In Situ Construction of Ag/TiO2/g-C3N4 Heterojunction Nanocomposite Based on Hierarchical Co-Assembly with Sustainable Hydrogen Evolution

The construction of heterojunctions provides a promising strategy to improve photocatalytic hydrogen evolution. However, how to fabricate a nanoscale TiO2/g-C3N4 heterostructure and hinder the aggregation of bulk g-C3N4 using simple methods remains a challenge. In this work, we use a simple in situ construction method to design a heterojunction model based on molecular self-assembly, which uses a small molecule matrix for self-integration, including coordination donors (AgNO3), inorganic titanium source (Ti(SO4)2) and g-C3N4 precursor (melamine). The self-assembled porous g-C3N4 nanotube can hamper carrier aggregation and it provides numerous catalytic active sites, mainly via the coordination of Ag+ ions. Meanwhile, the TiO2 NPs are easily mineralized on the nanotube template in dispersive distribution to form a heterostructure via an N–Ti bond of protonation, which contributes to shortening the interfacial carrier transport, resulting in enhanced electron-hole pairs separation. Originating from all of the above synergistic effects, the obtained Ag/TiO2/g-C3N4 heterogenous photocatalysts exhibit an enhanced H2 evolution rate with excellent sustainability 20.6-fold-over pure g-C3N4. Our report provides a feasible and simple strategy to fabricate a nanoscale heterojunction incorporating g-C3N4, and has great potential in environmental protection and water splitting.


Introduction
With the development of semiconductor photocatalysis [1][2][3][4], semiconductor photocatalysts ushered in a glorious era of the photocatalytic water decomposition of H 2 [5][6][7][8]. Graphitic carbon nitride is currently a prominent semiconductor material that has garnered tremendous attention in academia due to its low cost, environmental friendliness, suitable band structure and impressive physical and chemical stability [9][10][11]. However, the major bottlenecks, such as small specific surface areas and, especially, grievous charge recombination, will generate inferior photocatalytic activity.
The surface plasmon resonance effect (SPR) is expected to enhance the photocatalytic energy conversion efficiency in the construction of composite photocatalytic materials due to its unique light-substance interaction characteristics, which has attracted much attention in recent years. Metal

Monitoring of Photocatalytic Hydrogen Evolution Activity
The photocatalytic water-splitting experiments were performed in a 10 mL quartz test tube equipped with a rubber stopper. A 350 W xenon lamp with a 400 nm optical filter was regarded as a light source at a distance of 10 cm away from the photocatalytic reactor. In a typical photocatalytic experiment, 10 mg catalyst was dispersed in 5 mL aqueous solution containing 0.5 mL triethanolamine as a sacrificial agent. Additionally, 0.5 mL potassium tetrachloroplatinate (II) (K 2 PtCl 4 ) was added into the hybrid solution with 1 wt % of Pt cocatalyst to photocatalyst. To provide an anaerobic environment, the reaction system was adequately removed from the dissolved oxygen by discharging Ar for 30 min and then sealing with paraffin. In order to keep the sample in suspension status, the photocatalyst was stirred continuously by a magnetic stirrer during the whole experiment. The reaction was monitored for 4 h, and 0.2ml of gas was sampled at intervals of an hour through the septum to analyze by gas chromatography (TRACE 1300, Thermo Fisher Scientific, Waltham, MA, USA, TCD, 5Å molecular sieve column).

Characterization
The morphology of the Ag/TiO 2 /g-C 3 N 4 composite sample was observed by a transmission electron microscope (TEM, HT7700, High-Technologies Corp., Tokyo, Japan) and a high-resolution transmission electron microscope (HRTEM, JEM-2100F, JEOL, Tokyo, Japan). X-ray diffraction (XRD) was obtained on an X-ray diffractometer equipped with a CuKα X-ray radiation source and a Bragg diffraction apparatus (SMART LAB, Rigaku, Tokyo, Japan). The thermogravimetric (TG) method was carried out under an Ar atmosphere using a NETZSCH STA 409 PC Luxxsi simultaneous thermal analyzer (Netzsch Instruments Manufacturing Co., Ltd., Ahlden, Germany). The FTIR spectrum of the photocatalyst was recorded by Fourier infrared spectroscopy (Thermo Nicolet Corporation, Hemel Hempstead, UK), including KBr flakes. UV-visible diffuse reflectance spectra were examined on a UV-visible spectrophotometer (Hitachi U3010, Tokyo, Japan) using barium sulfate (BaSO 4 ) as the substrate. The Brunauer-Emmett-Teller (BET) method was carried out to measure the specific surface areas and pore size distribution on an ASAP 2460 system. X-ray photoelectron spectroscopy (XPS) analyses were achieved via the Thermo Scientific ESCALab 250Xi (Thermo Fisher Scientific, Waltham, MA, USA) with 200 W monochromatic Al Kαradiation. The Hitachi F-4500 fluorescence spectrometer (Hitachi Ltd., Tokyo, Japan) was carried out by monitoring the steady-state fluorescence spectra of the nanocomposites. The time-resolved fluorescence spectrum was acquired on an Edinburgh Instrument F900, for which the above excitation wavelengths are 360 nm.

Results and Discussions
Herein, as shown in Figure 1, we elaborately designed the Ag/TiO 2 /g-C 3 N 4 heterostructure by a small-molecular co-assembly of AgNO 3 , Ti(SO 4 ) 2 , melamine in hot solution (70 • C) and further calcining. Heterostructures can form spontaneously in the designed soup. Specifically, AgNO 3 can direct the π-stacked structures of melamine into orientationally-organized, one-dimensional nanotubes by a multi-scale synergy of interactions, including the coordination effect and electrostatically-enhanced hydrogen bonds. Meanwhile, TiO 2 nanoparticles can be easily incubated and self-mineralized on the nanotubes due to the high protonation reactivity of H + from Ti(SO 4 ) 2 with the amine of melamine molecule.
The features of as-synthesized samples were researched by TEM. The assembly Ag/MA exhibits nanotube structures with a smooth surface, upon which the mean diameter can reach several hundred nanometers, as shown in Figure 2a. However, only a few nanometers of Ag nanoparticles were found in Ag/MA, as seen in Figure 2i, which may be associated with a limited amount of silver. For all the Ag/TiO 2 /MA nanocomposites, there are numerous TiO 2 NPs uniformly assembled on the Ag/MA nanotubes as seen in Figure 2b-d. Due to the strong reactivity of H + generated by the hydrolysis of Ti(SO 4 ) 2 under hydrothermal conditions, it can easily be protonated with a large number of basic groups Nanomaterials 2020, 10, 1 4 of 13 in melamine to form TiO 2 NPs and in situ anchor simultaneously on the Ag/MA NTS. Interestingly, the mineralized TiO 2 NPs gradually become larger by increasing the content of Ti(SO 4 ) 2 as showed in Figure 2j-f. assembled on the Ag/MA nanotubes as seen in Figure 2b-d. Due to the strong reactivity of H + generated by the hydrolysis of Ti(SO4)2 under hydrothermal conditions, it can easily be protonated with a large number of basic groups in melamine to form TiO2 NPs and in situ anchor simultaneously on the Ag/MA NTS. Interestingly, the mineralized TiO2 NPs gradually become larger by increasing the content of Ti(SO4)2 as showed in Figure 2j-f. This indicates that the growth of TiO2 NPs can be readily manipulated in the assembly process by simply adjusting the precursor mass. After the thermal treatment, all the Ag/TiO2/g-C3N4 present porous structures because of the release of NH3 molecules, as seen in Figure 2e-h.   generated by the hydrolysis of Ti(SO4)2 under hydrothermal conditions, it can easily be protonated with a large number of basic groups in melamine to form TiO2 NPs and in situ anchor simultaneously on the Ag/MA NTS. Interestingly, the mineralized TiO2 NPs gradually become larger by increasing the content of Ti(SO4)2 as showed in Figure 2j-f. This indicates that the growth of TiO2 NPs can be readily manipulated in the assembly process by simply adjusting the precursor mass. After the thermal treatment, all the Ag/TiO2/g-C3N4 present porous structures because of the release of NH3 molecules, as seen in Figure 2e-h.   Ag/TiO 2 /MA-2, (d) Ag/TiO2/MA-3, (e) Ag/g-C 3 N 4 , (f) Ag/TiO 2 /g-C 3 N 4 -1, (g) Ag/TiO 2 /g-C 3 N 4 -2, (h) Ag/TiO 2 /g-C 3 N 4 -3, (i) size histograms of Ag nanoparticles in Ag/g-C 3 N 4 , (j) TiO 2 nanoparticles in Ag/TiO 2 /g-C 3 N 4 -1, (k) TiO 2 nanoparticles in Ag/TiO 2 /g-C 3 N 4 -2, (l) TiO 2 nanoparticles in Ag/TiO 2 /g-C 3 N 4 -3.
This indicates that the growth of TiO 2 NPs can be readily manipulated in the assembly process by simply adjusting the precursor mass. After the thermal treatment, all the Ag/TiO 2 /g-C 3 N 4 present porous structures because of the release of NH3 molecules, as seen in Figure 2e-h.
For the representative morphology of Ag/TiO 2 /g-C 3 N 4 -2 as shown in the Figure 3A TEM image, AgNPs and TiO 2 NPs of varying sizes are scattered across the g-C 3 N 4 NTS. The HRTEM image Nanomaterials 2020, 10, 1 5 of 13 ( Figure 3B) shows that the different stacking distances are 0.22, 0.35 and 0.335 nm, corresponding to the (111) lattice spacing of Ag, anatase TiO 2 (101) crystal plane and (002) lattice plane of g-C 3 N 4 , respectively, which verify the successful co-assembly of Ag, TiO 2 and g-C 3 N 4 [32,33]. Additionally, the element mapping images ( Figure 3B) of the C, Ti, Ag and O elements in the Ag/TiO 2 /g-C 3 N 4 -2 make it obvious that the Ag element is distributed on the full of the g-C 3 N 4 NTs surface, but the Ti element is not. Therefore, it further indicates that there is a certain accumulation between Ag and TiO 2 NPs, which may be beneficial for efficient plasmonic resonant energy transfer between g-C 3 N 4 NTs and TiO 2 NPs by the Ag bridge to accelerate charge separation in photocatalytic reactions. Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 13 Ag/TiO2/g-C3N4-1, (k) TiO2 nanoparticles in Ag/TiO2/g-C3N4-2, (l) TiO2 nanoparticles in Ag/TiO2/g-C3N4-3.
For the representative morphology of Ag/TiO2/g-C3N4-2 as shown in the Figure 3a TEM image, AgNPs and TiO2 NPs of varying sizes are scattered across the g-C3N4 NTS. The HRTEM image ( Figure 3b) shows that the different stacking distances are 0.22, 0.35 and 0.335 nm, corresponding to the (111) lattice spacing of Ag, anatase TiO2 (101) crystal plane and (002) lattice plane of g-C3N4, respectively, which verify the successful co-assembly of Ag, TiO2 and g-C3N4 [32,33]. Additionally, the element mapping images (Figure 3b) of the C, Ti, Ag and O elements in the Ag/TiO2/g-C3N4-2 make it obvious that the Ag element is distributed on the full of the g-C3N4 NTs surface, but the Ti element is not. Therefore, it further indicates that there is a certain accumulation between Ag and TiO2 NPs, which may be beneficial for efficient plasmonic resonant energy transfer between g-C3N4 NTs and TiO2 NPs by the Ag bridge to accelerate charge separation in photocatalytic reactions. Furthermore, TGA was carried out to confirm the mineralization amount of TiO2 for the Ag/TiO2/g-C3N4 HNCs from Figure 4. All the samples of Ag/TiO2/g-C3N4 HNCs have significant weight loss at 480-700 °C, which may be correlated with the decomposition of the g-C3N4 part. So, in other words, the remaining TiO2 contents for Ag/TiO2/g-C3N4-x (x = 1, 2, 3) are about 7.47%, 28.62% and 57.66% by subtracting the residue contents of Ag, respectively. Amazingly, the incubated content of TiO2 is linearly relying on the mass of Ti(SO4)2 by feeding in the co-assembly process, as shown in Figure 4b. All the above results adequately prove that it is easy to achieve the controllable surface morphology and depositing amount of TiO2 NPs mineralized on g-C3N4 in the whole assembly evolution. Furthermore, TGA was carried out to confirm the mineralization amount of TiO 2 for the Ag/TiO 2 /g-C 3 N 4 HNCs from Figure 4. All the samples of Ag/TiO 2 /g-C 3 N 4 HNCs have significant weight loss at 480-700 • C, which may be correlated with the decomposition of the g-C 3 N 4 part. So, in other words, the remaining TiO 2 contents for Ag/TiO 2 /g-C 3 N 4 -x (x = 1, 2, 3) are about 7.47%, 28.62% and 57.66% by subtracting the residue contents of Ag, respectively. Amazingly, the incubated content of TiO 2 is linearly relying on the mass of Ti(SO 4 ) 2 by feeding in the co-assembly process, as shown in Figure 4b. All the above results adequately prove that it is easy to achieve the controllable surface morphology and depositing amount of TiO 2 NPs mineralized on g-C 3 N 4 in the whole assembly evolution. Ag/TiO2/g-C3N4-1, (k) TiO2 nanoparticles in Ag/TiO2/g-C3N4-2, (l) TiO2 nanoparticles in Ag/TiO2/g-C3N4-3.
For the representative morphology of Ag/TiO2/g-C3N4-2 as shown in the Figure 3a TEM image, AgNPs and TiO2 NPs of varying sizes are scattered across the g-C3N4 NTS. The HRTEM image (Figure 3b) shows that the different stacking distances are 0.22, 0.35 and 0.335 nm, corresponding to the (111) lattice spacing of Ag, anatase TiO2 (101) crystal plane and (002) lattice plane of g-C3N4, respectively, which verify the successful co-assembly of Ag, TiO2 and g-C3N4 [32,33]. Additionally, the element mapping images (Figure 3b) of the C, Ti, Ag and O elements in the Ag/TiO2/g-C3N4-2 make it obvious that the Ag element is distributed on the full of the g-C3N4 NTs surface, but the Ti element is not. Therefore, it further indicates that there is a certain accumulation between Ag and TiO2 NPs, which may be beneficial for efficient plasmonic resonant energy transfer between g-C3N4 NTs and TiO2 NPs by the Ag bridge to accelerate charge separation in photocatalytic reactions. Furthermore, TGA was carried out to confirm the mineralization amount of TiO2 for the Ag/TiO2/g-C3N4 HNCs from Figure 4. All the samples of Ag/TiO2/g-C3N4 HNCs have significant weight loss at 480-700 °C, which may be correlated with the decomposition of the g-C3N4 part. So, in other words, the remaining TiO2 contents for Ag/TiO2/g-C3N4-x (x = 1, 2, 3) are about 7.47%, 28.62% and 57.66% by subtracting the residue contents of Ag, respectively. Amazingly, the incubated content of TiO2 is linearly relying on the mass of Ti(SO4)2 by feeding in the co-assembly process, as shown in Figure 4b. All the above results adequately prove that it is easy to achieve the controllable surface morphology and depositing amount of TiO2 NPs mineralized on g-C3N4 in the whole assembly evolution.  on the N 1s spectrum belong to representative peaks of g-C 3 N 4 , mainly containing sp2-coordinated N from the C=N-C group, sp2 hybridized N in the N-(C) 3 groups and terminal imino group (C-NH-) in heptazine rings, respectively. Another signal peak located at 396.5 eV, corresponding to a Ti-N bond, suggests that the mineralized TiO 2 might possess interactions with g-C 3 N 4 . Two peaks at 456.9 and 459.3 eV on the Ti 2p spectrum signify the existence of the N-Ti bond [21,22], also including the Ti 2p 3/2 peak at 458.2 eV with the Ti 2p 1/2 peak at 464 eV of two TiO 2 characteristic bands, respectively. In the O 1s spectrum, the peaks at 529.8 and 530.4 eV originate from the Ti-O bond and O-H bond, respectively. The above analysis suggests that the TiO 2 NPs are firmly chemically bound to the surface of g-C 3 N 4 NTs. For the Ag 3d spectrum, the two characteristic peaks of Ag 0 at the Ag 3d 5/2 and Ag 3d 3/2 states can be observed from 367.9 eV and 373.6 eV, respectively. As shown in Table 1, the Ti components obviously increased from 1.25% to 9.73% calculated from the cumulative area of XPS peaks for Ag/TiO 2 /g-C 3 N 4 -1 to Ag/TiO 2 /g-C 3 N 4 -3, while Ag and N components slightly decreased oppositely. In addition, the XRD patterns of TiO 2 NPs, pure g-C 3 N 4 , Ag/g-C 3 N 4 NTs, Ag/TiO 2 /g-C 3 N 4 -X HNCs in  [36]. Remarkably, the peaks of TiO 2 become stronger, while the peak intensity of g-C 3 N 4 gradually becomes weaker with increasing concentrations of the precursor Ti(SO 4 ) 2 for Ag/TiO 2 /g-C 3 N 4 heterojunction nanocomposites, suggesting that TiO 2 NPs can be continually assembled onto the Ag/g-C 3 N 4 NTs [37,38]. . Thermogravimetric analysis (TGA) curves (a) of the as-constructed Ag/g-C3N4, (green); Ag/TiO2/g-C3N4-1, (red); Ag/TiO2/g-C3N4-2, (blue); Ag/TiO2/g-C3N4-3, (pink); (b) relationship of loading content of TiO2 with Titanic sulfate mass. X-ray photoelectron spectroscopy (XPS) generally reveals the chemical interfacial bonding state of various composites. The results of Ag/TiO2/g-C3N4-2 HNCs for XPS analysis are displayed in Figure 5a-e. The C 1s spectrum exhibits three different peaks located at 282.8, 284.8 and 288.3 eV are ascribed to the calibration of XPS measurement, sp2 hybridized C atoms in the lattice (C-N bond) with the coordinate N-C=N of the s-triazine rings, respectively. The peaks situated at 397.3, 398.5, 399.4 and 403.4 eV on the N 1s spectrum belong to representative peaks of g-C3N4, mainly containing sp2-coordinated N from the C=N-C group, sp2 hybridized N in the N-(C)3 groups and terminal imino group (C-NH-) in heptazine rings, respectively. Another signal peak located at 396.5 eV, corresponding to a Ti-N bond, suggests that the mineralized TiO2 might possess interactions with g-C3N4. Two peaks at 456.9 and 459.3 eV on the Ti 2p spectrum signify the existence of the N-Ti bond [21,22], also including the Ti 2p3/2 peak at 458.2 eV with the Ti 2p1/2 peak at 464 eV of two TiO2 characteristic bands, respectively. In the O 1s spectrum, the peaks at 529.8 and 530.4 eV originate from the Ti-O bond and O-H bond, respectively. The above analysis suggests that the TiO2 NPs are firmly chemically bound to the surface of g-C3N4 NTs. For the Ag 3d spectrum, the two characteristic peaks of Ag 0 at the Ag 3d5/2 and Ag 3d3/2 states can be observed from 367.9 eV and 373.6 eV, respectively. As shown in Table 1, the Ti components obviously increased from 1.25% to 9.73% calculated from the cumulative area of XPS peaks for Ag/TiO2/g-C3N4-1 to Ag/TiO2/g-C3N4-3, while Ag and N components slightly decreased oppositely. In addition, the XRD patterns of TiO2 NPs, pure g-C3N4, Ag/g-C3N4 NTs, Ag/TiO2/g-C3N4-X HNCs in Figure 5f [36]. Remarkably, the peaks of TiO2 become stronger, while the peak intensity of g-C3N4 gradually becomes weaker with increasing concentrations of the precursor Ti(SO4)2 for Ag/TiO2/g-C3N4 heterojunction nanocomposites, suggesting that TiO2 NPs can be continually assembled onto the Ag/g-C3N4 NTs [37,38]. (f) X-ray diffraction (XRD) patterns of the g-C 3 N 4 , Ag/g-C 3 N 4 , Ag/TiO 2 /g-C 3 N 4 -1, Ag/TiO 2 /g-C 3 N 4 -2, Ag/TiO 2 /g-C 3 N 4 -3 samples.  Figure S1 describes the FTIR spectra of all samples. The wide absorption peak around 500 cm −1 can be traced to the Ti-O-Ti bond vibration mode of TiO 2 and the peak intensity enhances with an increasing concentration of Ti(SO 4 ) 2 , which accords well with the result of XRD [39]. Several strong peaks ranging from 1200 cm −1 to 1600 cm −1 belong to the characteristic stretching vibration pattern in the hybrid aromatic CN. The characteristic band centered around 800 cm −1 belongs to the breathing modes for the triazine ring [40]. Moreover, the broad peak in the range of 3000 cm −1 to 3500 cm −1 pertains to the stretching vibration for the NH or NH 2 groups. Nitrogen adsorption-desorption isotherms were usually used to estimate the specific surface area of the samples [39,40]. As shown in Figure 6, bulk g-C 3 N 4 possess a relatively small specific surface of 15.344 m 2 g −1 , while the specific surface areas gradually expand to 18, 23, 34 and 71 m 2 g −1 for Ag/g-C 3 N 4 NTs, Ag/TiO 2 /g-C 3 N 4 -x HNCs (x = 1, 2, 3), respectively [41][42][43][44]. All of the Ag/TiO 2 /g-C 3 N 4 -x HNCs display the existence of mesopores around 12.5 nm, and the gradual narrowing of the peak may be related to the growth of TiO 2 NPs based on the BJH pore-size distribution curves in Figure S2, which are coincident with the TEM results. These results suggest that the construction of hierarchical self-assembly is beneficial in inhibiting the accumulation of g-C 3 N 4 , and thus enlarging their specific surface area at different scales. Additionally, the porous tubular structures were conducive to the fast transfer of carriers to the surface, and further increase the photocatalytic performance. Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 13 Figure 5. X-ray photoelectron spectroscopy (XPS) spectra of the fabricated Ag/TiO2/g-C3N4-2 (a-e) showing elements C/N/Ti/O/Ag in the structure; (f) X-ray diffraction (XRD) patterns of the g-C3N4, Ag/g-C3N4, Ag/TiO2/g-C3N4-1, Ag/TiO2/g-C3N4-2, Ag/TiO2/g-C3N4-3 samples.  Figure S1 describes the FTIR spectra of all samples. The wide absorption peak around 500 cm −1 can be traced to the Ti-O-Ti bond vibration mode of TiO2 and the peak intensity enhances with an increasing concentration of Ti(SO4)2, which accords well with the result of XRD [39]. Several strong peaks ranging from 1200 cm −1 to 1600 cm −1 belong to the characteristic stretching vibration pattern in the hybrid aromatic CN. The characteristic band centered around 800 cm −1 belongs to the breathing modes for the triazine ring [40]. Moreover, the broad peak in the range of 3000 cm −1 to 3500 cm −1 pertains to the stretching vibration for the NH or NH2 groups. Nitrogen adsorptiondesorption isotherms were usually used to estimate the specific surface area of the samples [39,40]. As shown in Figure 6, bulk g-C3N4 possess a relatively small specific surface of 15.344 m 2 g −1 , while the specific surface areas gradually expand to 18, 23, 34 and 71 m 2 g −1 for Ag/g-C3N4 NTs, Ag/TiO2/g-C3N4-x HNCs (x = 1, 2, 3), respectively [41][42][43][44]. All of the Ag/TiO2/g-C3N4-x HNCs display the existence of mesopores around 12.5 nm, and the gradual narrowing of the peak may be related to the growth of TiO2 NPs based on the BJH pore-size distribution curves in Figure S2, which are coincident with the TEM results. These results suggest that the construction of hierarchical self-assembly is beneficial in inhibiting the accumulation of g-C3N4, and thus enlarging their specific surface area at different scales. Additionally, the porous tubular structures were conducive to the fast transfer of carriers to the surface, and further increase the photocatalytic performance. Figure 6. Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption isotherms of pure g-C3N4, Ag/g-C3N4, Ag/TiO2/g-C3N4-1, Ag/TiO2/g-C3N4-2 and Ag/TiO2/g-C3N4-3.
Generally, the UV-vis diffuse reflectance spectra, transient steady-state spectrometer, enabled us to evaluate the photophysical behaviors of carriers in the photoconductors. The light absorption ability was distinctly revealed by UV-vis absorption spectra. In the visible light region, all the photocatalysts presented the broadened absorption bands and possessed steep curves in the 375-450 nm spectral range (except TiO2 NPs), as shown from Figure 7a, which was related to the band gap transition of semiconductors. Due to the optical interaction by combining g-C3N4 with TiO2, the visible light response of the heterogeneous photocatalyst diminishes with the increase of Figure 6. Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption isotherms of pure g-C 3 N 4 , Ag/g-C 3 N 4 , Ag/TiO 2 /g-C 3 N 4 -1, Ag/TiO 2 /g-C 3 N 4 -2 and Ag/TiO 2 /g-C 3 N 4 -3.
Generally, the UV-vis diffuse reflectance spectra, transient steady-state spectrometer, enabled us to evaluate the photophysical behaviors of carriers in the photoconductors. The light absorption ability was distinctly revealed by UV-vis absorption spectra. In the visible light region, all the photocatalysts presented the broadened absorption bands and possessed steep curves in the 375-450 nm spectral range (except TiO 2 NPs), as shown from Figure 7a, which was related to the band gap transition of semiconductors. Due to the optical interaction by combining g-C 3 N 4 with TiO 2 , the visible light response of the heterogeneous photocatalyst diminishes with the increase of mineralized TiO 2 NPs. The peak red shift observed from Figure 7a for composite materials resulted from the presence of narrow band gap C 3 N 4 and probably from the plasmonic properties of silver nanoparticles. In addition, Nanomaterials 2020, 10, 1 8 of 13 high light absorption capacity does not always mean excellent photocatalytic activity, but also depends on enough low photoinduced electron-hole recombination rates.
is seen that Ag/TiO2/g-C3N4-2 HNCs decayed much faster than pure g-C3N4 and Ag/g-C3N4 NTs, which may result in interfacial electron transfer after the construction of a heterojunction. Significantly, the average lifetime of Ag/TiO2/g-C3N4-2 HNCs was about 1.096 ns, which was shorter than the bulk g-C3N4 (6.15 ns) with Ag/g-C3N4 NTs (2.409 ns), confirming the relatively better separation of photogenerated carriers. These results further demonstrated the high separation efficiency of interfacial carriers for Ag/TiO2/g-C3N4-2 HNCs, which may promote enhanced hydrogen production activity associated with the formation of good heterostructures [45][46][47].  It is known that the peak intensity in steady-state fluorescence spectra was closely related to the detached capability of the photo-generated carrier. According to the steady PL spectra in Figure 7c, all of the Ag/TiO 2 /g-C 3 N 4 -x HNCs present much lower emission peak intensity compared with bulk g-C 3 N 4 , and almost no PL emission was monitored over Ag/TiO 2 /g-C 3 N 4 -3, which is probably because TiO 2 NPs dominated in the composition. The damped intensity of Ag/TiO 2 /g-C 3 N 4 -x HNCs indicates their better photo-generated electron-hole separation, and thus their recombination was greatly hindered, which could be benefited from the design of heterostructure with Ag SPR effects. Transient fluorescence spectrum of the photocatalysts was measured to ulteriorly inspect the charge carrier's separation and transfer efficiency from Figure 7d. The emission decay curves of all samples presented exponential attenuation, and could be fitted by the double-exponential formula [30,43]. It is seen that Ag/TiO 2 /g-C 3 N 4 -2 HNCs decayed much faster than pure g-C 3 N 4 and Ag/g-C 3 N 4 NTs, which may result in interfacial electron transfer after the construction of a heterojunction. Significantly, the average lifetime of Ag/TiO 2 /g-C 3 N 4 -2 HNCs was about 1.096 ns, which was shorter than the bulk g-C 3 N 4 (6.15 ns) with Ag/g-C 3 N 4 NTs (2.409 ns), confirming the relatively better separation of photogenerated carriers. These results further demonstrated the high separation efficiency of interfacial carriers for Ag/TiO 2 /g-C 3 N 4 -2 HNCs, which may promote enhanced hydrogen production activity associated with the formation of good heterostructures [45][46][47].
The photocatalytic performance of the Ag/TiO 2 /g-C 3 N 4 -x HNCs for the H 2 production reaction was executed upon solar-simulated light ( Figure S3). As given in Figure 8, the bulk g-C 3 N 4 purely generated about 3.1 µmol of H 2 after 4 h of illumination. When Ag was introduced to assemble with MA, the calcined sample Ag/g-C 3 N 4 NTs produced about 24.3 µmol of H 2 and the value nearly 8-fold more than bulk g-C 3 N 4 , due to Ag SPR effects and the formed porous nanotube structures.
photocatalyst, the obtained generated H2 amount shows slight values in comparison with these cases with the photocatalyst, as shown in Figure 8. These consequences mean the significant improvement of photocatalytic activity in samples thanks to the nanoscale structure of g-C3N4 and the well-established heterojunction in Ag/TiO2/g-C3N4. Specifically, one-dimensional, tube-like g-C3N4 with the increased specific surface area can effectively avoid carrier aggregation, and provide many more catalytic active sites [48][49][50][51]. Moreover, the heterogeneous structure with nanometer size can reduce the charge diffusion length and effectively facilitate the separation process of charge carriers at the interface, which is also confirmed with the steady-state and transient fluorescence spectrum. In view of the above results, the presently prepared new heterojunction materials provide an exploration of new catalyst fields such as water oxidation and self-assembled composites [52][53][54][55][56][57][58][59][60][61][62][63].

Conclusions
In summary, we provide a simple and feasible strategy to construct a nanoscale heterojunction model with controllable morphology based on the co-assembly of coordination donor (AgNO3), inorganic titanium source (Ti(SO4)2) and g-C3N4 precursor (Melamine). The obtained Ag/TiO2/g-C3N4 heterojunction nanocomposite possessing self-organization of multiple kinds of When incubating AgNO 3 , Ti(SO 4 ) 2 with MA to form a composite by one-pot co-assembly and calcining their precursors, the obtained Ag/TiO 2 /g-C 3 N 4 -1 HNCs generated about 31.75 µmol of H 2 , which is 10.2 times more than pure g-C 3 N 4 , and slightly higher compared with the Ag/g-C 3 N 4 NTs, benefiting from the constructed Ag/TiO 2 /g-C 3 N 4 heterojunction. The Ag/TiO 2 /g-C 3 N 4 -2 HNCs presents a substantially enhanced H 2 evolution amount of 64 µmol, which leads to a 20.6-fold increase on pure g-C 3 N 4 and much superior to Ag/g-C 3 N 4 NTs. However, the inconspicuous hydrogen precipitation activity of Ag/TiO 2 /g-C 3 N 4 -3 HNCs may derive from the abundant defects which bring poor photo adsorption and further suppress the carrier transfer process by the introduction of high TiO 2 content. It should be noted that triethanolamine is a typical sacrificial agent and hole trapping agent often used for photocatalytic hydrogen production. In our present reference system of hydrogen evolution during sacrificial agent photolysis in the absence of a photocatalyst, the obtained generated H 2 amount shows slight values in comparison with these cases with the photocatalyst, as shown in Figure 8. These consequences mean the significant improvement of photocatalytic activity in samples thanks to the nanoscale structure of g-C 3 N 4 and the well-established heterojunction in Ag/TiO 2 /g-C 3 N 4 . Specifically, one-dimensional, tube-like g-C 3 N 4 with the increased specific surface area can effectively avoid carrier aggregation, and provide many more catalytic active sites [48][49][50][51]. Moreover, the heterogeneous structure with nanometer size can reduce the charge diffusion length and effectively facilitate the separation process of charge carriers at the interface, which is also confirmed with the steady-state and transient fluorescence spectrum. In view of the above results, the presently prepared new heterojunction materials provide an exploration of new catalyst fields such as water oxidation and self-assembled composites [52][53][54][55][56][57][58][59][60][61][62][63].

Conclusions
In summary, we provide a simple and feasible strategy to construct a nanoscale heterojunction model with controllable morphology based on the co-assembly of coordination donor (AgNO 3 ), inorganic titanium source (Ti(SO 4 ) 2 ) and g-C 3 N 4 precursor (Melamine). The obtained Ag/TiO 2 /g-C 3 N 4 heterojunction nanocomposite possessing self-organization of multiple kinds of features leads to the superior photocatalytic hydrogen production efficiency and sustainability, which is a 20.6-fold increase compared to pure g-C 3 N 4 . The excellent catalytic performance may have benefited from the following aspects: (1) one-dimensional porous structure, accelerating the separation of charge carriers and providing more reactive sites for photocatalytic process; (2) tight interface, reducing the interfacial charge diffusion migration length and immensely utilizing photogenerated charges; (3) heterostructure, facilitating photogenerated electron-hole pair separation and resulting in improved quantum efficiency with enhanced photocatalytic activity.
Our work highlights the critical role of the structure nanosizing in the heterojunction system and develops a realistic method to build a platform for potential applications including water oxidation and CO 2 reduction on the utilization of solar energy.