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Crafting Mussel-Inspired Metal Nanoparticle-Decorated Ultrathin Graphitic Carbon Nitride for the Degradation of Chemical Pollutants and Production of Chemical Resources

Jingsheng Cai, Jianying Huang, Shanchi Wang, James Iocozzia, Zhongti Sun, Jingyu Sun, Yingkui Yang, Yuekun Lai,* and Zhiqun Lin*

DOI: 10.1002/adma.201806314

challenges over the last decades.[1] Modern commercial dyes offer superior color sta-bility because of the high degree of aroma-ticity and extensive conjugation present in their chemical structures.[2] Some such dyes are both environmentally persistent (i.e., do not readily break down on their own) and carcinogenic. Many such organic pollutants, including methylene blue (MB), phenol, bisphenol A, and so on, are utilized extensively in various fields where their unintended release into the environment poses a great threat to both human health (causing or contributing to deformities, and several debilitating and fatal conditions) and the natural environ-ment (contributing to the destruction of the ozone layer, the generation of photo-chemical smog, and global warming).[3]

As a clean chemical oxidant with only water and oxygen as byproducts, hydrogen peroxide (H2O2) has been widely used in a variety of industrial fields owing to its low cost and minimal environmental impact, and widespread use in several industries including biosciences (disinfection), envi-ronmental remediation (organic break-

down), and chemical processing (pulp bleaching).[4] In addition, H2O2 has received much attention as a substitute for hydrogen in fuel cells,[5] offering a superior alternative to hydrogen and

The development of efficient photocatalysts for the degradation of organic pollutants and production of hydrogen peroxide (H2O2) is an attractive two-in-one strategy to address environmental remediation concerns and chemical resource demands. Graphitic carbon nitride (g-C3N4) possesses unique electronic and optical properties. However, bulk g-C3N4 suffers from inefficient sunlight absorption and low carrier mobility. Once exfoliated, ultrathin nanosheets of g-C3N4 attain much intriguing photocatalytic activity. Herein, a mussel-inspired strategy is developed to yield silver-decorated ultrathin g-C3N4 nanosheets (Ag@U-g-C3N4-NS). The optimum Ag@U-g-C3N4-NS photocatalyst exhibits enhanced electrochemical properties and excellent performance for the degradation of organic pollutants. Due to the photoformed valence band holes and selective two-electron reduction of O2 by the conduction band electrons, it also renders an efficient, economic, and green route to light-driven H2O2 production with an initial rate of 0.75 × 10−6 m min−1. The improved photocatalytic performance is primarily attributed to the large specific surface area of the U-g-C3N4-NS layer, the surface plasmon resonance effect induced by Ag nanoparticles, and the cooperative electronic capture properties between Ag and U-g-C3N4-NS. Consequently, this unique photocatalyst possesses the extended absorption region, which effectively suppresses the recombination of electron–hole pairs and facilitates the transfer of electrons to participate in photocatalytic reactions.

Environmental Remediation

J. S. Cai, Prof. J. Y. Huang, Prof. Y. K. LaiNational Engineering Research Center of Chemical Fertilizer Catalyst (NERC-CFC)College of Chemical EngineeringFuzhou UniversityFuzhou 350116, P. R. ChinaE-mail: [email protected]. S. Cai, Dr. Z. T. Sun, Prof. J. Y. SunSoochow Institute for Energy and Materials InnovationS (SIEMIS)Key Laboratory of Advanced Carbon Materials and Wearable EnergyTechnologies of Jiangsu ProvinceSoochow UniversitySuzhou 215006, P. R. China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201806314.

S. C. Wang, Prof. Y. K. LaiNational Engineering Laboratory for Modern SilkCollege of Textile and Clothing EngineeringSoochow UniversitySuzhou 215123, P. R. ChinaDr. J. Iocozzia, Prof. Z. Q. LinSchool of Materials Science and Engineering Georgia Institute of TechnologyAtlanta, GA 30332, USAE-mail: [email protected]. Y. K. YangSchool of Chemistry and Materials ScienceSouth-Central University for NationalitiesWuhan 430074, P. R. China

With the rapid industrial developments of the 21st century only expected to continue, burgeoning energy demands and global environmental deterioration have been recognized as two main

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other fuel gasses because of its safety and ease of transport and storage,[6] and the ability to be used in one-compartment cells for electricity generation.[7] However, traditional methods for large-scale H2O2 production (such as anthraquinone autoxida-tion, oxidation of alcohols, and electrochemical synthesis) often require energy intensive processes as well as the consump-tion of, and generation of. dangerous chemicals during the multistep reactions making them less than ideal from a green chemistry standpoint.[8] Furthermore, the H2O2 produced by these conventional processes is often contaminated requiring further energy and chemical investment as well as waste gen-eration to produce a usable product.[9]

Therefore, it is important to develop an economical, efficient, and sustainable technology for the synthesis of H2O2. As an alternative strategy to address these issues, the light-driven gen-eration of H2O2 through light-responsive semiconductors from only water and molecular oxygen could be an ideal pathway for the clean production of solar fuels. This process utilizes inex-haustible and sustainable solar energy to produce hydrogen peroxide without producing any organic pollutants or carbon emissions.[10] Thus, it is of great importance to seek efficient, stable, and scalable visible-light-responsive semiconductors for use in pollutant degradation under mild conditions.

As an useful member of the 2D nanomaterials’ family, graphitic carbon nitride (g-C3N4) has garnered considerable attention in the fields of photocatalysis, energy conversion, light-emitting devices, and tribological coatings.[11] Graphitic carbon nitride’s superior physicochemical properties, including large surface areas, high aspect ratios, fast carriers transport, and strong reduction ability of photoexcited electrons under vis-ible-light irradiation, combine to produce a material with out-standing visible-light photocatalytic performance and excellent stability. Since the pioneering work reported by Wang et al. in 2009, which used g-C3N4 for water splitting under visible-light irradiation,[12] much effort has been invested to improve its photocatalytic activity including metallic/nonmetallic elemental doping, enlarging the specific surface area, functionalization with metal nanoparticles (NPs), control of morphology, and the development of heterostructures.[13] It has also been docu-mented that parameters such as the precursor (such as urea and melamine) and the thermal treatment conditions (including the temperature, reaction time, and atmosphere) used for the preparation of g-C3N4 greatly affect the photocatalytic activity.[14]

As an essential neurotransmitter and hormone present in most organisms, dopamine (3,4-dihydroxyphenylethylamine) plays an important role as a chemical messenger in mam-mals.[15] Inspired from the composition of bioadhesive proteins in marine mussels and shellfish, dopamine is well known as a mimic of the specialized adhesive foot protein, Mytilus edulis foot protein-5 (Mefp-5), which can spontaneously polymerize to form a polydopamine (PDA) coating under a weakly alka-line aqueous medium in ambient conditions.[16] This useful phenomenon has been widely employed in almost all surface modification applications, such as antifouling, dye removal, antibacterials, self-healing hydrogels, and lithium-ion-battery separators, because of its versatile adhesion properties, simple fabrication, outstanding biocompatibility, and hydrophilicity.[17] Covalent crosslinking with amines, catechol, and metal coordi-nation is responsible for the mechanical reinforcement of the

adhesive materials in mussels, and further endows PDA layers with a versatile platform for growing and tightly adhering almost all organic and inorganic substrates, such as metal–nonmetal oxides, various semiconductors, noble metals and synthetic polymers. Importantly, PDA is both an environmentally friendly and biocompatible material enabling its use in living systems.[18]

Motivated by the above advantages, this work employs a post gas etching (PGE) technology to prepare ultrathin g-C3N4 nanosheets (U-g-C3N4-NS) through the self-modification of polymeric units via the successful thermal treatment of bulk g-C3N4 (B-g-C3N4) under ambient conditions. During the process, the g-C3N4 can be thermally converted from bulk into ultrathin nanosheets by overcoming the weak van der Waals force between layers. Next, highly controllable 2D U-g-C3N4-NS decorated with dispersed Ag NPs were successfully prepared by the one-pot self-polymerization mediated redox of dopamine (Figure 1). This report systematically demonstrates their prom-ising applications in the visible-light photodegradation of both colorful dyes (MB) and colorless (phenol) aqueous organic con-taminants, as well as the visible-light-induced H2O2 production. The exfoliated U-g-C3N4-NS consistently show improved photo-catalytic activity over B-g-C3N4 (bulk). Compared with pure B-g-C3N4, the resultant Ag@ U-g-C3N4-NS composites are favorable for the photodegradation of aqueous organic contaminants. A series of experiments were conducted to understand the role of photocatalytic performance in visible-light photodegrada-tion. In addition to degradation pathways, Ag@ U-g-C3N4-NS nanocomposites also provide a clean and sustainable way for the production of H2O2 from accessible feedstocks (water and oxygen) without any sacrificial carbon-containing organic elec-tron donors (such as alcohols) under visible-light irradiation. The improved photocatalytic performance can be attributed to the large specific surface area, and exposed new edges and active sites of the 2D U-g-C3N4-NS. In addition, the surface plasmon resonance (SPR) effect of Ag NPs and the effective separation and transfer of photogenerated carriers are due to the coop-eration between Ag NPs and U-g-C3N4-NS. This strategy is a simple, robust, and generalizable approach to the photocatalytic degradation of organic pollutants and light-driven hydrogen peroxide (H2O2) production with dual purposes of environ-mental remediation and useful chemical production.

The morphologies and microstructures of the samples are characterized by scanning electron microscopy (SEM) and trans-mission electron microscopy (TEM). Figure S1a (Supporting Information) shows B-g-C3N4, indicating a solid agglomerate several micrometers in size. After PGE treatment, the resulting U-g-C3N4-NS exhibit greater ductility and reduced sizes (Figure S1b,c, Supporting Information) suggesting the effective exfoliation of B-g-C3N4 into U-g-C3N4-NS. After PDA coating, of U-g-C3N4-NS, surface modification with Ag(I) salt enabled the formation of Ag NPs due to the presence of amines and cat-echol groups on the surface (the chemical structure is shown in Figure S2 in the Supporting Information). A large amount of Ag NPs with diameters in the range of 10–30 nm is gener-ated on the nanosheets. Other noble metals (Au and Pt) were also successfully grown on the U-g-C3N4-NS (Figure S3, Sup-porting Information). Figure S1d–f (Supporting Information) compares the top view of Ag@ U-g-C3N4-NS with different con-centrations of Ag+ ions (0.5, 1.0, and 1.5 mg mL−1, respectively)

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subsequently reduced for 30 min after PDA modification. Sporadic Ag NPs anchored on the surface of U-g-C3N4-NS formed when immersed in 0.5 mg mL−1 of silver nitrate solution (Ag precursor) with an average size of 20–25 nm (Figure S1d, Supporting Information). For an increased concen-tration of 1.0 mg mL−1 of Ag+ ions, large amounts of Ag NPs were homogeneously and uniformly distributed on the surface of U-g-C3N4-NS and with an average NP diameter of 17 nm (Figure S1e, Supporting Information), the slight shrunken nan-oparticles may result from the more nucleation sites and the lower Ag precursor concentration than 1.5 mg mL−1. When the precursor concentration increases to 1.5 mg mL−1, clusters of Ag NPs aggregated and the average diameter increased to some extent to ≈30 nm (Figure S1f, Supporting Information). These trials revealed an optimum growth of distinct NPs on U-g-C3N4-NS samples at a precursor concentration of 1.0 mg mL−1 with a reaction time of 30 min as revealed by UV–vis diffuse reflec-tance spectrum (UV-DRS), photocurrent response, and electro-chemical impedance spectroscopy (EIS) analysis. Apart from the precursor concentration, parameters such as the reduction time were also investigated to optimize the amount and mor-phology of the Ag NPs (Figure S4, Supporting Information).

Figure S5a,b (Supporting Information) compared the SEM images of the as-prepared Ag@ B-g-C3N4 and Ag@ U-g-C3N4-NS. Different from the coral-like B-g-C3N4, it clearly indi-cates that the surface of the single layer U-g-C3N4-NS is loose, porous and no longer smooth as a result of the PGE treatment.

Figure S5c (Supporting Information) shows the energy disperse spectroscopy (EDS) of a wide area of the Ag@ U-g-C3N4-NS sample confirming that the compound is composed of O, C, N, and Ag elements (Si comes from the silicon substrate) with corresponding atomic ratios of 3.302%, 44.677%, 37.087%, and 1.915%, respectively. Additionally, the selected-area mapping of the C, N, and Ag elements supports an even distribution of the Ag NPs on the surface of the U-g-C3N4-NS (Figure S5d, Sup-porting Information).

The surface chemical composition of the as-prepared photocatalysts was obtained by X-ray photoelectron spectros-copy (XPS) analysis. From Figure 2 the XPS survey spectra revealed the element composition of the samples. Compared with the pure U-g-C3N4-NS and the PDA modified U-g-C3N4-NS (both contain C 1s and N 1s peaks), a new Ag 3d peak is observed in Ag@ U-g-C3N4-NS. For an in-depth analysis of the C 1s, N 1s, and Ag 3d peaks, the corresponding high-res-olution XPS spectra of the samples are shown in Figure 2b–f. In Figure 2b,c, the XPS C 1s core-level spectrum of U-g-C3N4-NS can be Gaussian curve-fitted into two peak components corresponding with binding energies of 284.5 and 288.1 eV. These are attributed to CC and CN species, respectively, and the N 1s peaks at 398.6, 399.8, and 401.3 eV corre-spond to CNC, NC3, and NH bonding, respectively.[19] For U-g-C3N4-NS-PDA, new peaks at 285.9 and 287.6 eV in C 1s appeared corresponding to CO and CO bonding (Figure 2e) and a new peak at 400.3 eV in N 1s corresponding

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Figure 1. Stepwise representation of the route to Ag NP–decorated ultrathin g-C3N4 nanosheets (last column), and the corresponding molecular structures and photographs of the related resultants in each step.



to CNH3+ bonding (Figure 2f) indicating the presence of PDA.[20] As shown in Figure 2d, the high-resolution spec-trum of Ag 3d was fitted to two distinct peaks (Ag 3d5/2 and Ag 3d3/2) with a wide gap of 6.0 eV indicating that Ag exists in the Ag0 state (metallic) on the surface of the U-g-C3N4-NS.[21] All these spectra confirm the successful preparation of Ag@ U-g-C3N4-NS.

The TEM and high-resolution TEM (HRTEM) measurements were conducted on the various morphologies of the as-prepared Ag@ U-g-C3N4-NS. Figure 3a–c shows that the formation of highly uniform Ag NPs tightly adhered on the surface of U-g-C3N4-NS due to the reduction ability and metal-binding affinity of the catechol groups in the PDA coating. The red dashed box in Figure 3b show that the single U-g-C3N4-NS layer is extremely thin. The HRTEM image reveals clear lattice fringes with an interplane distance of 0.234 nm corre-sponding to the (111) lattice space of metallic Ag0.[22] These results qualitatively agree with the XPS data (Figure 3e,f). The narrow Energy dispersive X-ray (EDX) element mapping (Figure 3d,g–j) was used to verify the presence and distribu-tion of the main elements of the Ag@ U-g-C3N4-NS. The C and N distributions are uniform and continuous, resembling the morphology of U-g-C3N4-NS. In contrast, the distribution of Ag NPs is discrete, indicating a hierarchical heterostructure with good dispersion on the surface of U-g-C3N4-NS. All of these results confirm the successful synthesis of the Ag@ U-g-C3N4-NS composite photocatalyst.

The X-ray diffraction XRD) patterns of B-g-C3N4, U-g-C3N4-NS, U-g-C3N4-NS-PDA, and Ag@ U-g-C3N4-NS are shown in Figure 4a. For B-g-C3N4, U-g-C3N4-NS, and U-g-C3N4-NS-PDA, the diffraction peak at 27.4° in all samples corresponds to the interlayer stacking of aromatic segments indexed as the (002)

peak of the stacking of the conjugated aromatic system.[23] For Ag@ U-g-C3N4-NS, four additional peaks appeared at 38.1°, 44.2°, 64.4°, and 77.4°, in agreement with the standard diffraction peaks of Ag (JCPDS no. 65-2871), corresponding to the (111), (200), (220), and (311) planes of Ag0, respectively.[22] The results qualitatively agree with the TEM measurements as well. The UV-DRS tests enable the measurement of the optical absorbance property of the samples. As shown in Figure 4b, the B-g-C3N4 has an absorption edge in the visible-light region with a wavelength of about 460 nm. After the PGE treatment, the as-prepared U-g-C3N4-NS clearly displayed an enhanced absorption in the visible and near-infrared regions implying the thickness of the g-C3N4 primarily contributed to the wave bands.[24] However, PDA modification has a slight influence on the optical properties of PDA-g-C3N4. More importantly, Ag@ U-g-C3N4-NS exhibits an much stronger absorption intensity in the range of 200–2000 nm than the others. Additionally, the absorption spectrum of Ag@ U-g-C3N4-NS is mildly redshifted. The enhanced absorption could be attributed to the special construction of the U-g-C3N4-NS, and the local-ized surface plasmon resonance (LSPR) of the Ag NPs, which is essential for photocatalytic activity.[25] The UV-DRS absorp-tion spectra of the as-prepared Ag@ U-g-C3N4-NS with different Ag NP loadings (0.5, 1.0, and 1.5 mg mL−1 of Ag+) are shown in Figure S6a (Supporting Information). It was found that Ag@ U-g-C3N4-NS-1.0 exhibited superior photoabsorption from UV to near-infrared.

To investigate the migration, transfer, and recombination processes of photogenerated electron–hole (e−–h+) pairs in the photocatalysts, photoluminescence (PL) spectra measurements were also conducted (Figure 4c). It was found that the emission peaks of all the samples center around 448 nm with the order of

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Figure 2. a) XPS survey spectra of U-g-C3N4-NS, U-g-C3N4-NS-PDA, and Ag@U-g-C3N4-NS. High-resolution XPS spectra of b) C 1s and c) N 1s for U-g-C3N4-NS, d) Ag 3d for Ag@U-g-C3N4-NS, and e) C 1s and f) N 1s for U-g-C3N4-NS-PDA.



intensity as follows: B-g-C3N4 > B-g-C3N4-PDA > U-g-C3N4-NS > U-g-C3N4-NS-PDA > Ag@ U-g-C3N4-NS. The PDA-modified B-g-C3N4 and U-g-C3N4-NS have a lower emission intensity when compared to bare B-g-C3N4 and U-g-C3N4-NS, both sug-gesting a lower photogenerated carrier recombination. The results can be attributed to PDA with semiquinone and qui-none functional ligands, which act as an electron acceptor. The prepared U-g-C3N4-NS exhibits a lower PL emission intensity with respect to B-g-C3N4, suggesting a lower radiative recombi-nation of photoexcited electrons and holes. Further, the intro-duction of Ag NPs on the surface of the U-g-C3N4-NS leads to the lowest PL emission intensity, confirming the role of Ag in trapping the photogenerated e− and providing more efficient separation of e−–h+ pairs. The PL spectra of Ag@ U-g-C3N4-NS with different loading of amounts of Ag NPs are shown in Figure S6b (Supporting Information). It was found that Ag@ U- g-C3N4-NS-1.0 exhibited the lowest intensity. The excited stated

lifetime of Ag@ U-g-C3N4-NS was shorter than U-g-C3N4-NS and B-g-C3N4 (Figure S7, Supporting Information), suggesting a lower recombination and higher separation efficiency of electron–hole pairs, which lead to weaker photoluminescence and higher photocatalytic activity.

To further support the efficient separation of photogenerated e−–h+ pairs and the electrical conductivity of the samples, EIS and photocurrent response tests were carried out.[26] As shown in Figure 4d, the semicircle on the EIS plots of Ag@ U-g-C3N4-NS was the smallest among all the samples, suggesting the separation and transfer efficiency of photogenerated e−–h+ pairs are greatly increased by the interfacial interaction between U-g-C3N4-NS and Ag NPs. A similar result is found in photo-electric response tests, which are recorded for several on–off cycles of intermittent irradiation with the photocurrent intensity remaining kept steady and reproducible. Both U-g-C3N4-NS and U-g-C3N4-NS-PDA showed an enhanced photocurrent density

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Figure 3. a–c) TEM and e,f) HRTEM images of Ag@U-g-C3N4-NS from different visual angles. d,g–j) EDX mapping on the selected range of the Ag@U-g-C3N4-NS.



as compared to B-g-C3N4. Ag@ U-g-C3N4-NS (1.496 µA cm−2) exhibited an even greater enhancement in photocurrent den-sity ≈3.13 times that of B-g-C3N4 (0.478 µA cm−2; Figure S8a, Supporting Information). The photocurrent generation stems primarily from the separation and diffusion of photogenerated e−–h+ pairs from the inner structure of the photocatalyst to the free charge acceptors on its surface and in the electrolyte. Therefore, the enhanced photocurrent of Ag@ U-g-C3N4-NS can be attributed to the 2D structure of U-g-C3N4-NS, which reduced charge transfer path length and improves charge mobility and transfer. The EIS and photocurrent response tests of Ag@ U-g-C3N4-NS with different loading amounts of Ag NPs are shown in Figure S8b,c (Supporting Information). In order to compare the photoresponse performance of the catalysts, the incident photon-to-electron conversion efficiencies (IPCE) were collected (Figure S9, Supporting Information). Clearly, Ag@ U-g-C3N4-NS possessed higher IPCE values in the visible region with a wider range than original g-C3N4, suggesting the improved utilization of visible light.

The photocatalytic performance of Ag@ U-g-C3N4-NS was evaluated by its photodegradation of MB and phenol under visible-light irradiation. Before all the photocatalytic reac-tion, adsorption–desorption experiments were carried out on photo catalysts. Figure 5a summarizes the degradation rate of MB by the samples under visible-light irradiation. In the pres-ence of B-g-C3N4, a MB degradation of only 3.5% was observed under visible-light irradiation over 40 min. The photocatalytic activity is greatly enhanced for PGE-treated U-g-C3N4-NS with a degradation rate of nearly 39.2% observed. A slight increase was observed after PDA modification. All Ag@ U-g-C3N4-NS

composites exhibited a higher photodegradation rate of MB than that of the bare U-g-C3N4-NS. Among them, the Ag@ U-g-C3N4-NS-1.0 exhibited the highest photocatalytic activity with an MB degradation of 88.5% in 40 min under visible-light irradiation, which is resulted from a larger amount of the exposed active areas and sites. However, increasing the Ag content beyond this optimum concentration decreased the photocatalytic activity. This is explained by the fact that Ag serves as a sink for elec-trons to prevent charge recombination at low loading, but it forms a new charge recombination center and covers g-C3N4 at higher loading. Similarly, the photodegradation activities of the samples for phenol under visible-light irradiation were also conducted, and the time-dependent degradation rates are summarized in Figure 5b. Only 9.8% of phenol can be photo-degraded by B-g-C3N4 in 80 min. For comparison, U-g-C3N4-NS can photodegrade 49% in the same time under same lighting conditions. After loading Ag NPs, Ag@ U-g-C3N4-NS-1.0 exhibited the highest photodegradation activity of phenol with 89% photodegradation in 80 min. It suggests that U-g-C3N4-NS posses higher photocatalytic performance than B-g-C3N4 owing to the more exposed new edges and higher active sites of U-g-C3N4-NS. Additionally, Ag NPs can also greatly enhance the catalytic activities of U-g-C3N4-NS. For practical industrial applications, the recyclability and stability of photocatalysts are another important factors to be considered. As shown in Figure 5c,f, after successive cycles (five times) of photodegrada-tion tests, no obvious decrease in the photocatalytic activity can be seen after being irradiated for 200 min for MB and 400 min for phenol degradation suggesting excellent photocatalytic sta-bility of Ag@ U-g-C3N4-NS. With the photocatalytic degradation

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Figure 4. a) XRD patterns of B-g-C3N4 (1), U-g-C3N4-NS (2), U-g-C3N4-NS-PDA (3), and Ag@U-g-C3N4-NS (4). b) UV-DRS absorption spectra of the as-synthesized Ag@U-g-C3N4-NS and the related products during the formation. c) PL spectra of B-g-C3N4, B-g-C3N4-PDA, U-g-C3N4-NS, U-g-C3N4-NS-PDA, and Ag@U-g-C3N4-NS. d) EIS Nyquist plot of the samples.



continued, more produced O2•− supplements the reactor, which

may adhere on the surface of the catalytic species. As a result, the binding between the organic contaminant macromole-cules and the catalyst becomes stronger, which can inactivate the reaction. Moreover, we compared the photodegradation efficiency of Ag NPs-decorated ultrathin g-C3N4 with that of commercial TiO2 (P25) for MB and phenol under visible-light irradiation. As expected, Ag@ U-g-C3N4-NS exhibited a much better performance than commercial P25 (Figure S10, Sup-porting Information).

To further understand the stability of the samples, XRD pat-terns of the fresh and five-time-cycled Ag@ U-g-C3N4-NS were examined (Figure S11, Supporting Information). No obvious changes occurred on the crystal structures were observed in any samples after five cycles. Moreover, the element content and the valence of Ag@ U-g-C3N4-NS before and after five-time photodegradation of phenol were also tested (Figure S12, Supporting Information). Similarly, all of them well retained, signifying that the Ag@ U-g-C3N4-NS is especially stable during and after the photodegradation reaction. To further uncover the

photocatalytic degradation mechanism of Ag@ U-g-C3N4-NS and reveal the contribution of the primarily active radical spe-cies during the photodegradation reaction, different charge scavengers were introduced into the photodegradation reaction to identify which specific radicals are generated based on the corresponding drop in the degradation efficiency due to the radical quenching. As shown in Figure 5d, the photocatalytic performance of Ag@ U-g-C3N4-NS decreased slightly when tert-butanol (.OH scavenger) was added into the photoreaction system. However, the introduction of K2S2O8 (e− scavenger), ammonium oxalate (h+ scavenger) and 1,4-benzoquinone (O2

•− scavenger) impacted significantly on the degradation rate of MB under visible-light irradiation. A poorer photocatalytic degrada-tion performance suggests a negative impact by the scavenger during the reaction and consequently the specific radical types generated. The results indicate that e−, h+, and O2

•− play the most important role as oxidative species in the photodegrada-tion process. Charge transfer from the conduction band (CB) of g-C3N4 to Ag can retard the recombination of photogenerated electron–hole pairs. Ag NPs in the Ag@ U-g-C3N4-NS catalysts

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Figure 5. Photodegradation efficiency of a) MB and b) phenol under solar irradiation for different times on all samples. Reusability of Ag@U-g-C3N4-NS for the photocatalytic degradation of c) MB and f) phenol under solar irradiation with five times of cycling uses. d) Influence of different scavengers during the photocatalytic degradation of MB in the presence of Ag@U-g-C3N4-NS under visible-light irradiation. e) Light-driven H2O2 production over different catalysts in 70 min. f) Formation rate constants (Kf, red symbol) and decomposition rate constants (Kd, blue symbol) for H2O2. g) H2O2 production is accomplished by photocatalysis. h) The photocatalytic decomposition of H2O2 (C0 = 3 × 10−3 m) over the samples. Reaction conditions: H2O (100 mL), catalyst (0.10 g, 1 g L−1), O2-equilibrated, 100 mW cm−2, pH = 3, 25 °C.



can scavenge the photogenerated electrons instantly to form a Schottky barrier, thus reducing the recombination of photo-generated electron–hole pairs and improving the availability of holes. Meanwhile, as the photogenerated electrons transfer to the surface of metallic Ag, the Fermi level of Ag shifts from 0.4 eV versus normal hydrogen electrode (NHE) to a more negative potential than the standard redox potential of O2/O2

•− (−0.046 eV vs NHE) due to the matched energy levels of Ag and g-C3N4 (Figure S13, Supporting Information).

The photocatalytic generation of H2O2 was conducted in an O2-equilibrated acidic aqueous solution (pH = 3) under sunlight irradiation (a Xe lamp with 100 mW cm−2 power density). Time-dependent light-driven H2O2 production with different photocatalysts is shown in Figure 5g. It was found that U-g-C3N4-NS produced H2O2 (0.414 × 10−6 m min−1) more effectively than B-g-C3N4 (<0.03 × 10−6 m min−1) with the rate of H2O2 production significantly enhanced after PGE treat-ment. Ag@ U-g-C3N4-NS efficiently produced H2O2 with an ini-tial (the first 10 min under the visible light) production rate of more than 0.75 × 10−6 m min−1, a value significantly higher than pure B-g-C3N4. The Ag@ U-g-C3N4-NS-1.0 photocatalyst showed the best photocatalytic activity with a H2O2 production rate of 1.975 × 10−6 m min−1 under light irradiation. This enhancement in photocatalytic performance is likely attributed to the higher specific surface area of U-g-C3N4-NS providing an increased number of photocatalytic sites and the optimal loading of Ag NPs restricting the recombination of e−–h+ pairs. The control experiments are shown in Figure S14 (Supporting Informa-tion). No H2O2 can be detected when the oxygen environment is substituted with argon gas supporting the fact that oxygen is indispensable for the photocatalytic production of H2O2. Furthermore, in the absence of either photocatalyst or visible light, H2O2 could also not be detected supporting the impor-tance of a photocatalyst in H2O2 production (Figure 5e). Light-driven H2O2 production also depends on the pH of the solution because the formation of H2O2 is a proton-coupled electron transfer process.[27] The reaction pH was also varied (pH = 1, 3, and 5) in an aqueous solution of HClO4 in the presence of Ag@ U-g-C3N4-NS-1.0 photocatalyst (Figure S15, Supporting Information). When the pH = 1, only 10.2 × 10−6 m of H2O2 was produced in 80 min in the presence of the photocatalyst. When the pH increased to 3, roughly seven times the amount of H2O2 is produced over 80 min. A further increase to pH = 5 decreased amount of H2O2 production suggesting that a higher pH leads to faster H2O2 decomposition. We also drew a “vol-canic curve” to reflect the influence of the pH of the solution on the H2O2 production (Figure S16, Supporting Information). Obviously, the amount of produced H2O2 reached the max-imum at pH = 3. With the increase of pH, light-mediated deg-radation of H2O2 outpaces the photocatalytic production in the presence of a less proton-rich environment, leading to a lower H2O2 production. When the pH is below 3, the excess pro-tons may induce the gradual oxidization of the produced H2O2 into H2O.

The gradual reduction in the rate of light-driven produc-tion of H2O2 with time (Figure 5g) suggests that the pho-tocatalytic production of H2O2 is hindered by its decom-position rate at higher concentrations of H2O2. The in situ decomposition of H2O2 by all samples should also be taken

into consideration during the overall light-driven production of H2O2. The produced H2O2 can be degraded by its reaction with both the CB electron and the valence band (VB) hole as soon as it is produced necessitating the need to remove produced H2O2 from the reaction area (Equations (1)–(4))[28]

+ + = + ⋅+ −H O H e H O OH2 2 2 (1)

+ + =+ −H O 2H 2e 2H O2 2 2 (2)

+ = + ⋅+ +H O h H HO2 2 2 (3)

+ = ++ +H O 2h O 2H2 2 2 (4)

Therefore, the decomposition behavior of H2O2 in the pres-ence of the photocatalysts was also investigated under the same conditions with an original H2O2 concentration of 3 × 10−3 m. As shown in Figure 5h, there is a 13% H2O2 decomposition over B-g-C3N4 after 60 min, a value lower than that over U-g-C3N4-NS (31%), Ag@ U-g-C3N4-NS-0.5 (37%), Ag@ U-g-C3N4-NS-1.0 (42%), and Ag@ U-g-C3N4-NS-1.5 (39%). In the absence of any catalysts, the decomposition of H2O2 was less than 5% after 60 min of illumination.

The formation and decomposition of H2O2 on all photo-catalysts proceed through two competitive pathways owing to the thermodynamic instability of H2O2 at room temperature. To further understand the mechanism of H2O2 generation, we separately evaluated the rate constants for H2O2 formation (Kf; (× 10−6 m) min−1) and decomposition (Kd; min−1) by assuming zero-order and first-order kinetics, respectively[29]

{ }[ ] )(= × − − ×H O / 1 exp2 2 f d dK K K t (5)

Values of Kf and Kd were estimated by fitting the data in Figure 5g to Equation (5) and the results are presented in Figure 5i. Samples 1–6 are represented by B-g-C3N4, U-g-C3N4-NS, U-g-C3N4-NS-PDA, Ag@ U-g-C3N4-NS-0.5, Ag@ U-g-C3N4-NS-1.0, and Ag@ U-g-C3N4-NS-1.5, respectively. After PGE treatment, the Kf value for g-C3N4 increases from 0.086 × 10−6 to 0.548 × 10−6 m min−1 and the Kd value increases from 0.0065 to 0.0079 min−1. The results indicate that the increased specific surface area of g-C3N4 can enhance H2O2 formation and sta-bilize the formed H2O2 under light irradiation. The Kf value continuously increased after Ag NP loading reaching a max-imum for Ag@ U-g-C3N4-NS-1.0 (Kf = 1.975 × 10−6 m min−1) representing a 3.6-fold enhancement relative to U-g-C3N4-NS (Kf = 0.548 × 10−6 m min−1) while the Kd value also increased but less significantly with only a 45.6% enhancement relative to U-g-C3N4-NS (from 0.0079 to 0.0115 min−1). This result indi-cates that the Ag NPs preferentially photocatalyze the forma-tion of H2O2 over its decomposition.

To study the stability of the Ag@ U-g-C3N4-NS-1.0 photo-catalyst for the light-driven production of H2O2, the photocata-lyst was separated from the solution after use and washed and dried. Subsequently, the photocatalyst was reused in a new system for monitoring the generation of H2O2 under the same conditions. As shown in Figure S17 (Supporting Information), the light-driven H2O2 production photocatalyst performance

Adv. Mater. 2019, 31, 1806314



of the recycled Ag@ U-g-C3N4-NS-1.0 demonstrated a slight degradation contribution (<5%) when compared with the fresh sample after five cycles. The nontrivial repeatability of the photocatalytic activity provides strong evidence of the stability.

In summary, a facile and green strategy for the production of Ag@ U-g-C3N4-NS via PGE treatment of B-g-C3N4 and the uniform growth of Ag NPs via a mussel-inspired dopamine polymerization modification on the surface of single U-g-C3N4-NS layers is discussed. The Ag@ U-g-C3N4-NS photocatalyst exhibits enhanced electrochemical properties and excellent per-formance as a visible-light photocatalyst for the degradation of MB and phenol. In addition, it offers an efficient, economic, and green route for the light-driven production of H2O2 from water and oxygen in the absence of organic charge scavengers. The improved photocatalytic performance can be attributed to the large specific surface area, more exposed new edges, and active sites of the 2D U-g-C3N4-NS, the SPR effect of the Ag NPs combined with the effective separation of photogenerated carriers and the reduced pathlength of the electrons both within and on the surface of Ag@ U-g-C3N4-NS composites.

Experimental SectionPreparation of Bulk g-C3N4 (B-g-C3N4): B-g-C3N4 was synthesized by

direct thermal polymerization of urea in a muffle furnace.[30] Typically, 10 g of urea powder was transferred into an alumina crucible with a cover and heated from ambient to 550 °C at a rate of 1 °C min−1 under ambient atmosphere and maintained for 2 h. The obtained yellow B-g-C3N4 product (around 0.9 g) was naturally cooled to ambient and ground into powder, washed with ultrapure water several times, collected by filtration and finally dried at 60 °C for 6 h.

Synthesis of Single Layer Ultrathin g-C3N4 Nanosheets (U-g-C3N4-NS): The single layer U-g-C3N4-NS were prepared as follows. First, 1.5 g of the as-prepared B-g-C3N4 was uniformly spread in an alumina crucible to ensure sufficient contact with the atmosphere, and the samples were then heated to and maintained at 550 °C with a heating rate of 2 °C min−1 for different periods of time ranging from 1 to 3 h. The yellow B-g-C3N4 powder was exfoliated gradually yielding a white product of U-g-C3N4-NS layers. Second, for fewer layered U-g-C3N4-NS, the naturally cooled samples were collected and dispersed into 500 mL of ultrapure water and further dispersed by sequential ultrasonication for 8 h. Finally, the samples were washed with ultrapure water several times, collected by filtration, and dried at 60 °C for 6 h.

Preparation of Polydopamine-Modified Single Ultrathin g-C3N4 Nanosheets (U-g-C3N4-NS-PDA): U-g-C3N4-NS-PDA was obtained by the addition of a solution of oxytyramine hydrochloride (98%) to an aqueous dispersion of single layer U-g-C3N4-NS at 80 °C for 24 h. Briefly, 200 mg of single layer U-g-C3N4-NS was added to 320 mL of water, and the suspension was dispersed by sequential ultrasonication for 30 min. Next, 100 mg of dopamine hydrochloride was added and the mixture was stirred at room temperature for 1 h. Next, 80 mL of 50 × 10−3 m Tris-HCl solution (pH = 8.5) was added under vigorous stirring at 80 °C for 24 h. The color of the solution changed from faint yellow to dark black. The solution was cooled to room temperature and filtered with a 0.45 µm membrane filter to obtain U-g-C3N4-NS-PDA samples. The black solid was washed, redispersed, and dialyzed (molecular weight cutoff: 7000 g mol−1) against water for 3 days to remove excess dopamine. The black powders were recovered by filtration and dried at 60 °C for 6 h.

Decoration of Ag NPs on the Surface of Layer U-g-C3N4-NS-PDA (Ag@ U-g-C3N4-NS): Ag NPs were anchored onto the surface of the PDA-modified single layer U-g-C3N4-NS by an in situ reduction in the presence of numerous functional groups (OH and NH2) present in the surface PDA coating.[31] In detail, 0.5 mL of single layer U-g-C3N4-NS-PDA

(3 mg mL−1) solution and 1.25 mL of AgNO3 (10, 20 and 30 mg mL−1) solution were mixed and diluted to 25 mL with water and stirred for 30 min at room temperature. After reaction, the resulting Ag@ U-g-C3N4-NS samples were separated from the suspension by centrifugation (3500 × g, 10 min) with water several times, collected by filtration, and finally dried at 60 °C for 6 h. The straightforward procedure for the production of Ag@ U-g-C3N4-NS is summarized in Figure 1.

Characterization of the Ag@ U-g-C3N4-NS: The structure and morphology of all samples were investigated by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) at 3.0 keV. Before each measurement, samples were prepared by dropping casting and drying on silicon wafers. The microstructure, composition, and presence of Ag were further confirmed by using TEM (FEI Tecnai G-20 operated at 200 keV) and HRTEM. EDX spectrometry was employed for elemental analysis. XPS was performed with a Kratos Axis-Ultra HAS instrument equipped with a standard and monochromatic source (Al Kα, 1486.6 eV) and the binding energies were normalized to the signal for C 1s at 284.5 eV. The crystal phases of all samples were identified using XRD with Cu Kɑ radiation (Philips, X’pert-Pro MRD). UV-DRS was recorded in the range of 200–2000 nm at room temperature using a UV-3600. PL spectra of the samples were measured using a FLSP920 Edinburgh Fluorescence spectrometer.

Photo-Electrochemical Testing: The electrochemical performance of different samples during the formation of Ag@ U-g-C3N4-NS was investigated by photocurrent response and EIS with a standard three-electrode system utilizing a PGSTAT302N electrochemical workstation (AUTOLAB, Switzerland). The Ag/AgCl electrode and Pt wire were used as the reference and counter electrodes, respectively. The working electrode was fabricated by immobilizing samples on conductive fluorine-doped tin oxide (FTO) glass by a casting method. Briefly, 10 mg of sample was dispersed in a solution (800 mL of ethanol combined with 200 mL of 0.5% Nafion), and then sonicated for 30 min. The resultant suspension was then cast dropwise onto the FTO surface. The FTO electrodes immobilized with the samples were connected to a copper tape and used as the working electrode. Na2SO4 solution (0.1 m) was used as the electrolyte (pH = 7.0). The working electrodes were irradiated by a GY-10 Xenon lamp with an irradiation separation distance of 15 cm. The focused incident light intensity on the flask was ≈100 mW cm−2. EIS measurements were performed in both dark and illuminated states at open-circuit voltages over a frequency range from 105 to 10−1 Hz with a potential amplitude of 5 mV to characterize the interfacial properties of the electrodes.

Photodegradation of Aqueous Organic Contaminants: The photodegradation performance of samples was investigated by evaluating the decomposition of MB and phenol with an initial concentration of 10 mg L−1 (pH = 8.5) using a PS-GHX photochemical reactor. A 300 W xenon lamp was utilized as the simulated visible-light resource with a separation distance of 60 mm to the quartz reaction tube. The specific photodegradation test proceeded by the following steps. A 50 mL aqueous solution with 10 mg mL−1 initial concentration of MB and phenol was stirred in the dark in presence of the desired photocatalyst at a concentration of 0.5 g L−1 for 30 min to reach the necessary adsorption–desorption equilibrium. At each photoirradiation time interval (8 and 20 min for MB and phenol, respectively), 3 mL of the suspension was collected and centrifuged to separate the photocatalysts and the clear supernatant liquid. Finally, the concentration of MB or phenol in the supernatant was analyzed via a UV–vis spectrophotometer (Hitachi, UV-1080, Japan).

Photocatalytic H2O2 Production: The photocatalytic activity of all the samples was evaluated by the activation of molecular oxygen under a 300 W xenon lamp at 100 mW cm−2 under magnetic stirring. During each photocatalytic test, 0.10 g of Ag@ U-g-C3N4-NS was dispersed in 100 mL of distilled water in a container (1 g L−1 Ag@ U-g-C3N4-NS) by ultrasonication for 10 min. The pH value of the suspension was adjusted to 3 by the addition of HClO4 solution (1 m). The bottle was sealed with a rubber septum cap with a gas inlet and outlet device. The solution was first purged by oxygen bubbling while stirring in the dark for 60 min to ensure the adsorption–desorption equilibrium

Adv. Mater. 2019, 31, 1806314



among the catalyst, dissolved oxygen, and water before light irradiation. During the light-driven reaction, 4.0 mL of the suspension was taken from the reaction cell at specific time intervals, and centrifuged to remove the catalyst. The amount of generated H2O2 was determined by redox titration with KMnO4. After reaction completion at 70 min, the photocatalysts were recovered by centrifugation, washed thoroughly with water, and dried under N2 flow. To investigate the decomposition behavior of H2O2 over the photocatalysts, 1 g L−1 of the sample was dispersed in H2O2 solution (initial concentration: 2 × 10−3 m, pH = 3) and irradiated for 60 min under continuous stirring.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsJ.S.C. and J.Y.H. contributed equally to this work. The authors thank the National Natural Science Foundation of China (21501127, 51502185). The authors also acknowledge the funds from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Project for Jiangsu Scientific and Technological Innovation Team (2013).

Conflict of InterestThe authors declare no conflict of interest.

KeywordsAg nanoparticle, graphitic carbon nitride, hydrogen peroxide production, photocatalysis, polydopamine

Received: September 29, 2018Revised: January 13, 2019

Published online: January 30, 2019

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