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Preyaphat Wongchaiya, [Thi Kim Ngan Nguyen](https://orcid.org/0000-0001-8935-1306), Pornapa Sujaridworakun, Siriporn Larpkiattaworn, [Tohru S. Suzuki](https://orcid.org/0000-0001-9458-6863), [Tetsuo Uchikoshi](https://orcid.org/0000-0003-3847-4781)

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[Fabrication of g-C3N4 films with enhanced mechanical and charge transfer properties by electrophoretic deposition and subsequent citric acid modification](https://mdr.nims.go.jp/datasets/dd5b6cd9-612e-4117-9cb5-1cf4e21fe5e7)

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Fabrication of g-C3N4 films with enhanced mechanical and charge transfer properties by electrophoretic deposition and subsequent citric acid modificationAdvanced Powder Technology 35 (2024) 104460Contents lists available at ScienceDirectAdvanced Powder Technologyjournal homepage: www.elsevier .com/locate /aptFabrication of g-C3N4 films with enhanced mechanical and chargetransfer properties by electrophoretic deposition and subsequent citricacid modificationhttps://doi.org/10.1016/j.apt.2024.1044600921-8831/� 2024 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan.⇑ Corresponding authors at: Department of Materials Science, Faculty of Science,Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330,Thailand (P. Sujaridworakun).E-mail addresses: NGUYEN.Thikimngan@nims.go.jp (T.K.N. Nguyen), pornapa.s@chula.ac.th (P. Sujaridworakun), uchikoshi.tetsuo@nims.go.jp (T. Uchikoshi).Preyaphat Wongchaiya a,b, Thi Kim Ngan Nguyen c,⇑, Pornapa Sujaridworakun a,d,e,⇑,Siriporn Larpkiattaworn f, Tohru S. Suzuki b, Tetsuo Uchikoshi b,⇑aDepartment of Materials Science, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, ThailandbResearch Center for Electronic and Optical Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japanc International Center for Young Scientists, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japand Photocatalysts for Clean Environment and Energy Research Unit, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, ThailandeCenter of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, 254 Soi Chula 12, Phayathai Road, Wang Mai, Pathumwan, Bangkok 10330, Thailandf Expert Centre of Innovative Materials, Thailand Institute of Scientific and Technological Research, 35 Mu 3 Technopolis, Tambon Khlong Ha, Amphoe Khlong Luang, PathumThani 12120, Thailanda r t i c l e i n f o a b s t r a c tArticle history:Received 30 January 2024Received in revised form 16 April 2024Accepted 24 April 2024Available online xxxxKeywords:Graphitic carbon nitrideCitric acidElectrophoretic depositionChemical interactionOptoelectronic deviceA low-cost 2D metal-free material based on a graphitic carbon nitride (g-C3N4) film functionalized with acitric acid molecule has been successfully fabricated by Electrophoretic Deposition (EPD) and thermaltechniques, aiming to evaluate its suitability for optoelectronic devices. The thickness-controlled g-C3N4 film having a good mechanical property was successfully performed. The chemical stability andphotostability of the citric-modified g-C3N4 films deposited on indium tin oxide (ITO) glass were inves-tigated. New chemical links were clarified such that it could possibly become a bridge for enhancingcharge transport. The modified samples exhibited a significant increase in their photocurrent response,reaching 25 lA/cm2 at a thickness of about 20 lm. Electrochemical impedance spectra (EIS) verifiedthe enhanced conductivity, efficient charge transfer, and reduced electron-hole recombination rate.This research revealed a facile synthetic route and environmentally benign materials, thereby suggestingpromising prospects for their application in the optoelectronic field.� 2024 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of PowderTechnology Japan.1. IntroductionGlobal warming has prompted research on optoelectronicmaterials for renewable energy. Their versatile applications insolar cells, lighting, energy storage, and greenhouse gas monitoringhave addressed urgent environmental challenges [1,2]. However,commonly used optoelectronic materials, such as GaAs, InAS, CdSe,PbS, and InSb [3–7], have the following drawbacks: high cost, scar-city, and potential environmental and social concerns linked totheir extraction and mining processes. The metal-free semiconduc-tor known as graphitic carbon nitride (g-C3N4) has garnered signif-icant attention within the optoelectronics field due to itsdistinctive attributes. Possessing a robust thermal and chemicalstability along with an optimal bandgap for effective visible lightabsorption, g-C3N4 has emerged as a standout candidate. More-over, its cost-effectiveness and synthesis feasibility using abun-dant, eco-friendly elements, make it an attractive semiconductoralternative [8,9]. The g-C3N4 has demonstrated its effectivenessas a photocatalyst for water splitting, pollutant degradation, andhydrogen production. Moreover, its utility extends to areas suchas photovoltaics, gas sensing, and energy storage systems [10–13]. Nevertheless, the majority of bulk g-C3N4 application remainsconstrained by its low surface area, high electron-hole pair recom-bination rates, and limited charge transfer propensity [14]. Toovercome these issues, various approaches are employed, includ-ing morphological modifications [15], elemental doping [16], andthe fabrication of g-C3N4-based heterostructures [17]. Further-more, chemical functionalization via covalent or noncovalentinteractions is extensively acknowledged and utilized methodolo-gies for tailoring the g-C3N4 properties [18]. Vidyasagar et al. [19]http://crossmark.crossref.org/dialog/?doi=10.1016/j.apt.2024.104460&domain=pdfhttps://doi.org/10.1016/j.apt.2024.104460mailto:NGUYEN.Thikimngan@nims.go.jpmailto:pornapa.s@chula.ac.thmailto:pornapa.s@chula.ac.thmailto:uchikoshi.tetsuo@nims.go.jphttps://doi.org/10.1016/j.apt.2024.104460http://www.sciencedirect.com/science/journal/09218831http://www.elsevier.com/locate/aptP. Wongchaiya, T.K.N. Nguyen, P. Sujaridworakun et al. Advanced Powder Technology 35 (2024) 104460employed a post-functionalization approach to introduce the 2,5-thiophene dicarboxylic acid (TDA) moiety into the defect sites ofg-C3N4. Within the electronic structure of g-C3N4-TDA, anelectron-accepting AC@O thiophene segment is linked to the g-C3N4 unit via an amide (ANHAC@O). This configuration enhancesthe interfacial charge transfer and fosters efficient separation ofthe charge carriers, significantly contributing to its heightenedphotocatalytic performance. However, in its powdered form, itsrange of applications is constrained.Fabricating g-C3N4 films is vital due to their non-self-standingnature, requiring substrates or supports for shape maintenance.Despite common methods, such as dip coating [20], drop-casting[21], and spin coating [22], the films exhibit an unpredictablethickness and low photocurrent response. On the other hand, tech-niques, such as chemical vapor deposition (CVD) [23] and atomiclayer deposition (ALD) [24], offer precise control over the deposi-tion process. However, these methods can be complex and expen-sive. They may not be suitable for large-scale production. Theelectrophoretic deposition (EPD) method is preferred for fabricat-ing g-C3N4 films due to its exceptional ability to precisely depositg-C3N4 particles onto a substrate, allowing for a controlled thick-ness. This technique involves the application of an electric fieldto force charged g-C3N4 sheets within a suspension toward theelectrode, resulting in their controlled and targeted depositiononto the substrate [25]. Han et al. recently synthesized porousnanosheets of g-C3N4 using glyoxal-treated melamine as a precur-sor and fabricated their films on fluorine-doped tin oxide (FTO)substrates by EPD. Remarkably, the porous g-C3N4 nanosheetsexhibited a significant photocatalytic activity for photodegradationof isopropyl alcohol (IPA) and gaseous nitric oxide (NO). Notably,their photocurrent density reached 3.5 lA/cm2, surpassing thevalue of 0.76 lA/cm2 observed for g-C3N4 films [26]. In addition,Phoon et al. fabricated mesoporous graphitic carbon nitride(GCN) films by using EPD that showed a high efficiency in photode-grading tetracycline (TC) antibiotics and exhibited a photocurrentresponse of about 2 lA/cm2 [27]. However, the photocurrentresponse of the g-C3N4 films remains relatively low, as indicatedby studies [28–30], attributed to deficient adhesion between theg-C3N4 and ITO glass, resulting in detachment during electrochem-ical analyses. Thus, organic molecule modification of the g-C3N4films becomes essential for improving adhesion and fostering asurface charge transfer, charge separation, and reduced carrierrecombination rates.Citric acid (C6H8O7), a natural organic polycarboxylic acid withthree carboxyl groups, is commonly employed as a non-toxic andcost-effective precursor. It serves as cross-linker agents [31], disin-fectant, environmental remediation [32], and sterilizing agents[33,34]. Studies have investigated its potential as a cross-linkeragent to enhance the physical and mechanical properties of mate-rials like cellulose [35], starch [36], and wood [37]. Furthermore,citric acid (CA) serves as a reagent, either alone or most frequentlyin combination with nitrogen-containing compounds, for the syn-thesis of Carbon Dots (C-dots). This process yields C-dots withdiverse structures and optical properties [38].In this study, the g-C3N4 powder was synthesized by the pyrol-ysis of urea [39]. Subsequently, a g-C3N4 film was fabricated on ITOglass by the EPDmethod using a dispersing medium of acetone andiodide. The study aims to enhance adhesion between the g-C3N4and ITO substrates along with improving the surface charge trans-fer and photocurrent response. This is achieved by functionalizingg-C3N4 films with citric acid moieties and optimizing the syntheticconditions. The results of this study show that modification withcitric acid improves the physical stability and photoelectric con-version properties of g-C3N4 films prepared by the EPD method,which is important information for optoelectronic deviceapplications.22. Material and methods2.1. Synthesis of g-C3N4The g-C3N4 powder was prepared by the simple pyrolysis ofurea (98 %, Sigma-Aldrich). In the procedure, a total of 10.0 g ofurea powder was weighed, then placed in an alumina crucible withan outer diameter of 46 mm and height of 36 mm, which was sub-sequently covered. The crucible was subjected to heating in anelectric furnace (Koyo Thermo Systems Co., Ltd., Japan) at the rateof 5.0 �C/min in the presence of air, reaching a temperature of550 �C, and maintained at this temperature for 4 h. After thecompletion of the heating process, the resulting product had apale-yellow color. The BET specific surface area of the synthesizedg-C3N4 powder was 86.9 m2/g.2.2. Electrophoretic deposition of g-C3N4 layer on ITO glassThe g-C3N4 layer was fabricated on an ITO glass substrate usingthe EPD method. A schematic diagram of the fabrication of CA-modified g-C3N4 film by EPD and subsequent thermal process isshown in Fig. 1. To ensure stable suspensions, 0.5 g of g-C3N4 pow-der was dispersed in 50 ml of acetone (Nacalai Tesque, Inc., Japan)and subjected to ultrasonication for 1 h using an ultrasonicationhomogenizer. Subsequently, a 100 ll solution of iodine (ACS grade99.5 %, Sigma-Aldrich), used as an effective dispersant agent forelectrostatic stabilization in EPD suspensions and to induce a pos-itive surface charge on the g-C3N4 particles, was added to theslurry. The sonication process was then continued for an additional20 min. The zeta potential of g-C3N4 changed from �6.56 mV to+34.4 mV with the addition of iodine. The particle size of g-C3N4ranged from 0.5 to 4 lm with and without the addition of iodine.For the deposition process, an ITO glass substrate with an area of2 � 3 cm2 (Geomatec Co., Ltd., Japan; 6.15–7.27 Ohm/sq) wasemployed as the cathode (deposition electrode), while astainless-steel sheet was used as the anode (counter electrode),with a fixed distance of 10 mm maintained between the two elec-trodes. Based on preliminary investigations, the optimal conditionsfor the EPD method were determined. A potential of 25 V wasapplied across the electrodes, and the deposition was carried outfor a duration of 10 s. Finally, the coated samples were dried inan oven at 50 �C for one hour to ensure complete evaporation ofthe solvent and achieve a uniform g-C3N4 layer on the ITO glasssubstrate.2.3. Surface modification of g-C3N4 layer with citric acidCitric acid (98 %, FUJIFILM Wako Pure Chemical Industries, Ltd.,Japan) was dissolved in acetone (CH3COCH3, Nacalai Tesque, Inc.,Japan) to prepare solutions with various concentrations of 10, 25,50, 75, and 100 g/l. Subsequently, 80 ll of these citric acid solu-tions were drop onto the as-prepared g-C3N4 layer on the ITO glasssubstrate. The samples were then placed on a hot plate and heatedat 120 �C for a duration of 1 h to allow for solvent evaporation andenhance adhesion between the citric acid and g-C3N4. Followingthe initial heating step, the samples underwent a further heattreatment at temperatures of 200 �C, 250 �C, 300 �C, and 350 �Cfor an additional 2 h. This extended heat treatment aimed toinduce specific changes or reactions in the citric acid-g-C3N4 sys-tem, and to study the temperature-dependent effects on the result-ing material. The prepared g-C3N4-CA samples, heated at 300 �Cand modified with a citric acid concentration of 50 g/L, werelabeled as follows: g-C3N4-CA-300 �C and g-C3N4-CA 50 g/l, respec-tively, based on the heating temperature and citric acid concentra-tion used in the fabrication process.Fig. 1. Schematic overview of CA modified g-C3N4 film fabrication via thermal process.P. Wongchaiya, T.K.N. Nguyen, P. Sujaridworakun et al. Advanced Powder Technology 35 (2024) 1044602.4. Measurement of photocurrent responsePhotocurrent measurements were performed using an electro-chemical analyzer (VSP-300 Potentiostat, BioLogic Science Instru-ments, France). The experimental setup consisted of a standardthree-electrode cell, in which the prepared samples served as theworking electrode, a Pt sheet acted as the counter electrode, andan Ag/AgCl electrode functioned as the reference electrode. A0.1 M Na2SO4 (Wako Pure Chemical Industries, Ltd., Japan) aqueoussolution was used as the electrolyte. The applied potential was1.23 V, and the active area of the working electrode was 4 cm2.To generate light illumination, a 300 W compact Xenon lightsource (MAX-303, Asahi Spectra Co., Ltd., Japan), positioned10 cm from the reactor, was utilized. The electrochemical impe-dance spectroscopy (EIS) was performed using a potentiostat. TheNyquist plots were measured at 1.23 V (vs. Ag/AgCl) with an ACamplitude of 10 mV, frequency of 0.01–100,000 Hz under a300 W compact Xenon light source.2.5. CharacterizationThe morphology of the samples was characterized using a FieldEmission Scanning Electron Microscope (FE-SEM, S-4800, HitachiHigh-Tech Corporation) operated at an acceleration voltage of5 kV. To analyze the crystal structure of the samples, X-ray Diffrac-tion (XRD) patterns were measured at room temperature using aGrazing Incidence X-ray Diffractometer (GI-XRD, Rigaku Corpora-tion, Japan). The measurements were conducted in the 2-thetaangle range of 10–45�, utilizing Cu Ka radiation (k = 1.5418 Å) at45 kV, 200 mA. The step size for the scans was set at 0.02� witha scan speed of 1�/min, and the incident angle was set to 0.5�.The chemical bonding in the samples was investigated using aFourier Transform Infrared Spectrometer (FT/IR-4100, Jasco Inter-national Co., Ltd., Japan). The samples were analyzed in thewavenumber range of 400–4000 cm�1, which allowed for identifi-cation of the characteristic vibrational modes associated with dif-ferent chemical bonds. To measure the absorbance properties ofthe films, a UV–Vis spectrophotometer (V-650, Jasco InternationalCo., Ltd., Japan) was employed. The measurements were carriedout in the wavelength range of 200–800 nm at the scan rate of3400 nm/min. The luminescence spectra of the samples wererecorded at room temperature using a NanoLog spectrofluorometer(HORIBA Instruments Incorporated, Japan) equipped with a 450 Wxenon arc lamp. The excitation wavelength was 370 nm, and theemitted light was measured to analyze the luminescent propertiesof the samples. Additionally, the binding energy of the samples wasdetermined using X-ray Photoelectron Spectroscopy (XPS) by a PHIQuantera SXM instrument (Ulvac-PHI) employing Al Ka radiationat 15 kV. The binding energies were calibrated with respect tothe C1s peak of the adventitious carbon at 285 eV. This techniqueprovided information about the chemical state and electronicstructure of the samples.3. Results and discussionFE-SEM images were employed to analyze the morphology andmicrostructure of the g-C3N4, and g-C3N4-CA films deposited on theITO glass. Fig. 2 displays the SEM images corresponding to differentheating conditions during the film fabrication process. The SEMimages of the g-C3N4 film (Fig. 2a) and the g-C3N4-CA film(Fig. 2b) show a slightly aggregated structure with two-dimensional sheet-like formations that have irregular edges andslight curls. These sheets are extremely thin, measuring only afew nanometers in thickness, and a range in size from tens to hun-dreds of nanometers. The images also highlight the high porosity ofthe films. When the g-C3N4-CA film undergoes additional heattreatment at 200 �C and 300 �C (Fig. 2c and d, respectively), therewas no noticeable change in the particle shape and size by the acidtreatment, indicating that the presence of citric acid has a minimalimpact on the microstructures of the g-C3N4, as observed in theimages. Additionally, Fig. 2e and f show a cross-section view ofthe g-C3N4 and g-C3N4-CA-300 �C films on the ITO glass, respec-tively, revealing a film thickness of 28 lm for g-C3N4 film. How-ever, the reduction of the thickness to about 20 lm wasobserved after the induced citric acid and heating process. Thethermal treatment released adsorbed solvent molecules, creatingcross-linking between the citric acid and g-C3N4. The film thicknessplays a pivotal role in the generation and transportation of chargecarriers [40], subsequently influencing the photocurrent response.Nevertheless, although the photocurrent magnitude rises with theFig. 2. FE-SEM images of EPD films containing a) g-C3N4 and b) g-C3N4-CA c) g-C3N4-CA 200 �C d) g-C3N4-CA 300 �C, Cross-section of e) g-C3N4 f) g-C3N4-CA-300 �C, with aconstant CA concentration of 75 g/l.P. Wongchaiya, T.K.N. Nguyen, P. Sujaridworakun et al. Advanced Powder Technology 35 (2024) 104460thicker films, excessively thick films during the electrochemicalmeasurements can lead to detachment from the ITO substrate.Thus, determining the optimum film thickness is critical.The X-ray diffraction (XRD) analysis of the g-C3N4 film on theITO glass revealed two prominent diffraction peaks at 2h of 13.0�and 27.4�, which align with the JCPDS 87-1526 database (Fig. 3)[41]. The peak observed at 2h = 13.0� corresponds to the (100)crystal plane of g-C3N4, indicating the in-plane structural arrange-ment. The strong peak at 2h = 27.4� corresponds to the (002) crys-tal plane of g-C3N4, representing the characteristic interlayerstacking structure. The X-ray diffraction (XRD) pattern of g-C3N4-CA without additional heat treatment shows clear peaks that cor-respond to citric acid (CA). The indication of the characteristicXRD peaks confirms the presence of CA in the sample. When theg-C3N4-CA sample was heated to 200 �C on higher, an unidentifiedphase occurred. This occurrence could be attributed to the poten-tial melting of the citric acid and its interaction with the g-C3N4phase or transformation into another form. To understand theFig. 3. X-ray diffraction (XRD) patterns of the films on ITO glass obtained atdifferent calcination temperatures with a constant CA concentration of 75 g/l.4changes that the citric acid underwent during heating from200 �C to 350 �C, please refer to Fig. S1, which illustrates the melt-ing of citric acid and the formation of an amorphous phase, as indi-cated by the broad peak observed in Fig. S1a. Furthermore, inFig. S1b, the sample exhibited an apparent gel-like appearanceand a high degree of stickiness at 200 �C. At 250 �C, it transformedinto a fine powder with a brownish texture. Finally, at 300 �C and350 �C, the sample transitioned into coarse particles with a blackcoloration. The thermal decomposition of citric acid is a multi-step process leading to the formation of intermediate products liketrans-aconitic acid and citraconic anhydride [42,43]. Citric acid ini-tially melts at 153 �C, followed by dehydration at 175 �C, resultingin aconitic acid. As the temperature continues to rise, decarboxyla-tion reactions occur, yielding methyl maleic anhydride or citra-conic anhydride [44]. These intermediates, formed during thedecomposition process, may subsequently undergo chemical inter-actions with the g-C3N4 material during the heat treatment. Theseinteractions have the potential to induce unknown peaks in thecrystallographic structure of the sample due to the thermaltreatment.The FTIR spectra presented in Fig. 4a reveals the chemical bond-ing vibrations of pure citric acid (CA), pure g-C3N4, and g-C3N4-CAfilms with various CA concentrations without additional heat treat-ment. The results showed that the g-C3N4-CA film at a concentra-tion of 10 g/L exhibited a lower intensity peak corresponding toCA. Concentrations exceeding this value distinctly showed thepresence of the CA peak in the g-C3N4 matrix. In Fig. 4b, the spectraof the pure CA heated at 300 �C, g-C3N4 and g-C3N4-CA films heatedat 200–350 �C were analyzed. In the spectra, multiple absorptionpeaks were observed in the range of 1200–1640 cm�1, which canbe attributed to the stretching vibrations of C-N and C@N bondspresent in the CN aromatic repeating units of the g-C3N [45]. Fur-thermore, absorption peaks at 810 cm�1 indicated the characteris-tic out-of-plane vibrations of the triazine/s-triazine aromaticrepeating units [46]. Moreover, the FTIR spectra of the g-C3N4-CA-200 to 350 �C samples showed the presence of peaks associatedwith g-C3N4. However, the peak corresponding to citric acid (CA) isnot clearly observed, suggesting that the CA component may haveundergone some changes or reactions during the heating process.Fig. 4. FTIR of g-C3N4 and g-C3N4-CA on ITO glass obtained at a) by varying the CA concentration, without additional heat treatment, and b) various calcination temperaturesat 200–350 �C.P. Wongchaiya, T.K.N. Nguyen, P. Sujaridworakun et al. Advanced Powder Technology 35 (2024) 104460To gain further insight into the transformation of citric acid, theFTIR spectrum of pure citric acid heated at 300 �C (Fig. S2) wasexamined. Comparing the FTIR absorption spectra within the1900–1600 cm�1 range of citric acid (CA) before and after heating,distinctive changes were observed. The two C@O stretching bandsin the FTIR spectrum of the pure citric acid (Fig. 4a), located at1749 and 1703 cm�1, exhibit a different relative intensity and shiftwith respect to CA-300 �C (Fig. S2). These findings are consistentwith observations made in a previous study [47], highlightingintense absorption bands characteristic of citraconic anhydride at1774 cm�1 (C@O stretching of anhydride carbonyl) and1844 cm�1 (C@O antisymmetric stretching). This suggests that,after heating, the citric acid component likely transforms into ita-conic or/and citraconic anhydride [43]. These anhydrides possess acarbonyl group that can potentially form a covalent bond with theN-atom of g-C3N4. The introduction of theAC@Omoiety connectedto the g-C3N4 unit via an amide (ANHAC@O) linkage plays a piv-otal role in augmenting charge separation between electrons andholes, thereby improving the light absorption properties andimproving the photocurrent response.The X-ray photoelectron spectroscopy (XPS) analysis of g-C3N4and g-C3N4-CA-300 �C provided information about the surfacecomposition and bonding of these materials. In the high-resolution C 1s spectrum (Fig. 5a) of g-C3N4, two distinct carbonbonding states were observed. The peak observed at 284.9 eVcan be attributed to the CAC bond of the surface adventitious car-bon. On the other hand, the peak at 288.3 eV indicated the pres-ence of sp2-bonded carbon (NAC@N) [48]. For the g-C3N4-CA-300 �C, the C1s binding energies could be fitted to four peaks.Two prominent peaks appeared at 286.2 and 288.0 eV, which canbe attributed to the CAO and C@O bonds, respectively [49]. Fig-ure S3 illustrates the comparative XPS spectra of g-C3N4 and g-5C3N4-CA-300 �C, both exhibiting similar overall features. However,a notable difference is observed in the ratio of the peak height at288.3 eV to that at 285.0 eV between the two samples. Specifically,the peak height ratio for g-C3N4 is 12:1, whereas for g-C3N4-CA-300 �C, this ratio is 3:1. This significant change in the peak heightratio indicated that g-C3N4-CA-300 �C contains a concentration ofCAC functional groups that is four times higher compared to thepure g-C3N4. The increase in the number of CAC bonds can beattributed to the presence of citric acid during the modificationof g-C3N4-CA-300 �C. In the O1s spectra (Fig. 5b) of g-C3N4, twomain peaks were observed. These corresponded to the externalAOH group or water molecules adsorbed on the surface andNAO bonds at 531.9 eV and 533.1 eV, respectively [50]. Interest-ingly, a new peak appeared at 532.2 eV for the g-C3N4-CA-300 �C,which can be attributed to the formation of the C@O species, indi-cating a chemical change caused by heating the material with citricacid. By analyzing the N 1s spectrum (Fig. 5c), four fitted peakswere observed for g-C3N4, corresponding to different nitrogen spe-cies. The binding energies of these peaks were measured at398.6 eV, 399.1 eV, 401.0 eV, and 404.6 eV. These peaks can beattributed to CAN@C (sp2-hybridized nitrogen), NAC3 (sp3-hybridized nitrogen), CANAH (amino group from the surfaceuncondensed bridging N atom), and the p–p* excitations betweenthe stacking interlayers, respectively [51]. Regarding the g-C3N4-CA-300 �C, there was a peak shift to 400.6 eV compared to the pris-tine g-C3N4 at 401.0 eV. This decrease in binding energy indicated ashift towards lower energies corresponding to the NA (C@O) bond,suggesting that, after heating at 300 �C, an interaction occurredbetween the citric acid and g-C3N4. The disparities observed inthe XPS data between the g-C3N4 and g-C3N4-CA-300 �C providedevidence that, after heating at 300 �C, the carbonyl (C@O) groupsof citric acid can form bonds with the NH2 group of the triazineFig. 5. XPS spectra of g-C3N4 and g-C3N4-CA-300 �C.P. Wongchaiya, T.K.N. Nguyen, P. Sujaridworakun et al. Advanced Powder Technology 35 (2024) 104460ring structure of g-C3N4. This indicated a chemical interactionbetween the citric acid and g-C3N4, which could potentially serveas a bridging mechanism for enhancing the charge transport.The UV–vis absorption results demonstrated the tunability ofthe optical properties of the g-C3N4 and g-C3N4-CA. In Fig. 6a, thespectrum of g-C3N4 exhibited an absorption band in the range of200–460 nm and in agreement with previous reports [52]. How-ever, when the film was modified with citric acid and subjectedto heat treatment, the absorption wavelength of g-C3N4-CA shiftedtowards higher wavelengths in range from 450 and 550 nm. Mean-while, an absorption in the region of 600–800 nm was observed,indicating an improved visible light utilization ability. Addition-ally, the electronic structure of g-C3N4-CA might also have beenaltered due to the incorporation of citric acid and the thermaltreatment process. These modifications have yielded diverse band-gap characteristics as shown in Fig. 6b. Specifically, the pristine g-C3N4 shows an energy bandgap of 2.84 eV, falling between thebandgap values of g-C3N4-CA without heating, which is 2.81 eV,and g-C3N4-CA at elevated temperatures. The g-C3N4-CA-200 �Cand g-C3N4-CA-250 �C exhibited an equivalent bandgap of2.83 eV. Conversely, g-C3N4-CA-300 �C and g-C3N4-CA-350 �Cdemonstrated wider bandgaps at 2.87 eV and 2.86 eV. However,the introduction of citric acid onto the g-C3N4 films slightlyaffected the energy bandgap. In Fig. S4, the UV–Vis spectra ofg-C3N4 and g-C3N4-CA on the ITO glass were obtained at variouscitric acid (CA) concentrations after heating at 300 �C. As theresults, the absorption wavelength of the heated g-C3N4-CA filmsslightly increased with the increase of the citric acid content com-pared to that observed for g-C3N4 on the ITO glass sample. Hence,the incorporation of citric acid into the g-C3N4 films has a negligi-ble impact on their light absorption properties.In Fig. 6c, The PL spectra showed the processes of charge migra-tion, transfer, and separation in the films. The intensity of the emis-sion peaks corresponds to the number of photons emitted at aspecific energy. A higher photoluminescence emission peak indi-6cates a higher rate of electron-hole pair recombination [53]. Theobtained PL spectra showed that all samples, including the pureg-C3N4 and g-C3N4-CA samples, exhibited an emission within therange of 420–600 nm, which correlated with the 450-nm absorp-tion edge of g-C3N4. However, a significant decrease in the PLintensity was observed in the g-C3N4-CA-300 �C sample comparedto both the pure g-C3N4 and g-C3N4-CA sample. This observationaligns with the FTIR and XPS findings, suggesting that the NHA(C@O) bond serves as a bridge for carrier transfer and reducesthe electron-hole recombination.Heat treatment plays a crucial role in inducing a chemical inter-action between the citric acid and g-C3N4, leading to robust adhe-sive films with improved mechanical properties. To assess this,g-C3N4 films, g-C3N4-CA films, and g-C3N4-CA films subjected toheat treatments at 200, 250, 300, and 350 �C were evaluated fortheir mechanical characteristics using manual removal by fingers.The results (Fig. S5) revealed that the pure g-C3N4 films exhibitedan insufficient adhesion to the ITO glass, making them easilyremoved by the simple touch of a finger. In contrast, the other sam-ples exhibited a strong adhesion, remaining firmly in place withoutany signs of detachment. Although the g-C3N4-CA films exhibited astrong adhesion, they experienced detachment from the ITO glassduring photocurrent measurements in the Na2SO4 electrolyte sys-tem. The necessity for heat treatment arises from the inadequatecuring at 120 �C to produce films with a robust adhesion. Thus,the g-C3N4-CA films were subjected to an additional heat treat-ment in the temperature range of 200–350 �C and showed anenhanced adhesion and stability during the measurements. Sinceiodine decomposes at temperatures between 50 and 140 �C [54],it would not affect g-C3N4 itself. However, the addition of citricacid (CA) and subsequent heat treatment would introducehydroxyl-containing functional groups on the film, which wouldform chemical bonds with the functional groups on the ITO sub-strate and improve the adhesion of the g-C3N4 film. Moreover,the g-C3N4-CA film heated at 400 �C can be easily removed withFig. 6. UV–vis spectra of g-C3N4-CA on ITO glass obtained at a) different calcination temperatures, with a constant CA concentration of 75 g/l, b) band gap energy of the thinfilms and c) photoluminescence (PL) spectra of g-C3N4 and g-C3N4-CA on ITO glass before and after heating at 300 �C.P. Wongchaiya, T.K.N. Nguyen, P. Sujaridworakun et al. Advanced Powder Technology 35 (2024) 104460a gentle touch of a finger, and at 450 �C, all of the g-C3N4 and CAwere burnt out and disappeared off the ITO glass.The separation of electrons and holes was investigated by pho-tocurrent measurements. When the films were exposed to a lightsource, there was a noticeable rise in the photocurrent. Thisincrease is indicative of the migration of photo-generated electronswithin the bulk materials, which consequently leads to the gener-ation of the photocurrent during light irradiation. Fig. 7a shows thephotocurrent response curves, while Fig. 7b displays the linearsweep voltammetry (LSV) plots of the prepared samples. The influ-ence of the heat treatment temperature on the photocurrentresponse of the films was investigated at 200, 250, 300, and350 �C. The results indicated that g-C3N4-CA heated at 300 �Cexhibited the highest photocurrent response compared to theother temperatures. This suggests that a calcination temperatureof 300 �C provided optimal conditions for efficient photocurrentgeneration in the citric acid-modified g-C3N4 film. This could beattributed to the robust physical properties and the strongcrosslinking between CA and g-C3N4 through the NHA (C@O) bond,acting as a bridge for charge transport. Consequently, this canenhance the separation of electrons and holes, thereby contribut-ing to improved photocurrent response characteristics. Fig. 7cshows that the rising time and decaying time of the photocurrentresponse are 0.18 and 0.2 s, respectively.To investigate the influence of the citric acid concentration onthe film stability, the g-C3N4 films were crosslinked with varyingCA concentrations (10, 25, 50, 75, and 100 g/L), then heat-treatedat 300 �C (Fig. S6a). Surprisingly, the films modified with CA at10 g/L underwent cracking upon immersion in Na2SO4 electrolyte(Fig. S6b), implying inadequate crosslinking with g-C3N4 due tothe low citric acid content. Conversely, g-C3N4 films modified with7CA at concentrations of 25, 50, 75, and 100 g/L exhibited improvedadhesion to the g-C3N4-ITO glass, preventing detachment duringthe photocurrent measurements. Photocurrent response curves(Fig. S7a) and linear sweep voltammetry (LSV) plots (Fig. S7b)revealed that at a 75 g/L CA concentration, the films exhibitedthe highest photocurrent response among the lower CA concentra-tions. This suggests the optimal citric acid concentration for max-imizing the photocurrent in the g-C3N4-CA films. Notably, at the100 g/L CA concentration, the photocurrent response exhibited adistinct shape, possibly due to excessive citric acid. This is sup-ported by Fig. S6a, illustrating non-smooth and streaky film mor-phologies. The streaky appearance indicated incompleteintegration of excess citric acid into the film matrix, resulting inuneven distribution and compromised film quality.The investigation of charge transfer in the citric acid-modifiedg-C3N4 was conducted using electrochemical impedance spectra(EIS) (Fig. 7d), and the EIS parameters were obtained by fittingthe experimental data with the equivalent circuit (Fig. S8). Theobserved trend in the semicircle diameters (g-C3N4-CA-250 �C >g-C3N4-CA-200 �C > g-C3N4-CA-350 �C > g-C3N4-CA-300 �C) signi-fied variations in the conductivity, charge carrier migration effi-ciency, and electron-hole pair recombination [55]. Smallerdiameters correspond to an enhanced conductivity, efficient chargetransfer, and reduced recombination rate. Among the samples,g-C3N4-CA-300 �C exhibited the smallest semicircle diameter, sug-gesting that citric acid-modification and thermal treatment syner-gistically improved the conductivity, accelerated the chargetransfer, and decreased the electron-hole recombination. Theresults support the investigation of the photocurrent response.The introduction of citric acid and heating is speculated to createC (acid) and N (g-C3N4) bonds with the NH2 groups, fosteringFig. 7. A) photocurrent response, b)linear sweep voltammetry (LSV) plots of the films at different calcination temperatures, c) enlarged rising and decaying edges of thephotocurrent response of the g-C3N4-CA-300 �C, and d) EIS Nyquist plots of citric acid-modified g-C3N4 films.P. Wongchaiya, T.K.N. Nguyen, P. Sujaridworakun et al. Advanced Powder Technology 35 (2024) 104460charge transfer and reducing recombination. Moreover, the hydro-gen bonding between the compositions also speeds up the chargetransfer. This modification is expected to facilitate efficient chargecarrier migration, yielding higher photocurrents. In essence, thisstudy highlights the impact of tailored citric acid modificationand thermal treatment on the charge transfer properties withing-C3N4 with implications for an enhanced photocurrent response.4. ConclusionsCitric acid-modified g-C3N4 films were successfully prepared,and the citric acid played a crucial role in enhancing the film sta-bility and improving the photocurrent response. The modifiedsamples exhibited a porous structure, optical absorption, reducedelectron-hole recombination rates, and enhanced charge transfer,resulting in a high photocurrent response. Under optimal reactionconditions, the best sample, g-C3N4-CA-300 �C at a citric acid con-centration of 75 g/L, achieved a photocurrent of approximately0.025 mA/cm2. This high photocurrent response can be primarilyattributed to the strong adhesion, efficient transfer of the chargecarriers and reduced electron-hole recombination. Therefore, citricacid-modified g-C3N4 holds significant promise as anenvironmentally-friendly material for various optoelectronicapplications.CRediT authorship contribution statementPreyaphat Wongchaiya: Conceptualization, Data curation, For-mal analysis, Investigation, Methodology, Validation, Visualization,Writing – original draft, Writing – review & editing. Thi Kim Ngan8Nguyen: Conceptualization, Data curation, Formal analysis, Inves-tigation, Methodology, Supervision, Validation, Visualization, Writ-ing – original draft, Writing – review & editing. PornapaSujaridworakun: Conceptualization, Funding acquisition, Projectadministration, Resources, Software, Supervision, Writing – review& editing. Siriporn Larpkiattaworn: Conceptualization, Fundingacquisition, Methodology, Project administration, Supervision,Writing – review & editing. Tohru S. Suzuki: Data curation,Resources, Software, Writing – review & editing. Tetsuo Uchi-koshi: Conceptualization, Funding acquisition, Methodology, Pro-ject administration, Resources, Supervision, Writing – originaldraft, Writing – review & editing.Declaration of competing interestThe authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.AcknowledgementsThe authors would like to thank Drs. M. Nemoto, _I. N. G.Özbilgin and T. Hiroto of National Institute for Materials Science(NIMS) for their help with the instrumental analyses. One of theauthors, P. Wongchaiya, would like to express her deep gratitudeto the following two institutions for supporting her research activ-ities at NIMS through their educational programs: the Develop-ment and Promotion of Science and Technology Talents Project(DPST) Scholarship, Institute for the Promotion of Teaching Scienceand Technology (IPST), Ministry of Education, Thailand, and theInternational Cooperative Graduate Program (ICGP), NIMS, Japan.P. Wongchaiya, T.K.N. Nguyen, P. Sujaridworakun et al. Advanced Powder Technology 35 (2024) 104460Appendix A. Supplementary materialSupplementary data to this article can be found online athttps://doi.org/10.1016/j.apt.2024.104460.References[1] S. Mokkapat, C. Jagadish, III-V compound SC for optoelectronic devices, Mater.Today 12 (2009) 22–32.[2] X. Zhang, J. Shao, C. Yan, R. Qin, Z. Lu, H. Geng, T. Xu, L. Ju, A review onoptoelectronic device applications of 2D transition metal carbides and nitrides,Mater. Des. 200 (2021) 109452.[3] J. Yoon, S. Jo, I.S. 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