Jasmine
Bezboruah‡
,
Devendra
Mayurdhwaj Sanke‡
,
Ajay
Vinayakrao Munde
,
Palak
Trilochand Bhattad
,
Himadri
Shekhar Karmakar
and
Sanjio S.
Zade
*
Department of Chemical Sciences, Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, Nadia 741246, West Bengal, India. E-mail: sanjiozade@iiserkol.ac.in
First published on 9th November 2023
Visible light-driven photoelectrochemical (PEC) urea oxidation using inorganic/organic nano-heterostructure (NH) photoanodes is an attractive method for hydrogen (H2) production. In this article, inorganic/organic NHs (TiO2/PDIEH) consisting of a N,N-bis(2-ethylhexyl)perylene-3,4,9,10-tetracarboxylic diimide (PDIEH) thin layer over TiO2 nanorods (NRs) were fabricated for the PEC urea oxidation reaction (UOR). In these NHs, a PDIEH layer was anchored on TiO2 NR arrays using the spin-coating technique, which is beneficial for the uniform deposition of PDIEH on TiO2 NRs. Uniform deposition facilitated adequate interface contact between PDIEH and TiO2 NRs. TiO2/PDIEH NHs achieved a high current density of 1.1 mA cm−2 at 1.96 VRHE compared to TiO2 NRs. TiO2/PDIEH offers long-term stability under light illumination with 90.21% faradaic efficiency. TiO2/PDIEH exhibits a solar-to-hydrogen efficiency of 0.52%. This outcome opens up new opportunities for inorganic/organic NHs for high-performance PEC urea oxidation.
In 1972, the discovery of TiO2 electrodes as a photoelectrocatalyst by Fujishima and Honda opened the door for PEC studies.10 Later, extensive research on TiO2 in different forms, like TiO2 nanorods (NRs),11 nanotubes,12 and nanoparticles,13,14 as photocatalysts was carried out. Cho et al. reported branched TiO2 NRs for PEC H2 production.15 Niu et al. reported corrugated nanowire TiO2 as a versatile photoanode for PEC alcohol and water oxidation.16 Cho et al. reported codoping TiO2 nanowires with tungsten and carbon for enhanced PEC performance.17 Hwang et al. reported a TiO2/BiVO4/SnO2 triple-layer photoanode with enhanced PEC performance.18 Duan et al. reported a TiO2 nanowire/microflower photoanode modified with Au nanoparticles for efficient PEC water splitting.19 Park et al. reported photocatalytic UOR on TiO2 in water and urine.20 In addition to increasing the conductivity, TiO2 has been doped and blended with various metals21–23 and non-metals.24–26 To functionalize doped/undoped TiO2, various organic semiconductors have gradually emerged as beneficial candidates for designing TiO2 NH photoanodes.27–29
Perylene diimide (PDI) derivatives have received a lot of attention as organic semiconductors (OSCs)30,31 in the fields of fluorescent solar collectors,32 organic photovoltaics (OPVs),33 and organic field-effect transistors (OFETs).34 PDI derivatives possess excellent thermal and optical stability and good carrier transport properties.35,36 The relatively facile and reversible reduction process of PDI derivatives is important in many applications.37 The relatively simple and reversible reduction of PDIs is important in many of these applications. They are widely used in basic studies on photoinduced energy and electron-transfer processes because of their facile reductions, combined with their easily identifiable excited states, anions, and dianions via absorption spectra.30,31 Due to their fused-ring structures and strong intermolecular forces, they are poorly soluble in organic solvents. N-Alkyl substitution improves the solubility of PDIs in various organic solvents and enables the integration of PDI derivatives into NHs.30 To improve the photocatalytic activity of NHs, Zhang et al. reported NH composites of various PDI derivatives with TiO2.38 A TiO2 nanotube and a PDI were combined in an organic/inorganic heterojunction for PEC water splitting.39
This paper focuses on the engineering of inorganic/organic NHs with N,N-bis(2-ethylhexyl)perylene-3,4,9,10-tetracarboxylic diimide (PDIEH) anchored on TiO2 NRs (represented as TiO2/PDIEH henceforth (Fig. S3†)). To synthesize PDIEH, commercially available PDA was used as the starting material and modified at an anhydride functional group with 2-ethylhexylamine (Scheme 1). The PEC properties of TiO2/PDIEH NHs towards PEC urea oxidation reaction (UOR) were investigated using various electrochemical measurements. A stability test was done by performing a chronoamperometry experiment. The morphological study was done by field emission scanning electron microscopy (FE-SEM).
The absorption spectrum of PDIEH recorded in chloroform showed multiple peaks located at 429 nm, 457 nm, 489 nm, and 525 nm (Fig. 2a) which match the reported spectrum.42,43 The absorption bands at 457, 489, and 525 nm are attributed to the 0–0, 0–1, and 0–2 electronic transitions, respectively.42 The spectral intensity of the 0–0 transition is smaller than that of the 0–1 transition. This might be due to the existence of a face-to-face-stacked dimer of PDI in the solvent.42 PDIEH showed maximum absorbance at 525 nm, which corresponds to the vibronic progression of the first S0–S1 transition, making PDIEH a good candidate as an organic semiconductor to be blended with TiO2 for PEC application.44 TiO2 barely absorbs any light in the visible spectrum, as shown in Fig. 2b. Fig. 2c displays the absorption spectra of PDIEH thin films on FTO (FTO/PDIEH) and TiO2/PDIEH NHs. TiO2/PDIEH NHs exhibited absorption in wide visible regions corresponding to the absorbance of PDIEH. FTO/PDIEH shows higher absorption intensity compared to TiO2/PDIEH NHs because of the higher concentration of PDIEH in FTO/PDIEH than in TiO2/PDIEH NHs. The absorption spectra of TiO2/PDIEH NHs were also recorded when changing the thickness of PDIEH on TiO2 (Fig. S6†). On increasing the thickness of PDIEH, no absorption is observed because it blocks the incident beam of the UV-vis spectrophotometer and no absorption is observed by the detector. The photoluminescence (PL) spectrum of PDIEH in CHCl3 exhibited emission peaks at 609 nm and 635 nm (Fig. S7a†). The PL spectrum for TiO2 NRs (Fig. S7b†) shows an emission peak at 413 nm arising from the radiative recombination of self-trapped excitons localized within the octahedral TiO6 matrix and oxygen vacancies (VO).45 The peak at about 441 nm is due to VO with two trapped electrons, and the peaks at 469 nm and 483 nm are due to VO with a single trapped electron center.22,46 The peak at 527 nm is ascribed to oxygen vacancy-related trap states.22 The PL spectra of PDIEH thin film on FTO (FTO/PDIEH) and TiO2/PDIEH NHs are shown in Fig. S7c.† TiO2/PDIEH NHs exhibit emission around 645 nm corresponding to the PDIEH as revealed from the emission spectrum of FTO/PDIEH. The Kubelka–Munk plots of PDIEH (Fig. 2d), TiO2 NRs (Fig. 2e), FTO/PDIEH (Fig. 2f), and TiO2/PDIEH NHs (Fig. 2g) were used to calculate their respective optical band gaps (Eg). The Eg calculated for PDIEH, TiO2 NRs, FTO/PDIEH, and TiO2/PDIEH NHs were 2.28 eV, 3.2 eV, 2.05 eV, and 1.87 eV, respectively.
XPS was carried out to analyze the constituent elements of TiO2 (Fig. S8†) and TiO2/PDIEH NHs (Fig. 3). Fig. 3a shows that the XPS spectrum of C 1s for the NHs consists of three peaks at 283 eV, 284.9 eV, and 287.11 eV. The small peak at 283 eV is a signature of sp2 hybridized CC carbon.47,48 The 284.9 eV peak corresponds to benzenic and adventitious carbon.47 The 288.8 eV peak corresponds to the carbon in the carbonyl group (CO).49 The high-resolution XPS spectrum of N 1s (Fig. 3b) consists of three peaks centered at 402.96 eV, 400.51 eV, and 397.53 eV. The high binding energy 402.96 eV peak is attributed to the X-ray oxidized charged nitrogen bipolarity.47 The 400.51 eV peak is assigned to the imide ((OC)–N–(CO)) functional group.50 The 397.93 eV binding energy peak is related to the nitrogen involved in Ti–N bonds.47 The Ti 2p XPS spectrum (Fig. 3c and S8a†) consists of two doublet peaks centered at 459 eV and 464.73 eV representing the Ti 2p1/2 and Ti 2p3/2 states of Ti4+.51 The XPS of Ti 2p for both TiO2 and TiO2/PDIEH reveals the same type of Ti cluster in both materials. The O 1s spectrum (Fig. S8b†) of TiO2 depicts a 530.11 eV peak, corresponding to lattice oxygen (OL) in the TiO2 matrix. The secondary peak centered at 532.21 eV corresponds to oxygen vacancies (VO).47,49 The XPS of O 1s from TiO2/PDIEH NHs (Fig. 3d) shows small shifts in the OL peak (530.91 eV) and VO peak (532.81 eV) which might be due to the interaction of PDIEH with TiO2, as the peak centered at 532.81 eV is attributed to the carbonyl (CO) group present in the organic compound. Moreover, the small peak at 534.32 eV corresponds to the C–O group occurring in PDI due to the resonance phenomenon in the imide bond.49 The above discussion reveals that there is covalent interaction between TiO2 NRs and PDIEH.
Fig. 3 High-resolution peak-fitted XPS spectra of (a) C 1s, (b) N 1s, (c) Ti 2p, and (d) O 1s for TiO2/PDIEH NHs. |
The chronoamperometry (i–t) experiments of TiO2 NRs and TiO2/PDIEH NHs reveal that both photoanodes have good photostability (Fig. S9b†). In the i–t experiments of TiO2/PDIEH NHs, a gradual increase in the photocurrent density at the initial stage is observed which may be related to some surface phenomenon like rapid charge transfer at the electrode/electrolyte interface in the initial stages. FT-IR spectra of TiO2/PDIEH NHs (Fig. S10†) before usage and after 10 hours of the PEC stability test also confirm the stability of TiO2/PDIEH NHs. After the stability test, FT-IR spectra reveal that the structure of PDIEH has not changed. In PEC water oxidation, O2 gas is generated over the anode. This experiment was performed in 0.5 M KOH solution. Fig. S12a† shows the quantities of O2 collected using the TiO2 NR and TiO2/PDIEH NH photoelectrodes. The amounts of oxygen gas collected for the TiO2 NR and TiO2/PDIEH NH electrodes are 1.9 μmol cm−2 and 4.55 μmol cm−2, respectively. Both photoanodes exhibited a stable O2 generation ability. The faradaic efficiencies (FE) (details are given in ESI†) calculated for each photoelectrochemical OER are 55% and 81.3% for the TiO2 NR and TiO2/PDIEH NH electrodes, respectively (Fig. S12b†). The increment in the oxygen production in TiO2/PDIEH NH electrodes could be attributed to the nanoheterostructure formation. The H2 production capabilities of the photoanodes were tested by running an i–t experiment under constant light irradiation for 60 min (Fig. 5a). In PEC UOR, H2 was generated at the cathode. The amount of H2 generated was measured using an inverted-burette technique (Fig. S6a†). Fig. 5b shows the quantity of H2 collected after each 15 min at 0.96 VRHE. The quantities of H2 collected after 60 min for the TiO2 NR and TiO2/PDIEH NH electrodes are 3.32 μmol cm−2 and 10.2 μmol cm−2, respectively. The generation of H2 over the Pt wire electrode with the TiO2/PDIEH photoanode can be seen in Video S1.† The calculated FE for the TiO2 NR and TiO2/PDIEH NH electrodes are 58.85% and 90.21%, respectively (Fig. 5c). The FE of TiO2 is almost half that of TiO2/PDIEH NHs because the recombination rate of the photogenerated electrons and holes is high over TiO2,25,59,60 which reduces the urea oxidation tendency of bare TiO2 (LSV and i–t stability studies), resulting in a decrease in H2 production on the cathode. This results in a lowering of the faradaic efficiency in the case of bare TiO2. Hence, blending the semiconductor with TiO2 is required to suppress the charge recombination rate. TiO2/PDIEH NHs cannot acquire 100% FE because, during PEC UOR, N2 and CO2 are produced as a product of UOR and, at the same time, O2 generated from urea assists in water oxidation in urea solution.61 The urea assists water oxidation and consumes some fraction of the photogenerated holes to generate O2; this generated O2 reacts again with generated H+ to form water.62 This inhibits the FE from reaching 100%. The PEC catalytic activity of the TiO2 NR and TiO2/PDIEH NH electrodes towards PEC UOR was also computed from their solar-to-hydrogen (STH) conversion efficiencies (ηSTH). The ηSTH is the chemical (H2) energy generated against the incident light, calculated using eqn (S2).†63 The ηSTH calculated for the TiO2 NR and TiO2/PDIEH NH electrodes are found to be 0.17% and 0.52%, respectively (Fig. 5d). All the above results explain that TiO2/PDIEH NHs have better PEC catalytic activity toward UOR compared to TiO2 NRs.
Mott–Schottky (M–S) plots for the PDIEH exhibit both negative and positive slopes versus the applied potential, revealing the simultaneous n-type and p-type nature of PDIEH (Fig. 6a).64 Considering the initial positive slope at a lower potential, n-type PDIEH shows a flat band potential (Vfb) of 0.16 VRHE. The M–S plots of TiO2 NRs (Fig. 6b) and TiO2/PDIEH NHs (Fig. 6c) exhibit positive slopes, expressing their n-type nature.49 The decrease in the slope of TiO2/PDIEH compared to that of TiO2 is due to a significant increase in charge carrier density after forming NHs. The decrease in the M–S slope increases its charge donor density, resulting in enhancement of the electron charge concentration in the conduction band (CB).47 This enhancement of electrons in the CB moves the Fermi level towards the CB edge.65 Here, the Vfb for the TiO2 and TiO2/PDIEH electrodes are 0.35 VRHE and 0.07 VRHE, respectively. The Vfb of an n-type semiconductor corresponds to the CB edge.66 The Nyquist plots of the photoanodes provide information about their charge transfer resistance (RCT) (Fig. 6d). RCT is used to study the kinetics enhancement and separation efficiency of charge carriers of the electrodes.53 The equivalent circuit is used to explain the Nyquist plots (Fig. 6e). Since the charges generated in the photoanode undergo bulk recombination and surface recombination, an equivalent circuit with several resistors was used to distinguish them. Rs is the series resistance at the interface between FTO and the photoanodes, RB is the bulk resistance in photoanodes, and CPB is their capacity. Also, Rct is the charge-transfer resistance at the interface between the photoanodes and electrolytes, and CPct is the constant phase element that represents the dielectrics of the electrical double layer at the electrode/electrolyte interfaces.67 The detailed values of resistance obtained from the fitted data of the equivalent circuit are given in Table S1.† The obtained resistance values reveal that, after the formation of the NHs, there is a decrease in the equivalent series resistance (RS), bulk resistance (RB), and RCT values. The drop in RB indicates a rise in the ion-conducting channels. The fall in RCT indicates the fast and better charge transfer at an electrode–electrolyte interface in NHs.68
For a quantitative analysis of the PEC UOR, the applied bias photon-to-current efficiency (ABPE) was calculated using eqn (S3).†69Fig. 7a exhibits the maximum ABPE (%) values of both photoanodes at different VRHE. TiO2 NRs exhibit 0.11% ABPE at 0.72 VRHE, while TiO2/PDIEH NHs show 0.25% ABPE at 0.63 VRHE. Compared to TiO2 NRs, the TiO2/PDIEH NHs exhibit higher ABPE due to the TiO2 NR being decorated with a visible-light active organic semiconductor which greatly improves the photoresponse under visible light. The ideal regenerative cell efficiency (ηIRC) is calculated from the LSV plots in Fig. 7b (replotted from LSV plots in Fig. 3c) using eqn (S4).† The maximum power densities (Pmax) for TiO2 NRs and TiO2/PDIEH are indicated by the green and pink shaded areas, respectively. The ηIRC% values calculated for TiO2 and TiO2/PDIEH are 0.16% and 0.25%, respectively. Interestingly, the obtained ηIRC% and η% values of each photoanode are quite comparable. Table S2† compares the TiO2/PDIEH NHs' UOR performance to a few recently published UOR results. This comparison shows that TiO2/PDIEH NHs is a better photocatalyst than those reported in the past. LSV measurements for the photoelectrodes were also carried out in 0.5 M aq. Na2SO3 to understand the enhanced surface charge transfer efficiency (ηCT). Na2SO3 acts as a hole scavenger and helps to investigate the ηCT. Owing to the low activation energy and fast kinetics for the oxidation of SO32− species, the ηCT for SO32− oxidation can be presumed to be 100%.70,71ηtrans is calculated using eqn (1),71 where and are the photocurrent densities obtained in 0.5 M aq. KOH and 0.5 M aq. Na2SO3, respectively.
(1) |
Fig. 7 (a) The η% plots, (b) LSV vs. RHE plots, and (c) surface charge transfer efficiencies of TiO2 NRs and TiO2/PDIEH NHs. |
Fig. 7c displays that the TiO2 NR photoanode shows a maximum ηCT value of ∼51%, whereas the TiO2/PDIEH NH photoanode exhibits a higher maximum ηCT value of ∼96.99%.
Fig. 8a shows the PEC cell diagram with expected charge flow in the TiO2/PDIEH NH photoanode for solar-driven UOR and H2 generation. The following equations describe the chemical reactions at the electrodes.72,73
Anode: CO(NH2)2 + H2O → 6H+ + CO2 + N2, E0 = 0.37 VRHE |
Cathode: 6H+ + 6e− → 3H2, E0 = 0 VRHE |
Total: CO(NH2)2 + H2O → CO2 + N2 + 3H2, E0 = 0.37 VRHE |
Fig. 8 (a) PEC cell diagram with expected charge flow in the TiO2/PDIEH NH photoanode for UOR. (b) The energy diagram based on the Vfb. |
In this PEC cell, TiO2/PDIEH forms a type-II heterojunction with a staggered gap. Upon light irradiation, TiO2/PDIEH is excited by gaining photons, resulting in the photogeneration of electron (e−) hole (h+) pairs in the CB and valence band (VB) of PDIEH, respectively. As the CB of PDIEH is located at a higher energy level than that of TiO2, the photogenerated e− in the CB of PDIEH transfers to the CB of TiO2. At the same time, the h+ travels in the reverse direction, i.e., from the VB of TiO2 to the VB of PDIEH. The photogenerated h+ in the VB of PDIEH is positive enough to carry out UOR to produce CO2, N2, and H+.74,75 The e− in the CB of TiO2 moves rapidly at the Pt electrode and reduces H+ to produce H2. The working principle of the TiO2/PDIEH NHs is illustrated by the energy diagram (Fig. 8b) showing the e− transfer from the CB of PDIEH to the CB of TiO2. The band diagram is constructed from the obtained Vfb and Eg values of PDIEH and TiO2. The conduction band potential (VCB) of TiO2 was measured as 0.25 VRHE (assuming 0.1 negatives of its Vfb (0.35 VRHE))76 and the valence band potential (VVB) of TiO2 was obtained as 3.45 VRHE from the Eg value (3.2 eV). Similarly, the VCB of PDIEH is considered to be 0.06 VRHE (assuming 0.1 negatives of its Vfb (0.16))76 and the VVB of PDIEH was obtained as 2.34 VRHE, considering the Eg value (2.28 eV).
Footnotes |
† Electronic supplementary information (ESI) available: Synthesis of TiO2, synthesis of PDIEh, NMR. See DOI: https://doi.org/10.1039/d3na00294b |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2023 |