Letter pubs.acs.org/NanoLett
Secondary Branching and Nitrogen Doping of ZnO Nanotetrapods: Building a Highly Active Network for Photoelectrochemical Water Splitting Yongcai Qiu,†,‡ Keyou Yan,†,‡ Hong Deng,‡,§ and Shihe Yang*,†,‡ †
Nano Science and Technology Program and ‡Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China § School of Chemistry and Environment and Key Laboratory of Electrochemical Technology on Energy Storage and Power Generation of Guangdong Higher Education Institutes, South China Normal University, Guangzhou 510006, China S Supporting Information *
ABSTRACT: A photoanode based on ZnO nanotetrapods, which feature good vectorial electron transport and network forming ability, has been developed for efficient photoelectrochemical water splitting. Two strategies have been validated in significantly enhancing light harvesting. The first was demonstrated through a newly developed branch-growth method to achieve secondary and even higher generation branching of the nanotetrapods. Nitrogen-doping represents the second strategy. The pristine ZnO nanotetrapod anode yielded a photocurrent density higher than those of the corresponding nanowire devices reported so far. This photocurrent density was significantly increased for the new photoanode architecture based on the secondary branched ZnO nanotetrapods. After N-doping, the photocurrent density enjoyed an even more dramatic enhancement to 0.99 mA/cm2 at +0.31 V vs Ag/AgCl. The photocurrent enhancement is attributed to the greatly increased roughness factor for boosting light harvesting associated with the ZnO nanotetrapod branching, and the increased visible light absorption due to the N-doping induced band gap narrowing of ZnO. KEYWORDS: Branched ZnO nanotetrapods, N-doping, photoanode, water splitting
O
(NTs) have grabbed great attention owing to the peculiar architecture consisting of four rod-shaped structures joined at tetrahedral angles to a central core. In our previous reports, we have shown that the ZnO NTs could be prepared on a large scale by the oxidative metal vapor transport deposition technique.20 The photoanode film derived from the ZnO NTs delivers an overall power conversion efficiency up to 3.27% in dye-sensitized solar cells (DSSCs).21 However, the loose packing of ZnO NTs with different sizes brings numerous large cavities, which compromise the DSSC performance. One solution to this problem is to fill the large cavities with SnO2 nanoparticles and indeed, a large enhanced performance has been achieved.22 By the same token, the ZnO-based PECs for hydrogen generation from water splitting also require a photoanode with a large roughness factor (RF) in addition to a rapid electronic transport path and high light absorbance. Although nitrogen-doped ZnO/N nanowires were used to ensure rapid electronic transport and efficient separation of electrons and
ver the past few decades, the application of metal oxides as photoanodes in photoelectrochemical cells (PEC) for hydrogen generation from water splitting has been explored extensively, primarily because of their physical and chemical stability, seminconducting properties, low cost, and easy availability.1−8 For the water-splitting application, nanometer-sized metal oxides are particularly relevant on account of their high surface-to-volume ratio and short diffusion length for carrier transport compared with their bulk counterparts. Recently, considerable efforts have been invested to exploring new photoelectrode architectures in place of conventional nanoparticles in order to further understand the inner working system and enhance the device performance.9−11 For example, vertically oriented TiO2 nanotubes/wires arrays provide an optimal material architecture for photoelectrochemical devices because of their lower recombination losses and vectorial charge transport perpendicular to the charge collecting substrates.12−14 However, these arrays-based photoelectrodes generally land poor power conversion efficiency due to their relatively low surface area and low visible absorption. Zinc oxide (ZnO) has a unique crystalline structure, a direct wide band gap (3.37 eV), and a large exciton binding energy (60 meV), making it useful in piezoelectric transducers, gas sensors, photocatalysts, and photovoltaics.15−19 ZnO nanotetrapods © 2011 American Chemical Society
Received: October 23, 2011 Revised: December 2, 2011 Published: December 12, 2011 407
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holes and to enhance visible light absorption, a low RF largely limited the PEC performance.23 In this report, we have prepared a photoanode made of the branched N-doped ZnO NTs for PEC water splitting. First, the ZnO NTs with each arm being >1 μm were selectively prepared based on our previous work (Figure 1).20 The ZnO NTs were
Scheme 1. Three Main Steps Used for Growing Branched ZnO NTsa
Figure 1. (A) SEM image of the ZnO NTs film. Scale bar is 10 μm. (B) Typical TEM image of the ZnO NTs. Scale bar is 500 nm. The film shows the porous structure due to the special packing of the ZnO NTs.
easily assembled into a film ∼20 μm in thickness with a large open structure (Figure 1A). To fill those cavities so as to increase RF, higher level branches of each arm were constructed. Second, in order to capture more sunlight extending from ultravoilet to visible region, nitrogen doping was implemented through thermal nitridation in ammonia gas flow. This was inspired by previous efforts devoted to the studies of n-type nitrogen-doped ZnO film finding a significant red shift in light absorption.24−27 The bulk of this work is based on the perception that the backbone network of the ZnO NTs proffer a highway for rapid electron transport and the elaboration on the periphery expand the interface with the electrolyte for maximizing reaction sites and widen the electromagnetic spectrum of absorption. These features are expected to reduce charge recombination and therefore enhance device performance.21 Summarizing from the above, several design benefits can be envisaged for such a photoanode: (1) the thin arm diameter and the highly branched structure of our post treated ZnO nanotetrapods promise a decent RF; (2) the branched ZnO nanotetrapods are amenable to assembly into a highly connected network with excellent electron transport and mechanical strength;21 and (3) nitrogen-doping of the ZnO branched nanostructures could induce a large shift in the absorption spectra toward the visible light region and therefore enhance PEC performance. Because our vapor-transport grown ZnO NTs are generally bounded by high-quality surfaces, they cannot be directly coated with ZnO nanoparticle seeds by some commonly used methods such as spin coating, drop-casting, and so forth.28,29 We adopted a multistep approach to address this problem. Specifically, the additional branching of the as-prepared ZnO NTs was achieved by three main steps, as shown in Scheme 1. The first step involved etching of the surfaces of ZnO NTs. We noticed that it was difficult to deposit ZnO seeds on the smooth surfaces of the as-prepared ZnO NTs (Figure 1B). In particular, the thermal decomposition of Zn(AC)2 as reported previously in ref 30, did not result in the deposition of ZnO seeds onto the surfaces of the NTs arms. Our solution was to pre-etch the surfaces of the ZnO NTs before formal ZnO deposition, which was found to be effective. To eliminate the
a
(1) Surface etching of the ZnO NTs for 5 min, followed by nucleation of ZnO spines for 60 min; (2) nucleation/growth of more ZnO seeds; and (3) growth of ZnO branches.
effect of interfacial resistance between FTO and ZnO layers, a thin ZnO layer was deposited on the FTO substrate by Zn(AC)2 thermal decomposition treatment prior to the deposition of the main ZnO NTs layer by the doctor-blading method.21 The resulting film was then immersed in a stirred aqueous solution of (0.5 M) NaOH and (0.06 M) Zn(AC)2 at 80 °C for 60 min, followed by rinsing with ethanol and drying in air. Shown in Figure 2 are electron microscopic images that track the course of surface etching and nucleation on the ZnO NTs. After etching for 5 min by the solution of NaOH and Zn(AC)2, the surfaces of the became rough with numerous tiny lips and holes (Figure 2C,D) in sharp contrast to those of the pristine ZnO NTs (Figure 2A,B). On prolonging the reaction time for 60 min, we could find small ZnO spines sparsely grown on the ZnO NTs (Figure 2E,F). Interestingly, the spines are almost parallel to each other with lengths ranging from 20 to 100 nm, indicating their epitaxial relationship with the parental arms. The ratio of NaOH to Zn(AC)2 appears to be important for the successful etching and nucleation: (1) high concentration of NaOH (0.5 M) without Zn(AC)2 easily broke the ZnO NTs, leading to fast and complete etching away of the NT framework; and (2) diluting NaOH while adding in Zn(AC)2 in an appropriate ratio could form the nice ZnO spines presumably via Zn(OH)2 at an appropriate temperature. In addition, the SEM images in Figure 2G,H also reveal that the arm surfaces of ZnO NTs are rough with the sprouted spines. Such small spines could act as the sites for nanowire branching as will be shown below. In the following step, additional ZnO seeds were nucleated and grown on the ZnO NT surfaces, which are necessary for growing dense ZnO branches. Such extensive branching of the NTs is desirable for capturing more sunlight. After treatment 408
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Figure 2. (A,C) Low-magnification and (B,D) high-resolution TEM images of the ZnO NTs before (A,B) and after (C,D) etching by a solution of NaOH and Zn(AC)2 for 5 min. (E) Low- and (F) high-magnification TEM images of ZnO NTs after the etching for 60 min. (G) Low- and (H) high-magnification SEM images of ZnO NTs after the etching for 60 min. Scale bar is 50 nm for (A,C), 5 nm for (B), 20 nm for (D), 200 nm for (E), 50 nm for (F), 1 μm (G), and 100 nm for (H).
Figure 3. (A) Low-amplification and (B) high-amplification SEM images of the branched ZnO NTs without seed coating. (C) SEM image of ZnO NTs with seed coating. (D) SEM image showing secondary ZnO branches growing on NTs after one round of growth. (E) SEM image of secondary ZnO branches growing on NTs after three rounds of growth. (F) Typical TEM image of one arm of the branched ZnO NTs after one round of growth. (G) High-resolution (HR) TEM image taken from the circled region in (F). (H,I) SAED patterns of the arm and one branch portions in (G), respectively, taken from FFT. Scale bar is 1 μm for (A), 100 nm for (B), 1 μm for (C), 1 μm for (D), 1 μm for (E), 100 nm for (F), and 5 nm for (G).
NTs could only grow sparsely branched ZnO nanorods (Figure 3A,B). We believe that the longer nanorods were formed from the development of the little irregular spines in the aqueous solution of NaOH and Zn(AC)2 mentioned above. In contrast, high-quality hierarchical branched ZnO NTs could be generated with the ZnO seeds coating process, as expected (Figure 3C). A typical SEM image of the sample (Figure 3D) shows secondary ZnO branches growing on the NTs with lengths ranging from 50 to 300 nm. However, numerous cavities are still clearly observable in the film because the ZnO branches are still too short after only one round of branching growth (Figure 3C). Since in the precursor solution ZnO nanowires tend to grow along the c-axis direction,28 we envisage that the
with the Zn(Ac)2 solution, the film was dried and sintered at 350 °C in order to achieve a better connection between the ZnO NTs and ensure the formation of a mechanically and electrically robust network of ZnO NTs. To introduce more growth sites for the ZnO seeds on the arm surfaces of the ZnO NTs for branching more dense nanowires, we repeated three times the procedure of thermal decomposition of Zn(AC)2 at 350 °C. Finally grown were the matured ZnO branches by immersing in an aqueous solution of 25 mM Zn(NO3)2 and 25 mM hexamethylenetetramine (HMT) at 92 °C for 3−5 h. Without another ZnO nanoparticle's coating process through thermal decomposition of Zn(AC)2 at 350 °C, the backbone of ZnO 409
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fresh solution could induce the growth of longer ZnO nanowires. To test this idea for filling the remaining space in the film, the hydrothermal process was repeated three times, each in a fresh aqueous mixture (25 mM Zn(NO3)2 and 25 mM HMT) to grow longer ZnO branches. Indeed, the ZnO branches have grown to lengths up to 1 μm, resulting in a large increase in RF, as excepted (Figure 3E). It is noteworthy that the new generation of ZnO branches are almost parallel to each other and make an angle of ∼110° with the arm of the ZnO NTs, as revealed by TEM images in Figure 3F and Supporting Information Figure S1. To elucidate the growth mechanism, we captured a high-resolution (HR) TEM image in a joint region (the encircled region) of an arm of ZnO NTs and a branch with the common zone axis of [1120̅ ] (see Figure 3G). The fast Fourier transform (FFT) patterns of the arm and the branch portions are shown in Figure 3H,I in a row. The images show that both structures are high-quality single crystals and grow in the [0001] direction, indicating that the primary arm still retains the original structure and the branches are epitaxially generated on the surface of the primary arm. It is notable that with the same zone axis of [112̅0], the angle between [0001] directions of the arm and the branch is approximately 110°, in accordance with the SEM observation in Figure 3C. Since in the wurtzite structure of a ZnO crystal, the angle between the (001) and (302̅) planes is 109.82°, it is suggested that the (001) planes of the branch originate from the (302̅) planes of the arm with the common zone axis of [112̅0].31 The nitrogen-doped ZnO product could be achieved by thermal nitridation at 530 °C for 30 min. The color of the film changed from white to yellowish gray, indicative of the successful doping of the N elements, which was also verified by diffuse reflectance, Raman, and XPS spectra (Figure 4). First, we observed a red shift in light absorption wavelength for the film of branched ZnO NTs from the band edge absorption at 350 nm (3.54 eV) of the as-prepared ZnO NTs to the visible region at 500 nm. An even larger red shift up to 650 nm was observed for N-doped branched ZnO NTs (Figure 4A). The smaller red shift should be ascribed to carbon-doping on the surface of the branched ZnO NTs originating from the thermal decomposition of Zn(AC)2 at 350 °C. In this connection, let us draw a comparison between the three high-resolution C1s XPS spectra in Figure 4C: (1) the middle strong sharp peak at 284.5 eV can be attributed to free carbon from contamination, (2) the higher energy band at 288.5 eV can be ascribed to the absorbed CO or C−O bonds from incomplete decomposition of Zn(AC)2 at 350 °C, and (3) the lower energy shoulder band at 283.5 eV suggests the presence of carbon atoms in the carbide form, indicating carbon substitution for oxygen and formation of Zn−C bonds on the surface of branched ZnO NTs.32 It should be noted that the film derived from the branched ZnO NTs shows stronger light scattering in the near-infrared wavelength region, which could be beneficial to dye-sensitized solar cells.33 Second, Raman spectra provide some new information. A peak at 275 cm−1 appears in the Raman spectrum of the N-doped product but not in the as-synthesized ZnO NTs and the branched ZnO NTs (Figure 4B). In fact, this Raman peak has been observed previously when N was incorporated into ZnO.34,35 The peak at 436 cm−1 corresponds to the E2H mode of ZnO. E2H is stronger than the 580 cm−1 band, indicating a good crystallinity of the pristine ZnO NTs. The broad peak at 580 cm−1 is a combination of the E1LO and A1LO modes, which
Figure 4. (A) Diffuse reflectance spectra of the film made of pure ZnO NTs (blue solid line), branched ZnO NTs (red dashed line), and the N-doped branched ZnO NTs (green dotted line). (B) Raman scattering spectra of the pure ZnO NTs, branched ZnO NTs, and the N-doped branched ZnO NTs. (C) High-resolution C1s and N1s peak of the pure ZnO NTs, branched ZnO NTs, and the N-doped branched ZnO NTs.
is usually enhanced by disorder.34 In N-doped ZnO lattice, N-induced disorder leads to a much stronger 580 cm−1 peak and a near disappearance of the E2H mode. Finally, to quantitatively determine the N concentration of the N-doped branched ZnO NTs, we have recourse to high-resolution XPS. It has been realized that N can be incorporated into ZnO 410
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when extrapolating 1/C2 to zero, the intercept at the X-axis corresponds to the flat band potential (VFB), which is estimated to be −0.56 V vs Ag/AgCl (+0.05 V vs the reversible hydrogen electrode (RHE)). By comparison, the VFB value is similar to that of a ZnO/N nanowire array (−0.58 V),23 but exhibits a ∼0.1−0.2 V negative shift from that of ZnO nanoparticles on account of the N-doping effect.36 In regard to the nitrogen dopant density (ND) of the N-doped branched ZnO NTs, the following equation can be used for calculation
lattices in at least two different states: one is (N2)O that means a N2 molecule occupying a position on the oxygen sublattice and should be a shallow double donor, the other is NO that means a N atom occupying an O site.35 As shown in Figure 4C, the core level spectrum of the N 1s region presents a broad peak with a full width at half-maximum of ∼4 eV ranging from 396.5−402.3 eV, which is almost absent before thermal nitridation. With the peak being absent at 404 eV, we can rule out the chemical state of (N2)O in our product.36,37 By fitting the experimental line profile, a doublet centered at 399.5 and 398.4 eV was identified. Plausibly, the former feature is ascribed to the characteristic energy losses or chemisorbed NHx due to incomplete decomposition of ammonia, while the latter binding energy feature lies in between the typical binding energy found for zinc nitride (∼396−397 eV) and NO type species (above 400 eV), indicative of the existence of the oxynitride (O−Zn− N).36,37 The XPS results are consistent with the Raman results, both of which demonstrate successful N-doping into the branched ZnO NTs. The overall N concentration was calculated to be ∼3.5%. The N-doped branched ZnO NTs films were investigated as photoanodes in PECs for hydrogen generation from water splitting. PEC characterizations were performed in a 0.5 M Na2SO4 electrolyte solution buffered to pH ∼ 7.0 with phosphate buffer solution using a typical three-electrode electrochemical cell configuration. To understand the intrinsic electronic properties, for example, capacitance, of the N-doped branched ZnO NTs in the electrolyte solution, we performed electrochemical impedance measurements in the dark coupled with Mott−Schottky (MS) analysis commonly used to determine both dopant density and flatband potential at semiconductor/liquid contacts.38 In flat electrodes the capacitance per unit area of surface is
⎡ 1 d 2 ⎢ C2 ND = ⎢ e0 εε0 ⎢ dV ⎣
( )
⎤−1 ⎥ ⎥ ⎥ ⎦
A positive slope was determined from the Mott−Schottky plots (1/C2 versus V) at various frequencies, indicating the n-type nature of the N-doped branched ZnO NTs. From the known dielectric constant of ZnO (ε = 10) and the permittivity of vacuum (ε0 = 8.85 × 10−14 F/cm), ND was estimated to be ∼3.5 × 1018 cm−3 at the frequency of 5 kHz. This dopant density is comparable to the typical value of 1017−1018 cm−3 observed for other ZnO films.19,23 Photoelectrochemical measurements were performed in a three-electrode electrochemical cell configuration using the pure ZnO NTs and the N-doped branched ZnO NTs deposited on FTO as the working electrode, a platinum coil as the counter electrode, and Ag/AgCl as the reference electrode. The photocurrent density versus applied voltage curves were recorded in the dark and under irradiation with the AM 1.5G simulated solar light (1 sun, 100 mW/cm2) and are shown in Figure 6A. Dark scan linear sweep voltammagrams from −0.4 to +1.3 V versus Ag/AgCl showed an almost negligible current in the range of 10−7 A/cm2. When illuminated by the standard simulated solar light, the pure ZnO NTs yielded a photocurrent density of 0.046 mA/cm2 at +0.31 V versus Ag/AgCl (0.92 V vs RHE). More impressive, with the branched ZnO NTs the photocurrent density reached 0.12 mA/cm2 at that applied voltage. Significantly, the N-doped branched ZnO NTs showed a large enhancement in photoresponse with a photocurrent density of 0.99 mA/cm2 at the same applied voltage. Interestingly, there is no saturation of photocurrent observed in the whole potential scan range, clearly suggesting efficient charge separation in N-doped branched ZnO NTs upon illumination. The efficiency (η) for a water splitting photoelectrode that requires an applied voltage can be evaluated using the equation39
⎡ 2 kT ⎤ = − − ( V V ) ⎢ ⎥ FB e0 εε0ND ⎣ e0 ⎦ C2 1
where e0 is the electron charge, ε is the dielectric constant of ZnO, ε0 is the permittivity of vacuum, Nd is the dopant density, V is the potential difference across the semiconductor spacecharge region, VFB the flatband potential, and kT/e0 is a temperature-dependent correction term. As shown in Figure 5,
η=
I(1.23 − Vapp) Plight
where Vapp is the applied voltage versus RHE, I is the externally measured current density, and Plight is the power density of the illumination. The potential was measured against an Ag/AgCl reference and converted to RHE potential by using the equation E(RHE) = E(Ag/AgCl) + 0.1976 V + 0.059 pH. The result is displayed in Figure 6B. One observes a maximum photoelectrode efficiency of 0.016% for the pristine ZnO NTs at applied voltage of +0.86 V vs RHE. Under the same condition with the branched ZnO NTs we obtained a maximum efficiency of 0.045% at applied voltage of +0.82 V versus RHE. Significantly, the N-doped branched ZnO NTs yielded a maximum efficiency of 0.31% at applied voltage of +0.92 V versus RHE, which is 19 times higher than that for the pristine ZnO
Figure 5. Mott−Schottky plots of the film made of the N-doped branched ZnO NTs in the dark at frequencies of 3 (black solid line), 5 (red dashed line), and 7 kHz (blue dotted line) and an AC current of 5 mV with a three-electrode system. Dashed lines represent the extrapolated lines from the linear portion of the Mott−Schottky plots. 411
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Figure 6. (A) J−V curves recorded for pristine ZnO NTs in the dark (black line), pristine ZnO NTs (blue solid line), branched ZnO NTs (red dashed line), and N-doped branched ZnO NTs (green dotted line) with a scan rate of 10 mV/s and with the AM1.5G simulated solar light at 100 mW/cm2. (B) Photoconversion efficiency of pristine ZnO NTs (blue solid line), branched ZnO NTs (red dashed line), and N-doped branched ZnO NTs (green dotted line) as a function of applied potential. The potential was measured against the Ag/AgCl reference and converted to RHE potential according to E(RHE) = E(Ag/AgCl) + 0.1976 V + 0.059 pH. (C) IPCEs for pristine ZnO NTs (squares), branched ZnO NTs (cycles), and N-doped branched ZnO NTs (triangles), measured in the wavelength range from 350 to 600 nm at applied voltage of +0.50 V vs Ag/AgCl. (D) Photocurrent retention performance over 3500 s of pristine ZnO NTs (blue solid line), branched ZnO NTs (red dashed line), and N-doped branched ZnO NTs (green dotted line) at applied voltage of +0.55 V versus Ag/AgCl under illumination.
NTs. This efficiency is approximately 7 times higher than that for the ZnO/N nanowire array when compared on the same footing.23 The obtained efficiency for the N-doped branched ZnO NTs is also much higher than the recently reported values for undoped and N-doped ZnO films.40,41 These results thus benefit from the unique features of our N-doped branched ZnO NTs photoanode: a large increase in roughness factor and a large red shift in the absorption toward the visible region. To decipher the enhanced overall photoelectrochemical conversion of the N-doped branched ZnO NTs, we performed incident-photon-to-current-conversion efficiency (IPCE) measurements as a function of incident light wavelength for pure, branched and N-doped branched ZnO NTs (Figure 6C). IPCE is expressed as IPCE = (1240I)/(λPlight), where I is the photocurrent density (mA/cm2), λ is the incident light wavelength (nm), and Plight (mW/cm2) is the power density of monochromatic light at a specific wavelength. In comparison with the pristine ZnO NTs, the branched ZnO NTs showed substantially enhanced IPCE and a little red shift toward 450 nm. Significantly, the N-doped branched ZnO NTs presented more enhanced IPCE and a more red shift toward 500 nm, which is in accordance with their J−V and diffuse reflectance characteristics. For instance, the IPCE of the pristine ZnO NTs,
branched ZnO NTs, and N-doped branched ZnO NTs at the incident wavelength of 400 nm are 1.7, 3.6, and 10.1%, respectively. The observed enhanced photocurrent density could be ascribed to the improved absorption toward lower energy wavelength range and a large increase in roughness factor. To assess the stability of the PECs, we also performed an amperometric I−t study on the pristine ZnO NTs, branched ZnO NTs, and N-doped branched ZnO NTs, which were assembled as the photoanodes, under the 100 mW/cm2 illumination at +0.55 V versus Ag/AgCl. To draw a clear comparison, all the plots of photocurrent versus time were rescaled to those of photocurrent retention versus time. As shown in Figure 6D, after 3500 s of operation the photocurrent density of the pristine ZnO NTs has decayed by ∼3.1%, the branched ZnO NTs by ∼4.6%, and the N-doped branched ZnO NTs by ∼12.1%. Two possible reasons can be considered for the stability differences. First, the increased surface area due to the secondary branching would increase the exposure of the ZnO NTs to photo-oxidation processes, leading to the slightly increased instability. Second, the slow surface photocorrosion could cause leaching of the N-dopant from the N-doped branched ZnO NTs and accelerate the photocurrent decay. Counter measures against the photocurrent decay could involve 412
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depositing an ultrathin protective layer of materials such as TiO2 and Al2O3 on the ZnO surface.42 In conclusion, we have successfully prepared the branched ZnO NTs by a three-step protocol consisting of etching, seed nucleation, and branch growth processes. When tested as a PEC photoanode, the secondary branched ZnO NTs could attain an improved photocurrent density of 0.12 mA/cm2 at +0.31 V versus Ag/AgCl (+0.92 V vs RHE). Significantly, the N-doped branched ZnO NTs accomplished by thermal nitridation in an ammonia flow could yield a dramatically increased photocurrent density of 0.99 mA/cm2 at the same applied voltage. This photocurrent density and the associated photoelectrode efficiency are significantly higher than those for the branched ZnO NT photoanode, much more so for the pristine ZnO NT photoanode, thanks to a large increase in the roughness factor and a large shift in the absorption spectra of the ZnO toward the visible light region. As a whole, our work has opened a promising avenue to developing novel PEC photoanodes for photocatalytic water splitting.
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ASSOCIATED CONTENT S Supporting Information * Additional information and figure. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. ACKNOWLEDGMENTS This work was supported by the HK-RGC General Research Funds (GRF No. HKUST 604809 and 605710). REFERENCES
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dx.doi.org/10.1021/nl2037326 | Nano Lett. 2012, 12, 407−413
