Structural, optical and photocatalytic applications of biosynthesized NiO nanocrystals

ABSTRACT NiO is one of the most important candidates for semiconductors metal oxide nanocrystals by the arrangement of photocatalytic application. However, the photocatalytic performance of biosynthesized nanocrystals using Aspalathus linearis (Burm.f.) R. Dahlgren has not been investigated yet. In this contribution, we synthesize α-Ni(OH)2 using an A. linearis. A heat treatment of the α-Ni(OH)2 is carried out at 300–400°C for 2 h at normal air yields. Furthermore, we have characterized the structural, optical and photocatalytic activity of the samples. The optical results indicate that biosynthesized nanocrystals exhibit UV–visible light absorption and a narrow range distribution of intense green light (518.95 nm) emission, which decreases significantly as annealing temperature increases. The bandgap energies of the sample annealed at 300–400°C shift to lower photon energy, compared to bulk NiO (∼ 4 eV). Moreover, the photocatalytic experimental results reveal that NiO nanocrystals enable color switching of methylene blue. GRAPHICAL ABSTRACT


Introduction
The semiconductor-metal oxide nanostructures have gained considerable attention from the scientists worldwide due to their promising applications in energy supply and environmental remediation for photoreversible color-switching materials (1)(2)(3)(4)(5). Nickel oxide nanocrystallites has a rock salt structure which exhibits the characteristics of a p-type semiconductor (6). NiO nanostructures are chemically stable and has good electrical conductivity as well as magnetic and optical properties, which make it a promising material for technological applications such as antiferromagnetic materials (7), electrode materials for lithium-ion batteries (8), electrochemical supercapacitors (9), gas sensors adsorbents (10), optical amplifier and tunable laser (11), photovoltaic devices (12) and electrochromic materials (13,14). Methods such as sol-gel method (15), solvothermal route (16), hydrothermal (17), thermal decomposition (18), precipitation method (19), and plants such as Moringa oleifera (20), Tamarix serotine (21), Callistemon viminalis (22) and Agathosma betulina (23) allow the synthesis of NiO nanostructures. Among them, TiO 2 is one of the excellent metal oxide photocatalysts for water splitting and oxidative degradation of organic pollutants (24,25). Irradiated with sufficient photon energy, an electron (e − ) of TiO 2 moves out of its energy level, leaving behind a hole (h + ) (26)(27)(28). The hole reacts with adsorbed OH − or H 2 O on the semiconductor surface to produce (•OH). The electron reduces the molecules present oxygen into (•O 2− ), •OH and •O 2− active species destroy the toxic compounds. Fortunately, electrons and holes recombine mostly, leading a decrease of photocatalytic activity or emigrate into the surface-bound of the semiconductor photocatalyst. Thus, Wang et al. (29) synthesize TiO 2 nanocrystals enabling color switching of methylene blue (MB) in response to light. Their surfacebound molecules scavenge the photogenerated holes of TiO 2 nanocrystals upon UV irradiation and the surviving emigrated electrons reduce MB to colorless leuco methylene blue (LMB), which recovers dramatically its blue spectrum upon visible light. Recently, Han et al. (30) reported the visible light color-switching system of coupling the photoreduction activity of Sn 2+ -doped SnO 2−x nanocrystals with the electrochromic properties of a redox system. The self-doping of Sn 2+ in SnO 2−x redshifts their absorption to visible region and simultaneously produces oxygen vacancies which effectively scavenge the photogenerated holes and thus enable color switching of redox dye using light irradiation. However, there is no report of NiO nanocrystals enabling color switching of Methylene in response to light source under ambient conditions.
In this work, we have reported NiO nanocrystals through α-Ni(OH) 2 by the green novel approach and environmentally friendly pathway using Aspalathus linearis and propose a mechanism of their formation. In our knowledge, we investigated the structural, optical and catalytic performance of NiO nanocrystals in a detailed report.

Materials and methods
In this work, we used A. linearis leaf extract as a reduction/chelating agent. Commonly known as Rooibos, A. linearis is a plant originally found in the Western Cape province of South Africa. It possesses antioxidant, antiaging, anticancer, antidiabetic and antiinflammatory properties and is used for treating cardiac arrhythmias, colic, diarrhea, asthma and hypertension (31) as well in nanomaterials production. The most bioactive compounds reducing/ chelating have been reported in our previous works (28)(29)(30)(31)(32)(33)(34)(35). In typical extraction, high-quality dried flowers of A. linearis leaves were weighted and cleaned with cold distilled water. Ten grams were added to 250 ml of deionized water for 48 h; the obtained extraction was filtered using Whatman paper filter paper. The extracted A. linearis leaves dye pH was ∼5 at room temperature (RT). In the typical biosynthesis of NiO nanocrystals, to the 200 ml of the filtered reddish-brown aqueous extract obtained was added 4 g of [Ni(NO 3 2 ]. The resulting light green products were thermally treated at 300°C and 400°C for 2 h. The samples will be designed as B_NiO_300°C and 400°C B_NiO_400°C where B means biosynthesized. A schematic diagram of the formation of NiO nanocrystals using A. linearis is shown in Figure 1. The major functional groups in products were examined using Fourier transform-infrared absorption spectrometer (Mattson Instruments, Inc., Galaxy 7020A). The crystalline structure of NiO confirmed by powder diffraction, a Bruker D8-Advance diffractometer with Cuk α radiation (λ = 0.15406 nm). JEOL JEM 4000EX electron microscopy unit operating at a 400 kV with a point-topoint resolution of 0.17 nm was conducted to investigate the microstructure of the biosynthesized NiO matrix. The UV-VIS absorption spectra of NiO nanocrystals were performed by using an Agilent 8453 spectrophotometer. Photoluminescence (PL) spectra were measured at RT on a Cary Eclipse Fluorescence Spectrophotometer.

Photocatalytic activity
The photocatalytic activity of biosynthesized NiO nanocrystals system of (B_NiO_W_MB) was tested at RT in a photocatalytic reactor chamber equipped with UV lamps emitting in the range (100-280 nm) located ∼5 cm above the reactor. For the photocatalytic experiment, 5 mg of NiO catalysts was dispersed in 50 ml of 10 mgl −1 MB aqueous solutions containing in 50 ml Becher. The initial pH of the solution was 6.5 for all experiments. Prior to UV illumination, the system was stirred by a magnetic stirrer in dark for 30 min to ensure the adsorption equilibrium of MB. In addition to that, during UV exposure the system is kept homogeneously. At the specific time intervals between 30 min, about 2 ml of the solution was taken from the reaction chamber by a syringe and analyzed with the Agilent 8453 spectrophotometer. The spectra of the sample annealed at 400°C were similar to the specimen thermally treated at 300°C within the considered spectral region. The spectrum (a) shows the typical features α-Ni(OH) 2 , which are described in detail elsewhere (36). The broad absorption band spectrum centered at 3405 cm −1 corresponds to the O-H of hydrogen-bonded water molecules which exist the interlamellar space of α-Ni(OH) 2 (37). The band at 1643 cm −1 is assigned to bending vibrations of the interlayer water molecules (38,39). The weak band at 2380 and 994 cm −1 are attributed to intercalated CO 2− 3 and NO − 3 (40,41). The presence of 1390 cm −1 arises from the interaction of the samples with the KBr in the pellet (42). In the lower wavenumber, the two bands appearing at 715 and 495 cm −1 are attributed to δ OH and ν(Ni-O) vibrations, respectively (43)(44)(45). After annealing at 300°C for 2 h, the δ OH disappeared and a strong band at 449 cm −1 corresponding to the stretching vibration of Ni-O (46) was observed, which clearly reveal the complete dehydration α type of hydroxides, leading to the formation of NiO nanostructures. Figure 3(a) shows the XRD spectrum of the synthesized nanosamples dried at 200°C, which clearly exhibits the characteristic diffraction of α-Ni(OH) 2 (JCPDS card no. 38-0715) (47,48). While annealed temperature is raised up to 300°C, the specimens manifest a uniform crystal structure with five diffraction peaks centered 37.24°, 43.27°, 62.87°, 75.41°and 79.40°, as shown in Figure 3(b). These diffraction peaks are ascribed respectively to crystallographic reflections from (111), (200), (220), (311) and (222) planes of NiO (space group Fm3m, JCPDS card no 00-047-1049) and are in agreement with several reported results (49)(50)(51). The broad peaks indicate the nanoscaled nature of the synthesized NiO nanostructure. The deduced average crystallite size of the NiO particles annealed at 300°C and 400°C, calculated by the Scherer equation (52) was about 6 and 10 nm, respectively. Figure 4(a) shows the TEM image of B_NiO_300°C. It is observed that B_NiO_300°C consists of quasi-spherical shape nanoscaled particles with size around 7 nm, as shown in Figure 4(e). Once rising the annealed temperature up to 400°C, the growth direction of the nanocrystals occurs (Figure 4(c)). The size of B_NiO_400°C is around 14 nm, as shown in Figure 4(f). One was to note for B_NiO-300°C, most of the lattice fringes were not discernable, which indicate the presence of defects in some zone of B_NiO_300°C. However, the lattice is clearly seen from the HRTEM image as shown in Figure  4(d), in which the interplanar distance is about 0.24 nm and consistent with the d spacing of (111) of fcc NiO. The reason for that part of the defect has been altered by the annealing.   Figure 5(a) displays the UV-visible absorption spectrum of biosynthesized NiO nanocrystals. B_NiO_300°C exhibits the absorption maximum ranges from 190 to 237 nm with the long shoulder that extends to visible region when compared to B_NiO_400°C, which showed a noticeable shift toward a shorter wavelength. It is observed that absorption intensity of B_NiO decreases with an increase in annealing temperature. Furthermore, the extension spectrum of visible region is related to the presence of oxygen vacancies in B_NiO nanocrystals. Wang et al. (53) reported similar findings regarding the ZnO nanoparticles, where oxygen species extend to visible region absorption. However, to decrease the absorption peak with an increase in annealing temperature is due to increasing the crystalline size and the reduction effect of crystal defects. The B_NiO_300°C sample captures the photons energy efficiently and exhibits a strong absorption in UV region. The B_NiO_400°C sample possesses a larger particle size to exhibit the weak UV absorption since higher annealing temperature allows improving their structure crystallinity, which may lead to decreasing the bandgap energy (9). This is in accord with XRD and HRTEM analysis, the annealing effect which crystalline has increased, the defects and the grain boundary density of nanograins are decreased, subsequently, the scattering of the carrier effect at grain voids has decreased (54). The absorption bandgap E g is determined by the following equation (26):

Optical studies
where α is the absorption coefficient, hυ is the photon energy, B is a constant related to the material expansion and n (which is equal to 2 for a direct transition or 1/2 for an indirect transition) is determined by quantum selection rules. In this work, we chose n = 2 for the reason that it gives a good fit on the photon energy axis. Figure 5(b) shows the plot (α hυ) 2 vs. hυ for the NiO nano samples annealed at 300°C and 400°C. According to Equation (1), the optical bandgap the NiO nanosamples were estimated through the extrapolation of the linear region on the energy axis as shown in Figure 5 (b). The NiO nanocrystals annealed at 300°C and 400°C possess a bandgap value of 3.4 and 3.5 eV, respectively. Thus, the biosynthesized NiO nanocrystals are the potential candidate for optoelectric applications. It is established that the nanoscaled semiconductor system shows a blue shift in UV-visible studies by the cause of quantum confinement effects. However, one of the energies gaps of B_NiO nanosamples are smaller than the bulk product (4 eV) (55), which is noted possibly as a result of news energy levels in the bandgap region due to the presence of vacancies or chemical defects (56).
To examine the nature of the defect and ion deficiencies, RT Photoluminescence measurements of B_NiO nanosamples under 257 nm excitation wavelength were conducted. Figure 6(a) showed the PL spectra of B_NiO nanosamples in the wavelength range of ∼258-800 nm. Let us note that the relative intensity of narrow edge distribution of green emission of B_NiO_300°C sample at 518.95 nm (∼2.4 eV), which decreases about 64% for the sample B_NiO_400°C. The diminution in PL intensity of B_NiO_400°C at 518.95 nm compared to B_NiO_300°C is attributed to the filling of oxygen voids during heating treatment. Kim et al. (57) reported similar findings in ZnO nanostructures where green emission about 518 nm is attributed to the oxygen vacancies. Second, the effect of Ni 2+ vacancies is not evident, as shown in Figure 6(a). In   (58)(59)(60)(61). In this report, the biosynthesized NiO nanoparticles were treated of visible emission bands of B_NiO_400°C nanosample at 431 nm (2.87 eV), 460 nm (3.08 eV) and 586 nm (2.11 eV) were enhanced compared to B_NiO_300°C sample. Hence, these later visible emission peaks are attributed to the electronic transition of Ni 2+ . The color change of the biosynthesized NiO (greenish to black) can also be justified by the existence of nickel vacancy and or/interstitial oxygen defects. However, in the presence form of O 2− vacancies, the intensities of the visible emissions due to the intraionic transition are negligible compared to the green luminescence originated to the oxygnen vacancies as shown in Figure 6(a).

Photocatalytic application
The light responses of biosynthesized NiO nanocrystals were recorded at functional irradiation time. As shown in Figure 7(a), upon the UV exposure of methylene without B_NiO, slight decreases of absorption peak are recorded after 120 min. It is worth noting that 3% of MB is bleached and recolouration process of MB is not observed. The MB bleached detection is negligible by MB degradation dye, it is seen that the B_NiO_300°C and B_NiO_400°C showed obvious photo-bleaching following the recombination effect of MB, as shown in Figure 7(b-f). We required 90 min of UV irradiation so as to bleach about 46% with B_NiO_300°C, while it took only 30 min with B_NiO_400°C. Thus, the photobleaching ratio of B_NiO_400°C is about three times as that of B_NiO_300°C, which indicated that the heating can improve the bleaching effect of the biosynthesized NiO nanoparticles because photogenerated electronhole pairs are generated less on the surface morphology of photocatalytic, act as a poor crystallinity and the crystal orientation growth would be imperfect (26). The heating effect which leads to a perfect crystal pattern of B_NiO_400°C has a good crystallinity (narrow XRD peaks in Figure 3(b)). As we have highlighted in Section 1, upon UV light irradiation with sufficient equal energy, a mass of electrons and holes are generated in the matrix. The huge amount of electrons and  holes would recombine at recombination centers immediately once the irradiation stopped working and a little amount would diffuse to the surface of OH radical (62). Therefore, the recombination of the photogenerated electrons and holes has been prevented as much as bleaching reaction be favored. Upon UV, B_NiO_400°C reproduced huge electron-hole recombination of charge photocatalyst compound B_NiO_400°C , leading to the right reduction of MB to its colorless LMB. However, to compare the B_NiO_300°C that the fast reduction could not happen for a long duration due to lower content of oxygen vacancy to scavenge the photogenerated hole, which leads to recombination of electrons and holes. In fact, with the oxygen-deficient photocatalytic semiconductor, the oxygen vacancies scavenge the photogenerated holes and subsequently promote the reduction of MB to its colorless leuco (5). On the other hand, the higher photogenerated electronhole ratio of surface morphology and lower oxygen vacancies of B_NiO_400°C nanosample compared to B_NiO_300°C are the reason that the bleaching ratio of B_NiO_400°C is about three times as that of B_NiO_300°C. In this case, it is observed from the Figure 7(a,b) that UV irradiation of MB without NiO photocatalysts cannot reverse the decoloration process. Furthermore, the photo-oxidation quenching process (63) is also negligible at the limitation of experimental condition. To our knowledge, with the presence of NiO photocatalysts, while increasing the UV exposure irradiation time, the reactions were found to occur in the reverse direction of recoloration effect. Thus the recoloration process act by the self-catalyzed oxidation of LMB. Similarly to our findings, the recoloration effect of self-oriented oxidation of leuco methylene blue was reported elsewhere (29). To our understanding, NiO nanocrystals absorb the visible light irradiation and excite LMB to LMB* which reacts the dissolved oxygen species of MB to enhance the recoloration ratio of B_NiO_300°C due to its larger oxygen vacancy content which leads to increase the absorption of visible range spectrum as shown in Figure 5(a).

Conclusion
In summary, we have successfully prepared α-Ni(OH) 2 via A. linearis leaf extract by the heating treatment of nickel hydroxide in normal air atmosphere at 300°C to allow acquiring single phase NiO nanocrystalline matrix with average particles size of 14 nm as supportable of FTIR, XRD and HRTEM investigation. The biosynthesized NiO nanocrystals exhibited UV-visible light absorption and a narrow range distribution of intense green light (518.95 nm) emission. We reported the photocatalytic performance of MB, which shows that the biosynthesized NiO nanocrystals enable color switching of Methylene in response to a light source under ambient conditions. Furthermore, in this work, based on the device orientation of photo-rewritable printers, gas sensing and optoelectronics requires to be carried out and the results obtained compared with existing literatures.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This research program was generously supported by the National Research Foundation of South Africa (NRF), iThemba LABS-National Research Foundation, the UNESCO-UNISA Africa Chair in Nanosciences-Nanotechnology, College of Graduate Studies, University of South Africa, the Centre for Science and Technology of the Non-Aligned and Other Developing Countries (NAM S&T Centre), New Delhi [NAM-05/74/ 2015], the Centre for Nano Science and Nanotechnology, K.S. Rangasamy College of Technology, Tiruchengode 63721, Tamil Nadu, India, the Abdus Salam ICTP via the Nanoscience African network (NANOAFNET) as well as the African Laser Center.