Multifunctional and smart Er2O3–ZnO nanocomposites for electronic ceramic varistors and visible light degradation of wastewater treatment

In this proposed study, erbium (Er3+)-doped ZnO nanocomposites were prepared through the effective, basic, and green combustion method. The significant effects of Er dopants on the structural, morphological features, dielectric, and optical behaviors of the pure ZnO matrix as well as Er2O3–ZnO nanostructured materials were investigated applying X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transformation infrared (FT-IR) spectroscopy, and UV–Vis spectrophotometer techniques. These results showed that the synthesized Er2O3–ZnO nanocomposites are well polycrystalline. The Er2O3–ZnO nanocomposites are almost uniformly distributed on the surface morphologies. Furthermore, UV–Vis diffuse reflectance spectroscopy, AC electrical conductivity, and dielectric properties’ current–voltage characteristics were utilized to examine the influence of erbium doping on the optical properties, energy bandgaps of the proposed Er2O3–ZnO nanostructured powder. The tested nano-samples were applied for the visible light photodegradation of p-chlorophenol(4-CP) and p-nitrophenol (4-NP). The Er-doped ZnO ratio affects the photocatalytic activity of the ZnO matrix. This current research substantiated that more than 99.5% of 4-CP and 4-NP were photodegraded through 30 min of irradiation. Four times, the Er:ZnO nanocatalysts were used and still displayed an efficiency of more than 96.5% for 4-CP and 4-NP degradations in the specified period of 30 min. The as-prepared Er2O3–ZnO nanostructures are considered novel potential candidates in broad nano-applications from visible photocatalytic degradation of waste pollutants to the electronic varistor devices.


Introduction
The nanocrystals are promising candidates to be used as a technological material in different applications, such as bioimaging, solar cells, and tunable lasers, which eventually have a strong dependence on controlling the nanocrystals' properties through adding impurities as a dopant (Michalet et al. 2005). It is reported that the impurities could potentially adjust structure morphology, magnetic, optical, and electronic properties of bulk semiconductor materials. Material science research mostly focuses on different types of semiconductors taking advantage of their exceptional optical and electronic characteristics. Last decades, the research of semiconductor materials with one-dimensional(1D) had engrossed extremely intense attention due to the electrical, physical, chemical, and magnetic characteristics with dimensionality-dependent in nanoscale. The different 1D nanoscale semiconductor has been produced and also investigated, including III-V group, carbon group, II-VI group, as well as oxide group materials (Calarco et al. 2005;Iijima and Ichihashi 1993;Vayssieres 2003;Mor et al. 2006). Among those various unique 1D nanoscale oxides, zinc oxide (ZnO) materials have been considered as a great potential candidate to own a structure of n-type wurtzite, considerable energy of exciton binding with 60 meV, and a broad-ranging optical energy bandgap of 3.37 eV at ambient temperature. The outstanding transparent and piezoelectric properties of ZnO semiconductor materials have extended the range of high technology in various applications, including transparent electrodes and wave filters of surface acoustic (Lee et al. 2004a, b). Both zinc interstitials or oxygen vacancies perform significant roles in n-type semiconductor dopants, mainly affecting chemical and physical properties (Zeng et al. 2010;Djurisic et al. 2010;Unalan et al. 2008;Willander et al. 2009).
In recent decays, ZnO 1D nanostructures (NSs), including nanotubes, nanorods, nanowires (NWs), and nanobelts, are promising candidates to be applied widely in technological applications for nanoscale photoelectronic devices, such as light-emitting diodes (LEDs), piezoelectric nanogenerators, gas sensors, photovoltaic cells, photodetectors, biosensors, and field emission devices Pacholski et al. 2002;Law et al. 2005;Dalal et al. 2006;Hatch et al. 2013;Tseng et al. 2003). The intentional ratios of dopants to ZnO materials as a host matrix encourage dramatic variations in the consistent morphological, transport, electrical, and optical properties. Mainly, the ionic radius and electronegativity of the doping component affect the composite growth process. For ZnO materials, effective doping is reached as Zn +2 ions are substituted with higher valence elements.
Lately, air and water pollutions have been considered as the main environmental issues that required more intensive efforts to motivate the continuous essential practical environmental research. ZnO semiconductors have been recognized to be attractive photocatalysts due to the large efficiency of catalytic, environmental sustainability, and reasonable price (Kamat 1993;Pelizzetti and Serpone 1986). ZnO nanomaterials have been developed as effective catalysts for water detoxification due to the efficient generation of H 2 O 2 , as well as high mineralization and reaction rates. Also, ZnO has a larger amount of dynamic positions with large surface reactivity (Carraway et al. 2004). Although chlorophenols have been utilized in pesticide and antiseptics manufacture, formed in the chlorine bleaching procedure throughout papermaking, shaped throughout the chlorination of drinking water, and manufactured in chemical, textile, and pharmaceutical engineering, they cause mouth burning, lung damage, and headache and disturb the digestive tract as well as the immune system according to the type and dose of exposure (Michalowicz and Duda 2007). Meanwhile, nitrophenols were produced using the phenol reaction and nitrite ions in water with solar light, formed throughout pesticide production and degradation, generated via the electronic and metallurgic manufacturing utilized in solvents, plastic, and dyes production (US EPA 2016).
Many research studies report that doping ZnO 1D-NSs have certainly enhanced its physical properties, including enlarged transparency, improved conductivity, amplified ferromagnetic behavior, and shortened work function (Wang et al. 2013;Yuan et al. 2008;Chu et al. 2007;Ruankham et al. 2011). It is well acknowledged that III group, such as boron (B-), gallium (Ga-), aluminum (Al-), and indium (In-), is considered as the furthermost appropriate dopants for the ntype of ZnO materials, that are found in transparent conducting oxides (TCO) which display great optical transparency and large conductivity (Kim et al. 2014;Xu et al. 2004;Chen et al. 2010a, b;Lin et al. 2007;Bae et al. 2005). Several studies have stated n-type doping of ZnO 1D-NS materials through using titanium (Ti-), hydrogen (H-), and as well as elements of group VII, like chlorine (Cl-) and fluorine (F-) (Chang et al. 2011;Huang et al. 2012;Luo et al. 2011). Nevertheless, it is challenging and unstable to produce p-type dopants of ZnO 1D nanostructured materials due to indefinite mechanisms. However, elements of group V, such as arsenic (As-), nitrogen (N-), antimony (Sb-), and phosphorus (P-), are potentially promising p-type doping for ZnO 1D NSs, where p-type ZnO materials achieved via potentially replacing those elements with oxygen (Lee et al. 2004a, b;Chavillon et al. 2012;Chen et al. 2010a, b). In comparison, group IB atoms, including copper (Cu-), gold (Au-), and silver (Ag-), are used to get p-type ZnO materials, while group I atoms, such as lithium (Li-), potassium (K-), and sodium (Na-), have been reported to replace Zn sites (Xing et al. 2008;Li et al. 2011;Jayakumar et al. 2006;Zhang et al. 2010). Recently, enhancing the luminescence properties of semiconductors has been widely achieved via using rare-earth elements as dopants. It has been reported that ZnO1D-NSs materials as a matrix could be successfully doped with filler elements of rare-earth atoms, such as erbium (Er-), europium (Eu-), lanthanum (La-), and dysprosium (Dy-) (Lo et al. 2011;Wang et al. 2011;Jia et al. 2009;Wu et al. 2006). During the last years, many photodetectors have been prepared using large bandgap semiconductors, like SiCN, diamond, SiC, II-VI compounds, and III-V compounds, where that UV-photodetectors have been fabricated with either p-n junction structures or metal-semiconductor-metal (MSM) (Chang et al. 2003;Yang et al. 2010;Afsal et al. 2012;Hsu and Lu 2012;Nie et al. 2013).
Different researches on ZnO doping have been investigated in several available reports of Ce-, In, and Ga-dopants (Sofiani et al. 2006;Zironi et al. 2012;de L 2003). The ZnO microstructure changes have been considered in detail due to various dopant elements, including Sn-, In-, Al-, Fe-, and Cu-(Paraguay-Delgado et al. 2000). Dopants play significant roles in semiconductor devices, which have encouraged the study of semiconductors' properties and promising applications (Alivisatos 1996). Rare-earthelements-doped semiconductor nanocrystals have unique chemical and physical properties in addition to their special phosphors of high efficiency and low degradation (Rakov et al. 2003). It has been found that II-VI group semiconductors are exclusive host materials for optically energetic impurities doping at ambient temperature (Ishizumi and Kanemitsu 2005).
The rare-earth Er ions-doped ZnO nanostructures can be an extremely multifunctional nanomaterial with simultaneous semiconducting, optical, and electromechanical properties. Among various lanthanides, erbium (Er-) elements gain enormous critical attention, taking advantage of its massive potential in different applications (Divya et al. 2015;Khataee et al. 2015).
Kai Sheng Yu et al. described the actual methyl blue (MB) photodegradation utilizing Er:ZnO photocatalyst below UV light (Yu et al. 2013a, b and c). Also, below UV light, A Khataee et al. grow excellent crystalline of Er-doped ZnO nanocomposites using the sonochemical approach for the purpose of photocatalytic degradation of sensitive orange 29 dye (2015). J. Gao et al. synthesized Er-doped ZnO nanocomposites utilizing the liquid boiling approach, where Er:ZnO was considered as photocatalysts for acid red B dye's photodegradation below sunlight . Up to the current reported scientific research, there are not muchreported studies incomplete clear portrayal regarding effects in morphological, structural, electrical, optical, dielectric, energy bandgaps, and nonlinear optical (NLO) properties of induced Er ions as rare-earth-doped ZnO composited nanomaterials, because of the difficult doping growth processes (Julian et al. 2005;Falcony et al. 1988;Ortiz et al. 1987;Voort et al. 1992;Gu et al. 2004). Additionally, procedure optimization is required to develop a full understanding of doping mechanisms, which is considered one of the main scopes of this current research.
In this proposed research, undoped zinc oxide (ZnO) and erbium-doped ZnO nanocomposites (Er 2 O 3 -ZnO) have been synthesized using an effective, low cost, easy, and ecofriendly combustion technique. The synthesized nano-ZnO has different ratios as a matrix with different concentrations of erbium ions. The morphological, structural, optical, electrical, as well as photocatalytic performances of the Er 2 O 3 -ZnO nanostructured materials were studied. The surface morphology and structure analysis of Er 2 O 3 -ZnO nanocomposites have been characterized by employing a scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier transformation infrared (FT-IR) spectroscopy technique. The optical diffused reflectance and their related parameters of the as-prepared Er 2 O 3 -ZnO nanocomposites were measured via Shimadzu UV-Vis-NIR spectrophotometer in a range of light wavelength between 200 and 1600 nm. Besides, the AC electrical conductivity, dielectric constant, and current-voltage characteristics were investigated in detail. The photocatalytic efficiencies of methylene blue, phenol, and rhodamine B degradations were examined for all synthesized Er 2 O 3 -ZnO nanostructured materials. The photocatalytic efficiency enlarged as the erbium doping concentrations increased.

Growth of Er-doped ZnO nanomaterials
In the proposed work, the main target is to produce ZnO nanoparticles (NPs) doped with different erbium (Er ions) concentrations via the low-cost combustion method. Primarily, 5 g of Zn(NO 3 ) 2 . 6H 2 O was mixed and crushed well in crucibles, made of ceramic, with 1 g of gum acacia. Then, separately adding eight various erbium concentrations, in the range of 0.001-5 g to the earlier mixture, which was labeled as So, S1, S2, S3, S4, S5, S6, and S7, respectively, as shown in Table 1. After that, 5 mL of distilled (DI) water was used to dissolve the previous mixtures. For almost 2 h, the combinations of Er 2 O 3 -ZnO nanocomposite were heated at a temperature of 600°C and then left to be cooled down to the ambient temperature. In this preparation technique, gum acacia has been working as a fuel to support the ZnO structure's change from a highly crystalline structure to a nanoscale structure through increasing their constituents inside the matrix. A handheld hydraulic press was utilized to shape a pellet with a diameter of 13 mm and a thickness of 1 mm. The final product of ZnO nanoparticles doped with erbium at several concentrations is presented in Fig. 1.

Devices and measurements
X-ray diffractometer (XRD) is an effective approach to study the prepared ZnO nanostructure's structural morphology with different erbium doping ratios. In this work, XRD data was attained through Shimadzu LabX-XRD-6000, applying filtered radiation of CuK α (λ = 1.5406 Ǻ) at room temperature. The attained XRD results were matched with the programmed software (pdf-2 library) in XRD Shimadzu, where the diffraction angle (2θ) ranges between 5 and 80°. In this research study, the SEM was JSM-6360 type with a 20-kV operating voltage, which was utilized to examine the surface morphologies of all produced Er 2 O 3 -ZnO nanocomposites. Additionally, the optical diffused reflectance spectra of the produced Er 2 O 3 -ZnO nanostructured materials were measured using a 3600 UV/Vis/NIR spectrophotometer (Shimadzu, Japan), the wavelength ranged from 200 to 1600 nm, the step scan was 5 nm, and the reference sample as barium sulfate. To measure the Fourier transform infrared (FT-IR) spectra of the prepared Er 2 O 3 -ZnO nanocomposites, DXR FT-IR Spectrometer, THERMO SCIENTIFIC, was applied within the wavenumber range from 400 to 4000 cm −1 . The Er 2 O 3 -ZnO nano-samples were subjected to the FT-IR analysis through primarily preparing 100 mg of potassium bromide with finely 2% of the mixtures crushed the Er 2 O 3 -ZnO nano-powders.
At a constant temperature, the prepared Er 2 O 3 -ZnO nanocomposite samples' electrical measurements were accomplished utilizing a computerized digital Keithley 4200-SCS with a broad range of frequency between 3 kHz and 10 MHz. The distinctive circuit of controllable PID was utilized to measure the current-voltage at ambient temperature. The electrical circuit comprises of a pico, digital ammeter that is DPM-111/SVS (Labs Inc., USA) model, a power supply that operates at high voltage from the EHT-11/SVS (Labs Inc., USA) model, and a PID controlled oven that has the PID-200 model. The operating system is connected to a distinct oven, where a holder with double probes is linked to it. In this current research, ZnO doped with various ratios of erbium ions was pressed in 10 tons on a solid disc to have a disc dimension of 1-mm thickness and as well 13-mm diameter.

Photodegradation measurements
The photocatalytic performance and the degradation of typical pollutant p-chlorophenol and p-nitrophenol in various conditions were assessed as a type of colorless pollutant for the synthesized nanomaterials. The visible light irradiation was performed using a wooden visible photocatalytic reactor planned by the group of I.S. Yahia in NLEBA at Ain Shams University (ASU) in Egypt. The irradiation source is 14 lamps (i.e., wavelength greater than 420 nm and power = 18 watts) (Mai et al. 2020). These photocatalytic experiments were used to test Er 2 O 3 -ZnO nanoparticles against phenol derivates. An aqueous solution of 100 ml was formed in water containing 300 ppm p-chlorophenol and p-nitrophenol separately. The phenolic solution was applied for photodegradation using 0.01 g of pure ZnO and various Er 2 O 3 -ZnO loading. For the proper homogeneity of the photocatalyst and the equilibrium of adsorbent desorption, the resulting suspension was stirred for 30 min before light irradiation. At a standard period, 5 ml Fig. 1 The as-prepared Er 2 O 3 -ZnO nanocomposites with a different erbium doping concentration of Er 2 O 3 :ZnO solution was taken from the photoreactor, followed by centrifugation for 10 min at 3000 rpm to remove all powder suspension. After that, UV-Vis spectrophotometry has been applied to analyze the solution. Distilled water was utilized for baseline correction.
In XRD patterns, a small shift in 2θ to the left diffraction angles for the (002) plane has been noticed for the undoped ZnO material. This angles' movement directs the effective integration of erbium doping in the ZnO as well as the existence of the second phase in Er 2 O 3 -doped ZnO nanocomposites as increasing the Er concentrations. XRD analysis showed that the contents of erbium dopant affected the crystalline structure of the combustion Er-doped ZnO nano-powder noticeably and affected the formation of Er 2 O 3 phase, which was started to be found with increasing erbium concentrations. A. Khataee et al. investigated the structural morphology of the Er-doped ZnO nano-powders with different ratios of erbium dopants (Khataee et al. 2015). In this current study, the XRD approach was utilized for investigating both the chemical composition and structural morphology of the combustion synthesized Er 2 O 3 -doped ZnO nanostructures. The found XRD data effectively confirmed the substitution of Er-REE ions in the ZnO lattice. Table 1 illustrates the calculated mean values of the grain size in the scale of nanometers, the dislocation density (η) in (nm) 2 , and the lattice strain (ε) for all phases of the manufactured pure ZnO with different erbium doping levels on the Er 2 O 3 -ZnO nanocomposites. Here, Scherrer's equation was applied to investigate the XRD and determine the crystallinity size (D) of the as-prepared Er 2 O 3 -ZnO nanostructured samples as Furthermore, with the doping difference, the average values of the crystallite size of all prepared ZnO-Er 2 O 3 nanocomposites are changed in the range of 63-26 nm. Those obtained data are in a great match with the published grain size with 15 to 19 nm on average for Pr-doped ZnO nanoparticles, which were synthesized and investigated by D. Venkatesan et al. (2010).
Individually, from the next Eqs. (2) and (3), the average values of dislocation density (η) and lattice strain (ε) of pure ZnO and Er 2 O 3 -doped ZnO nanocomposites were computed (Callister 1997;Pathinettam Padiyan et al. 2002): where the X-ray wavelength (λ) is in the unit of nm, the diffraction angle is θ in the degree unit, and β is the full width at half maximum (FWHM) in the unit of a radian. From the data of both Table. 2 and Fig. 2, it is understandable that the Er 2 O 3 -ZnO phases do not occur in low doping, which was appeared as the dopant concentration was increased. The dislocation density in nanomaterials is an imperfection related to the lattice's misregister in one part of the crystal concerning another part.

Scanning electron microscopy images
Scanning electron microscopy (SEM) was employed to examine the as-prepared erbium-doped ZnO nanostructures' growth topographies. Figure 3 demonstrates the SEM images to investigate the structure morphologies of the studied Er 2 O 3 -ZnO nanocomposites with different erbium doping ratios ranging from 0.001 to 5 g (i.e., So to S7, respectively). The variations in the structure morphology for the undoped ZnO and erbium-doped ZnO nanostructures were noticeable. The SEM images of the Er 2 O 3 -ZnO nanocomposite powders generally displayed the uniform distribution of Er-ZnO spherical nanoparticles. The erbium ions create small-sized grains and promote the crystal nucleation rate, while erbium grains' trapping stops grain development. The cause of this issue might be due to the variation in ionic radius between zinc and erbium. With the increase in erbium doping concentrations, both the shape and the size of the Er 2 O 3 -ZnO nanoparticles differed. As a result, the obtained SEM findings agreed very well with the XRD analysis. The morphology analysis illustrates that the grain sizes' calculated mean values were ranging from 70 to 86 nm for Er-doped ZnO nanostructured materials and around 105 nm for pure ZnO nanoparticles. In this present research, the attained SEM data excellently matched with that published SEM results by K. Yu et al. (2013a, b, c). The structural morphology of ZnO and Er:ZnO was investigated, and the distribution of the grain size was evaluated through Manual Microstructure Distance Measurement software. On average, the particle diameter was between 40 and 60 nm based on the Er ratios, where some of those particles clearly had a spherical shape. The SEM images concluded that the grain size partially declined with increasing the erbium concentrations in ZnO.

Optical characterization of Er-doped ZnO nanocomposites
Fourier transformation infrared spectroscopy FT-IR spectroscopy was employed to examine the considered Er 2 O 3 -ZnO nanostructured samples' vibrational properties since it is an extremely sensitive characterization technique and can contribute to XRD findings. Figure 4 presents the FT-IR spectra, which define the relation between the light absorbance and the wavenumber in the range 400-7000 cm −1 for the Er 2 O 3 -ZnO nanostructured at different Er doping concentrations. It can be noticeable that the erbium doping affects both the shape and the intensity of the main absorption bands and that changes were more obvious in the Er 2 O 3 -ZnO curves than the undoped ZnO. In the FT-IR spectral range, there is an overlapping of IR lines, which contribute to the phases of both ZnO and Er 2 O 3 . There were small improvements in the optical transparency at the low doping concentrations respecting the undoped ZnO sample. With increasing the erbium ratios, the transmittance of ZnO-Er 2 O 3 samples displayed lower values.
The highest doping ratios of Er 2 O 3 (S7) have the maximum absorbance, which excellently agreed with the found results of the structure morphology from both XRD and SEM analysis.  (2016). FT-IR spectra illustrated few peaks located at 501 cm −1 , 1515 cm −1 , and 3621 cm −1 , where the sharp strongest peak at 501 cm −1 confirms the ZnO formation, and the wide peak at 1515 cm −1 is related to OH bond (Sonik Sonik et al. 2016). Also, there was a small and sharp peak presented at 3621 cm −1 , which was assigned to OH vibration stretching mode in Er:ZnO nanocomposites. To conclude, the absorption intensity increased as increasing the Er doping ratios, where that was perfectly matched with these current FT-IR data.

Optical diffused reflectance
The optical diffused reflectance (ODR) measurement is considered a traditional technique used to collect optical data on the proposed Er 2 O 3 -ZnO nanostructured composite powder through the reflectance characteristics. For semiconductor materials, the optical energy bandgaps and the absorption coefficient are two factors that can be achieved from ODR analysis.  Fig. 5, it is obvious that there were no many differences between diffuse reflectance (ODR) of all studied samples in wavelength of the range from 200 to 370 nm. After that, the ODR noticed a sharp increase in the wavelength ranges from 370 to 410 nm, producing the optical bandgap. The diffuse optical reflectance (ODR) spectra demonstrated almost straight curves with no significant changes with the light wavelength between 410 and 700 nm. It was clear that as the increasing Er concentrations ion ZnO matrix, three valleys appeared at around 660 nm, 530 nm, and 480 nm wavelength locations. The intensity of those reflection valleys increased and became sharper as the concentration ratios of erbium increased. The obtained ODR results in this current study were in great agreement with the optical application of Er:ZnO nanoparticles by S. Bhatia et al. using UV-Vis spectrophotometer (UV Vis 2600/2700-Shimadzu) (Sonik Sonik et al. 2016). The illustrated single peak at 537 nm in all Er:ZnO nanocomposites was corresponding to the phase of hexagonal wurtzite.

So
In conclusion, the absorbed light via the deliberate samples produces the optical absorption bandgaps (SowriBabun et al. 2013;Zhang et al. 2008). Figure 6 demonstrates that the studied Er 2 O 3 -ZnO nanostructured samples have a little small absorption index values (k), between 8.5 × 10 −4 and 1 × 10 −3 , due to the non-absorbent surface area of the nano-powders, and the Er 2 O 3 -ZnO nano compounds are predisposed to the incident light. Similarly, this figure illustrates maximum absorption values at a light wavelength of about 380 nm, characteristic for electronic transitions from band to the band (π→π*) (El-Kabbany et al. 2014;Brütting 2006). Kai-sheng Yu et al. optically characterized ZnO and Er/ZnO nanoparticles in the wavelength range of 200-800 nm, where the strongest absorption peak for pure ZnO is located at 363 nm (2013b). The light absorption in the visible region was less compared to the UV range, where the absorption band was because of the electron transition between VB and CB. The FT-IR spectra of Er-doped ZnO nanocomposites illustrated a somewhat redshifted at 375 nm band, indicating the ineffective impurity peak due to Er doping.
The following equations of Tauc's model were used to compute the optical bandgaps E g of the prepared Er 2 O 3 -ZnO nano-powders by (Kumar et al. 2013;Aydın et al. 2013) as where F(R) represents the material reflectivity applying the model of Kubelka-Munk, and R is known that is immediately recorded using the UV-Vis spectrophotometer. The coefficient of molar absorption is known as K, and the scattering quantity is defined as S. In Eq. (6), α is known as the absorption index, t is the material thickness inside the holder, and as well as in Eq. (7), hυ is the photon energy, υ is the photon frequency of electromagnetic (EM) radiation, h is Planck's constant, A is the band tailing factor where its values ranged from 1 × 10 5 to 1 × 10 6 cm −1 .eV −1 (Davis and Mott 1970), and E g is the optical energy bandgap of the studied nanomaterials. In the previous Eq. (7), the values are either n = ½ for direct optical bandgaps or n = 2 for indirect bandgaps transition. Then, the material direct and indirect bandgaps can be while the following equation is expressed the material optical indirect bandgap as Both Figs. 7 and 8 showed the individually plotting of (αhυ) 1/2 and (αhυ) 2 dependent on the photon energy (hυ) for Er 2 O 3 -ZnO nano-powder samples at several erbium concentrations. The direct and indirect optical bandgaps have been respectively found through the intercept line for the curves, in Figs. 7 and 8, to the x-axis where α 1/2 and α 2 reach to zero. As illustrated in Table 3

Dielectric behavior and AC electrical conductivity of Er-doped ZnO nanocomposites
The complex dielectric function ε * (ω) simplifies the material's direct response to electromagnetic radiation (EM) and also controls the propagation behavior of EM waves through the investigated media. As a result, it is critical to characterize the nature and the origin of the dielectric function for the asproposed Er 2 O 3 -ZnO nanostructured by using the following equations (Mansour et al. 2010): and Here, the real component of the dielectric constant is ε 1 (ω), whereas ε 2 (ω) is the imaginary component. The variation of dielectric constant ε 1 (ω) and dielectric loss ε 2 (ω) versus the applied frequency ranging from 3 × 10 3 to 10 7 Hz are shown in Figs. 9 and 10.
As shown in low-frequency regions from Fig. 9, the dielectric constants decline significantly as the incident frequency (lnω) increases up to around 14. The dielectric constants decrease slightly with the rise of frequency. Enhancing the dielectric constants with a frequency reduction discloses that Er 2 O 3 -ZnO nanostructures determine large interfacial polarization in the region of low frequency. At higher-frequency values, the dielectric constant values increase and get the maximum value of approximately 35 as the lnω value reached 13. Moreover, the dielectric constant values reduction is faster in the Er 2 O 3 -ZnO nanostructures with higher doping ratios of the erbium ions. To confirm, the values of both the dielectric constant and dielectric loss were larger in the region of low frequency comparing to those values in the high-frequency region. Those found dielectric data in this current research greatly agreed with the available results by P. Maji et al. (2015). The dielectric constants of Zn (NO3) 2 nanoparticles were observed to be declined with the polarization. In general, the reduction in the dielectric constant values for the investigated nanocomposites is assigned to the mechanism of dielectric polarization.
Also, the dielectric loss values reduce to the lowest possible point and then enlarge once more. Generally, reducing the values of the dielectric constant for the considered materials is linked to the polarization mechanism. To underline, dielectric polarization appears as ionic, dipolar, or interfacial electronic as an external electric field is practical to the Er 2 O 3 -ZnO nanomaterials. It can be concluded that ionic and electronic polarizations were dominant in the high range of frequency. In the region of low frequency, the maximum value of the dielectric loss ε 2 (ω) could be understood through the interfacial polarization mechanism due to charge carriers, which establish and limited by defects and grouped in the dielectric medium. When the frequency becomes higher, the ε 1 (ω) starts to rise, getting the dipole oscillation's advantages, which can quickly rotate (Jonscher 1977).
It is significant to investigate the frequency influences on the AC electrical conductivity to describe the Er 2 O 3 -ZnO nanocomposite materials' conduction mechanism at different erbium doping ratios. To compute the AC conductivity of the material under process, the next equations were applied (Murowski and Barczynski 1995;Bhattacharyya et al. 2001;Jali et al. 2007;Bobnar et al. 2004): Here, the total AC of electrical conductivity is noted as σ Total AC (ω), A is the effective area, and Z is known as the impedance. Also, respectively, σ DC (ω → 0) and σ AC (ω) are electrical conductivities of the DC and AC, respectively, where ω is represented the angular frequency, and s is donated the frequency exponent. The semiconductors showed AC values of the electrical conductivity between 10 × 10 −9 and 10 × 10 3 Ω −1 cm −1 (Brütting 2006). Equation (11) indicates that the AC electrical conductivity plays a major role in manybody interaction, explaining the power-laws universal behavior. The frequency exponent (s) is a vital parameter that illustrates the multi-interaction between the material impurities and charge carriers. The s value depends on the frequency  (2013). The values of the ZnO electrical conductivity decreased as the concentrations of the Fe dopants increased, as illustrated in Fig. 11. The attained values of activation energy were related to the optical energy levels of defects in the samples' bandgaps. The electrical conductivity of the Fe:ZnO nanoparticles were negatively affected by those defect levels.

Varistor applications of Er 2 O 3 -ZnO nanocomposites
The voltage-current (V-I) plots of undoped ZnO and doped with erbium varistor nano-ceramics at various Er contents are illustrated in Fig. 12a-h. The studied materials demonstrate a nonlinear conductive property in the applied voltage range. All specimens displayed highly nonohmic behavior. This plot clearly showed that the conduction features could be separated into two zones: a linear range of high impedance below the knee point voltage and a nonlinear range of a low impedance above the knee point voltage. C.W. Nahm found that the knee sharper of all plots among the two ranges is characterized as the improved nonlinear behavior (Nahm 2009).
The V-I characteristics exhibited by all samples-based varistor ceramics consist predominantly of pre-breakdown, breakdown, and upturn regions (Knecht and Meith 1979). The varistor action takes place in the breakdown region. In this region, the applied voltage is a highly nonlinear function of the current and can be described by the empirical law as in Kobayashi et al. (1978): where V is the applied voltage, I is current, K is a constant depending on the geometry and manufacturing process, and α is the nonlinear coefficient. The nonlinear coefficient values α are obtained from the inverse slope of the lnV-lnI curves at any value of voltage, as illustrated in Fig. 13.
It is recognized that the sharp knee point among the linear region and the breakdown field is the better nonlinear properties, V ss is known as the threshold voltage, α is defined as the nonlinear coefficient, R gb is the grain boundary resistance that is calculated from the Cole-Cole plots of the impedance measurements. The leakage current (I R ) is determined by the V-I plots. The parameters of AC impedance have been used to analyze the complex impedance (Z* = Z' + iZ") using the next expressions by Jonscher (1993): where the selected angular frequency is written as ω, the vacuum capacitance is defined as c o , as well as both the real and imaginary components of the dielectric permittivity are defined as (ω', ω" = ω' tanδ), where they could be calculated via the following functions (Jonscher 1993): The resistance (R b ) is simulated through the average square technique, the CPE values are specified via Q, and the parameter (c) is evaluated using the variance among the theoretical and experimental data. The CPE impedance is written as The angular frequency that is dependent on both real and imaginary components of different specimens' complex impedance is shown in Fig. 14a and b. The observed values of the real portion of the complex impedance are large in the region of lower frequency. It gradually decreases as increasing the angular frequency that is related to the increase of the AC electrical conductivity (see Fig. 14a) and keeps constant at the region with high frequency. This is associated with the polarization of dielectric characteristics. Similar performance is found in the tendency of variation in the value of imaginary components of the complex impedance (see Fig. 14b). At the higher-frequency region, the experimental impedance data becomes frequency independent. To understand the illustrated results in Fig. 14c, it is necessary to propose an equivalent circuit to extract grain boundary resistance and capacitance values for pure ZnO and the various erbium contents-doped ZnO-based ceramic varistor systems. It is noticed that all ZnO systems-based ceramic varistor have a quasi-semicircle that are related to the contribution of the grain boundary. In this way, the equivalent model of the electrical circuit was utilized to determine the resistance of grain boundary (R gb ) and capacitance (Q) in parallel for various studied specimens.
The effect of doping erbium doping levels on ZnO-based ceramic varistors' nonlinear coefficient is very complex. Figure 15a displays the difference between the leakage current and the grain boundary resistance for varistor samples with various erbium contents. Obviously, the leakage current is decreased with increasing the erbium contents. This is attributed to the increase in the homogeneous distribution and the activation energy of the imperfect quantity of varistor specimens. Based on the fitted impedance data and further investigating erbium doping's effect, the variation of the capacitance values for different specimens was illustrated in Fig. 15a. The variation of the capacitance showed an arrangement concerning the threshold voltage plots. The variation revealed an increase in the capacitance values.
Obtaining the grain boundary resistance (R gb ) was possible for all the investigated varistors because the impedance spectroscopy approach tolerates the resistive separation for contribution among the grain boundary and the trap state. From the relevant impedance data, the obtained grain of boundary resistance (R gb ) and the leakage current for the studied ceramic varistors are represented in Fig. 15b. Clearly, the leakage current declines and shows a little change. The increase of erbium   Fig. 13 The linear relation between lnI and lnV of the as-prepared Er 2 O 3 -ZnO nanostructured at various erbium ratios contents leads to an increase in the mean required energy for the homogeneous distribution in the ceramic varistor samples and for electrons overcoming the Schottky barrier. As a result, it is necessary to certify a large level of active barriers to the grain boundary and increase the resistance and avoid leakage currents to achieve higher values with a nonlinear coefficient. A graphical description of the nonlinear coefficient α and the threshold voltage (V ss ) according to the erbium contents in ZnO-based varistors is illustrated in Fig. 15c. It was clear that the variation in threshold voltage initially increased, followed by a decrease, and lastly increased as the erbium doping levels increased because of the reduction in the grain size of the ZnO-based varistors. From Fig. 3(c), the nonlinear coefficients of erbium-doped ZnO varistors firstly increased, then decreased, and lastly increased with various erbium contents. With increasing the level doping of erbium fillers, a similar variation of nonlinear coefficient α with the threshold voltage was observed. This revealed an upside-down U-shaped curvature, which was considered an unsymmetrical behavior. Comparable published results were conveyed. The significant improvement effect of erbium contents on the obtained nonlinear coefficients for the varistors was related to the microstructure uniformity and narrowed grain size distribution. Generally, the difference of the leakage currents was reverse to the variation on the nonlinear coefficients.

Photocatalytic activity
The use of rare-earth ions, as metal doping, delivers electron traps that defeat recombination between electron and hole. Then, the holes will shift to the catalyst surface and oxidize the organic compounds that have been adsorbed by Dutta et al. (2005). The Er dopants over the ZnO catalyst nanomaterials Eropumam ions gave a strong potential of oxidation reduction, leading to the photogeneration of electronic defects as well as the recombination rate of electron hole. The surface favors the creation of positive holes; the dynamic reaction center will be considered. Thus, the outstanding production of holes has successfully removed phenolic compounds, such as the same case for MB by Ju-ka and Pivrikas (2008). Figure 16a and b showed a corresponding plot between pchlorophenol concentration and p-nitrophenol versus time of illumination through changing Er concentration, respectively. Extensively, the percentage of ZnO doped with Er nanoparticles increases photocatalytic activity until 1 g of Er-doped ZnO (S5). The 1 g of Er-doped ZnO (S5) displays a remarkable drop in absorption as the irradiation duration increases. Figure 17a and b illustrates the photodegradation percentages as a function of irradiation time. The % of degradation was evaluated using Eq. (18).
The C o is the initial methylene blue concentration; Ct is the concentration at the chosen time. As the degree of Er doping increases by up to 1 g doped of Er (S5), the proportion of degradation increases significantly. However, the percentage of degradation (2.5 g, 5 g of Er (S5, and S6), respectively) declined as increasing the dopant concentration further.
The obtained results show that 1 g of Er-doped ZnO (S5) is essential for complete degradation (99.1%) in 30 min of solution p-chlorophenol and 20 min for the p-nitrophenol solution. Thus, 1 g of Er-doped ZnO (S5) is the perfect photodegradation composition of p-chlorophenol and p-nitrophenol. However, photocatalytic studies have shown that the increase has reduced the photocatalytic effects with doping separately and have decreased by an increase of 1 g of the Er in the ZnO matrix (S5). This effect was attributed primarily to the fact that the light penetrated the ZnO suppress electronhole pairs through the space-charge layer. It was decreased to a degree where the ratios of rare-earth ions were amplified to a certain amount (Wahl et al. 2003). Therefore, the dopant ion concentration is optimal; the thickness of the space-charge layer and the extent of the light dispersion at this concentration is comparable. Moreover, the pairs recombination of electron hole have increased by too large several rare-earth ions in the surface of ZnO, thereby the photocatalytic activity (Zhang and Zeng 2012).
Kinetic study of the photocatalytic procedure First-order kinetics is possible to exhibit heterogeneous photocatalytic reactions amongst ZnO nanoparticles and organic compounds with water solubility (Samoila et al. 2017). In this study, the authors, therefore, supposed that the concentration decay of p-chlorophenol and p-nitrophenol follow the pseudofirst-order kinetic model, k [min −1 ], calculated by nonlinear regression (Petrisor Samoila et al. 2017). For this kinetical analysis, the system's equilibrium has been achieved first via a 30-min dark mixture of oxidic materials and phenolic solutions. The initial concentration of phenolic solutions (C o = 300 ppm) during the balance phase. After this phase, the UV-illumination activated, and the photocatalysis reaction progressed with the decline of the phenol concentration. Figure 18a and b shows ln(C o /C) versus the irradiation time.
It was initiated that the pseudo-first reaction for each sample can be applied for the p-chlorophenol and p-nitrophenol degradations, respectively. The insertion in the ZnO matrix of Er ions as rare-earth elements produced improved photocatalytic properties' studied materials. Figure 18a and b using the nonlinear regression method was fit for the pseudo-first-order kinetic model in the presence of ZnO-based materials. Table 4 recapitulates the kinetic parameters for the p-chlorophenol and p-nitrophenol degradations. The most common expression used to define the kinetics of a heterogeneous procedure that includes aspects of both catalysis and adsorption phenomenon is the Langmuir-Hinshelwood model (LH). The initial reaction rate of k can be expressed according to the LH approach, according to C o ; the initial dye concentration, as indicated by Sobana and Thirumalai (2016):  The pseudo-first-order constant values were higher according to the kinetic profiles, with an increase of the loading up to 1 g of Er in the matrix. The fitted line reflects the LH model prediction. K's maximum values were 0.147 min − 1 and 0.158 min − 1 , respectively, for the pchlorophenol and the p-nitrophenol. These studies confirmed the photodegradation of the waste materials by different concentrations of Er-doped ZnO nanostructured. The p-chlorophenol and p-nitrophenol molecules are physisorbed or chemisorbed on the ZnO-NPs surface during photocatalytic activity. The Er-doped ZnO nanocomposites are supposed to enhance photocatalytic efficiency through surface defects. Also, the Er ions can serve as an efficient electron scavenger for trapping electrons in the conductive band, leading to reduce the risk of recombining electron hole (Khatamian et al. 2012). In this process, the responsive species are H 2 O 2 , O 2 − , and HO 2 , where this scavenger research was operated to identify responsible organisms for dye degradation. The Er ions combined with ZnO are responsible for capturing exciting electrons from the ZnO conduction band to the Er-doped nanoparticles. The free electrons number declines at higher Er concentrations, which causes photodegradation to decrease.
Two mechanisms may explain the enhanced photocatalysis in the presence of Er ions. Based on the luminesce mechanism, the photocatalyst absorbance and bandgap significantly affect the photocatalysis performance, with the addition of the Er dopant, the bandgap of ZnO shifts. Also, Er dopants can serve as the scavenger for superoxide ions to react and decrease the recombination rate of electron-hole pairs throughout the photocatalytic procedure (Khataee et al. 2013). Moreover, the Er:ZnO nanocomposites can generate an electron-hole pair surfacing barrier and a narrower zone of charging space in the ZnO lattice. However, an increase in the Er material contributes to a rise in the spatial charge layer by growing the diffusion depth of ultrasonic irradiation into ZnO nanoparticles. The electron-hole recombination could be increased, and the efficiency of photocatalysis decreased. Furthermore, the excess Er dopant will cover the catalyst surface and increase electron-hole centers' recombination (Yan et al. 2005). Depending on the results, 1% Er-doped ZnO is chosen for the next experiments as the best catalyst. Table 5 shows that Er-doped ZnO has better photocatalytic performances than other nanoparticles that have been documented in the previous work. It was reported in recent studies that the performance of zinc oxide in visible photodegradation for   (Dindar and Icli 2001). Nearly all of the work was performed using artificially visible or microwave irradiation in the presence of zinc oxide (Parida and Parija 2006). This study aims to use the sun and ZnO's ability to degrade phenol by efficiently radiating solar light. R. Kuma et al. found that the Ce:ZnO nanoparticles displayed significant photocatalytic behavior (2015). The degradation percentage improved to 99.5% as increasing the Ce concentration; however, photocatalytic degradation decreased further when Ce concentration increased. Therefore, the prepared Ce-doped ZnO particles exhibit significant photo-catalytic degradation at an optimum Ce-(3.28%) as an optimum concentration in 70 min only. K. Yu et al. reveals the XRD spectrum's characteristic peaks in the Er-doped ZnO nanostructures (2013). Under ultraviolet, the Er-doped ZnO absorbed more light and the fluorescence emission intensity increased by the Er doping in the ZnO. The degradation of the solution methylene blue (MB) showed that ZnO has significantly improved its photocatalytic activity through doping.  Considering that the main energy source is sunlight, photocatalytic activities were assessed in the samples by evaluating the photocatalytic decolorization of the methyl violet dye. For the synthesized sample, a catalyst dose of one gram per letter (1 g/L) with a starting concentration of 4.5 × 10 −4 g/L, the photocatalytic decolorization for methyl violet dye was illustrated the beneficial pH of 9.0. The photocatalytic decolorization efficiency of Mn:ZnO as well as Ag:ZnO was substantially larger than that of the pure ZnO matrix. In visible light, the addition of Mn and Ag metals improved the ZnO photocatalytic activities because of their ability to raise the defects in the ZnO surface. In effect, the transfer of optical absorption to the visible wavelength region, Yanqing Zong et al. reported that 11.0 mol% of Eu:ZnO nanoparticles had the maximum photocatalytic performance of methylene blue (MB) through the effective separation of photogenerated carriers (Yanqing et al. 2014). The photocatalytic performance of Nd-doped ZnO was investigated by A Phuruangrat through the decomposition of the UV-activated methylene blue (MB)(2018). The best photocatalytic activity for MB dye degradation was the improved 3 wt% Nd-doped ZnO by 3.95 × 10 −3 min −1 . The photocatalysis was also discussed of the UVactivated Nd:ZnO samples. The greatest activity with 98% degradation efficiency for 10 ppm MB degradation was found by U. Alam et al. (2018).
K.M. Parida et al. prepared ZnO with different precipitating agents using different methods. The solar light in set reactors toward the 4-nitrophenol oxidation was used to perform photocatalytic reactions (2006). He concluded that the 15-min photodegradation using microwave radiated samples in 0.6 g/L showed 92% and 98% of 4-nitrophenol, which keeps 3 h and 4 h under the sun. Thus, the ZnO-irradiated microwave is effectively used to oxidize and reduce organic and inorganic pollutants. In the case of stability and activity, irradiated microwave samples are better than commercial ZnO. Kausar Rajar et al. showed a simplistic approach to synthesizing zinc oxide nanoparticles by microwaves that assisted chemical precipitation (2018). It has been proven that in the absence of initiators such as H 2 O 2 , 98.5% of 4-CP was degraded in 20 min, reflecting a good prospective use as photocatalyst for the ZnO NP. Four times, the ZnO nanocatalysts were recycled, which displayed an efficiency of up to 95.5% for 4-CP catalytic degradation in the definite time of 20 min.

Photocatalytic degradation mechanism
The excited electrons are transferred to the conduction band (CB) when solar light is irradiated on the valence band (VB). The load pairs resulted in regulating the performance of the ZnO photocatalytic. Fast recombination of the photo charges, however, will reduce photocatalytic performance. Besides, the high-energy level of the ZnO surface, which reduces the photocatalytic activity, could easily be packed into large aggregates. Therefore, removing the recombination of electron hole is significantly high to enhance the photocatalytic efficiency of the ZnO matrix. Optically, the formed electrons react with oxygen besides decrease [ . OH] radical (Szabó-Bárdos et al. 2003). It is usually assumed that the electrons transferred from the ZnO surface and absorbed O 2 molecules limit the photocatalytic mechanism of electron-hole pair rate for semiconductors (Haruta 1997;Xu et al. 2010 (Sun et al. 2008). The [ . OH] can, therefore, be understood in the nanoparticles existence of Er-doped ZnO as follows: The proposed products of photocatalytic degradation The photocatalytic oxidation of phenolic compounds as the commonly reported organic compounds included three processes as regular steps. Firstly, Er-doped ZnO's uptake of the phenolic solution molecules occurred to attract them close to the catalytic sites. The second process involved the effective absorption of visible light photons by the composite grains to achieve multiple electron excitation. This is followed by the charge transfer interactions among the composite and the organic molecules, resulting in the oxidizing species' degrade the organic contaminants (Rana et al. 2013). The oxidative mineralization of phenolic compounds took place through several steps through either chromatographic or spectroscopic findings. First, it involves creating extremely toxic quinones, followed by the perfumed ring's opening, which leads to aliphatic acid creation and lastly to carbon dioxide (Borras et al. 2007;Liu et al. 2008). As seen in Scheme 2, the major pathways of oxidation reaction (perceived intermediates) are demonstrated during the 4-CP oxidation following the consecutive attack on the hydroxyl radicals. Chemical analysis has illustrated that this intermediate is not accumulated during the long irradiation period centralizing the mineralization of the original solution of 4-CP. In 4-chlorophenol and 4-nitrophenol, the hydroxyl radical may attack the para position results hydroquinone (HQ). It is subjected to oxidation reduction to produce benzoquinone (BQ). Similarly, if the hydroxyl radical attacks the ortho position, 4-chloro/nitrocatechol will be formed. The latter product has been transformed into hydroxy hydroquinone (HHQ) via oxidation through another hydroxyl radical, leading to chlorine or nitro radical removal. The hydroxybenzoquinone (HHQ) undergoes oxidation and reduction to form hydroxybenzoquinone (HBQ). The hydroxyl radicals' attack on BQ and HBQ results in aliphatic compounds that are also oxidized into aliphatic compounds with smaller molecular weight and then finally degraded into CO 2 and H 2 O.
Scheme 2 Photocatalytic degradation pathway of 4-CP and 4-NP for the as-prepared Er 2 O 3 -ZnO nanostructured with different erbium doping concentrations

Photocatalyst recyclability and stability
Finally, we performed the 1% Er-doped-ZnO test to reuse catalytic material in ideal conditions. A contrast was carried out between fresh material and reused material (with optimum conditions). The photodegradation outcomes are illustrated in Fig. 19a and b; the reused photocatalyst's stability was verified by the results showing a positive recovery of the 1% Er-doped ZnO (S5)-used catalyst. The observed efficiency of degradation with the reused catalyst 1% Er-doped ZnO (S5) exceeded the estimated level of 96.5%.

Conclusion
In this proposed work, erbium (Er-) effects as a dopant in the host ZnO matrix, combined efficiently in nanostructured powder, were successfully synthesized using the combustion technique. Compared to the conventional techniques, the combustion-friendly chemical approach has the uniqueness of the high yield, low price, simple, and effective procedures, and the ability to accomplish high purity in synthesizing oxide nano-powders. The produced Er 2 O 3 -ZnO nanostructured materials displayed excellent structural morphology, optical, electrical, dielectric, and photocatalytic characteristics compared to the pure ZnO nanoparticles. The XRD data and SEM analysis prove the prepared Er 2 O 3 -ZnO nanocomposites' polycrystalline structure and the nanoscale surface morphology, respectively. Noticeably, the surface morphology of Er-doped ZnO nano-powder has a practically uniform distribution. The optical absorption characterization discloses that the Er 2 O 3 :ZnO nanostructures have a direct bandgap. The direct optical bandgaps for the Er 2 O 3 -ZnO nanocomposites ranged between 2.88 and 3.19 eV, and on the other hand, the indirect optical transitions were evaluated to be from 3.2 to 3.35 eV. From the electrical characterization, the ZnO electrical conductivity decreased as the concentrations of the dopants increased. Also, the leakage current is decreased with increasing the erbium contents. The increase of erbium contents leads to an increase in the mean required energy for the homogeneous distribution in the ceramic varistor samples and electrons overcoming the Schottky barrier. The significant improvement of Er dopants on the obtained nonlinear coefficients for the varistors was related to the microstructure uniformity and narrowed grain size distribution.
The successful visible photocatalytic activity was demonstrated in ZnO doped with different concentrations of Er nanoparticles. The Er dopants played a vital role in improving the mineralization of transparent phenolic compounds via photocatalysis. The effective carrier transport through Er-functionality enhances the impact on responsible photodegradation activities. The process of charge transformation, which enhances their catalytic efficiency besides finds its part in catalytic applications, was demonstrated by designing the defect of the catalyst's band function through rare-earth impurities. Doping of Er-to ZnO has been shown to modify the ZnO's optical characteristics and greatly improve the photocatalytic activity. The Er-doped ZnO nanocomposites have attracted great consideration, taking advantage of their excellent potential for practical nano-technological applications in electronic varistor devices and environmental applications.
Availability of data and materials In this current research, the datasets obtained and analyzed are available with the corresponding author, "Dr.Thekrayat AlAbdulaal." Author contribution Thekrayat AlAbdulaal: formal analysisequal, investigationequal, methodologyequal, visualizationequal, writing original draftequal, writingreview and editingequal  Fig. 19 The recycling data for the photodegradation using 1% Erdoped ZnO (S1) of (a) pchlorophenol and (b) pnitrophenol