nanomaterials Article Influence of Cocatalysts (Ni, Co, and Cu) and Synthesis Method on the Photocatalytic Activity of Exfoliated Graphitic Carbon Nitride for Hydrogen Production Adeem Ghaffar Rana 1,2 , Michael Schwarze 3, *, Minoo Tasbihi 3 , Xavier Sala 4 , Jordi García-Antón 4 and Mirjana Minceva 1, * 1 Biothermodynamics, TUM School of Life Sciences, Technical University of Munich, Maximus-Von-Imhof-Forum 2, 85354 Freising, Germany 2 Department of Chemical, Polymer and Composite Materials Engineering, University of Engineering and Technology (UET), Lahore 39161, Pakistan 3 Department of Chemistry, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany 4 Departament de Química, Unitat de Química Inorgànica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain * Correspondence: michael.schwarze@tu-berlin.de (M.S.); mirjana.minceva@tum.de (M.M.); Tel.: +49-8161716170 (M.M.) Abstract: Exfoliated graphitic carbon nitride (ex-g-CN) was synthesized and loaded with non-noble metals (Ni, Cu, and Co). The synthesized catalysts were tested for hydrogen production using a 300-W Xe lamp equipped with a 395 nm cutoff filter. A noncommercial double-walled quartz-glass reactor irradiated from the side was used with a 1 g/L catalyst in 20 mL of a 10 vol% triethanolamine aqueous solution. For preliminary screening, the metal-loaded ex-g-CN was synthesized using the incipient wetness impregnation method. The highest hydrogen production was observed on the Citation: Rana, A.G.; Schwarze, M.; Ni-loaded ex-g-CN, which was selected to assess the impact of the synthesis method on hydrogen Tasbihi, M.; Sala, X.; García-Antón, J.; Minceva, M. Influence of Cocatalysts production. Ni-loaded ex-g-CN was synthesized using different synthesis methods: incipient wetness (Ni, Co, and Cu) and Synthesis impregnation, colloidal deposition, and precipitation deposition. The catalysts were characterized by Method on the Photocatalytic X-ray powder diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption using the Brunauer– Activity of Exfoliated Graphitic Emmett–Teller method, and transmission electron microscopy. The Ni-loaded ex-g-CN synthesized Carbon Nitride for Hydrogen using the colloidal method performed best with a hydrogen production rate of 43.6 µmol h−1 g−1 . Production. Nanomaterials 2022, 12, By contrast, the catalysts synthesized using the impregnation and precipitation methods were less 4006. https://doi.org/10.3390/ active, with 28.2 and 10.1 µmol h−1 g−1 , respectively. The hydrogen production performance of nano12224006 the suspended catalyst (440 µmol m−2 g−1 ) showed to be superior to that of the corresponding Academic Editor: Joon Ching Juan immobilized catalyst (236 µmol m−2 g−1 ). Received: 13 July 2022 Keywords: graphitic carbon nitride; water splitting; hydrogen production; nickel; cocatalyst deposition Accepted: 11 November 2022 Published: 14 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- Energy and environmental challenges are emerging with industrial and economic iations. development [1–3]. Industrialization and increasing population are the main reasons behind the energy and environmental crises. Most of the world’s energy demand is fulfilled by nonrenewable sources (petrol, diesel, and coal), which are becoming depleted [4,5]. Copyright: © 2022 by the authors. Additionally, during the past few decades, the increased consumption of fossil fuels has led Licensee MDPI, Basel, Switzerland. to severe environmental issues, such as global warming and climate change. As a result, This article is an open access article researchers are putting more effort into developing renewable energy sources, the only distributed under the terms and alternative to ensure sustainable development. Solar light, wind, biomass, hydro, and conditions of the Creative Commons geothermal energy sources are eco-friendly sources of renewable energy [6–9]. Among Attribution (CC BY) license (https:// them, sunlight energy can be stored in the chemical bonds of a fuel (e.g., hydrogen, H2 ) creativecommons.org/licenses/by/ through artificial photosynthesis [4,10–12]. Hydrogen, an alternative renewable energy 4.0/). source, can release considerable energy without emitting greenhouse gases; water is the Nanomaterials 2022, 12, 4006. https://doi.org/10.3390/nano12224006 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022, 12, 4006 2 of 13 only by-product of hydrogen combustion. Hence, photocatalytic H2 generation through solar light-driven water splitting using semiconductor materials that convert solar energy to chemical energy has become a promising approach [13]. Consequently, the development of a stable, effective, and inexpensive catalyst for the hydrogen evolution reaction (HER) has become a challenging and important research topic [14]. A wide variety of semiconductor-based photocatalysts, such as TiO2 [15], ZnO [16,17], Bi [18], CdS [19], and g-CN [20], have been investigated in the last few decades. The more conventional (i.e., TiO2 and ZnO) have advantages and disadvantages regarding stability and nontoxicity [21,22]. Conversely, they typically have large bandgaps and a high rate of electron–hole recombination, which results in low solar-to-fuel efficiency [23]. Among semiconductor materials, graphitic carbon nitride (g-CN) has attracted considerable atten- tion since Wang et al. first reported its use for water splitting [14,24]. g-CN, a visible-light photocatalyst composed of N, C, and H, has attracted interest because of its extensive ap- plication in CO2 reduction [25], pollutant degradation [23], organic synthesis reactions [26], and water splitting [27]. These nitrogen-rich materials are inexpensive, abundant, and easy to synthesize [22,28]. The thermal decomposition of nitrogen-rich precursors, such as melamine, urea, thiourea, cyanamide, or dicyanamide, is used to synthesize g-CN in the form of tri-s-triazine sheets. Moreover, the nontoxic g-CN, which can be activated by visible light because of the low bandgap energy (2.7 eV), possesses chemical, electronic, and thermodynamic stability [22,23,29]. However, the photocatalytic performance of bulk g-CN is low because of the low specific surface area (10 m2 /g), low availability of active sites, low adsorption and absorption, and rapid recombination of photogenerated electron-hole pairs. Several strategies can be adopted during the photocatalyst design to improve the surface and optical properties of g-CN to overcome these problems. These include metal and nonmetal doping, morphology control, compositing with other semiconductor materials, and exfoliation [22,30]. From these strategies, exfoliation is a fast, efficient, and easy method to improve a given material and its optical properties by increasing its surface area [23]. Moreover, doping can help improve the electronic properties of materials by introducing more electron-hole pairs and minimizing charge recombination [8,22,24,30]. Modifying the catalyst with a cocatalyst enhances the performance of materials sig- nificantly. Immobilization of a cocatalyst on the surface of the semiconductor material is one of the efficient and essential methods to accelerate the separation efficiency of electron– hole pairs generated in the process, consequently enhancing the overall photocatalytic performance of the semiconductor material. Therefore, adding cocatalysts enhances hy- drogen production in a water-splitting reaction. Noble and non-noble metals can play this role [1,6,13,21,31]. Noble metal cocatalysts typically enhance the kinetics of the reaction at low overpoten- tials and help induce charge separation from the semiconductor to the cocatalyst [13]. Noble metals, such as platinum (Pt) [32], gold (Au) [33], ruthenium (Ru) [34], silver (Ag) [35], and palladium (Pd) [36], are usually the most common cocatalysts for improving the photocatalytic performance [1]. However, noble metals are scarce, which prevents their practical implementation on a large scale. Hence, researchers are trying to find a cocatalyst that can replace these noble metals, such as non-noble metals, metal oxides, and metal sulfides [21,31]. From all the non-noble metals available for HER, nickel-based cocatalysts have attracted attention because of their low price, stability, and high activity [1]. Recently, Ni metal and Ni-based compounds (Ni2 P, Ni(OH)2 , NiN3 , NiB, NiS, and Ni3 C) have shown significant HER performance when used as cocatalysts in g-CN-based photocatalytic systems. Ni can play a similar role to noble metals, improving the separation efficiency of electron–hole pairs [1,13]. The cocatalyst loading on the support material can be achieved using diverse synthetic methods. The synthesis methods determine the structure, dispersion, and size distribution of the cocatalyst, which affects the activity of the final hybrid material. In recent years, various methods have been proposed to synthesize supported nanoparticles. However, few studies have compared different synthesis methods Nanomaterials 2022, 12, 4006 3 of 13 for a given support and a given photocatalytic processes. Therefore, rationally choosing the most suitable method remains a challenge [37]. This study evaluated the effect of non-noble metal (Ni, Cu, and Co) cocatalyst loading on exfoliated g-CN (ex-g-CN) for hydrogen production. The hybrid photocatalyst was initially prepared using the incipient wet impregnation (IWI) method and the superior photocatalytic activity of Ni (over Cu and Co) was observed. After this preliminary screening, the best-performing Ni-based hybrid photocatalyst was prepared using one of the following methodologies: (i) precipitation deposition method (PRDM), (ii) IWI method, and (iii) colloidal deposition method (CM). The effects of the synthesis method on the HER photocatalytic performance were studied with triethanolamine (TEOA) as the sacrificial agent under simulated solar light irradiation using a 300-W xenon lamp with a 395-nm cutoff filter. Moreover, the effects of the catalyst exposure to the reaction medium (immobilization or suspension) were studied for practical applications. 2. Materials and Methods 2.1. Chemicals Melamine (C3 H6 N6 , 99%) and dihydrogen hexachloroplatinate(IV) hydrate, 99.9% (metal basis) (H2 PtCl6 ·H2 O) were purchased from Alfa Aesar (Haverhill, MA, USA). Nickel sulfate hexahydrate (NiSO4 ·6H2 O), copper sulfate pentahydrate (CuSO4 ·5H2 O), and cobalt sulfate monohydrate (CoSO4 ·H2 O) (Merck, Darmstadt, Germany), sodium hydroxide (NaOH), ethanol (C2 H5 OH), methanol (CH3 OH), cyclohexane (C6 H12 ), 1-butanol (C4 H10 O), and ascorbic acid (C6 H8 O6 ) were obtained from VWR (Radnor, PA, USA). Triton X-100 (C14 H22 O(C2 H4 O)n (n = 9–10)) was supplied by Sigma-Aldrich (Darmstadt, Germany). TEOA (C6 H15 NO3 ) was acquired from Sigma-Aldrich (Darmstadt, Germany). All chemicals were used as received. 2.2. Synthesis of ex-g-CN Bulk carbon nitride (g-CN) was synthesized with prethermal decomposition of melamine using the procedure established in a previous study [23]. Briefly, a closed crucible with melamine was placed into a muffle furnace (Carbolite Gero, GPC 1200, Derbyshire, UK) for thermal decomposition. The heating program consisted of two steps: heating to 450 ◦ C with a gradient of 2 ◦ C min−1 , and this temperature was kept for 2 h. The sample was then heated to 550 ◦ C at a rate of 2 ◦ C min−1 , maintaining this temperature for 4 h. Bulk g-CN was crushed in a mortar and pestle, rinsed with ultrapure water, and dried overnight at 80 ◦ C. Carbon nitride (ex-g-CN) was exfoliated from g-CN by placing g-CN in an open crucible inside a muffle furnace for 2 h at 500 ◦ C using a heating ramp of 2 ◦ C min−1 . The ex-g-CN was obtained after thermal treatment. 2.3. Cocatalyst Loading onto ex-g-CN Incipient Wet Impregnation The IWI method was used to synthesize Ni, Cu, Co, and Pt-loaded ex-g-CN catalyst (2 wt.% theoretical loadings) for the preliminary screening of hydrogen production. The Ni- based catalyst synthesized using this method was called NiIWI /ex-g-CN [38]. According to stoichiometric calculations, the required amount of a respective salt precursor was added dropwise to the ex-g-CN. The beaker containing the catalyst powder was placed in a sonicator for better dispersion. The material was dried overnight at 80 ◦ C before further use. 2.4. Precipitation Deposition Method NiPRDM /ex-g-CN (2 wt.% theoretical Ni loading) was synthesized using the PRDM [39]. The respective amount of the nickel salt precursor was added to 300 mL of milli-Q water. The solution was maintained at pH 9 using 0.1M NaOH. At a stable pH, ex-g-CN was added with continuous stirring. The deposition–precipitation procedure was conducted at Nanomaterials 2022, 12, 4006 4 of 13 70 ◦ C at a constant pH for 2 h, and the slurry was stirred and dried overnight at the same temperature. The catalyst was washed and dried at 80 ◦ C under vacuum. 2.5. Colloidal Deposition Method NiCM /ex-g-CN (2 wt.% theoretical Ni loading) was synthesized using the CM. The microemulsion system for synthesizing Ni nanoparticles consists of two phases, e.g., the water and oil phases. The nickel salt precursor, surfactant (Triton X-100), and co-surfactant (1-butanol) were present in the water phase. The oil phase was cyclohexane. Two mi- croemulsions were prepared for the synthesis: (1) with Ni salt and (2) with a reducing agent (ascorbic acid). The synthesis process was conducted in a glass reactor with stirring. Emulsion 2 contained ascorbic acid, cyclohexane, butanol-1, triton X-100, and water. Emul- sion 1 contained the nickel salt precursor, cyclohexane, butanol-1, triton X-100, and water. Emulsion 2 was added slowly to the reactor containing emulsion 1. The mixture was stirred at 700 rpm for 30 min at room temperature to form the colloidal stabilized Ni nanoparticles and then stirred for 2 h at room temperature. The ex-g-CN support was then added to the mixture with vigorous stirring to deposit the Ni nanoparticles onto the surface of ex-g-CN. The suspension was stirred further for 2 h at 55 ◦ C. After the process, the reactor was cooled to room temperature. The solid was centrifuged and washed several times with acetone. Finally, the catalyst was dried at 80 ◦ C under vacuum. 2.6. Characterization The Brunauer–Emmett–Teller (BET) surface area was determined from a nitrogen adsorption–desorption experiment at 77 K (quadrasorp, Quantachrome, Boynton Beach, FL, USA). The crystalline phases were examined using X-ray diffraction (XRD, Mini Flex 600C, Rigaku, Tokyo, Japan) was conducted using CuKα radiation at a voltage, current, and spin speed of 40 kV, 15 mA, and 80 rpm, respectively, in the range between 3◦ and 60◦ 2θ with a step size of 0.0075◦ . Transmission electron microscopy (TEM, JEM-ARM300F2) images were obtained using a probe corrected with a cold FEG emitter (JEOL Ltd., Tokyo, Japan) operated at 300 kV with a camera length of 10 cm. The acquired and evaluated high- angle annular dark-field images represent a detection angle of 54–220 mrad. The image contrasts were formed mainly by Rutherford scattering and were correlated to the atomic number. X-ray photoelectron spectroscopy (XPS, Phoibos 150 analyzer, SPECS GmbH, Berlin, Germany) was conducted under ultra-high vacuum conditions (base pressure 5 × 10−10 mbar) using a monochromatic Al Kα X-ray source (1486.74 eV). The energy resolution measured from the FWHM of the Ag 3d5/2 peak for a sputtered silver foil was 0.62 eV at the Catalan Institute of Nanoscience and Nanotechnology in Barcelona. Ni loading in Ni-loaded ex-g-CN samples was determined using inductively coupled plasma optical emission spectrometry (Perkin-Elmer Optima 4300DV model system) in the Chemical Analyses Service of the Universitat Autònoma de Barcelona. 2.7. Photocatalytic Experiments The photocatalytic experiments were performed in a noncommercial side-irradiated double-walled quartz-glass reactor (shown in a previous work [40]) with a maximum volume of 35 mL. In general, 20 mg of the prepared photocatalyst was placed into the reactor, and 20 mL of an aqueous TEOA solution (10 vol% TEOA) was added. The solution was flushed with Ar for 15 min to remove oxygen, and the reactor was closed with a rubber septum. The reactor was connected to a thermostat (ministat 125, Huber, Germany, Tset = 19 ◦ C) and placed onto a magnetic stirrer in front of a 300-W Xe lamp equipped with a 395-nm cutoff filter. The distance between the lamp and reactor was 10 cm. The lamp was switched on, and after irradiating the intensively stirred photocatalyst suspension for the desired time, a gas phase sample was taken with a gas-tight syringe. The gas sample was analyzed by gas chromatography (Agilent Technologies 7890 A) equipped with a thermal conductivity detector. 395-nm cutoff filter. The distance between the lamp and reactor was 10 cm. The lamp was switched on, and after irradiating the intensively stirred photocatalyst suspension for the Nanomaterials 2022, 12, 4006 desired time, a gas phase sample was taken with a gas-tight syringe. The gas sample was 5 of 13 analyzed by gas chromatography (Agilent Technologies 7890 A) equipped with a thermal conductivity detector. Thehydrogen The hydrogenproduction productionphotocatalytic photocatalyticactivity activitywas wascalculated calculatedas asfollows: follows:  (μmol (𝑉 − 𝑉 ) ⋅ 𝐻 (𝐺𝐶) 𝐻 −1g −h 1 ) =(V0 − VL ) · H2 ( GC ) (1)  H2 µmol g h = 𝑚 ⋅𝑡 ⋅𝑉 (1) mcat · t R · Vm where V0 is the total volume of the reactor, VL is the volume of the solution, H2 is the where amount V 0ofisHthe total volume of the reactor, VL is the (g) 2 detected using GC, mcat is the amount volume of the tsolution, of catalyst, H2 is the R is the irradiation amount of H 2 detected using GC, m cat is time, and Vm is the molar volume of hydrogen. the amount (g) of catalyst, t R is the irradiation time, and Vm is the molar volume of hydrogen. 3. Results 3. Results 3.1. Catalyst 3.1. Catalyst Characterization Characterization Beforeassessing Before assessingthe theas-prepared as-preparedphotocatalysts photocatalystsfor forthetheHER, HER,they theywere werecharacterized characterized using the different techniques. Optical properties were characterized using the different techniques. Optical properties were characterized using UV-Vis using UV-Vis and and photoluminescence (PL) photoluminescence (PL) spectroscopy spectroscopy (Figures (Figures S1S1 and and S2). S2). The The bandgap bandgap energy energy for for bulk bulk andexfoliated and exfoliated g-CN g-CN were were 2.42 2.42 and and 2.62 2.62 eV eV showing showing light light absorption absorption in in the the visible visible range. range. Afterloading After loadingwith withCo, Co,Ni, Ni, andandCu,Cu, thethe bandgap bandgap energy energy waswas2.6, 2.6, 2.5, and 2.5, and 2.4respectively. 2.4 eV, eV, respec- tively. The optical properties remained after cocatalyst loading. In The optical properties remained after cocatalyst loading. In the case of different methodsthe case of different methods to load Ni onto ex-g-CN, bandgap energy was constant at to load Ni onto ex-g-CN, bandgap energy was constant at about 2.5 eV. PL spectroscopy about 2.5 eV. PL spec- troscopythat showed showed g-CNthat g-CN materials materials absorbedabsorbed the maximum the maximum in the of in the range range of 430–460 430–460 nm. The nm. The chemical chemical composition composition was studied was studied using using FTIR (Figure FTIR (Figure S3) and S3)spectra and spectra are typical are typical for g-CNfor g-CN materials. materials. Peaks Peaks at 806 cm −1 can at 806 cmbe−1 can be attributed attributed to triazine to triazine units, whereas units, whereas the bands the strong strong bands between between 1636 and16361242 andcm1242−1 cm −1 belong belong to the to the C=NC=N and and C–Nbonds C–N bondsofofheterocyclic heterocyclic rings. rings. Therewere There werenonopeak peakalterations alterationsandandnewnewpeaks peaksafter afterthe theaddition additionofofthe thecocatalyst. cocatalyst. 3.2. 3.2. X-ray x-RayDiffraction Diffraction The The phasestructures phase structuresofofg-CN, g-CN,ex-g-CN, ex-g-CN, and and Ni-loaded Ni-loaded ex-g-CN ex-g-CN were examined were examined using us- XRD and the diffractograms are shown in Figure 1. All samples showed ing XRD and the diffractograms are shown in Figure 1. All samples showed the charac- the characteristic peak teristic 27.2◦ at at peak 2θ27.2° and 2θ a weaker peak atpeak and a weaker 13◦ 2θ. The2θ. at 13° strong peak was The strong peakindexed to the to was indexed (002) the plane, a characteristic interlayer stacking peak of aromatic (002) plane, a characteristic interlayer stacking peak of aromatic g-C N systems. Moreover, 3 4 g-C3N4 systems. Moreo- the weak peak ver, the waspeak weak assigned to the (100) was assigned to crystal the (100)plane, which crystal was plane, attributed which to the repeated was attributed to the tri-s-triazine units. The decrease in intensity and the slight shift in the peaks were repeated tri-s-triazine units. The decrease in intensity and the slight shift in the peaks wereattributed to exfoliation attributed to and Ni loading. exfoliation and NiThere were no loading. Therepeaks were of Ni no due peaks to of theNi low Nito due content. the lowThe Ni XRD patterns of cobalt and copper loaded ex-g-CN showed similar content. The XRD patterns of cobalt and copper loaded ex-g-CN showed similar peaks with no visible peaks peak with of norespective visible peakmetal (Figure S4). of respective metal (Figure S4). Figure 1. XRD patterns of g-CN, ex-g-CN, and Ni-loaded ex-g-CN synthesized using the incipient wet impregnation (IWI) method, colloidal method (CM), and precipitation deposition method (PRDM). 3.3. Brunauer–Emmett–Teller The specific surface areas were measured to reveal any change in the structural features of carbon nitride before and after exfoliation. The surface area of ex-g-CN, obtained from Figure 1. XRD patterns of g-CN, ex-g-CN, and Ni-loaded ex-g-CN synthesized using the incipient wet impregnation (IWI) method, colloidal method (CM), and precipitation deposition method (PRDM). 3.3. Brunauer–Emmett–Teller Nanomaterials 2022, 12, 4006 6 of 13 The specific surface areas were measured to reveal any change in the structural fea- tures of carbon nitride before and after exfoliation. The surface area of ex-g-CN, obtained from the N2 adsorption-desorption curves (Figure S5), was significantly higher than that the N2 adsorption-desorption of g-CN (169.3 m2/g vs. 5.4 m2/g,curves (Figure S5), respectively). Thewas significantly surface higher areas of the than that Ni-loaded of ex-g- g-CN (169.3 2 2 CN, Ni PRDM, m Ni/gIWI, vs. and5.4 Nim CM/g, respectively). were The 65 84.6, 69.8, and surface m2/g,areas of the Ni-loaded respectively. ex-g-CN, The reduction in Ni , Ni , and Ni were 84.6, 69.8, and 65 m 2 /g, respectively. The reduction in the the surface area is probably due to blockage of the pores with the loaded metal particles. PRDM IWI CM surface area is probably due to blockage of the pores with the loaded metal particles. 3.4. Inductively Coupled Plasma (ICP) Mass Spectrometry (MS) 3.4. Inductively Coupled Plasma (ICP) Mass Spectrometry (MS) The theoretical loading of Ni for all samples was 2%. The Ni content of the NiPRDM, The theoretical loading of Ni for all samples was 2%. The Ni content of the NiPRDM , NiIWI, and NiCM, determined using ICP-OES, were 1.7, 1.2, and 1.4 wt.%, respectively, cor- NiIWI , and NiCM , determined using ICP-OES, were 1.7, 1.2, and 1.4 wt.%, respectively, cor- responding to Ni deposition yields of 85%, 60%, and 70%, respectively. The Ni loading responding to Ni deposition yields of 85%, 60%, and 70%, respectively. The Ni loading ratio ratio on the surface of g-ex-CN was high compared with that of Pt. The surface charge of on the surface of g-ex-CN was high compared with that of Pt. The surface charge of Ni was Ni was positive, and g-CN was negative, whereas the surface charge of Pt was negative. positive, and g-CN was negative, whereas the surface charge of Pt was negative. Because Because of the negative charge of Pt, the deposition yield for Pt obtained in earlier inves- of the negative charge of Pt, the deposition yield for Pt obtained in earlier investigations tigations was consistently lower (below 50%) [41,42]. The copper and cobalt actual load- was consistently lower (below 50%) [41,42]. The copper and cobalt actual loadings were ings were 1.7% and 1.4%, respectively (theoretical loading in both cases was 2%). 1.7% and 1.4%, respectively (theoretical loading in both cases was 2%). 3.5. X-ray 3.5. x-Ray Photoelectron Photoelectron Spectroscopy Spectroscopy XPS measurements XPS measurements were were conducted conducted to to investigate investigate thethe composition composition of of the the Ni-loaded Ni-loaded ex-g-CN photocatalysts. ex-g-CN photocatalysts.As Asshown shown in in Figure Figure2, the XPSXPS 2, the survey spectra survey confirmed spectra the pres- confirmed the ence of C, presence ofN, C, O, N, and Ni.Ni. O, and Ni Ni was was visible inin visible the thesample sampleprepared preparedusing usingthe the CM. CM. Figure 2b 2b presentsthe presents thehigh-resolution high-resolutionspectra spectraof ofNi Ni 2p. 2p. Four Four significant significant peakspeaks were were observed observed at at 852.3, 852.3, 857.2, 870.1, and 873.5 eV. The peaks at 852.3 and 870.1 eV correspond 857.2, 870.1, and 873.5 eV. The peaks at 852.3 and 870.1 eV correspond to metallic Ni. These to metallic Ni. These peaks were peaks were clearly clearly visible invisible in theprepared the sample sample prepared using the using CMthe (NiCM CM ),(Ni but ), butwere CMthey they were either either reduced reduced in intensity in intensity (NiPRDM ) or )absent (NiPRDM or absent (Ni)IWI (NiIWI for) for thethe other other two two samples, samples, even even if if Ni Ni waswas detected detected using using ICP-MS. ICP-MS. TheThe other other twotwo peaks peaks at 857.2 at 857.2 andand 873.1 873.1 eVeV correspond correspond to to 2+ Ni Ni. 2+The . The two two peaks peaks atat 862.5and 862.5 and876.8 876.8eVeVarearesatellite satellitepeaks peaksofofNi 2+2+.. Ni Ni Ni always always showed showed strong strong satellite peaks approximately 6 eV above the main electronic lines [31,43–47]. The satellite peaks approximately 6 eV above the main electronic lines [31,43–47]. The XPS XPS peaks peaksofofcobalt cobaltand andcopper copperloaded loaded ex-g-CN ex-g-CN (Figure S6) confirmed (Figure S6) confirmedthe presence of these the presence of species in different these species oxidation in different states.states. oxidation Figure 2. Figure 2. XPS profiles profiles of of the the obtained obtained samples samples for for (a) (a) survey survey and and (b) (b) Ni Ni 2p 2p scan. scan. 3.6. 3.6. Morphology Morphology The morphology of the Ni-based catalysts synthesized using different methods was investigated using SEM (Figure 3). The catalysts synthesized using the CM were studied further using TEM (Figure 4). All catalysts showed a stacked morphology, and EDX analysis (Figure 3) confirmed the Ni loading. In Figure 4, TEM images of the Ni-based catalyst synthesized using the CM at different magnifications are presented together with the corresponding EDX elemental mapping. A flat-sheet structure with thin layers was observed. EDX mapping showed that the synthesis method yielded Ni species loaded on The morphology of the Ni-based catalysts synthesized using different methods was investigated using SEM (Figure 3). The catalysts synthesized using the CM were studied7 of 13 Nanomaterials 2022, 12, x FOR PEER REVIEW further using TEM (Figure 4). All catalysts showed a stacked morphology, and EDX anal- Nanomaterials 2022, 12, 4006 ysis (Figure 3) confirmed the Ni loading. In Figure 4, TEM images of the Ni-based catalyst 7 of 13 synthesized using The the CM at different morphology of themagnifications Ni-based catalystsare synthesized presented together with the using different cor- was methods responding EDX elemental investigated mapping. using A flat-sheet SEM (Figure 3). The structure with thin layers catalysts synthesized wasCM using the observed. were studied EDX the mapping ex-g-CN showed further surface. that using TEM the(Figure TEM imagessynthesis and4).EDXmethod All yielded catalysts spectra showed Ni species a stacked of cobalt loadedloaded on the morphology, and copper ex-g- andex-g-CN EDX anal- CN surface. TEMysisimages and (Figure 3) EDX spectra confirmed the Niof cobalt loading. and In copper Figure are shown in Figure S7. They do not show nanoparticles on the surface. loaded 4, TEM ex-g-CN images are shown of the Ni-based catalyst in Figure S7. They synthesized do notusing showthe CM at different nanoparticles onmagnifications the surface. are presented together with the cor- responding EDX elemental mapping. A flat-sheet structure with thin layers was observed. EDX mapping showed that the synthesis method yielded Ni species loaded on the ex-g- CN surface. TEM images and EDX spectra of cobalt and copper loaded ex-g-CN are shown in Figure S7. They do not show nanoparticles on the surface. Figure 3. SEM images and EDX spectra of Ni-loaded ex-g-CN synthesized using the (a) incipient Figure 3. SEM images wet impregnation (IWI) and EDX(b)spectra method, ofmethod colloidal Ni-loaded ex-g-CN (CM), synthesizeddeposition and (c) precipitation using themethod (a) incipi- ent Figure 3. wet impregnation (PRDM). SEM method, (IWI) images and EDX (b) spectra of colloidal Ni-loaded method ex-g-CN (CM), and synthesized using the (c) precipitation (a) incipient deposition wet impregnation (IWI) method, (b) colloidal method (CM), and (c) precipitation deposition method method (PRDM).(PRDM). Figure 4. TEM images at different resolution and EDX mapping (from top left image) of NiCM. Figure 4. TEM images at different resolution and EDX mapping (from top left image) of NiCM . Figure 4. TEM images at different resolution and EDX mapping (from top left image) of NiCM. 3.7. Photocatalytic H2 Production Activity The photocatalytic activity of the synthesized hybrid materials was studied for hy- drogen production. First, the metal-loaded ex-g-CN catalysts synthesized using the IWI method were screened. The best-performing non-noble metal was chosen for further study. Second, the best-performing non-noble metal-based catalyst was synthesized with different methods, and the effects of the synthesis method on H2 production were studied. Further- more, the effects of the catalyst concentration and catalyst exposure to reaction medium (immobilized or suspended) on hydrogen production were studied. The reactions were Nanomaterials 2022, 12, 4006 8 of 13 performed in the presence of TEOA as a sacrificial electron donor, and the HER activities were evaluated under visible-light irradiation (λ > 396 nm). 3.8. Screening Experiments Screening experiments were performed for the synthesized catalysts; the results are presented in Figure 5a. Almost negligible hydrogen was produced for ex-g-CN without a co- catalyst after 6 h [48]. Ni (2% loading) performed best with 84.5 µmol h−1 g−1 hydrogen pro- duced, followed by 2% loading on cobalt (12.3 µmol h−1 g−1 ) and copper (0 µmol h−1 g−1 ). The catalyst performed in the following order: Ni > Co > Cu = ex-g-CN [48–50]. Pt (2% and 0.5% loading) outperformed all non-noble metal-based photocatalysts for hydrogen produc- tion (Figure 5a). Further experiments were performed with the best-performing non-noble Nanomaterials 2022, 12, x FOR PEER REVIEW 9 of 13 metal Ni system. Figure 5. (a) Comparison Figureof5.the photocatalytic (a) Comparison H2 evolution of the photocatalytic with with H2 evolution different differentnoble and noble and non-noble non-noble met- als; (b) photocatalytic H2 production activities of different Ni-based samples under visible-light ir- metals; (b) photocatalytic H2 production radiation activities for 6 h; (c) effect of different of the catalyst Ni-based concentration samples on the hydrogen under visible-light production; (d) photocata- irradiation for 6 h; (c) effect lytic H2of the catalyst evolution concentration of suspended and immobilizedonNithe hydrogen CM-based production; photocatalysts (d) photo- and analogous immo- bilized Pt-based photocatalyst; and (e) re-utilization of the catalyst (estimated error 10%). catalytic H2 evolution of suspended and immobilized NiCM -based photocatalysts and analogous immobilized Pt-based photocatalyst; and (e) re-utilization of the catalyst (estimated error 10%). Nanomaterials 2022, 12, 4006 9 of 13 3.9. Effect of the Ni Loading Method on Hydrogen Production After selecting Ni as a cocatalyst for further investigations, the effects of the loading method on H2 production were studied, and the results are presented in Figure 5b. Ni- loaded ex-g-CN synthesized using the CM performed best, with a H2 production rate of 43.6 µmol h−1 g−1 . The presence of Ni nanoparticles on the material surface (confirmed by TEM, vide supra) could explain the higher photocatalytic activity of this system. The catalysts synthesized using the impregnation and precipitation methods were less active, with H2 production rates of 28.2 and 10.1 µmol h−1 g−1 , respectively. In addition to XPS, although ICP and EDX verified the presence of Ni, no nanoparticles were observed by TEM, which could explain the lower activity of these catalysts. A plausible explanation could be that the Ni precursor is still present on ex-g-CN after synthesis and was reduced to Ni during irradiation (via photodeposition). Further experiments were performed using the best-performing NiCM . 3.10. Effect of Photocatalyst Concentration The effects of photocatalyst concentration on hydrogen production were examined by varying the mass of NiCM between 10 and 50 mg, keeping the liquid volume (20 mL) constant, resulting in a variation of the photocatalyst concertation between 0.5 and 2.5 g/L. Figure 5c shows the hydrogen produced as a function of photocatalyst concentration. No hydrogen production occurred in the absence of the photocatalyst. Hydrogen produc- tion initially increased with increasing photocatalyst concentration but decreased at high photocatalyst concentrations after reaching a maximum of approximately 1.5 g/L. The amount of hydrogen produced was 37.1, 43.6, 50, and 37.2 µmol h−1 g−1 for 10 (0.5 g/L), 20 (1 g/L), 30 (1.5 g/L), and 50 (2.5 g/L) mg of catalyst, respectively. Hydrogen production increased with increasing amount of photocatalyst because of the availability of more active sites for reaction until 2.5 g/L of catalyst. Further increases in photocatalyst concentration decreased hydrogen production because of the hindered light absorption and scattering. 3.11. Immobilized vs. Suspended Photocatalysts Photocatalyst immobilization is essential for technical applications. Therefore, its in- fluence on photocatalytic H2 production was studied. The photocatalyst was immobilized onto a filter paper using the method reported by Schwarze et al. [51], and the hydrogen production from the immobilized and suspended catalysts was compared. The results are presented in Figure 5d and were compared with those for the analogous Pt-based immobi- lized photocatalyst. The same amount of suspended and immobilized catalysts was used in all cases. The suspended Ni-based photocatalyst (440 µmol m−2 h−1 hydrogen) performed better than the immobilized Ni-based photocatalyst (236 µmol m−2 h−1 hydrogen). Immo- bilization led to an approximately 50% loss of photocatalytic hydrogen production activity. This activity loss was expected because not all particles contribute to the reaction when the photocatalyst particles are immobilized as a thin film. The photocatalyst particles deep in the film layer will not be irradiated. Optimizing the film thickness of the catalyst is the key to improving the activity and later use on a larger scale. Furthermore, the immobilized photocatalyst experiments were conducted with a sunlight simulator equipped with an air mass 1.5 global filter with a lower total intensity. The Pt immobilized catalyst produced 1516 µmol m−2 g−1 H2 for only 0.5% loading. Although Pt clearly outperformed Ni as a cocatalyst when combined with ex-g-CN, the use of Ni provides an alternative non-noble metal-based system with good photocatalytic activity. 3.12. Recycling of the Catalyst The recycling of the suspended NiCM system was demonstrated by performing three cycles of hydrogen production after subjecting the photocatalyst to washing and drying. The amount of hydrogen produced by the photocatalyst increased slightly for at least three consecutive cycles (Figure 5e). In the colloidal method, a reducing agent was used to reduce the Ni salt precursor to Ni nanoparticles. Some residual Ni salt precursor might be Nanomaterials 2022, 12, 4006 10 of 13 adsorbed and reduced during the first irradiation cycle. As a result, more active centers are available for the second run, leading to a higher activity. This assumption was proven by the third run that showed the same activity as the second run. Therefore, the material remained stable, and the slight activation observed potentially arise from changes in the surface composition of the Ni-based cocatalysts under reductive turnover conditions (i.e., changes in the NiO/Ni ratio). 4. Conclusions Non-noble metals (Ni, Cu, and Co) were evaluated as cocatalysts loaded onto ex- g-CN for hydrogen production. The preliminary study on Ni, Cu, and Co synthesized using the IWI method showed that Ni had seven times higher hydrogen production than Co, whereas Cu had little catalytic activity. The Ni-loaded ex-g-CN synthesized using three different synthesis methods was compared: incipient wetness impregnation, colloidal deposition, and precipitation deposition. The NiCM performed best because of the high loading of nanoparticles on the surface of ex-g-CN confirmed using TEM and XPS. The morphology, structure, and surface properties of the Ni-loaded ex-g-CN photocatalysts were characterized using TEM, XRD, XPS, BET, and ICP, and tested for hydrogen production using a 300 W xenon lamp equipped with a 395 nm cutoff filter. Hydrogen production increased with increasing photocatalyst concentration up to 1.5 g/L. A further increase in the photocatalyst concentration led to lower activity because of hindrance and light scattering. The suspended catalyst performed better than a filter- paper-immobilized counterpart. However, immobilization can be helpful for scale-up and industrial applications. The reduced light penetration onto the film formed upon immobilization leads to the loss of photocatalytic activity because not all particles contribute to the reaction. Furthermore, Ni-loaded ex-g-CN proved to be a stable catalyst that can be recycled and reused without significant loss of activity. Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/nano12224006/s1, Figure S1: UV-Vis Spectra of synthesized cat- alysts; inset tauc plots, Figure S2: PL spectra of g-CN and ex-g-CN, Figure S3: FTIR spectra of all synthesized catalysts, Figure S4: XRD patterns of Co- and Cu-loaded ex-g-CN, Figure S5: Adsorption and desorption curves of g-CN and ex-g-CN; inset particle size distribution, Figure S6: XPS profiles of the obtained samples for (a) Co 2p and (b) Cu 2p, Figure S7: TEM and EDX images of Co and Cu-loaded ex-g-CN. References [23,52,53] are cited in Supplementary Materials. Author Contributions: Conceptualization, A.G.R., M.T., M.S. and M.M.; methodology, A.G.R. and M.S.; software, A.G.R. and M.S.; validation, A.G.R.; formal analysis, A.G.R., M.S. and M.T.; investiga- tion, A.G.R., M.T. and M.S.; resources, M.S. and M.M.; writing—original draft preparation, A.G.R.; writing—review and editing, X.S., M.S., J.G.-A. and M.M.; visualization, A.G.R.; supervision, X.S., J.G.-A. and M.M.; funding acquisition, M.S., M.T., J.G.-A., X.S. and M.M. All authors have read and agreed to the published version of the manuscript. Funding: M. Schwarze and M. Tasbihi acknowledge support funded by the Deutsche Forschungsge- meinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy—EXC 2008/1 (UniSysCat)—390540038. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: X. Sala and J. García-Antón gratefully acknowledge the Ministerio de Ciencia, Innovacion y Universidades, and FEDER (PID2019-104171-RB-I00). X. Sala thanks ICREA for the ICREA Academia award 2020. Conflicts of Interest: The authors declare no conflict of interest. Nanomaterials 2022, 12, 4006 11 of 13 References 1. Sun, C.; Zhang, H.; Liu, H.; Zheng, X.; Zou, W.; Dong, L.; Qi, L. Enhanced activity of visible-light photocatalytic H2 evolution of sulfur-doped g-C3 N4 photocatalyst via nanoparticle metal Ni as cocatalyst. Appl. Catal. B Environ. 2018, 235, 66–74. [CrossRef] 2. Sordello, F.; Minero, C. Photocatalytic hydrogen production on Pt-loaded TiO2 inverse opals. Appl. Catal. B Environ. 2015, 163, 452–458. [CrossRef] 3. Bi, G.; Wen, J.; Li, X.; Liu, W.; Xie, J.; Fang, Y.; Zhang, W. Efficient visible-light photocatalytic H2 evolution over metal-free g-C3 N4 co-modified with robust acetylene black and Ni(OH)2 as dual co-catalysts. RSC Adv. 2016, 6, 31497–31506. [CrossRef] 4. 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