A TiO2 nanorod and perylene diimide based inorganic/organic nanoheterostructure photoanode for photoelectrochemical urea oxidation

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.


Introduction
Human society mainly uses exhaustible energy sources, like natural gas, natural oil, and coal. 1 The combustion of these fuels produces environmental pollutants as a by-product, thus creating an environmental threat.Signicant research efforts are being devoted to the development of clean and renewable energy sources as a replacement for traditional energy sources.In addition, urea is widely used as a signicant nitrogen source in industry and agriculture. 2Urea is produced in large quantities from bio-waste and is widely used in the chemical industry. 3rea from agriculture, industries, and domestic waste enters fresh water and acts as a pollutant.Treating urea in wastewater is essential due to the rising water pollution caused by urea discharge. 3,4Urea-containing wastewater can produce green hydrogen energy and reduces environmental pollution through the urea oxidation process.The process of producing hydrogen gas from urea-containing wastewater is promising from the standpoint of sustainable energy.This H 2 energy is considered a potential fuel with high efficiency and no emission of harmful chemicals. 5,6The photoelectrochemical (PEC) urea oxidation reaction (UOR) has been proven to be an effective approach for H 2 generation with low overpotential and energy input. 7,8heoretically, the urea electrolysis voltage is only 0.37 V RHE . 9his makes PEC urea oxidation a vital method for H 2 generation.UOR not only generates H 2 but also reduces environmental pollution.
In 1972, the discovery of TiO 2 electrodes as a photoelectrocatalyst by Fujishima and Honda opened the door for PEC studies. 10Later, extensive research on TiO 2 in different forms, like TiO 2 nanorods (NRs), 11 nanotubes, 12 and nanoparticles, 13,14 as photocatalysts was carried out.Cho et al. reported branched TiO 2 NRs for PEC H 2 production. 15Niu et al. reported corrugated nanowire TiO 2 as a versatile photoanode for PEC alcohol and water oxidation. 16Cho et al. reported codoping TiO 2 nanowires with tungsten and carbon for enhanced PEC performance. 17wang et al. reported a TiO 2 /BiVO 4 /SnO 2 triple-layer photoanode with enhanced PEC performance. 18Duan et al. reported a TiO 2 nanowire/microower photoanode modied with Au nanoparticles for efficient PEC water splitting. 19Park et al. reported photocatalytic UOR on TiO 2 in water and urine. 20][29] Perylene diimide (PDI) derivatives have received a lot of attention as organic semiconductors (OSCs) 30,31 in the elds of Nanoscale Advances PAPER uorescent solar collectors, 32 organic photovoltaics (OPVs), 33 and organic eld-effect transistors (OFETs). 34PDI derivatives possess excellent thermal and optical stability and good carrier transport properties. 35,36The relatively facile and reversible reduction process of PDI derivatives is important in many applications. 37The 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 electrontransfer processes because of their facile reductions, combined with their easily identiable excited states, anions, and dianions via absorption spectra. 30,31Due 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. 30To improve the photocatalytic activity of NHs, Zhang et al. reported NH composites of various PDI derivatives with TiO 2 . 38A TiO 2 nanotube and a PDI were combined in an organic/inorganic heterojunction for PEC water splitting. 39his 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 TiO 2 NRs (represented as TiO 2 / PDIEH henceforth (Fig. S3 †)).To synthesize PDIEH, commercially available PDA was used as the starting material and modied at an anhydride functional group with 2-ethylhexylamine (Scheme 1).The PEC properties of TiO 2 /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 eld emission scanning electron microscopy (FE-SEM).

Results and discussion
The FESEM images of the TiO 2 NRs reveal uniformly distributed TiO 2 NRs grown vertically on the FTO substrate with a rectangular surface (Fig. 1a).The thin coating of PDIEH over TiO 2 NRs can be seen in the FESEM images of the TiO 2 /PDIEH NHs (Fig. 1b).Element mapping showed the even distribution of Ti and O elements in TiO 2 NRs (Fig. 1c-e).Furthermore, the energy-dispersive X-ray (EDX) spectrum of TiO 2 NRs (Fig. 1f) showed Ti and O elements, conrming the successful formation of TiO 2 NRs.Fig. 1g shows the cross-sectional FESEM image of the TiO 2 NRs, which reveals an average size of ∼1 mm.The thick coating of PDIEH of ∼0.3 mm on TiO 2 NRs is seen in the crosssectional image of TiO 2 /PDIEH NHs with a strong interface between TiO 2 NRs and PDIEH (Fig. 1h).Fig. S4a and c † show the top-view and cross-sectional FESEM images of TiO 2 /PDIEH NHs with the ∼0.6 mm thick PDIEH layer and Fig. S4b and d † show the top-view and cross-sectional FESEM images of TiO 2 /PDIEH NHs with the ∼0.8 mm thick PDIEH layer.Fig. S5a † shows the PXRD pattern of powder PDIEH, revealing its crystalline nature.A comparison of the X-ray diffraction (XRD) patterns for both photoanodes (TiO 2 NRs and TiO 2 /PDIEH NHs) indicates the formation of NHs (Fig. S5b †).The XRD pattern of TiO 2 /PDIEH NHs shows additional peaks around 10°(enclosed in a red rectangle) corresponding to PDIEH.The Raman spectrum of PDIEH showed multiple peaks located at 1084 cm −1 (C-H bending vibrations), 1301 cm −1 (ring stretching), 1380 cm −1 (ring stretching), 1454 cm −1 (ring stretching), 1570 cm −1 (C]C stretching), 1587 cm −1 (C]C stretching), and 1612 cm −1 (C]C stretching), matching well with reported values (Fig. S4c †). 40,41he 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. 42,43The absorption bands at 457, 489, and 525 nm are attributed to the 0-0, 0-1, and 0-2 electronic transitions, respectively. 42The 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-toface-stacked dimer of PDI in the solvent. 42PDIEH showed maximum absorbance at 525 nm, which corresponds to the vibronic progression of the rst S 0 -S 1 transition, making PDIEH a good candidate as an organic semiconductor to be blended with TiO 2 for PEC application. 44  . 45The peak at about 441 nm is due to V O with two trapped electrons, and the peaks at 469 nm and 483 nm are due to V O with a single trapped electron center. 22,46The peak at 527 nm is ascribed to oxygen vacancy-related trap states. 22The PL spectra of PDIEH thin lm on FTO (FTO/PDIEH) and TiO 2 / PDIEH NHs are shown in Fig. S7c.† TiO 2 /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), TiO 2 NRs (Fig. 2e), FTO/PDIEH (Fig. 2f), and TiO 2 /PDIEH NHs (Fig. 2g) were used to calculate their respective optical band gaps (E g ).The E g calculated for PDIEH, TiO 2 NRs, FTO/PDIEH, and TiO 2 /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 TiO 2 (Fig. S8 †) and TiO 2 /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 sp 2 hybridized C]C carbon. 47,48The 284.9 eV peak corresponds to benzenic and adventitious carbon. 47The 288.8 eV peak corresponds to the carbon in the carbonyl group (C]O). 49The 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. 47The 400.51 eV peak is assigned to the imide ((O]C)-N-(C]O)) functional group. 50The 397.93 eV binding energy peak is related to the nitrogen involved in Ti-N bonds. 47The Ti 2p XPS spectrum (Fig. 3c and S8a †) consists of two doublet peaks centered at 459 eV and 464.73 eV representing the Ti 2p 1/2 and Ti 2p 3/2 states of Ti 4+ . 51The XPS of Ti 2p for both TiO 2 and TiO 2 /PDIEH reveals the same type of Ti cluster in both materials.The O 1s spectrum (Fig. S8b †) of TiO 2 depicts a 530.11 eV peak, corresponding to lattice oxygen (O L ) in the TiO 2 matrix.The secondary peak centered at 532.21 eV corresponds to oxygen vacancies (V O ). 47,49 The XPS of O 1s from TiO 2 /PDIEH NHs (Fig. 3d) shows small shis in the O L peak (530.91 eV) and V O peak (532.81 eV) which might be due to the interaction of PDIEH with TiO 2 , as the peak centered at 532.81 eV is attributed to the carbonyl (C]O) 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. 49The above discussion reveals that there is covalent interaction between TiO 2 NRs and PDIEH.

Photoelectrochemical (PEC) studies
All PEC measurements of TiO 2 NR and TiO 2 /PDIEH NH photoanodes were conducted in a three-electrode system on a CHI 660D electrochemical workstation (details are given in ESI †).Note that to convert the potential value (V Ag/AgCl ) measured against the Ag/AgCl reference electrode into the reversible

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hydrogen electrode (RHE), eqn (S1) † was employed.Fig. 4a shows the linear sweep voltammetry (LSV) graphs in aq.0.5 M KOH solution.The TiO 2 NRs and TiO 2 /PDIEH NHs exhibit photocurrent densities (J ph ) of 0.29 mA cm −2 and 0.48 mA cm −2 at 1.965 V RHE .This increment in J ph aer the formation of NHs is due to fast charge transmission at the electrode/electrolyte surface. 49A thin layer of PDIEH facilitates the absorbance of the photon within a wide-range of visible light that results in better charge separation.The PEC water oxidation onset potential is the potential in the LSV plot recorded in KOH solution at which the slope of J ph meets the dark current. 51The water oxidation onset potentials shown by the TiO 2 NR and TiO 2 /PDIEH NH photoanodes are 0.20 V RHE and 0.30 V RHE , respectively (Fig. 4a) and exhibit anodic shis of 0.1 V in the onset potential of the TiO 2 /PDIEH NH photoanode compared to the TiO 2 NRs.It is worth here that, despite having an anodically shied onset value compared to the TiO 2 NR photoanode, the TiO 2 /PDIEH NH photoanode has a signicantly higher J ph than the TiO 2 NR photoanode.The anodically shied onset value for the TiO 2 /PDIEH NH photoanode suggests sluggish oxidation kinetics at a lower potential, which is probably attributable to the unfavorable surface properties toward PEC water oxidation arising from surface charge recombination at lower potential. 52Fig. 4c shows the LSV graphs in KOH solution with urea.The TiO 2 NRs and TiO 2 / PDIEH NHs exhibit J ph of 0.42 mA cm −2 and 1.1 mA cm −2 at 1.965 V RHE .This increment in J ph of both photoanodes in the presence of urea is due to UOR over the photoanodes. 53The fastchopped light illumination LSV curve was recorded in KOH solution without (Fig. 4b) and with urea (Fig. 4d), conrming the fast and reproducible light sensitivity of the photoanodes.
The PEC urea oxidation onset potential (V op ) is the potential in the LSV plot at which the slope of J ph meets the dark current. 54A PEC system with high J ph and low V op exhibits higher efficiency toward UOR. 55From the LSV graph of TiO 2 NRs and TiO 2 /PDIEH in KOH solution with urea, the V op for TiO 2 NRs is found to be 0.40 V RHE , which is higher than the standard PEC urea oxidation V op (0.37 V RHE ). 56This higher V op of TiO 2 NRs towards PEC UOR reveals its lover activity towards PEC UOR.The V op for TiO 2 /PDIEH NHs was found to be 0.24 V RHE , which is lower than the standard PEC urea oxidation V op (0.37 V RHE ). 56TiO 2 /PDIEH NHs exhibit a cathodic shi of 0.2 V compared to TiO 2 NRs, indicating improved photocatalytic activity towards UOR aer forming NHs.The improvement in the performance aer forming NHs is due to the active surface, which allows fast charge transmission at the electrode/electrolyte surface. 49From the results, it can be estimated that TiO 2 /PDIEH NHs is not an appropriate catalyst for the water oxidation reaction, while TiO 2 NRs is not an appropriate catalyst for UOR. 57The thickness of the semiconductor layer in type II NHs is of great importance to understanding the electron transfer, which is important for optimizing the efficiency of the PEC system.The effect of changes in coating thickness on the PEC response of NHs was studied.Fig. S9a † shows the LSV plots of TiO 2 /PDIEH NHs with variable thicknesses of the PDIEH layer which reveal that with increasing PDIEH thickness, the J ph decrease.This decrease in J ph is observed because with the increasing thickness, the exposure of the TiO 2 to light decreases and thus the photogeneration of electron-hole pairs decreases.Additionally, this thick layer lacks the proper inter-junction contact between semiconductors which decreases the better charge transfer between NH layers. 58he chronoamperometry (i-t) experiments of TiO 2 NRs and TiO 2 /PDIEH NHs reveal that both photoanodes have good photostability (Fig. S9b †).In the i-t experiments of TiO 2 /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 TiO 2 / PDIEH NHs (Fig. S10 †) before usage and aer 10 hours of the PEC stability test also conrm the stability of TiO 2 /PDIEH NHs.Aer the stability test, FT-IR spectra reveal that the structure of PDIEH has not changed.In PEC water oxidation, O 2 gas is generated over the anode.This experiment was performed in 0.5 M KOH solution.Fig. S12a † shows the quantities of O 2 collected using the TiO 2 NR and TiO 2 /PDIEH NH photoelectrodes.The amounts of oxygen gas collected for the TiO 2 NR and TiO 2 /PDIEH NH electrodes are 1.9 mmol cm −2 and 4.55 mmol cm −2 , respectively.Both photoanodes exhibited a stable O 2 generation ability.The faradaic efficiencies (FE) (details are given in ESI †) calculated for each photoelectrochemical OER are 55% and 81.3% for the TiO 2 NR and TiO 2 /PDIEH NH electrodes, respectively (Fig. S12b †).The increment in the oxygen production in TiO 2 /PDIEH NH electrodes could be attributed to the nanoheterostructure formation.The H 2 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, H 2 was generated at the cathode.The amount of H 2 generated was measured using an inverted-burette technique (Fig. S6a †).Fig. 5b shows the quantity of H 2 collected aer each 15 min at 0.96 V RHE .The quantities of H 2 collected aer 60 min for the TiO 2 NR and TiO 2 /PDIEH NH electrodes are 3.32 mmol are produced as a product of UOR and, at the same time, O 2 generated from urea assists in water oxidation in urea solution. 61The urea assists water oxidation and consumes some fraction of the photogenerated holes to generate O 2 ; this generated O 2 reacts again with generated H + to form water. 62 This inhibits the FE from reaching 100%.The PEC catalytic activity of the TiO 2 NR and TiO 2 /PDIEH NH electrodes towards PEC UOR was also computed from their solar-to-hydrogen (STH) conversion efficiencies (h STH ).The h STH is the chemical (H 2 ) energy generated against the incident light, calculated using eqn (S2).† 63 The h STH calculated for the TiO 2 NR and TiO 2 / PDIEH NH electrodes are found to be 0.17% and 0.52%, respectively (Fig. 5d).All the above results explain that TiO 2 / PDIEH NHs have better PEC catalytic activity toward UOR compared to TiO 2 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). 64Considering the initial positive slope at a lower potential, n-type PDIEH shows a at band potential (V  ) of 0.16 V RHE .The M-S plots of TiO 2 NRs (Fig. 6b) and TiO 2 /PDIEH NHs (Fig. 6c) exhibit positive slopes, expressing their n-type nature. 49e decrease in the slope of TiO 2 /PDIEH compared to that of TiO 2 is due to a signicant increase in charge carrier density aer 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). 47This enhancement of electrons in the CB moves the Fermi level towards the CB edge. 65Here, the V  for the TiO 2 and TiO 2 / PDIEH electrodes are 0.35 V RHE and 0.07 V RHE , respectively.The V  of an n-type semiconductor corresponds to the CB edge. 66he Nyquist plots of the photoanodes provide information about their charge transfer resistance (R CT ) (Fig. 6d).R CT is used to study the kinetics enhancement and separation efficiency of charge carriers of the electrodes. 53The 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.R s is the series resistance at the interface between FTO and the photoanodes, R B is the bulk resistance in photoanodes, and CP B is their capacity.Also, R ct is the charge-transfer resistance at the interface between the photoanodes and electrolytes, and CP ct is the constant phase element that represents the dielectrics of the electrical double layer at the electrode/electrolyte interfaces. 67he detailed values of resistance obtained from the tted data of the equivalent circuit are given in Table S1.† The obtained resistance values reveal that, aer the formation of the NHs, there is a decrease in the equivalent series resistance (R S ), bulk resistance (R B ), and R CT values.The drop in R B indicates a rise in the ion-conducting channels.The fall in R CT indicates the fast and better charge transfer at an electrode-electrolyte interface in NHs. 68or a quantitative analysis of the PEC UOR, the applied bias photon-to-current efficiency (ABPE) was calculated using eqn (S3).† 69

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photoresponse under visible light.The ideal regenerative cell efficiency (h IRC ) is calculated from the LSV plots in Fig. 7b (replotted from LSV plots in Fig. 3c) using eqn (S4).† The maximum power densities (P max ) for TiO 2 NRs and TiO 2 /PDIEH are indicated by the green and pink shaded areas, respectively.The h IRC % values calculated for TiO 2 and TiO 2 /PDIEH are 0.16% and 0.25%, respectively.Interestingly, the obtained h IRC % and h% values of each photoanode are quite comparable.Table S2 † compares the TiO 2 /PDIEH NHs' UOR performance to a few recently published UOR results.This comparison shows that TiO 2 /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.Na 2 SO 3 to understand the enhanced surface charge transfer efficiency (h CT ).Na 2 SO 3 acts as a hole scavenger and helps to investigate the h CT .Owing to the low activation energy and fast kinetics for the oxidation of SO 3 2− species, the h CT for SO 3 2− oxidation can be presumed to be 100%. 70,71h trans is calculated using eqn (1), 71 where J H2O ph and J Na2SO3 ph are the photocurrent densities obtained in 0.5 M aq.KOH and 0.5 M aq.Na 2 SO 3 , respectively.
Fig. 7c displays that the TiO 2 NR photoanode shows a maximum h CT value of ∼51%, whereas the TiO 2 /PDIEH NH photoanode exhibits a higher maximum h CT value of ∼96.99%.
Fig. 8a shows the PEC cell diagram with expected charge ow in the TiO 2 /PDIEH NH photoanode for solar-driven UOR and H 2 generation.The following equations describe the chemical reactions at the electrodes. 72,73ode: CO In this PEC cell, TiO 2 /PDIEH forms a type-II heterojunction with a staggered gap.Upon light irradiation, TiO 2 /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 TiO 2 , the photogenerated e − in the CB of PDIEH transfers to the CB of TiO 2 .At the same time, the h + travels in the reverse direction, i.e., from the VB of TiO 2 to the VB of PDIEH.The photogenerated h + in the VB of PDIEH is positive enough to carry out UOR to produce CO 2 , N 2 , and H + . 74,75The e − in the CB of TiO 2 moves rapidly at the Pt electrode and reduces H + to produce H 2 .The working principle of the TiO 2 /PDIEH NHs is illustrated by the energy diagram (Fig. 8b) showing the e − transfer from the CB of PDIEH to the CB of TiO 2 .The band diagram is constructed from the obtained V  and E g values of PDIEH and TiO 2 .The conduction band potential (V CB ) of TiO 2 was measured as 0.25 V RHE (assuming 0.1 negatives of its V  (0.35 V RHE )) 76 and the valence band potential (V VB ) of TiO 2 was obtained as 3.45 V RHE from the E g value (3.2 eV).Similarly, the V CB of PDIEH is considered to be 0.06 V RHE (assuming 0.1 negatives of its V  (0.16)) 76 and the V VB of PDIEH was obtained as 2.34 V RHE , considering the E g value (2.28 eV).

Conclusions
In summary, an inorganic/organic nano-heterostructure was designed and synthesized by coating the surface of TiO 2 nanorods with a thin layer of PDIEH via the spin-coating technique.The spin-coating method established a PDIEH coating of uniform thickness.The resultant TiO 2 /PDIEH NHs as photoanodes were responsive to visible-light illumination and demonstrated an ultrahigh J ph of 1.1 mA cm −2 at 1.96 V RHE compared to that of TiO 2 NRs derived from PEC urea oxidation.TiO 2 /PDIEH NHs exhibit a PEC urea oxidation onset potential of 0.24 V RHE , which is very low compared to that of the standard PEC V op (0.37 V RHE ).The combination of PDIEH and TiO 2 NRs dramatically improved the PEC performance by enhancing light absorbance.The TiO 2 / PDIEH NHs also have high PEC stability under continuous light irradiation.Engineering these inorganic/organic NHs by taking advantage of the high electrical conductivity of the TiO 2 NRs and the structural exibility of the PDIEH gives rise to increased activities and is a promising strategy for the systematic design of the next generation of UOR photoelectrocatalysts.
Fig. 7a exhibits the maximum ABPE (%) values of both photoanodes at different V RHE .TiO 2 NRs exhibit 0.11% ABPE at 0.72 V RHE , while TiO 2 /PDIEH NHs show 0.25% ABPE at 0.63 V RHE .Compared to TiO 2 NRs, the TiO 2 /PDIEH NHs exhibit higher ABPE due to the TiO 2 NR being decorated with a visiblelight active organic semiconductor which greatly improves the

Fig. 5
Fig. 5 (a) i-t plots recorded at 0.96 V RHE in 0.5 M KOH + 0.5 M urea solution.(b) The amounts of H 2 gas collected at different time intervals.(c) Faradaic efficiency plots.(d) Solar-to-hydrogen (STH) conversion efficiencies.

Fig. 8
Fig. 8 (a) PEC cell diagram with expected charge flow in the TiO 2 / PDIEH NH photoanode for UOR.(b) The energy diagram based on the V fb .