NOVELTY - Preparing titania nanotubes (I), comprises: providing a titanium anode and an inert cathode, where the anode and the cathode in electrical contact through an electrolyte solution comprising at least 1 mM chloride and having a pH of 1-7; and establishing a voltage between the anode and the cathode to form (I) at the anode.    USE - (I) is useful in hydrogen sensor, hydrogen generator and solar cell (all claimed) for the production of hydrogen. (I) is useful as strengthening agent in composite materials and components of microfabricated device.    ADVANTAGE - The process provides: rapid production of (I) with diameter of 25 nm, high aspect ratio (at least 10); and highly organized arrays of titania nanotubes. (I) have a length of at least 1 mu m, preferably at least 50 mu m or 200 nm.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a process of adjusting the carbon content of (I) (0-40 atom% of carbon), comprising: preparing a first set of nanotubes, where the electrolyte solution comprises a mixture of an inorganic acid and an organic acid; determining the carbon content of the nanotubes; and preparing a second set of nanotubes with a different mixture of an inorganic acid and an organic acid, where increasing the proportion of the organic acid relative to the inorganic acid increases the carbon content of the second set of nanotubes, and decreasing the proportion of the organic acid relative to the inorganic acid decreases the carbon content of the second set of nanotubes;    (2) a process of pre-conditioning a titanium surface for the formation of (I) in a chloride-containing electrolyte solution comprising: providing the titanium anode and the inert cathode, where the anode and cathode in electrical contact through an electrolyte solution comprising 0.25-1 wt.% of hydrofluoric acid; and establishing the voltage of 10-20 V between the anode and the cathode for 5-30 minutes, where the titanium surface is pre-conditioned for the formation of titania nanotubes in a chloride-containing electrolyte solution;    (3) many titania nanotubes made by anodization of titanium in the electrolyte solution; many titania nanotubes comprising a mole fraction of at least 50% titania and having an aspect ratio of at least 10;    (4) a kit for preparing (I) and instructions for preparing (I) using a user supplied chloride-containing electrolyte solution, the titanium anode or cathode; and    (5) a solar cell, a hydrogen sensor and a hydrogen generator comprising an array of titania nanotubes fabricated by the process.