X-ray diffraction patterns were obtained on a PW1800 diffractometer with CuKα radiation (Kα = 0.15405 nm) and Ni-filtered. The nanostructures of the samples were analyzed by scanning electron microscopy (SEM; S-4160 Hitachi) and transmission electron microscopy ( Philips EM 208s) at an accelerating voltage of 100 kV. The photoluminescence (PL) spectrum was recorded at room temperature using a Xe lamp with an excitation wavelength of 280 nm by a Cary Eclips-fluorescence spectrophotometer. The degradation process was analyzed by monitoring dye absorption intensity at the maximum absorption wavelength using a UV/vis (Perkin Elmer, Lambda 35). Fourier transform infrared (FT-IR) spectra were recorded on TENSOR27 spectrophotometer in KBr pellets. Thermal studies were performed using Mettler-Toledo TGA/SDTA-851, thermogravimetric analyzer in air at a heating rate of 10˚C/min. Atomic absorption spectrometric measurements were done using a GVC atomic absorption spectrometer.

All chemical materials were purchased from Merck Chemical Company and used without further purification. In a typical synthesis, 1 mmol of Folic acid and 2 mmol of NaOH pellets were dissolved in 5 mL of water to give a homogenous solution. Then this solution was added drop wise into 5 mL of aqueous solution containing 1 mmol Ni(CH₃COO)₂, 4H₂O under magnetic stirring for 2 h. The resultant solution was refluxed for 12 h. The solid product was filtered, washed with distilled water to remove the impurities, and then dried in a vacuum oven at 60˚C for 12 h. Finally, the obtained precursor was calcined at 400˚C for 3 h. Deionized water was used for making all solutions. The calcined solid product at 400˚C contains: Ni: 60.55%.

Photocatalytic activity of the prepared NiO as photocatalyst was evaluated by the degradation of Congo red (Scheme 1). 0.05 g of the as-prepared nano NiO was poured into 100 mL aqueous solution of 3 ppm Congo red, in a glass reaction cell. A 30 W UV-C lamp was used as light source. The distance between the UV lamp and the glass reaction cell was fixed at 8 cm. Air was bubbled into the solution throughout the entire experiment to provide a constant source of dissolved oxygen. The dye solution was continuously stirred with a magnetic stirrer. Prior to irradiation, the set up was kept in the dark for approximately 15 minutes in order to reach an adsorption/desorption equilibrium among the photo catalyst particles, Congo red and atmospheric oxygen. During irradiation, the degradated dye was sampled in regular intervals. The photocatalytic degradation was monitored by measuring the absorbance of the soltion samples with UV-vis spectrophotometer.

XRD analysis of the as synthesized nickel folate compound with folic acid and Ni(acac)₂ designated as Ni precursor, followed by heating at 100˚C, 200˚C, 300˚C, 400˚C for 3 h. are shown in Figures 1(a)-(e), respectively. Based on the obtained results at room temperature up to 200˚C amorphous phases were formed, while metal oxide crystallization occurred after heat treatment at 200˚C. By increasing temperature up to 300˚C and finally 400˚C the NiO as a crystalline phase is observed. The diffracttion angle and intensity of the characteristic peaks of the samples at 300˚C and 400˚C (Figures 1(d) and (e)) are well consistent with those of the standard JCPDS card no (04-0835). The main diffraction peaks were observed at 2θ, 37.13˚, 43.53˚, 63.11˚, 75.80˚ and 79.22˚. All diffracttion peaks can be indexed to the pure NiO crystalline phase (space group: Fm3m), no impure peaks are observed in the XRD pattern. The average particle size was about 4.8 - 5 nm determined from XRD pattern parameters of the NiO powder according to the sherrer Equation (1) [15].

where b (FWHM; full-width at half-maximum or halfwidth) is in radians and q is the position of the maximum of diffraction peak (200) and l is the X-ray wavelength (1.5406 Å for CuKα).

The FT-IR spectra of folic acid, as prepared sample after heated at 100˚C, 200˚C and calcined at 400˚C are shown in Figures 2(a)-(d) respectively. The broad peak at ca. 3450 - 3400 cm^–1 in all four spectra are representative of OH stretching vibrations of template and adsorbed water. The vibration bands at 2946 and 2869 cm^–1 are due to the CH₂ groups of folic acid. The FTIR of folic acid (Figure 2(a)) shows vibration at the regions 1694, 1449, 1333 - 1352 and 600 cm^–1 due to the ν_(C=O), ν_as(COO-), ν_s(COO-) and C-OH of folic acid and C-O bending vibration respecttively [36]. By increasing temperature to 100, 200˚C (Figures 2(b) and (c)), the vibration modes of folic acid has been decreased, but at 400˚C the peaks due to the CO folate disappeared because of the decomposition of folic acid but appearing a relatively broad peak around 1200 cm^–1 should be due to the some C-N or C-OH vibrations of adsorbed specious on NiO surfaces during the calcination time, but by increasing temperature up to 900˚C this peak is disappeared. Two new peaks were also found at 490 and 445 cm^–1 (Figure 2(d)), these peaks undoubtedly assigned to Ni-O stretching as was reported earlier by other researches [36,37].

The SEM images of as prepared nickel-folic acid hybrid with molar Ratio 1/1 after refluxing for 6 h, 12 h, followed by calcinations at 400˚C designated as NiO400 and EDX of NiO400 are given in Figures 3(a)-(d) respectively. As seen in Figure 3(a), the SEM image of as prepared sample does not show any obvious morphology but by increasing reflux time from 6 h to 12 h the particle

size appeared as 48 nm (Figure 3(b)). The best results was obtained after calcination at 400˚C with particle size 21 - 22 nm. It was also observed increasing the amount of the folic acid from 1/1 to 5/1 the size of the nanoparticles increases and increasing the reflux time, the particle size decreases. The photographs show that the calcination temperatures are important factor on particle size and distribution of the nanoparticles. The higher calcination temperature, results the smaller the particles and the more good distribution.

Therefore the NiO₄₀₀ has been used for further studies. The EDX of NiO₄₀₀ is also shown in Figure 3(d). The interesting point is no peaks due to the impurity is appeared.

The transmission electron microscopy (TEM) photographs of NiO₄₀₀ has been given in Figure 4(a). The size of the nanoparticles obtained from the XRD diffraction patterns are in close agreement with the TEM studies which show sizes of about 4 - 5 nm. The selected area diffraction (SAED) pattern of the NiO₄₀₀ is shown in Figure 4(b), which shows the good crystalinity of the nanoparticles.

In order to reveal the changes that occurred during heat treatment of the precursor powders, TGA, and DTA analysis of as prepared sample was carried out from 30˚C to 750˚C in atmosphere (Figures 5(a)-(b)). According to

the TGA curve Figure 5(a), the major part of the weight loss seems to occur below 400˚C. The TGA curve shows a weight loss of 8% in the temperature range of 60˚C - 140˚C due to the evaporation of the absorbed water. An exothermic peak at approximately 342˚C occurs in DTA (Figure 5(b)), which might be associated with the conversion of precursor to NiO and also the decomposition of the organic residues. The total process has the weight loss of about 48.63%.

The nitrogen adsorption-desorption isotherms and pore size distribution of NiO calcined at 400˚C are shown in Figures 6(a)-(b) respectively. Curved inflations at P/P˚, 0.4 - 1 is related to the pressure of capillary condensation based on BHJ calculation. The obtained results indicate that the NiO₄₀₀ with surface area 138.1 m²/g, pore volume 0.09 cm³/g, and pore diameter 2.83 nm is formed.

The UV-vis spectra of NiO bulk, NiO₄₀₀ and NiO₁₀₀ are shown in Figures 7(a)-(c) respectively. Observing a maximum absorption at the region 300 nm for the NiO₄₀₀ is interesting point. The photoluminescence spectra of NiO₄₀₀ shows a maximum UV emission at 381 nm by

excitation beam at 280 nm, the obtained results are consistent with those reported before [38,39].

The photocatalytic activity of nanoporous NiO was prepared by nickel acetate, sodium hydroxide and folic acid as template followed by heating at 100˚C and calcination at 400˚C designated as NiO₄₀₀ were evaluated by the degredation of Congo red (CR) solutions (Figure 8). The UV-vis spectrum of CR shows two maximum absorption at the regions 347 and 497 nm. By illuminating UV light to the aqueous CR solution the two bands due to the azo linkage and naphthalene ring are decreased in the time 135 min. UV light illumination of the aqueous Congo red solution in presence of prepared nano structured NiO cause the absorption bands of the dye in the visible region to decrease with time and finally to disappear, indicating the destruction of its chromophoric structure in the vicinity of the azo-linkage. This is accompanied by a parallel decrease of the intensities of the bands in the ultraviolet region located at 347 nm, attributed to the naphthalene ring. It was observed by increasing time up to 135 min with NiO₄₀₀ the degradation were completed.

The photodegredation percentage versus photodegredation time in the presence of the mentioned catalyst and the illuminating CR without catalyst are shown in Figure 9. Based on the obtained results without using the catalyst 20% of Congo red was photodegreded after 1 h, but using the catalyst increases the degredation up to 90% at the same time. The photocatalytic efficiency was expressed in terms of percent of degradation from the following equation:

where, C₀ represents the initial concentration of the CR, C is the concentration after illuminating by UV-vis light,

A₀ is the initial absorbance, and A is the variable absorbance. Without any catalyst, only a slow decrease in the concentration of Congo red was detected under UV irradiation. The addition of catalysts leads to obvious degradation of organic dyes. The reusability of NiO₄₀₀ was checked by consecutive repeated adsorption of the dye and testing the photoactivity of the sample for the new degradation cycle (Figure 10). This procedure was repeated three times on the same sample as described for Congo red. It was found that photo degradation efficiency of CR slightly decreases with the recycling and remains almost constant after successive cycles, which is more than 90% of that of the first recycling times.