NiO/Ag heterostructure: enhanced UV emission intensity, exchange interaction and photocatalytic activity

Abstract

The effect of direct Ag coating on the optical and catalytic properties of NiO flake-like structures has been investigated in detail in this work. Ag nanoparticle decorated NiO flakes were synthesized using a facile two-step process. The phases and crystal structure were examined using an X-ray diffractometer, whereas the morphologies of the synthesized nanostructures were investigated using FESEM and HRTEM. Interfacial interaction of the synthesized heterostructures was examined using Fourier transformation infrared and Raman spectroscopy. Importantly the LO phonon and magnon scattering mode of NiO are significantly affected by the Ag coating. The band gap of NiO is found to be lowered (direct band gap: 3.52 to 2.78 eV and indirect band gap: 2.72 to 0.50 eV) due to the change in valence band energy originating from the charge transfer process between NiO and Ag. A UV emission that depends on the crystal field splitting of the Ni d orbitals is observed to possess a blue shift along with their enhancement of intensity. Variation of the magnon scattering mode also reveals that the enhancement of exchange interaction with NiO nanoflakes causes the blue shift of the UV emission. The nonradiative part corresponding to this UV emission which is parameterized using the Huang-Rhys factor gets enhanced due to the lowering of the electron–phonon interaction. The photocatalytic activity of the synthesized heterostructures was evaluated using the catalytic decomposition of the dye methylene blue in aqueous solution. The results show that the NiO/Ag heterostructures exhibit a much higher degradation rate than virgin NiO flakes that might be from improved separation of the photogenerated electrons and holes. In particular, the pseudo-first-order degradation rate constant of NiO/Ag is found to be 10 times greater than bare NiO.

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Introduction

Nanostructures of transition metal oxides are some of the most nurtured oxides in recent times due to their exotic physical and chemical properties, completely different from bulk, and their application aspects in various optoelectronic applications.^1–3 In this context, it may be stated that NiO being an antiferromagnetic p-type semiconducting material has also proven it’s potential use in the fields of catalysis,⁴ battery cathodes,⁵ gas sensors,⁶ fuel cell electrodes,⁷ photoelectrochemical water splitting,⁸ dye sensitized solar cell⁹ etc. due to its suitable electrical, optical and magnetic properties. Various morphologies of NiO have been synthesized previously by many researchers in order to tune their electronic structure and related properties. As an example, NiO nanofibers and nanorods are found to exhibit effective glucose sensitivity in alkaline medium due to their high surface to volume ratio.^10,11 Recently, heterostructure formation using organic as well as inorganic materials has been found to be another effective method by which various surface related properties of nanostructures are able to be modified.^12–19

Like ZnO and TiO₂, a few heterostructures have already been synthesized with NiO and their related properties have been explored. For example, the processes of photogeneration, separation and recombination of charge carriers have been investigated at the heterointerfaces of NiO/P3HT, NiO/CH₃NH₃PbI₃, NiO/acetonitrile, NiO/water etc. and their effect on photoelectrochemical activity has been examined.²⁰ The coaxial carbon nanofibers/NiO core–shell structure is found to be a potential anode material for lithium ion batteries.²¹ Pt–NiO nanoplate array/reduced graphene oxide nanocomposite exhibits non-enzymatic glucose sensing ability.²² The enhanced HCHO sensing properties of α-Fe₂O₃@NiO nanofibers have been investigated by Cao et al.²³ Compared to ZnO, much less effort has been made to investigate noble metal–NiO heterostructures. In this regard, Yu Ding et al. synthesized NiO/Ag heterostructures for blood–glucose sensing.²⁴ In this context it may be stated that Raman scattering, luminescence properties, and the catalytic activity of ZnO are significantly tuned by Ag, Au and Pt coatings.

Till now, no effort has been made to explore the optical and catalytic properties of NiO/Ag heterostructures. In this paper, we have investigated the effect of a NiO/Ag heterostructure on the optical and catalytic properties of a NiO flake. Here, the heterostructure is synthesized using a two-step process. The interfacial interaction is studied using Fourier transform infrared and Raman spectroscopy. The charge transfer process between NiO and Ag has been proposed from optical characterizations. To explore the effect of charge transfer on optoelectronic properties, the photocatalytic activities of the as prepared heterostructures have been tested using methylene blue. In summary, it may be mentioned that though there exist several methods to modify the optical and magnetic properties of NiO, here we have first illustrated that heterostructure formation with a noble metal may be another method to modify the optical properties of NiO, but magnetic exchange interaction may also be tuned.

Experimental

Synthesis of NiO/Ag heterostructure

Materials. Nickel nitrate hexahydrate [Ni(NO₃)₂·6H₂O], urea (Co(NH₂)₂), hexamethylenetetramine (HMTA, (CH₂)₆N₄) and oleylamine were purchased from Sigma-Aldrich. Silver nitrate (AgNO₃), toluene, 2-propanol and ethanol were obtained from MERCK. All of the chemicals are of analytical grade and used without further purification. Triply distilled deionized water obtained from a Milli-Q water purification system (Millipore) was used throughout the experiment. The NiO/Ag heterostructure synthesis involves a two-step process, where a porous NiO nanoflake was prepared using a hydrothermal route followed by calcinations. In the next step, the heterostructure was formed using a solution phase chemical method by which the growth of silver nanoparticles was carried out on the surface of the NiO nanoflakes.

Synthesis of the NiO nanoflakes

It is a typical synthesis procedure where 1.57 g of Ni(NO₃)₂·6H₂O, 1.57 g of urea and 0.785 g of HMTA (the molar ratio is 1 : 1 : 0.5) were added to 90 mL of DI water. The green mixture was stirred for 1 hour at room temperature to make it homogenous. Then the mixture was transferred into a 100 mL Schott bottle and was kept in a hot air oven, maintained at 90 °C. After 6 hours of reaction, a dense, light green precipitation appeared in the bottle which was filtered and washed repeatedly using distilled water and ethanol. Light green powder was obtained after drying at 40 °C for 4 hours. The green powder was calcined at 450 °C for 2 hours in a furnace (marked as NFO).

Formation of the hetero-structure

In the next step, the NFO (1.0 mmol) sample was used as a seed layer for the synthesis of silver nanoparticles where AgNO₃ was reduced to metallic silver with the help of 5.8 mL of oleyamine in 80 mL of toluene.²⁵ Two samples were prepared by fixing NiO : AgNO₃ with molar ratio 1 : 1 (1.0 mmol, named as NAG1) and 1 : 2 (2.0 mmol, named as NAG2). The mixtures were taken into a 100 mL Schott bottle, and then with the help of an oil bath, the solution was heated up to 85 °C for 8 hours, under vigorous stirring. The samples were collected using centrifugation, followed by drying at room temperature. The NAG1 sample appeared grey and for the NAG2 the grey color became paler. The synthesized products were preserved for further characterizations.

Characterization

The crystalline phases of the synthesized samples were investigated using X-ray diffraction (XRD) using the CuKα emission (λ = 1.54 Å, Rigaku-Ultima-III). The morphology and elemental analysis of the samples were examined with FESEM (S – 4800, Hitachi) and EDX (JEOL JSM 6300, Oxford-7582) respectively. High resolution transmission electron microscopy (HRTEM, JEOL) was used to take a high magnified image, selected area electron diffraction (SAED) and lattice fringes. Attachment processes of the silver on NiO flakes were studied using Fourier transform infrared (FTIR, SHIMADZU IR Prestige 21) and Raman spectroscopy (Raman spectrometer alpha 300, Witec, laser excitation wavelength 530 nm). Optical properties of the synthesized samples were investigated using a JASCO V650 spectrophotometer whereas room-temperature photoluminescence (PL) spectra of the samples were recorded using a JASCO-S8200 spectrofluorometer.

Photocatalytic study

Methylene Blue (MB, aqueous stain solution, Merck) was used as a representative dye pollutant to evaluate the photocatalytic activity of the as prepared samples. For the catalytic investigation, a stock solution was prepared by dissolving 52 μL of MB into 100 mL of DI water. 20 mL of the as prepared stock solution was taken into a glass beaker and 20 mg of NFO, NAG1 and NAG2 were added to the solutions. Before degradation in the presence of UV light, the mixture solution was kept under dark conditions for 1 hour without further agitation to achieve an adsorption–desorption equilibrium. To monitor the degradation process, the mixture solution was stirred at different time intervals viz. 30, 90, 150, 210 minutes under radiation. The decanted solution was collected via centrifugation. The degradation process was monitored using absorption spectroscopy, measured with the help of a JASCO V650 spectrophotometer within the range of 250–750 nm. The powdered samples were dried and were kept for phase analysis if there are any phase changes.

Results and discussion

Crystal structure and microstructure of NiO/Ag heterostructure nanocrystals

The diffraction patterns, obtained from the X-ray diffractometer, of NFO, NAG1 and NAG2 are shown in Fig. 1. The peaks (marked by #), identified at 2θ values of 37.28, 43.30, 62.92, 75.44 and 79.39 correspond to the reflections from the (111), (200), (220), (311), and (222) crystal planes of cubic NiO [JCPDS data card no. 78-0643], and are observed for all synthesized samples.

Fig. 1 XRD spectrum of NFO, NAG1, and NAG2.

For NAG1 and NAG2 four additional peaks (marked by *), measured at 2θ values of 38.12, 44.28, 64.43 and 77.47 are attributed to the reflections from the (111), (200), (220) and (311) planes of the fcc Ag peaks [JCPDS data card 040783]. No peaks other than Ag and NiO are present in the diffraction patterns, indicating the purity of the synthesized materials. In addition, no significant shift in the diffraction patterns of NAG1 and NAG2 i.e. changes in the lattice parameter are noticed, indicating no formation of a Ag_xNi_1−xO solid solution. The existence of peaks corresponding to both NiO and Ag confirms the coexistence of NiO and Ag within the synthesized nanohybrids.²⁶ The average crystallite size of the NiO flakes, calculated from the well-known Scherrer’s relation (0.9λ/β cos θ), where β represents the FWHM of the corresponding XRD peak, are found to be 25 nm. The crystallite sizes of the Ag nanoparticles were found to be 35 and 36 nm for NAG1 and NAG2 respectively indicating that the particle size is getting enhanced by the introduction of Ag.^26–28 The detailed reaction mechanism has been discussed in the ESI.†

In order to obtain more insight into the microstructure and morphology of the as-prepared samples, FESEM and TEM were employed. As investigated using FESEM (shown in Fig. 2(a)), the morphology of NFO is found to be a well-defined 3D flake-like structure hierarchical in nature with good monodispersity between flakes. From the high magnification FESEM image of NFO (Fig. S1 in ESI†), it is clear that the widths and thicknesses of the flakes are ∼600 and 4.38 nm respectively. Selected areas for the elemental mapping of NAG1 and NAG2 using EDS are presented in Fig. S2(a) and (b) in the ESI† respectively and Fig. S2(c) and (d) in the ESI† show their oxygen, nickel and silver mappings. EDS results, obtained at various parts of the synthesized samples, indicate that the prepared samples are mainly composed of nickel, oxygen and silver.

Fig. 2 (a): FESEM image of NFO, (b): TEM image (c) lattice image and (d): SAED pattern of the NAG2 sample.

It can be observed from the figures that the NiO/Ag systems remain in the 3D flake-like structure the same as NFO and varied amounts of the silver nanoparticles with diameters ∼32 nm are found on the surfaces of the NiO flakes. Ag percentage measured at different positions of the samples is not found to vary significantly i.e. the as prepared samples are found to be homogeneous. Ag concentration, measured using EDS, is found to be 3.83 and 9.34 (atomic%) for NAG1 and NAG2 respectively i.e. it may be concluded that with increasing percentage of Ag, deposition of Ag on the NiO flake increases. Careful analyses of the images predict that with increasing Ag content coarsening of the nanoflake surfaces increases. In addition, uniform distribution of Ag (blue dots in Fig. S2(c) and (d)†) on the EDS also supports the homogeneity of the synthesized samples.

The structure of the synthesized samples was investigated further using HRTEM. Fig. 2(b) shows the typical HRTEM image of the as prepared NAG2 sample. The image depicts the unique flake-like morphology of NiO and the homogeneous decoration of Ag nanoparticles on the NiO flakes. It may be also inferred that individual Ag nanoparticles are well adhered onto the surface of the flake. In addition, a few aggregations of Ag nanoparticles are also observed. One typical high magnification TEM image of an individual Ag on the NiO nanoflake is presented in Fig. 2(c), it illustrates a well defined lattice fringe having interplanar spacing (d) ∼ 2.39 Å corresponding to the (111) plane of fcc Ag.^29,30 Therefore it may be concluded that the TEM images of NAG2 have full consistency with the obtained SEM results. The SAED pattern recorded at the interface of NiO and Ag corresponding to sample NAG2, is shown in Fig. 2(d) it illustrates a mixture of the NiO [220] and [200] zone diffraction spots and the Ag [111] and [311] zone diffraction spots. The mixed patterns originating separately from cubic NiO and fcc Ag are attributed to the Ag nanoparticles attached on the surface of the NiO flake-like structure in agreement with the XRD data. Similar types of SAED pattern were obtained previously by Warule et al. for ZnO/Ag heterostructures.³¹

As represented in the FTIR spectra (Fig. S3, ESI†), no characteristic peak corresponding to the Ag–O bond is observed, while the band, appearing at 569,³² has been observed for NFO and is assigned to the stretching vibration of the tetrahedral Ni–O cluster within an anisotropic environment.^33,34 Presence of this peak in all samples suggests that there is no structural change within the NiO. But the stretching vibrational energy gets significantly reduced with increasing silver concentration i.e. there exists a strong coordination interaction between the NiO and Ag nanoparticles that favours adsorption of Ag on NiO.³⁵ Reduction of the vibrational energy may be attributed to the charge transfer between them.³⁶

Fig. 3 illustrates the Raman spectra of the synthesized heterostructures. As depicted in the figure, the Raman spectrum of NFO consists of three peaks located at 505, 1142 and 1538 cm⁻¹ which are assigned to the longitudinal optical (LO) one-phonon (1P) mode, the longitudinal optical (2LO) two-phonon (2P) mode^37–39 and the two-zone boundary magnons (2M) scattering mode.^39,40 In this context, it may be stated that first order Raman scattering could not be observed as k = 0 phonons in the cubic phase transform as Γ₄⁻.⁴¹ Thus the observed first order LO mode might be originating from the parity-breaking imperfections near the flake-surfaces.^39,42 It is further noticed that the intensity of the LO peak increases in the presence of Ag.

Fig. 3 Raman spectra of NFO, NAG1 and NAG2.

This enhancement may be assigned to the enhanced parity-breaking of phonons due to the local electric field that acts on the flakes originating from the electrons of the Ag nanoparticles. Analysis of the LO mode illustrates that the Raman shift corresponding to the LO mode increases from NFO to NAG1 to NAG2 (shown in Fig. S4, ESI†) i.e. the strength of the LO mode gets enhanced. Such enhancement may be ascribed to the charge transfer from Ag to NiO (discussed later). Careful analysis of the magnon scattering reveals that the Raman shift for 2M scattering increases with respect to Ag decoration on the NiO flake-surfaces (shown in Fig. S4, ESI†). It was established earlier that the next-nearest-neighbour antiferromagnetic exchange (J) interaction (180°) within NiO possesses a much higher value than the nearest-neighbour (90°) ferromagnetic interaction and is responsible for type-II antiferromagnetism with spin S = 1.⁴³ Raman shift (γ_2M) for 2M scattering is related to exchange interaction by the following equation^44,45

γ_2M = 4JZS − 2J

where Z (=6) represents the number of next-nearest-neighbors. Thus the enhancement of Raman shift corresponding to 2M scattering is attributed to the enhancement of J due to the enhanced charge at the oxygen sites (discussed later). In addition, two peaks at 1330 and 1401 cm⁻¹ and the single peak at 1366 cm⁻¹ have been observed for NAG1 and NAG2 respectively. For any infinitely extended perfect crystal, phonons at the centre of the Brillouin (l = 0) zone contribute to the Raman scattering, as determined by the momentum conservation rule between phonons and incident light. However for nanostructured materials, the selection gets relaxed and phonons with l ≠ 0 contribute to the Raman scattering, resulting in new vibration modes. In the case of metal–semiconductor nanoheterostructures, there exists two theories namely electromagnetic theory and chemical theory to explain the modified Raman spectrum.^46,47 Electromagnetic theory illustrates the effect of excitation of localized plasmon on Raman scattering, whereas chemical theory relies on the charge transfer between metal and semiconducting materials.⁴⁸ Now within our present NiO/Ag heterostructure, the enhanced Raman scattering intensity is attributed to the charge transfer from Ag into NiO after forming the NiO–O⋯Ag bond. Therefore it may be concluded that a polarizing field would arise at the NiO/Ag heterostructure due to the transfer of charge that would activate a few acoustic modes of vibration in the high frequency region. The appearance of a Raman scattering peak for NAG1 and NAG2 may be ascribed to the multiple – acoustic phonon scattering mode, developed from the polarization field during the growth of the NiO/Ag heterostructure.⁴⁹

Optical properties and electron–phonon interactions within the NiO/Ag heterostructure

Optical properties of the NiO/Ag heterostructure were examined using UV-Vis spectroscopy and photoluminescence (PL) spectroscopy taking identical amounts of the samples. As depicted in Fig. 4, absorption curves of the NiO/Ag nano-flake heterostructures are similar in nature to those of bare NiO nano-flake structures. The direct band gap (E_g) of the NiO/Ag heterostructure was calculated using the well-known equation

α = A(hν − E_g)^1/2/hν (A(hν − E_g)²/hν, for indirect band gap),

where α, ν, and A represent the absorption coefficient, frequency of incident light and constant respectively.⁵⁰ As presented in the inset of Fig. 4, the direct band gap of the bare NiO nano-flake is found to be 3.52 eV, whereas the indirect band gap (not shown) is calculated to be 2.72 eV.⁵¹ Absorption onset of bare NiO, identified at 400 nm, is noticed to be situated at an energy lower than the intrinsic band-gap of bare NiO nanostructures, confirming an indirect transition within the structures. However, for the Ag decorated NiO nanostructures, it has been realized that the absorption of the heterostructure is not just the superposition of the absorption spectra of their individual single-component materials. In addition, the red shift of the onset of the absorption spectra and band gap (direct 3.20 and 2.78 eV, indirect 1.65 and 0.50 eV), obtained for NAG1 and NAG2, is attributed to the strong interfacial electronic interaction between the NiO flakes and Ag nanoparticles due to interdispersion of Ag nanoparticles into the NiO flakes.⁵² In this context it may be stated that the first principle study reveals that the bottom of the conduction band and top of the valence band originate from Ni 3d and O 2p states respectively and the absorption edge of NiO originates from the ligand (O 2p) to metal (Ni 3d) charge transfer.⁵³ Our previous FTIR and Raman spectroscopic study revealed that there exists a strong coordination interaction within the synthesized heterostructures. Since the electronegativity difference between O (3.44) and Ag (1.93) is larger than the difference between Ni (1.91) and Ag, the interaction is mostly mediated via O i.e. O 2p orbital is getting affected significantly within the heterostructures and is responsible for the variation in band gap. In addition, it may also be stated that due to the very low electronegativity difference between Ni and Ag, the Ni 3d orbital is not getting affected significantly (also discussed later). This may also be explained in a different way: due to the lower work function of the Ag nanoparticles (4.26 eV)³⁰ than NiO (5.23 eV)⁵⁴ electron transfer occurs from the Ag nanoparticles into the NiO nanostructures (schematic band diagram is shown in Fig. 5) causing the variation of the valence band potential (E_vb) and may be calculated from the relation E_vb = X − E_e + 1/2E_g, where X (=5.7 eV) is the electronegativity of the materials (geometric mean of the electronegativity of the constituting atoms)⁵⁵ E_e (=4.5 eV) represents the energy of a free electron in the hydrogen scale. As presented in Fig. S5, ESI,† it is noticed that E_vb comprised of O 2p orbitals decreases.⁵⁶ Since electrons have a negative charge potential energy for electrons with O 2p orbitals gets enhanced. Therefore the band gap reduction is attributed to the increased valence band potential due to electron–electron repulsion.

Fig. 4 UV-Vis absorption spectrum of the samples (inset: band gap of NFO sample).

Fig. 5 Schematic band diagram of the heterostructure.

The luminescence properties of NiO were investigated previously by many researchers. In this context it may be stated that two bands (361 and 392 nm) in the UV region due to the near band edge emission, two bands (436 and 467 nm) in the visible region due to the surface oxygen vacancy and green emission (560 nm) due to the cation vacancy, interstitial oxygen trapping etc.^51,57 were observed in NiO. As revealed by Qi et al., the luminescence spectrum of NiO nanostructures also consists of a single peak at 391 nm (3.17 eV) and one peak at 467 nm (2.66 eV).⁵⁸ Activation energy and lifetime corresponding to these transitions were examined by Guerra et al.⁵⁹ Interestingly, NiO nanowires exhibit a strong orange peak (660 nm) and relatively weak red peak (680 nm), though a UV emission peak corresponding to the near band edge emission has been found to be absent.⁶⁰ In the present paper, two luminescence peaks, measured at 360 nm in the UV region and 429 nm in the visible region, have been observed for NiO (shown in Fig. 6(a)).

Fig. 6 Photoluminescence spectra of (a) NFO (b) NAG1 (c) NAG2 sample, (d) variation of the FWHM of the UV emission.

The former one is assigned to the near band edge emission, whereas the later one is attributed to the surface oxygen vacancy.^51,57 As presented in Fig. 6(b) and (c), the emission spectra of NAG1 and NAG2 also consist of two peaks in the UV and visible region respectively. Careful analysis of the spectra reveals that the UV emission peak shifts towards a lower wavelength i.e. blue shift in the UV emission after Ag decoration (plotted in Fig. S6, ESI†). In contrast to red shift of the band gap of NiO/Ag heterostructure, we have observed blue shift in this emission spectrum. Therefore, the present emission in contrast to the p–d charge transfer is referred to as the intersite d–d crystal field transitions within the Ni²⁺ ions.^61–63 Charge transfer from Ag to oxygen (as revealed from a previous study) increases crystal field splitting due to electrostatic coulomb interaction between the O 2p-orbitals and Ni 3d-orbitals according to molecular orbital theory i.e. blue shift of UV emission peak may be attributed to the charge transfer from Ag to oxygen as discussed earlier. This phenomenon is supported from 2M Raman scattering. According to discussion by Anderson, the crystal field splitting (10Dq) is related to J for NiO by the following equation^64,65

Our previous Raman study reveals that J increases in the presence of Ag decoration i.e. it may be concluded that crystal field splitting gets enhanced and causes blue shift of the UV emission. In contrast to the UV emission, visible emission exhibits non-linearity, initially its emission energy decreases and finally gets enhanced. The initial red shift may be assigned to the p–d charge transfer mechanism as described earlier. The later one is still to be investigated. For better clarity of the shifts in the luminescence spectrum, an enlarged version of it is represented in Fig. S6, ESI.† The striking point to mention here is that the full width at half maxima (FWHM) corresponding to the UV emission monotonically decreases with respect to Ag decoration (shown in Fig. 6(d)). It was realized earlier that the FWHM of the emission spectra significantly depends on the nonradiative transition which by consequence is determined by the electron–LO phonon interaction. In this context, the Huang-Rhys factor (S) is defined to attribute the electron–LO phonon interaction and was established to be proportional to the square of FWHM corresponding to the luminescence spectra.⁶⁶ Mathematically, the S-factor is expressed by the following expression:

Where, , ħω_LO and represent the electron–LO phonon interaction strength, energy of the LO vibrational mode and electron density respectively. Our previous Raman study indicates that the strength corresponding to the LO vibrational energy increases with Ag incorporation, thus the reduction of FWHM i.e. S-factor may be ascribed to the enhancement of electron–LO phonon within NiO due to the transfer of charge as discussed earlier. Therefore the shrinkage of FWHM leads to a decrement of the nonradiative transition; hence enhancement in the intensity corresponding to UV emission is caused by increased radiative transition.

Photocatalytic activity within the NiO/Ag heterostructure

The photocatalytic activity of the NiO/Ag heterostructure was examined by monitoring the photodegradation of MB under UV light irradiation. Fig. 7 illustrates the variations of MB concentration with UV light irradiation time using NFO, NAG1 and NAG2 as photocatalysts. The concentration of MB was determined from absorbance spectra and the degradation efficiency was calculated via the following equation

η = (C₀ − C)/C₀ × 100

where C₀ is the initial concentration of the dye and C is the final concentration of the dye after UV irradiation.

Fig. 7 MB degradation under UV irradiation (a) NFO (c) NAG1 (d) NAG2, (b) showing the magnified image of the NFO graph.

The efficiency of the NFO, NAG1, and NAG2 is found to be 2.75%, 41% and 70% (after 210 minutes) respectively. Fig. S7(b)† shows the linear relationship between the ln(C/C₀) versus ‘t’ justifying the fact that the reactions followed pseudo 1^st order kinetics. Now according to the Langmuir–Hinshelwood kinetic model, pseudo 1^st order rate constant (K_app) may be calculated using the following equation⁵⁶

ln(C − C₀) = K_app × t

The values of K_app are presented in Fig. S7(c).† It is evident from the above data that the K_app monotonically increases from the NFO to NAG2 samples implying that the addition of Ag on NiO surfaces has significantly increased the catalytic properties of NiO. The basic mechanism behind the observed photocatalytic process involves the excited electrons in the conduction band getting trapped by the electron absorbing agents such as the adsorbed O₂ and generating superoxide anion radicals (O²⁻). At the same time, holes in the valance band produce hydroxyl radicals (˙OH).³⁰ Our previous optical studies revealed that the electrons tunnel through the heterostructure formed between the metallic Ag and semiconducting NiO due to the difference in their work function. When such heterostructures are irradiated using UV light with an energy greater than the band gap energy of NiO, electrons are excited from the valence band to the conduction band of NiO. Since the energy of the conduction band of NiO, as presented in Fig. 5, is higher than that of the Fermi energy of Ag, excited electrons migrate from the conduction band of NiO into the Fermi energy of Ag through an interfacial charge transfer. Thus the enhanced value of K_app is attributed to the electron–hole separation within the heterostructure. In this context it may be stated that photocatalytic degradation of methyl orange (MO) was studied using NiO/TiO₂ and Ag/TiO₂ heterostructures. It is interesting to note that the K_app, obtained for our sample (NAG2) is approximately three times smaller than the maximum K_app obtained by them, but our synthesized sample possesses comparable K_app with that of P25.⁶⁷ It is also noted that 3% Ag coated TiO₂ nanoparticles degrade almost 90% MO within 200 minutes.¹² But our synthesized NAG2 sample degrades 70% MB within 210 minutes. The difference in the rate constant may be attributed to either the difference in the degradation rate for MO and MB or the difference in degree of charge separation by the synthesized nanostructures.

Conclusions

In summary, flake-like NiO/Ag heterostructures have been fabricated by a two-step method. NiO flakes were initially synthesized using a hydrothermal process. Highly dense Ag nanoparticles were then grown on the top of the NiO flakes using a chemical method. FESEM and HRTEM studies confirm the formation of such heterostructures. Strong interfacial interactions between the NiO flakes and Ag nanoparticles are also supported by an FTIR spectroscopic investigation. LO optical phonon and magnon scattering modes within NiO flakes were examined using Raman spectroscopy. It has been observed that Ag attachment has a strong influence on these scattering modes due to charge transfer between oxygen and silver atoms. Specifically, it may be stated that Ag attachment significantly alters the exchange interaction within NiO due to the transfer of charge. It is also noted that the band edge gets red shifted in NiO/Ag heterostructures. UV emission is mostly determined by d–d crystal field splitting of Ni d that gets blue shifted originating from the variation of exchange interaction. Photocatalytic degradation of methylene blue in aqueous solution has been investigated in detail by employing pristine NiO and Ag decorated NiO nanostructures. Apparent photocatalysis reaction rates for different heterostructures have been found to follow the order of NAG2 > NAG1 > NFO implying the significant photostability-dependent photocatalysis behavior of the NiO/Ag heterostructures.

Acknowledgements

One of us (SM) wishes to thank the Council for Scientific and Industrial Research (CSIR), Government of India, for awarding him a Senior Research Fellowship during execution of the work. We also acknowledge the University Grant Commission, Govt of India for university with potential scheme (UPE - II).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09432e

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