Toward Efficient Preconcentrating Photocatalysis: 3D g-C3N4

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Energy, Environmental, and Catalysis Applications

Towards Efficient Preconcentrating Photocatalysis: 3D g-C3N4 Monolith with Isotype Heterojunctions Assembled from Hybrid 1D & 2D Nanoblocks Yingfeng Xu, Qiaoqi Guo, Le Huang, Huajun Feng, Chen Zhang, Haiqun Xu, and Meizhen Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09290 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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ACS Applied Materials & Interfaces

Towards Efficient Preconcentrating Photocatalysis: 3D g-C3N4 Monolith with Isotype Heterojunctions Assembled from Hybrid 1D & 2D Nanoblocks Yingfeng Xu,a,b,* Qiaoqi Guo,a,b Le Huang,a,b Huajun Feng,a,b Chen Zhang,c,* Haiqun Xud, and Meizhen Wang a,b aSchool

of Environmental Science and Engineering, Zhejiang Gongshang University,

Hangzhou 310013, P. R. China. bZhejiang

Provincial Key Laboratory of Solid Waste Treatment and Recycling, Hangzhou

310013, P. R. China. cState

Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, P. R. China. dSchool

of Food Science and Biotechnology, Zhejiang Gongshang University, Hangzhou

310013, P. R. China. *Email: [email protected]; [email protected]

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Abstract The macroscopic integration of microscopic catalyst is one of the most promising strategies for photocatalytic technology in facing practical applications. However, in addition to the unsatisfactory photoactivated exciton separation, a new problem restricting the catalytic efficiency is the unmatched kinetics between the reactant diffusion and the photochemical reaction. Here, we report an isotype heterojunctional 3D g-C3N4 monolith, which is assembled from the hybrid building blocks of the nanowires and nanosheets. Benefiting from its hierarchically porous network and abundant heterojunctions, this catalytic system exhibits inherently promoted efficiency in light absorption and exciton separation, thus leading to a desirably improved photocatalytic performance. Furthermore, thanks to the structural and functional advantages of the constructed g-C3N4 monolith, a novel strategy of preconcentrating photocatalysis featuring the successive filtration, adsorption and photocatalysis have been further developed, which could technically coordinate the kinetic differences and result in over-ten-time enhancement on the efficiency compared with the traditional photocatalytic system. Beyond providing new insight into the structural design and innovative application of the monolithic photocatalyst, this work may further open up novel technological revolutions in sewage treatment, air purification and microbial control, etc. Key words: macroscopic assembly, monolithic g-C3N4, isotype heterojunction, adsorption, preconcentrating photocatalysis,

INTRODUCTION In the past few decades, the global environmental degradations and energy crises have been stimulating plenty of researches on photocatalysis, which hopefully offer a sustainable way to alleviate these growing issues.1-5 However, there are full of challenges for the traditional photocatalytic technologies towards the practical applications, primarily limited by some inherent problems in the micro-sized semiconductor-based photocatalysts6,7: (i) Unsatisfactory photoconversion efficiency due to the limited light adsorption and inefficient separation of the photoexcited carriers.8-10 (ii) Paradox between the freely-dispersed and immobilized implementation of these micro/nano-scaled photocatalysts. Generally, the introduction of the bulk support benefits a convenient recuperability, but it will diminish the photocatalytic activity due to unavoidable obstructions on the light transmission and mass transportation.11, 12 (iii) Kinetic mismatch among the reactant diffusion, surface adsorption 2


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and the subsequent photochemical reaction.13 Especially in the case of low concentrated reactant, due to the short lifetime of the photoinduced radicals,14-16 massive and highlydispersed photocatalysts are indispensable for a shortened diffusion length between the reactant molecules to catalyst surface. Thus, developing a tailored photocatalytic system is of great urgency to comprehensively address these problems aforementioned. The macroscopic assembly of the low-dimensional building blocks into three dimensional (3D) porous integration is an emerging strategy to construct novel catalytic systems,17-19 which not only retains the individual functionality but supplements some attractive structural functions.20 As demonstrated in the well-investigated porous aerogels of metal oxides21-24and chalcogenides,25, 26these monolithic photocatalytic systems benefit flexible recoverability and macroscopical operability.18, 27 Moreover, the 3D interpenetrated porous structure significantly promotes the solar energy adsorption through the well-defined multireflection of the incident light.28, 29 Accordingly, the integrated catalyst system is one of the most promising strategies towards practical photocatalytic implement, the remained problems are the inefficient separation of the photoexcited electron-hole pairs and the mismatch reaction dynamics.30, 31 Constructing photocatalytic heterojunction between two semiconductors with matched electronic structure is an efficient strategy for promoting the exciton dissociation.32-35 However, there are high-density strains and defects at the interface of the conventional junction, which generally origin from the lattice mismatch between the anisotype substances.36 These disadvantages essentially restrict a desirably high mechanical strength of the bulk assembly, and even oppositely produce new recombination centers to quench exciton pairs.37, 38 Instead of the conventional interfacial contact at nanoscale, we recently have proposed a co-condensation strategy to prepare the covalently bonded heterojunction without defined interface,39 demonstrating an improved photogenerated exciton transfer and separation. Accordingly, considering the necessity of the similar structural parameters and abundant surface dangling groups for realizing a co-condensed heterojunction, the perfect case is that these individual components are highly congeneric or even derived from the same substance.40, 41 In view of the above considerations, in this work, we present a novel 3D monolithic gC3N4 assembled from the 1D nanowire-2D nanosheet binary building blocks with isotype heterojunctions, which exhibits much promoted light absorption and exciton utilization. Moreover, based on such a hierarchically porous g-C3N4 monolith, we then develop a filtration-supported preconcentrating strategy to realize an efficient photocatalytic 3



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technology, thus essentially spanning the kinetic difference between the reactant adsorbing and photochemical process in practical applications. RESULTS AND DISCUSSION Design, Synthesis and Structure of the 3D isotype g-C3N4 Monolith. Up to now, there have been sporadic reports on preparing unitary 3D g-C3N4 monolith, such as template method26 and colloidal chemistry strategy,42 but it is still of great difficulty to realize a macroscopic assembly among different low-dimensional g-C3N4 building blocks with isotype heterojunction. Inspired by the well-investigated 3D graphene oxide, which can be obtained through a facile chemical cross-link due to the abundant surface functional groups (-OH) of graphene oxide sheets.43, 44To obtain a desirable 3D g-C3N4 monolith with isotype heterojunction, we designed a multistep fabrication process starting from the initial g-C3N4 powders synthesized by the traditional thermal polymerization of urea.45 As illustrated in Figure 1a, to obtain the targeted building blocks with different energy band structures, part of the powder was thermally etched in the solution of sodium carbonate, the 2D g-C3N4 nanosheets was then significantly decorated with much enriched porosity and surface functional groups of -NH2 and -OH. On the other parallel step, the remained powder was adequately hydrolyzed in the NaOH solution into 1D g-C3N4 nanowires (Figure S1), which were also endowed a very high density of functional groups in contrast to their parents (Table S1). Based on these binary low-dimensional building blocks, the g-C3N4 monolith with desirable isotype heterojunctions could be successfully constructed through a thermal co-condensation on their mixture (Figure 1a and S2). The as-prepared 3D g-C3N4 monolith is yellowish foam with visibly macroscopic pores. As further revealed in the scanning electron microscopic (SEM) images (Figure 1b), in addition to the notably porous surface, the inside feature of the g-C3N4 monolith also presents a 3D-interconnected porous structure, where the binary building blocks mutually build the network with tens of micrometers in the pore size (Figure 1c). According to the theoretical “resonator effect”, these connective micron-sized pores and channels benefit to increase the reflection and scattering of the incident light, thus enhancing the efficiency of solar energy adsorption.18

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Figure 1. (a) Schematic diagram of the fabrication process for 3D monolithic g-C3N4 with isotype heterojunctions from the as-synthesized 1D and 2D building blocks, whose morphologies are shown in the corresponding TEM images, respectively, scale bar 200 nm. The digital picture shows a cylindrical g-C3N4 monolith, scale bar 1 cm. (b, c) SEM images of the as-prepared 3D g-C3N4 monolith. Scale bars, 100 μm for (b) and 10 μm for (c). (d) Representative TEM image showing the joints between the mesoporous nanosheets and the nanowires in the constructed 3D g-C3N4, scale bar 100 nm. Moreover, the close-up view of the representative nanosheet-nanowires joint by transmission electron microscopy (TEM) has confirmed rich nanopores in the 3D g-C3N4 system (Figure 1d and S2), including the mesopores in the pretreated nanosheets and the macropores formed by the interlaced nanowires. Compared with the nanosheets and nanowires, 3D g-C3N4 demonstrates significantly enhanced surface area, which should be resulted from the present hierarchical porous structure and thus contribute to the matter adsorption (Figure S3).46 Taken together, the confirmed hierarchical porous structure of the 3D g-C3N4 monolith implies its improved light energy utilization and reactant transfer in photocatalytic applications. To provide a fundamental insight into the co-condensation process, the chemical states of C, N and O elements in the individual building blocks and the corresponding 3D assembly were studied by the X-ray photoelectron spectroscopy (XPS), respectively. As shown in 5



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Figure 2a, both of the NaCO3 etching treatment (2D nanosheets) and NaOH-mediated hydrolysis process (1D nanowires) are confirmed to introduce abundant functional -OH and -NH2 groups on the surface of the g-C3N4. The much richer surface dangling groups in 1D g-C3N4 nanowires indicate their lower degree of polymerization than that of 2D nanosheets, which can be further confirmed by the much more H content from the quantitative elemental analyses (Figure 2b, Table S1). Fortunately, the content ratio of -OH and -NH2 in the 1D nanowires is higher than that in 2D nanosheets (Figure 2a, Table S1). Thus, instead of a homo-condensation, the dehydration reaction of these two complementary groups could more preferentially take place between 1D and 2D building blocks (Figure 2c). And the hetero-assembly process essentially ensures the high-density isotype heterojunctions formation in the final 3D product. Moreover, we found that neither the individual nanowires nor nanosheets units could construct a freestanding 3D structure under the same fabrication process (Figure S4). This result confirms the necessary roles of 2D nanosheets as the structural skeletons and 1D nanowires as the cross-linking agent for constructing the desirable 3D isotype g-C3N4 monolith. Notably, after the hydrothermal treatment, as characterized by the Fourier-transform infrared (FTIR) spectroscopy (Figure 2d) along with the XPS results, the densities of both of C-NH2 and C-OH in 3D assembly are significantly diminished in comparison to the original 1D nanowires, indicating a successful dehydration reaction and the covalent bonding formation of C-N between the building blocks. The efficient covalent cocondensation ensures the tight combination between these low-dimensional blocks as revealed in the TEM image (Figure 1d and S2), and essentially leads to a good mechanical flexibility of the 3D g-C3N4 monolith. It possesses a more-than-23 % elastic compressibility with a modulus value of 7.1 kPa (Figure 2e), which is comparable to most commercial organic foams.

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Figure 2. (a) XPS spectra of C 1s, N 1s and O 1s recorded for 1D, 2D and 3D monolithic g-C3N4, respectively. (b) Quantitative elemental analyses of C, H, O and N in 1D, 2D and 3D monolithic g-C3N4. (c) Schematic diagram of the proposed co-condensation process between the 1D and 2D building blocks into 3D g-C3N4 with isotype heterojunction. (d) FTIR spectra of the 1D, 2D and 3D monolithic g-C3N. (e) Compressive stress-strain curve of the as-prepared 3D g-C3N4 monolith. The cycle and arrow indicate the onset point for the irreversible mechanical damage. (f) XRD diffraction patterns of the 1D, 2D and 3D monolithic g-C3N, the insert is the magnification of the corresponding region. Furthermore, judging from the X-ray diffraction (XRD) patterns (Figure 2f), in comparison with the 2D building blocks, all the peaks are found to slightly shift to largeangle direction in the final 3D product. Different from the powder of nanosheets, when they serve in a 3D matrix, the decreased degree of freedom will increase the planar pressure stress in the nanosheets, thus leading to the observed large-angle shift of the XRD peaks. The result implies the good combination among the building blocks in the 3D g-C3N4 monolith. More importantly, a certain amount of dangling -NH2 and -OH groups are confirmed to remain in the final 3D g-C3N4 (Figure 2a and 2d), which are theoretically helpful for reactant adsorbing through the hydrogen bond and/or electrostatic interaction. Optical and Spectroscopic Characterizations. We then investigated the electronic 7



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band structure of the constructed 3D isotype g-C3N4 monolith in comparison to that of the 1D and 2D building blocks. Judging from the Ultraviolet-visible (UV-vis) diffuse reflectance spectra (Figure 3a), the enhanced light absorption of 3D g-C3N4 monolith, compared with its mashed powder form, indicates that the “resonator effect” resulted from these connective micron-sized pores and channels could effectively increase the solar energy utilization. Meanwhile, the intrinsic absorption edge of the g-C3N4 nanowires show a blue shift compared to that of the nanosheets, which confirms the bandgap broadening caused by decreasing the degree of polycondensation. Through measuring the valance band (VB) XPS spectra (Figure 3b) and the Mott-Schottky plots (Figure 3c), the shift of valence band maximum (VBM) and conduction band minimum (CBM) of nanowires, compared with nanosheets, further confirm the different electronic band structure. Accordingly, such difference in the electronic band structure between the nanosheets and nanowires provides the possibility to create the staggered gap-type heterojunctions at their interfaces, which is expected to promote the dissociation of photo-generated excitons. In comparison to the steady-state photoluminescence (PL) spectra of the individual nanosheets (2D), nanowires (1D) and their mechanical mixture (2D&1D) (Figure 3d), 3D isotype g-C3N4 monolith demonstrates a significantly reduced emission peak intensity, indicating the more efficient photo-excited electrons and holes (e-/h+) separation resulted from the formation of isotype heterojunction structure by the co-condensation treatment. Furthermore, as shown in the time-resolved PL spectra (Figure 3e), the average PL lifetime for the 3D isotype g-C3N4 monolith is much longer than that for the individual lowdimensional building blocks and the mechanical mixture (2D&1D). These results evidently suggest the significantly suppressed radiative recombination of photoexcited electrons and holes in the as-prepared 3D isotype g-C3N4 monolith, which benefits from the band-alignment isotype heterojunctions inside as illustrated in Figure 3f. Additionally, it is worth mentioning that the UV-vis absorption intensity is also significantly enhanced in the 3D isotype g-C3N4 monolith compared to the mashed powder counterpart (Figure S5). The main reason is theoretically attributed to the rich channels in the 3D hierarchically porous structure, which can significantly increase the reflections and scattering of incident light, thus enhancing the absorption efficiency of solar energy.18 Taken together, the constructed 3D isotype g-C3N4 monolith with the promoted light absorption and improved exciton separation forebodes a significant enhancement in the photocatalytic performance. 8


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Figure 3. (a-e) UV-vis diffuse reflectance spectra (a), XPS valance band spectra (b), MottSchottky plots (c), steady-state PL emission spectra (d), and time-resolved transient PL decay spectra (e) of the 1D, 2D, their mixture (1D&2D), and 3D isotype monolithic g-C3N4. (f) Schematic illustration of the isotype heterojunction formed at the contact interface of the g-C3N4 nanosheets and nanowires. Photocatalytic Performance Investigation. To evaluate the photocatalytic performance of the as-constructed 3D isotype g-C3N4 monolith, the classic model reaction of gaseous CO2 reduction was then employed. As shown in Figure 4, S6 and S7, all the samples exhibit the photocatalytic CO2 reduction activities with the oxidation of H2O, even the nanowires with a low degree of polymerization. In comparison to the low CO generation rate (1.4 μmol h-1) for the simply-mixed powders of nanosheets and nanowires, the mashed powders derived from the 3D C3N4 parent shows a more-than-three-time promotion in the CO production efficiency (5.1 μmol h-1), evidently confirming the structural advantage with abundant isotype heterojunctions in improving the photoexcited charge carriers separation. Moreover, the further enhanced CO productivity can be observed for the 3D isotype C3N4 monolith compared to its mashed powder derivative (Figure 4, S6), which indicates the morphological advantage with hierarchical porous network in enhancing the incident light absorption. Very excitingly, the 3D C3N4 monolith and its mashed powder was found to have an ability of deep reduction to generate CH4 in addition to the common CO. However, negligible CH4 could be detected in the reaction systems of 1D nanowires, 2D nanosheets 9



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and their simple mixture under the same conditions (Figure 4 and S6). In spite of a superior CH4 productivity for the 3D isotype C3N4 monolith, its CH4 selectivity is almost comparative to the mashed powder counterpart (Figure 4), which implies that the unexpected CH4 production primarily benefits from the microstructure instead of the 3D morphology. Judged from the adsorption isotherms of CO2, benefiting from the rich functional groups of -NHx, -OH and porous structure, 3D isotype C3N4 demonstrate much higher CO2 adsorption capacity than that of 1D nanowires, 2D nanosheets and their simple mixture (Figure S8), which could change the thermodynamic and kinetic parameters of the CO2 reduction reaction.47 Accordingly, the introduction of isotype heterojunctions, functional groups and porous structure could improve the photocatalytic CO2 reaction kinetics to produce CH4. All these results confirm a superior photocatalytic performance of the 3D isotype C3N4 monolith, which comprehensively benefits from its structural and morphological advantages.

Figure 4. Evolution rates of CO, CH4 and the CH4 selectivity in the photocatalytic CO2 reduction by using the g-C3N4 nanowires, g-C3N4 nanosheets, mixture of the 1D, 2D, their mixture (2D&1D), 3D isotype monolithic g-C3N4 (3D) and the corresponding mashed 3D g-C3N4 powder (mashed 3D). 3D isotype g-C3N4 Filter-Mediated Preconcentrating Photocatalysis. Considering the rich -NH2 and -OH groups remained in the as-prepared g-C3N4 monolith, which can provide efficient charge sites or hydrogen bonding sites in the aqueous solution, the 3D isotype g-C3N4 with a high specific surface is supposed to process a good adsorbing ability to these water-soluble dye molecules. Fortunately, the shape of the final 3D isotype g-C3N4 can be flexibly adjusted by using different casting container. Based on the well-designed 10


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3D isotype g-C3N4 monolithic filter disc, we then evaluated its adsorbing efficiency on a model contaminant of methylene blue (MB) through a home-made filtration setup (Figure 5a). Under the negative pressure provided by a vacuum pump, the MB-containing water could fluently penetrate the filter layer, and the initial blue colour become much lighter through a single filtration. Most of the MB molecules could be efficiently adsorbed and concentrated by the 3D isotype g-C3N4 filter, and endow the whole disc present a deep blue colour (Figure 5a-c). To further quantitatively evaluate the filtration efficiency and adsorption limit, the absorbance of the filtrated solution was monitored by successively treating 400 mL of 20 mg L-1 MB-containing water. Due to an accumulated shielding on the active sites with the increasing MB adsorption, the filtration efficiency was found to gradually decrease following a pseudo-first-order kinetic model (Figure 5e and S9), which indicates a direct surface attraction through simple physical interactions instead of multistep process. The limiting adsorbing capacity of MB was determined to 78 mg g-1 for the 3D isotype monolithic g-C3N4 filter, and the filtration efficiency at its median adsorbing value of 39 mg g-1 is more than 80.12 %. More excitingly, the blue MB-adsorbing 3D isotype g-C3N4 disc was revealed to undergo an effective decoloration and recover to its original yellow gradually under the irradiation of AM 1.5, indicating the efficient MB photocatalytic degradation by 3D isotype C3N4 monolith (Figure 5c and S10). Judging from the 13C and 1H solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) results, after the administration of filtration, adsorption and photo-degradation, except the little increase in the structure water, the chemical states of the carbon and hydrogen in the 3D isotype g-C3N4 filter almost remained unchanged, further confirming its structure robustness. It is notable that no signals related to the MB molecules can be detected in the MB-adsorbing filter after sufficient light irradiation, indicating their complete photodegradation. Moreover, the completely disappeared characteristic -CH3 and -C=C groups in the FTIR spectrum after an abundant AM 1.5 irradiation further confirm that the adsorbed MB molecules on the 3D isotype gC3N4 disc can be completely photodegraded (Figure 5f). Therefore, these qualitative results preliminarily imply the constructed 3D isotype g-C3N4 to be not only a qualified filter, but also an excellent monolithic photocatalyst mediating the degradation of the surfaceadsorbing organics.

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Figure 5. (a) Digital photograph of the setup used for the 3D isotype monolithic g-C3N4 mediated filtration. The much lighter colour of MB solution after filtration indicates the efficient adsorption of MB by the designed 3D g-C3N4. (b-d) Pictures of the initial 3D isotype monolithic g-C3N4 filter (b), after the filtration of MB-containing water (c) and the further illumination treatment (d). The recovered colour of MB-adsorbing 3D g-C3N4 filter after illumination indicates the efficient photocatalytic degradation for reuse. (e) Relative concentration of MB (blue bar) filtered by the same 3D isotype monolithic in nine consecutive times and the corresponding filtration efficiency (red). (f) FTIR spectra of the initial 3D isotype g-C3N4 filter (initial), after the filtration of MB-containing water (filtra.) and further illumination treatment (illumin.). (g, h) Solid-state 13C (g) and 1H (h) MAS NMR spectra of the initial and MB-absorbed 3D isotype monolithic g-C3N4 filter after illumination treatment. Asterisks denote spinning sidebands. The inset in (g) shows the corresponding sources of the 13C NMR signals in 3D isotype g-C3N4. It is worth mentioning that the 1D nanowires and 2D nanosheets have specialized benefits in the system. The former with very rich functional groups presents a remarkable adsorbing capacity, but a low photocatalytic efficiency due to the broadened band gap (Figure S11 and S12). Oppositely, despite a relatively high photodegradation efficiency, the individual 2D units could not role as a good absorber because of their limited functional group density (Figure S11). So, the complementary expertise of adsorption and photocatalysis integrated in the 3D isotype g-C3N4 monolith enables the technologic feasibility of a filtrating preconcentration-supported photocatalytic strategy. As illustrated in Figure 6a, in comparison to the traditional photocatalytic system with the catalyst fully-dispersed in the solution, the 3D g-C3N4 monolith-mediated preconcentrating photocatalytic strategy has lots of advantages. Most of all, during the alternant filtrating and photocatalysis process, the low-concentration contaminant can be efficiently concentrated on the monolithic catalyst, thus significantly promoting the 12


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utilization of the photoactivated excitons with a very low free path length. Moreover, such a two-stage technology can not only avoid the kinetic difference between the low diffusion efficiency of contaminant and the fast photochemical reaction in a large-scaled dispersion system, but reduce the light energy loss reflected by the massive liquid medium. In addition to a more flexible administration, the novel strategy can get out of the limitation of the continual light illumination, which restricts the practical application of traditional photocatalysis when employing natural sunlight. Through a simulation experiment of purifying 4 L of MB-contaminated water (20 mg L-1), we further investigated the efficiency advantage of the preconcentraing photocatalysis against the traditional one. In brief, the initial MB-contaminated water was divided into ten cycles of (400 mL per cycle) filtration and photocatalysis by using the same 3D isotype gC3N4 filter disc. Notably, it takes only several minutes for each filtrating process, and within 2 hours for the MB-concentrated 3D isotype g-C3N4 filer to photodegrade the adsorbed MB under a subsequent AM 1.5 illumination (Figure 6, 5f-h and S10). Then, the filter can recover to the excellent adsorption ability for MB (Figure 6). After such a ten-cycle treatment, which takes 20 hours in total, about 95 % of the MB molecules has been removed and photodegraded from the 4 L water (Figure 6 and S13). And the structure of 3D isotype g-C3N4 was well remained after such a ten-cycle treatment (Figure 6b, S14). In contrast, in the traditional photocatalytic case with mashed isotype g-C3N4 powderdispersed 4 L of MB-contaminated water (20 mg L-1), the photodegradation process is much less efficient (Figure 6, S15). Through a mathematical fitting and estimation based on the first order kinetics equation, the time for degrading 95 % of the MB molecules in the water is calculated to be more than 220 hours. Excitingly, for the practical application of photodegradation on the medium-dispersed contaminant, the proposed 3D isotype g-C3N4 monolith-mediated preconcentrating photocatalytic strategy is over ten times more efficient than traditional catalyst-dispersed photocatalysis. Predictably, the efficiency can be further improved by employing more than one filter disc in the alternant filtrating and photocatalytic process.

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Figure 6. (a) Schematic diagram showing the implementary differences between the 3D isotype g-C3N4 monolith-mediated preconcentrating photocatalysis and the traditional gC3N4-dispersed photocatalysis. (b) Efficiency comparison on treating 4 L of MBcontaminated water between the traditional photocatalysis (tradition. photocata.) and the proposed preconcentrating photocatalytic technology (preconcen. photocata.), with the corresponding traditional photodegradation fitting curve and the remaining MB in water after filtration. The insert images are 3D g-C3N4 monolith before and after the 10-cycle test.

CONCLUSIONS In summary, by employing the binary low-dimensional g-C3N4 building blocks of nanosheets and nanowires, we have constructed a novel 3D isotype g-C3N4 monolith with unique isotype heterojunctions. Comprehensively benefiting from its 3D-interconnected porosity and abundant heterojunctions inside, the incident light absorption and exciton separation are significantly enhanced in comparison to the common g-C3N4 powder counterparts, thus leading to a superior photocatalytic performance. Furthermore, based on the structural and functional advantages of the constructed g-C3N4 monolith, a technical strategy featuring the successive filtration, adsorption and photocatalysis has been demonstrated, which could essentially coordinate the kinetic differences. In addition to 14


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providing a novel synthetic methodology and the functional 3D isotype g-C3N4 monolith, such a monolithic catalyst-mediated preconcentrating photocatalysis may open up numerous technological innovations in sewage treatment, air purification and microbial control, etc.

METHODS Materials and reagents. Dicyandiamide (C2H4N4, 99%) was obtained from Sigma-Aldrich, U.S.A. Sodium hydroxide (NaOH, 99%), sodium carbonate (Na2CO3, 99%), and methylene blue (MB, 96%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Deionized water used throughout the experiments was prepared using ELGA water purification system (PURELAB Classic). Synthesis of 2D C3N4 nanosheets. 2D C3N4 nanosheets with porous structure were prepared by etching bulk C3N4 powder in Na2CO3 solution. Briefly, bulk C3N4 powder was synthesized by placing 2 g of dicyandiamide in an alumina crucible, which was heated to 550 °C for 4 h in a muffle furnace. Then, the obtained C3N4 powder (500 mg) was dispersed into 1 M Na2CO3 aqueous solution (50 mL) in a 100 mL Teflon-lined stainless-steel autoclave, which was kept at 60 °C for 2 h. After cooling down to room temperature, 2D porous C3N4 nanosheets were collected by centrifuged, washed with water several times, and vacuum-dried at 353 K. Synthesis of 1D C3N4 nanowires. 1D C3N4 nanowires were prepared by hydrolysing bulk C3N4 powder in alkaline conditions. Briefly, the as-synthesized bulk C3N4 powder (500 mg) was dispersed into 6 M NaOH solution (50 mL) under ultrasonication for 1 h, which was stirred at 60 °C for 10 h. Then, 1D C3N4 nanowires were collected by centrifuging and further purified by dialysis (membrane cutoff: 3500 Da) against water until neutral to completely remove the residual NaOH. Synthesis of 3D C3N4 monolith, mashed C3N4 and C3N4 mixture. 3D C3N4 monolith was fabricated by the condensation of -NH2 and -COH active groups on the surface of 2D C3N4 nanosheets and 1D C3N4 nanowires. Typically, 200 mg of 2D C3N4 nanosheets and 200 mg of 1D C3N4 nanowires were dispersed into 30 mL aqueous ethanol solution (90 vol%) under 30 min sonication (Q700 Probe, QSonica, Newtown, CT, USA, 5 W). Then, the mixture was transferred into a Teflon-sealed autoclave and maintained at 150 °C for 12 hours. After 15



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cooling to room temperature, the as-prepared CN gel was freeze-dried to keep intact 3D structure. And the diameter of prepared 3D C3N4 monolith could be controlled by the autoclave shape for practical applications. Mashing the as-synthesized 3D C3N4 monolith into powder form was used to obtain mashed C3N4. C3N4 mixture powder was obtained by grounding the powder of 1D C3N4 nanowires and 2D C3N4 nanosheets with the same ratio (1D : 2D = 1:1) of as-synthesized 3D C3N4 monolith. Material characterizations. Transmission electron microscope (TEM) and high resolution TEM (HRTEM), were carried out on a JEM-2100F electron microscope. Scanning electron microscopy (SEM) images, EDX mappings and spectroscopy were obtained on a fieldemission scanning electron microscope (Magellan 400, FEI Company). X-ray diffraction (XRD) patterns were performed on a powder diffractometer (Bruker D8 ADVANCE) at Cu Kα (λ = 0.154056 nm). Nitrogen adsorption and desorption isotherms were measured at Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All samples were degassed at 250 °C for 3 h prior to the nitrogen-adsorption measurements. Fourier transform infrared (FTIR) spectra were obtained on a Bruker Tensor II FTIR spectrometer. The UVVis diffuse reflectance spectra were measured on Hitachi U4100 UV-Vis-NIR spectrometer. Photoluminescence (PL) spectra and Time-resolved PL spectra were obtained on a FLSP920 fluorescence spectrophotometer (Edinburgh) with an excitation wavelength of 400 nm, and the emission wavelength of 450 nm. The quantitative analysis of C, N, H, O content in the samples was performed on a VARIO EL III microanalyzer. X-ray photoelectron spectroscopy (XPS) experiments were performed on a Thermo Scientific ESCALAB 250 spectrometer with a monochromated Al Kα source (hν = 1486.6 eV). Accurate binding energies (± 0.1 eV) are determined with respect to the position of the adventitious C1s peak at 284.8 eV. The deconvolution of the C, N and O peaks was performed through a software XPSPEAK version 4.1, and the energy scales of valance band XPS (VBXPS) were aligned by using the Fermi level of the XPS instrument. The mechanical property of 3D C3N4 monolith was conducted in a single-column mechanical testing system (Instron-5566) at a constant loading speed of 1.5 mm min−1, and the sample was cut into small cylinders by a scalpel (Φ=2 cm, L=1.5 cm) for measurement. The CO2 adsorption isotherms of all these samples were obtained on a Micromeritics ASAP 2020 static volumetric analyzer at room temperature. Prior to adsorption analysis, each sample was dried in an oven at 150°C for 6 h and degassed for 4 h below 26.7 Pa at 150 °C. Photocatalytic CO2 reduction measurements. The photocatalytic activities of different 16


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catalysts were evaluated by measuring their photocatalytic CO2 reduction performance, which were conducted in a top-irradiation-type quartz reaction vessel (volume: 160 mL) under the irradiation of a 300 W Xe lamp with an Air Mass 1.5 G filter (AM 1.5 G, ca. 100 mw cm-2). Typically, 100 mg of the catalyst was placed on the bottom of the quartz reactor. Then, the reactor was bubbled with high purity CO2 (99.99%), passed through a water bubbler, for 20 min to remove air completely and produce a mixture of CO2 and water vapor. After that, the mixture was irradiated by the Xe lamp at a constant temperature of 15 °C under stirring. The gas products were analyzed by a gas chromatograph (GC-7900, Techcomp Corp. China). The product was quantified using external calibration standards with a calibration curve. During each measurement, 1 mL of gas product was periodically withdrawn from the reactor and analyzed by the gas chromatography. To minimize the experimental errors and ensure the credibility of the results, every cycling of the photocatalytic hydrogen evolution experiment was repeated for three times. Methylene blue adsorption measurements. To estimate the MB adsorption capacity of 3D C3N4 monolith, 400 ml of MB solution (20 mg/L) permeated through 400 mg of the catalyst via a home-made filtration setup under vacuum for nine consecutive times. Each time, the adsorption of the MB inside the catalyst was estimated by measuring the concentration of MB solution before (initial) and after permeation (filtration) by the UVVis-NIR spectrometer. The MB adsorption capacity (Γ) and filtration efficiency (η) could be calculated as follows:

（1）

（2） where n is the number of consecutive filtration cycle during the experiment, C0 is the initial concentration of MB, Cn is the remaining concentration of MB after the n consecutive filtration, V is the volume of feeding solutions, and m is the mass of filter.48 Efficiency measurement of the preconcentrating photocatalysis. To estimate the preconcentrating photocatalytic MB degradation performance, at first, 400 ml of MB solution (20 mg/L) permeated through 400 mg of 3D C3N4 monolith via the home-made filtration setup under vacuum. Then, the MB-adsorbing 3D g-C3N4 monolith were illuminated by a 300 W Xe lamp with an Air Mass 1.5 G filter (AM 1.5 G, ca. 100 mw 17



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cm-2), which was then freeze-dried, mashed and examined by UV-Vis-NIR, FTIR and solidstate NMR spectra. The solid-state

13C

and 1H MAS NMR spectra of the samples were

recorded on a Varian Model VNMRS-400WB spectrometer with a spinning rate of 4 kHz. The 13C chemical shift were referenced to neat TMS using the secondary reference of the adamantane CH2 peak at 38.48 ppm. In contrast, 1D C3N4 nanowires and 2D C3N4 nanosheets were treated under the same conditions. Photodegradation of 4 L MB solution (20 mg/L) were carried out to compare the efficiency of proposed preconcentraing photocatalysis with that of traditional photocatalysis. In the traditional photocatalysis, mashed isotype g-C3N4 (400 mg) was dispersed into 4 L MB solution (20 mg/L), illuminated by the same 300 W Xe lamp under stirring. Then, 10 ml of aliquot was collected and centrifuged to obtain a transparent solution in certain time, measured by the UV-Vis-NIR spectrometer to calculate the residual MB concentration. While, in the preconcentraing photocatalysis, the 4 L MB solution (20 mg/L) was divided into ten-cycle (400 mL per cycle) filtration and photocatalysis by using the same 3D isotype C3N4 monolith (400 mg). In each cycle, the 400 mL of MB solution was first filtered by 400 mg of 3D C3N4 monolith, and the MB concentration was measured before and after the filtration by the UV-Vis-NIR spectrometer. Then, the MB-adsorbing 3D C3N4 monolith was illuminated by the 300 W Xe lamp, and measured by UV-Vis-NIR spectrometer to evaluate the remaining MB in it. ASSOCIATED CONTENT Supporting Information Additional material includes the structural characterizations of 3D g-C3N4 monolith and its building blocks, pore structure of 3D g-C3N4 monolith, detailed results in the photocatalytic CO2 reduction, and the additional evaluations of the adsorption, photocatalysis and recovery in different samples. This material is available free of charge via the Internet at http://pubs.acs.org. The authors declare no competing financial interests AUTHOR INFORMATION Corresponding Author *[email protected] 18


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*[email protected]

ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (21805243), the Science and Technology Planning Project of the Science and Technology Department in Zhejiang Province (2018C37057), the Science and Technology Planning Project of the Science and Technology Department in Zhejiang Province (2019C03110). REFERENCES (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69-96. (2) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z. Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science 2015, 347, 970-974. (3) Ran, J.; Jaroniec, M.; Qiao, S. Z. Cocatalysts in semiconductor-based photocatalytic CO2 reduction: achievements, challenges, and opportunities. Adv. Mater. 2018, 30, 1704649. (4) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Photocatalytic reduction of CO2 on TiO2 and other semiconductors. Angew. Chem. Int. Edit. 2013, 52, 7372-7408. (5) Li, Y.; Ouyang, S.; Xu, H.; Wang, X.; Bi, Y.; Zhang, Y.; Ye, J. Constructing solid– gas-interfacial fenton reaction over alkalinized-C3N4 photocatalyst to achieve apparent quantum yield of 49% at 420 nm. J. Am.Chem. Soc. 2016, 138, 13289-13297. (6) Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J. Nanophotocatalytic materials: possibilities and challenges. Adv. Mater. 2012, 24, 229-251. (7) Dumortier, M.; Haussener, S. Design guidelines for concentrated photoelectrochemical water splitting devices based on energy and greenhouse gas yield ratios. Energy Environ. Sci. 2015, 8, 3069-3082. (8) Wenderich, K.; Mul, G. Methods, mechanism, and applications of photodeposition in photocatalysis: a review. Chem. Rev. 2016, 116, 14587-14619. (9) Marschall, R. Semiconductor composites: strategies for enhancing charge carrier separation to improve photocatalytic activity. Adv. Funct. Mater. 2014, 24, 2421-2440. (10)Wu, K.; Zhu, H.; Liu, Z.; Rodríguez-Córdoba, W.; Lian, T. Ultrafast charge separation and long-lived charge separated state in photocatalytic CdS-Pt nanorod heterostructures. J. Am. Chem. Soc. 2012, 134, 10337-10340. (11)Dhananjeyan, M.; Mielczarski, E.; Thampi, K.; Buffat, P.; Bensimon, M.; Kulik, A.; Mielczarski, J.; Kiwi, J. Photodynamics and surface characterization of TiO2 and Fe2O3 photocatalysts immobilized on modified polyethylene films. J. Phys. Chem. B 2001, 105, 12046-12055. (12)Xu, Y.; Zhang, C.; Lu, P.; Zhang, X.; Zhang, L.; Shi, J. Overcoming poisoning effects of heavy metal ions against photocatalysis for synergetic photo-hydrogen generation from wastewater. Nano Energy 2017, 38, 494-503. (13)Zhu, S.; Wang, D. Photocatalysis: Basic principles, diverse forms of 19



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Toward Efficient Preconcentrating Photocatalysis: 3D g-C3N4

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