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Intermediate-mediated strategy to horn-like hollow mesoporous ultrathin g-C3N4 tube with spatial anisotropic charge separation for superiorphotocatalytic H2 evolution

Chengyin Liua, Hongwei Huanga,⁎, Liqun Yeb, Shixin Yua, Na Tiana, Xin Duc, Tierui Zhangd,Yihe Zhanga,⁎

a Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, School of Materials Science and Technology, China University of Geosciences,Beijing 100083, Chinab Key Laboratory of Ecological Security for Water Source Region of Mid-line Project of South-to-North Water Diversion of Henan Province, College of Chemistry andPharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, Chinac Research Center for Bioengineering and Sensing Technology, Department of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing100083, Chinad Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190,China

A R T I C L E I N F O

Keywords:Graphitic carbon nitrideHollow mesoporous ultrathin tubesHydrogen productionCO2 reductionPhotocatalysis

A B S T R A C T

Metal-free graphitic carbon nitride (g-C3N4) has triggered huge interests for converting solar energy into fuels.However, direct-calcination derived bulk g-C3N4 always suffers from low surface area and high recombination ofcharge carriers, prompting attempts to foster g-C3N4 nano/microstructures to achieve high performance.Conventional routes, like templating method, always yields g-C3N4 with tedious morphology and requires post-treatment. Here we release the first report on development of horn-like hollow mesoporous ultrathin (HHMU) g-C3N4 tubes via first forming a horn-like Br-containing intermediate followed by further decomposition trans-formation under co-pyrolysis of melamine and substantial NH4Br. The multiple-superiorities achieved here(hollow/mesoporous/ultrathin/horn-like) allows g-C3N4 high surface area, drastically boosted bulk charge se-paration, carrier density and surface charge transfer efficiency. This advanced g-C3N4 thus casts outstandingphotocatalytic performance for H2 evolution with an apparent quantum efficiency (AQE) of 14.3% at420± 15 nm, far exceeding most of reported g-C3N4. HHMU g-C3N4 also delivers a strengthened photocatalyticCO2 reduction activity into CO and CH4. Selective photo-deposition results provide an in-depth insight intocharge movement behavior and high photo-reactivity that the photo-generated electrons migrate to the outershell and holes prefer to transfer onto the inner shell of HHMU g-C3N4 tubes, thus achieving efficient spatialanisotropic charge separation. The current study may furnish a reference towards developing efficient tactics forintegrally advancing g-C3N4 for renewable energy generation, and disclose a new perspective into promotingcharge separation via microstructure design.

1. Introduction

Solar energy not only directly provides light for illumination, butalso prominently transfers to chemical energy via natural photosynth-esis [1–3]. Largely inspired by the wisdom of nature, many semi-conductor materials have been found as ideal photocatalysts, which candirectly harvest solar energy for chemical energy transformation, e.g.H2 evolution or CO2 reduction into renewable fuels [4,5].

Graphitic carbon nitride (g-C3N4), as a potent metal-free and visible-light active semiconductor material, spares no effort to exhibit its

charming characteristics, involving facile synthesis, thermal stability,special electronic structure, and abundant resources on earth [6–10].Nonetheless, it also suffers from the problems of low surface area [11]and high recombination of charge carriers [12]. For the sake ofachieving satisfactory photocatalysis of g-C3N4, copious efforts forfabrication of advanced nano/microstructures have been made, cov-ering nanosheets [13,14], hollow nanospheres [15], nano/micro-tubes[16], and porous structure [17]. The above distinctive structures giverise to promoted photoinduced charge separation and more reactivesites for efficient photocatalysis and unusual physicochemical

http://dx.doi.org/10.1016/j.nanoen.2017.10.031Received 6 June 2017; Received in revised form 29 August 2017; Accepted 12 October 2017

⁎ Corresponding authors.E-mail addresses: hhw@cugb.edu.cn (H. Huang), zyh@cugb.edu.cn (Y. Zhang).

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Available online 14 October 20172211-2855/ © 2017 Elsevier Ltd. All rights reserved.

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properties. Howbeit, the reported g-C3N4 always presents tediousmorphology, either thin nanosheet, or hollow structure, or porousstructure. For instance, supramolecular self-assembly or protonation ofprecursors are demonstrated to afford hollow tubes, whereas incapableof creating pores [18,19]. On the other hand, meso/macroporous g-C3N4 reported so far are usually realized by hard or soft template-as-sisted or liquid-phase means [20–23], but without formation of hollowor lamella structures. Besides, templating-method always needs ha-zardous hydrogen fluoride to remove or leaves carbon residue, resultingin environmental injuries or counting against photoactivity. Therefore,construction of advanced g-C3N4 structures comprising all the abovemerits (hollow/mesoporous/ultrathin) with a facile, gentle and scalableapproach for superior photocatalytic performance is thus a charmingsubject and meanwhile challenges researchers.

Recently, ammonium halides serving as gas templates show hugepotentials to yield g-C3N4 products with different morphologies, such asmicroporous g-C3N4 (by thermal polymerization of 2 g dicyandiamideand 1 g NH4I) [24], g-C3N4 thin nanosheets (by thermal polymerizationof 10 g urea and 0.1 g NH4Br) [25] and mesoporous g-C3N4 nanosheets(by thermal polymerization of 4 g melamine and 51 g NH4Cl) [26]. It isimportant to note that the different synthetic conditions (e.g. differentprecursors and reactants ratio) all have significant impact on the mi-crostructure of final g-C3N4 products. Thus, it is very attractive to ex-plore novel g-C3N4 structures by manipulating the ammonium halidesin the thermal polymerization process in a controllable way.

Herein, we develop an unprecedented horn-like hollow mesoporousultrathin (HHMU) g-C3N4 tube via a one-pot co-pyrolysis route utilizingmelamine and substantial ammonium bromide (NH4Br) as precursors.To unearth the hollow tube formation mechanism, in-depth explora-tions on different synthetic parameters/environments (e.g. reactantsratio, heating rate, and substitution of NH4Br with other ammoniumsalts, like NH4Cl and NH4HCO3) in thermal polymerization process areconducted. It is uncovered that melamine reacting with ammoniumbromide gives birth to a horn-like Br-containing intermediate at first,then forming HHMU g-C3N4 tube after further pyrolysis. The HHMUstructure endows g-C3N4 with profoundly enhanced photocatalyticperformance towards H2 evolution from water and CO2 reductioncompared to bulk g-C3N4, with a high H2 evolution apparent quantumefficiency (AQE) of 14.3% at (420±15) nm. Systematic surveys oncharge movement behaviors disclose that the efficient spatial aniso-tropic charge separation occurs between the outer and inner shells ofHHMU g-C3N4 tubes, accounting for the huge enhancement in photo-catalytic activity. This work may further our understanding for fabri-cation of robust g-C3N4-based photocatalysts with advanced nanos-tructure.

2. Experimental details

2.1. Synthesis

The two raw materials of ammonium bromide and melamine wereof AR grade and without other processing. The modified CN-Br andpristine g-C3N4 were synthesized by a one-step pyrolysis treatment ofmelamine (1 g) and a certain amount of ammonium bromide. The twomaterials were mixed and ground thoroughly in an agate mortar andthen placed in crucibles, which were put into furnace and heated at500 °C with the rate set as 1.7 °C per minute. When the temperaturecooled down to room, the yellow powder was the purpose sample. Thesamples with molar ratios of ammonium bromide to melamine of 10,15, 30 and 50 were signed as CN-Br-1, CN-Br-2, CN-Br-3, and CN-Br-4,respectively. For comparison purpose, mesoporous g-C3N4 nanosheetsor spongy-like C3N4 are also prepared by thermal polymerization ofammonium chloride/ammonium bicarbonate and melamine with amolar ratio of 30:1 according to the reference [26]. To study the in-fluence of heating rate on the morphology of g-C3N4, the samples werealso prepared by pyrolysis of ammonium bromide and melamine (30:1)

with heating rates of 1.2 and 2.7 °C per minute.

2.2. Characterization

The phase structures of g-C3N4, CN-Br-1, CN-Br-2, CN-Br-3, and CN-Br-4 were determined by X-ray diffraction (XRD) on Bruker with Cu Kαradiation (40 kV/40 mA). The Fourier-transform infrared (FTIR)spectra were recorded by a Bruker spectrometer in the frequency rangeof 4000–450 cm−1. The scanning electron microscopy (SEM) (S-4800)was measured to study the morphology, and the microstructures wereinvestigated by the transmission electron microscopy (TEM) (JEM-2100JEOL, Japan). UV–vis diffuse reflectance spectra (DRS) were recordedby UV–vis spectrophotometer (Varian Cary 5000). The specific surfaceareas of samples were measured by Brunauer–Emmett–Teller (BET)nitrogen adsorption-desorption (USA, Micromeritics ASAP 2460).Thermogravimetric (TG) analysis is performed on a Labsys TGDTA16(SETARAM) thermal analyzer. The photoluminescence (PL) spectrawere measured by a Hitachi F-4600 fluorescence spectrophotometer.The surface properties were surveyed by X-ray photoelectron spectro-scopy (XPS), with 150 W Al Kα X-ray irradiation (Thermo ESCALAB250, USA). The surface photovoltage (SPV) spectra were recorded witha home-built apparatus, which combined with a lock-in amplifier(SR830) and a light chopper (SR540). The fluorescence microscopicimage was recorded by Nikon instruments N-SIM E super-resolutionmicroscope, and the AFM image was measured by Bruker dimensionicon atomic force microscopy. Organic elemental analysis (OEA) ismeasured by Elementar Analysensysteme GmbH vario EL instrument.

2.3. Photocatalytic H2 evolution

Water splitting for H2 evolution experiment was conducted in aPerfect Light Labsolar-Ⅲ (AG) photoreactor (Pyrex glass) connectedwith a closed-cycle gas circulation system. In a typical experiment,50 mg photocatalyst was dispersed in 100 ml solution containing 10 mllactic acid and 1 wt% Pt cocatalyst. The mixed solution was bubbledwith N2 for 30 min to ensure anaerobic condition and illuminated30 min with ultraviolet light before visible light irradiation to measureH2 evolution. The incident light wavelength was provided by a 300 WXe lamp with a 420 nm filter, and the reaction condition was kept atroom temperature. The generated gas was analyzed by Labsolar-Ⅲ (AG)gas chromatography furnished with thermal conductivity detector, andthe high-purity argon was carrier gas.

The apparent quantum efficiency (AQE) for H2 production wasmeasured by a series of band-pass filters (420, 450, 500, 550 and600 nm). The light intensities were measured by a PLS-MV2000 pho-toradiometer, and the values are 5.1, 6.2, 6.1, 5.6, 5.8 mW cm−2 for theXe lamp with 420, 450, 500, 550 and 600 nm filters, respectively. Theirradiated surface area was about 33 cm2, and the peak width of all theband-pass filter is 15 nm. The equation of AQE was calculated by thefollowing:

AQY (%) = number of evolved H2 molecules × 2 × 100/number ofincident photons

2.4. Photocatalytic CO2 reduction

Photoreduction activity for CO2 conversion was conducted by aclosed PLS-SXE300 Labsolar-IIIAG gas system and the total volume ofreaction is 350 ml. Firstly, a certain quality of NaHCO3 was added and50 mg photocatalyst was uniformly dispersed on the watch glass.Secondly, H2SO4 (4 M, 5 ml) solution was injected into the vacuum-treated gas system to react with NaHCO3 and 1 atm CO2 gas was ob-tained. The light source was supplied by a 300 W high pressure Xe lamp(PLS-SXE300, China) and temperature of the system was maintained ataround 20 °C (DC-0506, China). 1 ml gas was sampled and analyzed bya GC9790II gas chromatograph.

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2.5. Photo-depositions of Pt and MnOx on HHMU g-C3N4 tubes

H2PtCl6 and MnCl2 were used as metal precursors of Pt and MnOx inthe photo-deposition experiments, respectively. 50 mg of HHMU g-C3N4

photocatalyst was dispersed in 100 ml deionized water containing10 wt% of H2PtCl6 or MnCl2. Then, the suspension was illuminated byUV light (a 500 W Hg lamp) for 0.5 h under continuous stirring. Afterthe in-situ photo-deposition, the resulting precipitates were collected,and washed with deionized water and ethanol thoroughly, and dried at80 °C.

2.6. Photoelectrochemical measurements

The Photoelectrochemical measurements were analyzed by anelectrochemical workstation using a three-electrode system. The pre-pared electrode coated on IFO film was located in 0.1 mol L−1 Na2SO4

aqueous electrolyte as working electrode. Platinum wire was used asthe counter electrode and saturated calomel electrode (SCE) as the re-ference electrode. The transient photocurrent was measured using a30 s on-off light cycle at a bias voltage of 0 V. Methylviologendichloride (MVCl2, 1 mM) as a fast electron scavenger was added intothe electrolyte to survey the surface charge transfer efficiency.Electrochemical impedance spectra were collected in the frequencyrange from 1 to 105 Hz with a 5 mV sinusoidal AC voltage. The Mott-Schottky plots were measured at the frequency of 100 Hz to determineband gap level. The visible light source came from a 300 W Xe lampwith an ultraviolet filter (λ>420 nm).

3. Results and discussions

Horn-like hollow mesoporous ultrathin (HHMU) g-C3N4 tubes areobtained by co-pyrolysis of melamine and NH4Br with a molar ratio of1:30, and the schematic synthesis process is illustrated in Scheme 1 andFig. S1. Here, NH4Br plays a critical role in formation of this uniquestructure. Bulk g-C3N4 synthesized from direct calcination of melamineshows a chunk morphology (Fig. 1A and B). In contrast, the introduc-tion of substantial NH4Br as a co-precursor gives rise to the very distinctmicrostructure. The as-prepared g-C3N4 with melamine/NH4Br ratio of1:30, namely CN-Br-3, presents well-developed horn-like hollow tub-ular structure (Fig. 1D and E). From the magnified TEM image of anisolated hollow tube (Fig. 1F), one can see plenty of mesoporous on thelamellar shell, which are confirmed by the BET pore-distribution curve(Fig. 1I). Fig. 1G shows the fluorescence microscopy image of HHMU g-C3N4, which clearly exhibited strong fluorescence signal stemming fromelectronic transition. It confirms the formation of uniform horn-likehollow g-C3N4 tubes. Atomic force microscopy (AFM) image (Fig. 1H)of an exfoliated sheet indicates a thickness of ~ 3.5 nm for the shell ofg-C3N4 tube, which comprises ~ 8 atomic layers, revealing the ultra-thin tubular shell. To investigate the influence of NH4Br content onmorphology, a series of g-C3N4 products with different molar ratios ofNH4Br to melamine were analyzed. When the molar ratio of melamineto NH4Br is 1/10 or 1/15, the as-synthesized CN-Br-1 or CN-Br-2

samples contain some hollow tubes. But they are not uniform and in-complete (Fig. S2a and b). With increasing the NH4Br ratio to 50 times(CN-Br-4), the tubular g-C3N4 tends to agglomerate more (Fig. S2c), andimportantly most of the hollow tubes turn to solid rods. As the mor-phology of samples undergoes great change, N2 adsorption/desorptionisotherms (Fig. 1I) are recorded. These curves are between type II andIV together with an obvious hysteresis loops belonging to porous ma-terials [27–29]. The specific surface area of CN-Br-3 is 58.2 m2 g−1,which is about 4 times that of bulk g-C3N4 (14.0 m2 g−1) (Table S1).Due to containing some solid tubes, CN-Br-4 has a lower BET surfacearea (45.1 m2 g−1) than CN-Br-3, in agreement with the morphologychange according to SEM and TEM.

For the sake of uncovering the crucial action of NH4Br in thesynthesis process, a series of experiments have been made. Firstly,morphology of melamine and NH4Br was studied (Fig. S3). Obviously,both are irregular particles with size of dozens of microns, and notubular structure appears, indicating that formation of HHMU g-C3N4 isnot resulted from templating action of melamine or NH4Br. Then, westart to focus on the intermediate calcination process of CN-Br-3 toexplore the formation mechanism of HHMU tubes. The explored tem-perature range was set from 200 °C to 450 °C at a heating rate of 1.7 °Cper min. When the temperature reaches 300 °C, the product was foundconsisting of chunk core covered by plenty of particles (Fig. 2A). EDXresults demonstrate that C, N and Br are detected on the outer particles,while the chunk core mainly contains Br. Based on the size of rawmaterials (Fig. S3), it is speculated that NH4Br is covered by melamine.With the temperature rising to 400 °C, great change occurs in mor-phology, and the products all change to horn-like samples (Fig. 2B). Ithas homogeneous distribution of C, N and Br elements on the surface.Importantly, XRD patterns (Fig. 2C) and FTIR spectra (Fig. 2D) ofmelamine, NH4Br and CN-Br-3 synthesized at different temperaturesobviously reflect that the chemical reaction has not occurred before300 °C, and a new phase (Br-containing intermediate) appear at 350 °C.This as-formed intermediate decomposes until 500 °C and then trans-forms to HHMU g-C3N4. For further confirmation, we compared theXRD patterns of HHMU g-C3N4 and bulk g-C3N4 synthesized at 400 °C,450 °C and 500 °C [6] (Fig. 2E). It is important to note that the XRDpatterns of intermediates generated in the HHMU g-C3N4 formationprocess is very different from that of the intermediates produced duringthe formation of bulk g-C3N4 (direct calcination of melamine). Based onthe above experimental results, we can conclude that NH4Br partici-pates in the polymerization process of melamine and forms a horn-likeBr-containing intermediate.

TG curves of melamine, NH4Br and melamine/NH4Br mixture (1:30in molar ratio) further verify the above conclusion and provide moredetailed information for formation of the Br-containing intermediate(Fig. 2F, G and H). The thermal polymerization temperature of mela-mine is about 340 °C and NH4Br completely decomposes at 360 °C.However, a very distinct TG curve was observed for the melamine/NH4Br mixture, in which a new decomposition temperature at 325 °Cappears with a gradual weight loss until 500 °C. It reveals that a newintermediate was formed at 325 °C. Considering that CN-Br-3 synthe-sized at 400 °C contains Br and the temperature is higher than thecomplete decomposition temperature of NH4Br (360 °C), the specifictemperature of 325 °C can be regarded as the formation temperature ofthe Br-containing intermediate. Thus, the synthesis mechanism can bespeculated as follows. Melamine first reacts with substantial NH4Br toproduce a horn-like Br-containing intermediate at 325 °C, and thenform HHMU g-C3N4 tube with the release of Br and other gases. Heremoderate evolved gas in the pyrolysis reaction is the main reason forformation of the well-developed hollow and mesoporous structure (CN-Br-3), and excessive gas would cause collapse of horn-like hollowstructure, leading to generation of solid rod, as observed for CN-Br-4. Inaddition, the heating rate was also taken into account. HHMU g-C3N4

was obtained at a heating rate of 1.7 °C/min, and the higher or lowerheating rates (2.7 °C/min and 1.2 °C/min) all bring about fragmentary

Scheme 1. Schematic illustration for formation of horn-like hollow mesoporous g-C3N4

tubes (CN-Br-3).

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tubular structure (Fig. S4), which indicates that the heating rate is alsoa vital factor for generating this unique structure. Furthermore, poly-merization of melamine with two other ammonium salts NH4Cl andNH4HCO3 at identical preparation conditions (molar ratio of melamineto ammonium salts 1:30 and heating rate of 1.7 °C/min) was compared.For the NH4Cl case, sheet-like g-C3N4 products were obtained, andspongy g-C3N4 was formed for the case of NH4HCO3 (Fig. S5). Theseresults manifest that introduction of NH4Br is vital for formation ofhorn-like tubular g-C3N4 and appropriate synthetic parameters give riseto the well-developed hollow and mesoporous structure.

The crystalline structure of as-obtained g-C3N4 was then char-acterized. All the XRD peaks (Fig. 3A) are well indexed into the graphitephase carbon nitride (JCPDS No. 87-1526), and the two characteristicdiffraction peaks at 13.2° and 27.6° correspond to the (100) and (002)

peak, suggesting that the addition of substantial NH4Br does not alter g-C3N4 crystalline phase [30]. But it is worth noting that the relativeintensity of the two typical peaks, namely (100) and (002) peaks se-parately corresponding to the repeating of inplanar units and stackingof conjugated triazine plane, undergo large change (Fig. 3B). The in-tensity of (100) peak first increase and then decrease, and (002) peak isjust the opposite. This interesting phenomenon indicates that stackingdegree of triazine ring along z direction first decreases and then in-creases, fitting the SEM findings from chunk particle to hollow tube andthen to solid rod. The C/N ratios of bulk g-C3N4 and HHMU g-C3N4 havebeen measured by organic elemental analysis (OEA), and the results arelisted in Table S2. The C/N molar ratio of HHMU g-C3N4 is 0.59, whichis larger than that of pristine g-C3N4 (0.55) and closer to the theoreticalvalue (0.75), indicating a higher polymerization degree of HHMU g-

Fig. 1. SEM images of (A) g-C3N4 and (D) HHMU g-C3N4 tube (CN-Br-3). TEM images of (B) g-C3N4 and (E) CN-Br-3. (C, F) The magnified TEM images of a single HHMU g-C3N4 tube. (G)Confocal fluorescence microscopic image of CN-Br-3. (H) AFM image and thickness of HHMU g-C3N4 tube shell. (I) Nitrogen adsorption-desorption isotherms of g-C3N4 series and poredistribution curves of CN-Br-3.

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C3N4. It is corresponding to the increased intensity of (100) peak inXRD pattern of HHMU g-C3N4. Fourier transform infrared (FTIR)spectra (Fig. S6) and XPS survey spectra (Fig. S7) of g-C3N4 and CN-Br-X (X = 1, 2, 3, 4) demonstrated that the morphology change does notcause obvious alteration of chemical bond, and the slight shift inbinding energies of C and N may be attributed to the surface statechange. Besides, No Br element can be detected in the final HHMU g-C3N4 (Fig. S7d), excluding Br doping in HHMU g-C3N4 and meanwhileconfirming the generation of Br-containing intermediate. DRS (Fig. 3C)show that the absorption edge of modified samples undergoes succes-sive blue shift compared to bulk g-C3N4, which is consistent with thecolor variation from yellow to creamy white (insets in Fig. 3C). Whereasthe photoabsorption in visible region (> 500 nm) was enhanced, whichis due to the multiple reflection and scattering resulted from the horn-like hollow mesoporous structure. The energy bandgaps derived fromthe formula αhν = A(hν-Eg)n/2 [31] (n = 4 for indirect transitions)were calculated ranging from 2.76 to 2.86 eV for the bulk g-C3N4 and

CN-Br-X (X = 1, 2, 3, 4), respectively. The enlarged bandgap (Fig. 3D)can be put down to the nanocrystalline nature by shifting the conduc-tion and valence band edges in opposite directions [32,33] and is fur-ther identified by the blue shift of emission band by 16 nm (Fig. S8)[34,35]. The VB XPS (Fig. 3E) and Mott-Schottky plots (Fig. S9) wereemployed to reflect the change of conduction band and valence band.Based on the above-mentioned experiments, the energy band diagramof g-C3N4 and CN-Br-3 are illustrated to clearly show the band structureevolution (Fig. 3F).

The photocatalytic performance of CN-Br-X and g-C3N4 was eval-uated through H2 evolution (Pt cocatalyst) and CO2 reduction undervisible light irradiation (λ>420 nm). Metallic Pt serving as a cocata-lyst could quickly extract electrons from the interfaces of semi-conductors, prolonging the lifetime of charge carrier and reducing therecombination rate of electron-hole pairs. Besides, Pt shows the lowestactivation energy for hydrogen evolution [36]. Based on the abovereasons, Pt as cocatalyst can be largely beneficial to promoting

Fig. 2. (A) SEM images of melamine/NH4Br mixture (1:30 in molar ratio) calcined at 300 °C with EDS spectra at different areas. (B) SEM images of melamine/NH4Br mixture (1:30 inmolar ratio) calcined at 400 °C with EDS spectrum. (C) XRD patterns and (D) FTIR spectra of melamine, NH4Br and CN-Br-3 synthesized at different temperatures. (E) The comparison ofXRD patterns of bulk g-C3N4 and CN-Br-3 synthesized at 400, 450 and 500 °C. TG curves of (F) melamine, (G) NH4Br and (H) melamine/NH4Br mixture.

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photocatalytic activity. The photocatalytic H2 evolution activity showsa trend of first increase and then decrease, achieving maximum for CN-Br-3 (Fig. 4A). The H2 production rate of CN-Br-3 reaches67.7 µmol h−1 (Fig. 4B), which is almost 6 times higher than that ofpure g-C3N4. The wavelength-dependent apparent quantum efficiency(AQE) (Fig. 4C) on account of water splitting matches well with the DRScurve, giving impressive data with a AQE of 14.3% under irradiation at420 nm. This value is also higher than most of the previously reportedg-C3N4 with different morphologies, such as nano/microtubes [37–40],hollow spheres [41], nanospheres [42], nanosheets [43,44], helicalnanorods [45] (Table S3). Four consecutive H2 production operationspresents that the HHMU g-C3N4 keeps steady hydrogen yield under thesame experimental condition (Fig. 4D), indicating the durability andhigh stability. The photocatalytic activity is further investigated byreduction of chemically inert CO2 into CO and CH4 (Fig. 4E). CN-Br-3demonstrates a benign CO evolution rate of 0.54 µmol h−1 and bits ofCH4, which is also higher than that of pure g-C3N4. It is noteworthy that

the H2 evolution (Fig. 4F) activity of CN-Br-3 far exceeds that of me-soporous CN-Cl nanosheets, confirming the advantage of the currentunique morphology of CN-Br-3. The above water splitting and CO2 re-duction tests evidence that as-formed horn-like hollow mesoporousultrathin g-C3N4 achieves outstanding and stable photocatalytic per-formance for energy generation.

A reason for the high activity of the samples can be accredited to the4-fold enhanced surface area compared to pristine g-C3N4. But this casealong with the slightly altered band structure is not enough to engenderthe great enhancement in photocatalytic activity. So, we assume thatthe crucial role is the high efficiencies of photo-generated charge se-paration and movement, which are very beneficial for photocatalyticactivity. For verification, a series of photoelectrochemical tests wereelaborately designed and conducted over bulk g-C3N4 and CN-Br-3.Both of them can generate steady and reversible photocurrent, and thedensity of CN-Br-3 is 3.85 times that of bulk g-C3N4 (Fig. 5A and B).Here, methylviologen dichloride (MVCl2), as a fast electrons scavenger,

Fig. 3. (A) XRD patterns and (B) peak intensity of (100) and (002) of g-C3N4, CN-Br-1, CN-Br-2, CN-Br-3, and CN-Br-4. (C) UV–vis diffuse reflectance spectra for g-C3N4 and CN-Br-X (X =1, 2, 3, 4) and the corresponding color. (D) Band gaps of g-C3N4 and CN-Br-X (X = 1, 2, 3, 4). (E) XPS valence band spectra of g-C3N4 and CN-Br-3. (F) Schematic band structures of g-C3N4

and CN-Br-3.

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was added into electrolyte (0.1 M Na2SO4) to survey the surface chargetransfer efficiency (ηtrans) [46]. The photocurrent can be defined by thefollowing Eq. (1),

=J J η η ηH O max abs sep trans2 (1)

When adding electron scavenger MV2+ to electrolyte, the surfacecharge transfer is very fast and the ηtrans approximately reaches 100%.The photocurrent can be voiced as Eq. (2) [46].

=+J J η ηMV

2max abs sep (2)

Where, Jmax, ηabs, ηsep are unchanged for both JH2O and JMV2+. There-

fore, the surface transfer efficiency (ηtrans) can be counted as followedEq. (3),

=+η J J/trans H O MV

22 (3)

The photocurrent density of g-C3N4 increases from 0.67 µA cm−2 to2.0 µA cm−2, while that of CN-Br-3 increases from 2.58 μA cm−2 to

6.21 μA cm−2. This increment can be put down to easier reduction ofMV2+ than water molecules. So, ηtrans of bulk g-C3N4 and CN-Br-3 wascalculated to be 33.5% and 41.5%, respectively. This result disclosesthat CN-Br-3 possesses higher surface charge transfer efficiency than g-C3N4. Namely, carrier diffusion distance was shortened along theHHMU tube, which empowers faster electron injection into the redoxcouple in the photocatalytic reaction.

Furthermore, the density of charge carriers as a significant para-meter was determined. With existence of the fast electron acceptor, thephotocurrent onset potential in a voltammograms usually reflects thequasi Fermi level of majority carriers [47]. Since there is hardly anyoverpotential for the reduction of fast electron acceptor MV2+, chargecarrier can migrate to the external circuit for photocurrent generationonce the applied bias reaches the quasi Fermi level [47]. As shown inFig. 5C, the open-circuit potential of CN-Br-3 (−0.213 V vs. SCE) is0.075 V positive than that of bulk g-C3N4 (−0.288 V vs. SCE). In thequasi Fermi level, the carrier density difference between CN-Br-3 and g-

Fig. 4. (A) H2 production curves and (B) apparent rate constants for H2 evolution of pure g-C3N4 and CN-Br-X (X = 1, 2, 3, 4) samples under visible light (λ>420 nm). (C) Wavelength-dependent AQY and DRS spectrum of CN-Br-3. (D) Stability test of CN-Br-3 for four cycling H2 evolution. (E) Time-resolved CO2 reduction rate of g-C3N4 and CN-Br-3 under simulatedsolar light (300 W Xe lamp). (F) H2 evolution over CN-Br-3 and CN-Cl sample under visible light irradiation (λ>420 nm).

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C3N4 can be settled according to the Nernst Eq. (4) [48,49],

− =E E kTI N Nn( / )/ef1 f2 f1 f2 (4)

The 0.075 V shift corresponds to 18.1 (Nf1/Nf2) times higher carrierdensity in the HHMU tube, which is greatly beneficial to improvingphotocatalytic water splitting and CO2 reduction.

Surface photovoltage (SPV) spectra (Fig. 5D) originating fromchange in surface potential barriers are useful to reflect the separationextent of charge carriers. CN-Br-3 exhibits a stronger peak around380–450 nm than pure g-C3N4, demonstrating a more efficient separa-tion efficiency of the bulk charge carriers for HHMU g-C3N4 tube [50].Furthermore, electrochemical impedance spectra (EIS) and lifetime (τ)of injected electrons (Fig. S10) results also confirm the efficacious se-paration and transfer of charge carriers in CN-Br-3. Moreover, CN-Br-3exhibits the lowest PL emission intensity at 365 nm among the series ofphotocatalysts, manifesting its mostly reduced charge recombinationrate [51]. The above results provide solid evidence that more efficientcharge separation and migration occurred in the unique horn-likehollow mesoporous ultrathin g-C3N4, contributing to the remarkablyenhanced photocatalytic performance.

In order to disclose the mechanism and origin of efficient chargeseparation for HHMU g-C3N4 experimentally, the photo-induced de-position experiments of Pt and MnOx on HHMU g-C3N4 were conducted,as Pt and MnOx can indicate the reduction and oxidation active site ofphotocatalysts, respectively. The corresponding TEM images for Pt andMnOx photo-deposition are shown in Fig. 6 and Fig. S11. The TEMimages of Pt-HHMU g-C3N4 (Fig. 6A-C) demonstrated that the plenty ofPt particles are deposited on the surface of outer shell of the HHMU g-

C3N4 tubes, which means that there are abundant reduction active siteson the outer shell of HHMU g-C3N4 tubes that could effectively capturethe photo-generated electrons. Especially, we can see that a largeramount of Pt particles is distributed on the horn tips of HHMU g-C3N4,indicating that the horn tips are enriched with more abundant reduc-tion active sites (Fig. 6B). This feature highlights the special advantageof the horn-like structure of HHMU g-C3N4, which differs from the re-ported commonly isodiametric tubes. Besides, the MnOx photodeposi-tion experiments demonstrated that the outer shell of HHMU g-C3N4

tubes displays smooth surface and no MnOx were detected after 10%MnOx deposition (Fig. 6D-F). It implies that the oxidation active sitesare not distributed on the outer shell, and are speculated to be mainlyon the inner shell of HHMU g-C3N4 tubes. As illustrated in Fig. 6G, withlight irradiation, the photo-generated electrons would migrate to theouter shell and holes prefer to transfer to the inner shell of HHMU g-C3N4 tubes. Namely, efficient spatial anisotropic separation of chargecarriers is realized, which eventually significantly promotes the pho-tocatalytic activity of HHMU g-C3N4 tubes, and accounts for the re-markable H2 evolution performance of HHMU g-C3N4.

4. Conclusions

In summary, horn-like hollow mesoporous ultrathin g-C3N4 tubeshave been synthesized via a one-pot co-pyrolysis strategy. In the com-plex thermal polymerization process, a horn-like Br-containing inter-mediate was first generated, and finally yielding the HHMU g-C3N4

tubes with further decomposition. This unique structure endows g-C3N4

with efficient spatial anisotropic charge separation between the outer

Fig. 5. Photocurrent density of g-C3N4 and CN-Br-3 under visible light (A) without and (B) with 0.001 M methylviologen dichloride (MVCl2). (C) Linear sweep voltammetry (LSV) withMVCl2 and (D) SPV spectra of g-C3N4 and CN-Br-3.

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and inner tube shells, thus resulting in greatly promoted bulk chargeseparation, charge carriers density (18-times increase) and surfacetransfer efficiency. Benefiting from these merits, this advanced g-C3N4

material presents profoundly enhanced photocatalytic performance forH2 evolution (67.7 µmol h−1) with a high AQE of 14.3% at420±15 nm, and CO2 reduction into CO (0.54 µmol h−1) and CH4.This work affords a robust g-C3N4 material for H2 evolution and CO2

conversion, and the detailed uncovered polymerization mechanismenables the smart design on g-C3N4-based materials by a facile co-pyrolysis route.

Acknowledgment

This work was supported by the National Natural ScienceFoundations of China (Grant No. 51572246 and No. 51672258), theFundamental Research Funds for the Central Universities (292016153).

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.nanoen.2017.10.031.

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Chengyin Liu is currently a Ph.D. candidate in the Schoolof Materials Science and Engineering, China University ofGeosciences (Beijing). She obtained his bachelor degree inMaterials Science and Engineering, China University ofGeosciences (Beijing) in 2014. Her research interests focuson the design and synthesis of layered photocatalysts forenvironmental purification and energy generation.

Hongwei Huang received his Ph.D. in 2012 from TechnicalInstitute of Physics and Chemistry, Chinese Academy ofSciences. Currently, he is an Associate Professor at BeijingKey Laboratory of Materials Utilization of NonmetallicMinerals and Solid Wastes, School of Materials Science andTechnology, China University of Geosciences (Beijing). Hiscurrent research mainly focuses on the design and synthesisof layered nanomaterials and functional crystals and theirapplications for environment & energy.

Liqun Ye obtained his Ph.D. degree from WuhanUniversity, China, in 2013. At present, he leads EngineeringTechnology Research Centre of Henan Province for SolarCatalysis, Nanyang Normal University, China. He was aResearch Assistant at The Chinese University of Hong Kongfrom 2015 to 2016. His current research concentrates onthe synthesis of 2D photo-functional materials and theirapplications in the fields of environment remediation andnew energy production.

Shixin Yu is currently a Ph.D. candidate in the School ofMaterials Science and Engineering, China University ofGeosciences (Beijing). He obtained his bachelor degree inMaterials Science and Engineering, China University ofGeosciences (Beijing) in 2015. His research interests focuson the design and synthesis of layered photocatalysts forenergy generation.

Na Tian is currently a Ph.D. candidate in School ofMaterials Science and Engineering, China University ofGeosciences (Beijing). She obtained her bachelor degree inMaterials Science and Engineering, China University ofGeosciences (Beijing) in 2013. Her research interests focuson the design and synthesis of photocatalysts for environ-mental purification, H2 generation and CO2 reduction.

Xin Du received his Ph.D. in 2012 from Technical Instituteof Physics and Chemistry, Chinese Academy of Sciences.Currently, he is an Associate Professor at School ofChemistry and Biological Engineering, University ofScience and Technology Beijing. His current researches in-clude controllable preparation of nanomaterials with hier-archical (multi-stage) structure and its application in nano-drug gene delivery vector, multi-functional self-cleaningnano-coating, nano-catalyst, sewage treatment and otherfields.

Tierui Zhang is a full professor at the Technical Institute ofPhysics and Chemistry, Chinese Academy of Sciences. Heobtained his Ph.D. degree in Chemistry in 2003 at JilinUniversity, China. He then worked as a postdoctoral re-searcher in the labs of Prof. Markus Antonietti, Prof. CharlF.J. Faul, Prof. Hicham Fenniri, Prof. Z. Ryan Tian, Prof.Yadong Yin and Prof. Yushan Yan. His current scientificinterests are focused on the surface and interface of micro/nano-structures, including photocatalysis, electrocatalysis,thermocatalysis, and the controllable synthesis, self- as-sembly, and surface modification of colloidal inorganicnanostructures.

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Yihe Zhang is a Professor at School of Materials Scienceand Technology, China University of Geosciences (Beijing)and leads the Beijing Key Laboratory of MaterialsUtilization of Nonmetallic Minerals and Solid Wastes,School of Materials Science and Technology, ChinaUniversity of Geosciences (Beijing). Zhang received his PhDfrom Technical Institute of Physics and Chemistry, ChineseAcademy of Sciences in 2005, and undertook a VisitingScholar, Postdoctoral Fellow and Research Fellow in CityUniversity of Hong Kong and The Hong Kong PolytechnicUniversity from 2003 to 2009. His current research focuseson nanomaterials, composites and their applications forenvironment and energy.

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