Ni doped ZnS nanoparticles as photocatalyst: Can mixed phase be optimized for better performance?

Accepted Manuscript Title: Ni Doped ZnS Nanoparticles as Photocatalyst: Can Mixed Phase be Optimized for better Performance? Author: Ekta Shah Jayraj V. Vaghasiya Saurabh S. Soni C.J. Panchal Priya S. Suryavanshi Mukesh Chavda Hemant P. Soni PII: DOI: Reference:

S2213-3437(16)30391-8 http://dx.doi.org/doi:10.1016/j.jece.2016.10.031 JECE 1311

To appear in: Received date: Revised date: Accepted date:

2-9-2016 22-10-2016 28-10-2016

Please cite this article as: Ekta Shah, Jayraj V.Vaghasiya, Saurabh S.Soni, C.J.Panchal, Priya S.Suryavanshi, Mukesh Chavda, Hemant P.Soni, Ni Doped ZnS Nanoparticles as Photocatalyst: Can Mixed Phase be Optimized for better Performance?, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.10.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ni Doped ZnS Nanoparticles as Photocatalyst: Can Mixed Phase be Optimized for better Performance? Ekta Shah,[a] Jayraj V. Vaghasiya,[b] Saurabh S. Soni, [b] C. J. Panchal,[c] Priya S. Suryavanshi,[c] Mukesh Chavda[d] and Hemant P. Soni * [a] [a]

Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara- 390 002, Gujarat, India.

[b]

Department of Chemistry, Sardar Patel University, V.V.Nagar, Gujarat, India.

[c]

Department of Applied Physics, Faculty of Technology and Engineering, The Maharaja Sayajirao University of Baroda, Vadodara-390001, Gujarat, India.

[d]

Department of Applied Physics, Polytechnic, The Maharaja Sayajirao University of Baroda, Vadodara, Gujarat, India.

*Author for correspondence: [email protected] Ph. +91-0265-2795552

Graphical TOC

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We have demonstrated that on doping the type of semiconductor can be changed and also phase change can be induced which directly affect the photocatalytic activity of the material.

Highlights 

Amount and location of dopant are crucial factors for photocatalytic performance.



The type of semiconductor (n or p) matters.



Photocatalytic activity of ZnS NPs is correlated with crystallinity and type of semiconductor.



Reversal of semiconductor form n- to p- type is achieved by doping excess Ni2+ ions.



At 10% dopant level, the phase change initiated and local pockets of wurtzite phase formed among the zincblende phase. The mixed phases show enhanced photocatalytic efficiency.

Abstract ZnS nanoparticles with Ni2+ as dopant ions, have been synthesized and characterized by X-ray Diffraction, Inductive Coupled Plasma-Atomic Emission spectroscopy and Transmission Electron Microscopy. UV-Vis absorption and photoluminescence spectroscopies were employed to investigate different photo-processes occurring on exposure to uv-vis light. The synthesized

2

NPs were evaluated as photocatalyst for the removal of the color of aqueous solution of rhodamine B (RhB) dye as test solution under 250 W High Pressure Mercury Vapor lamp. MottSchottky, Cyclic Voltammetery, and hot-probe measurements suggest the change in the ‘type’ of semiconductor from n to p as well as phase change (cubic to wurtzite) take place on increasing the amount of dopant to 10 %. The newly appeared phase forms ‘local pockets’ in an overall cubic matrix of ZnS NPs, this way created defects at the local grain boundaries of dissimilar phases. These provide active sites for the adsorption of dye molecules, in turn, improve the efficiency of the material as photocatalyst.

KEYWORDS photocatalyst; doping; n-type semiconductor; p-type semiconductor; phase change; crystallinity.

1. Introduction Industrialization is one of the markers of progress for any country. Although it generates employment and other opportunities, nevertheless, it also poses plenty of problems related to natural resources that need to be tackled. Water pollution is one of such problems as many industries dump its effluent into the natural water bodies. These harm the environment and aquatic lives. Various physical, chemical, and biological methods are available to treat polluted water before being released outside. Physical methods (like absorption, adsorption, membrane technologies) and chemical methods (like coagulation, flocculation) have limitations in that these can only concentrate and settle the waste. On the other hand, biological methods (using a specific bacterial culture) demand extra care and special conditions, besides cost. Destructive techniques such as incineration of the concentrated waste or chemical oxidation lead to additional problem of air pollution. The type of treatment adopted by an industry depends on many factors such as its size, production rate, public or private sector involvement and last but not the least profit consideration. Many innovative and low-cost techniques are proposed and implemented nowadays to tackle such problems.

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Photocatalysis by semiconducting material is one of techniques to abate water pollution [1]. Besides being effective and low-cost, it also offers almost complete mineralization of the pollutants. When exposed to a light of suitable wavelength, it absorbs energy sufficient to excite electrons in HOMO of the valence band (VB) to LUMO of the conductance band (CB) and generate electron-hole pairs in the respective bands. These charge carriers have a natural tendency to recombine either in a radiative or in a non-radiative manner. The photocatalytic efficiency of the material depends on the longevity of these charge carriers and the dynamics of these charge carriers is governed by thermodynamic and kinetic parameters. For example, defects due to anion or cation vacancies (lattice defects), impurities present at interstitial sites, shallow and deep surface states, etc., play an important role in trapping the charge carriers before recombination, in turn, extending their longevity [2]. These isolated electrons and holes have a thermodynamic tendency to diffuse toward the surface and initiate redox processes [3]. Amongst these charge carriers on the surface, electrons are easily available to the adsorbed molecules (pollutants or dissolved oxygen present in water) leading to generation of free radicals – a reactive species which can initiate a sequence of secondary chain reactions. On the other hand, the holes present on the surface oxidize the adsorbed molecules and generate cationic species capable to initiate a sequence of secondary chain reactions. Another parameter, which controls the photocatalytic efficiency, is the band-gap between valence band (VB) and conductance band (CB) of the semiconducting material. The band-gap can be tailored according to end-applications by different methods [4]. For example, it can be increased by decreasing the particle size of the material and, at nano domain, becomes definite and discrete [5, 6]. The band-gap can be decreased by introducing suitable dopant ions, particularly from transition metal or lanthanide group. Judiciously selected dopant ions add extra energy levels between the VB and the CB of the host [7, 8]. The physical properties of the semiconducting material can be tuned by reducing the particle size along with appropriate doping. Many research groups are engaged in improving the photcatalytic performance of semiconducting materials; for instance, Schneider’s group recently developed Cu-doped ZnS quantum dots and coupled it with anatase TiO2 nanoparticles to form TiO2/Cu:ZnS nanocomposite. They observed an enhanced photocatalytic performance compared to either pure TiO2 or undoped TiO2/ZnS nanocomposite [9].

Choi’s group

synthesized ternary nanohybrid composite, ZnS–Ag2S wrapped with reduced graphene oxide (RGO) using hydrothermal method without any surfactant and evaluated the photocatalytic

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ability using the oxidation of Rhodamine B (RhB) under simulated sunlight irradiation. The superior photocatalytic ability of ZnS–Ag2S–RGO compared to bare ZnS, was ascribed to an efficient charge transfer from ZnS to Ag2S and graphene sheets. The recyclability results also demonstrated the excellent stability and reliability of the ZnS–Ag2S–RGO [10]. Recently, L. Wang et al. synthesized Mn doped ZnS microspheres with hexagonal crystal system and evaluated their photocatalytic activity in terms of H2 production from water and reducing Cr6+ under visible light irradiation. They attributed the enhancement in activity to the expansion of light absorption and the increase in life-time of photogenerated carriers [11]. S. Li. et al. synthesized CdS nanowires and generated shallow surface states by light Ni2+ doping. Using Surface Photovoltage Spectroscopy (SPV), Transient Photovoltage (TPV) measurement and photoluminescence (PL) spectroscopy, they demonstrated that these shallow and deep surfacestates make the photogenerated charge carriers separate, leading to the enhanced photocatalytic efficacy of the material [12]. It was observed that photocatalytic performance of the semiconducting material is boosted in presence of co-catalyst [13], by entrapping the material in polymeric film [14] etc. Zinc Sulphide (ZnS) is a semiconducing material from II-VI class. It finds application in phosphor, light emitting diode (LED), sensor and laser [15, 16]. It has a band-gap of 3.5 - 3.7 and 3.7 - 3.8 eV for zincblende and wurtzite phases respectively [17]. Advantage of using ZnS semiconductor is that it can be synthesized easily by simple wet chemical method and at nano level, its particle size, surface and other physical properties can be easily controlled using capping ligands like polyvinylpyrrolidine (PVP), mercapto propanoic acid, acrylic acid, methacrylic acid, etc [18, 19]. Pouretedal et al. reported ZnS nanoparticles (NPs) doped with Mn2+, Ni2+, and Cu2+ as nano-photocatalyst for the degradation of colored pollutants [20]. They showed that transition metal ions (dopants) enhance the efficiency of the photocatalyst. Bang et al. reported hexagonal Ni2+ doped ZnS NPs as promising photocatalyst [21]. Chen et al. reported the solvo-thermal synthesis of well-dispersed ZnS nanorods with efficient photocatalytic properties [22]. Chen J et al. synthesized various Ni doped ZnS photocatalysts by hydrothermal method and used for photocatalytic reduction of CO2 to produce methyl formate in methanol. They concluded that Ni-doped ZnS nanoparticles exhibit improved performance for forming methyl formate due to trapping of charge carriers on the surface of the photocatalyst, dopant Ni2+

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could act as a temporary trapping site of photoinduced electrons and greatly suppressed the recombination of electron-hole pairs [23]. In this study, our strategy is first to modify and control the band gap in ZnS semiconductor by reducing the particle size to nano level (quantum dots) making it possible to generate electrons and holes (charge carriers) as separate as possible. Secondly, the separated charge carriers can be trapped at pre-constructed shallow and deep surface states that are produced by dopant Ni2+ ions. In general, it is extremely difficult to obtain metastable phase of ZnS (Wurtzite) in a pure single crystalline form by controlling reaction parameters. Thermodynamic factors favor stable zincblende phase of ZnS on crystallization. The bulk ZB ZnS can be transformed to the WZ phase above 1020 °C [24]. The phase-transition temperature decreased with decreasing particle size, however, it is difficult to bring it down to 200 °C and below [25]. Instead, in an attempt to synthesize ZnS NPs in pure single crystalline WZ phase by solution route method (which favors ZB phase) [26], using either by doping or using biological molecules (bolaamphiphilic peptides) as templates [27], it may be possible to form WZ subdomains within ZB matrix (polytypism) [28]. We have demonstrated that transition of the phase from zinc blande (ZB) to wurtzite (WZ) can be initiated on increasing the percentage of dopant ions in the host matrix. By this way, mixed phase in a sole material can be generated. We propose that the grain boundaries of these mixed phases can trap the charge carriers and enhance the photocatalytic activity. The photocatalytic activity of the synthesized NPs has been evaluated using aqueous solution of RhB, as a test solution, under uv-vis radiation from HPMV lamp.

2. Experimental 2.1 Material Analytical grade zinc acetate dihydrate (Zn (CH3COO)2·2H2O), nickel acetate dihydrate (Ni (CH3COO)2·2H2O), Sodium sulfide Na2S.xH2O, Rhodamine B (RhB), terpthalic acid (S. D. Fine Chemicals Ltd., India), 2-Mercaptoethanol (HSCH2CH2OH), sodium hydroxide (NaOH) - Loba Chemie, India were used without further purification.

2.2 Method

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In our previous study, we had reported the synthesis of ZnS NPs with different weight percent of Ni doping [29]. Briefly it can be described as: solutions of Zinc acetate (0.025 mol dissolved in 25 mL methanol) and nickel acetate (1.386 x 10-3 mol dissolved in 25 mL methanol for 5 %) were mixed under continuous stirring followed by addition of mercaptoethanol (0.025 mol). The pH of the mixture was maintained to 10.2 using an aqueous solution of NaOH (1 M) throughout the reaction. Then, the temperature of the reaction mixture was raised to 70 oC and aqueous solution of sodium sulfide (25 mL, 0.025 mol) was injected immediately. The reaction mixture was continuously stirred for 3h to form a homogenous stabilized dispersion at the same temperature. After 3h, the mixture was cooled and centrifuged at 8000 rpm. The precipitates were washed with deionised water several times and then dried at 100 oC to obtain a ZnS:Ni2+ powder sample. Other samples of ZnS NPs with different dopant concentration were prepared by taking nickel acetate concentration 2.778 x 10-3 mol (for 10.0 %). Similarly, pristine (without dopant) sample of ZnS NPs was also synthesized. Photocatalytic efficiency of the synthesized material was evaluated under uv-vis light from 250 W HPMV lamp [30]. The lamp was placed inside the double wall glass tube surrounded by circulating cold-water. The whole assembly was kept in another glass cylinder of the photoreactor containing reactants and magnetic needle. Subsequently, 400 mL of aqueous solution of RhB (10-4 M, 47.902 ppm) was mixed with 5 mg synthesized NPs at RT, pH 6.5 and thoroughly mixed in dark for 60 min. Subsequently, the mixture was transferred to the photo-reactor (Fig 1). At every 10 min, the sampling (5 mL) of the irradiated solution was carried out up to 180 min. Then, the solution in photo-reactor was centrifuged at 10,000 rpm to remove the catalyst. The concentration of RhB present in the solution was determined spectrophotometrically. 2.4 Characterization The X-ray powder diffraction (XRD) analysis of the samples was carried out by using a Guinier X-ray powder diffractometer (Bruker D8) with Cu Kα radiation, λ=0.15418 nm. Particle size was determined by TEM (Philips Tecnai 20). The compositional analysis of the powder samples was carried out by using Inductive Coupled Plasma-Atomic Emission spectroscopy (ICP-AES) analysis (Perkin-Elmer Optima 3300 RL). The samples were degassed at 80 oC prior to Brunauer–Emmett– Teller (BET) measurements. The BET specific surface area (SBET) was

7

determined by nitrogen adsorption (Micrometrics ASAP 2020, USA) via a multipoint BET method using the adsorption data in the relative pressure (P/Po) range of 0.0–1.0. Desorption isotherm was used to determine the pore-size distribution using the Barret– Joyner–Halendar (BJH) method, assuming a cylindrical pore model. The nitrogen adsorption volume at the relative pressure (P/Po) range of 0.997 was used to determine the pore volume and average pore size. The solid sample was dispersed in water, sonicated for 10 min to record UV-Vis absorption spectra on a Perkin Elmer Lambda 35 UV-Vis spectrometer. PL spectra were recorded on a Jasco FP-6300 spectrophotometer using Xenon lamp as an excitation source at 280 nm. All measurements were made at 25oC in deionized water. The MS and CV studies were performed using Electrochemical Analyzer (CHI600E, CH instruments, Inc., Austin, TX). The semiconductor electrode on ITO glass was used as a working electrode. Before making a layer of ZnS NPs (pristine as well as doped), the ITO glass was degreased for 10 min using a surfactant solution, followed by rinsing with distilled water and alcohol for 15 min in an ultrasonic bath. The ZnS NPs (pristine or with dopant) were suspended in an organic polar solvent (ethanol: DMSO, 1:0.2) and the resultant solution was spin-coated on clean substrates at 3000 rpm for 60 sec, and the spin-coated film was then kept in an oven at 50˚C for 20 min. A saturated calomel electrode (SCE for CV and MS) was used as reference and platinum wire was used as a counter electrode. The aqueous solution of KCl (0.1 M) was used as a supporting electrolyte. The cyclic voltammetry was carried out at 25 mV/s scan-rate. The type (n/p type) of the semiconducting material was determined by the hot-probe method.

3 Results and discussion The X-ray diffraction patterns of the synthesized ZnS and ZnS: Ni2+ NPs are shown in Fig 2. The XRD peaks are broad as compared to bulk ZnS due to the nano regime [31]. Three major peaks at 2θ values of 29.06, 48.37, and 57.42o correspond to (111), (220), and (311) planes, respectively. It has been observed that out of these three main features, the peaks corresponding to (311) merged in background. The XRD patterns for pristine and 5 % ZnS NPs samples are well-matched with the standard cubic ZnS (for pristine and 5 % Ni2+ doped, JCPDS card No. 790043). At 10 % concentration of Ni+2, the broad peak corresponded to (111) planes in cubic

8

phase, became distorted and (101) shoulder became more intense along with the (002) and (100) peaks. This distortion of peaks clearly indicates that phase changes from cubic sphalerite to hexagonal wurtzite (JCPDS card No. 79-2204 for 10 % Ni+2). On the basis of the Full Width at Half Maximum (FWHM) of major XRD peak (111) and the Debeye-Scherrer formula [32], the average crystallite size was calculated and in the range of 4-5 nm. Further, the XRD pattern reveals that the pristine ZnS NPs are amorphous in nature. On the other hand, on increasing the amount of dopant, the crystallinity increases. At 10 % Ni2+ concentration, the pattern is distinct and manifests the cubic phase. This behavior can be explained in terms of the readjustment of lattice structure to minimize the potential energy. As the size of Ni2+ (72 pm) and Zn2+ (74 pm) is almost similar, on doping, Ni2+ ions may fill the vacant Zn2+ sites and thereby decrease the substitutional lattice defects [33].

The amount of dopant ions in the samples are analyzed using ICP-AES analysis shown in Table S1. The nitrogen adsorption–desorption isotherms for synthesized ZnS NPs, their calculated BET surface area and pore volume are presented in Fig. 3 and Table 1. It can be observed from Fig. 3 that the isotherms of pristine ZnS NPs and that with 10 % dopant are of type IV (BDDT classification) with H4 type of hysteresis loops in which the two branches are nearly horizontal and parallel over a wide range of p/p°. The quantity of gas adsorbed, in this case, increases with increase in relative pressure (P/Po). The Type H4 loop is often associated with narrow slit-like pores [34]. The sample of ZnS NPs with 5 % Ni as dopant shows isotherm of type IV (BDDT classification) having hysteresis loop of H2 type in the range of 0.5 < P/Po< 1.0 (Fig. 3B). This suggests the presence of pores with narrow necks and wide bodies (ink-bottle pores) in a mesoporous material. The adsorption average pore width (4V/A by BET) values indicate that the pristine ZnS NPs are mesoporous while those with 5 and 10 % dopant possess both meso as well as macropores [35]. It can be observed from Table 1 that on doping, the BET surface area decreases initially, however, it increases gradually on raising the amount of dopant. The BET surface area and pore size studies reveal that the material is suitable for catalytic activities.

9

The morphology of the nanoparticles is studied by using TEM analysis (Figure 4). The ZnS NPs (both pristine and doped with 10 % Ni) are almost spherical having a size in the range of 80 -120 nm. The inset of the figure (SAED pattern) supports the observation from XRD that the sample having 10 % dopant is more crystalline than the pristine ZnS NPs. The UV absorption spectra (200 to 400 nm) of pristine and ZnS: Ni2+ samples in aqueous solution are shown in Fig 5. The absorption spectrum of pristine ZnS NPs reveals a sharp onset followed by sharp edge indicates the narrow particle size distribution. The absorption edge displays a blue shift (270 nm) compare to the bulk ZnS (335 nm) due to quantum confinement effect [36] indicates broadening of the band gap and discreteness of the bands. This increases the longevity of charge carriers at room temperature resulting in sharp band-edge transition in the UV region. All spectra show absorption maxima on an average at 270 nm. Dustan et al. explained these bands due to excess Zn2+ ions [37]. It can be seen that on increasing the amount of Ni2+ (up to 5 %) ions the intensity of absorption at 270 nm increases. This is due to the bandto-band electronic transitions. However, at 10 % dopant concentration, the intensity of the same is reduced due to crystallinity of the material [33]. At 10 % dopant level, the Ni2+ ions not only provide extra energy levels but also fulfill the lattice defect sites. This leads to an increase in crystallinity whereby the host’s energy levels become discrete resulting into a decrease in bandto-band transition in this region. XRD results also support this argument. The band gap was calculated using the modified Kubelka-Munk function. The direct band gap energy was obtained from a plot of (αhν)2 vs photon energy (hν) using the relationship [38] (𝛼ℎ𝑣)2 = 𝐴(ℎ𝑣 − 𝐸𝑔 )

(1)

where, hν is the photon energy, α, the absorption co-efficient (α=4πk/λ, k is absorbance, λ is wavelength in nm), Eg, the band gap energy, and A, a constant. The value of direct band gap energy was determined by extrapolating the straight-line portion of the curve to the hν-axis (Table S1). Direct band gap increases from3.68 eV (bulk) to 4.43 eV (pristine NPs) and then decreases to 4.29 eV on doping Ni2+ ions. The decrease in the band gap on doping indicates that Ni2+ ions provide extra energy levels between VB and CB of host [39]. Figure 6 shows the PL spectra of synthesized NPs with excitation wavelength of 280 nm. The PL spectrum shows two predominant peaks, one in the violet region at 330 nm and the other in the blue region at 425 nm. In case of pristine ZnS NPs, the peak at 330 nm has high intensity due to radiative recombination

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processes between electrons (in CB) and holes (in VB). The peak at 425 nm is due to defect sites (absence of Zn2+ or S2- ions in ZnS lattice sites) and radiative transition of electrons from shallow trap states (ST) near the CB to sulfur vacancies (Vs) near the VB [40]. On doping ZnS with 5 % Ni2+, the intensity of PL peak at 330 nm increases while at 425 nm decreases. This observation indicates that doping of the impurity ions produces more defects and provides the extra energy levels between VB and CB. This results into faster radiative recombination processes [29]. The almost quenching of the peak at 425 nm with 10 % dopant concentration indicates trapping of electrons in ST leaving free holes in the VB [41]. The charge carriers trapped into such defects decrease their recombination rate and enhance longevity. Generally, this situation is favorable for photocatalysis. Due to separation of electrons and holes for a longer time have a thermodynamic tendency to diffuse towards the surface, which can further be exploited for photocatalytic reactions [42]. Hence, from this study it is established that tuning between crystallinity and defects is possible by varying the amount of dopant. Optimum performance in term of photocatalytic activity can be achieved by controlling the amount of impurity in the host material. 3.4 Photocatalytic performance of pristine ZnS and Ni doped ZnS NPs. Photocatalytic activity of synthesized NPs was evaluated spectrophotometrically in term of change in absorption intensity (λmax at 555 nm) of RhB in aqueous solution. The % decolorization efficiency of samples was calculated as

% 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 100 ×

[𝐴𝑜 –𝐴] 𝐴𝑜

= 100 ×

[𝐶𝑜 – 𝐶] 𝐶𝑜

(2)

where, Ao, A, Co, and C are initial absorbance, absorption after irradiation at various time intervals, initial concentration of solution, and concentration of dye after irradiation at various time intervals, respectively. The color of the aqueous solution of RhB removed negligibly (due to the presence of •OH free radical generated from water) in 180 min when irradiated under uv-vis light. Fig 7 shows the time-dependent uv-vis absorption spectra of RhB during photo irradiation in presence of synthesized ZnS NPs at every 10 min time interval and Fig 8 compares the color removal efficiency of the same (see Figures S1 and S2). The decrease in intensity of λmax at 555 nm indicates removal of the dye under experimental conditions. It is observed (Figure 7a) that pristine ZnS NPs remove the color of RhB slowly within the given period (upto 120 min) and at the end of 120 min, it removes almost 65 % of the dye. The 5 % Ni doped ZnS NPs removes the

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color of the dye same as pristine ZnS, however, after 10 min a sudden fall of absorbance from 0.280 to 0.175 a.u. is observed (Figure 7b). Almost 62 % of the dye was removed within the first 50 min. In case of 10 % Ni, the same amount of dye is removed within first 20 min and almost 70 % dye at the end of 80 mins (Fig 7c). The photo removal rates of RhB for all the samples are compared in Fig S3. The plot of ln Co/C vs irradiation time is a straight line and passes through the origin (Figure S3 inset) indicates the first-order decay kinetics. The calculated rate constant (k) for pristine, 5 and 10 % Ni doped ZnS NPs are 0.00788, 0.0483, and 0.0548 min-1, respectively. It is obvious from the results that photocatalytic efficiency of ZnS NPs for removing the color of the dye increases at 10 % dopant level. In order to test the stability of the synthesized nanoparticles as photocatalyst, same reaction was carried out under similar reaction conditions for three cycles. We found no significant change in catalytic activity among these cycles. 3.5 Mechanism of photocatalytic activity Semiconductor nanoparticles act as light harvesting material. On exposure to light of wavelength equal to the band gap energy, the electrons present in VB excite to CB and produce h + and e- in VB and CB of the host, respectively. These charge carriers should have sufficient life-time to diffuse towards the surface and initiate oxidation/reduction reactions with the surface adsorbed reactant molecules before involving in the radiative and/or non-radiative recombination processes [41, 43, 44]. The role of dopant ions is important here: they provide extra energy levels and this way reduce the band gap of the semiconductor and bring it to the visible or near UV region, increase/decrease surface defects, increase the shallow or deep trap states, act as reaction sites for substrate molecules on the surface, etc. In the present work, it is observed from optical and kinetic studies that 10 % Ni doped ZnS NPs show highest photocatalytic activity. Chauhan et al. evaluated the photocatalytic activity of Cu and Fe doped (3, 5 and 10 mol %) ZnS NPs using methylene blue dye. They concluded that 3 mol % of dopant concentration was required for the optimum performance of the catalyst [45, 46]. From these studies, it is proved that an optimum dopant concentration is required for better performance of the catalyst [47]. The present work shows that optimum performance is achieved at 10 % Ni concentration (actual 5.24 wt % according to ICP-AES data) in ZnS NPs. The ability of a dopant to act as a trap site or to intervene the recombination of charge carriers depends on its location in the host (at substitutional or interstitial sites, on the surface or in the depth of the material), their actual

12

amount in the host, particle size, etc. Figure 9 describes the mode of interaction of RhB with the surface of nanoparticles. Zn or Ni ions present on the surface may coordinate by vacant d orbital with electron-rich oxygen atoms or aromatic ring of RhB (by π bond-metal interaction). At nanoscale, the high surface to volume ratio increases the probability of a dopant to be present on the surface. This makes the interfacial diffusion of charge carriers easier and thereby resulting in a faster reaction rate [48].

To understand the effect of dopant ions on the Fermi level of the host semiconductor and in turn, mechanism of the photocatalytic activity, Mott-Sckottky (MS) measurements were carried out. For the purpose, the space-charge capacitance of the semiconductor surface was measured by varying the applied potential bias across the semiconductor in contact with electrolyte surface. Flatband potential was calculated from the plot of reciprocal of square of the space charge capacitance (CSC) vs applied potential bias using the MS equation [49]

1 𝐶𝑆𝐶2

2

= 𝜀𝜀

0 𝑁𝑑

[(𝐸 − 𝐸𝐹𝐵 ) − 𝐴2

𝑘𝑇 𝑒0

]+𝐶

1

𝐻

(3)

2

where CSC is the space charge capacitance of semiconductor, ε dielectric constant of ZnS, ε0 the electric permittivity in vacuum, e0 the elementary charge constant, Nd the donor density, A surface area, k Boltzmann constant, T the absolute temperature, E the electrode applied voltage, Efb the flat band potential and CH, the Helmholtz (double layer) capacitance of electrolyte solution. Since the value of double layer capacitance is 2-3 order of magnitude larger than the space charge capacitance, the contribution of 1/CH2 term is negligible for the calculation of total capacitance. The donor density (Nd) was also calculated from the slope of the linear region using the equation [49]

𝑁𝑑 = −(𝑒

2

0 𝜀0 𝜀

)(

𝑑(1⁄𝐶 2 ) 𝑑𝐸

)

−1 (4)

It is known that MS analysis is valid under the charge carrier depletion condition within the semiconductor [50, 51]. In case of n-type semiconductor, depletion region arise (positive charges are dominant on the surface of semiconductor near the semiconductor-electrolyte interface) at

13

potential bias positive of the flatband potential and vice versa for p-type. Due to charge depletion the band edges curve upward in case of n-type semiconductor and downward for p-type semiconductor. Intense electric field exists at the interface due to this. Hence, maximum separation of charge can be achieved on irradiation under this condition, which is favorable for photocatalytic activity. It can be observed from the Figure 10 that for each sample under study, we kept the wide range of applied potential bias (-0.2 to 1.4 V vs SCE) such that transition from depletion to flatband condition can be achieved. It was observed that pH of the electrolyte solution can directly affect the measurement of Efb [52]. Hence, NaH2PO4 buffer was used during the experiment to keep pH constant and 1M KCl was added as supporting electrolyte to increase the conductivity of the solution and maintain the potential difference across the interface. The positive slope of the line (linearly fit in the depletion region) for pristine and with 5 % dopant confirm the n-type nature of the semiconductor. Flatband potential was calculated on extrapolating the straight line to X-axis (1/C2 = 0) and donor density (in case of n-type) or acceptor density (in case of p-type) was calculated using equation 4. These values are tabulated in Table 2. If donor density calculated from this equation is considered as electron density (or negative charge density) then it can be seen that sufficient charge is available per unit volume of the pristine semiconductor surface to form depletion region and the Fermi level was established at -0.006 V (Efb, vs NHE). Now, on doping 5 % Ni in n-type semiconductor the donor density increases to an order of magnitude. This results into intensification of depletion layer and in turn, positive charge of semiconductor near the interface when keep in contact with electrolyte solution. Therefore, the electrode acts as photoanode and does the oxidation of adsorbate. The higher value of Efb (0.194 V vs NHE) in this case shows that more applied potential bias is required to raise the depletion region to the Fermi level which would be equal to redox potential of the solution. On increasing the amount of dopant to 10 %, it can be observed from the Fig. 10 that the slope of the straight line becomes negative suggesting p-type nature of the semiconductor. This is also corroborated from the carrier density data (-1.70 x 1019 per cm3) indicating acceptor (hole) density in semiconductor. This situation requires more potential bias, compared to previous case to achieve the Fermi level from depletion region.

14

The results of the MS analysis can be correlated with those of XRD. We proposed from XRD (and DSC analysis, Ref 26) analysis that at 0 and 5 % dopant level, ZnS exists in zincblende (ZB) phase in which majority of S-2 ions remain exposed on the front of the surface due to which a layer of negative charge is formed on the surface of semiconductor. Now when this type of semiconductor encounters the electrolyte solution, movement of charge between semiconductor and solution takes place. In case of n-type, the electrons will be transferred from electrode into the solution and positive charge exists on the surface of electrode. Hence, depletion region is generated in an open circuit condition which is a characteristic of n-type. On increasing the level of dopant to 10 %, we proposed the presence of pockets of wurtzite (WZ) phase in which Zn2+ ions are present on the faces of crystal lattice and form the layer of positive Zn2+ ions at the surface near the interface. It is possible that high amount of WZ phase exists on the surface due to high surface to volume ratio for NPs. When this semiconductor comes in contact with electrolyte solution, in an open circuit condition, positive holes are transferred to the solution and negatively charged electrons are exist on the surface of semiconductor and form the depletion layer, characteristic of p-type. These situations are picturized in Fig. 11. Now if mixed phases (ZB and WZ) exist under the experimental condition, one more type of interface due to grain boundries among the different phases come into existence. Opposite charges are present at the adjacent grain boundary due to which overall charge depletion is created in the core of the grains. A strong spatial dependence of all mobile charged defects is induced due to formation of potential barrier at these interfaces. These lead to defect profiles that differ from those in the grains outside of the space charge regions [53- 55]. Cyclic voltammograms of the ZnS (with and without doping) NPs as working electrode are shown in Fig 12. It is noted that pure ZnS NPs electrode exhibit cathodic current under negative applied potential. In dark, the negative potential, compared with the flat band potential, generates an accumulation layer and as a result the electrode of ZnS can act as a (dark) cathode that can reduce oxidized species of solution. This behaviour confirms the n-type character of pristine ZnS NPs [56]. As the Ni2+ doping increases, along with cathodic current at negative potential, an additional peak with anodic current at positive applied potential is observed (for 10 % Ni in ZnS NPs). The higher anodic current and the lower cathodic current, signify that 10 % or higher doping with Ni2+ decreases the n-type conductivity and increases the p-type conductivity of

15

semiconductor. Hence, the surface adsorbed dye molecules encounter large number of holes on the surface due to high surface to volume ratio in nano regime. In order to confirm the nature of semiconductor observed in CV analysis, hot-probe analysis was carried out [57]. The semiconductor shows positive potential at the hot-probe terminal for n-type while reverse is true for p-type (Table 3). The analysis shows that pristine ZnS and 5 % Ni doped ZnS NPs are of n-type semiconductors while 10 % Ni doped ZnS NPs of p-type, in agreement with the CV data. This may be due to filling of S2- vacancies by Ni2+ ions that decreases the availability of electrons in ZnS NPs [58]. At 10 % dopant level, the excess Ni2+ ions collected more on the surfaces either at interstitial vacancies or remain adsorbed at residual valances due to the high surface-to-volume ratio, thereby providing sites for dye molecules to be adsorbed on the surface. Further, we could correlate this observation with that of our previous study where we reported that at 10 % dopant concentration, phase change is observed in the material due to lattice strain. We proposed the presence of ‘local pockets’ of wurtzite phase within the overall cubic phase of host matrix. These local pockets may also provide sites for the adsorption of the dye molecules. Dong et al by the density functional theory of first-principles proposed that the distortion of ZnS4 tetrahedron in wurtzite ZnS resulted in the production of spontaneous polarization and internal electric field, which was beneficial for the transfer and separation of photogenerated electrons and holes beneficial to photocatalytic activity [59]. We could not increase the amount of dopant ions more than 10 % (5.72 % actual) by the experimental method reported in this study. This study should be considered as a model, Ni2+ may not be able to reduce the band gap and bring it to the visible region. However, this can be done by changing the dopant ions. Hence, these observations invite further research in this area.

4 Conclusion ZnS NPs (pristine and doped with 5 and 10 % Ni2+) were synthesized by simple wet chemical method. The XRD patterns suggest that at 10 % dopant concentration the phase of ZnS changes from cubic sphalerite to hexagonal wurtzite. In addition, at 10 % dopant concentration, the intensity of PL at 425 nm is diminished and that of at 330 nm increases compared to pristine NPs

16

due to trapping of charge carriers at shallow trap states indicating enhancement of photocatalytic efficiency. The actual amount of dopant and their location in the material are factors that govern the photocatalytic performance. The dopant ions play a role to (1) alter the band gap (2) shift the nature of semiconductor from n to p-type (3) coordinate with organic reactants on the surface and (4) induce phase change in a material, synergistically enhance the efficiency of the catalyst to make the aqueous solution colorless. The present study shows that the character (n/p type) of the semiconductor is an important parameter affecting the photocatalytic activity. At 10 % dopant level, the phase change initiated and local pockets of wurtzite phase formed among the zincblende phase. The mixed phases show enhanced photocatalytic efficiency. The photocatalytic activity of the synthesized NPs was evaluated using aqueous solution of RhB prepared in ‘pure water’, as a test solution, under uv-vis radiation from HPMV lamp. However, the results may change when this system of NPs applied to actual industrial effluent and, also, local pockets of ZB in WZ phase or 1:1 mixture of both phases may give significant results. These invite further research in the field.

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21

Fig. 1. Design of photochemical reactor used for this study for the removal of RhB from its aqueous solution in presence of synthesized photocatalyst.

22

(b)

(a)

(c)

Proposed presence of slight amount of different phase as defect

(d)

(e)

Fig. 2. XRD pattern of ZnS NPs. (a) pristine (b) 5.0 % (c) 10.0 % Ni as dopant ions (d) standard cubic ZnS pattern as per (JCPDS card No. 79-0043) and (e) standard wurtzite ZnS as per (JCPDS card No. 79-2204). Reprinted from, Materials Research Bulletin 48 (2013) 2259–2267, Copyright 2013, with permission from Elsevier Ltd.

23

(A)

(B)

24

(C)

Fig. 3. Nitrogen adsorption–desorption isotherms for (A) pristine (B) 5.0% Ni and with (C) 10 % Ni as dopant in ZnS NPs. (a)

25

(b)

Fig. 4. TEM images with particle size histogram of (a) pristine and (b) doped ZnS:Ni (10 %) NPs (insets shows the SAED pattern).

26

Fig. 5. UV absorption spectra of (a) ZnS undoped (b) ZnS:Ni2+(5.0 %) and (c) ZnS:Ni2+(10.0 %). Reprinted from, Materials Research Bulletin 48 (2013) 2259–2267, Copyright 2013, with permission from Elsevier Ltd.

Fig. 6. Photoluminescence spectra of ZnS NPs samples: (a) ZnS undoped (b) ZnS:Ni2+(5.0 %) and (c) ZnS:Ni2+(10.0 %). Reprinted from, Materials Research Bulletin 48 (2013) 2259–2267, Copyright 2013, with permission from Elsevier Ltd.

27

(a)

Wavelength (nm)

(b)

a:0 min b:10 min c:20 min d:30 min e:40 min f:50 min g:60 min h:70 min i:80 min

0.25

Absorbance(a.u.)

0.20

0.15

0.10

(C)

0.05

0.00

500

550

600

650

Wavelength(nm)

700

28

Fig 7.The photocatalytic removal of RhB dye from the aqueous solution in presence of (a) pristine (b) 5 % Ni2+ and (c) 10 % Ni2+ doped ZnS NPs.

Fig. 8. Comparison of the performance of pristine, 5 and 10 % Ni doped ZnS NPs as photocatalyst for RhB dye with respect to irradiation time under HPMV lamp.

29

Fig. 9. Surface interaction of Ni2+ doped cubic ZnS NPs with RhB dye molecules. The green, red, and blue balls represent Ni, Zn, and S ions, respectively. Pristine ZnS ED1 EN5 5% Ni2+/ZnS EN10 10% Ni2+/ ZnS

12

C-2 (cm4 F-2)

3.0x10

12

2.5x10

12

2.0x10

12

1.5x10

12

1.0x10

-0.2

0.0

0.2 0.4 0.6 0.8 Potential (V vs. SCE)

1.0

1.2

1.4

Fig. 10. Normalized Mott–Schottky plots for pristine, 5.0 and 10 % Ni2+ doped ZnS NPs.

30

Fig.11. Schematic diagram of ZnS NPs as semiconductor electrode and its interaction with electrolyte solution. (a and c) electrode with ZB phase of ZnS NPs and electrode with ZB phase of ZnS NPs in contact with electrolyte solution having 0 and 5 % dopant level (b and d) electrode with WZ phase of ZnS NPs and electrode with WZ phase of ZnS NPs in contact with electrolyte solution. (c) a

Current, 

i

(b)

EN5

(a) ED1

i

-1.5

EN10

c

-1.0

-0.5 0.0 0.5 Potential, V vs SCE

1.0

1.5

31

Fig. 12. Cyclic Voltammogram of (a) pristine (b) 5 %Ni2+ and (c) 10 % Ni2+ doped ZnS NPs as a working electrode in aqueous solution.

32

Table 1. BET surface characterization of as-synthesized ZnS NPs. Parameters

Pristine

ZnS:Ni (5 %)

ZnS:Ni (10 %)

SBET (m2/g)

433.41

49.12

151.87

Average pore diameter (Å)

105.34

55.33

55.26

58.14

49.87

Adsorption average pore width 28.42 (4V/A by BET): Å Pore volume (cm3/g)

0.180

0.0048

0.016

Total volume in pores (cm3/g)

0.45

0.066

0.136

Total area in pores (m2/g)

18.49

22.38

62.22

33

Table 2. Mott-Schottky parameters of synthesized ZnS NPs. % Ni as dopant

Slope x 1012

Donor densities (Nd) cm-3 x 1019

Efb vs. SCE (vs. NHE) V

0

1.460

1.09

-0.25(-0.006)

5.0

0.817

1.94

-0.05(0.194)

10.0

-0.933

-1.70

1.21(1.454)

34

Table 3. Evaluation of semiconductor type by hot-probe measurements. Sr.No.

Sample

Voltage (V)

1

ZnS (pristine)

+0.37a

2

ZnS/Ni2+ 5 %

+0.26a

3

ZnS/Ni2+ 10 %

-0.24b

a: positive potential for n-type semiconductor and b: negative potential for p-type semiconductor

35