Anomalous Photocathodic Behavior of CdS within ... - ACS Publications

6774

J. Phys. Chem. C 2009, 113, 6774–6784

Anomalous Photocathodic Behavior of CdS within the Urbach Tail Region Agnieszka Podborska,† Bartłomiej Gaweł,† Łukasz Pietrzak,‡ Iwona B. Szyman´ska,§ Jeremiasz K. Jeszka,‡ Wiesław Łasocha,†,| and Konrad Szaciłowski*,†,⊥ Centrum Nanochemii Nieorganicznej nanoInchem, Wydział Chemii, Uniwersytet Jagiellon´ski, ul. R. Ingardena 3, 30-060 Krako´w, Poland, Centrum Badan´ Molekularnych i Makromolekularnych Polskiej Akademii Nauk, ul. H. Sienkiewicza 112, 90-363 Ło´dz´, Poland, Wydział Chemii, Uniwersytet Mikołaja Kopernika, ul. Gagarina 7, 87-100 Torun´, Poland, Instytut Katalizy i Fizykochemii Powierzchni Polskiej Akademii Nauk, ul. Niezapominajek 8, 30-239 Krako´w, Poland, and Wydział Metali Niez˙elaznych, Akademia Go´rniczo-Hutnicza, al. A. Mickiewicza 30, 30-059 Krako´w, Poland ReceiVed: NoVember 6, 2008; ReVised Manuscript ReceiVed: February 10, 2009

A series of cadmium sulfide materials have been prepared using microwave-assisted hydrolysis of cadmium-thiourea complexes in water and nonaqueous solvents. Materials are characterized using X-ray diffraction, scanning electron microscopy, diffuse reflectance spectroscopy, and various photoelectrochemical techniques. Sulfur-doped samples of CdS exhibit a pronounced photoelectrochemical photocurrent switching effect within low-energy tails of their absorption spectra. A mechanism of the photocurrent switching process is presented on the basis of experimental investigations and quantum-chemical calculations. Introduction A bottom-up approach is a promising alternative for construction of electronic nanodevices.1-5 Molecular electronics and molecular logic devices may constitute a feasible alternative for classical, silicon-based electronics.6-10 Among new materials suitable for construction of nanoscale switches and other devices, wide band gap semiconductors are materials of choice because of their stability and functional versatility.11-13 Wide band gap semiconductors have found numerous applications in photovoltaic cells,14-20 electrochromic displays,21 and photocatalysis.22-26 Recently, several applications of nanocrystalline semiconductors as optoelectronic switches and logic gates were reported.11,27-36 Photoactivity of wide band gap semiconductors can be significantly enhanced via bulk or surface modification. This may result in photosensitization, novel luminescent properties, a change in (photo)chemical reactivity, and others.12,14,20,26,30,37-40 There are two main strategies of semiconductor modification: bulk doping and surface modification. In principle, bulk doping yields materials of higher chemical and photochemical stability, while surface modification offers enormous versatility of systems due to unification of cooperative properties of solids with the structural versatility of molecules. The surface complex formed via chemisorption of various transition metal complexes on surfaces of mesoporous and nanocrystalline wide band gap semiconductors usually plays the role of light-harvesting antenna, while the semiconducting structure acts as a charge separation device and provides mechanical support for the photosensitizer.14,41,42 Classical semiconductor devices are built on the basis of bulk modification of semiconducting structures via p- and n-doping. On the other hand, almost all of the wide band gap optoelec* Corresponding author. E-mail: [email protected]. Fax +4812 632 0515. † Uniwersytet Jagiellon´ski. ‡ Centrum Badan´ Molekularnych i Makromolekularnych, Polskiej Akademii Nauk. § Uniwersytet Mikołaja Kopernika. | Instytut Katalizy i Fizykochemii Powierzchni, Polskiej Akademii Nauk. ⊥ Akademia Go´rniczo-Hutnicza.

tronic devices reported so far are built on the basis of surfacemodifiedtitaniumdioxide11,27-34 andsemiconductorcomposites.35,36,43-52 In most of the systems, the photocurrent switching processes areassociatedwithselectiveexcitationofap-typecomponent35,36,46-49,51,52 or activation of alternative electron transfer pathways,11,28-34,43-45,50 usually involving oxygen or other electron acceptors. Photocurrent switching phenomena for other semiconductors (CdS, CdTe, and Se) are rarely reported, and there are no mechanistic data on these processes.53-58 In the case of CdS monocrystals, these effects are usually attributed to surface contamination and damages, especially associated with elemental sulfur resulting from oxidation of the surface. Similar effects were observed after contamination with aluminum.58 Single phase, bulk-doped semiconductors should offer superior performance and stability with respect to surface modified semiconductors and semiconducting composites. There are several nanoelectronic devices based on switchable wide band gap semiconductor structures.59-66 Furthermore, numerous oxide semiconductors are used as novel memory components67,68 and for flexible macroelectronic devices.69 On the other hand, photocurrent reversal processes in solar cells result in decreased photocurrent efficiencies and poor overall performance.54 These parasitic processes should be avoided, which implies a good understanding of the underlying photoelectrochemical processes. Therefore, this study concentrates on the photoelectrochemistry and switching phenomena of cadmium sulfide. Cadmium sulfide is an n-type semiconductor, mainly due to sulfur vacancies.70 It can be conveniently prepared by several chemical methods. Simple precipitation of CdS from aqueous solutions using soluble sulfides may lead, however, to unhomogeneous samples of variable crystallinity. These materials may require subsequent thermal processing in order to yield crystalline materials. Other methods require various soluble cadmium coordination compounds with sulfur-containing ligands. Controlled decomposition of these complexes results in highquality nanocrystalline powders and thin films.71 Chemical bath deposition,72 electrodeposition,73 and self-propagating combus-

10.1021/jp809794s CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009

Photocathodic Behavior of CdS in the Urbach Tail

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6775

tion74 are widely used for preparation of cadmium sulfide, while thiourea is commonly used as a source of sulfur. In this paper, we report on the synthesis, optical, and electrochemical properties of nanostructured sulfur-doped cadmium sulfide, together with a mechanistic interpretation of the photocurrent switching effect in these materials. Experimental Section Materials. Cadmium sulfide samples were prepared via microwave heating of a propylene glycol (45 mL) solution of cadmium acetate hexahydrate (1 g) with ethanolamine (5 mL) as complexing agent and thiourea (0.28-5.7 g, depending on desired stoichiometry) under a reflux condenser. Solutions were stirred with a vigorous stream of air, which at applied reaction conditions, induced partial oxidation of thiourea (and/or sulfide resulting from its hydrolysis) and formation of elemental sulfur, forming off-white precipitates on the surface of the reaction mixtures. Synthesis was conducted in a microwave reactor constructed on the basis of the household microwave oven Samsung MW87L (Samsung, Thailand). The oven was equipped with a borosilicate glass tube extending into its active space. On the end of the tube, spherical or pear-shaped flasks can be mounted using standard 14/23 grinded glass joints. The oven reactor was equipped outside with a reflux condenser and a reagents/inert gas/oxygen supply system. Typically 50 s of 850 W microwave irradiation at 2.45 GHz was sufficient to complete the reaction. Products were separated by centrifugation, washed five times with a large volume of water, and vacuum-dried at room temperature. All reagents were supplied by Fluka (Switzerland). Instrumentation. Scanning electron microscope images were taken using Jeol JSM -5500LV apparatus working in the secondary electron mode with an accelerating voltage of 10 kV or on SEM-LEO 1460V operating at 20 kV. The samples were attached to the microscope stage using silver or graphite paint, and their conductivity was high enough to dissipate an electric charge so that evaporation of an additional metal layer was not necessary. Energy dispersion X-ray (EDX) analysis was performed on an EDX Quantax 200 spectrometer with an XFlash 4010 detector (Bruker AXS) operating at 25 kV. Samples were characterized by X-ray powder diffraction (XRPD). X-ray diffraction measurements were performed on a Philips X’Pert Pro diffractometer with a Bragg-Brentano geometry using Co KR radiation (λ ) 1.78897 Å). The XRPD patterns were collected at 298 K, with a 2θ range of 2-80°, a time per step of 1.5 s, and a scan step size of 0.025° 2θ. Phase compositions were calculated using a Rietveld software package QUANTO,75 which is designed to automatically estimate the weight fraction of crystalline phases in mixtures. The peak shape was fitted using a modified Pearson VII function. The background of each profile was modeled by a fifth-order polynomial Am(2θ)m, where m is integer varied from 0 to 5. The mean dimension d of crystallites was estimated from the most intense X-ray peak for each phase, using the Scherrer’s equation

d ) Kλ/β cos θ

(1)

where K is a constant related to the crystallite shape (0.9) and β is the pure integral breath of the powder reflection free of the broadening due to instrumental contributions. This calibration was performed by means of the diffraction pattern of a standard Al2O3 powder. Powder diffraction patterns were simulated with Mercury 1.4.2 software (Cambridge Crystallographic Data Centre, U.K.).

Diffuse reflectance spectra were recorded on a Lambda 12 (PerkinElmer, U.S.A.) spectrophotometer equipped with a 5 cm integration sphere. Barium sulfate of spectral purity was used as a reference material. Fluorescence spectra were recorded on a LS45 (PerkinElmer) instrument operating with an 8 nm bandpass. Samples of cadmium sulfide were suspended in tetraethylene glycol prior to the measurement. Spectra were corrected for reflected and scattered light using a four-point baseline correction. Photoelectrochemical measurements were performed on a BAS CV-50W electrochemical analyzer (Bioanalytical Systems, U.S.A.), and an aqueous 0.1M KNO3 solution was used as an electrolyte throughout all the experiments. Working electrodes were prepared on indium tin oxide (ITO)-covered polyethylene terephthalate (PET) film (Aldrich) with a surface resistivity of 35 Ω/sq. Films were deposited from aqueous suspensions of semiconducting powders via a cast coating technique. Films were dried with a stream of hot air and used for measurement immediately after preparation. In order to avoid the influence of the semiconductor layer thickness, the electrodes were illuminated through the conducting support. Photocurrents were recorded using a classical three-electrode setup with a platinum wire counter electrode and a Ag/AgCl reference electrode. A high-pressure xenon lamp XBO150 (Osram, Germany) equipped with a computer-controlled monochromator and shutter was used in all photocurrent measurements. Photocurrent action spectra were recorded using pulsed illumination of the photoelectrodes at potentiostatic conditions. Photoelectrodes were conditioned at measurement potentials for 60 s. This procedure helps to keep the dark currents low (less than 0.2 µA within the whole potential range) and allows us to eliminate the effect of dark current fluctuations. Conduction band edge potentials were determined using the Berańek-Pleskov method76-78 and Roy method (for stoichiometric samples only).79 A computer-interfaced high-power 465 nm light-emitting diode (LED) matrix (Light Engine Uno Plus Air, Enfis, U.K.) was used as a light source. In the former method, the potential generated at a semiconductor photoelectrode on illumination was recorded as a function of light intensity within 136-5000 mW of the diode output power. At sufficiently high light intensity, the photovoltage reaches a plateau value and does not change with increasing light intensity. The plateau potential equals the conduction band edge potential.77,78 In the latter method, a 40 mg sample of semiconductor powder was suspended in 70 cm3 of 0.1M aqueous potassium nitrate and sonicated for 5 min. Thirty milligrams of methylviologen bis(hexafluorophosphate) was added, and the resulting mixture was acidified with 1 cm3 of concentrated perchloric acid. The suspension was placed in a rectangular glass vessel equipped with a combined pH electrode, platinum foil electrode (area 2.5 cm2), and reference Ag/AgCl electrode (FLEXREF, World Precision Instruments, U.S.A.). The vessel was vigorously purged with argon and irradiated with the full light of an HBO 200 mercury high-pressure lamp. The solution was titrated with a 0.1M solution of Na2CO3, using a Medipan 610 B.S computercontrolled infusion pump (Medipan, Poland) equipped with calibrated Hamilton syringes and a custom-built interface. Photopotentials were measured against a Ag/AgCl reference using a BM-811 digital voltammeter (Brymen, Taiwan). Calculations. Quantum chemical calculations were performed on Cd38S38+i (i ) 0-7) clusters of a cubic cadmium sulfide (hawleyite)80 structure. Doping sulfur atoms were randomly placed within clusters, and the geometry of the resulting structures were optimized using the MM/UFF method implemented with the Arguslab 4.1 package (Planaria Software,

6776 J. Phys. Chem. C, Vol. 113, No. 16, 2009

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TABLE 1: Composition, Structural, and Spectroscopic Parameters of Various CdS Samples f

a

1 5 10 15 20

xb

ηc

d (nm)d

Eg (eV)e

EU (meV)f

Xg

0.85 0.89 0.95 1.02 1.13

0.20 0.32 0.34 0.35 0.37

7 5 5 6 5

2.43 2.48 2.49 2.53 2.52

90.2 90.5 94.6 95.5 96.7

13.15 13.21 13.90 14.05 14.25

a

Ratio of thiourea:Cd2+. b Molar ratio of S:Cd, according to the EDX analysis. c Fraction of the hexagonal phase (grenockite). d Average crystallite size, according to Scherrer’s equation (eq 1). e Optical band gap energy. f Urbach energy (eq 3). g Structural disorder parameter (eq 4).

U.S.A.).81,82 Electronic structures of these optimized clusters were determined using the Hyperchem 7.0 package (Hypercube, U.S.A.) on a ZINDO/1 level of theory, while percentage compositions of molecular orbitals and density-of-states spectra were calculated using the AOMix program and Gaussian envelopes with 0.5 eV widths.83,84 Results and Discussion Synthesis. Heating of alkaline aqueous solutions containing cadmium ions, complexing agents (ammonia, citrate, edta, etc.), and thiourea results in precipitation of crystalline CdS as powder. Furthermore, all surfaces in contact with the solution are covered by thin semi-transparent layers of CdS. These powder preparations are crystalline but highly inhomogeneous (Table 1). Therefore, microwave heating was applied, and water was replaced by polar organic solvents with high boiling points (ethylene glycol and propylene glycol). These solvents are good microwave absorbers, and microwave heating is very efficient. Small amounts of water necessary for hydrolysis of Cd2+-thiourea complexes were introduced with hydrated cadmium salts. This approach also facilitates control over the growth of CdS particles, their aggregation, and sample morphology.85 Purging with air proved to be necessary to obtain materials exhibiting photocurrent switching. This indicates the important role of oxidation processes (presumably the formation of elemental sulfur) in the synthesis of the studied materials. EDX analysis reveals an increasing S:Cd ratio with an increasing thiourea concentration in the reaction mixture (Table 1), while the nitrogen and oxygen content is virtually the same. These results do not indicate the overall stoichiometry of the samples but provide information on chemical composition of the surface layers of the sample. Element distribution mapping indicated high homogeneity of all the studied samples, thus, excluding the presence of larger sulfur particles in the studied samples. The analytical results for carbon, nitrogen, and oxygen contents are rather inaccurate due to significant overlap of peaks but indicate a constant concentration of these elements within all the studied samples. This in turn indicates that the surface adsorption of thiourea cannot be responsible for the increasing sulfur content. Structure and Morphology. All of the studied samples consist of two forms of CdS: hexagonal (grenockite, symmetry group P63mc) and cubic (hawleyite, symmetry group F4j3m). Despite different crystal systems, Cd-S distances and density of both compounds are very similar (vide infra). Detailed analysis of diffraction peak shapes revealed variable contributions of hexagonal and cubic forms, depending on the thiourea: Cd2+ ratio (Figure 1). Because of peak broadening, resulting from very small crystallite dimensions, only the approximate phase composition of each sample could be calculated (Table

Figure 1. Rietveld plots for CdS (f ) 1) obtained in (a) microwave synthesis compared with simulated powder diffraction patterns of (b) hawleyite and (c) grenockite. Peaks marked with a star result from a sample holder.

1). It is clearly noticed, however, that an increasing concentration of thiourea in the reaction mixture does not significantly influence the size of the crystallites, presumably because of the high concentration of chelating ligands in the reaction mixture. On the other hand, an increased thiourea:Cd2+ ratio results in an increased content of hexagonal CdS in the final product. This may suggest involvement of both thiourea and its oxidation products (primarily sulfur) in the formation of CdS nanoparticles. Figure 2 shows scanning electron micrographs of CdS prepared in an aqueous solution (Figure 2a) and in propylene glycol (Figure 2b-d) at various thiourea:Cd2+ ratios (f). It is clearly seen that the material prepared in an aqueous solution with f ) 5 (Figure 2a) forms spherical aggregates (about 2 µm in size) of thin, plate-like nanocrystals, growing radially from the center. The thickness of the nanocrystals can be estimated as not more than 50 nm. The material grown at the same f in propylene glycol (Figure 2c) has a different morphology. Platelike objects, 50-200 nm, arranged nearly parallel to each other in layers are obtained. Their thickness seems to be on the order of 20-50 nm (estimated from images of layer edges), but the shape is not well defined, probably because of the adsorbed layer of propylene glycol and chelating amines. Synthesis in propylene glycol gives some structures of morphology intermediate between the two former cases when f is reduced from 5 to 1 (Figure 2b). Most of the material has a morphology similar to that in Figure 2b, but in some places perpendicular platelets also grow from larger structures (marked by an arrow in Figure 2b). As the ratio f increases, the crystal size decreases, and their shape is less defined. For f ) 20 (Figure 2d), the material is almost featureless. Individual crystals are hardly seen. However, their size is estimated to be decreased about two times as compared with the sample prepared at f ) 5. It was also observed that the charge dissipation gets worse as f is increased, which indicates decreasing conductivity of the obtained materials. This indicates pronounced structural distortions within CdS crystallites and/or the presence of a dielectric material (e.g., elemental sulfur) in studied samples. Different morphology of the crystals obtained in propylene glycol suggests different a nucleation and growth mechanism. In propylene glycol, crystals are grown independently with very limited aggregation, while in water secondary nucleation dominates. Optical Properties. Absorption spectra of the investigated materials are shown in Figure 3. Exact values of the absorption



Figure 2. Scanning electron micrographs of CdS prepared in an (a) aqueous solution and in (b-d) propylene glycol at various Cd2+:thiourea ratios; (b) 1:1, (a,c) 1:5, and (d) 1:20.

of isolated nanoparticles of comparable size (over 3.5), probably due to aggregation. All of the spectra show pronounced low-energy tails. It is noticed that materials prepared in a high excess of thiourea exhibit more intense tails. Various effects like thermal fluctuations, structural disorders, and surface states may contribute to the absorption band tail.88 All of these should result in various spectral tails generally described as E

R(E) ∝ e-( E0 ) , 1/2 e n e 2 n

(2)

where E is measured away from the band edge, Eg.

R ) R0e Figure 3. Diffuse reflectance spectra of various CdS samples. Inset shows transformed DR spectra, indicating changes in band gap energy with increasing f.

coefficient (R) cannot be derived from diffuse reflectance (DR) spectra, but the Kubelka-Munk function (RKM)86 can be used instead, as it is directly proportional to the absorption coefficient with an S-1 proportionality factor (S, scattering coefficient).87 In samples diluted in an nonabsorbing solid medium, the scattering coefficient is wavelength independent. Bulk CdS and CdS nanoparticles are generally believed to be direct semiconductors. The energy gap of the bulk material is 2.42 eV, and in the case of nanoparticles, it is size dependent and can reach 3.7 eV. Analysis of the absorption edge of the studied materials (inset in Figure 3) yields band gaps of ∼2.4-2.5 eV. Exact values are listed in Table 1. It is noticed that band gap values slightly increase with an increasing f value, while the dimensions of the crystallites are virtually unchanged, as estimated from the powder diffraction patterns. These values are slightly higher than those of bulk CdS but lower than those

hV-E0 EU

) R0KMSe

hV-E0 EU

(3)

Classical Urbach tails are characterized with n ) 1,89 while higher values are observed in the case of Halperin-Lax tails in heavily doped, compensated semiconductors.90 In the classical treatment of semiconductor absorption spectra, Urbach energy represents the thermal disorder in semiconducting crystals,91,92 and the spectral shape can be derived from the Hamiltonian for the interaction between the Frenkel exciton and the lattice vibrations.93,94 The shape of the tails recorded can be well-fitted with the Urbach equation (eq 3),95 which clearly indicates involvement of structural distortions in optical properties of the studied CdS samples. Any involvement of surface states in the spectral tail should result in tails characterized with n ) 2 (eq 2), as distribution of both donor and acceptor states are described with Gaussian statistics. The resulting transition is best described by the Gaussian envelope.96-99 Thus, the linear exponential shape of the spectral tail clearly indicates a significant contribution by structural distortions (induced by phonons and dopant atoms) and rather excludes the contribution of surface states. Urbach energies (EU) are on the order of 90 meV and tend to increase with an increasing f ratio (Table 1). Urbach energy can be in turn interpreted as a collective influence of thermal


Podborska et al.

fluctuations (associated with phonons) and lattice disorders and impurities on the valence and conduction band edges, according to Cody’s extension of the Urbach-Martienssen model (eq 4)100

EU )

(

Ep Ep X + coth 2σ0 2kT

)

(4)

where Ep is the lattice phonon energy, σ0 quantitatively describes the ionicity of the crystal lattice, and X is defined as the ratio of the mean square deviation of atomic positions caused by structural disorder to the zero-point uncertainty in the atomic positions.100 In the case of cadmium sulfide, the average phonon energy Ep equals 26 meV (resulting from optical phonons of energies 5.5, 28, 29, 31, and 38 meV), while the σ0 coefficient equals 2.2.100 These figures result in Urbach energy associated with the thermal fluctuations at a room temperature of 12.4 meV, while the fraction associated with structural disorders ranges from 77.7 to 84.2 meV for materials with f ) 1 and f ) 20, respectively. On the other hand, single-crystal CdS is characterized by a very low Urbach energy of 16 meV, which corresponds to X ) 0.61.101 This figure approximates the thermal limit of 12.4 meV (Urbach energy for X ) 0). Conversely, microcrystalline CdS powders and chemically deposited thin films are characterized with a high EU of 160-200 meV (X ) 25-32).102 Urbach energies determined for studied CdS samples indicate the formation of only moderately disordered crystals during microwave synthesis. Increasing structural disorders of CdS synthesized in increasing concentrations of thiourea suggests that excess sulfur (in the form of sulfide anions, hydrosulfide anions, or sulfur atoms) is incorporated in the interstitial positions of the CdS lattice. Surface modification of CdS crystals (with sulfide, hydrogen sulfide, thiourea, amines, or glycols) should not account for any changes in the Urbach energy. Furthermore, oxidative conditions maintained during synthesis should exclude the first two dopants, so sulfur inclusions (atomic or nanoscale aggregates) seem to be responsible for the observed increase of Urbach energies, EU (Table 1). The lack of any diffraction data indicating the presence of crystalline sulfur suggests atomic dispersion of excess sulfur within the lattice of CdS. In order to estimate the influence of excess sulfur atoms on the geometry of the CdS lattice, a series of hawleyite-CdS clusters, containing 38 cadmium ions, 38 sulfide anions, and a variable number of neutral sulfur atoms, were modeled using MM/UFF combined with the ZINDO/1 technique (Figure 4a). The average bond length for a stoichiometric Cd38S38 cluster was found to be 2.42 Å. This is slightly shorter than the real Cd-S bond in hawleyite (2.52 Å). The Cd-S bond length in the Cd38S39 cluster varies from 2.42 to 2.59 Å, and the maximal Cd-S distance increases with an increasing number of neutral sulfur atoms (Cd38S40, 2.70 Å; Cd38S41, 2.80 Å). An introduction of interstitial sulfur atoms results in local expansion of the lattice at the site of doping, while Cd-S bonds in more distant parts of the lattice are slightly contracted. This observation also supports the change in phase composition with increasing doping concentration. The grenockite (hexagonal CdS) lattice is more relaxed and can easily adopt dopant atoms in the interstitial positions. Therefore, with an increasing excess of thiourea and an increased sulfur dopant concentration, the content of the grenockite phase should increase (Table 1). The influence of surface-adsorbed thiourea on the electronic properties of the CdS powders was ruled out using ZINDO calculations of the electronic structures of the Cd38S38(thiourea)n (n ) 1-5) clusters. The thiourea highest occupied molecular

Figure 4. Geometry of a hawleyite Cd38S39 cluster (a) and the contour of the molecular orbital associated with interstitial sulfur atom calculated using the ZINDO/1 approach (b). Red ball in the center of the cluster represents the doping sulfur atom.

orbitals (HOMO) can be found ∼3 eV below the Cd38S38 HOMO, while the lowest unoccupied molecular orbitals (LUMO) are found ∼1 eV above the LUMO of the Cd38S38 cluster. Such an arrangement of molecular orbitals in respect to the frontier orbitals of the CdS cluster results in a negligible influence of surface molecules on the electronic properties of semiconductor crystals. On the other hand, strong dependence of spectral and photoelectrochemical properties are observed, which suggests sulfur doping of microwave-synthesized CdS. The influence of sulfur doping on the electronic structure of cadmium sulfide was tested in a series of Cd38S38+i clusters (i ) 0-5). The calculated Eg values (4.9 eV) for all of the Cd38S38+i clusters do not significantly depend on dopant concentration. This figure is significantly higher than the observed bulk material band gap value (2.4 - 2.5 eV) but is consistent with other reported computational and experimental results for small CdS103,104 and other cadmium chalcogenide clusters.105,106 Such high band gap energy values may be substantiated with the quantum size effect:107-109 the average diameter of all of the model clusters does not exceed 1.2 nm. In all the studied cases, however, the HOMO-LUMO gap is much smaller and amounts 2.30 (i ) 0), and with an increasing number of dopant atoms, the gap increases gradually to 2.72 eV (i ) 5). In studied clusters, the HOMO and LUMO orbitals cannot be regarded as band edges, as they are mostly associated with surface states and/or dangling bonds. The introduction of sulfur dopants (i > 0) in the interstitial positions do not affect the calculated band edge potentials. The band diagram of a Cd38S38 cluster shows two main pseudo bands: a valence band (VB), composed mostly from sulfur orbitals, and a conduction band (CB), composed mostly from cadmium orbitals (Figure 5a). A small contribution of sulfur orbitals to the CB (and cadmium orbitals to the VB) implies a significant covalent character to the sulfur-cadmium bond, which is a consequence of only a small difference in Pauling’s electronegativities of these elements (0.89). Just below the edge of a conduction band, a small group of electronic levels can be associated with CdS surface states, which may act as shallow traps and play an important role in the interfacial electron transfer processes. With doping, new electronic levels are generated (Figure 5b-d). With an increasing i, the number of empty states just below the CB edge increases; they originate from empty 3d orbitals of sulfur atoms in interstitial positions. Corresponding molecular orbitals are strongly overlapped with 3d orbitals of sulfur atoms within the CdS lattice (Figure 4b). Therefore, sulfur interstitial inclusions are considered as polysulfide species,110 which are known as good electron transfer mediators111 and can be involved in one-electron redox processes.112 It cannot be



Figure 5. Total (solid lines) and partial (dotted and dashed lines) densities of states calculated for hawleyite (a) Cd38S38, (b) Cd38S39, (c) Cd38S40, and (d) Cd38S41 clusters, using the ZINDO/1 method.

excluded that sulfur is located at the grain boundaries as molecular sulfur (S8) as well. Photoelectrochemical Properties. A pseudo-Fermi level potential measured using the Roy’s79 method equals -0.410 mV [versus a normal hydrogen electrode (NHE), pH 7] for stoichiometric CdS samples. This value is pH-dependent and decreases with increasing pH by 0.030 V per pH unit (eq 5).79

EF ) E0 - k(pH - pH0)

(5)

ExactlythesamevaluewasobtainedusingtheBerańek-Pleskov76-78 method. Cadmium sulfide photoelectrodes were illuminated with monochromatic light (465 nm) from high-power LED matrices, and the resulting photovoltage was recorded as a function of the diode light output (Figure 6a). With increasing light intensity, the photoelectrode potentials decrease and at sufficiently high illumination intensity assumes the plateau. The plateau potential can be in turn be considered as a pseudo-Fermi level potential at nonequilibrium photostationary conditions.76,78 This potential is identical with previously reported data31,113,114 and supports the equivalence of the Berańek-Pleskov77,78 and Roy79 methods of pseudo-Fermi level potential determination under photostationary conditions. The latter method performed with series of electrolytes of different pH values allows an independent determination of the Nernstian k factor, which describes variation of the band edge potential with pH. For the stoichiometric sample (f ) 1), the k parameter equals 27 mV, which is consistent with the literature for cadmium sulfide (30 mV).31 Increasing the concentration of thiourea during CdS synthesis results in materials with pseudo-Fermi level potentials (EF, Table 2) systematically shifted in the cathodic direction. The most dramatic decrease is observed on transition from neat to doped material; the introduction of dopant atoms results in a cathodic shift of 15 mV. This is in marked contrast with typical photocorrosion behavior, which results in anodic shifts of pseudo-Fermi level potential.58 Similar changes are observed in the k factor. Its value for neat CdS is 27-30 mV per pH unit, while with doping, the value increases dramatically and reaches 55-66 mV per pH unit (Figure 6b). These observations

Figure 6. Dependence of (a) open circuit photopotential on light intensity for various CdS samples and (b) pH dependence of plateau photopotentials.


Podborska et al.

TABLE 2: Electrochemical Parameters of Various CdS Samples fa

EF (V)b

EVBE (V)c

k (V)d

1 5 10 15 20

-0.41 -0.56 -0.58 -0.60 -0.66

2.02 1.92 1.91 1.93 1.86

0.027 0.050 0.050 0.052 0.066

a

c

Ratio of thiourea:Cd2+. b Pseudo-Fermi level potential at pH 7. Valence band edge potential at pH 7. d Nernstian factor (eq 5).

clearly indicate dramatic changes in the surface reactivity of these materials. The pseudo-Fermi level potential for neat, monocrystalline chalcogenide crystals is usually pH independent, but can be shifted by chemisorption of chalcogenide anions,115 thiols,116 and other chalcogen organic compounds117 (i.e., potential determining ions).76,118,119 Increased pH dependence of the pseudo-Fermi level potential indicates significant changes in the chemical reactivity of the surface. The introduction of interstitial sulfur atoms, as indicated by quantum-chemical modeling (Figure 4), is engaged in the formation of pseudopolysulfide entities within the anionic sublattice of CdS. Molecular orbitals associated with these entities easily reach the surface of CdS nanoclusters, thus modifying its chemical reactivity. Furthermore, the introduction of interstitial sulfur atoms into the CdS lattice must result in the change of the properties of the main trapping centers, which in turn should influence the photoelectrochemical and luminescent properties of this material. In neat CdS, the only reaction that can be considered as a trapping process is the reduction of surface cadmium centers, proceeding at potentials lower than -0.403 V versus NHE (eq 6)120

Cd2+ + 2e-aCd0

(6)

The standard potential of this process corresponds to the potential of the conduction band edge of neat cadmium sulfide (Table 2). The sulfur dopant, in the form of molecular sulfur or polysulfide anions, may generate additional shallow traps within the band gap. Elemental molecular sulfur can act as single electron acceptor in two subsequent reduction steps (eqs 7 and 8)112,121

S8 + e-aS8S8-

-

+e

aS82-

(7) (8)

On the other hand larger polysulfides undergo spontaneous homolytic scission yielding redox-active radicals (Eq. 9-10) e.g.:110

S52-aS2•- + S3•-

(9)

S3•- + e-aS32-

(10)

Independently on the nature of the trap, their energies should fall within about -0.3 ( 0.1 V versus NHE.112,122 As sulfur dopant atoms are engaged in the formation of polysulfide-like structures (Figure 4), sulfur doping should result in the formation of shallow traps (0.2-0.4 eV below the edge of the conduction band). The changes in the electron trapping processes can be monitored using fluorescence spectroscopy. Excitation at 2.637 eV (470 nm) results in relatively strong fluorescence of studied CdS samples (Figure 7). Detailed analysis of band shapes reveals the influence of dopants on the electronic structure of CdS

nanocrystals. Stoichiometric cadmium sulfide shows three Gaussian bands within the emission peak (Figure 7a): a weak high-energy band at 2.294 eV, a strong low-energy band at 2.053 eV, and the weakest energy band at 2.175 eV. The high-energy component is associated with the fundamental band gap transition, while the low-energy component is associated with the luminescence from the trapping states.123-127 Sulfur doping induces significant modification of the spectral band shape of the CdS emission. In the case of S-doped CdS (f ) 5, f ) 10, and f ) 15), an additional intense luminescence peak is observed at 1.93-1.97 eV (Figure 7b,c,d). Moreover, the intensity of this peak decreases with an increasing f, and simultaneously the intensities of the peaks at ∼2.05 and ∼2.17 eV increase. These results indicate the luminescence from the trapped states is associated with the emissions from: (i) the S-vacancy donor to the VB, peak ∼1.95 eV, (ii) the Cd interstitial donor to the VB, peak ∼2.05 eV, and (iii) the S interstitial to the VB, peak ∼2.17 eV. These spectral changes indicate that the CdS samples with f > 1 are strongly distorted and contain high concentrations of structural defects (sulfur and cadmium interstitials), while an increasing f results in the decreased concentration of sulfur vacancies. This further substantiates the p-type photoelectrochemical behavior of sulfur-doped cadmium sulfide and implies that the shallow traps must be associated with sulfur-rich moieties (eqs 7, 8, 9, and 10). Photocurrent Switching. Stoichiometric cadmium sulfide (f ) 1) generates an anodic photocurrent within a wide potential window upon illumination within its whole absorption spectrum (Figure 8a). With decreasing photoelectrode potential, lower photocurrent efficiencies are observed due to hampered electron injection to the conducting electrode at photoelectrode potentials approaching the Fermi level. With doping, the photoelectrochemical characteristics of the photoelectrodes prepared from CdS powders is dramatically changed (Figure 8b,c). With decreasing photoelectrode potential, the intensity of the anodic photocurrent decreases, similarly to the undoped material, but at a certain potential the photocurrent polarity is reversed. Negative polarization of the photoelectrode results in the generation of cathodic photocurrents. Furthermore, an increase in doping results in decay of the overall photoelectrochemical performance, i.e., decreased mean photocurrents. On the other hand, an increase in doping results in more pronounced photocurrent switching and an increase in the photocathodic response at the expense of photoanodic currents (Figure 8b,c). At the potentials close to the switching point, variation in the incident light wavelength also results in photocurrent switching (Figure 8d). This indicates that the sulfur-centered traps are directly populated via optical excitation. This is consistent with the dependence of the Urbach tail on the doping level. At significantly low potential (E ≈ 0.2 V versus NHE, Figure 9b-d), high-energy excitation populates the conduction band, thus yielding anodic photocurrents, while low-energy excitation populates only the trapping sites responsible for cathodic photocurrents. This effect is very similar to the previously described photoelectrochemical photocurrent switching (PEPS) effect observed in surface-modified wide band gap semiconductors (TiO2 and CdS).11,29-34,43,128 All of the above experimental and computational investigations allow elucidation of the photocurrent switching mechanism. Generation of anodic photocurrents at neat CdS results from VBfCB transitions, followed by interfacial electron transfer to the metallic electrode. Involvement of Cd2+/0 trapping sites cannot be excluded in this case (Figure 9a). Introduction of sulfur dopants results in a noticeable decrease of band edge



Figure 7. Corrected fluorescence spectra of CdS samples with (a) f ) 1, (b) f ) 5, (c) f ) 10, and (d) f ) 15, together with their deconvolution into Gaussian components. Fundamental transition is indicated in blue, while emission from trap levels is indicated in red.

Figure 8. Potential-dependent photocurrent action spectra recorded for various CdS samples in the presence of molecular oxygen: (a) f ) 1, (b) f ) 5, and (c) f ) 10. Potentials are referenced to the NHE electrode. (d) Photocurrent action spectrum is recorded for material with f ) 10 at -100 mV versus NHE in oxygen-saturated electrolyte.


Podborska et al.

Figure 9. Mechanism of photocurrent switching at CdS photoelectrodes: (a) undoped CdS (f ) 1), (b) sulfur-doped CdS (f ) 20) in photoanodic, and (c) photocathodic regimes. At potentials matching the energy of the dopant levels, wavelength-dependent photocurrent switching occurs (d).

potentials (Table 2) with only minute changes in band gap energy (Table 1). At photoelectrode potentials higher than the potentials of the dopant levels, the doped materials behave like normal n-type semiconductors generating anodic photocurrent upon excitation (Figure 9b). In both cases, electric neutrality of semiconducting particles can be attained via oxidation of external electron donors from the electrolyte or via oxidation of surface sulfhydryl groups (photocorrosion). In the absence of sacrificial electron donors (e.g., sodium 3-mercaptopropanesulfonate), photocorrosion induces a decrease in the photoelectrochemical performance and renders undoped materials switchable. At sufficiently low photoelectrode potentials, trapped electrons cannot be transferred to the conducting support (Figure 9c). In this case, the only way to generate photocurrent is a reduction of an electron acceptor present in solution. Therefore, in the presence of oxygen cathodic photocurrents are observed. Careful tuning of the photoelectrode potential allows for the observation of wavelength-dependent photocurrent polarity (Figure 9d). Selective excitation of a semiconductor photoelectrode within fundamental absorption or an Urbach tail results in a photoanodic and photocathodic response, respectively (Figure 8d). Photocurrent switching on wavelength changes and dual emission features in fluorescence spectra indicate that thermal population of intraband sulfur-centered states is rather difficult (wavy arrows in Figure 9b,c). Conclusions and Outlook This novel synthetic method toward stoichiometric and sulfurdoped nanocrystalline cadmium sulfide results in semiconducting materials of unusual properties. Peculiar photoelectrochemical characteristics of sulfur-doped cadmium sulfide in principle should allow construction of optoelectronic logic devices analogous to the devices based on surface-modified titanium dioxide, as the photocurrent switching characteristics are almost identical.32,33,43 Therefore, this simple chemical system can be regarded as a simple functional model of logic devices. Information can be supplied to the system by means of light pulses and photoelectrode potential, while processed information is retrieved in the form of current pulses. This behavior is quite unique for chemical logic systems and allows easy communication between various electronic silicon-based devices and chemical logic systems. Furthermore, the application of photoelectrochemical processes mitigates the problems of concatenation and interfacing. The input and output signals of these molecular devices are easily understood by classical electronic devices. Acknowledgment. The authors thank Dr. Przemysław Kolek for assistance in quantum chemical modeling of CdS clusters

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