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Molecular-Level Understanding of Selectively Photocatalytic Degradation of Ammonia via Copper Ferrite/N-Doped Graphene Catalyst under Visible Near-Infrared Irradiation Hang Zhang, Yang Zhou, Shou-Qing Liu * , Qin-Qin Gu, Ze-Da Meng and Li Luo * Jiangsu Key Laboratory of Environmental Functional Materials, School of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, Suzhou 215009, China; [email protected] (H.Z.); [email protected] (Y.Z.); [email protected] (Q.-Q.G.); [email protected] (Z.-D.M.) * Correspondence: [email protected] (S.-Q.L.); [email protected] (L.L.); Tel.: +86-512-6841-5070 (S.-Q.L.) Received: 18 August 2018; Accepted: 15 September 2018; Published: 20 September 2018







Abstract: Developing photocatalysts with molecular recognition function is very interesting and desired for specific applications in the environmental field. Copper ferrite/N-doped graphene (CuFe2 O4 /NG) hybrid catalyst was synthesized and characterized by surface photovoltage spectroscopy, X-ray powder diffraction, transmission electron microscopy, Raman spectroscopy, UV–Vis near-infrared diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The CuFe2 O4 /NG catalyst can recognize ammonia from rhodamine B (RhB) in ammonia-RhB mixed solution and selectively degrade ammonia under visible near-infrared irradiation. The degradation ratio for ammonia reached 92.6% at 6 h while the degradation ratio for RhB was only 39.3% in a mixed solution containing 100.0 mg/L NH3 -N and 50 mg/L RhB. Raman spectra and X-ray photoelectron spectra indicated ammonia adsorbed on CuFe2 O4 while RhB was adsorbed on NG. The products of oxidized ammonia were detected by gas chromatography, and results showed that N2 was formed during photocatalytic oxidization. Mechanism studies showed that photo-generated electrons flow to N-doped graphene following the Z-scheme configuration to reduce O2 dissolved in solution, while photo-generated holes oxidize directly ammonia to nitrogen gas. Keywords: molecular recognition; selective photocatalysis; N-doped graphene; copper ferrite; Z-scheme configuration; Ammonia

1. Introduction The selectively photocatalytic oxidization of specific pollutants in practical multicomponent systems such as waters contaminated by organics and ammonia is of significance for the removal of specific pollutants. The World Health Organization recommends that the total amount of ammonia (NH3 ) in drinking water should not exceed 1.5 mg/L [1]. Therefore, the development of the photocatalysts with the specific response to NH3 is very vital for the removal of ammonia in water treatments, which involves the molecular recognition to NH3 . CuFe2 O4 is an interesting material with the band gap of 1.7 eV due to its unique electronic configuration of valence shell (Cu 3d10 4s1 ) [1]. It has been widely applied in magnetic memory, high-frequency devices, sensors, drug delivery, anode materials, and catalysts owing to its advantages of environmental benignity, moisture insensitive, high dispersion, high reactivity, low price, large abundance of Cu and easy separation with an external magnet [2–5]. CuFe2 O4 was coupled Catalysts 2018, 8, 405; doi:10.3390/catal8100405

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price, large abundance of Cu and easy separation with an external magnet [2–5]. CuFe2O4 was coupled to 8,graphene [6], TiO2 [7], AgBr [8] and Ag3PO4 [9] to fabricate composites for degrading Catalysts 2018, 405 2 of 19 organic pollutants. Wang and co-workers utilized CuFe2O4 as a photocatalyst to selectively degrade methylene blue in the presence of methylene orange, rhodamine B and rhodamine 6G, and the to graphene [6], TiO AgBr degradation [8] and Ag3 PO to fabricate composites forsites degrading organic 2 [7], 4 [9]specific authors attributed the selective to the interaction of active of catalyst with pollutants. Wang and co-workers utilized CuFe O as a photocatalyst to selectively degrade methylene the methylene blue molecule [10]. To the best2 of4 our knowledge, however, the coupling of CuFe2O4 blue in the presence graphene of methylene orange, rhodamine B and rhodamine 6G, and the to nitrogen-doped (NG) for the selective photocatalytic oxidization of authors NH3 hasattributed not been the selective degradation to the specific interaction of active sites of catalyst with the methylene blue reported. molecule [10]. To best of our knowledge, however, the coupling CuFe nitrogen-doped 2 O4 to Graphene is the a two-dimensional sp2-hybridized carbon material of with unique properties such as graphene (NG) for the selective photocatalytic oxidization of NH has not been reported. 3 excellent charge transport, outstanding transparency, huge specific surface area, high mechanical Graphene is a two-dimensional sp2-hybridized material with unique properties such strength and superior thermal conductivity, so it wascarbon often used as a co-catalyst [11–17]. Moreover, as excellenttailoring charge transport, huge specific surfacecan area, high mechanical molecular (nitrogen outstanding atoms were transparency, doped into graphene framework) module its intrinsic strength and superior thermal conductivity, so it was often used as a co-catalyst [11–17]. properties to meet the rapidly increasing demand for practical applications in various Moreover, fields. For molecular tailoring doping (nitrogen doped into graphene framework) canand module example, nitrogen canatoms tailorwere its electrical properties, open a band gap allowitsitintrinsic to show properties to meet the rapidly increasing demand for practical applications in various fields. semiconducting properties. As a result, nitrogen doping can significantly improve the catalytic For example, nitrogen doping can tailor its electrical properties, open a band gap and allow it to activity toward photocatalytic reactions due to the enhanced electron transportation show from semiconducting a result, nitrogen doping canofsignificantly improve the catalytic activity semiconductorsproperties. to NG andAsthe reduced recombination the photogenerated electron–hole pairs toward dueCuFe to the enhanced electron transportation from semiconductors [18–20].photocatalytic In the work, reactions we coupled 2O4 to NG to prepare CuFe2O4/NG hybrid catalyst, it is to NG and the reduced recombination of the electron–hole pairs [18–20]. In the expected that the Cu-based hybrid catalyst canphotogenerated recognize ammonia via coordination effect since the work, we coupled to prepare CuFe2 O134. /NG hybrid catalyst, it is expected the 2 O4 to3)NG formation constantCuFe of Cu(NH 42+ approached 1.1 × 10 This large constant implies a strong that trend to Cu-based hybrid catalyst can recognize ammonia via coordination effect since the formation constant form a complex between NH3 molecules and Cu sites, and also capturing capacity of NH3 from 2+ approached 1.1 × 1013 . This large constant implies a strong trend to form a complex of Cu(NHsolutions. 3 )4 aqueous between NH3 molecules and Cu sites, and also capturing capacity of NH3 from aqueous solutions. 2. Results and Discussion 2. Results and Discussion 2.1. X-ray Photoelectron Spectroscopic Characterization 2.1. X-ray Photoelectron Spectroscopic Characterization X-ray photoelectron spectroscopic (XPS) determination indicated the as-prepared NG sample is X-ray photoelectron spectroscopic (XPS) determination indicated the as-prepared NG sample composed of C, N and O elements (Figure 1A), in which C 1s, N 1s and O 1s peaks appeared at is composed of C, N and O elements (Figure 1A), in which C 1s, N 1s and O 1s peaks appeared at ~284.6, 400.0 and 534.0 eV, respectively. The percentages of C, N and O atoms are 83.24%, 7.84% and ~284.6, 400.0 and 534.0 eV, respectively. The percentages of C, N and O atoms are 83.24%, 7.84% and 8.92% in NG, respectively. The high-resolution XPS spectra of N, C, and O elements revealed the 8.92% in NG, respectively. The high-resolution XPS spectra of N, C, and O elements revealed the presence of the N 1s peaks at 398.7, 399.9, and 401.9 eV, which corresponded to the pyridinic, presence of the N 1s peaks at 398.7, 399.9, and 401.9 eV, which corresponded to the pyridinic, pyrrolic, pyrrolic, and graphitic nitrogen atoms, respectively [21–23]. The pyridinic and pyrrolic nitrogen and graphitic nitrogen atoms, respectively [21–23]. The pyridinic and pyrrolic nitrogen atoms are atoms are bonded with two carbon atoms and donate one or two p-electron to the aromatic bonded with two carbon atoms and donate one or two p-electron to the aromatic π-system. Graphitic π-system. Graphitic nitrogen, also called “quaternary nitrogen”, indicates that nitrogen atoms have nitrogen, also called “quaternary nitrogen”, indicates that nitrogen atoms have substituted the carbon substituted the carbon atoms in graphene layers [24]. The further studies showed the percentage of atoms in graphene layers [24]. The further studies showed the percentage of pyridinic, pyrrolic, pyridinic, pyrrolic, and graphitic nitrogen atoms was 25.06%, 68.84% and 6.10%, respectively, which and graphitic nitrogen atoms was 25.06%, 68.84% and 6.10%, respectively, which is consistent with is consistent with those reported [25,26]. The atom ratios of copper to iron in both pure CuFe2O4 and those reported [25,26]. The atom ratios of copper to iron in both pure CuFe2 O4 and CuFe2 O4 /NG CuFe2O4/NG samples were also determined, the results are 1:2.17 and 1:2.31, respectively, samples were also determined, the results are 1:2.17 and 1:2.31, respectively, indicating very close to indicating very close to the 1:2 stoichiometry. the 1:2 stoichiometry.

Figure 1. A wide-scan XPS spectrum of as-synthesized NG sample (A) and the high-resolution spectrum of N 1s in NG sample (B).

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Figure 1. 405 A wide-scan wide-scan XPS XPS spectrum spectrum of of as-synthesized as-synthesized NG NG sample sample (A) (A) and and the the high-resolution high-resolution3 of 19 Catalysts 2018, 1. 8, Figure A spectrum of of N N 1s 1s in in NG NG sample sample (B). (B). spectrum

2.2. X-ray X-ray Powder Powder Diffraction Diffraction Characterization Characterization 2.2. Figure the X-ray powder diffraction (XRD) patterns the as-prepared Figure 2 22presented presented the X-ray X-ray powder diffraction (XRD)ofpatterns patterns of the theCuFe as-prepared 2 O4 /NG, Figure presented the powder diffraction (XRD) of as-prepared CuFe O and NG samples, with CuFe O exhibiting the typical peaks of spinel ferrites with five O444/NG, /NG, CuFe CuFe22O O44 and and NG NG samples, samples,2 with with CuFe22O O44 exhibiting exhibiting the the typical typical peaks peaks of of spinel spinel ferrites ferrites 4 CuFe22O CuFe ◦ ◦ ◦ ◦ prominent peaks occurring at 2θ = 30.28 35.75 57.80 57.80° and 62.83 . These diffraction peaks are with five five prominent prominent peaks occurring occurring at 2θ 2θ ==, 30.28°, 30.28°,, 35.75°, 35.75°, 57.80° and 62.83°. 62.83°. These diffraction peaks with peaks at and These diffraction peaks indexed to Bragg planes (220), (311), (511) and (440), respectively. CuFe O is the major crystal are indexed to Bragg planes (220), (311), (511) and (440), respectively. CuFe 2 O 4 is the major crystal 2 4 are indexed to Bragg planes (220), (311), (511) and (440), respectively. CuFe2O4 is the major crystal phase, [27,28]. Peaks Peaks at at phase, which which is is in in good good agreement agreementwith withJCPDS JCPDS25-0283 25-0283for forthe thecubic cubicspinel spinelCuFe CuFe222O O444 [27,28]. phase, which is in good agreement with JCPDS 25-0283 for the cubic spinel CuFe O ◦ ◦ 2θ ofof CuO. The diffraction peak at 2θ = 32.90°[plane [plane 110] 110]and and39.11 39.11°[plane [plane111] 111]suggests suggestsaaasmall smallfraction fraction of CuO. The diffraction peak 2θ == 32.90 32.90° [plane 110] and 39.11° [plane 111] suggests small fraction CuO. The diffraction peak ◦ is indexed to Bragg plane (002) assigned to N-doped graphene. The peak at 30.28◦ from the 26.43 at 26.43° is indexed to Bragg plane (002) assigned to N-doped graphene. The peak at 30.28° from at 26.43° is indexed to Bragg plane (002) assigned to N-doped graphene. The peak at 30.28° from the CuFe22O O444/NG /NGsample, sample,as seenin incurve curveaaain in Figure Figure 2, 2, is is attributed attributed to to plane plane (220) (220) of of CuFe O444 [29,30]. [29,30]. O /NG sample, asasseen seen in curve in Figure CuFe222O [29,30]. CuFe CuFe But, the the peak sample. It that NG NG component component makes makes peak did did not not appear appear in inthe thesingle singleCuFe CuFe222O O444 sample. It showed showed that But, peak did not appear in the single CuFe O CuFe O more regular. In addition, plane (002) assigned to N-doped graphene did not appear in O444more more regular. regular. In In addition, addition, plane plane (002) (002) assigned assigned to to N-doped N-doped graphene graphene did did not not appear appear in in the the CuFe22O the CuFe sample, because the amount of is NG is very small in the CuFe sample. CuFe O44/NG /NG sample, because the amount amount of NG NG is very small in the the CuFe O44/NG /NG sample. The 2 O4 /NG 2 O4 /NG CuFe 22O sample, because the of very small in CuFe 22O sample. The The average diameters (D) of the as-prepared CuFe O and CuFe O /NG crystal sizes are 12.1 and average diameters (D) of the as-prepared CuFe 2 O 4 and CuFe 2 O 4 /NG crystal sizes are 12.1 and 14.5 2 4 2 4 average diameters (D) of the as-prepared CuFe2O4 and CuFe2O4/NG crystal sizes are 12.1 and 14.5 14.5 respectively, which were determined using Scherrerequation equationD Kλ/(Wcosθ) at at aa nm, nm, respectively, which were determined using thethe Scherrer equation DD=== Kλ/(Wcosθ) Kλ/(Wcosθ) at nm, respectively, which were determined using the Scherrer ◦ diffraction angle angle of /NG crystalsis is enlarged the surface surface of 35.75 35.75°(2θ). (2θ). The The average averagesize sizeof ofCuFe CuFe222O O444/NG /NG crystals crystals is enlarged by by the diffraction of 35.75° (2θ). The average size of CuFe O 2+ 3+ ions on NG sheets. 2+ or 3+ self-assembly of Cu or Fe 2+ 3+ Fe ions on NG sheets. self-assembly of Cu or Fe ions on NG sheets. 311 311

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Figure2.2. 2.XRD XRDpatterns patternsof ofthe theas-prepared as-preparedCuFe CuFe22O 2O O444/NG /NG (a), (a), CuFe CuFe222O O (b) and NG (c) samples. Figure O444(b) (b)and andNG NG(c) (c)samples. samples. Figure XRD patterns of the as-prepared CuFe /NG

2.3. Transimission 2.3. Transimission Transimission Electron Electron Microscope Microscope Characterization Characterization 2.3. Electron Microscope Characterization Figure 3A,B O particles were on layered structure of NG Figure 3A,B 3A,B showed showed that that the the CuFe CuFe222O O444 particles particles were were dispersed dispersed on on aaa layered layered structure structure of of NG NG Figure showed that the CuFe dispersed sheets. The high-resolution transmission electron microscopy (HRTEM) image displayed the lattice sheets. The high-resolution transmission electron microscopy (HRTEM) image displayed the lattice sheets. The high-resolution transmission electron microscopy (HRTEM) image displayed the lattice fringes. The d-spacing d-spacing value value between between the the adjacent adjacent lattice lattice fringes fringes is is 0.25 0.25 nm nm (Figure (Figure 3C), which is fringes. The The 3C), which which is is fringes. d-spacing value between the adjacent lattice fringes is 0.25 nm (Figure 3C), characteristic of (211) spinel planes [27,28]. The size of CuFe O crystals looked uniform; the diameter 2 4 characteristic of of (211) (211) spinel spinel planes planes [27,28]. [27,28]. The The size size of of CuFe CuFe22O O44 crystals crystals looked looked uniform; uniform; the the characteristic of most crystal particles is distributed about 25–30 nm, which is in thewhich scope is of in thethe average particle diameter of most crystal particles is distributed about 25–30 nm, scope of the diameter of most crystal particles is distributed about 25–30 nm, which is in the scope of the size of CuFe crystals that 2was estimated XRD. 2 O4 size average particle size of of CuFe CuFe O44 crystals crystals thatby was estimated by by XRD. XRD. average particle 2O that was estimated

Figure 3. 3. TEM image of of CuFe CuFe22O O444/NG /NG samples (A,B) and HRTEM image of of CuFe CuFe222O O444/NG /NG sample (C). TEM image /NGsamples samples(A,B) (A,B)and andHRTEM HRTEM image /NGsample sample(C). (C). Figure

2.4. Ultraviolet-Visible Ultraviolet-Visible Near-Infrared Near-Infrared Diffuse Diffuse Reflectance Reflectance Spectroscopy Spectroscopy 2.4.

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UV–visible near-infrared diffuse reflectance spectra were measured to elucidate the enhanced 4 of 19 2.4. Ultraviolet-Visible Near-Infrared Diffuse Reflectance photocatalytic activity of the CuFe 2O4/NG hybridSpectroscopy catalyst. Compared with curve b (CuFe2O4) in Figure 4, curvenear-infrared a (CuFe2O4/NG) underwent a red-shift, indicating the CuFe 2O4/NGthe hybrid catalyst UV–visible diffuse reflectance spectra measured to enhanced UV–visible near-infrared diffuse reflectance spectra were were measured to elucidate elucidate the enhanced can harvest more incident light energy. photocatalytic of the the CuFe CuFe22O O44/NG /NG hybrid curve bb (CuFe (CuFe22O O44)) in photocatalytic activity activity of hybrid catalyst. catalyst. Compared Compared with with curve in Two Tauc-curves of the CuFe 2O4/NG and CuFe 2O4 catalysts for the direct transition were Figure 4, curve a (CuFe 2 O 4 /NG) underwent a red-shift, indicating the CuFe 2 O 4 /NG hybrid catalyst Figure 4, curve a (CuFe2 O4 /NG) underwent a red-shift, indicating the CuFe2 O4 /NG hybrid catalyst obtained usingincident transformation data from Figure 4 and are presented in Figure 5, respectively. can harvest light can harvest more more incident light energy. energy. Extrapolation of linear portions of2O curves axisthe to direct zero (ytransition = 0) gave were a band Two Tauc-curves of of the the CuFe CuFe Othe 4/NG and towards CuFe2O4absorbance catalysts for Two Tauc-curves 2 4 /NG and CuFe2 O4 catalysts for the direct transition were gap (Eg)using of direct transitions. The direct gap44estimated for the CuFe 4 sample isrespectively. equal to 1.70 obtained transformation data from Figure and in Figure 5,5, respectively. obtained using transformation data fromband Figure and are are presented presented in2O Figure eV, which is in very good agreement with that reported [31]. As expected, the band gap for the Extrapolation aa band Extrapolation of of linear linear portions portions of of the the curves curves towards towards absorbance absorbance axis axis to to zero zero (y (y == 0) 0) gave gave band CuFe 2 O 4 /NG sample shifted to 1.50 eV, the red-shift of 0.20 eV exhibited a strong electron-orbital gap direct band gap estimated for for thethe CuFe 2O4 sample is equal to 1.70 gap (E (Egg))of ofdirect directtransitions. transitions.The The direct band gap estimated CuFe 2 O4 sample is equal to interaction between NG and CuFe 2Owith 4 andthat a widened absorption scope of solar irradiation. fact, eV, which is in very good agreement reported [31]. As expected, the band 1.70 eV, which is in very good agreement with that reported [31]. As expected, the bandgap gap for forInthe the the absorption edge for the CuFe 2 O 4 /NG composite catalyst was extended to 870 nm as indicated CuFe electron-orbital in CuFe22O O44/NG /NGsample sampleshifted shiftedtoto1.50 1.50eV, eV,the thered-shift red-shiftof of0.20 0.20 eV eV exhibited exhibited aa strong strong electron-orbital curve a in Figure 4. This wavelength falls in the scope of near-infrared light irradiation, which interaction interaction between between NG NG and and CuFe CuFe22O O44 and andaa widened widened absorption absorption scope scope of of solar solar irradiation. irradiation. In In fact, fact, the edge CuFefor 2for O4/NG composite catalyst can utilize near-infrared irradiation foras the absorption the CuFe CuFe 2O composite catalyst was to indicated therevealed absorption edge the O4/NG /NG composite catalyst wasextended extended to870 870 nm nm asphotocatalysis. indicated in in Catalysts 2018, 8, x FOR PEER REVIEW

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curve Figure4. 4. This wavelength inscope the scope of near-infrared light irradiation, which curve aa ininFigure This wavelength fallsfalls in the of near-infrared light irradiation, which revealed 1.6 revealed the CuFe 2 O 4 /NG composite catalyst can utilize near-infrared irradiation for photocatalysis. the CuFe2 O4 /NG composite catalyst can utilize near-infrared irradiation for photocatalysis. 1.4 1.6 1.2

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near-infrared diffuse reflectance spectraspectra of CuFeof /NG2(a), CuFe(a), (b) catalysts. Figure 4. 4. UV–visible UV–visible near-infrared diffuse reflectance O4/NG 2O4 (b) 2 O4CuFe 2 O4CuFe catalysts.

Figure 5. Tauc-plots for direct transition of CuFe2 O4 /NG (a), CuFe2 O4 (b) catalysts. Figure 5. Tauc-plots for direct transition of CuFe2O4/NG (a), CuFe2O4 (b) catalysts.

2.5. Molecular Recognition and Selective Photocatalysis 2.5. Molecular Recognition and Selective Photocatalysis 5. Tauc-plots for direct transition ofofCuFe 2O4/NG (a), CuFe2O4 (b) catalysts. Figure 6Figure presented the separate degradation ammonia and RhB using CuFe2 O4 /NG as the Figure 6 presented the separate degradation of ammonia and RhB using CuFefor 2O4ammonia /NG as the photocatalyst under visible near-infrared irradiation. The degradation ratios of 96.3% 2.5.photocatalyst Molecular Recognition and Selective Photocatalysis visible near-infrared irradiation. The degradation ratios of 96.3% for ammonia and of 63.6% forunder RhB were achieved, respectively, at 5 h under visible-near-infrared irradiation in and of 63.6% for RhB the wereseparate achieved, respectively, at 5 h under irradiation Figure 6 presented degradation of ammonia andvisible-near-infrared RhB using CuFe2O4/NG as the in

photocatalyst under visible near-infrared irradiation. The degradation ratios of 96.3% for ammonia and of 63.6% for RhB were achieved, respectively, at 5 h under visible-near-infrared irradiation in

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100 mg/L ammonia solution alone and 50 mg/L RhB solution alone (for the blank or controlled tests, 100see mg/L ammonia solution alone and 50 mg/LThe RhBfacts solution (forphotocatalyst the blank or controlled tests, Figure S1 in Supplementary showalone that the degrade both 100 mg/L ammonia solution aloneMaterials). and 50 mg/L RhB solution alone (for the blank orcan controlled tests, seeammonia Figure S1and in Supplementary Materials). The facts show that the photocatalyst can degrade both is RhB in a single-component solution. The degradation for organic pollutants see Figure S1 in Supplementary Materials). The facts show that the photocatalyst can degrade both ammonia and RhB a single-component solution. The [32,33]. degradation for organic pollutants consistent with theinreferences reported by Qu and Wang In a mixture of ammonia and is RhB ammonia and RhB in a single-component solution. The degradation for organic pollutants is consistent consistent with the references reported by Qu and Wang [32,33]. In a mixture of ammonia and RhB with the same concentration of ammonia and RhB as that in Figure 6, however, the degradation ratio with the references reported by Qu and Wang [32,33]. In a mixture of ammonia and RhB with the same with same concentration of ammonia and RhB as degradation that in Figureratio 6, however, degradation ratio as forthe ammonia reached 92.6% at 6 h whereas the for RhBthe decreased to 39.3% concentration of ammonia and RhB as that in Figure 6, however, the degradation ratio for ammonia forshown ammonia reached 92.6% at 6 h whereas the degradation ratio for RhB decreased to 39.3% as in Figure 7. The high degradation ratio for ammonia but low one for RhB in the mixed reached 92.6% at 6 h whereas the degradation ratio for RhB decreased to 39.3% as shown in Figure 7. shown in Figure 7. The high degradation ratio for ammonia but low one for RhB in the mixed solution confirms the CuFe2O4/NG photocatalyst prefers to degrade ammonia in mixed solution The high degradation ratio for ammonia but low one for RhB in the mixed solution confirms the solution confirms the CuFe 2O4/NG photocatalyst prefers to degrade ammonia in mixed solutionThe containing ammonia and organic compounds under visible near-infrared irradiation. CuFe2 O4 /NG photocatalyst prefers to degrade ammonia in mixed solution containing ammonia and containing ammonia and organic compounds under visible near-infrared irradiation. Thethe phenomenon also occurred between ammonia and methyl orange. The preference indicates that organic compounds under visible near-infrared irradiation. The phenomenon also occurred between phenomenon also occurred between ammonia and methyl orange. The preference indicates that the CuFe2O4/NG photocatalyst can selectively eliminate ammonia. ammonia and methyl orange. The preference indicates that the CuFe2 O4 /NG photocatalyst can CuFe2O4/NG photocatalyst can selectively eliminate ammonia. selectively eliminate ammonia.

Figure 6. Photocatalytic degradation of ammonia (a) and RhB (b) in 100 mg/L ammonia-N solution Figure6.6.Photocatalytic Photocatalyticdegradation degradationofofammonia ammonia(a) (a)and andRhB RhB(b) (b)in in100 100mg/L mg/Lammonia-N ammonia-Nsolution solution Figure alone and in 50 mg/L RhB solution alone, respectively. The CuFe 2O4/NG catalyst of 0.1 g was used in alone and in 50 mg/L RhB solution alone, respectively. The CuFe O /NG catalyst of 0.1 g was usedinin 2 4 alone andvolume in 50 mg/L alone, respectively. The CuFe2O4/NG catalyst of 0.1 g was used 50 mL withRhB pH solution 9.5 under visible light irradiation. mLvolume volumewith withpH pH9.5 9.5under undervisible visiblelight lightirradiation. irradiation. 5050mL

Figure 7. Selective degradation of ammonia (a) and RhB (b) in a mixed solution containing 100 mg/L Figure 7. Selective degradation of ammonia (a) and RhB (b) in a mixed solution containing 100 mg/L ammonia-N plus 50 mg/L RhB. other conditions are(b) the as those in Figure 6. Figure 7. Selective ofThe ammonia and RhB insame asame mixed ammonia-N plusdegradation 50 mg/L RhB. The other(a) conditions are the as solution those in containing Figure 6. 100 mg/L ammonia-N plus 50 mg/L RhB. The other conditions are the same as those in Figure 6.

2.6. Effect of NG Content 2.6. Effect of NG Content

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2.6. Effect of NG Content In order to obtain the optimal degradation conditions, the mass ratio of NG to CuFe2O4 was In order the to8, obtain theREVIEW optimal degradation the mass ratio of NG CuFe was 2O optimized, results shown in Figureconditions, 8 (see detailed information in to Figure S2 Catalysts 2018, x FOR PEER were 6 of 194in optimized, the results were shown in Figure 8 (see detailed information in Figure S2 in Supplementary Supplementary Materials), the mass percentage of 6% for NG to CuFe2O4 resulted in an optimal Materials), the ratio mass percentage for NG to conditions, CuFe resulted in anthe optimal degradation In order to obtain theatoptimal degradation the massthat ratio of NG to2O CuFe 2O4catalyst was ratio degradation (96.3%) 5ofh.6% Comparative studies CuFe 4/NG 2 O4 showed optimized, the results were in Figure 8 (see detailed Figure in (96.3%) at 5 the h. Comparative studies showed that the CuFe catalyst highest possessed highest activity forshown ammonia, compared with those ofinformation the CuFe2Opossessed 4in and CuFethe 2S2 O4/rGO 2 O4 /NG Supplementary Materials), thewith massthose percentage of 6% for NG to CuFe 2O 4 resulted in an optimal activity for (see ammonia, compared of the CuFe O and CuFe O /rGO catalysts (see Figure S3 catalysts Figure S3 in Supplementary Materials). Therefore, NG can enhance the photocatalytic 2 4 2 4 degradation ratio (96.3%) at 5 h. Comparative studies showed that the CuFe 2O4/NG catalyst of CuFe2OMaterials). 4. inactivity Supplementary Therefore, NG can enhance the photocatalytic activity of CuFe2 O4 . possessed the highest activity for ammonia, compared with those of the CuFe2O4 and CuFe2O4/rGO catalysts (see Figure S3 in Supplementary Materials). Therefore, NG can enhance the photocatalytic 100 activity of CuFe2O4. 90

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Figure 8. 8.Dependence CuFe22OO4/NG photocatalyst Figure Dependenceofofphotocatalytic photocatalytic activity activity for for the the as-prepared as-prepared CuFe photocatalyst on on 4 /NG NG / % NG percentage. NG percentage. Figure 8. Dependence of photocatalytic activity for the as-prepared CuFe2O4/NG photocatalyst on

2.7. Degradation Kinetics NG percentage. 2.7. Degradation Kinetics The different of ammonia ammoniawere wereused usedasasdesired desired while dosage of 0.1 differentinitial initialconcentrations concentrations of while thethe dosage of 0.1 g g 2.7.The Degradation Kinetics catalyst withpH pH9.5 9.5was was kept constant. degradation kinetic catalystinin50.0 50.0mL mL solution solution with kept constant. The The degradation kinetic curvescurves for the for Theammonia different initial concentrations of ammonia were used9.asThe desired while theln(C dosage of 0.1linearly g the various concentrations were shown in Figure parameter /C ) is various ammonia concentrations were shown in Figure 9. The parameter ln(C0/C0t) ist linearly catalyst in 50.0 mL solution with pH 9.5 was kept constant. The degradation kinetic curves for the proportional pseudo-firstorder orderkinetic kineticequation equation proportionaltotothe theirradiation irradiationtime timet,t, following following aa pseudo-first various ammonia concentrations were shown in Figure 9. The parameter ln(C0/Ct) is linearly proportional to the irradiation time t, following ln(C0/Cat) pseudo-first = Kapp t + b order kinetic equation (1)

ln(C0 /Ct ) = Kapp t + b (1) ln(C app t +can b be estimated as 0.3224 h−1, the standard (1) 0/Ct) = Kk The average value of the apparent rate constant app The average of the apparent rate constant k can be estimated as 0.3224 h−1 , the standard error isThe equal to value 0.01278. average value of the apparent rate constant kapp app can be estimated as 0.3224 h−1, the standard errorerror is equal to 0.01278. is equal to 0.01278. 3.0 25 mg/L 50 mg/L 25 mg/L 75 mg/L 50 mg/L 100 mg/L

3.0

2.5 2.5

75 mg/L 100 mg/L

Ln(C / C ) Ln(C0 /0Ct) t

2.0 2.0

1.5 1.5

1.0 1.0

0.5

0.5

0.0

0.0

0

0

1

1

2

3

2

3

4

4

5

5

Time / h

Time / h

Figure Dependence ofofln(C ln(C 0/Ct) on irradiation time in 50 mL solutions at pH 9.5 during the Figure 9. 9.Dependence on irradiation irradiationtime time solutions at 9.5 pH during 9.5 during Figure 9. Dependenceof ln(C 0/Ctt)) on in in 50 50 mLmL solutions at pH the the 0 /C photocatalytic degradation of ammonia at different concentrations 25, 50, 75, 100 mg/L with g g photocatalytic degradation ammoniaat at different different concentrations ofofof 25, 50, 75, 100 mg/L with 0.10.1 g 0.1 photocatalytic degradation ofofammonia concentrations 25, 50, 75, 100 mg/L with CuFe 2 O 4 /NG catalyst. 2O4/NG catalyst. CuFeCuFe catalyst. 2 O4 /NG

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2.8. 2.8. Stability StabilityofofCuFe CuFe22OO4/NG Catalyst 4 /NGCatalyst The The cyclic cyclic tests tests were were performed performed in in order order to to evaluate evaluate the the catalytic catalytic stability stability during during aa series series of of experiments. experiments. The The catalyst catalyst of of 0.1 0.1 g CuFe22O O44/NG /NGwas wastested testedininsix sixconsecutive consecutiveexperiments experimentsusing usingthe the fresh fresh ammonia ammonia solutions. solutions.The Thereaction reactiontime timewas wasabout about55hh for for each each run. run. At Atthe theend end of of the the previous previous experiment, experiment,the thecatalyst catalystwas was collected collectedusing usingan an external external magnetite, magnetite,and andthen then separated separatedand andwashed washed with times. It was observed that that the sixth photocatalytic degradation ratio with deionized deionizedwater waterfor forthree three times. It was observed the sixth photocatalytic degradation of ammonia usingusing the the same CuFe 2O4/NG catalyst stillstill achieved 92% (Figure ratio of ammonia same CuFe catalyst achieved 92% (Figure10), 10),showing showing the the 2 O4 /NG CuFe verystable. stable.The The spent catalyst out6 after 6 runs and it was CuFe22O O44/NG /NGcatalyst catalyst is very spent catalyst was was takentaken out after runs and it was measured measured TEM to As seen Figure S4 in Supplementary the by TEM toby confirm theconfirm stability.the Asstability. seen in Figure S4 in in Supplementary Materials, theMaterials, nanoparticles nanoparticles were still sheets, value the d-spacing value of 0.25that nm the indicated that the were still distributed ondistributed NG sheets,on theNG d-spacing of 0.25 nm indicated adjacent lattice adjacent of (211)remained. spinel planes remained. fringes oflattice (211) fringes spinel planes

1 2 3 4 5 6

100

(C0-Ct) / C0 / %

80 60 40 20 0 0

10

20

30

40

Time / h Figure 10. 10. Cyclic Cyclic tests tests for for checking checking CuFe CuFe22O O44/NG /NGstability. stability.The Thevolume volumeofofthe thetest testsolutions solutionsisis50 50mL, mL, Figure which contained contained 100 100 mg/L mg/Lammonia-nitrogen ammonia-nitrogenatatpH pH9.5 9.5under undervisible-near-infrared visible-near-infraredirradiation. irradiation. which

2.9. Identification Identification of of Products Products 2.9. AccordingtotoChio’s Chio’sand and Butler’s investigations [1,34], ammonia oxidized into N2 and According Butler’s investigations [1,34], ammonia was was oxidized into N 2 and NO3− − (NO − ) by two paths. One is through a series of · NH , · NH, N H NO intermediates, 2 paths. One is through a series of ·NH2, ·NH, N2H 2 x+y(x+y =0,1,2)2 intermediates, x+y(x+y =0,1,2) giving out N2 (NO32−) by two − intermediates, giving out N as a consequence. The other is to form the HONH and NO2generating 2 2 − as a consequence. The other is to form the HONH2 and NO2 intermediates, finally NO3−. − − − finally generating NO . NO or NO ions will appear the absorption band in the wavelength 2 NO3− or NO2− ions will 3appear 3the absorption band in the wavelength range of 200 to 260 nm if there − or NO − in aqueous solution [33] (see Figures S5 and S6 in range of 200 to 260 nm if there exists NO 3 2 − − exists NO3 or NO2 in aqueous solution [33] (see Figures S5 and S6 in Supplementary Materials). Supplementary Materials). In our case the ultraviolet–visible spectroscopic measurements displayed that there is not any In our band case the ultraviolet–visible spectroscopic measurements that thereprocess is not any absorption from 200 nm to 260 nm except noise wave duringdisplayed the photocatalytic as absorption band from 200 nm to 260 nm except noise wave during the photocatalytic process as shown shown in Figure 11, suggesting neither nitrite nor nitrate were formed during the degradation in Figure 11, suggesting neither nitrite nor nitrate were formed during the degradation process since process since NO2− and NO 3− can generate the absorption peaks at 211 nm and 206 nm, respectively − and NO − can generate the absorption peaks at 211 nm and 206 nm, respectively [34–36]. NO 2 3 [34–36].

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0 0 1 1 2 2 3 3 4 4 5 5

0.7 0.7 0.6 0.6

Absorbance Absorbance

0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 200 200

300 300

400 400

Wavelength / nm Wavelength / nm

500 500

600 600

Figure 11. Ultraviolet-visible absorption spectra of ammonia solution during photocatalytic Figure 11. 11. Ultraviolet-visibleabsorption absorptionspectra spectra of of ammonia ammonia solution solution during during photocatalytic photocatalytic Figure Ultraviolet-visible degradation process at 0,0, 1,1,2,2,3,3,4, 4,5 5h, h, respectively. Experimental conditions as follows: the degradation process at respectively. Experimental conditions as follows: degradation process at 0, 1, 2, 3, 4, 5 h, respectively. Experimental conditions as follows: the concentration of ammonia-nitrogen, 100 mg/L; catalyst mass of CuFe 2 O 4 /NG, 0.1 g; solution volume: the concentration of ammonia-nitrogen, 100 mg/L; 0.1 g; volume: solution 4 /NG, concentration of ammonia-nitrogen, 100 mg/L; catalystcatalyst mass ofmass CuFeof 2OCuFe 4/NG,2 O 0.1 g; solution 50 mL with pH 9.5. volume: 50 pH mL with pH 9.5. 50 mL with 9.5.

In order to to confirm confirm the the product product of of oxidized oxidized ammonia, ammonia, the the detection detection of of nitrogen nitrogen gas gas (N (N222))) was was In order was In performed during the photocatalytic degradation of ammonia in the sealed photocatalytic reaction performed during the photocatalytic degradation of ammonia in the photocatalytic reaction performed during the photocatalytic degradation of ammonia in the sealed photocatalytic reaction system mentioned previously, previously, in which which aaa 50 50 mL mL aqueous aqueous solution solution containing containing100.0 100.0mg/L mg/L NH NH333-N -N was systemmentioned previously, in in which 50 mL aqueous -Nwas was system solution containing 100.0 mg/L irradiated under visible-near-infrared light, the mixed gas of oxygen and argon was cycled, and the irradiatedunder undervisible-near-infrared visible-near-infraredlight, light,the themixed mixedgas gasof ofoxygen oxygenand andargon argonwas wascycled, cycled,and andthe the irradiated released 2 detected with gas chromatograph. The results were displayed in Figure 12. During released N was detected with gas chromatograph. The results were displayed in Figure 12. During the released N22 was detected with gas chromatograph. The results were displayed in Figure 12. During the visible-near-infrared light irradiation, the peak of O 2 gas in the sealed reaction system was visible-near-infrared light irradiation, the peak of O gas in the sealed reaction system was declining the visible-near-infrared light irradiation, the peak2 of O2 gas in the sealed reaction system was declining with irradiation irradiation time, while the peak of boosting N22 gas gas was was boosting with the the irradiation time, with irradiation time, whiletime, the peak ofthe N2 gas was with the irradiation time, indicating the declining with while peak of N boosting with irradiation time, indicating the generation of N 2 gas during the photocatalytic decomposition of ammonia. generation of N gas during the photocatalytic decomposition of ammonia. 2 indicating the generation of N2 gas during the photocatalytic decomposition of ammonia.

Intensity/ Intensity/a.a.u.u.

50 50 40 40

8 8

O2 O 2

Intensity/ Intensity/a.a.u.u.

t0 t0 t2 t2 t4 t4 t6 t6 t8 t8

60 60

6 6

N N22

4 4 2 2 0 0

30 30

5 5

6 Time6/ min Time / min

20 20

7 7

10 10 00 -10 -10

00

22

44 Time min Time // min

66

88

Figure 12. 12. Gas Gas chromatograms chromatograms during during the the photocatalytic photocatalytic degradation degradation of of ammonia ammonia under under visible visible Figure Figure 12. Gas chromatograms during the photocatalytic degradation of ammonia under visible near-infrared light irradiation at t = 0.0, 2.0, 4.0, 6.0 and 8.0 h, respectively. near-infrared light irradiation at t = 0.0, 2.0, 4.0, 6.0 and 8.0 h, respectively. near-infrared light irradiation at t = 0.0, 2.0, 4.0, 6.0 and 8.0 h, respectively.

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2.10. Molecular Molecular Recognition Recognition Evidence Evidence 2.10. In order explore the mechanism for the for selective degradation of ammonia, spectroscopic In ordertoto explore the mechanism the selective degradation ofRaman ammonia, Raman measurements were performed before and after the CuFe O /NG catalyst was immersed in NH 2 4after the CuFe2O4/NG catalyst spectroscopic measurements were performed before and was3 solution and mixed solution, respectively. Figure 13 displayed the Raman spectra of the the immersed in NH NH33-RhB solution and NH3-RhB mixed solution, respectively. Figure 13 displayed CuFe O /NG catalyst (a) itself and NH adsorbed on the CuFe O /NG catalyst (b) and both NH 2 4 3 2 4 Raman spectra of the CuFe2O4/NG catalyst (a) itself and NH3 adsorbed on the CuFe2O4/NG catalyst3 andand RhBboth adsorbed on RhB the CuFe catalyst As to the assignments of the Raman shifts of 2 O4 /NG (b) NH3 and adsorbed on the CuFe(c). 2O4/NG catalyst (c). As to the assignments of the the CuFe component in4 the composite catalyst, they catalyst, were listed Tablelisted 1 based on the FeO4 2 O4 of Raman shifts the CuFe2O component in the composite theyinwere in Table 1 based symmetry [37]. on the FeO4 symmetry [37]. −1 appeared in Raman Besides the the Raman /NGitself, itself, aa new new peak peak at Besides Raman peaks peaks of of CuFe CuFe22O O44/NG at 1100 1100 cm cm−1 appeared in Raman spectra both (b) and (c) after the CuFe O /NG catalyst was immersed in NH solution and NH33-RhB -RhB 2 4 3 spectra both (b) and (c) after the CuFe2O4/NG catalyst was immersed in NH3 solution and NH −1 was assigned to the bending vibration of mixed solution, respectively. This Raman shift at 1100 cm −1 mixed solution, respectively. This Raman shift at 1100 cm was assigned to the bending vibration of NH33-H -H22O O complex Thus, it unambiguously revealed that NH3that was NH adsorbed on the CuFe O4 /NG NH complex[38]. [38]. Thus, it unambiguously revealed 3 was adsorbed 2 on the catalyst in both cases. CuFe2O4/NG catalyst in both cases. For RhB RhB Raman Raman shifts, shifts, it it is is known known that that the the spontaneous spontaneous Raman Raman shift shift range range of of RhB RhB is is from from 500 500 to to For − 1 − 1 1700 cm cm−1 [39], [39],thus, thus,the theshift shiftatat 602 cm from Raman spectrum (c)Figure in Figure 13 was attributed to −1 from 1700 602 cm Raman spectrum (c) in 13 was attributed to the the C–C bending vibration of xanthene ring in the molecular structure of RhB [40,41]. For the NG C–C bending vibration of xanthene ring in the molecular structure of RhB [40,41]. For the NG −1 as seen in curves (a) and (b), component, there there were were two component, two Raman Raman shifts shifts located located at at 1372 1372 and and 1580 1580 cm cm−1 as seen in curves (a) and (b), respectively, which were attributed to D (defect-induced mode) and G (in-plane vibration mode)mode) bands. respectively, which were attributed to D (defect-induced mode) and G (in-plane vibration −1 ) derived from the G (1610 cm−1 ) of the pristine graphene, which also confirmed The G band (1580 cm −1 −1 bands. The G band (1580 cm ) derived from the G (1610 cm ) of the pristine graphene, which also that nitrogen wereatoms incorporated into graphene [42,43]. Compared with curve (b), confirmed thatatoms nitrogen were incorporated into framework graphene framework [42,43]. Compared with the significant shifts occurred of D and G bands after the CuFe O /NG catalyst was immersed in 2 4 curve (b), the significant shifts occurred of D and G bands after the CuFe2O4/NG catalyst was −1 NH -RhB mixed solution, as seen curve (c). The D and G bands were shifted from 1372 and 1580 cm 3 immersed in NH3-RhB mixed solution, as seen curve (c). The D and G bands were shifted from 1372 −1 respectively, which indicated the strong interaction between NG sheets and to 1308 and −1 to cm and 1580 cm1560 1308 ,and 1560 cm−1, respectively, which indicated the strong interaction between RhB, thereby RhB being adsorbed onbeing NG sheets. It is reasonable that the interactionthat between NG NG sheets and RhB, thereby RhB adsorbed on NG sheets. It π-π is reasonable the π-π and RhB occurs because thereRhB are occurs π bondsbecause in theirthere planar RhB structure, interaction between NG and arearomatic π bondsmoieties in their [44,45] planar (for aromatic moieties see Figure S7 in Supplementary Materials). However, compared with curve (a) in Figure 13, no of [44,45] (for RhB structure, see Figure S7 in Supplementary Materials). However, comparedshift with the D and G bands appeared in curve (b) after the CuFe O /NG catalyst was immersed in the single 2 4 curve (a) in Figure 13, no shift of the D and G bands appeared in curve (b) after the CuFe2O4/NG NH3 component solution. It suggested NH3 wassolution. adsorbed preferentially CuFe 2 Oadsorbed 4 moiety, catalyst was immersed in the single NH3that component It suggested thaton NH 3 was but not NG, in the composite catalyst. preferentially on CuFe2O4 moiety, but not NG, in the composite catalyst. 216

285 656

Intensity / a.u.

602 397 490

1560

1308

NH3

693

c

684 620 241 180

472 340

725 1100

NG sheets

722

D 1372

170 223

0

1580

378 565

500

G b 1610

a 1000

1500

2000

Raman shift / cm-1 Figure 13. Raman spectra sample (a), (a), the /NG sample sample after after being being Figure 13. Raman spectra of of the the CuFe CuFe22O O44/NG /NG sample the CuFe CuFe22O O44/NG immersed in 100 mg/L NH -N solution for 4 h (b) and the CuFe O /NG sample after being immersed 3 2 4 immersed in 100 mg/L NH3-N solution for 4 h (b) and the CuFe2O4/NG sample after being immersed in 100 100 mg/L mg/LNH NH3-N and50.0 50.0mg/L mg/L RhBmixed mixedsolution solutionfor for4 4h h(c). (c). 3 -Nand in RhB

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Table 1. Assignments of Raman shifts for CuFe2 O4 /NG. Table 1. Assignments of Raman shifts for CuFe2O4/NG. Mode a* b c

Mode T (F2g) T (F2g) T/L a *170 b 180 c -

223 170 241 180 - 216

T/L ν2

ν2 ν4

285 223 285 241 285 216

285 378 378 285 378 378 285 397 397

ν4

ν4 ν4 452~472 452~472 472 472 489 489

ν3

ν3 ν3 565684 565684 - 656

ν1 ν3 684 722 684 725 656 693

ν1 722 725 693

565 565 ** (a), (a),CuFe CuFe 2O4/NG; (b), CuFe2O4/NG after being immersed in 100 mg/L NH3 solution; (c), 2 O4 /NG; (b), CuFe2 O4 /NG after being immersed in 100 mg/L NH3 solution; (c), CuFe2 O4 /NG after CuFeimmersed 2O4/NG after immersed in 100RhB mg/L NH 3 and 50 mg/L RhB mixed solution. being in 100being mg/L NH mixed solution. 3 and 50 mg/L

In sample (to (to avoids In order order to to confirm confirm ammonia ammonia adsorbed adsorbed on on CuFe CuFe22O O44,, the the single single CuFe CuFe22O O4 sample avoids interference of nitrogen nitrogenfrom fromNG) NG)was wassynthesized synthesized and immersed in 100 mg/L 3 -N solution interference of and immersed in 100 mg/L NH3NH -N solution for 4 for 4 adsorption, h for adsorption, was out taken and washed with deionized water three to h for then itthen was ittaken andout washed with deionized water for threefor times totimes remove ◦ C in a vacuum chamber for XPS remove ammonia adsorbed physically. Finally, it was dried at 60 ammonia adsorbed physically. Finally, it was dried at 60 °C in a vacuum chamber for XPS measurement. For the the sake sake of O44 component component that that was was not measurement. For of contrast, contrast, other other CuFe CuFe22O not immersed immersed in in NH NH33 solution was also used for XPS measurement. Figure 14 displayed the high-resolution XPS spectrum solution was also used for XPS measurement. Figure 14 displayed the high-resolution XPS spectrum of of N N 1s 1s in in the the CuFe CuFe22O O44 sample sample after after itit was was immersed immersed in inaa100 100mg/L mg/L NH NH33-N -N solution. solution. It It indicated indicated evidently the presence presence of of nitrogen nitrogen atoms, atoms, whereas whereas an an N N 1s 1s peak evidently the peak was was not not detected detected out out when when the the CuFe O sample was not immersed in such 100 mg/L NH -N solution. The dissimilarity implies CuFe22O44sample was not immersed in such 100 mg/L NH3-N3 solution. The dissimilarity implies that that ammonia adsorbed CuFe when wasimmersed immersedinin ammonia ammonia solution. solution. 4 sample ammonia was was adsorbed on on thethe CuFe 2O42 O sample when it itwas Deconvolution XPS spectrum of Nof 1s N may ascribed to the Ebto of the NH3Eb (399.97 eV), copper-NH Deconvolution ofofthe the XPS spectrum 1sbemay be ascribed of NH 3 (399.97 eV),3 − [46], which− implied ammonia is partly oxidized in air atmosphere. (398.30 eV) and NO copper-NH3 (398.302eV) and NO2 [46], which implied ammonia is partly oxidized in air atmosphere. 399.62 403.70 398.30 Intensity / a.u.

399.97

392

396

400 404 Binding energy / eV

408

412

Figure 14. High-resolution XPS spectrum sample after it was immersed in 100 spectrum of of N N 1s 1s on on the the CuFe CuFe22O44 sample mg/L NH33-N -Nsolution. solution. mg/L NH

Furthermore, between NH3NH and or Fe(III) identified by XPS technique. Furthermore, the thecoordination coordination between 3 Cu(II) and Cu(II) or was Fe(III) was identified by XPS The high-resolution XPS measurements of Cu 2p andofFeCu 2p2p in the were as 2O technique. The high-resolution XPS measurements andCuFe Fe 2p in4 sample the CuFe 2O4 conducted sample were shown in Figures 15 and 16 before (a) and after (b) the CuFe O sample was immersed in 100 mg/L 4 the CuFe2O4 sample was immersed conducted as shown in Figures 15 and 16 before (a) and after2 (b) NH solution, respectively. Very interestingly, the shifts of Eb 933.10 Cu 2p eV 3/2for (0.75 3 -Nmg/L in 100 NH3-N solution, respectively. Very interestingly, theatshifts ofeV Ebfor at 933.10 CueV) 2p and at 953.06 eV for Cu 2p 1/2 (0.50 eV) were observed [47], implying that the chemical surroundings 3/2 (0.75 eV) and at 953.06 eV for Cu 2p 1/2 (0.50 eV) were observed [47], implying that the chemical of copper in the sample were after it was immersed 100 immersed mg/L NH3in-N100 solution. 2 O4in surroundings of CuFe copper the CuFe 2Oaltered 4 sample were altered after itinwas mg/L However, the shifts of Eb at foratFe 2p 3/2 724.10 eVatfor Fe 2p NH3-N solution. However, the710.56 shifts eV of Eb 710.56 eV and for FeEb2pat3/2 and Eb 724.10 eV1/2 for were Fe 2p not 1/2 2+ observed as shown in Figure 15 [48]. It is well known that the formation constant of Cu(NH ) 3 4of were not observed as shown in Figure 15 [48]. It is well known that the formation constant is up to3)1.1 × up 1013to. Thus, it 13is. Thus, reasonable that the CuFe catalyst can recognize ammonia 2 Othe 4 /NG Cu(NH 42+ is 1.1 × 10 it is reasonable that CuFe 2O4/NG catalyst can recognize via a coordination effect between copper atoms in the atoms catalyst in ammonia; is ammonia via a coordination effect between copper inand the nitrogen catalyst atoms and nitrogen atomsit in concluded that the coordination interaction occurred between copper, but not iron and ammonia. ammonia; it is concluded that the coordination interaction occurred between copper, but not iron

and ammonia.

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Cu Cu 2p 2p 3/2 3/2 932.35 932.35

Cu Cu 2p 2p 1/2 1/2

Intensity // a.u a.u.. Intensity

933.10 933.10

952.56 952.56 953.06 953.06

Cu Cu satellite satellite peaks peaks

(b) (b) (a) (a)

930 930

940 950 940 950 Binding Binding energy energy // eV eV

960 960

Figure Cu component component in in the the CuFe CuFe222O samplebefore before(a) (a)and Figure 15. 15. High-resolution High-resolution XPS XPS spectra spectra of of Cu O444sample sample before (a) and after after (b) solution, respectively. mg/L NH33-N -N solution, respectively. (b) itit was was immersed immersed in in 100 100 mg/L mg/L NH NH -N solution, respectively. 3

Fe Fe 2p 2p 3/2 3/2

Fe Fe 2p 2p 1/2 1/2

Intensity // a.u. a.u. Intensity

(b).CuFe (b).CuFe22O O44-NH -NH33

(a).CuFe (a).CuFe22O O44

710.56 710.56

700 700

724.10 724.10

710 720 730 710 720 730 Binding Binding energy energy // eV eV

740 740

Figure 16. 16. High-resolution High-resolutionXPS XPS spectra CuFe before (a) and Figure spectra of Fe 2p in the CuFe 22O sample before (a) after (b) 4 sample Figure 16. High-resolution XPS spectra ofof FeFe 2p2p in in thethe CuFe O442 O sample before (a) and and afterafter (b) (b) adsorbing ammonia. adsorbing ammonia. adsorbing ammonia.

2.11. Reaction Mechanism 2.11. 2.11. Reaction Reaction Mechanism Mechanism The surface photovoltage spectroscopy (SPV) was utilized to explore the transfer of the The The surface surface photovoltage photovoltage spectroscopy spectroscopy (SPV) (SPV) was was utilized utilized to to explore explore the the transfer transfer of of the the photo-generated holes and photo-generated electrons in order to elucidate the degradation mechanism photo-generated photo-generated holes holes and and photo-generated photo-generated electrons electrons in in order order to to elucidate elucidate the the degradation degradation of ammonia. As known, the surface photovoltage is defined as the illumination-induced change in the mechanism mechanism of of ammonia. ammonia. As As known, known, the the surface surface photovoltage photovoltage is is defined defined as as the the illumination-induced illumination-induced surface potential [49], being equal to the difference between the surface potential under illumination change in the surface potential [49], being equal to the difference between change in the surface potential [49], being equal to the difference between the the surface surface potential potential and the surface potential in dark given by Equation (2) under under illumination illumination and and the the surface surface potential potential in in dark dark given given by by Equation Equation (2) (2) SPV − Vs(dark) SPV Vs (ill) (ill) (dark) SPV===Vs Vs (ill) − − Vs Vs (dark)

(2) (2) (2)

As are concerned, aa positive response of means that the As far far as the band-to-band transitions are concerned, positive response of SPV SPV means thatthat the faras asthe theband-to-band band-to-bandtransitions transitions are concerned, a positive response of SPV means photo-generated holes move to irradiation side of photo-generated holes move to the the irradiation side ofofthe the sample, whereas the the photo-generated holes move to the irradiation side thesample, sample,whereas whereasthe the photo-generated photo-generated electrons electrons move into the the bulk bulk of of the the sample That is, is, the material is an an n-type n-type electronsmove moveinto sample [50,51]. [50,51]. That the semiconductor semiconductor material materialis semiconductor in this case. On the contrary, a negative response represents a p-type semiconductor. semiconductor semiconductor in in this this case. case. On Onthe thecontrary, contrary, aa negative negative response responserepresents representsaap-type p-typesemiconductor. semiconductor. In In the the present present case, case, the the SPV SPV responses responses measured O444sample sample were shown in Figure 17. measured for for the the CuFe CuFe222O samplewere wereshown shownin inFigure Figure17.

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50 50

Vb= +0.1 V Vb= -0.1 V Vb=No +0.1 biasV Vb= -0.1 V No bias

c

40

c

Photovoltage / mV Photovoltage / mV

40 30

a

30

a

20

20

b

10

b

10 0

0 -10 300 -10 300

400

500

600

700

/700 nm 500 Wavelength 600 Wavelength / nm

400

800 800

900 900

Figure 17. SPV spectra of the CuFe2O4 sample biased by an external electrical field at zero (a), +0.1 V field at at zero (a),(a), +0.1 V (b) Figure 17.−0.1 SPVVspectra spectra ofthe theCuFe CuFe22O O44 sample samplebiased biasedby byan anexternal externalelectrical electrical field zero +0.1 V (b) and (c). of andand −0.1 V (c). (b) −0.1 V (c).

Curve (a) in Figure 17 presents a positive response under incident light illumination, suggesting Curve (a) in in Figure Figure 17 17 presents presents positive response under incident light illumination, suggesting Curve (a) aa positive under incident light illumination, suggesting that the photo-generated holes move to the response illuminated surface of the CuFe 2O4 sample. Applying a that the photo-generated holes move to the illuminated surface of the CuFe O sample. Applying 44 sample. that the photo-generated move the illuminated of the CuFe2to 2Othe Applying positive bias of 0.1 V to holes the CuFe 2O4to sample suppressedsurface the holes moving surface, resultingaain positive bias of ofSPV 0.1Vresponse Vto tothe theCuFe CuFe sample suppressed the holes moving the surface, resulting 2O positive bias 0.1 O 44 sample the to to the surface, resulting a decreased (b); 2whereas a suppressed negative bias ofholes 0.1 Vmoving promoted the holes moving toin the a decreased SPV response (b); whereas a negative bias of 0.1 V promoted the holes moving to the aindecreased SPV response (b); whereas a negative bias of 0.1 V promoted the holes moving to the surface, leading to an increased SPV response (c). It can be concluded from these facts that the surface, leading totoanan response (c). in It(c). can concluded from these facts that the spectra CuFe Ofor 4 surface, leading increased SPV response Itbe can be concluded from these facts that 2the CuFe2O 4 material isincreased an n-typeSPV semiconductor the case [52,53]. The XPS valence band material an n-type an semiconductor in the case in [52,53]. The[52,53]. XPS valence band spectra for the CuFe2for OThe 4 CuFe O4 is material n-type semiconductor the case The XPS valence spectra the 2CuFe 2O4 andisNG semiconductor materials have been determined as shown inband Figure 18A,B. and NG semiconductor materials have been determined as shown in Figure 18A,B. The valence bands the CuFe2O 4 and are NGequal semiconductor materials been2Odetermined as shown in Figure The valence bands to 1.7 eV and 1.4 eV have for CuFe 4 and NG [54], respectively. The18A,B. conduction are equalbands to 1.7equal eV equal and for CuFe and [54], The conduction The bands areonequal 2 O4eV valence are toeV 1.7 eVand and−0.1 1.4 CuFe 2Orespectively. 42O and NG [54], respectively. conduction bands are to 1.4 0.2 eV eVforNG for CuFe 4 and NG, respectively, based their tomeasurements 0.2 eV and −0.1 eV for CuFe O and NG, respectively, based on their measurements of band gaps. 2 4 bands are equal to 0.2 eV and −0.1 eV for CuFe 2 O 4 and NG, respectively, based on their of band gaps. measurements of band gaps. A

B

A

Intensity / a.u. Intensity / a.u.

Intensity / a.u. Intensity / a.u.

B

1.70 eV

1.40 eV 1.40 eV

1.70 eV

-2 -2

-1 -1

0 0

1

2

3

4

5

6

-2

7 -2

-1 -1

0

1

0 1 / eV Binding energy

1 Binding 2 energy 3 6 7 /4eV 5 Binding energy / eV Binding energy / eV Figure 18.18. XPS valence band spectra of CuFe /NG (A) Figure XPS valence band spectra of CuFe 4/NG (A)and andNG NG(B). (B). 2 O24O

2 2

3 3

Figure 18. XPS valence band spectra of CuFe2O4/NG (A) and NG (B).

UV–visible near-infrared diffuse reflectance spectrum of NG sheets beenhas measured as shownas UV–visible near-infrared diffuse reflectance spectrum of NGhas sheets been measured inshown Figure 19. As seen, the absorption edge of NG sheets extended to near-infrared region, the insert UV–visible diffuse reflectance ofextended NG sheets has been measured as in Figurenear-infrared 19. As seen, the absorption edge ofspectrum NG sheets to near-infrared region, the indicated a direct band gap of 1.50 eV for the as-synthesized NG sheets, corresponding to 826 nm shown Figure 19. As seen, thegap absorption edge NG sheets extended near-infrared region,to the insertinindicated a direct band of 1.50 eV forof the as-synthesized NGto sheets, corresponding 826 near-infrared incident light. As EdaAs Chai have postulated that isolated sp2 nanodomains areto likely insert indicated a direct band gap ofand 1.50 eV the as-synthesized NG corresponding 826are nm near-infrared incident light. Eda andfor Chai have postulated thatsheets, isolated sp2 nanodomains to exhibit quantum confinement-induced semiconducting behavior [55,56]. The local energy gaps of nm near-infrared light. As Eda and Chai have postulated that isolated[55,56]. sp2 nanodomains are likely to exhibitincident quantum confinement-induced semiconducting behavior The local energy π-π* transition vary depending the size, shape of these sp2 domains. The domains. smaller likely toofexhibit quantum confinement-induced semiconducting [55,56]. The local energy gaps π-π* then transition then vary on depending on theand size,fraction shapebehavior and fraction of these sp2 this sp2 domain, the higher the outcome of the energy gap [53]. The calculated energy gap between gaps π-π* transition then vary the size, and fraction these sp2 domains. Theofsmaller this sp2 domain, thedepending higher theon outcome of shape the energy gap [53].ofThe calculated energy HOMO and this LUMO a cluster 37 rings ∼of 2 eV, and this energy gap progressively increases to The smaller sp2 for domain, theof higher the isoutcome of the gap [53]. The calculated energy gap between HOMO and LUMO for a cluster 37 rings is energy ∼2 eV, and this energy gap progressively ∼increases 7 eV for atosingle benzene ring benzene [57]. Therefore, is reasonable that the NG sheets gap between HOMO andaLUMO for a cluster 37it rings is ∼2 eV, and thisas-synthesized energy ∼7 eV for single ringof[57]. Therefore, it is reasonable thatgap theprogressively as-synthesized work as a semiconductor with a band gap of 1.50 eV in the present case. The conduction band of NG increases to ∼7 eV as fora asemiconductor single benzenewith ringa[57]. it eV is reasonable that case. the as-synthesized NG sheets work bandTherefore, gap of 1.50 in the present The conduction

NG sheets work as a semiconductor with a band gap of 1.50 eV in the present case. The conduction

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band of NG sheets can thereby be estimated as −0.10 eV based on its band gap of 1.50 eV and valence band of 1.40 eV measured in Figure−18B. sheets be estimated 0.10 eVasbased band of 1.50 eVofand of band ofcan NGthereby sheets can thereby beas estimated −0.10 on eV its based ongap its band gap 1.50valence eV and band valence 1.40 eV measured in Figure 18B. band of 1.40 eV measured in Figure 18B. (h)2 / eV2 nm2-2 2 (h) / eV nm-2

16 12

16 12

Absorbance Absorbance

8

8 4 0

1

4 0

200 200

400 400

600

800

1

2 3 4 Photon energy / eV 2 3 4 Photon energy / eV

1000

Wavelength / nm 600 800 1000

1200 1200

1400 1400

Figure 19. UV–visible near-infrared diffuse Wavelength reflectance spectrum of the NG sample. Tauc-plots for / nm the direct band-gaps of the NG sample. of of thethe NGNG sample. Tauc-plots for the Figure 19. UV–visible UV–visible near-infrared near-infrareddiffuse diffusereflectance reflectancespectrum spectrum sample. Tauc-plots for direct band-gaps of the NGNG sample. the direct band-gaps of the sample. 0

Under consideration of the standard electrodes (E for N2/NH3 redox is 0.057 V vs. NHE, E0 for 0 beNsuggested O2/HUnder 2O is 1.23 V vs. NHE), a Z-scheme mechanism can as demonstrated in Scheme consideration of redox is 0.057 0.057 V V vs. NHE, E00 for for1 Under consideration of the the standard standard electrodes electrodes(E (E0for for N22/NH /NH33 redox is vs. NHE, E [2,8]. O /H2 Oisis1.23 1.23 V V vs. vs. NHE), cancan be be suggested as demonstrated in Scheme 1 [2,8].1 O 22/H2O NHE),aaZ-scheme Z-schememechanism mechanism suggested as demonstrated in Scheme [2,8].

Scheme1.1.Z-scheme Z-schemephotocatalytic photocatalyticmechanism mechanismofofCuFe CuFe 2O /NG. The photo-generated photo-generated holes holes leave leave on on Scheme 2O 44/NG. CuFe22O O44 to tooxidize oxidizeNH NH 3 N to2 ,N 2, while the photo-generated electrons on the conduction band CuFe while the photo-generated electrons on the conduction band of CuFe 3 to 2 Oof 4 Scheme 1. Z-scheme photocatalytic mechanism of CuFe2O4/NG. The photo-generated holes leave on flow sheets Z-scheme to reduce O2 molecules underO visible-near-infrared CuFeto2ONG 4 flow toalong NG the sheets along configuration the Z-scheme configuration to reduce 2 molecules under CuFe2O4 to oxidize NH3 to N2, while the photo-generated electrons on the conduction band of light irradiation. visible-near-infrared light irradiation. CuFe2O4 flow to NG sheets along the Z-scheme configuration to reduce O2 molecules under visible-near-infrared light irradiation. As mentioned previously, NH3 molecules were selectively adsorbed on the CuFe2 O4 component,

As mentioned previously, NH3 molecules were selectively adsorbed on the CuFe2O4 component, while while RhB RhB molecules molecules were were dominantly dominantly adsorbed adsorbed on on the the NG NG sheets sheets by by π-π π-πinteraction. interaction. Therefore, Therefore, As mentioned previously, NH 3 molecules were selectively adsorbed on the CuFe2O4 component, NH molecules were oxidized by photo-generated holes on the valence band of copper ferrite 3 NH3 molecules were oxidized by photo-generated holes on the valence band of copper ferriteto toN N22.. while RhB molecules were dominantly adsorbed on the NG sheets by π-π interaction. Therefore, + +holes ++ valence band of copper ferrite to N NH3 molecules were oxidized by photo-generated the 2. 2NH 3 + N22 +on + 6H 6H (3) 2NH 6h6h ==N (3) 3+ + 2NH 3 + 6h+ = N2 + 6H (3) At the thesame sametime, time,photo-generated photo-generated electrons reduce dissolvedin insolution solutiontotoHHO 2O2 through At electrons reduce OO22dissolved 2 2 through − O2−··first , then continue to [58]. O first, continue to reduce reduce H H22O O22to toH H2O 2 At 2 O [58]. thethen same time, photo-generated electrons reduce O2 dissolved in solution to H2O2 through − O2−·first, then continue to reduce H2O2 to H2O O2[58]. (4) + e = O2−· O2 + e − = O2 − · (4) (4) + e+− += eO− 2=−·H2O2 (5) O2−·O+22H O2 − · + 2H+ + e− = H2 O2 (5) (5) O2−·+ 2H+ + e− = H2O2

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H2 O2 + 2H+ + 2e− = 2H2 O

(6)

Thus, the overall photocatalytic reaction can be formulated as follows in Scheme 1. 2NH3 + 3/2O2 = N2 + 3H2 O

(7)

3. Experimental Section 3.1. Synthetic Procedures 3.1.1. Synthesis of N-Doped Graphene Hummers’ method was initially adopted to synthesize graphene oxide (GO) as our group has previously synthesized [59] and NG was synthesized as the reference [25]. The as-synthesized GO (0.1000 g) was ultrasonically dispersed in 25.0 mL of deionized water. Urea (30.0000 g) was dissolved in 25.0 mL of deionized water under stirring. The urea solution was added dropwise to the GO suspension solution under stirring. Subsequently, deionized water of 10 mL was added in the mixture and ultrasonicated for 2 h. After that, the mixture was transferred to a Teflon-lined stainless-steel autoclave with a volume of 60 mL, sealed and heated to 170 ◦ C for 12 h to form NG. Finally, the resulting product was filtered, washed and dried in a vacuum chamber for use. 3.1.2. Synthesis of CuFe2 O4 /NG A one-step hydrothermal route has been used for preparing CuFe2 O4 /NG samples [60]. Cu(NO3 )2 ·3H2 O (1.2080 g, 0.005 mol) and Fe(NO3 )3 ·9H2 O (4.0400 g, 0.01 mol) were separately dissolved in 10.0 mL of deionized water. NG (0.072 g, 6.0% of the CuFe2 O4 mass) was dispersed in 10.0 mL of deionized water by an ultrasonic vibrator. The Cu(II) and Fe(III) solutions were added to NG suspension solution under stirring. NaOH (1.6 g, 0.04 mol) was dissolved in 10.0 mL of deionized water, and this solution was added dropwise to the mixed suspension solution described above under continuous stirring. Deionized water was also added to the suspension to obtain a final volume of 60 mL. The suspension solution was then transferred to a 100 mL Teflon-lined stainless-steel autoclave that was subsequently sealed and maintained at 180 ◦ C for 10 h. The solution was cooled to room temperature and filtered to obtain CuFe2 O4 /NG precipitates. The CuFe2 O4 nanoparticles can be formed by reaction Equation (4). The products were rinsed thrice with water to remove excess NaOH and other electrolytes and dried at 60 ◦ C in a vacuum chamber for use. Cu(NO3 )2 + 2Fe(NO3 )3 + 8NaOH = CuFe2 O4 + 8NaNO3 + 4H2 O

(8)

3.2. Photovoltage Characterization Surface photovoltage spectra were recorded using a lock-in amplifier (SR830, Stanford Research Systems, Sunnyvale, CA, USA). The measurement system was composed of a 500 W xenon lamp, a monochromator (SBP500, Zolix Instruments Co., Ltd., Beijing China), a lock-in amplifier with a light chopper (SR540, Stanford Research Systems, Sunnyvale, CA, USA), and a sample chamber. The xenon lamp emitted incident photons with various wavelengths, which then passed through the monochromator to provide monochromatic light. The light was chopped with a frequency of 23 Hz, and its intensity depended on the spectral energy distribution of the lamp. The monochromator and the lock-in amplifier were controlled by a computer. The input resistance of the lock-in amplifier was set as 10 MΩ. SPV spectra were recorded by scanning from low to high photon energy. A UV–cutoff filter (λ > 420 nm) was employed at incident photon energy hv < 2.14 eV (λ > 580 nm) to remove the frequency and double the amount of light generated by the grating monochromator (doubled frequency of λ > 580 nm). The system was calibrated by a DSI200 UV–enhanced silicon detector to eliminate possible phase shift not correlated with the SPV response; thus, any phase retardation reflected the kinetics of SPV response [61].

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3.3. Molecular Recognition and Selective Photocatalysis Photocatalytic experiments were conducted under visible-near-infrared irradiation (λ > 400 nm). A 300 W UV–visible lamp (OSRAM, Munich, Germany) was used as a light source. Photocatalytic degradation was performed in a 100 mL beaker at room temperature (25 ± 2 ◦ C). The distance between the lamp and test solution was approximately 10 cm, and the wall of the beaker was shielded from surrounding light by aluminum foil. Visible-near-infrared light was allowed to pass through a λ > 400 nm cut-off filter covering the window of the beaker; this filter absorbed UV light and allowed visible-near-infrared light of λ > 400 nm to pass through. In a typical photocatalytic experiment, 50 mL of test solution was used. The NH3 solutions were prepared according to the desired concentrations, and the CuFe2 O4 /NG catalyst of 0.1 g was used for the photocatalytic experiments. NaHCO3 -Na2 CO3 (0.1 mol/L) buffer was used to control the pH of the test solutions. For the selective photocatalytic degradation, the CuFe2 O4 /NG catalyst of 0.1 g was immersed in NH3 -RhB mixed solution for 2 h first, then the visible-near-infrared light source was turned on for the photocatalytic tests. A double-beam TU-1901 spectrophotometer (PGENERAL Instrument Limited-liability Company, Beijing, China) was used to determine the concentration of NH3 by reaction with Nessler reagent during the photocatalytic process. Nessler reagent is an alkaline solution of dipotassium tetraiodomercurate(II), this reagent was prepared by dissolving 10 g of HgI2 and 7 g of KI in water, adding to NaOH solution (16 g NaOH in 50 mL of water), and then diluting with deionized water to 100 mL. The reagent was stored in dark bottles and diluted properly before analysis. NH3 reacts with this reagent to yield colored solutions via Reaction (2) previously. As the absorbance of the solutions showed a maximum value at 392 nm, the absorbance was measured at the wavelength of 392 nm. The procedures for the stability tests are follows: The CuFe2 O4 /NG catalyst was sunk by a magnetic field outside due to the CuFe2 O4 magnetism after the last test finished. And then, the supernatant solution tested was poured out, the solid catalyst was kept in. Finally, a 50 mL fresh NH3 solution was poured into the reactor. Subsequently, the next test was carried out again. 4. Conclusions The composite photocatalyst composed of copper ferrite and N-doped graphene enables to recognize ammonia from NH3 -RhB mixed solution. The measurements of gas chromatography show that the composite photocatalyst oxidizes NH3 selectively to non-toxic N2 under visible near-infrared light irradiation, thereby fulfilling the removal of nitrogen, the effect solar use and purification of water. The mechanism studies indicate that the photo-generated electrons flow to N-doped graphene following the Z-scheme configuration to reduce O2 dissolved in solution. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/8/10/405/s1, Figure S1: Blank test. (a) visible-near-infrared irradiation without the photocatalyst; (b) 0.1 g CuFe2 O4 /NG catalyst used in solution without irradiation; (c) 0.1 g CuFe2 O4 /NG catalyst used in solution under visible-near-infrared irradiation. The solution used is a 50 mL solution containing 100 mg/L ammonia-nitrogen at pH = 9.5. Figure S2: Effects of NG percentage on the photocatalytic activity of the as-prepared CuFe2 O4 /NG photocatalyst. 0.1 g photocatalyst in 50 mL solution at pH 9.5 containing 100 mg/L ammonia-nitrogen. Figure S3: Photocatalytic degradation of ammonia using different catalysts: CuFe2 O4 /NG (a), CuFe2 O4 /rGO (b), and bare CuFe2 O4 (c) in 100 mg/L ammonia-N solution, respectively. The catalyst of 0.1 g was added in 50 mL solution containing 100 mg/L ammonia-N with pH 9.5 under visible-near-infrared irradiation. Figure S4: HRTEM image of the CuFe2 O4 /NG catalyst after it was tested for 6 runs. Figure S5: The absorption spectra of nitrate and nitrite standard solutions containing 1 mg L−1 of nitrogen. It is from the reference (see reference: D. L. Miles, C. Espejo, Comparison Between an Ultraviolet Spectrophotometric Procedure and the 2,4-Xylenol Method for the Determination of Nitrate in Ground waters of Low Salinity, Analyst, 1977,102, 104–109). Figure S6: The calibration curve of NO3 − in the concentration range of 0.2–5.0 mg/L. Figure S7: The chemical structure of Rhodamine B. Author Contributions: H.Z., Y.Z. and Q.-Q.G. conducted the experiments and collected the data. S.-Q.L. designed the experiments and reviewed the article. Z.-D.M. analyzed the data and discussed the results. L.L. wrote draft. All authors discussed the results and commented on the manuscript.

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Funding: This research was funded by the National Natural Science Foundation of China (Nos. 21576175, 51502187), the key industrial prospective program of Jiangsu Science and Technology Department (BE2015190), the Natural Science Foundation of Jiangsu Province of China (No. BK20141178), the General Program of the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 16KJB150036), the Earmarked Nanotechnology Fund of the Bureau of Science and Technology of Suzhou City (No. ZXG201429) and Collaborative Innovation Center of Technology and Material of Water Treatment. Conflicts of Interest: The authors declare no conflict of interest.

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