Structural and Photoelectrochemical Properties of GaP Nanowires

ARTICLE pubs.acs.org/JPCC

Structural and Photoelectrochemical Properties of GaP Nanowires Annealed in NH3 Wen Wen,† Azhar I. Carim,† Sean M. Collins,† Michelle J. Price,‡ Sabrina L. Peczonczyk,† and Stephen Maldonado*,†,‡ †

Department of Chemistry and ‡Program in Applied Physics, University of Michigan, 930 North University, Ann Arbor, Michigan 48109-1055, United States

bS Supporting Information ABSTRACT: Nitrogen alloyed gallium phosphide (GaP1
xNx) nanowires have been prepared by annealing gallium phosphide (GaP) nanowires in flowing NH3(g) at 750
C. X-ray diffraction patterns and electron microscopy showed that changes in the annealing conditions afforded controlled alloying of N without effecting a complete conversion to gallium nitride (GaN). Raman measurements on nanowire films and individual nanowires highlighted intense new signatures, consistent with symmetry reduction from N incorporation in the zincblende lattice. The resultant optical properties and photoresponse of the GaP1
xNx nanowire films were investigated by wavelength-dependent diffuse reflectance and photoelectrochemical measurements, respectively. Diffuse reflectance measurements showed progressively lower reflectivity of visible light for nanowire films annealed in increasingly higher levels of NH3(g), indicating an increased light absorption. Corresponding photoelectrochemical measurements of the GaP1
xNx nanowires revealed an increased quantum efficiency, relative to GaP, for energy conversion of light with wavelengths longer than 545 nm. The presented data set thus identifies a methodology for improving the solar energy conversion properties of GaP nanowire film photoelectrodes for visible light.

’ INTRODUCTION New semiconductor photoelectrode materials are needed to advance artificial photosynthetic systems1 for solar energy conversion/storage. For photoelectrochemical cells that store incident sunlight in energy-rich chemical fuels, candidate materials should demonstrate two important properties. First, the photoelectrode material should absorb and convert an appreciable portion of the solar spectrum. This constraint implies that the semiconductor photoelectrode passes a relatively high photocurrent density under illumination at the level of AM 1.5.2 Second, the thermodynamics of fuel-forming reactions require that the semiconductor photoelectrode generates a cell electromotive force greater than the standard potential of the reaction. For solar water splitting, the cell voltage requirement is 1.23 V at standard conditions.3 The mechanistically complex processes for bond formation/scission in water splitting demand an additional overpotential that increases the total needed photoinduced voltage to g1.7 V.4,5 Midsized bandgap semiconductors are naturally suited to simultaneously satisfy both requirements and are thus appropriate for solar-powered fuel generation systems.6 Gallium phosphide (GaP) has long been considered as a candidate semiconductor for photosynthetic photoelectrochemical applications since its midsized bandgap intrinsically allows for large photovoltages under illumination.7 Unfortunately, short minority carrier diffusion lengths typical in GaP artificially lower the capacity for collection of long-wavelength light by planar GaP r 2011 American Chemical Society

photoelectrodes. However, two separate strategies have been explored to increase the attainable photocurrent density with GaP photoelectrodes. Our recent efforts have demonstrated that GaP photoelectrodes with short minority carrier diffusion lengths can exhibit improved photoresponse characteristics for photon energies as low as the bandgap energy if a high aspect ratio, rather than a planar, form factor is used.8,9 Separate work by Turner et al. has shown that the bandgap energy of GaP can be lowered slightly through alloying GaP with N to form GaP1
xNx (0 e x e 1).10 This work also demonstrated the excellent corrosion resistance of GaP1
xNx in aqueous electrolytes,10 an additional feature attractive for practical water electrolysis systems. To date, GaP photoelectrodes that have both a high aspect ratio and an appreciable level of N have not been examined as possible solar energy conversion/storage materials. The advantage of combining these two strategies is that the high aspect ratio form factor increases the capacity for efficient light energy capture/conversion out to the bandgap wavelength and that N alloying lowers the energy threshold for optical energy capture and conversion. Long nanowire films naturally embody one type of high aspect ratio form factor and have been accordingly studied as a viable photoelectrode design for widescale use.11,12 Herein, we Received: August 22, 2011 Revised: October 3, 2011 Published: October 24, 2011 22652

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The Journal of Physical Chemistry C show data illustrating that GaP1
xNx nanowire films can be prepared by annealing GaP nanowire films in NH3(g) under conditions that do not result in a total conversion to gallium nitride (GaN).13 In contrast to a previous report on GaP nanowires with only an outer shell enriched with N,14 we present data illustrating N incorporation throughout the nanowire volume. In addition, we report data detailing the physicochemical and photoelectrochemical properties of GaP1
xNx nanowire films. In the context of Raman scattering as a probe for the local structural order in III
V semiconductors following ion irradiation,15 doping,16,17 and alloying with either isoelectronic cationic or anionic substitutional impurities,18
20 we report and interpret the rich and unusual features of the Raman spectra of GaP nanowires following treatment with NH3(g). The sum materials analyses are used to assess this methodology for producing alloyed GaP nanowire films, and the prospects for preparing high efficiency, GaP-based photoelectrodes capable of sustaining large photovoltages and high photocurrents are discussed.

’ EXPERIMENTAL SECTION GaP Nanowire Film Preparation. A small amount (ca. 15 mg) of ground GaP powder (99.99% metal basis, Sigma-Aldrich) was placed on a quartz platform which was then inserted into a quartz tube (diameter 2.2 cm, length 66 cm) placed inside a split, single zone tube furnace (Thermolyne F79345). The source powder was located in the center of the heated zone. A growth substrate was loaded into the same quartz tube approximately 18.41 cm from the source powder. Prior to loading, the growth substrates were coated with a thin (18 MΩ cm, Barnstead Nanopure III purifier) and with GaP sections etched in concentrated H2SO4 (doubly distilled 18 M, Sigma-Aldrich) for 30 s and rinsed with distilled water. For N alloying, the as-prepared GaP nanowire films were subsequently annealed in a quartz flow tube system (diameter 4.4 cm, length 90 cm) at temperatures ranging between 600 and 750
C in flowing NH3(g). The system was initially purged of O2(g) by flowing Ar(g) (Cryogenic Gases) at 450 sccm for 30 min at room temperature. The temperature was then raised to the desired set point temperature, and NH3(g) (Cryogenic Gases) was introduced into the gas stream. The total flow rate of Ar(g) and NH3(g) was kept at 150 sccm for 4 h. The ratio of

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NH3(g):Ar(g) was used to control the level of nitrogen incorporation. Upon completion, the NH3(g) flow was stopped, and the system was allowed to cool radiatively to room temperature over a time period of 4 h. Materials Characterization. Samples for transmission electron microscopy were prepared by first sonicating GaP nanowire films in ethanol for 15 s to dislodge individual nanowires from the growth substrate. These suspensions were then cast onto holey carbon-coated copper TEM grids and characterized with a JEOL 3011 transmission electron microscope operated at 300 kV. For collection of electron diffraction patterns, the specimens were tilted and aligned to a low index zone axis as determined by the observation of Kikuchi diffraction patterns. Scanning electron micrographs of nanowire films were taken separately with a FEI Nova 200 Nanolab and Phillips XL30FEG. X-ray diffraction data were acquired with a Rigaku Ultima IV diffractometer equipped with a Cu Kα source, a steel sample support plate, and a scintillation counter detector. A grazing angle geometry was employed to eliminate the contribution of reflections from the underlying substrate. The sample position and incident angle were first aligned by monitoring the transmission of the incident X-rays with respect to the stage height and incident angle and then precisely adjusted using the reflected X-rays so that the beam exactly entered the receiving slit. During pattern collection, the incoming X-ray beam was fixed at an incident angle of 0.3
with respect to the sample plane, while the detector was swept through the full 2θ range. Diffuse reflectance measurements of representative nanowire samples annealed at 750
C under various NH3-containing ambients were taken using a 4 in, four-port Newport integrating sphere coated with Spectraflect. Monochromatic light ((4 nm tolerance) was generated from the output of a 150 W Xe arc lamp (Newport) coupled to a quarter-turn, single-grating monochromator (Newport) equipped with 1.24 mm wide rectangular slits. Reflected light was collected with a Thorlabs S120 Si photodiode detector coupled to the integrating sphere. The incident light was chopped at 15 Hz, and the photodiode signal was measured with a Stanford Research Systems SR830 lock-in amplifier. A quartz beamsplitter was used to direct a portion of the incident monochromatic beam to a second Newport 70316NS photodiode detector monitored by a Newport lock-in amplifier (Merlin) to correct for variation in lamp intensity. The system was controlled via a custom LabVIEW program. Raman spectra of as-prepared and nitrogen-alloyed GaP nanowire films on Si substrates were obtained with a Renishaw inVia Raman spectrometer equipped with a Leica microscope, an Olympus SLMPlan 20 objective (numerical aperture = 0.35), holographic notch filter, a 1800 lines/mm grating, and a RenCam CCD detector in a 180
backscatter geometry. A 514.5 nm Ar+ laser (Laser Physics 25s) and a 632.8 nm HeNe laser (Renishaw RL633) were used as excitation sources with radiant fluxes of 500 and 390 μW, respectively, at the sample. The incident excitation was linearly polarized, and no polarizing collection optic was used for spectral acquisition. For single nanowires, polarization optics were used to collect Raman spectra at 514.5 nm excitation. Dispersions of individual nanowires were prepared by removing the nanowire films from the substrate by sonication in ethanol. The suspensions were then drop-cast onto glass microscope slides. A Leica N Plan 100 objective (numerical aperture = 0.90) was used to excite and isolate the signal from localized regions on discrete nanowires. A λ/2 plate was used to align the polarization direction of the excitation either parallel or 22653

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Figure 1. (a) Cross-sectional view of a scanning electron micrograph of a GaP nanowire film grown on a Si(100) substrate. (b
d) X-ray diffractograms of GaP nanowire films annealed in (b) pure NH3(g) at T = 800
C, (c) pure NH3(g) at T = 750
C, and (d) NH3(g):Ar(g) (1:5) at T = 750
C.

perpendicular to the long axis of the nanowire. Concomitantly, a linear polarizer was placed between the notch filter and the grating to isolate the Raman scatter with polarization directions either parallel or perpendicular to the long axis of the nanowire. The convention used to define the polarization condition for each spectral acquisition is diagrammed in Figure S3 (Supporting Information). X-ray photoelectron (XP) spectra were acquired with a PHI 5400 analyzer using a Mg Kα (1253.6 eV) source without a monochromator at a takeoff angle of 54.6
. Spectra were recorded without charge neutralization at a base pressure of 545 nm and progressively decreased at longer wavelengths. After annealing in NH3(g) at T = 750
C, the nanowire film electrodes exhibited different quantum yield wavelength dependencies at wavelengths longer than 545 nm. Annealing in 1:20 v/v NH3(g):Ar(g) caused a systematic suppression of the quantum yield values in the range of wavelengths shown in Figure 6b. Annealing GaP nanowire films under slightly more concentrated NH3(g) conditions effected an increase in the overall quantum yield values. Nanowire films exposed to an ambient containing 1:9 v/v NH3(g):Ar(g) during annealing showed the uniformly largest quantum yields between 550 and 680 nm. GaP nanowire films that had been annealed with 1:5 v/v NH3(g)/Ar(g) were noticeably less photoactive. The inset to Figure 6b summarizes the observed trend for quantum yield values as a function of NH3(g) in the annealing ambient. The collective data indicated an optimum condition for annealing in the presence of NH3(g) for effecting increased light harvesting at longer wavelengths than the bandgap energy of pristine GaP.

’ DISCUSSION The cumulative data support the contention that annealing GaP nanowires in effluent gas containing NH3(g) is a viable route for producing GaP1
xNx nanowires. The observation of reflections in the X-ray diffraction data for only zincblende GaP,

the absence of GaN phonon modes in the Raman spectra of individual nanowires and nanowire films, and the selected area electron diffraction patterns with reflections for only zincblende GaP show that N is alloyed into the zincblende lattice of GaP without conversion to wurtzite GaN in dilute NH3(g) at T = 750
C. The specificity of XP spectra for surface analysis, in conjunction with the difficulties in quantitative analysis of nonplanar sample interfaces,45 preclude a precise determination of the GaP1
xNx alloy composition produced from each annealing trial. Nevertheless, high-resolution N 1s X-ray photoelectron spectra do indicate that the near surface regions of the annealed GaP nanowires likely contain substitutional N atoms covalently coordinated rather than appreciable amounts of NOx oxides or interstitial N2 species. The transmission electron micrographs indicate that structural changes induced by annealing specifically in the presence of NH3(g) are inhomogeneous throughout the entire volume of the GaP nanowires. The transmission electron micrographs show that annealing GaP nanowires in effluent gas containing 5% v/v of NH3(g) largely disrupts just the surface. Annealing in effluent gas with higher contents of NH3(g) caused the emergence of distinct crystalline domains throughout the nanowires, consistent with the idea that these regions are rich with N atoms. The lack of sensitivity toward polarization conditions in the Raman spectra from individual GaP1
xNx nanowires indicates that the new spectral features induced by N incorporation do not have any specific orientation with respect to the long axis of the nanowire. The increase in the intensity of the new Raman spectral features tracks with the NH3(g) content in the effluent gas, as does the domain size evident in the transmission electron micrographs. Taken together, the transmission electron micrographs and the polarized Raman spectra thus suggest that N alloy formation is highly localized in regions randomly located throughout the nanowire. Many of the emergent features in the Raman spectra of GaP nanowires following high-temperature treatment with NH3(g) have not previously been reported for GaP1
xNx alloys and therefore merit specific discussion. For GaP1
xNx alloys, several changes in the Raman spectra (with respect to the spectra for pristine GaP) have been previously noted, including the common observance of the LO(X) mode.14,46
50 Often referred to as the “X” or defect mode, this spectral feature has been consistently 22658

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The Journal of Physical Chemistry C observed in GaP1
xNx samples at room temperature irrespective of the method of sample preparation. Depending on the method of alloy preparation, a separate “local vibrational mode” near 500 cm
1 which shows resonance enhancement has been noted46,47 and described as the vibrational frequency of isolated Ga
N bonds surrounded by Ga
P bonds.46 The observation of this mode typically requires a high concentration of N in the lattice and/or low temperatures (T < 20 K).48,50 The local vibrational mode was not observed in the GaP1
xNx nanowires at any excitation wavelength (λ = 514, 633, and 785 nm). A related mode at 437 cm
1 for N2 (rather than N) replacing P in the anionic sites in the lattice has also been speculated for related GaAs1
xNx alloys51 but was also not observed in the Raman spectra for the GaP1
xNx samples prepared here. However, the red shift of the LO(Γ) mode is consistent with a substitutional, rather than an interstitial, impurity in the host lattice.52 Moreover, the room temperature Raman spectra of the NH3(g)treated GaP nanowires did show numerous (up to 11) assignable Raman modes for GaP nanowires after annealing in NH3(g) that have not been observed previously for GaP1
xNx or related III
V N alloys. The appearance of Raman signals at frequencies corresponding to phonon modes at the Brillouin zone boundaries suggests defect-activated phonon scattering from nonuniformly distributed impurities.20 Substitutions within the lattice destroy translational symmetry and relax the wave vector selection rule for Raman scattering (i.e., q 6¼ 0), allowing the observation of formally forbidden acoustic and optical phonon modes.16,17,20 Randomly substituted N atoms at anionic sites within GaP have been proposed previously to activate specifically phonon modes from the X point of the Brillouin zone,20,47 consistent with the identification of TA(X), LO(X), and TO(X) modes (and their combinations) in the spectra reported here. The absence of a dependence on polarization of these features in the spectra in Figure 3c stands in contrast to that reported for epitaxial GaP1
xNx thin films47 but is consistent with a randomized incorporation within the identified domains in the transmission electron micrographs. The observation of several additional phonon modes from the L point of the Brillouin zone could indicate that multiple types of sites within the zincblende lattice are occupied by guest species or could simply mean that the defect-induced phonons arise from a point with a high density of states near both the X and L points.52 Additional (low-temperature) Raman analyses are needed to better distinguish these possibilities. The available bulk and surface materials characterizations, in conjunction with separate photoelectrochemical data, indicate that all NH3(g)-based annealing conditions do not equivalently effect solar energy conversion properties in GaP. The optical properties inferred from the diffuse reflectance measurements indicate a steady increase in the absorbance of long-wavelength visible light with an increasing fraction of NH3(g) in the effluent gas. These data are generally in accord with prior reports of slight bandgap lowering (with respect to GaP) in GaP1
xNx alloys.53
55 However, the photoelectrochemical responses at these same wavelengths did not show a similar monotonic increase in photoresponse at wavelengths longer than 545 nm. Instead, the photoelectrochemical data clearly indicate an optimal set of annealing conditions, with a steep decrease in quantum yields for optical energy conversion after annealing in gas mixtures with a high NH3(g) content. These observations imply that increased levels of N in GaP nanowires simultaneously increase the total absorbing power of light at λ > 545 nm but also decrease the

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quality of the electrical properties relevant to the capture of photogenerated carriers. For materials annealed with higher contents of NH3(g), the material degradation observed in the transmission electron micrographs and the preponderance of GaN-specific signatures in the Raman spectra and XRD patterns suggest that severe disruption of the GaP lattice negatively impacts the electrical properties. However, under processing conditions that facilitated uniform N incorporation without major structural damage, the collected data do indicate that annealing in NH3(g) can increase the photoresponse of GaP nanowires at wavelengths below the formal bandgap energy. The findings of this work show that alloying N into GaP nanowires through annealing in dilute NH3(g) reproducibly redshifts the onset of photocurrent by visible light by more than 100 nm. The possibility that additional postsynthetic routes for introducing N atoms into GaP nanostructures (e.g., annealing with hydrazine or at lower pressures) could result in similar or further enhancement in photoresponses at long wavelengths remains an open question. Since aspects including the electrically active dopant density,56 surface passivation, and nanowire packing density were not explicitly “optimized” in these nanowire film photoelectrodes, the absolute energy conversion efficiencies of the photoelectrodes studied here were accordingly low in comparison to recent studies of nanostructured n-GaP which showed better cumulative energy conversion efficiencies.8 Nevertheless, the data shown here illustrate that optimally prepared GaP1
xNx nanowires could prove to be useful photoelectrode materials capable of sustaining large photovoltages and large photocurrents under normal solar insolation. Coupled with recent advances in methods to modify the surface chemistry and energetics of Ga-based III
V semiconductors through covalently attached surface groups,57 such materials should prove useful for photoelectrochemical applications for solar energy capture and conversion.

’ SUMMARY GaP nanowire films were annealed in flowing gas containing varied levels of NH3(g) at elevated temperatures, resulting in incorporation of N atoms into the lattice of individual GaP nanowires. In effluent Ar(g) streams with a high (>20% v/v) NH3(g) content, X-ray diffraction patterns and Raman spectra separately indicated a conversion of zincblende GaP to wurtzite GaN at T g 750
C. At T = 750
C, annealing in dilute NH3(g) effected incorporation of substitutional N atoms into the GaP nanowires without compromising the zincblende structure. Changes in the polarized and nonpolarized Raman spectra following treatment with NH3(g) were consistent with disorderactivated phonon modes from substitutional N atoms at anionic sites in the zincblende lattice. Separate X-ray photoelectron spectroscopy further supported the notion that the incorporated N is substitutional rather than interstitial or in the form of oxidizing NOx groups. The nonuniformity did not appear to hamper visible-light absorption since increased absorptivity of visible light at wavelengths longer than 545 nm was observed in nanowire films treated with higher fractions of NH3(g). However, the quantum yields for optical to electrical energy conversion in a ferrocene-based regenerative photoelectrochemical cell did not show the same monotonic increase following annealing, implying that the electrical properties suffered after exposure to high levels of NH3(g). The present data thus identify a specific 22659

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The Journal of Physical Chemistry C set of processing conditions that improve the solar energy conversion properties of GaP nanowire film photoelectrodes at wavelengths longer than 545 nm. Application of these findings on separately optimized GaP nanowire photoelectrodes should prove useful in assembling an efficient photoelectrochemical cell with the simultaneous capacity for appreciable photocurrents and photovoltages.

’ ASSOCIATED CONTENT

bS

Supporting Information. A table of Raman spectra features of GaP nanowire films treated at various conditions, Raman spectra of the films highlighted in different wavenumber regions, Raman spectra collected with multiple excitation wavelengths of NH3-treated nanowires, polarized Raman spectra of individual nanowires, transmission electron micrographs of GaP nanowires annealed in argon, high-resolution transmission electron micrograph of GaP nanowires treated with NH3, and high-resolution X-ray photoelectron data in the form of Ga 3d spectra are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: 734-647-4750. E-mail: [email protected]. Home Page: http://www.umich.edu/∼mgroup/.

’ ACKNOWLEDGMENT M.J.P. acknowledges a Department of Energy Office of Science Graduate Fellowship. The authors thank the National Science Foundation (grant No. DMR-1054303) for support of this work. The JEOL 3011 TEM used in this work is maintained by the University of Michigan Electron Microbeam Analysis Laboratory through NSF support (DMR-0315633). ’ REFERENCES (1) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons, Inc.: New York, 2001. (2) Heller, A.; Chang, K. C.; Miller, B. J. Am. Chem. Soc. 1978, 100, 684. (3) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (4) Turner, J. A. Science 1999, 285, 687. (5) Bolton, J. R.; Strickler, S. J.; Connolly, J. S. Nature 1985, 316, 495. (6) Semiconductor Electrodes; Finklea, H. O., Ed.; Elsevier: Amsterdam, 1988. (7) Aspnes, D. E.; Studna, A. A. Phys. Rev. B 1983, 27, 985. (8) Price, M. J.; Maldonado, S. J. Phys. Chem. C 2009, 113, 11988. (9) Hagedorn, K.; Collins, S.; Maldonado, S. J. Electrochem. Soc. 2010, 157, D588. (10) Deutsch, T. G.; Koval, C. A.; Turner, J. A. J. Phys. Chem. B 2006, 110, 25297. (11) Maiolo, J. R., III; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Atwater, H. A.; Lewis, N. S. J. Am. Chem. Soc. 2007, 129, 12346. (12) Garnett, E.; Yang, P. Nano Lett. 2010, 10, 1082. (13) Sukegawa, T.; Katsuno, H.; Kawaguchi, S.; Kimura, M.; Tanaka, A. Appl. Surf. Sci. 1997, 117/118, 536. (14) Seo, H. W.; Bae, S. Y.; Park, J.; Yang, H.; Kang, M.; Kim, S.; Park, J. C.; Lee, S. Y. Appl. Phys. Lett. 2003, 82, 3752. (15) Berg, R. S.; Yu, P. Y. Phys. Rev. B 1987, 35, 2205.

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