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Figure 2 shows FTIR spectra of gelatin composites:

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FTIR of hydrogel samples (a-d) samples 1&#;4 respectively.

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Bands assigned to characteristic stretching frequency (wavenumber, cm&#;1), with increasing wt. % CoCl2, band intensity at  cm&#;1&#; cm&#;1 increased due to multiple OH groups. Vibration bands below  cm&#;1 confirmed high force constant in Co-oxygen and Co-nitrogen bonds. Weak bands symmetric stretching NH, CH at  cm&#;1. The band at &#; cm&#;1 became much weaker, blue shifted to  cm&#;1 on binding through N, O. Bands at 649, , and  cm&#;1 due to stretch C&#;N, N&#;N, C&#;=&#;N bonds respectively slightly shifted, intensified as doping increased electron cloud. Bands at , ,  cm&#;1: C&#;N stretching, N&#;H deformation on binding CoCl2. Bands weakened and slightly shifted in position on doping. Intense band at  cm&#;1 due to organic moiety of gelatin: AgNPs Bands at 520&#;537 cm&#;1 confirmed Co&#;N, Co&#;O bonds, and C-N. Peaks (NH2: symmetric, anti-symmetric NH and CN stretching. Intense band at &#; cm&#;1: C&#;=&#;C bond. Native gelatin retained in composites, peaks at , to , , , , , , 557 cm&#;1 due to O&#;H, C-H, O&#;H, C-N, C-O(H), C&#;O&#;C stretching respectively. Co(II) ion binding gelatin via electron donors heteroatoms via N, O atoms. High delocalized electron density attained by dispersed AgNPs on gelatin polymeric matrix υC=N, affected by interaction between Co(II) and π-electrons of C&#;=&#;O. Band at &#; cm&#;1due to stretching vibration C&#;=&#;N bonds. Coordination bond formed between ketonic C&#;=&#;O group of gelatin and Co(II) ion. Bonding includes Cl&#; ion and coordinating water molecules. The most intense vibrations bands of sample confirmed strong bonding43.

Figure 3 (a-c) showed SEM surface micrographs: surface morphology of gelatin composites showed (2D arranged chains at a small spacing distance) on binding Co(II) ions. Microstructure changed from semi crystalline and more appearance capsule shape into crystalline structure in the presence of AgNPs.

SEM micrographs: (a) gelatin, (b) sample 1 and (c) sample 4 respectively.

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Gelatin exhibited continuous polymeric chains. This structure changed on doping by cobalt chloride and decoration by AgNPs. Smooth structure due to high cross-linking between polymer chains. N atoms of gelatin binding oxygen atom via intra molecular H.B. and Van der Waals interaction gave uniform small micrometer particle size17. 3D cross-linking increased surface area. PMMA chemically grafted hydrophilic functional groups of gelatins. Doping and grafting gelatin improved its polymer shell giving network hydrogel. NH, C&#;=&#;O, OH increases water entrapment into capillary porous.

Gelatin properties modified on doping. Sample 4 showed the most intense vibrational bands in FTIR spectra, so it was selected for further investigation. SEM images show changes in surface morphology of gelatin composites compared to native gelatin indicating grafting of dopants CoCl2 and AgNPs to backbone chains of gelatin.

pXRD patterns, Fig. 4 showed pure single phase semi-crystalline gelatin composites have crystalline and amorphous domains with increased surface area. Gelatin was responsible for the formation of a transparent polymer film. pXRD diffraction pattern revealed the formation of gelatin composites containing CoCl2 and AgNPs with characteristic diffraction peaks at 2θ 24°, 23° and 38°, 47°, 64° respectively13,14,15.

pXRD pattern of hydrogel: (a) S1 and S4 respectively.

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Amorphous composites improved catalytic activity.

Figure 5 showed pXRD pattern foe AR8 dye after 5th repeated cycles reusibility.

pXRD pattern of hydrogel S4 after 5 recycle reusability.

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Retained pXRD pattern of hydrogel confirmed reusability for many repetitions due to: Chemical Stability (unchanged chemically after degradation organic pollutants).

Mechanical stability. Hydrogels must maintain their mechanical integrity throughout the swelling and de-swelling by aqueous solutions during photo catalysis and photo stability against photo degradation.

Thermal stability of gelatin composite sample4 is confirmed in Fig. 6. In TGA. Curves, weight loss: below 200 °C due to dehydration; above 200 °C for gelatin units&#; degradation accompanied with phase change15,19.

TGA, DTA and DSC thermograms of gelatin composite sample 4.

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Thermal behavior confirmed doping gelatin by cobalt chloride and AgNPs. TGA curves showed slight weight loss due to dehydration and loss both lattice and coordinated water molecules. DTA curves showed high Tm 240.71 °C confirmed thermal stability. In DSC, sample and reference sample have same or different masses and are kept at the same temperature. Energy given or removed from sample to maintain ΔTsample-reference, equal zero. The power energy change in the sample measured as a function of heat flow. Thermal transition changes specific heat and alters the power signal. Exothermic or endothermic peaks areas proportional to ΔH. Heat capacity C (heat Q absorbed by closed system of constant composition (dV&#;=&#;dN&#;=&#;0) on heating 1 K44:

$${\text{C }} = {\text{ Q }}/{\text{DT}}$$

(7)

Heat capacity at constant pressure CP differs than CV because Cv depends on internal energy and work done on volume expansion. For solids at low temperatures CP &#; CV, when two pans heated, heat absorbed by the sample-temperature plot represent DSC curves. Heat flow: (heat, q supplied per unit time, t. Heating rate is temperature increase per unit time, t.

$$CP = \frac{heat \, flow}{{heating\, rate}} = \frac{\frac{q}{t}}{{\frac{\Delta T}{t}}} = \frac{q}{\Delta T}$$

(8)

DSC represented in Fig. 7 explores thermal transitions: glass temperature Tg, crystallization, and melting. Above Tg: CP of sample increased and measured at the middle of the incline. Tcrystallization: above T g, mobility improved as kinetic energy increased atomic motion in ordered crystalline arrangements release heat.

DSC thermal transitions: glass temperature Tg, crystallization, and melting.

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The area under the crystallization peak gave exothermic ΔH crystallization transition). Endothermic melting above Tc at Tmelting that remains constant until complete melting, Area under peak equals latent heats of melting ΔH melting. Glassy and crystallization transition involves no peaks confirming the thermal stability of the gelatin composite, Table 2.

$${\text{C}}_{{\text{p}}} = {\text{ a T }} + {\text{ b }} = \, \alpha {\text{T}}^{{3}} + \, \gamma {\text{ T}}$$

(9)

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Above 100 °C, almost all sample showed glassy transition followed by crystallization and melting32. Small CP indicated the weak thermal conductivity of the samples confirming uses reduce thermal effect exothermic photo degradation.

The hydrogel composite withstand waste heat from exothermic reactions occur during photo catalysis without significant thermal degradation.

The main factors: pH, temperature, dose, contact time, and initial dye concentration affecting experimental %Re of AR8 dye in dark and light identified from Plackett&#;Burman statistical analysis. Five independent variables were screened in six combinations, each variable organized, high (&#;+) and (&#;&#;) level. Averages %Re of duplicates determinations taken as responses45:

$${\text{Factor observed effect }}\% {\text{R }} = \overline{\% R}_{ + 1} - \overline{\% R}_{ - 1}$$

(10)

\({\stackrel{-}{\%R}}_{+1}\), \({\stackrel{-}{\%R}}_{-1}\) are high and low setting respectively.

$${\text{Main average variable effect }} = \, \left( {\sum \% {\text{R}} + \, - \sum \, \% {\text{R}} - } \right)/{\text{number of trials }}\left( {\text{N}} \right))$$

(11)

% R0.0 means no effect. Positive and negative main effects indicate variables near or apart from optimum respectively, Table 3. Control factors with effects magnitude qualified and statistically significant effects determined. Optimal conditions determined by combining levels of factors had the highest main effect. Student's t-test explored the statistical significance of regression coefficients of variables45.

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Parameters enhanced %Re are dose initial concentration and time. Temperature and pH were not affected. Strong electrostatic interaction on the surface of positively charged composite in solution below its pH of zero PZC (5.3) did not alter %Re indicated that gelatin composites act by photocatalysis, not adsorption. The surface charge of gelatin composite at different pH did not affect %Re. %Re was independent of temperature. Statistical parameters optimized in Table 425.

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In light, both Ci and light intensity control %R.

The Fig. 8 UV&#;Vis. absorption spectra of native gelatin and gelatin composites due to electronic transitions from the ground state into the excited state.

UV&#;Vis. absorption spectra: a) gelatin and gelatin composites S1-S4 respectively.

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Sample 4 showed three absorption peaks at 262, 314 and 715 nm. Weak absorption at 715 nm is due to intra-molecular charge transfer. The spectral redshift of sample 4 (curve e) improved photo catalytic activity. Intensity of UV&#;Vis. absorbance bands depend on wt.% Co(II) ion for same chromophores42.

$${\text{ fraction absorption }}\alpha = \frac{2. \times absorbance A}{{{\text{sample thickness}} t}}$$

(12)

Figure 9 showed concentration dependence of band gap Eg that controlled UV-absorption coefficient depends on photon energy \(h\upsilon\) :

$$\alpha h\nu = A\left( {h\upsilon - {\text{Eg }}} \right)^{r} \,{\text{ or }}\,\left( {\alpha {\text{h}}\nu } \right)^{{2}} = {\text{ A}}\left( {{\text{h}}\nu - {\text{Eg}}} \right)$$

(13)

where ν frequency of incident radiation inversely proportional to wavelength (λ); A: constant depend on reduced masses of electron&#;hole pair and refractive index of the composite material and exponent r depends on nature of electronic transition; r&#;=&#;2, ½ for indirect transition and for allowed direct transition respectively42.

Tauc&#;s plots calculation band gap (a-d) for samples (1&#;4) respectively.

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Doping by CoCl2 and AgNPs increased band gap of gelatin composites from (1.82 to 1.95) due to separation of charge (electron&#;hole) charge carries. All composite samples have suitable low Eg to be used as photo catalysts for dye degradation.

Linear portion of last curve extrapolated to x-axis to find the intercept (band gap). Low Eg (1.9&#;2.04 eV) confirmed good absorbance in Vis. Region indicated increase density of states available for electrons occupation and high photo catalytic activity42.

Figure 10 shows the nonlinear photo luminesce (PL) activity of gelatin composites on excitation at wavelength 340 nm. Optical activity: at 325&#;380 nm is attributed to charge transfer from gelatin ligand to Co(II) ion44.

Photo lamination spectra of gelatin composites.

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Gelatin composites had large values of molar extension coefficients and long wavelength emission, Table 5.

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Sample 4 contained 20 ppm AgNPs (improved optical properties) showed the best optical activity. This composite sample showed intense absorption in Vis. region of electromagnetic radiation.

Catalytic efficiency reached 95% within 20 min. Cobalt dopant inhibited electron&#;hole recombination, decreasing Eg and improved photocatalytic properties, increased delocalization of electron density, and enlarged surface area23,44.

For 300 ppm AR8 dye completely degraded into colorless solution after 90 min. contact time. Kinetics first-order model fitted photodegradation, R2&#;=&#;0.. The ratio of residual concentration (Ct) to initial concentration (Co) decreases rapidly with irradiation time, Fig. 11.

Variation of Ct/Co in photodegradation AR8 dye on the composite hydrogel.

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Gelatin composite efficiently photo degrades AR8 dye without self-splitting because of inherent attractive forces retarding solubility. Hydrogel is an efficient photocatalyst in Vis. region light-driven photodegradation of AR8 with higher %Re than conventional La photocatalyst for other pollutants19.

Figure 12 shows a first-order plot for photodegradation data to a pseudo first-order kinetic model.

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Linear fitting of photodegradation data to pseudo first-order kinetic.

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Good straight line slope (correlation coefficient R2) indicating that photodegradation of AR8 dye on gelatin composite followed pseudo-first order kinetically. Amount degradation unfitted to zero-order and second-order kinetic equations (R2 less than 0.95).

The reactant AR8 dye exists in too small concentration relative to the concentration of gelatin composite photocatalyst. The slope of the straight line equals rate constant k1 5.13*10&#;2 min&#;1. The good straight line indicated degradation rates of AR8 dye increased with increasing irradiation time with dye decolonization more rapidly than La-perovskite photocatalyst for MB dye42.

Gelatin composite reused after immersion on 1.0 M NaOH solutions. Adsorbed hydroxyl removed dye molecules from active sites. Fig. proved hydrogel had 60% for 5th consecutive photo degradation. Biocompatible PMMA increased mechanical strength. Light stability: device tested under illumination without elevated temperatures. Heat stability: device tested without illumination at high temp (T). Shelf stability: without illumination at room T. Atmosphere humidity of stability tests not indicated. Light soaking tests (continuous 1-sun illumination) damp-heat stressors (85 °C, 85% relative humidity (RH)): Composites 90% initial wt. after 60 h in ambient atmosphere (20&#;30% RH) under 0.7-sun illumination. High wt.% PMMA 95% initial wt. photo stability: composite kept 100% wt. in dark, 96% on exposure inert atmosphere 10.488 h at 25 °C, illuminated at 1 sun in N2 at 25 °C for 100 h, wt. drop to 90% due heterogeneous interfacial layers. Lifetimes: composite retain 91 wt. % after one year aging. Phase stability: SEM micrograph, Figure after Vis. light illumination showed no phase segregation. There is no lattice Instabilities induced by strain.

Degradation mechanism

Composite photocatalyst in an aerated aqueous solution contains AR8 irradiated under Vis. photon energy (hν), λmax. 600 nm&#;&#;&#;electron (e&#;)-positive hole (h+) pairs adsorbed on catalyst surface giving OH radical. Excited electron in the conduction band (CB) of gelatin composite affects adsorbed oxygen molecules from water at composite/solution interface giving superoxide radical O2.&#;. Doping gelatin by Co(II) ion created structural defects and holes giving active oxygen species (O2&#;, O&#;). Holes decrease Fermi level causing CB-VB characteristic with electron&#;hole pair.

h+ in VB of catalyst react with O2.&#; generating OH. Oxidize AR8 dye46.

$${\text{O}}_{2}^{ - } + {\text{ H}}^{ + } \to {\text{ HOO}}$$

(14)

$${\text{HOO}}^{.} + {\text{ e}}^{ - } \to {\text{ HO}}_{{2}}^{ - }$$

(15)

$${\text{HOO}}^{ - } + {\text{ H}}^{ + } \to {\text{ 2H}}_{{2}} {\text{O}}_{{2}} \left( {{\text{H}}_{{2}} {\text{O}}_{{2}} = {\text{ 2OH}}^{ - } } \right)$$

(16)

$${\text{H}}_{{2}} {\text{O}}_{{2}} + {\text{ e}}^{ - }_{{({\text{CB}})}} \to ^{ - } {\text{OH }} + {\text{OH}}$$

(17)

$${\text{H}}_{{2}} {\text{O}}_{{2}} + {\text{ h}}\nu \, \to { 2} {\text{OH}}$$

(18)

$${\text{AR8 }} + {\text{ OH}}^{ \bullet } \left( {{\text{Radical}}} \right) \, \to {\text{ CO}}_{{2}} + {\text{ H}}_{{2}} {\text{O }} + {\text{NH}}_{{4}} + \, + {\text{ NO}}_{{3}}^{ - } + {\text{ SO}}_{{4}}^{{ - {2}}}$$

(19)

Optically active composites are with high carrier dynamics (e-hole) separation enhanced photo degradation.

This mechanism could be schematically represented as following image (Fig. 13).

The degradation mechanism.

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Holes generated in VB of catalyst oxidize dye and react with super-oxides radical, generating OH. radicals degraded dye via one-electron redox reaction forming CO2, sulphate, nitrate, and H2O. Nitrogen compounds NH4+, and NO3&#; recovered by using porous clay alumina silicate adsorbent to be used in agricultural fertilizers.

Photocatalytic activity of AgNPs on gelatin composite exceeded that of removal methylene blue (MB) over nano sized La based photocatalyst under irradiation by Vis. Light (catalytic efficiency 69% in 100 min. illumination time)47.

AR8 extensively polluted wastewater from textile industries and its degradation has not been reported by photocatalysts such as metal oxides and semiconductors. Efficient semiconductor catalysts with low Eg absorb in Vis. light irradiation at ambient temperature and pressure42. Insoluble thermally stable gelatin composite is low cos. non-toxic and easily prepared by facile low cost methods.

Electrical properties confirmed photocatalytic properties. The rough surface of the gelatin composite (sample 4) has many cavities with variable particle sizes and shapes that improve catalytic activity, Table 6.

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Dopants CoCl2 and AgNPs improved photocatalytic activity by enhancing the hole-doping level and decreasing Eg. Large surface area composite enhanced photodegradation of dye. All constituents of gelatin composite are safe and have no toxicity on ecosystem48, Fig. 12.

Detected ions (Co (II) and Ag(I) leaching from gelatin composite during photo degradation below undeletable limitsAR8 dye ensure safety and environmental compatibility. No ion replenishment needed. Regeneration of photo catalysts required activity over prolonged periods for ensuring economic and environmental sustainability. Regeneration restoring active sites and structural integrity after fouling or consumption during photo catalysis via: washing photo catalyst with distilled water to remove adsorbed dye; Photo activation by light, simply exposing them to UV or Vis. light depending on photo catalyst&#;s activation range) help breaking down adsorbed pollutants, essentially self-cleaning active sites. In some cases, a mild chemical treatment dissolve any strongly adsorbed dye molecules. This could involve mild acids, bases, or oxidizing agents that do not damage the photo catalyst itself. Annealing at 60 °C in vacuum oven 1.0 h regenerate photo catalyst by decomposing adsorbed dye organic compounds. Ultrasonic waves dislodge dye molecules from active sides, cleaned with 50 mL 1.0 M NaOH. High pH causes desorption of dye molecules. Regeneration by 4.0 M NaCl increases ionic strength by factor 0.4 causing desorption of dye molecules. Each regeneration process must be tailored to specific photo catalyst and its application, considering factors such as contaminants nature of the, hydrogel composition, and economics of regeneration process. Regeneration frequency and the number of effective regeneration cycles a photo catalyst can endure are critical factors that determine the lifecycle and cost-effectiveness of the photo catalyst.

Photodynamic activity of gelatin composites

Gelatin composite showed good optical activity: Absorption bands at 299&#;401 nm assigned to intra ligand metal charge transfer transition. Bands at 439&#;558 nm due to magnetic Oh geometry ((µeff. 4.11 B.M).

Optically active photosensitizer (S) molecules absorb and are excited by Vis. light photon (h \(\upsilon\)) energy. Excited S*molecule rapidly interact with inert triplet 3O2 molecular oxygen in water producing reactive free radicals reactive oxygen species (ROS): singlet oxygen, hydroxyl radicals (OH), and superoxide (O2&#;) ions through charge transfer. ROS radicals contribute to oxidative damage of AR8 dye. Singlet oxygen 1O2 species rapidly attack organic dye molecules. Energetic 1O2 is very short-lived and rapidly relaxes to 3O2 after dye oxidation.

S* had: efficient ISC and high T-state quantum yield (ΦT) and long τ allow interact with the oxygen of water.https://en.wikipedia.org/wiki/Photodynamic_therapy&#;cite_note-:0&#;5 An electron in an S moleculehttps://en.wikipedia.org/wiki/Electron is excited to a higher-energy orbital, elevating the chromophore from So into short-lived, electronically excited singlet state (Sn). Chromophore* loses energy by rapid decay through vibrational and rotational sub-levels in Sn via internal conversion (IC) to populate S1 before rapid relaxation to So. Radiative fluorescence (F) decay (S1&#;&#;&#;So), lifetimes (τF.10&#;9&#;10&#;6 s.) spin allowed transitions S&#;&#;&#;S or T&#;&#;&#;T conserve spin multiplicity of the electron. S1 undergoes spin inversion and populate lower-energy first excited triplet state (T1) via (ISC) spin-inversion forbidden transition followed by second spin-forbidden to depopulate excited triplet state (T1) by decaying So (phosphorescence (P) (T1&#;&#;&#;So). (τP 0.001&#;1 s.) Longer than τF23. Interaction excited T-S* with So (3O2); spin allowed transition-excited state photosensitizer: (3 S*&#;&#;&#;1P S*, 3O2&#;&#;&#;1O2)49. Photocatalytic activity of excited molecule depends on τT and ΦT of S*control photodynamic activity50, Fig. 14.

Jablonski diagram for absorption, fluorescence, and phosphorescence49.

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Transition metal Co(II) ion had high quantum yield (ΦT) and nano sec. τT showed no self-quenching of light photons before conversion 3O2 into 1O250. High thermal stability of molecular structure. The dark red color confirmed that optically active chromophores interact with light photons50.

Dimensional stability explained improved performance of hydrophilic composites (water absorption and thickness swelling) by aqueous dye solution is essential event for photo catalysis in aqueous solutions, Fig. 15. Water easily penetrates polymer chain of hydrogel. Doping by AgNPs and CoCl2 decreased swelling by restriction chains movement.

Electrical properties of gelatin composites

Total conductivity σtot and dielectric parameters εˋ, εˋˋ calculated by using data of impedance (Z), capacitance (C), resistance (R), and phase angle (ϕ) at any frequency Fig. 16 illustrates dielectric properties of solid-state sample.

Representation variation dielectric constant with frequency.

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εo static permittivity at DC, ε&#; optical permittivity for very high frequencies of light oscillators. Determine distribution parameter (α) macroscopic relaxation time τo, molecular relaxation timeτ, τo using relation:

$${\text{U}}/{\text{V }} = \, ({\text{wt}}_{{\text{o}}} )^{{{1} - {\text{a}}}}$$

(20)

where, U distance between εo, observed point, V distance between that point and ε&#;:

$${\text{Angular frequency}},{\text{w}} = {\text{ 2pn}}$$

(21)

where α: 0&#;1. Extent distribution τ increases with increasingα, τo and decreases in heating.

$${\text{Temperature affect}}:{\text{t}} = {\text{t}}_{{\text{o}}} {\text{exp }}\left( {{\text{E}}_{{\text{o}}} /{\text{KT}}} \right)$$

(22)

Constant τo characterizes relaxation time (single oscillation of dipole in a potential well, Eo free energy of activation for dipole relaxation, τ: average most probable spread relaxation time.

σtot constant at low frequency range for all samples and obeys a power relation at high-frequency range51:

$${\rm {\sigma }}_{{{\rm {tot}}}} = {\rm {\sigma }}_{{{\rm {dc}}}} \; + \;{\rm {\sigma }}^{\prime } \left( {\rm {\omega }} \right)$$

(23)

where σdc conductivity independent on frequency (extrapolation σtot at ω&#;=&#;0) σˋ(ω) is Ac conductivity. Frequency dependence fitted using a power low:

$$\upsigma^{\prime } \left( \upomega \right)\; = \;{\rm {A}}\;\upomega^{S}$$

(24)

$${\rm {Dielectric}}\;{\rm {constant}}\;{\rm {\varepsilon }}^{\prime } \; = \;\frac{t}{A}.\frac{C}{{{\rm {\varepsilon }}_{o} }}$$

(25)

where C, t, and A are capacitance, thickness, and cross-sectional area of the sample respectively and εo 8.85&#;×&#;10&#;12 F/m permittivity of free space.

Figure 17 illustrates plot εˋ against frequency at different temperatures. Decease εˋ with increasing frequency reflects dielectric properties, ions motions, and polarity of composite. Ions rotate around their negative sites at short-distance transport (hop out of sites with low free energy barriers and pile up at sites with high free-energy barriers (ΔG activation impeding ions diffusion that vary from site to site gives different ionic motions). In electric field ion motions contribute to dielectric response cause AC conductivity. Variation dielectric properties with frequency indicating polarity. Due to dipole polarization, when AC frequency increases, ε decrease at high frequency, dielectric behavior represented in Fig. 17.

Dielectric and electrical parameters, depend on frequency, σ (0- 26*0&#;6S/m. σmax 10.13*106&#;26.04*106 Hz51. Dielectric constant ε&#; decreased on increasing frequency due to dipole polarization and semi-conducting features on hopping mechanism.

log σtot (ω) against frequency F, Hz at room temperature showed AC conductivity increases on increasing frequency due to impedance decrease, Fig. 18.

Dependence of AC conductivity on temperature and frequency.

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Good electrical conductivity and magnetic properties (effective magnetic moment 4.11 B.M) improved dye degradation into simple inorganic anion51. Variation AC conductivity with temperature, Fig. 19 indicated semiconducting properties and dielectric permittivity properties of metallic gelatin composite showed semiconducting behavior52.

Total (AC&#;+&#;DC conductivity of hydrogel sample 4.

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AgNPs enhanced AC electrical conductivity 2.3 \(\times {10}^{4}\) Ohm.cm&#;1 due to high electron density. This behavior explored that gelatin composites are semiconductors that have empty CB and full valence Band (VB) in electronic structure.

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