nanomaterials Article 2D/2D Heterojunction of TiO2 Nanoparticles and Ultrathin G-C3N4 Nanosheets for Efficient Photocatalytic Hydrogen Evolution Ruifeng Du 1,2 , Baoying Li 3, *, Xu Han 4 , Ke Xiao 1,2 , Xiang Wang 1,2 , Chaoqi Zhang 1,2 , Jordi Arbiol 4,5 and Andreu Cabot 1,5, * 1 Catalonia Energy Research Institute—IREC, Sant Adrià de Besòs, 08930 Barcelona, Spain; ruifengdu@irec.cat (R.D.); kexiao@irec.cat (K.X.); xwang@irec.cat (X.W.); czhang@irec.cat (C.Z.) 2 Departament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, 08028 Barcelona, Spain 3 Shandong Provincial Key Laboratory of Molecular Engineering, State Key Laboratory of Biobased Material and Green Papermaking, School of Chemistry and Chemical Engineering, Qilu University of Technology, Shandong Academy of Sciences, Jinan 250353, China 4 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain; xuhan@irec.cat (X.H.); arbiol@icrea.cat (J.A.) 5 ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain * Correspondence: libaoying@qlu.edu.cn (B.L.); acabot@irec.cat (A.C.) Abstract: Photocatalytic hydrogen evolution is considered one of the promising routes to solve the en- ergy and environmental crises. However, developing efficient and low-cost photocatalysts remains an unsolved challenge. In this work, ultrathin 2D g-C3 N4 nanosheets are coupled with flat TiO2 nanopar- ticles as face-to-face 2D/2D heterojunction photocatalysts through a simple electrostatic self-assembly method. Compared with g-C3 N4 and pure TiO2 nanosheets, 2D/2D TiO2 /g-C3 N4 heterojunctions Citation: Du, R.; Li, B.; Han, X.; Xiao, exhibit effective charge separation and transport properties that translate into outstanding photo- K.; Wang, X.; Zhang, C.; Arbiol, J.; catalytic performances. With the optimized heterostructure composition, stable hydrogen evolution Cabot, A. 2D/2D Heterojunction of activities are threefold and fourfold higher than those of pure TiO2, and g-C3 N4 are consistently ob- TiO2 Nanoparticles and Ultrathin tained. Benefiting from the favorable 2D/2D heterojunction structure, the TiO2 /g-C3 N4 photocatalyst G-C3 N4 Nanosheets for Efficient yields H2 evolution rates up to 3875 µmol·g−1 ·h−1 with an AQE of 7.16% at 380 nm. Photocatalytic Hydrogen Evolution. Nanomaterials 2022, 12, 1557. https:// Keywords: hydrogen evolution; 2D/2D heterojunction; charge separation doi.org/10.3390/nano12091557 Academic Editor: Nikos Tagmatarchis 1. Introduction Received: 8 April 2022 Accepted: 28 April 2022 Owing to the abundance of low-cost solar energy, the numerous uses of hydrogen and Published: 4 May 2022 its advantages as an energy carrier, the photocatalytic generation of hydrogen is a highly appealing process [1,2]. However, the cost-effective photogeneration of hydrogen requires Publisher’s Note: MDPI stays neutral high activity and stable photocatalysts, development of which has been a long-standing with regard to jurisdictional claims in goal. Over the past decades, numerous semiconductors have been tested as photocatalysts published maps and institutional affil- for hydrogen evolution. Among them, titanium dioxide (TiO2 ) has received special atten- iations. tion owing to its stability, high abundance, low toxicity, being the earliest to be discovered and becoming the first to be industrialized [3]. Nevertheless, due to its wide bandgap and relatively fast charge recombination rate, its applicability has been strongly limited. Nu- Copyright: © 2022 by the authors. merous strategies have been proposed to improve the photocatalytic performance of TiO2 , Licensee MDPI, Basel, Switzerland. facilitating charge separation and promoting efficiency and activity, [4–7] including the This article is an open access article control of its particle facets and morphology [8–11], its modification with cocatalysts [12–15] distributed under the terms and and its coupling with other semiconductors to form heterostructures [16–24]. conditions of the Creative Commons Graphite carbonitride (g-C3 N4 ) with a layered structure similar to graphite, high Attribution (CC BY) license (https:// chemical stability and low cost has received increasing interest in recent years [25–27]. In creativecommons.org/licenses/by/ particular, as a polymeric semiconductor, g-C3 N4 has been recently reported as a promising 4.0/). candidate photocatalyst due to its unique structure and electronic characteristics, with a Nanomaterials 2022, 12, 1557. https://doi.org/10.3390/nano12091557 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022, 12, 1557 2 of 11 2.7 eV bandgap that allows absorbing part of the visible spectrum [28,29]. Additionally, two-dimensional (2D) g-C3 N4 nanosheets, benefiting from a huge specific surface area and a suitable band structure, have shown especially interesting properties and offer an excellent platform to produce heterojunctions with other semiconductors [30–33]. Recently, 2D/2D heterojunctions have been demonstrated to provide great advantages to improve charge separation [34,35]. 2D/2D heterojunctions simultaneously maximize the interface and surface areas, i.e., the charge transfer between the two materials and the interaction with the media, which can potentially improve photocatalytic activities. In the present work, we target improving photocatalytic hydrogen production using 2D/2D heterojunctions. In this direction, we report the first synthesis of 2D/2D TiO2 /g- C3 N4 heterostructures. Such composite materials are produced from the electrostatic assem- bly of 2D anatase TiO2 flat nanoparticles synthesized through a simple colloidal method with 2D ultrathin g-C3 N4 . The produced heterostructures are tested as photocatalysts for hydrogen evolution under simulated solar light irradiation. The excellent hydrogen evolution performance obtained after optimizing the weight contents of TiO2 and g-C3 N4 within 2D/2D heterojunction are rationalized using photoluminescence, photocurrent and impedance spectroscopy analysis. 2. Experiment Synthesis of bulk g-C3 N4 (bCN) and ultrathin g-C3 N4 (uCN): Bulk g-C3 N4 powder was synthesized by thermal polymerization of urea. Briefly, 10 g of urea (99%, Acros Organics) was placed into a ceramic crucible. The crucible was covered and heated to 550 ◦ C at a ramp rate of 2 ◦ C min−1 for 4 h under air atmosphere. After cooling to room temperature, the resulting light-yellow solid was ground with the mortar to obtain the bulk g-C3 N4 powder. To obtain ultrathin g-C3 N4 (uCN), bulk g-C3 N4 (2.0 g) was placed in a covered ceramic crucible, and it was heated to 520 ◦ C with a ramp rate of 5 ◦ C min−1 for 2 h under air atmosphere to obtain a light-yellow powder. Synthesis of TiO2 nanosheets: Titanium dioxide nanoparticles were prepared using a colloidal method. All the syntheses were performed using standard airless techniques [36,37]. Typically, 10 mL of oleylamine (OAm, 80–90%, Acros Organics, Geel, Belgium), 10 mL of octadecene (ODE, 90%, Sigma-Aldrich, Burlington, MA, USA) and 1 mL of oleic acid (OAc, 90%, Sigma-Aldrich, Burlington, MA, USA) were loaded in a three-neck flask and degassed under vacuum at 120 ◦ C for 1 h while being strongly stirred using a magnetic bar. Then, 300 mg of TiF4 (99%, Sigma, Burlington, MA, USA) was added in a mixed solution of 2 mL OAm, 3 mL OAc and 6 mL ODE and sonicated for 0.5 h to prepare a precursor solution. Subsequently, under nitrogen atmosphere, 10 mL of the precursor solution were slowly added to the reaction flask, which was then heated to 290 ◦ C at a rate of 5 ◦ C min−1 and maintained for 1 h. The solid product was centrifuged and washed with acetone and hexane three times. The particles were finally dispersed in hexane at a concentration of 10 mg/mL. Ligand removal from TiO2 nanoparticles: In a typical process, 10 mL of a TiO2 dis- persion in hexane (2 mg/mL) was combined with 10 mL acetonitrile to form a two-phase mixture. Then, 1 mL of a HBF4 solution (48%, Sigma-Aldrich, Burlington, MA, USA) was added. The resulting solution was sonicated until the particles transferred from the upper to the bottom layer. The surface-modified particles were washed with ethanol and a 1 mol/L sodium hydroxide (85%, Sigma-Aldrich, Burlington, MA, USA) aqueous solution three times to remove the residual fluoride ions and ligands. The particles were then washed with water to adjust the PH close to neutral. Finally, the particles were dispersed in 10 mL of water with a small amount of DMF. Synthesis of 2D/2D TiO2 /ultrathin g-C3 N4 (TiO2 /uCN) composite: TiO2 /uCN het- erojunctions were produced by an electrostatic self-assembly method. Briefly, 20 mg of as prepared ultrathin g-C3 N4 was dissolved in 10 mL of ultrapure water and sonicated for 1 h. The solution was then mixed with an ethanol solution of ligand-removed TiO2 nanoparticles with a weight ratio of 1:2, 1:1 and 2:1. The mixed solution was stirred for Nanomaterials 2022, 12, 1557 3 of 11 24 h after 1 h of sonication. The obtained composite was collected by centrifuging, it was washed with ethanol 2 times, and it was finally dried at 60 ◦ C for 12 h. The collected mate- rials were named T1 /uCN2 , T1 /uCN1 and T2 /uCN1 based on the different TiO2 /ultrathin g-C3 N4 weight ratios. TiO2 /bulk g-C3 N4 (T/bCN) samples were prepared using the same procedure. For photocatalytic measurements, 1 wt% of Pt was loaded on the surface of the photocatalysts by a photoreduction method. Photocatalytic Hydrogen Evolution Procedure The photocatalytic hydrogen evolution experiments were carried out in a Perfect Light Labsolar-III (AG) photoreactor (Pyrex glass) connected to a closed-loop gas circulation system. In a typical experiment, 20 mg photocatalyst was dispersed in 100 mL aqueous solution containing 10 mL methanol and 1 wt% Pt cocatalyst (40 uL 25.625 mmol/L H2 PtCl6 aqueous solution). The mixed solution was bubbled with N2 for 30 min to ensure anaerobic state and illuminated 30 min with UV light before simulated solar light irradiation to ensure the complete loading of Pt. The incident light was provided by a 300 W Xe lamp with an AM 1.5 filter, and the reaction conditions were kept at room temperature. The resulting gas was analyzed by a Labsolar-III (AG) gas chromatograph equipped with a thermal conductivity detector, with high-purity argon as the carrier gas. 3. Result and Discussion TiO2 /g-C3 N4 heterostructures were obtained by the electrostatic assembly of TiO2 nanoparticles and ultrathin g-C3 N4 nanosheets (Figure 1, see Experimental section for details). Colloidal TiO2 nanoparticles were produced in the presence of OAm and OAc using TiF4 as the Ti precursor. As shown in Figure 2a, low-resolution TEM images exhibited the TiO2 particles to have a flat square morphology with a side length of 30–50 nm and a thickness of about 5–10 nm. g-C3 N4 nanosheets were produced by the thermal etching of bulk g-C3 N4 . As observed by scanning electron microscopy (SEM, Figure S1a,b) and trans- mission electron microscopy (TEM, Figure 2b) characterization, bCN and uCN displayed significantly different morphologies. The uCN showed a thin nanosheet-based structure pointing at the occurrence of a layer etching during the thermal process. Figure S1c displays Nanomaterials 2022, 12, 1557 4 of 12 the nitrogen adsorption–desorption isotherms of bCN and uCN, which further proved uCN (85.7 m /g) to be characterized by a larger specific surface area than bCN (46.3 m2 /g). 2 Figure1.1. Schematic Figure Schematicillustration illustrationof ofthe theprocess processused usedto toproduce produce2D/2D 2D/2D TiO /uCNcomposite. TiO22/uCN composite. Nanomaterials 2022, 12, 1557 4 of 11 Figure 1. Schematic illustration of the process used to produce 2D/2D TiO2/uCN composite. Figure Figure 2. 2. Representative Representative TEM images images of of (a) (a) TiO TiO22 nanoparticles; nanoparticles;(b) (b)g-C g-C33N N44 nanosheets nanosheetsand andTT11/uCN /uCN11 composite composite with with representative representative (c) (c) low and (d) low and (d) high high magnification. magnification. (e,f) (e,f) HRTEM HRTEM images images of /uCN11. . of TT11/uCN A magnified detail A magnified detail (top (topright) right)ofofthethe orange orange squared squared region region in HRTEM in the the HRTEMimageimage and itsand its corre- corresponding indexed power spectrum (bottom right) is shown, revealing the TiO2 anatase phase sponding indexed power spectrum (bottom right) is shown, revealing the TiO2 anatase phase (space group = I41/amd) with a = b = 3.7840 Å, and c = 9.5000 Å. TiO2 lattice fringe distances were (space group = I41 /amd) with a = b = 3.7840 Å, and c = 9.5000 Å. TiO2 lattice fringe distances were measured to be 0.233 nm, 0.352 nm and 0.348 nm at 41.30° and 139.38°, which could be interpreted measured as to beTiO the anatase 0.233 nm, 0.352 nm and 0.348 nm at 41.30◦ and 139.38◦ , which could be interpreted as 2 phase, visualized along its [1–11] zone axis. the anatase TiO2 phase, visualized along its [1–11] zone axis. To positively charge the surface of the TiO2 particles, enable their dispersion in an To positively charge the surface of the TiO particles, enable their dispersion in an aqueous solution and promote charge transfer 2with the media; the organic ligands at- aqueous solution and promote charge transfer with the media; the organic ligands attached tached to the particle surface were removed using HBF4 (Figure S2). As observed by to the particle surface were removed using HBF4 (Figure S2). As observed by zeta-potential zeta-potential analysis, while the g-C3N4 nanosheets were negatively charged (V = −33.8 analysis, while the g-C3 N4 nanosheets were negatively charged (V = −33.8 mV), after mV), after ligands removal the TiO2 particles were positively charged (V = +18.6 mV), ligands removal the TiO2 particles were positively charged (V = +18.6 mV), which enabled which enabled the the electrostatic electrostatic self-assembly self-assembly of the two components of the two components [38]. Indeed,[38]. whenIndeed, when combining combining solutions of the two types of material, a light-yellow precipitate was formed. solutions of the two types of material, a light-yellow precipitate was formed. The precipitate was composed of large uCN nanosheets containing numerous nanoparticles attached to their surface. TEM analyses showed these nanoparticles lie flat on the surface of uCN, forming 2D/2D heterostructures (Figure 2c,d). High resolution TEM (HRTEM) further confirmed these nanoparticles are TiO2 with good crystallinity (Figure 2e,f). SEM-EDS elemental maps (Figure S4) displayed a homogeneous distribution of C, N, O and Ti, demonstrating a uniform distribution of TiO2 particles on the uCN surface at the microscale. On the other hand, quantitative EDX analyses showed the TiO2 :CN weight ratio to be close to that of the nominal combination of each phase: TiO2 :CN = 0.47 for T1 /uCN2 ; TiO2 :CN = 1.1 for T1 /uCN1 and TiO2 :CN = 1.9 for T2 /uCN1 , obtained from mixing 1:2, 1:1 and 2:1 mass ratios of particles, respectively (Figures S5–S7). Figure 3a displays the X-ray diffraction (XRD) patterns of bCN, uCN, TiO2 and T/uCN samples. The XRD peaks at 25.2◦ (101), 38.0◦ (004), 47.7◦ (200) and 54.8◦ (211) are associated with the anatase TiO2 phase (JCPDS No. 21-1272) [39]. Additonally, the characteristic diffraction peaks at 13.1◦ and 27.4◦ correspond to the (002) and (100) planes of g-C3 N4 (JCPDS No. 87-1526) [40]. The characteristic diffraction peaks of both TiO2 and g-C3 N4 can be observed in all the composites samples, confirming the coexistence of anatase TiO2 and g-C3 N4 . T/uCN samples. The XRD peaks at 25.2° (101), 38.0° (004), 47.7° (200) and 54.8° (211) are associated with the anatase TiO2 phase (JCPDS No. 21-1272) [39]. Additonally, the char- acteristic diffraction peaks at 13.1° and 27.4° correspond to the (002) and (100) planes of g-C3N4 (JCPDS No. 87-1526) [40]. The characteristic diffraction peaks of both TiO2 and Nanomaterials 2022, 12, 1557 g-C3N4 can be observed in all the composites samples, confirming the coexistence of an- 5 of 11 atase TiO2 and g-C3N4. Figure 3. (a) Figure 3. (a) XRD XRD patterns patterns of of TiO TiO22,, uCN uCN and and T/uCN. (b) XPS T/uCN. (b) XPS survey survey spectrum spectrum of of TiO TiO22,, uCN uCN and and T/uCN; high-resolution XPS T/uCN; high-resolution XPS spectra spectra atat the the regions regions (c) (c) C C 1s, 1s, (d) (d) N N 1s, 1s, (e) Ti 2p and (f) O 1s. The The X-ray X-rayphotoelectron photoelectronspectroscopy spectroscopyspectra of TiO spectra , uCN of 2TiO and T/uCN are displayed 2, uCN and T/uCN are dis- in Figure 3b–f. As observed from the survey XPS spectrum, besides Ti, C, O and N, a played in Figure 3b–f. As observed from the survey XPS spectrum, besides Ti, C, O and residual amount of F from the TiF4 precursor used to prepare the TiO2 particles was also N, a residual amount of F from the TiF4 precursor used to prepare the TiO2 particles was present in the final material (Figure 3b). The high-resolution C 1s XPS spectrum of uCN also present in the final material (Figure 3b). The high-resolution C 1s XPS spectrum of showed two main contributions at 288.2 eV and 284.8 eV, which were assigned to C-(N3) uCN showed two main contributions at 288.2 eV and 284.8 eV, which were assigned to and C–C/C=C, respectively (Figure 3c). Compared with pure uCN, the peak for C-(N3) of C-(N3) and C–C/C=C, respectively (Figure 3c). Compared with pure uCN, the peak for the T1 /uCN1 sample was slightly shifted to 288.2 eV. The high-resolution N 1s XPS spectra C-(N3) of the T1/uCN1 sample was slightly shifted to 288.2 eV. The high-resolution N 1s were deconvoluted using three contributions at binding energies of 398.1 eV, 499.4 eV XPS spectra were deconvoluted using three contributions at binding energies of 398.1 eV, and 400.5 eV for uCN and 398.1 eV, 499.6 eV and 400.7 eV for T/uCN (Figure 3d). These 499.4 eV and 400.5 eV for uCN and 398.1 eV, 499.6 eV and 400.7 eV for T/uCN (Figure 3d). three contributions were assigned to N-(C2 ), N-(C3 ) and N-Hx groups of the heptazine These three contributions were assigned to N-(C2), N-(C3) and N-Hx groups of the hep- framework. The small shifts detected for C and some of the N components might be related tazine to framework. a certain degreeThe small shifts of charge detected between for C the TiO and some of the N components might 2 and the CN phases. Figure 3e displays be related to a certain degree of charge between the the high-resolution Ti 2p XPS spectra of TiO2 and T/uCN. TiO2 Both and the CN phases. samples Figure show two 3e strong displays the high-resolution Ti 2p XPS spectra of TiO 2 and T/uCN. Both samples show peaks at approximately 458.7 and 464.5 eV, which are assigned to the Ti 2p3/2 and Ti 2p1/2 levels of Ti within a TiO2 environment. The high-resolution O 1s XPS spectra of TiO2 and T/uCN were fitted with two peaks at 530.4 eV and 531.8 eV, which were associated with oxygen within the TiO2 lattice and oxygen-containing surface adsorption groups such as surface hydroxyl, respectively (Figure 3f). The UV-vis spectra showed the UV absorption edge of TiO2 particles and uCN nanosheets at about 390 nm and 445 nm, respectively (Figure 4a). T/uCN composites showed a similar onset absorption edge as uCN but an increased absorption below 400 nm related to the presence of the TiO2 component. All TiO2 and T/uCN samples presented a small absorption in the range 500–800 nm related to a small amount of F ion doping. According to the Kubelk–Munk function, the band gaps of TiO2, uCN and T1 /Ucn1 samples were calculated at about 3.02 eV, 2.62 eV and 2.65 eV, respectively (Figure 4b). According to Mott–Schottky analyses (Figure 4c,d and Figure S3), the flat band poten- tials of TiO2 and uCN were −0.36 V and −0.86 V vs. the normal hydrogen electrode (NHE). The valence band (VB) XPS spectra of TiO2 and uCN showed the valence band maximum (VBM) to be located at 2.89 eV and 2.46 eV from the Fermi level, respectively. Since the flat band potentials are approximately equal to the Fermi level [41,42], the VBM was located at 2.53 eV and 1.60 eV with respect to the NHE for TiO2 and uCN, respectively. Then, taking into account the calculated band gaps (Eg = Evb − Ecb ) [43], the conduction band minimum (CBM) was located at 0.49 and −1.02 for TiO2 and uCN, respectively. Figure 4f displays and Ti 2p1/2 levels of Ti within a TiO2 environment. The high-resolution O 1s XPS spectra of TiO2 and T/uCN were fitted with two peaks at 530.4 eV and 531.8 eV, which were as- sociated with oxygen within the TiO2 lattice and oxygen-containing surface adsorption groups such as surface hydroxyl, respectively (Figure 3f). Nanomaterials 2022, 12, 1557 The UV-vis spectra showed the UV absorption edge of TiO2 particles and 6uCN of 11 nanosheets at about 390 nm and 445 nm, respectively (Figure 4a). T/uCN composites showed a similar onset absorption edge as uCN but an increased absorption below 400 nmenergy-level the related to the presence diagram of the for calculated TiOTiO 2 component. All TiO2 and T/uCN samples pre- 2 and uCN samples. According to this scheme, when combining uCN with TiO2 , a type500–800 sented a small absorption in the range nm related II heterojunction to a small is formed, amount electron involving of F ion doping. According to the Kubelk–Munk function, the band gaps of TiO 2, uCN and transfer from the uCN to the TiO2 particles. Besides, it is predicted that within such het- T1/Ucn1 samples erostructure, were calculated photogenerated at about electrons 3.02 move eV, 2.62 toward theeV andphase TiO 2.65 eV, andrespectively (Fig- photogenerated 2 ure 4b). holes toward the uCN, respectively. Figure4.4. (a) Figure (a)UV-vis UV-visabsorption absorptionspectra. spectra.(b) (b)Kubelka–Munk-transformed Kubelka–Munk-transformedfunction functionofofTiO TiO2 ,2,uCN uCNand and TT11/uCN /uCN11.. (c,d) (c,d)Mott–Schottky Mott–Schottky plots plots of uCN (c) and TiO Valence band TiO22 (d). (e) Valence band XPS XPS spectrum spectrumof ofTiO TiO22 anduCN. and uCN.(f) (f)Diagram Diagramof ofthe theband bandstructure structureof ofTiO anduCN. TiO2 2and uCN. To analyze the According photocatalytic analyses to Mott–Schottky activity towards (Figureshydrogen 4c,d and S3),generation, all thepotentials the flat band samples were of TiOloaded 2 and uCNwithwere 1 wt% platinum −0.36 as cocatalyst. V and −0.86 Figurehydrogen V vs. the normal 5 displays the photocatalytic electrode (NHE). The hydrogen valence band generation (VB) XPS from bCN,ofuCN, spectra TiO2TiO and2 and uCNTiO 2 /uCN showed thecomposites valence band for 4maximum h under simulated (VBM) to be solar light at located and using 2.89 methanol eV and 2.46 eVasfrom a sacrificial the Fermi agent. level,Figures S8 andSince respectively. S9 andthe Table S2 show the chromatogram plots and the linear fitting of the flat band potentials are approximately equal to the Fermi level [41,42], the VBM was lo- standard hydrogen curve cated for gas chromatography, at 2.53 eV and 1.60 eV with whichrespect show our measurement to the NHE for TiO error 2 andis less uCN, than 0.2%. respectively. For TiO , a high hydrogen evolution rate (HER) up to 1449 µmol · g −1 ·h−1 was obtained. Then, taking 2 into account the calculated band gaps (Eg = Evb − Ecb) [43], the conduction Additionally, band minimum a notable (CBM)HER waswas also obtained located at 0.49 and from uCNfor −1.02 (801 TiOµmol ·g−1 ·h−1 ), well above 2 and uCN, respectively. −1 ·h−1 ), which is consistent with the larger surface area provided that of bCN Figure (599 µmol 4f displays the·genergy-level diagram calculated for TiO2 and uCN samples. Ac- by the thin-layered structure of cording to this scheme, when combining uCN. All the TiO uCN 2 /uCN with composites TiO2, a type displayed a significantis II heterojunction HER formed, involving electron transfer from the uCN to the TiO2 particles. Besides,obtained improvement with respect to pure TiO 2 or uCN. The highest HERs were it is pre- with thethat dicted TiOwithin 2 /uCNsuchcomposites having a 1:1 heterostructure, weight ratio of photogenerated the two move electrons components, toward reaching the TiO2 aphase HER andof 3875 µmol·g−1 ·h− photogenerated 1 , which is 2.7 and 4.8 times higher than that of TiO and holes toward the uCN, respectively. 2 uCN,To respectively. The observed synergistic effect obtained when mixing analyze the photocatalytic activity towards hydrogen generation, all the samples both materials is related to the transfer and thus separation of photogenerated carriers at the 2D/2D were loaded with 1 wt% platinum as cocatalyst. Figure 5 displays the photocatalytic hy- heterojunctions, which prevents their recombination. Table S3 provides a comparison of drogen generation from bCN, uCN, TiO2 and TiO2/uCN composites for 4 h under simu- the activity obtained here with those of previous published works, demonstrating the lated solar light and using methanol as a sacrificial agent. Figures S8 and S9 and Table S2 outstanding activity provided by the 2D/2D TiO2 /uCN heterojunction. As a reference, we also measured the HER of TiO2 /bCN composites with the optimized weight ratio 1:1 (T1 /bCN1 ). As observed in Figure 5c and Figure S7, the HER of T1 /bCN1 also showed an obvious improvement with respect to that of pure TiO2 and bCN, but the highest HER values were well below those of 2D/2D T/uCN heterojunctions having extended surface and interface areas. Nanomaterials 2022, 12, 1557 7 of 12 Nanomaterials 2022, 12, 1557 show the chromatogram plots and the linear fitting of the standard hydrogen curve 7for of 11 gas chromatography, which show our measurement error is less than 0.2%. Figure 5. (a) Photocatalytic hydrogen generation on bCN, TiO2 and T/uCN samples during four hours under simulated solar light illumination. (b) Photocatalytic hydrogen peroxide generation rate Figure 5. (a) Photocatalytic hydrogen generation on bCN, TiO2 and T/uCN samples during four of bCN, TiO2 and T/uCN samples. (c) H2 production rate contrast between T1 /uCN1 and T1 /bCN1 . hours under simulated solar light illumination. (b) Photocatalytic hydrogen peroxide generation (d) Wavelength-dependent rate of bCN, TiO2 and T/uCN AQY of T1 /uCN samples. (c) 1H. 2 production rate contrast between T1/uCN1 and T1/bCN1. (d) Wavelength-dependent AQY of T1/uCN1. The apparent quantum yield (AQY) of the process was evaluated under 380 nm (4.51 mW·cm−2 ) and 420 nm (12.14 mW·cm−2 ) irradiation (Table S4, see details in the For TiO2, a high hydrogen evolution rate (HER) up to 1449 μmol·g−1·h−1 was ob- SI). For T1 /uCN1 , the AQY at 380 nm and 420 nm was estimated at 7.61%−1 and 2.64%, tained. Additionally, a notable HER was also obtained from uCN (801 μmol·g ·h−1), well respectively, which is consistent−1 with UV-vis spectroscopy results (Figure 5d). above that of bCN (599 μmol·g ·h−1), which is consistent with the larger surface area Figure 6a displays the positive photocurrents measured from uCN, TiO2 and TiO28/uCN Nanomaterials 2022, 12, 1557 provided by the thin-layered structure of uCN. All the TiO2/uCN composites displayedofa 12 samples under simulated solar irradiation. All the composite T/uCN, electrodes displayed significant HER improvement with respect to pure TiO2 or uCN. The highest HERs were significantly higher photocurrents than pure TiO2 and uCN, especially the T1 /uCN1 elec- obtained with the TiO2/uCN composites having a 1:1 weight ratio of the two components, trode that showed the highest −1 photocurrents, fourfold higher than those of uCN and TiO . −1, which is 2.7 and 4.8 times higher than that of TiO2 2 reaching of uCN a HER and of 3875 μmol·g TiO2.confirms This result ·h further confirms ancharge improvement of the chargewith separa- This result further an improvement of the separation/transport the and uCN, respectively. tion/transport with the The observed formation of synergistic the 2D/2D effect obtained when mixing both ma- heterojunction. formation of the 2D/2D heterojunction. terials is related to the transfer and thus separation of photogenerated carriers at the 2D/2D heterojunctions, which prevents their recombination. Table S3 provides a com- parison of the activity obtained here with those of previous published works, demon- strating the outstanding activity provided by the 2D/2D TiO2/uCN heterojunction. As a reference, we also measured the HER of TiO2/bCN composites with the opti- mized weight ratio 1:1 (T1/bCN1). As observed in Figures 5c and S7, the HER of T1/bCN1 also showed an obvious improvement with respect to that of pure TiO2 and bCN, but the highest HER values were well below those of 2D/2D T/uCN heterojunctions having ex- tended surface and interface areas. The apparent quantum yield (AQY) of the process was evaluated under 380 nm (4.51 mW·cm−2) and 420 nm (12.14 mW·cm−2) irradiation (Table S4, see details in the SI). For T1Figure /uCN1,6.the AQY at 380 nm and 420 nm of was estimated at 7.61% and 2.64%, respectively, Figure 6. (a) (a) Photocurrent Photocurrent response responsecurves curves ofbCN, bCN,TiOTiO2 2and andT/uCN T/uCN samples; samples; (b) (b) electrochemical electrochemical which is impedanceconsistent impedance spectroscopy with UV-vis spectroscopy (EIS) spectroscopy (EIS) Nyquist Nyquistplots plotsof results ofbCN,TiO (Figure bCN,TiO2 2and 5d). andTT1 1/uCN /uCN11 sample; (c) TRPL decay of of bCN,Figure bCN,TiO 6a T/uCN TiO22 and and displays T/uCN the positive photocurrents measured from uCN, TiO2 and samples. samples. TiO2/uCN samples under simulated solar irradiation. All the composite T/uCN, elec- trodes Electrochemical displayed significantly Electrochemical impedance impedance higher photocurrents spectroscopy spectroscopy (EIS)than (EIS) waspure was further further TiOemployed 2 and uCN, employed to especially to identify identify the the the T1/uCN charge charge 1 electrode transport transport that showed dynamics. dynamics. Figure Figure the 6bhighest 6b displaysphotocurrents, displays the the Nyquist Nyquist plot fourfold plot of thehigher of the impedance impedancethan those spectra spectra of of TiO TiO22,, uCN uCN andand TT11/uCN /uCN . Consistent 1. 1Consistent with with previous previous results, results, thethe T1 /uCN T1/uCN 1 electrode 1 electrode pre- presented sented a much smaller arc radius than the other two samples, confirming a much lower a much smaller arc radius than the other two samples, confirming a much lower charge charge transfer transfer resistance resistancewithwiththe theformation formationof ofthe the2D/2D 2D/2D TiO TiO22/uCN heterojunction [44]. /uCN heterojunction [44]. A strong photoluminescence (PL) peak was obtained under 370 nm light excitation from the uCN sample at about 455 nm, which is ascribed to the radiative band-to-band recombination of photogenerated charge carriers. When incorporating increasing amounts of TiO2, the PL intensity of T/uCN was progressively quenched (Figure S10). Additional time-resolved PL (TRPL) spectra under 365 nm light excitation (Figure 6c) Nanomaterials 2022, 12, 1557 8 of 11 A strong photoluminescence (PL) peak was obtained under 370 nm light excitation from the uCN sample at about 455 nm, which is ascribed to the radiative band-to-band recombination of photogenerated charge carriers. When incorporating increasing amounts of TiO2 , the PL intensity of T/uCN was progressively quenched (Figure S10). Additional time-resolved PL (TRPL) spectra under 365 nm light excitation (Figure 6c) allowed calculat- ing significantly longer PL lifetimes (4.72 ns) for T1 /uCN1 samples than for TiO2 (3.15 ns) and uCN (3.51 ns), which points at an effective separation of photogenerated charge carriers within the TiO2 /uCN heterostructures [45]. Based on the above results, the photocatalytic mechanism displayed in Figure 7 is proposed for hydrogen generation in T/uCN heterojunction photocatalysts. While both TiO2 and uCN can generate electrons and holes under simulated solar light irradiation, the photogenerated electron–hole pairs in pure TiO2 and uCN rapidly recombine, resulting in moderate HERs. Through the formation of a 2D/2D T/uCN heterostructure, the photogen- erated electrons remain or are transferred to the TiO2 CB because the TiO2 CBM is located 0.53 eV below that of CN. Similarly, photogenerated holes remain or are driven to the uCN VB, which is located 0.93 eV above that of TiO2 . Electrons at the TiO2 CB migrate to the platinum, which has a larger work function, thus a lower Fermi level, from where they are transferred to adsorbed H+ to produce H2 . On the other hand, holes react with sacrificial methanol at the CN surface. Consequently, the photocatalytic hydrogen evolution process using sacrificial methanol can be described as follows: hv uCN/TiO2 → uCN/TiO2 e− + h+  (1) uCN/TiO2 e− + h+ → uCN h+ + TiO2 e−    (2) TiO2 e− + Pt → TiO2 + Pt e−   (3) 2H+ + e− → H2 (4) Nanomaterials 2022, 12, 1557 9 of 12 2h+ + CH3 OH → CH2 O + 2H+ (5) Figure 7. Schematic diagram of photocatalytic hydrogen production production over over T/uCN T/uCN photocatalyst. photocatalyst. Finally, Finally, the the stability stability of ofthe theTT11/uCN photocatalyst in /uCN11 photocatalyst in hydrogen hydrogen evolution evolution conditions conditions under under simulated solar light irradiation was measured through five four-hour cycles. simulated solar light irradiation was measured through five four-hour cycles. As As shown in Figure S11a, after this 20 h of reaction, the photocatalytic performance shown in Figure S11a, after this 20 h of reaction, the photocatalytic performance was hardly was reduced, provingproving hardly reduced, the excellent stabilitystability the excellent and reusability of the Tof and reusability 1 /uCN 1 1photocatalyst. the T /uCN1 photo- Additionally, catalyst. Additionally, as displayed in Figure S11b,c, SEM and XRD analysis of after as displayed in Figure S11b,c, SEM and XRD analysis of the catalyst 20 h the cata- lyst after 20 h photocatalytic hydrogen generation reaction demonstrated the morphol- ogy and crystallographic structure of the material to be stable under photocatalytic reac- tion conditions. Nanomaterials 2022, 12, 1557 9 of 11 photocatalytic hydrogen generation reaction demonstrated the morphology and crystallo- graphic structure of the material to be stable under photocatalytic reaction conditions. 4. Conclusions In summary, we detailed the synthesis of 2D/2D T/uCN heterojunctions from ultra- thin g-C3 N4 (uCN) and colloidal TiO2 nanosheets through an electrostatic self-assembly approach. The highest hydrogen generation rate was achieved from T/uCN compos- ites with a 1:1 mass ratio of the two components. The photocatalytic performance for H2 production was increased in the following order: bCN < uCN < TiO2 < T1 /uCN2 < T2 /uCN1 < T1 /uCN1 . The enhanced performance was attributed to the unique 2D/2D type II heterojunction architecture that simultaneously maximized the surface area to interact with the media and the interface between the two materials. The face-to-face interfacial contact between ultrathin layers of g-C3 N4 and the faceted TiO2 provided fast separation of photogenerated charges inside the composites, reducing recombination and thus increasing the apparent quantum yield. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12091557/s1, Figure S1: SEM image of (a) bulk g-C3N4 and (b) ultrathin g-C3N4, (c) N2 adsorption-desorption isotherms of bCN and uCN; Figure S2: FTIR spectra of OAC, OLMA and TiO2 before and after ligands remove; Figure S3: Zeta potential distribution spectrum of TiO2 after ligands removal (a) and uCN (b); Figure S4: SEM image and EDS compositional maps of a T1 /uCN1 composite; Figure S5: SEM image of T1 /uCN2 and corresponding EDS spectrum; Figure S6: SEM image of T1 /uCN2 and corresponding EDS spectrum; Figure S7: SEM image of T1 /uCN2 and corresponding EDS spectrum; Figure S8: Chromatogram plots for 0.5 mL of standard hydrogen injected every half hour; Table S1: Gas Chromatography Peak Processing Data based on figure S8; Figure S9: Standard hydrogen curve for gas chromatography; Table S2: Exponential decay-fitted parameters of fluorescence lifetime of uCN, TiO2 and T1 /uCN1 ; Figure S10: Photocatalytic hydrogen generation amount on bCN, TiO2 and T1 /bCN1 during 4 h under simulated solar light irradiation; Table S3: Photocatalytic hydrogen production about TiO2 /g-C3 N4 based catalysts; Table S4: The AQE values with different incident light wavelengths for T1 /uCN1 ; Figure S11: (a) Stability cycles of the T1 /uCN1 for H2 evolution under simulated solar light irradiation; (b) TEM image of T1 /uCN1 after 20 h photocatalytic H2 evolution reaction and (c) XRD pattern of T1 /uCN1 before and after 20 h photocatalytic H2 O2 evolution reaction. Author Contributions: In this work, R.D. designed the experiment and the nanocomposites, prepared all the materials and conducted XRD, SEM, ZEM-EDS, TEM characterization, photoelectrochemical measurements and wrote the first draft of the manuscript. B.L. conducted the photocatalytic hydrogen evolution test and TRPL test. K.X., C.Z. and X.W. significantly contributed to the result discussion. X.H. and J.A. participated in high-resolution TEM characterization. A.C. conceived and guided the project and supervised the work. The manuscript was corrected and improved by all authors. All authors have read and agreed to the published version of the manuscript. Funding: R.D.: K.X., X.H., X.W. and C.Z. thank the China Scholarship Council for the scholarship support. IREC and ICN2 acknowledge funding from Generalitat de Catalunya, projects 2017 SGR 1246 and 2017 SGR 327, respectively. The authors thank the support from the project NANOGEN (PID2020- 116093RB-C43), funded by MCIN/AEI/10.13039/501100011033/ and the project COMBENERGY (PID2019-105490RB-C32) from the Spanish Ministerio de Ciencia e Innovación. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and is funded by the CERCAProgramme / Generalitat de Catalunya. Baoying Li greatly appreciates the financial support from the National Natural Science Foundation of China (Nos. 22171154 & 21801144), the Youth Innovative Talents Recruitment and the Cultivation Program of Shandong Higher Education. This study was supported by MCIN with funding from the European Union NextGenerationEU (PRTR- C17.I1), Generalitat de Catalunya and by “ERDF A way of making Europe” by the “European Union”. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. 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