Nitrophenol Chemi-Sensor and Active Solar Photocatalyst Based on Spinel Hetaerolite Nanoparticles

In this contribution, a significant catalyst based on spinel ZnMn2O4 composite nanoparticles has been developed for electro-catalysis of nitrophenol and photo-catalysis of brilliant cresyl blue. ZnMn2O4 composite (hetaerolite) nanoparticles were prepared by easy low temperature hydrothermal procedure and structurally characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) and UV-visible spectroscopy which illustrate that the prepared material is optical active and composed of well crystalline body-centered tetragonal nanoparticles with average size of ∼38±10 nm. Hetaerolite nanoparticles were applied for the advancement of a nitrophenol sensor which exhibited high sensitivity (1.500 µAcm−2 mM−1), stability, repeatability and lower limit of detection (20.0 µM) in short response time (10 sec). Moreover, hetaerolite nanoparticles executed high solar photo-catalytic degradation when applied to brilliant cresyl blue under visible light.


Introduction
Environmental pollution has received extensive interest because of the uncertain consequences on human health and living organisms [1,2]. Industrial activity causes many environmental problems by discharging vast quantities of toxic compounds into the environment. Many hazardous waste sites have been created by gathering pollutants in soil and water for many years [3,4]. Therefore inspection of health risky pollutants in water and environment is an urgent demand for pollution controlling option. Thus easy, efficient and low cost processes for the recognition and detoxification of organic pollutants in aqueous solutions are needed to check and protect water resources and food supplies.
Several instrumental techniques exist for the recognition of organic pollutants, but are of less significance on the basis of efficiency and expense. However, sensor technology plays an important role in environmental safety and water treatment [5,6]. Thus for environmental and health safety, it is essential to produce simple, reliable, and inexpensive sensors for the detection of pnitrophenol (p-NP) in water, because nitrophenols are hazardous and toxic pollutants with inhibitory and biorefractory nature and have adverse effect on living organisms. Nitrophenols have been widely used in the manufacture of pesticides, dyes and pharmaceuticals. p-NP is a toxic derivative of the parathion insecticide and is carcinogenic, hazardous, mutagenic and toxic (cytotoxic and embryo-toxic) to mammals [7]. Due to high solubility and stability of p-NP in water, it has been found in freshwater, marine environments and has been detected in industrial wastewaters.
Thus detection and monitoring of nitrophenols are crucial for environmental pollution control and industrial applications. Various chromatographic and spectroscopic techniques have been utilized for the detection and finding of hazardous solvents but their use is limited due to complication and sluggishness. Electrochemical sensors have achieved immense interest in the recognition and quantification of unsafe compounds since it is uncomplicated and rapid operation, response and recognition [8]. The sensitivity and selectivity of electrochemical sensor strongly dependent on the size, structure and properties of electrode materials and thus semiconductor nanostructured materials have received much importance and has extensively been utilized as a redox mediator in various sensors [9].
Photo-catalysis is also one of the low cost processes for detoxification of lethal organic compounds but the lack of active catalysts excluded this process from a wide range of applications. Among different photo-catalysts, TiO 2 and ZnO have proven to act as dynamic photocatalysts and play pivotal roles in the detoxification of lethal organic compounds [10,11]. However these photo-catalysts only encourage photo-catalysis upon irradiation by UV light because they absorb in the UV region. For solar photocatalysis, a photo-catalyst must promote photo-catalysis by irradiation with visible light because solar spectrum consists 46% of visible light while the UV light is only 5-7% of the solar spectrum. Thus it is an urgent demand to develop an active photocatalyst which can promote photo-catalysis in the visible region. Manganese oxides with various crystalline structures have been explored for various applications such as catalysis and electrode materials for lithium cathodes [12,13]. However, to enhance the various properties of manganese oxide to meet the increasing needs for different applications, the features of manganese oxide must be modified. One of the main noteworthy methods to amend the characteristics of these nanomaterials is the introduction of doped materials in the parent system because recently doped metal oxides have shown excellent properties in various applications [14].
Therefore in this study, spinel (AB 2 O 4 ) like zinc doped manganese oxide (spinel hetaerolite) has been synthesized by a simple low-temperature hydrothermal process and characterized by XRD, FESEM, XPS, FTIR, XPS and UV-vis. spectroscopy. From an application point of view, spinel hetaerolite was investigated as a sensor substance for the discovery of p-NP and solar photo-catalyst for the degradation of brilliant cresyl blue under visible light. To the best of our knowledge, this is the first detail of a spinel heterolite as a nitrophenol sensor and solar photocatalyst for brilliant cresyl blue.

Synthesis of hetaerolite nanoparticles
An equi-molar aqueous solution was prepared by dissolving 1.36 g of ZnCl 2 and MnCl 2 .4H 2 O in 100 ml distilled water and further titrated by NH 4 OH solution to increase the pH above 10.0. This high basic solution was then shifted to a Teflon autoclave and kept at 150.0uC for 16 hours in an oven. Finally a black product was obtained by washing the precipitate. The precipitate was dried and calcined at 400.0uC for 5 hours.

Possible growth of hetaerolite nano-particles
In the case of manganese oxides formation, it has been reported that manganese gets reduced to Mn 3+ or Mn 2+ states and generates lamellae structure which then appear in the form of rod like structure to reduce the surface energy [15][16][17]. After hydrothermal process it crystallizes to form MnO-OH nanorods. The calcination of MnO-OH gives Mn 3 O 4 due to dehydration and oxygen out diffusion at high temperature [18]. However the incorporation of a Zn precursor has great influence on the reduction of manganese. It enhances the reduction process of Mn by first reducing to Mn +3 state and further reduce Mn +3 to Mn +2 state which result in the formation of ZnMn 2 O 4 nanoparticles. Schematically probable growth process of ZnMn 2 O 4 nanoparticles is given in Figure 1.

Characterization
JEOL Scanning Electron Microscope (JSM-7600F, Japan) was used to analyze the morphology of the prepared material while the crystallography was studied by X'Pert Explorer, PANalytical Xray diffractometer. The XPS spectrum was recorded in the range of 0 to 1350 eV by using a Thermo Scientific K-Alpha KA1066 spectrometer (Germany). For compositional, optical and degradation studies of the nanomaterial, FT-IR and UV spectrum were record by a PerkinElmer (spectrum 100) FT-IR spectrometer and PerkinElmer (Lambda 950) UV-visible spectrometer. For sensing study, I-V measurements were carried out using an electrometer (Kethley, USA).

Fabrication of chemical sensor
Hetaerolite nanoparticles were coated as a thin film on the surface of a glassy carbon electrode (GCE, surface area 0.0314 cm 2 ) and dried at 60.0uC for 12 hours. Time delaying and response time were 5.0 sec and 10.0 sec, respectively. The sensing ability of the hetaerolite nanoparticle modified GCE was evaluated using nitrophenol in a similar way as we published earlier [19,20].

Physiochemical characterization of hetaerolite nanoparticles
FESEM was utilized to explore the morphology and size of hetaerolite. The morphology of hetaerolite was illustrated by FESEM which are shown in Figure 2 (a-c). It is evident from the FESEM images that the synthesized material is composed of particles having average diameter of ,38610 nm. These nanoparticles were prepared in large quantity possessing a spherical shape.
The crystallographic information of the hetaerolite nanoparticles were corroborated by X-ray diffraction (Figure 2 (d)). All the characteristic diffraction peaks coincided with those for well-crystalline distinct body-centered tetragonally structured of ZnMn 2 O 4 . X-ray diffraction peaks of the samples are in outstanding agreement and excellent accordance with the JCPDS card no. 077-0470 [21]. According to the JCPDS card, the synthesized product is a body-centered tetragonal phase ZnMn 2 O 4 with cell parameters of a = 5.72 Å and c = 9.24 Å and space group of 141/amd(141). All diffraction peaks were only related to ZnMn 2 O 4 without any impurity peaks and thus the synthesized product therefore consist of pure ZnMn 2 O 4 crystals. The size of hetaerolite nano-particles (38 nm) suggested by FESEM was also verified and supported by Scherrer formula.

D~0:9l=b cos h
Where l is the wavelength of X-ray radiation, b is the full width at half maximum (FWHM) of the peaks at the diffracting angle h. XPS was analyzed to study the composition of hetaerolite nanoparticles and the graphs of hetaerolite nano-particles are depicted in Figure 3. Hetaerolite nano-particles XPS spectrum exhibited peaks at 530.6, 642.6, 654.2, 1022.1 and 1045.1 eV which are responsible for O 1 s, Mn 2p 3/2 , Mn 2p 1/2 , Zn 2p 3/2 and Zn 2p 1/2 peaks, respectively. The spin-orbit splitting between Mn 2p 3/2 and Mn 2p 1/2 is 11.6 eV while 23.0 eV is observed among Zn 2p 3/2 and Zn 2p 1/2 . These results are comparable to the values report earliar [18,22]. The above results are consisted with XRD which confirms the synthesis of ZnMn 2 O 4 .
The chemical structure of the hetaerolite nano-particles was also examined by FTIR analysis which was recorded in the range of 400,4000 cm 21 and shown in Figure 4  The photocatalytic performance of a photocatalyst can be evaluated by its optical properties, one of the essential requirements for a photocatalyst. Thus optical properties of hetaerolite nano-particles were scrutinized by using a UV-Vis. spectrophotometer and the spectrum is shown in Figure 4 (b). In UV/visible absorption method, energy band gap of nanomaterials can be acquired by analyzing their optical absorption. UV-vis absorption spectra exhibited a broad absorption peak and showed band gap energy equal to 2.91 eV which is calculated by Tauc's formula. The relation between absorption coefficient (a) and the incident photon energy (hu) is given by the equation [24].

ahu~A(hu{E g ) n
where A is a constant, E g is the band gap of the material and the exponent n depends on the type of transition, n = 1/2, 2, 3/2 and 3 corresponding to allowed direct, allowed indirect, forbidden direct and forbidden indirect, respectively. Taking n = 1/2, we have calculated the direct energy band gap from the (ahu) 1/n vs. hu plots (Figure 4 (c).

Chemical sensing properties
The hetaerolite nano-particles were utilized for the recognition of p-nitrophenol in aqueous media in order to study their chemical sensing properties toward p-nitrophenol [25,26]. I-V technique was used to measure the electrical response hetaerolite nano- particles sensor for p-nitrophenol which is shown in Figure 5 and Figure 6.
I-V curves for the glassy carbon electrode without coating (without hetaerolite nano-particles) and after coating (with hetaerolite nano-particles) were measured and shown in Figure 6 (a). The hetaerolite coated glassy carbon electrode (black square) illustrates not as much current response in contrast to the naked glassy carbon electrode (gray square), which might be attributed to resistance originated by hetaerolite nano-particles along with binders coated on the electrode surface [27]. Figure 4 (b) illustrates electrical reactions of the hetaerolite nano-particles without p-nitrophenol (gray-dotted line) and with 100.0 mL p-nitrophenol (dark-dotted line) in 0.1 M phosphate buffer solution (pH = 7.0). It is observed from the Figure 6 (b) that by adding the target chemical, hetaerolite nano-particles demonstrate a noteworthy enhancement in electrical current that reveals the sensitivity of hetaerolite nano-particles to p-nitrophenol. Consequently by injection of analyte, the augmentation in electrical response shows that hetaerolite nano-particles exhibit a fast and sensitive reply to the target chemical which might be due to speedy redox reaction (electron exchange) and fine electrocatalytic oxidation properties of the hetaerolite sensor. p-Nitrosophenol generally undergo reduction and produces phydroxylaminophenol. In second step, oxidation of p-hydroxylaminophenol takes place which give rise to 4-nitrosophenol and the subsequent reversible reduction.
The influence of p-nitrophenol concentration on the electrical reaction of hetaerolite nano-particles was examined by consecutive addition of p-nitrophenol in the range of 50.0 mM to 1.0 M into 0.1 M PBS solution (pH = 7.0) and the graph is portrayed in Figure 6 (c). Enhancement of the electrical current with rising pnitrophenol concentration is observed which designates that the hetaerolite nano-particles conductivity was enhanced by the increase in the concentration of the target chemical [28]. The mechanism of p-nitrophenol sensing is graphically shown in Figure 5.
Calibration curve (Figure 6 (d)) was plotted from the difference in target concentration. The calibration curve portrays two sensitivity areas; the region at inferior concentrations (physisorption process) is linear up to 0.005 M with correlation coefficient (R) of 0.7599. The sensitivity is determined from the slope of the lesser concentration section of calibration curve, which is 1.500 mA.cm 22 .mM 21 . The linear dynamic range reveals from 50.0 mM to 0.05 M and the detection limit was estimated, based on signal to noise ratio (S/N), to be 20.0 mM. Above 0.005 M concentration the sensor become saturated due to chemisorption method which could be due to the lack of free hetaerolite nanoparticles sites for p-nitrophenol adsorption [28].

Solar photocatalytic performance of hetaerolite
nanoparticles. The solar photo-catalytic performance of hetaerolite nanoparticles was evaluated by degrading brilliant cresyl blue under solar light [29]. In this study two sets of photo-catalytic reaction were carried out utilizing hetaerolite nanoparticles. Irradiation of brilliant cresyl blue under visible light degraded a little amount of dye at pH 7 without the catalyst (hetaerolite nanoparticles) indicating a photolysis reaction. Photo-catalytical degradation of brilliant cresyl blue solution was carried out in the presence of hetaerolite nano-particles under visible light irradia- tion at different pHs and the effects of pH on the photo-catalytic degradation of brilliant cresyl blue studied under solar light irradiation [6]. Hetaerolite nanoparticles showed efficient activity for degradation of brilliant cresyl blue at different pH under solar light irradiation.
An aqueous suspension of brilliant cresyl blue was irradiated with solar light in the presence of hetaerolite nanoparticles and this led to the alteration of absorbance with irradiation time [8]. Figure 7 (a) display transformation in absorption for the photocatalytic degradation of brilliant cresyl blue at different time intervals which showed decline in absorption strength. It is clear that the absorbance at 595 nm steadily reduces with increase in irradiation time. Figure 7 (b) illustrates the transformation in absorbance with change in irradiation time for the brilliant cresyl blue in the presence and absence of hetaerolite nano-particles. Irradiation of brilliant cresyl blue solution in the presence of hetaerolite nano-particles leads to decline in absorption intensity. Figure 7 (c) demonstrates % degradation of brilliant cresyl blue in the presence and absence of hetaerolite nano-particles with respect to irradiation time (min). Degradation (%) graph shows that 27%, 52% and 83% of brilliant cresyl blue is degraded at pH 5, 8 and 10, correspondingly in the presence of hetaerolite nano-particles after 120 minutes of irradiation time while in the absence of hetaerolite nano-particles, no apparent loss of brilliant cresyl blue might be seen.
Hetaerolite nanoparticles were further compared with TiO 2 which is a dynamic photo-catalyst and displayed an imperative role in the detoxification of polutants [9]. However TiO 2 only promotes photo-catalysis upon irradiation by UV light because it absorbs only in the UV region. Hetaerolite nanoparticles promote  photo-catalysis by irradiation with visible light because the solar spectrum consists of 46% visible light while the UV light is only 5-7% in the solar spectrum. Thus it is vital to grow an active photocatalyst which can promote photo-catalysis in the visible region.
3.2. Effect of pH. The influence of pH on the visible light photocatalytic degradation of brilliant cresyl blue was examined in pH range 5-10 and compared with the brilliant cresyl blue degradation at various pH in the presence of hetaerolite nanoparticles (Figure 7). It exhibited the same trend of degradation at all pH but exhibited high photo-catalytic degradation at pH 10 as compared to pH 5 and 8. The results showed that the rate of decomposition of brilliant cresyl blue increases with increase in pH and at pH 10, brilliant cresyl blue was degraded 83% in the presence of hetaerolite nano-particles. The photocatalytic performance of hetaerolite nano-particles was attributed to the surface electrical properties, which facilitate the brilliant cresyl blue adsorption. It is beneficial for the promotion of a visible light generated charge carrier i.e. electron to the surface which leads to formation of hydroxide radical. Thus high pH makes the surface of hetaerolite nano-particles negatively charged and also supports the formation of OH 2 radicals [30]. Both these factors favor the attraction and degradation of brilliant cresyl blue. It is clear from the results that pH has a substantial influence on the photocatalytic degradation of brilliant cresyl blue and hetaerolite nano-particles synthesized by a very straightforward synthesis process exhibit considerable solar photo-catalytic activity towards brilliant cresyl blue degradation. This demonstrates the applicability of the solar photo-catalyst towards organic pollutants and the potential to extend the applications to environmental pollutants.
3.3. Reaction kinetics of photo-degradation. In order to realize the degradation behavior, we studied the degradation pattern of brilliant cresyl blue by Langmuir-Hinshelwood (L-H) model. Langmuir-Hinshelwood (L-H) model well defines the relationship among the rate of degradation and the preliminary concentration of brilliant cresyl blue in photo-catalytic reaction [31]. The rate of photo-degradation was calculated by using Equation 1: Where r represents the degradation rate of brilliant cresyl blue, K r is the reaction rate constant, K is the equilibrium constant, C is the reactant concentration. When C is very diminutive, then KC is insignificant and Equation 1 turns into first order kinetic. Consider preliminary conditions (t = 0, C = C 0 ), Equation 1 turn into Equation 2.
3.4. Mechanism of photo-degradation. In the current study, a heterogeneous photo-catalysis method was employed for the degradation of brilliant cresyl blue. Briefly, when hetaerolite nano-particles were exposed to light having energy the same or superior to the band gap of nanoparticles, the development of an electron and hole pair occurs on the surface of hetaerolite nanoparticles. If charge partition is sustained, subsequently the electron hole pair reacts with brilliant cresyl blue in the presence of oxygen. Hydroxyl radicals (OH N ) and superoxide radical anions (O 2 N2 ) are assumed to be the major degrading mediators (oxidizing species)  and oxidative reactions result in the degrading (oxidation) of the brilliant cresyl blue. The whole mechanism of photo-activity of hetaerolite nano-particles is depicted in Figure 8 [30,31].

Conclusion
Well-crystalline body-centered tetragonal hetaerolite nanoparticles based p-nitrophenol electrochemical sensor has been fabricated. Hetaerolite nano-particles were produced by a hydrothermal process. The featured structural characterizations proved that the manufactured hetaerolite is optically active, wellcrystalline, body-centered tetragonal nanoparticles. Sensing toward p-nitrophenol was executed which showed excellent sensitivity and limit of detection with quick response time. Moreover the solar photo-catalytic property of hetaerolite nanoparticles was utilized in the degradation of brilliant cresyl blue. Thus it is concluded that the hetaerolite nanoparticles are an attractive sensor material and active photo-catalyst for accomplishing a proficient chemical sensor and photo-catalyst for application within water resources and health monitoring.