Pristine and Graphene-Quantum-Dots-Decorated Spinel Nickel Aluminate for Water Remediation from Dyes and Toxic Pollutants

: Pristine nickel aluminate and the one decorated with graphene quantum dots were prepared via a cost-e ﬀ ective co-precipitation method. Both were fully characterized by thermogravimetry (TGA), di ﬀ erential scanning calorimetry (DSC), attenuated total reﬂectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray di ﬀ raction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and UV–Vis techniques. The photocatalytic activity of nickel aluminate under simulated solar light irradiation was demonstrated towards potential pollutants, including a series of dyes (rhodamine B, quinoline yellow, eriochrome black T, methylene blue), toxic phenol and fungicide (thiram). Further profound enhancement of the photocatalytic activity of nickel aluminate was achieved after its decoration with graphene quantum dots. The mechanism of the photocatalytic degradation in the presence of the NiAl 2 O 4 / graphene quantum dots (GQDs) composite was investigated; hydroxyl radicals were found to play the leading role. This work o ﬀ ers new insight into the application of the conjunction of the inorganic spinel and the carbon nanostructure (i.e., GQDs), but also provides a simple and highly e ﬃ cient route for potential water remediation from common pollutants, including dyes and colorless harmful substances. Moreover, we succeeded in improving the photocatalytic performance of NiAl 2 O 4 by decorating it with GQDs. We presented the physicochemical characterization of the obtained photocatalyst alongside studies of its photocatalytic activity towards rhodamine B and phenol degradations. The mechanism of the photocatalysis in the presence of the NiAl 2 O 4 / GQDs composite was studied using the TPA method and a series of scavengers. Hydroxyl radicals were found to play a leading role in the photocatalytic activity of the investigated composite. This work not only o ﬀ ers new insight into the application of the conjunction of the inorganic spinel and the carbon nanostructure (i.e., GQDs), but also provides a simple and highly e ﬃ cient route for potential water remediation from common pollutants, including dyes and colorless harmful substances. Moreover, the synthesized composite exhibited multifunctionality, which will be further investigated in an upcoming paper.


Introduction
Nanocrystalline spinel aluminates with the general formula of MAl 2 O 4 (M = Ni, Zn, Mn, Co, Mg, etc.) attract research interest due to their versatile properties. Aluminates have high thermal stability, mechanical resistance, hydrophobicity and low surface acidity. Nickel aluminates are one of the most important aluminate materials, and have been studied for their many applications, including electrochemical sensing [1], pigments [2], catalysts [3][4][5][6][7], photocatalysts [8][9][10][11][12], magnetic [13] and refractory materials [14]. NiAl 2 O 4 has also attracted attention as a hydrogen storage material [15,16], oxygen carrier in combustion loop reactors [17] and as a component of supercapacitor electrode materials [18]. Very recently, attention has been focused on its photocatalytic performance [10][11][12]. First reports described the utilization of nickel-aluminum layered hydroxides for carbon dioxide conversion [19,20] and for dye degradation [21,22]. However, to the best of our knowledge the photocatalytic activity of the nickel aluminate of the spinel structure was demonstrated for the first time in 2015 by A. Sobhani-Nasab et al. [8]. The catalyst was synthesized via sol-gel method, to then be applied for methyl orange degradation under visible light irradiation. M. Rahimi-Nasrabadi et al. [9] performed analogous experiments using ultraviolet light. The photocatalytic activity of the nickel aluminate spinel against a series of dyes (i.e., rhodamine B (RhB), methylene blue (MB), Briefly, 0.62 mmol of citric acid monohydrate and 0.85 mmol of the crystalline spinel were heated to 200 • C for 30 min until the transparent liquid changed color through yellow to amber. Subsequently, the heating temperature was reduced to 140 • C, and 10 mL of deionized water were added. The obtained mixture was heated under stirring until the complete evaporation of water.

Methods
DSC and TGA analyses were performed by Thermal Analyzer TGA/DSC 1 (METTLER TOLEDO, Giessen, Germany) with a heating rate of 15 • C·min −1 under a nitrogen environment with a flow rate of 20 mL·min −1 . All runs were carried out from 25 to 1550 • C. The measurements were made in alumina crucibles with lids.
The powder X-ray diffraction data were measured at 293 K using a SuperNova diffractometer (Rigaku, The Woodlands, TX, USA) with a charge-coupled device (CCD) and a Cu-Kα radiation source at a150 mm sample-to-detector distance.
Scanning electron microscopy images were recorded by secondary-electron SEM with the use of an INSPECT S50 scanning electron microscope (FEI, Hillsboro, OR, USA). The accelerating voltage of the electron beam was 15 keV and the working distance was 10 mm. Images were also obtained with a TEM system (FEI Teknai T20 G2 X-TWIN, Hillsboro, OR, USA) operating at 200 kV, equipped with an LaB 6 source.
The ATR-FTIR spectra (3200-500 cm −1 ) were obtained using a Nicolet Model 6700 FT-IR spectrometer with a DTGS detector (Thermo Scientific, Madison, WI, USA). The crystal-diamond spectra were obtained with 4 cm −1 resolution, and 32 scans for each sample spectrum were obtained. Diffuse reflectance UV-Vis spectra (DRS) were recorded on a Jasco V-30 UV-Vis/NIR spectrophotometer (Jasco, De Meern, Netherlands) equipped with an integrating sphere 60 mm in diameter using BaSO 4 as a reference.
The UV-Vis spectra were recorded with a HITACHI U-2800A UV-Vis spectrophotometer (Hitachi, Tokyo, Japan) equipped with a double monochromator and a single-beam optical system (190-700 nm). A SUNTEST CPS+ (ATLAS, Mount Prospect, IL, USA) solar simulator apparatus was used to perform photocatalytic degradation experiments. The emission spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan): excitation bandwidths 5.0 nm; emission bandwidths 5.0 nm; scan speed 1200 nm·min −1 .

Terephthalic Acid Probe Method
The generation of hydroxyl radicals as a consequence of the irradiation of the aqueous suspension of NiAl 2 O 4 catalyst with the simulated solar light was examined using the terephthalic acid (TPA) probe method [35]. The nickel aluminate particles (2 mg·mL −1 ) were dispersed in a 3 mmol·L −1 TPA solution prepared in a 10 mmol·L −1 NaOH solution. Afterwards, the obtained suspension was sonicated for 10 min and exposed to sunlight for 2 h while vigorous stirring continued. After a given time the suspension was centrifuged, and the fluorescence emission spectrum was measured at the excitation wavelength of λ = 312 nm.

Reactive Species Scavenging
The generation of electron holes, hydroxyl radicals and electrons was determined by treating the reaction suspensions with ammonium oxalate (AO), isopropyl alcohol (IPA) and dimethyl sulfoxide (DMSO) as respective scavengers [36].

Structural and Morphological Study
Thermogravimetric studies were performed to examine the temperature required for the formation of the crystalline form of nickel aluminate. Figure 1 presents DSC, TGA and derivative thermogravimetry (DTG) curves of the as-synthesized NiAl 2 O 4 before annealing. The TGA curve shows distinct mass loss in the temperature range of 230-400 • C, represented by the peak on the DTG curve at 284 • C. In the same temperature window, exothermic peaks on the DSC curve were attributed to the decomposition of the Ni(Ac) 2 ·4H 2 O and the following structural ordering of the nickel aluminate spinel phase. Therefore, it was concluded that 400 • C should be the lowest temperature used for the annealing to obtain the stable final inorganic crystalline product. Accordingly, the as-synthesized nickel aluminate was divided into three portions, which were annealed respectively at 400, 600 and 800 • C.

Reactive Species Scavenging
The generation of electron holes, hydroxyl radicals and electrons was determined by treating the reaction suspensions with ammonium oxalate (AO), isopropyl alcohol (IPA) and dimethyl sulfoxide (DMSO) as respective scavengers [36].

Structural and Morphological Study
Thermogravimetric studies were performed to examine the temperature required for the formation of the crystalline form of nickel aluminate. Figure 1 presents DSC, TGA and derivative thermogravimetry (DTG) curves of the as-synthesized NiAl2O4 before annealing. The TGA curve shows distinct mass loss in the temperature range of 230-400 °C, represented by the peak on the DTG curve at 284 °C. In the same temperature window, exothermic peaks on the DSC curve were attributed to the decomposition of the Ni(Ac)2·4H2O and the following structural ordering of the nickel aluminate spinel phase. Therefore, it was concluded that 400 °C should be the lowest temperature used for the annealing to obtain the stable final inorganic crystalline product. Accordingly, the as-synthesized nickel aluminate was divided into three portions, which were annealed respectively at 400, 600 and 800 °C. XRD images of the NiAl2O4 annealed at 400, 600 and 800 °C ( Figure 2) showed that the contribution of the spinel structure increased with increasing applied temperature. Nickel oxide was observed at lower temperatures (400 and 600 °C), as indicated by diffraction patterns assigned to  XRD images of the NiAl 2 O 4 annealed at 400, 600 and 800 • C ( Figure 2) showed that the contribution of the spinel structure increased with increasing applied temperature. Nickel oxide was observed at lower temperatures (400 and 600 • C), as indicated by diffraction patterns assigned to [200] and [220] lattices (JCPDS No. 47-1049). The pure spinel crystalline form of nickel aluminate (well-matched with JCPDS No. 44-0160), with no remaining cubic NiO, was obtained after annealing at 800 • C. The apparent crystallite diameter (D c ) of NiAl 2 O 4 particles was found to be of 3, 4 and 8 nm for samples annealed at 400, 600 and 800 • C, respectively. The latter numbers were calculated according to the Scherrer equation: D c = kλ/βcosθ, where β is the full width at the half maximum of the diffraction peak, k is the empirical constant (0.9), θ is the angular position of the diffraction peak, and λ is the wavelength of the X-ray source (here 1.5405 Å). The observed increase of crystallite sizes with the increase of the annealing temperature was in agreement with the findings of others reported for inorganic semiconductors (e.g., nickel oxide) [37].
Water 2019, 11, x FOR PEER REVIEW 5 of 15 the diffraction peak, k is the empirical constant (0.9), θ is the angular position of the diffraction peak, and λ is the wavelength of the X-ray source (here 1.5405 Å). The observed increase of crystallite sizes with the increase of the annealing temperature was in agreement with the findings of others reported for inorganic semiconductors (e.g., nickel oxide) [37]. ATR-FTIR spectroscopy studies were undertaken to confirm the purity of nickel aluminate nanoparticles and to investigate the presence of the functional groups on their surface. ATR-FTIR spectra of the samples annealed at 400, 600 and 800 °C were registered in the range of 500-3600 cm −1 (Figure 3). The bands observed at low frequencies within 500-700 cm −1 were attributed to the stretching vibrations of Ni-O, Al-O and Ni-O-Al bonds [8,38]. Moreover, the bands observed in the range of 3200-3500 cm −1 indicated the presence of the O-H surface bonds on the catalyst surface. The bands depicted in Figure 3 were observed for the samples annealed at all applied temperatures, from 400 to 800 °C. However, their intensity increased alongside the increase of the applied annealing temperature, indicating well-developed crystalline structures for nickel aluminate annealed at 800 °C.  ATR-FTIR spectroscopy studies were undertaken to confirm the purity of nickel aluminate nanoparticles and to investigate the presence of the functional groups on their surface. ATR-FTIR spectra of the samples annealed at 400, 600 and 800 • C were registered in the range of 500-3600 cm −1 ( Figure 3). The bands observed at low frequencies within 500-700 cm −1 were attributed to the stretching vibrations of Ni-O, Al-O and Ni-O-Al bonds [8,38]. Moreover, the bands observed in the range of 3200-3500 cm −1 indicated the presence of the O-H surface bonds on the catalyst surface. The bands depicted in Figure 3 were observed for the samples annealed at all applied temperatures, from 400 to 800 • C. However, their intensity increased alongside the increase of the applied annealing temperature, indicating well-developed crystalline structures for nickel aluminate annealed at 800 • C.
Water 2019, 11, x FOR PEER REVIEW 5 of 15 the diffraction peak, k is the empirical constant (0.9), θ is the angular position of the diffraction peak, and λ is the wavelength of the X-ray source (here 1.5405 Å). The observed increase of crystallite sizes with the increase of the annealing temperature was in agreement with the findings of others reported for inorganic semiconductors (e.g., nickel oxide) [37]. ATR-FTIR spectroscopy studies were undertaken to confirm the purity of nickel aluminate nanoparticles and to investigate the presence of the functional groups on their surface. ATR-FTIR spectra of the samples annealed at 400, 600 and 800 °C were registered in the range of 500-3600 cm −1 ( Figure 3). The bands observed at low frequencies within 500-700 cm −1 were attributed to the stretching vibrations of Ni-O, Al-O and Ni-O-Al bonds [8,38]. Moreover, the bands observed in the range of 3200-3500 cm −1 indicated the presence of the O-H surface bonds on the catalyst surface. The bands depicted in Figure 3 were observed for the samples annealed at all applied temperatures, from 400 to 800 °C. However, their intensity increased alongside the increase of the applied annealing temperature, indicating well-developed crystalline structures for nickel aluminate annealed at 800 °C.  The morphology of the NiAl 2 O 4 particles was evaluated based on SEM analysis. Figure 4 demonstrates that as-synthesized nickel aluminate formed agglomerates with particle sizes in the range of 50-200 µm. However, after annealing the size of the agglomerates decreased with increasing temperature. The highest homogeneity was observed for the material annealed at 800 • C ( Figure 4(4a,b)).
The chemical composition and purity of the synthesized nickel aluminate were evaluated using EDX analysis. As shown in Figure 4(1c-4c), Ni, O and Al were the only observed elements in all of the registered curves. Moreover, the decrease of the intensity of the peak attributed to the O element was noticed after annealing. This was due to the formation of the crystalline form of the spinel structure.
Water 2019, 11, x FOR PEER REVIEW 6 of 15 The morphology of the NiAl2O4 particles was evaluated based on SEM analysis. Figure 4 demonstrates that as-synthesized nickel aluminate formed agglomerates with particle sizes in the range of 50-200 μm. However, after annealing the size of the agglomerates decreased with increasing temperature. The highest homogeneity was observed for the material annealed at 800 °C ( Figure  4(4a,b)).
The chemical composition and purity of the synthesized nickel aluminate were evaluated using EDX analysis. As shown in Figure 4(1c-4c), Ni, O and Al were the only observed elements in all of the registered curves. Moreover, the decrease of the intensity of the peak attributed to the O element was noticed after annealing. This was due to the formation of the crystalline form of the spinel structure.

GQDs and NiAl2O4/GQDs Composite
The pristine GQDs exhibited structure of high porosity, as shown in SEM images ( Figure  5(1a,b)), as distinguished from the crystalline NiAl2O4 (Figure 4(4a,b)). Therefore, an increase in the porosity of the NiAl2O4/GQDs composite compared to the pristine spinel was observed, as shown in Figure 5(2a,b). TEM images of GQDs particles ( Figure 5(1c,d)) demonstrated their uniform sizes

GQDs and NiAl 2 O 4 /GQDs Composite
The pristine GQDs exhibited structure of high porosity, as shown in SEM images ( Figure 5(1a,b)), as distinguished from the crystalline NiAl 2 O 4 ( Figure 4(4a,b)). Therefore, an increase in the porosity of the NiAl 2 O 4 /GQDs composite compared to the pristine spinel was observed, as shown in Figure 5(2a,b). TEM images of GQDs particles ( Figure 5(1c,d)) demonstrated their uniform sizes ranging from 2 to 7 nm. However, due to profound differences in the size of GQDs and metal oxide particles, carbon nanostructures could not be distinguished in the TEM image of the NiAl 2 O 4 /GQDs. Nevertheless, their presence in the composites led to an increase in the dispersity of the spinel nanoparticles ( Figure 5(2c)). The TEM image of the pristine NiAl 2 O 4 ( Figure 5(3)) shows an agglomerated structure with particles having an average size of 20 nm. On the other hand, the particles of the NiAl 2 O 4 /GQDs ( Figure 5(2c)) had smaller diameters (between 7-10 nm) and appeared to be separate from each other. The carbon content in GQDs and in the NiAl 2 O 4 /GQDs composites was examined by EDX (see Figure 6). This showed that GQDs presented a moderate oxygen content of 16%. The value of the latter is relevant for photocatalytic activity since surface oxygen groups contribute to the photocatalytic activity on defect sites [39]. ranging from 2 to 7 nm. However, due to profound differences in the size of GQDs and metal oxide particles, carbon nanostructures could not be distinguished in the TEM image of the NiAl2O4/GQDs. Nevertheless, their presence in the composites led to an increase in the dispersity of the spinel nanoparticles ( Figure 5(2c)). The TEM image of the pristine NiAl2O4 ( Figure 5(3)) shows an agglomerated structure with particles having an average size of 20 nm. On the other hand, the particles of the NiAl2O4/GQDs ( Figure 5(2c)) had smaller diameters (between 7-10 nm) and appeared to be separate from each other. The carbon content in GQDs and in the NiAl2O4/GQDs composites was examined by EDX (see Figure 6). This showed that GQDs presented a moderate oxygen content of 16%. The value of the latter is relevant for photocatalytic activity since surface oxygen groups contribute to the photocatalytic activity on defect sites [39].  The UV-Vis diffuse reflectance spectrum (Figure 7) of the crystalline NiAl2O4 (after annealing at 800 °C) displayed a significant absorption in the ultraviolet spectrum range. Additionally, absorption ranging from 2 to 7 nm. However, due to profound differences in the size of GQDs and metal oxide particles, carbon nanostructures could not be distinguished in the TEM image of the NiAl2O4/GQDs. Nevertheless, their presence in the composites led to an increase in the dispersity of the spinel nanoparticles ( Figure 5(2c)). The TEM image of the pristine NiAl2O4 ( Figure 5(3)) shows an agglomerated structure with particles having an average size of 20 nm. On the other hand, the particles of the NiAl2O4/GQDs (Figure 5(2c)) had smaller diameters (between 7-10 nm) and appeared to be separate from each other. The carbon content in GQDs and in the NiAl2O4/GQDs composites was examined by EDX (see Figure 6). This showed that GQDs presented a moderate oxygen content of 16%. The value of the latter is relevant for photocatalytic activity since surface oxygen groups contribute to the photocatalytic activity on defect sites [39].  The UV-Vis diffuse reflectance spectrum (Figure 7) of the crystalline NiAl2O4 (after annealing at 800 °C) displayed a significant absorption in the ultraviolet spectrum range. Additionally, absorption The UV-Vis diffuse reflectance spectrum (Figure 7) of the crystalline NiAl 2 O 4 (after annealing at 800 • C) displayed a significant absorption in the ultraviolet spectrum range. Additionally, absorption in the visible region, due to the d-d transition of Ni(II) and Al(III) was seen. As typical for the normal spinel structure with the tetrahedrally coordinated Ni(II) in the NiAl 2 O 4 lattice absorption, a maximum around 650 nm was found. However, a presence of the inverse spinel structure was also revealed, as indicated by the absorption appearing around 380 and 770 nm. This is known to arise from the octahedral Ni(II) ions [40]. Based on the extrapolation of the linear part of the Kubelka-Munk vs. energy plot, the energy bandgap was calculated to be 2.9 and 2.5 eV for NiAl 2 O 4 and the NiAl 2 O 4 /GQDs composite, respectively. The calculated E g value of the pristine nickel aluminate was close to that reported for spinel (see Table 1). Meanwhile, the synthesized NiAl 2 O 4 /GQDs composite showed a significantly narrower band edge, which corresponded to 470 nm. This wavelength was in the solar spectrum range of the highest intensity [41], indicating a significant potential to harvest renewable solar energy. in the visible region, due to the d-d transition of Ni(II) and Al(III) was seen. As typical for the normal spinel structure with the tetrahedrally coordinated Ni(II) in the NiAl2O4 lattice absorption, a maximum around 650 nm was found. However, a presence of the inverse spinel structure was also revealed, as indicated by the absorption appearing around 380 and 770 nm. This is known to arise from the octahedral Ni(II) ions [40]. Based on the extrapolation of the linear part of the Kubelka-Munk vs. energy plot, the energy bandgap was calculated to be 2.9 and 2.5 eV for NiAl2O4 and the NiAl2O4/GQDs composite, respectively. The calculated Eg value of the pristine nickel aluminate was close to that reported for spinel (see Table 1). Meanwhile, the synthesized NiAl2O4/GQDs composite showed a significantly narrower band edge, which corresponded to 470 nm. This wavelength was in the solar spectrum range of the highest intensity [41], indicating a significant potential to harvest renewable solar energy.

Photocatalytic Activity Study
The photocatalytic activity of the NiAl2O4 nanoparticles was tested against a series of potential water pollutants. These included a series of dyes (i.e., RhB, QY, EB and MB), along with PH and the commonly used fungicide TM. The degradation efficiency is illustrated in Figure 8A as a decrease of the residual concentration ratio (Ct/C0) of each compound during the time of irradiation with the simulated solar light. All examined model contaminants were found to decompose under the applied conditions. The degradation of all model pollutants followed pseudo-first-order kinetics. Therefore, based on the plots presented in Figure 8B, the pseudo-first-order rate constants were calculated and are compared in Table 2. The determined k values increased in the following order: RhB < QY < EB < PH < TM < MB. Among dyes, the most resistant turned out to be RhB, while MB decomposed the

Photocatalytic Activity Study
The photocatalytic activity of the NiAl 2 O 4 nanoparticles was tested against a series of potential water pollutants. These included a series of dyes (i.e., RhB, QY, EB and MB), along with PH and the commonly used fungicide TM. The degradation efficiency is illustrated in Figure 8A as a decrease of the residual concentration ratio (C t /C 0 ) of each compound during the time of irradiation with the simulated solar light. All examined model contaminants were found to decompose under the applied conditions. The degradation of all model pollutants followed pseudo-first-order kinetics. Therefore, based on the plots presented in Figure 8B, the pseudo-first-order rate constants were calculated and are  Table 2. The determined k values increased in the following order: RhB < QY < EB < PH < TM < MB. Among dyes, the most resistant turned out to be RhB, while MB decomposed the easiest. The resistance to photocatalytic decomposition of MB was close that of TM. Tetramethylthiuram disulfide, unlike PH-which represents aromatic compounds-underwent photo-oxidation easily.
Water 2019, 11, x FOR PEER REVIEW 9 of 15 easiest. The resistance to photocatalytic decomposition of MB was close that of TM. Tetramethylthiuram disulfide, unlike PH-which represents aromatic compounds-underwent photo-oxidation easily.  The photocatalytic activity of the NiAl2O4/GQDs was examined towards RhB as a representative dye and towards PH (representative of toxic compounds forming colorless aqueous solutions). Each of the chosen model pollutants from the two examined groups exhibited the most resistance to degradation. The results of the photocatalytic studies obtained in the presence of the synthesized composite were compared with those performed using pristine spinel (see Figure 9).  The photocatalytic activity of the NiAl 2 O 4 /GQDs was examined towards RhB as a representative dye and towards PH (representative of toxic compounds forming colorless aqueous solutions). Each of the chosen model pollutants from the two examined groups exhibited the most resistance to degradation. The results of the photocatalytic studies obtained in the presence of the synthesized composite were compared with those performed using pristine spinel (see Figure 9). To examine the mechanism of the photocatalytic activity of NiAl2O4/GQDs, a composite hydroxyl radical generation probe method with TPA was applied. Figure 10 shows fluorescence spectra as observed for the supernatant solution of the NiAl2O4/GQDs catalyst suspension irradiated with terephthalate (TP) for various durations. A strong fluorescence emission peak was observed at λem = 426 nm. This was assigned to the formation of an adduct (hTP) between TP and hydroxyl radical (Scheme 1), indicating the formation of • OH species in the irradiated suspension. The intensity of the observed emission peak increased linearly within the irradiation time, as shown in the inset of Figure  10. To examine the mechanism of the photocatalytic activity of NiAl 2 O 4 /GQDs, a composite hydroxyl radical generation probe method with TPA was applied. Figure 10 shows fluorescence spectra as observed for the supernatant solution of the NiAl 2 O 4 /GQDs catalyst suspension irradiated with terephthalate (TP) for various durations. A strong fluorescence emission peak was observed at λ em = 426 nm. This was assigned to the formation of an adduct (hTP) between TP and hydroxyl radical (Scheme 1), indicating the formation of • OH species in the irradiated suspension. The intensity of the observed emission peak increased linearly within the irradiation time, as shown in the inset of Figure 10.  In order to investigate which other active species were contributing to the photocatalytic activity of NiAl2O4/GQDs, a series of experiments with established scavengers was performed. Ammonium oxalate (AO), isopropyl alcohol (IPA) and dimethylsulfoxide (DMSO) as electron hole, hydroxyl radical and electron scavenger, respectively, were separately mixed with the reactant mixture containing RhB and NiAl2O4. RhB was subjected to photocatalytic degradation under simulated solar light. As shown in Figure 11, the biggest influence on the photocatalytic degradation of RhB was observed in the presence of hydroxyl radicals. However, since AO (being the hole scavenger) also had a significant influence, it indicated that hydroxyl radicals were generated involving both valence band holes and conduction band electrons. The smallest effect was observed in the presence of DMSO, which may point to the instant reaction of the electrons in the conduction band after excitation of the semiconductor. These observations indicated the low electron-hole recombination effect in the synthesized catalyst. The suggested mechanism of the photocatalytic degradation of the organic pollutants in the presence of the NiAl2O4/GQDs composite is presented in Scheme 2. It shows that after GQDs harvest the sunlight, they give rise to the generation of the electron-hole pairs. The same phenomenon occurs in NiAl2O4 since it also absorbs light from the visible spectrum range. Subsequently, the electrons injected in the conduction band of NiAl2O4 may react with oxygen and lead to the generation of hydroxyl radicals, as shown in the Scheme 2. GQDs prolong the recombination rate of the charge carriers. They also contribute to harvesting the sunlight and are responsible for the adsorption of the pollutants, which ultimately decompose.  In order to investigate which other active species were contributing to the photocatalytic activity of NiAl2O4/GQDs, a series of experiments with established scavengers was performed. Ammonium oxalate (AO), isopropyl alcohol (IPA) and dimethylsulfoxide (DMSO) as electron hole, hydroxyl radical and electron scavenger, respectively, were separately mixed with the reactant mixture containing RhB and NiAl2O4. RhB was subjected to photocatalytic degradation under simulated solar light. As shown in Figure 11, the biggest influence on the photocatalytic degradation of RhB was observed in the presence of hydroxyl radicals. However, since AO (being the hole scavenger) also had a significant influence, it indicated that hydroxyl radicals were generated involving both valence band holes and conduction band electrons. The smallest effect was observed in the presence of DMSO, which may point to the instant reaction of the electrons in the conduction band after excitation of the semiconductor. These observations indicated the low electron-hole recombination effect in the synthesized catalyst. The suggested mechanism of the photocatalytic degradation of the organic pollutants in the presence of the NiAl2O4/GQDs composite is presented in Scheme 2. It shows that after GQDs harvest the sunlight, they give rise to the generation of the electron-hole pairs. The same phenomenon occurs in NiAl2O4 since it also absorbs light from the visible spectrum range. Subsequently, the electrons injected in the conduction band of NiAl2O4 may react with oxygen and lead to the generation of hydroxyl radicals, as shown in the Scheme 2. GQDs prolong the recombination rate of the charge carriers. They also contribute to harvesting the sunlight and are responsible for the adsorption of the pollutants, which ultimately decompose. In order to investigate which other active species were contributing to the photocatalytic activity of NiAl 2 O 4 /GQDs, a series of experiments with established scavengers was performed. Ammonium oxalate (AO), isopropyl alcohol (IPA) and dimethylsulfoxide (DMSO) as electron hole, hydroxyl radical and electron scavenger, respectively, were separately mixed with the reactant mixture containing RhB and NiAl 2 O 4 . RhB was subjected to photocatalytic degradation under simulated solar light. As shown in Figure 11, the biggest influence on the photocatalytic degradation of RhB was observed in the presence of hydroxyl radicals. However, since AO (being the hole scavenger) also had a significant influence, it indicated that hydroxyl radicals were generated involving both valence band holes and conduction band electrons. The smallest effect was observed in the presence of DMSO, which may point to the instant reaction of the electrons in the conduction band after excitation of the semiconductor. These observations indicated the low electron-hole recombination effect in the synthesized catalyst. The suggested mechanism of the photocatalytic degradation of the organic pollutants in the presence of the NiAl 2 O 4 /GQDs composite is presented in Scheme 2. It shows that after GQDs harvest the sunlight, they give rise to the generation of the electron-hole pairs. The same phenomenon occurs in NiAl 2 O 4 since it also absorbs light from the visible spectrum range. Subsequently, the electrons injected in the conduction band of NiAl 2 O 4 may react with oxygen and lead to the generation of hydroxyl radicals, as shown in the Scheme 2. GQDs prolong the recombination rate of the charge carriers. They also contribute to harvesting the sunlight and are responsible for the adsorption of the pollutants, which ultimately decompose. The reusability of NiAl2O4/GQDs was studied in four successive recycling experiments for the photocatalytic degradation of RhB. The catalyst was separated from the reaction suspension by centrifugation, washed with ethanol and water (four times each) and dried in the oven at 100 °C. As shown in Figure 12, NiAl2O4/GQDs retained its photocatalytic activity after four successive experimental runs. A slight decrease was observed after the first use. However, in consecutive runs the photocatalytic activity remained unchanged and retained 96% of its original efficiency. The reusability of NiAl2O4/GQDs was studied in four successive recycling experiments for the photocatalytic degradation of RhB. The catalyst was separated from the reaction suspension by centrifugation, washed with ethanol and water (four times each) and dried in the oven at 100 °C. As shown in Figure 12, NiAl2O4/GQDs retained its photocatalytic activity after four successive experimental runs. A slight decrease was observed after the first use. However, in consecutive runs the photocatalytic activity remained unchanged and retained 96% of its original efficiency. The reusability of NiAl 2 O 4 /GQDs was studied in four successive recycling experiments for the photocatalytic degradation of RhB. The catalyst was separated from the reaction suspension by centrifugation, washed with ethanol and water (four times each) and dried in the oven at 100 • C. As shown in Figure 12, NiAl 2 O 4 /GQDs retained its photocatalytic activity after four successive experimental runs. A slight decrease was observed after the first use. However, in consecutive runs the photocatalytic activity remained unchanged and retained 96% of its original efficiency.

Conclusions
Highly efficient nickel aluminate nanoparticles with spinel crystal structures were successfully synthesized via a simple and cost-effective co-precipitation method. A comprehensive study of the photocatalytic performance of the degradation of different water pollutants, including a series of dyes (i.e., rhodamine B, quinoline yellow, eriochrome black T, methylene blue), phenol and fungicide (thiram) under simulated solar light irradiation was carried out in this study. Moreover, we succeeded in improving the photocatalytic performance of NiAl2O4 by decorating it with GQDs. We presented the physicochemical characterization of the obtained photocatalyst alongside studies of its photocatalytic activity towards rhodamine B and phenol degradations. The mechanism of the photocatalysis in the presence of the NiAl2O4/GQDs composite was studied using the TPA method and a series of scavengers. Hydroxyl radicals were found to play a leading role in the photocatalytic activity of the investigated composite. This work not only offers new insight into the application of the conjunction of the inorganic spinel and the carbon nanostructure (i.e., GQDs), but also provides a simple and highly efficient route for potential water remediation from common pollutants, including dyes and colorless harmful substances. Moreover, the synthesized composite exhibited multifunctionality, which will be further investigated in an upcoming paper.

Conclusions
Highly efficient nickel aluminate nanoparticles with spinel crystal structures were successfully synthesized via a simple and cost-effective co-precipitation method. A comprehensive study of the photocatalytic performance of the degradation of different water pollutants, including a series of dyes (i.e., rhodamine B, quinoline yellow, eriochrome black T, methylene blue), phenol and fungicide (thiram) under simulated solar light irradiation was carried out in this study. Moreover, we succeeded in improving the photocatalytic performance of NiAl 2 O 4 by decorating it with GQDs. We presented the physicochemical characterization of the obtained photocatalyst alongside studies of its photocatalytic activity towards rhodamine B and phenol degradations. The mechanism of the photocatalysis in the presence of the NiAl 2 O 4 /GQDs composite was studied using the TPA method and a series of scavengers. Hydroxyl radicals were found to play a leading role in the photocatalytic activity of the investigated composite. This work not only offers new insight into the application of the conjunction of the inorganic spinel and the carbon nanostructure (i.e., GQDs), but also provides a simple and highly efficient route for potential water remediation from common pollutants, including dyes and colorless harmful substances. Moreover, the synthesized composite exhibited multifunctionality, which will be further investigated in an upcoming paper.

Funding:
We gratefully acknowledge the financial support from the Polish Ministry of Science and Higher Education under subsidy granted to the Faculty of Biology and Chemistry, University of Bialystok for R&D and related tasks aimed at development of young scientists and PhD students and for maintaining the research potential of the Faculty of Biology and Chemistry, University of Bialystok. Diffractometer, IR spectrometer, SEM and TEM microscopes, DSC and TGA instruments, UV-Vis/NIR spectrophotometer and spectrofluorometer were funded by EU, as part of the Operational Programme Development of Eastern Poland, projects Nr POPW.01.03.00-20-034/09-00 and POPW.01.03.00-20-004/11-00.