Brian Seger

TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide

Graphene oxide suspended in ethanol undergoes reduction as it accepts electrons from UV-irradiated TiO(2) suspensions. The reduction is accompanied by changes in the absorption of the graphene oxide, as the color of the suspension shifts from brown to black. The direct interaction between TiO(2) particles and graphene sheets hinders the collapse of exfoliated sheets of graphene. Solid films cast on a borosilicate glass gap separated by gold-sputtered terminations show an order of magnitude decrease in lateral resistance following reduction with the TiO(2) photocatalyst. The photocatalytic methodology not only provides an on-demand UV-assisted reduction technique but also opens up new ways to obtain photoactive graphene-semiconductor composites.

Using TiO2 as a Conductive Protective Layer for Photocathodic H2 Evolution

Surface passivation is a general issue for Si-based photoelectrodes because it progressively hinders electron conduction at the semiconductor/electrolyte interface. In this work, we show that a sputtered 100 nm TiO(2) layer on top of a thin Ti metal layer may be used to protect an n(+)p Si photocathode during photocatalytic H(2) evolution. Although TiO(2) is a semiconductor, we show that it behaves like a metallic conductor would under photocathodic H(2) evolution conditions. This behavior is due to the fortunate alignment of the TiO(2) conduction band with respect to the hydrogen evolution potential, which allows it to conduct electrons from the Si while simultaneously protecting the Si from surface passivation. By using a Pt catalyst the electrode achieves an H(2) evolution onset of 520 mV vs NHE and a Tafel slope of 30 mV when illuminated by the red part (λ > 635 nm) of the AM 1.5 spectrum. The saturation photocurrent (H(2) evolution) was also significantly enhanced by the antireflective properties of the TiO(2) layer. It was shown that with proper annealing conditions these electrodes could run 72 h without significant degradation. An Fe(2+)/Fe(3+) redox couple was used to help elucidate details of the band diagram.

Nitrogen Doped Sr2Ta2O7 Coupled with Graphene Sheets as Photocatalysts for Increased Photocatalytic Hydrogen Production

In this work we present the synthesis of a new type of nitrogen-doped tantalate, Sr(2)Ta(2)O(7-x)N(x), which exhibited significantly increased visible light absorption and improved photocatalytic hydrogen production by 87% under solar irradiation, compared with its undoped counterpart Sr(2)Ta(2)O(7). The photocatalyst also exhibited a strong capability in photoinduced reduction of exfoliated graphene oxide (GO) to graphene sheets. By using graphene as a support for a Pt cocatalyst, a new type of composite containing graphene-Pt and Sr(2)Ta(2)O(7-x)N(x) was designed, which demonstrated an additional ∼80% increase in hydrogen production and an quantum efficiency of 6.45% (∼177% increase from pristine undoped Sr(2)Ta(2)O(7)) due to the efficient charge carrier separation on the photocatalyst. This work suggests that graphene can play an important role as an electron transfer highway, which facilitates the charge carrier collection onto Pt cocatalysts. The method can thus be considered as an excellent strategy to increase photocatalytic hydrogen production in addition to a commonly applied doping method.

Recent Development in Hydrogen Evolution Reaction Catalysts and Their Practical Implementation.

The past 10 years have seen great advances in the field of electrochemical hydrogen evolution. In particular, several new nonprecious metal electrocatalysts, for example, the MoS2 or the Ni2P family of materials, have emerged as contenders for electrochemical hydrogen evolution under harsh acidic conditions offering nearly platinum-like catalytic performance. The developments have been particularly fast in the last 5 years, and the present Perspective highlights key developments and discusses them, along with hydrogen evolution in general, in the context of the global energy problem.

Hydrogen Production Using a Molybdenum Sulfide Catalyst on a Titanium-Protected n+p-Silicon Photocathode†

A low-cost substitute: A titanium protection layer on silicon made it possible to use silicon under highly oxidizing conditions without oxidation of the silicon. Molybdenum sulfide was electrodeposited on the Ti-protected n(+)p-silicon electrode. This electrode was applied as a photocathode for water splitting and showed a greatly enhanced efficiency.

2-Photon tandem device for water splitting: comparing photocathode first versus photoanode first designs

Within the field of photocatalytic water splitting there are several strategies to achieve the goal of efficient and cheap photocatalytic water splitting. This work examines one particular strategy by focusing on monolithically stacked, two-photon photoelectrochemical cells. The overall aim of the analysis is to compare the relative merits of two fundamentally different designs: one, where the photoanode is the large bandgap material (light-facing side), and the other, where the photocathode is the large bandgap material. Even though the former design is often shown in the literature, the present analysis shows that the latter design has several advantages. This is particularly true when considering designs that incorporate protection layers to protect the photoabsorbers. A high throughput computational screening was used to filter materials databases in search of candidates with the correct properties. These results show that without protective layers there are scarcely any materials which seem viable as photoabsorbers whereas with protection layers there are significantly more candidates. Since the protection layer (and redox catalysts) on the light facing side should not interfere with light absorption, this is the more difficult side to optimize. Nevertheless, by using TiO2 as a transparent cathode protection layer in conjunction with known H2 evolution catalysts, protection is clearly feasible for a large bandgap photocathode. This suggests that there may be promising strategies for photocatalytic water splitting by using a large bandgap photocathode and a low bandgap photoanode with attached protection layers.

Protection of p(+)-n-Si Photoanodes by Sputter-Deposited Ir/IrOx Thin Films.

Sputter deposition of Ir/IrOx on p(+)-n-Si without interfacial corrosion protection layers yielded photoanodes capable of efficient water oxidation (OER) in acidic media (1 M H2SO4). Stability of at least 18 h was shown by chronoamperomety at 1.23 V versus RHE (reversible hydrogen electrode) under 38.6 mW/cm(2) simulated sunlight irradiation (λ > 635 nm, AM 1.5G) and measurements with quartz crystal microbalances. Films exceeding a thickness of 4 nm were shown to be highly active though metastable due to an amorphous character. By contrast, 2 nm IrOx films were stable, enabling OER at a current density of 1 mA/cm(2) at 1.05 V vs. RHE. Further improvement by heat treatment resulted in a cathodic shift of 40 mV and enabled a current density of 10 mA/cm(2) (requirements for a 10% efficient tandem device) at 1.12 V vs. RHS under irradiation. Thus, the simple IrOx/Ir/p(+)-n-Si structures not only provide the necessary overpotential for OER at realistic device current, but also harvest ∼100 mV of free energy (voltage) which makes them among the best-performing Si-based photoanodes in low-pH media.

Iron-Treated NiO as a Highly Transparent p-Type Protection Layer for Efficient Si-Based Photoanodes

Sputter deposition of 50 nm thick NiO films on p(+)-n-Si and subsequent treatment in an Fe-containing electrolyte yielded highly transparent photoanodes capable of water oxidation (OER) in alkaline media (1 M KOH) with high efficiency and stability. The Fe treatment of NiO thin films enabled Si-based photoanode assemblies to obtain a current density of 10 mA/cm(2) (requirement for >10% efficient devices) at 1.15 V versus RHE (reversible hydrogen electrode) under red-light (38.6 mW/cm(2)) irradiation. Thus, the photoanodes were harvesting ∼80 mV of free energy (voltage), which places them among the best-performing Si-based photoanodes in alkaline media. The stability was proven by chronoamperometry at 1.3 V versus RHE for 300 h. Furthermore, measurements with electrochemical quartz crystal microbalances coupled with ICP-MS showed minor corrosion under dark operation. Extrapolation of the corrosion rate showed stability for more than 2000 days of continuous operation. Therefore, protection by Fe-treated NiO films is a promising strategy to achieve highly efficient and stable photoanodes.

MoS2—an integrated protective and active layer on n+p-Si for solar H2 evolution

A new MoS2 protected n(+)p-junction Si photocathode for the renewable H2 evolution is presented here. MoS2 acts as both a protective and an electrocatalytic layer, allowing H2 evolution at 0 V vs. RHE for more than 5 days. Using a MoSx surface layer decreases the overpotential for H2 evolution by 200 mV.

Formation of a p–n heterojunction on GaP photocathodes for H2 production providing an open-circuit voltage of 710 mV

Photocatalytic water splitting for the sustainable production of hydrogen using a two-photon tandem device requires careful optimization of the semiconductors used as photon absorbers. In this work we show how the open-circuit voltage of photocathodes for the hydrogen evolution reaction based on p-GaP was increased considerably by sputtering of different n-type metal oxides on the surface and thereby forming an effective p–n heterojunction. Both n-TiO2 and n-Nb2O5 increased the VOC of the photocathodes, with the latter giving an ultimate VOC of 710 mV using Pt as the cocatalyst. This value is unprecedented for a p-GaP-based HER photocathode operating in an acidic electrolyte under simulated 1 Sun illumination. An additional, but highly significant benefit of a TiO2 layer is that it provides a remarkable operational stability of more than 24 h under constant operation. It was found that TiO2 and Nb2O5 overlayers, which were characterized by high donor density, caused a large built-in potential drop that is located almost exclusively in the p-type substrate. The large built-in potential drop in the GaP effectively separates charge carriers driving photogenerated electrons toward the surface of the electrode to perform the HER. According to this result, a further careful choice of materials having specific properties, such as optimal carrier concentration and band positions, could potentially increase VOC even more, paving the way for the realization of a non-assisted two-photon solar water splitting device.