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N-doped carbon [email protected] facilitated heterostructure of TiO2 polymorphs for efficient photoelectrochemical water oxidation
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N-doped carbon dots@layer facilitated heterostructure of TiO2 polymorphs for efficient photoelectrochemical water oxidation Chan-Wei Hsu a, Chia-Hsun Li a, Lei Zhang b, Shih-Yuan Lu a,∗ a b
Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu 30013, Taiwan School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan Anhui 232001, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2018 Revised 1 August 2018 Accepted 3 August 2018 Available online xxx Keywords: Tio2 nanorod arrays Polydopamine N-doped carbon dots C-doped anatase Tio2 Photoelectrochemical performance
a b s t r a c t A cocktail strategy was developed to fabricate sandwich nanostructure of hydrogenated C-doped anatase TiO2 nanocrystals/N-doped carbon dots@layer/rutile TiO2 nanorod array as an efficient photocatalyst for photoelectrochemical water oxidation. The one-dimensional, single crystalline nature of the innermost rutile TiO2 nanorod facilitates charge transport. The middle N-doped carbon dots@layer offers dual functionalities, with the N-doped carbon dots as the photosensitizer to harvest long wavelength lights and the N-doped carbon layer as the conductive layer for fast charge transport toward the current collector. The outermost hydrogenated C-doped anatase TiO2 nanocrystals serve to increase the visible light absorption and to form a type II heterostructure with the innermost rutile TiO2 nanorods to enahnce the charge separation. An enhancement of 86% in photocurrent density was achieved at 1.23 V (vs. RHE) under illumination of simulated sun light of 100 mW/cm2 by the sandwich nanostrcuture, as compared to that of the plain rutile TiO2 nanorod array. The enhancement was boosted to 228% under visible simulated sun light (λ > 400 nm) illumination. The photoconversion efficiency and photocurrent density retention rate were significantly improved from 0.23% and 60% for the plain rutile TiO2 nanorod array to 0.65% and 84% for the sandwich nanostructure, respectively. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Hydrogen production from photoelectrochemical (PEC) water splitting catalysed with semiconductor photoelectrocatalysts is considered a promising renewable energy technology and has drawn a great deal of research attention [1]. For design of a successful photoelectrocatalyst, several key factors have to be taken into consideration, including light absorption, charge separation, and charge transport. It is however unlikely to fulfill all desired requirements with single component catalysts, and thus composite catalysts are sought after to improve the photoelectrochemical efficiency of the catalyst [2]. TiO2 , because of its high photocatalytic activity, chemical stability, low toxicity, and low cost, is still considered the most promising material for photoanodes in photoelectrochemical devices. Its most detrimental drawback is its large energy bandgap, photoresponsive only in the UV region, severely restricting its overall photoconversion efficiency and thus its practical applications [3]. Consequently, a wide range of approaches has been developed to improve on the situation, including doping, bandgap engineering,
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (S.-Y. Lu).
defect engineering, and heterostructure construction. [3,4] Among them, heterostructure constructed from a type II junction involving usage of a narrow bandgap material, offers not only enhanced charge separation, but also enlarged light absorption spectra [3,4]. Carbon dots, a new member of the versatile carbon family, because of their unique properties, including low cytotoxicity, excellent biocompatibility, and high mobility for photo-induced electrons, [5–8] have attracted extensive research attention and been applied to a wide spectrum of applications, such as chemical sensing, bio-sensing, bio-imaging, nanomedicine, electrocatalysis, and photocatalysis [9]. Moreover, nitrogen doped carbon materials gain increasing popularity for their ability to induce charge delocalization and adjust the work function of carbon, both enhancing the functionality of carbon dots as an effective photosensitizer [10]. Zhang et al. prepared N-doped carbon dot decorated TiO2 nanoparticles and applied them in photodegradation of pollutants [10]. The photocatalytic efficiency was significantly improved because of the effective absorption and utilization of visible light achieved with the N-doped carbon dots serving as an effective photosensitizer [10]. Dopamine is an important neurotransimitter, existing in the central nerve system of mammals. It can self-polymerize in an alkaline solution to form a dense, conformal polydopamine layer on a wide range of substrates. Furthermore, the amino group
https://doi.org/10.1016/j.jtice.2018.08.007 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: C.-W. Hsu et al., N-doped carbon dots@layer facilitated heterostructure of TiO2 polymorphs for efficient photoelectrochemical water oxidation, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.007
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contained in dopamine often leads to N-doping of the resulting carbon upon carbonization of the polydopamine layer [11]. It is thus a popular templating material for fabrication of novel conductive carbon nanostructures [12,13]. In this work, we developed a simple polymerize-and-calcine process to successfully fabricate a novel composite nanostructure of N-doped carbon dots well dispersed in N-doped carbon layer, termed N-doped carbon dots@layer, from dopamine. This composite carbon nanostructure offers dual functionalities, with the N-doped carbon dots serving as a photosensitizer for absorption and utilization of long wavelength light and the N-doped carbon layer as an electron conducting medium [14]. This unique property can find potential applications in a wide spectrum of photocatalytic devices, including photoelectrochemical water oxidation. Hydrogenated TiO2 , the so-called black TiO2 , has attracted a great deal of research attention in recent years. The hydrogenation of TiO2 nanocrystals generates a thin disorder layer that induces band tail states to up-lift the effective valence band toward the vacuum level to shrink the bandgap of the TiO2 nanocrystals for absorption of visible light, turning the colour of the TiO2 nanocrystals from white to black [14]. This thin disorder layer also offers trapping sites for photo-induced charge carriers to promote charge separation and thus the photocatalytic activity of the TiO2 nanocrystals [15]. Chen et al. prepared black TiO2 through hydrogen reduction of anatase TiO2 at 200 °C in 20 bar hydrogen for 5 days [15]. The product powders exhibited outstanding photocatalytic activities [15]. Although successful, the process requires high pressures and high concentrations of hydrogen, which is dangerous and not amenable to large scale applications. Zeng et al. developed a low concentration hydrogen reduction process for black TiO2 production, operated in a 5%H2 + 95%Ar atmosphere of ambient pressure [16]. The product powders exhibited excellent photocatalytic activity toward degradation of formaldehyde, attributable to the effective utilization of long wavelength light and enhanced charge separation provided by black TiO2 . In this work, we developed a cocktail approach for the material and nanostructure design of composite heterostructured semiconductor photocatalysts to boost the efficiency of photoelectrochemical water oxidation for hydrogen generation [17]. The cocktail approach involves 1D single crystalline rutile TiO2 core for guided fast charge transport and enlarged functioning surfaces, [18] N-doped carbon dots well dispersed in N-doped carbon layer as the middle layer offering long wavelength absorption [10] and fast transport for photo-induced charges, and decoration of hydrogenated C-doped anatase TiO2 nanocrystals as the outermost layer to offer long wavelength absorption and to form a type II heterostructure with the rutile TiO2 core for enhanced charge separation. This composite heterostructured semiconductor photocatalyst is expected to exhibit enhanced light harvesting, charge separation, and charge transport, and thus much improved photoconversion efficiencies over the plain rutile TiO2 nanorod array electrode. The N-doped carbon dots@layer was synthesized through carbonization of self-polymerized dopamine layer coated on the surface of the rutile TiO2 nanorods. This simple and safe polymerize-and-calcine process gives rise to a unique composite nanostructure of well dispersed N-doped carbon dots in N-doped carbon layer offering dual functionalities, unlike the carbon layer only case achieved with hydrothermal processes of high pressures [19–21]. The present polymerize-and-calcine process can be readily applied to other photocatalytic systems. In addition, coupling hydrogenated C-doped anatase TiO2 nanocrystals with single crystalline rutile TiO2 manorods to form a type II heterostructure for charge separation enhancement, facilitated with the N-doped carbon layer, is a novel application of TiO2 polymorphs. With construction of the sandwich nanostructure, the photocurrent density of the rutile TiO2 nanorod array was boosted by 86% at 1.23 V
(vs. RHE) under simulated sun light illumination, and by 228% at 1.23 V (vs. RHE) under illumination of visible simulated sun light of λ > 400 nm. 2. Experimental section 2.1. Materials Hydrochloric acid (HCl), titanium(IV) isopropoxide (TTIP), dopamine hydrochloride (C6 H3 CH2 CH2 NH2 (HO)2 ·HCl), tris(hydroxymethyl)-aminomethane (TRIS), and ethylene glycol were used as received without further purification. 2.2. Fabrication of rutile TiO2 nanorod array The rutile TiO2 nanorod array was fabricated on FTO glass, termed R-NRs standing for rutile-nanorods, with a hydrothermal method. Briefly, 0.48 mL TTIP was dissolved into 30 mL aqueous HCl solution (15 mL deionized (DI) water mixed with 15 mL concentrated HCl (37%)). The mixture, together with a piece of FTO glass, was then transferred into a Teflon lined steel autoclave of a capacity of 50 mL. The autoclave was heated in an oven set at 150 °C for 5 h and let cool naturally to room temperature. The rutile TiO2 nanorods coated FTO glass was rinsed with DI water and calcined at 500 °C for 3 h in air for later use. 2.3. Fabrication of N-doped carbon dots@layer/R-NRs The rutile TiO2 nanorod coated FTO glass was dipped vertically into 200 mL TRIS aqueous buffer solution (0.002 M) with the pH value of the solution adjusted to 8.5 by using concentrated HCl. An amount of 80 mg of dopamine hydrochloride was dissolved in the above solution under gentle magnetic stirring. The system was then kept still for 15 h at room temperature to allow in situ coating. The coated sample was rinsed with DI water and annealed at 500 °C, with a heating rate of 1 °C/min, for 3 h in argon to form the N-doped carbon dots@layer/rutile TiO2 nanorod array, termed N · C/R-NRs. 2.4. Fabrication of hydrogenated C-doped anatase nanocrystals/N · C/R-NRs The hydrogenated C-doped anatase TiO2 nanocrystals (H–C · ANPs) were first synthesized following the procedures developed by Zeng et al. [16] Briefly, 1 mL TTIP was first mixed thoroughly with 25 mL ethylene glycol, and then added slowly into 50 mL DI water under stirring for 30 min. The mixture was transferred to an autoclave for hydrothermal treatment at 180 °C for 10 h to afford the light brown C-doped anatase TiO2 nanocrystals. Non-doped white anatase TiO2 nanocrystals were also prepared for comparison with the same procedures described above, except for the addition of ethylene glycol. These C-doped anatase TiO2 nanocrystals were thermally treated at 300 °C in a 5%H2 + 95%Ar atmosphere for 2 h to afford the black hydrogenated C-doped anatase TiO2 nanocrystals. An amount of 0.01 g H–C · A-NPs was dispersed in 99.8% absolute ethanol, and then deposited onto the N · C/R-NRs with spin coating at 40 0 0 rpm for 30 s. The product was calcined at 400 °C for 1 h in argon to complete the construction of the sandwich nanostructure, H–C · A-NPs/N · C/R-NRs. 2.5. Characterizations The crystalline phase of the sample was investigated with powder X-ray diffraction measurements (XRD, Ultima IV, Rigaku, Japan). The morphology and crystalline structure were observed
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Scheme 1. Schematic of fabrication procedures for sandwich nanostructure H–C · A-NPs/N · C/R-NRs: (a) FTO glass, (b) rutile TiO2 nanorods grown on FTO glass (R-NRs), (c) N · C/R-NRs before carbonization, (d) N · C/R-NRs, (e) H–C · A-NPs, (f) H–C · A-NPs/N · C/R-NRs.
with a field-emission scanning electron microscope (FE-SEM) (Hitachi SU8010) and a transmission electron microscope (TEM, JEOL JEM 30 0 0F). The UV − visible spectra were acquired with a Hitachi U-3300 UV − visible spectrometer under the diffuse-reflection mode using an integrating sphere (P/N 2J2-0175). The photoluminescence (PL) spectra were recorded using a Hitachi F-4500 fluorescence spectrophotometer at an excitation wavelength of 300 nm. The chemical status of elements was analyzed with X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 50 0 0 Versaprobe II). 2.6. Photoelectrochemical measurement For the PEC measurements, an aqueous solution containing 1 M KOH was used as the electrolyte. The sample served as the photoanode and a Pt sheet of size 1 × 1 cm2 was employed as the counter electrode. An Hg/HgO (in 1 M NaOH, RE-61AP) electrode suitable for alkaline environment was used as the reference electrode. The light source was provided with a solar simulator (Newport, AM 1.5, 100 mW/cm2 ), and a cut-off filter (λ > 400 nm) was used to provide the visible light source. The PEC performances of the sample electrode were measured using a CHI electrochemical analyzer (CHI600E). The photoconversion efficiency (η) was calculated based on the following equation: [22]
η (% ) =
Jph × (1.23 − |Emeas − Eoc | ) I0
× 100%
(1)
where Jph is the photocurrent density achieved at the applied potential, Io is the power density of the incident light (100 mW/cm2 ), Emeas is the potential applied to photoanode versus the reference electrode, Eoc is the open circuit potential of the photoanode under illumination. All measured potentials were converted to refer to RHE (reversible hydrogen electrode) according to the following Nernst equation: 0 ERHE = ERE −61AP + 0.0592 × pH + ERE −61AP
(2)
where ERHE is the potential vs RHE, ERE-61AP is the measured po0 tential vs Hg/HgO, and ERE−61 = 0.118 V at 25 °C. AP 3. Results and discussion 3.1. Materials characterizations Scheme 1 shows the fabrication procedures for the sandwich nanostructure of hydrogenated C-doped TiO2 nanocrystals/N-doped carbon dots@layer/rutile TiO2 nanorod array, H–C · A-NPs/N · C/RNRs. The single crystalline rutile TiO2 nanorod array was grown on an FTO glass with a hydrothermal process followed by annealing in air atmosphere. The nanorod array was coated with an N-doped
Fig. 1. Photographs of (a) rutile TiO2 nanorod array coated FTO glass (R-NRs), (b) R-NRs after polydopamine coating, (c) N-doped carbon dots@layer coated R-NRs (N · C/R-NRs).
carbon dots@layer through carbonization of the first formed polydopamine thin layer on the nanorod surface. Finally, hydrogenated C-doped anatase TiO2 nanocrystals, prepared from hydrogenation of C-doped TiO2 nanocrystals, were coated onto the surface of the N·C/R-NRs via a spin-coating process to complete the construction of the sandwich nanostructure. The TiO2 nanorods grown on the surface of the FTO glass was confirmed to be of rutile phase, JCPDS 88–1175, with the XRD pattern shown in Fig. S1 [23]. The morphology was observed to be of nanorod array as clearly evident from the SEM image of Fig. S2. The nanorods, with a length of about 2.1 μm, appeared to be with a square cross section of a side length of around 150 nm. If examined closely with a TEM, each nanorod is in fact composed of bundled nanorods of much smaller diameters of around 5 nm. The rutile TiO2 nanorod array coated FTO glass appeared white as shown in Fig. 1(a), and turned black after the coating of the polydopamine layer. The color turned white again as the polydopamine was carbonized into the N-doped carbon dots@layer. The N-doped carbon dots@layer was characterized with Raman spectroscopy. The resulting Raman spectrum, as shown in Fig. 2, exhibits two characteristic peaks located at 1369 and 1587 cm−1 . The 1369 peak is contributed by the d-band of the carbon material, attributable to the presence of amorphous carbon and the disorders caused by the N-doping, whereas the 1587 peak is characteristic of the G-band of the carbon material resulting from the E2g mode of graphite through vibration of graphitic sp2 carbon. [24,25] The G-band intensity is significantly higher than that of the d-band, indicating the high degree graphitization of the carbon material, beneficial for the involved charge transport. Interestingly, significant graphitization was achieved at only 500 °C, [26] possibly because of the thinness of the carbon layer, only 8 nm as shown in Fig. 3a, enabling easy and fast graphitization. The Ndoping was confirmed with the high resolution XPS spectrum of N1s shown in Fig. S3. The characteristic binding energy peak located at around 400 eV for N1s can be further de-convoluted into
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Fig. 2. Raman spectrum of N · C/R-NRs.
three constituent peaks of 398.6, 399.9, and 401.2 eV. The 398.6 and 399.9 peaks are attributable to the presence of pyrridinic-like and pyrrolic-like N along the edge of some graphitic carbon nanostructure such as carbon dots, whereas the 401.2 peak is characteristic of quaternary N resulting from replacing graphitic carbon with N. [20,24] The more pronounced 399.9 peak implies a significant amount of carbon dots present in the N-doped carbon dots@layer. The carbon coating layer is dense and uniform with a thickness of around 8 nm as can be seen from the TEM image shown in Fig. 3a, forming the core-shell structure of N-carbon dots@layer/ rutile TiO2 nanorod array. Further close examination of the carbon layer reveals the presence of carbon dots of size of 3.3 ± 0.4 nm (Fig. 3b). Layered lattice fringes were also observed to confirm the significant graphitization of the carbon layer. The inset of Fig. 3b shows the lattice fringes of a carbon dot and gives an interlayer distance of 0.22 nm, in good agreement with the d-spacing of crystalline planes of {11¯ 00} of graphite [24]. This unique carbon dots@layer nanostructure, fabricated with the polymerize-andcalcine process developed in this work, offers critical dual functionalities to enhance the light absorption, charge transport, and charge separation and thus to boost the photoelectrochemical efficiency of the sandwich nanostructure.
Fig. S4 shows photographs of anatase TiO2 product powders prepared under different synthetic conditions: un-doped and unhydrogenated white products, C-doped but un-hydrogenated light brown products, and C-doped and hydrogenated black products. The apparent change in product colour confirms the successful preparation of the hydrogenated C-doped anatase TiO2 nanocrystals. These hydrogenated C-doped TiO2 nanocrystals were characterized with XRD to be of anatase phase (JCPDS 21–1272) [16] as shown in Fig. S5, and were with sizes of 9 ± 4 nm as determined from the TEM image presented in Fig. S5a. An interlayer distance of 0.35 nm was determined from the TEM image of Fig. S5b, in good agreement with the d-spacing of the crystalline planes of (101) of anatase TiO2 . Fig. S6 shows the UV–visible reflectance spectrum of the hydrogenated C-doped anatase TiO2 nanocrystals. The low reflectance exhibited in the long wavelength region indicates the visible light harvesting capability of the hydrogenated C-doped anatase TiO2 nanocrystals. These hydrogenated C-doped anatase TiO2 nanocrystals were decorated onto the surface of the N-doped carbon dots@layer coated rutile TiO2 nanorods through spin coating to complete the construction of the sandwich nanostructure H–C·A-NPs/N·C/R-NRs. The C-doping of the anatase TiO2 nanocrystals was confirmed with the high resolution XPS characterization of H–C·A-NPs/N·C/R-NRs on C1s as shown in Fig. 4. The characteristic binding energy peak of C1s was de-convoluted into 6 constituent peaks, including peaks located at 284.8, 285.6, 286.2, 287.0, 288.7, and 289.2 eV, attributable to the presence of the C–C, C–N, C–O, C = O, C–O-Ti, and O–C = O bonds. [27] All constituent peaks, except for the 288.7 one attributed to the C–O-Ti bond, were contributed by the N-doped carbon dots@layer, with the C–C bond of the carbon as the main contributor, the C–N bond from the N-doping of the carbon coming next, and the C–O, C = O, and O–C = O bonds from the minor oxidation of the carbon, particularly along the graphitic edge, the least. The peak at 288.7 eV, contributed by the C–O-Ti bond, confirms the C-doping of the anatase TiO2 nanocrystals. [28] Fig. 5a shows the SEM image of an individual H–C·A-NPs/N·C/RNR, with the nanosized H–C·A-NPs clearly visible to be uniformly distributed on the surface of the nanorod. Fig. 5b gives the edge shot of the nanorod, revealing the sandwich structure of the product, with the rutile TiO2 nanorod as the core, the N-doped carbon dots@layer as the middle layer, and the hydrogenated C-doped anatase TiO2 nanocrystals as the outermost layer. 3.2. Optical properties The main consideration of the present design on materials and nanostructure is to improve the visible light harvesting and charge
Fig. 3. (a) TEM image of N-doped carbon dots@layer coated on rutile TiO2 nanorod. (b) HRTEM image of N-doped carbon dots dispersed in N-doped carbon layer. Inset shows lattice fringes of a single carbon dot.
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whereas the one centred at 468 nm is caused by the metal-ligand charge transfer from Ti4+ to the oxygen anion of the TiO6 8− complex. [29] Evidently, with incorporation of the N-doped carbon dots@layer and hydrogenated C-doped anatase TiO2 nanocrystals, the PL intensity decreases, an indication of the enhanced charge separation. A significant PL quenching was achieved through the coating of the N-doped carbon dots@layer, whereas further decoration of the hydrogenated C-doped anatase TiO2 nanocrystals results in less pronounced PL quenching. The heterostructures created between the rutile TiO2 nanorods and the N-doped carbon dots@layer and with decoration of the outermost hydrogenated C-doped anatase TiO2 nanocrystals do significantly enhance the charge separation for boosting the photoelectrochemical efficiency on water oxidation. 3.3. Band structure analysis
Fig. 4. High resolution XPS spectrum of C1s of H–C · A-NPs/N · C/R-NRs.
separation of the plain rutile TiO2 nanorod array to enhance its photoelectrochemical efficiency on water oxidation. The introduction of the N-doped carbon dots@layer is particularly critical for achieving the above mentioned goal, with the N-doped carbon dots serving as the photosensitizer to enhance the absorption of long wavelength light and the N-doped carbon layer as the conductive layer to facilitate not only charge transport but also to attract photo-induced electrons for charge separation enhancement. Fig. 6a shows the reflectance spectra of the R-NRs, N·C/R-NRs, and H–C·A-NPs/N·C/R-NRs samples. The three spectra were recorded on a same R-NRs sample, sequentially coated first with the N·C layer followed by the H–C·A-NPs layer so that the spectra can be directly compared. Evidently, with incorporation of the N-doped carbon dots@layer and hydrogenated C-doped anatase TiO2 nanocrystals, the reflectance at the visible light region decreases, thus increasing the visible light utilization. A significant visible light harvesting enhancement was achieved through the coating of the N-doped carbon dots@layer, whereas further decoration of the hydrogenated C-doped anatase TiO2 nanocrystals leads to only minor additional improvements, likely because of the limited loading amount of the hydrogenated C-doped anatase TiO2 nanocrystals. Fig. 6b shows the PL spectra of the three structures excited with a 300 nm light. Two main emission peaks were observed. The one centred at 415 nm is the excitonic emission of rutile TiO2 ,
The band structure of the sandwich nanostructure was constructed to further explain the charge separation enhancement. Here, the Fermi level is taken as the reference point for the band structure construction. With the bandgap energies and valence band positions of the constituent semiconductors available, one can construct the band structure. First, the valence band position of rutile TiO2 has been determined to be 2.67 eV below the Fermi level. [30] As to the hydrogenated C-doped anatase TiO2 nanocrystals, the valence band position was determined with valence band XPS to be 1.52 eV below the Fermi level, with the band tail state [31] located at 0.53 eV below the Fermi level as shown in Fig. 7a. The bandgap energies of the rutile TiO2 nanorods and the hydrogenated C-doped anatase TiO2 nanocrystals were determined from the Tauc plots, derived from the corresponding UV-visible absorption spectra, to be 3.09 and 3.34 eV, respectively as shown in Fig. 7b. [31–34] With the valence band position and bandgap energy data available, the conduction band positions of the rutile TiO2 nannorods and the hydrogenated C-doped anatase TiO2 nanocrystals can be determined to be 0.42 and 1.82 eV above the Fermi level, respectively. The band structure of the sandwich nanostructure can thus be constructed as shown in Fig. 7c. Evidently, the rutile TiO2 nannorods and the hydrogenated C-doped anatase TiO2 nanocrystals form a type II heterostructure facilitated with an intermediate conductive carbon layer. The photo-induced electrons from both the rutile TiO2 nannorod and the hydrogenated C-doped anatase TiO2 nanocrystal domains tend to migrate to the much more conductive carbon layer to be transported toward the current collector, whereas the photo-induced holes from the rutile TiO2 nannorod domain tend to migrate toward the hydrogenated C-doped anatase TiO2 nanocrystal domain to proceed with the water oxidation re-
Fig. 5. (a) SEM image of individual H–C · A-NPs/N · C/R-NRs and (b) TEM image of edge of individual H–C · A-NPs/N · C/R-NRs.
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Fig. 6. (a) UV-visible reflectance spectra and (b) PL spectra of R-NRs, N · C/R-NRs, and H-C · A-NPs/N · C/R-NRs.
Fig. 7. (a) Valence band XPS spectrum of H–C · A-NPs. (b) Tauc plots of R-NRs and H–C · A-NPs. (c) Band structure of H–C · A-NPs/N · C/ R-NRs.
action. This way, the separation of the photo induced charges, electrons and holes, is significantly enhanced. 3.4. Photoelectrochemical properties Fig. 8a and 8b show the photocurrent density vs. applied potential (vs. RHE) curves of the R-NRs, N · C/R-NRs, and H–C · ANPs/N · C/R-NRs photoanodes under illumination of simulated sun light of AM1.5 G at 100 mW/cm2 and visible simulated sun light of wavelengths greater than 400 nm, respectively. [35] The cur-
rent densities measured in dark were also included for comparison. Several points can be made from the figures. First, the dark current densities can be safely neglected, excluding the possible interferences from leakage currents, electrolytic water splitting, etc. Second, the introduction of the N-doped carbon dots@layer onto the surface of the plain rutile TiO2 nanorods results in pronounced improvements, boosting the photocurrent densities from 0.42 to 0.72 mA/cm2 at 1.23 V (vs. RHE) under the simulated sun light illumination. Decoration of the hydrogenated C-doped anatase TiO2 nanocrystals further raises the photocurrent densities from 0.72 to
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Fig. 8. Photocurrent density vs. applied potential (vs. RHE) curves of R-NRs, N · C/R-NRs, and H–C · A-NPs/ N · C/R-NRs under illumination of (a) simulated sun light at 100 mW/cm2 and (b) visible simulated sum light.
Fig. 9. (a) Optical response curves, (b) photoconversion efficiency curves, and (c) long term stability curves of R-NRs, N · C/R-NRs, and H–C · A-NPs/N · C/R-NRs.
0.8 mA/cm2 . This trend is consistent with those of the reflectance reduction and PL quenching, signifying the critical contribution of the N-doped carbon dots@layer and hydrogenated C-doped anatase TiO2 nanocrystals. The total improvement in photoelectrochemical efficiency as quantified by the photocurrent density enhancement achieved at 1.23 V (vs. RHE) is 86% (0.8 vs. 0.42 mA/cm2 ) over the plain rutile TiO2 array electrode. The improvement is even larger if the measurements were conducted under visible simulated sun light illumination. As shown in Fig. 8b, the photocurrent density at 1.23 V (vs. RHE) increases from 25 to 65 and then to 82 μA/cm2 with subsequent incorporation of the N-doped carbon dots@layer and hydrogenated C-doped anatase TiO2 nanocrystals. The total
improvement is 228% (82 vs. 25 μA/cm2 ) over the plain rutile TiO2 nanorod array electrode. These improvements may be attributed to the incorporation of the N-doped carbon dots as the photosensitizer and hydrogenated C-doped TiO2 nanocrystals for enhanced light harvesting, the introduction of the N-doped carbon layer as the conductive layer for accelerated charge transport and induced charge separation, and formation of a type II heterostructure for further enhanced charge separation. The optical responsiveness of the three photoanodes, R-NRs, N · C/R-NRs, and H–C · A-NPs/N · C/ R-NRs, was further characterized with a three cycle optical on-off test at 1.23 V (vs. RHE) as shown in Fig. 9a. All three photoanodes exhibits fast responses
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to and stable photocurrent outputs under light illumination, and the dark currents are negligible. The photoconversion efficiencies of the three photoanodes were evaluated and compared in Fig. 9b. The maximum photoconversion efficiency and the optimal applied potential to achieve the maximum photoconversion efficiency are two important parameters to investigate. High photoconversion efficiency and low optimal applied potential are desired for the photoanode. It is evident from the figure that the photoconversion efficiency increases and the optimal applied potential decreases with subsequent incorporation of the N-doped carbon dots@layer and the hydrogenated C-doped anatase TiO2 nanocrystals, with the maximum photoconversion efficiency and optimal applied potential improving from 0.23% at 0.8 V (vs. RHE) for the R-NRs electrode to 0.56% at 0.55 V for the N · C/R-NRs, and to 0.65% at 0.53 V for the H–C · A-NPs/N · C/ R-NRs electrode. The long term stability of the three photoanodes, when operated at the optimal applied potential, 0.8 V (vs.RHE) for the R-NRs electrode, 0.55 V for the N · C/R-NRs electrode, and 0.53 V for the H–C · A-NPs/N · C/ RNRs electrode, was monitored for a continuous operation of 5 h. The results shown in Fig. 9c reveal that, in terms of the photocurrent retention rate at the end of the 5 h operation, the stability of the plain rutile TiO2 nanorod array electrode, was improved by 33% with the incorporation of the N-doped carbon dots@layer and by 40% with the additional incorporation of the hydrogenated C-doped anatase TiO2 nanocrystals. Evidently, the Ndoped carbon dots@layer plays the key role in enhancement of not just the photoelectrochemical efficiency but also the photoelectrochemical stability for the plain rutile TiO2 nanorod array electrode [36].
4. Conclusions A cocktail approach was developed for the design of materials and nanostructure of photoanodes of photoelectrochemical water oxidation to achieve significant enhancements on the photoelectrochemical efficiency and stability of the photoanode. Low cost and low toxicity materials, such as TiO2 and carbon, were chosen for the purpose. The design aims to achieve the goal through improvements on visible light harvesting, charge transport, and charge separation. These means were realized with incorporation of the N-doped carbon dots, N-doped carbon layer, hydrogenated C-doped anatase TiO2 nanocrystals, and formation of a type II heterostructure between the rutile TiO2 nanorods and the hydrogenated C-doped anatase TiO2 nanocrystals. The cocktail approach was proven successful with pronounced enhancements on the photoelectrochemical efficiency and stability of the photoanode achieved, 86% more photocurrent density at 1.23 V (vs. RHE) and 40% higher stability at 0.53 V (vs. RHE) under illumination of simulated sun light at 100 mW/cm2 . The present work is a successful demonstration of the cocktail strategy for development of efficient photocatalysts for photoelectrochemical water oxidation, combining different functionalities offered by different materials into one suitably configured composite heterostructured photoanode. The present approach can be readily applied to other material systems, although cautions should be taken on material compatibility and nanostructure assembly to realize a successful integration of compatible and complementary constituent materials.
Acknowledgements This study was financially supported by the Ministry of Science and Technology of Taiwan under grant MOST 105-2622-8-007 −009.
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2018) 1–9
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N-doped carbon dots@layer facilitated heterostructure of TiO2 polymorphs for efficient photoelectrochemical water oxidation Chan-Wei Hsu a, Chia-Hsun Li a, Lei Zhang b, Shih-Yuan Lu a,∗ a b
Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu 30013, Taiwan School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan Anhui 232001, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2018 Revised 1 August 2018 Accepted 3 August 2018 Available online xxx Keywords: Tio2 nanorod arrays Polydopamine N-doped carbon dots C-doped anatase Tio2 Photoelectrochemical performance
a b s t r a c t A cocktail strategy was developed to fabricate sandwich nanostructure of hydrogenated C-doped anatase TiO2 nanocrystals/N-doped carbon dots@layer/rutile TiO2 nanorod array as an efficient photocatalyst for photoelectrochemical water oxidation. The one-dimensional, single crystalline nature of the innermost rutile TiO2 nanorod facilitates charge transport. The middle N-doped carbon dots@layer offers dual functionalities, with the N-doped carbon dots as the photosensitizer to harvest long wavelength lights and the N-doped carbon layer as the conductive layer for fast charge transport toward the current collector. The outermost hydrogenated C-doped anatase TiO2 nanocrystals serve to increase the visible light absorption and to form a type II heterostructure with the innermost rutile TiO2 nanorods to enahnce the charge separation. An enhancement of 86% in photocurrent density was achieved at 1.23 V (vs. RHE) under illumination of simulated sun light of 100 mW/cm2 by the sandwich nanostrcuture, as compared to that of the plain rutile TiO2 nanorod array. The enhancement was boosted to 228% under visible simulated sun light (λ > 400 nm) illumination. The photoconversion efficiency and photocurrent density retention rate were significantly improved from 0.23% and 60% for the plain rutile TiO2 nanorod array to 0.65% and 84% for the sandwich nanostructure, respectively. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Hydrogen production from photoelectrochemical (PEC) water splitting catalysed with semiconductor photoelectrocatalysts is considered a promising renewable energy technology and has drawn a great deal of research attention [1]. For design of a successful photoelectrocatalyst, several key factors have to be taken into consideration, including light absorption, charge separation, and charge transport. It is however unlikely to fulfill all desired requirements with single component catalysts, and thus composite catalysts are sought after to improve the photoelectrochemical efficiency of the catalyst [2]. TiO2 , because of its high photocatalytic activity, chemical stability, low toxicity, and low cost, is still considered the most promising material for photoanodes in photoelectrochemical devices. Its most detrimental drawback is its large energy bandgap, photoresponsive only in the UV region, severely restricting its overall photoconversion efficiency and thus its practical applications [3]. Consequently, a wide range of approaches has been developed to improve on the situation, including doping, bandgap engineering,
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (S.-Y. Lu).
defect engineering, and heterostructure construction. [3,4] Among them, heterostructure constructed from a type II junction involving usage of a narrow bandgap material, offers not only enhanced charge separation, but also enlarged light absorption spectra [3,4]. Carbon dots, a new member of the versatile carbon family, because of their unique properties, including low cytotoxicity, excellent biocompatibility, and high mobility for photo-induced electrons, [5–8] have attracted extensive research attention and been applied to a wide spectrum of applications, such as chemical sensing, bio-sensing, bio-imaging, nanomedicine, electrocatalysis, and photocatalysis [9]. Moreover, nitrogen doped carbon materials gain increasing popularity for their ability to induce charge delocalization and adjust the work function of carbon, both enhancing the functionality of carbon dots as an effective photosensitizer [10]. Zhang et al. prepared N-doped carbon dot decorated TiO2 nanoparticles and applied them in photodegradation of pollutants [10]. The photocatalytic efficiency was significantly improved because of the effective absorption and utilization of visible light achieved with the N-doped carbon dots serving as an effective photosensitizer [10]. Dopamine is an important neurotransimitter, existing in the central nerve system of mammals. It can self-polymerize in an alkaline solution to form a dense, conformal polydopamine layer on a wide range of substrates. Furthermore, the amino group
https://doi.org/10.1016/j.jtice.2018.08.007 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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contained in dopamine often leads to N-doping of the resulting carbon upon carbonization of the polydopamine layer [11]. It is thus a popular templating material for fabrication of novel conductive carbon nanostructures [12,13]. In this work, we developed a simple polymerize-and-calcine process to successfully fabricate a novel composite nanostructure of N-doped carbon dots well dispersed in N-doped carbon layer, termed N-doped carbon dots@layer, from dopamine. This composite carbon nanostructure offers dual functionalities, with the N-doped carbon dots serving as a photosensitizer for absorption and utilization of long wavelength light and the N-doped carbon layer as an electron conducting medium [14]. This unique property can find potential applications in a wide spectrum of photocatalytic devices, including photoelectrochemical water oxidation. Hydrogenated TiO2 , the so-called black TiO2 , has attracted a great deal of research attention in recent years. The hydrogenation of TiO2 nanocrystals generates a thin disorder layer that induces band tail states to up-lift the effective valence band toward the vacuum level to shrink the bandgap of the TiO2 nanocrystals for absorption of visible light, turning the colour of the TiO2 nanocrystals from white to black [14]. This thin disorder layer also offers trapping sites for photo-induced charge carriers to promote charge separation and thus the photocatalytic activity of the TiO2 nanocrystals [15]. Chen et al. prepared black TiO2 through hydrogen reduction of anatase TiO2 at 200 °C in 20 bar hydrogen for 5 days [15]. The product powders exhibited outstanding photocatalytic activities [15]. Although successful, the process requires high pressures and high concentrations of hydrogen, which is dangerous and not amenable to large scale applications. Zeng et al. developed a low concentration hydrogen reduction process for black TiO2 production, operated in a 5%H2 + 95%Ar atmosphere of ambient pressure [16]. The product powders exhibited excellent photocatalytic activity toward degradation of formaldehyde, attributable to the effective utilization of long wavelength light and enhanced charge separation provided by black TiO2 . In this work, we developed a cocktail approach for the material and nanostructure design of composite heterostructured semiconductor photocatalysts to boost the efficiency of photoelectrochemical water oxidation for hydrogen generation [17]. The cocktail approach involves 1D single crystalline rutile TiO2 core for guided fast charge transport and enlarged functioning surfaces, [18] N-doped carbon dots well dispersed in N-doped carbon layer as the middle layer offering long wavelength absorption [10] and fast transport for photo-induced charges, and decoration of hydrogenated C-doped anatase TiO2 nanocrystals as the outermost layer to offer long wavelength absorption and to form a type II heterostructure with the rutile TiO2 core for enhanced charge separation. This composite heterostructured semiconductor photocatalyst is expected to exhibit enhanced light harvesting, charge separation, and charge transport, and thus much improved photoconversion efficiencies over the plain rutile TiO2 nanorod array electrode. The N-doped carbon dots@layer was synthesized through carbonization of self-polymerized dopamine layer coated on the surface of the rutile TiO2 nanorods. This simple and safe polymerize-and-calcine process gives rise to a unique composite nanostructure of well dispersed N-doped carbon dots in N-doped carbon layer offering dual functionalities, unlike the carbon layer only case achieved with hydrothermal processes of high pressures [19–21]. The present polymerize-and-calcine process can be readily applied to other photocatalytic systems. In addition, coupling hydrogenated C-doped anatase TiO2 nanocrystals with single crystalline rutile TiO2 manorods to form a type II heterostructure for charge separation enhancement, facilitated with the N-doped carbon layer, is a novel application of TiO2 polymorphs. With construction of the sandwich nanostructure, the photocurrent density of the rutile TiO2 nanorod array was boosted by 86% at 1.23 V
(vs. RHE) under simulated sun light illumination, and by 228% at 1.23 V (vs. RHE) under illumination of visible simulated sun light of λ > 400 nm. 2. Experimental section 2.1. Materials Hydrochloric acid (HCl), titanium(IV) isopropoxide (TTIP), dopamine hydrochloride (C6 H3 CH2 CH2 NH2 (HO)2 ·HCl), tris(hydroxymethyl)-aminomethane (TRIS), and ethylene glycol were used as received without further purification. 2.2. Fabrication of rutile TiO2 nanorod array The rutile TiO2 nanorod array was fabricated on FTO glass, termed R-NRs standing for rutile-nanorods, with a hydrothermal method. Briefly, 0.48 mL TTIP was dissolved into 30 mL aqueous HCl solution (15 mL deionized (DI) water mixed with 15 mL concentrated HCl (37%)). The mixture, together with a piece of FTO glass, was then transferred into a Teflon lined steel autoclave of a capacity of 50 mL. The autoclave was heated in an oven set at 150 °C for 5 h and let cool naturally to room temperature. The rutile TiO2 nanorods coated FTO glass was rinsed with DI water and calcined at 500 °C for 3 h in air for later use. 2.3. Fabrication of N-doped carbon dots@layer/R-NRs The rutile TiO2 nanorod coated FTO glass was dipped vertically into 200 mL TRIS aqueous buffer solution (0.002 M) with the pH value of the solution adjusted to 8.5 by using concentrated HCl. An amount of 80 mg of dopamine hydrochloride was dissolved in the above solution under gentle magnetic stirring. The system was then kept still for 15 h at room temperature to allow in situ coating. The coated sample was rinsed with DI water and annealed at 500 °C, with a heating rate of 1 °C/min, for 3 h in argon to form the N-doped carbon dots@layer/rutile TiO2 nanorod array, termed N · C/R-NRs. 2.4. Fabrication of hydrogenated C-doped anatase nanocrystals/N · C/R-NRs The hydrogenated C-doped anatase TiO2 nanocrystals (H–C · ANPs) were first synthesized following the procedures developed by Zeng et al. [16] Briefly, 1 mL TTIP was first mixed thoroughly with 25 mL ethylene glycol, and then added slowly into 50 mL DI water under stirring for 30 min. The mixture was transferred to an autoclave for hydrothermal treatment at 180 °C for 10 h to afford the light brown C-doped anatase TiO2 nanocrystals. Non-doped white anatase TiO2 nanocrystals were also prepared for comparison with the same procedures described above, except for the addition of ethylene glycol. These C-doped anatase TiO2 nanocrystals were thermally treated at 300 °C in a 5%H2 + 95%Ar atmosphere for 2 h to afford the black hydrogenated C-doped anatase TiO2 nanocrystals. An amount of 0.01 g H–C · A-NPs was dispersed in 99.8% absolute ethanol, and then deposited onto the N · C/R-NRs with spin coating at 40 0 0 rpm for 30 s. The product was calcined at 400 °C for 1 h in argon to complete the construction of the sandwich nanostructure, H–C · A-NPs/N · C/R-NRs. 2.5. Characterizations The crystalline phase of the sample was investigated with powder X-ray diffraction measurements (XRD, Ultima IV, Rigaku, Japan). The morphology and crystalline structure were observed
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Scheme 1. Schematic of fabrication procedures for sandwich nanostructure H–C · A-NPs/N · C/R-NRs: (a) FTO glass, (b) rutile TiO2 nanorods grown on FTO glass (R-NRs), (c) N · C/R-NRs before carbonization, (d) N · C/R-NRs, (e) H–C · A-NPs, (f) H–C · A-NPs/N · C/R-NRs.
with a field-emission scanning electron microscope (FE-SEM) (Hitachi SU8010) and a transmission electron microscope (TEM, JEOL JEM 30 0 0F). The UV − visible spectra were acquired with a Hitachi U-3300 UV − visible spectrometer under the diffuse-reflection mode using an integrating sphere (P/N 2J2-0175). The photoluminescence (PL) spectra were recorded using a Hitachi F-4500 fluorescence spectrophotometer at an excitation wavelength of 300 nm. The chemical status of elements was analyzed with X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 50 0 0 Versaprobe II). 2.6. Photoelectrochemical measurement For the PEC measurements, an aqueous solution containing 1 M KOH was used as the electrolyte. The sample served as the photoanode and a Pt sheet of size 1 × 1 cm2 was employed as the counter electrode. An Hg/HgO (in 1 M NaOH, RE-61AP) electrode suitable for alkaline environment was used as the reference electrode. The light source was provided with a solar simulator (Newport, AM 1.5, 100 mW/cm2 ), and a cut-off filter (λ > 400 nm) was used to provide the visible light source. The PEC performances of the sample electrode were measured using a CHI electrochemical analyzer (CHI600E). The photoconversion efficiency (η) was calculated based on the following equation: [22]
η (% ) =
Jph × (1.23 − |Emeas − Eoc | ) I0
× 100%
(1)
where Jph is the photocurrent density achieved at the applied potential, Io is the power density of the incident light (100 mW/cm2 ), Emeas is the potential applied to photoanode versus the reference electrode, Eoc is the open circuit potential of the photoanode under illumination. All measured potentials were converted to refer to RHE (reversible hydrogen electrode) according to the following Nernst equation: 0 ERHE = ERE −61AP + 0.0592 × pH + ERE −61AP
(2)
where ERHE is the potential vs RHE, ERE-61AP is the measured po0 tential vs Hg/HgO, and ERE−61 = 0.118 V at 25 °C. AP 3. Results and discussion 3.1. Materials characterizations Scheme 1 shows the fabrication procedures for the sandwich nanostructure of hydrogenated C-doped TiO2 nanocrystals/N-doped carbon dots@layer/rutile TiO2 nanorod array, H–C · A-NPs/N · C/RNRs. The single crystalline rutile TiO2 nanorod array was grown on an FTO glass with a hydrothermal process followed by annealing in air atmosphere. The nanorod array was coated with an N-doped
Fig. 1. Photographs of (a) rutile TiO2 nanorod array coated FTO glass (R-NRs), (b) R-NRs after polydopamine coating, (c) N-doped carbon dots@layer coated R-NRs (N · C/R-NRs).
carbon dots@layer through carbonization of the first formed polydopamine thin layer on the nanorod surface. Finally, hydrogenated C-doped anatase TiO2 nanocrystals, prepared from hydrogenation of C-doped TiO2 nanocrystals, were coated onto the surface of the N·C/R-NRs via a spin-coating process to complete the construction of the sandwich nanostructure. The TiO2 nanorods grown on the surface of the FTO glass was confirmed to be of rutile phase, JCPDS 88–1175, with the XRD pattern shown in Fig. S1 [23]. The morphology was observed to be of nanorod array as clearly evident from the SEM image of Fig. S2. The nanorods, with a length of about 2.1 μm, appeared to be with a square cross section of a side length of around 150 nm. If examined closely with a TEM, each nanorod is in fact composed of bundled nanorods of much smaller diameters of around 5 nm. The rutile TiO2 nanorod array coated FTO glass appeared white as shown in Fig. 1(a), and turned black after the coating of the polydopamine layer. The color turned white again as the polydopamine was carbonized into the N-doped carbon dots@layer. The N-doped carbon dots@layer was characterized with Raman spectroscopy. The resulting Raman spectrum, as shown in Fig. 2, exhibits two characteristic peaks located at 1369 and 1587 cm−1 . The 1369 peak is contributed by the d-band of the carbon material, attributable to the presence of amorphous carbon and the disorders caused by the N-doping, whereas the 1587 peak is characteristic of the G-band of the carbon material resulting from the E2g mode of graphite through vibration of graphitic sp2 carbon. [24,25] The G-band intensity is significantly higher than that of the d-band, indicating the high degree graphitization of the carbon material, beneficial for the involved charge transport. Interestingly, significant graphitization was achieved at only 500 °C, [26] possibly because of the thinness of the carbon layer, only 8 nm as shown in Fig. 3a, enabling easy and fast graphitization. The Ndoping was confirmed with the high resolution XPS spectrum of N1s shown in Fig. S3. The characteristic binding energy peak located at around 400 eV for N1s can be further de-convoluted into
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Fig. 2. Raman spectrum of N · C/R-NRs.
three constituent peaks of 398.6, 399.9, and 401.2 eV. The 398.6 and 399.9 peaks are attributable to the presence of pyrridinic-like and pyrrolic-like N along the edge of some graphitic carbon nanostructure such as carbon dots, whereas the 401.2 peak is characteristic of quaternary N resulting from replacing graphitic carbon with N. [20,24] The more pronounced 399.9 peak implies a significant amount of carbon dots present in the N-doped carbon dots@layer. The carbon coating layer is dense and uniform with a thickness of around 8 nm as can be seen from the TEM image shown in Fig. 3a, forming the core-shell structure of N-carbon dots@layer/ rutile TiO2 nanorod array. Further close examination of the carbon layer reveals the presence of carbon dots of size of 3.3 ± 0.4 nm (Fig. 3b). Layered lattice fringes were also observed to confirm the significant graphitization of the carbon layer. The inset of Fig. 3b shows the lattice fringes of a carbon dot and gives an interlayer distance of 0.22 nm, in good agreement with the d-spacing of crystalline planes of {11¯ 00} of graphite [24]. This unique carbon dots@layer nanostructure, fabricated with the polymerize-andcalcine process developed in this work, offers critical dual functionalities to enhance the light absorption, charge transport, and charge separation and thus to boost the photoelectrochemical efficiency of the sandwich nanostructure.
Fig. S4 shows photographs of anatase TiO2 product powders prepared under different synthetic conditions: un-doped and unhydrogenated white products, C-doped but un-hydrogenated light brown products, and C-doped and hydrogenated black products. The apparent change in product colour confirms the successful preparation of the hydrogenated C-doped anatase TiO2 nanocrystals. These hydrogenated C-doped TiO2 nanocrystals were characterized with XRD to be of anatase phase (JCPDS 21–1272) [16] as shown in Fig. S5, and were with sizes of 9 ± 4 nm as determined from the TEM image presented in Fig. S5a. An interlayer distance of 0.35 nm was determined from the TEM image of Fig. S5b, in good agreement with the d-spacing of the crystalline planes of (101) of anatase TiO2 . Fig. S6 shows the UV–visible reflectance spectrum of the hydrogenated C-doped anatase TiO2 nanocrystals. The low reflectance exhibited in the long wavelength region indicates the visible light harvesting capability of the hydrogenated C-doped anatase TiO2 nanocrystals. These hydrogenated C-doped anatase TiO2 nanocrystals were decorated onto the surface of the N-doped carbon dots@layer coated rutile TiO2 nanorods through spin coating to complete the construction of the sandwich nanostructure H–C·A-NPs/N·C/R-NRs. The C-doping of the anatase TiO2 nanocrystals was confirmed with the high resolution XPS characterization of H–C·A-NPs/N·C/R-NRs on C1s as shown in Fig. 4. The characteristic binding energy peak of C1s was de-convoluted into 6 constituent peaks, including peaks located at 284.8, 285.6, 286.2, 287.0, 288.7, and 289.2 eV, attributable to the presence of the C–C, C–N, C–O, C = O, C–O-Ti, and O–C = O bonds. [27] All constituent peaks, except for the 288.7 one attributed to the C–O-Ti bond, were contributed by the N-doped carbon dots@layer, with the C–C bond of the carbon as the main contributor, the C–N bond from the N-doping of the carbon coming next, and the C–O, C = O, and O–C = O bonds from the minor oxidation of the carbon, particularly along the graphitic edge, the least. The peak at 288.7 eV, contributed by the C–O-Ti bond, confirms the C-doping of the anatase TiO2 nanocrystals. [28] Fig. 5a shows the SEM image of an individual H–C·A-NPs/N·C/RNR, with the nanosized H–C·A-NPs clearly visible to be uniformly distributed on the surface of the nanorod. Fig. 5b gives the edge shot of the nanorod, revealing the sandwich structure of the product, with the rutile TiO2 nanorod as the core, the N-doped carbon dots@layer as the middle layer, and the hydrogenated C-doped anatase TiO2 nanocrystals as the outermost layer. 3.2. Optical properties The main consideration of the present design on materials and nanostructure is to improve the visible light harvesting and charge
Fig. 3. (a) TEM image of N-doped carbon dots@layer coated on rutile TiO2 nanorod. (b) HRTEM image of N-doped carbon dots dispersed in N-doped carbon layer. Inset shows lattice fringes of a single carbon dot.
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whereas the one centred at 468 nm is caused by the metal-ligand charge transfer from Ti4+ to the oxygen anion of the TiO6 8− complex. [29] Evidently, with incorporation of the N-doped carbon dots@layer and hydrogenated C-doped anatase TiO2 nanocrystals, the PL intensity decreases, an indication of the enhanced charge separation. A significant PL quenching was achieved through the coating of the N-doped carbon dots@layer, whereas further decoration of the hydrogenated C-doped anatase TiO2 nanocrystals results in less pronounced PL quenching. The heterostructures created between the rutile TiO2 nanorods and the N-doped carbon dots@layer and with decoration of the outermost hydrogenated C-doped anatase TiO2 nanocrystals do significantly enhance the charge separation for boosting the photoelectrochemical efficiency on water oxidation. 3.3. Band structure analysis
Fig. 4. High resolution XPS spectrum of C1s of H–C · A-NPs/N · C/R-NRs.
separation of the plain rutile TiO2 nanorod array to enhance its photoelectrochemical efficiency on water oxidation. The introduction of the N-doped carbon dots@layer is particularly critical for achieving the above mentioned goal, with the N-doped carbon dots serving as the photosensitizer to enhance the absorption of long wavelength light and the N-doped carbon layer as the conductive layer to facilitate not only charge transport but also to attract photo-induced electrons for charge separation enhancement. Fig. 6a shows the reflectance spectra of the R-NRs, N·C/R-NRs, and H–C·A-NPs/N·C/R-NRs samples. The three spectra were recorded on a same R-NRs sample, sequentially coated first with the N·C layer followed by the H–C·A-NPs layer so that the spectra can be directly compared. Evidently, with incorporation of the N-doped carbon dots@layer and hydrogenated C-doped anatase TiO2 nanocrystals, the reflectance at the visible light region decreases, thus increasing the visible light utilization. A significant visible light harvesting enhancement was achieved through the coating of the N-doped carbon dots@layer, whereas further decoration of the hydrogenated C-doped anatase TiO2 nanocrystals leads to only minor additional improvements, likely because of the limited loading amount of the hydrogenated C-doped anatase TiO2 nanocrystals. Fig. 6b shows the PL spectra of the three structures excited with a 300 nm light. Two main emission peaks were observed. The one centred at 415 nm is the excitonic emission of rutile TiO2 ,
The band structure of the sandwich nanostructure was constructed to further explain the charge separation enhancement. Here, the Fermi level is taken as the reference point for the band structure construction. With the bandgap energies and valence band positions of the constituent semiconductors available, one can construct the band structure. First, the valence band position of rutile TiO2 has been determined to be 2.67 eV below the Fermi level. [30] As to the hydrogenated C-doped anatase TiO2 nanocrystals, the valence band position was determined with valence band XPS to be 1.52 eV below the Fermi level, with the band tail state [31] located at 0.53 eV below the Fermi level as shown in Fig. 7a. The bandgap energies of the rutile TiO2 nanorods and the hydrogenated C-doped anatase TiO2 nanocrystals were determined from the Tauc plots, derived from the corresponding UV-visible absorption spectra, to be 3.09 and 3.34 eV, respectively as shown in Fig. 7b. [31–34] With the valence band position and bandgap energy data available, the conduction band positions of the rutile TiO2 nannorods and the hydrogenated C-doped anatase TiO2 nanocrystals can be determined to be 0.42 and 1.82 eV above the Fermi level, respectively. The band structure of the sandwich nanostructure can thus be constructed as shown in Fig. 7c. Evidently, the rutile TiO2 nannorods and the hydrogenated C-doped anatase TiO2 nanocrystals form a type II heterostructure facilitated with an intermediate conductive carbon layer. The photo-induced electrons from both the rutile TiO2 nannorod and the hydrogenated C-doped anatase TiO2 nanocrystal domains tend to migrate to the much more conductive carbon layer to be transported toward the current collector, whereas the photo-induced holes from the rutile TiO2 nannorod domain tend to migrate toward the hydrogenated C-doped anatase TiO2 nanocrystal domain to proceed with the water oxidation re-
Fig. 5. (a) SEM image of individual H–C · A-NPs/N · C/R-NRs and (b) TEM image of edge of individual H–C · A-NPs/N · C/R-NRs.
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Fig. 6. (a) UV-visible reflectance spectra and (b) PL spectra of R-NRs, N · C/R-NRs, and H-C · A-NPs/N · C/R-NRs.
Fig. 7. (a) Valence band XPS spectrum of H–C · A-NPs. (b) Tauc plots of R-NRs and H–C · A-NPs. (c) Band structure of H–C · A-NPs/N · C/ R-NRs.
action. This way, the separation of the photo induced charges, electrons and holes, is significantly enhanced. 3.4. Photoelectrochemical properties Fig. 8a and 8b show the photocurrent density vs. applied potential (vs. RHE) curves of the R-NRs, N · C/R-NRs, and H–C · ANPs/N · C/R-NRs photoanodes under illumination of simulated sun light of AM1.5 G at 100 mW/cm2 and visible simulated sun light of wavelengths greater than 400 nm, respectively. [35] The cur-
rent densities measured in dark were also included for comparison. Several points can be made from the figures. First, the dark current densities can be safely neglected, excluding the possible interferences from leakage currents, electrolytic water splitting, etc. Second, the introduction of the N-doped carbon dots@layer onto the surface of the plain rutile TiO2 nanorods results in pronounced improvements, boosting the photocurrent densities from 0.42 to 0.72 mA/cm2 at 1.23 V (vs. RHE) under the simulated sun light illumination. Decoration of the hydrogenated C-doped anatase TiO2 nanocrystals further raises the photocurrent densities from 0.72 to
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Fig. 8. Photocurrent density vs. applied potential (vs. RHE) curves of R-NRs, N · C/R-NRs, and H–C · A-NPs/ N · C/R-NRs under illumination of (a) simulated sun light at 100 mW/cm2 and (b) visible simulated sum light.
Fig. 9. (a) Optical response curves, (b) photoconversion efficiency curves, and (c) long term stability curves of R-NRs, N · C/R-NRs, and H–C · A-NPs/N · C/R-NRs.
0.8 mA/cm2 . This trend is consistent with those of the reflectance reduction and PL quenching, signifying the critical contribution of the N-doped carbon dots@layer and hydrogenated C-doped anatase TiO2 nanocrystals. The total improvement in photoelectrochemical efficiency as quantified by the photocurrent density enhancement achieved at 1.23 V (vs. RHE) is 86% (0.8 vs. 0.42 mA/cm2 ) over the plain rutile TiO2 array electrode. The improvement is even larger if the measurements were conducted under visible simulated sun light illumination. As shown in Fig. 8b, the photocurrent density at 1.23 V (vs. RHE) increases from 25 to 65 and then to 82 μA/cm2 with subsequent incorporation of the N-doped carbon dots@layer and hydrogenated C-doped anatase TiO2 nanocrystals. The total
improvement is 228% (82 vs. 25 μA/cm2 ) over the plain rutile TiO2 nanorod array electrode. These improvements may be attributed to the incorporation of the N-doped carbon dots as the photosensitizer and hydrogenated C-doped TiO2 nanocrystals for enhanced light harvesting, the introduction of the N-doped carbon layer as the conductive layer for accelerated charge transport and induced charge separation, and formation of a type II heterostructure for further enhanced charge separation. The optical responsiveness of the three photoanodes, R-NRs, N · C/R-NRs, and H–C · A-NPs/N · C/ R-NRs, was further characterized with a three cycle optical on-off test at 1.23 V (vs. RHE) as shown in Fig. 9a. All three photoanodes exhibits fast responses
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to and stable photocurrent outputs under light illumination, and the dark currents are negligible. The photoconversion efficiencies of the three photoanodes were evaluated and compared in Fig. 9b. The maximum photoconversion efficiency and the optimal applied potential to achieve the maximum photoconversion efficiency are two important parameters to investigate. High photoconversion efficiency and low optimal applied potential are desired for the photoanode. It is evident from the figure that the photoconversion efficiency increases and the optimal applied potential decreases with subsequent incorporation of the N-doped carbon dots@layer and the hydrogenated C-doped anatase TiO2 nanocrystals, with the maximum photoconversion efficiency and optimal applied potential improving from 0.23% at 0.8 V (vs. RHE) for the R-NRs electrode to 0.56% at 0.55 V for the N · C/R-NRs, and to 0.65% at 0.53 V for the H–C · A-NPs/N · C/ R-NRs electrode. The long term stability of the three photoanodes, when operated at the optimal applied potential, 0.8 V (vs.RHE) for the R-NRs electrode, 0.55 V for the N · C/R-NRs electrode, and 0.53 V for the H–C · A-NPs/N · C/ RNRs electrode, was monitored for a continuous operation of 5 h. The results shown in Fig. 9c reveal that, in terms of the photocurrent retention rate at the end of the 5 h operation, the stability of the plain rutile TiO2 nanorod array electrode, was improved by 33% with the incorporation of the N-doped carbon dots@layer and by 40% with the additional incorporation of the hydrogenated C-doped anatase TiO2 nanocrystals. Evidently, the Ndoped carbon dots@layer plays the key role in enhancement of not just the photoelectrochemical efficiency but also the photoelectrochemical stability for the plain rutile TiO2 nanorod array electrode [36].
4. Conclusions A cocktail approach was developed for the design of materials and nanostructure of photoanodes of photoelectrochemical water oxidation to achieve significant enhancements on the photoelectrochemical efficiency and stability of the photoanode. Low cost and low toxicity materials, such as TiO2 and carbon, were chosen for the purpose. The design aims to achieve the goal through improvements on visible light harvesting, charge transport, and charge separation. These means were realized with incorporation of the N-doped carbon dots, N-doped carbon layer, hydrogenated C-doped anatase TiO2 nanocrystals, and formation of a type II heterostructure between the rutile TiO2 nanorods and the hydrogenated C-doped anatase TiO2 nanocrystals. The cocktail approach was proven successful with pronounced enhancements on the photoelectrochemical efficiency and stability of the photoanode achieved, 86% more photocurrent density at 1.23 V (vs. RHE) and 40% higher stability at 0.53 V (vs. RHE) under illumination of simulated sun light at 100 mW/cm2 . The present work is a successful demonstration of the cocktail strategy for development of efficient photocatalysts for photoelectrochemical water oxidation, combining different functionalities offered by different materials into one suitably configured composite heterostructured photoanode. The present approach can be readily applied to other material systems, although cautions should be taken on material compatibility and nanostructure assembly to realize a successful integration of compatible and complementary constituent materials.
Acknowledgements This study was financially supported by the Ministry of Science and Technology of Taiwan under grant MOST 105-2622-8-007 −009.
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