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Graphitic Carbon Nitride Impregnated Niobium Oxide (g-CN/NbO) Type (II) Heterojunctions and Its Synergetic Solar-Driven Hydrogen Generation Ibrahim Khan, Nadeem Baig, and Ahsanulhaq Qurashi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01633 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 7, 2018

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ACS Applied Energy Materials

Graphitic Carbon Nitride Impregnated Niobium oxide (gC3N4/Nb2O5) Type (II) Heterojunctions and its Synergetic Solardriven Hydrogen Generation Ibrahim Khan1, Nadeem Baig 2, Ahsanulhaq Qurashi1 * 1

Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and

Minerals, Dhahran, 31261, Saudi Arabia. 2

Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran, 31261,

Saudi Arabia. *

Corresponding author. Tel.: +966 (0)138607063. Email: [email protected].

ABSTRACT: Graphitic carbon nitride (g-C3N4) based catalysts are evolving in energy harvesting applications due to their robustness, nontoxicity, and most important photocatalytic efficiencies. In this work, we successfully engineered g-C3N4/Nb2O5 type (II) heterojunction via pulse sonochemical technique based on opposite charge-induced hetero-aggregation on the surface. The agglomerated spherical Nb2O5 nanoparticles (NPs) having diameter 30-40 nm observed on the lamellar surface of g-C3N4 in FESEM images. The XRD and XPS analysis confirm the orthorhombic phase and formation of the g-C3N4/Nb2O5 heterostructure. The FTIR spectra of g-C3N4/Nb2O5 show characteristic poly s-triazine bands from 1250 to 1650 cm-1. Moreover, g-C3N4/Nb2O5 exhibited the lower bandgap value of 2.82 eV as compared to Nb2O5 (3.25eV) with significant redshift and enhance visible light absorption. The Mott-Schottky (MS) analysis confirms the formation of heterojunction between g-C3N4 and Nb2O5, with significant band shifting towards lower hydrogen evolution reaction (HER) potential. The g-C3N4/Nb2O5 heterojunctions showed many folds enhanced photocurrent response from photoelectrochemical (PEC) water splitting, and the value reached to – 0.17 mA/cm2 with good stability and insignificant dark photocurrent at 1.0 V vs RHE. The electrochemical impedance spectroscopic (EIS) measurements further elucidate the suppression of photogenerated electrons/holes as the radius of the semicircle significantly decreased in case of heterojunction formation. The enhanced photocatalytic hydrogen generation by the heterostructures could be attributed to the 1

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effective formation of heterojunctions between the g-C3N4 and Nb2O5 semiconductors, causing the migration of the photogenerated electrons and holes, hence increasing their lifetimes.

KEYWORDS: Nb2O5 NPs; g-C3N4; Pulse Sonication; Photocurrent density; PEC Water Splitting

1. INTRODUCTION Photoelectrochemical (PEC) water splitting is a well-established technique for energy harnessing from naturally available water and solar light. Fujishima et al. Initially explain the PEC water splitting reaction on the TiO2 photoelectrodes.1 Since then enormous research has been carried out in search of best catalyst and many important semiconductors and metallic oxides have been utilized to achieve this goal including ZnO,2,3 WO3,4 V2O5,5 Fe2O3,6 and Nb2O5 7

etc. Though these oxides showed significant photocurrent densities, however, the cost and

stability towards photooxidation limit their industrial implications in pristine form. More recently, much attention is shifted towards nanocomposites and heterostructures formation to obtain the desired properties and to overcome these issues. Many important hybrid materials such as BiVO4,8,9 Fe3O4/ZnO,10 and Fe2O3/TiO2,11,12 were successfully engineered and tested for PEC water splitting. Additionally, some non-metal carbon-based photocatalysts such as CNTs, graphene,13,14 graphene oxide,15–17 and carbon black

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were also incorporated/combined with

semiconductors for enhanced photocatalytic applications. Though, carbon-based metal oxide catalysts found to be relatively stable and cheaper. Yet, much efforts required to produce industrially feasible catalyst with high efficiency and stability. This can be possible if the optoelectronic properties of these materials are tuned towards better electron transport and sufficient light absorption in the desired spectral region with minimal charge recombination.19 Among carbon-based catalysts, graphitic carbon nitride (g-C3N4) has been recognized as a valuable organic semiconductor, which is obtained from nitrogen rich urea, cyanamide, dicyanamide, and melamine precursors.20–23 The indirect band gap of g-C3N4 is ∼2.7 eV, which

is the favorable range for photocatalytic reactions such as reduction step in water splitting and artificial photosynthesis

24,25

or oxidation step in the degradation of organic moieties.26–28

However, the low surface area of bulk g-C3N4 is the major concerns in addition to electron/hole 2


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pairs charge recombination, which strictly reduce their photocatalytic efficiency. Many strategies employed to enhance its photocatalytic performance by overcoming the stated issues, among which the coupling of g-C3N4 with another semiconductor is a feasible approach. Carbon nitride selectively form type (II) heterojunctions, which has a definite band alignment with the counter materials.29–32 In type (II) heterojunction, the potential of the conduction (CB) and valence bands (VB) of the constituent semiconductors are lower than the combined semiconductor. Type (II) heterojunctions have various advantages such as the photogenerated charge transport between the combined semiconductors is feasible with longer photogenerated excitons lifetimes, which lead to enhanced PEC performance.29 Among various metal oxides and semiconductor materials, niobium oxide (Nb2O5) can be an efficient choice for type (II) heterojunction formation with gC3N4 due to (i) its proper optical band positions as compared to g-C3N4, (ii) tunable band gap value (∼3.2 eV), (iii) high surface acidity, (iv) larger specific surface area, and (v) sufficient photoactive nature.7,33–35 Therefore, it has been coupled with many important photoactive materials for PEC applications. The previous devoted studies on the synthesis of g-C3N4/Nb2O5 heterostructures suggested long synthesis times and lack of control of heterostructure formation with non-uniform morphology, which can lead to insufficient photocatalytic activity. Therefore, a suitable synthetic protocol for g-C3N4/Nb2O5 with controlled morphology and particle size still remains a matter of great research. In this regard, the ultra sonochemical assisted technique offers an interesting pathway to obtain g-C3N4/Nb2O5 heterostructures with control morphology in relatively shorter time. The sonochemical method is fast, accurate and simple. The bubbles produce during sonication expand due to cavitation and burst with a sufficient amount of energy. This energy is transfer to precursor molecules, which disperse well and recombine to produce the product with larger surface area and uniform composition of the constituent atoms.8,36 Additionally, g-C3N4/Nb2O5 heterojunctions are yet to test for the PEC water splitting applications to the best of our knowledge. Therefore, the purpose of this work is to synthesize gC3N4/Nb2O5 heterostructures via the ultra-sonochemical method and to evaluate their PEC water splitting for the first time. 2. EXPERIMENTAL SECTION 2.1. Preparation of the lamellar g-C3N4 and Nb2O5 nanoparticles 3



As discussed earlier the g-C3N4 material was prepared by the thermal condensation polymerization of organic-based Melamine (C3H6N6) by a reported method. In a typical Experiment, 20 g of melamine (Sigma Aldrich 99%) was placed in a rectangular shaped alumina crucible. The crucible covered partially and introduced inside the tubular muffle furnace (OTF1200X, MTI Corporation) with regular air supply at 600 °C for 4 h at the ramping speed of 5 °C/min. The yellow powder was obtained and labeled as g-C3N4. Nb2O5 white powder was produced using niobium (V) ethoxide (Nb(OCH2CH3)5) from Sigma Aldrich with 99.99% purity and NH4OH solution (precipitating agent) by sol-gel method. Pluronic-123 (P123) polymer was used as a gelling agent and the %wt ratio was maintained at 1 (Nb):10 (P123). P123 is a symmetric triblock copolymer constitutes of poly(ethylene glycol) chain bonded with poly(propylene glycol) chain followed by bonding with another poly(ethylene glycol) chain. The precursors were dissolved in ethanol and ultrasonicated for 1 hr through Ultra/probe Sonicator (SONICS-4000 Vibra-Cell) operated at 20 amperes at 50oC. The pulses were provided at 10s ON and 3s OFF cycles. After ultrasonic probe treatment, the solution was transferred to a sealed Teflon lined autoclave and kept at 120oC for 12h at the ramping speed of 10 °C/min. The white gel product was washed and dried at room temperature to achieve white powder. The powder was transferred to a crucible and calcined at 550 °C to achieve the final product. 2.2. Preparation of g-C3N4/Nb2O5 heterostructure The surface charge-induced hetero-aggregation technique adapt for the synthesis of gC3N4/Nb2O5 type (II) heterojunctions.37 The pulse sonication is additionally applied for homogeneous intermixing of lamellar g-C3N4 and Nb2O5 NPs. In this technique, the opposite surface charge is the main driving force, which causes electrostatic interaction. The pH value plays important role in controlling the positive surface charge on g-C3N4. It is reported that at lower pH (3.6 in our case), the layered g-C3N4 acquire positive charge on the surface due the incomplete condensation of amino groups.37 Conversely, Nb2O5 NPs acquire negative charge, due to the presence of surface hydroxyl groups. Figure 1 demonstrates the synthesis of the gC3N4 /Nb2O5 heterojunctions schematically. Typically, the calculated mass of g-C3N4 and Nb2O5 (3:1) were transferred to a 200 mL beaker containing 130 mL of deionized (DI) water and the beaker was kept under continuously stirring. 0.5 M Nitric acid HNO3 was slowly added dropwise 4


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to the stirring solution until pH 3.6. The pulse sonication was applied for 30 minutes to completely intermix the constituents. After pulse sonication, the product was centrifuged and washed several times to achieve the neutral pH and dried in the fume hood at 50 °C.

+ + ++ + + + + + + + + +

+

Figure 1. Schematic diagram of the synthesis process of the g-C3N4/Nb2O5 heterostructure. 2.3. Characterization of the materials The physicochemical characteristics of the prepared materials were explored via various analytical techniques. The morphologies of pristine and composite samples were exploded with a JEOL JSM-6701F field emission scanning electron microscope (FE-SEM). The phase and unit crystal confirmations were carried out via X-ray diffraction (XRD) using a Shimadzu XRD 600 diffractometer inbuilt with nickel (Ni)-filtered Cu Kα radiation, at 2θ from 5 to 70°. The scan speed and step width were kept at 0.02° and 3° min−1, respectively in continuous scan mode. Surface chemical investigations were carried out with x-ray photoelectron spectroscopy (XPS) using PHI 5000 Versa-Probe II spectrometer, equipped with Al Kα radiation source. The C1s (E = 284.7 eV) level was used as a standard under10−8 mbar of vacuum. A resolution was kept at 1 eV, with 5 scans were run for survey spectra. The high-resolution spectra were recorded at a resolution of 0.1 eV, with 30 scans. The XPS data analysis was performed using Avantage software. Fourier transform infrared (FTIR) and Raman spectra were recorded from 3500 to 500 cm−1, and 1400 to 50 cm−1, using a Thermo Scientific Nicolet NEXUS 870 FT-IR70 and iHR320 Horiba Spectrometers, respectively. The optical properties were measured with the help of Agilent Cary 5000 UV-Vis-NIR spectrophotometer. The UV-Vis/diffuse reflectance spectrometry (DRS) spectra were recorded from 200 to 800 nm. Magnesium oxide (MgO) circular stab was used as a reflectance standard for calibration. The photoluminescence (PL) 5



spectra were recorded via FluoroLog-3 using 450W Xenon lamp standard R928 PMT detector that covers the full range from UV to near-IR. The excitation wavelength was kept at 330 nm. 2.4. Film Fabrication of Photoelectrodes Photoelectrochemical (PEC) water splitting performance of the layered g-C3N4, pure Nb2O5 NPs and g-C3N4/Nb2O5 heterostructures investigated using fluorinated tin oxide (FTO) conducting glass substrates (support electrodes). The FTO glass substrates were washed with the mixture of ethanol and DI water in the bath sonicator for 10 min and then dried at 105 oC. 10 mg is taken from each sample and dissolved in 1mL ethanol containing 0.005 mL nafion. The mixture is sonicated until acquiring full homogeneity. For the fabrication of each electrode, 40 µL of sample solution is deposited over the pre-treated FTO substrate. The substrate is allowed to dry at 80 oC for 4hrs. These electrodes were used in 3-electrodes electrochemical transparent cell having 0.5 M Na2 SO4 (pH = 7.2) electrolyte. The platinum (Pt) and Ag/AgCl electrode served as a counter electrode and reference electrode, respectively. The periodic ON/OFF photocurrent density performance was assessed under regularly chopped light irritation. The light ONOFF cycles were kept at 20 s for a scanned (in case of I-V curve measurements) and constant potential of −1.0 V (in case of I-t curve measurements) (vs. RHE). The Electrochemical impedance spectroscopy (EIS) was initiated at the open circuit potential (OCP). A sinusoidal AC perturbation of 5 mV was practiced to photoelectrode over the whole frequency range 0.01–105 Hz at an open circuit potential under 1-SUN solar illumination. The Mott-Schottky plots were derived from impedance potential tests conducted at 100 Hz. The latest model of Metrohm Autolab Potentiostat (PGSTAT302N) is used for PEC experiments. Moreover, for measuring the photocurrent response an artificial solar light source i.e. Oriel sol 3A class AAA solar simulatorNewport (IEC/JIS/ASTM certified) was used. The power of the lamp (450 W Xenon) was calibrated with a silicon photodiode at 100 mW.cm−2(1-SUN) at the point of the light incident. The Air Mass 1.5G Filter (AMF) was supplied at the 2 × 2 inch output aperture for radiation beam. 3. RESULTS AND DISCUSSION

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3.1. Characterization of the morphological, structural, optical properties Figure 2a,b demonstrates the morphological features of pristine Nb2O5 samples at low and high resolution. The micrographs reveal agglomerated identical spherical shaped morphologies and each Nb2O5 NP exhibit the diameter of 30-40 nm approximately. Similarly, Figure 2c,d indicates the micrographs of g-C3N4/Nb2O5 heterostructure at lower and higher magnification. The existence of layered g-C3N4 and Nb2O5 NPs can be clearly seen. Additionally, agglomerated Nb2O5 NPs are quasi-homogeneously distributed on the laminar surface of g-C3N4.

Figure 2. Low and high-resolution SEM images of Nb2O5 NPs (a, b), and g-C3N4/Nb2O5 heterostructures (c, d). To elucidate the formation of heterojunction TEM investigation were carried out. Figure 3a,b represents the low and high resolution TEM images of the as-synthesized g-C3N4/Nb2O5 heterostructure, respectively. The Figure 3 clearly indicates relatively spherical Nb2O5 NPs entrenched on the surface of g-C3N4 sheets. The 30-40 nm avg size of Nb2O5 NP is in good agreement with the SEM images. High magnification TEM image (fig 3 b) indicates spherical Nb2O5 nanoparticles are well dispersed on the surface of g-C3N4 sheets making conceivable interface between the two materials. 7



These results also confirm that layered agglomerates comprised of Nb2O5 NPs in intimate interaction with the g-C3N4 lamellar sheets. A distinguishing feature was the reasonable distribution of NPs over the g-C3N4 nanosheets as indicated in red arrows in figure 3 (a). This is suggesting that the pulse ultra-sonochemical energy maximize the overall distribution of the NPs over the nanosheets.

Figure 3. Low and High Resolution TEM images of g-C3N4/Nb2O5 heterostructures (a, b) The XRD patterns of the pure Nb2O5 NPs and g-C3N4/Nb2O5 heterojunction are provided in Figure 4. Typical XRD pattern of the Nb2O5 NPs exhibit diffraction peaks corresponding to the orthorhombic Nb2O5 with high crystallinity and lattice constants of a = 3.607 Å and c = 3.925 Å. The results are matching with Joint Committee on Powder Diffraction Standards (JCPDS) # 30-0873. The characteristic 001, 110 and 101 peaks are centered at 22.8 o, 28.77 o, and 36.95 o, respectively. The XRD pattern of pure Nb2O5 NPs does not show any extra peaks, suggesting the high purity of the product.38 The XRD patterns of the g-C3N4/Nb2O5 heterojunctions diffraction showed peaks with reducing intensities and slight broadenings, which is attributed to the amorphous nature of C3N4 with lower crystallinity as compared to pristine Nb2O5.39 The broader peak with a shoulder near 29o is composed of characteristic (002) C3N4 and (110) Nb2O5 peaks. The composite peaks also show small shifts (inset Figure 4), which can be related to the strain caused by polymeric C3N4 in the unit cell of Nb2O5 and strong intermixing due to high energy pulse sonication. Nevertheless, the diffraction pattern of g-C3N4/Nb2O5 heterojunctions exhibit the same peaks, which suggested that the overall phase is unchanged from the pure orthorhombic Nb2O5.37 8


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Figure 4. XRD patterns of as-synthesized Nb2O5 NPs and g-C3N4/Nb2O5 heterostructure To investigate the composition and chemical environment of g-C3N4/Nb2O5 heterostructure, XPS measurements were performed (Figure 5a-e). The XPS survey is provided in Figure 5a, which was used to selectively magnify the binding energy regions of O, N, C and Nb with deconvolution to understand the chemical nature of the contributing elements. The deconvoluted high-resolution spectrum of O(1s) in g-C3N4/Nb2O5 heterostructure (Figure 5b) demonstrates three sub peaks, which can be attributed to Nb-O bond at 529.57 eV, the characteristics oxygen peak in metal hydroxyl or oxalate groups derived from the precursor of niobium at 531.06 eV, and the most important high energy C-O peak at 534.01 eV, respectively. The O-H peak is also observed in the FTIR spectrum of the heterostructure. The intensified XPS spectra in Figure 5c,d show N(1s) and C(1s) peaks, that arise from the lamellar g-C3N4 phase of the g-C3N4/Nb2O5 heterostructure. Deconvoluted N(1s) spectrum show two subpeaks that can be assigned to sp2 nitrogen in the C-N-C bond of poly s-triazine ring centered at 398.57 eV, and ending amino groups (C-NH3) at 401.35 eV, correspondingly.40 The C(1s) high-resolution spectrum exhibit two peaks that are centered at 284.72 and 291.32 eV, respectively. The weak intensity peak at 284.72 eV attributed to characteristic sp2 C-C bonds, which is used as a standard to calibrate the XPS binding energies of all species. The intense C(1s) peak at 291.32 eV matched with N-C-N coordination of expected triazine rings existed in lamellar g9



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C3N4. Finally, the deconvoluted Nb(3d) spectrum (Figure 5c) display two subpeaks at 206.78 and 209.386 eV, which is corresponded to Nb(3d5/2) and Nb(3d3/2) chemical states. The peaks values are varied slightly from the reported values of pure Nb2O5,41 which establishes the interaction between the atoms present on the Nb2O5 and g-C3N4 surfaces that resulted in the formation of effective heterojunctions between them. Additionally, no extra peaks are observed during deconvolution, which suggested the high purity of the sample.

(a)

Figure 5. High resolution deconvoluted XPS spectra of the as-synthesized g-C3N4/Nb2O5 heterostructure: (a) Survey (b) O(1s), (c) N(1s), (d) C(1s), and (e) Nb(3d). 10


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Figure 6 shows the FTIR spectra of the as-synthesized pristine Nb2O5 NPs and gC3N4/Nb2O5 heterostructure. The Nb2O5 NPs spectral peaks (black line) are in good agreement with the available literature.42,43 The bands located at 593, 870, 1376 cm−1 originated due to Nb–O stretching vibrations, respectively, while band at 788 cm−1 is attributed to Nb–O–Nb bending vibration. The band observed at 1629 cm−1 is due to the adsorbed bending vibrations of moisture, while the broadband at 3415 cm−1 are due to the stretching vibration of the hydroxyl groups. Some important variations are observed in the FTIR spectrum of g-C3N4/Nb2O5 heterostructure (red line), as new peaks arose and some peaks of Nb2O5 are masked. Nevertheless, it contains the characteristic peaks of both materials i.e. group of peaks in the range 1250 to 1650 contain characteristic out-ofplane bending vibration of stretching modes of C-N heterocycles (heptazine rings).44 Similarly, the bands located at ~815 and 890 cm−1 can be assigned to deformation mode of N-H bonds. A broadband at ~ 3225 cm−1 corresponded to stretching vibrations of amino groups present at the end of the g-C3N4 structure with some contribution of adsorbed OH stretching. A shoulder peak at 1101 cm−1 and an intense peak at 900 cm−1 were assigned to the stretching vibrations of Nb– O–C

and Nb = O, respectively. It can be seen that the FTIR spectra of the heterostructured

samples exhibited characteristic peaks of both phases (g-C3N4 and Nb2O5), with negligible shifts.

Figure 6. FTIR spectrum of FTIR spectrum of Nb2O5 NPs and g-C3N4/Nb2O5 heterojunction 11



The optical properties of these materials are of prime importance to predict the probability of their photocatalytic performance. Figure 7 provides the DRS/UV-Vis spectra of the as-fabricated photoelectrodes. These plots are obtained by using Tauc`s equation, knowing the fact that g-C3N4 the band gap value of g-C3N4/ Nb2O5 are indirect band gap materials.45,8 The band gap value of the pure Nb2O5 is 3.25 eV, which is in the reported range.46 Whereas, the gC3N4/ Nb2O5 heterostructure shows significant band gap energy drop to 2.82 eV, which was anticipated due to the lower band gap of the incorporating material. The inset in Figure 7 revealed the absorption spectrum and as expected the g-C3N4/ Nb2O5 heterostructure show dominant visible light absorption with absorption edge 460 nm as compared to pristine Nb2O5 NPs, which exhibit absorption at 400 nm, approximately. Consequently, the g-C3N4/ Nb2O5 heterostructure could be sufficiently activated under visible irradiation due to redshift and good absorption capacity.

Figure 7. Tauc`s plot obtained from UV–vis diffuse reflectance spectra data for g-C3N4, and the g-C3N4/Nb2O5 heterostructures. Photoluminescence (PL) spectroscopy is an important analytical technique, which originated from the recombination of photogenerated electrons/holes. The PL intensity of each sample could reflect the recombination rate of generated charge carrier pairs.44 Figure 8 provides the PL spectra of 12


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pure g-C3N4 and g-C3N4/ Nb2O5 samples excited at 330 nm. The PL fluorescence peak of pure g-C3N4 and g-C3N4/Nb2O5 centered on ~460 nm. The PL intensities of the samples are in the decreasing order as g-C3N4 > g-C3N4/ Nb2O5. The lower PL intensity of g-C3N4/ Nb2O5 heterojunction photoanode is signifying that the hybrid sample has lower photogenerated electrons/holes recombination rate. These results are in good agreement with the photocurrent measurements, EIS and Mott-Schottky (MS) results, which further confirm that the heterojunction interface is the key to promote the enhancement of separation rate of the photo-generated hole-electron pairs, thereby significantly improving PEC water splitting efficiency which we will discuss in the later part of the manuscript in details.

gC3N4 gC3N4/Nb2O5

Figure 8. Room temperature PL spectra of Nb2O5, g-C3N4 and gC3N4 / Nb2O5 nanostructures. 3.2. Evaluation of photoelectrochemical water splitting properties The PEC water splitting properties of bare Nb2O5, lamellar g-C3N4 and g-C3N4/Nb2O5 heterojunctions were explored by various electrochemical techniques assisted by solar light. Linear sweep voltammograms (LSV) (Figure 9a) were obtained under the periodic chopped 13



light. It is evident from the LSV spectra that the materials are dominantly showing negative photocurrent response, which can be related to the hydrogen generation due to water reduction step.47-49 For bare Nb2O5 and g-C3N4 samples, the photocurrent density remains very low or negligible even at high voltage (generally Nb2O5 is oxygen evolution reaction (OER) catalyst and also showed significant current under UV light in OER, however in this case shows negligible hydrogen evolution in visible light which might be due to impurities from native reaction precursors). While in case of the g-C3N4/Nb2O5 heterojunction, a significant increase is observed in the photocurrent density beyond -0.6 V vs RHE, which reach to saturation at 1.1 V and remains stable until -1.5 V. The most important feature is the dark photocurrent, which is insignificant in all cases until -1.0V, because the photoelectrochemical effect is dominant until -1.2 V. However, beyond that the dark photocurrent arises as well, especially for g-C3N4/Nb2O5 heterojunction. This can be attributed to the electrochemical and related thermal processes, which become dominant at higher voltage.50,51 Figure 9b shows the comparative photocurrent densities of all samples obtained at -1.0 V vs RHE under periodic ON/OFF light cycles. Pristine g-C3N4 and Nb2O5 photoanodes showed photocurrent density values of -0.02 and -0.05 mA/cm2 at the given voltage, respectively. However, as expected g-C3N4/Nb2O5 heterojunction photoanode exhibited higher photocurrent density of - 0.17 mA/cm2. This indicated that more electrons have been created by this material and transferred efficiently. The photocurrent density is almost 3-folds of the pure Nb2O5. These finding suggest that g-C3N4/Nb2O5 heterojunction possessed superior PEC performance due to effective visible light absorption and enhanced electron/hole separation properties as compared to their pristine counterparts. Figure 9c describes the long-term photostability performance of the samples for 1000 s. In all cases, the photocurrent densities remain stable, with insignificant decrease observed in their photocurrent densities. Therefore, the heterojunction material has shown significant resilience and has more resistance against photocorrosion. Figure 9d shows the electrochemical impedance properties of the Nb2O5 NPs and gC3N4/Nb2O5 heterojunction materials in term of semi-radial Nyquist plots obtained by electrochemical impedance spectroscopy (EIS). EIS analysis is helpful to predict the charge separation capacity of the electrode materials. Usually, each semi-circular plot in the diagram 14


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signifies a resistance in the form of impedance during charge transfer course for each material. The larger radius correlates to a higher charge-transfer resistance at the electrode interface with low charge separation and vice versa.40 Consequently, it is obvious from the Figure 9d that the radius of the g-C3N4/Nb2O5 Nyquist plot is smaller than pristine Nb2O5. This indicated that the charge transfer resistance and charge recombination are lower in the case of heterojunction material and therefore the PEC performance of g-C3N4/Nb2O5 will accordingly maximize leads to higher photocurrent density. In order to elucidate the flat band position and charge carrier density of the g-C3N4/Nb2O5 heterojunction, Mott–Schottky (MS) plots are acquired in Na2SO4 (0.5M) solution at 100 Hz (Figure 9e). As expected, both the plots showing positive slopes, which signify n-type semiconductor character of the photoanodes i.e. electrons are the majority carriers. Moreover, the slope of Nb2O5 is greater and at more negative voltage than the g-C3N4/Nb2O5, which implies their high electron donor density. The slope line on extrapolation touch the quasi-linear regions at -0.207 V, and -0,179 V, which are the flat band potentials (Vfb) of Nb2O5, and g-C3N4/Nb2O5, respectively. The remarkable shift of Vfb values in case of g-C3N4/Nb2O5 suggest that the band bending took place with the incorporation of g-C3N4, which is essential for effective charge transfer between heterojunction as in the present study. The enhanced electrical conductivity can increase the lifetime of photoexcitons, giving rise to a decrease in the electron−hole recombinations.

15



Figure 9. PEC water splitting measurements of g-C3N4, Nb2O5 and g-C3N4/Nb2O5 photoanodes (a) Linear voltammetry sweeps (I-V) at 5 mV/s under chopped illumination, (b) Periodic photocurrent density (I-t) plot at 1.23 V (vs RHE), (c) Stability testing under continuous light, (d) EIS plots for interfacial charge transfer detection under light, and (e) Mott-Schottky plots collected at 100 Hz 16


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3.3. Proposed Charge Transfer and Water Splitting Mechanism In

order

to

propose

the

PEC

water

splitting

mechanism

of

the

g-

C3N4/Nb2O5 heterojunctions, the band gap values, PL spectra, and PEC measurements can be helpful with some literature background. Figure 10 provides the schematic of water splitting reaction. It is widely reported that in type (II) heterojunction the band positions of g-C3N4 are higher than metal oxides counterparts,52,53 thus we predicted the water splitting mechanism based on the cited principle. Briefly, in the initial step, the artificial solar radiations generate electronhole pairs or photoexcitons in in our case. Conversely, Nb2O5 cannot be excited efficiently due to its larger band gap value i.e. 3.25 eV. The g-C3N4 owned more negative CB potential and hence the photoinduced electrons drifted from their CB to the CB of Nb2O5 via heterojunction interface. These electrons take part in the PEC reduction of water into H2 molecule. Due to this drifting, the photogenerated charge recombination is suppressed significantly, resulting in remarkably enhanced higher PEC water splitting performance for the g-C3N4/Nb2O5 heterojunction as compared to pristine Nb2O5 and g-C3N4. These results are in agreement with the PL, MS, and EIS plots, which showed a similar trend in the case of the heterostructure photoanode.

Nb2O5 ECB - - - - - - -

- -

Triazine unit of g-C3N4

PEC Water Splitting Mechanism

-

Electrons (e-)

+

Holes (h+)

- ECB - - - - - - - - -

gC3N4 Nb2O5

Eg = 2.70 eV

Eg = 3.25 eV + + + + + + + EVB +

+

+ + + + + + + EVB

Figure 10. The schematic of PEC water splitting reaction over g-C3N4/Nb2O5 type (II) heterojunction 4. CONCLUSIONS 17



In summary, type (II) heterojunction was successfully fabricated from g-C3N4 and Nb2O5 NPs via pulse sonication and opposite charge-induced hetero-aggregation at control pH. The SEM indicated the distribution of Nb2O5 NPs on the lamellar g-C3N4 nanosheets. The XRD confirm orthorhombic phase of the sample, while the XPS values of Nb, C, N, and O show a slight shift from the pristine values suggesting the heterojunction formation. The optical results via DRS and PL suppression indicated the lowering of band gap from 3.25 to 2.82 eV and maximization of charge electrons/holes pair charge separation in case of g-C3N4/Nb2O5. The EIS further confirm the suppression of charge recombination due to enhanced charge transfer. The PEC water splitting results suggested a 3-fold increase in the photocurrent density of gC3N4/Nb2O5 from Nb2O5 with high stability and insignificant dark photocurrent. The improvement of the PEC performance of g-C3N4/Nb2O5 can be attributed to successful type (II) heterojunction formation, which leads to facile migration of photogenerated charge carriers between g-C3N4 and Nb2O5 and ultimately suppresses the charge recombination and longer lifetimes of the electron/hole pairs. In summary, the combination of g-C3N4with Nb2O5 to produce a type (II) heterostructure is a good strategy to overcome important challenges in photocatalysis and the same approach can be adapted to produce new C3N4 based photocatalyst for energy harvesting applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: EDX analysis and elemental mapping of the g-C3N4/Nb2O5. ACKNOWLEDGMENTS This project was funded by the National Plan for Science, Technology, and Innovation (MAARIFAH) – King Abdulaziz City for Science and Technology – through the Science and Technology unit at the King Fahd University of Petroleum and Minerals (KFUPM) – the Kingdom of Saudi Arabia, award number (13-NAN1600-04).

18


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Nb2O5

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