n-GaAs BAND-EDGE REPOSITIONING BY MODIFICATION WITH METALLOPORPHYRIN = POLYSILOXANE MATRICES {

Tetra(-4-pyridyl)porphyrinatomanganese(III)sulfate, MnP, (in the forms of Mn and Mn mixture), was embedded into a polysiloxane polymer matrix and attached to the surfaces of n-GaAs wafers. The n-GaAs=polymer=MnP system was annealed under nitrogen and used for photoelectrochemical study in water=LiClO4=Fe(CN) 3 6 = Fe(CN) 6 system. The results indicated a positive shift in the value of the flat-band potential of the semiconductor due to MnP. This was manifested by shifting the values of the dark-current onset potential and the photo-current open-circuit potential towards more positive values. These findings are potentially valuable in future applications of solar energy in hydrogen and oxygen production from water.


INTRODUCTION
Monocrystalline semiconductor electrodes are known catalysts for conversion of light into electric and chemical energies [1][2][3][4][5][6][7].Controlling the semiconductor characteristics at the solid=liquid interface, associated with PEC studies, is one potentially important objective.Shifting the positions of the flat-band potential is important for photoelectrochemical (PEC) applications, viz. the position of the conduction band (CB) and the valence band (VB) edges determines the energetics and kinetics of charge transfer at the solid=liquid interface.This in turn will affect the ability of the semiconductor surface to oxidize or reduce other chemicals in contact with it.For example, n-GaInP 2 needs about 300 mV lowering in its flat-band potential, in order to effectively oxidize water [4].It is assumed that shifting the flat-band potentials of a given semiconductor (SC) can be achieved by attaching charged ions to its surface.Positive charges may cause a positive shifting (lowering) in the value of the flat-band potential.This is due to the stabilization of the molecular orbitals within a given energy band by the neighboring positive charges.However, if the positive charges are attached only to the surface, the lowering will occur only in the Space Charge Layer, SCL, and will mostly involve the flat-band potential (V fb ).Scheme (I) shows how attachment of cations, to a SC surface, may lower its V fb values.The Scheme also shows how such lowering will kinetically enhance hole transfer from the VB accumulation region to the other species with oxidation potentials slightly lower than the unmodified VB V fb level.On the other hand, negative charges should yield a negative shifting (destabilizing) in the band-edge positions.SCHEME 1 In earlier studies, we have modified n-GaAs and n-GaInP 2 with MnP cations, at a submonolayer coverage level, by chemically attaching metalloporphyrines onto the SC surface through an anchoring ligand via a quaternization reaction [3].It was found that the MnP cations shifted the positions of the SC flat-band positions towards more positive values.However, the earlier technique did not resolve the technical difficulties associated with the peeling out of the monolayer coverage.
The main goal of this work is to permanently control the n-GaAs band-edge positions at the solid=liquid interface in PEC systems.n-GaAs was chosen here for investigation purposes as a model SC.Future works will follow using n-GaInP 2 and other SC materials, which are good candidate photocatalysts for light-driven water splitting into hydrogen and oxygen.

Materials
LiClO 4 , K 3 Fe(CN) 6 , K 4 Fe(CN) 6 , and the 5,10,15,20-tetra(-4-pyridyl)-21H,23H-porphine, (H 2 TPyP), were purchased from Aldrich.The MnP complex mixture was prepared from H 2 TPyP as described in the literature [8].The mixture contained complexes of both the Mn II and the Mn III , as depicted from the electronic absorption spectra in the visible region.Indiumtinoxide (ITO) was purchased from Delta Technologies, Ltd. as one-side coated 1.5 mm thick transparent glass, with 100 O=square.Mono-crystalline single surface mirror polished Si-doped (3.2 Â 10 17 cm À 3 for the front and 9.9 Â 10 17 cm À 3 for the tail) n-GaAs wafers (Crystal Specialties, Intl.) were used.The monocrystalline plates, 50 mm roundshaped, were 0.35 mm thick with h100i orientation.The mobility was 2.8 Â 10 3 cm 2 =V-s in the front and 2.3 Â 10 3 cm 2 =V-s in the tail.The wafer was cut into smaller wafers.Indium metal back contact was made as thin uniform layers.The wafers were then annealed at 350 C under clean N 2 atmosphere.The SC electrodes were fabricated as follows: the small square n-GaAs wafer ( $ 0.1 cm 2 ) was mounted to a copper plate (with a copper wire welded to it) using a thin uniform layer of Ga-In eutectic.The back-side and the edges of the electrode, together with the copper wire, were then insulated with an epoxy resin and allowed to dry overnight under nitrogen.The dried sealed electrodes were then rinsed repeatedly with methanol to remove any possible oily solvent contaminants.
Etching of the n-GaAs window was conducted using a Br 2 =methanol (1.0%V=V) solution [9].The electrode was dipped inside the etching solution for 5 seconds, immediately rinsed with de-ionized water and then with methanol.The electrode was then dried using nitrogen stream.

Electrode Modification
The etched SC electrode surface was then modified as follows: A dilute solution of the commercial, R.T.V. polysiloxane (À ÀOÀ ÀSiR 2 À ÀOÀ À) n (vended as a paste in acetic acid), was prepared by dissolving 0.05 g of the paste in 100 mL of dichloromethane.Similarly, methanol solutions of MnP (0.1 g, 1.38 Â 10 À 4 mol, in 10 mL) were prepared.The MnP= Polysiloxane solutions were prepared by adding the MnP solution to the Polysiloxane solution, in a 1:4 V=V ratio, respectively.0.1 mL of the stock solution (containing 4 Â 10 À 5 g polysiloxane and 2 Â 10 À 4 g MnP) were pipetted onto the n-GaAs polished surface.The organic solvents were then allowed to evaporate, leaving a transparent thin layer of the MnP=Polysiloxane matrix on the SC surface.The polymer matrix was calculated to be 4.0 mm thick, containing 2.0 Â 10 À 4 g of MnP (its concentration in the matrix being 6.9 mmol=cm À 3 ).With thicker films, the polymer matrix was insulator and allowed no current to be measured.Therefore, unless otherwise stated, all experiments were conducted using the 4.0 mm thickness MnP=Polysiloxane coated n-GaAs after annealing under nitrogen.

Equipment
Solid-state electronic absorption spectra were measured on a Shimadzu UV-1601 spectrophotometer, as solid thin films of MnP=Polysiloxane on quartz slides.The Soret band at $ 460 nm resembled literature values for other MnP spectra measured in solid state [8] and in solution [10].Infra-red spectra was measured, as thin films of MnP=Polysiloxane, on a Shimadzu FTIR-8201 PC spectrophotometer.Current-potential measurements were performed using a Princeton Applied Research (PAR) Model 263A computer-controlled potentiostat.The Mott-Shottky plots were measured with a SRS 830 DSP lock-in amplifier, a SRS DS345 function generator and a PAR Model 263A potentiostat.All measurements were conducted in a thermostated one-compartment three-electrode electrochemical cell with SCE electrode, a platinum counter electrode and the SC working electrode.LiClO 4 (0.10 M) was used as a supporting electrolyte, and Fe(CN) 3À=4À 6 (0.05 M K 3 Fe(CN) 6 , 0.05 M K 4 Fe(CN) 6 ) as a redox couple in freshly distilled water with (pH 6.5).A nitrogen gas stream was kept flowing above the solution, throughout the experiment, to minimize n-GaAs BAND EDGE contamination with air.Illumination was carried out using a 50 W Xenon lamp equipped with a concentrating lens, placed at defined distance from the working electrode.A 10 mm thick water filter was placed between the lamp and the electrode.The intensity of light at the electrode surface was measured with a correlated light-meter and was 0.1013 W cm À 2 .

Cyclic Voltammometric Measurements
Cyclic voltammometry was measured on a PAR 263A potentiostat, in a one-compartment cell, using platinum electrodes, SCE electrode, in water=LiClO 4 under nitrogen atmosphere with a scanning rate of 100 mV=s.

Capacitance-Voltage
Measurements of capacitance vs. applied potential were conducted using same electrochemical cell, described above, in water=LiClO 4 Fe 2þ =Fe 3þ system.Mott-Schottky M-S plots were constructed, for a potential scanning over a range of À 0.5 to þ0.5 V (vs.SCE) at a scan rate 100 mV=s and 10 kHz frequency.

RESULTS AND DISCUSSION
The water=LiClO 4 =Fe 2þ =Fe 3þ systems are not the best solution for n-GaAs.They give poor current-potential (J-V) plots, in the dark and under illumination.Researchers use alternative systems for the J-V measurements of n-GaAs in aqueous media.Se 2À =Se 2À 2 redox couples are known to give better J-V plots for n-GaAs electrodes.The water=LiClO 4 =Fe 2þ =Fe 3þ system was intentionally employed in this work, so as to see if the modification has any effect on the shapes of the n-GaAs J-V plots.Figure 1 indicates that the attachment of the MnP= Polysiloxane matrix to the n-GaAs surface significantly enhanced the J-V plots while using the Fe 2þ =Fe 3þ redox couple system in water.This was attributed to the extra stabilization of the n-GaAs surface by the matrix.The modification method described here has a very important technical advantage over other ones described in the literature [6][7][11][12][13][14][15][16][17].In other earlier reports, the SC surface was modified with conjugated polymeric matrices.The polymer layers were found to peel off.Efforts have been made to improve the adherence of the polymer to the SC surface.Researchers often scratch the n-GaAs polished surfaces to prevent the polymer matrix peeling.In this work, the polymer matrix was permanently adhered to the polished n-GaAs surface without any need for scratching.When the n-GaAs=MnP= Polysiloxane system was annealed at 100 C the adherence was enhanced and a clear robust glassy solid layer appeared on the surface.When thick matrices of MnP= Polysiloxane were used, the J-V plots showed poor current readings, indicating that the matrix conductivity was low.Therefore, thin matrices were made to lower the resistance.Optimum matrix thickness was 4.0 mm.With such thickness, well-defined uniform J-V plots were obtained.

Dark-Current Measurements
Dark J-V plots were measured for the naked (etched), Polysiloxane-coated (n-GaAs= Polysiloxane) and n-GaAs=MnP=Polysiloxane electrodes.Figure 1 shows that the MnP-modified SC surface (after annealing) has a shifting in the dark-current onset potential, V onset , compared to the un-modified surface.The value of the shift was about 200 mV, towards a more positive potential value (positive shift).Such a positive shift is an indicator of a lowering in the value of the flat-band potential (V fb ).The lowering in the value of the V fb dictates that only a less negative (more positive) applied onset potential is needed for the dark-current to occur, when using all other conditions the same.Scheme 2 describes how, in the dark, the value of the onset potential for the modified n-GaAs is less negative than that for the naked n-GaAs.The lowering in the value of V fb is mainly due to the MnP positive charges entrapped inside the polysiloxane matrix adhered to the SC surface.SC surfaces modified with MnP-free polysiloxane matrix did not show a lowering in the value of the V fb .This is because the polysiloxane matrix free of positive charges.The effect of solution ions on the position of V fb has been reported [3][4][5][18][19].
The V fb lowering was well manifested in annealed n-GaAs=MnP=Polysiloxane electrode.With no annealing, no significant shifting in V fb was observed.Therefore, and the annealing process is necessary to permanently bring the MnP positive ions into direct contact with the SC surface at the SC=matrix interface.

Photo-Current Measurements
The J-V plots were measured for the modified and unmodified n-GaAs surfaces under high intensity polychromatic illumination with a Xe lamp, in the water=LiClO 4 =Fe 2þ =Fe 3þ system described above.Figure 2 shows that the annealed n-GaAs=MnP=Polysiloxane surfaces exhibited a shifting in the value of the open-circuit potential V oc towards a more positive (lower) position.The 120 mV lowering in the value of the V oc is a good indication of a lowering in the value of the V fb .Such a lowering is due to the MnP positive charges in a close proximity to the SC surface.In the absence of MnP, no shifting in the value of the V oc was observed.The lowering in the value of the V oc by attachment of positive charges has already been observed in similar photoelectrochemical systems [3] using sub-mono-layer of MnP chemically anchored to the SC surface.The positive shifting of the value of the SC V oc by modification with positive charges is theoretically accepted.With modification, the band bending is lowered as a result of V fb lowering (stabilizing).Therefore, a less negative (more positive) applied bias is therefore needed to flatten the bands, and consequently to inhibit the photocurrent, in case of the modified SC.The value of V oc in case of modified SC will in fact be lowered (positively shifted) and will indicate a lowering in the value of the V fb .
The value of the lowering in the V oc is smaller than the value of the dark-current V onset .This is not unexpected.Literature [1][2]19] shows that exposure of naked SC electrodes, to light, causes a positive shift in the value of V oc as a result of hole accumulation in the space charge region.Such positive charge accumulation would cause a shift in the value of the V oc .

Capacitance Measurements
Values of the V fb were directly measured for the modified and the unmodified n-GaAs electrodes.Capacitance (C) vs. potential (V) plots were measured in water=LiClO 4 =Fe 2þ =Fe 3þ systems in the dark and under illumination.Mott-Schottky (M-S) plots of C À 2 vs. V were constructed in order to directly measure V fb values [20][21][22].n-GaAs BAND EDGE

In the Dark
Figure 3 shows the Mott-Schottky plots measured for the annealed modified and unmodified n-GaAs wafers.The value of V fb in case of n-GaAs=MnP=Polysiloxane is shifted towards a more positive value, by up to 700 mV, relative to that of the unmodified n-GaAs.The attachment of the polymer, without MnP, did not cause positive shifting.These results indicate that the positive shift in the value of V fb is a result of only MnP ions. Figure 4 shows M-S plots measured for the modified and unmodified n-GaAs electrodes without prior annealing.In the case of the non-annealed n-GaAs=MnP=Polysiloxane electrodes, the value of the V fb was positively shifted by only about 240 mV.Again, with no MnP, no shift in the value of the V fb was observed.
Figures (3 and 4) show that annealing n-GaAs=MnP=Polysiloxane surfaces gives better interaction between the SC and the MnP.This was manifested by the larger shift in the V fb value in case of annealing.
The n-GaAs surface modification enhanced the SC surface in another way as appears from Figures ( 3 and 4).The slope of the modified n-GaAs wafer gave a value for the measured doping density, DD, (3.26 Â 10 17 cm À 3 ) consistent with the authentic value (3.26 Â 10 17 cm À 3 ) of the semiconductor.The un-modified surface showed rather less accurate values, ranging between 3.7 Â 10 17 and 5.24 Â 10 17 cm À 3 .The accuracy in the measured value of the DD, for the modified n-GaAs surface, is due to the added surface stability.The unmodified n-GaAs degrades in the aqueous system LiClO 4 =Fe 3þ =Fe 2þ employed here.The lack of the surface stability is presumably the reason for the inaccurate values of DD measured for the unmodified n-GaAs in the dark.

Under Illumination
M-S plots were measured for the modified and unmodified n-GaAs surfaces in the water=LiClO 4 =Fe 2þ =Fe 3þ system.The effect of n-GaAs surface modification was more pronounced.The low stability of the unmodified n-GaAs surface made it difficult to measure the M-S plots under illumination.No linear M-S plots could be constructed for the unmodified surface.The Polysiloxane-modified surface was also not stable enough to show linear plots.
Only did the annealed n-GaAs=MnP=Polysiloxane electrode show soundly linear M-S plots under illumination with room light, Figure 5.The non-annealed modified surface showed but to a lesser extent linear M-S plots, as shown in Figure 5.

Cyclic Voltammometry
Thin films of MnP=polymer were cast onto a pre-cleaned platinum gauze and used as a working electrode.This was to measure the cyclic voltammograms of the MnP inside the polysiloxane matrix.The oxidation peak at (À 100 mV) and the reduction peak at (À 230 mV), against SCE, were observed.These values are consistent with earlier values of MnP cation measured as homogenous solutions [23] and as solid films [24].The MnP-oxidation peak (À 100 mV) is slightly higher than the VB V fb value of the modified n-GaAs, lower than À 120 mV.Therefore, the lowering in the modified SC V fb , associated with MnP=Polysiloxane, is very important.This allows the holes, generated in the accumulation region of the VB, to transfer upward to the Mn II P ion which will give

FIGURE 2
FIGURE 2 Effect of attachment of MnP=polymer on the photocurrent-potential characteristics for (a) unmodified n-GaAs, (b) polymer-modified n-GaAs and (c) Mn=polymer-modified n-GaAs electrodes.All measurements were conducted under illumination using a Xe lamp (illumination intensity at the electrode surface 0.1013 W cm À 2 ) in water=LiClO 4 =Fe(CN) 3À 6 =Fe(CN) 4À 6 at 25 C.

FIGURE 3
FIGURE 3 Effect of n-GaAs surface modification on the value of its flat-band potential, after annealing each electrode, using (a) unmodified, (b) polymer-modified and (c) MnP=polymer-modified electrodes.All measurements were conducted in the dark in water=LiClO 4 =Fe(CN) 3À 6 =Fe(CN) 4À 6 at 25 C.

FIGURE 4
FIGURE 4 Effect of n-GaAs surface modification on the value of its flat-band potential, without annealing the electrodes, using (a) unmodified, (b) polymer-modified and (c) MnP=polymer-modified electrodes.All measurements were conducted in the dark in water=LiClO 4 =Fe(CN) 3À 6 =Fe(CN) 4À 6 at 25 C.

FIGURE 5
FIGURE 5 Effect of annealing the MnP=polymer-modified n-GaAs electrodes in Mott-Schottky plots under room light.Plot (a) for the annealed electrode shows a better and more linear than plot (b) with no annealing.All measurements were conducted in water=LiClO 4 =Fe(CN) 3À 6 =Fe(CN) 4À 6 at 25 C.