e À /h + pairs. Hence, the NiS 2 layer is a promising photocatalyst for the photodegradation of destructive organic pollutants.