Rare-Earth Metals-Doped Nickel Aluminate Spinels for Photocatalytic Degradation of Organic Pollutants

: Visible-light-activated photocatalysts based on samarium-doped, europium-doped, and gadolinium-doped nickel aluminates (SmNA, EuNA, GdNA) were synthesized. The spinel crystalline structures of the doped mixed metal oxides were demonstrated by X-ray di ﬀ raction (XRD) and transmission electron microscopy (TEM) analysis. The presence of the rare-earth metals (REMs) was conﬁrmed by the energy-dispersive X-ray (EDX) studies. Ultraviolet-visible-near-infrared (UV-Vis-NIR) spectra revealed that the REMs-doped catalysts absorb in the full solar spectrum range covering both visible and near infrared wavelengths. Scanning electron microscopy (SEM) visualized the profound morphological alterations of the doped nickel aluminate samples. Consequently, the pore volume and the Brunauer-Emmett-Teller (BET) surface area decreased, while nanoparticles sizes increased. Fourier-transform infrared spectroscopy (FTIR) exposed that surfaces of REMs-doped nickel aluminates are rich in hydroxyl groups. Finally, the photocatalytic performance was notably increased through doping nickel aluminate (NA) with REMs; the highest activity was observed for EuNA.

The rare-earth metals have been already proposed as dopants for spinel double oxides other than nickel aluminate. The first report dates back to 2001 when magnetic properties of CoFe 2 O 4 doped with series of lanthanide ions (Ce, Sm, Eu, Gd, Dy, Er) were studied [16]. Shortly afterwards, magnesium titanium ferrites doped with Nd, Gd, and La were prepared [17]. In 2005, Peng et al.

Structural and Morphological Study
The phases and structures of the prepared pristine and REM-doped nickel aluminate (NA) were examined by powder X-ray diffraction. The XRD diffractograms of spinel NiAl 2 O 4 (JCPDS 10-0339), γ-Al 2 O 3 (JCPDS 50-0741) and NiO (JCPDS 89-7131) were included for comparison. As depicted in Figure 1, NA presents a spinel structure; its diffraction pattern coincides with the spinel nickel aluminate NiAl 2 O 4 as (111), (220), (311), (400), (511), and (440) crystallographic planes are represented by the peaks at 2θ values of 19.1, 31.4, 37.0, 45.0, 59.7, and 65.5 [36]. The corresponding reflections are also present in the patterns registered for SmNA, EuNA, and GdNA, and thus the latter exhibit spinel nickel aluminate structure as well. Additionally, we observed the diffraction patterns arising from the NiO phase. The latter occur at the 2θ values of 37.2, 43.3, and 62.8 and correspond to (111), (200), and (220) planes, respectively [37]. The additional contribution of the cubic NiO is present in both pristine and in the doped NA samples. Nevertheless, the lowest content of the NiO phase is present in SmNA, and is increasing in the following order SmNA < NA < GdNA < EuNA. Additionally, the XRD peak widths revealed the small sizes of the crystallites of all the prepared samples; further confirmed by TEM studies.
The Tauc plots depicted in Figure 2a allowed us to compare the energy of inner electron transitions present in the pristine nickel aluminate and in the REM-doped catalysts. Indeed, we observed multiple linear zones, which point to the series of transitions arising from possible rotational and vibrational changes, fluorescence, phosphorescence, and the presence of trapping sites. The extrapolation of the linear parts of the Tauc plots in the low energy region revealed the pronounced difference of the pristine nickel aluminate band gap and the REM-doped semiconductor samples. While the least energetic transition for NA corresponds to 1.70 eV, the transitions of the REM-doped NA range from 0.42 for SmNA and 0.39 eV for EuNA to 0.30 eV for GdNA (Table 1). Therefore, incorporation of the rare-earth metal cations into the inorganic framework contributed to the significant decrease of the transition energy reaching the NIR solar spectrum range. The UV-Vis-NIR reflectance spectra confirmed the nickel aluminate spinel structure of the prepared catalysts ( Figure S1). The band maximizing at 366 nm arises from both LMCT (ligand-to-metal charge transfer) and octahedral Ni(II) 3A 2g -3T 1g transitions [38][39][40]. The octahedrally coordinated Ni(II) species are also manifested through the bands at 518 and 675 nm [41]. In turn, shoulders occurring at 580 and 617 nm arise from the tetrahedrally coordinated Ni(II) ions. Upon NA doping with the REMs cations the bands attributed to the O h Ni(II) appeared to decrease, while those assigned to T h Ni(II) slightly increased. Therefore, we demonstrated that Sm(III), Eu(III), and Gd(III) cations preferred to be octahedrally coordinated.
Catalysts 2020, 10, x FOR PEER REVIEW 3 of 13 also manifested through the bands at 518 and 675 nm [41]. In turn, shoulders occurring at 580 and  617 nm arise from the tetrahedrally coordinated Ni(II) ions. Upon NA doping with the REMs cations  the bands attributed to the Oh Ni(II) appeared to decrease, while those assigned to Th Ni(II) slightly increased. Therefore, we demonstrated that Sm(III), Eu(III), and Gd(III) cations preferred to be octahedrally coordinated.  The FTIR spectroscopy analysis allowed us to prove the spinel nickel aluminate structure of the prepared materials and to investigate the presence of the functional groups on the catalysts' surfaces (see Figure 2b). The pristine and REM-doped nickel aluminates possessed the intense and broad bands with maxima at 3456 cm −1 . The latter demonstrated the presence of a considerable amount of hydroxyl groups-an attractive feature of photocatalysts [42,43]. Furthermore, the bands which occurred at 1639 cm −1 arose from the H-O-H deformation vibrations of water molecules. Since the calcination of the catalysts underwent at 800 °C, and thus water should have been eliminated, we attributed the presence of the adsorbed water molecules to the absorption from the atmosphere during the IR pellets formation [44,45]. Importantly, all samples revealed the bands in the 400-1000  also manifested through the bands at 518 and 675 nm [41]. In turn, shoulders occurring at 580 and 617 nm arise from the tetrahedrally coordinated Ni(II) ions. Upon NA doping with the REMs cations the bands attributed to the Oh Ni(II) appeared to decrease, while those assigned to Th Ni(II) slightly increased. Therefore, we demonstrated that Sm(III), Eu(III), and Gd(III) cations preferred to be octahedrally coordinated.  The FTIR spectroscopy analysis allowed us to prove the spinel nickel aluminate structure of the prepared materials and to investigate the presence of the functional groups on the catalysts' surfaces (see Figure 2b). The pristine and REM-doped nickel aluminates possessed the intense and broad bands with maxima at 3456 cm −1 . The latter demonstrated the presence of a considerable amount of hydroxyl groups-an attractive feature of photocatalysts [42,43]. Furthermore, the bands which occurred at 1639 cm −1 arose from the H-O-H deformation vibrations of water molecules. Since the calcination of the catalysts underwent at 800 °C, and thus water should have been eliminated, we attributed the presence of the adsorbed water molecules to the absorption from the atmosphere during the IR pellets formation [44,45]. Importantly, all samples revealed the bands in the 400-1000  The FTIR spectroscopy analysis allowed us to prove the spinel nickel aluminate structure of the prepared materials and to investigate the presence of the functional groups on the catalysts' surfaces (see Figure 2b). The pristine and REM-doped nickel aluminates possessed the intense and broad bands with maxima at 3456 cm −1 . The latter demonstrated the presence of a considerable amount of hydroxyl groups-an attractive feature of photocatalysts [42,43]. Furthermore, the bands which occurred at 1639 cm −1 arose from the H-O-H deformation vibrations of water molecules. Since the calcination of the catalysts underwent at 800 • C, and thus water should have been eliminated, we attributed the presence of the adsorbed water molecules to the absorption from the atmosphere during the IR pellets formation [44,45]. Importantly, all samples revealed the bands in the 400-1000 spectrum range, where the characteristic metal-oxygen stretching vibrations from the inorganic framework were expected. Indeed, we observed the vibrational modes for Ni-O, Al-O, and Ni-O-Al bonds, i.e., at 494, 603, 734, and 803 cm −1 , respectively. The bands at 494 and 734 cm −1 we assigned to the octahedral (O h ) and tetrahedral (T h ) coordinated metal-oxygen bonds, respectively [38,46]. Additionally, the bands occurring at 1415 and 1523 cm −1 arose from the stretching vibrational modes associated with Al-OH bonds [46]. In turn, the band present at 1106 cm −1 we attributed to the Ni-OH stretching vibrations [35]. Moreover, in the spectra of SmNA, EuNA, and GdNA samples the tiny N-O stretching vibrations at 1384 cm −1 arose [47]. They resulted from the residual nitrogen coming from the nitrate precursors of the rare-earth metal ions.
The TEM studies ( Figure 3) pointed out that crystallite sizes were in order of nanometers. The broadest XRD peaks appeared for EuNA, which is in line with the smallest sizes of the same sample revealed by TEM analysis. The EuNA nanoparticles had a diameter of ≈5 nm and single particles can be differentiated in Figure 3(3b). Meanwhile, the nanoparticles of GdNA exhibited diameters in the range of 5-10 nm, and single particles were more difficult to distinguish. Similarly, particles of SmNA tended to aggregate and had diameters between 5 and 15 nm. The biggest sizes of particles were demonstrated by the pristine NA, which were in the range 10-20 nm. The d-spacing values calculated for the reflections visualized by HRTEM images (Figure 3(1c-4c)) confirmed the presence of cubic NiO (d 111 = 0.24 nm) in the REM-doped NA with the highest content observed for EuNA. In the HRTEM images of all the prepared samples ( Figure 3(1c-4c)) the interplanar spacing corresponding to the spinel nickel aluminate is visualized; d 111 = 0.46 nm.
The elemental compositions of the synthesized REM-doped materials were examined by the energy dispersive X-ray (EDX) mapping analysis ( Figure 4). The presence of nickel, aluminum, and oxygen in all samples was confirmed. Moreover, the respective lanthanides were found in SmNA, EuNA, and GdNA samples. range of 5-10 nm, and single particles were more difficult to distinguish. Similarly, particles of SmNA tended to aggregate and had diameters between 5 and 15 nm. The biggest sizes of particles were demonstrated by the pristine NA, which were in the range 10-20 nm. The d-spacing values calculated for the reflections visualized by HRTEM images (Figure 3(1-4c)) confirmed the presence of cubic NiO (d111 = 0.24 nm) in the REM-doped NA with the highest content observed for EuNA. In the HRTEM images of all the prepared samples ( Figure 3(1-4c)) the interplanar spacing corresponding to the spinel nickel aluminate is visualized; d111 = 0.46 nm.  The elemental compositions of the synthesized REM-doped materials were examined by the energy dispersive X-ray (EDX) mapping analysis ( Figure 4). The presence of nickel, aluminum, and oxygen in all samples was confirmed. Moreover, the respective lanthanides were found in SmNA, EuNA, and GdNA samples. Subsequently, the morphologies were investigated using SEM technique. The images of the pristine and REM-doped NA samples are depicted in Figure 5. At the microscale, NA spinels form a compact porous sponge-like structure with many channels (Figure 5(1a-c)). The morphology of REMdoped NA samples ( Figure 5(2a-c,3a-c,4a-c)) differs significantly from the morphology exhibited by pristine catalyst, namely, a tendency to agglomerate is observed. Separate aggregates of REM-doped NA samples can be distinguished and their sizes increase in the following order, EuNA < SmNA < GdNA. The latter tendency correlates well with the sizes of nanoparticles of the respective materials derived from TEM investigations. Afterwards, this relation was corroborated by the physisorption analysis. Thus, the morphology of the reported catalysts should be translated to the specific surface area. Therefore, the nitrogen adsorption-desorption studies were performed.
The surface area estimated based on BET isotherms reached the highest value for the pristine Subsequently, the morphologies were investigated using SEM technique. The images of the pristine and REM-doped NA samples are depicted in Figure 5. At the microscale, NA spinels form a compact porous sponge-like structure with many channels (Figure 5(1a-1c)). The morphology of REM-doped NA samples ( Figure 5(2a-2c,3a-3c,4a-4c)) differs significantly from the morphology Catalysts 2020, 10, 1003 6 of 13 exhibited by pristine catalyst, namely, a tendency to agglomerate is observed. Separate aggregates of REM-doped NA samples can be distinguished and their sizes increase in the following order, EuNA < SmNA < GdNA. The latter tendency correlates well with the sizes of nanoparticles of the respective materials derived from TEM investigations. Afterwards, this relation was corroborated by the physisorption analysis. Thus, the morphology of the reported catalysts should be translated to the specific surface area. Therefore, the nitrogen adsorption-desorption studies were performed.

Photocatalytic Activity Study
The photocatalytic activities of the pristine and Sm, Eu, and Gd-doped nickel aluminates, i.e., NA, SmNA, EuNA, and GdNA, were examined through the phenol degradation over the given photocatalyst and under the simulated visible light irradiation. The efficiency of the photocatalytic degradation was calculated according to the following formula η = (1 − Ct/C0) × 100%, where C0 and Ct denote initial and final concentrations, respectively. The photocatalytic degradation efficiencies, together with the kinetic parameters, i.e., constant rates and the degradation half-life times, are collected in Table 2. Figure 6a depicts the phenol degradation within 2 h of the irradiation over the prepared catalyst. The catalysts doped with the rare-earth metals demonstrated better photocatalytic The surface area estimated based on BET isotherms reached the highest value for the pristine spinel sample, 13.9 m 2 g −1 . This value was within range obtained by others, i.e., 6.9-21 m 2 g −1 [33,39]. The subsequent NA doping with the REM contributed to the decreases in BET surface area for SmNA, EuNA, and GdNA (Table 1). In fact, EuNA exhibited the highest BET surface area among all doped spinel samples; 11.1 m 2 g −1 . This value was only slightly smaller from the BET surface area of the pristine NA. In turn, GdNA exhibited a two-times smaller specific surface area of 6.9 m 2 g −1 , whereas the BET surface area of SmNA reached 9.7 m 2 g −1 . Moreover, EuNA exhibited the highest pore width and correspondingly the highest pore volume. Accordingly, SEM images showed the biggest distances between agglomerates for EuNA catalyst ( Figure 5(3a-3c)). On the other hand, GdNA had the highest tendency to agglomerate as visualized by SEM studies (Figure 5(4a-4c)). Therefore, the increase in size of the doped catalyst agglomerates translated into a decrease of BET surface area. Indeed, REM cations possess high ionic radii (Sm 3+ , 0.958 Å; Eu 3+ , 0.947 Å; Gd 3+ , 0.938 Å) [48], considerably higher with respect to Ni 2+ (0.69 Å) [48] and Al 3+ (0.535 Å) [48]. Therefore, the substitution of the nickel cations generates the defects on cationic lattice sites and additionally may force oxygen vacancies [49][50][51]. Consequently, the induced structural disorder is responsible for the increased particles sizes and thus smaller surface area. Similarly, the herein observed decrease of NA surface area after doping with REM aligned well with the findings of others for doped spinels. Namely, Friedrich et al. [33] demonstrated, that BET surface area of nickel aluminate decreases after doping with nobium from 17 to 8 m 2 g −1 , respectively.

Photocatalytic Activity Study
The photocatalytic activities of the pristine and Sm, Eu, and Gd-doped nickel aluminates, i.e., NA, SmNA, EuNA, and GdNA, were examined through the phenol degradation over the given photocatalyst and under the simulated visible light irradiation. The efficiency of the photocatalytic degradation was calculated according to the following formula η = (1 − C t /C 0 ) × 100%, where C 0 and C t denote initial and final concentrations, respectively. The photocatalytic degradation efficiencies, together with the kinetic parameters, i.e., constant rates and the degradation half-life times, are collected in Table 2. Figure 6a depicts the phenol degradation within 2 h of the irradiation over the prepared catalyst. The catalysts doped with the rare-earth metals demonstrated better photocatalytic performance with respect to the pristine spinel aluminate. The EuNA catalyst showed the highest photocatalytic activity. Phenol degradation over EuNA reached 93%. Correspondingly, decomposition of phenol in the presence of EuNA was described by the highest rate constant (0.0220 min −1 ) and the lowest half-life time (32 min) ( Table 2). The photodegradation efficiency decreased from EuNA in the following order, EuNA < SmNA < GdNA < NA. Nevertheless, the photocatalytic performances of the doped samples were similar. The SmNA and GdNA samples allowed for 90 and 89% (Table 2) of phenol degradation, respectively. Accordingly, the half-life times estimated for SmNA and GdNA reached 36 and 37 min ( Table 2). On the other hand, the photocatalytic performance of the pristine NA was considerably lower compared to the rare-earth metal-doped nickel aluminates. Notably, the efficiency of the phenol degradation over pristine NA decreased to 79% (Table 2). Correspondingly, in the given conditions, rate constant and half-life time equaled 0.0136 min −1 and 51 min (Table 2), respectively. Subsequently, we examined the mechanism responsible for the enhancement of the photocatalytic performance with the REM-doped nickel aluminate samples. With this in mind, we conducted the series of photocatalytic experiments in the presence of selected scavengers. For that purpose, we employed ammonium oxalate (AO), dimethylsulfoxide (DMSO) and isopropyl alcohol (iPrOH). Figure 6b-d shows that after addition of isopropyl alcohol, degradation efficiency with the presence of all REM-doped spinels decreased to the highest extent. Notably, the ηfor the EuNA decreased from 93% to 24% ( Table 2). As Figure 6a demonstrates, EuNA showed the highest photocatalytic activity among all examined photocatalysts. Therefore, addition of isopropyl alcohol affected ηof EuNA to the highest extent. Photodegradation efficiencies of SmNA and GdNA in the presence of iPrOH decreased from 90% to 53% and from 89% to 65%, respectively. Accordingly, we concluded that generation of hydroxyl radicals was crucial for the photocatalytic performances of REM-doped nickel aluminate catalysts (Scheme 1). Nevertheless, hydroxyl radicals may be formed with the contribution of both electron holes in valence band (VB) and electrons in conduction band (CB). The photocatalytic experiments conducted with the addition of AO and DMSO pointed out that hydroxyl radicals are generated at both VB and CB. As exemplified for EuNA, addition of AO and DMSO decreased ηby 43% and 55%, respectively. On the other, applied scavengers had smaller effects on SmNA and GdNA photocatalytic performances, as they exhibited smaller activity with respect to EuNA. Thus, we hypothesized that lower amount of hydroxyl radicals was produced at their surface.
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 13 Nevertheless, all rare-earth metal-doped nickel aluminate samples revealed higher photocatalytic activity compared to the pristine NA. The trivalent cations, Sm(III), Eu(III), and Gd(III), doped into the nickel aluminate spinel structure, were able to harvest the electrons generated after the photocatalyst excitation. Therefore, they prevent recombination of the electron-hole pair, and thus allowed for producing the hydroxyl radicals. The latter, subsequently, led to the decomposition of the model pollutant-phenol.  Nevertheless, all rare-earth metal-doped nickel aluminate samples revealed higher photocatalytic activity compared to the pristine NA. The trivalent cations, Sm(III), Eu(III), and Gd(III), doped into the nickel aluminate spinel structure, were able to harvest the electrons generated after the photocatalyst excitation. Therefore, they prevent recombination of the electron-hole pair, and thus allowed for producing the hydroxyl radicals. The latter, subsequently, led to the decomposition of the model pollutant-phenol. Nevertheless, all rare-earth metal-doped nickel aluminate samples revealed higher photocatalytic activity compared to the pristine NA. The trivalent cations, Sm(III), Eu(III), and Gd(III), doped into the nickel aluminate spinel structure, were able to harvest the electrons generated after the photocatalyst excitation. Therefore, they prevent recombination of the electron-hole pair, and thus allowed for producing the hydroxyl radicals. The latter, subsequently, led to the decomposition of the model pollutant-phenol.

Synthesis of NiAl 2 O 4
Nickel aluminate catalyst was prepared by a coprecipitation method. In a typical experiment nickel and aluminum were coprecipitated from an aqueous nitrate solution (5 mmol·L −1 ) by adding an aqueous ammonia (1 mol·L −1 ) solution to produce a pH of about 8. The molar ratio of Al/Ni in the solution was 2.0. The precipitates were collected by filtration, washed with water, and dried at 100 • C for 6 h. The as-synthesized nickel aluminate was subjected to calcination in air at 800 • C for 3 h to obtain the crystalline spinel, which was labeled as NA.

Synthesis of Sm, Eu, and Gd-doped NiAl 2 O 4
The REMs-doped NA samples were prepared according to the procedure described for the synthesis of NiAl 2 O 4 . Additionally, the nitrate of the corresponding rare-earth metal (5 mmol·L −1 ) was mixed with the nickel and aluminum nitrates following the 0.05:1:2 molar ratio. The obtained samples of Sm, Eu, and Gd-doped NA were labeled as SmNA, EuNA, and GdNA, respectively.

Characterization of Photocatalysts
The powder X-ray diffraction data were measured at 293K using a SuperNova diffractometer (Rigaku, The Woodlands, TX, USA) with a CCD detector and a Cu-Kα radiation source at the 158 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. EDX TEM images were obtained by a transmission electron microscope system (FEI Teknai T20 G2 X-TWIN, Hillsboro, OR, USA), operating at 200 kV, equipped with a LaB 6 source.
Diffuse reflectance UV-Vis spectra 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.
Nitrogen adsorption/desorption measurements were performed using an ASAP2020 analyzer (Micromeritics Corp., Norcross, GA, USA) at liquid N 2 temperature. Before the experiment all samples were degassed for 20 h at 130 • C under reduced pressure.
The FTIR spectra were recorded in the transmission mode in the range 400-4000 cm −1 with a Nicolet 6700 spectrometer (Thermo Scientific, Madison, WI, USA) at room temperature. The spectra were collected at a resolution 4 cm −1 .
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).
The SUNTEST CPS+ (ATLAS, Mount Prospect, IL, USA) solar simulator apparatus was used to perform photocatalytic degradation experiments. SUNTEST CPS+ emits irradiation in the range of 380-700 nm.

Photocatalysis Experiments
The photocatalytic degradation experiments were investigated in a 50 mL glass cell. The SUNTEST CPS+ solar simulator apparatus was applied as the irradiation source. The photon flux of solar-simulated radiation was equal to 2.32 × 10 −6 Einstein s −1 for 250 W m −2 . The photocatalytic experiment was conducted under the room temperature (24 • C). The reaction mixture consisted of 20 mL of the phenol (PH, 10 −4 mol·L −1 ) aqueous solution and a photocatalyst (1.5 g·L −1 ). Prior to photocatalytic experiments, the catalyst was settled in suspension for 60 min in the dark for the adsorption-desorption equilibrium. All solutions were prepared using deionized water, which was obtained by Polwater apparatus (Polwater, Cracow, Poland). The photocatalytic degradation of PH followed pseudo-first-order kinetics; thus, the photocatalytic rate constant was determined according to the following equation: ln(C 0 /C t ) = kt, where C 0 -initial concentration, C t -concentration after time t, k-kinetic rate constant, t-time. The half-life time, t 1/2 , was determined from the following equation: t 1/2 = ln2/k.

Reactive Species Scavenging
The generation of electron holes, hydroxyl radicals, and electrons was determined by treating the reaction suspensions with ammonium oxalate (AO), dimethyl sulfoxide (DMSO), and isopropyl alcohol (IPA) at concentrations of 0.5 mg mL −1 each of the respective scavengers [12]. The chemicals outlined above easily react with electron holes, free electrons, and hydroxyl radicals, respectively, and thus are known as their scavengers. The irradiation excites the semiconductor photocatalyst; hence electron holes appear in the valence band (VB), whereas free electrons are injected into the conduction band (CB). Moreover, in the aqueous environment, hydroxyl radicals are generated with the contribution of VB (Scheme 1). In turn, electrons injected to the CB produce the superoxide radicals, which subsequently convert into hydroxyl radicals. Thus the decrease in the photodegradation efficiency in the presence of the given scavenger can clearly indicates the importance of each radical species for the photocatalytic degradation and thus point to the mechanism, as depicted in Scheme 1.

Conclusions
Herein, we designed visible-light-activated photocatalysts by utilizing the synergism of the Sm(III), Eu(III), and Gd(III) ions and the spinel nickel aluminate (NA). We examined the structural and morphological features of the pristine NA and the REMs-doped NA, i.e., SmNA, EuNA, and GdNA. The introduced REMs cations allowed for the utilization of NIR solar spectrum range, while NA exploited the visible part of the sunlight. Furthermore, the incorporation of the REMs cations contributed to the decrease of the electron-hole pair recombination. As a result, we succeeded in harvesting the broad range of the solar spectrum, and moreover, in enhancing the efficiency of the photocatalytic reaction. The obtained materials presented the enhanced potential to decompose phenol, the model toxic pollutant. The EuNA showed the highest photocatalytic performance. In that manner, we demonstrated the feasibility of synthesizing the Vis-NIR-activated photocatalysts through the doping of NA with the REMs cations.