In dye-sensitized solar cells, highly ordered TiO2 nanotube arrays provide superior electron transport. However, their low surface area limits the amount of dye loading and thus the photocurrent. In the present work, a hydrothermal treatment of the TiO2 nanotubes was carried out to form TiO2 nanoparticles on the tube walls, thereby increasing the surface area for a higher amount of dye loading. The nanotube arrays were prepared by electrochemical anodization and subsequently hydrothermally treated in water at 90 °C. Using the same nanotube length (i.e., 6.5 μm), but different treatment durations, it was found that nanotubes under hydrothermal treatment for 45 min yielded the best photovoltaic performance, due to the combined merits of a high surface area and vectorial electron transport. Under the same treatment duration (i.e., 45 min), but using different nanotube lengths, nanoparticle formation was found to be accelerated in the longer tubes. The parts of the tubes near the bottom were constantly filled with nanoparticles, which limited cell efficiency to about 2.2% when the length was over 16.5 μm. Accordingly, a further efficiency enhancement of up to 3.5% was achieved with tubes of 16.5 μm by adjusting the duration of the hydrothermal treatment.