Synthesis and enhanced photocatalytic property of Ni doped ZnS nanoparticles

Highlights

• Nanocrystalline Ni doped ZnS was prepared by Solid state reaction method.

• Phase; mean crystallite size and lattice parameters were determined by XRD analysis.

• Direct band gap energies were evaluated from UV–Vis analysis.

• From PL, the emission peaks were investigated.

• Ni doped ZnS with high-energy (1 1 1) facet showed good photocatalytic activity.

Abstract

Pure and Ni (0.5–2.0%) doped ZnS nanoparticles were prepared by an inexpensive solid state reaction method. The structural, functional, optical, morphological and chemical compositions of the products were characterized by XRD, FT-IR, UV–Vis, PL, SEM with EDX and TEM analyses. The X-ray diffraction results confirmed that the polycrystalline nature with cubic crystal structure of the nanoparticles. Also, using these data, crystallite size, dislocation density, micro-strain, stacking fault and lattice constant were calculated. The functional group associated with the vibration of a molecule was investigated by FTIR spectroscopy. The optical band gap was increased from 3.58 to 3.97 eV with increasing Ni dopant concentrations. The SEM and TEM images depict the nanosized particles with spherical shape morphology. The elemental composition of Ni-ZnS nanoparticles was examined by EDX analysis. The PL emission spectra show an intensity quenching upon Ni doping and exhibit green emission in the visible region. The photocatalytic activity results indicated that the Ni doping enhanced the photocatalytic activity of ZnS. Thus, Ni-ZnS could be effectively used as photocatalyst for degradation of environmental pollutant Methylene Blue dye.

Introduction

Semiconductor nanocrystals are defined as materials consisting of hundreds to thousands of atoms. This field of material science has rapidly been developing and attracted extensive studies during the past two decades due to their unique structure, electronic, magnetic, and optical properties originated from their large surface-to-volume ratio and quantum confinement (Afzaal et al., 2010, Fang et al., 2011c). Doping of transition metals or rare earth ions is an effective way to adjust the color output of the semiconductor nanostructures, which is important for their applications in LEDs, lasers, etc. (Didosyan et al., 2004, Kikkawa and Awschalom, 1999, Zutic et al., 2004). ZnS is a commercially important II–VI semiconductor having a wide bandgap of 3.67 eV (bulk) and large exciton-binding energy (∼40 meV). A number of reports are available on the magnetic and luminescence properties of ZnS NPs with different transition metals (TM) with many conflicts (Bhargava et al., 1994, Borse et al., 1999, Divya et al., 2011, Kumar et al., 2013, Poornaprakash et al., 2016a, Poornaprakash et al., 2016b, Reddy et al., 2007, Reddy et al., 2014, Sambasivam et al., 2008, Sambasivam et al., 2009). Poornaprakash et al. took Fe, Co, and Ni ions as dopants into ZnS NPs, since these three ions are important TMs with abundant electron shell structures with ionic radius less than Zn, which means that these dopants can easily penetrate into the ZnS crystal lattice (Poornaprakash et al., 2017).

The environmental hazard is mainly associated with toxic and non-biodegradable wastes; which are responsible for different types of pollution i.e. soil, water and air pollution. Amongst this water soluble formulations are more dangerous as they create direct impact on living beings (Encinas et al., 1996). Mainly organic dyes in waste waters coming out from textile and other industries have the major contribution to this. Many dyes are nontoxic themselves but when they mix with water; they easily form highly toxic complexes in the water waste and thus pollute water. For example, when azo dyes dissolve in water their breakdown or intermediate products are benzidine, naphthalene and other aromatic compounds, which are carcinogenic or mutagenic. Approximately, 50–70% of the dyes are aromatic azo compounds and some of the azo dyes and their degradation products such as aromatic amines are highly carcinogenic (Pouretedal and Keshavarz, 2011). Also mutagenesis, teratogenesis, carcinogenesis, respiratory toxicity and reduced fertility in humans have been reported for many of the dyes. The textile industries contain considerable amounts of azo dyes, and huge amount of inorganic salts. So, this issue is needed to be monitored closely and sort out urgently.

In the recent year, photocatalytic processes that decompose organic contaminant into simple inorganic species have attracted high attention to sort out various aqueous environmental pollution issues. II–VI compound semiconductors with direct band gap exhibit high potential as effective photocatalyst due to ability of rapid generation of electron-hole by absorption of photons with energy equal to or more than to its band gap. Such generated electron-hole pairs can produce free radicals in the system to redox the compounds absorbed on the surface of a photocatalyst. The resulting free radicals (OH) are a very efficient oxidizer of organic materials and can degrade pollutants in the medium. Hence, developments of novel II–VI group semiconductors as photocatalyst and tailoring of existence ones for decoloration of toxic organic dyes have received considerable attention. Amongst the various II–VI compound semiconductor, zinc sulfide (ZnS) has been extensively focused for this application due to its high chemical stability, non-toxicity and environmental safety nature. It exists in two crystalline forms, namely sphalerite (cubic phase) with a band gap of 3.66 eV and wurtzite (hexagonal phase) with a band gap of 3.77 eV at room temperature (Acharya et al., 2013, Fang et al., 2011a, Ong and Chang, 2001). The fundamental interest in ZnS is due to the presence of polar surfaces, excellent transport properties, good thermal stability and high electronic mobility, etc. In addition to this, ZnS based system has been widely explored as a photocatalyst, due to its high energy conversion efficiency, the relatively negative redox potential of its conduction band carbon dioxide reduction and splitting for H₂ evolution (Fang et al., 2011b, Wang et al., 2015, Zhang et al., 2011, Zhao et al., 2007). The rate of oxidation and reduction in the photocatalytic activities decide its photocatalytic efficiency.

In order to improve the photocatalytic efficiency, photogenerated electron-hole pair recombination is needed to be delayed. During the past several decades foreign elements (transition metals and nonmetal elements) as dopant have been attempted to develop visible light driven photocatalysis, which introduce impurity levels in the forbidden band and results in the enhanced absorption in visible region. There are various reports on enhancement in photocatalytic efficiency of ZnS by adding metals as dopants e.g. Mn²⁺ (Ashkarran, 2014, Chitkara et al., 2011), Co²⁺ (Tong et al., 2015, Chena et al., 2010), Ni²⁺ (Rajabi and Farsi, 2015b), Cu²⁺ (Chauhan et al., 2014), Cd²⁺ (Jia et al., 2011). However, it is expected that photocatalytic activities drastically decrease as the formation of recombination centres for photogenerated e⁻ and h⁺. Few report claims decrease in photodegradation rate of ZnS with dopants. Still there are dilemmas about the role of dopants in photocatalytic efficiency of ZnS. So the questions whether (i) such doping can be successfully applied to photocatalysis, (ii) the photocatalytic activity is possible in presence of visible light under natural atmospheric conditions; all these queries are still unanswered. This makes us curious to investigate doping effect on ZnS. Rohini and Smita selected the isovalent metals cations Mn²⁺, Co²⁺, Cu²⁺ and Cd²⁺ randomly as dopants. By fine variation of their percentage starting from 1 to 10% at atomic scale, their effect on photocatalytic activities of ZnS are systematically studied for two model dyes Cango Red (CR) and Malachite Green (MG). According to our knowledge it is a first attempt to study photocatalysis activity of doped-ZnS nanoparticles system for CR and MG dye under natural environmental conditions i.e. without irradiating in UV–Vis radiation or intense solar light (Rohini and Smita, 2016).

Lately, semiconductor nanoparticles (NPs) have pulled in an extraordinary interest in view of their size, tunable structural and optical properties emerging of quantum confinement effect. As a standout amongst the most imperative semiconductors zinc sulfide (ZnS) has been exceptionally referred to for quite a while as an adaptable and phenomenal phosphor host material and it has a wide band-gap of 3.8 eV and a small Bohr radius (2.4 nm), which make it a superb contender for investigating the natural recombination processes in dense excitonic systems (Dong et al., 2007, Navaneethana et al., 2010). ZnS is a critical inorganic material for an assortment of potential applications including optical coatings, solar panels, opto-electronic modulators, photoconductors, sensors, transducers and photocatalytic applications. Broad investigation of photocatalytic properties has been performed in different transition metal ions such as oxide and sulfide nanoparticles (Arsha Kusumam et al., 2016, Kiruthiga et al., 2014, Rajabi and Farsi, 2015a, Zeng et al., 2016). In particular, ZnS nanomaterial has attracted wide attention, because of its unique band structure and high capability to decompose the organic pollutants. Hamid Reza Rajabi and Mohammad Farsi have prepared ZnS quantum dots by chemical preparation method, which exhibited a high photocatalytic activity on methylin violet (Rajabi and Farsi, 2016). Chen et al. (2013) have reported that the as-synthesized ZnS rods displayed the largest photocatalytic activity in the degradation of methyl orange. Clearly, the structure of photocatalyst is the fundamental factor to influence the performance, and ZnS photocatalyst with different structures will have potential applications in energy and environment areas. A number of methods have been explored to synthesize ZnS crystals, such as solvothermal synthesis (Chai et al., 2014), thermal decomposition method (Niasari et al., 2010), sol-gel (Bhattacharjee et al., 2002), microwave irradiation (Jiang and Zhu, 2004), co-precipitation method (Thielsch et al., 1996), gas phases condensed (Sanchez-Lopez and Fernandez, 1998) and solid state reaction method (Lu et al., 2004). In the recent past, solid state reaction method has been widely and successfully used as an ideal method to prepare inorganic nanoparticles (Park et al., 2014). In the present work, we have synthesized pure and Ni (0.5–2.0 at.%) doped ZnS nanoparticles using a simple solid state reaction method at low temperature utilizing zinc acetate dihydrate and thioacetamide as precursor materials. The prepared samples are characterized by various analytical tools such as XRD, FTIR, UV–VISIBLE, PL, SEM with EDX and TEM analyses. In addition, the photocatalytic properties of pure and Ni²⁺ doped ZnS nanoparticles have assessed deliberately on degradation of methylene blue dye (model pollutant) under sunlight illumination.